Stationary Phases in Gas Chromatography

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JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 48

Stationary Phases in Gas Chromatography

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JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 48

Stationary Phases in Gas Chromatography Harald Rotzsche HULS Aktiengesellschaft, Zentralbereich FE, Marl, F. R. G.

ELSEVIER Amsterdam - Oxford - New York

- Tokyo

1991

This book is distributed in all non-socialist countries by ELSEVIER SCIENCE PUBLISHERS B.V. Sara Burgerhartstraat 25 P. 0. Box 211, loo0 AE Amsterdam, The Netherlands

Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLiSHlNG COMPANY INC. 655 Avenue of the Americas New York, NY 10010, U.S.A.

Library of Congress Cataloging-In-Publlcatlon Data Rotzsche, Harald. Stationary phases in gas chromatography / Harald Rotzsche. 424 p. 16,5 x 24,O cm. chromatography library ; v. 48) Includes bibliographical references and index. ISBN 0-444-98733-9 1. Gas chromatography. 2. Stationary phase (Chromatography) I. Title. II. Series. QD79.C45R673 1990 543’.0896-d~20

- (Journal of

90-3962 CiP

ISBN 0-444-98733-9(Vol. 48) ISBN 0-444-41616-1(Series)

@ Akademische Verlagsgesellschaft Geest Cr Portig K.-G., Leipzig, 1991 Licensed edition for Elsevier Science Publishers B.V., 1991 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the copyright owner. Printed in Germany

Contents

. . . . . . . . . . . . . . . . . . . . . . . .

1

. . . . . . . . . . . . . . . . . . . . . . .

2

1.

Introduction

2.

Basic Concepts

2.1.

Basic Components of a Gas Chromatographic System

2.2. 2.3. 2.3.1. 2.3.2. 2.3.3. 2.3.4. 2.3.5. 2.4. 2.5. 2.6. 2.7. 2.8.

Raw Data Measured from the Chromatogram Derived Basic Chromatographic Parameters . . . . . . . . . . . Retention Volume Terms . . . . . . . . . . . . . . . . . . Distribution Terms . . . . . . . . . . . . . . . . . . . . Temperature Dependence of Distribution and Retention Terms . . . . Parameters Characterizing the Emciency of Columns . . . . . . . Parameters Characterizing the Separation . . . . . . . . . . . Flow of Gases in a Gas Chromatographic Column and Formation of Bands Thermodynamic Bases of Gas Chromatography . . . . . . . . . The Quality of Chromatographic Separation . . . . . . . . . . . The Time of Analysis . . . . . . . . . . . . . . . . . . . Definition of Symbols Used (Table 1) and List of Essential Relationships (Table 2) . . . . . . . . . . . . . . . . . .

3. 3.1. 3.1.1. 3.1.2. 3.1.3. 3.1.4. 3.1.5. 3.1.6. 3.2. 3.3. 3.3.1. 3.3.2. 3.3.3. 3.3.3.1.

. . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . Packed Columns . . . . . . . . . . . . . . . Column Materials . . . . . . . . . . . . . . Column Dimensions . . . . . . . . . . . . . Preparing of the Packing and Packing Procedures . . . .

The Chromatographic Column

Column Conditioning . . . . . . . . . . . . Column Testing . . . . . . . . . . . . . . Pre-columns . . . . . . . . . . . . . . . Micro-Packed Columns . . . . . . . . . . . Open-Tubular Columns . . . . : . . . . . . Tube Material . . . . . . . . . . . . . . Column Dimensions . . . . . . . . . . . . Wall-Coated Open-Tubular (WCOT) Columns . . . . Properties and Pre-treatment of the Inner Tube Surfaces and the Formation of Films . . . . . . . . . . 3.3.3.2. Coating Procedures . . . . . . . . . . . . . 3.3.3.3. Bonding and Cross-Linking of Stationary Phases . . . 3.3.3.4. FilmThicknessoftheStationaryPhase . . . . . . 3.3.3.5. Quality Tests of WCOT Columns . . . . . . . . 3.3.4. Porous-Layer Open-Tubular (PLOT) and Support-Coated Open-Tubular (SCOT) Columns . . . . . . . . 3.3.4.1. PLOT Columns (strictly) . . . . . . . . . . . 3.3.4.2. SCOT Columns . . . . . . . . . . . . . . 3.3.4.3. Thick-Layer Columns . . . . . . . . . . . . Properties and Comparison of the Main Column Types . 3.4. 3.4.1. Packed Columns . . . . . . . . . . . . . . 3.4.2. Micropacked Columns . . . . . . . . . . . . 3.4.3. Support-coatedopen-tubularcolumns . . . . . . 3.4.4. Wall-coated Open-tubular Columns . . . . . . .

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3 4 4 7 9 10 15 17 21 28 33

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35

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2

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43 43 43 45 47 51 52 53 53 54 55 56 57

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57 63 64 71 73

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75 76 76 77 77 78 78 79 79

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VI 4. 4.1. 4.1.1. 4.1.2. 4.1.3. 4.1.4. 4.1.5. 4.2. 4.2.1. 4.2.2. 4.2.3. 4.2.4. 4.2.5. 4.2.6. 4.2.1. 4.2.8. 4.2.9. 5. 5.1. 5.1.1. 5.1.2. 5.5. 5.2.1. 5.2.2. 5.2.3. 5.3. 5.4. 5.4.1. 5.4.2. 5.4.3. 5.5. 5.5.1. 5.5.2. 5.6. 5.6.1. 5.6.2. 5.6.3. 5.6.4. 5.6.5. 5.6.6. 5.6.7. 5.7. 5.7.1. 5.1.2. 6

.

6.1. 6.2. 6.2.1. 6.2.2. 6.2.3.

Contents

. . . . . . Intermolecular Forces . . . . . . . . . . . . London-type Dispersion Forces . . . . . . . . Orientation Forces and Hydrogen Bridge Bonds . . Induction Forces . . . . . . . . . . . . . Donor-Acceptor Interactions . . . . . . . . . Further Intermolecular Forces . . . . . . . . Quantities for the Description of Interactions . . . Characterization of Stationary Phases

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.

80

FurtherQuantitiesfortheDescriptionoftheInteractions . . . . . . . . .

80 80 81 83 83 83 84 84 84 88 89 94 98 99 100 101

Solid Stationary Phases (In part together with W.Engewald and J.P6rschmann)

103

Selectivity Coefficient . . . . . . . . . . . . . . . . . . . . . Retention Index . . . . . . . . . . . . . . . . . . . . . . . Polarity . . . . . . . . . . . . . . . . . . . . . . . . . . Rohrschneider and McReynolds Constants . . . . . . . . . . . . . Thermodynamic Criteria of Polarity . . . . . . . . . . . . . . . The Molecular Retention Index . . . . . . . . . . . . . . . . . The Selectivity Index . . . . . . . . . . . . . . . . . . . . . The Infrared Spectroscopic Frequency Shift . . . . . . . . . . . . .

. . . . . . . .

. Classification of Adsorbents . . . . . . . . . . Classification AccordingtoChemicalStructure . . . Classification According to Geometrical Structure . . Carbon Adsorbents . . . . . . . . . . . . . Graphitized Thermal Carbon Black (GTCB) . . . . . Activated Charcoal . . . . . . . . . . . . . Carbon Molecular Sieve . . . . . . . . . . . BoronNitrideandMolybdenumDisulphide . . . . . . . . Adsorbents with Hydroxylated and Dehydroxylated Surfaces . . . Silica Gels . . . . . . . . . . . . . . . . . . . . Aluminium Oxide . . . . . . . . . . . . . . . . . Porous Glasses . . . . . . . . . . . . . . . . . . Porous Organic Polymers . . . . . . . . . . . . . . . Porous Polyaromatic Beads without Additional Functional Groups . Polar Porous Polymers . . . . . . . . . . . . . . . . Substances Forming Inclusion Compounds . . . . . . . . . Zeolitic Molecular Sieves . . . . . . . . . . . . . . . Bentonites . . . . . . . . . . . . . . . . . . . . Tri-o-thimotide . . . . . . . . . . . . . . . . . . Desoxycholic Acid . . . . . . . . . . . . . . . . . Werner Complexes . . . . . . . . . . . . . . . . . Benzenesulphonates . . . . . . . . . . . . . . . . Urea and Thiourea . . . . . . . . . . . . . . . . . Modified Adsorbents . . . . . . . . . . . . . . . . Modification with High-boiling Liquid and Solid Substances . . . Modification with Non-porous Ionic Adsorbents . . . . . . .

. . . . . . Adsorbents for Bonding Reactions . . . . . . . . Bonding Reactions . . . . . . . . . . . . . . Chemically Bonded Phases with S i - 0 - C Bonds . . . Chemically Bonded Phases with Si-CH2RBonds . . . Chemically Bonded Stationary Phases

Chemically Bonded Phases with Si-0-Si

Bonds

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103 104 106 108 108 113 113 114 114 114 119 120 120 123 128 131 131 134 136 136 136 137 137 137 138 140 142 143 144 144 147 148

VII

Contents 6.3. 6.4.

Properties and Characterization of Chemically Bonded Phases Outlook and Prospects for Chemically Bonded Phases . .

. . . . . . . . . . . . . . . .

7.

The Solid Support

7.1. 7.2. 7.3. 1.4. 7.4.1. 1.4.2. 1.4.3. 7.4.4. 1.4.5. 7.5. 1.6. 1.1. 7.8. 1.9.

The Particle Size and Shape . . . . . . . . . The Surface Area . . . . . . . . . . . . Activity of the Original and of the Coated Solid Support Diatomite Supports . . . . . . . . . . . . Acid washing . . . . . . . . . . . . . . Alkali treatment . . . . . . . . . . . . . Silylation . . . . . . . . . . . . . . . Coating of the Support with a Non-extractable Layer . Deactivation of Diatomite by Deposited Pyrocarbon . Synthetic Silica-based Supports (Volaspher and Quartz) Silica Gel . . . . . . . . . . . . . . . Micro Glass Beads and Porous Layer Beads . . . . Fluorocarbon Supports . . . . . . . . . . Other Support Materials . . . . . . . . . .

8.

Liquid Stationary Phases

8.1. 8.1.1. 8.1.2. 8.1.3. 8.1.4. 8.1.5. 8.1.6. 8.1.7. 8.1.8. 8.1.9. 8.2. 8.2.1. 8.2.2. 8.3. 8.3.1. 8.3.2. 8.3.3. 8.3.4. 8.3.5. 8.3.6. 8.3.7. 8.3.8. 8.3.9. 8.3.10. 8.3.11. 8.3.12. 8.3.13. 8.4. 8.4.1. 8.4.2. 8.4.3. 8.4.4. 8.4.4.1. 8.4.4.2. 8.4.4.3.

General Properties of Liquid Stationary Phases . . . . . . . . . . . . Chemical Inertness . . . . . . . . . . . . . . . . . . . . . . Vapour Pressure. Thermal Stability and Maximum Operation Temperature . . Molecular Weight . . . . . . . . . . . . . . . . . . . . . . Viscosity and Minimum Operating Temperatures . . . . . . . . . . . Film Formation . . . . . . . . . . . . . . . . . . . . . . . Solubilizing Power . . . . . . . . . . . . . . . . . . . . . . Purity and Homogeneity . . . . . . . . . . . . . . . . . . . . Selectivity . . . . . . . . . . . . . . . . . . . . . . . . . Specifications of Stationary Phases . . . . . . . . . . . . . . . Hydrocarbons . . . . . . . . . . . . . . . . . . . . . . . . Aliphatic Hydrocarbons . . . . . . . . . . . . . . . . . . . . Aromatic Hydrocarbons . . . . . . . . . . . . . . . . . . . . Silicones . . . . . . . . . . . . . . . . . . . . . . . . . Poly(dimethylsi1oxanes) . . . . . . . . . . . . . . . . . . . . Poly(dimethylsi1oxanes)containing Small Amounts of Vinyl Groups . . . . Poly(dialkylsi1oxanes) . . . . . . . . . . . . . . . . . . . . .

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. . . . Poly(chloroary1siloxanes)and Poly(chloralkylsi1oxanes) . Polyester Silicones . . . . . . . . . . . . . . Chiral Polysiloxanes . . . . . . . . . . . . . Mesogenic Polysiloxanes . . . . . . . . . . . .

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.

.

.

Aminoalkyl-Substituted Polysiloxanes . . . . Other Siloxane Stationary Phases . . . . . . Alcohols. Ethers and Carbohydrates . . . . . Monohydric Alcohols . . . . . . . . . . . Polyhydric Alcohols . . . . . . . . . . . Saccharides . . . . . . . . . . . . . . Polyglycols and Poly(alky1ene oxides) . . . . . Poly(ethy1eneoxides) [Poly(ethyleneglycols)] . . Other Poly(alky1ene oxides) . . . . . . . . Poly(oxyalky1ene) Derivatives . . . . . . .

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160 160 161 164 167 171 172 172 177 177 171 180 181 182 184 186

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. . . . . . . . . . . . . . . . . . . Poly(methyltrifluoropropylsi1oxanes) . . . . . Cyanoalkyl-Substituted Polysiloxanes . . . . . Poly(methylphenylsi1oxanes)

Carborane Siloxanes

155 158

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. .

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186 186 186 188 189 190 193 194 195 195 196 196 200 201 207 214 215 216 224 227 229 233 236 236 240 242 242 246 246 246 248 249 251 255 256

VIII 8.4.5. 8.5. 8.5.1. 8.5.2. 8.5.3. 8.5.4. 8.5.5. 8.6. 8.7. 8.8. 8.8.1. 8.8.2. 8.9. 8.10. 8.11. 8.12. 8.13. 8.14. 8.14.1. 8.14.2. 8.14.3. 8.14.4. 8.14.5. 8.15. 8.15.1. 8.15.2. 8.15.3. 8.15.4. 8.15.5. 8.16. 8.17.

Contents Aromatic Ethers . . . . . . . . . . . . . . . . . . . Esters . . . . . . . . . . . . . . . . . . . . . . . Esters of Dicarboxylic Acids (Phthalates. Adipates and Sebacates) . . Phosphates . . . . . . . . . . . . . . . . . . . . Stearates. Oleates . . . . . . . . . . . . . . . . . . Acetates. Citrates . . . . . . . . . . . . . . . . . . Polyesters . . . . . . . . . . . . . . . . . . . . . Nitriles and Nitrile Ethers . . . . . . . . . . . . . . . Nitro Compounds . . . . . . . . . . . . . . . . . . Amines . . . . . . . . . . . . . . . . . . . . . . Aliphatic Amines and Imines . . . . . . . . . . . . . . Aromatic Amines . . . . . . . . . . . . . . . . . . Amides . . . . . . . . . . . . . . . . . . . . . . Heterocyclics . . . . . . . . . . . . . . . . . . . . Sulphur Compounds . . . . . . . . . . . . . . . . . Fluorine Compounds . . . . . . . . . . . . . . . . . Fatty Acids and their Salts . . . . . . . . . . . . . . . salts . . . . . . . . . . . . . . . . . . . . . . . Silver Nitrate . . . . . . . . . . . . . . . . . . . . Dissolved Alkali Metal Halides . . . . . . . . . . . . . . Eutectics . . . . . . . . . . . . . . . . . . . . . Molten Organic Salts . . . . . . . . . . . . . . . . . Complexes of Some Transition Metals with N-Duodecylsalicylaldimine and Methyl n-octylglyoxime . . . . . . . . . . . . . . . Chiral Stationary Phases . . . . . . . . . . . . . . . . Peptide Stationary Phases . . . . . . . . . . . . . . . . Diamide Stationary Phases . . . . . . . . . . . . . . . Ureide Stationary Phases . . . . . . . . . . . . . . . . Metal Complexes . . . . . . . . . . . . . . . . . . Recommendable Chiral Stationary Phases and Some Applications . . Liquid Crystals . . . . . . . . . . . . . . . . . . . Mixed Stationary Phases . . . . . . . . . . . . . . . .

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259 261 262 265 266 261 261 210 214 215 215 211 211 218 219 280 282 283 283 283 284 285 286 286 281 288 288 289 289 291 296

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302

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302

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303 308 311

9.

Selection of Stationary Phases

9.1. 9.2.

General Recommendations for Choosing a Suitable Stationary Phase Choosing Stationary Phases for Special Separation Problems with Regard to the Desired Selectivity . . . . . . . . . . . Preferred Stationary Phases . . . . . . . . . . . . . . Approaches to Stationary Phase Selection . . . . . . . . .

9.3. 9.4.

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352

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319

General Subject Index

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393

Stationary Phase Index

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401

Literature Author Index

JOURNAL OF CHROMATOGRAPHY LIBRARY A Series of Books Devoted to Chromatographic and Electrophoretic Techniques and their Applications Although complementary to the Journal of Chromatography, each volume in the Library Series is an important and independent contribution in the field of chromatography and electrophoresis. The Library contains no material reprinted from the journal itself.

Other volumes in this series Volume 1 Volume 2 Volume 3 Volume 4 Volume 5 Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Volume 11 Volume 12 Volume 13 Volume 14 Volume 15 Volume 16 Volume 17

Chromatography of Antibiotics (see also Volume 26) by G.H. Wagman and M.J. Weinstein Extraction Chromatography edited by T. Braun and G. Ghersini Liquid Column Chromatography. A Survey of Modern Techniques and Applications edited by Z. Deyl, K. Macek and J. Janhk Detectors in Gas Chromatography by J. SevEik Instrumental Liquid Chromatography. A Practical Manual on High-Performance Liquid Chromatographic Methods (see also Volume 27) by N.A. Paffls Isotachophoresis. Theory, Instrumentation and Applications by F.M. Everaerts, J.L. Beckers and Th.P.E.M. Verheggen Chemical Derivatization in Liquid Chromatography by J.F. Lawrence and R.W. Frei Chromatography of Steroids by E. Heftmann HPTLC- High Performance Thin-Layer Chromatography edited by A. Zlatkis and R.E. Kaiser Gas Chromatography of Polymers by V.G. Berezkin, V.R. Alishoyev and I.B. Nemirovskaya Liquid Chromatography Detectors (see also Volume 33) by R. P. W. Scott Affinity Chromatography by J. Turkovh Instrumentation for High-Performance Liquid Chromatography edited by J.F.K. Huber Radiochromatography. The Chromatography and Electrophoresis of Radiolabelled Compounds by T. R.Roberts Antibiotics. Isolation, Separation and Purification edited by M.J. Weinstein and G.H. Wagman Porous Silica. Its Properties and Use as Support in Column Liquid Chromatography by K. K. Unger 75 Years of Chromatography-A Historical Dialogue edited by L. S. Ettre and A. Zlatkis

X

Journal of Chromatography Library

Volume 18A Electrophoresis. A Survey of Techniques and Applications. Part A: Techniques edited by Z. Deyl Volume 18B Electrophoresis. A Survey of Techniques and Applications. Part B: Applications edited by Z. Deyl Volume 19 Chemical Derivatization in Gas Chromatography by J. Drozd Volume 20 Electron Capture. Theory and Practice in Chromatography edited by A. Zlatkis and C. F. Poole Volume 21 Environmental Problem Solving using Gas and Liquid Chromatography by R.L. Grob and M.A. Kaiser Volume 22A chromatography. Fundamentals and Applications of Chromatographic and Electrophoretic Methods. Part A Fundamentals edited by E.Heftmann Volume 22B Chromatography. Fundamentals and Applications of Chromatographic and Electrophoretic Methods. Part B: Applications edited by E. Heftmann Volume 23A Chromatography of Alkaloids. Part A: Thin-Layer Chromatography by A. Baerheim Svendsen and R. Verpoorte Volume 23B Chromatography of Alkaloids. Part B: Gas-Liquid Chromatography and High-Performance Liquid Chromatography by R. Verpoorte and A. Baerheim Svendsen Volume 24 Chemical Methods in Gas Chromatography by V.G. Berezkin Volume 25 Modem Liquid Chromatography of Macromolecules by B.G. Belenkii and L.Z. Vilenchik Volume 26 Chromatography of Antibiotics Second, Completely Revised Edition by G.H. Wagman and M.J. Weinstein Volume 27 Instrumental Liquid Chromatography. A Practical Manual on High-Performance Liquid Chromatographic Methods Second, Completely Revised Edition by N.A. Parris Volume 28 Microcolumn High-Performance Liquid Chromatography by P. Kucera Volume 29 Quantitative Column Liquid Chromatography. A Survey of Chemometric Methods by S.T.Balke Volume 30 Microcolumn Separations. Columns, Instrumentation and Ancillary Techniques edited by M.V. Novotny and D. Ishii Volume 31 Gradient Elution in Column Liquid Chromatography. Theory and Practice by P.Jandera and J. ChurlEek

Journal of Chromatography Library

Volume 32 Volume 33

Volume 34 Volume 35

Volume 36 Volume 37

Volume 38

XI

The Science of Chromatography. Lectures Presented at the A. J. P. Martin Honorary Symposium, Urbino, May 27-31,1985 edited by F. Bruner Liquid Chromatography Detectors. Second, Completely Revised Edition by R.P.W. Scott Polymer Characterization by Liquid Chromatography by G. Glockner Optimization of Chromatographic Selectivity. A Guide to Method Development by P. J. Schoenmakers Selective Gas Chromatographic Detectors by M. Dressler Chromatography of Lipids in Biomedical Research and Clinical Diagnosis edited by A. Kuksis Preparative Liquid Chromatography edited by B.A. Bidlingmeyer

Volume 39A Selective Sample Handling and Detection in High-Performance Liquid Chromatography. Part A by R. W. Frei and K. Zech Volume 39B Selective Sample Handling and Detection in High-Performance Liquid Chromatography. Part B by K. Zech and R.W. Frei Volume 40

Aqueous Size-Exclusion Chromatography by P. L. Dubin

Volume 41A High-Performance Liquid Chromatography of Biopolymers and Biooligomers. Part A: hinciples, Materials and Techniques by 0. Mikes Volume 42

Quantitative Gas Chromatography for Laboratory Analyses and On-line Process Control by G. Guiochon and C.L. Guillemin

Volume 43

Natural Products Isolation. Separation Methods for Antimicrobials, Antivirals and Enzyme Inhibitors edited by G.H. Wagman and R. Cooper

Volume 44

Analytical Artifacts. GC, MS, HPLC, TLC and PC by B. S. Middleditch

Volume 45A Chromatography and Modification of Nucleosides Analytical Methods for Major and Modified Nucleosides HPLC, GC, MS, NMR, UV and m - I R edited by C. W. Gehrke and K. C. T.Kuo Volume 45B Chromatography and Modification of Nucleosides Biological Roles and Function of Modification edited by C.W. Gehrke and K.C.T. Kuo

XI1

Journal of Chromatography Library

Volume 45C Chromatography and Modification of Nucleosides Modified Nucleosides in Cancer and Mormal Metabolism Methods and Applications edited by C.W.Gehrke and K.C.T.Kuo Volume 45D Chromatography and Modification of Nucleosides Comprehensive Database for RNZ and DNA Nucleosides Chemical, Biochemical, Physical, Spectral and Sequence edited by C.W.Gehrke and K.C.T.Kuo Volume 46

Ion Chromatography: Principles and Application by P.R. Haddad and P.E. Jackson

Volume 47

Trace Metal Analysis and Speciation edited by I.S.Krull

Volume 48

Stationary Phases in Gas Chromatography by H. Rotzsche

Volume 49

Gas Chromatography in Air Pollution Analysis by V.G. Berezkin and Yu. S. Drugov

Every chemist is regularly confronted with samples of which the composition, purity, or concentration of by-products or impurities should be determined, and he/she will have to find the most appropriate analytical method. For volatile or vaporizable constituents of the sample, gas chromatography will often be the method of choice. However, like with any other physical method, reliable results can only be obtained when sufficient knowledge of the potential and limitations of the technique is available. The analytical result is influenced primarily by the “heart of the gas chromatograph”, i.e., by the separation column and the stationary phase, and the analyst has to decide carefully which column type and stationary phase to select for the problem at hand. One of the aims of this book is to make the chemist familiar with the numerous stationary phases and column types, with their advantages and disadvantages, to help him/her select the most suitable phase for the type of analytes, and to give him/her detailed information on the chemical structure, physico-chemical behaviour, experimental applicability, physical data of liquid and solid stationary phases and solid supports, which otherwise could only be collected with difficulty from various treatises, handbooks, prospectuses and scientific papers. To understand the processes occurring in the separation column and to offer a manual both to the beginner (for whom a synopsis on practical and theoretical principles could be of help) and to the seasoned chromatographer (for whom it is convenient to have basic equations and physical correlations at hand), one chapter in this book is devoted to basic theoretical aspects. Further, as the effectiveness of the stationary phase can only be considered in relation to the column type, a chapter on different column types and the arrangement of the stationary phase within the column is included. Another aim of this book is to stimulate the development of new and improved standardized stationary phases and columns, in order to improve the reproducibility of separations, as well as the range of applications. This book is also intended for physical chemists, who do not normally deal with chromatography. They can find suggestions for and references to physico-chemical investigations by gas chromatography, particularly in the field of intermolecular interactions and surface studies. It is always risky to write a monograph on a rapidly expanding field-one only needs to think of the development of fused-silica columns and of cross-linked stationary phases and the rapidly growing amount of relevant literature during the last years. Therefore, I hesitated for quite some time to follow the suggestion made some 10 years ago by Professor J. F.K.Huber of the University of Vienna, whom I thank kindly for his proposal and confidence, to write a book on stationary phases in gas chromatography. On the other hand, this task seemed to be attractive, because in our institute and in our chemical plants we have had to deal with problems of stationary phases since our first gas chromatographic separations in 1957, as the samples to be analysed often comprised low- and very-high-boiling, reactive, even offensive, and/or instable compounds. Therefore, we had to develop stationary phases which would withstand such heavy strains and hence we gained quite a lot of experience in synthesizing, evaluating and applying stationary phases. This book is not intended to be a review but rather a working instrument. The literature has been critically evaluated and cited only to a limited extent (which is nevertheless quite comprehensive).

XIV

Preface

I tried to adhere to a systematic representation and nomenclature as far as possible; however, some deviation is intended to serve a better understanding. The order in which the stationary liquid phases are presented may appear to be arbitrary. Except for the hydrocarbons, which are discussed first because of their use as reference liquid phases, the order only indicates to some degree the importance and frequency of application of the phases. Many of the product names, registered designs and patents referred to in this book are in fact registered trademarks or proprietary names even though specific reference to this fact is not made in the text. Therefore, the appearence of a product name, to simplify matters usually without designation as proprietary, is not to be interpreted as a representation by the publisher and the author that it is in the public domain. Any book devoted to a specific scientific area is based on the theoretical and experimental efforts of not only the author, but also his teachers, colleagues and co-workers. I shall always gratefully remember Professor Richard Miiller and the late Professors K. Schwabe and A. Y. Kiseleu for their scientific style which I had the opportunity and pleasure to get acquainted with during the years of collaboration and which had a great influence on so many chemists. It is a pleasure for me to acknowledge the contributions of and helpful discussions with Professor W.Engewald, Dr.J.Porschmann and Dr. D. Glindemann. With gratitude I acknowledge the experimental assistance, in some cases lasting for decades, by my co-workers and colleagues in- and outside of my former department. I only mention here H. Hahnewald. W.Schwenke, J. Grasshog Dr. K. -D. Miiller, S. Langer, Dr. J. Schlapa and H. Steimann. In addition, the service of the technical information staff, especially of the scientific libraries of the Chemical Works Nunchritz and Pharmaceutical Works Dresden, and the diligent typing of the manuscript by the two secretaries, Mrs. K. Weisse and Mrs. K. Kurowski, is gratefully acknowledged. My particular appreciation is due to the publishers for their patience, for the linguistic revision and for the really pleasant cooperation. I wish to thank Huls AG for their support during the final phase of completing this book and Chemical Works Nunchritz for their consent to the publication. Finally, I would like to express particular thanks to my wife, who by her undertaking of many duties and by her patience and understanding made it possible to write this book. Essen, October 1989 Troisdorf, March 1991

HARALD ROTZSCHE

1.

Introduction

Hardly another field in analytical chemistry has devoloped during the past 30 years as rapidly as chromatography. The success of this method is due to its extraordinary performance with regard to separation efficiency and speed. Chromatography enables the analytical chemist to determine the composition of complicated mixtures consisting of individual compounds. At first sight, the principle is relatively simple. In a chromatographic column or bed, repeated transitions of the constituents to be separated take place between two phases, one of which is stationary, has a large surface and retains the compounds more or less, and the other is mobile and carries the compounds along the stationary phase. Provided that the kind or degree of interactions between the individual constituents and an appropriate stationary phase are different, they undergo different retardations in their transport with the mobile phase in a sequence of dynamic solution or/and adsorption equilibria. Although the entire chromatographic process involves sorption equilibration in addition to convective transport and diffusion, the separation is essentially determined by the first of these three terms, resulting in differences in the distribution ratios between both the phases. In gas chromatography, the mobile phase is a gas, thus having a high diffusion coefficient, and the interaction between molecules and the influence of the nature of usually applied gases, if any, on the distribution ratio is very low. This is the reason why selecting the most appropriate stationary phase is especially important in gas chromatography, whereas in liquid chromatography the distribution ratio, i.e., the separation, can be influenced by both the phases. Here the analytical chemist has a further variance: he can select a suitable stationary phase as well as an appropriate mobile phase, which may be varied between non-polar and strongly polar liquids and liquid mixtures. If we return to gas chromatography, the separation occurs in a column, and the sample components must be sufficiently volatile at the column temperature in order to be transported by the carrier gas, i.e., by the mobile phase. Owing to the retardation by the stationary phase, the individual components move with different velocities through the column, and separate bands develop, which are detected, characterized and measured at the column outlet. Depending on the type of column (packed column, packed capillary or microcapillary column, wall-coated open-tubular column) the stationary phase can be an adsorbent, a solid support impregnated with a liquid phase, a thin film of a liquid phase distributed on the inside wall of a capillary tube or a porous layer on the inside wall of the open tube, coated with a liquid phase. As the separability of mixtures is essentially controlled by sorption and desorption processes of its constituents in the interface of the mobile and stationary phases, numerous conditions, in addition to the temperature and the carrier gas flow-rate, have to be considered and optimized, e.g., the particle size and its distribution, the pore size and poresize distribution of the solid support, the type of adsorbent and liquid stationary phase, viscosity, film thickness and interaction forces of the liquid phase. Such practical aspects of stationary phases in gas chromatography will be emphasized in detail in this book.

2.

Basic Concepts

The reader of this book is assumed to be familiar with gas chromatography. Nevertheless, some basic principles, both theoretical and practical, are outlined in t h i s chapter and are intended to provide sufficient help for the beginner and to constitute a useful review for the advanced gas chromatographer. On the other hand, they are not considered to replace a gas chromatographic textbook, and much less are they to enable an excursion into complex gas chromatographic systems or theories. It seems useful, to start with a general definition of the topic. Beginning with chromatography as the general term, one can define it as “the mass transport through a two-phase system being selective for mass transfer. The mass transport is effected by the relative movement of the phases, one of which is compact and the other fluid.” [l]. Gas chromatography, as a special kind of chromatography, can be characterized as follows: “Gas chromatography, abbreviated to GC, comprises all chromatographic methods in which the moving phase is gaseous. The other phase is stationary and may be either a dry, granular solid or a liquid supported by the granules or by the wall of the column, or both. Separation is achieved by differences in the distribution of the components of a sample between the mobile and stationary phases, causing them to move through the column at different rates and from it at different times.”

PI.

The following sections of this chapter go into detail and contain the most important relationships applicable in gas chromatography.

2.1.

Basic Components of a Gas Chromatographic System

Most gas chromatographers have applied the elution method because of its advantages over other gas chromatographic techniques (e.g., frontal analysis, displacement analysis, vacant0 chromatography). Because of their minor importance, these special techniques are not discussed here. The elution technique, introduced into gas chromatography by Cremer [3], is characterized by a continuous flow of the mobile phase through the column, in which the separation of the components of a sample takes place. The individual constituents, together with the mobile phase, leave the column at different times and are detected by a sensitive detection system, which records their concentration in the mobile phase as a function of time, c = f ( t ) , elapsed between sample injection and emergence at the end of the column. The time corresponding to the concentration maximum of a component is its retention time. A diagram of a gas chromatograph is shown in Fig. 1. The essential part of any gas chromatograph is the column (C),which contains the stationary phase. The carrier gas (GIrepresents the mobile phase and is generally delivered from a gas cylinder containing the compressed gas. Constancy of the flow-rate is achieved by a flow controller (F),and the flowrate is measured by a flow meter. An injection system (I)preceding the column permits the sample to be directly introduced and vaporized in the gas stream. The column is followed by a detector ( D ) , in which the separated sample constituents produce electrical analogue signals. After passing an amplifier, in the case of ionization detectors, these signals are displayed on a strip-chart recorder (R),resulting in chromatograms showing individual peaks if the separation was successful. In modem and more expansive apparatus, the detectorlamplifier is connected via an analog-digital converter to a more or less sophisticated data

3

2.2. Raw Data Measured from the Chromatogram

system (9,processing two types of data: the already mentioned retention times, which are used for component identification, and the signal intensities (connected with the amounts of the sample constituents passing the detector), which are utilized, after detailed calibrations, for the determination of the concentrations of the components present in the original sample. Devices for temperature control, frequently with a temperature programmer, and measurement are symbolized by (0). R

r

Fig. 1. Diagram of gas chromatograph. G compressed gas cylinder containing the carrier gas; CG carrier gas cleanup; F flow controller, including flow meter; I injection system; C column; D detector, including supply and amplifrer; R recorder; S data system, including interface, integrator and computer; 0 column oven, including temperature control, measure and programmer

Raw Data Measured from the Chromatogram

2.2.

Although rather simplistic, it may be useful to specify the raw experimental data resulting directly from the gas chromatogram. Fig. 2 shows a one-component chromatogram (apart from the “air peak”) displayed on a common recorder, indicating a compound which was moni-

A

( h 4J L

0 +

Ln

0

Fig. 2. Gas chromatrogram showing the raw gas chromatographic data. t k = tR - tM adjusted retention time Symbol Description 0, start injection point, zero, origin, start y peak width at the inflection points tM mobile time wl peak width at half height tR retention time ?vb peak width at base

4

2. Basic Concepts

tored, after having passed the column, in the mobile phase by the detector. The first parameter usually measured from the chromatogram is the retention time, tR, which represents the time period between the instant of sample injection at (I) (Fig. 1) or Start (Fig. 2) and the emergence of the peak maximum (Fig.2) of a compound retained by the stationary phase. In addition to retention time, sometimes other terms are used for rR, e.g., total retention time of a solute and gross retention time. Unretained compounds, i.e., those not solved or adsorbed by the stationary phase, also need time to pass from (I)to (D). The same transit time through the column and the equipment is required by the mobile phase and it is therefore called the “mobile time“, tM. Other frequently used terms are gas hold-up time and air peak time (as air often represents an unretained substance). Obviously, the part of the retention time due to the residence of the retained compound in the stationary phase is

r;

= t,

- tM

(1) or, in other words, tk represents that portion of time resulting from interactions of the molecules of this compound with the stationary phase, and is characteristic of this compound. Thus, tk is the basis of almost all gas chromatographic calculations (except for the plate number). tk is called the “adjusted retention time”. As other expressions occasionally applied to t k , such as corrected retention time, net retention time and reduced retention time, could be misinterpreted and would confuse the gas chromatographer, only the terms tR = retention time, tM = mobile time, and t k = t R - tM = adjusted retention time

are used in this book. These retention times either can be measured on the recording in units of length and converted into minutes and seconds or the times themselves can be determined directly if a data system is available. The mobile time, being an important value, can be timed in the same way, taking the air peak as the basis, if the stationary phase does not retain air (this generally applies to liquid phases above or near room temperature but not necessarily to solid stationary phases) and if an appropriate detector, e.g., a thermal conductivity detector, is used. With a flame ionization detector methane is often used instead of air. One has to be aware, however, that the measured values are not correct at column temperatures below 50°C.Another possibility is to calculate the mobile time tM [4, 51. Further raw data obtainable from the chromatogram are the peak height, i.e. the distance between the baseline and the maximum peak height, and the peak width at various proportions of the maximum peak height. Three characteristic widths should to be mentioned here (Fig. 2): y = peak width at the inflection points, w, = peak width at half-height, we =peak width at the base (distance between the points of intersection on the baseline by the tangents to the curve through the inflection points).

2.3.

Derived Basic Chromatographic Parameters

2.3.1. Retention Volume Terms Retention Volume, V, The volume of carrier gas flowing through the column during the retention time, tained compound is

zR, of a re-

2.3. Derived Basic Chromatographic Parameters

5

VR = f R F c . (2) F,[mllmin] =volumetric flow rate at the column temperature T, [K] and the column outlet pressure po [MPa]. When measuring the flow rate (e.g. by means of a soap bubble flow meter, determination of the stream of carrier gas volume passing per time unit, stagnation dynamic pressure meter) we do not obtain F, for the column temperature, but Fa at ambient temperature at the column outlet. F. should therefore be corrected for at least the column temperature and, if water is present in the flow meter, for dry carrier gas conditions [5] to obtain F,: Tc Pa-Pw Fc= FaTa Pa

(3)

where column temperature [K], ambient temperature [K], pa = ambient pressure, h = partial pressure of water at ambient temperature (both pa and h are expressed in the same units). T, T,

= =

Mobile Volume, VM The carrier gas volume corresponding to the mobile time, tM, of a non-retained substance is

VM = fMFc.

(4)

Adjusted Retention Volume, VR The carrier gas volume corresponding to the adjusted retention time, t k , is

Vk = tkF,. As eqn. (1) rk = fR - t M , we can write Vk = ( f R - fM) FC= f R F c - tMFc. Substituting the appropriate expressions from eqns. (2) and (4), we obtain

vk= V R -

VM. (6) V i contains, in analogy to t k , only those contributions of retention resulting from interactions with the stationary phase. Hence it is the basis for gas chromatographic separation and identification calculations. Corrected Retention Volume, The carrier gas volume corresponding to the retention volume VR when corrected for gas compressibility is

V::= V R j (7) where j is the James and Martin factor 161: _ _ P2-1 and P=-. Pi j = -3 2 ~ 3 - 1 Po pi = inlet pressure; it shouldbe pointed out here that gauges often do not indicate the true inlet pressure but the difference in pressure from ambient. Therefore, the ambient pressure has to be added to the indication.

6

2. Basic Concepts

= ambient Dressure at the column outlet. (jvalues in dependence on pilpo can be taken from tabular compilations, e.g. in

-~n”

174).

The jfactors which apply to the generally used carrier gases (He, H2, N2, Ar) take into consideration that the carrier gas is compressible and makes it possible to obtain the average linear gas velocity, i,from the velocity measured at the column outlet. Net Retention Volume, VN This is the carrier gas volume corresponding to the adjusted retention volume when corrected for the gas compressibility. From eqns. (l), (5) and (6), it follows that V,

=

V i j = (V, - V d j = (tR - tM)j F,.

(8)

Corrected Mobile Volume, PM The corrected mobile volume or the corrected retention volume of a non-retained compound, according to eqns. (4)and (7), is given by p,=tMFcj=

VMj.

Specific Retention Volume,

(9)

V,

In order to obtain an absolute retention parameter, independent of the amount of the stationary phase, the specific retention volume is calculated from the net retention volume reduced to 273,16K and 0,1013MPa and referred to 1 g of the effective stationary phase:

where

mL = mass of effective stationary phase or in gas-liquid chromatography the mass of effective liquid phase [s], T, = column temperature [K]. Being an absolute term, V, does not depend on the apparatus and operating conditions used. It seems to be an ideal parameter, but its determination is difficult if higher demands on precision are made, since measuring, e.g., the flow-rate and the amount of the effective stationary phase is generally not as accurate as necessary. This is why relative retention parameters (Section 2.7) have proved to be more advantageous. In contrast to gas-liquid chromatography, in gas-solid chromatography the retention volume is proportional to the surface area of the adsorbent in the column. Therefore, a different specific magnitude is commonly used in gas solid chromatography, V,. Just as when calculating V,, the net retention volume VN reduced to 273,16K and 0,1013MPa is divided by the amount of stationary phase, in this instance the adsorbent, but additionally by its specific surface area. Specific Retention Volume in Gas-Solid Chromatography, V, :

V*=-

273,16

VN ~

m,*&

Tc

where

mA = mass of effective stationary phase or in gas-solid chromatography of adsorbent [sl,

7

2.3. Derived Basic Chromatographic Parameters

SA = specific area of adsorbent [m*.g-’].

273’16, V, is connected with V, as follows:

AS mA corresponds to mL, and V, = mL

Tc

and is no longer a volume per gram but a volume per square metre (cm’.m-*).

2.3.2.

Distribution Terms

The retardation of a compound in the gas chromatographic column is a result of the repeated distribution steps between the stationary phase and the mobile phase. In gas-liquid chromatography the retardation is mainly implemented by a sequence of dissolution and volatilization steps to which the volatile compound is subjected in the non-volatile stationary liquid. In gas-solid chromatography retardation is due to repeated adsorption-desorption steps. These steps are characterized by the distribution of the volatile molecules between the two phases and the term describing this distribution is called the distribution coefficient

K=

concentration in the stationary phase concentration in the mobile phase

(13)

‘

The numerical value of this quantity depends on the units in which the concentrations are formulated. In gas-liquid chromatography it is a common practice to define the partition coefficient: KL =

KL=;

solute concentration in the stationary liquid phase solute concentration in the mobile gas phase

7

CL

where cL= mL/ VL, c, = mc/VG.

mL, mo = masses of sample in the liquid and gas phases, respectively, VL,VG = volumes of liquid and gas phases, respectively, in the whole column. The partition coefficient is a fundamental quantity, being constant over wide ranges of concentration provided that they are low. It depends only on the solute-stationary liquid system and on temperature and is connected to the retention terms by VL’KL where VL is the volume of the liquid stationary phase. As V =- VN 273’16 it follows that mL Tc VN=

VN= VB-mL.-

‘C

273,16 ’

and

(15)

8

2. Basic Concepts

From

-_ m L - eLit follows that VL

v, =-K L eL

273,16 Tc

where 4~ is the density of the liquid phase at the column temperature. It can easily be seen that sample components with different partition coefficients leave the gas chromatographic column in order of increasing K, values and are thus separated. It should be remarked in passing that KL, if corrected for the concentrations by the activity coefficients

where

aL = solute activity in the liquid stationary phase, = solute activity in the gas phase, aM k,h =activity coefficients of the sample in the liquid and gas phases, respectively, this partition coefficient has become a thermodynamic quantity and permits e.g., calculations of heat contents from the temperature dependence of retentions [8]. In gas solid chromatography the concentration ratio according to eqn. (13) is formed by the sample concentration on the surface of the solid adsorbent divided by the sample concentration in the mobile gas phase:

xL

and is called the adsorption coefficient. As cs, unlike cL[eqn.(14)], has the dimensions of mass of adsorbed sample per unit adsorbent surface area:

where @

=mass of adsorbed sample,

mA =mass of adsorbent, S, =specific surface area of the adsorbent

and cM= *, K, is not dimensionless in contrast to KL in GLC,but has, with the quantities VO used here for concentration and area, the dimensions cm’ m-2 and is CoMected to the retention (cf. eqn. (12)) by

-

vs=-= vN mA

Ks.

sA

Compared with KL, which according to eqn. (15) is KL = VN/VL, the distribution coefficient Ks is not referred to the volume but to the surface area of the stationary phase present in the column. The retardation of a compound being carried through the column by the mobile phase can be expressed by the ratio

9

2.3. Derived Basic Chromatographic Parameters

where k is the capacity ratio. Applying the retention volume terms, k can be shown to be

This term k expresses how many times longer the compound is retarded in the column by the stationary phase than a completely insoluble compound, e.g., air. As a consequence, in gas-liquid chromatography k is the ratio of the fraction of soluble molecules at equilibrium in the liquid and in the gas phase at a certain time. It is an important quantity, being characteristic of a particular column. The capacity ratio, sometimes denoted retention capacity, depends strongly on the partition coefficient [eqn.(23)] and hence on the temperature, and is directly proportional to mL, the mass of liquid phase, provided that adsorptive interactions can be neglected [eqn.(25)]. Hence it is an expression for the effective amount of stationary phase per unit column length. KL=/?*k,

(23)

where VL = volume of the liquid phase, VG = volume of gas phase in the whole column

=

Vk.

and mL/VL= eL,where & is the density of the liquid phase, we obtain

This means that the capacity ratio is directly proportional to the amount of the liquid stationary phase, as just stated. The proportionality factor /?, termed the phase ratio (i.e., the ratio of mobile to stationary phase, V,/VJ is a column parameter and changes not only from column type to column type but also from column to column even if prepared and operated under the same conditions. As KL is constant for a given stationary phase and is independent, as already mentioned, of the column parameters (but depends on the temperature, see eqn. (26)!), the product k [eqn. (23)] is constant. As a consequence, if the phase ratio is decreased (e.g., by applying a higher degree of loading in packed columns), the capacity ratio must increase, and vice versa. /?a

2.3.3.

Temperature Dependence of Distribution and Retention Terms

The partition coefficient KLdepends strongly on the column temperature:

and so does the capacity ratio k, according to eqn. (23), where k is shown to be directly proportional to KL:

10

2. Basic Concepts C

logk=-++. (27) TC Hence, an increase in k can be achieved by drecreasing the column temperature. As V, is directly proportional to KL [eqn. (16)], the logarithm of the specific retention volume is likewise related to the absolute column temperature. This temperature dependence can be taken advantage of when calculating enthalpies from retention values [8-lo]: log v, = AH' 2,30 RT, +constant

(28)

where enthalpy of vaporization of the solute from an infinitely dilute solution in the investigated liquid stationary phase, R = gas constant. AH: is the difference of two enthalpies: AH:

= molar

AH?= AH,- AH,. (29) AH, = enthalpy of vaporization of 1 mole of pure solute, AH, = excess enthalpy of mixing of 1 mole of solute with the liquid stationary phase at infinite dilution. AH? is dependent on the interaction of the solute with the stationary liquid and on the structure of the liquid. If log V, values are plotted against 1/T, a straight line of slope AHbIR is obtained. It should be mentioned that there is a restriction, because AH? decreases slowly with increasing column temperature over a wide temperature range and the curve is therefore not exactly a straight line. When AHv % AHE (valid in non-polar solutehon-polar liquid systems) the enthalpy of vaporization of the solute can be achieved from the relationship between V, and 1/T [eqn.(28)]. On the other hand, if AH, is known from published data, the excess enthalpy can be calculated by plotting log V, against V T , , measuring AH'/R and applying eqn. (29) [lo] to obtain AH, [lo]. From the sign and magnitude of AHE, conclusions can be drawn about the type and strength of the interactions in the solute-solvent system concerned. By analogy with eqn. (28), which applies to gas liquid chromatography, the temperature dependence of retention that holds for gas-solid chromatography can be derived [ll]:

log v, =

2,30 RT,

+ constant

where AH,, is the partial (differential) molar enthalpy of adsorption (approximating to an infinitesimally small surface coverage of the adsorbent by the adsorbed compound).

2.3.4.

Parameters Characterizing the Ef'ficiency of Columns

Owing to diffusion, convection and delayed mass transfer processes, the residence times of individual molecules of the chromatographed compound diverge more or less from the average. This spreading results in a time-dependent concentration change corresponding to a Gaussian distribution curve (Fig. 2). Assuming that the chromatographic peak (Fig. 2) has an ideal Gaussian profile, its width is characterized by its standard deviation, u, which is a function of band spreading in the column, and the following relationships can be derived:

11

2.3. Derived Basic Chromatographic Parameters

(31)

w,=2u where y is the peak width at the inflection points (Fig. 2).

(32) where wB is the peak width at the base [distance between the intersection points of the tangents (drawn to the inflection points) width the baseline (Fig. 2)]; and wB=4a

where w, is the peak width at half-height (Fig.2). As both u and % can be obtained from the chromatogram only with difficulty it is better to measure the peak width at half-height, %, representing 2.355 u [eqn. (33)]. The quality of a gas chromatographic column is determined by the band spreading in the column and can be expressed as

(+) 2

n=

(34)

where n = number of theoretical plates, u = standard deviation,

rR = retention time. The term “number of theoretical plates” stems from distillation theory, but it should be emphasized that the values of n in chromatography and distillation have been defined completely differently. As a rule of thumb, [12] nDist. dnGC . (35) According to mathematical theory, different equations for determining the number of theoretical plates in practice have been developed, two of which are cited here:

Y:(

n = 16 - , [13] and

y:(

n = 8 ln2 -

y:(

= 5,545 -

[14, 151.

(37)

These equations could be shown to be identical and to be traceable to eqn. (34) [16]. Most commonly eqn. (37) is applied. The number of theoretical plates is dependent on the length of the column. By analogy with distillation (but again note eqn. (35)!) the height equivalent to a theoretical plate has been introduced in order to enable column efficiencies to be compared independent of the column length:

where h = height equivalent to a theoretical plate [cm or mm], L = column length [cm or IIM according to h], n = number of theoretical plates. In eqns. (34), (36) and (37) the retention time tR is used which in eqn. (1)was stated to consist of two terms, the mobile time t M and the adjusted retention time 2;. Hence, eqn. (34) can be written as

12

2. Basic Concepts

Obviously, tM does not contribute to the efficiency of the column. It was therefore eliminated [17], to give

which is called the number of effective plates. Substituting tk for tR in eqns.(36) and (37), we obtain N = 16 2

(5)

and N

= 5,545 (-$)2.

This term N is more appropriate for the evaluation of a gas chromatographic column than n, the number of theoretical plates. According to eqn. (38), where the height equivalent to a theoretical plate is calculated from the column length divided by the number of theoretical plates, a new term is necessary if n is replaced by N :

H = -L N

where H is the height equivalent to an effective plate. The height equivalent to an effective plate, H, and N, the number of effective plates, should be applied instead of h and n especially when comparing different column types. N and n can be correlated by using eqns.(21) (tM = t)R/k), (34a) and (39), and we obtain

n = N(?)’

=N

(1 +

k) 2

(43)

where k = t ) R / t M = capacity ratio. By converting eqn. (43) the relationship between the numbers of effective and theoretical

plates can be clarified:

n * k2 = N ( l + k ) 2 [18]. From eqns. (43) and (42) the following equations can be easily derived:

(43 a)

and inversely H = h l( +kk ) .

(44 a)

The value of h or H is one of the criteria in estimating the efficiency of a column and depends on several variables, one of which is the average carrier gas velocity. This linear velocity (in the direction of the column axis) can be determined by

13

2.3. Derived Basic Chromatographic Parameters

where linear average carrier gas velocity, column length, tM mobile time, and is related to the plate number by a fundamental relationship, the Van Deemter equation, derived in 1956 [19]:

E L

= = =

h=A

B +Y + CP U

(46)

where A, B and C are complex terms. Mathematically, this equation can be described by a hyperbole (Fig. 3) having a minimum which represents the carrier gas velocity at which the height equivalent to a theoretical plate is minimal and conversely the column efficiency is maximal, i.e., the carrier gas velocity is optimal.

'opt.

Average linear gas velocity U in crn.s-'

Fig. 3. Van Deemter curve (hhdependence)

The empirical terms A, B, and C are composite constants consisting of direct physical parameters, e.g., diffusion constants in the liquid and gas phases, capacity ratio and adsorptiondesorption constants. The optimal carrier gas velocity can be deduced from eqn. (46) to be

g.

iopt =

(47)

In practice, it is worth working at a slightly higher velocity than the optimum, i.e., slightly to the right from the minimum of the hyperbole (Fig. 3), as h is then affected only negligibly and the retention time and hence the analytical effort can be decreased, as can be seen from the relationship between retention time and carrier gas velocity in eqn. (48), where tR is inversely proportional to K L L According to eqn.(45), tM = T. From tR = tM + t ; [eqn.(l)] it follows that tR = 1+ r;. As U

tX = k . tM

U

L L [eqn. (21)], we obtain c,=T+ kt,. Replacing rM by T again we obtain U U

14

2. Basic Concepts

The term A represents the effect of the multiplicity of the gas paths, whereas B is an expression of the longitudinal gas diffusion. C, which is composed of C = Cl + C,, is the resistance to mass transfer, related to the diffusion process in the liquid (CJ and gas phases (Cd. In Fig. 4 the Van Deemter curves for packed and open tubular columns are compared. Obviously, the slope of the ascending part of the hyperbole representing the resistance to mass

open-tubular column

uopt.

uopt.

Ip.d

(0.t.c.)

Fig. 4. Van Deemter curves for a packed and an open-tubular column with the contributions of the A-, B- and C-terms and with the optimum practical gas velocity (after Struppe [23]) transfer, C, is smaller for capillary than for packed columns. As a consequence, the dependence of h on the carrier gas velocity is less distinct for capillaries, and thus this column type, according to eqn. (48), enables higher velocities to be used (without a serious loss of efficiency) and hence shorter analysis times to be achieved. Further, there does not exist a multiplicity of the gas path whereby the A term can be omitted, and eqn. (46) varies to become B h=k_+Cii U

(49)

with the restriction that it is not valid for packed columns. This is the so-called Golay equation [20, 211, valid for open tubular columns. Two further points should be mentioned here. First, in the hyperbolic part left from the optimal carrier gas velocity the B term strongly affects the efficiency, and this region has to be avoided. Second, at higher velocities B can be neglected, and eqns. (46) and (49) change to

15

2.3. Derived Basic Chromatographic Parameters ~~

h=A

+ Cii,

(46 a)

(49 a) with the restriction that they are valid only for higher carrier gas velocities. It was proposed [22, 231 to define an optimum practical velocity above which the loss of efficiency is tolerable and eqns. (46a) and (49a) are valid. This value is the average linear velocity, ii, for which the term C is ten times higher than B. In Fig.4 the contributions of the A, B and C terms to the height equivalent to a theoretical plate for packed and open-tubular columns, as a function of carrier gas velocity, are demonstrated. The magnitudes of h and H discussed hitherto are based on the assumption of the chromatographic peak having an ideal Gaussian profile. In practice, however, tailed peaks are often observed as a consequence of internal and extra-column processes leading to peak asymmetry. Apart from recourse to automated data acquisition systems or on-line computers, correct n and h values can be obtained with reasonable accuracy and precision for tailed peaks by a relatively simple graphical method [24], which cannot be described in detail here. It is based on an exponentially modified Gaussian equation [25]. h = Cii

2.3.5.

Parameters Characterizing the Separation

Being an analytical separation technique, gas chromatography is applied to separate individual constituents present in more or less complex mixtures. Hitherto, we have discussed only single peaks. Let us now consider the separation of two compounds, in addition to the unretained air. As already mentioned, absolute retentions, e.g., V, or VN,can be measured with only limited accuracy. Relative data are less inaccurate and less dependent on changes in, e.g., flow-rate or amount of stationary phase. Here corrections, otherwise necessary, can be omitted with the exception of the column temperature (because of the temperature dependence of the partition coefficient). Hence, relative retentions depend only on the pair of compounds taken in relation to each other,.on the special stationary phase and on the temperature. The relative retention, r2,1ris defined as the ratio of the two adjusted retention times of components 1 and 2:

‘X,

r2,1 = t’ Rl

where r2,

‘b ‘

R1

=

1

=

relative retention of compound 2 relative to compound 1, adjusted retention times of compounds 1 and 2.

As a consequence of above relationships, r also corresponds to the ratios of net and specific retention volumes, partition coefficients and capacity ratios:

For the same stationary phase the magnitude of r2, is constant and independent of the kind and dimension of the column, provided that the column temperature is the same when measuring tl; and the conditions do not change during the analysis. Eqn.(51) makes it clear that the relative retention is a ratio of dynamic processes taking place in the column and also a ratio of thermodynamic terms corresponding to an equilibrium state, both describing the separation. In qualitative gas chromatographic analysis, relative retentions are often used for the

16

2. Basic Concepts

identification of unknown peaks because of the mentioned advantages. It is appropriate to relate the unknown peak to a standard: ri* = tki tXS

(51a)

where t ,’

=

adjusted retention time of the compound to be identified,

tL = adjusted retention time of a standard

and to choose a standard, the retention time of which is not too far from that of the compound. Numerous reference substances have been proposed, often of the same type as the investigated compounds. They should be eluted near the centre of the complex chromatogram, with tk values smaller or larger than the tk, and the relative retentions, qS, having values smaller or larger than unity. As the many published standards are not related to one another, the general application of relative data for the identification of unknown compounds offers only limited possibilities, especially as they have not always been determined under comparable or exact conditions. In order to overcome these obstacles satisfactorily it would be advisable to use an only standard. However, this standard would be advantageous only in those cases, where the column temperature permits its elution within a reasonable time. One of the proposed standards is n-nonane [26], the retention time of which could be far from optimum if the compound of interest had a much higher or much lower boiling point. It was therefore suggested to apply homologues of n-nonane, the relative retentions of which can easily be related to that of the standard n-nonane [27]. Relative retentions have not only been used for identification purposes, but also for characterizing the separation efficiency. The term most often used is the separation factor, a, determined from two adjacent peaks, the separation of which is the most problematic:

Obviously, a is identical with r, with the restriction that the numerator of the ratio [eqn.(52)] always contains the adjusted retention time of the later eluted compound, so a is always larger than unity. A value a = 1means that there is no separation at all ( t f l= tR1). The separation of two adjacent peaks depends not only on their adjusted retention times but also on the standard deviations, u, of the peaks, for it can easily be realized that peaks having larger u values._expressed,e.g., in W~ values (peak width at base, eqn. (32)) (Fig. 2), are separated worse than narrow-disperse concentration profiles. The peak resolution, R,,an expression describing the degree of separation, takes account of the column efficiency and is defined as

where tRz - tR1 = tX, - tX1 (since by subtraction tM is eliminated) and is expressed as Ar. For two adjacent peaks eqn. (53) can be reduced by w B 1 - wB2 to give

As can be concluded from Fig. 2, a sufficient separation is achieved if AtR L 4 a, R hence be-

coming 2 1 (provided WE2 s 4 u, Fig. 2). If AtR 2 6 u, R becomes 2 1.5 and the separation is almost complete. For further terms characterizing the separation and resolution, see Section 2.7., and for characterizing the resolution in non-ideal situations, see [27a].

2.4. Flow of Gases in a Gas Chromatographic Column and Formation of Bands

2.4.

17

Flow of Gases in a Gas Chromatographic Column and Formation of Bands

After injection into the gas chromatograph, the sample compounds are carried through the column by the carrier gas, the mobile phase. In a dynamic process each sample component is distributed (according to its interaction possibilities with the stationary phase) between volume elements of the mobile and stationary phase. One can imagine the column being divided into numerous sections (theoretical plates) and the more or less rapid movement through the column being accomplished by a sequence of transport and equilibrium steps. By a large number of such successive steps the components are spatially separated from each other, provided that their interactions with the stationary phase and their properties, especially their vapour pressures, differ sufficiently. If the separation is successful, each compound is eluted purely (with regard to the other components) but mixed with the carrier gas in form of a more or less broadened band, and the elution of the individual bands takes place successively with respect to time. The peak broadening is caused by several parameters, a few of which are discussed later. First, let us examine the carrier gas flow-rate. Its volume flow-rate, Fc [ml/min], is measured according to eqn. (3) considering the column temperature and, if using a soap-bubble flow meter, the partial pressure of water: Fc = Fa T P - P w (for the meaning of the parameters, Ta Pa see eqn. (3)). Another possibility for characterizing the gas flow-rate is its linear velocity, which from the point of view of the gas chromatographic process should be used rather than the flow-rate. Both terms can be related by

where

~0 = velocity at column outlet,

rc

=

column inner radius,

& = interparticle porosity (without regarding the pore volume), &

= interparticle volume, V, = L ma = volume of the column. For empty tubes, which means even for wall-coated open tubular columns, & = Vc because of the lack of particles, and E,, becomes unity. Hence

uo =

FC

(valid only for wall-coated open tubular columns). For packed columns E,, is found to be about 0,4 [28]. Assuming the carrier gas to show ideal behaviour, the outlet velocity for packed columns is ~ 9 1 uo =

x PT-PZO 2 q M goL PO

where x = column permeability (independent of the nature of the carrier gas), pi = inlet pressure, po = outlet pressure,

(57)

18

2. Basic Concepts

tfM = mobile phase viscosity,

L

= column

length,

eo = interparticle porosity.

This is valid, however, only if the velocities are not too high. The permeability of a column determines the value of the inlet pressure necessary to obtain reasonable analysis times and optimum separation conditions. By converting eqn. (57), x is found to be x = 2 uotfM eoL

Po Pi-Po

which is valid for packed columns. With the open tubes, where the outlet velocity has been shown [32] to be

(valid for open tubes), we obtain with the use of eqn. (57a) and substitution of uo by eqn. (58), x = r: eo/8 and for eo = 1, as in open tubes the whole volume [eqn. (56)] is available, % becomes [32]

(valid for wall-coated open-tubular columns). The permeability of a column determines the value of the inlet pressure necessary to obtain reasonable analysis times and optimum separation conditions. Obviously, for packed columns x depends on the particle size and the density of the packing. By using the Kozeny-Carman equation [30, 311, valid for non-compressible media, the column permeability, x,, related to the velocity in an empty tube can be discribed: x, =

dz e: 1808; (1 - eo)2

(59)

where x, = column permeability related to the velocity in an empty tube, dp= average particle diameter of the solid support or adsorbent, 0, = factor describing the deviation of the particle shape from spherical. At least approximately, this equation can be applied to gas chromatographic systems. Hence, the permeability is seen to be proportional to the square of the average particle diameter, but additionally to depend on the packing density (because e0 = &, i.e., the higher is &, the interparticle volume, the less dense is the column packing). In order to obtain efficient columns, the density should be as high as possible, even if it affects the column permeability. The nature of the particles is less dominant than the shape, represented by ap.Spherical particles with Bp = 1 permit twice the permeability of, e.g., Kieselguhr particles of comparable size having 0, values of around 0.7 because of their irregular surfaces. The evaluation of eqns. (57a) and (58a) for the selection of column types and packings will be discussed in a later chapter. Let us return to the carrier gas velocity. From eqns. (57) and (58), it can be concluded that, as p o is almost constant (atmospheric), the gas outlet velocity uo increases considerably faster than the inlet pressure, owing to the gas decompression. The outlet velocity does not reflect, however, the real situation existing in the column. Owing to the compressibility of the mobile phase, at each point of the column the velocity, density and pressure of the gas phase differ, with increasing velocity, e.g., towards the column outlet. The relationship between the

19

2.4. Flow of Gases in a Gas Chromatographic Column and Formation of Bands

outlet velocity, uo, and the already repeatedly used average linear gas velocity (eqns.(45-49)) is given by U = uo.j. where j is the compressibility correction factor (James and Martin factor [6]),where

3 P2-1

i = 2- P3-1

(60)

and P = - Pi

Po

(see eqn. (7)). The flow of the carrier gas is caused by the pressure difference between the column inlet and outlet, pi - p o , and the velocity is not constant either along the column or across the entire cross-section. According to eqn. ( 4 9 , P can be measured by P = UtMand uo by substituting ii in eqn. (60): li 2L P 3 - 1 u --=(61) Oj 3fM P*-1‘ If eqns. (45) and (61) are used, il and uo can be calculated by measuring the column length, the mobile time and the absolute column inlet and outlet (ambient) pressure. This method is simpler and much more accurate [33] than calculating the velocities from the flow rate. At the beginning of this section the band broadening was mentioned. All causes cannot be reviewed in detail, but as peak broadening strongly affects separation and resolution, some causes should be discussed here. In eqn. (46), the meaning of the constants A , B and C is very complex. Numerous investigations, beginning with the basic ones by Van Deemter, Zuiderweg and Klinkenberg [19] for packed colums and by Golay [20] for open-tubular columns, have been carried out [34-371. As a summary, the following equations will be discussed briefly here. Packed columns with liquid stationary phases:

h=2Mp+

2hD,+ 1+6k+llk2 P 96(1+ k)2

#P I ”‘

Dg

2k dili ( 1 + k)2 kd

k d2P 1 3 ( 1 + k)’ Dl .

+ -2

Open-tubular columns with liquid stationary phases:

20 h=-+ u

1+6k+llk2 A r 2 P+2k dlli+ 2k 24(1 + k)’ Dg ( 1 -t k)’ kd 3(1 + k)’

4

Packed columns with solid stationary phases:

h=2Mp+

2 h D g + 1 + 6 k + l l k 2 .x.-e+d21i 2k dali li 96(1 -t k ) 2 D, ( 1 + k)’ kd

where = numerical factor desribing the packing, = mean particle diameter, yN = labyrinth factor, a numerical factor describing path length, obstruction Dg = diffusion coefficient of a solute in the gas phase, Dl = diffusion coefficient of a solute in the liquid phase, dl = mean film thickness of the liquid phase,

A d,

kd, k; = desorption coefficients, r, = column inner radius k = capacity ratio,

factor,

20

2. Basic Concepts = average linear carrier gas velocity, da = mean thickness of the adsorption layer, x = dimensional constant.

Considering the term A = 2Mp (eqns.(62) and (64)), this source of peak broadening can be attributed to the different path lengths and gas flow velocities in the porous packing representing a channel system. The d f i s i o n in the pores which can be described by the coefficient D,,,,, = M, U [37] must be taken into account for the distribution of the solute (or adsorbate). The term B = 2 yN D, (packed columns) (eqns. (62) and (64)) is related to the axial molecular diffusion of the solute (or adsorbate) in the space between the particles and to the sinuosity of the paths the molecules have to cover. Considering the term B = 2 0 , (open-tubular columns), owing to the lack of particles, there is no labyrinth between them, and the correction factor for the axial molecular d f i s i o n is y~ = 1, as there are no extended pathways of the solutes. The term C = C, + C, + C, is a composite term, the composition of which depends on the type of column applied and on the kind of stationary phase. C describes the contribution of the resistances to liquid- and gas-phase mass transfers, related to the difision in the gas and liquid phases, to peak broadening.

c,

=

1 + 6 k + l l k Z ' X .dZ 9 96(1 + k)2 4

c,

=

1+6k+llkZ & 24(1+ k ) z Dg

and

(packed columns) (open tubular columns)

do not represent, in contrast to B, the longitudinal but the lateral diffusion in the mobile phase. Like B, C, is a contribution of the resistance to gas phase mass transfer to the peak broadening. C, is of importance for all types of columns, but especially for open tubular columns, as r, is generally larger than the particle radius, and both r, and d,/2 are squared terms in the equation. Bohemen and Purnell [36] substituted r, by the particle radius. They have considered the packed column as a system of capillaries of radius d,/2.

and

c -!t I-

2k

4 3 ( 1 + k)'

are expressions which, in the case of liquid stationary phases for both packed and open-tubular columns, take into consideration the phenomena diminishing the mass transfer rate of solutes from the stationary phase to the mobile phase and the inverse. The desorption coeflicient, k d , takes into account that the transport of the solute molecules depends on their concentration on the liquid stationary phase/gas (mobile) phase interface, which in turn is determined by an adsorption/desorption process (K = k,/kd where k, = adsorption coefficient). If kd--* 0 3 , which means that sorption equilibrium is established instantaneously, C, becomes zero and we obtain for packed columns (where C, is neglected) the well known Van Deemter equation; kd is related to 4 and dl [37]. The C, term is proportional to the square of the film thickness of the liquid phase, i.e., reducing the film thickness on the particles and on the walls of capillaries will strongly reduce C, and hence reduce that part of peak broadening caused by the resistance to the mass trans-

2.5. Thermodynamic Bases of Gas Chromatography

21

fer in the liquid phase. Unfortunately, the surface properties of the particles or of the inside wall of the tubes limit the decrease in the film thickness. 2k*d, For solid stationary phases (adsorbents), Cl,= -. k)d is applied rather than C,, as (1 + k ) z commonly there is a gas-solid rather than a gas-liquid interface (but several exceptions may occur, which cannot be dealt with here), and C, is not applicable. Solid stationary phases with a uniform surface offer rapid adsorption-desorption steps, and Ci is very small. If the surface is not uniform, a heterogeneity factor (>1) has to be applied, the magnitude of which depends on the distribution of energy centres. To summarize the complicated relationships between column efficiency, expressed as the height equivalent to theoretical plate, h, and the average linear carrier gas velocity can be described in general form as follows: For packed columns with liquid stationary phases: h =A

+ BU + (C, + c, + Cd I7 Y

(624

For packed columns with solid stationary phases: h =A

+ -=BU + (C, + Ci)I7

For open tubular columns:

B h = Y + (C, U

2.5.

+ c, + CJ E

Thermodynamic Bases of Gas Chromatography

The basic relationships between retention values and thermodynamic magnitudes have already been discussed in the Section 2.3.2. The essential term is the partition coefficient, KL (liquid stationary phases) or K, (solid stationary phases). Let us first deal with the thermodynamics in gas-liquid chromatography. In eqn. (21) we have seen that the capacity ratio is k = fk/fM, i.e., the ratio of the adjusted retention time to the mobile time. At equilibrium, k represents the ratio between the fraction (or number) of molecules residing in the liquid and gas phases at that moment: k = nL n0

(65)

If we consider nL, the number of solute molecules present in the liquid stationary phase, it can be neglected in comparison with the number of the molecules of the stationary liquid itself, n,, as the solution is (ideally) infinitely dilute: nL= xn,, where x is the molar fraction of the solute and n, is the number of molecules of the stationary liquid and can be expressed as

where eL and M are the density and the molecular weight, respectively, of the stationary phase, and VL its volume in the column. The number of sample molecules residing, at equilibrium, in the (ideal) gas phase is

22

2. Basic Concepts

where p is the partial pressure of the sample over its solution in the stationary liquid. According to Raoult’s law, the partial pressure over a solution is equal to the product of the saturated vapour pressure of the pure compound, PO, and the molar fraction, x, i.e., p = pox, and is valid for ideal solutions. For real solutions the Lewis activity has to be used instead of x, and a = 9, where y is the activity coefficient and the superscript zero means the limiting value at infinite dilution. Thus, p = poy”x, and no becomes

no

=

POYOX vo RT

Substituting nL in eqn. (65) by eqn. (66) and no by eqn. (67) we obtain

and, owing to eqn. (23a), where KL = k Vo/VL,we obtain for the partition coefficient

The relative retention, r2,1, is an expression describing the separation of two compounds, 2 and 1. Combining eqn.(51), where rz, = KLZ/KLl,with eqn.(69), we obtain, because eL, R , T and M characterize the stationary phase and column temperature and are the same for both compounds,

or, as rz,l = V k / V k l (eqns. (3,(8) and (51))

-

This fundamental relationship, often used in its logarithmic form: log rz, 1 = log

V’

VX1

= log

4 + log L!J P2 YOZ 0

was found by Heringion [38]. It can easily be seen that the separation, expressed by the logarithm of the ratio r2,1 or Vkl VRl,is determined by two terms, one of which is specific for the compounds under investigation (logpylp9 and cannot in practice be changed within the range of column temperatures, whereas the other term (log y!/y$ offers the possibility of strongly influencing the separation by utilizing selective properties of the stationary liquid. The separation can be achieved, with regard to these two compounds 1 and 2, either by taking advantage of differences of their vapour pressures, or by choosing a selective stationary liquid (as f’ is determined by the properties of both the solute and the stationary liquid) or by both. It has been shown [39-411 that the activity coefficients diverge considerably from unity, depending on the type of stationary liquids and solutes, over a range of more than two orders of magnitude. Incidentally, the y” value is only approximate. The accurate limiting activity coefficient can be calculated by adding two complex terms [4]. Despite some restrictions that cannot be discussed here, the determination of the activity coefficient offers the possibility of calculating excess enthalpies and entropies:

23

2.5. Thermodynamic Bases of Gas Chromatography

p E = R T l n yo where pE, the excess chemical potential, is related to the Gibbs-Helmholtz equation:

AGE= AHE- T A P

(71) (72)

where ACE = excess molar free energy, A P = excess (or mixing) enthalpy of 1 mole of the solute with the stationary phase at infinite dilution, excess partial molar enthalpy, A P = excess partial molar entropy.

Using (72a)

pE = ACE

we can combine eqns. (71) and (72) to give AHE- T A S E = R T h y o . (73) If we assume that A P and A P are constant within a small range of temperature and differentiate eqn. (73) with respect to 1/T we obtain

with the consequence that varying the temperature and measuring the change in y" permits the calculation of excess partial molar enthalpies. The excess chemical potential used in eqn. (71) only refers to a dilute real solution (75) p E= p - pideal = R T h y taking only into consideration the deviation of a real dilute solution from ideal behaviour. However, in a gas chromatographic system we meet with a distribution between a gas and a liquid phase, and we therefore have to consider the two phases, as follows. Gas phase (with regard to a real mixed phase): pi' = pi'

+ RT ln ai = p! + RT In xi + RT In y:i

(76)

where pf = chemical potential of component i in a real gaseous mixed phase, pf = chemical potential of the pure gaseous component i in the state of an ideal

gas, ai = activity of component

i in the real gaseous mixed phase,

xi = molar fraction of i in the gaseous mixed phase, yo. = activity coefficient of i in the gaseous mixed phase. 8'.

Liquid (stationary) phase (with regard to a real dilute solution):

R T ~ ~ Y ~

p ) = p: + R T ~ U= )p ; + R T ~ X + ;.

(77)

where pi = chemical potential of component i in a real dilute solution, p ) = chemical potential of the pure component i in an ideal dilute a: = activity of component i in the real dilute solution,

solution,

x ) = molar fraction of component i in the solution, yfi = activity coefficient of component i in the solution. If it is assumed that the partition of i between the liquid and gas phases is at equilibrium, the two chemical potentials are equal, i.e., p f = pf , and using eqns. (76) and (77) we can write p)

+ RT h x ) + RT lnyyi = & + RT h x g + RT h y i i .

24

2. Basic Concepts

Substituting RT In y by the excess chemical potential [eqn. (791,we obtain p)

+R

T ~ X+;pf

= pf

+R

T ~ +Xp ; .~

With Api = pf - p! and A p E= p i - p t , we have

+A

~ =ERT (hX; - h x f )

and

K,, the partition coefficient related to the molar fractions, is defined by K ,

= x,!/x!, and

thus

Analogous to eqn. (72a), we can substitute Ap, and Ape by the corresponding free enthalpies,

AG = AH - TAS, and for 1 mole we obtain AHV+ AHe - T(AS' + AS') K, = exp RT

where AH" and A P are the enthalpy of vaporizing 1 mole of pure solute and the excess (or mixing) enthalpy of 1 mole of solute with the stationary liquid at infinite dilution, respectively, and where AS" and ASe are the corresponding entropies. A P is small compared with AH", and the sum of both AHS= AHv + AHE is the molar vaporization enthalpy of the solute from an infrnitely dilute solution in the stationary liquid. Hence, the vapour pressure of a component is an essential value for the gas chromatographic analysis. AHS is of the order of 42 kJ/mole. Until now, we have only discussed energies of one component. In eqn. (52) the separation of two compounds has been characterized by the separation factor a = f,&/tl;l, which in this investigation can be taken to be identical with rz, in eqn. (70) and hence with the ratio K z / K l in eqn. (51)

i.e., the separation can be described by the ratio of the partition coefficients. Combining eqn. (51b) with eqn. (79) and with eqn. (78), the logarithmic form, respectively, and substituting Api + ApE by AG for both compounds, which are separable when a > 1, we obtain In az, Kz - 1 = In -- -(AGz - AG1) and representing the difference AGz - AG1 by A(AG), we can K1 RT formulate

,

RT ln az,= A(AG),

(81) where A(AG) is the difference between the molar free energies of the vaporization of each of compounds 1 and 2 from their infinitely dilute solution in the stationary liquid. It could be shown that values of the separation enthalpy A(AG) of about 10 Joule/mole are sufficient for separating compounds, in spite of the fact that AG (the molar free energy of vaporization from the infinitely dilute solution in the stationary phase) itself has values of ca. 4 . lo4Joule/ mole. Thus, by gas chromatography small thermodynamic differences can be made visible even though they are based on thermodynamic effects four orders of magnitude higher [42]. Let us reconsider the partition coeficient. In eqn. (79) we used the partition coefficient K,,which is related to the molar fractions, whereas in eqn. (14) K L is the partition coefficient in relation to the concentrations in the gas and liquid phases (in mass per unit volume).

2.5. Thermodynamic Bases of Gas Chromatography

25

They are connected by the equation

where

VmOl,,= molar volume in the mobile (gas) phase, Vmol,I = molar volume in the stationary (liquid) phase. Hitherto we have neglected the influence of the interface between the liquid and gas phases. However, considering the large area of the surface film usually applied in gas chromatography in both packed and open-tubular columns, we have to improve eqn. (15) by a term taking into consideration the adsorption processes on the surface film in a simplified manner [43-45]: VN = VL. KL + ALKA

(83)

where

AL = surface area of the liquid phase, KA = adsorption coefficient of the gas-liquid interface, VL = volume of the liquid stationary phase, KL = bulk liquid partition coefficient. Such processes are mainly caused by polar molecules of the stationary liquid or of the sample components. Additionally, adsorption effects on the liquid-solid interface (stationary liquid/ solid support) may occur, and also between sample molecules and those parts of the liquid phase situated near the liquid-solid interface which, compared with the bulk liquid phase, can be regarded as being modified by the solid support's surface activity. These phenomena contribute to the retention, and eqn. (83) becomes m

n

KLiVLi +

VN = i=1

KAjAAj? j- 1

where KLiis the partition Coefficient between the mobile and gas phases, VLiis the volume of the stationary phase of the i-type (macro- or micro-layer of the liquid on the surface of the solid support, liquid phase in the micropores, etc.), KAj is the partition coefficient between the stationary phase and the interface of the j-type and A , is the area of the j-type in the column, e.g., gas-solid, liquid-solid, gas-liquid. We shall now briefly discuss the thermodynamics of gas-solid chromatography. In eqns. (20) and (30) we have defined the adsorption coefficient and the partial molar adsorption enthalpy, respectively

and log v,

=

2,30 RT,

+ constant*

where AHA approximates to infinitesimally small surface coverage of the adsorbent by the adsorbed compound. In physical adsorption which, in contrast to chemisorption, is considered here, Van der Waals forces between the adsorbent and adsorbate become effective in the boundary layer, and AHA values of <SO kJ mole-' occur (whereas in chemisorption AHA values of >500 kJ mole-' would be observed). Nevertheless, the AHA values in gas-solid chromatography in most instances are higher than comparable AH: values in gas-liquid chromatog-

26

2. Basic Concepts

raphy, GSC thus requiring generally column temperatures more than 100°C higher. The concentration of compound i on the surface of the adsorbent can be expressed by

r-

%a&.

SA

where nil&. = number of adsorbed moles of i , s,, = surface area of the adsorbent, or, in other dimensions, by the adsorption molality:

r - niab. mA

where mA is the mass (weight) of adsorbent. Applying the specific surface area, &[m2 g-'I:

we obtain

With unimolecular localized adsorption, a maximum surface concentration rmm, or rLm,, maximum adsorption molality, can be achieved, and the real coverage can be expressed by 8,the degree of coverage:

r or r' are functions of temperature and pressure or concentration of the sorbate, respectively: or r, r' = g(c, 0. (85) This function is usually determined empirically by plotting r or r' at constant temperature

r, r' = p(p, t )

P

Fig. 5. Typical Langmuir adsorption isotherm

r,-

nirb m.4

[mole.g-'] adsorption molality, p equilibrium pressure of the sorbate

2.5. Thermodynamic Bases of Gas Chromatography

27

against pressure or concentration of the sorbate, i.e., by a system of adsorption isothermes. A typical Langmuir adsorption isotherm is shown in Fig. 5 . The curve, having a linear part only at low pressures, rises continuously with increasing pressure, asymptotically approaching the limiting value rku corresponding to unimolecular coverage of the adsorbent by the sorbate. An equation describing the isotherm can be derived using kinetics: If 8 is that part of the surface covered with sorbate molecules, then 1 - 8 is the free part of the surface. The adsorption rate can be expressed by d l a & .= ap(1 - 8)SA dt where p is the pressure of the sorbate and the desorption rate by dRidcs= b&A dt. At equilibrium dni,& = dnlde8,.and eqn. (86) can easily be developed: @=-

UP b + up a

where a and b are constants. If a’ = - is applied, we obtain b @=-

alp 1+ a l p

and the Langmuir isotherm is [eqn. (84d)l a’P r = rim--1+ a ’ p

‘

From eqn.(87), it can be concluded that at higher pressures r ‘ becomes rk,, and that at very low pressures r‘ is approximately proportional to the pressure, as can be seen in Fig.6. This linear region can also be described by the Henry equation: n i r h = KHP

where

KH= Henry constant [mole. m-2 * Pa-’], = partial pressure of i. KH is related to the partition coefficient used in eqn. (20), K s , by p

K --.Ks

”- RT

Eqns. (86)-(88) are valid only if the adsorption sites have equal or t least similar adsorption potentials. As a consequence of inhomogeneities of the surfaces and of adsorbate-adsorbate interactions that we have not taken into account in the above relationships, numerous investigations have been carried out resulting in modified equations that have been described elsewhere [46-491. Let us finally consider the temperature dependence of the adsorption. From phase equilibrium, where pi = p f , 01; is the chemical potential of i on the surface of the adsorbent and pf is the chemical potential of the pure gaseous compound i in the state of an ideal gas), eqn. (89) can be developed [50]:

28

2. Basic Concepts

where AHA is the partial molar adsorption enthalpy. This relationship is analogous to the Clausius-Clapeyron equation. Again assuming that the gas behaves ideally and applying eqns. (18), (10)-(12) and (20), for constant coverage we obtain

Eqn. (89a) can also be easily derived from eqn. (30): log v, =

2,30 RT,

+ constante.

From the measured adsorption isotherms, the partial molar adsorption enthalpy can be determined by means of eqn. (89), and from the measured V, values adsorption enthalpies are obtained by eqn.(30) or (89a), which represent very low degrees of surface coverage of the investigated adsorbent.

2.6.

The Quality of Chromatographic Separation

In Section 2.3.4 we dealt with parameters characterizing the separation of two compounds. The resolution of two adjacent peaks has been defined by eqn. (53). This relationship for R,, the resolution, only reflects the quality of separation and does not express the causes that result in the measured R value. Other parameters describing the separation are the relative retention rz, (eqn. (50)) and the separation factor a (eqn. (52)). It has been stated that for a given stationary phase, without regard to the influence of temperature, these parameters are constant for the compounds under investigation. Hence, r and a,being characteristic of the stationary phase used, offer the possibility of a correlation between the separation effect and the type of stationary phase if they are correlated with the band spreading (or standard deviation) expressed as column efficiency via the number of theoretical plates necessary for a desirable separation quality. As indicated in Section 2.3, almost complete separation and resolution are attainable when the retention difference of two adjacent peaks is equal to or larger than approximately six times the standard deviation: AfR 2 3 (61+ 62) (90) [this value corresponds to a resolution R,= 1.5 eqn. (53)]. Let us now develop relationships containing only parameters that express the selectivity of the column (AtR or, better!, rz, the separation efficiency (a, peak width, or, better!, n or N) and the solubility (GLC)or adsorbability (GSC) of compounds 1 and 2 (k). Such correlations for the description of the separation quality are necessary as r (or a) alone is not sufficient, because the column quality also depends on the column efficiency. Starting with eqn.(90), giving nearly complete separation (ca.6 a),we can write instead of ArR fu

- rR1=

3 ( q + 62).

This can be rearranged to give

(2, J:

-- 1 = 3 - + - . fR1 a 2

If the fraction -on the right-hand side is reduced to higher terms by tRz

ru/fu,

it follows that

29

2.6. The Quality of Chromatographic Separation

1 = 3 - + Y i h

-t~

I:(

fR1

fR2

tR1

AS f R / U = J;; (eqn. (34)) and, for two adjacent peaks, nl = n2, we can write

or 2

Eqn. (91) provides information on the number of theoretical plates required for a certain ratio of retention times in order to achieve a largely complete separation of two compounds, the separation of which is difficult. If we use N, the number of effective plates, instead of n, and N =

r'Y $

(eqn.(39)), substi-

tuting At = f R Z - fR1 by A f = f l ; - tA1 (which are identical because the mobile time disappears) [51] we obtain

As tl;/tAl

= r2,1,

this can be reduced to

r2, + 1 ~ = 9 r2,1 ( -1) . Assuming that n is used more often than N,we shall continue to deal with eqn. (91). In Chapter 2 the adjusted retention time, &, has been stated repeatedly to be a more characteristic parameter than the retention time, tR. Therefore, the ratios tWlfR1in eqn. (91) have to be altered: --

-

fR1

f&

+ tM

tkl

+ tM

'

Multiplying by fkl/tkl leads to f' + fM R z fR2 --

-

Gl

fk1

fR1

+ tM .

4 1

Substituting f M / f k l= l/kl and I

tR2 -tR1

-

+kl 1 ' 1+kl

r2.1

tk/tkl= r2.

we obtain

30

2. Basic Concepts

Reducing to higher terms by kl/kl it follows that

-_t R z - rz*lkl+ 1 tR1 kl + 1 . This expression substitutes both ratios

tRz/tR1

in eqn. (91) and we obtain

1+ r2,Ikl + 1

kl + 1 If the ratio kl + l/kl

+ 1 is used in place of 1, we can write

Now the fractions may be added or subtracted and simplified and we obtain

We can imagine the capacity ratio kl = f k l / t Mto be of great importance (in addition to relative retention) for the required number of theoretical plates. An example is given below to illustrate this point. If r2, = 1.1, the required plate number is 4761 if kl = 10, but 33 489 if kl is only 0.5. Another expression describing the efficiency of a chromatographic column is the separation number or, in its original German name, Trennzahl (n)[52,53]. As a pair of major (or standard) peaks, two consecutive members of a homologous series are chosen, e.g., CI4 and Cls n-alkanes or slightly lower or higher: t & z + 1)

Tz=

wH(Z+ 1)

- t;(a + wH(Z)

-1.

(94)

where t i ( z + 1) t R ., B. wH(z+ WH(Z)

adjusted retention time of two consecutive n-alkane

} = homologues with 2 and 2 + 1carbon atoms

1

= peak

widths at half-height.

(It does not matter whether t i or t R is used, because the difference in the numerator in eqn. (94) eliminates tM if tR is used). The Trennzahl (which must not be confused with the resolution!) indicates how many peaks can be placed and yet be separated (4,71 cr separation) between two consecutive n-alkane homologues that, by definition (see Chapter 4), have a chromatographic separation of 100 retention index units. The minimum TreMzahl required for the separation of two compounds having retention indices Il and I2 (see Section 4.2.2) can be calculated by the equation [S41

This is valid if the column is not overloaded, if the concentrations of compounds 1 and 2 are similar and if only the k range of 1 and 2 is considered. The separation value 4,71 u corre-

31

2.6. The Quality of Chromatographic Separation

sponds to a resolution Rs = 1.177. It can easily be seen that for an index difference Z2-Zl of 5 the magnitude of TZminhas to be 19. For example, well packed columns have 7Z values up to 30, and open-tubular columns from 20 to more than 80. A further expression for the number of peaks that might be placed between peaks 1 and 2 while guaranteeing a 4 d separation (corresponding to a resolution RS = 1.00) for every peak is the effective peak number (EPN)[ 5 5 ] :

where 1 and 2 are successive n-alkanes. However, eqn. (96) has been derived assuming n to be the same for each compound and that it is strictly valid only for the specially applied pair of n-alkanes. Simplifying eqn. (96) and substituting '(" wB2

- '") by Rs (eq. (53)) we obtain + wB1

EPN= Rs2.1- 1 . (964 Possibly the most important relationship that correlates factors on which the separation is based (selectivity r and capacity ratio k,being an expression for the distribution) with factors of band spreading (a) counteracting the separation will now be derived. Applying eqn.(53) and assuming wB2to be equal to wB1(an approximation not always verified!), we can write

The right-hand side of eqn. (97) substitutes wB2 in eqn. (36):

(k) 2

, we can write

Reducing eqn. (98) to higher terms by

'- , )' (?). 2

n2 = 16 R:(

, tRZ

tRl

Dividing both the numerator and denominator of the first squared term in eqn. (98a) by tkl, we obtain

tL tL-ti1

ti2

AL ti1

$2

k1

+

til

=

*

lR1

IRl

As t&,lt& = r2, we can write tL

-

fL - t i 1

12.1

-1. If in the second squared term in eqn. (98) tRZis expressed by nominator are divided by tM, we obtain 12.1

-_tRZ - & + tM = tL

tL

k+tM & + I tM

tM

& tM

-

tM

tL tM

*

tL + tM and nominator and de-

32

2. Basic Concepts

tRz - kz + 1 However tLltM= k2 and we then have -- -. th kz

These two modified terms are substituted into eqn. (98)

Instead of n2 we can write nrC,,, the number of theoretical plates that are required to achieve a resolution Rs for the two peaks specified by rz, and k2 (56):

Eqn. (99) permits the determination of the number of theoretical plates necessary to attain a certain resolution R between the two peaks under investigation. From eqn. (99) we obtain the resolution

Eqn. (100) states first that the resolution for a given peak pair is proportional to the square root of n and, as n = L / h (eqn.(38)), to the square root of the column length. Second, the capacity ratio affects the resolution more the smaller it is, i.e., the closer to the air peak the compounds are eluted the smaller is the term kzlk2+ 1 and hence the worse is Rs.In other

( kzl )I

words, the reciprocal squared term - in eqn. (99) increases strongly, thus demanding considerably more plates to achieve the same resolution as with higher k values, i.e., low capacity ratios should be avoided in complicated separation problems. Third, we can conclude from eqn. (100) that the selectivity of a stationary phase, expressed here by rz,1, strongly influences the resolution. We shall often return to this important relationship in later chapters. The derivation of this and similar relationships has repeatedly been the subject of thorough investigations [57-601. Because each of the resolution equations depends on special approximations, it has been proposed to re-define the term resolution [61] by

Instead of eqn. (53) it should be emphasized that the magnitude of R, is on no account identical with the above discussed R values. This can easily be realized if one compares eqn. (58) with eqn. (101) and assuming (wB2+ wBl) to amount to about 8 u, hence R, is found to be about the four times Rs.From eqn. (101) another resolution can be derived [61]:

A final relationship describing the separation and revealing the number of peaks that can be placed between two reference peaks, taking into consideration the band spreading (depending on the retention time) and a 4u separation (R = 1.0) can be mentioned here:

log 21.2

=

p

fi+2 log fi-2

2.6. The Quality of Chromatographic Separation

33

where N is the effective plate number:

($) 2

N=

=5.545

(39)

(5) 2

.

(41)

This Z value [62] is an expression characterizing the separation power of a chromatographic column for compounds (of the same class) having boiling points within the boiling point interval of the two reference compounds 1 and 2. The value Z - 1 is the number of peaks that can be placed between peaks 1 and 2. The 2 value is only a relative measure as it is valid for the investigated homologous series and cannot be assigned to other homologous series [63]. If N,the effective number of theoretical plates, has not been determined, Z can be calculated by eqn. (103a) from the adjusted retention times and the peak widths at half-height: 21,2 =0,678

(&+A) log+. wHZ wHl kl

(103a)

It has been shown [63] that if an approximation is applied, Z1,2may be expressed as

and, as In r2,

= A(AG2*1) (eqn. 82), the

RT

2 value is directly proportional to the free enthalpy

of separation.

The Time of Analysis

2.7.

Hitherto we have not dealt with an important factor, especially in industrial chromatographic analysis, namely the time the analysis will require. The numerous influences on the analysis time [amount of stationary phase, column type and length, number of theoretical plates, retention time interval between the first and last eluted compound, column temperature, crux of the separation problem, particle diameter (packed columns) or capillary diameter (opentubular columns, etc.)] cannot be discussed in detail here, and only a few correlations between some chromatographic parameters and the minimum analysis time will be mentioned. In eqn. (21) we have defined k = f k / t M . The adjusted retention time is substituted by the retention time: (105) According to eqn. ( 4 9 , the average linear carrier gas velocity is correlated with the column length and the mobile time, L Z=-

fM

and fR

=

fM

L

and f M = -

L U

in eqn. (105) is substituted by the right hand side of eqn. (106) and we obtain

7(k + 1). With eqn.(38), h = L / n or L = hn, and we can substitute L by hn and obtain

34

2. Basic Concepts

Now the expression for n (eqn.(99)) is substituted into eqn.(l07) and we obtain, using the appropriate subscripts [64], ropt,=--.16R:(+-) h2 ii

rz. 1 -

(k2+ 1)’ k2’

which is the retention time necessary to separate a pair of peaks with relative retention rz, and a desired resolution Rs in a column having a height equivalent to a theoretical plate and a capacity ratio, both referred to the second peak of the pair, at a linear carrier gas velocity ii (on which h2 depends!). This basic relationship is primarily thought to optimize the analysis time and to show the factors to which the gas chromatographer has to pay attention: fmt, using a column having a minimum h value at a maximum possible carrier gas velocity; second, selecting a stationary phase giving walues for the most critical substance pairs that are as far from unity as possible; third, as the term (k + l)’/k2 has minimum at k = 2 [64], the capacity ratio of the most critical pair should be brought close to this value by appropriate selection of the stationary phase, column temperature and column type; and fourth, demanding a resolution R of the critical pair that is only as high as necessary and not as high as possible. Returning to the carrier gas velocity (eqn. (108)), it can be realized that increasing ii means decreasing topt., provided that h is not seriously affected. The column efficiency (or, using a corresponding term, h,& has been shown (see Section 2.3.3) not to be seriously reduced when ii is increased to a magnitude called the optimum practical gas velocity (OPGV) [65]. The value of OPGV is higher than the optimum carrier gas velocity kpt., thus shortening the analysis time without a substantial decrease in efficiency. Depending on the column cross-section, temperature, film thickness and column temperature, it varies widely. As a guide, OPGL is about 1 m/min for packed columns and between 3 and 80 m/min for open-tubular columns [66]. A similar expression for the optimization of the analysis time has been developed [61]:

where topt. is the optimum retention time of compound 2. Increasing the linear gas velocity to values too far from the OPGV has been shown to leave no margin, as the ratio hliiapproximates to a limit for ii+ a. The minimum analysis time, again taking into consideration two compounds the separation of which is difficult, can be calculated by a simple equation [67]:

The time required for a gas chromatographic analysis naturally depends on the retention time of the last eluted compound, rR1, and can be calculated by [68]

where k, is the capacity ratio of the last eluted compound. Further details, especially of the time normalization chromatographic method, which is used for optimizing column length and temperature, are given in Section 3.1.2.

2.8. Definition of Symbols and List of Essential Relationships

2.8.

35

Definitions of Symbols Used (Table 1) and List of Essential Relationships (Table 2)

Table 1. Definitions of Symbols Used Symbol

Eqn. No.

Correlation

A = Idp

B =2bD8 B = 20, CL= mL/V,, C, = molVo

c = c, + c. + c,

2kd. (1 + k)' kh c,=& 1 + 6 k + l l k 2 Dg 9 6 ( l + k)2

c:

Definition

Activity coefficient Activity coefficient in the liquid stationary phase Activity coefficient in the mobile gas phase Term describing the multiplicity of the gas phase in packed columns Surface area of the liquid phase Term describing the longitudinal gas difhsion in packed columns (or in open-tubular columns) Concentration in the liquid stationary phase Concentration in the mobile gas phase Concentration in the solid stationary phase Term describing the resistance to mass transfer in the column Term describing the diminution of the mass transfer rate of solute from the stationary phase to the mobile phase and inversely As above

=

=& 1 + 6 k + l l k 2 D,

c c d2 =l

2 4 ( 1 + k)l

2k D, 3 (1 + k)'

Term representing the lateral diffusion in the mobile phase (packed columns) Term representing the lateral diffusion in the mobile phase (open-tubular columns) Term describing the diminution of the rate of mass transfer Mean thickness of the adsorption layer Mean film thickness of the liquid phase Mean particle diameter Diffusion coefficient of a solute in the gas phase Diffusion coefficient of a solute in the liquid phase

36

2. Basic Concepts

Table 1 (continued) Symbol

Cornlation

Eqn. No.

Dcfmition

Effective peak number Volumetric flow-rate at ambient temperature Volumetric flow-rate at T, [K] and column outlet pressure p o WPa] AGe = AHE- TASe AG2 - AG1

AH: = AHV- AHe AHS = AHV+- AHE

Excess molar free energy Separation enthalpy Height equivalent to a theoretical plate Height equivalent to an effective plate Partial molar adsorption enthalpy Excess partial molar enthalpy (of 1 mole of solute at infinite dilution in the liquid phase) Partial molar vaporization enthalpy Molar vaporization enthalpy of the solute from an infinitely dilute solution in the liquid stationary phase Vaporization enthalpy of the pure solute Retention index James and Martin factor Capacity ratio Adsorption coefficient Desorption coefficient Capacity ratio of the last eluted compound Distribution coefficient Partition coefficient between the stationary phase and the interface Henry constant Partition coefficient (GLC) Thermodynamically based partition coefficient (GLC) Partition coeEcient (GSC) Partition coefficient related to the molar fractions (GLC) Column length

37

2.8. Definition of Symbols and List of Essential Relationships

Table 1 (continued) Symbol

Eqn. No.

Correlation

Defmition

Mass (weight) of the effective adsorbent Mass of effective liquid stationary phase Mass of the solute in the gas phase Mass of the solute in the liquid phase Mass of the sorbate on the surface Molecular weight Number of theoretical plates Number of moles adsorbed Number of moles in the gas phase

P VG n, = -

RT

QLVLX nL = xns = -

M

p = p o x (ideal solution) p = p o y o x (real solution)

Number of moles in the liquid phase Number of moles of the liquid stationary phase Number of effective plates Partial pressure of the sample over its solution in the liquid stationary phase Ambient pressure Inlet pressure Ambient pressure at column outlet Saturated vapour pressure of the pure compound at the column temperature Partial pressure of water at ambient temperature Optimum practical gas velocity Ratio of inlet to outlet pressure in the James and Martin factor Relative retention Column inner radius Relative retention with respect to a standard Particle radius Gas constant Resolution of two peaks (representing two compounds)

38

2. Basic Concepts

Table 1 (continued) Symbol

Cornlation

Eqn. No.

Definition

Resolution related to the standard deviation of the fmt peak of a pair of peaks Surface area of an adsorbent Specific surface area of an adsor. bent Excess partial molar entropy Vaporization entropy of the pure solute Separation number (Trennzahl) Mobile time Minimum retention time Optimum retention time Retention time Adjusted retention time Required analysis time Difference of the retention times of two compounds Column temperature Trennzahl (separation number) Average linear carrier gas velocity Cmier gas velocity at column outlet Optimum carrier gas velocity Volume of the column tube Specific retention volume (GLC)

VN = q j = (tn - th3iF,

v; = v, - v, %= VRj

Volume of the gas phase Interparticle volume Volume of the liquid phase Mobile volume Corrected mobile volume Molar volume in the gas phase Molar volume in the liquid phase Net retention volume Adjusted retention volume Corrected retention volume Specific retention volume (GSC)

39

2.8. Definition of Symbols and List of Essential Relationships Table 1 (continued) Symbol

Correlation

WB

WE

WH

WI

Eqn. No.

= 4u

Peak width at base Peak width at half-height

= 2,355

a

w, = 2 a

Peak width at inflection points Molar fraction

X

2

Z=

Defmition

log r fi+2

log -

Z value characterizing the separation

fi-2

Separation factor (th> tAl)

a

B Y YO

Y: YL

YM

YN

r

r 80 ?lM

BP x

1 P PE P? P?

Pt

Phase ratio of a column k

Activity coefficient Activity coefficient at infinite dilution Activity coefficient in the gaseous mixed phase Activity coefficient in the liquid stationary phase Activity coefficient in the gaseous mobile phase Numerical labyrinth factor Concentration of an adsorbed compound i on the surface Concentration of an adsorbed compound i on the surface (adsorption molality) Interparticle porosity Viscosity of the mobile phase Factor describing the deviation of the particle shape from spherical Column permeability Numerical factor describing the packing Chemical potential' Excess chemical potential Chemical potential of a compound i in a real gaseous mixed phase Chemical potential of a pure gaseous compound i in the state of an ideal gas Chemical potential of a compound i in a real dilute solution

2. Basic Concepts

40 ~

Table 1 (continued) Egn. No.

Detinition

Pt

(77)

eL

(16), ( 2 9 , (66), (691, (68) (34), (34a), (39), (90). (101) (64)

Chemical potential of a pure compound i in an ideal dilute solution Density of the liquid phase

Correlation

Symbol

U X

Standard deviation Dimensional constant

Table 2. List of Essential Relationships and Definitions Eqn. No.

Equation

Defdtion

Adjusted retention time Adjusted retention volume Calculation of the volumetric flow rate Calculation of the net retention volume Specific retention volume (GLC) Specific retention volume (GSC) Correlation of V, with the partition coefficient Correlation of the partition coefficient with the phase ratio and capacity ratio Definition of the capacity ratio Definition of the phase ratio log v, = 2,30RTc constant

Correlation of V, with thermodynamic quantities

log v, = -t constant 2,30RTc

Correlation of V, with adsorption quantities

AH,"

+

n = 16

h = -L n

($)2

'):(

= 5,545

Calculation of the number of theoretical plates Height equivalent to a theoretical plate

41

2.8. Defmition of Symbols and List of Essential Relationships

Table 2 (continued) Eqn. No.

Equation

N = 16

($)’5,545 (6)’

Dchition

Calculation of the number of effective plates

=

Linear average carrier gas velocity Relative retention Separation factor Permeability of packed columns Permeability of wall-coated opentubular columns Van Deemter equation h=214+

2*Dg -+U V

D.

9 6 ( l + k)’

k

Bl a

+--(1 +2kk)’

Dependence of h on the carrier gas velocity for packed columns with liquid stationary phases

Xd’B 1 + 6 k + l l k Z

C, P

dia

kd

1

c.-a

--

d:P + 3 (12k+ k)’ DI c,-a

Dependence of h on the carrier gas velocity for wall-coated open-tubular columns

rta +-- 2k diu h = 20, y + 1+6k+llk2 u 24(1 + k)’ Dg (1 + k)’ kd Bl u

-

+ 3(12k+ k)* d:p 4 c, a

(70)

Separation equation

vx, P? YY log r2,1= log -= log (I+ log -

(70a)

Herington relation, separation equation

RT In rz,l= A(AG)

(81)

Correlation between the relative retention and the separation enthalpy Correlation between V, and the partition coefficient taking adsorption processes on liquid surfaces into account Resolution of two adjacent peaks

vx,

P2

?J!

(83)

R, =

2A t R +

wBI

(53)

42

2. Basic Concepts

Table 2 (continued) Egn. No.

Equation

Defhition

Expression describing the separation quality by nrsguirsd for given rZel and &l values of a column Expression describing the separation quality

(A)

. , = ' a4 ( - )

fq

= (1

+ kJ LU

Y

Fundamental relationship for selecting gas chromatographic columns and parameters Calculation of the required plate number to obtain a certain resolution of two adjacent peaks Similar expression to eqn. (loo), but with the base resolution (other than eqn. (53)):

Expression for optimizing the analysis time by selecting optimum parameters (on the basis of eqn. (100)) Calculation of the analysis time on the basis of the last eluted compound

3.

The Chromatographic Column

The success of a gas chromatographic analysis depends above all on the chromatographic column in which the separation process takes place. This part, often described as the "heart" of the chromatograph, can be designed as the pivot of the gas chromatographic system. The choice of its type, dimensions and stationary phase determines the feasibility, quality and duration of the analysis. In their original work, James and Murtin used columns prepared from tubes of 4 mm I.D. with lengths of 1.2-3.3 m, packed with a stationary phase consisting of a liquid distributed on the surface of relatively inactive porous solid particles. Since then, packed columns have been applied which, as early as 1958, could be optimized to an efficiency of 30000 theoretical plates for a 16 m length and an I.D. of 2.2 mm [70]. By the introduction of more inert supports and very low loadings, the range of application of packed columns could be extended to the analysis of steroids and other biologically important compounds [71]. An extraordinarily significant breakthrough in increasing the column performance is due to Go&, who was the first to report on open-tubular columns [72], a column type revolutionizing high-resolution gas chromatography and offering the facility for achieving column efficiencies of many hundreds of thousands of theoretical plates. Further important steps in column technology were the development of support-coated open-tubular columns [73] with increased sample capacity, of glass [74] and even flexible fused-silica open-tubular columns [75] and of cross-linking the liquid phase on the inside walls of open-tubular columns [76]. A detailed report on the historical development of columns was given by E m [77]. Before dealing in detail with stationary phases themselves, the column, although not being a major topic in this book, should be discussed in more detail than any other part of the chromatograph, as it is the vessel containing the stationary phase. Its function is to expose the stationary phase to the mobile phase such as to guarantee an optimum separation by maximum repeated partition steps between these two phases. There are several possibilities for achieving this goal, each having pros and cons. They will be considered in this section, including the description, where necessary, of pre-treatments and other preparation steps. We shall consider the details of different column types successively.

3.1.

Packed Columns

Originally, these classical columns had lengths of 1-3 m and inner diameters of 4-6 mm and were packed with coated supports having particle diameters of 0.2-0.4 mm. The column materials were copper, aluminium, stainless steel, and glass. As a result of numerous investigations and of the development of detectors of higher sensitivity, column inner diameters and particle diameters could be decreased and column lengths were increased, resulting in highperformance columns. At present the commonly used classical columns have the following characteristics. 3.1.1.

Column Materials

The most commonly used column materials are stainless steel, glass, quartz, nickel and polytetrafluorethylene (PTFE). It is important to note that the gaseous or vaporized sample com-

44

3. The Chromatographic Column

ponents come into contact with the column walls (and the walls of the connecting tubing). Hence, on metallic inner surfaces undesirable effects might occur, especially at higher temperatures, such as decomposition, conversion or adsorption of labile or polar compounds. Such substances should be analysed using glass, especially borosilicate glass, or quartz columns. On the other hand, metallic columns are more advantageous because they are more rigid, have good thermal conductivity and are more easily handled and processed. Nevertheless, the choice of the column material should primarily depend on the type and properties of the sample compounds. Columns made of stainless steel are predominantly applied. They are rigid and commercially available with various lengths and diameters. Before being used they should be washed with ethyl acetate, methanol and distilled water. Subsequently they should be filled with 50% nitric acid and allowed to stand for 10 min, then rinsed with distilled water to neutrality and with methanol and acetone. Finally they are dried in a stream of nitrogen. Lable compounds may be decomposed, e.g., steroids and pesticides, presumably owing to the presence of transition metals in the steel alloys. Glass columns are more inert provided that they do not contain alkali metals or calcium on the inner surface. In the analysis of labile compounds the inner surface must be treated by leaching the metal ions with hydrochloric acid, washing with distilled water to neutrality, then with methanol, drying, filling with a 5% solution of dimethyldichlorosilane in dried toluene, allowing to stand for several hours, draining, rinsing several times with dry toluene and methanol and drying. Another procedure for deactivating glass surfaces [78] involves heating for 4 h at 90°C with concentrated nitric acid, cooling, rinsing with distilled water and allowing to stand for 1h at ambient temperature with concentrated ammonia solution. The column is then rinsed with water and allowed to stand for 1h at ambient temperature with glacial acetic acid, then rinsed again thoroughly with distilled water. Ammonia treatment results in an activated porous surface, fit for the subsequent silylation, and acetic acid removes metal oxides from the surface. The active sites are subsequently blocked with a 20% solution of octadecyltrialkoxysilane in tert.-butanol and diacetone alcohol. Instead of the trifunctional silane, difunctional silanes (dimethyldichlorosilane) and siloxanes and monofunctional silanes (trimethylchlorosilanes), silazanes (hexamethyldisilazane, tetraphenyldimethyldisilazane) can also be used, the success of the procedure depending on the structure of the sample components and the conditions applied, especially temperature [79-871. Soda-lime glass columns, containing more than 20% of sodium oxide + calcium oxide, are less suitable for analysing polar and labile substances than columns made from borosilicate glasses which, however, also contain alkali, at lower levels, but also requiring the above treatment. The treatment with dimethyldichlorosilae is necessary in order to deactivate silanol groups, which are always present on glass surfaces and exhibit harmful adsorption effects. @or a detailed discussion of such surface phenomena and treatments see Section 3.5.1.) With the exception of aggressive fluorine compounds, most organic and inorganic samples, especially those containing strongly polar or sensitive constituents, can be better analysed in glass columns than in columns made of other materials. They also have, moreover, the advantage of being transparent and thus to allow the packing of the column to be inspected and cavities to be seen. A disadvantage is their fragility, especially in the column oven when using hydrogen as the carrier gas, because of the risk of explosion. Glass-lined metal tubing (GLT) columns (Scientific Glass Engineering, Melbourne, Australia) consist of an inner borosilicate glass tube encased in a stainless-steel sleeve, resulting in a column that retains the chemical inertness of conventional glass columns but which can withstand far more careless handling and offer better uniformity of diameter than can be expected from conventional glass tubes. By applying a temperature of about 800°C it can be coiled. The maximum column temperature is otherwise 500°C.

3.1. Packed Columns

45

Nickel columns, if pre-treated in the same way as stainless-steel columns, are almost as inert as glass columns, and steroids, alkaloids, analgesics, barbiturates and catecholamines are readily analysed [88]. PTFE columns are most suitable for investigating aggressive substances, e.g., HCl, Cl,, HF and ClF3, since this material is chemically inert. Nevertheless, caution is required, because owing to their processing, such tubes might be microporous, and traces of ambient air might enter the column and give rise to undesirable effects (oxidation, additional N2and 0,peaks [891). Even organic compounds may be adsorbed and retarded [90]. Owing to its small surface energy and to its relative neutrality [90], quartz offers certain advantages. The most important application of this material in gas chromatography, however, is in open-tubular columns. A detailed discussion is given in Section 3.4.1. Columns made of tantalum, tantalum-tantalum oxide or zirconium-zirconium oxide are appropriate for the analysis of corrosive samples [91]. Finally, it should be mentioned that deactivation even of metallic surfaces has often been carried out with alkylchlorosilanes or dazanes. A temporary improvement may occur, as chemisorption and adsorption sites seem to have been neutralized. However, one should be aware that if a reaction of metallic OH groups with the S i - C l groups of the chlorsilane has taken place, the resulting metal-0-Si bonds are hydrolyzable, and traces of water, which cannot easily be avoided, will soon re-establish the previous state.

3.1.2.

Column Dimensions

For solving not too complicated separation problems, a column length of 1-3 m is sufficient, corresponding, depending on the quality of the packing, to a number of theoretical plates between 500 and 6000, or with high-performance columns even 10000. Simple analyses, e.g., of sample components the boiling points of which are far apart, may be carried out with even shorter columns, having lengths of 0.3-1 m. For the solution of complicated problems gas chromatography offers two possibilities: increasing the efficiency (n) or the selectivity (r), of which the latter, if realizable, is always favoured. A pure or mixed stationary phase giving the best r values for the compounds under investigation is packed into a column of the required length. Originating from eqns. (38) and (93) describing the almost complete separation of the most crucial solute pair, an expression for the required minimum column length Lhcan be derived:

The general column length is 1-6 m. Longer columns are needed for the separation of multicomponent mixtures. Lengths exceeding 12m for packed columns are hardly suitable. It should be emphasized again, and illustrated by an example, importance of the selectivity: let k be m, then when rz,l= 1.05 and h = 0.05 cm a column length of 7.56 m would be necessary, whereas with r2,1= 1.20 only 0.54 m would be required. The resolution can be optimized by varying the column length and temperature by a method called length-temperature time normalization chromatography [92, 931. The basic equation of time normalization is

where L is the column length, r7 is the average linear carrier gas velocity, k is the capacity ra-

46

3. The Chromatographic Column

tio and subscripts A and B identify two sets of chromatographic conditions. As we have seen

L

in eqn. ( l l l ) , the term Y (1 + k ) corresponds to the retention time, and eqn. (112) only exU

presses the fact that tIu = fRB, i.e., the retention time of a certain compound is the same on both columns. If the carrier gas velocity remains constant and if the columns are operated in the vicinity of the Van Deemter equation, we can draw the following conclusions and apply the following relationships. As column B is lengthened with respect to A, the temperature at which B is operated must be higher than that for A in order to decrease the value of kB, and thus keep eqn.'(ll2a) true equality. The required capacity ratio, at any length L,which will keep tR constant, i.e., normalize the system, is calculated from eqn. (112) (subscripts B will henceforth be dropped):

The temperature required to give that k can be obtained from eqn. (114) [94]:

T=

TA 1 - (NAH")T A In (Wkd

(1 14)

where T

R AHV

= absolute temperature, = gas constant, = molar vaporization enthalpy

of the last eluted compound, i.e., the normal-

ized one. For each length change, be it an increase or a decrease, the normalizing temperature can be easily calculated via eqns.(ll3) and (114). AH" can either be approximated from Pictet-Trouton's rule or according to eqns. (28) and (29) from runs on column A at two different temperatures [8]. Gnishka [92] has shown that the optimum capacity ratio is given, using the fundamental relationship of eqn. (lOO), by

where r2.1 is a pair of substances difficult to separate, kptis the optimum capacity ratio giving the maximum resolution, a is a constant that can be determined [95] by running two analyses:

at different teperatures and b is another constant. Once the optimizing k value is known, eqns. (113) and (114) allow the calculation of the column length and temperature that will yield kptand thus maximize the resolution. A convenient procedure is as follows [93]. Install a column of any convenient length in the oven, operate at any sensible temperature, find roughly the velocity for the maximum efficiency and observe the resolution. If it is not sufficient, change the temperature for a second run. This will allow the calculation of the required parameters to solve the left-hand side of eqn. (115). This in return will yield the optimized capacity ratio, kpt, thus allowing the calculation, via eqns.(ll3) and (114). of the maximized lengths and the temperature at which it should be run. Then install in the chromatograph a column of the above length and operate it at the normalized temperature just calculated. The resolution thus obtained should be maximum at the analysis time. The two parameters temperature and length of the column are changed

3.1. Packed Columns

41

concurrently. Although one might be tempted just to increase the column length or decrease the temperature independently, Grushku [931 has shown that frequently a reduction in the column length and temperature might substantially increase the resolution at a constant analysis time. A limitation of the above equations is that they can only be used to give an indication of the parameters to be optimized and not to calculate their actual magnitudes, as complex and not binary mixtures usually have to be separated and as, ideally, the optimization should take into consideration both the column parameters (type, length, diameter, stationary phase, phase ratio) and the operating parameters (temperature, carrier gas nature and velocity). The column inner diameter of classical packed analytical columns ranges from 1.5 mm to 6 mm. Although the limits between the dimensions of classical packed and microbore columns are controversially defined, in this book only columns with diameters d, 5 1 mm are called microbore columns. An important parameter concerning d, is the particle diameter, d , , of the packing material, and the ratio d,ld, should not be lower than 0.03 or higher than 0.3. Each value offers different advantages, depending on the aim of the analytical application. Columns with a ratio of d,/d, near 0.3 are highly permeable, owing to the column permeability, which is proportional to the square of the particle diameter, according to eqn. (59). On the other hand, the efficiency is almost inversely proportional to the mean particle size (which is present explicitly and implicitly in eqn. (62)), and hence smaller particles and columns with narrower bores would be more advantageous. This has been emphasized by Huber et al. [96]. An important restriction in attempting to prefer fine particles is the higher column inlet pressure. Therefore, short columns have been used in such instances. An important factor that should not be overlooked, is the load. At the same impregnation rate, the load is directly proportional to the column cross-section and hence to the square of the inner diameter, i.e., load = constant d : . This is why higher sample weights can be analysed on columns with larger diameters. The sample weight ranges from approximately 3 mg for 5-mmcolumns to 1 mg for 3-mm columns, if other dependences (on temperature, weight of stationary liquid, and type of sample) are disregarded. Hence numerous factors have to be considered when choosing a suitable column diameter. Suitable column inner diameters might be 2-3 mm if the columns are packed with support materials of particle size dp = 0.12-0.15 111111. The shape of the column is most often a helix. Metal columns can easily be coiled by an appropriate device [97]. For the various forms, tubing connectors, etc., see ref. [97].

3.1.3.

Preparing of the Packing and Packing Procedures

In packed columns, the packing may itself be the stationary phase, if it is an adsorbent (GSC),or an appropriate solid support, being as inert as possible, which may be coated with a liquid stationary phase (GLC).Both the adsorbents and the inert solid carriers, and also the liquid stationary phases, will be discussed in later chapters, whereas the coating of the support and the packing of the column with the stationary phase will be dealt with here.

The carrier has to be coated with a liquid film that is as uniform as possible. The wettability of a surface can be expressed in terms of the “critical surface tension” yc [98], which is a value above which liquids with surface tensions yi show a finite contact angle (8) on a smooth surface. The value of y, can be obtained from a plot of cos 8 vs. yi, which is usually a straight line; the intercept with the Cos 8 = 1 line giving yc [99]. On a rough surface, as exists on the usual porous support particles, the (macroscopic) contact angle is decreased (and cos 8 increased) because the liquid phase penetrates into the scratches, holes and pores. The degree of roughness is characterized by rf, the roughening factor [loo]:

48

3. The Chromatographic Column

cos 8’

rr=-=-cos8

s’ s

where 8 = contact angle on the smooth and B on the rough surface, s = macroscopic surface area, s‘ = microscopic surface area. This equation indicates that a porous surface, because s’> s and hence cos 8’ > cos S, can better be coated than a smooth surface. We shall return to this fact when discussing open-tubular columns and column support materials. The weight of the stationary liquid (mJ that is coated on the support material to give a certain impregnation rate depends on several factors. Basically, the weight has to be chosen so as to avoid agglomeration; the material must not be sticky. The area and structure of the support determine the impregnation rate. High impregnation rates have two advantages: the residual activity of the support is less troublesome, and the sample weight can be increased. On the other hand, some disadvantages have to be considered: the efficiency decreases, the h value depends strongly on the gas velocity and the analysis time increases. As a guide for diatomaceous-type supports, 5-20% by weight is advantageous; for other materials, see Chapter 7 and ref.[101]. The optimum value for a certain separation problem can be found [lo21 as follows. The most appropriate stationary liquid is coated on to the support using an impregnation rate of about 5% by weight, and the stationary phase is packed into the column, which serves as a test column. The column temperature that gives the optimum resolution of the investigated sample constituents is determined and applied to the actual column, which will give, independent of the impregnation rate, the most advantageous resolution. The impregnation rate, on the other hand, is selected so as to give the required analysis time. Decreasing the weight of liquid is limited, however, because of possible adsorption effects on the uncovered surface, on the solid-liquid interface and in the pores, which contribute to the retention of polar compounds. However, provided that the support has been well deactivated previously, low impregnation rates are successful even in the analysis of high-boiling compounds. This applies to diatomaceous-type supports, previously deactivated, impregnated in the range ca. 0.1-3% [102]. Nevertheless, one must be aware of the following phenomena. The real stationary phase is not a single-phase but a multiphase system, and the net retention volume is not only determined by solution processes obeying eqn. (15), but additionally by adsorption on the gas-liquid [43] and gas-solid [lo31 interfaces and by adsorption exchanges between liquid and sample molecules interacting with the active sites of the support [104]. Such effects have a relatively greater influence with low impregnation rates because of the lower V, values and hence the relatively higher contributions of the additional effects expressed by the second term of eqn. (83 a) [45]: m

(for definitions, see Section 2.5). The adsorption on the liquid surface is favoured by the adsorption power of the support, which becomes apparent with very thin films. Such a modified layer may develop if a support with a specific surface of a.3 m2/g is less coated than 1% [105]. The minimum possible liquid loading depends not only on the activity of the support, but also on the polarity of the liquid and sample molecules, i.e., when polar compounds are investigated by using non-polar stationary phases, the danger of adsorption processes has to be taken into account. This applies especially to the calculation of retention indices, and differences in experimentally determined retention indices can often be attributed to a non-uni-

3.1. Packed Columns

49

form coating. Retention index differences of 5-10 have been observed with badly deactivated supports [106], whereas relatively inactive supports when coated with 2 2 % of non-polar poly(dimethylsi1oxane)did not indicate a retention index dependence on the liquid loading; merely a decrease from 2 to 1 resulted in a retention index increase of 10 units [107]. Finally, it should be pointed out that if the dimensions of the impregnation rate are as usual weight/ weight, then supports with higher bulk densities (glass beads, Chromosorb G) would contain a larger amount of liquid phase per unit support surface area, and also per unit column packing volume, than supports with lower bulk densities. Hence the degrees of impregnation should rather be related to the support’s surface area in order to be comparable. Supports with a small specific surface area and porosity (glass beads, quartz particles) must be loaded with less than 2% (w/w). There are many coating techniques. Generally the liquid stationary phase is dissolved in an appropriate solvent, from which it is deposited as a film on the support by means of one of the following procedures. The degree of impregnation (the loading) is given either in weight of liquid per 100 g of support (percent by weight) or in weight of liquid per unit packing (i.e. percent liquid stationary phase referred to the weight of impregnated support). Provided that the surface area of the carrier is known, it can also be a basis for the impregnation rate [108], the dimensions of which in this instance would be weight of liquid per square metre of support surface area. Before outlining the various coating procedures, it should be mentioned that detailed discussions have not been included, but only summaries of the principles, with short evaluations and descriptions of techniques to be recommended. For details the reader is referred to a textbook [lo91 and original papers. Filtration technique. The stationary liquid is dissolved in a suitable solvent and the support is added and briefly mixed. After filtration the residue is dried, e.g., by the fluid-bed technique [110-1131. This method is recommended with low loadings (<3%, w/w), the value of which ensues from the difference in added liquid and residual liquid contained in the filtrate. Frontal technique. The support is packed into the column and a solution of the stationary liquid is passed through the packed column until the concentrations of added and draining solution are equal [114]. The solvent is expelled from the column by heating in an inert gas flow. Evaporation technique. A weighed amount of stationary liquid to be coated is dissolved and, after addition of the support, the solvent is evaporated. The simplest way to carry out the evaporation is to use a water-bath and subsequently a steam-bath and a drying cupboard. However, although this method is often applied, it cannot be recommended, for several reasons. The main reason is the danger of oxidation of the liquid stationary phase. If we consider that a small amount of liquid is spread over a large surface area and present as a very thin film, in presence of oxygen partial oxidation of the liquid is likely to occur. For example, a packing with a loading of 5% (w/w) is prepared for a 6 m X 3 mm I.D. column, corresponding to a volume of ca. 42 cm3. Assuming that the support would have a packing density (in the column) of 0.5 g/cm3 and a specific surface area of 4 m2/g,the 1.05 g of stationary liquid would cover a surface of 84 mz. It can be calculated that the average film thickness would be ca. 12 MI, provided that the surface area is covered completely, and we can imagine this thin film to be exposed to atmospheric oxygen at oven temperatures, as often used, above 200°C and hence easily subject to oxidation processes with serious consequences in view of the quality of the packing. Therefore, the evaporation should be performed out of contact with air. A short-cut method con-

50

3. The ChromatographicColumn

sists in evaporating the solvent in a rotary evaporator. At ambient temperature a water-jet vacuum is applied for a few minutes until the support no longer evolves air bubbles. The vacuum is filled up with nitrogen, the bath temperature is elevated to ca. 40°C, then the pressure is reduced cautiously while rotating the flask and the solvent is withdrawn. Once the material is flowing freely, the rotation is stopped and the residual solvent is issued, maintaining the vacuum, at a bath temperature of 80°C [115]. Vacuum-vibration technique The stationary liquid is heated to 20-50K above its maximum operating temperature and by vacuum and vibration impregnated on the support [116]. A maximum loading of 14%is attainable and the occurrence of degradation products is unavoidable. Recommended procedures (a) Prior to coating, the support should be dried in a round-bottomed flask for 8 h at 300°C under vacuum. The cooled flask is vented with dry nitrogen and the solution of the liquid stationary phase is added. Residual oxygen is removed by a partial vacuum, until air bubbles no longer occur. The solvent is mainly evaporated by rotary evaporation and frnally under vacuum at a temperature of 100°C below the maximum operating temperature of the liquid phase overnight [117]. (b) In a special flask (hedgehog flask) the support mixed with the solution of the liquid stationary phase is cautiously evacuated and degassed and after ca. 10min disconnected from the vacuum pump and connected with a rotary evaporator. The temperature of the heating bath should be 20°C above the solvent's boiling temperature. Pure nitrogen is introduced into the flask at ca. 1Ymin. The solvent is vaporized while slowly rotating the flask. A visible reflux should be maintained, thus rinsing the suspension downwards from the walls of the flask [118]. (c) The support is placed in a Pyrex tube (480 X 23 mm) fitted with a sintered glass disc, closed at one end and evacuated at room temperature by connecting the other end to a vacuum pump. After several minutes, the tube is placed in a furnace at 450°C for 4-24 h while maintaining the vacuum, and removed from time to time and shaken to mix the powder. After heating, the tube is cooled to room temperature while still under vacuum. The tap connected to the pump is closed and the dissolved liquid is run into the Pyrex tube via a funnel connected to the tap. This tap is opened and the solution flows over and coats the powder. The tube is allowed to stand overnight. Then the cap, which closed the other end of the tube, is removed and, after the upper surface of the solution has reached the carrier surface, the remaining liquid is forced out under a gentle stream of nitrogen until the solid becomes freeflowing. The purpose of this procedure is to avoid the formation of water, which when using coated support material not been subjected to vacuudheat treatment could be eliminated by condensation of silanol groups to form siloxane bridges by heating in the column. This water could be trapped underneath the liquid phase and thus might account for the gradual deterioration of some types of packed columns [119]. Determination of the degree of impregnation The simplest way to determine the loading is Soxhlet extraction using the same solvent as applied in the coating step [120]. It should be noted that a polar solvent might leach soluble constituents from the support, thus falsifying the impregnation values. The loading with organic stationary liquids can be established by a simple ash determination provided that the liquid is quantitatively oxidizable at 800°C to form only volatile products of combustion such as C02 and H20[121]. Silicone liquid loadings cannot be investigated by this procedure; instead C and H can be determined by elemental analysis.

3.1. Packed Columns

51

It should not be neglected to run a blank with the uncoated carrier, especially ifit has been previously treated with organochlorosilanes or -silazanes. Carboranesiloxanes or organomet a l k phases can be determined by a boron or metal determination, respectively. A method that can be applied to most organic and to poly(dimethylsi1oxane) phases is based on the vaporization and degradation of the stationary liquid phase with exclusion of oxygen. With a temperature programme from ambient to 800°C within 2 h the sample is heated in a flow of suprapure nitrogen. After a further hour the “vaporization” is finished, without the formation of carbon. As water would affect the results, the non-impregnated support has to be analysed in the same way [122]. Finally, it should be pointed out that the determination of the degree of impregnation should be carried out only when the stationary phase was previously conditioned, because during the conditioning residual solvent and vaporizable constituents of the liquid phase could be released, which therefore would not contribute to the retention and must not be taken into account when, e.g., calculating V,. Details concerning the conditioning are given in Section 3.1.4. Packing procedure In a simple way U-tubes are packed by connecting both ends with small funnels through which small portions of the impregnated support are poured alternately while vibrating and slightly tamping until the upper surfaces of the packing can be seen 1 mm underneath the column ends. The ends are loosely closed with deactivated glass-wool in such way that the particles of the packing will be held back but the resistance to flow will not be increased. Packing of coiled columns has to be carried out differently, by tap-filling the precoiled column with a nitrogen flow at the column inlet and a vacuum applied at the column outlet [117, 1231. As the permeability of the packing decreases during its growth, the gas flow-rate produced by the vacuum also decreases, and the packing procedure is prolonged. Therefore, commercially available devices for pressurized packing should be used. To overcome the disadvantage of such appliances, that columns packed in this way are inefficient, a more favourable device has been designed for pouring the stationary phase slowly and continuously under a gas pressure into the column, so ensuring a homogeneous and tight packing [124]. It is especially suitable for laboratories in which many columns have to be packed. Coated PTFE supports should be packed into the column at 0°C to avoid electrostatic charges [125], using vibration [126]. A further recommended procedure applies pressure and ultrasonics. It has been developed especially for the optimum packing of columns with a coated support of the Volaspher type [127]. Finally, it should be emphasized that after finishing the filling of the columns, the pressure and vacuum must be reduced only slowly, as otherwise the tight packing would be loosened and the efficiency diminished. The efficiency of packed columns may also be improved if the packing is completed after having been heated to the maximum column temperature (which depends on the liquid stationary phase) [117].

3.1.4.

Column Conditioning

If the column is used for performing analyses immediately after the packing, then the retention times, selectivity and efficiency will change gradually and the detector will be contaminated. If the packing is not packed densely enough it will become tighter under the influence of the carrier gas flow. Solvent residues, volatile constituents of the liquid stationary phase and its degradation products will evaporate to various extent. At higher temperatures, the stationary phase might be degraded or cross-linked. In order to avoid these and other difficulties, any gas chromatographic column has to be conditioned before operation. Basically, the

52

3. The Chromatographic Column

stationary phase is heated, either in a special tube [128] or in the column itself, whilst (purified, especially oxygen-free) carrier gas is passed through it at a higher temperature than the subsequent maximum operating temperature. When the column itself is used, its effluent end has to be disconnected from the detector. In detail, the following procedures can be recommended: (a) The column containing the packed stationary phase is installed in the oven of the gas chromatograph at ambient temperature; it must not be connected to the detector. A flow of purified carrier gas, which especially has to be oxygen-free, is begun at a flow-rate of 10-20 ml/min. Purging the column for 30-45 min allows air to be removed from the column, which might otherwise cause oxidation of the stationary phase. The column is then heated at 60" for 30 min and subsequently programmed at 2 Umin to the desired upper limit. This temperature, the value of which should be 25OC above the desired operating temperature (but not exceeding the upper temperature limit of the stationary phase), is maintained for at least 12 h. The length of the conditioning time depends on the type of stationary phase, the impregnation rate, the maximum temperature of the analysis relative to the upper temperature limit of the stationary phase and on type and necessary sensitivity of the detector being used, and can require as long as 1 week if extreme parameters (operation at the maximum temperature limit of the stationary phase and at maximum sensitivity of the detector) are inevitable [129]. (b) The packed column is COMeCted with the injector, but not with the detector, [both as in (a)], and a flow of purified carrier gas is begun at the same flow-rate as in (a). After 30 min at ambient temperature the column temperature is raised to 60°C and maintained there for 1 h. After cooling to room temperature the carrier gas flow is stopped, the effluent column end is closed gas-tight and the column is heated to the upper temperature limit of the stationary phase and maintained for 14 h. The column is then cooled, the tube seal is disconnected and the carrier gas is allowed to flow through the column, which is heated to a temperature 20°C below the upper temperature limit of the stationary phase for further 14 h. This method eleminates troublesome surface activity from the support and column walls, especially when using glass columns packed with silicone stationary phases [130]. A shortcoming of both methods (a) and (b) is the lack of control of weight loss of stationary phase, as the relatively high weight of the column generally does not allow exact weighing. 3.1.5.

Column Testing

The column walls and connections must not permit any trace of carrier gas to pass as otherwise losses of carrier gas and sample would occur and, if the carrier gas is hydrogen, an explosion hazard may arise. The column is checked by sealing the effluent end and applying first an Nzpressure which, after interrupting the N2supply, must remain constant for a longer period, and then repeating the procedure with a Hzpressure. By the determination of the permeability, the mobile time, the efficiency, the selectivity, the time of analysis and the degree of impregnation according to the previous chapters and sections, a decision can be made as to whether the column comes up to expectations or preferably should be re-prepared. The peak shape, especially of polar compounds on non-polar stationary phases, indicates residual adsorption sites in the system, and deactivation procedures or a change of support would be required unless the amount of sample is too high for the available loading (in this instance the peak shape, however, would differ from the unsymmetric shape caused by adsorption phenomena). Further details concerning this subject will be dealt with when discussing open tubular columns (Section 3.3) and support materials (Chapter 7).

3.1. Micro-Packed Columns

53

The capacity of the column for the amount of sample can be established by determining the number of theoretical plates as a function of increasing sample volume. If the efficiency is only 90% of the value found for the smallest sample volumes, one has arrived at the maximum capacity. The upper temperature limit and its determination will be described in a subsequent chapter. The packing density (g/ml) can be calculated from the amount of packing and the volume of the column, which was previously determined with any liquid the density of which is known. If water is applied, the remarks in Section 3.1.1. should be considered.

3.1.6.

Pre-columns

A special type of packed columns are pre-columns. These are short columns, having lengths between 2 and 20 cm, and with inside diameters between 2 and 8 mm. They can, generally, be backflushed and are easily interchangeable. Their function consists in saving the column (and detector) from slowly or not eluting sample constituents and from changes in selectivity caused by strongly polar sample constituents having low vapour pressures, in that these are eliminated before arriving at the column by backflushing. Further, the analytical column can be saved from losses of stationary phase in the initial part of the column by using a pre-column with a high degree of impregnation. Moreover, they enable samples that contain nonvaporizable residues, e.g., polymers or salts, to be analysed without changing the quality of the analytical column. It should be borne in mind that the pre-column diameter affects the efficiency of the main column. The higher the pre-column inside diameter, the greater is the loss of efficiency of the column. This loss can partially be diminished by using of smaller diameter solid support particles, but a better method is to increase the pre-column temperature [13Oa]. Pre-columns are packed with an inactive support impregnated with 20-30% of an appropriate silicone phase, and they are connected gastight with the analytical column and operated isothermally at the highest possible temperature, the magnitude of which depends, of course, on the problem to be solved. The principle and installation have been described by Kaiser [131]. Conventional packed columns, if coupled with open-tubular columns, can be considered as pre-columns if they are used to remove the main sample components, thus allowing the trace components to be optimally separated in the capillary column. For trace analysis, this is an important possibility. Both column types are complementary to one another, the packed column with its high sample capacity (2-3 orders of magnitude higher than that of capillary columns) and the open-tubular column with its outstanding performance.

3.2.

Micro-Packed Columns

Owing to their very promising parameters of mass capacity, phase ratio and resolution with respect to the required analysis time, micro-packed columns have increased in importance. Their main features are microbore tubes with inside diameters (4)between 0.3 an 1mm and lengths of ca. 1-15 m, packed with particles of 0.007-0.3 mm diameter (dp).If the ratio of particle diameter to column inside diameter is d,/d, = 0.25, the columns can be packed regularly and densely. For dp/dc> 0.25, the packing becomes increasingly irregular and less dense. Transitions between both classical packed and micro-packed columns and regular and irregular micro-packed columns obviously occur, and an exact classification must therefore be arbitrary. Nevertheless, packed columns ought to be subdivided according to the following

54

3. The ChromatographicColumn

Table 3. Characteristics of Packed Columns Classical packed columns

Regular micro-packed columns

Imgular micro-packed columns

Tube i.d. (d,) [mm] Particle size (d,)

22 0.12-0.30

0.3-1.5 0.04-0.3

0.3-0.5 0.05-0.15

1-1 dP -

0.04-0.10

0.07-0.25

0.25-0.5

> 1000

5-20 0.4-1.2

<15 < 10 OOO*)

1-10 0.15-0.40 5 15 5 50 000

Simple separation problems; trace analysis; coupled techniques with spectrometry

Multi-component analysis; high-resolution coupled technique with MS; trace analysis

Short columns permit high-speedanalyses; coupled techniques with MS

dc

Sample capacity Cg] h [-I L [ml n Preferred applications

>o.s

s6 5 15000

9 n = 30000 for 16 m length obtained by Scott (70) is the exception, not a standard

parameters, as their merits and hence their application fields differ from one another, as shown in Table 3. It should be realized, however, that the values are only guidelines. The term “packed capillary columns”, used repeatedly in the chromatographic literature, has been deliberately avoided in this book in order to prevent misunderstandings. In this we agree with Stnrppe, who also suggested a subdivision into these three types of packed columns [132]. Owing to their high efficiency combined with a sufficient sample capacity, micro-packed columns offer excellent possibilities for the analysis of multi-component mixtures and trace constituents, and, especially, for coupling with mass spectrometry. They further combine high efficiency with short analysis times, thus permitting high-speed analyses. The stationary phases, as in classical packed columns, consist of both adsorbents (GSC)and liquid phases on supports, the particle sizes of which are given in Table 3. With smaller particle diameters (< 160 pm) the above packing procedures have to be modified by using ultrasonic vibration, maintaining an inert gas pressure [133-1351 or at least using electromechanical vibration of 20-100 Hz [136-1381. Apart from the detailed advantages a disadvantage should not be ignored, namely that the pressure drop, depending on particle size and packing density, ranges from 0.03 to 0.3 MPa, thus permitting, in the case of the higher values that occur with smaller particles, only relatively short columns. However, this disadvantage is not serious: owing to the higher efficiency and to the essentially lower C, term of the Van Deemter equation (eqn. (63)) compared with classical packed columns, even short micropacked columns yield high numbers of theoretical plates (e.g., up to 5000-10000 per metre [117, 1351 if operated at high pressures), and higher carrier gas velocities can be applied (lower C, term because of the smaller particle diameters) without seriously increasing the height equivalent to a theoretical plate, both advantageous conditions for high-speed analyses. It should be noted merely that longer columns packed with small particles require modified injection devices applicable to gas inlet pressures above 0.5 MPa [117, 1351.

3.3.

Open-Tubular Columns

Since the first detailed reports by Coluy [20, 211 on open-tubular columns, i.e., tubes that do not contain a packing, and in which the liquid phase is coated on the inside wall and the

3.3. Open-Tubular Columns

55

shape of the free gas cross-section is maintained over the whole length of the column, this gas chromatographic technique has developed considerably and its applicability has widened tremendously. This development is based on the fact that gas chromatography with open-tubular columns represents the most efficient method for separating mixtures of vaporizable constituents regarding information content, resolution and analysis time, and there is no doubt that the future of gas chromatography, disregarding preparative GC, special cases of trace analysis and coupling methods if larger amounts of the sample constituents are required, will lie in the application of open-tubular columns with either thin films or thin layers.

3.3.1.

Tube Material

Essentially, the tube used for the preparation of open-tubular columns consists of stainless steel, glass or fused silica glass, each having advantages and shortcomings. Stainless steel: This is commercially available as tubing with the required internal and external diameters, can easily be bent and connected to the other parts of commercial gas chromatographs, has good thermal conductivity and does not break. On the other hand, the quality of its inner wall is often rough, dirty and active towards polar and labile compounds. This disadvantage is especially important with wall-coated columns, whereas in support-coated columns the inner surface, being covered with porous particles, cannot be reached by the sample molecules. Recently, a rather inert metal column has been developed by Chrompack, HT-SIMDIST, especially for high-temperature application (to 450 "C) [138a]. Glacis: Glass is cheap, easily deformable and has a homogeneous surface. Nevertheless, because of some disadvantages, its application has been restricted to highly skilled chromatographers: glass capillaries are fragile and their dead volume-free connection to the other parts of the gas chromatograph is complicated. The main restriction is the poor wettability, i.e., the difficulty of forming a coherent and stable film of stationary phase on the inner wall of the glass tube, and only after about ten years of intensive investigations by numerous chromatographers did glass open-tubular columns become generally accepted. The production of glass tubes can be implemented by means of a glass capillary drawing machine, invented by Desly et al. [139]. Most often soda-lime glasses are drawn, as capillaries made from this glass type are more elastic than those from borosilicate glass, although the latter are more inert. Fused-silica glacis: Fused-silica glass is a column tube material with outstanding properties. It exhibits flexibility and great inertness, the latter being due to the low metal oxide content compared with soda-lime and borosilicate glasses. Unlike normal glasses with a wall thickness of about 0.3 mm, fused-silica glass capillaries are drawn to give a very thin wall of about 0.05 mm thickness and externally coated, by analogy with the fibre optics process, with an appropriate polymer, e.g., polyimide, to impart flexible tensile strength. The outside coating is thermally stable up to 340°C and has a film thickness of about 0.05 mm. Two types of vitreous silica glasses have been used as starting materials: (a) Conventional fused quartz derived from Brazilian quartz rock crystals by electric or flameCusion [76]. It consists of ca. 99.99%SiO, with a few hundred ppm of N 2 0 3as the main contaminant [76], the content of which can be lowered by further chemical purfication. (b) Fused silica has been produced by gas-phase hydrolysis of a very high purity silicon tetrachloride and fusion of the S O z formed. This synthetic product contains only lppm of impurities. The drawing process is carried out at the melting temperature, which ranges, depending on the composition, between 1700 and 2100°C. An appropriate drawing machine has been described by Lipsky et al. [76].

56

3.3.2.

3. The Chromatographic Column

Column Dimensions

Stainless-steel capillaries for gas chromatography commonly range in length from 5 to 100 m and have outside diameters from 1.0 to 1.6 mm, most often 1.59 mm (1/16 in.), and the inside diameters may vary from 0.05 to 1.55 mm, but commonly only from 0.2 to 0.3 mm (wall coated) or from 0.2 to 0.5 mm (support-coated open-tubular columns). It should be emphasized that the column diameter must be constant all over the column length, thus requiring good column drawing technology. From eqn. (63), it can be derived that the column inside diameter or radius, r,, strongly influences the efficiency (expressed by h), i.e., increasing r, increases h, and hence decreases the efficiency. On the other hand, increasing r, means increasing the amount of stationary phase in the column, because VL= 2 nr, L d,

(118)

(provided r, a dJ, where VL = volume of the stationary phase, L = column length and 4 = average film thickness of the stationary phase, and a larger amount of stationary phase permits the amount of sample to be increased without overloading the column. This is important when coupling techniques are to be applied or trace analyses are to be carried out. A further fact should not be neglected, namely the influence of variations in the column diameter on the phase ratio. This term has been defmed by eqn. (23):

As VG can be expressed by VG = nra L

(118a)

[provided r, % dl because otherwise VQ would be VQ= TI (r, - dd2* L] we obtain, on combining eqns.(23), (118) and (118a),

With jl= K L / k (eqn. (24)) we obtain from eqn. (119)

This means that the smaller the column radius, the higher will be the capacity ratio, k = tlR/tM, or, in other words, increasing the column radius would increase the phase ratio and decrease k (as KLdepends only on the stationary phase and the temperature and does not depend on the column dimensions). Decreasing k would, however, according to eqn. (99), require more theoretical plates to achieve the same resolution of two adjacent peaks. As a consequence, the column diameter has to be suited to the analytical problem. As an average, standard column diameters of 0.25-0.30mm (wall-coated) or 0.5 mm (supportcoated columns) can be recommended, especially for screening purposes. When volatile compounds have to be analysed, the phase ratio must be decreased, which can be achieved by increasing the film thickness or decreasing the column diameter, and vice versa when analysing high boiling samples, where the phase ratio has to be increased. The diameters of glass and fused-silica glass capillary columns are similar to those made of stainless steel. The inside diameter commonly ranges from 0.10 mm to 0.20 mm for high resolution analyses (only split mode) and for mass selective detectors. For split/splitless injec-

3.3. Open-Tubular Columns

57

tion, column diameters of 0.25-0.32 mm can be recommended. In spite of their lower separation efficiency (about one third compared with columns of 0.18mm I.D.), wide bore columns (0.53-0.75 mm I. D.) are favoured for trace analysis (owing to their expanded sample capacity of ca. 2000 ng, i. e., a factor of >20 at comparable film thickness!), for high speed analyses in the case of less complicated separation problems and for packed column conversion. Depending on the inside diameter, the outer diameter is 0.20-0.29 mm (0.10-0.18 mm I. D.), O . ~ ~ I I I I I I (0.25 mm I. D.), 0.40-0.50111111 (0.32I.D.) and 0.66(0.53I. D.). Returning to the column length, which, as it is directly proportional to the number of theoretical plates [eqn.(38)] and to the analysis time [eqn.(lll)], is an essential column parameter, we must carefully consider necessity and possibility. In order to save time when analysing a large number of samples, the column length should be chosen only so as to achieve the necessary resolution of the worst separable pair of sample constituents. This possibility, which saves analysis time contrary to the concept of "open-tubular columns = exceeded columns length = superior efficiency", has often been overlooked. Thus, unless very complex mixtures have to be analysed, relatively short columns can also solve many less complicated problems, in a shorter time. For calculating the required column length and for length-temperature time normalization, see Section 3.1.2.

3.3.3.

Wall-Coated Open-Tubular (WCOT) Columns

Wall-coated open-tubular (WCOT) columns, introduced into gas chromatography by Golay in 1958 [20, 721, contain the stationary phase in form of a thin film deposited on the more or less smooth internal surface of the tube wall. This doubtless most promising column type in addition to the support-coated open-tubular columns, has been developed particularly by Diksfra and De Goey [140], Desty [141], Kaiser and Sfruppe [142], Scoff [143], Condon [144], Zlatkb and Louelock [145], Schomburg [146], Grob [147], Tesaiik and Novofny [148], Eftre [149] and Guiochon [150]. 3.3.3.1.

Properties and Pre-treatment of the Inner Tube Surfaces and the Formation of Films

An essential requirement for high separation efficiency is the formation of a thin and uniform film along the total length of the column, as otherwise the formation of droplets might occur, which would decrease the efficiency of the column to unacceptably low levels, and problems connected with active sites would be even more in evidence. The applicability of a WCOT column stands or falls with the homogeneity and stability of this film on the wall of the column. The wettability of the surface depends, in one respect, on the surface energy of the tube wall, and in the other on both the surface tension of the liquid stationary phase and the (mostly unknown) interfacial tension between liquid and solid. A measure of wettability is the cosine of the contact angle, which is correlated with the mentioned quantities by eqn. (121) [90, 981: Ys = Y L S + YL cos @

(121)

where ys = surface energy of the solid, 12. = surface energy of the liquid stationary phase,

interfacial tension between liquid and solid, 0 = contact angle. In order to wet a surface sufficiently, 0 must be as low as possible and cos 0 as near as possiy~~ =

58

3. The ChromatographicColumn

ble to unity, and ys must be higher than fi. We have seen in Section 3.1.3 that the contact angle can be decreased by roughening the surface [eqn. (117)],and hence the film formation can be improved. Let us now consider the different surfaces. Stainless steel has relatively rough walls, which is advantageous with regard to film formation, as we have just seen. However, this material is active towards polar and labile compounds, and the activity must, prior to or simultaneously with the coating procedure, be reduced, e.g., by the addition of surfaceactive agents [151].Because of their lower activities, siliceous tube materials have been preferred for numerous gas chromatographic applications with the exception, perhaps, of the analysis of hydrocarbons, which is most often carried out with stainless-steel capillaries. Nevertheless, one should be aware of the fact that even glass and to some extent also fusedsilica glass have some activity. Its structure, composition and history of temperature treatment during the manufacture affect its behaviour, and especially its surface, the composition and structure of which influence column performance much more than the bulk glass characteristics [152],and this will be discussed in detail now. If we consider glass surfaces, we can expect metal and boron ions, which can serve as Lewis acid sites, causing adsorptive tailing. Owing to the substantially minor concentrations of these ions on fused-silica glass surfaces, this material is originally more inert [75].To improve the surface of soda-lime and borosilicate glasses, the oxides of metals (Ca, Mg, Al, Fe) and boric oxide have to be removed by leaching or etching procedures. Leaching involves, according to Grob and Grob [154],extensive hydration of the SiOz lattice, which permits soluble ions to be extracted with an appropriate solvent. The procedure consists in, e.g., filling of the tube with 20% HC1(90%of the tube length), sealing of the inlet and outlet and heating for 14 h at 160°C.After cooling the tube it is rinsed with water to neutrality [155].The surface is now a silica gel, which has to be dehydrated, but without removing most of the silanol groups because these are needed for the subsequent deactivation steps (preferably silylation). Therefore, the tube is purged with nitrogen and dried, applying a vacuum at temperatures between 150°C [156]and 300°C [155, 1571. In contrast to leaching, in the etching process the entire surface, including the lattice ions, is attacked and more or less dissolved [154].Etching reagents are gaseous HF or fluorine compounds that liberate HF at elevated temperatures, e.g., 2-chloro-1,1,2-trifluoroethyl methyl ether or NH4HF2[158,1591. The reagent is introduced, the column ends are sealed and the column is placed in an oven, the temperature gradient of which is small, heated (in the case of the ether) to 400°C and maintained at this temperature for 24 h. By the reaction of HF with the glass surface, silicon fluorides are formed, which are decomposed in the closed system to form silicon dioxide, which in turn is deposited in the form of whiskers. This uniform layer of silica whiskers provides, owing to the lack of the metal ions, and to the increased surface area and roughness, a suitable substrate for the liquid stationary phase. Wettability and the achievable (increased) sample capacity favour this procedure and its results. On the other hand, its disadvantage may not be overlooked. In particular, the subsequent, necessary deactivation seems to be difficult [160]. So far we have only considered the conversion of glass surfaces by leaching, rinsing and etching processes to produce a pure silica surface. The physical and chemical state-porosity, ruggedness, SiOH content and entrapped residual reagents-cannot be expected to be the same in each treated column. This is the reason why on the one hand excellent columns can be produced from conventional glasses, but on the other hand some columns are obtained that have to be rejected, as differences in surface density and porosity may affect the chromatographic behaviour [1611.Fused-silica glass columns, in contrast, possess only negligible, if any, Lewis acid sites and exhibit by nature a pure silica surface without any previous treatment and have a high degree of uniformity from column to column. The development of the fused-silica column has thus offered a better understanding of the role of silica surfaces with respect to deactivation and coating procedures, and treatments prior to the coating pro-

59

3.3. Open-Tubular Columns

cess are much more predictable and reproducible and less expansive than those required for conventional glass columns. Nevertheless, owing to the presence of surface SiOH groups, there are active sites on fusedsilica column surfaces and also leachedhinsed or etched conventional glass column surfaces. In an excellent survey, Jennings dealt with such surface phenomena, their origins and methods of deactivation [161]. Hydroxyl groups occur on those lattice silicon atoms, whose interatomic distances preclude the formation of additional siloxane linkages or which are formed after re-reaction of highly strained siloxane bridges with water. Different types of such silanol groups can be present: Lone silanols (a), vicinal silanols (b), geminal silanols or silanediols (c). OH

HO

I

- Si

HO

I -Si-0-5-

I

\ /

I

I (b)

(0)

OH

OH OH

(d

Free silanols are acidic and can react with ion-pair electron donors and hence they exhibit strong adsorption sites. When the interatomic distances of adjacent oxygen atoms are less than 2.8 A (2.8 10-lo m or 0.28 nm), the silanol groups are hydrogen-bonded to one another

(d)

(e)

[161] (d) and exhibit only weak or no adsorption, unless, through interaction with water, they become strong adsorption sites [162, 1631 (e). Heating to ca 165°C results in the desorption of water, which in turn can be readsorbed on cooling. Condensation reactions of spatially appropriate silanol groups to form siloxane bridges,

take place between 150 and >800"C. At first, on increasing the temperature from ambient to ca. 200"C, water from the condensation of geminal hydroxyl groups (c) is degassed, followed by water stemming from types (b), (d) and (e). This dehydration is reversible on heating to 400°C and partially even to 800"C, and increases with increasing temperature. Even above 800°C silanol groups still disappear, and strained, reactive siloxane bridges form. This reaction becomes reversible once again at much lower temperatures and exposure to room air and moisture [164]. We can now conclude that there exist in each glass column (after having removed Lewis acid sites) and fused-quartz and fused-silica glass columns (where Lewis acid sites, if any, would exist in negligible concentrations) a series of gradations from completely free silanols to fully bound silanols and from non-reactive to highly strained, very reactive siloxane linkages and bridges. These surface properties and compositions determine the possibility and degree of reactive and adsorption tendencies and also wettability. A column, just fused, exhibits a very high surface energy and hence can, according to eqn. (121), easily be wetted. On the other hand, a high surface energy means a high concentration of active sites, which has to be avoided in order to produce an inert column. Between these oppositing demands a compromise has to be reached. This is the goal of the following column surface pretreatments, and of the recently developed coating procedures described below and in Section

60

3. The Chromatographic Column

3.5.2., which avoid droplet formation by using gum phases, by bonding phases and by immobilizing the stationary liquid by means of cross-linking reactions. Because of their lesser importance in present, several pre-treatments, even though not long ago frequently applied and investigated, will be mentioned only briefly. In addition to the carbonization of methylene chloride and deposition of the formed carbon black on the inner column walls [165], the controlled deposition of sodium chloride [166, 1671 and barium carbonate [168] have been thoroughly investigated. In lieu of salt deposits, which have been found to decrease the column activity to a certain extent, thin layers of silica can be deposited by the hydrolysis of silicon tetrachloride at high temperatures [169]. These layers increase the surface area and wettability, but simultaneously the column activity is distinctly increased, for obvious reasons, and subsequent deactivation treatments are in any event necessary. Procedures meant to block active sites by surface active groups, by polar polymers or by thermal degradation products of polyethylene glycols sometimes seem to be successful (e.g. ref. [170]). However, the thermal stability is limited, and the column activity is frequently regenerated at elevated temperatures; further, abstraction of reactive sample constituents may occur, and the retention behaviour may be affected. One of the most promising ways of deactivating the inner walls of glass and fused-silica glass columns (which must be free from Lewis acid sites!) is chemical reaction of the surface silanols and highly strained siloxane bridges with thermally stable silylating reagents or other organosilicon compounds when reacted at relatively high temperatures. Let us first consider the true silylation reaction. A reactive silanol reacts with a silylating reagent to form silicon-oxygen-silicon bonds or, in other words, the active hydrogen atom of the SiOH is exchanged by a silyl group:

I I

-SOH

+ A-SiR3

+

I I

-Si-0-SiR3

+ HA

where A-Si is a reactive group (Cl-Si, CH,O-Si, HN-Si, etc.) and R is alkyl (most frequently CH3)or phenyl or, occasionally, if A is C1, a further C1 atom. For details of such reactions, see refs. [171-174, 85 and 1551. Most often hexamethyldisilazane is applied: 2 SiOH + (CH3),SiNHSi(CH3), --* 2 SiOSi(CHJ3 + NH, which, at high temperatures [85], in addition to silylation, might due to attack on the lattice by the liberated ammonia form fresh SiOH groups. However, this seems innocuous, as at such lattice points where the reaction SiOSi + NH, + SiOH + H2NSi might have taken place, in successive reactions, including water from condensation processes, excessive silylation might occur: SiNH2+ H20 + SiOH + NH3 and 2 SiOH would react as above to form 2 SiOSi(CH3),. Hexamethyldisilazane, of course, can also react with water to form hexamethyldisiloxane: (CH3)3SiNHSi(CH3)3 + H20 + (CHJ3SiOSi(CHJ3 + NH3 but this phenomenon is not of great consequence because of the excess of the reagent, unless the amount of ammonia formed is to be measured for elucidating the silylation rate. In lieu of hexamethyldisilazane, its phenyl and fluoroalkyl substitution products, R3SiNHSiR3 (R = CH,, CF3CH2CH2,CsHS),have been applied, which, when using modified conditions, also deactivate well [155, 156, 1751and provide a wettable surface for weakly and medium polar phases, as they increase the surface energy of the column walls [compare eqn.(l21)]. If organic compounds are to be silylated, often a mixture of hexamethyldisilazane and trime-

61

3.3. Open-Tubular Columns

thylchlorosilane is used [173] because of its higher silylation power. This is also true for silylating surfaces under mild conditions. However, such conditions are not sufficient for good deactivation, and high-temperature silylation should rather be used as the presence of additional trimethylchlorosilane is not necessary. Chlorosilanes themselves react with silanols, the reaction rate depending on the number of chlorine atoms in the molecule: CH3SiCI3 > (CH3)2SiC12 % (CH3)3SiC1. Monofunctional chlorosilanes R3SiCI (R = alkyl, phenyl, most often methyl) react according to

I I

I + C1SiR3 + -Si-O-SiR3

-SOH

+ HCl

I

the liberated hydrogen chloride being suspected of affecting the silica lattice. With dimethyldichlorosilane, one reaction course corresponds to the above-mentioned equation:

c1 I I I --SiOH + C12Si(CH3)*4 -Si-OSi(CH3)2 I I

+ HCI

one reactive Sic1 group surviving and being capable of consecutive undesirable reactions. However, as dimethyldichlorosilane is a difunctional chlorosilane, it may react with two properly spaced silanol groups: VH3

OH

OH

I

I

-Si-Si

I

CI

-+

1

-I

CI

\

0-Si-0

/ Si

__c

/ \

CH3

I

CH3

I

I

CH3 -

I1

+ 2 HCI

1

I

This type of reaction is also applicable to a trifunctional chlorosilane, but here unreacted C1 atoms will survive to a greater extent: I

CI

I

-Si-0-Si-CH? I I CI

and

CI I 0-Si-0

I

I

CH3

I

and have to be removed, as with dimethyldichlorosilane, by an additional reaction with moisture or methanol (producing new SiOH groups or hydrolyzable SiOCH3groups in the case of methanol). Regarding these restrictions, the following procedure seems more promising [155, 1611. After the leaching step (for opening of a maximum number of silanol groups) the column is thoroughly dried in order to remove traces of water (but without condensing silanol groups!) and rinsed with hexamethyldisilazane-diphenyltetramethyldisilazane(1:1) in two parts by volume of pentane, subjected to a vacuum, flame sealed and heated at 400°C for ca. 12 h. With fused-silica glass columns, the oven has to be purged with nitrogen in order to protect the polyimide that covers the outer walls of the column. One sealed column end is opened under toluene, which is allowed to fill some of the coils, then toluene is replaced with methanol and finally with diethyl ether. The amount of reacted reagent, recognizable by the ammonia liberated, is determined by measuring that part of the column which remains un-

62

3. The Chromatographic Column

filled when the pressure has equilibrated (as mentioned earlier, this only applies if water, which also could have reacted, can be excluded). According to Grob et al. [155], the proper degree of silylation has been reached when a third to a half of the column is filled with NH3, which prevents it filling with solvent. Further phenyl-substituted silylating reagents were used by Grob and Grob [176]. Deactivation can also be achieved using organosiloxanes. Schomburg et al. [177, 1781 coated dynamically the leached glass column with a poly(dimethylsiloxane),filled it with inert gas, sealed it and heated it at 450°C for 2-20 h. The degradation products formed under these severe conditions were considered to react with surface silanol groups or be adsorbed. The column was then opened and subjected to solvent extraction to remove that part of the decomposed polysiloxane which was not bound. It is well known in silicone chemistry that by thermal degradation of poly(dimethylsiloxanes), in addition to linear fragments of varying chain length, oligomeric cyclosiloxanes (hexamethylcyclotrisiloxane,octamethylcyclotetrasiloxane, and in decreasing amounts the higher homologues) are the main products that are formed. It can be assumed, and this would be analogous to ref. [179], where cyclosiloxanes were applied for deactivation of silanols on capillary walls from the start, that at elevated temperatures the cyclosiloxanes react with the silanols via ring opening of the cyclosiloxane and substitution of the reactive SiOH proton:

,

IjI.." P-EjiR2

- si -0

I

.... SiRZ I

0

I

0 -SiR2

--

I -Si-O-SiRZ

-0SiRZOSiR20H

I

on the surface. These groups exhibit less active sites and, on the other hand, create a better wettable surface for non- or weakly polar stationary phases, and are thermally stable with regard to chromatographic requirements. Similar ways of deactivating the inside wall and simultaneously improving the wettability have been the subject of recently published and current investigations [180- 1851. Murkides et al. [180] treated fused silica columns (0.20 mm i.d.) as follows. A plug of halfconcentrated hydrochloric acid ( ~ 1 8 % HC1 w/w) (10% of the column length) was forced through the tube by nitrogen until it had left the end of the tube. Both ends were carefully sealed and the capillary was heated at 140°C. The water may substantially increase the number of silanol groups on the surface and hence increase the reaction between these groups and functional groups of the consecutively applied organosiloxanes, being silanol-terminated or having become silanol-terminated by ring-opening reactions. The acid was displaced by one capillary length of dilute HCl (PH3), followed by one capillary length of methanol. Dehydration was carried out at 260°C for 5 h under a slow stream of nitrogen. The column was then dynamically coated (for deactivation!) (for true coating procedures see below) with 20% (w/v) solution of bis(cyanopropy1)cyclotetrasiloxane in CH2C12at a constant rate of 20 mm/s in order to cover the silica walls with an even film. After evaporation of the solvent (6 h), the tube ends were carefully sealed under vacuum, preventing any trace of air from entering the column. Chemical modification was achieved at 395°C (the outer surface of the fused-silica tube has to be taken into account, however, when choosing the temperature, as with a polyimide outside coating immediately after the drawing process its thermal stability is limited to ca. 340°C unless a protective gas (N,) is purged through the oven) for 2 h, the oven being heated and cooled slowly (5 Wmin). The column end was opened under methylene chloride, which was used to rinse the column free from excess of cyclosiloxane. The column was dried and was then ready for the true coating procedure with the stationary phase. In lieu of cyclosiloxanes, which have to undergo ring opening during the treatment prior to the reaction with surface silanols, silanol-terminated organosiloxanes can be directly applied for covalent bonding to the inner walls of the column.

3.3. Open-Tubular Columns

63

Verzele et al. [1811 synthesized an a,w-oligo(methylpheny1siloxane)diol CH3

I

HO-(Si-O)nH

I C6H5

and deactivated the tube by dynamically coating it with a 0.3% solution of the diol in CHzC12 at a speed of 2 cm/s. The capillary was evacuated, sealed and heated to 300°C at 3 K/min and held at this temperature for 15 h. Excess of reagent was removed as above [180]. Even with untreated fused-silica glass tubing exhibiting only relatively few silanol moieties on the surface, specially prepared non-polar, high-molecular-weight silanol-terminated polysiloxane stationary phases can be bonded covalently to the walls [182, 1831. Untreated fusedsilica glass columns were purged with nitrogen at room temperature before use to remove residual HC1 (unreacted Sic& present in the various procedures used in the drawing of narrow-bore tubing above 2000°C is converted by moisture into HCl, which clings to the inner surfaces of the tube unless preferentially removed [183]. For a 25-m column, all traces of hydrogen chloride disappeared, as indicated by moist pH paper, in ca. 5 min. Non-polar silanol-terminated poly(dimethylsi1oxanes) or weakly polar poly(methylphenylsi1oxane) mixtures with poly(dimethylsi1oxane) (5% phenyl), partially cross-linked by heat before use, were dissolved (30% in CHzC12for dynamic coating or 0.3-1.2% in pentane or CHzC12for statical coating). Traces of solvent were subsequently removed by gently purging the column with nitrogen for 2-3 h at 60°C and, with N2still flowing, the column was allowed to cool to ambient temperature. The ends were placed under vacuum for 30 min and then carefully sealed. The temperature of the column was slowly increased at 2-4"C/min to 370°C and maintained at this temperature for 5-15 h. After cooling, the seals were broken and the column was again purged with nitrogen for 1 h and rinsed with methylene chloride and pentane. The polymers, contacted directly to the untreated surface of the fused-silica capillary tubing at these high temperatures, readily deactivate and wet the column inner surface by distributing themselves as a homogeneous film throughout the whole length. Following the extraction with methylene chloride and or pentane, 85-95 % of the film appeared to be cross-linked in the case of partially gummified polymers prior to coating; if not previously cross-linked, only 50-60% of the condensed film was immobilized. From numerous results (e.g., refs.[177-186]), it can be concluded that no single pre-treatment can be used for the preparation of the surface even of as a unique glass as fused-silica which would enable one to coat it effectively with stationary phases of any polarity. Thus, surface pre-treatment has to be selected carefully, depending on the stationary phase to be employed. A step forward in this direction is the surface deactivation of untreated fused-silica glass with non-polar and polar prepolymers having various chemical compositions and subsequent cross-linking of the prepolymer to obtain the proper stationary phase film. 3.3.3.2.

Coating Procedures

As already emphasized in Section 3.1.3, a detailed discussion of coating procedures is not given in this book; only summaries of the principles and brief mentions of their pros and cons are included. For more details the reader is referred once again to handbooks (e.g., ref. [187]) and to the original papers cited. A practicable procedure is the dynamic technique introduced by Dijkstra and Goey [40]. The stationary phase is dissolved in a suitable, carefully purified solvent to give a concentration of 5-30% w/w. The solution is poured into a coating reservoir according to Schomburg and Husmann [188] together with mercury in order to produce reproducibly homogeneous films.

64

3. The Chromatographic Column

The coating device is connected with the tube, and by means of nitrogen, the flow-rate of which is controlled by a precision pressure regulator, a plug (10-30 cm long) of the impregnation solution, followed immediately by a mercury plug (3-15 cm long which corresponds to 0.02-0.11 g of mercury at a tube i.d. of 0.25 mm), is forced through the tubing at a velocity of 1-2 cm * s-*. The nitrogen flow is continued, even if both plugs have left the column, in order to remove the solvent; if the boiling point of the solvent is not too high, purging with nitrogen for 3 h at 60°C will be sufficient. For details of this method, see refs. [187-1911. The advantage of this method is that it is easily and quickly practicable. The disadvantages are that: The amount of the stationary phase in the column cannot be determined exactly; deviations of the inside diameter of the tube result in variations of the film thickness; and the efficiencies are lower than with static procedures. The static vacuum method first applied by Bouchd and Venele [191] consists in slowly vaporizing the solvent from the stationary phase solution in the column. The dilute solution (5l%),prepared with highly purified solvent, is degassed and sucked into the column. After complete filling, one column end has to be sealed, avoiding any air bubbles, e.g., by means of a mechanical device [192], and the column is placed in a suitable thermostat the temperature of which depends on the solvent. This is volatilized slowly, cautiously maintaining a vacuum. When the volatilization has finished, the sealed end is opened and traces of solvent and residues are removed by an inert gas stream. For details, see refs. [192-1981. The advantages of this method are that it is especially applicable to high-viscosity stationary phases, high-efficiency columns are produced; and the amount of stationary phase and the film thickness can be calculated. The disadvantage is that it is time consuming. The static evaporation procedure was introduced by Golay [20]. As above, the impregnation solution is sucked into the column, carefully avoiding evaporation of the solvent. After having sealed one end, the column, starting with the open end, is moved through a relatively hot zone into a thermostat, thus avoiding re-condensation of the already vaporized solvent. In the hot region the stationary phase film develops. Several parameters (velocity of the column movement through the heated zone, the applied temperature etc.) have to be optimized. For details, see refs. [20, 187 and 199-2031. The advantages of this method are that the amount of stationary phase in the column can be measured and the film thickness easily be calculated; viscous liquids can be coated; and it is not time consuming. The disadvantage is that occasionally inhomogeneous films are formed with low viscosity stationary phases (owing to non-uniform evaporation of the solvent).

3.3.3.3.

Bonding and Cross-Linking of Stationary Phases

Recent advances in bonding phases on to the inside walls of fused-silica glass columns and in immobilizing stationary phases in fused-silica glass columns have led to enhanced stationary phase stability. Owing to its outstanding advantages compared with the usual coated open-tubular columns, in all likelihood this method will, in conjunction with fused-silica glass column material, substantially extend the application of WCOT columns to increasing numbers of laboratories and fields. This is why this procedure, although still in mid-development, will be discussed here in detail. Let us first define the suitable chemical reactions taking place (a) on the wall surface between the surface silanol groups or strained siloxane bridges and appropriate stationary phase reactants, and (b) within the stationary phase itself prior to, during or after coating. Type (a), which we have already dealt with in the previous section, is the actual bonding reaction, which can be assumed to proceed according to one of the following simplified schemes:

65

3.3. Open-Tubular Columns

-SiOH

+

Si - R

HO-

I

I

-

I

I

R

+

CI

I

- Si-R

I

I

--

18

-

I I

R

R

I

-R

SiOH + H - 3

I

I

-

1 +(R2SiO)nt)

I

R

I

I

+ H2

-SiOSiR

R

-SiOH

+ HCI

-SiOSiR

R

I

H20

R

R -SiOH

+

-5iOSiR

I I

R

-

followed by ++I

R = C 3 z n + 1 , C ~ H> SClzHg, CnH7 CF3CHzCH2 CN(CHz)n, OH 9

9

*) cyclic Siloxanes **) thus linking two surface silanol groups, which otherwise would for steric reasons not react with them-

selves, via a short diorganosiloxane chain [small number n of D-units (R2SiO)].

\SI "'Si

/

\o/

/'

+

HOSiR,

\

-

OH

OSlR3

\ ISI - 0 - 5 - '

/ \

/

(highly strained) OH

\s/o\si/ H20 \o/ \ +

/

OH

\ I si -0 - si I/ /

\

/I

(being reactive os above shown) Si R2

66

3. The Chromatographic Column

PH

(being reactive as above shown)

*) Cyclic siloxane

Type (b) is a reaction taking place in the liquid stationary phase. Several terms have been used, e.g., curing, vulcanization, immobilization, cross-linking, in situ cross-linking, polymerization and condensation of prepolymers, free radical cross-linking, auto cross-linking, gummification, in situ vulcanization and production of non-extractable (or insoluble) films. Some terms stem from polymer chemistry, others from elastomer and rubber technology and two can be traced back to gas chromatography. In this book, we prefer in general to use those terms that describe the chemical reaction (cross-linking, polymerization, condensation) if the reaction itself is dicussed. The main reasons for the extensive investigations in this field are that on glass and fused-silica glass walls in open-tubular columns, column deterioration was often observed as a consequence of a physical rearrangement of the stationary phase (formation of droplets) at elevated temperatures and that the wettability of the surface with polar stationary phases, if any, was considerably obstructed. The first attempts to bond the phase to the surface in combination with cross-linking were carried out by Grob [165], Bossurf [203] and Mudani [204] and soon afterwards by Blornberg and co-workers [205-2071. So far, bonding and cross-linking has chiefly been restricted to silicone-type phases. This is based on the fact that these phases have been most widely used in gas chromatography because of their numerous merits, which will be discussed later in this book, and not least by the possibilities that they offer for tailormaking stationary phases. Bonding reactions have already been discussed and will only be described here when experimental details prior to the cross-linking reaction have to be given. Increasing the viscosity (i.e., increasing the chain length in the case of linear polymers) has been shown to improve film formation and film stability of liquid stationary phases [208], and polysiloxane gum phases with long chains have been synthesized, thus increasing the column efficiencies of coated fused-silica open-tubular columns [209-2 111. In spite of these improvements, the bulk phases are not yet immobilized, even when partially bonded to the surface. By introducing a slight degree of cross-linking, however, in the stationary phase itself, better immobilization can be achieved [205-207, 1211. Occasionally, a,o-(polysiloxane)diols have been partially bonded to the surface or/and partially coated and subsequently heat-cured after addition of a catalyst, e.g., tetramethylammonium hydroxide. Such coatings are less extractable, but the reaction having taken place is a polycondensation, leading to high-molecular-weight materials, and is not a desired cross-linking reaction. Cross-linking is a decisive step for immobilization. With the mentioned siloxanediols, tri- or tetrafunctional silanes, e.g., RSi(OR)3 or Si(OR)4, have to achieve a true cross-linking reaction (184, 185, 204-207) by condensation of hydroxy and, e.g., alkoxy groups with elimination of water, alcohol or ether and formation of Si-0-Si cross-links.

67

3.3. Open-Tubular Columns

Even if the thermal stability, due to the stability of the Si-0-Si bond, is outstanding, this type of cross-linked stationary phase has crucial disadvantages. High cross-linking levels, which give rise to diminished solubilities of sample molecules, and an undesirable activity of the column, are caused by residual SiOH and SiOCzHScontent [213].Further, residual alkoxy groups are susceptible to hydrolysis. Hence it is much more promising at present to use free radical generators to form carbon- carbon cross-links, as very little cross-linking (0.1-1%) is necessary to change long polymeric chains into insoluble rubbers, hence requiring only low levels of cross-linking agents [213].Free radicals can be generated by decomposition of organic peroxides, of azo compounds or by the use of high-energy radiation. The linking occurs according to the following scheme in the case of pure poly(dimethylsi1oxanes) (R' = radical):

)

CH3- Si - CH3

s I

R' R' I T

)

2R'

CH3 -5- CH3 -CH3 I

P

)

-Si -CH2

I

>

- CH2-Si

+2 R H

-CH3

I

Groups other than methyl present in the poly(diorganosi1oxane)molecule have a decisive effect on the cross-linking rate and the nature of the cross-linked product. If vinyl groups are present (in addition to the bulk of methyl groups), the methyl-to-methyl cross-linking is no longer dominant, but rather methyl-to-vinyl cross-linking [213,2141:

1

CH3-Si-CH I

R'

2

R'

2

CH3-Si -CH~-+CH3-Si-CH2-CH~-C~-Si-CH3

= CH2 T

}

I

I

I

P

P

P

+ R'

Because of the greater tendency of vinyl groups to cross-link, as is well known in silicone chemistry [215],lower levels of free radical generators are required, in order to achieve the same or even a higher degree of cross-linking. Numerous free radical generators have been used for the initiation of cross-linking. First, peroxides are most effective in forming insoluble stationary phases. Benzoyl peroxide, bis (2,4-dichlorobenzoyl) peroxide, dicumyl peroxide and di-tert.-butyl peroxide have, for example, been applied, of which dicumyl peroxide can be considered to give decomposition products that affect the column stability less than the other peroxides, and hence is the most often used. It would be beyond the scope of this book to review the numerous investigations in this area and therefore only a few bibliographic details are cited [210,212, 213, 216-2311. Second, in situ generation of free radicals can alternatively be accomplished by using azo compounds, e.g., azo-tert.-butane, azoisobutyronitrile, or azo-tert.-dodecane. Owing to its main decomposition products, nitrogen, isobutane and isobutene, which are non-polar and do not react with the polysiloxane chain, the azo-tert.-butane, in contrast to peroxides, cannot form acidic products which would attack the column. Unfortunately, it is less reactive than the peroxides and requires higher temperatures to obtain acceptable free radical generation rates, and hence comparable cross-linking [213].Nevertheless, apart from dicumyl peroxide, azo compounds can be used; for details see refs. [182,211, 213, 227 and 232-2371. A third procedure for cross-linking silicone, especially polar silicone, stationary phases consists in applying radiation, either gamma radiation from a 6oCosource [227,238, 2391 or accelerated electrons from a Van de Graaff generator [240].It offers the advantages that no chemicals with harmful effects have to be added, that the reaction takes place at ambient tempera-

68

3. The Chromatographic Column

ture and that the columns may be tested before cross-linking. Owing to the expansive equipment required, its application will be limited, however. More readily available is the equipment for a fourth method, in situ cross-linking with ozone [241, 2421. Immobilization can be achieved at low temperatures (ambient for dimethylsiloxanes) or medium temperatures (150°C for phenyl- or cyanopropyl-substituted siloxanes). Nevertheless, we feel that this reaction type should be treated with caution. Even if silicones have proved to be relatively resistant towards oxidation, the presence of ozone must have some effect. The absence of an infrared carbonyl band, or at least only a very weak absorption, observed after curing, [242], may not be admissible evidence, nor are the marginal differences in Kovhts retention indices [242]. Naturally, this also applies to peroxide-initiated cross-linking. More thorough investigations need to be carried out if small alterations, which could affect the analysis of labile or trace compounds, are to be recognized. A few cross-linking procedures will now be given in detail.

Procedure (a), according to Wright, Peaden and Lee [213]. Deactivation consists in rinsing the fused-silica capillary tubing with 5-10 ml of methanol at room temperature, then purging with Nz for several hours to remove any traces of methanol. Next, octamethylcyclotetrasiloxane (D4 in organosilicon chemistry nomenclature) is dynamically coated by filling ca. 20% of the column and then rapidly pushing the D4 plug through the column with Nz until it has been expelled. After sealing both ends, the column is heated for 2 h at 420°C (whilst protecting the polyimide outer coating with N2!),and subsequently the column is purged with Nz for about 30 min at 350°C to remove any residual D4. The column is statically coated. The long-chain poly(dimethylsi1oxane) containing vinyl groups is dissolved in purified pentane and the concentration is selected to give a film thickness of 0.1-0.5 pm. Solid dicumyl peroxide, dissolved in methylene chloride (l%),is added to the coating solution 30 min prior to coating to give a concentration of 0.28%.After the usual coating the column is purged with nitrogen, sealed and temperature-programed from 40 to 175°C at 4 W rnin and held at 175°C for 15 min. After cross-linking, the column is washed for 30-60 rnin with 5-10 ml of methylene chloride. Subsequently it is conditioned with a slow carrier gas flow for 1h at 40°C to desorb any residual solvent from the stationary phase left from the washing procedure and then temperature programmed to 260°C at O.S"C/min and held there for 8 h. When using azo-tert.-butane, which is a liquid at room temperature, the column is coated in the usual way, without the free radical generator prior to cross-linking. The coating is followed by saturation of tke stationary phase with the vapour of azo-tert.-butane by bubbling nitrogen through the azo compound and purging the coated column at 40°C for 2 h. Subsequently, for dynamic curing the column is attached to an argon manifold in an oven and heated at 5-1O"Umin to 220°C and maintained there for 15 min; the argon linear velocity is 10 c d s . After curing, the column is rinsed with 10-25 column volumes of methylene chloride-acetone (5050,v/v) and then conditioned with a rapid carrier flow for 30-60min at room temperature. After being reconnected to the argon manifold, the column is heated at 5"C/min to 350°C and held there for 4 h with an argon velocity of 25-30 cm/s. Procedure (b), according to Lee and co-workers [210, 2361. First, a mixture of dimethyldichlorosilane, methylphenyldichlorosilane, diphenyldichlorosilane, methylvinyldichlorosilane and 1,4-dimethyl-1,1,4,4-tetrachlorodisilethyleneis prepared for synthesizing a polymer with the desired phenyl and vinyl contents. Hydrolysis of the mixture is accomplished by dissolution in an equal volume of acetonitrile and adding an equal volume of water, followed by an equal volume of methylene chloride. The methylene chloride layer is then extracted with distilled water to neutrality. The methylene chloride is finally removed by gentle warming under a nitrogen purge. Polymerization

3.3. Open-Tubular Columns

69

is accomplished by adding 0.05 wt-% of the catalyst tetramethylammonium hydroxide and heating at 110-130°C until the viscosity of the polymer ceased to change. The catalyst is then rendered inactive and the polymer is end-capped by the addition of trimethylchlorosilane. As the polymerized product contains both low- and high-molecular-weight moieties, fractionations have to be performed in order to remove the lower-molecular-weight materials. This is achieved by dissolving the polymer mixture in four times of its volume of CHzClzand adding an equal volume of methanol, which causes the higher-molecular-weight moiety to precipitate. After repeating this precipitation four times, the final precipitate is collected. In case of a 50% phenyl-, 49% methyl-, 1%vinyl-polysiloxane, the solvent used to prepare the coating solution is ratio n-pentane:methylene chloride 2:1, v/v. An appropriate amount of the polymer is dissolved to give the desired film thickness. The column is coated statically. To give a film thickness of 0.25 pm in a 15-m column of 0.1 mm I.D. in a reasonable time, the coating temperature can be chosen to be as low as 28°C. After coating, the column is purged with nitrogen for 30 min at room temperature. For cross-linking, azo-tert.-butane is purged through the column for 2.2 h using a special purging device [236]. Cross-linking twice, each time after purging azo-tert.-butane from opposite ends of the column, increases the successful cross-linking rate. After azo-tert.-butane purging, the column is sealed and temperature programmed from 40 to 220°C at 4"C/min and held at 220°C for 1 h for cross-linking. After cross-linking twice, the column is normally temperature programmed from 40 to 250°C at 0.5 Wmin under a gas stream (N2or He) and held at 250°C overnight. The finished column is then evaluated as usual and subsequently washed with 50-100ml of methylene chloride by means of an HPLC pump. Washout data can be obtained from the difference in k values before and after washing. Using this procedure, nearly 100%non-extractability can be achieved. Procedure (c), according to Venele et a1 [181]. Although not a cross-linking reaction, the following procedure is described here for two reasons. First, because free radical generators are avoided, decomposition products cannot occur and hence cannot give rise to column activity. Second, the polarity and selectivity are well defined and equal to those of, e.g., OV-17, because the prepolymer does not contain any vinyl or tolyl groups, which otherwise are necessary to achieve cross-linking of phenylsiloxanes initiated by free radical generators. Further, it is easily practicable. A prepolymer 1, synthesized by alkaline hydrolysis of methylphenyldichlorosilane, is used to deactivate the previously leached fused silica column. Deactivation is achieved by dynamically coating the column with a 0.3% solution of prepolymer 1 in CH2C12at a velocity of 2 cm/s. The column is then evacuated, sealed and heated to 300°C at 3 K/min and held isothermally at 300°C for 15 h. Excess of reagent is removed by rinsing the column with 10 ml of CHzClz.Prepolymer 1 is further polymerized to a semi-gum, called prepolymer 2, by heat-curing under nitrogen whilst continuously controlling the viscosity to avoid complete gummification and loss of solubility. The deactivated column is then statically coated with a 0.1-0.3% (w/v) solution of prepolymer 2 in CHzClz.The silanol-terminated prepolymer 2, coated on the column wall, is immobilized by in situ gummification by heat-curing. This is achieved by heating the column repeatedly from 150 to 250°C at 3 K/min for 15 h under a low flow of carrier gas (0.1 ml/ min). The column is then rinsed with 5 ml of CHzClzand conditioned. The immobilization yield was 100%. Residual terminal silanol groups can be capped by hexamethyldisilazane at elevated temperatures. Procedure (d), according to Markides, Elomberg, Buijten and Wannman [223, 223al. Bis(cyanopropy1)dichlorosilane is hydrolyzed with acid catalysis to form octakis(cyanopr0py1)cyclotetrasiloxane [212], which is coated dynamically in a 20% (w/v) or 2-5% (w/v) [223a] solution in methylene chloride at a constant rate of 20 mm/s on the inner walls of a fused-silica column. Prior to the coating, the column has to be leached with 20% hydrochloric acid at

70

3. The Chromatographic Column

100°C for 12 h, rinsed and dried [224, 1551 as described in Section 3.3.3.1, as a fused-silica surface seems to contain too few silanol groups to be sufficiently modified by this cyclosiloxane. To avoid any contamination, leached glass vessels, high-purity chemicals and freshly prepared solutions should be used for the leaching procedure. The dynamic coating has to be performed carefully in order to form an even film along the length of the capillary, and no air or solvent residues should be present. During the dynamic coating, a buffer capillary is connected to the column end. Immediately after the coating plug has left the column, the flow of dry nitrogen passing through the column is drastically increased, and the evaporation of the solvent is allowed to proceed for 6 h. The capillary is then evacuated and both ends are sealed. The sealed capillary is heated in an oven to 395°C at 5 Wmin and held at the fmal temperature for 1.5 h, after which the oven is allowed to cool slowly to room temperature. One end of the column is opened under the surface of CHzClz, thus forming a plug with which the column is rinsed using nitrogen. Finally, the column is dried by flushing with dry nitrogen. Now, the column is modified for good wettability with polar cyanopropylsiloxane phases. Prepolymers with a high degree of cyan0 substitution are prepared from bis(cyanopr0pyl)dichlorosilane, methyl(toly1)dichlorosilaneand dimethyldichlorosilane by basic reversed hydrolysis (in methylene chloride solution, according to Patnode and Wilcock [223b]). A linear methyl(viny1)pentasiloxane is synthesized from methyl(viny1)cyclopentasiloxane by ring-opening with boiling butanol [243]; 1.7-3 mol-% of this vinyl siloxane is included in the reaction mixture to incorporate vinyl groups into the gums as methyl(viny1)siloxanesegments rather than single methyl(viny1)-siloxane units in order to facilitate cross-linking of gums containing bulky groups. Further, 0.5 mob% of 1,4-dimethyl-1,1,4,4,-tetrachlorodisilethylene is added [analogous to procedure (b)] in order to introduce a slight degree of crosslinking in the prepolymer. The synthesis is carried out in acetonitrile. Polymerization is carried out at 110°C for 10 min using ammonia solution as catalyst [215, 2441. The reaction is performed with stirring under nitrogen. The gum is dissolved and purified by washing with dilute hydrochloric acid, followed by water, to remove catalyst residues. Finally, after drying the gum with calcium sulphate, residual silanol groups are capped by reaction with 1,3-divinyltetramethyldisiloxanein refluxing acetonitrile for 6 h under an atmosphere of nitrogen. This polymer is dissolved in acetonitrile-diethyl ether (3:2), and 5% of dicumyl peroxide, calculated from the amount of stationary phase, is added to the coating solution. Before filling the column, the coating solution is filtered and centrifuged; only freshly prepared solutions are used. Coating is performed by the static method and with the column immersed in a horizontal position in a water-bath. The coated column is opened under an atmosphere of dry nitrogen, 1min before disconnecting the vacuum, and is then directly purged with dry nitrogen for 30 min. The gum phase is then cross-linked in situ by dynamic curing in a GC oven, programmed from 40 to 170°C at 5 Wmin, the final temperature being maintained for 40 min. During curing the column is rinsed with a slow stream of dry hydrogen (0.1 ml/min). After cross-linking, the column is rinsed with 5 ml of methylene chloride and conditioned in a gas chromatograph programmed to 250°C at 1Wmin. Cyanopropylsilicone rubber, cross-linked via Si-0-Si, can be obtained according to ref. [212], and trifluoropropylsilicone rubber with cross-links of the type Si-C-C-Si according to ref. [220]. Conclusions concerning bonding and cross-linking of stationary phases With the development of immobilized phases by bonding and in situ cross-linking reactions during the past 5 years, the overall quality of WCOT columns has been vastly increased. The following advantages have become apparent: enhanced film stability involving long column lifetimes and high efficiencies of both non-polar and polar stationary phases on the col-

3.3. Open-Tubular Columns

71

umn wall; minimal phase stripping from sample injection solvents, thus allowing the injection of large amounts of sample, including samples dissolved in both non-polar and polar solvents, even in water; the possibility of washing the columns to remove non-volatile compound deposits, provided these are soluble; higher film thicknesses while maintaining stable films can be chosen; in spite of cross-linking, no significant differences in diffusion coefficients compared with conventional stationary phases [245] have been observed; cross-linking suppresses the glass transition mechanism of poly(dimethylsiloxanes), thus allowing efficient separations of, e.g., CI-C5 hydrocarbons at sub-ambient temperatures, even at - 70°C; and reduction of column bleeding can generally be observed as a consequence of cross-linking (including conditioning) and end-capping [which converts terminal silanols (which would give rise to degradation reactions) into innocuous SiOSiR,-groups, in the case of silicone stationary phases]. There are also a few disadvantages: temporarily, the immobilization is, with the exception of poly(ethy1ene oxides), restricted to silicone phases at present, but as different functional groups can easily be attached to the siloxane skeleton (C6H5,CN(CH2),, CF3(CH&, C6HS, C6H4,etc.), a wide range of selectivity can be achieved; cross-linking of polar silicone phases is more difficult than cross-linking of poly(dimethylsi1oxanes) and generally requires additional vinyl or tolyl groups and higher amounts of the free radical generator, e.g., peroxides; and decomposition of the cross-linking initiators, especially of peroxides, may cause undesirable column activity. In spite of these disadvantages, it can once again be concluded that fused-silica glass capillaries of different diameters, containing bonded and/or cross-linked tailor-made stationary phases, of different film thicknesses will increase the applicability of WCOT columns tremendously, at the expense of packed columns.

3.3.3.4.

Film Thickness of the Stationary Phase

We have seen in eqns. (62) and (63) that it is preferable to choose a stationary phase film that is as thin as possible, as both the C, and C, terms, i.e., terms describing the resistance to mass transfer from and in the stationary phase, respectively, depend on the stationary film thickness, d , . The first term is proportional to d : , and the second term is proportional to d , . Hence a reduction in d, will result in a decrease in C, and, owing to the square dependence, a greater decrease in C,, hence reducing hmin,i.e., increasing the column efficiency. However, this is only one side of the picture. On the other side we have to take into account, that a reduction in d, leads to an increase in the phase ratio, B, as

p , 2Ldl

(119)

and a higher B value means, according to eqn. (24) ( B = K L / k ) ,a smaller k value (because KL is a constant). A smaller k, in turn, would require, corresponding to eqn. (99), more theoretical plates to achieve the same resolution. This would be especially serious when analysing low-boiling compounds, which exhibit very small capacity ratio values. Therefore, we have to reach a compromise depending on the tube material (which might be active!), on the column temperature (which can be reduced when the film thickness is reduced) and on the physical and chemical properties of the sample components. If we return to the phase ratio, B = VG/VL [eqn. (23)], it is clear, that it can easily be adjusted by changing either the film thickness (which is contained in VJ or the column diameter (expressed as V,). Volatile compounds, for example, can well be separated in columns with an increased film thickness of up to 5 pm, compensating for the lower separation efficiency by an increase in

72

3. The ChromatographicColumn

column length, whereas the separation of high-boiling compenents can be accomplished on shorter columns with relatively thin stationary phase films of dl = 0.1 pm [2471. Although hitherto the exception, open-tubular columns of inside diameter ~ 0 . mm 5 and with film thicknesses of d, = 2 to 7 pm, rendered possible by cross-linking, can be prepared, thus permitting the injection of larger sample amounts but nevertheless maintaining better performances than with packed columns, especially shorter analysis times, lower adsorption of polar compounds and higher thermal stability [248, 2491. The lower limit of the film thickness may be 0.1 pm; the usual range on glass or fused-silica glass walls is between 0.2 and 0.3 pm and on stainless steel walls owing to their activity, between 0.5 and 0.6 pm. Using the static coating method, the film thickness can easily be calculated from dl = 5r,c

(122)

where dl = film thickness [pm], r, = column radius [mm] and c = concentration of the stationary liquid phase in the coating solution [%, w/w], as all of the stationary phase being contained in the coating solution will remain in the colUmn. If the column is to be coated dynamically, the film thickness may be predicted by one of the following equations (Fairbrother and Stubbs [250]):

dl = rcc

4 4 + tl/n

where d, = film thickness [pm], r, = column radius [mm], u, = coating velocity [mm/s], 9 = viscosity of the coating solution

Pa. s],

[

yc = surface tension of the coating solution 10-

c

= concentration

4

of the stationary liquid phase in the coating solution [%, w/w] (Nouotny et al. [251])

or (Guiochon [252])

4=

134r c

u,q 3 ( 7 T .)

The best accord with experimental results is given by eqn. (124), except with very thin films or very low viscosities of the coating solution, where eqn. (125) is to be preferred (2521. The film thickness can, however, also be determined from gas chromatographic data (Crumers et al. [245]): d-I-

kr, 273 2YBeL T

where

k = capacity ratio, V, = specific retention volume of a solute at the column temperature T [K], eL = density of the liquid stationary phase at the column temperature.

13

3.3. Open-Tubular Columns

The values of r, and eL are known, T and k can be measured and V, can be obtained advantageously from, e.g., corresponding data from packed columns.

3.3.3.5.

Quality Tests of WCOT Columns

In addition to the usual determination of the values of k [eqn. (21)], B [eqns.(23) and (24)], n or N [eqns.(36), (37), (40) and (41)], the following characterizations of the column ought to be carried out: (a) Determination of hdn (smallest experimental h value by plotting h =f(@ at Copt.(see Fig. 4). (b) Calculation of the coating efficiency, i.e., the ratio of the theoretical and experimental minimum height equivalent to a theoretical plate [192]: CE =

hmh (theor.)

(exp.) using the k value of a standard compound, the selection of which depends on the operating temperature. hmin(theor.) can be calculated from hmin

hmin(theor.) = r,

JW,

r, = internal column radius, k = capacity ratio.

Hence it follows that

or, if CE is to be given as a percentage, (128a) Instead of CE, the term UTE (utilization of the theoretical best efficiency) has also been used, see, e.g., ref. [223]. At this point it should be noted, that eqn. (127) is a simplified expression, neglecting both the pressure drop in the column and C, [eqns. (62) and (63)]. Provided that the dif€usion coefficients are known, an expression developed by Cramers et al. [253] can be utilized, which has been proved to correspond better with the experimental data [252]. (c) Determination of the loading, i.e., of the amount of sample that can be injected without a serious loss of efficiency. An approximation is [254] SL [g] = 0.05 M .$(1

+ k)

*

(129)

where SL = sample loading = molecular weight of the standard compound, d, = column inside diameter, k = capacity ratio of the standard compound, which should be chosen such as to give k > 10.

M

A simple experimental means of obtaining the maximum solute load consists in injecting increasing amounts of sample and measuring the peak width at half-height until the value of

74

3. The ChromatographicColumn

wH [eqn.(33)] has increased to 5% above the average wH value. This value is the maximum solute load (MSL). (d) Determination of undesirable column activity. A serious problem, especially in the trace analysis of polar compounds, has been the possible residual column activity. The column therefore has to be checked by an appropriate test method. One must be aware of the fact, however, that deleterious effects might be caused by the injector, injector sleeves, the tube connectors, etc., unless these parts of the instrument have been properly deactivated. The inertness of a column can be tested by two approaches. Reversible adsorption can be observed by peak tailing of selected compounds having special functional groups. This tailing can be quantitied by an asymmetry factor [255]: A, =

a+b (a + b ) - ( b - a )

where A,

= asymmetry

(I

= front

factor, half of the peak, measured from the perpendicular drawn through the peak maximum, b = back half of the peak. A totally symmetrical peak would have, according to this equation, an A, value of 1.00, and tailing on the rear edge, being an indication of reversible adsorption would give asymmetry factors greater than 1.00. Irreversible adsorption and partial decomposition are even more detrimental when carrying out trace analyses. This effect, which would falsify the analytical results, can be detected by comparing the ratios of the peak areas of selected compounds with that of a hydrocarbon contained in the test mixture, as compounds being subject to such effects would exhibit reduced peak areas. This method holds true, however, only for the investigated concentration range. Hence it is recommended that as small amounts of the test mixture as possible are injected as at higher concentrations the effects might become less evident. The composition of the test mixture can be chosen according to Table 4.

Table 4. Standard Test Mixtures Schombug et al. 1178.2581

Rohrschneider I2561

MeReynolds

benzene ethanol methyl ethyl ketone nitromethane pyridine

benzene n-Clo, n-Cll, 1-butan01 n-C12alkanes 2-pentanone G o - , c11-, nitropropane C12-acid methyl pyridine esters 2-methyl-2-pen- 1-octanol tan01 n-octylamine 1-iodobutane 2,6-dimethyl2-octyne aniline 1,4-dioxane dicyclohexylcis-hydrindane amine 2,6-dimethylphenol

12571

Gmb

[W ~~~~

n-Clo, n-CI1-al- n-Clo, nkanes C12-alkanes 1-0ctanol c10-9c11-, Clz-acids octanediol methyl esters octanoic acid 1-octanol trichlorophenol nonanal nitrophenol 2.3-butane-diol n-Cs-, n-Clo-, 2,6-dimethylani- n-Clz-amines line naphthalene 2,6-dimethylphe- biphenyl no1 dicyclohexylamine 2-ethylhexaneacid

n-Cs-C12-alkanes dibutyl ketone nonanal nonylamine nitrohexane 4-tert-butylpyridine nonanol naphthalene 4-propylphenol

3.3. Open-Tubular Columns

75

Owing to different causes of adsorption, it is necessary, as can be seen from Table 4, to test compounds with different structures. In any event, an m i n e ought to be present in the test mixture, because amines are very sensitive indicators of adsorption effects. Pyridine, used in the test mixtures of Rohrschneider and McReynolds (Table 4), seems to be less suitable. This is not surprising, as both of these test mixtures were developed for other reasons (see Chapter 4) and not to evaluate adsorption. Nevertheless, both of these mixtures have also been applied for this purpose occasionally, because they are necessary for determining the column selectivity, and dominant adsorption phenomena may occur. (e) Long-Term Stability Test Depending on numerous parameters the column can be subjected to changes during the operation. Therefore, several checks ought to be carried out at suitable intervals, e.g., once per week. An easily measured value is the capacity ratio, k, of a standard, which would give information on eventual losses of the stationary phase. Occasionally the Kovhts retention indices (see Chapter 4) of the Rohrschneider or McReynolds test mixture ought to be measured. Changes in the retention index of any of the components would indicate changes in column selectivity. Finally, any increase in adsorption effects can be realized by injecting a standard mixture according to (d) (Table 4).

3.3.4.

Porous-Layer Open Tubular (PLOT) and Support-Coated Open-Tubular (SCOT) Columns

In order to increase the sample capacity of open-tubular columns while decreasing the stationary phase film thickness, porous layers were first prepared on the inside walls of the column tubing by Halasr and Horuath [260, 2611 and thoroughly investigated and made commercially available by Ettre et al. [262, 2631. These porous layers increase the surface area of the original geometric area of the inner walls of the tubing, thus increasing the amount of stationary phase and with it the load of the column, in spite of the simultaneous reduction of the film thickness, in order to increase the efficiency according to eqn. (63) (decreasing both the C, and C, terms). Compared with WCOT columns, the phase ratio is much smaller, because the amount of stationary phase will be 10- to 50-fold higher (maintaining the high permeability and the efficiency of WCOT columns!). An additional advantage of this type of column is that the layer surface can be better coated than the normal wall surface, as the contact angle on the rough surface to be wetted by the liquid is smaller than that on smooth surfaces [compare eqn. (117)l. Porous layers can be applied to the inner tube wall by static coating with a suspension of particles of, e.g., graphitized carbon black [264, 264a], activated charcoal [265], modified silica [266], silanized silicic acid [268, 268a], molecular sieves or Chromosorb-based support materials [263, 2671, porous polyaromatic polymers [267a].Another possibility consists in the formation of porous layers direct from the column wall material by etching procedures (especially in glass capillaries), which will lead to silica gel-type layers [269, 2701, or to silica whiskers [158] (discussed in Section 3.3.3.1). The layer may be very thin, or thick, may be more or less active, and may be coated with a liquid stationary phase or may be used unmodified as an adsorbent, e.g., for the separation of non-or weakly polar low-boiling compounds. All these feasibilites exist when using the general term porous-layer open-tubular (PLOT) columns. In order to take into account the type and the geometric dimensions of the layer, we shall subdivide this type of column into subgroups: PLOT columns (strictly), SCOT columns and thick-layer columns; although this cannot be claimed to be scientifically exact, it is expedient for practical purposes.

76 3.3.4.1.

3. The Chromatographic Column

PLOT Columns (strictly)

Principle of preparation. Open tubes, most often having an inside diameter of 0.5 mm, are coated with adsorbent particles finely dispersed in a vaporizable liquid, which is subsequently evaporated so as to leave the particles as a thin porous layer on the column walls. Occasionally, the porous adsorbent layer can be modified by previously dissolving, e.g., a polar stationary liquid, in a dispersion, leaving, after evaporation of the solvent/dispersion liquid, a wet porous layer. The tube has first to be rinsed with purified solvents, e.g., in the order methylene chloride, methanol, acetone and R113 (CFZC1-CFClz),and subsequently filled with the solvent used for the dispersion (e.g., R113, carbon tetrachloride, chloroform or another vaporizable liquid of relatively high density). The particles generally have sizes of about 1 pm or less but larger particles have also yielded good efficiencies provided that the particle size distribution is narrow. Usually, adsorbents are applied, e.g., graphitized carbon black, activated charcoal, silica or porous polymers. They are added to the vaporizable dispersing liquid and dispersed by rapid stirring or sonication. The dispersion is forced through the tube by means of pressurized nitrogen, similar to the static coating procedure described in Section 3.3.3.2,avoiding the formation of gas bubbles. After being filled, one end of the tube is sealed, and the tube is slowly, beginning with the unsealed end, drawn or pushed through an oven, the temperature of which depends on both the tube length and the boiling point of the liquid to be vaporized. When the evaporation is completed, the column is purged with nitrogen to remove residual liquid. The particles adhere to the column walls by Van der Waals forces. The thickness of the layer depends on the particle size and structure, and generally ranges from 1 to 30 pm. If a wet, modified porous layer is to be prepared, the stationary liquid is added to the dispersion prior to stirring. This procedure for preparing porous layers allows a fairly unrestricted selection of particle size, layer thickness and composition, whereas etching of the inner walls to form the layer, produces porous layers, the structure of which depends on the tube material [269,270].These chemically produced layers (e.g., by the action of ammonia water or sodium hydroxide solution) may also act as an active porous layer per se or, if coated with a stationary liquid, as a modified adsorbent. 3.3.4.2.

SCOT Columns

This type of column differs from the previous type in the porous material used and by the fact that the layer, formed on the inside column walls, will always be wetted with a stationary liquid. The particles consist of relatively inert column support material (e.g., Chromosorb) having d, 5 1 pm. Silanized silicic acid has also been used [267](e.g., Silanox 101). They are coated on to the inner walls of the tube, together with the stationary liquid, both dispersedl dissolved in an appropriate solvent having a density L 1.5 g/cm3 and a boiling point between 50 and lOO"C,e.g., R113 (1,1,2-trifluoro-1,2,2,-trichloroethane), analogously to the procedure described in Section 3.3.4.1. Two further coating procedures are described in detail below. (a) Two-step procedure (according to Van Hour et al. [267].A plug consisting of Silanox 101 (silanized silicic acid) dispersed in a dilute solution of a polar phase in chloroform is forced through the column at a rate of 5-8mWs. The dispersion is prepared, immediately prior to use, by dissolving 0.1g of polar phase in 100 ml of chloroform, then adding 1.0g of Silanox and sonicating the mixture at 35°C for 15 min. A plug of about 1 ml is propelled through the capillary, and the solvent is removed by a flow of nitrogen through the column for 2 h. A thin layer of the polar phase containing Silanox remains on the walls. In the second step, a solution of the polar phase in acetone is prepared free from dissolved gases by dissolving 72 mg of the polar phase in 40 ml of acetone and removing 10 ml of the

3.4. Properties and Comparison of the Main Column Types

77

solvent by vacuum. The capillary is immediately filled with the solution using reduced pressure. One end of the column is warmed to expel a drop of the coating solution and immediately sealed, avoiding trapping of air. The opposite end of the tube is connected to a vacuum pump, and the solvent is slowly evaporated at room temperature. A 20-m column requires 36-48 h for solvent evacuation. (b) One-step procedure (according to Chauhan and Darbre [271]) A glass capillary previously treated with hydrogen fluoride is deactivated with benzyl triphosphonium chloride and pre-coated with poly(dimethy1oiloxane). A solution/dispersion is then prepared by dissolving 5% stationary liquid phase and intensively dispersing 5% of Silanox 101 or Chromosorb R-6470-1. A plug of this dispersion having a length of one quarter of the column length is propelled through the column at a rate of 4 c d s . After this dynamic coating the solvent is removed by a flow of nitrogen at 15 ml/min for 4 h. These SCOT columns, which belong, besides WCOT columns, to the most efficient gas chromatographic columns, exhibit lengths generally in the range 10-40 m, most often 15-20 m. The inside diameter is usually 0.5 mm, occasionally lower (down to 0.25 mm), and the layer consisting of support stationary liquid has a thickness from less than 1to a few pm. Their advantages and disadvantages in comparison with WCOT columns are discussed in Section 3.4. Due to the development of cross-linked surface-bonded liquid stationary phases, the application of SCOT columns has considerably decreased. 3.3.4.3.

Thick-Layer Columns

A special case of porous layer open-tubular columns, when considering only the presence of a layer, is the thick-layer column. One could argue, however, over the group of columns to which this type ought to be assigned. Regarding the layer type, either the PLOT or the SCOT type would be possible; regarding large layer thicknesses, with respect to the column inside diameter, the term open-tubular would no longer hold good, and these columns ought to be considered as a kind of micro-packed column. Nevertheless, as they are often thought to fill a gap between open-tubular and packed columns and as their properties, disregarding the thickness of the layer, are essentially the same as those of PLOT and SCOT columns, they are listed here. Their preparation [272-2741 differs from that of the other open-tubular columns. A glass tube with an inside diameter of, e.g., 3 mm is loosely filled with relatively coarse particles (d, = 60-90 pm) and drawn over a steel core of 0.3 mm to give a capillary tube with an inside diameter of, e.g., 0.5 m. The particles adhere tightly to the walls, and form a layer the thickness of which is about 100 pm. The particle material may be an adsorbent, and thus the column can be used in the unimpregnated form, or it may be an appropriate support material, which has to be coated with a stationary phase. Owing to the essentially smaller phase ratio, the amount of sample injected can be considerably larger than that with either of the other open-tubular column types, hence facilitating the method of sample introduction. Conventional gas chromatographs are applicable, on-column injection can easily be arranged, and routine analyses are simple. Nevertheless, the decrease in efficiency caused by the thick layer must not be overloocked.

3.4.

Properties and Comparison of the Main Column Types

At the beginning it must be emphasized that each column type has advantages and disadvantages. An absolute answer to the question of which column type is to be preferred cannot be given, as the decision is dependent on the equipment available, on the expenditure that can be devoted to research and development and, especially, on the analytical problem to be

78

3. The ChromatographicColumn

solved. In order to facilitate such a decision, several characteristic values of the different column types are listed in Table 5 and the pros and cons and the field of application of the most important column types are discussed below. The magnitudes in Table 5 are only average values, and should not be thought to represent all experimental results that have been obtained so far: nor was it possible to take all dependences into account. Table 5. Characteristic Properties of Different Column Types

Phase ratio, B Characteristic /J values k i n

1-1

[cm/sI Optimum practical gas velocity [ c d s ] L [ml Column inside diameter d , &p,

1-1

Film thickness, dl, or layer thickness, dl, , Cm] Sample capacity Cg] Column permeability, x [lo-’ cm2] n (maximum) AZmh, Minimum retention index difference necessary for two compounds to be separated at optimum column length (R,= 1.5)

3.4.1.

Conventional packed Columns

Micropacked Columns

SCOT

PLOT

WCOT

Columns

Columns

Columns

10-30 15 0.5-2 2-10 8-12

10-100 50 0.2-1 10-20 20-50

50-150 80 0.5-1 10-20 20-80

20-150 80 0.5-2 10-20 20-80

80-500 250 0.2-1 8-15 15-30

2-12 2-4

1-15 0.3-1

10-30

10-30

0.5

0.5

10-100 0.05-0.7

d10.01-0.3

d10.01-0.1

d1,O.S-S

dl,0.5-30

d10.1-1

1000 1-10

1-50 5-40

0.3-10 750- 1000

0.3-10 500-800

10 000 10

50 000 3

60 000 2

60 000 2

0.02-2 10- 1500, mostly 200 500 000 0.5

Packed Columns

Advantages: Inexpensive, can be laboratory-made, easy to use, require only simple apparatus, high sample capacity, no injection problems. Shortcomings: High pneumatic resistance (low permeability), restricted column length, low efficiency, low resolution, long analysis time. Application fields: Routine analysis provided that the mixtures to be separated are not too complex; simple separation problems; trace analysis; coupled techniques; process gas chromatography.

3.4.2.

Micro-packed Columns

Advantages: High efficiency enables short columns to be operated at high gas velocities and hence high-speed analysis to be implemented. Inexpensive, can be laboratory-made, sufficient sample capacity. Shortcomings: Pressure drop generally between 0.03 and 0.3 MPa may cause instrumental problems; e.g., at 0.5 MPa a special injector is necessary. Application fields: Multi-component analysis, high-resolution coupled techniques, trace analysis, high-speed analysis using short but efficient columns, high performance gas solid chromatography.

3.4. Prouerties and ComDarison of the Main Column Types

3.4.3.

79

Support-coated Open-tubular Columns

Advantages: High efficiency at high gas velocity permits high-speed analysis; higher sample capacity, lower column bleed, simpler wettability, smaller phase ratios than WCOT columns; splitless injection possible; low detection limits. Shortcomings: Less inactive and shorter column lengths than WCOT columns. Application fields: High performance gas chromatography, high-speed analysis, trace analysis of complicated mixtures, multi-component analysis, high-resolution coupled techniques, analysis of low-boiling compounds.

3.4.4.

Wall-coated Open-tubular Columns

Advantages: Highest efficiencies; possibility of optimization of analysis by changing 4 , 4 and L;owing to the low flow resistance long columns are applicable; analysis time 1-2 orders of magnitude shorter than with conventional packed columns; high-resolution at short analysis times, separability of compound pairs having small retention index differences; lowest adsorption of polar compounds; wall-coated fused-silica open-tubular columns, when combined with the most advanced GC hardware and when the stationary phase is immobilized, offer an almost ideal gas chromatographic system permitting highly reproducible retention values. Shortcomings: Complicated injection, lower sample capacity and more unfavourable phase ratios than other GC columns; only limited trace analysis practicable ( 210 ppm for 4 5 0.5 pm). Application fields: General purpose GC separation system for screening complex mixtures of different structure compounds; high-speed, high-resolution analysis; analysis of very complex mixtures the separation of which would otherwise be impossible; highly reproducible retention index measurements that will offer the possibility of identifying unknown components only by means of a comprehensive retention index library.

4.

Characterization of Stationary Phases

4.1.

Intermolecular Forces

The high selectivity of gas chromatography is based on molecular interactions between the molecules to be separated and the molecules of the stationary phase, be it an adsorbent or a stationary liquid. The intermolecular forces occur between molecules the valencies of which are saturated. Hence the appertaining interaction energies are mostly two orders of magnitude lower than the energies of chemical bonds and are generally < 60 kJ mole-' for both stationary liquids and adsorbents. The molecular interaction can be subdivided into four general types: London-type disperson forces, orientation forces including hydrogen bridge bonds, induction forces and donor-acceptor forces.

4.1.1.

London-type Dispersion Forces

As a result of the investigations of London [275], these forces represent the main interactions of non-polar or weakly polar components. They result from induced dipole/induced dipole and multi-polar attractions, as each atom and each molecule, even if it does not exhibit a measurable dipole moment, is unsymmetric at each point in time because of the permanent movement of electrons and exhibits dipole moments that cancel out only in the temporal average. Such an electron system acts as a fluctuating dipole polarizing the electron systems of its neighbouring atoms and molecules, achieving an attraction force. Hence the interaction depends on the polarization and on the ionization potentials of the interacting partners (London [275]):

where a;,a; = polarizability of the coherent partners 1 and 2, Zpl, Zpz = ionization potentials of the coherent partners 1 and 2, fD = distance between the coherent partners' centres, EL = interaction energy resulting from London-type dispersion forces. This type of interaction is independent of temperature. Based on this London relationship, Korol [276] developed the dispersion index for non-polar stationary liquids containing methylene groups in a non-ordered concentration, interacting with the molecules to be separated:

(132)

where

Ri, Rj= atomic refraction of the atomic groups i and j, = sum of the Van der Waals radii of the atomic groups i and j. rG The dispersion index, 4, represents the energy of the interaction between two atomic groups i and j. The intermolecular interaction energy, in the case of predominant dispersion forces, e.g., beween sample components consisting of saturated hydrocarbons, with carbon atoms ex-

4.1. Intermolecular Forces

81

hibiting only a-bonds and molecules of the stationary liquid phase having the same structure, or between such hydrocarbons and graphitized thermal carbon black, ranges from 0.1 to 40 kJ/mole, depending on the molecular distances [r-6 in eqn.(131)] between the sample and stationary phase molecules.

4.1.2.

Orientation Forces and Hydrogen Bridge Bonds

Forces between dipolar molecules result from the spatial orientation of the interacting dipoles. That is, if this spatial orientiation of the stationary dipoles and the sample dipoles were the same, attraction and repulsion would counterbalance. However, as the dipole orientation with the lower potential energy is favoured statistically, an attraction results. Because of the thermal motion which has a contrary effect, it depends strongly on the temperature. From this interaction, energy is liberated [277]; (Keesom 1921):

where permanent dipoles of the dissolved molecules and of the molecules of the liquid stationary phase, respectively, = distance between these dipoles, r, T = absolute temperature. With increasing temperature, accordingly, the orientation forces decrease, and liquid stationary phases that are selective at low temperatures because of a high proportion of orientation interactions, become less or even non-selective at high temperatures. A particular case of this electrostatic orientation effect is the hydrogen bridge bond that occurs between proton donor groups X-H and proton acceptor groups Y: p,,

=

X-H...Y where X is a strongly electronegative atom, especially 0, N and F, the donor group X-H thus having a partially ionic character. In addition to these three types of atoms, X may also be their higher homologues, e.g., S, P, C1, Br and I, and also C; the strength of the resulting hydrogen bond in these instances, however, is only of the order of magnitude of normal orientation forces. The higher the electronegativity of X, the more positive H becomes in the X-H bond. The proton acceptor group, Y,may be either an electronegative atom having polarizable lone pair orbitals or an unsaturated or aromatic system with sr-electrons. In Table 6 common proton donor and acceptor groups are listed [278]. Depending on the presence of donor and/or acceptor groups, four types of molecules with different hydrogen bonding tendencies can be classified [278] (Table 7 a). Systems consisting of two groups, one of which representing the dissolved/adsorbed compound and the other the stationary liquid phase/adsorbent, may or may not form more or less strong hydrogen bonds as can be seen in Table 7 b. When combining two partners of Type I or 11, for example, the retention is increased by hydrogen bridge bonds. Hydrogen bonds are also formed between molecules of type I (or 11) and I11 or IV. If molecules of types 1-111 and IV are interacting, donor-acceptor complexes emerge, the strength of which depends on the acidity of the proton donor I, I1 or I11 and the basicity of the acceptor IV (which increases approximately in the order sr-electron systems < nitriles < ketones < ethers < amines) [278]. When donor and acceptor groups are present in type I and I1 molecules, donor-acceptor complexes are also formed within such systems. The nature of the hydrogen bond equilibria, however, is more complicated than that between type I11 and IV molecules [280].

82

4. Characterization of Stationary Phases

Table 6. Proton Donor and Proton Acceptor Groups (2781 Proton Donor Oroups-X-H

Proton Acceptor Oroups Y

Examples

Examples

a) Lone pair orbitals OH

Water, alcohols, phenols, carboxylic acids, silanols primary and secondary amines, pyrroles, amides thiols hydrohalic acids haloforms, ethinyl hydrocarbons, nitro or cyan0

NH SH Hal-H CH

\ \

/ O!

\

\

o/\

oximes, nitroso compounds, pyridine N-oxides nitro compounds

o/\

sulphoxides

-N=0/

-N= II

compounds with H-atoms in aposition PH

\ /

/C = 0

\o/ /s= \

water, alcohols, phenols, carboxylic acids, silanols, ethers ketones, carboxylic acids, esters

sulphones

phosphines fP=O \

/

phosphineoxides primary, secondary and tertiary amines, pyrroles, pyridines

-j \ NI

b) rr-electrons

-c=c-

alkynes

\

/

/

\

c=c

alkenes aromatics

-CENI

nitriles

Table 7a. Classification of Molecules According to the Presence or Absence of Proton Donor or Acceptor Groups [278, 2791 Type

H+-donor groups present

H+-acceptor groups present

Examples

I

Yes, several

Yes, several

I1

Yes

Yes

I11 IV

Yes No

No Yes

V

No

No

water, polyols, aminoalcohols, hydroxy acids, polyphenols,polybasic acids alcohols, phenols, monobasic carboxylic acids, oximes, primary and secondary amines, nitriles and nitro compounds with a-H atoms haloforms ketones, esters, ethers, alkenes, aromatics, tertiary amines, aldehydes, nitriles and nitro compounds without a-H atoms alkanes, perhalogenated hydrocarbons

83

4.1. Intermolecular Forces ~~~

Table 7 b. Strength of Hydrogen Bonds of Groups I-V

I

I1 111

IV

V

I

I1

111

Iv

V

Verystrong strong

strong strong medium-weak

medium medium-weak none weak none

medium medium weak none none

none none none none none

medium medium none

medium

none

In systems that contain only groups I11 or IV or V, hydrogen bridge bonds cannot be developed. This also holds for all the combinations of group V with molecules of the other groups (disregarding the self-association of type I and I1 molecules). Therefore, systems of this kind behave nearly ideally (activity coefficients =l),and the elution order of the dissolved compounds occurs in the order of decreasing vapour pressures at the column temperature. In all other systems in Table 7b, marked contributions to the intermolecular interaction are made by the formation of hydrogen bridges, and the hydrogen-bridge bond enthalpies range between 1.6 kJ.mole-' (S-H...n) and 38 kJ. mole-' (0-H ...N) [281]. Hence they are relatively high compared with other polar interaction enthalpies.

4.1.3.

Induction Forces

Induction forces represent only a relatively small portion of the Van der Waals forces. They were investigated by Delye [282] and are based on the fact that an electric field of a dipolar molecule induces an electrical moment in another, polarizable molecule. The resulting attraction is generally independent of temperature, as can be seen in the Debye equation

where 01

=

polarizability,

p = dipole moment, rj

4.1.4.

=

distance between the coherent partners' centres.

Donor-Acceptor Interactions

If one of two partners has a high electron affinity (acceptor) and the other a n-electron system with a low ionization energy (donor), a partial transfer from the loaded orbital of the donor to the vacant orbital of the acceptor is possible. Typical acceptors applied in gas chromatography include nitro aromatics, nitrile ethers and tetrachlorophthalate. They retain appropriate donors, e.g., aromatics and olefins, selectively.

4.1.5.

Further Intermolecular Forces

Coordinatively unsaturated stationary phases can retain selectively these sample components which are suitable ligands and form complexes the stability of which is low enough at column temperature to transport the volatile pre-ligands through the column. The underlying forces are ion-dipole interactions [283].

84

4. Characterization of Stationary Phases

4.2.

Quantities for the Description of Interactions

4.2.1.

Selectivity Coefficient

From eqn. (70) it can be derived that the separation even of two equally boiling compounds can be accomplished by utilizing the selective properties of stationary phases expressed by the selectivity term:

Provided that the vapour pressures of the two compounds are identical at the column temperature, the vapour pressure or volatility term becomes unity and r2,1becomes q,l, a quantity that has been called the (special) selectivity coefficient [284, 2851: r2,1= V L

= u2,1=

(+) y2

R1

(135) PY-P:'

This coefficient can be elucidated either by calculation from the adjusted or relative retentions of two equally boiling compounds or by interpolation from two homologous series in a log Vk/Tbdiagram (Tb = boiling points of the members of the two homologous series), as it is well known in isothermal gas chromatography that graphs of the logarithm of adjusted retentions against the boiling points of the members of a homologous series are linear. The hypothetical compounds 1and 2 would have, in the case of identical (hypothetical) boiling points, more or less widely differing retentions, the retention ratio being an expression of the differences in their interactions with the stationary phase. The more u deviates from unity, the more selective is the stationary phase for the separation of compounds 1 and 2 and, moreover, for the separation of the two homologous series, as equally boiling compounds do not belong to the same homologous series but have different constitutions. For details and tables, see ref. [286]. It should be mentioned that u owing to the temperature dependence of the orientation forces, frequently changes over a wide temperature range.

4.2.2.

Retention Index

From Equation 51b: and Equation 8 1:

K2 r2,1= az,l= -

K1 RTln a2,1 = A (AG)

it can be seen that differences between the molar free energies of vaporization of two compounds 1 and 2 from their infinitely dilute solution in the stationary liquid phase (i.e., the separation enthalpy), render possible the separation expressed as r2,1or E ~ , On ~ . the other hand, if the vapour pressures are not taken into consideration in eqn. (70), r2,1is an appropriate quantity for the description of the interaction differences between the two samples and the stationary phase. Therefore, one could be tempted to choose a standard compound (s) according to eqn. (51a) r\, = t)Ri/t)Rsthe interactions of which with the stationary phase are well known, in order to obtain the interaction differences of i and s with the stationary phase. This relative retention related to a standard has been widely applied for identification purposes. There is, however, a serious disadvantage when using only one standard: if the retentions of the sample compound and the standard differ greatly (and this will often be the case because

4.2. Quantities for the Description of Interactions

85

of the great temperature range of gas chromatography, the numerous stationary phases and the wide range of samples), the accuracy will be far from acceptable. Thus, a multitude of standards have been proposed, most often diversely interacting with the stationary phase. These problems could be overcome by the introduction of the retention index system proposed as early as 1958 by Koucits [287] and revised by himself [288] and by Etfre [289]. The retention index is defined as the carbon number (multiplied by 100) of a hypothetical n-alkane that would have an adjusted retention time identical with that of the sample compound of interest, and which is bracketed by two n-alkane standards:

where

4

=

Z t'

= =

tzz tX(Z+

1)

}

=

retention index of substance i, carbon number of an n-alkane, adjusted retention time of the substance, adjusted retention times of alkanes with carbon numbers z and (z + l), where

As

and

we can write the retention index eqn. (136) as (136a)

In addition to n-alkanes (general formula C,H2,+ Z)r which exhibit only dispersive interactions, other compounds (general formula C,H2,+ X ) can additionally interact with polar groups of the stationary phase, and the resulting retention is an indicator of the group X and also of the type of stationary phase. We shall return to this fact in the following section. Because of the great importance of the retention index in gas chromatography, we shall in eqn. (136) means that the discuss this quantity in detail. The restriction t X , 5 t ; I&(,+ compound of interest, i, is eluted just after the n-alkane of carbon number z and just prior to that of carbon number z + 1. As the selection of an n-alkane pair suitable for the compound in question and for the applied column temperature is not difficult, standards from other types of compounds can be abandoned. According to definition, the retention indices of n-alkanes are I = 100 (CH,), 200 (C2H6), 300 (n-C3H8), ... z 100 (n-C,H2,+ A compound with an adjusted retention time between that of n-octane and n-nonane would therefore have a retention index, I, between 800 and 900. In a homologous series the retention indices increase by 100 units for each CH2-groupon both polar and non-polar stationary phases. The retention indices are completely independent of the experimental equipment and operating conditions, with the following exceptions. First, according to definition, the retention indices of n-alkanes are the same on all stationary phases and independent of temperature. However, this does not hold for all other compounds; their retention indices depend on both the stationary phase used (because of the relationship of ?Xi [eqn.(Sla) and (136a)l with H [eqn.(70)] and fi, strongly depending on the

86

4. Characterization of Stationary Phases

stationary phase) and on the applied column temperature (because the orientation forces decrease with increasing temperature according to eqn. (133), through which y iand r4 are again effected. Second, the retention index depends to some extent on both the residual activity of the liquid-solid interface (solid support or column wall, respectively) used and the stationary phase loading or film thickness, respectively. Precautions can be taken against these restrictions by choosing a less active solid support or column wall and deactivating it prior to use and by avoiding very low loadings or film thicknesses, respectively [290]. The temperature dependence of Z can be calculated by the Antoine equation (291): Z=A+-

B

(137) T+C where A, B and C are constants, depending on the solute and the liquid stationary phase. Within the temperature range usually applied in gas chromatographic practice, the function Z = f(2') exhibits, to a first approximation, a straight-line portion that is longer for non-polar than for polar stationary liquids. It was shown that one unit per degree frequently applies [292] and that similarly boiling isomers with a boiling point difference of x K differ in their retention indices by 5 x units (2871. The dependence of Z on boiling point, 4 , for homologous series of hydrocarbons, ethers, aldehydes, ketones, esters and alcohols on numerous stationary phases has been investigated by many workers [293-2981 and was found to be described by the linear correlation [298] Z=arb+ b

where ti,

= boiling

temperatures of the homologues,

a, b = constants. The constant a, valid for the homologous series concerned, is related only to the functional

group X and is therefore independent of the stationary phase, whereas 6 is related to both the stationary phase and the functional group of the sample homologues. As 6 includes y, it will vary with temperature (temperature dependence of the orientation forces!), whereas the constant a will remain constant [298]. At first sight, the simple linearity of eqn.(138) seems to be strange, yet it is easily explicable. It is well known in isothermal gas chromatography that the relationship between the logarithmus of the adjusted retention times and the boiling temperatures is linear for homologous series, this behaviour being a consequence of the relationship between the specific retention volume (V& and the vapour pressure (Po) of a solute:

v, = 273,16 R

(139) Y0P0MP where Mp = relative molecular mass of the stationary phase and yo = activity coefficient of the solute, which can consisdered to be approximately constant for a given homologous series. Combining the Clausius-Clapeyron equation and Trouton's rule, it was derived [298] that hl vg= Arb + B (138a) i.e., a logarithmic dependence. Because, according to definition, the retention index is logarithmically correlated with the retention r& [eqn. (136)], the combination with (138a) and (8) results in direct linearity. The retention index, Z, of a compound can be calculated from basic factors (corresponding to the carbon skeleton) and increments [resulting from functional groups and their position relative to each other (methylene increment, homomorphic increment, phase increment, substitutions increment)]; for details, see ref. [299]. The exact calculation of the essential data and various mathematical methods for the calculation of the Kovhts retention index have

4.2.

Quantities for the Description of Interactions

87

been thoroughly investigated by Smith, Haken et al. [300, 3011, Ettre [302], Guardino et al. [303], Budahegy et al. [304] and others [305-3111. The reproducibility of I values is not only dependent on the mathematical method used for processing the experimental data, but also on these data themselves. The combination of a well designed experimental technique, an accurate timing mechanism (nearest tenth of a second) and an appropriate mathematical method will give an inter-laboratory reproducibility of one unit for larger values of Kovhts indices, and two units for indices below ca. 400 [300, 3011. Owing to the development of fusedsilica wall-coated open-tubular columns (described in detail in Chapter 3), when combined with advances in gas chromatography hardware, it is now practical to compile a retention index library that is reproducible enough for the qualitative analysis fo unknown compounds. Sadtler Research Laboratories introduced such a Gas Chromatography Retention Index Library of chemical compounds measured on fused-silica WCOT capillary columns (stationary phases: cross-linked methylsilicone, cross-linked 5% phenylmethylsilicone, polythylene glycol Carbowax 20M bonded phase and cross-linked polyethylene glycol Carbowax 20M) [312]. The reproducibility is one retention index unit for non-polar stationary phases and two units for polar phases. Hence the measurement of the retention index of an unknown on, e.g., two different columns, varying in stationary phase polarity and under the same column temperature conditions, is usually sufficient for the positive identification of an unknown that is in the retention index library [312]. Finally, it should be mentioned that we have only dealt with the isothermal (standard) Kovats retention index. For the analogous quantity in temperature programmed gas chromatography, see refs. [312-3141. The retention index in gas-solid chromatography can be defined, in the case of wide-pore or non-porous adsorbents, by the equation

SA = specific surface area of the adsorbent, m, = mass of the adsorbent.

The retention volumes apply to an infinitesimally low surface coverage of the adsorbent by the adsorbed compounds, i, and the n-alkanes. The differences in the logarithms in eqn.(l40) correspond to the relative retentions (for zero coverage): (140a) The dependence of the retention indices ZAi on molecular structure and on temperature is more complicated than the dependence of Z in gas liquid chromatography. These dependences of ZA, are even more complex than those of the Henry constant [315]:

(88a and 20) The retention indices ZA of numerous hydrocarbons with different structures and of their oxygen and chlorine derivatives have been determined on graphitized thermal carbon black and on hydrogenated thermal carbon black [316].

88 4.2.3.

4. Characterization of Stationary Phases

Polarity

At the beginning it must be emphasized that the terms “selectivity” and “polarity”, often used synonymously in gas chromatographic papers, are not synonyms in practice. As described in the previous sections, the selectivity is an expression for the different interactions between the solutes adsorbates and the stationary phase. In contrast, the polarity is a term for substances (both stationary phases and sample molecules) having a significant concentration of polar substituent groups and hence having a measurable dipole moment. Thus, for example, a non-polar stationary phase is not automatically non-selective. The situation is additionally complicated by the polarizability of one of the two or of both the partners, if one is considering the term polarity. Therefore, it is not surprising that correlations between adjusted retentions or retention indices and electrical quantities such as dipole moments or dielectric constants of the stationary phases and solutes/adsorbates have only rarely been observed. In fact, correlations occured only between those phases and samples the structures of which were relatively simple, and the portions of polar groups of which were not too high. Especially compounds with hydrogen bonds showed no regularity. On the other hand, when taking into account all types of interactions (which contribute to the retention) as a characteristic of polarity, we had to consider even n-alkanes to be polar, which is absurd. In Section 4.2.2 we stated that the retention index of a compound other than an n-alkane would be an indicator of its group X (in C,H2,+J) and also the type of stationary phase. On this basis, and considering that the polarity in gas chromatography has most often been used in connection with elution order, a gas chromatographic polarity, Pp,can be defined by the retention ratio of a polar or polarizable solute (adsorbate) R-X, compared with a non-polar solute (adsorbate) R-Hon the stationary phase in question related to a standard phase: (141) Pp= l o g ( V y / y ) - l o g ( V y v y ) , = polarity of the polar phase p, PP Vy,Vy = retention volume of RX and RH, respectively, on the polar phase, Vp = retention volume of RX and RH, respectively, on the non-polar standard phase, RX = polar solute, RH = non-polar solute. The retention ratio for another compound, RX’, on the stationary phase p is connected with the polarity Ppby

F,

l o g ( V ~ / V =~ A“’Pp )

+ log(Vy/Vy),

(141a)

Am’ = characteristic constant of the solute RX’. On the basis of eqn.(141), a polarity scale of stationary phases can be advanced if both an appropriate polar or polarizable compound RX and a non-polar compound RH are selected and their retentions are determined on all suitable stationary phases [317]. Rohrschneider selected butadiene as the polar compound RX and n-butane as the non-polar compound RH and chose squalane as the non-polar stationary phase, the polarity of which was assigned by definition to be P = 0. For the pair of compounds butadieneh-butane, mainly the olefinic interactions are reflected. Hence we cannot expect to obtain a general polarity, but rather a special olefinic polarity, Paler. A polarity scale on this basis was established in 1966 by Rohrschneider [317, 3181 for 29 stationary phases, with squalane Polcf= 0 and o,@’-oxydipropionitrile Polef= 100 bracketing the whole polarity range. The retention ratio of two successive (non-polar!) n-alkanes also depends on the polarity of the stationary phase; it is the smaller the more polar is the stationary phase [319, 3201: g z , z + 1 = VB(Z+1)/

VBW *

(142)

4.2. Quantities for the Description of Interactions

89

From this, the polarity can be expressed on an empirical scale by the logarithm of the relative retention (0 for squalane, 1 for BIB'-oxydipropionitrile).As Vg(z+l)/Vg(,) is not independent of the number of carbon atoms (the reasons for this fact are dealt with in Section 4.2.5), the polarity expressed by this term is not the same for any pair of n-alkanes. Koucits established a system of retention indices that offered the facility of characterizing liquid stationary phases by retention index differences of varied homologous series [287,292, 3211. For example consider a compound RX (where the group X is polar or polarizable) that is chromatographed on two stationary phases a and b, where the retention indices I' and Ib can be calculated from the chromatographic data. If the retention index on phase b, Ib,is higher than that on phase a, In, then phase b can be regarded as more polar than phase a, and the difference in the gas chromatographic polarity is I=Ib-I'. (143) This relationship is the essential basis for the following generalizations. If I of a given solute/adsorbate is higher on another stationary phase, then the polarity of that phase would be higher or, in the case of the same phase, the dissolved/adsorbed compound would be more polar. It follows that

I = ax,

(143 a)

a = polarity factor of the dissolved/adsorbed compound, x = polarity factor of the stationary phase.

On this basis, as absolute values of polarity cannot be obtained. Rohrschneider was the first to develop a system utilizing relative values [317, 322, 3231. Based on the non-polar series of nalkanes used to measure and calculate retention indices and based on the non-polar liquid stationary phase squalane, he applied certain probes differing in the type of polarity and POlarizability. This system was extended by McRtynolds [324]. Because of the importance of this system and its great applicability and widespread use, it will be discussed in detail in the following section.

4.2.4.

Rohrschneider and McReynolds Constants

Eqn. (143 a), using one probe and one stationary phase only, is not sufficient to describe the behaviour of any solute on each stationary phase. Rohrschneider [317, 322, 3231 found that the retention index differences consist of several contributions, which are always determined by two types of factor: those which are specific for the sample and do not change when the stationary phase is varied, and those which are specific for the stationary phase and do not depend on the solute concerned, but are related to a limited number of selected standard compounds. The first are symbolized by a, b, c, d and e, and the latter by x, y , z, u and s (the reason for the alphabetically irregular order of the stationary phase factors is that Rohrschneider originally used only three test samples (symbols x-z), and later realized that it is necessary to take two additional compounds into consideration, and that he wanted to maintain the symbols x, y and z of the phase specific constants as they had already been published). For a given system stationary phase - solute, we obtain from eqn.(143a), applying five factors in each instance:

I = ax + by + cz + du + es,

(143b)

where x, y, z, u and s are polarity factors characterizing the stationary phase and a. b, c, d and e are polarity factors characterizing the solute. If m probe solutes are chromatographed on each of n stationary phases, we obtain mn results, which describe 5m polarity factors of the

90

4. Characterization of Stationary Phases

sample solute and 5n polarity factors of the stationary phases in a system of mn equations with 5 (rn + n ) unknowns: Z i = ulxl + bty, + clzl + dlul + elsl, I: = u1x2+ btyz + clzz + dluz + els2, Zf, = ulxn+ bty, + clzn + dlu, + elsn, Z: = u2x1+ bzYl + c2zl + d2u1+ e2s1, Z;=U,X,,+ b~n+Cmzn+dmUn+emsn.

In order to solve this indeterminate system, Rohrschneider selected arbitrary, yet typical, values for the stationary phase polarity factors: (143 c) (143 d)

- zqualane

zphaae

= AZnitrometimnc/100

= nltromctbPDc

-

s = AZpMdi,,e/lOO

nl-

3

100

100

Hence it follows for a stationary phase j and for a dissolved compound i that AI{

AI&H6

= II] -

100

+

Az&,oH

bi

100

A&H,COC,H, +

ci

100

AI&H3NOl +

dl

100

+ ei-

Azis~,~ 100

. (143 h)

The calculation of all factors is very time consuming. If, however, the factors a, b, ..., e of a compound in question are determined on at least five different stationary phases, the polarity factors of which, x, y, z, u and s, have been established by means of the five sample solutes benzene, ethanol, methyl ethyl ketone, nitromethane and pyridine, then the behaviour of the compound in question on each stationary phase can be predicted, provided that the factors of which x, y, z, u and s are known and the equations for the constants a-e can be solved. As the stationary phase specific factors x...s represent Z / 1 0 0 values for the specific test substances on this stationary phase, the factors u, ., e of the five test substances used for the determination of the phase specific factors are constants by definition (Table 8).

..

Table 8. Solute Specific Constants of the Five Test Substances used by Rohmchneider

benzene ethanol methyl ethyl ketone nitromethane pyridine

a

b

C

100 0 0 0 0

0 100

0 0 100 0 0

0 0 0

d

c

0

0 0 100

0

0 0 0 0 100

Although Rohrschneider proposed both stationary phase-specific constants and solute-specific constants, the expression “Rohrschneider constants” is used almost exclusively for the

4.2.

91

Quantities for the Description of Interactions

stationary phase-specific factors x, y, z, u and s. The five test substances have been selected for their specific interactions: induction forces (acceptor), donor-acceptor forces hydrogen bridge bond (H+ donor) hydrogen bridge bond (H+ acceptor) induction forces (donor), orientation forces, donor-acceptor forces orientation forces

benzene: ethanol: methyl ethyl ketone: nitromethane : pyridine:

Let us repeat and summarize the meaning, origin and object of Rohrschneider constants. The I;::; values (Kovfits retention indices) are measured for five deliberately chosen solutes on the stationary phase in question and on squalane as a non-polar reference phase. The differential Kovats retention indices, AI, are calculated from eqn. (143 c-g), and the resulting five A I values are divided by 100 to give x, y, z, u and s. These Rohrschneider constants are intended to characterize the selectivity and/or polarity of the stationary phase in question. Although not absolutely correct, the term "constant" should be retained because it has become common practice. Determination of Rohrschneider constants [325-3281 (Example) Stationary phases: (1) 20% Ucon LB 550X polypropylene glycol on deactivated Kieselguhr (2) 20% Squalane on deactivated Kieselguhr Less than 1pl of the test substance (benzene for x, ethanol for y, methyl ethyl ketone for z, nitromethane for u and pyridine for s) is chromatographed at a column temperature of 100°C on polypropylene glycol and squalane. From their retention times and from the retention times of the appropriate n-alkanes, the retention indices are calculated (see Section 4.2.2). In our example, the following values were found: qolypropylcneglycol = 753 Isqualane = 649 enzcne benzene , x = (763 - 649). 0.01 = 1.14; Ipolypropylene glycol = 660 Isqualane = 384 ethanol ethMOl v = (660 - 384) * 0.01 = 2.76; pO&OpylCnCglycol'= 699 ISqULhlle methyl ethyl ketone = 53 zmethyl ethyl ketone z = (699 - 531) eO.01 = 1.68; Ipolypropylene glycol = 769 Imgualane nitromethane = nitromethane u = (769 - 457). 0.01 = 3.12; polmropylenealycol = 903, zwy@ = 695 Ipyridine pyndme s = (903 - 695) * 0.01 = 2.08. 9

9

9

3

9

4573

9

Hence the liquid stationary phase Ucon LB 550X polypropylene glycol can be characterized at 100°C by the Rohrschneider constants x = 1.14, y = 2.76, z = 1.68, u = 3.12 and s = 2.08. As the retention indices are temperature dependent (see Section 4.2.2), the Rohrschneider constants also depend on temperature. However, this effect can be neglected in practice because it affects at best only the second decimal place. Numerous Rohrschneider constants have been determined. They are listed in Section 9.4, using the publications of Supinu and Rose [329] and Huken [330]. As the retention times of the bracketing low homologous n-alkanes, necessary for the calculation of the Kovits retention indices, raise objections if (and this is an absolut requirement) they have to be determined accurately [300-3111, McReynolds proposed the application of higher homologous of three probe compounds: he recommended n-butanol instead of ethanol, 1-nitropropane instead of nitromethane and 2-pentanone instead of methyl ethyl ke-

92

4. Characterization of Stationary Phases

tone. Additionally, McReynolds investigated further typical compounds in order to characterize special interactions and found that the supplementary application of 2-methyl-2-pentanol and 1-iodobutane increases the accuracy of the prediction of the retention indices of branched-chain and halogen compounds, respectively. On the other hand, 2-octyne, dioxane and cis-hydrindane only negligibly improved the accuracy of prediction and can generally be neglected when characterizing stationary phases [331, 3321. McReynolds did not correlate the stationary phase specific constants with the sample-specific constants, as he intended only to characterize liquid stationary phases. Therefore, he did not need to divide AZ by 100 as Rohrschneider proposed because the sample-specific constants a-e were defrned as 100. This is why McReynolds could use the AZ value itself. Therefore, the Rohrschneider constants range between ca 0.10 and 13.00, whereas the McReynolds constants range from 10 to 1300 (see Chapter 4). In Table 9 the sample compounds proposed by McReynolds and the types of substances they represent are listed. Table 9. McReynolds Test Substances Symbol

Test Substances

Retention index on Squalane

Compound types Characterize

(I'puhe)

X Y 2

U S

H I

K L

M

benzene butand-1 2-pentanone (methyl n-propyl ketone) 1-nitropropane pyridine 2-methyl-2-pentanone 1-iodobutane 2-octyne 1,4-dioxane cis-hydrindane

653 590 62 I

aromatics,olefins alcohols, nitriles, acids ketones, ethers, aldehydes, esters, epoxides, dimethyl amino derivatives nitro and nitrile compounds N-heterocycles branched-chaincompounds, especially alcohols halogen compounds acetylenic compounds ethers, polyols polycyclic compounds, steroids

652 699 690 818 841 654 1006

The values of X and S correspond to the original 100-fold Rohrschneider constants, whereas for X Z and V the retention differences AZ = ZPolar - Inon-Pohr h ave been determined for the higher homologues of the Rohrschneider sample compounds. McReynoZh showed, after having evaluated 226 stationary phases, that the selectivity and gas chromatographic polarity of liquid stationary phases can be characterized by the differential Kovhts retention indices, AZ = zpolar - Inon-polar, measured for the solutes in Table 9 on the given stationary phase and on squalane as a non-polar reference stationary phase. By analogy with the previous examples, the meaning of the symbols X...M, again, is

x= Azbcnzcnc = Y = AZbutanol etc.

cnzcnc

= @a*e utanol - 1

- zbenzene qualane - qualane zbutanol

9

- 19

These so-called McReynolds constants, symbolized by the capital letters X...M, were determined by McReynolds [324] at 120°C instead of 100°C as used by Rohrschneider. By means of the stationary phase-specific Rohrschneider and McReynolds constants, polarity scales of stationary phases can be formulated. For example, they are graduated according to increasing X values and hence a gas chromatographic polarity scale (elution order on the stationary phases concerned) for aromatics and olefins (relative to alkanes) can be obtained. Similarly, a gas chromatographic polarity order for different stationary phases can be given for alcohols, acids by comparing the Y values, for ketones, ethers, aldehydes, esters (Z values), nitro and cyan0 compounds ( V values) and N-heterocycles (Svalues).

93

4.2. Quantities for the Description of Interactions

Hence the constants can be used to forecast whether different stationary phases will behave similarly or differently and hence to facilitate the choice of a stationary phase. This aspect will be discussed in detail later (Chapter 9). Hitherto, we have only considered the selective interactions, expressed by the different constants, and not the general polarity. However, attempts have been made starting from A1 values to find the polarity order of stationary phases towards an “average” functional group. For example, “general polarity” can be defined by

P,,,

=

x+

Y + z+ u+ s.

(144) In Table 10 several selected liquid stationary phases are listed with their McReynolds constants and arranged according to their general polarities. Table 10. Order of Gas Chromatographic Polarity Pkenof Some Selected Liquid Stationary Phases (General Polarity = Sum of the McReynolds Constants, X + Y + 2 + II + S) Stationary Phase

X

Y

Z

U

S

General polarity

methylsilicone oil OV-101 methylphenylsilicone oil OV-17 poly(oxyethyleneoxypropy1ene) Ucon LBSSOX methylphenylsilicone oil, OV-25 polypropylene glycol, Ucon 50 HB-2000 cyanoethylmethylsiliconeoil, XE-60 cyanopropylmethylphenylsiliconeoil, OV-225 Carbowax 600 diethylene glycol succinate cyanopropylsiliconeoil Silar 1OC tris(cyanoethoxy)propane cyanopropylsiliconeoil, OV-275 bis(cyanoethy1)fomamide

17 119 118 178 202 204 228 350 470 523 593 629 690

57 158 271 204 394 381 369 631 705 757 857 872 991

45 162 158 208 253 340 338 428 558 659 752 763 853

67 243 243 305 392 493 492 632 788 942 1028 1106 1110

43 207 206 280 341 361 396 605 779 801 915 849 1000

229 884 996 1175 1582 1785 1823 2646 3300 3682 4145 4219 4644

By P,,, , the predominant interactions altogether are considered. However, special interactions cannot be recognized by this term. For example, the polyethers with their hydrogen bridge bonding tendency exhibit relatively high Y values; hence the order of the Y values in Table 10 is not the same as the order based on P,,,. It can be stated that the general polarity is much less clear than the special Rohrschneider/McReynoldsconstants. Instead of the sum of the McReynolds constants, the polarity has also arbitrarily been defmed as the arithmetic mean of the constants:

1 P=r 1A l ; i

(145)

for i= benzene, 1-butanol, 2-pentanone, 1-nitropropane and pyridine [324, 3331. This “average McReynolds constant” consists of the average interactions of the stationary phase with an “average” functional group of a solute. On the basis of AZ values for the solutes ethanol, 1,4-dioxane and nitromethane, which are intended to characterize hydrogen-bonding interactions (ethanol as a proton donor and dioxane as a proton acceptor) and dipole interactions (nitromethane), stationary phases have been characterized by selectivity parameters [334-3371. Multiplying these AZ values by the logarithm of the retention increments of an additional methylene group in the solute, Snyder [335] obtained corrected logarithms of the partition coefficients the sum of which he defined as polarity P’.

94

4. Characterization of Stationary Phases

A series of solvents including some gas chromatographic stationary phases has been investigated, and polarities P' ranging from 0.1 for n-hexane to 10.2 for water have been calculated for the solvents and from 1.2 (squalane) to 6.8 (B,fl-oxydipropionitrile) for the stationary phases. The restrictions already mentioned when using the term polarity also apply to P,as this quantity only reflects the sum of special interactions and does not indicate the selection of a stationary phase for the numerous special separation problems. It has been shown [338] that in some instances the use of n-alkanes as the basis of Kovhts retention indices, I, and the Rohrschneider-McReynoldsconstants, AZ, fails owing to adsorption effects from highly polar stationary phases or from the column supports at low stationary phase loadings. The n-alkane standard series may exaggerate the relative polarity of the more polar stationary phases and n-alkanes are badly resolved on some highly polar columns. Therefore, another series of polar compounds such as alcohols or aldehydes should also be used together with n-alkanes [338, 3391.

4.2.5.

Thermodynamic Criteria of Polarity

In Section 4.2.2 we considered a more or less polar monofunctional solute with the general formula C,Hzz+]X, where X represents the polar group. We can, instead, write (CH,),(CH,),X, where m + n = z, and n is the only variable in a given homologous series. According to Martin [340] and Rorh and Nouak [333], the standard molar Gibbs function of sorption of a solute i (the free standard enthalpy in the partition equilibrium), AGo(i), represents the sum of the standard Gibbs function contributions corresponding to the individual groups of the solute i: AGo(i) = mAGo(CH3)+ nAGo(CHz) + AGo(x). Approximately, AGo(CH3)= AGo(CH2), and the'Kovbts retention index of the solute i on a given stationary phase, I,(eqn. (136), Section 4.2.2), can be expressed by [341]

4 = 100

[

z, + AQ(CH2)

I

where z, is the number of carbon atoms in the solute molecule. The retention index difference AZi for a solute i on a stationary phase y and on squalane (sq) is then given by

The ratio AGo(X)/AGo(CHz) will increase on the stationary phase y with increasing interactions of i with y, and hence the value of AZ,, the retention index difference of the solute i on stationary phase y with respect to squalane, is a measure of the extent to which non-dispersive intermolecular forces contribute to the retention of i. In Section 4.2.4 we defined the average McReynolds constant as the arithmetic mean of representative interactions between a stationary phase and five selected solutes, P = 1/5 AZ, (145). This expression, according to I

eqn. (146) with a thermodynamic basis, reflects the relative affinity of the stationary phase towards an "average" functional group and a methylene group of the solute (if present). The excess molar free energy of methylene, AGE(CH2) (partial molar excess Gibbs function of a solute in a solute - stationary phase system), has been suggested as a criterion of the polarity of stationary phases [342-3501. Let us consider two consecutive members of the series C,H2,+ ]X with in = (CH3),(CH2),,X and in+ = (CH,),(CH2),+ lX. Combining eqn.(71), p E = RTln yo, eqn. (72a), pE=AGE, and eqn. (139), V, = 273,16R/y0p0Mp,we obtain AGE

4.2. Quantities for the DescriDtion of Interactions

95

= RTln(273,16R/V~oMp), and for the two consecutive homologues in and in+ we obtain the partial excess Gibbs function of 1 mole of solute methylene, A&(CH2) [333]:

which is a measure of the resistance of the stationary phase to accomodate a methylene group. According to eqn. (72), AGE = AHE- TAS', A@ consists of the excess (or mixing) enthalpy of 1 mole of the solute with the stationary phase at infinite dilution, AHe, and of an entropic component (ASE= excess partial molar entropy). Provided that there is a linear relationship between AHe and ASE [351] and that, consequently, AHE is direct proportional to ASE(CH2)at a given temperature for a given solvent and a given homologous series of solutes (CH,),(CH,),,X, then AHE(CH2)and AGE(CH2) are also proportional to one another. Calculating an average excess molar free energy m ( C H 2 ) of methylene for several different types of solutes on a given stationary phase, we can expect to obtain a measure of the gas chromatographic polarity of the stationary phase concerned, as AGE(CH2)expresses the interactions other than dispersive between the solutes (CH3),,,(CHz),,Xand the stationary phase. Moreover, a correlation between and the average McReynolds constant P = 1/5 AZ can also be expected. Roth and Nouak [333] proved i

these assumptions and found a direct proportionality between m ( C H , ) and P and stated that P and m ( C H 2 ) are equivalent criteria of polarity for liquid stationary phases. However, if the direct proportionality between AHE(CH2)and ASE(CH2)is not fulfilled, then AGE(CH2) and AHEare no longer proportional in the solute - solvent system, and the partial excess enthalpy of 1 mole of methylene group, AHE(CH2),being based exclusively on the type and intensity of the intermolecular solute - solvent interactions, would be a more appropriate criterion of polarity [333]. Unfortunately, the determination of AHE(CH2) requires much more experimental work than m ( C H 2 ) . The assumptions we made, that AGE(CH2) is independent of the chain length n and AGo(CH3)= AGo(CH2) for compounds (CH3),(CH2),,X, are not completely correct. It has been shown that any homologous series exhibits a non-linear change in the logarithm of the specific retention volumes or retention indices with increasing carbon number [352-3541, and the strong deviation from linearity observed for the first five or six members of any series C,H2,+ 1Xhas been well known for a long time. It was found that the energy contribution per methylene group, AGE(CH2),is not constant but changes with the increasing carbon number z in any homologous series on both polar and non-polar stationary phases [355], owing to interactions between the methyl group and the functional group X or between two neighbouring CH3 groups [356]. This change can be taken into account according to Golounya et al. [355], in the following way. In Section 4.2.2, we have seen that the difference in log V, for any two neighbouring n-alkanes is equal to 100 index units (i.u.). Applying the relationship between AG and the specific retention volume and using Briggsian logarithms, we can write

The dependence of the specific retention volume on the number of carbon atoms, z, can be described by [357]

96

4. Characterizationof Stationary Phases

where a, b, c and d are coefficients that depend on the stationary phase and the column temperature. Combining eqns. (148 and 149) we obtain the change in energy equivalent to 1 i. u. on increasing the length of the carbon chain of n-alkanes from z - 1 to z:

[

AGi.u.= -0.023 RT b -

C

(z+d)(z-1+d)

1

(150)

'

It can easily be seen that only for high z values AGi.u. tends to become constant (-0.023 RTb). According to definition (see Section 4.2.2), any compound i with retention index Ii may be considered as a hypothetical n-alkane with a fractional number of carbon atoms, zi = IJlOO, and we obtain from eqn. (149) b Zi log V s i = a + -+ 100

C

and, on combination with eqn. (150), we obtain

[

AGi,U,=-0.023 RT b -

C

(A 100 + d) (A- 1 + d)

]'

Hence it follows that AG.u. depends on the two n-alkanes used for the calculation of the retention index. The total energy equivalent of the retention index on a n y stationary phase can be calculated from AGI = ZAGi.,,.. (153) Considering two stationary phases 1 and 2, one of which is polar (phase 2) and the other nonpolar (phase l), we can formulate the difference between the energy equivalents of compound i on these two phases AQ'J

= I:AGtu,

where A@'

= =

=

- I;AGt,

(154)

energy equivalent of the difference in the retention indices A I on the polar and non-polar phases, energies equivalent to 1 i.u. calculated by eqn. (152) for stationary phases 2 and 1, respectively, retention indices of substance i on the phases 2 and 1, respectively.

Golounya and Grigoryeua [355, 362b] have conclusively shown, that the AQ value of a compound on two stationary phases is a better measure than A I of the energy of interaction of the functional group X with the stationary phase, and that the non-linear variation of the dispersion energy with increasing carbon number in the homologous series C,HzZ+'X is taken into account. As the dispersion energy contributes mainly to the free energy of interaction between solutes and the stationary phases, the investigation of its variation on different phases offers comprehensive information on the energy contribution of the functional group and its specific interaction with the stationary phase. However, the constants b, c and d [eqn. (152)] have to be determined previously (for n-alkanes only!) on the stationary phases under investigation by using eqn. (149). The dependence of AQ itself on the carbon number z has been shown to be linear, starting with the first member in a n y aliphatic series CzHzz+,X. A further criterion of polarity based on the dispersive interactions between the methylene group and the stationary phase was proposed by swtik and Lhentap [358]. The difference in

97

4.2. Quantities for the Description of Interactions

retention times between adjacent n-alkanes gives the contribution of the methylene group only. According to eqn. (53): At,+ 1 = k ( r + 1) - rR(z) = t k ( z +

1)

- tk(z)

and according to eqns. (21, 23 and 24):

we obtain M t tk = -KL .

B

As AG? = -RTln KL(,> (80) we obtain from eqn. (155) for At,+

The ratio of neighbouring retention time differences is defined as the polarity criterion A: A=- At,+ 1

(157)

At, '

where

= tko) - t i ( , -

1).

After substitution of retention time differences by eqn. (156) and

- A@ M t -

division by 7 e

RT

the polarity criterion A can be expressed by

Eqn.(158) shows the relative changes between adjacent n-alkanes relative to a centrally positioned n-alkane with z carbon atoms, the exponential term expressing the contribution of one methylene group. With an increasing proportion of dispersive interactions of a solute-stationary phase system, the free energy change of a methylene group becomes larger and A will increase. Hence A is large for non-polar stationary phases, and with increasing polarity of the stationary phase A will decrease. A is related to the relative retention by A

= r,-

r,+l- 1

r, - 1

(159)

where r,+ = tk(,+I)/tk(,)and r, = tk(z)/tk(z-l).With increasing chain length r, approximates to r,+ 1 , and A becomes equal to the relative retention r, in eqn. (159). The determination of A is easy, as it requires only the evaluation of the retention time differences for adjacent n-alkanes. The temperature dependence of A can be desribed by b' -

A=a'eT (160) where d and b' are constants, and allows the polarity to be calculated at any temperature; the constants d and b' provide a quantitative description of stationary phase polarity [358]. Criterion A is large for non-polar stationary phases and low temperatures and diminishes for stationary phases with a large number of functional groups and with an increase in temperature.

98 4.2.6,

4. Characterization of Stationary Phases

The Molecular Retention Index

In order to characterize stationary phases, Evans proposed the molecular retention index, Me, which is based on the molecular retention index scale of relative retentions [359].The relative retentions (in molecular retention index units) are obtained from chromatograms based on n-alkanes as an internal standard. From the adjusted retention times t i the value of Me is calculated using the equation Me = M,+ 14.027 n

log tk - log f R l log tx(,+ ") - log tx,

where

1

= molecular weight of the n-alkane with z C atoms, = adjusted retention time of the investigated compound, = adjusted retention time of an alkane with z C atoms, tX(,+,) = adjusted retention time of an alkane with z n C atoms.

M,

rh rk,

+

As mentioned in the previous sections, the Kovhts retention index can be regarded to be 100 times the carbon number of a hypothetical n-alkane that would have the same retention as the investigated compound i, I = 100 Z', where ? is the number of carbon atoms in the hypothetical n-alkane CrHZr+ or (CHZ),+ 2H. The carbon number is substituted by the molecular weight: Me = 14.027 I'

+ 2.016 = 0.14027I +

2.016.

(162)

AMe is defied as the difference between the molecular retention index and the molecular weight of compound i (M,): AMe = Me - Mi.

(163)

According to eqns.(l61) and (162)and to definition, AMe is zero for all n-alkanes and for all chromatographic column parameters. As soon as a functional group X has been introduced, the values of Me and Mi will diverge. Polar groups that strongly interact with the stationary phase generally exhibit positive AMe values. For homologous series, AMe is constant. On polyethylene glycol, for example, the AMe-values for bromoalkanes range between 6.79 for 1-bromohexaneand 6.86for 1-bromodecane,and for the methyl esters of monocarboxylic acids from 48.87for methyl n-butanoate to 49.48 for methyl n-dodecanoate, with probably only random differences [3591. The temperature dependence can be described by the relationship AMee = AMeo + p.8

where AM% = value at "C (by extrapolation) AM% = value at BC, p* = constant, 8 = temperatue in "C. The temperature dependence of gas chromatographic polarity was considered in Section 4.2.5.Eqn. (163)also offers the facility to characterize the stationary phases in relation to selected functional groups at any temperature. The differences in AM% at different temperatures are signscant on special liquid stationary phases. On polyethylene glycol, for example, the functional groups X = Br shows a value AMe = 4.59 at 75°C and 9.03 at 125°C and for X = COCH3 AMe is found to be 65.72 at 75°C and 70.57 at 125°C. Significant AM% differences were observed on polyethylene glycols of different molecular weights, which hence possess different moieties of the hydroxy end-groups. The AMe values at 100°C of, e.g., 2-ethylhexan-1-01were found to be 116.11 on PEG 200,99.35 on PEG 400, 87.72 on PEG 1000 and

99

4.2. Quantities for the Description of Interactions

81.56 on PEG 4000 (for further details see refs. [359] and [360]. Further, AMe indicates the residual activity of the solid support. AMe is related to the McReynolds constant by the relationship AMe = Me - Mi = 0.14027 (AZi +

+ 2.016 - Mi

where McReynolds constant of the investigated standard compounds i used by McRtynolds, valid for the stationary phase concerned, = retention index of compound i on squalane, AMe = difference between the molecular retention index Me and the molecular weight Mi of compound i. This equation indicates that the molecular retention index Me and the derived term AMe vary numerically from the Kovhts retention index and from the Rohrschneider-McReynolds concept. Its advantage consists in distinctly expressing different interactions by evidently different numerical values. AZi

=

c AZ (see Section 4.2.4), and [361] proposed the term "McReynolds polarity index" c AMe, which expresses the sum of the 5

By analogy with the "general polarity"

Evans

Osbom

1

5

=

1

main polar interactions of the stationary phase in question.

4.2.7.

The Selectivity Index

As the molecular retention index has attracted little attention, Evans et al. [691] proposed to extend the Kovhts retention index system by the introduction of dispersion and selectivity indices. We have seen that both apolar (London dispersion) und polar forces (induction, orientation, hydogen bond, etc.) contribute to the overall retention, the former being the only forces between n-alkanes and the stationary phases, while the latter occur when functional groups are introduced into the CnHzn+z molecule of the analyte. We can assume that the dispersion forces are directly proportional to the molecular weight of the analyte, and the retention index [eqn. (136)] can be expressed by

where

ZM =retention index of a hypothetical n-alkane having the same molecular weight as the solute, designated the dispersion index, I* =carbon number equivalent of AMe [eqn. (163)], designated the selectivity index. The value of ZM can easily be calculated from IM =

M - 2.016 0.14026

where M is the molecular weight of the solute. This expression arises from the definition of retention index [see eqn. (136)] as Z = lOOi, where i is the number of carbon atoms of a hypothetical n-alkane CiHzi+ or (CH& + 2H with a molecular weight of 14.026 i + 2.016. ZM is regarded as equivalent to the contribution for dispersion forces to the overall retention. By definition, it is ZM = Z for n-alkanes, i.e., I* = 0 in eqn. (164). For all other solutes, the difference I* = Z - ZM [from eqn. (164)] expresses the effects of functionality and molecular shape. Its calculation is easy, as Z can be

4. Characterization of Stationary Phases

100

obtained from eqn. (136) and ZM as described. Strongly polar functional groups in solutes yield large positive I* values, e.g., nitroalkanes and cyanoalkanes, whereas substituents with screened electrons (bromo- and iodoalkanes) show large negative values. Structural (cyclic groups, chain branching) and steric influences also bear distinctly on I? Changes in the stationary phase composition are evidently indicated, and the parameter Z*, the selectivity index, can be expected to permit the characterization of stationary phases without the need for a reference phase such as squalane [691]. Values of the selectivity index I* are given in Section 9.4.

4.2.8.

The Infrared Spectroscopic Frequency Shift

The infrared spectroscopic solvent effect has been shown to be a suitable means for characterizing intermolecular forces [362]. Dividing the total cohesive forces into dispersion and association forces, the latter, interacting between polar functional groups of solute molecules and molecules of the liquid stationary phase, can be recorded spectroscopically. The excess chemical potential py of compound i is also assumed to be composed of contributions from association and dispersion forces. Taking into consideration eqn. (71), we obtain R T h y!

= py =

pp + p? + p;ter

(165)

where y:

=

activity coefficient at infinite dilution,

pp = association part of pE, p p = dispersion part of

#,

= steric

part of pE. Neglecting steric contributions and assuming that the non-polar standard compound does not contribute to the association forces, pp = 0. On combining eqn. (72), p E = AH - TAS, and the Herington relation, eqn. (70a), we obtain

where Vkiand Vk,

=

vxi

adjusted retention volumes of compound i and a non-polar standard, respectively,

=

relative retention of a polar solute,

V’Rst

dispersion contributions to the excess partial molar enthalpy of the standard solute, = dispersion contribution to the excess partial molar enthalpy of the AH? polar solute i, = association contribution to the excess partial molar enthalpy of the AH? polar solute i, As:, = relative excess entropy, = saturation vapour pressures of the pure standard compound st and P L P! compound i, respectively. The total relative excess entropy, composed of the term =

AH:

AS:,

where

=

-AS:

+ AS! + A S P

(167)

4.2. Quantities for the Description of Interactions

101

AS: - dispersion contribution of the excess entropy of the standard compound AS: and compound i, respectively, A S P = association contribution of the excess entropy of compound i, is assumed, for similar molecular weights of st and i, to be determined chiefly by the association contribution. To a good approximation, the association forces can be realized by means of the infrared spectroscopic frequency shift A v P , in accordance with the equation

where Avp

infrared spectroscopic frequency shift of a suitable frequency of the functional group X of a solute i, measured in squalane and in the stationary phase: = ,,;qualane - stat.liq.ph. [cm-'1 Vi E = empirical constant, P = constant consisting of the contributions of the dispersion forces to the excess enthalpy and the entropy and vapour pressure term, nj = number of the interacting groups per molecule of the stationary phase. =

1 j

Modifying Korol's method [363], the contributions of the dispersion forces to the excess enthalpy may be assessed from the molecular properties as follows [364]: AH? - AH:

=

-constant A D

where AD = Di - Dst. Di and Dst can be calculated from the equation n

m

where Dv = dispersion index (see eqn. (132) in Section 4.1.1), Kv = intramolecular shielding coefficient, = relative frequency of different functional groups in the molecule. Assuming T A P to be constant, we obtain log*

vxst

= A.

+ A l 1 nj AvhS + A2 AD

(170)

j

where Ao, A l and Az are empirical constants. The equations in this section offer the facilities for determining, at least roughly, the most important interactions in the solute-stationary phase system, provided that suitable infrared spectroscopic frequencies are available.

4.2.9.

Further Quantities for the Description of the Interactions

Burns and Hawkes classified liquid stationary phases according to scales for differences in polarity, in hydrogen bonding, and in acidity, all based on relative retentions [365]. They calculated the dispersive interactions from the refractive indices of both the stationary phase and the solute using the equation [366]

(nZ- 1) (n2+ 2)

8D = 30.1 -

where

102

4. Characterizationof Stationary Phases

r5,, = dispersive solubility parameter [J/mole41, n = refractive index. In order to differentiate between the interactions of permanent dipoles, dispersion interactions and acid-base effects, Bum and Hawkm used the retention of benzene relative to heptane as a measure of the intensity of permanent dipoles in the stationary phase, as benzene suffers an induced dipole and is retained by the consequent induced dipole - permanent dipole interaction, but is not subject to acid-base effects. With the restriction to this type of polarity, they defined a polarity index P with the elimination of dispersion contributions implicated in the benzeneh-heptane relative retention: n-heptane

-6.4-(dxD-4.7xd

D,

5 90 2.3 RT

-

where (6; - 4 . 7 ~ represents ~) the correlation between the relative retention term and the refractive index term [365]; d; is the average number of solvent molecules to which each solute molecule is bonded. The calculated values of x are 1.820 for hydrocarbons and 1.887 for methyl and methylphenylsilicones. In most instances, alcohol retention may be considered to be a suitable index of the basicity of the stationary phase. Hence the hydrogen bonding potential, caused by the ability of hydroxy groups of a solute to form hydrogen bonds with the stationary phase, can be indicated by an alcohol in relation to a compound showing the same dispersion and dipole-dipole interactions, which can therefore be eliminated. An appropriate pair of substances was considered to be n-butanol and ethyl acrylate (3651, and the hydrogen bonding potential H was given by

[

H = 10 0.1 +log

(

t6-butanol

)]

L Y l acnlate

(173)

The Lewis acidity of a stationary phase can be conveniently obtained from the retention of pyridine. However, there is no matching partner that, calculating a relative retention, would simultaneously eliminate the dipole of pyridine and its dispersion potential. Therefore, benzene was used to cancel the dispersion interaction, and the polarity index had to be used (with an empirical factor) to cancel the dipole-dipole interaction. However, the interaction with stationary phase dipoles induced by the pyridine dipole could not be taken into consideration when formulating the acidity index A :

[ (7 f2::)- L P - O . I ~ ] .

A = 5 log

22

(174)

The indices are listed in Section 9.4., Table 86. Approaches for characterizing liquid stationary phases by thermodynamic parameters other than those already discussed in previous sections have also been reported. Kurger et al. [367] calculated speciiic solubility parameters for dispersion, induction, orientation and hydrogen bonding interactions, and Rkby et al. [368-3701 developed a method for calculating the entropies and free energies of individual functional groups on sixteen representative stationary phases, From the partial molar Gibbs free energies of solution for particular test solutes (from partition coefficients corrected for interfacial adsorption) an approach which is claimed to be a better measurement of stationary phase selectivity than with the Rohrschneider-McReynolds model was developed by Poole et al. 406a, 406b.

5.

Solid Stationary Phases (In part together with W.Engewuld and J. Porschmunn)

Compared with the far more popular gas liquid chromatography gas solid chromatography (GSC) is the older technique, even when only the elution mode is considered [3], [6], and in several fields of application it is superior to the gas liquid variant. This applies not only to the analytical seperation of gases and low-boiling mixtures, which has been well established for decades, but also to the separation of stereoisomeric and structurelly isomeric hydrocarbons and of certain polar organic compounds and aromatics. In the recent past a distinct trend towards GSC has been observed. This trend may be attributed to two factors: fmt, new and improved adsorbents have been developed, and second, in order to exhaust the possibilities of the improved gas chromatographic hardware which permits e.g., better trace analyses, more reproducible temperature programming and better coupling techniques, both liquid and solid stationary phases of outstanding properties are required, among which tailor-made solid stationary phases are essential for solving many of the problems involved. Some advantages and disadvantages of the various adsorbents are mentioned prior to their detailed description. They are chemically stable and they offer rapid adsorption-desorption steps (eqn. (64) in Section 2.4), hence reducing the mass transfer term and enhancing the emciency of columns compared with the more adverse gas-liquid mass transfer. High flow rates of the carrier gas can be applied, hence reducing the analysis time. Many adsorbents can be used over a wide temperature range, and they show excellent selectivity for the separation of stereoisomers and isotopes. At high column temperatures there is generally no bleeding, and some of the adsorbents are insensitive to oxygen attack. On the other hand, with geometrical and chemical heterogeneity of the surface the efficiency of the adsorbents will be affected, and the adsorption isotherm r;,,,,, = p(c) (eqn. (85) in Section 2.5) will be strongly curved with only a very short straight section, resulting in tailing of peaks, in an undesirable dependence of the retention on the sample size and in incomplete recovery of the sample from the column. At elevated temperatures, the sample is exposed to the risk of being catalytically altered. Finally, the standardization of solid stationary phases is more difficult than that of liquid phases, and its neglect has often led to failures. Several basic practical and thermodynamic aspects have been dealt with in previous chapters and need not be repeated here.

5.1.

Classification of Adsorbents

At the beginning it should be emphasized that in contrast to other fields of application the adsorbents used for gas chromatographic analysis must not be subject to appreciable chemisorption of the sample types to be analysed. In order to provide a rapid and reversible establishment of equilibrium, only physical (Van der Wuuls) adsorption attributable to intermolecular forces, discussed in Section 4.1,should prevail. Interaction energies are composed of two terms, the specific surface energy and the specific surface area. The first term is determined by the chemical structure of the surface and is responsible for the intermolecular forces between the adsorbent and adsorbate and influences mainly the selectivity term (r2, - 1)/r2, in the resolution equation [eqn. (loo)] (see Section 2.6), whereas the other term is an expression of the geometrical structure of the adsorbents and influences the capacity term k,/(kz + 1) of eqn. (100).In the Kubin and KuEera theory of GSC, ignoring the particle diameter and flow-rate dependences and assuming that the adsorption takes place in the lin-

104

5. Solid Stationary Phases

ear region of the adsorption isotherm, the retention time and the peak width, defined as the first absolute moment pl and the second central moment p 2 , respectively, are functions of the quantities that characterize the chemical nature and texture of a solid packing [371-3751: P1 = fl(K,)

(175)

where K,

k, E’

= partition coefficient in GSC, = adsorption coefficient, = internal porosity of the particles

with respect to the pore space (which, contrary to e0, the interparticle porosity, was not considered in eqns. (55) and

(56), Dint= effective diffusion coefficient of the separated substances in the pores of the solid. Owing to the proportionality between K , and the specific retention volume in GSC, V, [eqn. (20)], the retention increases with increasing adsorption equilibrium constant, which in turn depends on the strength of the interaction of the compound to be adsorbedldesorbed with the solid phase. The differences in the K , values for solutes with different chemical structures will be large, hence accomplishing the separation (pl = f l ( K J ) . Strong adsorption invariably broadens the peak, as is apparent from eqn. (176) where K , is involved, and as is confirmed by experience, i.e., it affects the efficiency and separability. A principal means of improving the peak width consists in the selection of adsorbents with a favourable pore size distribution, as p2 [eqn. (176)] depends on E’ and on the reciprocal of the diffusion coefficient Dht in the pores of the solid. In narrow pores, e.g., with pore diameters 10 nm, the Knudsen diffusion is predominant, i.e., collisions of sample molecules with the pore walls take place more often than with other sample molecules or with molecules of the carrier gas. This special diffusion, the rate of which increases linearly with increasing pore diameter, is very slow (e.g., for n-hexane at 10 Torr, 4, = 0.0202 cm2/s, compared with the bulk diffusion coefficient for a hydrogen-n-hexane mixture of D = 0.5148 cm2/s [375, 3761). In wide pores, i.e., with pore diameters > 200 nm,the rate of difision is independent of the pore diameter and the effective diffusion coefficient is proportional to the bulk diffusion coefficient. To summarize, there are two essential characteristics of adsorbents by which they can be classified: their chemical structure and their geometrical structure.

5.1.1.

Classification According to Chemical Structure

Based on the chemical nature of the adsorbent surface, different kinds of interactions with different sample molecules can occur. Kbeleu, whose proposed classification [377, 378-3791 has been generally accepted and applied, subdivides adsorbents into three groups (1-111) and adsorbates into four groups (A-D). Adsorbents of type I Non-specific adsorbents, which do not have any functional groups or ions on the surface and hence are not capable of specifically interacting with adsorbates. The interaction with all types of sample molecules A-D proceeds non-specifically. Adsorbents of this type are saturated hydrocarbons (in crystalline or solid polymer modification or as a layer on a suitable supporting adsorbent), graphite or rare gas crystals. The most important adsorbent of this type is graphitized thermal carbon black (GTCB)which in its adsorption properties approaches an ideally non-specific adsorbent when prepared or pre-treated in a suitable manner.

5.1. Classification of Adsorbents

105

The adsorbed molecules are arranged in such way that they contact the highest possible number of surface atoms. Owing to their structure, which is similar to that of graphite, the inorganic adsorbents boron nitride (BN), and sulphides of some metals (e.g., MoSJ can be included in this group [379, 3801. Adsorbents of type I1 Specific adsorbents exhibiting positive partial charges localized on the surface. In addition to the dispersion interactions that occur on any adsorbent independent of its type, specific interactions develop, resulting in an orientation and localization of the adsorbate molecules at the sites with the highest charge. Especially salts, in which the positive charge is concentrated on cations of small radius whereas the negative charge is distributed over a relatively large volume, belong to this type (e.g., BaS04). Zeolites, the cations of which have small atomic volumes, whereas the negative charge is distributed over the inner bonds of a large complex anion formed from NO4- and Si04 tetrahedra, are also of this type [379]. However, the most significant representatives of this type are adsorbents with functional groups of protonated acids, such as hydroxylated silica gels, and with aprotic Lewis centres on the surface. Sample molecules of type A (saturated hydrocarbons, rare gases) are adsorbed non-specifically, as only dispersion forces can become effective. Molecules of type B, C and D can be adsorbed specifically. Type B include molecules with an electron density localized on some bonds or atoms: sr-bonds (unsaturated and aromatic hydrocarbons); functional groups, the atoms of which exhibit unshared electron pairs (ethers, ketones, tertiary amines, pyridine, nitriles); high quadrupole moments (N2molecules) The interaction between type B adsorbates and type I1 adsorbents occurs between the centres of higher electron density (sample molecule) and the positive charge of the adsorbent (for example, the acidic proton of hydroxylated silica gel or an appropriate cation (Li, Na, Mg, Ca) in zeolites or aprotic Lewis centre (Al, B) on the surface). Type C molecules have a localized positive charge on a metal atom and the excess of the electron density is distributed over adjacent bonds (organometallic compounds). Because of the high reactivity of many organometallic compounds and of the risk of chemisorption, there have been only a few investigations of this interaction. Type D molecules contain peripheral functional groups (OH, NH, etc.), the electron density of which is increased on one of the atoms (0,N) and diminished on the other (H). This group includes water, alcohols and primary and secondary amines. The specific interactions of type D adsorbates with type I1 adsorbents mainly involve forces between the positive charge centres of the adsorbent and the lone electron pairs of the 0 or N atoms of the sample molecules. Adsorbents of type I11 Specific adsorbents bearing centres of higher electron density on the surface. To this group belong polymers such as polyacrylonitrile, copolymers of vinylpyridine and divinylbenzene and polymers with C==O and -0groups on the surface. Porous polymers based on styrene-ethylvinylbenzene, cross-linked with divinylbenzene, varied by applying difierent polymerization initiators with various functional groups, may also be included in this group, even if non-specific dispersion forces preponderate. Type I11 adsorbents include also crystal surfaces formed by anions, and especially chemically modified adsorbents or non-specific adsorbents covered by a dense monolayer of suitable substances, hence creating negative charge centres on the surface. Adsorbents of type I11 interact non-specifically with adsorbates of type A and specifically

106

5. Solid Stationary Phases

with types such as B, C and D by forces between the negative charge on the adsorbent’s surface and the positive charge of the metal atom (C) or of the functional group’s (OH, NH) proton (D) or of the dipole or an induced dipole (B).

5.1.2.

Classification According to Geometrical Structure

We had stated that the geometry of the adsorbents influences especially the capacity term in eqn.(100). Hence the surface area should be as high as possible in order to increase this term. However, there are serious reservations. Increasing the surface area means either increasing the dispersity (with the consequence of an increase in heterogeneity due to the increasing contact points between the particles) or narrowing the pore diameters (with the disadvantage of Knudsen diffision). The outcome of numerous investigations in this field, among which especially the work of Kkeleu should be given prominence, has been that difficulties of this kind, having retarded the development of GSC for a long time, have been surmounted [379]. The role of the surface area can be derived from basic equations in Chapter 2. For infinitely small (zero) samples, the net retention volume VN,under equilibrium conditions, is equal to the Henry constant of the adsorption equilibrium [379]:

where c is the concentration of the sample in the gas phase. If we consider the total surface area of the adsorbent in the column, mASA, where m A = weight of the adsorbent and SA= specific surface area of the adsorbent, we obtain (from eqns. (11) and (20)), neglecting the temperature,

which is the adsorption coefficient (or the Henry constant referred to unit surface area of the adsorbent). From VN = KH we obtain the correlation of both constants KH and K s with the geometrical parameter:

v~ = KH = mASAVs = mASAKs.

(178) Hence VN, the net retention volume, can be influenced by both the column parameters (weight of the adsorbent m d and the geometrical characteristic of the adsorbent, its specific surface area SA.K,, however, can be influenced by the chemical nature and structure of both the interacting adsorbent and adsorbate, expressed by analogy with eqn. (189a) as the partial molar adsorption enthalpy [381]:

As AHA, the partial molar adsorption enhalpy, changes only slightly with the temperature, we can write [379]

or (18Oa)

5.1. Classification of Adsorbents

107

where ASA = partial molar adsorption entropy of the adsorbate for the transition from the standard state of the gas volume with concentration 8 into the standard adsorbate state with an adsorption concentration P. Eqn. (180a) shows the exponential dependence of K s on temperature, the third essential parameter in GSC, in addition to the adsorbent’s chemical structure and geometrical structure. Eqns. (178) and (180a) demonstrate that even adsorbents with small specific surfaces areas permit the separation of weakly adsorbable gases, provided that the column temperature is decreased accordingly, hence increasing K s and thus also its product with the surface area, mAS& [379]. The alternative, or better for completion when separating low boiling gases, is the use of adsorbents with small particle diameters and/or fine pores, hence increasing SAand m A S A . Kiseleo and Yashin [377] classified adsorbents geometrically as follows. Type 1

Type 2

Type 3 Type 4

Non-porous adsorbents Crystalline products with a smooth surface (sodium chloride, graphitized thermal carbon black, BN, MoS,) SAvalues 0.1- 12 m2/g. Uniformly porous adsorbents with wide pores Silica gels with pore diameters between 10 and 200 nm (but each silica gel product with narrowly distributed pore diameters!) (Porasil, Spherasil) and phases bonded chemically on silica gel (Durapak, etc.) as also some wide-pore styrene-divinylbenzene polymers (pore diameters 20-400 nm). Uniformly porous adsorbents with narrow pores Molecular sieves (zeolites), carbon molecular sieves, porous glasses, porous polymers. Pore diameters 10 nm. Irregularly porous adsorbents Active charcoal, alumina. Owing to the geometrical (and chemical!) heterogeneity [the pore diameters range from 2-20 nm (transition pores) up to >200 nm (macropores)], such adsorbents are not appropriate for GSC (with the exception of their use as enrichment materials), even though they were widely applied in the early years of gas chromatography.

This classification is based on the existence and size of the pores. Porous adsorbents differ from non-porous solids by a void structure shaped from a system of pores. This structure can be characterized, independent of the chemical composition of the adsorbent, by the following quantities [382]: - Specific surface area SA(geometric size of the pore wall area per gram of adsorbent). - Specific pore volume V, (total pore volume per gram of adsorbent). - Mean pore diameter dSo(average diameter of 50%of the pores; this value is identical with the maximum frequency only for a homogeneous pore size distribution). - Pore size distribution (distribution function d( V,)/d(dso)). They can be determined by gas chromatography [381], mercury porosimetry and reversed size exclusion chromatography [383]. Important for porous adsorbents is the ratio of their pore diameters to the diameters of the adsorbate molecules. If this ratio is high, i.e., the pore diameters are much larger than the molecule diameters, then the adsorption equilibrium is established rapidly. If the pore diameters are similar in size to the adsorbate molecules, then the adsorption rate depends on both the pore shape and the size of the molecules. In narrow pores the adsorbed molecule may interact with surface atoms of the opposite pore walls, and the exchange of molecules with the mobile phase is delayed. Hence the adsorption behaviour

108

5. Solid Stationary Phases

should be improved on replacing fine-pore materials with macroporous products because of the increased mass transfer rate. However, a large specific surface area, which is caused by the presence of micropores, will improve the capacity term [eqn. (loo)] and with it the resolution. As these demands are partially contradictory, a compromise has to be adapted which, depending on the analytical problem, has led to the many types of adsorbents dealt with in detail in this chapter. Special types of adsorbents are the so-called molecular sieves and substances that form inclusion compounds. The former separate molecules and atoms according to their size and shape by the micropore dimensions (holes, cavities), and the latter by the ability, owing to their suitable steric properties, to enclose (temporarily) spatially such “guest” molecules, the spatial dimensions of which correspond to the dimensions of the free cavity in the “host” material. These special adsorbents will also be discussed. For the effect of the pore diameter on the diffusive sample retention, see ref. [383a]. Finally, it should be noted that the particle diameter (or particle size distribution) of solid materials for gas chromatography, has not yet been covered. This will be treated when discussing solid supports (Section 7.1).

5.2.

Carbon Adsorbents

5.2.1.

Graphitized Thermal Carbon Black (GTCB)

Pioneering investigations on the properties and applications of this adsorbent were carried out especially by Kiseleu, as already mentioned [377], and by Halarz [384], Liberti [385] and Guiochon [386] and their co-workers, Studies of the sorption and separation properties showed that, according to the Kiselev type I classification, this adsorbent interacts almost exclusively with any adsorbate by dispersion forces, and it was stated that thermal carbon blacks are the most homogeneous adsorbents known [387]. A review of the theory and chromatographic applications was given by DiCorcia and Liberti [388]. The adsorption properties approximate those of the basal face of graphite, and the dominating feature is the high potential of dispersion forces resulting from the high concentration of force centres (carbon atoms) on the basal face of graphite. The strength of the interaction depends on the geometric structure of the molecules and the adsorbent surface and on the adsorbent-adsorbate distance. Because the adsorbate molecules can approach to the adsorbent plane only from one side, and adsorption on the flat homogeneous surface is most sensitive to the geometric structure of an adsorbate (GTCB belongs to adsorbents of type 1) the retention on such a non-specific adsorbent is determined predominantly by the steric structure of the adsorbate molecule. Based on semi-empirical atom-atom potentials of adsorbate-adsorbent intermolecular interactions, molecular statistical calculations of the thermodynamic characteristics of adsorption at very small (zero) surface coverage on GTCB could be carried out and the results were compared with the experimental thermodynamic values (the Henry constant) obtained by gas chromatography [3891. The possibility of transferring these potentials from a simple to a complex molecule was thoroughly investigated by Kiseleu, including the changes in the atom-atom potential with changes in the electronic configuration of the atoms in the molecule. The high sensitivity of the Henry constant to the geometry of molecules offers the possibility of determining molecular structure parameters for a molecule of unknown structure using experimental gas-solid chromatographic data and the known atom-atom potentials of adsorbate-adsorbent intermolecular interactions. It could be shown that any atom of the adsorbed molecule contributes an increment that diminishes with increasing distance of the atom from the equilibrium distance to the graphite plane, hence permitting the chromatographic separation of similar

5.2. Carbon Adsorbents

109

molecules that exhibit only small steric differences. The method for the determination of molecular structure parameters is named chromatoscopy, chromatostructural analysis or the adsorption method [389]. Returning to the structure of the adsorbent GTCB, it was found that it has the same structural parameters as graphite itself. The distance between adjacent carbon atoms in the hexagonal layers is 0.1418 nm and that between the layers is 0.3395 nm.GTCB is prepared by thermal decomposition of methane and subsequent heating to 3000°C in a reducing atmosphere, or directly by graphitizing carbon black at this high temperature. Crystal growth is effected by this graphitization up to a particle size of 500nm. During this process, volatile compounds are removed. Residual contaminants, which would cause asymmetric peaks and irreversible adsorption of polar adsorbates, can be largely eliminated by treatment with hydrogen at 1100°C after the graphitization [388]. The particles consist of polyhedra, the interfaces of which are formed essentially by the basal faces of the graphite crystals [381]. GTCB is a fine, light-weight powder with a specific surface area of S, = 6-12 m2/g. By shaking and screening, small pellets aggregate, merely by adhesion, which after fractionation can cautiously be packed into the column. Owing to the low stability of the beads, vibration and pressure surges must be avoided. Nevertheless, the efficiency of such columns is not satisfactory, but it can be improved by addition of viscous products (Apiezon L, silicones) in small amounts (0.01%)to the powder to increase the mechanical stability of the pellets [390, 3911. A further possibility consists in building up layers of pyrocarbon, formed by the pyrolysis of hydrocarbons, on the soft pellets. The pyrolysis occurs in a gas stream directed through a bed of graphitized carbon black. Mechanically stable macroporous adsorbents, Carbochrom A (from GTCB) and Carbochrom B (from non graphitized thermal carbon black), can be obtained by this procedure, and exhibit similar surface properties to GTCB [392, 3931. It is not possible and not intended to review the many analytical applications of this adsorbent (and of subsequent solid and liquid stationary phases) in this book. Therefore only some characteristic and perhaps interesting applications of GTCB are referred to here. Isomeric alkanes with primary, secondary and tertiary carbon atoms can be separated because shorter, more branched molecules offer fewer sites of contact with the adsorbent, as any molecule aligns with its longitudinal axis parallel to the surface and hence is eluted prior to less branched molecules [377, 394, 395, 406~1.Fig.6 illustrates the separation of fifteen isomeric octanes [396]. Isomeric cycloalkanes can differ in ring size, in the number, kind and position of the alkyl substituents and geometrically. The cyclohexane molecule, for example has a preferential chair conformation, and can contact the GTCB surface only with three of its six ring-carbon atoms, the other three ring-carbon atoms being more distant from the surface. For substituted cyclohexanes the greatest intermolecular interaction may be observed for isomers the substituents of which are all aligned equatorially, as Fig. 7a demonstrates for the geometric isomers of 1,2,4,54etramethyl cyclohexane [397]. Isomeric cis-trans-cycloalkanes can be separated as excellently [398-4001 as endo and exo isomers of substituted bicyclic hydrocarbons [401] and numerous structurally and sterically isomeric terpenes [377]. Applying micro-packed GTCB columns [137, 2961 will increase the column performance enormously, as can be seen in Fig. 7b, which illustrates the separation of all stereoisomeric perhydroanthracenes on a short (1.4 m) column within 12 min [402], in order of increasing flatness of the structures. These micro-packed columns can be directly coupled with mass spectrometers. As a result of numerous investigations on hydrocarbons with double and triple bonds, it can be concluded that on transition to double and triple bonds, the interaction between the carbon atoms of the adsorbate and the surface increases, but on the other hand, owing to the

110

0

5. Solid Stationary Phases

5

-

10

t,, in min

Fig. 6. Chromatogram of an isooctane mixture separated on micro-packed graphitized thermal carbon black (column) 2.7 m X 0.45 mm I.D.). Conditions: GTCB Sterling MT, 4 0.09-0.125 mm; It-, 0.21 mm (hydrogen); column temperature, 150°C; inlet pressure, SOOkPa; carrier gas, hydrogen. Peaks (Koviits retention indices in parentheses): 1 2,2,3,34etramethylbutane (619); 2 2,3,3-trimethylpentane (676); 3 2,2,4-trimethylpentane (679); 4 3-methyl-3-ethyl pentane (689); 5 2,3,4-trimethylpentannt (691); 6 3,3-Dimethylhexane (701); 7 2,2-dimethylhexane (706); 8 2,4-dimethylhexane (713); 9 3,4-dimethylhexane diastereoisomen (716); 10 3,4-dimethylhexane diastereoisomers (719); 11 3-ethylhexane (737); 12 2,s-dimethylhexane (743); 13 4-methylheptane (754); 14 3-methylheptane (759); I5 2-methylheptane (770) (by courtesy of Wekch et al. [396])

decrease in the number of hydrogen atoms in the adsorbate, this interaction decreases, in fact to a greater extent. In addition, however, the influence of the geometric structure may be superimposed on these interactions. For example, the chair form of cyclohexane, with only three of its carbon atoms being able to contact the graphite face, is less retained on GTCB than are cyclohexene and 1,3-cyclohexadiene,which have flatter structures and hence can interact more strongly with the surface owing to the shorter distance involved. This effect is most pronounced for benzene, which is eluted after the cycloolefms (but, owing to the smaller number of hydrogen atoms, prior to n-hexane!) (3811. The cis-isomers of unsaturated hydrocarbons, although higher boiling, show smaller retentions than the corresponding trans-isomers [394]. This does not hold, however, for large-ring cycloalkenes (XI,), where the trans-isomers are eluted first. Correlations between molecular structure and retention have been found for octynes and octadienes [403], linear and branched alkenes and alkanes [404] and alkylnaphthalenes [405]. If we consider alkylaromatics, the results of the investigations of Kiseleu and co-workers [379, 406-4081 seem surprising. In fact, monoalkylbenzenes are less retained than polymethylbenzenes with the same number of carbon atoms. This result is explained by the occurrence of rotational isomers of the alkylaromatics which cause both the side chain and the ring carbon atoms to be on average more distant from the surface than the methyl and ring carbon atoms

111

5.2. Carbon Adsorbents

0

6

30

18

Time in min

Fig. 7A. Separation of the geometric isomers of 1,2,4,5-tetramethyl cyclohexane (after Kiseleu and Yashin [397]). Conditions: GTCB column (2 m x 3 mm I.D.), column temperature 210°C; carrier gas helium, 35 ml.min-l (by courtesy of VEB Deutscher Verlag der Wissenschaften)

* t-a-t

,

t-s-t

7 c-a-c

I

0

I

I

2

4

I

I

I

I

1

6 8 1 0 1 2 1 4 Time in min

Fig. 7B. Separation of the isomers of perhydroanthracene on micro-packed GTCB (after Kiseleu, Nosamuu and Shcherbekouu [402]). Conditions: GTCB column 1.4 m X 0.5 mm I.D., 4 0.1-0.12 mm; column temperature, 240°C; carrier gas, hydrogen, 3 ml * min-l; pressure drop, 200 kPa (by courtesy of VEB Deutscher Verlag der Wissenschaften)

112

5. Solid Stationary Phases

of the polymethylbenzenes. The rotation of the methyl groups around the C-C bond facilitates the adsorption of those conformational isomers of the polymethylbenzenes the methyl groups of which are oriented such as to be at the minimum distance from the graphite surface [407-408, 3791. For halogen derivatives of the hydrocarbons, the partial molar adsorption enthalpy, AH,, is increased, compared with the unsubstituted hydrocarbons owing to the polarizability of the halogen compounds and depending on the radii and number of the halogen atoms [379, 409-41 11. When investigating oxygen containing organic compounds, it must be noted that traces of oxides on the GTCB-surface affect the retention of aldehydes, ketones, alcohols and esters (peak tailing, the retention depending on the sample size), owing to contributions of specific interactions [412]. Therefore, the GTCB must first be treated with a stream of hydrogen at 1100°C [379, 381, 388, 4121. Even amines can be analysed on this non-specific adsorbent [413]. Because of its low specific surface area (6-12m2/g) (which is also a requirement for the hitherto cited applications!) it is poorly suitable for the analysis of gases. For this type of application, a graphitized carbon black with a higher specific surface area (ca. 100 m2/g) has been developed and is commercially available (Carbopack B). In Table 11 the properties of some unmodified graphitized and partially graphitized carbon blacks are listed.

Table 11. Properties of Some Graphitized Thermal Carbon Blacks Commercial Name

Producer. Supplier

specific Surface Area [mZg-'1

Carbopack A

Supelco

Carbopack B Carbopack B-HT

Supe1co Supelco

Carbopack C Sterling FT

Supelco Cabot Corp.

Sterling MT Graphon, Spheron

Cabot Corp. Cabot Corp. Phase Separations

9.6 90-130

Carbochrom 1 Carbochrom 2 Graphitized Carbon type A1 Carbopack F

U.S.S.R. U.S.S.R.

7-10 80-100

Graphpac-GC

Alltech

15 100-110 9-12 6-30

Car10 Erba, Italy Supelco

6

Particle Diameter

Iml

Bulk Density

Comment

ig.cm-31

150-225

0.66

180-250 180-250

0.34

180-250 250-420 180-250 150-180

0.66

No longer supplied Hydrogen treated

No longer supplied

250-500 250-500 180-250 150-250

Experimental product [414]

5.2. Carbon Adsorbents

5.2.2.

113

Activated Charcoal

Charcoals are non-specific or at least poorly specific adsorbents, exhibiting numerous macroand transition pores of various diameters, and as a consequence of the large surface areas (800-1000 m2/g) they show a high adsorption activity [415]. Owing to the starting material and to the method of preparation [382], the charcoal surface is both chemically and geometrically inhomogeneous, hence affecting the peak form and the dependence of the retention on the sample size. In the past, activated charcoals were used for the separation of permanent gases and lowboiling hydrocarbons [416]. At present, their field of application is mainly limited to adsorptive enrichment of trace compounds.

5.2.3.

Carbon Molecular-Sieve

From specially prepared and selected polymers, e.g., poly(vinylidene chloride), active charcoals can be formed by thermal decomposition. Whereas for the usual charcoals (5.2.2) prepared from natural substances the porosity is chiefly determined by meso- and macropores, the pyrolysis of poly(viny1idene chloride) leaves a fine-pore structure with pore diameters of 1-3 nm. A high purity of the polymer and cautious decomposition are necessary in order to achieve chemically and geometrically homogeneous pores. With poly(viny1idene chloride), any contamination with poly(viny1 chloride) must be avoided, otherwise the surface of the carbon skeleton formed by the elimination of HC1 would become less homogeneous. Certain properties of these so-called saran active charcoals have been explained by the molecular sieve effect [417]. This adsorbent was first applied in gas-solid chromatography by Gvosdovich. Kiselev and Yashin [418] and Kaiser 1419,4201. As the partial molar enthalpies are substantially higher on this “carbon molecular sieve” than on GTCB, numerous gases can be analysed at room temperature. On the other hand, compounds with more than six carbon atoms cannot be desorbed without decomposition of the adsorbate. The specific surface areas, depending on the preparation, range between 100 and 1200 m2/g. These carbon molecular sieves, owing to their homogeneous surface and their non-specific (dispersive) interactions, belong to the type I adsorbents and permit the separation of even strongly polar compounds (which offer less pronounced dispersion forces). Prior to use, the carbon molecular sieve has to be conditioned (i.e., the adsorbed compounds, e.g., hydrocarbons, H20 or C02, have to be desorbed) in an oxygen- and moisturefree stream of helium above 200°C for > 10 h. The carrier gas used subsequently must also be purged of oxygen and of moisture. The field of application of this adsorbent is the trace determination of moisture in organic substances, of hydrocarbons in water [420], of sulphur and nitrogen-containing gases and of low-boiling hydrocarbons [421,422] and the analysis of lowmolecular-weight alcohols and fatty acids. In analogy with GTCB, unsaturated hydrocarbons are eluted before their n-alkane analogues. Another similar type of active carbon has been prepared by the electrochemical reduction of polytetrafluoroethylene (PTFE) with lithium amalgam at low temperature [423]. This is called “Jado” and it may also be classified as a type I adsorbent. In Table 12 some carbon molecular sieves and their properties are listed. Recently, Chrompack (The Netherlands) have introduced a fused silica column (Carbo PLOT 007) coated with a 25 pm layer of carbosieve, giving excellent separation of He, air, CO, CHI, C02 even at the ppm level.

114

5. Solid Stationary Phases

Table 12. Properties of Some Carbon Molecular Sieves ~~

~

Commercial Name

Producer, Supplier

[3 .&-*I

Carbosieve B Carbosieve C Sorptophase, CMS Spherocarb Carbosphere Jado

5.3.

specific Surface h a

Supelco Supelco Applied Science Labs. Analabs Chrompack Czechoslovakia

100 560 1000 1200 1000 2600

Particle Diameter

im

1.2 2-2.4 2-2.4 1.5 2.6 2.3

Particle shape

Bulk Density 1g.cm-31

irregular spherical spherical spherical spherical irregular

0.12-0.14 0.5-0.7

0.4-0.45 0.5

0.4-0.45

Boron Nitride and Molybdenum Disulphide

The lamellar structure of these crystalline substances resembles that of graphitized thermal carbon black, and their gas chromatographic and adsorptive behaviours are also similar. The only structural differences lie in the fact that with boron nitride at the edges of the hexagonal basal planes the carbon-atoms are alternately substituted by boron and nitrogen atoms, and with MoSz there are MoSz-laminatedlattices, each consisting of three lamellae, a central lamella consisting of Mo atoms and two external lamallae consisting of S atoms. Provided that the lattices are free of defects and the surfaces correspond exactly to BN and MoSz, respectively, these adsorbents exhibit essentially the properties of a non-specific adsorbent (type I). It should be noted that MoSz crystals are not chemically inert and that traces of oxygen in the carrier gas have to be removed [379,424-4251.

5.4.

Adsorbents with Hydroxylated and dehydroxylated Surfaces

5.4.1.

Silica Gels

Silica gel is an amorphous condensation product of polysilicic acid with a high specific surface area. It is generally prepared by neutralization of aqueous sodium silicate solutions with mineral acids, and varying the pH value and the stirring rate influences the growth of the polysilicic acid particles in the sol and the agglomeration density and pore diameters in the gel. After washing and heating, a hard porous silica gel is formed, the pore structure of which may be modified by an acidic or hydrothermal treatment. Silica gels may also be manufactured by flame hydrolytic processes (from Sic&), or by hydrolyzing monomeric tetraalkoxysilane and condensing and catalytically converting the hydrolysate into spherical silica. Depending on the reaction conditions, the parameters of texture may be varied within wide limits. The skeleton of the adsorbent consists of agglomerated globules of silica and contains both physisorbed water and silanol groups on the surface and within the bulk of the particles. Heating will not remove the strongly physisorbed water until a temperature of 180°C is reached [426, 4271 leaving extremely narrow pores (ultrapores) that are accessible to small molecules, primarily water. Calcining on exposure to air at 900-1000°C or in a steam atmosphere at 750-850°C will remove most of these fine pores [379]. It was suggested that a temperature of 300°Cwas necessary to remove all physisorbed water [428] and that physically bound

5.4.

Adsorbents with Hydroxylated and Dehydroxylated Surfaces

115

water was still being released at 600°C [429]. However, it has to be taken into account that above 200°C silica gel is already losing a significant number of OH groups by condensation reactions with elimination of water [430, 4311. Hence a temperature of about 180-200°C may be a useful compromise [422]. The hydrothermal treatment of silica gel widens the pores, may decrease the specific surface area from several hundred m2/g down to less than 20 m2/g, and a transition of the globular structure into a spongy structure takes place. At higher temperatures the formation of crystals in the amorphous bulk begins [379, 4331. The surface of silica gel, as a rule, is covered by hydroxyl groups to various extents. Their surface concentration is reproducible, provided that the conditions of the hydroxylation or dehydroxylation were constant, and it does not depend on the specific surface area [434]. Since the exact position of oxygen atoms and surface hydroxyl groups is not known, for the sake of simplification a planar surface model with standard bond distances may be considered, represented by twelve-membered silicon-oxygen rings [435]: I

I

0

0

0

I

I

I

0

I

0

This structure, when fully hydroxylated, can theoretically accomodate eight hydroxyl groups per 1nmz,whereas the experimental values range from 4 to 8 silanol groups per 1nm2surface area [435-4391. Different types of surface silanol groups are possible:

geminal

vicinal (bound)

isoloted(lone1

where the distance between two isolated SiOH groups is 20.5 nm. These structures hold true for dry silica surfaces. On “wet” surfaces (which contain physically adsorbed water) the following model has been proposed [440, 44Oa]:

116

5. Solid Stationary Phases

(after Bush et al. [4401) H I Q\H

H'

'i;' B 0LH

Y I

e-

-

3rd layer of weakly adsorbed water, loss between room temperature and 70°C, reversible, removed by solvents 2nd layer of weakly adsorbed water, loss complete at 120°C, reversible, removed by solvents 1st layer of strongly hydrogen bonded water, loss commences at 200°C and appears complete at 650°C, not removed by solvents Silanol groups lose water to produce siloxy groups, commences at 45OoC, complete at llOO°C, loss is likely to be irreversible.

Si

1 ' 0 0' 0 I

(after Scott [440a]) Additionally, a certain number of hydrogen-free bridges are also likely, especially after thermal treatment:

The surface silanol groups are primarily responsible for the specific adsorbent-adsorbate interactions of type 11, which form hydrogen bridge bonds with molecules of groups B and D. The SiOH groups exhibit strong acidic effects owing to their protonation [441], and the equilibrium constant for -Si-OH

# --Si-O-

+ H+

is of the order of lO-'O, an acid strength comparable to that of phenol [435, 4421. The hydroxylated surface of a pure silica gel (pure with regard to the absence of traces of aluminium or boron) behaves against group B and D molecules, including amines, not as a chemisorbent but only as a type I1 adsorbent, forming primarily hydrogen bridge bonds of different strength with these molecules [379]. Additionally, dipole-dipole, dipole-induced dipole and n-complex bond interactions may occur. For molecules containing several functional groups the geometry of their structure is important. Dioxane, although containing two ether groups, does not show the double interaction energy of an ether with only one C - 0 4 group, but only a slightly increased interaction, as the chair-form dioxane molecule can only be oriented with respect to the surface so that only one ether group can interact with the adsorbent, and the other group cannot contact the silanol groups to form a strong hydrogen bond [379]. It has been demonstrated by high-performance liquid chromatography that solute retention (aromatics) on silica gel depends only on the water and/or hydroxyl content of the silica gel and that the Si-0-Si groups contribute little or nothing to the solute

117

5.4. Adsorbents with Hydroxylated and Dehydroxylated Surfaces

retention [427]. This supports the results of Kiselev's numerous investigations on silica gel in GSC. Because of the importance of the surface silanol group as the major centre for adsorption on silica gel (and moreover also on other chromatographic surfaces, e.g., silica and glass column walls, silica-based and diatomaceous earth-based solid supports and chemically bonded phases), its determination will be discussed in detail. Hydroxyl groups on the surface have been characterized and determined by chemical methods [428, 436, 443-4461, by solid-state nuclear magnetic resonance spectroscopy [447-4491 and by infrared spectroscopy [437,440-441,450-4611. The infrared spectrum of silica shows strong absorption bands at 800, 1100 and 1200cm-', which can be assigned to fundamental silicon-oxygen vibrations, whereas surface hydroxyl groups and water absorb in the 3000-4000 cm-' region. The 3743-3750 cm-' absorption is due to the stretching vibration of isolated hydroxyl groups, the 3660-3680 cm-' band to weakly hydrogen-bonded hydroxyl groups and the 3400-3500 cm-I region to strongly hydrogen-bonded hydroxyl groups and/or adsorbed water. Unfortunately, the overlapping of bands for the 0-H stretching vibrations of bound silanols and physically adsorbed water in the region 3400-3750 cm-' has caused problems in the identification and determination of these groups. These problems can be relieved by evaluating the less intense combination and overtone bands in the near-infrared region (8000-4000 cm-'), and it could be shown that the combination (stretching and bending) bands for silanols and physically adsorbed water are easily resolved, resulting in more reliable quantitation [440, 4591. The sample slurried with carbon tetrachloride as an appropriate refractive index matching medium in near-IR and fundamental IR spectroscopy to reduce scattering was investigated in a specially developed near-IR cell by scanning the near-IR region from 2400 to 1200 nm (4167 to 8333 cm-'). In Table 13 the observed frequencies and their assigments, compiled and measured by Bush et al. [440], are given. Table 13. Assignment of IR-Spectral Bands of Silica to OH Functional Groups Functional

Band Assignment

BrOUP

Band frequencies [cm-'1 (references in parentheses) Vacuum

Silanol

Deuterated silanol Water

Bending (63 Stretching ( y 3 H-bonded stretching ( y J Combination (8, + v,) Overtone (2y3 H-bonded overtones (2y3 Deuterated stretching (yD)

870'453) 3750(452) 3500(452) 4450(458) 7326(462) 7140(462) 68 5 0(462) 2760(453)

Deuterated overtone (2yD) Bending (6,) Stretching (y,) Combination (6, + y,) Overtone stretching (2y,)

1635(453) 3400(452) 5260(462) 6850(462)

-

CCl,

Measured by Bush et al. I4401 in CC4

-

3700(461) 7220(462)

3670 3480 4515 7220 7140 6850 2710

5265(459) -

5362 1600 3200 5277 6850

-

Because the overtone band for the silanol (2y8:7220 cm-I) and combination band for water (6, + v, : 5277 cm-I) are completely resolved, they appear to be ideal for studying the silica surface. Applying the non-destructive sample preparation technique proposed by Bush et al. [440] in conjunction with their IR cell, which avoids exposure of the sample to atmos-

118

5. Solid Stationary Phases

pheric humidity and allows for in situ dosing of the silica gel and subsequent addition of CCl,,silica gel samples can be readily evaluated. Moreover, the degree of solid support deactivation (see chapter 7) and bonding of organic groups to silica-based matrices (chemically bonded stationary phases, Chapter 6) may be assessed by this technique. When removing the silanol groups from the surface of silica gel by heating, the specific intermolecular interaction is reduced. This general statement, however, is valid only when the material is chemically pure. The presence of aluminium or boron which form strong Lewis acid centres, on the silica surface, causes an increase in the bond energy with electron donor molecules. This effect, although occuring to a lesser extent also on hydroxylated silica, is especially noticeable on dehydroxylated silica surfaces, as the strong Lewis electron-acceptor centres between the siloxane groups are far more exposed, and molecules of triethylamine, for example are chemisorbed at these centres with an adsorption energy of 210 kJ .mole-' when there is less than 0.4% Al in the silica gel. Whereas the energy of a specific molecular adsorption on silanol groups, forming strong hydrogen bonds, would amount to 85 U mole-' and the non-specific adsorption on siloxane groups (after a strong dehydroxylation of a pure silica surface at 1100°C)would amount only to 42 kJ * mole-' [379]. The elution of group A molecules is not affected by traces of Al on the surface, whereas for group B and D molecules silica gel adsorbents that are free from Al (and B) have to be used. The presence of these contaminants would lead to asymmetric peaks for unsaturated and aromatic hydrocarbons, ethers and ketones and retard these compounds [463], and many amines and N-heterocycles are often not eluted at all as they are chemisorbed [433]. To summarize the numerous investigations on silica gel gas-solid chromatocraphic adsorbent, it can be stated that the silanol groups on its surface neither are too active nor do they cause insuperable problems due to the heterogeneity, and chemically pure silica gel does not chemisorb electron donor molecules. It behaves like a polar adsorbent of type 11, forming hydrogen bonds of different strength with suitable molecules. The dehydroxylation decreases the retention of such molecules, hence decreasing its specificity. Surface contaminants, especially aluminium and boron, must be avoided. A further requirement is the geometric homogeneity, i.e., the absence of fine pores and fractures, which would locally increase the nonspecific adsorption and hence cause peak asymmetry. Silica gel grades for application in GSC have been developed by PCchiney-Saint-Gobain, France. They were introduced by Guillemin et al. [464] and are among the most suitable chromatographic type I1 adsorbents. The commercial names of these porous silica beads are Spherosil (PCchiney) and Porasil (distributed by Waters Associates, USA).They are supplied in six grades, each being well characterized by the specific surface area, mean pore diameter and total pore volume, and are available as spherical beads with diameters ranging from 20 to 300 pm.These beads are perfectly rigid and, although considerably porous, they are incompressible. They are heat resistant up to 600"C, resistant to attribution and they do not swell in any liquid. Their adsorption properties were investigated by Felte, Smolkoua et al. [432,465]. Owing to their graduated porosity, their field of application ranges from gas analysis to the analysis of medium polarity organic compounds. It is advantageous to analyse high-boiling polar compounds on porous silica beads only when the adsorbent has been previously treated hydrothermally at 180°C and about 1 MPa water vapour pressure to achieve a completely hydroxylated surface [432]. Russian silica gels, named Silochromes, are chemically pure and geometrically homogeneous and have wide pores. They are prepared from non-porous highly dispersed aerosils [466]. Their mean pore diameters range, closely distributed, from 20 to 120 nm. The properties of some commercial silica gels (non-modified) are listed in Table 14.

119

5.4. Adsorbents with Hydroxylated and Dehydroxylated Surfaces Table 14. Properties of Some Commercial Silica Gels Name

Producer, Supplier

S.

dso

Spherosil XOA 400 Spherosil XOA 200 Spherosil XOB 075 Spherosil XOB 030 Spherosil SOB 015 Spherosil XOC 005 Porasil A Porasil B Porasil C Porasil D Porasil E Porasil F Silochrom 1 Silochrom 2 Silochrom 3 Silochrom S 80 Silochrom S 120 Aerosilogel MSA-1 Aerosilogel MSA-2 Silipor 600 Silipor 400 Silipor 300 Silipor 200 Silipor 075 Silipor 030 Silipor 015 Silikagel narrow-pore Silikagel wide-pore GC Grade Silica Gel Davison Grade 12

5.4.2.

Pechiney-Saint Gobain RhBne-Poulenc (France) Analabs, Supelco (U.S.A.) RhBne Poulenc (France), Waters Associates (U.S.A.), Supelco Applied Sci. Labs., Analabs (U.S.A.)

350-500 8 140-230 15 75-125 30 37-62 60 18-31 125 5-15 300 350-500 8 . 125-200 10-20 50-100 25-45 6-10 2-6 15-30

20-40 40-80 80-150 150 70-150

40-60 70-100 60-100 100-140 U.S.S.R. 15-30 60-90 Lachema Bmo (Czechoslo- 500-700 vakia) 350-490 250-340 120-240 50-115 20-48 10-19 Becker Delft (The Nether- 800 lands) 350 Applied Sci. Labs. (U.S.A.) 720-760

60-90 40-60 40-60 20-30 70-150 35-60

Sojuskhim reaktiv. (U.S.S.R.)

Pore volume

Shape

1 . ~ 1 3.g-q

W.g-9

0.7 - 1.O 0.7-1.0 0.5-0.7 0.5-0.7 0.5-0.7 0.3-0.5

spherical spherical spherical spherical spherical spherical spherical spherical

1.5

spherical spherical spherical spherical spherical

1.5 1.5 1.3 1.3 0.55 0.75 0.8

spherical spherical irregular irregular spherical spherical irregular

2.1

irregular irregular irregular irregular irregular irregular irregular

11.5 2.2

irregular irregular

Aluminium Oxide

It was discussed in the previous section that aluminium ions on the surface of an adsorbent strongly affect gas chromatographic separations, as exposed aluminium ions are Lewis acid (electron-accepting) sites and can react chemically with or chemisorb suitable molecules, and also cause severe tailing and high retention values. Owing to the existence of Lewis base and Lewis acid Eentres, the surface of alumina is heterogeneous, and so is the pore-size distribution. The y-modification of alumina, which is generally used, is prepared by calcining Al(OH)3 at 600-1000°C,leaving an adsorbent that, in addition to the mentioned Lewis centres, exhibits different types of hydroxyl groups, which in turn, together with sorbed water that normally covers the surface, additionally complicate the adsorption mechanism, as their

120

5 . Solid Stationary Phases

surface concentration cannot be adjusted reproducibly and depends strongly on the humidity of the carrier gas. Thus, in order to block the active centres, alumina has been coated fairly reproducibly in the column by moistening the carrier gas, e.g., by passing it over CuS04* 5 H 2 0 [467]or Na2S04* 10 H 2 0 [468].Owing to its shortcomings, alumina has only limited applicability today in GSC, e.g., for the analysis of volatile hydrocarbons [469].

5.4.3.

Porous Glasses

When heating alkaline borosilicate glasses up to about 1400°C, a homogeneous melt results which, on decreasing the temperature to 500-6OO0C,disintegrates into two phases, one consisting of nearly pure silicon dioxide and the other of alkali borate penetrating the silica skeleton like a coherent vascular system. After cooling, the borate phases is leached with a dilute acid, leaving a silica skeleton. Varying the composition of the original glass and the conditions of the subsequent treatments enables glasses with defined pore diameters ranging from molecular dimensions to 1pm to be tailor-made. Owing to the remaining Si02 skeleton, porous glasses belong to type I1 adsorbents. Their spongy structure, veined by pores the diameters of which are narrowly distributed and the same on the surface as in the centres of the particles, although seeming advantageous, has a shortcoming in the case of fine pores (ca. 1 nm). The diffusion into and out of the long pores proceeds slowly, resulting in a high mass transfer rate and hence increased hminvalues. On the other hand, owing to the defined pore structure, porous glasses show favourable properties similar to molecular sieves: permanent gases are separable on a porous glass with fine pores (1 nm), volatile hydrocarbons on a porous glass of pore diameter 3-10 nm and higher boiling hydrocarbons may be separated on a glass of pore diameter 30-50 nm [470].The most serious disadvantage of porous glasses, however, is the existence of residual boron oxide on the walls of the pores, which may cause chemisorption, and asymmetric peaks for compounds of groups B and D. This effect has already been discussed in more detail in previous sections. The disadvantage of the long pores passing through the whole particle can be avoided by using superficially porous glasses, which can be prepared by leaching with hydrochloric acid [471].The preparation, properties and applications of porous glasses have been reviewed by Janowski and Heyer [472,4731. Of the commercially available products only one is mentioned, viz., coming 7930 (dSo2.5 MI, S, 173 m2 g-l, pore volume 0.11 cm3 g-l) (Coming Glass Works, USA).

5.5.

Porous Organic Polymers

Porous polymers are synthesized by a heterogeneous cross-linking polymerization from different, generally aromatic monomers in the presence of an inert solvent, which leaves a more or less permanent porosity during the subsequent drying process. Owing to residual monomers and oligomers, to organic and inorganic agents necessary for regulating the particle size, to residual solvent and to acid sites created by oxidation during the polymer cleanup, commercial products cause batch-dependent retention variations, limiting the reproducibility. This is not surprising, as these products have been prepared primarily for use as ion exchangers and as catalyst systems. They have been used extensively in gas chromatography since their introduction by Hollis [474,475]in 1966 for the separation and trace analysis of a wide range of compounds, especially polar compounds such as diols, polyols, amines and water. For the analysis of these compounds they are undoubtedly superior to other stationary phases. They can be prepared by either bulk polymerization or bead polymerization. In the former method, the material is polymerized, then fractured, ground and separated into the required particle sizes. In the latter technique the polymer is prepared by suspension

5.5. Porous Organic Polymers

121

(bead) polymerization to give spherical particles [476]; this is the preferred method of synthesis. The basic principle involved is polymerization of a mixture of monomers which can be cross-linked in the presence of an inert and soluble component solvent. A wide variety of monomeric materials can be used. The inert solvent has to be soluble in the monomer mixture and should not be chemically bound to the polymer network during the polymerization reaction. Further in suspension polymerization it must be essentially insoluble in the suspending medium, which commonly is water. When polymerizing a mixture of a monofunctional (50-90%) and difunctional, cross-linking monomers (50- lo%), the growing polymer has only a limited swelling capacity, and microstructures of gel, formed within the droplets in the early stages of the polymerization, gradually grow together into a sponge-like structure with the inert solvent filling the space between the microstructures. Removal of the inert solvent after the polymerization leaves a cross-linked, highly porous structure. The permanent porosity after complete drying of the polymer can be observed with a light microscope, whereas the microstructures have been studied by electron microscopy [476]. Independent of various suitable monomers having different functional groups, a chloromethylation reaction, carried out after the polymerization, also allows the introduction of various functional groups on the pore surfaces of the particles to obtain derivatized porous polymer beads. In spite of its different preparation, a special poly(pheny1ene oxide), named Tenax-GC, is dealt with in this section, owing to its similar gas chromatographic behaviour. The field of application of porous polymers is wide, as the chemical and geometrical properties of the surface may be varied, the adsorption energy is relatively low, the thermal stability is sufficient (up to 200-300”C), the affinity for hydroxyl compounds is low and the sample capacity is high. Thus, they have been applied especially in the analysis of gases [477] and of highly polar, predominantly group D, compounds (water, carboxylic acids, alcohols, amines). Because a complete survey of the numerous applications of porous polymers cannot be given here, readers are referred to review papers [478-4831. Porous polymers based on styreneldivinylbenzene are generally considered when they do not exhibit polar functional groups, to be “non-polar” adsorbents. It should be borne in mind, however, that in addition to the non-specific interactions, interactions of suitable adsorbates with the n-electrons of skeletons containing phenyl and phenylene groups may also occur, even if the energy is weak. Thus, these adsorbents behave as type I11 adsorbents according to Kiselev’s classification. The predominant intermolecular interactions are determined mainly by dispersion forces, and only secondarily by induction forces [484-4861. The high density of n-electrons associated with the aromatic structure results in high capacity factors. CI8groups, covalently bonded, can shield the sorbates from the n-orbitals [486a]. Polar porous polymers containing cyano, nitro, carboxylate or hydroxy groups in varying concentrations may specifically interact with molecules of groups B and D and can be graded as strongly specific phases, mainly type 111, but, depending on the character of the functional group, partially also type 11. Peak tailing observed for compounds of group D (alcohols, carboxylic acids) has been attributed to unreacted vinyl groups on the surface which cause a certain degree of heterogeneity, resulting in non-linearity of the adsorption isotherm [487]. The retention mechanism on these adsorbents seems to differ from that on common adsorbents and stationary liquids. Obviously, in the separation process both adsorption and absorption take place simultaneously [379]. However, the ratio of the contributions of adsorption and dissolution may depend on several conditions: on the nature and geometric structure of the polymers and the samples and on the column temperature. For the same polymedanalyte systems at lower temperatures apparently adsorption prevails, whereas at higher temperatures, when the polymer begins to soften and the mobility of its single chains or segments increases, dissolution dominates [379]. In order to characterize special interactions of the different porous polymers with sample

122

5. Solid Stationary Phases

molecules of different structures, the Rohrschneider/McReynoldsconcept has been applied or the retention indices of benzene, tert.-butanol, butan-2-one and acetonitrile have been determined. In order to reveal the similarities and differences between the various porous polymer types and to assist the gas chromatographer to select the best stationary phase for a given separation or to replace a phase that is not readily available with a similar one, Caszello and D'Amato carried out comprehensive investigations on the classification of the selectivity of porous polymer beads [354, 488-4941. As discussed in Section 4.2.4, the standard reference non-polar liquid stationary phase for Rohrschneider's and McReynolds' classification is squalane. As sample compounds are retained more strongly on (solid) porous polymers than on common liquid phases, the column temperature has to be more than 50°C higher than with liquid stationary phases (for which the Rohrschneider-McReynolds concept was developed) in order to obtain acceptable retention times. Squalane, however, has an upper temperature limit of 100-120°C and can be used as a non-polar reference liquid for characterizing other stationary liquids at 10O-12O0C, but not for porous polymers at, e.g., 200"C, unless the adjusted retention times of the polarity probes and of n-alkanes are extrapolated from the thermally highest possible experimental values to 200°C by using Arrhenius plots to obtain q&,for the probe compounds and hence the desired I values, e.g., $:A;& [:: = It&'g[;$: zoooc, extrapolated from 120°C. The resulting accuracy of the AZ values may, owing to the unreliable qm,,,, 2ooec extrapolated from 120°C values, be fairly low. Other non-polar standards (graphitized thermal carbon black, Chromosorb 106) [493,495] are inferior to thermally stable saturated hydrocarbons, e.g., CB7H176(22,24-diethyl-19,29-dioctadecylheptatetracontane) molecular weight 1222,34, proposed by Riedo et al. [496] (commercial name Apolane-87) or the cheaper Apiezon MH (a hydrogenation product of Apiezon M, proposed by Haken and Vernon [497]), both applicable between 30 and 250-27O0C, corresponding to the range of application of porous polymer beads. The AI values obtained from measurements of the retention indices of the Rohrschneider/ McReynolds probes (benzene, ethanol, methyl ethyl ketone, nitromethane, pyridine, butanol, methyl propyl ketone and l-nitropropane), together with C5-C11n-alkanes, on both Apolane and the porous polymer under investigation at 200"C, allow a rough classification of the different porous polymer types. These values, together with other data, will be listed in the tables in this section when discussing the porous polymer types concerned. For further more detailed data see refs. [493] and [498-5001. Before using porous polymers, residual low-molecular-weight compounds (monomers, solvents, suspension stabilizers, etc.) and contaminants originating from processing and storage have to be removed. Several techniques have been suggested for the cleanup and conditioning of porous polymers either before or after column packing. Solvent extraction using a series of solvents such as acetone, followed by methanol, water, dilute hydrochloric acid, water, methanol and then drying using vacuum and heating, will free the porous polymer from the above-mentioned residues and extraneous materials, as recommended by Hollis [476). However, this procedure failed when applying Cekachrom types of porous polymer (see the following tables). Soxhlet extraction with acetone or a similar solvent, a high vacuum with heating overnight, or purging with an inert gas while heating in a flow-through system, have proved to be suitable purification procedures, provided that all traces of oxygen can be totally excluded, as these polymers will be attacked by oxygen to form C=O groups on the surface [SO11 or are oxidatively degraded [476]. In general, it is best to pack the column after conditioning, as porous polymers shrink on heating, causing cavities within the column, formed during the fmt thermal strain. The column packing method preferred by Hollis [476] consists in applying a vacuum to the column and using vertical vibration via a tamping rod until no further particles enter the column, and continuing the vibration for as long as 4 h or more. In this way efficiencies of more than 3000 plates per metre are attainable with porous polymers of 80-120 pm particle

Zzmene,

5.5. Porous Organic Polymers

123

diameter at a relatively broad Van Deemter minimum. The shrink-swell-behaviourof porous polymers has to be considered especially during temperature programming, where flow changes due to this property may occur. For the analysis of polar compounds a well deactivated gas chromatographic system has to be used.

5.5.1.

Porous Polyaromatic Beads Without Additional Functional Groups

Macroporous polymers of styrene and/or ethylvinyl-benzene cross-linked with divinylbenzene have been most popular since the first paper presented by Hollis at the Third International Symposium on Advances in Chromatography in Houston in 1965. The reaction schemes of their preparation are as follows [502]:

Varying the amount of the divinylbenzene cross-linker and the inert solvent controls the porosity. These types, often described as non-polar, owing to the presence of vinyl and phenyl groups on the surface, exhibit weakly specific intermolecular interactions with molecules of types B, C and D, but non-specific interactions with all types of adsorbates predominate. Other than the cross-linked vinylbenzene polymers, poly(2,6-diphenyl-p-phenyleneoxide), with the commercial name Tenax-GC, first applied by Van Wijk 1503-5041, exhibits pores with diameters in the range of 25-1500nm, and is hence geometrically inhomogeneous. However, its thermal stability (375OC) in conjunction with its relatively low specific surface area (20 mZ/g) enables higher boiling polar compounds to be investigated. Its molecular weight is >500000. As its ether oxygen is shielded by the phenyl and phenylene groups

5. Solid Stationary Phases

124 r

1

L this polymer belongs, similarly to styrene/ethylvinylbenzenebased polymers, to the weakly or nearly non-polar adsorbents, only specifically interacting by surface phenyl or phenylene groups, but predominantly interacting non-specifically. Its advantageous properties

-

short retention times, stable baseline up to >320”C, no irreversible adsorption of polar compounds, resistance against oxygen and water, high long-term stability,

have led to its widespread application in gas chromatography, especially for separating higher boiling alcohols, diols, aldehydes, ketones, phenols, amides and amines [505-5071. It is mainly applied however, in pre-columns and traps to concentrate organic compounds from their matrices and subsequently to desorb them very rapidly by heating [508]. Its properties have been thoroughly investigated by Sukodyllsk# et al. [509]. In order to achieve a sufficient efficiency, a column has to be packed as densely as possible and ought to be conditioned in a slow stream of carrier gas for ca. 1h at room temperature and subsequently heated at a rate of 6 Wmin to 350”C, the latter temperature being maintained for 1-2 h. If at the subsequent lower column temperature the baseline was not constant, the procedure should be repeated until a stable base line has been achieved. At the end of each temperature programme the column should be cooled slowly as otherwise holes would be formed, resulting in decreased efficiency. This applies especially to spiral columns. If such cracks could not be avoided, the column should be re-packed at the injector side or another column should be packed less closely. In Table 15 commercial products and their most important properties are listed. In order to assess the interactions with the most important functional groups represented by the Rohrschneider and McReynolds probe compounds, the A I values published so far [491, 493-4951 are listed in Tables 16 and 17. In view of the application of the porous polymers at higher temperatures, the A I values measured at 200°C relative to the Cg7hydrocarbon Apolane of the test substances benzene, ethanol, butan-1-01, methyl ethyl ketone, pentan-2-one, nitromethane, 1-nitropropane and pyridine are listed in Table 17 [494]. The retention indices of these compounds on Apolane at 200°C have been found [494] to be benzene 702, ethanol 397, butan-1-01 610, methyl ethyl ketone 542, methyl propyl ketone 636, nitromethane 500, 1-nitropropane 678 and pyridine 757. The negative values found for several compounds imply that the porous polymers are less “polar” than the reference liquid phases Apolane and squalane. Therefore, some alternative non-polar solid phases have been suggested as reference phases, e.g., Porapak Q and Chromosorb 106 as the least polar porous polymers or graphitized thermal carbon black [490, 493, 4951. Unfortunately, it is difficult to standardize adsorbents, and batch-to-batch variations of these solids may affect the retention data, thus not justifying their choice as reference phases. From Tables 16 and 17 it can be concluded that only Porapak Q, Chromosorb 106, Chromosorb 102 and ethylvinylbenzene-divinylbenzenepolymer (5050) can be considered to be predominantly non-specific, whereas the interaction of the other polyaromatics investigated so far, especially Chromosorb 103 and Porapak P, with the probes cannot be overlooked.

Table 15. Properties of Commercial Weakly or Non-Polar Polyaromatic Beads

Name

Composition

Source

s. [mZ. g-'1

Porapak P

PCl

250

0.27-0.43

500-850 30-40 300-400 15-25

7.5-10 300-400 8.5-9.5 300-400

250 275 250 250-275

0.35 0.25-0.34 0.25-0.34 0.25-0.34

ditto ditto

600-700 700-800

40-60 5

200-250 250

0.25-0.34 0.25-0.34

200-400 400-550

13 35

250

250

0.2-0.3 0.2-0.5

250-350 100-150

130 160

250 250

0.2-0.5 0.2-0.5

25-7500 (72) 4.5

375

0.31

Cekachrom 2 Cekachrom 3

Ethylvinylbenzene-Divinylbenzene Styrene-Ethylvinylbenzene-Divinyl-

Tenax

benzene 2,6-diphenyl-p-phenylene oxide

Akzo Research Lab. Enka

19-30

N.V. Styrene-Divinylbenzene-ethylvinyl-

k .~ m - 3 1

(depending on the particle shape and size)

ditto Johns-Manville ditto ditto

U. S.S.R. Chemiekombinat Bitterfeld ditto ditto

Synachrom

Bulk Density

7.5-10

Polysorb-1 Cekachrom 1

Chromosorb 105 Chromosorb 106

tw

, T (isothermal)

70-200

Waters Associates

Ethylvinylbenzene-Styrene-Divinylbenzene Ethylvinylbenzene-Divinylbenzene Styrene-Divinylbenzene Styrene-Divinylbenzene Styrene Cross-linked Polyaromatic Styrene Cross-linked Styrene-Divinylbenzene 60:40 Ethylvinylbenzene-Divinylbenzene

Porapak Q Chromosorb 101 Chromosorb 102 Chromosorb 103

dso

Lachema Brno

benzene S, = specific surface area dSo = mean pore diameter (average pore diameter of 50% of the pores)

520-620

300

126

5. Solid Stationary Phases

Table 16.McReynolds' Constants X, Y, Z, Cr, and S and General Selectivities CX...Sof Some Non-Polar Porous Polymers (Smith et al., [495])*) Polymer

X

Y

Z

U

S

.W

-24 -46 -41 -86

55 10 17 18

51 14 19 21

46 1 -4 13

27 -42 -42 -22

155 -53 -51 -8

~~

Styrene-Divinylbenzene-Ethylvinylbenzene55:40:5 Ethylvinylbenzene-Divinylbenzene5050 Porapak Q Chromosorb 102

*) Column Temperature: 140'C. Kovits retention indices on squalane [20%(w/w) on Oas Chrom Q at 140°C with 50mVmin N2J were: benzene 658, butan-1-01 590, pentan-2-one632, 1-nitropropane658. and pyridine 820 i.u.

Table 17. Al~~'.nc-values*) of Rohrschneider-McReynoldsPolarity Probes on Some Non-Polar Porous Polymers (after Castello and D'Amato [494]) ~~~~

Stationary Phase

Porapak P Porapak Q Chromosorb 101

Benzene

44

Chromosorb 105

-78 22 15 -54 61 55 -54

Chromosorb 106

-85

Chromosorb 102 Chromosorb 103

Ethanol

76 92 4 I1 12 151 161 41 54 -23

Butan-1-01 Methylethyl ketone

Methylpropyl ketone

Nitromethane

Nitropropane

88

98

96

141

130

-1 65

12 82

14 77

14 149 156 50 60 -11

32 141

32 133

-20 120 113 20

-18 101 94 18

55 64 8

56 66 11

_**)

-**)

49

52

-36

-20

Pyridine

91 112 -70 60 53 -35 128 119 -11 +13 -a4

*) Where two values are shown, the upper one refers to the unused and the lower one to the aged porous polymer (22 days at ZOO'C under a helium flow). **) Reacts with the stationary phase; very broad or multiple peaks.

CO~UUUI

Table 18. Preferred Applications of Non-Polar and Weakly Polar Polyaromatic Beads Type

Application

Chromosorb 101 Porapak P Chromosorb 106 Porapak Q Cekachrom 1

Permanent gases; alcohols; ketones; aldehydes; esters, glycols; alkanes

Chromosorb 103 Chromosorb 102,105 Cekachrom 2 Cekachrom 3 Tenax

Gases; low-molecular-weight and oxygenfunctional compounds; C1-Cs alcohols and C2-C5fatty acids; sulphur compounds, nitro compounds; N-oxides; organic compounds in water; determination of water in organic compounds; trace enrichment, e.g. of organosilicon compounds amines, amides, hydrazine, PH3,ASH,); alcohols; keBasic compounds (NH,, tones. No glycols, no nitro compounds! Aqueous formaldehyde solutions; aqueous acrylic acid derivatives; acetylene; trace enrichment Universally applicable; alcohols; amines; aqueous mixtures of low-molecularweight carboxylic acids Carboxylic acids; hydroxycarboxylicacids; aliphatic and aromatic amines; nitroanilines; phthalates; ethylene glycols up to 13 EO units; the most favourable porous polymer for trace enrichment, on-column enrichment and precolumns.

127

5.5. Porous Organic Polymers

Although not containing functional groups other than phenyl and vinyl, these polymers have similar A I values to porous polymers with polar functional groups, as will be shown in the next section. This may suggest a distinct surface concentration of vinyl or phenyl groups easily accessible to sample molecules. In order to decrease the influence of residual active sites, porous polymers have been treated, in analogy with silica-based solid supports used in gas liquid chromatography, with reactive silicon compounds, e.g., chlorosilanes and silazanes. Commercial products are Porapak P-S and Porapak Q-S,which are intended to reduce peak tailing of strongly polar compounds. One should be aware, however, that this deactivating effect is generally based on the substitution of active hydrogen from, e.g., OH, COOH, SH, NH2 and NH groups by organosilyl groups and that the corresponding 0-Si, S-Si, and N-Si groups formed are readily hydrolysable by moist carrier gas to re-form the original deleterious group. This does not apply to solid supports in GLC, as the silyl derivatives formed from silanol groups are hydrolytically stable. Hence the value of silanized organic polymers compared with the untreated type is limited. Gas chromatographic applications of non-polar porous polymers have ranged over a wide sphere. Some preferred applications are indicated in Table 18. The possibilites and problems of the application of porous polymers in gas analysis have been thoroughly discussed by Thompson [477]. Recently, fused silica capillary columns were coated with a 10-30 pm layer of a porous polymer, the surface of which is 300-840 mz g-l. Such porous layer open tubular (PLOT) columns can be prepared with more than 2000 theoretical plates per meter and can be used at up to 250°C without deterioration of the adsorbent [510]. They combine the unique retention characteristics of porous polymers with high resolution capillary gas chromatography. Fig. 8 shows the separation of CI-Cs alcohols in water.

-

12

I

0

I

in min

7

Fig. 8. Separation of C1-C5alcohols in water on a Pora PLOT Q column. Column, 10 m x 0.32 mm I.D.Pora PLOT Q; layer thickness 10 Fm; n = 20000; carrier gas, hydrogen; detection, FID Peaks: 1 methanol; 2 ethanol; 3 2-propanol; 4 1-propanol; 5 2-methyl-2-propanol; 6 2-butmol; 7 2-methyl-1-propanol; 8 1-butanol; 9 2-methyl-2-butanol; 10 2-pentanol; 1I 3-methyl-1-butanol; 12 1-pentanol; 13 4-methyl-2-pentanol; 14 2-ethyl-1-butanol (after De Z e e w et al. [SlOal) (by courtesy of International Scientific Communications)

128 5.5.2.

5. Solid Stationary Phases

Polar Porous Polymers

If monomers with different functional groups are introduced into the polymerizing batches, porous polymers with specific sorption properties can be prepared, differing from each other in the nature and amount of the functional groups. Together with styrene and/or ethylvinylbenzene and divinylbenzene, substituted styrenes with functional groups in the 4-position, 4-vinylpyridine, N-vinylpyrrolidone, or acrylic derivatives have been polymerized to give polar porous polymers. Further polar types have been synthesized by chemically modifying (e.g., nitrating) polyaromatic beads, and also cross-linked methacrylic acid-ethylene glycol copolymers, exhibiting a hydrophilic surface, polyimides and quinoxaline-based porous polymers have been prepared and applied. The polar porous polymers are, owing to the nature of the functional group (CN, COOR, NO2, pyridyl, etc.) type I11 adsorbents and interact due to the negative charges on their surfaces specifically with molecules of groups B, C and D. This specific interaction becomes apparent in Tables 20 and 21 showing the McReynolds constants. The properties of several polar porous polymers are listed in Table 19. In the Tables 20 and 21 the AI values of the Rohrschneider-McReynoldsprobes are listed for the polar porous polymers of Table 19 as far as they have been determined and published hitherto. As has been hinted at already, it can be concluded when comparing the AZ values of “nonpolar” and “polar” porous polymers that both Porapak R and N, although exhibiting additional polar/polarizable groups, are less polar than the “non-polar” Porapak P. Porapak S, which specifically interacts only weakly with aromatics and amines, may react with nitro compounds and should be applied with caution when analysing unknown mixtures because of the risk of not eluting all components. Porapak T, Chromosorb 107 and Chromosorb 108 are distinctly polar, but the polarity may change significantly owing to ageing processes. The most polar porous polymer commercially available is Chromosorb 104. One of the specially prepared porous polymers, poly(N4nylpyrrolidone) can be considered to be a porous organic polymer with an outstanding polarity. Although not suitable, owing to the high retentions, for the separation of medium, or high-boiling polar compounds, it can be advantageously applied for fractioning groups of compounds, e.g., trace amounts of non aromatic hydrocarbons in aromatics [514]. This property is based on the charge-transfer complex forTable 20. A I g ’ m e Values*) of Rohrschneider-McReynolds Polarity Probes on Some Commercial Polar Porous Polymers (after Casfello and D’Amato [494]) Stationary Phase

Bemene

Ethanol

Butan-1-01 Methylethyl ketone

Methylpropyl ketone

Nitromethane

1-Nitropropane

Porapak N Porapak R Porapak S Porapak T

-36 -54 -68 28 76 160

92 64 47 156 222 309

97 66 41 172 233 319

93 55 39 163 207 329

94 56 43 164 200 325

114 60

106 53

-**)

-**)

- 16 -44

230 327 481

-31 6 46 69

117 135 171 205

128 156 204 224

114 146 173 192

116 151 172 193

144 192 144 293

272 297 447 456 160 194 245 272

103 188 320 330 56 87 146 178

Chromosorb 104 Chromosorb 107 Chromosorb 108

Pyridine

13

*I Where two values arc shown, the upper one refers to the unused and the lower one to the aged porous polymer phase (22 days at 200°C under a helium flow). *’) Reacts with the stationary phase. resultins in very broad or multiple peaks.

Table 19. Properties of Polar Porous Polymers

Name

Composition Sty =styrene EVB = ethylvinylbenzene DVB = divinylbenzene

Source

s.

Sty-DVB with vinylpyrrolidine Sty-DVBwith vinylpyridine Ethylene glycol dimethacrylate Sty-DVB with vinylpyrrolidine DVB-acrylonitrile Acrylic ester, crosslinked Acrylic ester, crosslinked EVB-DVB with methyl acrylate

Cekachrom 5

EVB-DVB with methyl acrylate

Cekachrom 6 Spheron MD 30170

EVB-DVB-acrylonitrile copolymer Methacrylic acid-ethylene glycoldivinylbenzene Styrene-ethylene dimethacrylate Sty-dimethacrylic ester Sty-dimethacrylic ester Polyimide Polyphenylquinoxalene DVB-EVB-4-nitrostyrene

Spheron SE Separon SE Separon AE Polyimide Polyphenylquinoxalene SD-NO2

TmU (isothermal)

[m’.6-11

Porapak R Porapak S Porapak T Porapak N Chromosorb 104 Chromosorb 107 Chromosorb 108 Cekachrom 4

dso

Waters Associates Waters Associates Waters Associates Waters Associates Johns-Manville Johns-Manville Johns-Manville Chemiekombinat Bitterfeld ditto

450-750 300-590 200-450 225-500 100-200 400-500 100-200 150-250

ditto Laboratorni pfistroje

250-350 130-320

Laboratorni pfistroje Laboratorni pfistroje Laboratorni pfistroje

(511) (512) (487,495)

20-50

Iml

7-10 7-9 1-9 9 60-80 8-9 23-25 85 135

25 32-40

PCl

250 250 190 190 250 225 225 200 200 200 230

-

280 230 230

50

81

320 300

354

300

70 70 50 50

(50:5:45) ED-NO2 CEM HEM PYR PON

2’-

EVB-DVB (5050)nitrated with fuming HN03 Poly[(2-cyanoethyl)methacrylate]

Poly[(2-hydroxyethyl)methacrylate] Poly(4-vinylp yridine) Poly (N-vinylpyrrolidine)

= maximum column temperature after conditioning

(487,495)

24 15 51 50

76 61 80 58

190 220 250 220

Bulk Density k.m-’] (depending on the particle shape and size)

0.27-0.43 0.27-0.43 0.27-0.43 0.27-0.43 0.28-0.32 0.28-0.32 0.28-0.32

130

5. Solid Stationary Phases

mation between the pyrrolidone functional groups of the polymer and aromatic compounds. In Table 22 several characteristic applications of polar porous polymers are given. Very recently, a polar PLOT column was developed by Chrompack [514a]: Pora PLOT U. It exhibits a 10 pm layer of a polar porous polymer on fused silica column walls (column I. D. 0.32 and 0.53 mm, respectively) and renders possible the separation of low-boiling compounds maintaining reproducible retentions even in the case of widely differing humidity of the samples. Table 21. AISjF'Mcand AIYjYLmeValues of McReynolds Polarity Probes on some Specially Prepared Polar Porous Polymers (after Smith et al. [487, 4951 and Komers et al. [513]) Stationary Phase

A1

Benzene

Butan-1-01

Methylpropyl ketone

1-Nitropropane

Pyridioe

118 180 5720 522 476 156

126 167 4150 312 404 166

132 139 6000 566 603 292

89 148 5530 551 481 183

~~

SD-NO2 ED-NOZ PON HEM CEM PYR

A% A% Ar;g2 ditto ditto ditto

0

- 14 3180 275 269 58

Table 22. Characteristic Applications of Polar Porous Polymers Type

Referred Separations

Porapak R

Ethers, esters, nitriles, nitro compounds, amines, water in inorganic compounds Carbonyl compounds, n-/iso-alcohols, halogen compounds. No nitro compounds! Oxygen-containing compounds, formaldehyde in aqueous solutions Acetylene in Cz hydrocarbons, separation of NH,, C 0 2 , H20 The most polar commercial porous polymer. Nitriles, nitroalkanes, vinyl chloride, xylenols, NH, , SOz, COz, moisture in solvents Formaldehyde, S-compounds, medium polarity compounds Gases, polar compounds, e.g., alcohols, aldehydes, ketones, glycols, water Technical gas mixtures, mixtures of polar compounds (H20, alcohols, aldehydes, ketones, esters), aromatics and aliphatics in aqueous phase, chloroacetic acid derivatives Nitriles, nitroalkanes, aqueous H2S solutions, NH,, N, S and C-oxides, traces of water in aromatics Hydrocarbons, alcohols, glycols, carboxylic acids Alcohols, esters

Porapak S Porapak T Porapak N Chromosorb 104 Chromosorb 107 Chromosorb 108 Cekachrom 4 Cekachrom 6 Spheron SE Separon SE Separon AE Polyimide SD-NO2 ED-NOZ CEM HEM PYR PON

High-boiling compounds, polar compounds, e.g. alcohols, esters, aldehydes, ketones, pyrrolidones, specific interactions with unsaturated and polar compounds Aromatics, nitroaromatics, phenols, aniline derivatives Fatty acids, H 2 0 in carbonyl compounds, proton-accepting abilities Associates with various solutes by hydrogen bonds H 2 0 in aniline solutions and separation of aniline derivatives, proton-accepting coordination abilities Most polar porous polymer, charge-transfer complexing properties. Group analysis of trace non-aromatic fractions in aromatic hydrocarbons

131

5.6. Substances Forming Inclusion Compounds

5.6.

Substances Forming Inclusion Compounds*)

In addition to the interactions and the surface properties discussed above, rendering possible the chromatographic separation, some solid substances reveal, owing to their suitable steric structures, a special property viz., the ability to form inclusion compounds that can be utilized for the separation of substances with different molecular structures. This phenomenon is based on the spatial structure of a "host" compound with free cavities of dimensions that permit the temporary or permanent inclusion of appropriate "guest" molecules. The formation of inclusion compounds depends strongly on, in addition to parameters such as contact time, temperature, pressure and the presence or absence of a solvent, above all on the spatial dimensions of the "guest" molecules, on their interactions and, moreover, if the cavities are approachable by the "guest" molecules, on their interactions with the surfaces of the free cavities. These interactions are at the level of the usual intermolecular forces already discussed and correspond to the energetically most suitable mutual arrangement [515). Essential requirements for the application of solids with "host" structures as stationary phases in gas chromatography are stability and suitable dimensions of the host lattice. The cavity size and cavity shape (cage, channel, or layer) certainly determine the separation of compounds having spatially different structures, but the retention is also dependent on the chemical nature of the host compounds' inner and external surface. Thus, we might not often find a purely inclusion mechanism but the separation process might be composed of inclusion, screening and electrostatic contributions.

5.6.1.

Zeolitic Molecular Sieves

Natural and synthetic zeolites are aluminosilicates of the general formula MzlnOA1203 xSi02 yH20, where M = metal, chiefly an alkali (Na, K, Li) or alkaline-earth metal (Ca, Mg, Ba), R = valence

of the metal and x varies with the zeolite type [516];

x = 1.8-2.1 x = 2-3 x = 4-6

type A zeolite, type X zeolite, type Y zeolite,

x = 10 x = 30 x = 1000

mordanite type zeolite,

ZSM type zeolite, silicalite type zeolite

and y is a factor changing with the degree of hydration, with values between 0 and 7. Increasing the Si02content will increase the thermal stability and the acid resistance, whilst decreasing the hydrophilic character. The primary structural units of the crystalline lattice of aluminosilicates consist of tetrahedral Si04and A104 anions. The vertices of these tetrahedra are formed by oxygen ions, and the silicon or aluminium ions are in the centre. The tetrahedra containing aluminium are negatively charged, and these charges are balanced by the alkali or alkaline-earth metal ions. The Si04 and A104 tetrahedra form three-dimensional network structures being arranged into polyhedra with characteristic lattice types. The outstanding feature of the molecular sieves is their cavity structure consisting of a network of uniform large cavities connected by uniform micropores, the diameters of which fall in the range of molecular dimensions, hence enabling the cavities to be accessed only by suitable molecules. Thus, the dominant factors in separation are the size of the cavity or channel in the molecular sieve structure and the sizes and shapes of the sample molecules. The most important molecular sieves for GC are types A, X and Y. Their building principle can be made clear if we start from a cubic octahedron as a lattice unit, the vertices of which are occupied by Si04 and A104 tetrahedra, the ratio of Si/AI depending on the zeolite *) Cyclodextrins, also forming inclusion compounds (e.g. aromatics into the hydrophobic cavities), are not dealt with here, as these crystalline solids are of minor importance than their liquid derivatives which will be discussed in Section 8.4.3.

132

5. Solid Stationary Phases

type. Thus, an aluminosilicate skeleton will be obtained that is composed of four-membered and six-membered rings. This skeleton represents a sodalite unit, sometimes called truncated octahedra or 0-cages. Connecting the square areas of the 0-cages via cubes, we obtain a type A zeolite, the large cavity of which, or a-cage, having a diameter of 1.1 nm, is surrounded by eight cubic octahedra connected with six adjoining cavities via eight-membered windows of diameters 0.4-0.5 nm (depending on the charge and size of the cations) (Fig. 9). Each of the eight octahedra contains a smaller cavity. The differences between type A and type X and Y zeolites lie in the binding of the polyhedra: arranging the units via hexagonal prisms like the carbon atoms in a diamond, we obtain the faujasite-type structures X and Y. Each cubic octahedron (or obtuse regular octahedron) is CoMected with four other octahedra via six-membered oxygen rings, and four octahedra enclose a large cavity (or supercage) (diameter 1.3 nm)that is COMeCted with adjoining cavities via twelve-membered windows (diameters ca. l nm) (Fig. 10). Sodalite cage

\

iy ls;

s' 0

\

Schematic structure of a synthetic zeolite 4A, large with 8 sodalite units. I, I1 = positions of the cations

P0s.I

Fig. 10. Schematic structure of synthetic zeolite types X and Y, respectively (after Sch6llner [518a], by courtesy of Chemische

Gesellschaft der DDR)

The figures show that the structures of A, X and Y zeolites exhibit three-dimensional pore systems consisting of linked cavities. In zeolite A, monovalent cations can occupy positions within the eight ring windows and restrict the access to the large a-cage. Exchanging, e.g., Na+ ions by CaZt ions, however, removes Na+ ions from the eight ring positions, and the free diameter increases from 0.4 nm for NaA to 0.42-0.44 for CaA [517]. In Table 23 characteristic parameters of some zeolites are listed [518].

5.6. Substances Forming Inclusion ComDounds

133

Table 23. Characteristic Parameters of Some Zeolites (After Turn and Wan [518]) Zeolite')

Channel/cavity dimensions

Na+-A (4A)

main cavity 1.1 nm channel opening 0.4 nm main cavity 1.3 nm channel opening 0.8 nm main cavity 1.3 nm channel opening 0.8 nm

Na+-X (13X) Na+-Y (LZ-Y52)

Na+-mordenite (LZ-M5) (large pores) Silicalite (S-115)

- -

-

main channels 0.7 nm side channels 0.3 X 0.6 nm

-

main channels 0.52 X 0.57 nm side channels 0.54 nm

Nature of channellcavity system

SVAl ratio

Kinetic") diameter for adsorption [nml at room temperature

3-D channel system with cavities 3-D channel system with cavities 3-D channel system with cavities 2-D channel system pertinent one-dimen-

0.7-1.2

0.4

1-1.5

0.8

1.5-3

0.8

4.5-5

0.7

99%Si

0.6

sional cavities 3-D channel system no cavities

*) Designations in parentheses are Union Carbide product codes. **) Indicates approximately the largest sized molecule that can be accommodated in the voids of the

zeolite or the h e aperture

of the channels.

The adsorption and inclusion behaviour of zeolites is based on steric, thermodynamic and kinetic effects. The steric, geometric or sieve effect is characterized by the critical molecule diameters of the sample molecules on the one hand and by the critical pore diameters of the zeolites on the other. These diameters range from about 0.3 mm (zeolite 3A) to about 1nm (zeolites 1OX and 13X) for the channels in the molecular sieves. If the molecules of the compounds to be separated are spherical, the critical molecule diameter corresponds to the true diameter. However, for polyatomic molecules, the diameter perpendicular to the longitudinal axis has to be taken into account, which has, e.g., the same value for all n-alkanes (0.49 nm). The cavities can be accessed only by molecules with fitting diameters. That are smaller than the free aperture of the channels of the applied zeolite. For example, the channel system of zeolite NaA can be entered by NH3, HzO,H2,Ar,O2and Kr (molecular diameters < 0.35 nm), whereas Nz, SOz, CO, CH3, Xe, cyclopropane, propane and CF2C12(molecular diameters ca. 0.36-0.43nm) require the application of CaA. Isobutane, CF4, SF6, benzene, cyclohexane and neopentane, owing to their molecular diameters of 0.47-0.62 nm, can cross the large pores of mordenite, and (C4H9)N(0.82 nm) can only penetrate the channel system of NaX,CaX and NaY (5191. Consequently, by careful selection of the zeolite type it is possible to separate many gas mixtures. The thermodynamic effect of the zeolites is based on their sorption properties, which are essentially determined by the internal surface area of the molecular sieves, which for typical zeolite crystals is approximately 200 times larger than the external surface area [520]. The isomorphous replacement of silicon by aluminium atoms within the zeolite lattices results in a net negative charge concentrated on the oxygen ions between Al and Si, yet partially delocalized over the whole framework. The positive charge, necessary for electroneutrality, is centred on the exchangeable cations within the cages and channels. Owing to this ionic surface, zeolites belong to type I1 adsorbents. Hence the heat of sorption is composed of a non-specific term (dispersion energy contribution) and a specific term [resulting from surface interactions with sorbate molecules exhibiting permanent quadrupole moments (N,, CO, COJ or large permanent dipole moments (H20, NH3)]. The n bonds in alkenes and aromatic sorbate molecules also contribute significantly to the beat of sorption [520]. Some data may illustrate the dependence of the heat of sorption on the structure of the sorbates. On NaCaA the heat

134

5. Solid Stationary Phases

of adsorption for C3HBis 35 kJ/mol (non-specific term 35 kJ/mol, specific term 0 kJ/mol), yet for C3Hs it is 47 kJ/mol (35 kJ/mol non-specific, 12 kJ/mol specific) and for CH3NH2it is 80 kJ/mol (30 kJ/mol non-specific, 50 kJ/mol specific) [519]. Thus, ethane is eluted before ethylene. The heats of sorption in the cavities can be as much as eight times larger than that found for the corresponding smooth, planar surface of the same chemical composition, owing to the high spatial concentration of cations and negative charges in the voids. Increasing the cation density increases the heat of sorption (Nay > NaX > NaA [520]), and the contribution of the specific interactions is largest for Ca2+ zeolites [521]. Hydroxyl groups within the channels provide the active, protonic sites. Additionally, because of their electron acceptor properties, cationic species can act as Lewis sites, especially aluminium, the order of strength being A13+> Al (OH)'+ > Al(OH2)' [519]. The Kinetic effect of the zeolites can be utilized, just as the thermodynamic and steric effects, for gas chromatographic separations. The migration of the sorbates within the crystals has to be considered in addition to the intercrystalline flow. At a given temperature, the diffusion coefficient may change by many orders of magnitude for small changes in the critical diameters of the diffusing molecules, even if the heats of adsorption are equal [520]. The shape-selective diffusion is a new type of diffusion regime, the so-called configurational diffusion, occuring in very small pores the surface of which constantly influences the diffusing molecules, thus differing from the regular diffusion (pore sizes greater than the mean free path of the diffusing molecules) and the Knudsen diffusion (pore size less than the mean free path, but no shape-selective dfisivity) [519]. Summarizing, we can state that the selectivity of the zeolites results from the co-action of all three effects. Even if the size of the cavity or channel in the molecular sieve structure and the size and shape of the molecules to be separated are dominant factors for the separation, the retention values are far from being based only on molecular sieving. For example, exchanging a suitable cation for the zeolite lattice while retaining the lattice spatial parameters can affect the magnitude of electrostatic interactions during the separation and hence also the retention of the separated compounds. If we disregard water, the stability of inclusion compounds with zeolites is low at reasonable temperatures, i.e., the decomposition rate of the inclusion is of the same order as the formation rate, hence permitting the elution of the compounds listed in Table 25 in reasonable analysis,times. Hence there are many gas chromatographic applications of the zeolites. For the synthesis, the starting materials, e.g., sodium silicate and sodium aluminate, are mixed in the desired proportions and hydrothermally processed, requiring a high degree of supersaturation of the components in a gel phase for the nucleation of a large number of crystals [517], which, owing to their small particle size of about 1 pm, must be formed as grains. The ability of the zeolites to sorb considerable volumes of sorbates in place of the zeolitic water present in the crystals on formation requires the water to be removed by activating the humid zeolite at about 250°C in a carrier gas flow. In Table 24 some commercial products are listed, and in Table 25 some characteristic gas chromatographic applications are given. Recently, fused silica PLOT columns coated with molecular sieve 5A (layer thicknesses 30 and 50 pm, respectively) have been developed by Chrompack (The Netherlands). 5.6.2.

Bentonites

The essential constituent of bentonite, a mineral deposited in rich sediments in the U.S.A., is the aluminosilicate montmorrillonite, A12[Si,010](OH)z. nH20. This silicate exhibits a framework of layer lattices, the layer distances depending on the amount of water included be-

5.6. Substances Forming Inclusion Compounds

135

Table 24. Commercial Synthetic Zeolites Name

Type

Roducer

3A 4A 5A 13X LZ-Y52 LZ-M5 S-115 Zeosorb 4A Zeosorb 5A Zeosorb 5AZ Zeosorb 1OX Zeosorb 13X Zeolith KA Zeolith NaA Zeolith CaA Zeolith CaX Zeolith NaX Naloit 4 Calcit 5 Naloit 13 Pentasil ZSM-5 Pentasil ZSM-11

KNaA NaA NaCaA NaX NaY Na-mordenite Silicalite NaA NaMgA NaCaA

Union Carbide Union Carbide Union Carbide Union Carbide Union Carbide Union Carbide Union Carbide Chemiekombinat Bitterfeld Chemiekombinat Bitterfeld Chemiekombinat Bitterfeld Chemiekombinat Bitterfeld Chemiekombinat Bitterfeld U.S.S.R. U.S.S.R. U.S.S.R. U.S.S.R. U.S.S.R. Czechoslovakia Czechoslovakia Czechoslovakia Zeolit-Socony Mobil Zeolit-Socony Mobil

lox

NaX KNaA NaA NaCaA NaCaX NaX NaA NaCaA NaX Na-pentasil Na-pentasil

Table 25. Gas Chromatographic Applications of Some Zeolites Zeolite

Free Dimension (window space)

Possible guests

Characteristic Separations

“nl

Removing H 2 0 form organic solvents CzH6from higher homologous n-alkanes; C2H64ZH443H6 n-alkanes from iso-alkanes n-alkanes and n-alkenes from esters, ketones, ethers, halogenated hydrocarbons; n-alkanes from naphthenes and aromatics; H2-02--N2-CH4--CO; ; Ar-02-N2 He-Ne-H2-02-N2 ; contaminants in SF6 group separation of cycloalkanes-isoalkanes-n-alkanes butenes-butadiene; 02-NZ-NO40; Ar-N2-Kr4H4--CO-Xe

*)

Depending on the composition

tween the layers. A bentonite treated with dimethyloctadecylammonium salts has found broad applicability in gas chromatography under the name Bentone 34. Obviously, molecules with a suitable geometric shape can better be adsorbed between the layers of the expanded

136

5. Solid Stationary Phases

clay than those of a less suitable shape. In particular Bentone 34 can discriminate between isomers of many aromatic compounds and cycloalkane derivatives. For example, the m-isomer of the xylenes is adsorbed preferentially, giving the elution order p-, 0-,m-xylene. At a column temperature of 70"C, the selectivity for separation of m- and p-xylenes is so high that a 99,9%separation can be obtained using only 800 theoretical plates. Bentone 34 is coated on support materials normally used in gas-liquid chromatography, and is often modified with a silicone oil [523] in order to improve the peak symmetry.

5.6.3.

Tri-o-thimotide

A supersaturated solution of tri-o-thimotide in tritolyl phosphate is able to separate selectively straight-chain from branched-chain molecules by temporarily including and hence retaining longer the unbranched molecules. It is assumed that in a supersaturated solution tri-o-thymotide molecules cluster together in a manner similar to that in the crystal, producing channels (0.48 nm) and cages (0.69 nm) and that, additionally, some solid tri-o-thymotide is formed when applying a column at, e.g., 100°C, which is lower than the saturation temperature of the solution (180°C) [524, 5251, both crystalline forms acting as molecular sieves. The tri-o-thymotide solution in tritolyl phosphate was supported on Celite. Comparison of retentions obtained with such a solution/suspension and with a similar solution/suspension of the chemically similar di-o-thymotide, which does not give rise to inclusion compounds, demonstrated that n-alkanes and n-alkenes and also primary alcohols and aromatic and halogenated hydrocarbons were selectively retarded by a factor of 1.3-1.4.

0 pr'

/ /

Tri-o-thymotide

5.6.4.

Di-o-thymotide

Desoxycholic Acid

Desoxycholic acid, likewise, is known to form inclusion compounds with n-alkanes and, partially, n-alkenes. Thus, a solutionldispersion of this acid in tritolyl phosphate selectively retains these compounds. Unfortunately, however, the retention is accompanied by peak broadening [526]. The channels of the (solid) desoxycholic acid have diameters of about 0.5-0.6 nm [527] and hence are traversable by n-alkanes and some n-alkenes and n-alkanols [515].

5.6.5.

Werner Complexes

Werner complexes, e.g., Ni(4-methylpyridine),(NCS)2, owing to their crystal structures (cages), can form clathrates with various organic compounds. For the intended use in gassolid chromatography, the 4-Me-@ complex of nickel is dissolved in chloroform and coated on the solid support, e.g., Chromosorb P. The observed elution order can be explained by the

5.1. Modified Adsorbents

137

presence of permanent clathration holes in the host lattices, and linear molecules, which fit in such holes, are subjected to strong Van der Waals forces inside, leading to their greater retention compared with non-linear molecules. The shape-discriminating feature of the 4-Me-Py complexes especially holds good for aromatic molecules but is not restricted to them. At 80°C, the separation factor for p- and m-xylene is 2.42 [528], and the separation of 0-,m- and p-isomers of nitrophenol, nitroaniline, chloronitrobenzene and nitrotoluene and the separation of naphthalene derivatives could also be successfully carried out [529, 5301. Unfortunately, owing to the limited thermal stability of the complexes, their application in gas-solid chromatography is limited even at low temperatures (80-90°C).

5.6.6.

Benzenesulphonates

The selectivity of benzenesulphonates of some alkali metals (K, Na, Rb) used as solid stationary phases has proved to be governed by clathration, hydrogen bonding and interactions of the metal ion with lone pairs of electrons on, e.g., oxygen atoms of the sorbate. Holes or channels inside the crystal lattice of the sulphonates can be entered by diethyl ether, but not by diisopropyl ether with its bulky structure. Hence the latter, in spite of its 34°C lower boiling point, is eluted before diethyl ether. The sulphonates, coated on Chromosorb P and used at a column temperature of about 80°C must not be subjected to temperatures higher than about 140"C, as otherwise the holes or channels inside the crystal lattice will collapse [515]. Both of the other factors influencing the retention, viz., hydrogen bonding and interactions with the metal ion, permit excellent separations of isomeric alcohols and isomeric substituted phenols [531, 5321.

5.6.7.

Urea and Thiourea

Urea and thiourea can form adducts with suitable compounds changing the tetragonal urea lattice to a hexagonal arrangement with channels with diameters of 0.5 nm, allowing, e.g., the ready incorporation of n-alkanes [515]. The selectivity and stability of urea and its adducts as stationary phases in gas solid chromatography towards n-alkanes were reported by Marik and SmolkouCi [533, 5341. Owing to clathration occuring in the chromatographic column [e.g. 25% (w/w) of urea or thiourea on Chromosorb PI, the separation of isomeric alkanes or peffluoroalkanes is possible, resulting in the elution of the most highly branched structure first and the n-isomer last [534a].

5.7.

Modified Adsorbents

Several disadvantages of adsorbents used in gas solid chromatography have restricted this efficient method for a long time: chemical and geometric heterogeneity, resulting in a non-uniform heat of adsorption of sorbates along the surface, the void structure and the high atom concentration on the surface. With a sufficiently high concentration of the intermolecular force centres on the surface of inorganic adsorbents, the heat of adsorption of groups A and B molecules in the first adsorption layer exceeds the heat of condensation considerably. Consequently, higher boiling compounds cannot be analysed on these adsorbents. This applies to the analysis of organic compounds on the (organic) porous polymers, which strongly adsorb, e.g., hydrocarbons. In addition to the development of new adsorbents, several attempts to modify the surfaces of the adsorbents geometrically or chemically have proved to be successful [535].

138 5.7.1.

5. Solid Stationary Phases

Modification With High-boiling Liquid and Solid Substances

Adsorbed monolayers of large organic molecules adhere strongly to the surface of inorganic adsorbents and exhibit a lower vapour pressure than does a liquid film of the same organic compound on the same adsorbent. Hence, the good thermal stability of the adsorbent and low bleeding of the stationary phase remain nearly unchanged. There are fiuther advantages. The most active sites of the adsorbent’s heterogeneous surface are covered fitst, hence decreasing the specific surface energy. In addition to an improved peak shape (tailing reducing property of the modifier), a decrease in the heat of sorption and of the k-values can be observed. This enables the column temperature to be decreased, polar and higher boiling compounds to be analysed, trace analyses to be carried out, and gas chromatography-mass spectrometry to be combined. According to Purnell[536]and di Corcia and Liberti [537],this variant of gas solid chromatography is frequently designated; gas-liquid-solid chromatography (GLSC). It should be considered that solid modifiers can also be used successfully and that a monolayer on the surface behaves differently from both the bulk liquid and solid phase. It combines the high efficiency of GSC with the high selectivity of GLC. By varying the type and layer thickness of the modifying liquid phase, the selectivity of GLC can be adjusted over a wide range, and difficult separations can be achieved. The amount of liquid phase required results from the complicated influence of the layer thickness on the retention and from the amount necessary to block the active sites. For example, with Sterling FT graphitized carbon black (specific surface area 12 m2/g) about 0.2%,whereas with Carbopack B partially graphitized carbon black (specific surface area 110 m2/g) 0.6% of liquid phase is required to eliminate the heterogeneous sites [537]. The mass transfer rate of the van Deemter equation for GLSC columns lies between those for GSC and GLC columns. Therefore, increasing the layer thickness means a reduction in the efficiency [538]. The retention mechanism is determined, for mono- and oligo-layers, by adsorption-desorption processes. It is difficult to prepare GLSC column packings reproducibly. Usually, the modification is accomplished by dissolving the required amount of the liquid or solid phase in a suitable solvent, addition of the adsorbent and evaporation of the solvent. After drying, the desired size fraction is screened and packed into the column. The adsorbents most often modified are graphitized thermal carbon blacks and highly pure spherical silica gels. Owing to the high concentration of carbon atoms on the base region of graphite (at a relatively small Van der Waals radius of the carbon atom) the adsorption energies on the graphitized carbon blacks are relatively high compared with the non-specific adsorption on other adsorbents [539]. By modifying with non-polar polymolecular liquids, e.g., Apiezons, the Van der Waals distances of which are many times longer than the bond lengths of the C - C bonds, the non-specific interactions can be considerably decreased without affecting the efficiency and selectivity of the adsorbent [540]. The non-specific character of this type I adsorbent is retained. In contrast, graphitized thermal carbon black, if modified with polar liquid phases, changes into a specific adsorbent. For example, when using polyethylene glycol as a monolayer, the modified graphitized thermal carbon black, owing to the high electron density, becomes an adsorbent of type 111. Modification with nitroaromatics, owing to charge-transfer interactions, permits the analysis of aromatic hydrocarbons, and the separation of all C4-alkanes and -alkenes can be achieved with 0.19% picric acid modified Carbopack C. In Table 26 some graphitized thermal carbon blacks and their modifications are listed. As can be seen, especially di Corcia and his colleagues have comprehensively investigated the modification of graphitized carbon black with high-boiling liquid and solid organic phases. Their experiences was utilized by Supelco (U.S.A.) and surface-modified graphitized carbon

139

5.7. Modified Adsorbents Table 26. Modified Graphitized Thermal Carbon Blacks and Their Application Oraphitized

Modification

Commercially available

Maximum Application Column Temperature I'Cl

References

4%Carbowax 20M + 0.8%KOH 1%SP 1000') 0.3% Dexsil + 0.5% H3P04 TCEP*)

Supelco

220

Amines

15411

Supelco Supe1co

225 220

Organohalides Alkyl sulphides, thiols Low-molecularweight aliphatic carbonyl compounds Aqueous solutions of fatty acids, aldehydes and alcohols Low-molecularweight alcohols, organohalides, aldehydes, ketones Alcohols, esten, ketones C2-Cs acids

15421 (5421

Thermal Carbon Black

Carbopack B*) Carbopack B Carbopack B Carbopack B Carbopack B

3%Carbowax 20M + 2.4% trimesic acid

Carbopack C**) 0.2%Carbowax 1500

Supelco

175

Carbopack C

0.3%Carbowax 20M

Supelco

225

Carbopack C

Supelco

200

Carbopack C

0.3%Carbowax 20M + 0.1% H3P04 0.8%THEED))

Supelco

125

Carbopack C

0.1%SP 1000

Supelco

225

Carbopack C

0.19% picric acid

Supe1co

120

Carbopack C

0.2-0.4% 2,4,5,7-tetranitrofluorenone 0.5% THEED + 0.5% TCEP 4-8% polyphenyl ether or 3%Dexsil400 0.4% trimeric acid + 1%Carbowax 20M 1%polyethylene imine + 0.4%KOH 0.3-1% H3P04 + 10%F F e )

Carbopack C Carbopack C Carbopack C Carbopack C Sterling FT

-

Carbopack B has a specific surface area of 100 mZlg a specific surface area of 12 mz/g l) mixed melt from Carbowax ZOM and substituted terephthalic acid (AFPAP) *) 1.2.3-tris(cyanoethoxy)propane

')

**) Carbopack C has

3,

N,N,N',N.tetrahydroxVethylethylenediamine

4,

free fatty acid @oly(ethylene glycol)-di-?-nitrotcrcphthalatc)

15431 [544]

15451

W I [5461

Separates ethylene [547] oxide/ethylene glycol/ethylene chlorohydrin, glycols, ethylene oxide residues Phenols, isomeric al- 1548-5501 kylbenzenes, fluorocarbons Unsaturated C4hy[546] drocarbons, vinylchloride Aliphatic and aro[551,552] matic hydrocarbons Monoterpenes is531 C4-C40hydrocarbons

[554]

Traces of substituted phenols in H 2 0 Amines, diamines, basic compounds Monocarboxylic acids

(5551 [556] W71

140

5 . Solid Stationary Phases

blacks have been commercially offered since then. When applying solid modifiers, the adsorption takes place on the modified carbon black surface and also on the crystal surface of the excess modifier. Owing to their thermal stability (>3OO0C) and to their selectivity, metallic and metal-free phthalocyanines (Fig. 11) are interesting compounds for the modification of graphitized carbon blacks. Investigations by Vidol-M&ar and Guiochon [558] have shown that the phthalocyanines form dense monolayers on the surface and decrease the force centre of the original adsorbent.

Fig. 11. Copper phthalocyanine Additionally, small crystals of the phthalocyanine (Fig. 11) LLvelop weak but specific interactions, and high-boiling polynuclear aromatics, alcohols, phenols and amines can be separated with short analysis times. By varying the phthalocyanine, an appropriate selectivity for substances with n-electrons and lone electron pairs can be adjusted, as this affinity increases in the order metal-free < Cu- < Co-phthalocyanines. The modification of Carbopack C with

2,4,5,7-tetranitrofluorenone,

which can be coated on to the graphitized thermal carbon black from a methylene chloride solution, enables the retention of donor molecules (unsaturated and aromatic hydrocarbons) to be adjusted and the separation from alkanes to be achieved by varying the amount of the modifier [551]. Similarly to graphitized carbon black, pure silica gels can be modified by thin layers of liquid or solid phases, thus homogenizing the surface area and decreasing the surface energy of these adsorbents. Owing to the low surface activity of these extremely pure adsorbents, only very small values of the average layer thickness are necessary for the deactivation, 1-2 nm as a rough value. Typical modifiers are polyethylene glycol, bis(cyanoethy1) ether [559] and phthalocyanines [560]. The selectivity remains essentially unchanged on varying the surface area of the applied silica gel but maintaining the layer thickness constant. On the other hand, with a constant type of silica gel and hence surface area, on varying the layer thickness the selectivity is altered.

5.7.2.

Modification With Non-porous Ionic Adsorbents

Disregarding barium sulphate, boron nitride and molybdeneum disulphide, non-porous inorganic adsorbents are rarely used in the pure form as column packings. Their specific surface area is low, and so is their capacity. In order to increase the surface area, they are coated on silica gel, aluminium oxide or inert solid supports. Regarding the chromatographic superiori-

141

5.7. Modified Adsorbents Table 27. Applications of Salt-modified Adsorbents Salt

Adsorbent

Application

References

LiCl, NaCl

Silica gel

[566]

KC1, CsCI, Na2S04,LaCI,

NiC12, CoCl,, BaCl, MgC12, CoCl,, ZnClz LiF, NaF, KF,CsF

Silica gel, graphitized carbon black Silica gel, aluminium oxide Silica gel Silica gel Aluminium oxide

Unsaturated hydrocarbons, Organohalides aromatics Unsubstituted and halogenated aromatics Inert gases

Na3P04

Silica gel

NaCI, LaCI,, NazMoO,

Isomers of different families Aromatics Unsaturated/saturated compounds C1-C5hydrocarbons within 2 min

[567,568, 5691

W I I5651 P711

15721 [568]

ties of (pure!) silica as type I1 adsorbents, we shall deal only with the modification of this material. Particularly alkali and alkaline-earth metal halides and salts of the transition metals have been applied as modifiers. They are coated on the adsorbent as follows. An aqueous solution of the salt is mixed with the silica gel whilst heating until the water has evaporated. In order to obtain a homogeneous coverage, the mixture is heated to the melting point of the salt. It can be assumed that a chemical reaction proceeds between the surface silanol groups and the metal chlorides, forming a qualitatively new surface [561], which is more homogeneous than the surface of the salts themselves. Investigations by Scott [562] and Vidul-Mudjar and Guiochon [563] in this field indicated that the modified adsorbents can interact specifically and charge-transfer interactions with n-electron systems take place. Varying the cations and anions allows considerable adjustment of the selectivity [564]. Adsorbents modified with BaCl, (which has both BaZ+and C1ions on all faces and on the surface) or with CoC1, or NiClz (which exhibit a crystalline layer structure and faces and surfaces with mainly C1- ions) actually show differences in the specificity of the intermolecular interaction, but by modification with these three salts a more homogeneous surface is created, consisting preferentially of C1- ions, independent of the previous crystalline structure [561]. Owing to the greater specificity of BaCl,, the separation of butadiene from the isomers of butane and butene can be achieved on BaC1,-modified Silochrom at 50°C, whereas CoC1,-modified Silochroms require a column temperature below room temperature [565]. In Table 27 some applications of salt-modified adsorbents are listed. Complex compounds have also been utilized for modifying the surface. For example, (Ag pyridinez)N03interacts specifically with olefins [573], and silica gels modified with tetramminecopper(I1) sulphate, copper(I1)bisethylenediamine sulphate or copper(I1)bistriethanolamine sulphate are suitable for the separation of low-molecular-weight aliphatics and aromatics [574]. Metal complexes of disubstituted organophosphorus acids have been used in the separation of compounds with rr-electrons or heteroatoms [575]. Finally, it should be pointed that metal complexes, similarly to salts and other modifiers, coated on an adsorbent do not remove any heterogeneity completely but decrease the specific adsorption energy and increase the specificity even at small layer thicknesses.

6.

Chemically Bonded Stationary Phases

Organic or organosilicon groups can be chemically linked via substitution of the surface’s silanol protons directly with the surface of siliceous materials. This procedure has great importance in chromatography because in addition to both the removal of interfering active sites and homogeniziation of the surface, the selectivity of the adsorbent can be adjusted to a certain extent by the choice of the substituting groups. Other than in the case of modification, where the layers essentially adhere to the surface only by Van der Waals forces, the surface of the adsorbent can be covered with a monolayer or a bulky layer of attached, i.e., chemically bonded, ligands. Hence two advantages arise: these phases are, depending on the nature of the attached groups, thermally more stable and show lower bleeding than comparable conventionally coated phases, and the higher mass transfer rate affords lower hmi, values, improving the resolution and permitting higher flow-rates of the carrier gas without a distinct decrease in the efficiency. This improvement, compared with conventional stationary liquid phases, comes from the more uniform coating, which frequently exists as a monolayer, whereas the conventional coverage exhibits pools many molecular layers thick. Chemically bonded phases are predominantly applied in high performance liquid chromatography (HPLC) in the revened-phase mode (i.e., the stationary phase being non-polar and the mobile phase polar), and in ion-pair chromatography. The GC application of chemically bonded phases is relatively limited compared with their immense scope in HPLC. This may be caused by deficiencies in the reproducibility of the originally manufactured batches, which were applied almost exclusively in GC, and by the fact that in gas chromatography the selectivity can only be influenced by the stationary phase, which therefore has a greater importance than in HPLC, where an additional variance is available, namely the choice of the most appropriate mobile phase. Furthermore, liquid stationary phases are much greater competitors for chemically bonded phases in GC than in HPLC (where the mobile phase is liquid and hence hinders or prevents the application of a liquid phase soluble in the mobile phase). In liquid chromatography, the solvolytic stability of the chemically bonded phases is perhaps their greatest superiority. Returning to gas chromatography, it can be stated that as a result of numerous recent investigations on the adsorbents used as matrices, on the complex nature of the bonding reaction, on characterization of the nature of the surface after the bonding reaction and on the nature of commercially available bonded stationary phases, the reproducibility of their manufacture has been and may further be improved. Hence the goals of the development of stationary phases, viz., to prepare reproducibly a phase of high efficiency and to design a desired selectivity, are within closer reach even for bonded stationary phases than a few years ago. It seems likely that owing to recent progress in this field, the scope of application of bonded stationary phases will also grow in gas chromatography with packed and micro-packed columns. In Section 3.3.3.3. we dealt with bonding phases to the walls in open-tubular columns. This aspect will not be discussed again here, but rather column packings for packed and micro-packed columns consisting of an efficient silica matrix modified by chemically bonded layers will be considered. However, there is no denying that the development of chemically modified packings will benefit by the most recent investigations on capillary wall deactivation and bonding reactions, and vice versa.

6.1. Adsorbents for Bonding Reactions

6.1.

143

Adsorbents for Bonding Reactions

Disregarding a few exceptions, only silica-based adsorbents have been applied for chemically bonded stationary phases. On the surfaces of these type I1 adsorbents are silanol groups in a variety of forms (e.g., hydrogen-bonded, unassociated, geminal, vicinal, isolated) of different chemical reactivity. Further, the non-uniform distribution of the silanol and the presence of physisorbed water will additionally affect the physical accessibility and reactivity to bonding reagents. Even if traces of undesirable ions (especially A13+and B3+)are absent, the surface is nevertheless to some extent heterogeneous, owing to the existence of OH-rich areas or clusters that cause differences in the energies of interactions with a given sorbent depending on the location where the interaction takes place [616, 619-6211. These silanol clusters are unfavourable in several respects. First, when reacted with bonding reagents, the ligands formed will also be concentrated in clusters, hence giving rise to a dispersion in the interaction energy which in turn will cause a dispersion in the retention, i.e., loss of chromatographic efficiency. Second, owing to the differences in the area required for an 0-H and an O-Rgroup, the number of R groups will be lower than that of the reactive protons previously present in the clusters, i.e., silanol groups will remain unreacted. Third, the reactivity of the silanols is likely to depend on hydrogen bonding between silanols, which may be greater in OH-rich than in OH-poor areas. Therefore, it cannot be expected that a completely homogeneous surface will be obtained by chemical bonding, although much of this active site heterogeneity as a source of retention dispersion can be removed by the chemical modification of silica. The macro particles of porous silica used for the bonding reactions (and also for direct chromatographic application as a solid adsorbent of type 11) contain hundreds of thousands of elementary microparticles, which have diameters ranging from few nanometers up to about 1 pm. These macroparticles exhibit for gas chromatography average grain sizes between 100 and 250 pm but with a size distribution as narrow as possible. Theoretically if we assume a six-membered planaring system (see Section 5.4.1) a silanol concentration on the surface of 8 OH groups per nmz is possible, [57Oa] and experimentelly 4-7 OH groups per nmz have been found. Owing to the strong hydrogen bonding tendencies of the SiOH groups, multilayers of adsorbed water can be formed. For example, in equilibrium with an atmosphere at 50% relative humidity, the porous silica may contain three layers of adsorbed water. The first layer appears to be water strongly hydrogen bonded to the silanol groups and is not released completely until temperatures in excess of 600°C are reached. The two outer layers are hydrogen bonded to the first layer of water and to themselves and are more easily removed by heating to 120-150°C forming activated silica gel [576]. In addition to pure silica gels, also superficially porous sorbents, porous glasses and diatomaceous earth-type supports have been used as matrices for the bonding reactions. The superficially porous solids are prepared by coating cores, e.g., non-porous glass beads, impenetrable to gas, with a porous layer of highly dispersed (particle size d 1pm) silica (porous layer beads). Porous glass is prepared by treating alkaline borosilicate glass with acids and water so as to achieve a desired pore distribution and reactive silanols on the surface. (The disadvantage is the presence of boron ions on the surface!) The solid supports normally applied in gas liquid chromatography, show only a limited number of silanol groups on the surface. Hence the number of ligands after the bonding reaction is also limited. Further their surface is far from being homogeneous. Nevertheless, many a separation could be achieved successfully with columns packed with diatomaceous particles to which organic groups had been bonded. In Table 28 some materials applied as chemically bonded phases are shown.

144

6. Chemically Bonded Stationary Phases

Table 28. Matrices Applied as Chemically Bonded Phases Commercial Name

Specific Surface Area

Average Pore Diameter

[mz/el

Silica gel (SiOz)

Porasil C Porasil F Porasil B p-Porasil Spherasil Porasil S Supelcosil YWG (silica) Spherisorb RSiL spher. Superficially porous solids (ac- Corasil tive) Pellosil Vydac SC sil-x-I1 Perisorb A Silica V Superficially porous solids Zipax (slightly active) porous glass CPG 650 diatomaceous earth Chromosorb P, W (acidwashed) Celite 545

6.2.

50...100 2... 6 125...200 350...400 100 300 170 300 200

20...40 300 10...20

10 10 10

15 8 12 12 10...14 11 0.83

70 1...4 1

13 500 ...3500 1000

Bonding Reactions

Many papers have been published describing chemical reactions between the silanol surface and appropriate reactants in order to modify chemically the silica surface. In addition to esterifications and Grignard reactions, reactions with chloro- and alkoxysilanes have most frequently been carried out. Depending on the type of reaction, different types of chemically bonded phases are obtained. In an esterification, the organic groups are linked with the surface silicon atoms via S i - 0 4 bonds. If a Grignard reaction has taken place, S i x bonds are present, and chloro or alkoxysilanes react with surface silanols to form Si-0-Si bonds. We shall discuss these reactions in detail.

6.2.1.

Chemically Bonded Phases with Si-0-C

Bonds

These phases are mainly prepared by esterification of the acidic silanol groups with alcoholic groups: In addition to different alkanols, especially polyethylene glycols [577] and other compounds with terminal hydroxyl groups, e.g., 3-hydroxypropionitrile, have served as alcoholic reactants. Example of a method of preparation: Porous silica, e.g., Porasil C (specific surface area 50-80 mVg, average pore diameter 20-40 nm), is heated for 1h in concentrated sulphuric acid-nitric acid (1O:l v/v) at 120°C. After cooling, the acids are removed and the solid is washed with distilled water to neutrality

6.2. Bonding Reactions

145

and dried overnight at 150-200°C. A 50-ml volume of 3-hydroxypropionitrile is added to 20 g of the dry matter and the silanols are esterified by heating for 2 h at 180-200°C in an open Erlenmeyer flask. The suspension is cooled and filtered. The esterified material is washed with methylene chloride and extracted with this solvent for 6 h in a Soxhlet apparatus. Finally, it is vacuum-dried at 120°C for 6 h (133 Pa). Commercially available bonded phases of this type (Durapak types) show distinctly unreacted silanols [578]. According to the procedure described by Hulusz and Sebestiun [579], polyethylene glycols can also be linked to silica gel surfaces [580]. Hulusz and Sebestiun suggested that the attached groups could be viewed as bristles on the surface. We shall return to this model when discussing the structure and properties of the bonded phases. A further preparation method consists in reacting a chlorinated silica gel with an alcohol or an alcoholate: Octyl-modified silica gel is thermally stable up to 160°C and hexadecyl-modified silica gel up to about 180°C. If the surface silanols are reacted with phenyl isocyanate, we obtain an Si-O-C-NH-C6H5 modification, which is, however, sensitive to light and humidity

I1

0 and can only be used below 120°C. A special type of bonded groups via Si-0-C bonds are the crosslinked polyester acetals. The acid-washed solid support is added to a solution of the polyester acetal. The catalyst ptoluenesulphonic acid monohydrate splits the acetal bond, and at these split positions both the cross-linking reaction between the polymer chains and the reaction with the acidic surface silanols take place. Subsequent conditioning at 270°C yields a chemically bonded and relatively thermally stable phase. Such products were described as early as 1968 [582] and were improved by additionally applying diethylene glycol and dimethyl-l,4-~yclohexanedicarboxylate [583]. This type of a bonded phase does not resemble a “brush” with “bristles”. It is rather a gel-like space network, behaving similarly to the polymeric liquid phases usually coated on solid supports. Advantages of brush type phases with Si-02-bonds: They are thermally more stable than conventionally coated liquid stationary phases (e.g., with PEG 400 or fl,fl’-oxydipropionitrile) The bleeding of the packing is low. High mass transfer rates permit high carrier gas flow rates and rapid analyses. The silanol influence is low. Low hminvalues (0.4 mm) can be achieved with bonded substituted groups (e.g., polyethylene groups, cyanoalkyl groups), irrespective of the kind and amount of sample (10-8-10-3 g per component) and of the temperature. Disadavantages of phases of this type: Low hydrolytic stability (due to the reverse reaction in the equation written above). Essentially less thermally stable than phases with Si-0-Si and Si-C bonds. Unstable against subsequent silylation reactions [581]. Therefore, they can be applied at temperatures up to 140°C (propionitrile group) or 230°C (PEG group) using well dried carrier gases. The sample must not contain water, and silylations have to be carried out prior to the esterification. In Table 29 several S i - 0 4 chemically bonded phases and their properties are listed. It should be noted, however, that only a few phases are still available, as most of them have been discontinued by the manufacturers.

Table 29. Chemically Bonded Phases with S i - 0 - C bonds Adsorbent

Bonded Phase Structure

Porasil C

Ads.-Si-O-C8HI7

Spherosil XOB 075 Spherosil XOB 075 Porasil c Porasil C

Maximum Column Temperature b*.~-’l PCI

Specific Surfaceha

Reference or Commercial Name

Referred Application

Hydrocarbons (short analysis times)

50

160

Durapak n-Octane/

Ads.-Si-O-C16H3, Ads.-Si-C)-C.&l A~S.-S~+~~H,, Ads.-Si-O(CH2CH20),H

100 100 50 50

180

W I

180 200

Porasil F

Ads.-Si4(CHzCHzO),H

6

200

Porasil S

Ads.-Si4(CH2CH20).H

300

Chromosorb W,HC1- Ads.-Si4(CH2CHzO),H activated and reacted with Sic& Ads.-SiaH2CH2CN Porasil c

4

230

50

130

~ 3 1 GC-Bondapack Durapak Carbowax 400/ Porasil c Durapak Carbowax 4001 Porasil F Durapak Carbowax 4001 Porasil s PEG 20M [584 Ultrabond (Carbowax 20h4) Durapak OPN/Porasil C

50

120

Durapak Phenyl isocyanate/Porasil C

Separation of positional isomeric hydrocarbons

<4

210

[5851

Heterocycles, esters, alcohols, aldehydes, ketones, hydrocarbons

Porasil C

Ads.- SiOC - NH- C6H5 II 0

Chromosorb

Ads. - 5 O C H

, Q-CH2

I ,O-CH2/

\

180

Porasil c

Aromatics, pesticides

Alcohols, ketones, aldehydes, hydrocarbons Steroids, polynuclear aromatics

Separation of saturatedl unsaturated hydroCiUb0Il.S

/CH2

- O\

C

CHR ‘CH2 -0’

147

6.2. Bonding Reactions

6.2.2.

Chemically Bonded Phases with Si-CH2R Bonds

The organic groups are attached to the silanol substrate by the following reactions: [chlorination of t h e substrate 1

%icl

a

4

SiR

(Grignard r e a c t i o n )

[orgonolithium reaction]

Example (1) (after Pesek and Graham [SSl]) A 15 g amount of Porasil C in a round-bottomed flask is refluxed in a mixture of distilled thionyl chloride and dry toluene (1:3) for 12 h. Because of the released gases (HC1 and SOz) the apparatus is kept in a fume hood. The chlorinated Porasil becomes dark brownish, indicating the completeness of the reaction, as unattached Porasil is white. After cooling, the toluene and the unreacted thionyl chloride are poured off and the chlorinated Porasil is washed three times with toluene and once with dry diethyl ether, strictly avoiding any humidity which would cause the formed Sic1 groups to be hydrolysed immediately to re-form the original SiOH groups. Through a round-bottomed three-necked flask filled with a condenser, gas inlet, stirring bar, balance funnel and drying tube, a constant flow of helium is passed. After having removed all traces of moisture by heating, 25 ml of moisture-free diethyl ether are placed in the cold flask and 10 g of high-purity lithium wire are added in small pieces. The mixture of ether and metallic lithium is refluxed for 30 min. Subsequently, 20 g of l-bromooctadecane in 200 ml of moisture-free diethyl ether are slowly added to the boiling mixture. In the subsequent reaction, the lithium becomes golden-glittering and the ether solution becomes cloudy. The finish of the reaction becomes apparent by dulling of the lithium. The warm organolithium solution is then cautiously added to the chlorinated Porasil, and the suspension is stirred, during which the Porasil changes colour, from black to light brown. After being stirred for 1h, the ether containing the unreacted organolithium is poured off and the octyl-modified Porasil is washed three times with CCq, three times with toluene and finally three times with diethyl ether (in portions of 100 ml).The residue1 ether is evaporated and the product is dried overnight. The reaction steps correspond to types (a) and (c). Example (2) (after Pesek and Graham [SSl]) Instead of helium, nitrogen is passed through the equipment described for example (1). A 20 g amount of magnesium splinters together with 50 ml of moisture-free diethyl ether are placed in the flask and a solution of 20 g of 1-bromooctadecane in 200 ml of moisture-free diethyl ether is added. To start the reaction, a small crystal of iodine is sufficient. The reaction is controlled by dipping the flask into an ice-bath. When the reaction has finished, the

6. Chemically Bonded Stationary Phases

14A

ethereal solution of the organomagnesium compound is added to the chlorinated Porasil C and the mixture is stirred for 1h. Subsequently, excess Grignard reagent and diethyl ether are poured off and the modified Porasil is treated as in the previous example. The reaction proceeds according to (a) and 0). Advantages of phases of this type: They are resistant to hydrolysis and chemically stable. They can be thermally stressed up to about 240°C. Phases with polar groups exhibit low hh values. High mass transfer rates and low dependences of the linear flow rate of the carrier gas in the range from 2 to 6 cm s-* permit rapid analyses. Adjustable selectivity by the selection of appropriate organohalogen compounds.

-

Disadvantages of phases of this type: The presence of distinct amounts of residual silanol groups requires an additional silylation reaction. High expenditure on preparation.

6.2.3.

Chemically Bonded Phases with Si-0-Si

Bonds

Owing to the chemical and thermal stability of Si-0-Si-C linkages, the silylation treatment of surface silanol groups has attracted by far the greatest interest and has been the most successful. Considerable attention has been drawn to this type of reaction owing to the requirements of modern liquid chromatography, to bonding reactions in glass and quartz capillaries (WCOT columns) and to improvements in glass, quartz and diatomaceous earth deactivation and wettability. This modification reaction is based on early systematic investigations of Kkeleu and Shcherbukouu [586], who reacted silica gel with trimethylchlorosilane and thus decreased the adsorption energies, e.g., for hydrocarbons, and achieved linear adsorption isotherms. The reactants for the silanol groups on the adsorbent’s surface are chlorosilanes, silazanes or alkoxysilanes, the latter being less reactive. Recently, cyclosiloxanes have also been used, most often in conjunction with chlorosilanes or silazanes, in order to achieve a more or less marked “per”silylation,if the reaction is carried out at high temperatures. Whereas in the past apart from deactivation reactions, di- and trifunctional reactive silanes were preferred, today monofunctional reagents are used more widely, especially in commercial materials, as the silylation reaction can be controlled more easily and the modification can be arranged more uniformly. The substrates, e.g., silica gel, diatomaceous earth materials or porous glasses, more or less activated, are reacted with chloroalkoxysilanes according to the- following reaction schemes.

2-o

OH

I

149

6.2. Bonding Reactions

-

CI

?I

I

+

ZSi-OH

CI-Si-R

OH

I

Me3SiCL or (MelSihNH

(end-copping reaction1

I

I I

2

I

CL

1s, -o-sI I -R

CL

gi-0-Si--R

CL 0 - 9Me3

- dl-o-s'-R I

1

I

0 - SiMe3

OH

With non-adjacent silanol groups, only one linkage with the surface will occur: (b)

1

SI-OH

+ CI~SIR

1

(d)

Me

9. , %-OH

+

/

A

1

CI-Si-CI

I

Hzo heat-

Si-O-SiC12

1

I

R

$

Me

+

RzSiCL2

1

CL-si

I

R

0

CL

-R

,Si -0-5-0

4

Me

4

I

CI-Si-R

I

Me

R

9. I L , S I - 0 - Si -OH I 3 R

I ,Si -0 -Si -CL 4 I 3

CI

(h) %-OH+

In

slo1,5

Me

CL

+

I

Me

R

I

5-0

I

R anhydrcur

'

Me

9. -,Si-O-Si-CI

I

A

Y. ( f ) ,SI-OH

-

-%i-O--Si-R

a

-Si

I

Me

re I

Me

I

I

CL

-R

J

5 - 0 -Si

-0 -5 - R I

6. Chemically Bonded Stationary Phases

150

Instead of the chlorosilanes, alkoxysilanes may also be used. It must be noted however, that the reactivity decreases in the order trichloroorganosilanes 5 dichlorodiorganosilanes > monochlorotriorganosilanes> alkoxyorganosilanes. ( i ) $i-OH+

3 9 ,Si-OH 2

?

H N ,SIR3 ‘SiR3

+

---+Si-O-SiR3

2

3

+ HzN-SiR,

R3Si-NH2

+ NH3

-5i-O-SiR3

3

9 3

+ CzH5O-Si-CHz-Cnz-@CHz-P

d

I

P

OCZ I H5

( j ) /%-OH

I

OCZ H5

d i -0 -Si -CHz

: ; 1 I

-CH,-@CHz

--+ C Z H ~ O H +

b P P

b

(with a subsequent further reaction, bonding salts, e.g., CuBr, or CuCl,, to the diphenylphosphinic group). If the reactions are carried out according to (a), (g), (h), (i) or (j) and if the unreacted chlorosilanes or alkoxysilanes are completely removed before adding water in reactions of types (a) and (g), we obtain a predominantly monomolecular surface coverage, whereas in reactions (b)-(f) polymeric layers will be formed, and the superiority of monomolecular layers with their high mass transfer rates (and low h& values) will be lost. In addition to both the initial siliceous surface and the functionality of the silane (mono-, di- or trichloro-, or mono-,di- or trialkoxysilane), the length and type of substitution of the side-chain R distinctly influences the reaction rates. R may be CH3, CzHz, CbH13, C8Hl7, C16H33 C17H35, Cl8H37 C6Hs (CHhCl, C6HlzOz (CHz)zCN, (CHzhCN, (CH2)3NH2 etc. As a consequence of their original surface concentration and local distribution, several of the adsorbent’s silanol groups will remain unreacted and, together with the silanols formed in the course of reactions a)-g), will have to be removed as far as possible by a subsequent reaction with an appropriate silylating agent. Usually, because of its smallness, a trimethylsilyl donor is applied for this “end-capping” reaction, e.g., trimethylchlorosilane, hexamethyldisilazane or N,O-bis(trimethylsily1)acetamide. In addition to the desired bonding reaction (i), siloxane bridges of the silica matrix may be formed or cleaved by the liberated ammonia. Further, at higher temperatures already bonded organosiloxy groups may react with adjacent uncharged silanols, forming new siloxane bridges and possibly render accessible previously screened or sterically hindered silanols, which in turn can be silylated by excess reagent: 9

not accessible except ball molecules

I

1

1

adsorbent

1

now accessible

1

I

I

1 0‘’

adsor bent

151

6.2. Bonding Reactions

Possibly a further reaction may take place, indicated by the occurrence of the hydrocarbon RH at a high temperature of the silylation, forming a new siloxane bridge and eliminating RH, especially when R = C6HS[587]:

+ RH

adsorbent

It is not dflicult to prove that complete coverage of largely hydroxylated centres on siliceous surfaces by organosilyl groups cannot be expected for steric reasons. A liquid-phase or vapour-phase silylation, generally carried out at medium temperatures, however, does not include the maximum of accessible or reactive silanols. It has been shown [587] that increasing the temperature of the silylation reaction (i) will increase the number of silyl-substituted silano1 protons. The necessary temperature is ca. 350°C. Whereas originally only trimethylsilylation (monofunctional) reagents were applied for the persilylation of siliceous surfaces, recently also polar, potentially difunctional cyclosiloxanes have been shown to yield chemically modified surfaces [588]. This reaction has been carried out originally in order to achieve better spreading of polar liquid phases on the inner walls of open-tubular columns. However, it may also start an improved chemical modification of other siliceous surfaces, e.g., silica gels:

2

R

I

;Si -0 -Si

a

Si-0-Si

R/k

F R

R’ ‘0 / R-Si-R \

’0

/

R-S<-R

0

/

R-~-R

I

OH

The bonded group -(Si(R,)O),H may react with a further surface silanol group [588a], may attack a further cyclosiloxane or can be end-capped by an appropriate monofunctional reagent. Example (1) (after Aue and Hustings [SSS]) Activation of Chromosorb G or W: An 86.6 g amount of acid-washed Chromosorb is suspended in 500 ml of concentrated hydrochloric acid and refluxed for 4 h. The acid is poured off and the Chromosorb is filtered by suction, washed with distilled water to neutrality, suspended in 200ml of acetone and the acetone is withdrawn. Washing with acetone is repeated twice and the Chromosorb is dried in a vacuum desiccator. The activated Chromosorb is suspended in 250 ml of toluene containing 20 g of dimethyldichlorosilane and refluxed in the absence of moisture. The vaporizable components are distilled off in vacuo and the modified Chromosorb is suspended again in 150 ml of dry toluene

6. Chemically Bonded Stationary Phases

152

and 12.3 g of hexadecyltrichlorosilane are added. The suspension is cautiously boiled down in a rotary vacuum evaporator, slowly heated to 65°C in a water bath and evaporated to dryness in vacuo (133 Pa), then heated for 30 min at 65°C. The treated Chromosorb is subsequently surface polymerized for 25-35 h at 120°C in a fluid bed which is generated by an air flow to which 6-8 ml of water per hour are fed. The reactions correspond to reaction schemes (b) and (g). Example (2) (after h e and Hustings [589]) A solution of 0.679, of water in 30 ml of acetonitrile is added dropwise to a magnetically stirred solution of 7.522 g of methyltrichlorosilane in 50 ml of acetonitrile at room temperature. The mixture is allowed to stand for 30 min and is then added to the suspension of the surface-modified Chromosorb in 150 ml of toluene instead of the hexadecyltrichlorosilane in example (1)and treated as in that example [reaction schemes (c) and (g)], the polycondensation in the fluid bed taking place at a lower temperature. Residual silanol groups are silylated (end-capped) as follows: 100 g of the material are heated in a pressure tube in 100 ml of xylene, which contains 3% (w/w) of N,O-bis(trimethylsilyl)acetamide, at 140°C for 2 h. After cooling, the tube is opened, the liquid is poured off and filtered, the solid material is rinsed three times with 100 ml of dry benzene, suspended for 1h in dry methanol, filtered, extracted with methanol in a Soxhlet apparatus for 4 h and dried for 8 h at 80°C in a vacuum desiccator: I 2 -Si-OH

I

+ CH3 - C

/OSi(CH3)3

\ NSi(CH $3

-

I 2-Si-O-Si(CH3)3

I

+ CH3CONH2

Example (3) (after Guwdzik, Suprynowicz and Jaronic [590]) A 10 g amount of micro glass beads of controlled pore size ( d p0.15-0.20 MI, S,,71.5 m2/g, Pd 13 nm, total porosity 0.64 cm3/g) is dried for 24 h at 200°C and placed in a 100 ml autoclave containing a solution of the required chlorosilane in dry toluene. The autoclave is closed and maintained at 100°C for 10 h. After cooling it is decanted, washed five times with benzene and extracted in a Soxhlet apparatus with benzene, acetone and aqueous methanol for 6 h in each instance. 10-g amount of the chemically bonded phase is dried at 110°C for 2 h and suspended in 50 ml of a solution of 5% (v/v) hexamethyldisilazane in dry toluene in the autoclave, which is then maintained at 100°C for 10 h. Subsequently the phase is washed five times with benzene and finally with methanol, then filtered and dried. Example (4) (after Marshall, Srutler and Lochrniiller [SSl]) Silica gel is kept in an oven at 130°C for 1week before use. The reaction is carried out in glassware that has first been treated with a toluene solution of trimethylchlorosilane, then with pyridine, which is poured off and removed under vacuum. The glassware is baked at 130°C and assembled hot under a stream of dry nitrogen. An excess of the monofunctional silane in question, dissolved in dry toluene, is slowly added to a toluene suspension of the silica. The mixture is stirred rapidly and refluxed for 24 h. A subsequent end-capping reaction is carried out at room temperature, stirring an excess of the end-capping reagent in toluene for several hours. The chemically modified silica is filtered several times with methanol, methanol-water (1:1) and methanol, Soxhlet-extracted overnight with methanol, air-dried and then dried in an oven at 130°C for 20min. Example ( 5 ) (after WeLrch and Frank [587]) The silica sample is vacuum-dried at 180°C and placed on a frit in an ampoule. A suitable hexaalkyldisilazane, e.g., di-n-decyltetramethyldisilazane,is added and the ampoule is flushed with dry nitrogen, sealed, placed in a thermostat and heated at 4Wmin to a final

153

6.2. Bonding Reactions ~~~

~~

~

~~

~

temperature of 350°C which is maintained for 15 h. Subsequently, the ampoule is cooled, opened and the chemically bonded phase is washed several times with methanol and then dried. Example (6) (after Wusiak and Szczepaniak [592]) Polyphenylsiloxane is prepared by hydrolysis of phenyltrichlorosilane and condensation to form a hydroxyl rich polymer, soluble in benzene, with a molecular weight between 2000 and 4000. After drying at 180°C for 6 h, the silica is chlorinated under moisture-free conditions by using a solution of 30%SOClzin dry benzene for 12 h at room temperature. Then the solution is decanted and the solid is washed four times with 75-ml portions of benzene (for the reaction type see Section 6.2.1). The resulting material is contacted with 50 ml of a 20% benzene solution of the polymer and refluxed for 16 h. After washing it is extracted for 6 h with benzene and vacuum-dried. As a certain portion of the polymer silanols will not have reacted with the surface Sic1 groups, the remaining polymer OH groups have to be end-capped. The chemically bonded phase is finally dried at 150°C for 6 h. Advantages of Si-0-Si type bonded phases: They are resistant to hydrolysis, chemically stable and thermally stable, depending on the ligand, up to 270-370°C. Owing to their universal applicability, they are the most frequently used bonded chromatographic phases, especially in HPLC. Disadvantages: With reaction schemes (b)-(f), polycondensation may occur, forming thick layers which no longer retain the benefit of high mass transfer rates exhibited by bonded phases with monolayers. In Table 30 several Si-0-Si chemically bonded phases and their application are listed. Table 30. Chemically Bonded Phases with Si-0-Si Adsorbent

Bonded Phase Structure’)

bonds Maxunum Co- Reference or lumn Commercial Name Temperature

Referred Application

“CI

Porasil C

350

RI

GC-Bondapak

Aromatics, pesticides

WI

Hydrocarbons

Ads - S I - O - S I C ~ ~ H ~ ~ I

Chromosorb W

R R

I

320

Ads -Si-O-SiCleH37

W41

I

R

s

Spherosil XOB 075 Ads - s ~ - o - s ~ c , ~ H ~ ~

Hydrocarbons

I

SIA, SIB, SIC, SID 0.75 hydrophobic

R

W51

Hydrocarbons

W31

Separation of isomers (electron donors) (olefines, aromatics) by complexation GC

R

Celite

I

Ads - Si-O-SiC16

H33

I R Porasil C

I

Ads.-Si-O-Si(CH2)2C6H4CH2PPh2 I complexes of Cull, COII,Nil,

6. Chemically Bonded Stationary Phases

154 Table 30 (continued) Bonded Phase Structure")

Adsorbent

Maximum Co- Reference or lUm0 CommercialName Temperature

Referred Application

PCI

I

Nucleosil lOOO-7

Nucleosil 1000 (Pd 100 m) 7 C18 Hypenil-WP 30O-Octyl (pd 30 run)

Ads.-Si-O-SiClsH,,

I

Hypenil-WE' 300

I I I Ads.--Si--O--SiClsH3, I Ads.-Si-0-SiClsH3,

Zipax

Gas Chom Q

I Ads.-Si-O-SiClsH,,

W61

Hydrocarbons, pesticides

370

P971

Hydrocarbons

300

Bondapak Phenyl Hydrocarbons

I

I I

Porasil B

Ads.-si-o-siC6H5

I

Zipax

Permaphase (etherphase) [596,597]

Ads.-si&SiC6H1&,

I

I

Zipax

A~S.-S~--O-S~(CH~)~CN

300

IS991

I

Supelcosil (170 m2/g)

Ads.-Si-O--Si(CH,),

I

Supelcosil

Ads.-SiUSi(CH3)2CsH17

I

Supelcosil

Ads.-Si-O-Si(CH3)2C1(H,7

Supelcosil

Ads.-Si--O-Si(CH3)

Supelcosil Supelcosil Spherisorb

Ads.--Si-O--Si

Spherisorb

A~S.-S~-O-S~(CH~)~(CH~)~CN

1' Ads.

=

(C6H5),

A~S.-S~-O-S~(CH~)~(CH~),CN chiral urea

Ads.-Si-O-Si(CH,)2C18H3,

adsorbent

HPLC LC-8 (pd 10 run) LC-308 (Pd 30 IM) HPLC

LC-DP (Pd-10 mj HPLC LC-3DP (pd 30 m)HPLC LC-CN HPLC LC-(R) urea HPLC Spherisorb C18 HPLC bonded phase Spherisorb nitrile HPLC bonded phase

6.3. Properties and Characterizationof Chemically Bonded Phases

155

The Supelcosil phases, representing the numerous bonded phases normally used in HPLC, are also listed here because of their structure, which may be interesting for gas chromatographers for their potential use for micro-packed columns and as a suggestion for the future development of chemically bonded gas chromatographic phases, which should, of course, be based both on wider pore matrices and coincidentally larger particle sizes of the silica used for the bonding reaction.

6.3.

Properties and Characterization of Chemically Bonded Phases

We shall restrict ourselves to silica matrices and to ligands bonded via Si-0-C and Si-0-Si to the surface, and to silica gels exhibiting specific surface areas of, e.g., 60-300 m2/g and average pore diameters of 5 10 nm. The theoretically possible number of SiOH-groups (six-membered planar SiO ring system; compare Section5.4.1) will be 8 per nm2.Experimentally, 4-7 SiOH groups per nm2 have been determined for samples after prolonged dehydration in vacuo between 135 and 180°C. If micropores can be neglected, an average value of 5 SiOH groups per nm2 can be assumed. The concentration of silanol groups on the surfaces of either the native silica or the bonded phase strongly influences the chromatographic properties and its determination is therefore of great interest. Most frequently, infrared spectroscopy has been used to establish surface SiOH concentrations before and after bonding [600-6051 and to determine the degree and distribution of hydrogen bonding on the chemically modified surfaces [604,606-6081. As for chemically modified silica, unlike native silica, the pressed-disc technique cannot be applied because of the hydrophobic particles developing strong repulsive forces, and reflection spectra have to be measured [608]. The strong absorption band between 3200 and 3700 cm-' indicates the stretching vibration vOH in adsorbed associates and terminal water groups vHzO(3400-3690), in hydrogen-bonded silanols via hydrogen vOH (3320-3650), in silanol vapours vOH (3650-3680), in silanol vapours hydrogen bonded via silanol oxygen v (3715-3740) and in non-associated (isolated) silanol groups (3745-3750 cm-') [600, 608-6091, whereas the stretching vibration in the region 3100-2900 cm-' indicates the presence of attached organic groups (alkyl, alkenyl and aryl groups) [610]. For the determination of the surface concentration of silanol groups their reaction with a Grignard reagent (methylmagnesium iodide), with methyllithium or dimethylzinc [Zn(CH3)2.2THF]can be used and the amount of liberated methane is measured [611-6131. A further method consists in the treatment of native or bonded silica with D20 vapour undergoing H-D exchange reactions and subsequent mass spectrometric investigation of the gas phase (H20, HDO, D20) [614]. The surface concentration of organic groups (a,)can be calculated from the carbon content determined by elemental analysis (C, H, N) according to the equation of Berendsen et al. [608, 6151: 1000 * % c . M.w. a, = 1.2 . n, * SA where a, = surface concentration ~mol/m2], n, = number of carbon atoms in the attached group, SA = specific surface area of the matrix silica gel [m2/g], M.W.= molecular weight of the attached group [s/mol]. The determination of the C, H content may be carried out in a common elemental analyser at 900-1000°C. For complete combustion, a mixture of 75% (w/w) MnOz, 8% K2Cr20, and

156

6. Chemically Bonded Stationary Phases

17% WOq should, according to investigations of the author, be applied as oxidation catalysts [615a]. In addition to infrared spectroscopy and chemical analysis, luminescence, photoacoustic and NMR investigations have provideb important information about surface-ligand structure and segmental and total chain mobility. Luminescence studies have shown [591, 616-6181 that the distribution of the ligands attached to the silica surface is heterogeneous, even if monofunctional reagents have been used in its preparation, and that most of the bound chains exist in high-density regions (concept of ”rich patches”, “micro-droplets” or “clustering” [616,619-6211). As the topography of the chemically bonded phase is a key for a better understanding of interactions of sample with the bonded phase and hence for questions of chromatographic selectivity, direct studies of bonded groups on silica surfaces have received increasing attention. Originally, for ligands formed from monofunctional reagents and anchored singly on the surface, a view of rigid bristles, like on a brush, prevailed [622], followed by the folded alkyl blanket model [624, 6231. More recently, NMR investigations of adsorbed molecules and groups attached to silica surfaces have been carried out under dry conditions, by means of 29Siand I3C cross polarization-magic angle spinning NMR spectroscopy [620, 623, 625-631, 633-634, 634al. It has been observed that with alkylsilyl bonded groups molecular motion towards the end of the chain occurs, that the motion of the terminal methyl groups is greater than that of other carbons along the chain and that the motion of the methyl groups is dominated by end rotation [625, 6261. Further, it has been found that chemically bonded phenyl groups undergo changes in orientation with temperature and that the ligand mobility increases more the longer are the alkyl spacer arms used to make the phenyl groups more distant from the surface [629]. When trifunctional organosilicon reagents are applied for the bonding reaction, according the reaction scheme (b) orb), two types of attachment are possible, polymerization along the surface and bulk polymerization away from the silica network [623, 632, 6201. This multiple-type polymerization, already proposed earlier, could be directly proved by NMR investigations I6231 for the first time. The distance of the bound ligands is limited by their local requirements, which in any case are more spacious than those of the starting surface hydroxyl groups. Hence, the average number of five silanols per nm2, corresponding to a surface concentration of 8.3 pmol/m*, can only react to a certain degree, and a certain portion of truly residual surface OH groups will always be left unreacted. Table 31 shows some characteristic surface coverage values of silica gels bonded with different alkyl and aryl groups, calculated from the carbon values. The surface concentrations of the OH groups of silica gel before the bonding reaction range between 8 and 9.9pmoYm2 and decrease, at the optimum silylation temperature of 350°C, to eoH = 1pmoYmz. The average area required by one OH group amounts to 5 0.18 nm2. Residual silanols may be simply obtained by the separation of basic solutes such as pyridine and 2,6-dimethylpyridine, after the bonded phase to be tested has been packed into an HPLC column [636] or by gas chromatographic interaction with even hydrocarbon solutes [637]. The influence of the pore diameter has been neglected in our discussions, but is should be taken into account for more precise examinations [638]. It was shown [634a], that a hydrothermal treatment will increase the content of geminal silanol groups on the silica surface, which increase the monofunctional modification with octadecyl silanes. Although the surface energy of the initial silica is very high, the bonding of alkyl groups lowers the surface energy by reducing considerably the contribution of the dispersive surface energy component and by quenching its polar contribution [640]. If a hydrocarbon, e.g., nhexane, is adsorbed its adsorption increases with increasing chain length of the grafted alkyl group, and the type of isotherm transforms from type 2, indicative of strong lateral interactions of the adsorbed molecules, to type 1 (or Henry’s) typical of a dissolution process. Both

6.3.Properties and Characterizationof Chemically Bonded Phases

157

Table 31. Surface Coverage Values of Alkyl and Aryl Bonded Phases (after Kkeleu [610],Berendren et al. (6151, Frank [608] and WeLFch and Frank [587]) Bound group

a

[pmoVm2]

Average required Area by 1 bound group

[nm21

-

0,32 0,39 0,34 0,41 0,55 0,45

0,56 0,55

0,81 0,46 0,63 0,87 a, = surface concentration of the bound group

AH and AS decrease as the chain length increases. For alkyl = C,6r for example AH = 32 kJ/mol, i.e., a value very close to the heat of dissolution of n-hexane in hexadecane [640]. The differential heat of interaction remains constant provided that the same adsorption process has taken place, but changes when the molecular mobility of the chain varies suddenly (at the melting temperature) or progressively (in second-order transitions between 70 and 310 K, where the transition enthalpies and entropies are one quarter of the melting enthalpies and entropies [641]. It was found that surfaces with chemically bonded alkyl groups exhibit a dual nature of adsorption and partitioning behaviour, and a model was suggested which explains this nature in terms of both gas-solid and gas-liquid mechanisms, depending on the temperature and chain length of the attached groups [620, 642-6441. The surface energy, which is very low for Czo alkyl groups (49mJ/m2), approaches that of polyethylene (24-42 mJ/mz) as if the surface of the silica were completely covered or shielded by the alkyl chains, provided that a non-polar adsorbate is investigated [640]. It can be increased by bonding functional groups more polar than methyl groups at the chain ends, e.g., -(CH2),CF3, -(CH,) ,CN or -(CHz) ,NH2. The behaviour of the chemically bonded polymer phases corresponds neither to a real liquid, even if there are thermodynamic results indicative of it, nor to an adsorbent, although some tailing and an increase in retention with decreasing amount of sample can be observed. A similarity to the interactions in gel structures is obvious (e.g., swelling of rubber by benzene vapour), where a kind of inner adsorbate is formed. The chemically bonded polymer phases can be classified between GSC and GLC. They have more potential to interact with a dissolved/adsorbed sample than a two-dimensional surface area of an adsorbent such as silica gel, but are more strongly squeezed together, and therefore possess distinctly higher interaction energies, than a normal liquid [646]. In order to characterize the chemically bonded phase with regard to the structure of the ligand and its purity, its functionality, the degree of substitution and of endcapping, several techniques in addition to the already discussed spectroscopic methods have been developed. Alkali hydrolysis or fusion and subsequent gas chromatographic analysis of the silylated siloxane moieties [647, 650, 6521 or of the formed hydrocarbons, respectively, or in the case of substituted alkyl ligands of the respective substituted alkanes or their alkali-catalysed reac-

158

6. Chemically Bonded Stationary Phases

tion products [648] can indicate the nature of the attached groups. Hydrofluoric acid digestion [649, 6501 and pyrolysis, coupled with gas chromatography and/or mass spectrometry, can also be used [653-6551. In addition to the structure of the ligands and their purity, their distribution on the surface as mono-, di- and trifunctional bonded alkyl groups and monoand polymeric substitution may be recognized, and also the degree of endcapping when carrying out the mentioned chemical or thermal treatment followed by an appropriate separation and identification method.

6.4.

Outlook and Prospects for Chemically Bonded Phases

In gas chromatography, it is assumed that the linking with the inner walls of both glass and quartz capillaries will continue to be a focal point of the bonding reactions in order to achieve stable films of tailored thicknesses, which in turn permit tailored analysis speed, resolution, column capacity and reproducibility. The optimal bonding reagents, in addition to polyoxyethylenes, are expected to remain chlorosilanes, silazanes, alkoxysilanes or cyclosiloxanes with alkyl, trifluoroalkyl, cyanoalkyl, aryl and other groups by which the selectivity can be influenced and adjusted. With chemically modified silica microparticles, the main apdlication field will remain HPLC, with further selective functionalities of the attached groups. However, the possibilities of developing and using chemically altered silica particles in gas chromatography are by no means fully exhausted, in the opinion of the author. Medium-sized silica particles with particle diameters between 30 and 100pm, of course narrowly fractionated, and even microparticulat? silica (10, 15 or 20pm), with organic ligands, should offer several advantages (see Fig. 12). Especially high-temperature silylated particles treated with mono- and difunctional reagents (e.g., substituted alkyl and aryl monochlorosilanes and cyclosiloxanes), yielding chemically bonded phases with minimal residual OH groups, are expected to expand the applicability of bonded phases in packed, micropacked and PLOT columns. As the wet packing technique, owing to the solvent resistance of phases bonded via Si-0-Si can be applied as

9 6 2 1

0,s

0

Fig. 12. Separation of gaseous hydrocarbons. Column packing, microparticulate silica (4 10-15 pm); chemically bonded with Si-CI8H3,,; organic coverage 10%w/w; column dimensions, 150 mm X 2 mm I.D.;n = 2 * lo4m-l; temperature, ambient; peaks: I methane; 2 ethane; 3 ethylene; 4 acetylene; 5 propane; 6 propylene; 7 isobutane; 8 n-butane; 9 1-butene; 10 trans-2-butene; I1 cis-2-butene; I2 butadiene (after Chianghai et al. [656])

6.4. Outlock and Prospects for Chemically Bonded Phases

159

in HPLC, highly efficient columns may be prepared. For example chemically bonded phases with alkyl, cyanoalkyl and aminoalkyl groups, packed into columns by the unbalanced-density slurry technique, have been demonstrated [648] to exhibit more than 2 . lo4 plates per metre, hence permitting short columns, short analysis times and the separation of complicated mixtures. Although it may still seem exaggerated, the term “high-performance gas chromatography” [648] for micropacked columns, packed using this HPLC-technique and achieving short, densely packed columns of high efficiency, is considered to be justified.

7.

The Solid Support

In gas-liquid chromatography with packed columns, the mixtures to be investigated are separated on a packing which consists of a solid support material coated with a liquid phase that is expected to bring about the separation process. The role of the solid support is to permit the formation of a thin uniform film of the liquid phase and hence to create a suitable gas-liquid interface between the mobile phase (gas) and the stationary liquid phase, thus providing appropriate conditions for a rapid mass exchange between the two phases. Solid supports should be chemically inert, mechanically and thermally stable, and should exhibit a surface area that is large enough and shows an appropriate texture and pore structure to achieve separations in columns of reasonable length. The particle size, its distribution and the shape of the particles, as they strongly influence the pressure drop in the column, are further properties to be considered. Ideally, the solid support must not adsorptively or even chemically interact with any sample constituent. In reality, however, the conditions of pure gas-liquid chromatography, i.e., that the retention is exclusively determined by the bulk stationary liquid phase and that at the liquid-solid interface any reversible or irreversible adsorption of any component should not occur at all, may scarcely be achieved. Hence the adsorptivity, at best the residual adsorptivity may cause systematic errors in quantitative analysis, especially in the determination of trace amounts of polar compounds and in the case of low support loadings. Let us now discuss the properties and the most important demands upon a suitable solid support in detail and deal with the various materials used.

7.1.

The Particle Size and Shape

We have seen that the A-term of the Van Deemter equation as a source of peak broadening [eqn. (62)] depends on, in addition to A, the mean particle diameter d, of the solid support. Decreasing d, should decrease A and hence the height equivalent to a theoretical plate (h). However, d, is not independent of A, as smaller particles can only be packed less uniformly, and an increase in A would increase A and hence h. Thus, as a compromise, the smallest particle size has to be chosen at which this disadvantage is negligible. Further, the composed term C of the Van Deemter equation contains C,, the contribution of the resistance to gasphase mass transfer to the peak broadening, which depends on d i , also demanding smaller particles. Conversely, we have stated earlier that the column permeability is also proportional to the square of the average particle diameter [eqn. (59)] which means that larger particles would reduce the necessary inlet pressure and, in turn, smaller particles would increase the pressure drop along the column. Finally, the column diameter has to be taken into consideration. We have already shown that the ratio d,ld, should be lower than 0.3 and higher than 0.03. For classical packed columns (d, > 1.5 mm), as a compromise, suitable particle size ranges would be 0.12-0.15or 0.15-0.18 mm. Microbore columns, i.e., tubes with inside diameters between 0.3 and lmm, should be packed with particles having diameters between 0.05 and 0.15 mm. Not only the absolute size, but also the size distribution of the particles influences A and with it A, and it is clear that by packing a column with a material which in addition to having an average particle size of, e.g., 0.15mm contains essentially finer fractions, great flow irregularities will occur, as densely packed sites are situated besides loosely packed and hence more permeable sites. Thus, a range of particle diameters as narrow as possible has to be ap-

7.2. The Surface Area

161

plied. Generally, screening, elutriation, or air classification have proved their worth in limiting the size dispersion. Screening is carried out using standard test sieves, and the size grading can simultaneously be taken advantage of for the determination of the particle size distribution of the material. The particle size is reported according to the mesh openings of standard sieve series, and the term mesh refers to the number of openings per linear inch. The greater the mesh the smaller will be the particle that can pass through it; a range of 0.15-0.18mm would mean that the whole material can be passed through a sieve with a mesh width of 0.18mm (80mesh), but that it is completely retained by a 0.15mm sieve (100 mesh). Differences in the millimetre or micrometre designation are caused by different sizes of the wire constituting up the screen, hence the micrometre (or millimetre) designation is more suitable than the mesh designation for describing the particle range, though it has become an established practice to refer to solid supports in terms of mesh range. In order to permit the determination of the actual particle size from the different mesh data Table 32 lists the openings and mesh sizes most frequently used. Apart from sieving, the particle size analysis methods most commonly used are optical microscopy, sedimentation, photoelectric sensing zone analysis and the electrozone method. Stokesian methods (sedimentation, elutriation) can function, as well as light beam interference, in either liquid or gaseous media, the liquid medium being prevalent. The size fractionation of particles by hydrodynamic elutriation permits, e.g., the preparation of solid supports for micropacked columns (14.5 X 1.5 to 29.5 X 3 pm range) [656]. The electrozone method, however, has proved to be by far the most precise method, with regard to both fidelity and resolution, and is least affected by particle shape and roughness. It uses a small liquid resistor, formed by a suitably sized orifice in a non-conductor, through which the particle-bearing electrolyte flows [657]. For details concerning these methods of particle size analysis and the required instrumental equipment, see refs. [658-6591. It must be taken into consideration, that in the course of sieving, coating and column packing the particles may be abraded, producing undesirable finer fractions. Hence, the original particle size of the applied solid support is not necessarily identical with that of the column packing prepared from it, unless hard, and unfriable material is used and the impregnation and packing steps are carried out cautiously. As the shape of the particles based on natural products, e.g., diatomaceous earth, is irregular, the packing density of the column can be reproduced only with difficulty and the 1values of the Van Deemter equation are high. Further because these materials are relatively soft, the abrasion additionally affects the efficiency. Therefore, the development of synthetic spherical, hard solid supports, implemented by Merck, has greatly improved the situation in the field of solid supports. The spherical form permits a high packing density, and the packing can easily be carried out owing to the good flow properties of the material. Hence, low 1values can be obtained, and the reproducibility, nearly independent of the manner of packing the column, is high, and even micropacked columns can be prepared in good quality. This material (commercial name Volaspher) will be discussed in detail later.

7.2.

The Surface Area

Specific surface area and pore structure are important characteristics of the solid support for the formation of a thin film of the liquid phase, appropriate for the rapid mass exchange of the sample molecules between the mobile and the stationary phase. These features, in addition to the chemical structure and hence the activity of the surface area, are essential, as the efficiency of the separation is dictated by the uniformity and the total surface area of the liquid film.

162

7. The Solid Support

Table 32. Comparison of Standardized Sieves (MeshSizes) and Openings (Mesh Widths) I S 0 565 R 20') TOL394042) STROW 2644.80') DIN 4188')

OOST 3584 ASTM E161-70ASTM Ell-70 0J.S.S.R)6) (U.S.A.)') (U.S.A.)s) Mesh Meshwidth MeshNo. width [PI [mm]

Tyler 0J.S.A.) Mesh width

[PI

Tyler (U.S.A.) Mesh No.

BS 410-69 (U.K.)? Mesh wldth

[PI

BS 410-69 AFNOR NF X-11-50ls) Nominal SieveNo. MeshNo.

(U.K.)Iq

JIS 28801 (Japan) Mesh") width

[ P I

AFNOR NF X-501') Mesh width

1-

32

0,032 0,036 0,04 0,045 0,05

0,045 0,05

0,056 0,063 0,071

0,056 0,063 0,071

0,08 0,09 0s

0,08 0,09 0,1

38

400

38

400

38

45

325

43

325

45

17

0,112 0,125 0,14

0,125

0,16 0,18 02

0,16 O,l8 0,2

0,224 0,25 0,28

0,25

0,315 0,355 0,4

0,315 0,355 0,4

18 53

270

53

270

53

300

63

230

63

250

63

240

15

200

74

200

75

200

0,s

0,63 0,71 03

0s

0,63 0,7 0,8

53 19

62 14

20 90

170

88

170

90

170

88

21

106

140

104

150

106

150

125

120

124

115

125

120

150

100

150

100

150

100

105 22

125 149

23 177

80

175

80

180

85

177 24

2 10

70

208

65

212

12

250

60

246

60

250

60

297

50

305

48

300

52

354

210 25

250 291

26 45

351

42

355

44

350 27

420

40

417

35

42 5

36

500

35

495

32

500

30

595

30

5 90

28

600

25

0,45 0,56

44

350

707

420 28

500 590

29 25

701

24

710

22

710 30

Inlemational Standard ISO-TC24 4 British Standards Institution, BS 410-69, Nominal Aperture TOL 39404 Standard of Testing Siwca (O.D.R.) '4 British Standards Institution, BS 410-69, Nominal Mesh No. St. ROW 2644-80 (CMEA) Japanese Standard Specification JIS Z 8801 (metric opening) DIN 4188 Standard of Testing Sieves (F.R.O.) s, AFNOR NP X-SO1 French Standard *) OOST 3584 U.S.S.R. Standard 1' ASTM E 161-70 US-American Standard Specification (metric opening) ASTM E 11-70 US-Amencan Standard Specacalion (Sieve No.)

163

7.2. The Surface Area

Originally, to achieve good separations, the specific surface area was supposed to amount to at least 3 mVg. However, subsequent investigations showed that solid supports with a specific surface area of down to 0.5 m2/g are sufficiently effective and, additionally, exhibit lower undesirable adsorptivity. Generally, the application of supports with a specific surface area of >5 m2/g is disadvantageous, as certain polar sample constituents might be adsorbed and the adsorption isotherm would be non-linear, and hence unsymmetrical peak shapes and retention dependences on the amount of the sample would result.-Nevertheless, when used in the right way, increasing the specific surface area would give rise to shorter analysis times and high resolution even on short columns. According to Guillemin [660], the adjusted retention time to a first approximation, is proportional to the product of column length and specific surface area:

where

L

= column length,

u

= linear

velocity of the mobile phase,

K

= partition coefficient, S = specific surface area of

the support, urn= volume of the mobile phase, m,=weight of the support in the column, d, = film thickness of the liquid stationary phase. Hence it follows that it is possible to obtain identical adjusted retention times or separations, of superimposable peaks by balancing S and L in order to keep constant the product S - L. This holds true but can be utilized only for the analysis of non-polar or very weakly polar samples on polar stationary liquids. Similar results were obtained by Huber and Eisenbeiss [661]. Hence, rapid analyses are achievable for chlorinated hydrocarbons on, for example, Spherosil of 200 m2/g (d,, 25-4Opm) coated with 14% (),r-oxydipropionitrile in a 15 cm micropacked column): vinyl chloride, methylene chloride, benzene, 1,2-dichloroethane and 1,1,2-trichloroethane could be well separated within 65 s [660]. As the surface area of the most support materials is batch-dependent, it is of advantage to measure this parameter. This can be done by means of the well known BET method [662] or by a dynamic procedure [663]. In this manner the total surface area, composed of the inner and the outer components, is included. The outer surface area is formed from the walls of the interstices between the single particles, whereas the inner surface includes the walls of the visible and finer pores down to micropores of molecular dimensions. Particularly for surfaces with OH groups, as they are present on the usual solid supports of the diatomite type, a method has been developed that is based on the determination of the area of a monomolecular layer of the liquid stationary phase by inverse gas chromatography [664]. It is applicable to non-silylated supports exhibiting macropores (pore widths >50 nm)or/and mesopores (pore widths 2-50 nm). The pore size and its distribution are important parameters of a solid support, as they strongly influence both the efficiency and adsorption/desorption phenomena. Generally, supports exhibiting a large fraction of pores with radii between 0.2 and 2 pm have proved to be especially favourable. When coated, a large portion of the liquid phase can be taken up by this fine pore structure, and only a thin film will cover the residual surface area. The material appears to be dry, and the efficiency is sufficient. The latter deteriorates if pores with radii >1.5 pm predominate or if the amount of liquid phase is too large, so that the larger pores are also filled. These large liquid phase “pools”, owing to their depth, might have a smaller surface area/volume ratio than the narrower pores, hence increasing the holdup of the solute because of the longer diffusion length. Consequently, peak broadening oc-

164

7.The Solid Support

curs and the efficiency diminishes. Solid supports with a large-pore surface ought to be impregnated only weakly. On the other hand, a fine-pore material such as silica gel is hardly suitable as a solid support, as its deep and too narrow pores (pore radii 1-10 nm) fill up with the liquid phase, and the surface areahohme ratio is likewise so small that the mass exchange is delayed and the efficiency is decreased. The pore size of solids can be rapidly determined according to Hal& and Martin [665]. More recently, the availability of automated instrumentation has improved the state-of-theart and the pore distribution analysis for solids having capillaries of various types can be accomplished by gas adsorption measurements [666].

7.3.

Activity of the Original and of the Coated Solid Support

Ideally, the support, the function of which is to hold the stationary liquid phase on its surface as a thin, uniformly distributed film, should be inert. However, the conditions of “pure” gas liquid chromatography are often not implemented in practice. The non-inert behaviour of solid supports is reflected in their chemical activity and in adsorption effects. Various solid supports showed isomerization of terpenes, allenes and dienes, cis-trans isomerization of fatty acid methyl esters, chain isomerization of alkylbenzenes, dehydration of alcohols, conversion of benzoin into deoxybenzoin and benzil; and decomposition of amino acid derivatives, alkaloids, acid halides, chlorosilanes and aminosilanes. However, even liquid phases are exposed to chemical alterations at elevated temperatures when applied to non-inert supports, especially through dehydration of hydroxy-containing phases and catalytic autoxidation of hydrocarbons, phthalates and sebacates, ethers and polyethers and depolymerization reactions of organosiloxanes caused by the solid support. This can sometimes already be observed at a temperature of 150°C if the carrier gas contains traces of oxygen. Such chemical reactions depend on the surface area and the pH of the solid support, but especially on the presence of highly active catalytic sites (B, Al, Fe, Ca, Mg). Especially strongly acidic Lewis centres (B and Al) can act as acceptors of free electron pairs against donors of free electron pairs (-NH2, >NH, &O, -CN, -NO2, unsatu-

I

rated hydrocarbons) and may release coordination forces. Al and Fe atoms with an OH group may even cause chelate formation, provided that the solute contains two or more OH or NH2 groups spaced 2-3 carbon atoms apart (glycols, amino alcohols, etc.). This behaviour is not surprising, because the supports based on Kieselguhr, which are the materials applied most frequently, usually contain some iron and aluminium oxide (see Table 3 9 , in addition to further mineral constituents. The presence of these active adsorption centres with different activities brings about an energetic heterogeneity of the surface, which is the main reason for a non-linear adsorption isotherm. The second type of active centres present on silicic surfaces consist of silanol groups, adsorbed water and siloxane groups, which have been discussed in detail in Chapters 5 and 6. From the theoretical point of view, any surface has a certain adsorption potential. Hence it is impossible to develop a completely inert solid support. Nevertheless, adsorption on the support ought to be reduced to the minimum and the adsorption isotherm should be linear. It should be noted, that for, e.g., the reproducibility of retention data, a non-linear adsorption isotherm is more dangerous than the occurrence of any adsorption with a linear isotherm [668].For example, the retention times for polar sample-non-polar liquid phase systems depend on the sample size, relative retentions of pairs of solutes of different polarity change with variations in the impregnation rate of the stationary phase, and peaks of polar com-

7.3. Activity of the Original and of the Coated Solid Support

165

pounds show severe asymmetry (tailing). As the sample size or concentration is reduced, the retention time of the peak is increased. Such phenomena are detrimental to column performance and efficiency, quantitative analysis, determination of physico-chemical data, identification purposes and especially trace analysis. They will be appreciable if the liquid phase coated on such supports is non-polar, and may involve adsorption at both the support-gas and support-liquid interfaces. Data obtained on columns with a liquid loading of less than 5% are likely to show unrealistic retention times owing to solute-support interactions, if the support has not previously been deactivated [669]. In addition to the adsorption on the gas-solid interface, the adsorption on the gas-liquid interface has also to be taken into account when applying low impregnation rates. For example, with Chromosorb W HP (one of the efficient and inert supports in the Chromosorb series), the specific retention volume of a given sample at a given temperature decreases as the loading of the nematic liquid crystal phase BMBT [N,N’-bis(p-methoxybenzylidene)-a,a’-di-p-toluidine)increases to 2,5%, beyond which remains almost constant. This effect can be traced back to differences in the arrangement of the liquid phase. With low percentage loadings only surface films are formed, whereas at higher loadings the pores of the support are filled with bulk liquid, and the retention process is predominantly determined by the solubility of the probe in the bulk liquid [692]. When stationary liquid phases are used with diatomite supports, there may remain in the support surface uncoated cavities separated from the gas phase, causing undesired adsorptioddesorption processes with certain sample components. In addition to the bare gas-solid interface, adsorption may also occur at the support surface which is separated from the gaseous medium by a layer of liquid [669]. Further, when using a stationary liquid phase and solutes that can interact strongly with active adsorption centres, processes of adsorption exchange or competitive adsorption have to be taken into account, resulting in asymmetric peaks for small samples [670]. In order to determine and to take into consideration the adsorption part of the retention time, a method for calculating the non-linearity of the adsorption isotherm has been proposed by Korol et al. [668], using the empirical equation

VN = adjusted retention volume, h, = peak height, A l , B = constants, and where the slope of the VN vs. l/log hq plots characterizes the relative non-linearity of the adsorption isotherm. As specific interactions (orientation, hydrogen bonds, complex formation, etc.), unlike dispersion interactions, decrease rapidly with increasing temperature, the adsorption isotherm becomes more linear than at lower temperatures, and within a small column temperature range the relationship between the A values and column temperature can be approximated by a descending straight line [668]. Substances especially susceptible to tailing are (in decreasing order) polyamines, glycols, fatty acids, amines, alcohols, esters and ethers, and tailing decreases with increasing molecular weight within a homologous series. When polar phases are used, support effects on retention are not as significant as with nonpolar stationary liquids, provided that the liquid loading is sufficient to cover the support surface and that the support is wettable by the phase concerned. Another attempt to quantify support effects consists in measuring the retention data of part of the Rohrschneider or McReynolds probes or of other polar compounds on the uncoated and the coated support with low and high liquid loadings and calculating both the specific retention volumes or retention indices and the Rohrschneider/McReynolds constants [669, 671, 6781.

166

I. The Solid Support

There have been further proposals for determining the undesirable activity of the support. Generally, easily decomposable compounds are analysed on the impregnated support, the impregnation rate being within the usual range for classical packed columns (5-20%). In addition to fatty acid methyl esters [672] and alcohols and cholesterol [673], N,N-bis trimethylsilyl lysine trimethylsilyl ester was taken as an indicating compound, and the relative molar response relative to the internal standard n-octadecane was defined to be the decomposition index of TMS-lysine as a measure of residual active sites of the column packing [674] peak area TMS-lysine/ n (TMS-lysine) RMR = peak area octadecanel n (octadecane) where

RMR = relative molar response n = number of moles of the respective compound in the injected mixture. It has been found, however, that there are even more sensitive indicators of residual activity, for example sterically unhindered trimethylsilyl esters of higher fatty acids (behenic acid, stearic acid, myristic acid), and an actual activity has been proposed, which can be measured by a temperature programmed analysis of a mixture of a fatty acid TMS ester together with the adjacent well separated n-alkane (e.g., behenic acid TMS ester + octacosane) [675]:

where yfa.tms,r = actual activity of the chromatographic column for the fatty acid trimethyl-

silyl esters, i.e., at the moment of the test, Aakz = peak area of the adjacent n-alkane, Afa.ms,z=peak area of the fatty acid trimethylsilyl ester, Z = number of carbon atoms of the alkane and of the fatty acid trimethylsilyl ester, respectively. This test is applicable to all supports coated with thermally stable liquid phases, unless the supports have been coated with inorganic acids or bases or the liquid phase exhibits destinct amounts of OH or NH/NH2 groups. In addition to these indirect measurements for characterizing the activity or heterogeneity of surfaces, direct surface analyses have also been carried out. Electron spectroscopy for chemical analysis (ESCA)provides a simple method for the evaluation of the relative number of atoms in the surface region from 1-5 nm depth and may be used to determine elemental changes in the surface of solid supports occurring by deactivation treatments, i.e., by elimination of the troublesome elements (Al,B, Fe, Ca, Mg) from the surface [676]. Another direct method is the examination of the outer and inner surface areas of microporous diatomite support particles coated with a stationary liquid phase by scanning electron microscopy [677]. When investigating the liquid distribution pattern of non-polar squalane and polar polyethylene glycol 400 on surfaces featuring hydrophilic (untreated diatomaceous product) and hydrophobic properties (diatomaceous product treated with dimethyldichlorosilane),it could be shown that on the hydrophilic surface the polar phase is distributed in a continuous thin layer over the inner and outer surfaces of the sorbent grain. In contrast, on the hydrophobic surface the polar phase is mainly situated at the outer surface in the form of a thick, non-uniform layer, and has penetrated inside to only a small extent, and hence the liquid phase distribution along the diameter of the sorbent grain is non-uniform and can be considered to be “microdrop-like”on this poorly wettable (in the case of a polar stationary liquid phase) surface [677]. On the other hand, however, a non-polar phase (squalane) is uniformly distributed over the inner and outer surfaces of the hydrophobic support. The solid support investigated by Berez-

1.4. Diatomite Supports

167

kin et al. [677] was Chromaton N, but other diatomaceous support materials, e.g., Chromosorb G [679], and synthetic silicic products such as Volaspher [680] have also been examined, coated and uncoated, by scanning electron microscopy, revealing both the pore structure of the support and the distribution of the liquid phase on its surface. Summarizing, we can state that the support material should exhibit the following properties:

Particle size: 30-200 pm, but narrowly distributed, e.g., 100 k 15 pm, the lower values (30-50 pm) predominantly for micropacked columns. Particle shape: Irregular or (better) spherical. Pore size: Pores with radii between 0.2 and 2 pm should dominate. Surface: The specific surface area should be in the range 0.5-3 m2/g. Troublesome elements (B, Al, Fe, Ca, Mg) should be absent and, only Si-0-Si bridges, SiOH groups or converted SiOH groups (silylated to form Si-0-Six linkages) should be present. Wettability: The solid support should be easily wettable, i.e., the surface should be hydrophilic when it is to be coated with a polar liquid phase and should be hydrophobic when using a non-polar liquid phase. Stability: The material should be rigid such that in subsequent treatments (sieving, coating, packing) any formation of fine abrasion particles can be avoided. The material should be thermally stable up to at least 300°C. Impregnation rate: The material should permit both low (2-3%,w/w) and higher impregnation rates (20%,w/w). Inertness: The material should be as inert as possible in order to avoid any contribution to the retention by the solid support.

As especially an overall inertness is not easily achievable, special deactivation procedures have to be carried out in order to reduce or remove the residual activity. As the most suitable procedure and reagent or modifier are determined by the nature of the active sites and hence by the composition and origin of the solid support material, the deactivation steps will be described below when discussing the support materials concerned. For completeness it should be mentioned, that in exceptional cases active supports are also used on purpose. One must be aware, however, of the mentioned disadvantages and of the support’s contribution to the retention, which changes considerably with changing impregnation rate.

7.4.

Diatomite Supports

Most chromatographic supports have been prepared hitherto from diatomite, also called diatomaceous earth, diatomaceous silica or Kieselguhr. This material has a biological origin. It is composed of the skeletons of microscopic single-celled algae (diatomaceae), which have formed huge fossil deposits in the deeps of former waters in various parts of the world (Canada, U.S.S.R.,U.S.A., North Africa, Czechoslovakia, Germany), and is nowadays mined commercially to be processed for the production of filter aids, fillers and firebrick. According to Offenstein [681], about 10000 species may have contributed to the formation of the material, some of which have lived in fresh water and others in salt water (marine form). The diatomaceous earth consists mainly of silica (about 90%), the remainder up to 100% being metal oxides and water, apparently originating from marine sediments (clays). These mineral impurities (oxides of A,Fe, Mg, Ca, Na, K and others) are the main cause of the unsatisfactory inertness of the supports made of this material. The heterogeneity in both the morphology and the surface chemistry of the fine particles can be made visible by ESCA and scanning electron microscopy, as we have seen in Section 7.3. Fig. 13 shows a mi-

168

7.The Solid Support

Fig. 13. Type I Kieselguhr solid support. Scanning electron micrograph by Merck, enlargement 1:lOOO (after Ringsdorf [696]) (by courtesy of GIT-Verlag Damstadt, F.R.G.) crophotograph (1:lOOO) of rigid sintered Kieselguhr (1:1000), and one can recognize that the cell walls are perforated with many small pores (pore diameter in the pm range), responsible for the geometrically advantageous support properties. Two different treatments are commonly used to prepare the material for application as a gas chromatographic support. The first, analogous to the preparation of firebrick consists in calcining small chunks, mixed with clay binder, at temperatures above 900°C. The metal oxides, particularly iron oxide, form complex oxides giving the material a pinkish red colour. This support is harder and more compact than the original Kieselguhr. Its surface area is approximately 4 m2/g (1.9 m2/ml) and its packed density is 0.47 g/cm3. The maximum recommended loading without becoming too sticky to flow freely is 30wt-%. It is referred to as a type I support [681] and provides excellent efficiencies in the analysis of hydrocarbons and low polarity organic compounds, whereas it cannot be used for analysing polar samples. In the 1950s these type I supports were made from the Johns-Manville Sil-0-Cel C-22 firebrick under the name of Chromosorb P (€' = pink), by Coast Engineering Company under the name C-22 Super Support, and by Applied Science Laboratories under the name Gas Chrom R [681]. The type I supports commercially available formerly or at present are listed in Tables 33 and later in Table 35. In the second treatment, calcination of diatomaceous earth occurs, after mixing it with a small amount of sodium carbonate as a flux, in a rotary kiln above 900°C. The flux causes the particles to fuse together at the points of contact via sodium silicate glass, and the silica is partially converted into cristobalite. The resulting product is very friable and light-weight. It has a specific surface area of approximately 1m2/g, a packed density of 0.3 g/cm3 and can be loaded up to 25 wt-%. Owing to the conversion of iron oxide into a colourless complex of sodium iron silicate, the product is white. This so-called type I1 support is much less adsorptive than a type I support and can therefore, in contrast to the latter, be used for the analysis also of polar compounds. However, a restriction must be pointed out (which will be discussed in more detail later in this chapter), namely that neither type I1 or type I supports give satisfactory analyses of polar compounds without further treatment of the support. The material is very friable and must not be handled roughly during sieving, impregnation and column pack-

169

1.4. Diatomite Supports

Table 33. Type I: Diatomaceous Earth Supports Commercial Name

Roducer/Supplicr

Chromosorb P

Johns-Manville London, U.K. and New York, U.S.A. Johns-Manville London, U.K. and New York, U.S.A. Johns-Manville London, U.K. and New York, U.S.A. Johns-Manville London, U.K. and New York, U.S.A. Applied Science Laboratories, State College, PA., U.S.A. Coast Engineering Company, Gardena, U.S.A. Sterchamol-Werke Diisseldorf, F.R.G. Berlin-Chemie, Berlin-Adlershof, F.R.G. Rybitvi, Czechoslovakia Facts & Methods Scientific New Castle, Delaware, U.S.A. Phase Separation Ltd., Rock Ferry, U.K. JJ’s, U.K. Analytical Engineering Lab., Inc., Hamden, U.S.A. Analytical Engineering Lab., Inc., Hamden, U.S.A. Burrel Corp., Pittsburgh, U.S.A. Max & Baker, Ltd., U.K. Field Instruments Co., Ltd., U.K. Shimadzu Seisakusho Ltd., Kyoto, Japan Sojusreaktiv, U.S.S.R. Sojusreaktiv, U.S.S.R. Sojusreaktiv, U.S.S.R. Lachema, Bmo, Czechoslovakia Chemapol, Czechoslovakia Reanal, Budapest, Hungary

Chromosorb A Sil-0-Cel C 22 firebrick Chromosorb R Gas Chrom R C-22 Super Support S terchamol Porolith Rysorb Diatoport P Phase Sep HC, P Diatomite S Anakrom P, C 22, C 22 A Ana Prep. P Kromat FB Diatomaceous brick dust Hylon P Shimalite A, B, C, D Spherochrom 2,3 Dinochrom N INS 600 Chezasorb Porovina MH-1,2,3

ing, or it will be abraded and finer particles will be produced, diminishing the column efficiency. Type I1 supports first became available in the 1950s under the name Celite 545, and Johns-Manville produced Chromosorb W (W for white) from marine diatomite. In the 1960s the same manufacturer developed two further Chromosorb types, G and A. Chromosorb G was intended to exhibit a less adsorptive surface than a type1 support but to maintain the strength and good handling properties. Chromosorb A was designed to have the high stationary phase capacity of a type I support but with a lower adsorptivity and to be applied in preparative-scale chromatography [681].In Table 34, formerly and currently available type I1 supports are listed. On comparing type I and type I1 supports with regard to the surface area per unit bulk volume, which for comparison of the adsorptive nature of the supports is more relevant than the surface area per gram, we find that type I supports have about a six times greater surface area per unit volume than type I1 supports, which, because of the correlation between specific surface area and adsorptive properties, explains this negative property of type I supports. A further critical factor has to be considered, namely that the packing densities of the different supports vary considerably. Therefore, when choosing a certain impregnation rate, heavier supports, such as Chromosorb G (packed density 0.58g/cm3), will be coated with

170

7. The Solid Support

greater amount of liquid stationary phase per unit surface area and also per unit column packing than lighter supports, although the impregnation rates are the same. For example 10%on a type I and a type I1 support, both packed into columns of identical dimensions, will result in twice as much liquid phase in the type1 than in the type11 column. As a conseTable 34. Type 11: Diatomaceous Earth Supports Commercial Name

Pmduwr/Supplier

Chromosorb W, G, A, 750, C-48 560, Celite, Celite 535, Celite 545 Embacel Kieselguhr Gas Chrom A, CL, P, S, Z, Q Anakrom U, A, AB, ABS, AS, 545,545 A, Q,SD Ana Prep U, A, ABS Kieselguhr Merck Nr. 9697,9748-49,9751-53 Kieselguhr SK Diaphorit Dicalite Diatoport W

Johns-Manville London, U.K. and New York, U.S.A. May 8c Baker, Dagenham, U.K. Applied Science Labs., State College, PA., U.S.A. Analytical Engineering Laboratories, Hamden, CT., U.S.A. Merck, Darmstadt, F.R.G. Calofrig, Borovany, Czechoslovakia Berlin-Chemie, Berlin-Adlenhof, F.R.G. Dicalite, U.S.A. Fact & Methods Scientific, New Castle, DE., U.S.A. Phase Separations, U.K.

Phase Sep CL 0,HC, N, ABS, Universal B, C, Phase Chrom Q, Phase Prep A Spherochrom I Porochrom I, 11,111 Dinochrom Chromaton N, N Super Inerton, -super Varioport 30 Supelcoport, HD Diatomite C, CQ, M Gas Pak FS Kromat CE Supasorb Shimalite W

Sojusreaktiv, U.S.S.R. Sojusreaktiv, U.S.S.R. Sojusreaktiv, U.S.S.R. Lachema, Bmo, Czechoslovakia Lachema, Bmo, Czechoslovakia Varian Palo Alto CA, U.S.A. Supelco, Bellefonte, PA., U.S.A. JJ’s Chromatography, King’s Lynm, Norfolk, U.K. Regis Chemical, Chicago, IL., U.S.A. Burrell, Pittsburgh, PA., U.S.A. BDH Poole U.K. Shimadzu Seisakusho Ltd.,Kyoto, Japan

quence, the retention time in the former column will be about double that in the latter [681]. Hence the surface area and the packing density are important values for the choice of a proper impregnation rate. In Section 7.3., we examined the heterogeneity of the surface of diatomite supports. The adsorptive sites (mineral impurities, silanol groups and adsorbed water) are responsible for, e.g., degradation reactions and for tailing of polar compounds. Before use, the material therefore has to be treated so as to eliminate or at least to reduce the undesirable activity. Of the numerous proposed treatments, such as washing with acid and/or alkali, saturation of the active sites with an active agent (amines, amino alcohols, formamide, small amounts of a polar liquid phase, saturation of the carrier gas with a polar substance, frequent sample injection until all active sites are saturated, etc.), covering the active sites with an inert substance (detergent “Tide”, pyrolysed organic resins to form carbon, polytetrafluoroethylene, increased liquid loading etc.) and silylation of the silanol groups, only a few have proved effective and durable and have been applied routinely by the manufacturers of gas chromatographic supports. Efficient deactivation methods are removal of the mineral impurities from the surface of the support by acid or alkali washing, silylation of the surface silanol groups,

7.4. Diatomite Supports

171

coating of the support with a thin, non-extractable layer and deposition of pyrocarbon on the diatomite surface via a suitable organic polymer. 7.4.1.

Acid washing

Acid washing with hydrochloric acid is intended to leach iron, aluminium, etc., from the support surface in order to remove the active sites based on these impurities. It was claimed that Kieselguhr supports of high quality can be obtained by pretreating the raw support with boiling concentrated hydrochloric acid prior to silylation [682].By acid washing of commercial Chromosorb P at room temperature which can be considered to be less drastic, some of the iron, aluminium, calcium, etc., is removed from its surface [683],and the support becomes less active although the overall reduction of the metal content is small. Such products, in this example Chromosorb P AW (AW = acid-washed), remain pink, unless the purification has been carried out more drastically in form of, e.g., a preliminary wash with 6 M hydrochloric acid in a Soxhlet apparatus followed by the main clean-up with gaseous hydrogen chloride at ca. 850°C and subsequent washes with 6 M hydrochloric acid and distilled water [683]. This procedure has been shown to remove about 98% of the iron and lower percentages of other elements and to give a white material [684]. Chromosorb W, on the other hand, as a type I1 support, from the beginning exhibits less oxidic impurities on the surface. Hence it is not stupendous that a decrease by a similar treatment is less distinct. It was found that a simple treatment with distilled water to neutrality is sufficient to remove most of the iron and much of the sodium and aluminium from the surface. The elemental changes in the surface of Chromosorb W could be watched by means of electron spectroscopy for chemical analysis (ESCA), which evaluates the relative number of atoms in the surface region at depths from 1 to 5 nm with sufficient accuracy [676].Soaking in 3 M HC1 for 2 h with periodic stirring, collecting by filtration and washing to neutrality with distilled water resulted in a decrease in the aluminium content by 50%, in the sodium content by 80% and in nearly complete removal of iron [676]. A comparison of acid treatment with alkali-treatment shows that mineral contamination can be better removed by the former procedure. Moreover, for subsequent silylation prior to acid treatment is especially recommendable. Procedure 1 (for type I supports) (according to Aue et al. [684]). The support is extracted with 6~ HC1 in a Soxhlet apparatus. The main purification is effected at 850°C with gaseous HC1. Subsequently, the product is washed with 6 M HC1 and finally with distilled water to neutrality. Procedure 2 (for type I1 supports) (according to Smith et al. [6741) 100 g amount of the solid support, free from finer particles, is suspended in 250 ml of concentrated hydrochloric acid and allowed to stand for a few minutes. The acid is poured off and replaced with 150 ml of fresh concentrated hydrochloric acid. After stirring the suspension is allowed to stand overnight. The sediment is then shaken rigorously and, after the support has sedimented again, the supernatant liquid is decanted. The same procedure is repeated four times, in each instance using distilled water. The material is filtered on a Buchner funnel, washed several times with distilled water to neutrality, aspirated to dryness and kiln-dried Overnight at 150°C.

7.4.2.

Alkali treatment

Acid washing is sometimes followed by alkali washing. The latter, however, does not appear to improve the purity of the support surface. On the contrary, it seems to deposit iron selec-

172

I. The Solid Support

tively on the surface [676].On the other hand, alkali washing and even alkali deposition (e.g., KOH) is a good pre-treatment for supports that are to be used in the separation of basic compounds, such as amines, pyridines, quinolines, guanidines, melamines and epoxides. However, it must be pointed out that alkali remaining on the support may adsorb or react with various sample compounds. Commercially available alkali-washed supports are designated by BW or B.

7.4.3.

Silylation

Deactivation of active centres due to the presence of silanol groups is best accomplished through silylation where the hydrogen of the free SiOH group is replaced with organosilyl groups. As we have already discussed this topic in detail (see Section 6.2.3.), we shall deal only with the most frequently applied reactions of this type for deactivating diatomite surfaces. Two reagents predominate at present: hexamethyldisilazane and dimethyldichlorosilane. The first silylation reagent will mask silanol groups by the reaction 2 SiOH (surface silanols) + [(CH3)3Si]2NH+ 2 SiOSi(CH3), + NH3.

To ensure sufficient silylation with hexamethyldisilazane, the treatment demands high ternperatures. However on heating the sample obove 150"C, the concentration of surface hydroxyl groups may decrease, and the siloxane formed does not react with the silazane but would have a high adsorption ability. On the other hand, the ammonia formed according to the above reaction will react with surface siloxane groups at high temperatures to form SiOH and SiNHz, which in turn may be silylated. Hence the unfavourable effect of siloxane formation by heat may be compensated for. Procedure 1 (after Takuyama [685]) About 900g or less of the diatomite support, pre-treated by an appropriate acid washing procedure and well dried, are packed into the reactor tube (67mm I.D.) of a silylation reactor (Fig. 14) up to the level of 87 cm high. The Pyrex glass tube has a length of 1000 mm and is arranged vertically. The lowest part of the tube (20 mm) contains glass Raschig rings with a layer of glass-wool (10 mm) above. The reactor is situated in an aluminium sheath of length 1020 mm and thickness 0.5 mm. Additional heaters prevent cooling of the ends. Hexamethyldisilazane vapour is supplied from a modified 1-1 claisen flask in an oil-bath maintained between 135 and 140°C. The reactor is connected at its top to the vapour source and nitrogen reservoir via a three-way stopcock. Emuent hexamethyldisilazane, together with the silylation reaction product (hexamethyldisiloxane) and the contaminants, pass from the bottom of the tube to a receiving flask. By means of the main heater the temperature of reactor is increased from room temperature to 210°C while passing dry nitrogen through the reactor to remove adsorbed water. The nitrogen flow is then switched to the hexamethyldisilazane vaporizer. The oil-bath surrounding the vaporizer is then rapidly heated to a pre-set temperature of 134°C. When the temperature has reached 100°C, the nitrogen flow is stopped. In order to prevent condensation in the exposed reactor head, its temperature is kept above 150°C during the supply of the silylating agent. Its flow-rate should be high (400g/h) in the initial stage chosen. After more than 5 h, the nitrogen flow is switched to the initial path and residual hexamethyldisilazane vapour in the reactor is purged. The silazane collected in the receiver can be re-used. The reactor temperature is decreased to room temperature, and the silylated diatomite does not require any further deactivation before coating.

173

7.4. Diatomite Supports

neuiw ncyuiuI b c u

by 6 and Controller

Silicone O i l W

HMOS-Vaporizer Main Heater Regulated by C and Controller

r:: Additional Heater Controlled by Slidock

Glass Raschig Rings Part Additional Heater Controlled by SLidack

Aluminium Pipe

HMOs Receiving Flask Water Seal Fig. 14. Apparatus for the preparation of silylated Celite (after 7'uka.varna [6851). 1-6 thermocouples connected to temperature recorders; 4, 6 connected to temperature regulators; c thermocouple connected to a regulator to control the reactor temperature (dimensions in cm)

Procedure 2 (after Welsch and Frank [5871) This high-temperature silylation is less expensive and tedious, and seems, according to the investigations of the author [686],to be efficient and is especially appropriate for silylating smaller amounts of solid support. Originally, it was proposed only for deactivating glass capillaries and for chemically modifying silica gel for HPLC reversed phases [587]. The acidwashed solid support, vacuum dried at 180°C, is placed on a frit in an ampoule. Hexamethyldisilazane, the amount of which is governed by the amount of dry support in the ampoule (about 2-3% of the diatomite) is added and the ampoule is flushed with dry nitrogen, sealed, placed in a thermostat and heated at 4Wmin to the final temperature of 35OoC, which is maintained for 15 h. After cooling, the ampoule is cautiously (excess pressure!) opened, and the deactivated support is washed several times with methanol and finally dried. The second organosilicon compound, intended to provide a dense monolayer, dimethyldichlorosilane, has become the preferred reagent, especially by manufacturers of support materials. To a certain extent both chlorine atoms will react with vicinal silanols from the surface:

174

7.The Solid Support

I -Si

-OH

I

CI

+

0

I

-5 -OH I

CH3

‘d

/ \ CI CH3

-

I -Si -0 C,S H,i3

I 0

I -Si-0

/ \

I

CH3

In the most instances, however, two vicinal group may not be available and reactive chlorines may atoms remain after the treatment: CI

I

+

-SiOH

I

CH3

‘5’

/ \

CI

-

CI CH3

I

I/

- 5i -0-Si

CH3

\

I

CH3

Therefore, in a subsequent reaction step with methanol, the chlorine has to be replaced with a methoxy group by means of anhydrous methanol: I CI I -Si -0 -Si -CH3 I

I

+

CH30H

-

I

y

3

-5-0 -Si -CH3 + HCL

I

I

CH3

CH3

In addition to this supposed esterification course, a hrther mechanism was found [687]: y 3 CH3 -Si -0-

I

0

Cl

CH3

I

Si -CH3

I 0

I

CH -5 - CH3

I

OH

CyOH

0

7% 7% CHS-SiI

0- Si-CH3

I

This would mean that the Si-O-Si(CH3)2C1 bond formed during the reaction of surface silanol with dimethyldichlorosilane is cleaved, and the group -OSi(CH3)2C1 is substituted by the much less bulky methoxy groups, hence exposing the adjacent silanol group, which previously was shielded, giving rise to undesirable adsorption effects. Further the methoxy groups both at the surface silicon atoms and at the substituting groups, (CH3)2(CH30)Si-O-, are sensitive to hydrolysis, and the presence of water might form or re-convert silanol groups. It should be taken into account that residual adsorbed water present on the diatomite surface may cause additional hydrolysis/condensation reactions according to reaction (0 in Section 6.2.3, resulting in a linear polymer linked to the surface, which no longer has the properties of an inert solid support that does not contribute to the retention. This polymer may certainly show a deactivating effect by shielding residual SiOH groups to some extent; how-

7.4. Diatomite Supports

175

ever, the retention on such a support, impregnated with a liquid phase other than polydimethylsiloxane, will be composed of the contributions of both the liquid phase and the chemically bonded organosilicon groups and will no longer be uniform and easily calculable. The silylation has been carried out at room temperature in nitrogen as carrier [688] and in the liquid phase [674, 689, 6901. Three procedures are described below. Procedure 1 (after Sparagana et al. [690]) The acid-washed and well dried support (50 g) is allowed to react for 10 min in 250 ml of a 5% (w/w) solution of dimethyldichlorosilane in toluene. The liquid is removed and the material is rinsed with toluene, suspended in methanol for 5 min, the liquid is poured off and the product is rinsed with methanol and dried at 110°C. Procedure 2 (after Smith et al. [674]) In this more efficient procedure, l o g of the acid-washed and well dried support are suspended in 50 ml of a 5% (w/w) solution of dimethyldichlorosilane in toluene. The suspension is heated with stirring and the dimethyldichlorosilane/toluene is refluxed for about 30 min, then allowed to cool, the liquid is removed and the material is washed twice with toluene; the support should always be covered with solvent. In the same manner, it is washed twice with fresh, dried methanol and dried. The support is dried overnight in a glass dish in an explosion-proof kiln at 120°C. Procedure 3 (after Chuiko et al. [693] A 100-g amount of diatomite is calcined for 2 h at 300°C before evacuation for 1h at the same temperature, then reacted with 10 ml of a octamethylcyclotetrasiloxane(D4) - methyldichlorosilane (4:l) at 350°C for 1h. Excess of reagent is removed by evacuation for 1h at 250°C. Finally, the product is treated with 5 ml of methanol for 30 min at 250°C. The above silylation procedures, together with a preliminary acid wash, provide distinctly less adsorptive diatomite supports. It should be borne in mind, however, that silylated surfaces are hydrophobic rather than hydrophilic as they are normally when non-silylated. Hence, polar liquid phases such as glycerol, diglycerol, polyesters and silicones with a high content of cyano groups, such as OV-275, do not spread out on such hydrophobic surfaces. Therefore a uniform coating is not achievable, and the efficiency of these column packings is insufficient. Thus, for strongly polar phases the selection of an inert, acid washed, but nonsilylated support is advisable. The thermal stability of silylated supports does not exceed about 325°C. Further, at high temperatures, water and free acids, when injected on to these deactivated materials, will remove the surface silyl ether groups and regenerate the active silanol groups. In Table 35 several commercially available or formerly available diatomite supports and their properties are listed. In order to make the choice of an appropriate support easier, the following can be recommended. For the analysis of hydrocarbons any residual activity is not too critical. All types of liquid stationary phases can be coated on type I supports, the loadings ranging between 5 and 30%. An acid washed type is to be preferred, and DMCS treatment and high phase loadings will further reduce any troublesome activity for the analysis of weakly polar compounds, when applying non-polar stationary phases. With polar stationary phases, the supports should be only acid washed and not silylated. Pink types exhibiting hard surfaces provide the most efficient packings for hydrocarbons and compounds of low polarity. Recommended supports for this application are Chromosorb P AW, Anakrom A and Gas-Chrom A. For the analysis of medium and highly polar compounds and especially those which are sensitive (pesticides, steroids, drugs, bile acids, etc.), only top-quality support types should be used. This includes not only minimum batch-to-batch variation and elimination fines, but also maximum inertness and low fragility. Recommended supports are Gas-Chrom Q, Supelcoport, Chromosorb 750, Chromosorb GHP, Chromosorb WHP and Anakrom Q. However,

176

7. The Solid Support

Table 35. Properties of Diatomite Supports Commercial Name

Chromosorb W Chromosorb 750 Chromosorb G Chromosorb A Chromosorb R-6470-1 Chromosorb P Gas-Chrom A Gas-Chrorn Z Gas-Chrom Q Supelcon AW Supe1coport Anakrom AS Anakrorn Q Anakrom A Anakrom C22

Rc-trulmcnt or Rc-treated Type& Remark#

AW*), AW-DMCS**), HP***) AW-DMCS AW, AW-DMCS, HP Fine particle diatomite (4 1-4 pu) for PLOT columns NAW****), AW untreated, AW AW-DMCS AW-DMCS (better quality) AW, AW-DMCS AW-DMCS AW-DMCS AW-DMCS AW NAW, AW (type C22 A), AW-DMCS (type CZ2 AS)

Embacel Celite 545 Celite C29 924 Phase Sep P INS-600 Sterchamol only 68% SiOz Johns-Manville C22 firebrick Chezasorb Chromaton N Inerton Spherochrom 1 Spherochrom 2,3 Porochrom-I Porochrom-I1 Rysorb BLK only 72% SiOz (21%&OJ) Porovina only 63% SiOz (33%MzOd Porolith only 60% SiO' (30%Mzod

-- -

Packed Donaity

[m'ld

Wmll

rnl

1

0.22-0.24

1-3.5

0.8 0.5 2.7 6

0.3 0.58 0.48

1-3

4 4

0.47 0.48

0.5-1

Maximum Loading

1%

WlW]

15 7 5

25

2

30 20 25

1.4

1.1 1.14 0.45 5 10 5.83 3-4.9 1.4-2.4 1 0.4-0.6 0.8-2.1 4-15 0.1-1.5 0.5-2 7 0.8-2.5

'1 AW acid washed AW-DMCS add WMhed Md dimCthfldkhlO~UMeIrMtCd '"1 HP hi@ P C I ~ O ~ ~(a Mbetter C C W d e AW-DMCS). ""1 NAW no8 acid w&ed ")

Pore Diameter

specific surtacc Area

0.3

1

0.26 1-3.5 1.5

0.6 0-2 0.6 0.67 0.24

25 30 30 30 30 30 30 25 25 30

7.5. Synthetic Silica-based Supports (Volaspher and Quartz)

177

the superiority of these types for the analysis of polar and sensitive compounds is only valid when comparing diatomite supports with each other. Synthetic materials such as Vohasper (see Section 7.9,which largely avoid chemical heterogeneity from the beginning and which nevertheless show excellent mechanical stability, are distinctly more advantageous regarding interness.

7.4.4.

Coating of the Support with a Non-extractable Layer

In order to eliminate or to reduce the disadvantageous influence of the surface silanol groups on the support, a polymer with hydrogen bonding properties, deposited on the surface as a non-extractable layer, has proved to be efficient. According to Daniewski and Aue [683, 6941, the acid washed and subsequently alkali-washed diatomite is washed with distilled water to neutrality and dried in vacuo at 110°C.The bonded layer is formed by coating the dry support with Carbowax 20M in refluxing n-hexadexane (b.p. 287,5"C).After cooling and decanting off the high-boiling solvent, the material is leached for 3 days with methanol near boiling temperature in a highly efficient continuous extraction apparatus and finally dried. Most of the silanol groups are blocked and a diatomite support treated in this manner exhibits, after the usual impregnation with a liquid stationary phase, distinctly fewer active sites and hence less tailing effects.

7.4.5.

Deactivation of Diatomite by Deposited Pyrocarbon

We have dealt in detail with the properties of carbon surfaces, which, provided that the surface is sufficiently homogenous, belong to the type I adsorbents and show only dispersion interaction and excellent chemical resistance. It was found that a pyrocarbon, when completely covering the diatomite surface, does not significantly reduce the surface area of the diatomite support, and alters and improves the surface nature distinctly such that even carboxylic acids, amines and alcohols can be well separated on diatomaceous earth supports impregnated subsequently in the usual way. Procedure: At room temperature, phenol-formaldehyde monomer is dissolved in ethanol to give a stock solution of known composition. The diatomite support is added to form an 8% (w/w) a suspension of diatomite in the phenol-formaldehyde solution. The solvent is removed in a rotary evaporator without heating and the material is then heated in an argon atmosphere to 1200°C.During this temperature increase, the pheno-formaldehyde passes through a mesophase until the pyrocarbon is formed and the diatomite discolours to black. By this treatment the particles are strengthened and are hence less subject to attrition in the subsequent impregnation and packing step.

7.5.

Synthetic Silica-based Supports (Volaspher and Quartz)

We have discussed in detail the properties of diatomites which, owing to their natural origin, show batcfi-to-batch variations and, owing to the heterogeneity of the particle surface, show undesirable activity phenomena. Merck first developed a synthetic solid support, which, whilst maintaining most of the advantageous properties of diatomite supports, does not exhibit their disadvantages. Thus, Merck prepared a spherical material of high mechanical strength (Fig. 15) and good flowability. Its uniform pore structure (Fig. 16) permits the liquid

178

7. The Solid Support

Fig. 15. Volaspher particle. Scanning electron micrograph by Merck, enlargement 1:1500 (after Ringsdorf [696]) (courtesy of GITVerlag, Darmstadt, F. R. G.)

Fig. 16. Volaspher, coated with 5% (w/w)Dexsil.

Scanning electron micrograph by Merck, enlargement 1:4000 (after Ringsdorf [696]) (courtesy of GITVerlag Darmstadt, F.R.G.)

phase to be coated on the synthetic support with a homogeneous film thickness. This property, together with the possibility of packing the column very densely, results in high numbers of theoretical plates. However, perhaps the main superiority of this material is its more suitable chemical composition. In contrast to Kieselguhr, Volaspher types consist of about 99%silicic acid, the dif€erence from 100%being, according to investigations in our laboratories (6951, in order of decreasing concentrations, oxides of Na, Ca, Ti, Al, Mg, Fe etc. whereas diatomite supports consist only of less than 95% silicic acid. The spherical particles of d , = 100 pm have an average pore diameter of 25 pm, an inner surface area of 0.9 m2/ml and a packing density of 0.5 g/ml[696].Some properties of Volaspher are given in Table 36.

7.5. Synthetic Silica-based Supports (Volaspher and Quartz)

179

Table 36. Properties of Volaspher Specific surface area Packing density Average pore diameter Normal impregnation rate Maximum column temperature

- 1 m2/g

-0.5 g/cm3

-5 pm 3-15% 350°C 380°C 400°C

(A2) (Al) (A4)

The low content of metal oxides results in a more homogeneous surface area compared with diatomite, and the undesirable activity will mainly be restricted to interactions with the silanol groups. This holds especially for Volaspher Al. Its residual activity does not noticeably change within the temperature range from room temperature up to about 370°C and it can be utilized for the analysis of aqueous solutions of organic compounds, as the water peak is eluted nearly symmetrically, and as frequent injections of aqueous samples do not impair the efficiency. Even amines can be analysed on this support without addition of KOH prior to the impregnation with the liquid phase. However, carboxylic acids are not eluted without tailing, unless a small amount of an acid that is non-volatile (e.g., phosphoric acid up to 150"C), has been added to this support. Volaspher A2 is a weakly acidic to neutral support, deactivated by means of a silanizing agent, probably dimethyldichlorosilane. It can be applied for the investigation of all neutral, medium polar and weakly acidic compounds. It is thermally stable up to 350°C for normal analysis and up to 300°C for trace analyses when applying an electron capture detector, as decomposition products of the deactivating agent may affect the baseline above 300°C. Volaspher A4 has a neutral to weakly basic nature. The deactivation, which has probably been carried with hexamethyldisilazane, permits a universal application. This type is the most inert Volaspher support and yields the lowest heights equivalent to a theoretical plate. The thermal stability ranges up to 400°C. In addition to non-problematic families such as hydrocarbons, esters and halocarbons, some problematic compounds, e.g. amines, can also be analysed on this support material. This holds good for other polar compounds, too, especially in trace analysis, with the exception of carboxylic acids. Fig. 17 shows the separation of nonderivatized aromatic amines on 8% polyethylene glycol 20 000 on Volaspher A4. There is one further point in favour of Volaspher. In order to elucidate the influence of the solid support on the chemical stability of poly(dimethylsiloxanes), Volaspher A1 (the most "polar" Volaspher type) and Chromosorb G (a relatively inert diatomite support) were each coated with 3% (w/w) of OV 101 in our laboratory [697]. Both the numbers of theoretical plates and the capacity factors with respect to n-octadecane at 220°C were determined both immediately after impregnation and after temperature programming the 1-m column packed with the stationary phase from 100 to 350°C at 4Wmin, with a 10min isothermal hold at 35OoC,repeated three times. Whereas both the number of theoretical plates and the capacity factor remained unchanged on Volaspher, the plate number of the column coated with OV 101 on Chromosorb diminished by 27%and the capacity factor by 10%.These differences indicate that on the diatomite the organosiloxane is decomposed and hence the amount of liquid phase decreased, probably owing to the catalytic influence of the metal oxides on the surface area of the Chromosorb. In contrast to the diatomite support, the surface area of the Volaspher, with its negligibly low metal oxide content, does not affect the thermal stability of the poly(dimethylsi1oxane) within the investigated temperature range and under the other conditions used (e.g., the application of oxygen-free nitrogen as the carrier gas). When comparing diatomite and Volaspher supports, we feel that the latter comes nearer to the properties of an ideal support material and will substantially replace the widespread di-

7.The Solid Support

180

aJ

.-9 C

(I,

a .-

a

42 L

8

-

Lo

I

I

0

2

I

1

I

4

6

8

I

I

I

Fig. 17. Separation of non-derivatized aromatic amines on 8% (w/w)polyethylene glycol 20 000 on Volaspher A4, 100-120 mesh Glass column, 2x11 X 2 mm I.D., temperature programme 100-200°C at 6 K.min-I (after Ringsdog [696]) (courtesy of GITVerlag, Darmstadt, F.R.G.)

1 0 1 2 1 C

in min

atomite support materials, if more sophisticated analyses have to be carried out, provided that the liquid phase spreads well on its surface. This holds true for non-polar or weakly polar stationary phases (e.g., methylsilicones), but not for polar materials (e.g. polyglycols). Therefore, packings with stationary phases the surface tensions of which exceed ca. 24mJ/m2 should be prepared using diatomite supports. The advantages of this synthetic support material can best be utilized if a special flask (“hedgehog” flask) is used for the coating procedure [698] and if a special column packer, i.e., a vessel for filling GC columns under pressure, is used for the packing procedure [699]. Chemically similar to Volaspher is synthetic quartz powder. This material can be used for the analysis of hydrogen-bonded compounds, provided that only low column eficiencies are required. Because of the low specific surface area of quartz powder (ca 0.20 m2/g for a particle size of 100-150 pm),the kin value of this non-porous material is only ca. 3 mm. Moreover, it can only be coated very weakly (50.2%, w/w) with the usual liquid stationary phases [699a]. Hence, the sample size has to be chosen to be very small in order to avoid overloading the column.

7.6.

Silica Gel

Starting from silica gel, Kiseleu and Shcherbakouu [700]prepared uniform, regular supports by widening the unfavourable fine pores, yielding a more uniform pore size distribution and decreasing the surface area. They reacted silica gel with water in an autoclave and subsequently silylated most of the free silanol groups. The average pore diameter could be increased to ca. 0.4 pm.

Further possibilities for preparing Si02particles with suitable gas chromatographic properties have been proposed, starting from water glass [701] or ethyl silicate [702],but both the pore sizes (515 nm)and the specific surface areas (>200 m2/g) demonstrate that an undesirable adsorption contribution of this material has to be taken into account.

7.7. Micro Glass Beads and Porous Layer Beads

181

Some types with increased pore diameters are listed in Table 37. The last column of this table indicates that these materials cannot be classified as inert support materials. Table 37. Silica Gel Support Materials ~

Name

Specific Surface Area of the Usual Particle Size [m2/d

Pore Diameter

~~

Properties

[rml ~

Silica gel, geometrically modified 9

0.4

Silica gel, geometrically and chemically [(CH&SiO]modified Spherosil XOC 005

5

0.4

5-15

0.3

7.7.

~

distinct adsorption contributions even if heavily loaded with stationary phase lower adsorption contributions, but still measurable distinct adsorption contributions even if heavily loaded with stationary phase

Micro Glass Beads and Porous Layer Beads

Originally, glass beads were assumed to be inert, provided borosilicate or calcium-containing glass was not considered. However, adsorption affects cannot be neglected. They can be decreased, in analogy with the treatment of glass capillaries, by leaching and silylation processes. Owing to their low specific surface area, the impregnation rate has to be chosen to be very low. The maximum amount of liquid phase depends on the bead radius, the surface tension and the density of the liquid phase and ranges between 0.05 and 0.5%. It should be noted, however, that a 0.5% coating of glass beads, because of their relatively high packed density (1.6-2 g/cm3) corresponds to an impregnation rate of 5% on Chromosorb W (density ca. 0.22 g/cm3). By acid treatment of alkali silicate beads (80% S O z , 20%NazO) the surface area is roughened, and after silylation with dimethyldichlorosilane these beads can be impregnated with up to 0.4%,and h,,,h values of 0.29-0.42 mm can be achieved [703, 7041, which in the range 16-100 ml/min depend only negligibly on the carrier gas flow-rate. This fact and the low coating enable the retention time (which is determined by the amount of stationary phase and the flow-rate of the mobile phase) to be decreased by 40%, and the column temperature can be decreased to 250°C below the boiling point of the highest boiling component of a sample [705].Because of the smaller amount of stationary phase, the sample size must be smaller than usual, of course, in order to avoid overloading the column. Depending on the type of glass and pre-treatment, the surface areas range from 0.04 to 0.4 m2/gand the bin values from 0.29 to 6 mm (the last values showing this type of glass beads to be unsuitable). In Table 38 some commercial products are listed. Porous layer beads (superficially porous beads, solid core or pellicular supports), first proposed by Golay [706] and developed by Haldsz and Homath [707], consist of glass beads upon which is coated a thin layer of porous silica. The layer thickness is 0.5-3 pm. Compared with totally porous supports, the mass transfer rate is more favourable, and owing to the geometrically uniform shape of the particles, the 1 values are low, both effects resulting in high efficiencies of columns packed with this material, while keeping the possibility of high flow-rates without a severe reduction in efficiency. The main advantage in comparison with glass beads is, however, that coatings with up to 2%w/w of liquid stationary phases can be prepared and still remain free flowing, allowing higher sample capacities. It has been shown [708] that with the 30-pm (25-37 pm) particle fraction of Zipax, coated with 2%(w/w) of OV-1 Lpoly(dime-

7.The Solid Support

182 Table 38. Some Commercial Types of Glass Beads Name

Manufacturcr/Supplier

Coming Code 0201 (DMCS) GLC 100,110 (DMCS) Glass Beads (DMCS) Anaport Glass Beads (DMCS) Glass Beads Cera Beads (DMCS) Glass Beads Glassport M Glass Beads Glass Beads Mikroglaskugeln Glaskugeln (DMCS)

Coming Glass Works, U.S.A. Column Technology, U.S.A. Applied Science Labs., U.S.A. Analabs, U S A . 3M, U.S.A.

Pittsburgh Coming, U.S.A. Perkin-Elmer, U.S.A. Hewlett-Packard, U S A . Phase Separations, U.K. BDH, U.K. alaswerk Lauscha, F.R.G. Serva, F.R.G.

Table 39. Commercial Porous Layer Beads Name

RoducedSupplier

Specific Surface Arca

Average particle

[mz/d

[vml

40 30 50

Zipax CSP

DuPont, U.S.A.

0.8

Liqua-Chrom Perisorb B

Applied-Science Labs., U.S.A. Merck, F.R.G. Jasco, Japan

5-10

Jascosil WC-03

size

thylsiloxane)] and packed into a microbore column (1 mrn I.D.), 8000-10000 platedm can be obtained, the inlet pressures, however, being 13-26 atm (or 1.32-2.63 MPa). Commercial products are listed in Table 39. Zipax porous layer beads are distinguished by their low surface area and hence by a lower surface activity relative to other commercially available porous layer bead supports [709]. Sample capacities of 5 pg could be achieved, however [708].

7.8.

Fluorocarbon Supports

Fluorocarbon polymers have been widely used for the separation of strongly polar or reactive compounds. Initially, Teflon-1 (DuPont) was applied which, owing to its small specific surface area, proved inadequate and contributed to the opinion that such a material is not suitable as a support. However, subsequent work by Landault and Guiochon [710], Homing, Moscatelli and Sweeley [711], Stuszewski and Junak [712], Kirkland [713], Sakodynsky and Brazhnikow [714] and the author [715] revealed that values of 1-2 mm can be achieved, provided that the surface area of the support material does not fall below ca. 0.6 m2/g. Most fluorocarbon supports are made from Teflon-6 powder, a poly(tetrafluoroethy1ene) resin usually used as a moulding powder. The support particles consist of aggregates of many much smaller primary particles (0.2-0.5 pm), causing material of 180-250 pm particle size to have a fairly high specific surface area of 0.6-12 m2/g. Laboratory technicians are not enthusiastic about packing columns, or coating and screening such solid supports, as PTFE supports are soft and liable to electrostatic charging, causing them to aggregate and also to adhere to the walls of the column and of the accessories.

183

7.8. Fluorocarbon Supports

These problems can be circumvented by cooling the support to 0°C before use, and by replacing glass equipment by plastic materials. On cooling to 0°C the material becomes more rigid (one of the transition points of PTFE being 19°C) and, as the humidity is condensed, the static charge is reduced. The mechanical strength, according to studies of Sukodynski and Bruznikou [714, 728, 7291 and of the author [715, 716, 7181 is distinctly improved when the porous materials is thermally treated for 10-20 min between 300 and 340°C (caution: use a hood!). Fig. 18 shows a particle under a microscope. Using special sintering technology the shape of the particles becomes more uniform, as can be seen in Figs. 19 and 20, hence permitting a better packing and separation efficiency.

Fig. 18. Micrograph of untreated polytetrafluoroethylene particles. d, ca. 300 pm, enlargement 1:45

Fig. 19. Micrograph of specially sintered polytetrafluoroethylene particles. 4 ca. 200 pm, enlargement 1:lOO

Fig. 20. Micrograph of a thermally treated polytetrafluoroethylene particle. d p ca. 200 pm, enlargement 1500

184

I. The Solid Support

In addition to the transition at 19"C, a further change occurs at 30°C and, above 290" some trace decomposition takes place with the release of, among others, the extremely noxious peffluoroisobutene [(CF,),C = CF,]. Decomposition reactions owing to the extreme toxicity of peffluoroisobutene, (higher than phosgene!), become highly dangerous at temperatures above 327°C [1018]. Hence, such temperatures have to be avoided when handling the supports (also open flames and smoking are prohibited). The normal temperature range of the application of this support material, depending of course on the stationary phase, lies between room temperature and 250°C. The impregnation of this support can be carried out as usual, provided that the solvent is removed with a stream of nitrogen and not by heating, that stirring is carried out very gently and that electrostatic charging is avoided as far as possible by cooling as described above. Another possibility consists in the solvent coating technique [717]. As the free surface energy of this material is essentially lower than that of all other plastics (ysnPE = 15 mJ/m2 with a polar term 1mJ/m2), the use of stationary phases with high surface tensions (e.g., glycerol, or diglycerol) is not possible because they do not wet the solid support. Methylsilicone oils, hydrocarbons and poly- or peffluorinated compounds, however, have proved to be suitable stationary phases for this support material. The impregnation rate should not exceed 10% (w/w), even though stationary phase loadings up to 20% (w/w) have been reported. In addition to its greater thermal resistance compared with other plastic materials, another outstanding property of PTFE is its high resistance to chemical attack. It reacts only with molten alkali metals or with elemental fluorine, and hence it is by far the best support material for the analysis of free halogens, interhalogen compounds and other corrosive compounds (HF, HC1, C102F, ClO,F, UF6, NF,, NzF2,N2F4, SiF4, NOC1, NO,Cl, NOz, Sic&, PC&,BCb, BBr,, etc.) [715-7161. Moreover, compounds with considerable hydrogen bonding (HzO, NH,, hydrazine, amines, carboxylic acids, alcohols, etc.) and compounds sensitive to hydrolysis (aminosilanes, halides other than those mentioned above, easily decomposable compounds or their silyl derivatives) can be analysed on this support without peak tailing [715-7191, indicating that PTFE supports are the most inert and least polar supports. For this reason, PTFE supports have been applied for the determination of retention data as a function of stationary phase loadings, in order to exclude solute-support adsorption contributions [720]. Nevertheless, although PTFE supports do not interact with polar samples, differences in the retention of alkanes and benzene have been found which must be attributed to a matrix effect [720]. The supports seem to absorb n-alkanes within the polymer matrix (inclusion and capillary condensation, permeation processes). Therefore, the determination of retention indices, which is based on n-alkanes as reference materials, on PTFE supports may be problematic [720]. In Table 40 solid supports, based on PTFE, and their properties are listed.

7.9.

Other Support Materials

In a similar manner to PTFE, poly(trifluorochloroethy1ene) can be applied (Anaport Kel-F 300 LD, Kel-F 3081, Hostaflon-C2, Haloport K, Plascon 2300, Voltalef), even though it is not as inert as PTFE and is less thermally stable. For the separation of heavy hydrocarbons in crude oil (stationary phase, polyphenyl ether sulphone), dendritic sodium chloride has been applied, which is not too hygroscopic and not fragile [721]. However, with this material only low efficiencies can be obtained. Moreover, it is not as inert as is sometimes believed. For example, the retentions and partition coefficients of several amides are affected by alkali metal halides. This goes so far that the elution order of diacetamide and butyroamide on PEG 20M is reversed by the influence of LiCl[722]. (The reasons for this behaviour of alkali halides have been discussed in Chapter 5 . )

185

7.9. Other Support Materials Table 40. Solid Supports Based on PTFE Commercial Name

Manufacturer1 Supplier

Specific Surface Area

[mz/sl

Teflon-1 Teflon-6 Chrornosorb T Haloport-F Fluoropack-80 Columpack-T Anaport Tee Six Gas-Pak F Phase Sep T 6 Polychrom-1

DuPont de Nemours, U.S.A. DuPont de Nemours, U.S.A. Johns-Manville,U.S.A. Hewlett-Packard,U.S.A. Fluorocarbon, U.S.A. Fisher Scientific, U.S.A. Analytical Eugineering, U.S.A. Chemical Research Service, U.S.A. Phase Separations, U.K. U.S.S.R.

Packed Density [g/cm’]

Notes

Not suitable for GC

0.23 0,49

Suitable for GC

7.8 7.8 0.64 4.8 1.1

0.49 0.5 0.7

Screened from Teflon-6 Screened from Teflon-6

3-6

0.1

11

Specially screened and processed PTFE

Sintered PTFE-particles, more rigid than others

Hostaflon TF Teflon powder Shimalite F

Hoechst, F.R.G. Becker, The Netherlands Shimadzu Seisakusho, JaPan Fluoroplast-4powder U.S.S.R.

For the investigation of strongly polar low-boiling compounds, porous polyaromatic beads have proved advantageous. However, as they contribute considerably to the retention, even at impregnation rates of 10-20% (w/w), they are far from being true support materials. Although many separations have been carried out on coated porous polymers (e.g., [723-725]),the liquid phase does not distinctly improve the separation properties of the porous polymer. The effect may be compared with that which can be achieved on modifying an active adsorbent in gas solid chromatography. At first sight, the silylation of polystyrene, cross-linked with divinylbenzene, appears useless. However, residual phenolic hydroxy groups are silylated, and other active sites may be shielded by this treatment, but it should be taken into account that C-0-Si bonds, formed by this reaction, are sensitive to hydrolysis. Commercial silanzied products are Porapak P-S and Q-S and Supelpak-S. The undesirable activity is actually reduced, but the contribution of these materials, when applied as solid supports, to the partition coefficient surpasses that of the liquid phase. It has been shown by scanning electron microscopy that even with an amount of 10%(w/w) of a well-wetting liquid phase and of 25% of a liquid phase that does not properly wet the support, a substantial part of the liquid phase is accumulated on the periphery of the support grains and only a certain amount enters the pores (pore widths 0.3-0.4 lm). Hence, uncovered zones of the support will remain, which may be the reason for the distinct adsorption contribution [730]. Tenax GC [poly(2,6-diphenylphenylene oxide)], introduced by van W j k [726]and compared with several Porapak and Chromosorb types by Duemen et al. [727],also remains an adsorbent in its basic properties in spite of coating with a liquid phase. As these porous polymers have been discussed in detail in Chapter 5 , they will not be dealt with here. It will only be mentioned that by the impregnation with a liquid phase the specific surface area is decreased, and hence also the retention times, especially of higher homologues of various compound classes, can be decreased.

8.

Liquid Stationary Phases

8.1.

General Properties of Liquid Stationary Phases

It is obvious that a substance that is to be used as a liquid stationary phase must be chemically inert, should be non-volatile and thermally stable and has to possess some solvent power and selectivity for the sample components to be separated. Moreover, the liquid phase should cover the support surface (packed columns) or the inner wall (open-tubular columns) as uniformly and completely as possible and should achieve the separation as completely as necessary within as short a time as possible. To fulfil these and further requirements, the liquid stationary phase must possess certain properties, which are discussed below.

8.1.1.

Chemical Inertness

A liquid stationary phase must not react irreversibly with the carrier gas, with the solid support or with the sample components. Carrier gases often contain oxygen, at least in trace amounts. Hence, the risk of oxidation of the liquid phase exists, favoured by the large area the phase occupies as a thin film. Extreme care must therefore be used to keep oxygen out of a column packed or coated with any organic liquid phase when it is hot. The oxygen content should not exceed 5 ppm (v/v) in the carrier gas (commercial cylinders of the common carrier gases, Nz,H2and He, often contain >0.1% of Oz! Even the “oxygen-free” or “white spot” grade of nitrogen has an oxygen content of 10 ppm [732]). By means of an oxygen scavenger, consisting of activated manganese (II) oxideldiatomite, the concentration of O2 in nitrogen can be reduced to cO.1ppm (v/v) [733]. As active sites of the solid support may catalyze such reactions, its activity has to be removed or decreased by the methods described in the previous chapter. Otherwise, polymeric phases might be oxidized or depolymerized, and ester phases might be saponified. Chemical reactions with individual constituents of a sample or the liquid phase would alter the sample composition and falsify the gas chromatographic results. For example, the addition of phosphoric acid to the liquid phase gives rise to the danger of an undesirable reaction with alcoholic constituents of the sample (condensation, i.e., splitting off water involving the occurence of additional, (only formed in the column) compounds and the subtraction of basic compounds of the sample. On polyester phases the risk of structural changes of sterols exists, as observed by Lipsky and Landowne [731]. When potassium hydroxide is added to the liquid phase, which is frequently done in order to elute amines without tailing, acidic compounds will be subtracted, and silicone and polyester phases will be readily destroyed, especially at higher temperatures.

8.1.2.

Vapour Pressure, Thermal Stability and Maximum Operation Temperature

Even if highly volatile solvents are applicable, they are used only in exceptional circumstances because of the great expense involved. Generally, the maximum operating temperature is limited by the vapour pressure and by the thermal stability of the liquid phase. The

8.1. General Properties of Liquid Stationary Phases

187

loss in weight of the stationary phase due to vaporization or decomposition affects the column life and the retention time and detection of the separated compounds. The temperature values published in the chromatographic literature for the same stationary phase often differ considerably. This is not surprizing, because they depend on the analytical problem to be solved, on the column type, on the nature of the substrate, on the film thickness, on the column length, on the type of cure, on the purity of the carrier gas, on the column conditioning, on the detector type etc. As a rule of thumb, the maximum operating temperature of a vaporizable liquid phase is 70°C below its boiling point at a pressure of 13-67Pa, provided that the detector is not very sensitive. For ionization detectors the maximum operating temperature is even lower, between 90 and 150°C below the boiling point at the above pressure. For polymeric phases, their thermal stability is of greater importance than their vapour pressure, as the latter can be neglected for phases of higher molecular weight, provided that there are no residual monomers and oligomers dissolved in the polymer. In order to avoid interferences caused by such volatile impurities in the polymer, it is firmly recommended that a chromatographic-grade polymeric phase only is used, as technical-grade polymers usually do not meet the requirements. Even if the stationary phase does not contain any low-molecular-weight constituents, it will begin to bleed off the column above a certain temperature limit and the result will be a rise in and spiking of the baseline as the temperature increases, caused by the detector's response to volatile degradation products of the stationary phase. It should be emphasized that, if a well manufactured stationary phase of chromatographic grade is used, column bleeding does not occur, as is occasionally assumed, by residual low molecular weight components in the liquid phase, and attempts to flush these volatiles from the column at a temperature higher than the limit cannot be successful. On the contrary, baking the column out may damage the liquid phase and perhaps ruin the column [734],as by this blowing off of the liquid phase the retention times will be shortened, the resolution will become poorer and peak tailing will occur as a consequence of uncoated carrier or column wall sites brought about by the removal of the liquid phase. Qualitative and quantitative investigations of column bleeding have been carried out [732, 735-7431 for various polymeric phases, the results of which indicate that, in addition to temperature, the composition of the support surface, the purity of the carrier gas, the amount and the area of the liquid phase exposed to the carrier gas and especially the structure of the liquid phase (polymer backbone, nature of the side groups, nature and number of chain terminal groups) strongly influence the decomposition temperature and bleed rate. A further factor may be possible catalysis by the decomposition products themselves. Obviously, an exact maximum operating temperature cannot be assigned. For example, when using a plasma chromatograph or a G C - M S system, the bleeding will become apparent at distinctly lower temperatures than with insensitive detectors. As the test methods for the maximum operating temperature and for the conditioning are not standardized, it is difficult to compare the published values. A general test method consists in weighing an alkali-free glass tube packed with the stationary phase under investigation, coated on an appropriate solid support, before and after purging with the carrier gas at certain temperatures [744].Further common methods are thermogravimetry and differential thermogravimetry, the stationary phase being investigated as a bulk liquid or coated on a solid support, both under oxygen-free nitrogen. With these methods of measurement the weight changes of the investigated material are recorded as a function of the temperature at certain heating rates. As long as the permissible total weight loss is not standardized, one may define as a maximum limit the temperature at which either the weight loss amounts to 0.1%(w/w) or even to 1% (w/w), etc., the latter value evidently causing difficulties when using a sensitive detector, or the bleeding (weight loss) rate, calculated from the differential thermogravimetric result (dm/dT) as a function of the heating time, amounts to, e.g., lo-' g/min

188

8. Liquid Stationary Phases

(for packed columns). Further investigations will increase our knowledge of phase degradation and hence enable the values of the maximum operating temperature to be made more precise. As the nature of the degradation products and reactions naturally depend on the chemical nature and structure of the polymer, they will be discussed for the most important polymeric liquid phases when discussing the individual liquid stationary phases. The maximum operating temperatures, as far as they are known and with the imperfections just mentioned, will be given there. They are only suggested to serve as a general indicaton of the temperature limit.

8.1.3.

Molecular Weight

Both well defined compounds of a definite molecular weight and polymers exhibiting a molecular-weight distribution have been applied as liquid stationary phases. Individual chemicals were preferred in the early days of gas chromatography, as their batch-to-batch manufacture is readily reproducible and their purification is easy. However, owing to their volatility, they can be used only at low or moderate column temperatures and not over wide temperature ranges or in temperature-programmed gas chromatgraphy. Far more universally applicable are certain polymers, which are suitable over a wide temperature range, can be prepared reproducibly, may cover a large field of selectivity and are appropriate for both packed and open tubular columns. This holds especially for polyorganosiloxanes, which have continued to be the dominant types of liquid stationary phases for more than a quarter of a century. As the polymers used in the early gas chromatography were intended for various industrial applications, they contained, in addition to undesirable and critical constituents (catalysts, residual solvents, additives), also low-molecular-weightmoieties and often had a wide molecular-weight distribution. With the passage of time refined polymeric products replaced the technical products, the purification involving the removal of catalysts, etc., and especially of oligomeric moieties. When designed and manufactured exclusively for use as gas chromatographic stationary phases, the polydispersity of the polymers should be as low as possible. Molecular-weight distributions I1.3 have been shown to result in higher efficiencies = weight-average molecular-weight, = number-average molecular weight). Thus the well known poly(dimethylsi1oxane) OV-101has a ratio of = 1.3 [745] and several polyglycol phases have ratios of I 1.1, indicating a narrow molecular weight distribution, whereas, e.g., Carbowax 20M has a ratio of 2.95, exhibiting much too wide a distribution [746]. For separations of only low-boiling compounds to be carried out at low column temperatures, polymers of lower molecular weight are needed, as very high molecular-weight materials frequently give badly shaped (skewing or fronting) peaks at low temperatures. On the other hand, materials of too low molecular weight will cause excessive column bleeding. Because for silicones, for example, there is little change in the retention properties with molecular weight, a good compromise for a universal application would be a narrowly distributed organosiloxane material with a molecular weight between 10000 and 60000. As far as thermal stability is concerned, an increase in the molecular weight of silicones above ca. 20 000 does not have any positive effect. Much more important is the absence of any traces of acids, bases or Lewis acids and of silanol end groups when operated at higher temperatures. Lighter n-alkanes were shown to diffuse faster in a poly(dimethylsi1oxane) of M, = 400 000 and hence to cause less peak dispersion than in a poly(dimethylsi1oxane) of M, = 35 000,but the opposite effect, a somewhat hindered diffusion, was observed for higher molecular weight nalkanes [747]. Such effects, however, become negligible at temperatures above 200°C [748]. A special case is liquid phases for the coating of the inner walls of open-tubular columns. In this instance, high-molecular-weight stationary phases are to be preferred, as thin films of such materals on walls of low surface energy should be as viscous as possible, even at elevated temperatures, because otherwise there is the risk of physical rearrangement of the

(a,

&/a,,

a,,

a,/&

8.1. General Properties of Liquid Stationary Phases

189

phase, leading to a reduction in efficiency or even droplet formation when exposed to thermal stress (frequent change from low to high temperatures and vice versa, as is usual in gas chromatography). The film stability is sufficient, e.g., for poly(dimethylsiloxanes),above molecular weights of ca. 100000 and can be further enhanced by free-radical cross-linking, as was discussed in Chapter 3, of siloxanes containing small amounts of vinyl groups, that undergo such curing reactions. Contrary to expectations, Crarners et al. [749] did not find significant differences between diffusion coefficients for n-alkanes in chain-like conventional and in cross-linked liquid stationary phases. The influence of terminal groups in polymeric chains on the selectivity has not yet been discussed here. Considering high molecular weights, e.g. 100000, two end groups per chain would mean an influence of the interaction forces on the solute of about 0,1% compared with that of the links of the polymeric chain and may obviously influence the retention behaviour only negligibly. For molecular weights lower than about 5000 the situation is substantially different, especially with regard to polyethylene glycols. For example, the molecular retention index, a term that characterizes the liquid phase (see Chapter 4), depends strongly on the molecular weight: for 1-bromoheptane AMe is 12.20 for polyethylene glycol of molecular weight 200 (PEG 200), 6.93 for PEG 400, 3.50 for PEG 1000 and 1.64 for PEG 4000. Distinct differences have also been shown by Evans for other solutes, e.g. for 3-methylbutane-1-01 AMe = 121.76 (PEG 200), 102.22 (PEG 400), 89.82 (PEG 1000) and 83.30 (PEG 4000) [359]. Hence, the influence of terminal hydroxy-groups must no long be neglected and the molecular weight has to be taken into consideration when selecting such stationary phases and when reporting the results.

8.1.4.

Viscosity and Minimum Operating Temperatures

The application of a liquid stationary phase at very low temperatures is limited by either its melting point or its pour point. In the solid state, the "liquid" phase is no longer a liquid, and the heat of solution and of adsorption indicate that adsorption effects become responsible for the separation [750], where upon the efficiency is poorer than above the melting or softening point, resepectively [751]. Highly viscous liquid phases permit satisfactory efficiencies only with an increase in temperature, and with the consequential decrease in viscosity [752, 7531, because the resistance to mass transfer is proportional to the viscosity of the liquid phase. This restriction does not apply to non-polar linear polymers, especially poly(dimethylsi1oxanes), for which an increase in the chain length and hence in the viscosity only slightly influences the diffusity of small molecules [747, 7541. Moreover, they exhibit an exceptionally low dependence of viscosity on temperature. For example, if a mineral oil is heated from -20 to + 100°C, its viscosity decreases from 2200 to 6 mPa * s, i.e., by a factor of 367, whereas the viscosity of a methylsilicone oil in the same temperature range only decreases from 250 to 30 mPa .s, i.e., by a factor of 8. This property is extremely advantageous for gas chromatographic applications at both high and low temperatures may be'explained by the uncommon bonds. This flexible freedom of movement of the non-directional (d-p)sr type Si-0-Si (coiled helix), less rigid bonding structure and the larger atomic radius of silicon (0.118 nm) compared with carbon (0.077 nm) cause silicon compounds to occupy a greater molecular volume and to give a low cohesive energy density [755]. In situ cross-linking via vinyl groups does not appear to affect the specific retention volume, the diffusion and hence the efficiency, even if the viscosity of the cross-linked poly(dimethylsi1oxanes)has increased significantly [749, 7561. Raising the temperature will flatten the polymeric chains and increase the interaction between the chains, resulting in an incremental viscosity, which partially compensates the usual decrease in viscosity with increase in temperature.

190

8. Liquid Stationary Phases

There are various simple relationships between the average molecular weight and the viscosity of poly(dimethylsiloxanes), of which two have a broad range of validity: log qa = 1.43 log (M. W;) - 5.4 [757],

M.W.=

464 ( v ~ s ) O . * ~ ’ (2 + 0.0905 (q25)0.5’5)

(183)

[7581

where M.W. = molecular weight, ~ 4 0 = viscosity at 40°C (0.1 Pa * s), qzs = viscosity at 25°C (0.1 Pa * s). The unique viscosity-temperature coefficient (VTC value) of poly(dimethylsi1oxane) does not hold, however, for siloxane chains with large pendant side groups such as phenyl [759] or biphenyl [760]. However, the detrimental effect of these groups can be reduced by cross-linking the polymer via vinyl groups or by incorporating dimethyl-siloxane moieties into the polymer. In Table 41 various characteristic polymeric organosiloxanes and some of their thermal and mechanical properties are listed. For application in the region of near-ambient to sub-ambient temperatures, silicones are frequently more suitable than other liquid stationary phases. This holds good especially for cross-linked poly(dimethylsi1oxanes) and poly(ethylmethylsi1oxanes). The behaviour of liquid phases at low temperatures can best be estimated by differential scanning calorimetry, as solid-liquid-phase transitions can easily be realized. The phase to be investigated is rapidly cooled (160 Wmin) to 110 K. Cooling is immediately followed by slowly heating (10 K/min) and measuring the endothermic and exothermic effects. Starting with low temperatures (110 K) = - 163.15”C), at first the glass transition (T’J temperature is reached, the value of which is ca. - 126°C for poly(dimethy1siloxanes) and is lower for ethyl- and higher for phenyl- and fluoropropyl-substituted polysiloxanes (see Table 41). Tgexpresses the transition of amorphous moieties of the polymer structures at lower temperatures. It has been shown that cross-linking may suppress the glass transition mechanism [761]. When the heating is continued, an exothermic transition at -85 to -87°C for poly(dimethylsi1oxanes) of viscosity 5000 mPa s was observed [762], indicating a crystallization phenomenon, and in the region of “higher” temperature, between -33 and -53°C depending on the molecular weight, two endothermic transitions occur when investigating methylsilicones of viscosity 5000 mPa s [762]. The integration of the discontinuous region yields AH values, for OV-101, of 13 and 25 J g-’, respectively. This transition is due to the melting of crystalline moieties of the polymer. Below these temperature, the phase can no longer regarded as “liquid”, and near this point the efficiency of separation is drastically reduced compared with efficiencies above these ”melting”points. Cross-linking, again, largely suppresses freezing and melting phenomena and enables poly(dimethylsiloxanes),to be used down to -70°C without a significant reduction in efficiency, provided that the original viscosity is not too high [761]. In addition to especially cross-linked poly(dimethylsiloxanes), phenyl- fluoropropyl- and cyanoalkylsilicones have also proved to be applicable at subambient temperatureq7631.

8.1.5.

. Film Formation

A sufficient column efficiency can only be achieved if a film of liquid phase covers the surface of the solid support particles (packed columns) or of the inner tube wall (open-tubular columns) as uniformly and coherently as possible and if this film retains this homogeneity through numerous repeated heating and cooling cycles as is usual in gas chromatography.

8.1. General Properties of Liquid Stationary Phases

191

Table 41. Properties of Organopolysiloxanes*) Organopolysiloxane

Poly(dimethy1siloxane)

Poly(methylethylsi1oxane) Poly(diethylsi1oxane) Poly(80%dimethyl-20% diphenylsiloxane) Poly(90%dimethyl-10% methylphenylsiloxane) Poly(50%dimethyl-50% methylphenylsiloxane) Poly(methylphenylsi1oxane 1:l) Poly(50%methylphenyl50%diphenylsiloxane) Poly(methyl-3,3,3-trifluoropropylsiloxane) qZs

= dynamic

'IZS

M W.

[mPa. s]

11031 (weight average)

50 3.9 100 6.2 200 9.7 500 17.7 1000 28.8 1500 31 5000 51 10000 64 30000 94 50000 113 100000 142 1000000 315 2500000 478 10 000 000 1000 100 000 30 200

800

df'

Yn:

Y1

ern-)]

0.961 0.967 0.968 0.971 0.971 0.971 0.973 0.974 0.976 0.977 0.977 0.978 0.978 0.975

mF2]

20.8 20.9 21.0 21.1 21.2 21.2 21.3 21.5 21.5 21.5 21.5 21.6 21.6 21.6 23.1

0.59 0.60 0.60 0.60 0.61 0.61 0.61 0.61 0.61 0.61 0.61 0.62 0.62 0.62

Pourpoint ('Cl

- 69 -65 -65 -50 - 50 -49 - 48 -48 -43 -42 -41 - 39 -38 - 37

Ts ('CI

- 129 -128 - 128 - 127 -127 - 126 - 126 - 126 - 126 - 126 - 126 - 125 - 125 - 123 -135 - 143

0.95 1.05

-40

1.oo

- 73

-115

130

2.1

1.07

-51

- 100

550

2.6

1.092

- 20

- 86

10

1.150

+20

3,3 15

1.28 1.30

- 40 -30

120000 1000 10000

- 14 -71

viscosity at 25°C [mPa * s],

M.W.= molecular weight, d:' yI

= specific

= surface

gravity at 25°C [g .cm-)], tension [mJ. m-*],

viscosity at 100°C viscosity at 38°C Pour point ["C] characterizes lowtemperature properties Determination according to ASTM D 97, TB = glass transition temperature (determination by differential scanning calorimetry) ["C].

Vn:

= viscosity

(

temperature coefficient: 1 -

*) Values taken from 'Silicon Compounds", Petrarch Systems, Catalog 1986, from numerous literature data and from the author's measurements and calculations, partially together with H.Sleimann.

The formation of such a film depends on the ability of the liquid phase to wet the surface completely. The adhesion strength is determined by the liquid phase's molecular weight, rigidity, mobility (flexibility of polymer fragments) and chemical nature and by the nature and shape of the substrate. Dominant macroscopic parameters that particularly are responsible for good or bad wettability are the critical surface tension (or energy) of the substrate and the surface tension of the liquid phase. The surface tension of a liquid, M, which describes the cohesive forces, opposes its spreading on the particle or tube wall surface, whereas the adhe-

192

8. Liquid Stationary Phases

sive forces between the substrate’s surface and the liquid, expressed by the critical surface tension, yc, and being characteristic of the particular solid, endeavour to extend the liquid on the surface area. When the adhesive forces (yc) are greater than the liquid’s cohesive forces (d,the surface will be wetted completely (provided that the chemical nature of the solid surface and the liquid phase are not too different), and, in turn, if yl > yc, the surface cannot be wetted uniformly. In Section 3.1.3. we have seen that the contact angle, 8, for a liquid that entirely wets the surface is zero, and cos 8 = 1. With increasing 8 (and hence decreasing cos 8)the wettability of the surface decreases, and above 8 = 90” (i.e., cos 8 < 0) the surface cannot be wetted at all. If we now consider a rough surface, then the liquid phase may penetrate in the scratches and pores, and we have to take into consideration the roughening factor (see eqn. (117) in Section 3.1.3), cos B’/cos 8. As cos 8’ > cos 8, the contact angle on a rough surface, B,is smaller than @, it follows that a rough surface can be coated by liquids the surface tension of which is higher than that of the substrate more completely than a smooth surface. This is the main reason for the familiar phenomenon that Kieselguhr particles and metal capillaries can be coated uniformly more easily than glass or fused-silica capillary tubing. Glass capillaries can be roughened by etching procedures or by the deposition of microcrystalline inert inorganic compounds. Good roughening factors are cos @’/cos8 > 1.5. This method, to permit film formation also for’systemswith yl > ycby increasing the specific surface area at constant specific surface energy (but, owing to the higher spec& surface area, at a higher total surface energy!), is not the only approach. Theoretically, we can increase the critical surface energy ye, or we can select liquid phases exhibiting low surface tensions H. However, the former possibility conceals the risk of undesirable activity towards polar samples, and the latter would exclude the application of polar liquid phases that are necessary for numerous separation problems. As a compromise, surfaces have been chemically modified in order to enhance the film formation tendency whilst maintaining or increasing the state of deactivation. As we have discussed this aspect in detail in previous sections, we shall only deal with surface tensions here. The critical surface energy of fused silica ranges, depending on its processing and on subsequent adsorption and hydration possibilities, from 28 to 48 mJ m-z [764]. Therefore, even high-polarity organosiloxanes can in principle wet these surface. However, thin films are not stable at elevated temperatures, and deactivation (after hydroxylating the surface) with the usual monofunctional silylating reagents or with methyl- and vinylcyclosiloxanes and -silazanes and polysiloxanes at high temperatures (300-450°C) [173, 176-177, 209, 223, 61 1,764-7661 and also with cyclic and linear polymethylhydrosiloxanes at medium temperatures (250-300°C) [767-7681 leads to critical surface energies between 20 and 23 Such surfaces are easily wettable only by poly(dimethylsiloxanes), as can be concluded from Table 41, where their yl values range between 20.8 and 21.6 mJ * m-*, and from measurements by Bhatia et al. [769]. As the surface tension of liquid poly(dimethylsi1oxanes)is lower than the critical surface tension of their adsorbed fiis, they can spread over their own mono- and oligolayers and hence form the desired liquid stationary phase [770]. The surface energy of fused-silica-surfacescan be increased by high-temperature deactivation with phenyl-, trifluoropropyl- and 0-cyanoethyl-derivativesof the above-mentioned silylation reagents and siloxanes to values between 22 and 32 mJ . m-2, hence permitting a sufficient wettability with medium polarity liquid phases [209, 764, 771-7721. A further deactivation procedure, based on the pyrolysis of poly (ethylene oxide), results in very high surface energies of the deactivated fused silica (34-44 mJ * m-2), which now ought to be wettable also by more polar phases. However, the limited thermal stability of the hydrophilic deactivation layer (200-250°C) and its unfavourable wettability by hydrophobic nonpolar phases have restricted its common use [764, 773-7741. Before coating, modified surfaces can be characterized, e.&, by cross-polarization magic-angle spinning NMR-spectro-

-

8.1. General Properties of Liquid Stationary Phases

193

scopy, reflectance IR-spectroscopy, scanning electron microscopy and capillary rise measurements [764]. The formation and stability of films may be improved by the application of highly viscous “gum” phases and by bonding poly(organosi1oxane) phases to the inner wall of the capillary tubing and/or cross-linking such phases, which will hence be immobilized and will no longer be subject to contraction and to serious droplet formation. This topic has been discussed in detail in Section 3.3.3.3.

8.1.6.

Solubilizing Power

The liquid stationary phase must possess some solvent power for the mixtures to be examined in the column in order to prevent the sample components moving too quickly and possibly without being separated through the column. For the selection of an appropriate solvent it is generally valid that the solvent and the compounds to be dissolved in it ought to be chemically similar, according to the old experience “similia similibus solventur”. This experience can easily be explained thermodynamically. Dissolution takes place if the chemical potential of the system of more components is lower than that of the initial components. This is normally the case, owing to the usual increase in the entropy, unless strong interactions have a counteracting effect (i.e. if the cohesion energy of the initial components were higher than that of the mixture). Hence, for a good solvent power, the interactions between the “solvent” molecules themselves and between the solvent and solute molecules should be similar, and this holds especially for structurally similar molecules. We have seen in eqns. (71-73) that the chemical excess potential pE= RTln y and, as p E= AGE (excess molar free energy), and A G E= A H E- TASE,where = mixing enthalpy of 1 mole of the solute with the stationary phase at AHE infinite dilution (excess partial molar enthalpy), = excess partial molar entropy, and ASE A H E- T A S E= RTln 4, we can conclude that decreasing AHE optimizes the solubility (the entropy contribution being less open to influence). One should be conscious, however, of the important roll of AHE (and hence of yo) for the selectivity of a stationary phase, i.e. for the separability of equally or similarly boiling components, the AHE- (or yo) values of which should be as different as possible. Hence optimization of the separation means searching for a compromise between solubility and selectivity. The mixing enthalpy is the smaller, the greater is the chemical resemblence between the solute and the solvent. Solubility parameters can be calculated from increments G [775-7761 according to the equation (after Small [775]), where 6

=

solubility parameter ((4.187 J/cm’)O.’],

& = density of the liquid stationary phase [g/cm3],

Mo = base unit weight [e.g., (CH3)*Si0= 74.16 for poly(dimethylsiloxanes)]. Some increments are listed in Table 42. Solubility factors, taking into consideration apolar dispersion and polar orientation, electron acceptor, proton donor and basicity factors derived from McReynolds data, of 207 stationary phases for 240 solutes can be taken from ref. [777]. Solutes that are chemically similar to the liquid stationary phase will be eluted according to the order of their vapour pressures at column temperature (because of small/or negligible differences in their yo or AHE- values (73)). In conclusion, it is thought advisable to emphasize that the Kovlts’ retention indices or McReynolds constants, because they are relative data, are not suitable (and not thought) to

194

8. Liquid Stationary Phases

Table 42. Increments G for the Calculation of Solubility Parameters According to eqn. (185) (after Gliickner [776]) Structural Unit

Structural Unit

0

G

~~

=CO

(ketones)

215

400-

(esters)

3 10

-93

-0-

(ethers)

I0

-H

(variable)

six-membered ring

214 111 190 95-105

five-membered ring conjugation phenyl phenylene (0, m, p)

105-11s 20-30 735 658

133

-CHz-

I I I

28

-CH-

-c

I

-

4%

-CH= CHI=

-CN 4 1 4Clz 4x1,

-CF*-CF3 -S-

-SH -Si-

(in silicones)

80-100 410 270 427 657 150 214 225 315 -38

supply absolute solubilities of different liquid phases. As regards the Kovhts retention index, it is clear that the same retention index for a solute on two or more liquid phases does not necessarily mean that they are chromatographically equivalent. The retention index is a measure of the solute retention relative to n-alkanes, and the same Zrvalues of a solute i measured on different liquid phases may signify only exceptionally the same solubility (and hence retention volume) of the solute, and more frequently equivalent changes in the solubility of n-alkanes. Therefore, absolute retention parameters such as partition coefficients or specific retention volumes have to be used for comparison purposes instead, as they are expressions of the free standard enthalpy in the partition equilibrium, AG:ij [778]:

(Mi = molecular weight of compound i). With regard to the Rohrschneider/McReynoldsconstants, the non-polar dispersion contributions of retention remain unconsidered (A1 = Zpo'ar - Z n o n - P O ' ~ and hence, details of the solubility, in contrast to non-dispersive interactions of the solutes in the stationary phase, cannot be expected.

8.1.7.

Purity and Homogeneity

In the course of time, technical grade liquid stationary phases have rationally been replaced by refined products. The gas chromatographic grade phases generally (and should always) exhibit batch-to-batch reproducibility, monitored by Kovhts retention indices and McReynolds constants, and must not be composed of isomers, perhaps in unreliable concentrations. In order to enhance the thermal stability and to shorten the conditioning time, low-molecularweight moieties and basic, acidic and metallic catalyst residues must be removed from polymeric liquid stationary phases, and reactive terminal groups should be substituted in an end-capping reaction, as otherwise at higher temperatures there is the risk of depolymeriza-

8.1. General Properties of Liquid Stationary Phases

195

tion, beginning at the ends of the polymeric chains in a chain unzipping reaction. For highefficiency columns that are intended to be used for long periods, the liquid stationary phase must be especially purified and homogenized, and it is advisable to investigate it before use by, e.g., infrared (IR), nuclear magnetic resonance (NMR), and atomic absorption spectroscopy (AAS) and size exclusion chromatography (SEC). SEC will provide information on the molecular weight distribution and on the content of low-molecular-weight constituents, AAS on traces of metallic catalysts, and both IR and NMR spectroscopy on the presence of undesirable functional groups in the stationary phase. For example, silicones, polyethylene glycols and polyesters often contain traces of strong acids, Lewis acids and strong bases that would cause degradation of the stationary phase at higher temperatures. Cyano-containing organic and polysiloxane phases often show a distinct content of carboxamide groups, and many didecyl phthalate commercial products are mixtures of isomers. Apiezon L, a frequently applied stationary phase, consisting of branched aliphatic hydrocarbons, contains some (nonspecified!) aromatic and olefmic components and also carbonyl and carboxylic groups. These few examples, selected from numerous others, indicate that it is necessary to use only chromatographic liquid stationary phases, i.e., polymers and well defined compounds specially prepared for gas chromatographic applications.

8.1.8.

Selectivity

Because of its great importance for gas chromatographic separations, we have already thoroughly discussed this topic, and it is unnecessary to do so again. The reason that it is mentioned here at all is that this essential property belongs to the properties of liquid stationary phases dealt with in this chapter and must not be consigned to the background.

8.1.9.

Specifications of Stationary Phases

Beginning with Section 8.3., liquid stationary phases will be discussed in detail in this chapter, and suitable solvents and minimum and maximum recommended operating temperatures (see Sections 8.1.2. and 8.1.4.) will be given. However, the maximum temperatures are only guide values and most of them are valid only when using a thermal conductivity detector. For the more sensitive ionization detectors, phase bleeding might cause too high an ionization noise current and baseline drift, and the listed temperature should be lowered by ca. 50-90°C. In order to achieve greater clarity, the stationary phases have been arranged according to their chemical families in this chapter. As a result, overlapping cannot be avoided; e.g., bis (2-propionitrile) ether will be dealt with under the nitriles, although it also belongs to the ethers. Within the families, the liquid phases have been further subdivided into groups with similar selectivities, hence exhibiting similar separation properties. Naturally, this holds good merely for the selectivity term (rz, - l)/r2, in eqn. 100. The other terms in this equation (capacity and efficiency terms) have not been taken into consideration, as they depend on the practical behaviour of the liquid phases (solvent power, viscosity, molecular weight, temperature dependence of the viscosity, etc.). McReynolds constants have been listed for each group as far as they are known. They cannot predict the quality of a gas chromatographic separation, for the reasons just mentioned, and do not provide evidence on efficiencies, peak tailing, temperature dependence of the separation, etc. However, they provide the best possibility for comparing the selectivities of stationary phases and indicate which other phases could improve a separation. The constants have been listed using McReynolds’ [331-3321, Haken’s [780, 7821, Supina’s [329], Lurje’s [784], Itsikson’s et al. [785] and Korol’s data [781].

196

8. Liquid Stationary Phases

8.2.

Hydrocarbons

8.2.1.

Aliphatic Hydrocarbons

It is not surprizing that liquid stationary phases of this type are very good solvents for hydrocarbons, hence yielding, compared with other stationary phases of comparable viscosity, very high specific retention volumes, 4,for hydrocarbon samples. The partition coefficient for nalkanes is exclusively determined by London dispersion forces that act between CH3 and CH2 groups from both solvent and solute molecules. They increase, in a direct proportionality, with increasing molecular weight. Also with polar and polarizable solutes, dispersion interations occur caused by CH3, CH2 and CH groups of the solute and by contributions of its polar groups. Branching of isomeric alkanes seems to reduce the dispersion forces involved in retention due to orientation effects, and halogen substitution also lowers the dispersion interaction, presumably because a proportion of the halogen electrons are unable to participate in London dispersion interactions, whereas for cyclic solutes an enhanced dispersion interaction can be assumed [779]. One could be tempted to conclude, because of the occurrence only of dispersion forces for all types of molecules, that hydrocarbon liquid phases do not show any selectivity for the samples to be separated. Naturally, this holds true for hydrocarbon compounds, and they are generally eluted in order of their vapour pressures at the column temperature. However, owing to distinct differences in the magnitude of apolar forces between these apolar liquid phases and solutes of different polarity, hydrocarbon stationary phases can selectively separate sample constituents of different polarity, especially oxygen-containing from oxygen-free compounds. This fact is made use of for the determination of polar impurities in hydrocarbons. These stationary phases are, according to investigations of the author, also appropriate for the separation of peffluorocarbons from partially or non-fluorinated hydrocarbons. The retention increases with decreasing fluorine content and peffluoro compounds are eluted before the corresponding fluorine-free compounds [716]. Owing to their chemical inertness, polymolecular hydrocarbons are suitable liquid stationary phases for nearly all types of organic and for many inorganic compounds and even, disregarding free halogens and similar oxidizing substances, for some aggressive compounds. However, the main application of hydrocarbon liquid stationary phases, owing to their non-polarity, consists in their use as standard reference phases with "zero" polarity, especially for the determination of Rohrschneider and McReynolds constants, widely used for the characterization and comparison of liquid stationary phases. The most important reference phase is squalane, although it can be only used at moderate temperature (up to 120°C). An important disadvantage of non-polar stationary phases must not be overlooked. They are more susceptible to mixed retention mechanisms, caused by active sites on the column walls or solid support particles, than polar stationary phases, in the case of polar and polarizable solutes. It is for this reaon that the retention indices of such solutes can differ widely, are influenced by the sample, size stationary phase loading, type of support (packed columns) or film thickness and deactivation treatment (open-tubular columns), and that peak tailing occurs. In order to keep the variation in retention indices as small as possible, both the column loading and sample size should be high (e.g., at least 10%(w/w) loadings and mole per gram of stationary phase in the case of packed columns [786]). Squalane Structure: 743

(CH3)2CH-(CH2)3-

B.p.: ca. 375°C

CH3

I

7%

FH3

CH-(CHz) ,-CH-(CH2)4-CH--ICH2)3-CH-(CH2)3-CH

-

d:' : 0.805 g cm-3

(CH3)2

197

8.2. Hydrocarbons

Minimum column temperature: 20°C Maximum column temperature: 120°C Solvent: toluene, n-hexane Commercial names: Embaphase, Squalane (Perkin Elmer) For retention indices of McReynold's sample compounds on squalane at 120°C see Table 9 (Section 4.2.4.). Application: Standard reference phase for Rohrschneider and McReynolds constants. Hydrocarbons; inorganic and organometallic compounds; phosphorus, nitrogen and halogenated compounds; aldehydes and ketones. Suitable for both packed and open-tubular columns (e.g. [787], [788]. Apolane-87 (c87 hydrocarbon) Structure: c , ~ H ~ ~ c 2 H5 I

I

CH-(CH2)4

-C

Cl8H37

I

-(CH2)4 - C H

I

I

I

Cl8 H37

C2H5

Cl8H37

Molecular weight: 122,37 g . mol I. M.P.: 28-34°C Minimum column temperature: 35°C Solvent: n-hexane

di30 :

0.776 g

Maximum column temperature 260°C

Retention indices of McReynolds' sample compounds (after Haken and Vernon [789]). Column temperature "C

Ibenzcne

I,-butanol

12-pentanone

12-nitropropane

Ipyridinc

120 140 160 180 200

674 679 684 691 705

600 596 594 596 596

630 630 631 633 635

664 665 671 678 684

724 728 735 744 753

Kovats retention indices of many isomeric alkanes, alkenes, alkynes, alkanols, alkanones, halogenated alkanes, ethers, nitro compounds, nitriles, aromatics and heterocyclics have been reported by Korol' [790]. C8,-hydrocarbon (24,24-diethyl-19,29-dioctadecylheptatetracontane) was first prepared and reported by Riedo et al. [791] and investigated by Haken et al. [789, 7921 and Castello and DArnato [793]. It is supplied by Applied Science Division, Milton Roy Co. Although it is more "polar" than squalane, as can be seen in Table 43, C,,-hydrocarbon has the great advantage of being more thermally stable. Squalane does not show any alteration on heating to lOO"C, and is scarcely affected between 100 and 120°C. Above this temperature, however, chemical alteration has to be taken into account, whereas Apolane-87 can be used (in the absence of oxygen possibly contained in the carrier gas!) up to 260°C (thermal conductivity detector). Hence it is appropriate for use as a reference phase for McReynolds constants at appreciable higher temperature. However, other liquid stationary phases have also been proposed as non or only slightly polar reference phases, e.g., hydrogenated Apiezon M (Apiezon MH) (discussed below) and poly(diethylsiloxanes), both of which exhibit a similar low degree of polarity as Apolane-87. Moreover, poly(dimethylsiloxanes), the most commonly used liquid stationary phases, although more polar, can also be applied as standards for high-temperature characterizations of stationary phases, if the retention index difference A I are corrected for the difference IFEp - I;!::? (see Section 9.2).

198

8. Liquid Stationary Phases

Table 43. McReynolds Constants of Aliphatic Hydrocarbon Liquid Stationary Phases (at 120°C) Stationary Phase

X

Y

Z

U

S

H

I

K

L

M

Squalane Hexatriacontane Apolane-87 Apiezon L Apiezon M Apiezon N Apiezon I Apiezon H Apiezon W Paraffin oil Nujol Asphalt Convoil20 Oronite Polybutene 32 Oronite Polybutene 128

0 012 021 035 031 038 038 059 082 011 009 019 014 021 025

0 002 010 028 022 040 030 086 135 006 005 058 014 029 026

0 -003 003 019 015 028 027 081 099 002 002 014 008 024 025

0 001 012 037 030 052 049 151 155 007 006 021 017 042 041

0 011 025 047 040 058 057 129 154 013 011 047 021 040 042

0 0

0 010

0 002

0 005

0 008

016 012 025 023 046 090 002 002 021 010 018 014

036 032 041 042 053 093 012 009 016 015 024 029

011 010 015 015 023 042 002 002 005 005 008 008

033 028 043 042 081 109 009 006 021 014 040 043

033. 029 035 035 037 059 009 006 010 010 024 033

n-Hexatriacontane Structure: CHp(CH2)&H3 Bq.: 265OU1.33 lo2Pa Minimum column temperature: 76°C Solvent: n-hexane n-Tetracosane Structure: CH3(CHZ)&H3 B.P.: 370°C Minimum column temperature: 52°C Solvent: h-hexane n-Octadecane Structure: CH3(CH2)&H3 B.P.: 317°C; M.P.: 27°C Minimum column temperature: 30°C Solvent: n-hexane, toluene n-Hexadecane Structure: CH3(CH2)&H3 B.P.: 280°C; M.P.: 20°C Minimum column temperature: 20°C Solvent: n-hexane, toluene Triisobutylene Structure: mixture of three isomers:

4: 0.764 g .cm-3 Maximum column temperature: 150°C

d:' :0.779 g * cm-3 Maximum column temperature: 140°C

d:' :0.775 g * cm-3 Maximum column temperature: 55°C

do:0.775 g * cm-3 Maximum column temperature: 30°C

199

8.2. Hydrocarbons

B.P.: 175°C Minimum column temperature: 20°C Solvent: n-hexane, toluene

d:' : 0.759 g * cm-3 Maximum column temperature: 20"C, but with distinct volatilization

Paraffin oil (Nujol) Structure: mixture of alkanes, cycloalkanes, etc., of varying composition. B.p. about 350°C Minimum column temperature: 20°C Maximum colum temperature: 100°C. Paraffin wax Structure: mixture of linear and branched alkanes, solid at room temperature B.p. about 350"C, M.P.: between ca. 30 and 130"C, predominantly 50°C. Minimum column temperature: 55°C Maximum column temperature: 140°C Asphalt Structure: petroleum residues, consisting of high-molecular-weight hydrocarbons, mixed with small amounts of oxygen-, nitrogen- and sulphur-containing compounds. Properties are strongly batch-dependent. Maximum column temperature: 120°C Solvent: xylene Polyethylene Structure: (-CH2--CH2-),, only very small amounts of CH, groups, which are lower for low- than for high-pressure polyethylene. M. W.:high pressure type: 10 000- 50 000 low-pressure type: 50 000-300 000 Minimum column temperature: 1120°C (high-pressure polyethylene), 125-134°C (low-pressure polyethylene) Maximum column temperature: 240°C Solvent: xylene, toluene Apiezons Even if a small content of phenyl groups, olefinic, carbonyl carboxylic and ether groups has been indicated, Apiezons are dealt with here as they consist mainly of branched hydrocarbons of molecular weight up to 15,000, and as they show similar properties and interactions to the other liquid stationary phases of this group (Section 8.2.1) (compare Table 43). There are several types of Apiezon (Apiezon A, B, C, H, I, K,L, M,MH,N, T and W). As they are generally technical-grade products, the batch-to-batch reproducibility normally does not meet the requirements for modem gas chromatographic columns. Therefore, only a few types (Apiezon L and MH) are suitable, although many papers have been published on the gas chromatographic applications of, e.g., Apiezon M and N. Apiezon L Structure: branched aliphatic hydrocarbon mixture with small amounts of olefinic, aromatic, -0, COC, and COOH groups. Maximum column temperature (after condiMinimum column temperature: 50°C tioning) : 270°C (TCD) 200°C @ID) (identical with the liquid phase Perkin-Elmer Q)

200

8. Liquid Stationary Phases

Solvent: n-hexane, chloroform Apiezon MH (hydrogenated Apiezon M [789]) Structure (supposedly):

M. W.:950-1000 Minimum column temperature: 50°C Solvent: n-hexane, chloroform

Maximum column temperature: 250°C

Retention indices of McReynolds sample compounds (after Haken and Vernon [789] and Korol' [794]) Stationary Column Phase temperature ["CI

1benzene

11-butanol

12-pentanone

knitropropane

Ipyridine

ApiezonM 120 Apiezon 120 MH 140 160 180 200

680 674 678 684 691 705

611 600 594 593 593 593

643 634 633 634 634 638

680 670 67 1 677 683 691

744 729 732 740 748 757

Application: Standard reference phase for higher temperatures. Analysis of high-boiling aromatic hydrocarbons and numerous organic and inorganic compounds In Table 43, the McReynolds constants of several aliphatic hydrocarbon liquid stationary phases are listed. The symbols of the McReynolds' sample compounds refer to the capital letters used for the constants (compare Table 9., Section 4.2.4). Recommended selection: Squalane (at temperatures < 120°C) Apolane-87 or Apiezon MH (at temperatures between 100 and 250°C).

0.2.2.

Aromatic Hydrocarbons

Although this group generally shows similar gas chromatographic behaviour to the previous group, there are several characteristic differences, that are the reason for this separate treatment. First, aromatics are selectively retained, owing to the ability of compounds with chemically similar structures to form association compounds. This association tendency between an aromatic liquid phase and an aromatic solute is lowered if the latter contains alkyl groups on the ring system, which disturb the regular spreading of the sr-electron cloud by steric hindrance [795]. Hence, the presence of the alkyl groups decreases the selectivity. Second, the interaction of the liquid stationary phase with the sr-electrons causes electron acceptors to be retained more strongly than on non-aromatic hydrocarbons. Third, polar compounds may induce a dipole moment into liquid phase molecules containing phenyl and phenylene groups, and the resulting attraction effects a small increase in retention.

201

8.3. Silicones

Benzyldiphenyl Structure:

B.p.: ca 285"C/14,7 kPa Minimum column temperature: 60°C Maximum column temperature: 100°C. Solvents: acetone, chloroform Applications: aromatics, heterocyclics, halogenated compounds, alcohols.

Alkylnaphthalenes Structure: alkylated naphthalene, alkyl >C20H41 Minimum column temperature: ca. 50°C Maximum column temperature: 280°C Solvent: toluene Commercial name: Fluhyzon. Polyphenyl tar Structure: tar product containing a large number of phenyl and phenylene groups. M.W. 800-2 100 Maximum column temperature: 280°C Minimum column temperature: 230°C Solvent: toluene Stilbene trimer Structure: phenyl-substituted indane derivatives Minimum column temperature: ca. 60°C Maximum column temperature: ca. 260°C Solvent: toluene SP 525 (polyaromatic, Supelco) Minimum column temperature: 60°C Solvent: toluene

McReynolds constants (at 120°C): Y z U 225 255 253 368

X

Maximum column temperature: 270°C

S

320

H 190.

Recommended phases for this group: none, because more suitable products (phenyl ethers, phenylsilicones, trifluoropropylsilicones)have largely replaced the aromatic hydrocarbons used in the early days of gas chromatography.

8.3.

Silicones

Poly(organosi1oxane) liquid phases are the most popular and most commonly used stationary phases in gas chromatography. The reasons can be found by considering the requirements that a stationary phase has to meet, as discussed in Section 8.1. Silicones, in contrast to organic phases, ideally meet most of these requirements. Some of the most essential advantages that silicones exhibit for gas chromatographic applications are their chemical inertness, their advantageous thermal behaviour, the great range of selectivity that they cover owing to the possibility of introducing substituents of different structures and polarities into the siloxane back-bone, their good solubility for numerous solutes, the low surface tensions permitting good wettability of different substrates (solid supports, inner walls of glass and quartz open tubular columns) especially by poly(dimethylsi1oxanes) and the possibility of cross-linking after impregnation and hence of immobilizing the liquid film on the substrate. These unique properties have conferred a unique position upon

202

8. Liquid Stationary Phases

silicones in the field of gas chromatographic stationary phases. Therefore, they are dealt with in this book in greater detail than other liquid stationary phases. The term “silicone” applies to any organosilicon compound with the organic group (R,,SiOF)m, where n = 1,2 or 3 and m 2 2. The termination “... one” of silicone was coined by Wohler in 1857 to describe compounds having the empirical formula R2Si0, as he erroneously assumed that the oxygen was bound to Si by a double bond by analogy with organic ketones, R2C0. The correct terminology for such compounds is organosiloxanes and, for polymers, poly(organosi1oxanes).The number and type of the organic substituents are further specified, e.g., for exclusively methyl groups and unbranched-chain products, the term is poly(dimethylsiloxanes),

The repeating unit of this polymer is the dimethylsiloxane unit, referred to as the D unit C? I

(from “difunctional”). Monofunctional (M) units, e.g., CH3- Si -0-,

I

are used to termi-

CH3 nate a chain, and we have the well known methylsilicone liquid stationary phases (OV-1, OV101, SE-30GC, SP-2100, AS-100, PMS-100, SKT) (CH3)3Si-O--[Si(CH3)20] ,,Si(CHJ3, which can be abbreviated in organosilicon chemistry to MD,,M. In contrast to these trimethylsiloxy terminated poly(dimethylsiloxanes), silanol terminated poly(dimethylsi1oxane.s) HO-[Si(CH3),0],,H, are distinctly less stable, as they may undergo condensation and depolymerization reactions, especially at higher temperatures. They should therefore not be used as liquid stationary phases, unless end-blocked by an appropriate end-blocker (silylating agent) in a successive reaction. The starting materials for the synthesis of poly(organosi1oxane) stationary phases are most often diorganodichlorosilaes. By their hydrolysis, a mixture of low-molecular-weight organocyclosiloxanes and low-to high-molecular-weight silanol-terminated linear compounds is formed. The silicone layer is separated from the layer of aqueous hydrogen chloride, washed and neutralized, and the cyclosiloxanes are distilled off immediately or after catalytic (alkali) depolymerization of the entire neutralized organosiloxane layer, whereby small-ring cyclic compounds (mainly D3-Ds) are formed. An alkaline catalyst and an end-blocker are added to the cyclics, and by ring opening polymerization, high-molecular-weight organopolysiloxanes are produced, terminated with the end-blocker. The methyl groups in a silicone may be substituted, choosing the corresponding diorgano or organomethylchlorosilanes, by, e.g., phenyl, biphenyl, tolyl, vinyl, trifluoropropyl or cyanopropyl groups, the type and concentration of the substituting groups permitting a variety of selectivities to be adjusted and permitting consecutive reactions, e.g., cross-linking. With the exception of poly(ethy1ene oxides), the different types of non-,medium- and highly polar poly(diorganosiloxanes) and, additionally, the chiral and mesogenic polysiloxanes may be the only reasonable liquid phases for pure siliceous substrates [inner walls of fused silica and glass (treated to obtain a pure silica surfaces) capillaries and pure silica-based solid supports such as Volasphers]. Some outstanding properties (filmformation and stability, cross-linking ability, low glass transition temperatures, thermal stability and high diffisivity) of silicones have been discussed in Section 8.1 and in previous chapters. However, because of the impor-

8.3. Silicones

203

tance of these properties for the practical use of a liquid stationary phase, we shall deal with them in more detail. The energy of the Si-0 bond (ca. 370 kJ/mol[796]) and its polarity (40% ionic character [797]) in a polysiloxane chain are distinctly higher than the corresponding values in organic polymers ( C - C = 347 kJ/mol, C-0 = 350 kJ/mol [796], ionic character of C-0 = 20% [797]), and the Si-0 and C - C bond lengths are 1.6 and 1.54 A, respectively [798, 7991. The sums of the ionic radii (1.71 A) and of the atomic radii (1.83 A) of Si and 0 are higher than the measured value (1.63 A), the polar component and d,-p, bonds between Si and 0 being responsible for this difference. The polar and (d-p)n bonds, being non-directional, give rise to very open, flexible chain structures: the bond angles of between 130" and 151" (the latter value for hexamethyldisiloxane [goo]) lie between the values of 90" (which would be expected for an exclusively covalent a-bond) and 180" (which would be expected for a pure double bond), and the barriers to rotation are considerably lower in siloxane structures (Si--CH3, 6.7kJ/mol, Si-0, <0.8kJ/mol) than in a carbon backbone (C-CH,, 15kJ/mol, C-0, 11.3 kJ/mol) [801, 8021. The wide Si-0-Si bond angle and the low barrier to rotation around the Si-0-Si bonds strongly influence the physical behaviour of siloxanes: they occupy a larger molecular volume than do comparable organic compounds, which results in low cohesive energy densities and extremely low intermolecular forces, which in turn explain, e.g., the low surface tensions (see Section 8.1.5), the low viscosity-temperature-coefficients, the desirable low-temperature properties of poly(organosi1oxane) fluids and elastomers (lower glass transition temperatures than any other polymer, see Section 8.1.4 and the low values of the activation energy of viscous flow (Evis[kJ/mol]) for polyisobutylene M. W.3000 = 59, for polypropylene oxide M. W. 2000 = 38, for poly(methylphenylsi1oxane) M. W.5000 = 27, for poly(dimethylsi1oxane) M. W. 200000 = 17, for M2D,,M. W. = 1200 [803] = 16, for poly (methylethylsiloxane) = 19 and =33 for poly(methyltrifluoropropylsi1oxane) [802]. These properties contribute to the advantageous spreading behaviour and are especially desirable for quartz and glass surface coating purposes. When dissolved in an organic solvent or spread on surfaces, poly(dimethylsi1oxanes) are assumed to occur in coiled structures, and in the absence of solvents they have a helical configuration. The surface area per monomer unit in a spread or helix structure is 0.23 or 0.10 nm2, respectively [810]. For the thermodynamics of PDMS-solutions see ref. [826]. The presence of substituents other than methyl, e.g., phenyl, trifluoropropyl or 3-cyanopropyl, disturbes the helix structures and increases the temperature dependence of viscosity. The high thermal and oxidative stability, compared with organic polymers, is based on the high energy of the Si-0 bond, whereas the Si-C bond energy is lower and depends on the type of substituent: S i x bond energy [kJ/mol] = 310 for Si-CH3, = 260 for Si-C2HS and = 226 for Si-C4HSr i.e., the longer the substituting chain, the lower is the Si-C bond energy and hence the lower the thermal stability [803]. Provided that all traces of substances that catalytically promote thermal degradation reactions are reliably absent, poly(dimethylsi1oxanes) have been shown to be thermally stable up to 400°C [804]. By purification of endblocked poly(dimethylsi1oxanes) of various viscosities, e.g. 10 Pa * s, using procedures developed by Miiller [804] in the author's laboratory, highly pure products can be obtained, the thermogravimetric measurements of which between ambient temperature and 450°C provide evidence that up to 400°C in an inert pure nitrogen atmosphere the weight loss amounts to less than 0.3% (sample weight 4.3 mg, scan rate S"C/min)! [804] (see Figs.21 and 22). This result has been confirmed by Clamon and Semlyen [805] who also found that uniform cyclic and linear polydimethylsiloxanes do not begin to decompose until 400°C. If methyl groups are replaced by phenyl groups, the siloxane bond becomes stronger owing to the increase in the (d-p)n bond contribution, and the tendency for depolymerization reactions decreases. An important fact for good thermal stability is the absence of OH end groups, which cause depolymerization reactions starting from the chain ends. The decomposition rate of OH-

204

8. Liquid Stationary Phases

~ 0 0 - 1 0 0 0 0 ( N ~WT: l , 4,3235 mg , Scan Rote: 5.00deg/min

0.001 I

moo

I

80.00

I

I

I

I

120.00 160.00 2 ~ ~ 02aoo 0

I

I

2801~)

Temperature in

OC

3om

I

I

I

~0.00

~*LO.OO

TG

Fig. 21. Thermogravimetric analysis of a highly purified poly(dimethylsi1oxane) (NVG 700-10 000, Chemical Works Niinchritz, G.D.R.) compared with a technical grade poly(dimethylsi1oxane) of the same viscosity (10 000 CP= 10Pa * s) before removal of the low molecular weight species and of the catalyst (after Steimann and Miller [804]) terminated poly(dimethylsi1oxanes) is a factor of at least 30 higher than that of the corresponding end-blocked product [806]. However, the main factor causing decomposition reactions and hence undesirable column bleeding at higher temperatures is, owing to the ionic character of the Si-0 bond, the presence of traces (or even higher levels) of bases and acids, including Lewis acids. These catalytically acting substances may come from different sources. First alkaline catalysts are generally used for the polymerization of cyclosiloxanes to high viscosity fluids or gums. Any residues of such catalysts in the polysiloxanes would, even in parts per million amounts, cause continual depolymerization of poly(organosi1oxanes) to form volatile cyclosiloxanes, which are the compounds contributing predominantly to the bleed signal, and column stability will never be attained. Especially hexaorganocyclotrisiloxanes and octaorganocyclotetrasiloxanes are formed, as found in 1960 by G e m d et al. [807] and by the author [808]. The completeness of the removal or at least neutralization of the catalyst determines the quality of the siloxane phase. Some of the different commercial products will be evaluated according to results obtained from the literature or by the author, when discussing the individual liquid phases. Because of the wide variability of the catalysts applied during manufacture (alkali metal silanolates, tetramethylammonium hydroxide, tetrabutylphosphonium hydroxide, etc.), it is difficult for the chromatographer to select an appropriate purification step. The second source of deleterious compounds or sites is the composition of the supporting surface. Both the solid support surfaces and the capillary walls may strongly affect the silicone liquid phase, which here owing to the exceedingly thin film that is exposed to the supporting surface and to the carrier gas, is in a highly vulnerable state. There is the risk of the presence of traces of alkali metals on soda-glass capillaries, of traces of Lewis acids (Fe, Al

205

8.3. Silicones

\

OV-101, Meth.Silic.Oi1 ( N z ) WT: 1.88LOmg Scon Rate: 5.00deglmin OV-101, Meth. Silic. Oil [Air) WT: 1,7601 mg Scan Rote: 500deglmin NVG 700-10000, Meth.Silic.Oi1 (Nz) WT: 4.3235 mg

85.00

\

+

\.

Scan Rate: 5.00deg/min

1oaoo

-

c +J m ._

a] O V - I Meth. Silic. Oil IN21 WT: 3.9999mg Scan Rate: 5.00 deglmin b) O V - I Meth. Silic. Oil (N21 WT:2.1903 mg Scan Rate: 5.00 deglmin

3

s

cl OV-I Meth. Silic Oil (Air) WT: 2.2800mg Scan Rate: 5.

50.00 -

i -

0.00

'

I

I

I

1

I

I

I

I

Fig. 22 A, 22 B. Thermogravirnetric analyses of three specialty poly(dimethylsi1oxanes)(after Steimann. Miller and Rotzsche (804, 8271)

and B halogenides) on Kieselguhr supports and of acidic silanol groups or certain siloxane bridges. Each of these surface constituents may promote silicone degradation. For example, poly(dimethylsi1oxane)OV-101, coated on Volaspher A2 (which is essentially free from Lewis acids on the surface), does not show notable changes in the n, k and r values after re-

206

8. Liquid Stationary Phases

peated temperature programming between 100 and 350"C, whereas the same liquid phase, when coated on Chromosorb G (traces of Fe and Al on the surface!), is distinctly degraded, detectable by a decrease of more than 15%in n, k and r already after temperature programming three times between 100 and 350°C (4Wmin and isothermal heating at 350°C for 10 min) [809]. Further factors causing decomposition of silicone phases are exposure to air during and after the impregnation and the purity of the carrier gas (which ought to be free from oxygen and moisture). Oxidation can occur at the organic groups along the polysiloxane chain. Compared with hydrocarbon liquid phases, alteration of silicone phases by oxidation is negligible [8111. Nevertheless, oxygen should cautiously be kept away, in order to prevent changes in the Kovhts retention indices of polar compounds, even if the changes are only very small [8111. By infrared measurements on silicone liquid phases after extraction from column packings, which had been exposed to air for 8 h at 150°C immediately after impregnation, the poly(diorganosiloxanes), especially those containing trifluoropropyl groups, showed a few tenths of a percent of silanol groups which originally, of course, the liquid phase did not contain (812). Hence, it can be concluded that the oxidation takes place preferentially at the S i - C bonds between the organic groups and the siloxane backbone. This result, most pronounced for trifluoropropylsilicones, is supported by the fact that electron-accepting substituents in a silicone weaken Si-C bonds [803]. Further, the stability of silicone phases may be affected by the injection of acidic ar basic samples and by cross-linking agents, e.g., peroxides, which also form acidic decomposition products [813]. Contrary to the too general statement that is occasionally made, that the thermal stability of silicones would be enhanced and column bleeding would be decreased by cross-linking, there is no such a simple connection. Even if with three-dimensional structures the bonds have to be ruptured at two or three points for cleavage to form volatile compounds responsible for a rise in the baseline, this restriction for thermal degradation is overcompensated by two facts. First, the number of cross-links is insignificant compared with the lengths of vulnerable chain segments between the cross-links. Second, if cross-linking has been carried out too vigorously (type and concentration of peroxide, temperature, high-energy radiation), some Si-C bonds may have been split, and the resulting RSiOI.Strifunctional units would be less thermally stable than the previous D unit [803]. Nevertheless, cross-linking and bonding, as already stated in Section 3.3.3.3, of silicone stationary phases is very advantageous: the redistribution of the liquid phase on the surface at elevated temperatures is hindered, hence hindering the decrease in column efficiency; the bonded phase is not easily displaced from the substrate when strongly polar samples and large volumes of sample solvents are injected, and columns may be washed in order to remove non-volatile deposits. Extensive thermal degradation studies of silicones for gas chromatography have been carried out. In addition to cyclosiloxanes, several linear compounds, MD,M in the case of methylsilicones, have been found as decomposition products [745, 759, 807, 808, 813-8171, and the prediction of the decomposition of poly(organosi1oxanes) at various heating rates may be possible by means of equations derived from differential thermal analysis [818]. It must be emphasized once again, that the only way a gas chromatographer can improve a commercial silicone is to remove low-molecular-weight species (provided they are present) by conditioning the column, i.e., by purging with an inert gas at elevated temperatures for several hours. Conditioning, however, will by no means obviate the troublesome influence of catalytically acting contaminants in the chromatographic column. If we examine phenylsilicones with regard to their thermal behaviour, it can be stated that here also cyclic and linear siloxanes are formed at high temperatures. They contain methyl and phenyl substituents, the structure depending on the type of phenyl silicone, e.g., cis- and trans-[(CH3)(CsH5)SiO],(n = 3-5) or [(CH3)2SiO],[(C6H,)2SiO],, where n = 1-6 and rn = 1-3

207

8.3. Silicones

[815, 8171. Two obvious differences from poly(dimethylsi1oxanes) are evident. First benzene is formed in small amounts, the formation being based on the cleavage of the Si-phenyl bond, the rate of formation decreasing with increasing molecular weight [815]. Nevertheless, phenylsilicones, when free from residual catalysts, have thermal stabilities higher than those of methylsilicones [745] and are more stable to oxidation [811]. Empirical equations for the calculation of weight losses of poly(methylphenylsi1oxanes) depending on their composition have been developed by Ostrovsku and Charitonou [819]. Thermal degradation of cyanoalkyl- and fluoroalkylsilicones also produces primarily the corresponding cyclosiloxanes, but at lower temperatures than for methyl- and phenylsilicones. Additionally, poly(trifluoropropylmethylsi1oxanes) form distinctly 1,l-difluoropropane by Si-C bond cleavage [806]. In addition to the previously discussed outstanding properties of silicones as liquid stationary phases, one aspect has only been incidentally dealt with, although it is extraordinarily important, namely the selectivity. Owing to the various possibilities of introducing substituents of extremely different types into the siloxane backbone, there is the possibility of adjusting the selectivity widely between poly(diethylsi1oxanes) (nearly polyolefin-like) over poly(methylphenylsiloxanes), poly(methyltrifluoropropylsi1oxanes) and poly(methy1cyanopropylsiloxanes) as far as to chiral polysiloxane phases for the separation of optically active compounds and mesogenic polysiloxane phases capable of achieving separations of compounds, e.g., polycyclic hydrocarbons, that exhibit differences in their molecular geometry. In this section the silicones will be discussed in groups of similar structures and/or similar gas chromatographic separation properties, and the selectivity and polarity can be deduced from the McReynolds constants.

8.3.1.

Poly(dimethy1siloxanes)

These are by far the most widely used type of organosiloxanes as liquid stationary phases and generally have only very low polarity, hydrocarbons usually being eluted in order of decreasing vapour pressures (or increasing boiling points). For the separation of oxygenated compounds, however, they are fairly selective. Owing to their high thermal stability and their simultaneously advantageous behaviour at low temperatures, they are suitable for use at both low and high temperatures and especially for analysing samples the constituents of which have a wide boiling range. There is hardly a class of volatile compounds that has not yet been investigated on a dimethylsiloxane liquid stationary phases, either isothermally or temperature-programmed, whether in packed columns or in open-tubular columns, with excellent results, from hydrocarbons to steroids and drugs. Therefore, it is not intended here to survey the innumerable gas chromatographic applications. This type of phase should be regarded as the first liquid stationary phase to be used if there is little information on the sample composition. The general formula of poly(dimethylsi1oxanes) is where n, the number of dimethylsiloxane units, generally ranges for gas chromatographic purposes from ca. 80 to 35 000,according to the viscosity range from ca. 100 mPa s (liquid) to 10 kPa. s (gum) and a molecular weight range from ca. 6000 to 2.5 lo6. The densities range from 0.966 to 0.98 g . ~ m - the ~ , pour points from -65 to -35"C, the viscosity-temperature coefficients from 0.60 to 0.62, and the refractive indices from 1.4025 to 1.4035 (for surface tensions see Table 43). It is advisable to use only special chromatographic grade products, in order to avoid the mentioned disadvantages of commercial quality products that have been provided for industrial purposes (containing low-molecular-weight species, residual catalysts, OH end groups,

-

208

8. Liquid Stationary Phases

unfavourable molecular-weight distributions). Moreover, the highly refined materials should be handled carefully, especially avoiding exposure to alkalis, acids or Lewis acids, as such contaminants would cause decomposition of the methylsilicones at elevated temperatures. It should be mentioned here that even in the case of deactivated and fairly coated column walls, there remains the influence of the liquid-gas and liquid-solid interface, contributing, due to adsorption processcs, to the retention: V, =

m

n

i- 1

1-1

1KLiVLi+

K A ~ A A (equ. ~

83a). It was

found that the second term may amount to 3 % (n-pentane) and 7 % (methanol), respectively, of the retention (glass capillaries, deactivated with poly(ethy1ene glycol), coated with poly(dimethylsiloxane), film thickness 0.1-0.3 pm) [8341. In Tables 44-45 to properties of speciality grade and in Table 46 those of industrial and obsolete poly(dimethylsi1oxanes) are listed. The data have been taken from the excellent reviews of Haken [820-8221,from Itsikson, Berezkin and Haken [785],McReynolds [324],Korol [781],Petrarch Systems [810],Luje (7841 and Supina and Rose [783],from publications and investigations by the author (740,762, 824,8251 and from other miscellaneous sources. As can be seen in Table 45,the selectivities of all speciality methylsilicones for gas chromatography are very similar. The minimum operating temperatures (Table 44) depend, as already discussed in Section 8.1.4,on the glass transition temperature T,,on the freezing point and on the melting ranges of the silicone concerned. Fig. 23 shows the low-temperature behaviour of OV-101,studied by differential scanning calorimetry. The preliminary cooling took place at a cooling rate of 160 K/min. With increasing temperature (scan rate 10 K/min beginning at 110 K), the glass transition is reached at 147 K (- 126.15"C),and the freezing point at 187 K (-86.15"C). Here the degree of order of the subcooled materials distinctly increases and crystallites are formed. This higher order Table 44. Properties of Specialty Grade Poly(dimethylsi1oxanes) for Gas Chromatography Phase

Supplier')

Viscosity

Mw

[mm2's-']

Density k .cm-'1

Solvent'')

at 25°C

ov-101

ov-1

SP-2100 PMS-100 SKT NVG 700 ASL-100

SE-30GC OD-1

JXR

ov ov SP

1500 ca. 3 . lo6 600 100

su su

9~10~

CWN AS GE AN AS

104 1.2.104 9.5.106 gum gum

30000

20 000 6000 300000 60000 68 000 106

0.975 0.980 0.972 0.968 0.970 0.976 0.976 0.98

T,H,P T

T,H,P T,H,P T T, p

T,H,P T T T

Suggested Operating Temperature I"Cl Min

Max

-30 +30 -30 -30 +30 -30 -30 +50

+350 +330 +350 +250

+300 +400

+300 +300

*) OV = Ohio Valley Specialty Chem. Co, Marietta, OH, USA SP = Supelco. Inc., Bellefonte. PA, USA SU = Soviet Union: Rcakhim CWN =Chemical Works, VEB Nuenchritz, GDR AS =Applied Science Labs., State College, PA., USA GE = General Electric Co.. New York. N. Y., USA AN = Analabs. Inc.. North Haven, CT.. USA '9 T = toluene, H = n-hexane. P = n-pentane. Chloroform, which would also be suitable, cannot be recommended as it might contain acidic contaminations that would catalyze the decomposition of the polysiloxanes. ***) Fig. 22 shows the thermogravimetric analysis of three specialty grade poly(dimethy1siloxanes) in purified nitrogen. Samples of weight between 1.7 and 4.3 mg were heated at a rate of 5 Wmin from 40 to 440'C. The weight loss reaches 1%for OV-1 at 330'C. for OV-101 at 350°C and for NVG 700 at 420°C [827,804]. These values have been taken for the maximal operation temperatures in Table 44.

209

8.3. Silicones

Table 45. McReynolds constants of Specialty Grade Poly(dimethylsi1oxanes)

ov-1 ov-101 SP-2100 PMS-100 SKT ASI-100 SE-30GC OD-1 JXR

X

Y

Z

U

S

H

I

K

L

M

16 11 11 15 11 11 16 16 16

55

44 45 45 43 46 44 44 45 44

65 61 61 65 61 61 65 66 65

42 43 43 42 45 43 42 42 42

32 33

4 4

23 23

46 46

-2 -2

51

51 55 51

55 53 53 55

44 41 32 32 32 32

3

22

44

-2

(enthalpy change ca. -28 J/g, interchain distance in the crystalline lattice about 0.3 nm) only exists up to ca. 226 K, where a discontinuous region begins with two well defined endothermic transitions at 228 K (-45.15"C, enthalpy change ca. +13 J/g) and at 240 K (-33.15"C, AH = 25 J/g), which are due to the melting of the crystalline material [827]. It has been found that cross-linked poly(dimethylsi1oxanes) still generate 3000 platedm at - 70"C, whereas for an uncured material such as OV 1 it drops to a few hundred platedm [761]. This effect has been traced back to glass transition phenomena [761]. We believe, however, that cross-linking hinders the chains from arranging themselves in a crystalline order. This assumption is supported by the low-temperature behaviour of poly(dimethylsi1oxanes) containing as low a content of phenyl groups as 5%, as can be seen in Fig. 24. In contrast to pure PDMS,the few but bulky phenyl groups prevent the polysiloxane chains from achieving a crystalline state. It can be recognized that the glass transition occurs at 158 K (-115.15"C), but both freezing and melting phenomena, which would be visible by distinct exo- and endothermic discontinuities, are absent. Hence it can be concluded that cross-linking will also improve the low-temperature behaviour of silicone liquid stationary phases, as the chains are no longer freely movable and cannot reach the degree of orientation necessary for the crystallization. Owing to the missing increase of order when lowering the temperature, the change from liquid to solid behaviour of cross-linked poly(dimethylsi1oxanes) does occur not until vitrification, whereas a quasi-solid state is reached for non-crosslinked poly(dimethylsi1oxanes) far above this temperature, at the melting points of the crystalline polymer (i.e., at - 33°C for OV-101). Some immobilized, i.e., cross-linked and chemically bonded, poly(dimethylsi1oxane) liquid phases in open-tubular columns are listed in Table 47. Owing to the largely good immobilization and non-extractability, these phases, crosslinked and in some instances covalently bonded to fused-silica column walls, can be used with all types of injection techniques, including on-column injection, without a decrease in efficiency. However, if cross-linking affects the McReynolds constants only slightly, as can be seen in Table 47 (asterisk) compared with Table 45, a slight polarity change has been found, perceptible by an increase in Kovhts retention indices between 3 (for alkylbenzene) and 18 (for 2,6-dimethylphenol) [829]. It can be assumed that one explanation for the slightly increased retardation ability of cross-linked phases for polar solutes may be, that in addition to the desired cross-linking reactions, other reactions with the peroxidic initiator take place, causing (at least to a minor extent) oxidation and/or substitution reactions with the stationary phase. In order to demonstrate the applicability of bonded poly(dimethylsi1oxanes) even for trace analyses at high temperatures, the analysis of a drug mixture using a thick-film open-tubular column is shown in Fig. 25 (after Duffy [831]).

Table 46. Properties of Miscellaneous Commercial Poly(dimethylsi1oxanes) Frequently Used as Liquid Stationary Phases*) Phase

Supplier")

Viscosity [ m m 2 . s-'1 at 20'C if not indicated

Mw

Density

McReynolds mustants

(p.em-31

X

Y

Z

U

S

H

16

51

45

66

43

33

13 15 17 18 17

51 56 58

42 44 47 41 41

61 66 68 68 68

36 40 46 44 46

31 32 34 34 34

OthCrwise

DC 123 DC 200 DC 220 DC 330 DC 400 DC 401 DC 410 Silastic 401 Silastic 132 Silastic 152 PMS- 1000 PMS-1 10' PMS-1 lo6

DCC DCC DCC DCC DCC DCC DCC DCC DCC DCC

su su su

SKTN

su

E-300 E-301 F 111 Silicone M-430 L-45 L-46 SF-96 SF-96-200 SF-96-2000 w95 W 950 KS-1014 MS-200 MS-2211 SE-30 SE-30 Ultrauhase Viscasil

ICI ICI ICI ICI

ucc

ucc ucc ucc ucc ucc ucc

ucc MS MS GE GE GE

0.97 (25°C)

1012-1014 0.5-1.105 44 50

1012-1014 gum gum gum gum gum

103 105 166 gum 1012- 1014 1-2.5. lo6 0.5-1.105 gum 0.5-1 * 10' 0.5-1.105 0.5-1.105 200 2.10' gum gum gum 0.5-1.105 1012-1014 9.5.106 107

105

0.6-1. lo4 3.5.104 2.2.10s 0.75-7.5. lo4

3.5.104

51

58

0.98 0.98 0.98 0.915

0.974

15 16 16

55 51

44 44 45

66 65 66

40 44 43

32 32 33

16 16 14 14 14

57 56 53 53 53

45 45 42 42 42

65 65 61 61 61

43 41 37 31 37

33 33 31 31 31

57

45 44 44 44 45

66 66 65 65 66

43 40 42 42 43

33 32 32 32 33

56

0.96

1-2.5*106

0.966 (25'C)

16 15 16 16 16

56 55

55 57

v, Phase

Supplier..)

Silicone oil 81705 NM 1-200 NM 1-100000 NM 1-300000 NG-100 Lukooil M 100 Lukooil M 200 Lukooil M 500 Lukopren M 50 Perkin-Elmer C Perkin-Elmer Z Bayer M DMF-05 DMF-1 DMF-12 DMF-30 DMF-100

GE CWN CWN

SP-70

SP PS PS PS MB

PS 041 PS 045 PS 050 Embaphase Oil

CWN

CWN

cs cs cs cs PE PE B I I I I

Viscosity [ m m 2 . S-'] at 20°C if not indicated otherwise

MW

Density

McRcynolds constants

L.~ m - 3 1 X

Y

Z

U

S

H

16 16 16

57 55 51

45 44 45

66 65 66

43 42 43

33 32 33

14

57

45

66

43

33

0.970 0.970 0.970

200 1.105

3.105 gum 100 200

0.975

500

gum 9.47.106 1-2.5'10' 5 . lo4 - 3 - lo5 500 (25°C) 103 ( 2 s ~ ) 1.2.104 ( 2 5 ~ ) 3.104 ( 2 5 ~ ) 105 ( 2 5 ~ ) gum

100 104 2.5. lo6 300-400

6 . lo' 6.3.104 4.2.105

0.966

0.972 (25°C) 0.972 (25°C) 0.973 (25°C) 0.973 (25°C) 0.973 (25°C) 0.966 0.974 0.978 0.970-0.973

*) Most of these products have been applied successfully as liquid stationary phases for many years. They should nevertheless be replaced by products specially prepared for their gas chromatographic w e in order to improve the reproducibility from laboratory to laboratory, to abbreviate conditioning procedures and to enhance the thermal stability. Owing to the batch-to-batch dependence of such products, operating temperatures have not been given here. They may be similar to those in Table 44. but may also be worse. The solvents for these industrid products are the same as for specialty poly(dimethylsi1oxanes).i.e., toluene, n - h e w e or n-pentane. Diethyl ether may also be used. Dichloromethane or chloroform should be avoided as there is the risk of the presence of traces of hydrochloric acid, which might cause depolymerization of the siloxanes. ") Suppliers OV, SP. SU, CWN, AS. GE, AN as in Table 44. DCC = Dow Corning Corp., Midland, MI, U.S.A. ICI = Imperial Chemical Industries Ltd., U.K. UCC = Union Carbide Corp., New York, N.Y.. U.S.A. MS = Midland Silicones. Barry, U.K. GE =General Electric Co.. New York, U.S.A. CS, = Synthesia K o l i Czechoslovakia PE = Perkin-Elmer Corp., Norwalk, CT..U.S.A. B = Bayer Farbenfabriken AG. F.R.G. I = Metroark Private Ltd.. Calcutta, India PS = Petrarch Systems, H u h America, Bristol PA., U.S.A. MB = May and Baker, Dagenham, U.K.

212

8. Liquid Stationary Phases

ii0.00

1

1

in00

i5am

1

170.00

I

1

I

igaoo 210.00 230.00 Temperature in K

I

I

I

250.00 270.00 290.00 DSC

Fig. 23. Low-temperature investigation of OV-101 by differential scanning calorimetry. Apparatus: DSC-2 (Perkin-Elmer);Pre-cooling to 110 K at a cooling rate of - 160 K * min-l (after Steimann and Rotzsche [827])

Normalized Methyl Silicone Gum SE-52 ( 5 % Phenyll WT: 6.93 mg Scan Rate: 10.00deg/min 0 -0 W c

V

21.00 4 E c ._

0.00I

'

iiam

I

13000

I

I

15a00 im.00

I

I

I

i9000 210.00 23000 Temperature in K

I

I

1

2 5 ~ ~ x 1270.00 290.00 DSC

Fig. 24. Low-temperature investigation of SE 52 poly(dimethylsiloxane/diphenylsiloxane) (5% phenyl) by difFerentia1 scanning calorimetry. Apparatus: DSC-2 (Perkin-Elmer);Pre-cooling to 110 K at a cooling rate of - 160 K .min-' (after Steimann and Rotzsche [827])

213

8.3. Silicones Table 47. Commercial Cross-Linked Poly(dimethy1siloxane) Liquid Stationary Phases Phase

ManufacturerlSupplier

Chemically Bonded Crosslinked Phase

Film Thickness [pm]

me

Durabond DB-1 BP-1

ov-1 SPB-1, SE-30 SP-2100 CP-SiL 5 CB*)

J. & W Scientific Inc, Rancho Cordova, CA, U.S.A. Scientific Glass Engineering Ry. Ltd., Victoria, Australia Phase Separations Ltd., Queensferry, U.K. Supelco, Inc., Bellefonte, PA, U.S.A.

ov-1 ov-1 ov-1 ov-1 ov-101 ov-1 ov-1

HP-1 HP-101 Permaphase DMS

Chrompack Nederland B.V., Middelburg, The Netherlands Hewlett-Packard Co., Analytical Group, Palo Alto, CA., U.S.A. Perkin-Elmer Corp. Norwalk, CT, U.S.A.

Rt,-l

Restek Corp. Bellefonte, PA, U.S.A.

ov-1

RSL-150 RSL-160 BP-1 007-1 NB-1 NB-30

Alltech, Applied Science Labs., Deerfield, IL, U.S.A. S.G.E., Victoria, Australia Quadrex, New Haven, CT, U.S.A. HNU-Nordion LTD OY, Helsinki, Finland

ov-1 ov-1 ov-1 ov-1

*)

McReynolds constants: X

= 15;

ov-101 ov-1

0.1 0.25 0.25

1.5 3.0 0.5

0.25 0.25 0.12 0.2

1.0 0.30 0.4

1.2

0.25 0.3 0.10 0.25 0.50 1.00

1.0 5.0 1.50 3.00 5.00 7.00

0.1 0.25

1.0

r = 50; Z = 41; u = 60; s = 36

In conclusion we shall consider whether silicone greases, used in the early days of gas chromatography, would be of advantage or not. Silicone greases are silicone oils to which fillers, most often Aerosil (finely dispersed and silylated SiOz), have been added. It is obvious that these fillers will behave as adsorbents and hence will influence the selectivity and cause tailing, an increase in retention and even irreversible adsorption of certain solutes. The application of silicone greases (e.g., high-vacuum silicone grease, Silastic 132 and 152) is therefore undesirable.

8. Liquid Stationary Phases

2 14

I

3

1

X

rl-

I

!

Fig. 25. High-temperature analysis of a drug mixture. Conditions: Bonded poly(dimethylsi1oxane) column, 25 m X 0,53 mm I.D.,dp 5.0 pm;column temperature, 240"C, held for 1min; 240-270°C at 8 K.min-I, then held at 270°C; flow-rate (He), 5 ml.min-'; detection, FID;sample, drug mixture, each peak representing approximately 400 ng of compound. Peaks: 1 amobarbital; 2 pentobarbital; 3 secobarbital; 4 phenobarbital; 5 methadone; 6 cocaine; 7 codeine; 8 morphine (after DYm, [831]) (courtesy of International Scientific Communications)

8.3.2.

Poly(dimethy1siloxanes) containing Small Amounts of Vinyl Groups

One could be inclined generally not to use methylsilicones that contain vinyl groups, because their presence is detrimental to the quality of a liquid stationary phase even if property changes are not pronounced, (the latter is not surprising as the vinyl content often is usually not more than 1%).However, this slight modification is very important for the preparation of non-extractable liquid phases on fused silica surfaces, as the presence of vinyl groups facilitates cross-linking when peroxide initiators are used [759]. Vinyl groups may be bound to the terminal silicon atoms of the poly(dimethylsi1oxane) chains [vinyldimethyl or divinylmethyl terminated poly(dimethylsiloxanes)] or may occur within the chains themselves [poly(dimethylsiloxane vinylmethylsiloxane)]: Me Me I I Me3Si

-O+

Si -0

I

Me

*" I -0t Si

Si Me3

CH

The latter type is generally used for free-radical coupling and addition reactions between vinyl and methyl groups (initiator: dicumyl peroxide), n being d m .

215

8.3. Silicones

In Table 48 some commercially available vinyl containing poly(dimethylsi1oxanes) and their properties are listed. These polymers do not differ from the vinyl-free counterparts with regard to selectivity, as can be seen by comparing the McReynolds constants. For the properties related to practical use, the same restrictions have to be made as for the industrial silicones listed in Table 46. For example, the maximal operating temperatures may range between 200 and 300°C after conditioning depending on the batch quality of the product. Table 48. Commercial Vinyl-Modified Poly(dimethylsi1oxanes) and their Properties.) Name

Supplier")

Vinyl Density comonomer k 'cm-'I

[%I

DC-430 E-302 E-303 LC-430 Lukopren GlOOO NG-300 OV-1 vinyl SE-31 Se-33 Silastic DCC 430

SKTV SKTV-1 UC-W 96 UC-W 98 UC-W 982 UC-W 960 PS 422 PS 426 PS 428

DCC JCJ JCJ DCC

cs

McReynolds constants

X

Y

Z

U

H

L

16 16 11 16

54 54 54 54

45 45 45 45

65 43 65 43

22 32 33 43

-

65

42 43

16 11 16 16

54 54 54 56

45 45 45 44

65 61 65 66

43 42 43 43

32 33 32

-

46

16

55

45

66 42

33

-

61

S

-

CWN

ov

GE GE

1 1

su su

0.4 0.18 1 1 0.15 1 1 1 4-5

ucc

ucc ucc PS PS PS

*) Mer (820-8221. (7851 and **) Suppliers as in Tables 44

8.3.3.

Viscosity [mm2's-'] at 20 "C

0.98 0.975

[810] and 46.

Poly(dialky1siloxanes)

Materials containing alkyl groups other than methyl have up to now barely been applied as liquid stationary phases, but to the author, this seems undeserved. In his laboratory, poly(diethylsiloxanes) have been demonstrated to show a gas chromatographic behaviour that is to some degree dissimilar to that of poly(dimethylsi1oxanes) and may be advantageous in solving certain separation problems. The main restriction for their limited application may be their lower thermal stability. However, also here it must be taken into consideration whether the material has previously been purified sufficiently. For example, thermal stability up to 350°C for poly(diethylsi1oxanes) [832]and up to 270°C even for poly(octylmethylsi1oxanes) [833]could be achieved. This fact is assumed to be based on the unchanged rearrangement tendency of the silicon-oxygen bonds to form cyclic siloxanes compared with the dimethylsiloxanes, and to the less extensive rupture of Si-C bonds than suspected. However, this is only one reason for adopting these types of siloxanes; a second is the already mentioned change in selectivity, as follows. The McReynolds constants (Table 49) are lower than those of the poly(dimethylsiloxanes), i.e., polar interactions with sample compounds are lower and approximate those of squalane.

216

8. Liquid Stationary Phases

Table 49. Poly(diaky1siloxane)Liquid Stationary Phases*) Name

Supplier

SKTE type A

su

VKZh-94 su PES-V-2 su PES-S-1 (132-24) SU PES-S-2 (132-25) SU PES-5 su PS 140 PS PS 914 DC 730

PS DCC

Composition (alkyl K per total substituents)

Density

Viscosity

McReynolds constants

at ZO'C

X

CH3-92 CZH5-8 C2H5-100 CzHs-100 CZHS-100 CzHS-100 CzHS-100 CHj-50 CBH17-50 CZHS-100 CZHS-100

0.975

gum

0.949 0.949 1 f0.05 1 f 0.05 1 f 0.02 0.907

40-52 40-52 210-315 180-305 200-500 500-800

[B .~m-'] (mrn2.s-']

Y

Z

U

S

L

I

14

18

21

26

6

10

14

17

22

24

7

7 After refs.I785 and 8101. Solvents: toluene, n-hexane, n-pentane, diethyl ether, dichloromethane. Abbreviations of suppliers: as in Tables 44 and 46.

However, the higher alkyl concentration (two CH3CH2 groups per siloxane unit for poly(diethylsiloxanes) compared with two CH, groups for poly(dimethylsi1oxanes)) will increase the dispersion interaction, increase the specific retention volumes of organic solutes, especially hydrocarbons, and hence improve the separation efficiency for volatile samples and sample constituents. A third, and perhaps the most important, point arguing for these alkylsiloxanes, is the better wettability of silica surfaces by these phases compared with hydrocarbon liquid phases (the latter have higher surface tensions, e.g., squalane 30.0 mJ m-2, and poly(diethy1siloxanes) only ca. 26 mJ. m-2), and the cross-linkability allowing high column efficiencies to be produced (e.g., a cross-linked poly(n-octyl methylpolysiloxane) phase, 0.3 pm film thickness, 15 m column, k' (n-dodecane) = 18 gave an efficiency of 4300 platedm [833]). Therefore, the applicability of poly(dialkylsi1oxanes) as immobilized phases in open-tubular columns may be expected to advance.

8.3.4.

Poly(methylphenylsiloxanes)

After poly(dimethylsiloxanes), phenyl-containing polysiloxanes have been the most popular liquid stationary phases. As early as 1952, James and Martin reported DC 550, a phenylsiloxane (phenyl content 25%) as a liquid stationary phase in their pioneering work [6]. The poly(methylphenylsi1oxanes) cover a wide range of phenyl contents (4-75 mole %) and hence of selectivity, which is a function of the phenyl content. The selectivity is based on both a strong dispersion interaction and a high polarizability of the phenyl groups (highest dispersion index of all liquid stationary phases dD = 11.6 for poly(methylphenylsi1oxane) [835]), distinct increase in the specific retention volumes of the McReynolds probe solutes with increasing phenyl content up to at least 35%with the exception of the lower homologues of nalkanes (below n-dodecane [836,837]) and, above all, a dramatic increase in the McReynolds constants with increasing degree of phenyl substitution; we shall return to this topic). The phenyl group can be introduced as a methylphenylsiloxane and/or as a diphenylsiloxane unit, both by copolymerization of cyclic oligomers, e.g., methylphenylcyclosiloxanes,octaphenylcyclotetrasiloxane and/or octamethylcyclotetrasiloxane. The phenyl groups introduce rigidity in the polysiloxane chain and to a certain extent oxidation stability up to ca. 250°C, but above this temperature the silicon-carbon bond is ruptured, leading to cross-linking and gummification of the phenylsilicone.

217

8.3. Silicones

In Section8.3 (before Section8.3.1) we have already dealt with the thermal stability of phenylsilicones. However, it must be emphasized once again that technical-grade materials should not be used without thorough purification. Fig. 26, for example, demonstrates that the weight loss of the industrial product SE-52 [a poly(dimethylsi1oxane diphenylsiloxane) with 5% phenyl groups] is 1%at 150"C, 3% at 200"C, 5% at 250"C, 6% at 300°C and 7% at 350°C in the absence of air, supposedly caused by low-molecular-weight moieties. In Fig. 27 it can be seen that phenylsilicone oils (phenyl content ca. 33%) are not seriously affected even in air up to 280°C (weight losses <2%), but that above this temperature the decomposition distinctly increases. It has been found [838] that the presence of the cyclic oligomer pentamethylpentaphenylcyclopentasiloxane [CH3(C6HS)SiOl5, is responsible for the chromatographic differences between batches of technical-grade DC 710, as the structure of the ring results in unusual interactions, recognizable through large AZ values of polar samples, which in turn correspond to large differences in A(AG) for these solutes. However, as the A H values of the solutes in this phenylcyclosiloxane are very small, it can be concluded that the entropy term must be distinctly larger than those for other oligomeric phenylsiloxane species [838]. There are further peculiarities of phenylsilicones. For example, the dependence of A H for propyl chloride on the phenyl content shows an anomaly between OV-17 (50% phenyl), on which A H increases with increasing phenyl content as expected, and OV-22 (75% phenyl), on which A H is surprisingly lower [839]. Another example is the fact that the specific retention volumes or heats of solution of polar and aromatic [836] and heterocyclic [840] solutes do not increase continuously with increasing phenyl content; they pass through a maximum for OV-11 (35%phenyl) and OV-17 (50%phenyl) and decrease significantly for OV-25 (65%and 75%) phenyl. On the other hand, the Kovhts retention indices and AZ values of the polar solutes increases, unlike the V,,K and A H values, with increasing percentage of phenyl groups in the stationary phase. However, both Z and AZ values can be misleading when interpreting

4'

100.00

-\ '*A

-\

\,b)

SE - 52, Meth. Silic. O i l , a ) WT 3.b729rng Scan Rate: 5.00 deglrnin b) Wl: 2.71 mg cl WT: 2.64 rng

.r K OI ._

2 s

' cl

'i

50.00 -

d0.00

8000

120.00 160.03 200.00 260.00 280.00 320.00 360.00 d00.00 Temperature in OC TG

Fig. 26. Thermogravimetric analysis of SE-52. Temperature programme from 40 to 400°C (after Steirnann and Rotzsche [827])

218

8. Liquid Stationary Phases

60-

NM 4146 [ 25%Phenyl)

, \ " c ._

PFMS 4 ( SO%Phenyl)

$ 40-

NVM 4180 (33%Phenyll

2

r r m ._

s"

20 -

0.

Fig. 27. Thermogravimetric analysis of poly(methy1phenylsiloxanes) in air. Heating rate, 8 K.min-’: sample 1 .phenyl . content 33%:sample 2 phenyl content 33% (different strutture); sample 3 phenyl content 66%;sample 4 phenyl content 66%-(differentstructure) (after R i m WaI)

solute-phase interactions, as they are relative data based on n-alkanes, and I or A I changes may be based on changes in the solubility (or retention volume) of the solute or on equivalent changes in the solubility of the n-alkanes [836].The anomalous behaviour of OV-22 and OV-25 may be caused by steric effects due to the bulky diphenylsiloxane groups in these phases. In addition to high dispersion and solute dipole-stationary phase induced dipole interactions, a low level of hydrogen-bonding acceptor character of phenylsiloxanes has also been observed [841].The polarizabilities of different aromatic substituents bound to silicon increase in the order

[841].Owing to the greater number of mobile n-bonding electrons (ten in two rings), the biphenyl group has a higher occupied molecular orbital of higher energy and hence higher polarizability than the phenyl group with six It-bonding electrons in one ring. This property offers a further improvement in the selectivity of isomer separations for any type (electron acceptor or donor) of polar solute, as was demonstrated on capillary columns coated with cross-linked polysiloxane polymers containing such side groups [841].Although such phases are not yet commercially available, they are discussed here in order to stimulate such developments. Spreading on silica surfaces and cross-linking could be achieved by introducing additional octyl and vinyl groups, respectively, in the siloxane chain [841].The wettability of silica surfaces deteriorates with increasing aryl content, owing to the increasing surface tension: a poly(dimethylsiloxane/methylphenylsiloxane) with 10% phenyl groups has a surface tension of 21.8 mJ m2 and with 50% phenyl groups 31.4 mJ m - z. The densitites increase from 0.997 (10% phenyl) to 1.150g-cm-3 (75% phenyl), and the refractive indices from 1.4436 (10% phenyl) to 1.5825 (75% phenyl). The increasing rigidity can be discerned from the increase in both the glass transition temperature [T,= -126°C (0% phenyl), -115°C e

219

8.3. Silicones

(5% phenyl), - 86°C (50% phenyl)] and the pour point [from - 100°C (0% phenyl) to -20°C

(50% phenyl)] for comparable molecular weights. Unfortunately, the diffusion of organic compounds in phenylsiloxanes is significantly slower than in methylsiloxanes and increases with increasing phenyl content, hence causing a distinct loss of column efficiency compared with poly(dimethylsi1oxanes). In addition, there is an anomaly with diffusion in OV-25 (75% phenyl groups) compared with phenylsilicones with lower phenyl content [843]. We have already repeatedly discussed the requirement that a polymeric stationary phase should not contain low-membered moieties. However, even special stationary phases such as OV-17, PFMS-4 and OV-25 are not free from cyclic compounds, as could be shown by sizeexclusion chromatographic analyses [785], to say nothing of the technical-grade products (DC 710, SE-52, SE-54 etc.). The only stationary phases that are tolerably uniform are OV-7, OV-61, SP 2250 and PFMS-6, as was confirmed by NMR spectroscopic investigations [842]. These studies, together with other NMR and W investigations, have revealed that the assumed phenyl content as stated by the manufacturer, does not necessarily correspond to the actual phenyl content [842, 785, 8371. This discrepancy is partially caused by the influence of the terminal (CH3)& groups which especially for low-molecular-weight phenylsilicones distinctly reduce the entire phenyl content. Solvents usable for phenylsiloxanes with up to 50% phenyl groups are toluene, n-pentane, diethyl ether and dichloromethane (caution should be observed because of the possible presence of traces of hydrogen chloride!). With higher phenyl contents, only diethyl ether or dichloromethane is suitable.

I?= CH3 or C6H5

In Table 50 special phenylsiloxane phases of the general formula are listed, and Table 51 contains industrial phenylsiloxanes of the general formula

R3Si -0

Si-Ov S iO v Si -0 r k e

Q

6

Si-0

I CHZ CH II

where R may be CH, or/and CsHS.These formulae refer to both methylphenyl and diphenylsiloxy units in the tables, as far as it is known. The maximal operating temperatures of the products listed in Table 50 should not exceed 300°C. Poly(methylphenylsi1oxane) liquid phases are almost as widely applicable as poly(dimethylsi1oxanes). For example, hydrocarbons, phenols, steroids, pesticides, alkaloids, drugs and derivatized compounds (carbohydrates, amino acids) can be well separated, and it appears hardly necessary to present a characteristic chromatogram here. We have seen in Section 8.3.1 that the high viscosity poly(dimethylsi1oxanes) are at present the most widely used stationary phases for open-tubular glass and fused silica columns, as they provide the highest efficiencies. Nevertheless, there remain unresolved resolution problems for the separation of critical solute pairs or clusters, which might be solved on a column of different selectivity. Moreover, it would be desirable for the chromatographic peak identification to have available also polar or/and polarizable liquid stationary phases of high efficiency, i.e., coated on capillary columns. However, spreading and cross-linking of polari-

Table 50. Specialty Poly(methylphenylsiloxanes)*) Name

SupplieP)

% Phenyl

Structure

dto k.cm-3)

OV-73 OV-3') OV-7 OV-61

ov-11 OV-17 ov-223 OV-25') SP 2250 PFMS-43 PFMS-6b) PFMS-6b) ASI-50

ov ov ov ov ov ov ov ov SP su SU SU AS

W

*

Y

n

[%I

[%I

[%]

(avcwe)

94.5 80 60 67 30 -

-

-

20 40

5.5

-

-

33

70 100 70 50 100 100 100 100 100

-

high 120 100 high

30 50

12 25 22

-

11 12

-

nominal

effective

5.5

5.5

10 20 33 35 50 65 75 50

33.5 41 64

1.oo 1.02 1.09 1.06 1.09 1.13 1.15

50 50

35 58

1.12 1.03

55

3 After refs. [324,842,820-824 and 781-7871 and data from the manufacturers. ") Suppliers as in Tables 44 and 46. ') b,

Product is not free from low-molecular-weightmoieties, which may cause bleeding R is (CH3) in all instances except for PFMS-6, where R = C ~ H S .

Viscosity

McReynolds constants

[mm*.s-'] at 25°C

X

Y

Z

U

S

H

K

L

gum 500 500 50 000 500 1300 50 000 100 000

40 44 69 101 102 119 160 178 119 105 164 119 119

86 86 113 143 142 158 188 204 158 145 192 158 158

76 81 111 142 145 162 191 208 162 149 198 162 162

114 124 171 213 219 243 283 305 243 223 288 243 243

85 88 128 174 178 202 253 280 202 185 262 202 202

57 55 77 99 103 112 133 144 112

39 46 66 85 105 132 147 -

-

-

-

1000 50

112 112

84 120 164 184 228 251

-

165 236

-

Table 51. Industrial Poly(methylphenylsiloxanes)*) Name

supplier

w

1x1

DC510 DC556 DC702 DC703 DC550 DC710 SE-52 SE-54 SKTFV-803 Lestosil**) SKTA-1**) DFOK**) PFMS-5 SKTF-100 Lukosil MF Lukosil DF E 350 E 351 AR-20 NM4146 NM4180 NM4180 OV-17 Vinyl OV-73 Vinyl PS060 PS060.5 PS062 PS063 PS160 PS162 PS264

DCC DCC DCC DCC DCC DCC GE GE

su su su su su su CS CS

50

50 50

-

95 94

x

1x1

15 50

PS PS PS PS PS PS PS

94

1x1

25 50

z

-

nominal

-

-

-

XPhenyl

[lo-’]

IK1

2.1

1.5 2.0 2.5

5 10 25 25 25 50 5 5 4

1 5.5 1.5 5 20

10 50 100 50 50 5.8 0.2

2.1 2.6

25 33 33 5.5 5 20 5 25 50 75

di0

Viscosity

ig.rn-31

Immz.g-’]

1.00 1.07

-

1.07 1.10 0.98 0.98 1.09 1.04 1.13 1.15 1.03 1.09

2.0 28 30

50-1000 15-30 45 55 100-150 500 gum gum gum 35 gum

1.04 1.05 1.08 1.08

200 200 500 1000

0.98 1.05 1.00 1.07 1.11 1.15

100 170-250 30 000 125 500 100 000 gum

After refs. 1324. 329, 331-332, 780-785, 820-824 and 8421 and data from the manufacturers. Suppliers as in Tables 44 and 46, except W = Wacker Chemie. Munich, F.R.G.

*)

McReynolds constants

effective***)

1 0.15 300 1-30 0.4-2 0.2-1 27 12 48 200 50 25 50 5 1 5

25 33 33 100

93 95 80 90 50

Y

50 50 50 100 5 5 -

ICI ICI W CWN 75 CWN 61 CWN 67

ov ov

M.W.

structure

”)

X

Y

Z

U

S

25 31 77 76 74 107 32 33 29 73

65 77 124 123 116 149 72 12 13 124

89 80 189 189 178 228 98 98 97

57 118 142 140 135 190 67 67 68 189

126 126 117 153 65 66 62 122

86 142 145

130 176 181

134 182 186

181 267 272

154 233 240

32 33 32

72 72 72

65 66 65

98 98 98

67 67 67

60 80

H

42 79 90 89 81 170 44 46

See following text

“9 Regarding the content of end groups in the polymer

K

L

32 49

59 71 134

18

72 98 36 36

128 174 67 68 65 140 143 210 217

44 46 44

222

8. Liquid Stationary Phases

zable (aryl) and polar silicones cannot readily be achieved, as phenyl groups (and polar substituents) counteract spreading [surface tension at 50% phenyl groups 31.4 mJ * m -*,critical surface energy of a deactivated fused-silica surface 20-23 m J * m-z (depending on the deactivating group)!] and cross-linking, even at low phenyl contents. Three approaches have succeeded in achieving cross-linking: the incorporation of vinyl groups in the phenylsiloxane chain, end-capping of prepolymers with M-units containing vinyl and a certain extent tolyl substitution in phenylsilicone gum. Although numerous investigations and pioneering work have been carried out in this field, these already have been discussed in Section 3.3.3.3, so only few publications will be cited here. Kong et al. [236] prepared small-diameter fused-silica capillary columns with non-extractable 50%phenylpolysiloxane (50% phenyl, 49%methyl, 1%vinyl groups, azo-tert.-butane for cross-linking, wash-out O%!) of ca. 87000 theoretical plates for a column of 15 m X 0.1 mm I.D. and with SE-54 (5% phenyl, 1%vinyl groups; see Table 51) nearly 700 000 theoretical plates were generated by a 46 m x 50 pm I.D. column with a 0.25-pm film. The preparation of soda-glass capillary columns coated with in situ cured poly(dimethylsiloxane/diphenylsiloxane), which contained some vinyl substitution, or with in situ cross-linked poly(methyltolylsiloxane), for which vinyl groups were not necessary, was described by Bugten et al. [222, 8461. Further immobilization procedures for phenyl- and other arylpolysiloxanes have been described [210, 237, 181, 844-8481. Several fused-silica open-tubular columns with chemically bonded and cross-linked poly(phenylsi1oxanes have become commercially available, partially even in the wide bore and thick-film mode [849]. They are listed in Table 52. In order to increase the thermal stability further, special developments of aromatic siloxanes have been made. In Table 51 two products are mentioned, SKTA-1 and DFOK, which have been proposed for chromatography by Yudina et al. [850]. SKTA-1 has the formula Me

Me

Me

Me

and the structure of DFOK is

Me

Me

Jn

The upper temperature limit has been reported to be 350°C and the selectivity to be between those of OV-7 and OV-11 (see Table 51). Products similar to SKTA-1 have been offered by Petrarch Systems as PS 094

(M. W. 150 000) and a further disilphenylene polymer as PS 095, a dimethylsiloxane-silphenylene copolymer:

Me

Me

Me

223

8.3. Silicones Table 52. Commercial Capillary Columns Coated with Cross-linked Poly(methylphenylsiloxanes) Name

Supplier')

% Phenyl

Siloxane Units.') w

x

y

Similar to

Film Thickness Illml

z

Maximal Operating Temperature

PCI

Durabond DB-5 JW

94

Durabond DB-17 JW

-

100

94

10

Durabond DB608

JW

CP-Sil-8 CB

CP

CP-Sil-19 CB***) CP RS Rtx-5

1

5

-

-3

50

-

1-3

5

5

5

85 9 4 -

5

1-3 1

5

SE-54 SE-52 OV-17 SP-2250 SE-54 SE-52 OV-73 SE-54 SE-52 OV-1701 OV-5 SE-54 SE-52

Rt,-20

RS

79

-

20

1

20

OV-7

Rtx-35

RS

64

-

35

1

35

ov-11

Rt,-50

RS

-

99-

1

50

OV-17

Rtx-1701***)

RS

8 5 -

-

1

7

DB-1701***)

Jw

8 5 -

-

1

7

NB-54

NI

-

10-

1

5

NB- 1701***)

NI

8 5 -

-

1

7

Permaphase PVM S IS 4 Permaphase PVMSI 17 HP-5

PE

9 4 -

1

5

PE

-

HP

9 5 -

HP-17

HP

-

SPB-5 SPB-20 SPB-35 SPB-2250 GB-5

SP SP SP SP FB

9 5 79 64 -

100

100

5

-

1-3 5

-

5 20 35 999 5 5

1

50 5

1-3

50

1 1 1 1 1

20 35 50 5

c p = Chrompack, SP = Supelco. NI = Nordion PE = Pcrkin-Elmer. FB = Foxboro Co. HP= Hewlett-Packarb RS I Rest&

5

0.10, 0.25, 1.00 1.5, 3.0, 5.0 0.15,0.25, 0.50 1.0 1.6

300

0.10, 1.50 0.25,3.00 0.50, 5.00 1.00 0.10, 1.00 0.25, 1.50 0.05, 3.00 0.10,1.00 0.25, 1.50 0.50, 3.00

325 (0.10 pm)

0.10,O.SO

300 (0.10 pm) 260 (3.00 pm) 300 (0.10 pm) 260 (3.00 pm) 310 (0.10 pm) 290 (1.00 pm) 280 (0.10 pm) 240 (3.00 pm)

320 (0.10 pm) 280 (1.00 pm) 280 (0.10pm) 220 (1.00 pm) 300 260 300 260

0.25, 1.0 0.25, 1.0 0.25, 1.0 0.25, 1.0

*) JW = 1 Br W Scientific Inc., w , x, y . L refer to the general formula ***) 14%cyanopropylphenyl

' 9

280

0.12, 1.2 0.25, 5.0

0.25, 1.00 OV-1701 0.15, 1.00 0.25, 1.50 0.50, 3.00 OV-1701 0.15, 1.00 0.25 SE-54 0.10, 1.00 0.25 OV-1701 0.10, 1.00 0.25 SE-54 OV- 17 SP-2250 SE-54 SE-52 OV-17 SP-2250 SE-54 OV-7 0-11 OV-17 SE-54 SE-52 OV-73

300 260

300 280 280 260 300

used for Table 51.

224

8. Liquid Stationary Phases

More recently, copolymers containing both silsesquioxane units and dimethylsiloxane/methylphenylsiloxane units have been described by Yudina et al. [852]. The silsesquioxane moiety in this polymer, called Lestosil, is believed to have a ladder structure: R

R I

R I

I

I

I

R

R

where R

= C6HS,and

the whole polymer has the following composition:

7 -Si-0-

I

0

I

-9 -0-

Jn = 0-0.33, = CH,, CsHs, = 6-60, number of silsesquioxane units, = 10-350,number of poly(dimethyl/methylphenyl/diphenylsiloxane)units,

M.W. = 1000-30000. The McReynolds Constants, determined for different ratios x : y are (120°C): x:y

x

Y

z

U

S

5.6 2.8 2.0

65 79 102

110 126 170

120 190 151

160 184 211

108 262 239

This colourless sponge-like polymer is distinguished by a high thermal stability [by thermogravimetric analysis up to 420"C, applicable as a liquid stationary phase up to 400°C (FID)]. It can be dissolved in dichloromethane, chloroform, benzene or toluene. In addition to phenyl substitution, adamantyl substitution also improves the thermal stability and resistance to oxidation. Hence, such products might also be applicable in gas chromatography [865].

8.3.5.

Carborane Siloxanes

Linear poly(m- or p-carboranylenesiloxanes),the carborane group being BloHl,C2 and siloxane groups or chains bonded to both carbon atoms of the carborane [853,8641, exhibit a quasi-aromatic structure and behaviour and outstanding thermal stability. The boron content is generally in the ranges 20-25%. Carboranes can be prepared easily and inexpensively from NaBH, and BF3-etherate [854].The structure of the carborane (dicarbaclosododecaborane) group, to which different alkyVarylsiloxane chains are bonded via the two carbon atoms in the m-position of CBloHloC,shows Fig. 28.

225

8.3. Silicones

R

I

This m-borane with its six-coordinate carbon atoms is considered to be quasi-aromatic and may occur in several resonance states and may stabilize bonded groups (R)in the meta position against thermal degradation. The structure of dimethylsiloxane-m-carboranecopolymer is Me

Me

r i

I

1 --H

R

Rl

where R,RI= methyl, m = 1-5, n = 50. The product (m = 4) is known as Dexsil300. Corresponding types are KBS-1 (m = l), KBS-2 (m = 2) and KBS-3 ( m = 3) ( n = 1000) [855]. Thermogravimetric analysis reveals (Fig.29) that the weight loss up to 300°C is 1%,increases to 2% up to 350°C and reaches only 3% at 400°C; the resistance to oxidation exceeds even that of phenylsiloxanes, as can be seen in Fig. 29. Phenylated types (R = C,H,) with one or more phenyl groups are Dexsil400 (n = 5, M. W. 15000) and KBS-2F and KBS-3F (M.W. 105-106) [856]. They can be used even above 450°C.

100.00 a] Dexsil 300 GC in air WE 2.0078 mg Scan Rate: 5.00 deg/min

b) Dexsil 300 GC in N2 W k 2.52

4

‘b)

mg Scan Rate: 5.00 deglmin

c

._

P

8 50.00

t

a00

ltO.00

80.00

120.00 16000 200.00 240.00 280.00 320.00 Temperature in OC

360.00 400.00 TG

Fig. 29. Thermogravimetric analysis of Dexsil 300 GC in air and in nitrogen (after Steimann and Rotzsche [827])

226

8. Liquid Stationary Phases

Ldkina, who carried out much work on the development of silicone stationary phases in the Soviet Union, proposed triarylsiloxy terminated carborane-siloxanes [857-859, 7851:

where R1,R2and R3are alkyl-substituted or unsubstituted aryls, n = 3-24 and m = 1-8. They can be used in a wide temperature range from -30 to 400°C. An almost oxygen-insensitive carborane-siloxane that is less thermally stable (only up to 250°C) but exhibits an extremely high selectivity towards hydroxyl-containing compounds has been reported by Berezkin et al. [860]. This rubber-like material, soluble in toluene, dichloromethane and acetone, is called Silbor-1. A further Dexsil type, Dexsil410, owing to the presence of cyanoethyl groups (R = 2-cyanoethyl in structure I above), can specifically interact with alcohols, ketones, nitro compounds and N-heterocycles. The application of the carborane-siloxanes is especially determined by their outstanding thermal stability. Very recently, a carborane modified polysiloxane containing cross-linking units which enable immobilization on the inner walls of fused silica columns has been developed by SGE International, Pty., Australia. It is non-polar and known as HT 5 and can be operated on temperature programs to 480°C and still maintain a long lifetime. Obviously, this high-temperature application requires fused silica columns the external coating of which (traditionally polyimide) has been replaced by aluminium clad [86Oa]. Carboranesiloxanes are also used for the high-temperature analysis of high-boiling esters of carboxylic acids, and of aromatic amines, halogenated alcohols, polyphenyl ethers, siloxanes, glycerides and peptide derivatives [861] in packed columns and for the separation of volatile metal chelates and organometallic compounds in porous-layer open tubular (PLOT)columns [862]. Open-tubular columns coated with phenylsiloxanecarboranes (structures I and 11) of the Dexsil400 type have been reported for the separation of the butyl esters of trifluoroacetylamino acids [863]. In Table 53 some carborane-siloxanes and their selectivities are listed. Table 53. Characteristics of Some Carborane-Siloxane Stationary Phases Structure (I or ID

Name')

Operating Temperature

McReynolds constants

Raqge

X

Y

Z

U

S

H

K

PCl

Dexsil-300 Dexsil-400 Dexsil-410 Pentasil-350 KBS-2 KBS-3 KBS-2F KBS3F Silbor-1 PS-097 PS-098 5-SiB-1 5-SiB-1.5 HT-5**)

I

I, R = R 1 = CH3 40-400 I, R = C ~ H IR1= ( , CHI 50-375 I, R = 2-cyanoethyl, CH3, R1 = CH, 50-360 I, R = R1 =CHI I, R = R1= CHI I,R=CH~,R~=C~HJ I,R=CHI,R~=C~HS

40-450 40-450 40-450 40-450 20-350

41 80 103 148 96 55 72 107 118 168 123 71 286 174 249 171 16 3 121 131 162 74

25

111 688 175 234 224

I, R = R1= CHI I, R = CH,, R2 = CH, and C6HS I, R = R1= CHI, m = 2-2.5 50-480

9 Dexsils supplied by O h Cow. and Analabs, U.S.A.,M.W.9Doo-20oM), KBS and Silbor supplied by Reakhiim, U.S.S.R. PS supplied by Petrarcb Systems.U.S.A.Pentasil. 5-SiB-1, 5-SIB-1.5are similar types, supplied by Chemical Systems Inc.. U.S.A. **) Chemically bonded on PS-capillary.

227

8.3. Silicones

8.3.6.

Poly(methyltrifluoropropy1siloxanes)

Trifluoropropyl-substitutedpolysiloxanes are the only halogen-containing poly(organosi1oxanes) that have been widely used as liquid stationary phases. Chloroalkyl- and chloroarylsilicones, even if occasionally applied in gas chromatography, are of minor importance. Owing to the low electronegativity of silicon and the strongly electronegative nature of the fluorine

I I

atom, fluorine substitution in an a-or fl-position to the Si atom in an -Si-alkyl group

I

(-5 -CH-

I

I

F

I

R or -Si-CHZ-CHR I I F

weakens the Si-C

)

I I

bond and may cause its cleavage to form -SiF

and an alkene when this

fluoroalkylsilicone is thermally stressed. Therefore, y-substitution, which stabilizes the Si-C bond, is generally utilized, and 3,3,3-trifluoropropylmethylsiloxanesare the preferred industrial fluorosilicones. They are manufactured by base-catalyzed polymerization of the cyclotrisiloxane CH3 CF,-CH2-CH2-Si

‘ I I

-0-

3.

Prior to the gas chromatographic use, thorough purification of industrial products by washing with water and vacuum stripping is necessary, because residual base and traces of water would depolymerize the polymer to form the cyclic oligomers. Above 250°C this decomposition begins to proceed also in the absence of basic catalysts, and additionally 1,l-difluoropropene is formed to some extent by Si-C bond cleavage [866]. The first fluorosilicone introduced for gas chromatography by Van den Heuvel et al. [867] in 1961 was QF-1-0065 (= QF-1, FS-1265, see Table 54). The structure of most fluorosilicone liquid phases that are commercially available at present is Me

I

Me3Si - 0 t Si -0 -fSiMeg

I

Structure I

CH2

I

In addition, there are a few types containing further units

I Me

I

Me3 Si -0

Si -0

I

CH2

Me

I

+, f Si -0 I

Me

Me

I

jY+Si -0

II

CH2

CH2

CF3

Structure I1

CH

I

I

SiMe3

I

n

where the content of dimethylsiloxy units (y) determines the decrease in polarity and change in selectivity, and the low content of methylvinylsiloxy units (z) (1-2.5%) is necessary when

Table 54. Properties of Poly(methyltrifluoropropylsi1oxanes)and PoIy[methyltr~uoropropylsiloxanes(dimethylsiloxanes)methylv~ylsilox~es] *), **) Name

Supplier. manufacturer

Struc-

X

Y

ture

[%I

I%]

Z

1x1

Maximum operating Temperature

M.W.

rcl

ov

I

100

250

16

ov-202' OV-215' OV-215vP SP 2401' AS1 50 Me, 50%TFP FS-169

ov ov ov

I II1

SP AS

I I

100 100 98 100 100

250 250 250 250

10 200 200 2.6

su su su su su su su su su su su

I1

23 77

I1

23 77

FS-328 FS-16 FS-303 SKIFT-25 SKIFT-50. SKIFr-5OX' SKIFr-75 SKIFr- loo* NFS-100 QF-1 ( = QF-1-10065) FS-1265 LSX-3-0295 S i l a ~ t LS-24 i~ (= silastic DC-24) PS 181 PS 182 PS 183 PS 286

II

31

I I I1

DCC

I1 I1 I1 I I I

100 100 25 50 50 75 100 100 100

DCC DCC DCC

I I I1

100 100 99

PS

I I I I1

100 100 100 98

PS PS PS

2

290

69 150 160 300 300 300 300 300

75 50 50 25

Viscosity

McR~moldsconstants

[mZ.g-*j

x

Y

z

u

s

H

r

K

L

M

(25'C)

OV-210.

FS-169/300'

a. k.cm-']

146 238 358 468 310 206 139 056 283 060

1.284(25°C)10000 1.32 500 1.252 gum 1.284 gum 700 1.26

146 238 358 468 310 206 149 240 363 478 315 208

1.6

1.1

046 104 149 189 118

112

3.2

1.1

2-3

1.1

45-60 (20°C) 50-70 (20°C) 65-120 (20°C) 40(20"C) 1000-1500 gum gum gum gum gum gum 1000

055 116 169 215 137

130

087 161 239 296 194 179 272 419 527 366

186 337

047 160 212 268 230 066 132 192 247 158

148

143 232 350 456 303

278

10000

053 144 233 355 463 305 203 152 241 366 479 319 208 144 055 291 064 152 241 366 479 319 208 144 055 291 064

1

1.21 1.33 100-500 1.065 10-60 1.15 115-575 1.15 270-675 1.24 300-800 1.329 15-78 1.329 24 2.5

200

gum gum

1 2.35 4.6 14

1.25 1.28 1.30

056 056

146 238 358 468 310 206 146 238 358 468 310 206

144 233 355 463 305 203 136 053 280 059

300 1000 10 000

2

*) After ref. (324, 781-785, 820-824, 8421 and data from the manufacturers. Suppliers 8s in Tables 44 and 46.

**) According to structures I and 11; the values of X, Y and Z refer to the percentages of the units concerned in these structures. a = Specialty grade

229

8.3. Silicones

cross-linking reactions (coating of open-tubuar columns) are to be applied. The densities range, depending on the ratio x.y and on n, from 1.065 to 1.329 g cm-3. The surface tension (structure I, n = 1000) is 26.9 mJ * m-2. Column temperatures should not exceed 250°C unless substantial bleeding is allowable. Appropriate solvents are acetone and ethyl acetate. However, solutions should be used immediately and not be stored in glass vessels. The resistance to air is worse than that of phenylsilicones but better than that of comparable methylsilicones [870]. Some properties that might affect the gas chromatographic applicability of commercial fluorosilicones have been examined 18681, and it was found that most types contain, at least in trace amounts, terminal silanol groups, and some also a small content of cyclosiloxanes. By thermogravimetry, the maximal operating temperature (here the temperature up to which the weight loss in nitrogen is < 1%)was determined to be 250°C for FS-169 and 200°C for FS1265 [868]. The bulky side groups -CH2CH2CF3 are assumed to turn away from the supporting medium at low or medium temperatures and to turn towards the surface with increasing temperatures, hence altering the polarity and selectivity to a certain (small) extent. During cooling, the previous state is not reached immediately, and temperature-programmed analyses, if succeeding one another very quickly, may result in undesirable retention changes [869]. Poly(trifluoropropylmethylsi1oxanes)are moderately polar. Their selectivity is based on the pronounced acceptor character of the 3,3,3-trifluoropropyl group bound to the siloxane skeleton. Owing to their electron-accepting tendency, CF3 groups in fluorosilicones can interact with free electron pairs, hence preferentially retaining carbonyl and nitro compounds (McReynolds constants 2 and U!), and retention of steroids with different functional groups increases in the order ethers < hydroxy groups < esters < keto groups [871]. Infrared spectroscopic measurements have revealed that local association between sample OH groups (e.g., from methanol) and the stationary phase CF3 groups does not take place, and that for steric reasons only two F-C (from - C F J bonds are capable of simultaneously associating with the polar groups (e.g., >C=O, -NOz) of chromatographic samples [839]. Olefins and aromatics are well separated from aIkanes and cycloalkanes. Further applications are the separation of halogen compounds, carbohydrates, diastereoisomers, metal chelates, organo-silicon compounds, anticonvulsant drugs, nitroaromatics, polychlorinated biphenyls (PCBs), pesticides and especially aldehydes/ketones from other compounds. In Table 54 the characteristics of trifluoropropylsiloxanes that have been used or appear to be suitable for use as liquid stationary phases are listed. Specialty products for GC are indicated by a. Chemically bonded poly(methyltrifluoropropylsi1oxanes) have recently also been offered commercially by J. & W. Scientific as Durabond-210 fused silica capillary columns with film thickness: 0.10,0.25,0.50 and 1.00 pm and by BIO RAD/R.S.L. (Belgium) as RSL400, film thickness 0.25 and 1.2 pm, respectively. They can be thermally stressed up to 220°C.

-

8.3.7.

Cyanoalkyl-SubstitutedPolysiloxanes

Cyanoalkylsilicones belong to the few liquid stationary phases that combine high polarityhelectivity and reasonable thermal stability. Cyanoethyl- and cyanopropylpolysiloxanes exhibit both polar and polarizable characteristics and are among the most useful stationary phases with respect to polarity at both low and high temperatures. By linking both polar (CN-CHz-CH2-CHz-) and non polar (CH,-) groups to the siloxane chain, the selectivity and polarity can be adjusted to different values within the limits CH, = 100% and CN-CH2-CHz-CH2= 100% by control of the number of cyanoalkyl groups inserted during the preparation of the polymer. This basic principle and the corresponding silicones

230

8. Liquid Stationary Phases

containing methyl, cyanopropyl and phenyl groups (i.e., the cyanoalkylpolysiloxanes which are currently the most important polar thermally stable liquid stationary phases for both packed and capillary columns) were published in detail by the present author 28 years ago at the 4th International Gas Chromatography Symposium in June 1962 (41st Meeting of the European Federation of Chemical Engineering) in Hamburg [872-8741. This new class of polar stationary phases (patented to the author and his company in 1963 [874]), although SUCcessfully applied immediately after publication in many laboratories in the G.D.R., were not offered commercially to chromatographers until 1972 [875], by Applied Science Laboratories (Silar). Numerous products of this type of silicone have been proposed and developed since then, and in the author's view, owing to certain requirements for coating capillaries with such phases, to recent knowledge and analyticallspectroscopic possibilities and to improved preparation possibilities, some new tailor-made immobilizable cyanoalkylsiloxanes will subsequently be developed. The polymers can be prepared by combining and co-hydrolyzing the corresponding diorganodichlorosilanesor diorganodimethoxysilanesto give a polymer of the desired composition. After the removal of volatile byproducts, the prepolymers are polymerized by employing a basic catalyst (which afterwards must be removed or destroyed!) and end-capped [803, 877, 8781. Another route is via the preparation of the corresponding cyclosiloxanes, which are then polymerized at moderate temperatures, also with base catalysis, in the presence of a small amount of an appropriate hexaorganodisiloxane as a chain stopper [803, 8771. There are other methods of preparation that cannot be discussed here. Persilylation of open-tubular columns with 1,3-bis(3-cyanopropyl)tetramethyldisiloxane and coating with OV-240-OH resulted in highly polar columns, appropriate for the isomer specific determination of polychlorinated dibenzo-pdioxins and dibenzofurans [878a]. The high-temperature applicability, after condensation and cross-linking processes on the column wall, of hydroxy-terminated cyanopropylsilicones containing a high cyanopropyl content [878b, 878~1 confirms our results and predictions from 1963 with regard to high selectivity and reasonable thermal stability of cyanoalkylsilicones [872-8741. Cyanosiloxanes, specially designed to provide a good thermal stability and exhibiting a cyanopropyl content of as high as 70 %, were prepared, coated and cross-linked on fused silica capillary columns by SGE International Pty., Australia. They have a maximum operating temperature of 300 "C and are particularly useful in the analysis of fatty acid methyl esters and their corresponding geometric isomers [888]. Poly(cyanoalkylsi1oxanes) can be described by the general formula

FN y42

SiMe3

CH2

y+2

CH2

CN

CH2

CH2

I

I

I

CN

1

CN

CH2

I

CHz

CHz

1

CN

In reality, the special cyanoalkylsilixones have a less complicated structure than that formulated here, as generally only 1-3 different units t...z are contained in the product and not, as one might suppose, all of them. When discussing the individual stationary phases, the notation will refer to this formula. Similarly to trifluoro substitution, cyano substitution in the aposition also weakens the Si-C bond and hence reduces the thermal stability, whereas with

23 1

8.3. Silicones

substitution in either the 8-position (p-cyanoethylgroup, unit u in the above structure) or the y-position (y-cyanopropyl group, units w, x and y) the nitrilesilicone has adequate thermal and oxidative stability up to at least 250°C. The stability can be increased by introducing phenyl groups, and a homopolymer containing only phenylcyanopropylsiloxane units (x) i.e., with both the phenyl and the cyanopropyl group on the same Si atom, in addtion to a very small number of trimethylsilyl end-groups, may be thermally stressed even up to 300°C. Thermal degradation produces especially the corresponding substituted cyclotri- and cyclotetrasiloxanes, while a cleavage of the Si-C bonds from, e.g., cyanopropylsiloxanes, to form vinylacetonitrile or -butyronitrile, occurs only on the trace scale, i.e., the cyanopropyl groups seem to be relatively firmly anchored to the silicon atom [879]. This corresponds with investigations by the author, who in the course of the first gas chromatographic application of nitrilesilicones [872] examined whether by thermal degradation, which might arise in the column if overheating occurred, there would be a risk of the formation of volatile toxic decomposition products. This was tested with mice but they were found not to suffer any pathological changes when exposed to the volatile decomposition products [880]. With regard to thermal stability, there are no distinct differences between 0-cyanoethyl and y-cyanopropyl substitution. The main causes of bleeding below the maximum operation temperature are the presence of low molecular-weight moieties, of residual basic catalyst and of residual hydrogen chloride (even in ppm amounts) stemming from the hydrolysis of the chlorosilanes and being easily adsorbed by the CN group. These drawbacks with commercial products are much more pronounced for polar than for non-polar stationary phases. Therefore, the reproducibility between batches ought to be improved, and the synthetic routes used for preparation should be modified. For example, it could be shown that by avoiding diorganodichlorosilanes in the hydrolysis step and using the corresponding diorganodimethoxysilanesinstead, results, owing to the absence of HCl, in avoiding an undesirable side reaction, Si-CH2-CH2-CH2-CN

H20 HC,

* -Si-CH2-CH,-CH2-

C -NH2,

II Q

by which carboxamides are formed as an artifact of the synthesis. Such carboxamide groups, formed from the cyanoalkyl groups, have been found to be present in non-negligible amounts in several commercial products (OV-275 5%, SP 2310 and SP 2340 3%, SP 2330 2% [878]). As they affect the polarity and selectivity, their formation should be prevented by using more suitable synthesis paths. Further, these highly polar stationary phases should be handled very carefully, as they are susceptible to oxidation in the presence of air during coating procedures and during use; alkalis and air should also be excluded during storage. Cyanoalkylsiloxanes belong to the most polar liquid stationary phases. The cyano group, attached to the siloxane backbone via two or three-CH, groups, is dipolar and strongly electron attracting, hence displaying dipole-dipole, dipole-induced dipole and charge-transfer interactions. Moreover, the unshared electron pair on the nitrile nitrogen may form intermolecular hydrogen-bonds with suitable hydrogen-donor sample molecules, e.g., phenols. These properties lead to increased retentions for compounds bearing n-electrons and for alcohols, ketones and esters. Table 55 demonstrates the change in selectivity with change in content of cyanopropyl groups, even if the latter only amounts to maximum of 50% as in this example. At 100%cyanoalkyl substitution (OV 275), the separation factors would change even more distinctly than the values in table 55, confirming that nitrilesilicones are indispensable stationary phases owing to their selectivity. Table 55 indicates that the selectivity may be adjusted, within certain limits, according to the practical analytical requirements. For example, the separation of several chlorinated hydrocarbons can not be achieved either on the non-polar chemically bonded and cross-linked methylsilicone DB-1or on the medium polar cyano-

232

8. Liquid Stationary Phases

Table 55. Separation Factors of Compounds with Similar Boiling Points on Cyanopropyl-Substituted Polysiloxanes (after [873]) (Column temperature 100°C) Compound type.

Alkylbenzenes/n-allranes Alkylbenzenes/cyclohexanes Alkylbenzenes/cycloolefms Ketones/primary alcohols Primary alcohols/ethers Ketoneslethers Alkylformatedethers 08-1 30meters x 0.25mm i.0 1,0 micron film Split Injection ~:~O(.L,PL)

Ratio of methyl:phenyl:cyanopropylgroups 1OO:O:O

75:25:0

72:0:2S

SO:lS:3S

62.5:0:37.5

S0:O:SO

1.2 1.1

1.7 1.5

3.0 1.9

3.7 1.9 1.6 1.5. 1.4 2.1 1.8

4.1 2.4 2.0 1.3 1.7 2.3 1.8

6.1 3.0 2.2 1.3 2.1 2.8 2.1

-

-

-

2.1 0.4 0.8 0.9

2.6 0.4 1.0 1.3

1.5 1.5 2.2 1.8

08-1701 30meters x 0.32mrn i. 0 1.0 micron f i l m Split Injection 1:50(4yL)

DB -1301 30meters x 0.25mm i,D 1.0 micron f i l m Split Injection 1:50($ pL)

Fig. 30. Separation of halogenated hydrocarbons by adjusting the selectivity of cross-linked chemically bonded silicone phases. DB-1, poly(dimethylsiloxane), non-polar; DB-1701, poly[cyanopropylphenyl(l4%)/dimethyl(86%)siloxane],medium-polarity; DB-1301, polarity ranges between DB-1 and DB 1701; Peaks: I 1,l-dichloroethane; 2 dichloromethane; 3 chloroform; 4 1,2-dichloroethane; 5 1,1,2-trichloroethane; 6 bromodichloromethane (by courtesy of J&W Scientific, Division of Curtin Matheson Scientific, Inc.)

propylphenyl(l4%)dimethyl(86%)polysiloxane, as can be seen in Fig. 30, but by adjusting the selectivity of a the stationary phase by means of computer-calculated “window diagrams” (a method that is used to predict relative retentions of critical substance pairs on binary mixed stationary phases [881,882]), both the critical pairs 1-2 and 3-4 which coelute on DB-1 (1-2) and DB-1701 (3-4), can be well resolved. The specially developed stationary phase DB-1301 contains a smaller number of cyanopropylphenylsiloxane units, and the polarity/selectivity ranges between DB-1 and DB-1701 (the 1701 belonging to the most widely used liquid stationary phases for WCOT columns) and fills the gap for the separation of low- to medium-polarity compounds [883]. This stationary phase is chemically bonded and cross-linked just as the other two. Increasing the cyanoalkyl content means increasing the selectivity for the separation of aromatic from aliphatic hydrocarbons, as can be concluded from the McReynolds constant X (see Table 56). However, although higher Kovbts retention indices are measured for aromatics on highly cyanoalkyl-substituted siloxanes than on other stationary phases, the specific retention volumes of these aromatics are substantially lower than those on other phases. This phenomenon is caused by the unusually low solubility of n-alkanes (which are the reference

233

8.3. Silicones

compounds for Koviits indices and McReynolds constants) in poly(cyanoalkylsi1oxanes). If the cyanoalkylcontent is very high n-alkanes have a 10-100 times lower retention than in poly(dimethylsiloxanes), and evidence for sample overloading (peak fronting) and retention dependence of the sample size can be observed [871]. Nevertheless, apart from the dipole-dipole and dipole-induced dipole interactions and hydrogen-bonding potential, the dispersion component still prevails. Slower diffusion in highly cyanoalkyl-substituted siloxanes than in 50% phenyl- or trifluoropropylsiloxanesbroadens the peaks to some extent, hence causing lower efficiencies [843]. The main applications of poly(cyanoalkylsi1oxanes) are based on their outstanding selective properties. Unsaturated fatty acid methyl esters (FAME)have been the subject of numerous investigtions using these stationary phases, and positional and geometric isomers can be separated and identified (e.g., [877, 884, 885, 8871). Compared with polyester stationary phases, the cyanoalkylsiloxanes give enhanced cis-trans selectivity, the trans isomer being eluted earlier than the cis isomer, and an acetylene-olefin selectivity, the acetylenic FAME being eluted earlier than would be expected from data obtained on polyester phases [885]. Unsaturated wax esters with chain lengths of CzB-C44,containing 1-7 double bonds and differing in the degree and/or position of the unsaturation, were separated [886] and the transfatty acid content of foodstuffs has been successfully determined on poly(dicyanopropylsi1oxane) [887]. This bis(cyanopropy1)-substituted siloxane was patented by Kruppa and Coleman and Applied Science Labs./Silar Labs., in 1976 [888]. Phenols, hydroxybiphenyls [880], phenolethers, aromatic amines, alkaloids, butyl esters of diterpene resin acids, organochlorosilanes [892], and especially steroids and nitrogen heterocyclics have also proved to be well separated on these stationary phases. On chemically bonded and cross-linked cyanoalkyl stationary phases carbohydrate enantiomers were analysed as methyloxime pertrifluoroacetyl chiral derivatives to produce two peaks for each enantiomer, representing the syn 0 and anti (E) aldoximes [890], and sugars as trimethylsilyl derivatives [891]. In Table 56, the characteristics of poly(cyanoalkylorganosi1oxanes) which have been used as stationary phases are listed. Suitable solvents for these cyanoalkyl-substituted polysiloxanes are dichlormethane and methyl formate.

8.3.8.

Poly(chlotoarylsi1oxanes) and Poly(chloroalky1siloxanes)

In spite of sufficient thermal stability and of usable intermolecular interactions with esters and chlorobenzene derivatives, chloroarylsilicones have not grown in importance in gas chromatography. Their only moderate selectivity can be seen from the McReynolds constants in Table 57. The presence of chlorine linked to an aromatic group does no contribute substantially to the selectivity, and the commercially available products may easily be replaced with phenylsilicones containing 5-15% phenyl groups. In contrast to chlorophenylsiloxanes, the chloromethyl-substituted products exhibit a definite acceptor character with considerable enhancement of ketone retention similar to that observed with trifluoropropylsiloxanes,as was shown by Haken and Kritzler [893]. Aromatics and pyridine derivatives are also distinctly retained. Structures: Me Me I

I

I

I

I

Me3Si -O-(-Si-Og(-Si

Q

-O-),,-Si

Mes

I

Me

Cl

Comniercial names: DC 560 (Dow Coming Corp.) F50, F60 (General Electric) SP-400 (Supelco)

Table 56. Characteristics of Poly(cyanoalkylorganosi1oxane) Stationary Phases.) Supplier, r manu1x1 facturer

Name

silar 5CP Silar ICP Silar 9CP silar 1oc SP 2300 SP 2310a) SP 23308) SP 23408) OV-105 ov-1701 OV-225 OV-275') AN-600 NPS-25 NPS-50 NPS-100 NSKI-25 NSKT-33 NSKT-50 NSKT-100 p-NSKT-100 XF-1105 XF-1112 XF-1125 XE-60 XF-1150 0E-4118 ASI-25 Ph 25 C Y ~ O prowl PS 902 PS 906

u

u

[%I

w [K1

x [%I

100

SP SP SP SP

100 50 20

ov ov AN su su su su su su su su

M.W 110-~1

dto

b.cm-31

Viscosity (ZQO

1.13

50

50

20

80 100

1.12

50

80 100

10

90 85

14

1

50

50

8 100b)

I5

25

50

50

I5 61

100 25 33

1.5 1.2 1.3 83-830 81-870 93-930 113-1130 121-1210

50

50

100 100

GE GE GE GE GE

90 76

10 24

50 50

50 50

CWN

25

AS

z

1x1

[ ~ d . g - ~ ]

AS AS AS AS

ov ov

y

1.096 (25°C) 1.16 1.08 0.998 1.05 1.09

1500 gum 9000 gum

80 350 1340 gum gum gum gum

gum

1.08

gum

100 50

15 50

-

PS PS

100 100

4-5000

McReynolds constants

x

r

319 440 489 523 319 440 490 520 036

495 638 725 I55 495 638 125

z

446 605 631 659 446 605 630 151 659 108 093

228 369 338

u

s

631530 844613 913718 942801 631530 a44613 913118 942800 139086

H

I

K

L

M

319 492 401 268 603 225 566 459 292 696 256 584 480 298 122 261 319 492 566 584 014

492386 282

629 872 763 1106849 686 202 368 331 483361 291 502 451 122 261 231 135 215 251

644512 345244 363259

448 211 225

320 513 414 216 461 405

656541 584473

501 42 1

204 381 340 204 381 340 308 520 410

483361 289 493361 289 669528 401

228 369 338

492386 282

Table 56. (continued) Name

Supplier. I manu[%I facturer

u [%I

u

1x1

w

x

y

[%I

[%I

[%I

P

PS 908 PS 910 33CN-1 33CN-2 33CN-3 50CN-1 60CN1,2 6OCN-3-5 60CN-6 70CN-1 DB 13014

PS PS MA

10

MA

64 67d)

JW

92

7

33 33 3 33 50 60 2 60 60 3 75 1

DB 17019

JW

85

14

1

DB-6249

Jw

90

10

1

DB-2259

JW

CP-Sil19CB

CP

85

14

CP-Sil88 Permaphaseq CPMS/17001 Permaphaseq CPMS-225

CP PE

85

14

MA MA MA MA MA MA

PE

50 67c)

3

SC)

4OC)

37 2SC)

49

dP

k .~ m - 3 1

Viscosity

McReynolds constants

(20~) [m2.g-'l

X

Y

Z

U

H

S

I

K

L

M

1.096

90 50

503 33

M.W

1x1 [10-~1

50

1 1

2000

chemically bonded chemically bonded chemically bonded chemically bonded chemically bonded

100 49

50

1 1

chemically bonded chemically bonded

*) Atter 1324, 781-785, 810, 820-824, 8421 and data from the manufacturers and research chemists, Suppliers as in Tables 44 and 46, except MA (see (8771) and JW (J & W Scientific). The subscripts 4 4 u, w, x y and z refer to the general formula for nitrile silicones given at the beginning of this section @. 230). a) Distinct content of carboxamide groups (between 1 and 5%) [878]. b) 100% here means 100%cyanoalkyl-substituted, of which 60% is cyanopropyl and 40% cyanoethyl 1878. 8841. c) Methyltolylsiloxy units here instead of methylphenylsiloxy.

Me

e) Bis(to1yl)siloxy units here instead of methylphenylsiloxy. f) Fused-silica capillaries, polysiloxanes chemically bonded and cross-linked. Film thickness between 0.15 and 1 . 0 ~

d) Here from 1.4-bis~ydroxydimethylsilyl)benzenr

Me

I

-ki -@si -0 I

I

Me

Me

236

8. Liquid Stationary Phases

he

CI

Me

Me

CL

Commercial name: F-4050 (Dow Coming Corp.)

I”‘ 111 Me Si -0 - - Si -0-J

Me

I

(-Si-

(

I

m

0 -k-Si

Me3

I

Commercial name: Lukooil X 100, 200, 600

IV

7‘

MejSi-O-(Si-O--),SiMes

I CH2 Cl

No commercial product. Preparation from MeSiClz [893]

I

CHzCl

8.3.9.

Polyester Silicones

In order to combine the high selectivity of polyester stationary phases with the thermal stability of silicones, intermediates of polyethylene glycol succinate were reacted with appropriate a, w-functional poly(diorganosi1oxanes) [894]. However, the maximum operating temperatures could be increased only by 1O-2O0C, and the McReynolds constants declined by about 100 units for methyl- and phenylsilicone moieties. As the reproducibility of the composition may be unsatisfactory, these products did not succeed, in spite of some succesful separations of high-molecular-weight fatty acid esters, catecholamines, carbohydrate derivatives and steroids. Suppliers of polyester silicones include Applied Science Laboratories (Table 58). The operating temperatures range between 30-100°C (min) and 210-230°C (max.). Structure:

8.3.10. Chiral Polysiloxanes The direct separation of enantiomers on a chiral stationary phase is of considerable and growing importance. We shall discuss the corresponding stationary phases in Section 8.15; here only the covalent bonding of optically active moieties to the siloxane backbone is dealt with. This combination is intended to utilize the siloxane’s thermal stability and spreading

Table 57. Characteristics of Poly(chloropheny1- and chloromethyl siloxanes) *) Name

Supplier,

S~NC-

manufacturer

ture type

Chloroalkyll aryl substitution

Viscosity (20°C) Imm2.g-'l

d:' [g.~n-']

McReynolds constants X

Y

U

Z

S

H

I

K

L

M

49 36 49 49 49

24

35

69

07

24

35

69

07

1x1

SP GE GE GE DCC W DCC [893]

SP-400 F-50 Versilube F-60 F-61 DC-560 Chlorphenyl 0 F-4050 Khs-2-1 Khs-2-1w Lukwil X-100 Lukwil X-200 LUkOOil X-600

su su cs cs cs

I I I I I I

11

1.045

70(25"C) 63 500 75 20-1000

11 !i

1;

32 19 32 32 32

72 57 72 72 72

70 48

141 22

217 61

70 70

100 69 100 100 100

68 47 68 68 68

245 66

309 87

254 65

I0

I1

IV

50 1.03-1.04

I I 111 111 111

70-85

40 6 8

3 Suppliers as in Tables 44.46 and 51

Table 58. Characteristics of polyester silicone stationary phases Commercial Name

EGSS-X

EGSS-Y -EGSP-A EGSP-Z ECNSS-S ECNSS-M

R

Ch3 CHS C6HS C6HS

(CHz)zCN (CHz)zCN

m:n

1 1 1 1 1 1

McReynolds constants X

Y

Z

U

S

H

I

K

L

M

484 391 397 308 438 42 1

710 597 629 474 659 690

585 493 519 399 566 581

831 693 721 548 820 803

778 661 700 549 722 732

566 469 496 373 530 548

412 335

316 261 218 220 286 259

713 591

237 190

469

167

644

211

279 383

238

8. Liquid Stationary Phases

behaviour on the capillary column wall and to regulate the distance between the chiral centres and the siloxane backbone via appropriate spacers while maintaining the enantioselectivily of the optically active amino acid derivatives. Frank et al. [895, 8961 hydrosilylated an acrylic acid with methyldichlorosilane

F'

CI

Me-Si -H

+

I

H2C=C-CCOOH

I

HPC'6-

I

I

R

CL

Me-Si-CHZ-CH-COOH CL

I

R

and copolymerized, after hydrolysis, the 2-carboxyalkylmethylsiloxanewith cyclic dimethylsiloxanes to form a poly(dimethylsiloxane), the chain of which exhibited a certain number of methyl-2-carboxyalkylsiloxaneunits. The carboxy groups projecting from the chain were reacted with the amino groups of a suitable D- or L-amino acid or its peptides [897], of which L-valine-tee.-butylamide [898] proved successful. The separation of the chiral centres by several siloxane units is necessary in order to keep the L-valinamides at a distance and to prevent the formation of intramolecular hydrogen bonds, which would impart a quasi-crystalline structure to the polymer. The phase named Chirasil-Val by its inventors [896] has the following structure and AI values of the McReynolds samples (at 50°C) [899]: CH3

I

-0-Si-(

Ck,

H

CH3

-\/ C-CHj

o=c

CH3

I

-0-Si-b-O-si I CH3

I

-

4\2

H-C-CH3 /

/

NfH3 / CH-C*H I \ CH3

H-N

/"= / C-CH3 /\

CH3 CH3

Sample benzene n-butanol 2-pentanone 2-nitropropane pyridine 1,4-dioxan

AI 24 264 102 130 107 82

,

C-CH3

/\

CH3 CH3

This stationary is available as 25-m coated open-tubular columns from Applied Science Labs. The AEvalues, which cannot be designated as McReynolds constants (owing to their determination at 50"C), are relatively low, indicating the small polarity of the phase. The structure of a diastereomer complex formed with an 0-pentafluoropropionyl-L-lactamideis outlined in heavy types in the structure shown for the stationary phase. D- and L-isomers are well separated (the D-enantiomer being eluted first), and the separation is attributed to differences in the strength of the indicated hydrogen bonds between the sample and stationary phase as a consequence of the D-configuration's mutual hindrance of the alkyl groups to form a stable complex [900]. The stationary phase is thermally stable up to 200°C [899]. It allows the separation of all essential amino acids within 30 min and, by adjusting the polarity, it is possible to prevent overlapping of any pair of enantiomers with another. Moreover, such polysiloxanes with covalently bound amino acid (or peptide) groups can be used for the direct separation of optical antipodes of hydroxy acids, alcohols, amines and biphenyl derivatives without the necessity to prepare diastereoisomers. On an L-amino acid stationary phase the D-enantiomer is generally eluted first, because with the L-configuration the maximum number of hydrogen bridges between the stationary phase and the optically active sample can

239

8.3. Silicones

be formed, whereas this is not possible with the D-configuration for steric reasons (circles in blue colour in the structure above). For example, the auDvalues of the N,O-perbentafluoropropionyl) derivatives of the antipodes of numerous pharmaceuticals and metabolites on a 20 m X 0.3 mm I.D. Chirasil-Val column (glass) are 1.014-1.102. They still increae with decreasing column temperature. Chirasil-Val has been modified by phenyl groups [901], commercially available (15% phenyl substitution) as Heliflex Chirasil-Val (25 m X 0.31 mm I.D. fused-silica open-tubular column, d, 0.23 pm). Also here, the separation of the amino acid derivatives is dictated by the structure of the alkyl groups [902]. Instead of the hydrosilylation described above, two alternative methods have been adopted to prepare an appropriate siloxane reactant for chemically bonding the optically active amino acid derivatives. The first consists in the saponification of the nitrile groups of a cyanoalkylpolysiloxane, e.g., OV-225 or Silar 1OC (see Section 8.3.7.) to give the corresponding carboxyalkylpolysiloxane. The carboxy groups are then converted into the acid chlorides, which can be reacted with the optically active component [903, 9041. Because of the greater distance between the chiral centre and the siloxane chain (propyl rather than an ethyl group) or the content of phenyl groups (OV-225), or a higher content and shorter distance between the chiral groups (Silar lOC), the gas chromatographic properties differ distinctly from those of Chirasil-Val. The second method employs the reduction of the cyano groups in OV-225 to amino groups with lithium alanate and subsequent reaction with L-valine or L-leucine derivatives [905]. These stationary phases also show a retention behaviour deviating from that of the other phases, not least because of the long distance between the optically active centre and the siloxane chain (four CH2-groups between Si and the amino acid owing to the reaction I

-Si-(CH2)3CN

I

-- I

Si -(CH2)3 CH2

I

-NH2

-

I

-Si --ICH2) 3 -CH2 -NH I

I

-C -CH -NHR

II I O R

Further stationary phases have been prepared from XE-60 (containing SiCH2CH2CNgroups, see Section 8.3.7) via the carboxylic group SiCH2CH2COOHand its condensation with L-valine-(S or R)-a-phenylethylamide in dicyclohexalcarbodiimide and CHC13 [907, 9081. They are thermally stable up to 200°C. The sites for hydrogen bonding and their positions both in the stationary phase and in optically active samples, in addition to the usual properties, seem to determine the retention order. Further similar stationary phases have been prepared in order to study the influence of the number of dimethylsiloxane units between the active centres, and it has been confirmed that a phase with a separation of active centres by five dimethylsiloxane units seems to be optimum, whereas phases with only one unit or more than five units show much lower enantioselectivity [909]. Chrompack (chromatographic specialties supplier, The Netherlands) offer fused-silica open-tubular columns coated with either L-valine-(S or R)-a-phenylethylamide [910], covalently bound by condensation with -COOH groups (from SiCH2CH2COOH, formed by saponificaton of the nitrile groups in XE-60). Cross-linking procedures can also be successfully applied to these polymeric chiral phases, hence producing films that are stable against solvent rinsing and droplet formation at elevated temperatures [911, 9121. Recently, the direct enantiomeric resolution of primary, secondary and tertiary alcohols and of further compounds was achieved on optically active polysiloxanes [913, 9141. Further methods describing the preparation of WCOT columns with cross-linked chiral stationary phases, based on vinyl-, epoxy-, amino- and methacryl-modified trimethoxysilanes were recently published [914a and b]. Chiral polysiloxanes derived from (R,R)-tartrami&,have been successfully used for the direct resolution of enantiomeric diols [914c]. For commercially available stationary phases, see Section 8.15.

8. Liquid Stationary Phases

240

8.3.1 1. Mesogenic Polysiloxanes In addition to dispersion, dipole-dipole, induced dipole, charge-transfer and complexation interactions between solutes and stationary phases, liquid crystalline stationary phases offer an extra dimension for gas chromatographic separations, viz., interaction differences resulting from the molecular shape of the solute and the ordered arrangement of the liquid crystal molecules in the stationary phase, which can be utilized for difficult separations of geometric and optical isomers. This topic will be discussed later, and here we shall deal only with liquid-crystalline polysiloxane phases. Owing to the severe disadvantages of non-siloxane liquid crystal phases (poor efficiencies, thermal stabilities and solute permeability), successful attempts were made to bond mesomorphic groups on to a polysiloxane backbone, similarly to the previous chiral groups (Section 8.3.10), and hence to improve the thermal and spreading behaviour and the permeability for solutes. Moreover, cross-linkeability was sought and could be obtained while preserving the desired properties of the monomeric mesomorphic molecules. Finkelman et al. and Kong et al. were the first to report the use of mesomeric polysiloxanes as stationary phases in gas chromatography [915-9191. The mesomorphic moieties have generally been coupled to the polysiloxane skeleton via alkyl spacers, but polysiloxanes with the mesogene in the main chain have also proved successful [920]. The synthesis of mesogenic polysiloxanes is effected by the addition of the SiH groups from specially prepared polymethylhydrosiloxanes: Me

to suitable vinyl-terminated mesomorphic moieties (H2PtC1,catalyst), for example (after Markides, Lee et al. [921] and Jones, Lee et al. [922]: Me3 Si I

I

Q Me- Si - H

1

d

4

Si-

0:

Me- Si-

H2C=CH-(CH2

l

I

-

O

~

C

I1

-

~

0

c

H

H ~ C = C H - ( C H Z ~ - O ~ ! - O

Me

e O C H 3

Me3 Si I

Me- O\Si-

(CH2)I+2

0

- 0 w ! - o e O C H 3

Q

Me- Si - Me

0:

Me-?-

4

0 ( CHz), O C H e3--@ !-3(-0-2

Tn Me3 Si

= 1.

~

3

HlPtC16

0

I

where 1. m

O

+

Me-Si-Me Me-

0

241

8.3. Silicones

The mesomorphic moiety is decoupled from the methylsiloxane main chain by the hydrocarbon spacer. Structural variations [type and percentage of mesomorphic substitution, length and length differences of the functional group spacer ( 1 and m values) and type and length (n-value) of the polysiloxane chain] influence the gas chromatographic properties. By X-ray photography, ordered domains were found in both the crystalline and smectic states [923], and differences were found by differential scanning calorimetry (DSC) and thermodynamic investigations for methyl-, ethyl- and propyl-substituted polysiloxane main chains [924-9261. Changing m and 1 means changing the mesomorphic properties and transition temperatures. The phase properties usually change from nematic to a more ordered smectic form, where nematic liquid crystals are only structurally restricted in that the long axes of the mesomorphic moieties maintain a nearly parallel arrangement over macroscopic distances, whereas smectic properties distinguish themselves by more organized mesomorphic molecular packings arranged in layers and by a structured packing within the layer [921, 9271. It was found that the selectivity of a liquid crystalline phase, which increases with higher order, also increases with differing spacer lengths ( 1 =k m ) in the same polymer, while an increase in the polysiloxane chain length only increases the efficiency and film stability in open-tubular columns [921, 9271. The separation of geometric isomers results in an elution order according to the length-tobreadth ratio and planarity of the solute. By cross-linking with pure azo-tert.-butane, the smectic biphenylcarboxylate ester polysiloxanes described above exhibit elastic properties, the smectic order being maintained between 120 and 260°C (m = 1, 1 = 3) or 103 and 288°C (m,1 = 3) to become isotropic above the upper temperature without a transition to a nematic order for m,Is 3. Similar smectic liquid crystalline polysiloxanes have been prepared by Nishioka et al. [1122]. They contain the following units, bonded to the siloxane backbone: I CH 3 - S i -(CH2)30 - @ C O O w C O O C H $ H I

(CH3) C2H5

Tg - s 13OoC

0

Ts - N 219'C

I

TN-I

235°C

TN-I

30°C

I CH3-Si - ( C H 2 ) 3 0 M C 0 0 -@OCH3

I

0

I

CH3-Si -(CH2)30

COO~COOCH~CH(CH3)C2H5

I

0

I

They are reported to be useful for the analysis of polycyclic aromatic hydrocarbons. Attempts to incorporate mesogenic groups into the poly(dimethylsi1oxane) main chain proceeded successfully. Oligomeric SiH-terminated dimethylsiloxanes, i.e. Me

I

Me

I

H-SSi-O-(Si-O-),,Si-H

Me

I

1

I

I

Me

Me

Me

242

8. Liquid Stationary Phases

undergo addition (H,RC& catalyst) to mesogenic groups with terminal double bonds, e.g. CH2=CH-CH20-Z-OCH2-CH=CH2, where Z means - C 6 H 4 - C O O C 6 ~ 4 ~ ~ ~ ~ 6 ~ 4 or the biphenyl derivative, -C6H4COOC6H4-COOC6H4-. However, the smectic liquid crystal phases exhibit glass and clearing temperatures that are substantially lower than those for comparable mesogenic polysiloxanes with hydrocarbon spacers, and the phase transition temperature decreases with increasing length of the siloxane segments between the mesogens [920]. The utility of mesogenic polysiloxane phases has been demonstrated using fused-silica open-tublar columns coated statically for a wide variety of polycyclic aromatic and sulphur compounds. Especially for four-membered ring aromatics these phases show an unsurpassed selectivity, and the good efficiency over a wide temperature range for gum and cross-linked stationary phases offers excellent possibilities for the separation of various isomeric polycyclic aromatic compounds [921, 9271.

8.3.12 Aminoalkyl-SubstitutedPolysiloxanes Optically active amino-substitued stationary phases were described in Section 8.3.10. Simple aminoalkyl-substituted polysiloxanes containing moieties with primary, secondary and tertiary amino groups were studied by Huken et al. [928-9301, and it was shown that reactions of both primary and secondary amino groups with aldehydes, ketones and lactones occur, whereas reactions of these compound types with tertiary amino groups do not take place. Owing to the chemical reactivity of the stationary phases, their general use is restricted to abstraction columns (for carbonyl compounds). In Table 59 the composition and McReynolds constants of several substituted polymers and of monomers for chemically bonded stationary phases (by reaction of the trimethoxysilyl terminal group with the SiOH groups of the support material) are listed.

8.3.13. Other Siloxane Stationary Phases Poly(diorganosi1oxanes) containing, in addition to methyl, both 3,3,34rifluoropropyl and 2-cyanoethyl groups have been described by Luskinu et al. [931]:

he

where m = 0-20. n = 0-20 and = 1-20. They are highly selective for the separation o partially and perfluorinated compounds and can be applied at column temperatures up to ca. 230°C [931]. Low-phenylated poly(dimethylsi1oxanes) and poly(rnethylphenylsi1oxanes) into which pyrimidone moieties have been incorporated [932] have proved to provide useful selective interactions with underivatized barbiturates, which are dmcult to separate. The monomer to be incorporated as the pyrimidone moiety is 1,3-diallyl-5-ethyl-5-isoamylbarbituric acid:

243

8.3. Silicones

The content of this unsaturated barbiturate monomer in the final cross-linked polymer ranges from 4 to 50%. The incorporation proceeds by statically coating a solution containing SE-54 (94% dimethylsiloxane, 5%diphenylsiloxane, 1%methylvinylsiloxane), the diallylated barbituric acid and dicumyl peroxide, flushing the previously persilylated capillary column with dry nitrogen and subsequent curing. The presence of two ally1 groups in the barbiturate monomer facilitates cross-linking with the SE-54 matrix, resulting in non-extractable pyrimidonesilicones. On a 15 m X 0.23 rnm I.D. Pyrex column, persilylated with divinyltetramethyldisilazane and coated with SE-54 - pyrimidone (10:30)(the pyrimidone moieties having been incorporated by cross-linking at 140°C; catalyst 1.5% dicumyl peroxide), 21 out of 22 common (underivatized) barbiturates encountered in toxicological cases, including the dXi-

17

16

22

%

I

I

I

I

18

15

10

5

0

Time in min

Fig. 31. Chromatogram of 22 barbiturates on an SE-54 Pyrex column (15 m X 0.23 mm I.D.) with 4% incorporated pyrimidone. Column temperature 120°C for 2 min, programmed at 8 K.min-l to 240°C and held at 240°C for 3 min. Flow-rate (nitrogen) 16.3 cm. s-’. 1 barbital; 2 probarbital; 3 allobarbital; 4 aprobarbital; 5 ethylcrotylbarbital; 6 butobarbital; 7 butalbital; 8 amylobarbital; 9 nealbarbital; 10 pentobarbital; 11 vinbarbital; 12 quinalbarbital; 13 hexobarbital; 14 brallobarbital; 15 cyclopentobarbital; 16 ibomal; 17 methylphenylbarbital; 18 phenobarbital; 19 cyclobarbital; 20 5-ethyl-5-p-tolylbarbital; 21 heptabarbital; 22 reposal (af€erChow and Gaddy [932])

Table 59. Composition and McReynolds Constants of Some Aminoallcylalkoxysilanes and of Correspondingly Substituted Polysiloxanes (after Haken et al. [928, 9301) Commercial Name

Z 6020 A 1120 ICBM-603 89008 A 1100 88542 X-12-563 X-12-570

Manu-

Chemical Composition

[9.nn-31

DCC

1.01-1.04

B.p.

McReynolds constants

Wa)

X

rcl

PS DCC

NH~(CH~)~S~(OC~HS)I

0.942

SET

CH2=CH-CH2-NH+CH2)3Si(OCH3)3

0.975

146 (2) 259 (101.3) 217 (101.3) 122.5 (31) 107 (1)

SET

0

1.05

122 (0.67)

ucc SET

FH2 -cH2 ‘N

\

CHZ -CHz

S4150l41

di5

facture?)

ICI

/

(CH2 )3 - Si (OCH3 13

(CH3)2 N (CH2)2 - 0

I

(

r

z

u

s

247

700

393

454

433

145

426

226

313

297

323

653

441

593

555

072

539

129

511

469

-008

143

076

069

082

063 030

269 155

129 119

204 084

116 062

024

159

070

093

070

031

166

067

095

078

-5 -O-),,

I

Me

DC-530 DC-531

DCC DCC

Dc-535 MS-2560 X-22-857

DcC MS

SEJ

Copolymer Z 6020 and PDMS 50%solution of DC 530 in 2-propanol and aliphatic solvents**) poly(dimethylsi1oxane) with ca. 2% ethylenediamine moieties As above, but ca. 2.5%ethylenediamine moieties Me

PS 812

I

Me

I

(HZNCH~CH~CH~S~-O), (-Si-O-)n

I

I

Me

*) Manufacturers as in Tables 44 and 46, except SEJ (Shin-Etsu Chemical Industries, Tokyo, Japan). **) The decrease in the McReynolds constants might indicate a chemical alteration of DC 530 by reaction with propanol and/or traces of water, causing cleavage of the

containing group.

N-

245

8.3. Silicones

cult-to-separate pairs, can be separated within 21 min (Fig. 31). The thermal stability is assumed to be sufficient up to 300°C [932]. Polymetallorganosiloxanes,the principal structure difference from poly(organosi1oxanes) being the presence of divalent or higher valent metal atoms in the siloxane chains

I I

I

I

-Si-0-Si-0-Met-0-Si-

I

I

I

or -Si-0-Ji-0-Met-0-Si-.

I have been prepared. The introduction of the metal into the siloxane chain enhances the thermal stability by increasing the ionic nature of the Si-0 bond when the metal is more electropositive than silicon. The more ionic (or, with a covalent bond, the more polarized) is the bonding, the higher is the thermal stability. Additionally, the selectivity will, depending on the nature of the metal in question, be distinctly altered owing to its capability of interacting coordinatively with unsaturated compounds or compounds containing heteroatoms with lone electron pairs. Copper- and chromium-containing polymers have been shown to interact especially strongly with unsaturated compounds [592]. Similar polymers with Be or Cr in the chain have been recommended as stationary phases for the analysis of high-boiling compounds. These poly(beryl1ium- or chromium-diphenylsiloxanes)have the general formula

where met = Be or Cr, M. W. = 1500-2000 [933]. They are highly selective for alcohols, acids and nitrogen compounds and can be used at column temperatures up to ca. 400°C. Substituted phenylsiloxane polymers have been prepared by Brudrhaw et al. [1121] by the OCH2CH=CH2,

addition of x

CH2-CH=CH2 or

@CH=CH2to

methyldichlorosilane [catalyst and reactant HC(OCH3)3]to form (including esterification) the corresponding monomers O(CHZ)~S~(OCHJ)Z, @ (CHz)3Si(OCH~)~ and

x

I

I

CH3

CH3

(cH~hyi(0CH~)~. CH3

The corresponding polymers are formed by hydrolysis and polymerization. Very recently, a new crown ether, attached via spacer to a dimethylsiloxane backbone, was prepared and coated on a fused silica capillary column [934d]: FH3

y 3

y 3

CH3 -Si -O(Si-01,-(Si

-O-&

7H3 Si

-CH3

I

I

I

I

CHj

CH3

CH2

CH3

I

(CH2)lo

I

246

8. Liquid Stationary Phases

This liquid stationary phase, cross-linked on the column wall with azo-tert.-butane, has promising properties: good film forming ability, unique selectivity and applicability between 70 and 300 "C. The selectivity is based on the strong hydrogen-bonding force to alcohols. McReynolds constants of this new phase: X = 304, Y = 229, 2 = 141, U = 252 and S = 218.

8.4.

Alcohols, Ethers and Carbohydrates

Liquid stationary phases in this group are poor solvents for hydrocarbons, which hence can therefore be separated selectively from other organic compounds. Homologous alkanes, however are insufficiently separated from one another unless the stationary phase possesses a distinct content of methylene or phenylene units. Of great importance is the capability of the ether and hydroxy oxygen atoms in these phases to act as acceptor atoms for the hydrogen bond. Therefore, owing to the tendency to form hydrogen bridge bonds with alcohols, acids, phenols and primary and secondary amines, such compounds are more strongly retained in the column than those without hydrogen-donor atoms. However, the hydrogen atoms of hydroxylic stationary phases (alcohols, carbohydrates) may also form hydrogen bonds with receptive acceptor atoms of the solutes, hence increasing the retention of ethers, esters, ketones, aldehydes, tertiary amines, N- and 0-heterocyclics and cyclopropane derivatives. Moreover, substances with a n-electron system are also eluted later than these which cannot be selectively polarized.

8.4.1.

Monohydric Alcohols

Because of the vapour pressure, only high-boiling members are suitable, but also these should be substituted by low-molecular-weightpolyglycols. They offer certain advantages but only for the investigation of low-boiling aliphatic amines and for the separation of primary, secondary and tertiary alcohols. 1. Hexadecanol (cetyl alcohol) Structure: CH3(CH2)14CH20H B.p. : 189°C (2 kPa) dj9: 0.798 g ~ r n - ~

Min. col. temp.: 50°C Max. col. temp.: 90°C [935]

2. Octadecanol Structure: CH3(CH2)16CH20H B.p. : 210.5"C (2 kPa) d:9 : 0.812 g * cm-3

Min. col. temp.: 59°C Max. col. temp.: 90°C

8.4.2.

Polyhydric Alcohols

The inductive effect of the negative hydroxy group enhances the tendency of the hydrogenatoms to form hydrogen bonds. Hence, the specific retention values, V8, of alcoholic and phenolic compounds are predominantly determined by the heat of formation of the hydrogen

247

8.4. Alcohols, Ethers and Carbohydrates

bond. The outstanding selectivity for such solutes becomes apparent, e.g., in that methanol is eluted on diglycerol substantially later than tert.-butanol, which has a 35°C higher boiling point, and even later than ethanol. Stationary phases in this group are also suitable for the selective separation of substituted phenols, pyridine homologues and even stereoisomeric methylcyclohexanols. For application at higher temperatures, polyglycerol has been prepared from glycerol by reaction with l-hydroxy-2,3-epoxypropanein the presence of basic or acidic catalysts. It retains aldehydes more strongly than ketones, but otherwise resembles the polyethylene glycols. Glycerol Structure: CH20H CHOH CH20H B.p. : 2 90°C die: 1.260 g Commercial name: Perkin Elmer KA Digly cerol Structure: (CH20H CHOH CH2)20 d:O: 1.26 g McReynolds constants of Diglycerol: X 371, H 608, Y 876, I 245, Z 560, K 141, U 616, L 124, S 854, M 36. i-Erythritol (butanetetrol) Structure: (CH20H CHOH)2 B.p. : 330°C d-Sorbitol Structure: CH20H(CHOH)&H20H

Min. col. temp.: 20°C Max. col. temp.: 50°C

Min. col. temp.: 20°C Max. col. temp.: 100°C

Min. col. temp.: 120°C Max. col. temp.: 150°C Min. col. temp.: 110°C Max. col. temp.: 190°C

Min. col. temp.: 225°C Max. col. temp.: 230°C

Inositol Structure OH

Polyglycerol Structure:

CH$HCHOHCH~(OCHZCHOHCH~),,OCH2CHOHCH2OH

and HOCH2CH(OCH2 CH), OCHCH20H I 1 1 CHzOH CHzW CH20H

Min. col. temp.: 20°C Max. col. temp.: 200°C

248

8.4.3.

8. Liquid Stationary Phases

Saccharides

Stationary phases of similar selectivity to those described in the previous section are octakis (2-hydroxypropyl) sucrose (Hyprose S.P. 80), the y-lactone of galacturonic acid, etc. Their field of application is limited by the mostly high melting prints. Acylated g-cyclodextrins described by Schlenk et al. [936] resemble polyesters with regard to the selectivity. However, their high melting points (169°C for the propionyl and 201°C for the more polar acetyl derivative) do not allow lower column temperatures and, on the other hand, above 220°C the decomposition begins (cleavage of acetic acid with the acetate).

Structure: di0 :

CI2Hl4O3 (-OCHzCH CH3)e I OH

1.16-1.18 g .

Min. col. temp.: 20°C Max. col. temp.: 175°C

Commercial name: Hyprose SP-80 McReynolds constants of Hyprose SP-80: X 336, H 565, Y 742, I 310, Z 492, K 227, U 639, L 590, S 727, M 196. 0-Cyclodextrin propionate Empirical formula: C10sH154056 M.W.: 2312 M.p. : 169°C

Min. col. temp.: 170°C Max. col. temp.: 210°C

Recently, fused silica capillary columns coated with cyclodextrin derivatives have been commercially offered (ICT Handelsgesellschaft, Frankfurt, and Chromatography Service GmbH, Langerwehe, Germany). Though there are serious and unusual restrictions for the application (only split injection, sensitivity to thermal shock, extremely low sample load), these phases enable, due to their outstanding enantioselectivity, the chiral recognition of racemic amines, amino alcohols, alcohols, carboxylic acids, sugars lactones, epoxides etc. [934,934ab]. Based on native cyclodextrins (a-,0- and y-cyclodextrin) permethylated hydroxypropyl and dialkyl derivatives have been synthesized [934b-c], which are stable high-boiling liquids and, being chiral, can separate many enantiomers. If substituting the permethylated hydroxypropyl derivative by the dialkyl derivative, the enantioselectivity can be influenced and the elution order of the enantiomers can often be reversed. Seventy pairs of enantiomers could be resolved (934~1.Chiraldex-A-PH and -A-DA (A means a cyclodextrin, PH = permethylated hydroxypropyl, DA = dialkyl) should be applied for the analysis of smaller molecules and Chiraldex-G-PH and -G-DA (G= y cyclodextrin) for the analysis of larger molecules (cyclic and bicyclic diols, steroids, carbohydrates, multicyclic compounds).

249

8.4. Alcohols, Ethers and Carbohydrates

8.4.4.

Polyglycols and Poly(alky1ene oxides)

Apart from silicones, polyglycols are the most popular stationary phases for gas chromatography. They have a poor solubilizing power for aliphatic hydrocarbons, but they are to a certain extent selective for the separation of normal from branched-chain alkanes and of unsaturated from saturated hydrocarbons. Alkylbenzenes are retained selectively. The presence of both acceptor atoms (hydroxy and ether oxygen) and donor atoms (hydroxy hydrogen) for the hydrogen bond enables especially the lower members of the polyglycols to interact strongly not only with hydroxy compounds and primary amines, but also with compounds containing carbonyl-, sec.- and tert.-amino groups and heterocyclically bonded nitrogen and oxygen. As the hydrogen-bridge bond in all these instances is the major contributor to the attractive forces, distinct differences in selectivity do not occur for the above compound types. Hence aldehydes, ketones and ethers, if present simultaneously, are eluted approximately in order of their vapour pressures (boiling points). Polyethylene glycol 2000, for example, is only weakly selective for oxygen compounds. The essential factor for the selectivity of poly(ethylene glycols) is the concentration of the hydroxy groups accessible for the partition process [937]. As usual for polymers, the ratio of the number of end groups to the number of links decreases with increasing molecular weight. High-molecular-weight polyglycols (polyalkylene oxides) therefore posses only negligibly few remaining hydroxy-hydrogen atoms, and the proton acceptor properties of these stationary phases come to the fore, caused by the clusters of ether oxygen atoms, and hydrogen bonds only occur if the hydrogen atoms are provided by the solutes. Hence, poly(alky1ene oxides) exhibit excellent selectivity for primary, secondary and tertiary amines, as the formation of hydrogen bridge bonds and hence an increase in the retention volume become prevalent only with the primary amines, whereas with tertiary amines there are no corresponding hydrogen-atoms available. The strength of the interaction with secondary amines is intermediate between those with primary and tertiary amines. A similar behaviour is observed for low-molecular-weight polyglycols, provided that they are totally etherified. The dependence of the selectivity of non-etherified poly(alky1ene glycols) on the molecular weight is obvious, e.g. by the AMe values in Table 60.

Table 60. Dependence of the AMe Values on the Content of Terminal Hydroxy Groups in Poly(ethy1ene glycols) (after Evans [938]) (for AMe, see section 4.2.5) Approximate Molecular Weight of the Poly(ethy1enc glycol)

Solute

1-Bromoheptane Methyl n-octanoate n-Butylbenzene n-Pentyl cyanide 2-Ethylhexan-1-01 2-Methylbutan-1-01 *)

200

400

1000

4000

6AMe.)

12.20 59.16 66.70 111.10 116.11 121.76

6.93 49.43 60.64 102.64 99.35 102.22

3.50 43.44 56.61 94.11 87.12 89.82

1.64 40.19 54.73 89.52 81.56 83.30

10.56 18.57 11.97 26.58 34.55 38.46

8AMe = AMcpeo 2w - AMepeo 4m.

Whereas AMe for 3-methyl-butan-1-01, for example, is 122 on PEG200, it decreases via 102 (PEG 400) and 90 (PEG 1000) to 83 (PEG 4000) owing to the decrease in the number of hydroxy groups, which are especially responsible for the selectivity. These results were conf m e d by Huwkes et al. [939], who showed that the retention times, of, e.g., alcohols, do not

250

8. Liquid Stationary Phases

become independent of the molecular weight of the polyglycol, nor do they even approach this until the molecular weights are much greater than 6000 and probably greater than 20000. Poly(ethy1ene glycol) with molecular weight 40000 therefore appears to be an improvement over lower molecular weight polyglycols as a standard stationary phase. When lower membered polymers are used, perhaps by virtue of more suitable retention behaviour for the sample to be analysed, the reproducibility will vary from batch to batch, and standardization can only be achieved with tight control of the molecular weight distribution or the use of a single oligomer [939]. The influence of the hydroxy end groups is larger than that of the silicone end groups, owing to their high polarity. Therefore, the molecular weight distribution of polyglycols is of much greater importance than for all types of silicone stationary phases with their trialkylsiloxy end groups. First described by Wurlz in 1877, poly(ethy1ene glycols) were synthesized (patents in 1930) by polymerizing ethylene oxide using alkali metal hydroxides as catalysts. This is probably still the exclusive path of synthesis even today. A small content of a salt formed by the neutralization of the alkali metal catalyst is usually present, serving as a buffer and stabilizer of the polymer. The polymers are soluble in water, methanol, acetone and methyl formate, and even in dichloromethane or trichloroethylene with warming. With increasing molecular weight, the water solubility, hygroscopicity and solubility in organic solvents decrease, whereas the melting range, density and viscosity increase [940]. An important requirement that liquid stationary phases have to meet is thermal stability. The necessity to remove low-molecular-weightconstituents by conditioning has been repeatedly emphasized in this book. This also holds true for these stationary phases, as a few percent of volatile materials (formaldehyde, acetaldehyde, formic acid, acetic acid, etc.) have been detected [940-9431 in industrial products. However, the influence of the volatiles does not appear to cease with their removal, and it may be assumed that these compounds, together with hrther factors (catalytic action of the support material, oxygen and water present in the carrier gas) are responsible for decomposition processes as they can act as decomposition catalysts themselves at higher temperatures [940]. Ethers are well known to be susceptible to autoxidation, and so are poly(alky1ene oxides) with their numerous ether groups. Therefore, all traces of oxygen have to be removed from the carrier gas by an appropriate scavenger (e.g. manganese(I1)oxalate [944]) in order to prevent both oxidation itself and the formation of traces of acidic components which would catalyze the depolymerization. It was parts in shown [944] that reducing the oxygen concentration in the carrier gas from ca. "oxygen-free"-grade nitrogen to a nominal parts will reduce the oxidative degradation by a factor of nearly 5 and will raise the minimum temperature at which measurable degradation occurs within a 100-h period of operation from 160°C to about 200°C for the most-favoured PEG 20M,and the usually recommended l i t , variously stated to be between 200 and 250"C, is therefore excessively optimistic, unless an oxygen scavenger is used and a short column life or substantial degradation is allowed for [944]. These results could be endorsed by the thermogravimetric investigations of the author's team [945] on Carbowax 1000 and 20 000. In dry, pure nitrogen, the weight loss reached 1%and distinct degradation started for Carbowax 20M at ca. 250°C. and for Carbowax 1000 at 50°C (and increased by a further 1-2% for each 50°C in each case up to 250°C). Above 250"C, the weight loss increases distinctly (Carbowax20M) or dramatically (Carbowax 1000). In the presence of air, however, the temperature for a 1%weight less decreased to 190°C for (Carbowax 20M). For Carbowax 1000, drastic degradation starts at 140"C, and a weight loss of 1%is reached at 50"C, equal to the above-mentioned value. Moreover, in the chromatographic column the poly(alky1ene oxide) is even more exposed to the risk of oxidation and decomposition, because it is coated on the support or column wall and hence exhibits a much higher ratio of surface area to bulk liquid compared with the situation in thermogravimetry. Efficient drying of the carrier gas is important, as otherwise water molecules might act as

8.4. Alcohols, Ethers and Carbohydrates

251

cross-linkers between poly(alky1ene oxide) chains (hydrogen-bonds) which would cause retention changes with certain solutes [946], and as at high temperatures the water might be involved in degradation reactions in which carbonyls are formed [944]. In the same way, strong acids, particularly halogen acids, and Lewis acids have to be kept away from the polymeric alkylene oxides. 8.4.4.1.

Poly(ethy1ene oxides) IPoly(ethy1ene glycols)]

Structure: HO(CH2CH20),H surface tension 1.13 g ~ r n - ~ 42-45 mJ * m-2 (depending on the M. W.) (depending on the M.W.) Application of poly(ethy1ene oxides): The fields of application can be derived from their selectivity. Thus, the separation of primary, secondary and tertiary amines, of alcohols from hydrocarbons, ketones or aldehydes and of ethers or heterocyclics from hydrocarbons can often readily be achieved. They are suitable stationary phases for the investigation of all types of compounds that form hydrogen bridges, e.g., lower alcohols, solvent mixtures and samples containing small amounts of water, and are well established for the analysis of aldehydes, ketones, esters, sulphur compounds, halogenated compounds, ethereal oils and pesticides, to mention only a few applications. Table 61 shows the characteristics of polyethylene oxide stationary phases. The analysis of samples containing silylating reagents should be avoided because of the risk of silylation reactions with the stationary phase, and diorganoperoxides and organohydroperoxides must not be analysed on these ether stationary phases (formation of peroxides in the liquid phase). Solvents for poly(ethy1ene oxides): Methanol, acetone, methyl formate. Recommendable products: Carbowax 20M, Superox 20M; for special separation problems if necessary lower molecular weight species of HO(CH2CH20),H. Chemically Bonded Poly(ethy1ene oxide) Phases: It has been shown that most of the active sites on the solid supports can be blocked by treatment with Carbowax at elevated temperature, as described by Aue et al. [948-9501, and by the preparation of chemically bonded stationary phases [954]. One might therefore expect bonding of poly (ethylene oxides) on the glass or fused silica surfaces inside the capillaries to be easy. However, the surface tension of these polyethers is >41 causing a much worse spreading behaviour than that of methylsilicones (surface tension ca. 21 mJ * m-2), and hence limiting the stability of a uniform film. This is why the use of polyethers in open-tubular columns has been restricted in comparison with that of silicones. Moreover, the risk of traces of water and air in the carrier gas causing the stationary phase to degrade at fairly low temperatures 200-250°C is a further drawback. To overcome these problems, various approaches have been followed, in addition to using oxygen-free carrier gases and leak-tight apparatus, such as the use of high-molecular-weight polyethers (Superox 20M [951]) or in situ immobilization with a non-extractable layer of another stationary phase in order to obtain bonded poly(ethy1ene oxides) [952, 9531. Further procedures for immobilization have been reported, for example the use of a copolymer with ethylene oxide [955], covering the glass walls with a layer of graphitized carbon black, coating this layer with high-molecular weight glycols and cross-linking with dicumyl peroxide [956], forming partially covalent and partially hydrogen bonds with the glass wall, and frnally partially bonding and cross-linking of the poly(ethy1ene glycol) with poly(dimethylsi1oxane) [957,958]. An application of this type of phase is shown in Fig.32. Alkoxysilanes have proved to react first with the column wall and to serve as a coupling agent for a subsequent reaction d:' :

Table 61. Characteristics of Poly(ethy1ene oxide) or Poly(ethy1ene glycol) Stationary Phases.) M ~ U facturcr, supplier')

M.W.

n3

[IO-~] (approx)

(approx)

Min. oper. temp.

PCI

ucc ucc ucc ucc ucc ucc ucc ucc ucc

Carbowax 400 Carbowax 600 carbowax 1000 carbowax 2000 Carbowax 4000 Carbowax 6000 carbowax 10 000 carbowax 20 000C) Carbowax 40 000 Superox 20M Superox 0.1 Superox 0.6 Superox 4 Polyox 600Mb) Polyox 4000Mb)

ASL ASL ASL ASL

0.4 0.6 1 2 3-3.7 6-7.5 8-12 15-20 35-40 20 100 600 4000 600 4000

Polyethylene oxideb) Polyethylene oxide lOOT Polyethylene oxide 200T Polyethylene oxide 300T Polyethylene oxide 600T Polyethylene oxide 4Mio Polyethylene oxide 5Mio

EGA EGA EGA EGA EGA EGA

100 200 300 600 4000 5000

2 300 4 500 6 800 14 000 90 000 110000

BDH BDH BDH BDH Ciba

300 600 4000 5000

6 800 14000 90 000 110000

Polyethylene oxide') Polyethylene oxide 300T Polyethylene oxide 600T Polyethylene oxide 4M Polyethylene oxide 5M AD 264 Mer-2 Mer-21 Mer-35

ASL ASL

9 13 23 45 80 150 220 400 850 450 2 300 14 90 14 90

20 30 40 60 60 70 70 70 80 70

Max.

MeReynolds constants

opcr. temp. PCl

X

100 100 120 150 160 170 180 250 250 250 250 250 250

Y

Z

U

S

H

I

K

L

M

350 631 428 632 605 472 308 240 503 162 347 607 418 626 589 449 306 240 493 161 325 551 375 582 520 299 285 224 443 148 322 540 369 577 512 390 282 222 437 147 322 536 368 572 510 387 282 221 434 148

Table 61. (continued) Commercial Name

Polyox

WSR N 10 Polyox-100 Lubrol MO Oxidwachs Perkin Elmer K

Manufacturer suppliep)

M.W.

n7

[lo-’] (approx)

(approx)

Min. oper. temp. PCI

Max.

McReynolds constants

oper. temp. PCI

k .’

r

Z

U

S

H

I

K

L

M

ucc

sw ICI Buna PE

1500

*) After t324, 783. 784. 781, 824. 9691 ‘3 n = Number of ethylene oxide groups,

9 Abbreviations as in Tables 44 and 46, except EGA = Ega-Chemic; BDH = BDH Chemicals. Poole. U.K.; Buna = Chemical Works Buna, P.R.G b, Roducts of lower purity I9471 3 Carbowax ZOM, which is the most frequently used poly(ethy1ene glycol), is synthesized by joining three polymeric molecules of Carbowax 6OOO with a dicpoxide and may therefore contain residual traces of unreacted epoxide groups.

254

8. Liquid Stationary Phases

Table 62. Commercially available fused-silica open-tubular columns with chemically bondedlcross-linked lyglycol phases Name

Manufacturer supplief)

Durawax-l**)

J & W Scientific

Phase composition

10%(CHZCH~O), 90%(SiMe,O), Durawax-2**) J & W Scientific 25% (CH&H20), 75% (SiMe20), Durawax-3**) J & W Scientific 50%(CH~CHZO). 50%(SiMe20), Durawax-4**) J & W Scientific 85% (CHICH~O), 15%(SiMe20), Durabond-Wax J & W Scientific 100%(CH2CH20). CP Wax 40 M Chrompack Nederland 100%(CH,$HZCHzCH20), CP Wax 52 CB Chrompack Nederland 100%(CHZCH~O), Permaphase PEG Perkin Elmer 100%(CHZCH,O), (Carbowax 20M) cross-linked HP-2OM Hewlett Packard 100%(CHZCH20), Stabilwax Restek 100 % (CHZCH2O), Stabilwax DA

Restek

100 % (CH2CH20),

Stabilwax DB

Restek

100 % (CH2CH20),

*) ")

Film thickness [lyn]

References, Remarks

0.25,l.O

W l

0.25,l.O

W I

0.25,l.O

W91

0.25,l.O

W I

0.15,0.25,0.50,1.0 0.25 0.25,2.0 0.25,0.4,1.0

[960]

0.5,l.O 0.10,0.50 0.25,l.OO 0.10,0.50 0.25,l.OO 0.10,0.50 0.25,l.OO

(9641

PO-

[961, 9621 W31

For acidic compounds For basic compounds

See also Table 47. DistQctly less polar, caused by the content of poly(dimethy1siloxane) units.

with a suitable glycol [1165, 11661. In addition to the advantage of the non-extractability and film stability, bonded/cross-linked poly(ethy1ene oxide) phases exhibit further merits: they can also be used at lower temperatures than their conventional counterparts, as bonding/ cross-linking distinctly influences the phase transition behaviour and lowers the minimum operating temperature to ca. 20"C, and they are also suitable for prolonged use at fairly high temperatures (250°C) without the risk of too great a decrease in efficiency. On the other hand, the reproducibility and chemical stability of these phases might be inferior to that of the silicone bonded phases. However, the results, of the separation of alcohols, phenols and other strongly polar compounds are of great promise. Some commercially available columns of this type are listed in Table 62.

255

8.4. Alcohols, Ethers and Carbohydrates

I

0

5

I

I

10

15

I

20

I

25

Time in min Fig. 32. Separation of tricyclic antidepressants. Column, 25 m X 0.20 mm I.D. fused silica; Stationary phase, Superox-4 + 1,2,3-trivinyl-1,1,3,5,5-pentamethyltrisiloxane [50% (w/w)to Superox-41 (as a coupling agent) + dicumyl peroxide [3%(w/w)to Superox-41; Temperature programme, 150°C (1 min), raised at 20 K .min-' to 260°C; splitless injection, 10 ng of each component; Peaks: I amitriptyline; 2 imipramine; 3 cis-doxepin; 4 trans-doxepin; 5 nortriptyline; 6 desipramine; 7 protriptyline (after Pnybyeiel et al. [1167]) (courtesy of Dr.Alfred Huthig Verlag, Heidelberg, F.R.G.)

8.4.4.2.

Other Poly(alky1ene oxides)

Poly(propy1ene glycols) are distinctly less polar than the corresponding poly(ethy1ene glycols), and because of the vulnerable hydrogen atoms attached to the tertiary carbon atoms their thermal stability is also lower. Therefore, stationary phases of this type have been much less used than the poly(ethy1ene oxides). Also, a specially prepared poly(trimethy1ene oxide) HO(CH2CH2CH20) ,,H as an intermediate polar and more thermally stable stationary phase did not find widespread application [965].This also applies to the poly(ethy1ene oxide-propylene oxide) copolymers HO(CH,CH20)(CH2CH(CH3)O),,H.Ucon stationary phases are poly(alky1ene glycols) from Union Carbide, the symbol H or HB indicating water-soluble (250% ethylene oxide units): 50 HB = water-soluble, poorly soluble in light petroleum; 75 H = water-soluble, indissoluble in light petroleum; X = containing an antioxidant; the symbol LB (poorly or non-water-soluble) indicates 250% propylene oxide units (soluble in light petroleum ether. Structure of poly(propy1ene glycols): Min. col. temp.: 20-70°C HO(CHZCH(CHJ0) ,H die: 1g . Max. col. temp.: 125-200°C Commercial Names: PPG, P 425, P 1025, P 2025, (Union Carbide Corp., the number corresponds to the average

256

8. Liauid Stationary Phases

molecular weight) PPG 1000 (Thanol) PPG 2000 (Jefferson) Structure of poly(ethy1ene glycoVpropylene glycol) Min. col. temp.: 20°C Max. col. temp.: 200°C Commercial names: Pluracol P-2010 Pluronic F-68, F-88, L-35, L-61, L-64, L-81, P-65, P-84, P-85 (The units in the numbers indicate the percentage of ethylene oxide units when multiplied by 10 [e.g., L 61,l x 10 = 10%(CH2CH20),and the tens indicate the molecular weight (e.g., 6 in L61, M. W.1750) [947]. POlyglyCOl 15-200 Ucon 50-HB-280X, 660,2000, 3520, 5100, 1800X Ucon 75-H-90 000 (Union Carbide Corp.) Ucon LB-550X, 1715 The Pluronic types may be preferred, as in contrast to the Ucon types, they do not contain additives and exhibit a smaller molecular weight distribution [947]. Nevertheless, Ucon 50 HB-5100 belongs to the polyethers which have found widespread use for coating open-tubular columns [1165]. Solvents for poly(alky1ene oxides) are dichloromethane and methyl formate. The McReynolds constants are listed in Table 63.

HO(CH~CH~O),(CH~CH(CH~)O),H

8.4.4.3. Poly(oxyalky1ene) Derivatives

In order to alter the selectivity and to improve (insignificantly!) the thermal stability, one or both terminal hydroxy groups of the polyglycols (see Sections 8.4.4.1 and 8.4.4.2) have been etherified or esterified, respectively. Long-chain derivatization, for example, increases the retention of hydrocarbons and reduces that of alcohols, whereas strongly polar end groups enhance to some extent all polar interactions. The monoethers are considered in Table 63. The most important and most frequently used stationary phases of this group are FTAP and Carbowax 20M-TPA. General structure of poly(ethy1ene oxide) derivatives: RO(CH2CH20),R’

PO2

(I) R = R ’ = H O O C e i 0

poly(ethy1ene glycol) di-2-nitroterephthalate Commercial names: FFAP (free fatty acid phase), SP-1000 (Supelco) Carbowax 20M-dinitroterephthalate FFAP, as the name indicates, was originally synthesized for use as a liquid stationary phase in the analysis of free fatty acids. Later it proved to be generally applicable also for the analysis of other underivatized acids, of alcohols, amides, N-acylamino acids, phenols, all types of esters, ketones, lactones, etc. However, owing to the presence of the free carboxyl groups in R, it must be used with caution if compounds that may react with the terminal groups or that may be adsorbed on the stationary phase, are to be analysed (e.g., aldehydes, epoxides and triazines) [866-9691. The adsorption ability is reported to decrease or even to cease with continued use and storage of the coated stationary phase in the column [969]. This indicates chemical alterations to the stationary phase (possibly splitting of the carboxyl groups). The thermal stability of this stationary phase has been investigated [945], and it has been found that after the removal of volatiles (ca. 2%),which begins at 30”C, a distinct weight loss does

257

8.4. Alcohols, Ethers and Carbohydrates

Table 63. McReynolds Constants of Poly(propy1ene oxides) and of Poly(ethy1ene oxidelpropylene oxides) *) Commercial Name

X

Poly@ropylene oxide) 130 PPG 1025 131 PPG 1000 128 PPG 2000 Pluracol P-20 lod) 129 Poly(ethy1ene oxide/propylene oxide) 264 Pluronic F-68 d, 262 Pluronic F-88 ') Pluronic L-35 ') 206 144 Pluronic L-81d) 203 Pluronic P-65d, Pluronic P-85d, 201 207 POlyglyCOl 15-200') 177 Ucon 50-HB-280Xd) 193 Ucon 50-HB-660') 202 Ucon 50-HB-2000C)d) 198 Ucon 50-HB-3520') 214 Ucon 50-HB-5100b)d) 123 Ucon 5O-HB-l800X**)dl Ucon 75-H-90000 255 Ucon LB-550Xa) 118 132 Ucon LB-1715.')') Poly(trimethy1ene oxide) **I)176 *)

Y

z

u

s

290 314 294 295

174 185 173 174

258 277 264 266

230 243 226 221

182 214 196 191

465 461 406 314 394 390 410 362 280 394 381 418 275 452 271 297 354

309 306 257 187 251 247 262 227 241 253 241 278 161 299 158 180 217

488 483 398 289 393 388 401 351 376 392 319 421 249 470 243 115 298

423 419 349 249 340 335 354 302 321 341 323 375 212 406 206 235 303

331 327 286 211 276 271 289 252 265 277 264 301 179 32 1 177 201

H I -

K

L

M

110 106 106

101 098 099

2I35 194 195

046 045 046

229 227 177 120 174 172 179 151 166 173 169 185 101 220 096 109

184 183 148 108 146 145 150 130 141 147 144 155 095 180 091 100

363 359 296 212 289 285 301 256 214 289 278 316 181 348 177 199

115 114 085 055

083 082 086 065 015

080 080 086 045 110 040 046

After [781. 784, 824, 314, 783, 969). **) Presumably poly(propy1cne glycols). ***) [965] ') 10%ethylene oxide--90% b, 50% ethylene oxide-SO%

7 d,

proDylene oxide. proDylcnc oxide. Surface tension 35.7 mJ .m-' [947]. Presumably one terminal OH group etherified (alkyl).

not occur, either in the absence or in the presence of atmospheric oxygen, before 260°C. Distinct decomposition in nitrogen starts at 280°C and a dramatic weight loss in air occurs at 260°C. Chemically bonded FFAP in fused silica capillaries, dl = 0.25 pm, is commercially available from J & W, USA (DB-FFAP). poly(ethy1ene glycol) diterephthalate (11) R = R' = HOOC a C II 0

Commercial names: Carbowax-TPA, Carbowax 4000-TPA, 20M-TPA. This stationary phase has a similar thermal stability to FFAP and also a similar applicability. Possibly the commercial product is only a mixture of Carbowax and terephthalic acid, and the esterification takes place only in the column at higher temperatures [9471. (111) R = R' = HOOCCH2CHIC or HOOCCH=CHCLpoly(ethy1ene glycol) disuccinate or dimaleate] Commercial name: STAP (steroid analysis phase) The product was specially developed for the analysis of the trimethylsilyl derivatives of steroids.

258 (IV)

8. Liquid Stationary Phases

R = R = HOOC(CH2)3C-

poly(ethy1ene glycol) diglutarate

a

poly(ethy1ene glycol) disalicylate (VI)

poly(ethy1ene glycol) bis (-3,s-dinitrobenzoate)

R=R=OpY

pi-

OpN

0

0

(VII) R=R’=CH3-@;-

poly(ethy1ene glycol) ditosylate 0

poly(ethy1ene glycol) distearate Commercial name: Nonex 76

poly(ethy1ene glycol) dioleate Commercial names: Carbowax 400 dioleate Carbowax 4000 dioleate poly(ethy1ene glycol) di(octadecy1) ether Commercial names: Emulphor ON-870 Mulgofon ON-870 (XI)

R = 0zN

@!-

I

d = C17H35-C-II

poly(ethy1ene glycol) 3,5-dinitrobenzoate monostearate

0

OZN

0 (XII) R = cS&,04-, R’ or C H C-, l7

=

3s~10

c,&-c- II

poly(ethy1ene glyco1)sorbitane monooleate (Tween 80), poly(ethy1ene glyco1)sorbitan monostearate (Tween 61)

These phases have proved successful for the analysis of hydrocarbons (with boiling points up to 200°C) poly(ethy1ene glycol) mono(tridecy1) ether (XIII) R = H, R’ = C13H27 Commercial names: Surfonic TD-300 Tridox

(XIV) R = H, R’ = C16H33 or CI7H&-

II

0

poly(ethy1ene glycol) monooleate or monostearate Commercial name:Ethofat 60/25

8.4.

259

Alcohols, Ethers and Carbohydrates

(XV) R = H ,

R’=

(XVI) R = H, R’ =

C

B

H

,

~

c g H 1 g G

~

poly(ethy1ene glycol) mono-4-n-octylphenyl ether Commercial names: Triton QS-15 Igepal CO-630 Triton X-100 (m = 10) Triton X-305 (m = 30) poly(ethy1ene glycol) mono-4-n-nonylphenyl ether Commercial names: Antarox CO-990, Dowfax 9 N 9 (m = 9), 9 N 15 ( m = 15), 9 N40 (m = 40); Igepal CO 880 (m = 30), CO 990 (m = 100); Lutensol (BASF) ( m= 20); Renex 678; Surfonic N-300; Tergito1 NP-35, NPX-728.

One product type containing ethylene oxide units has found only negligible acceptance viz., Silicone-Carbowax copolymers, poly(dimethylsiloxane/ethylene oxides), HO[(CH,CH,O-),(SI(CH,),O--,I kH. A corresponding commercial product, OV-330, can be used between 20 and 250°C. It is distinctly less polar than Carbowax and soluble in dichloromethane, methyl formate and acetone. Recommendable products: FFAP, SP-1000, Carbowax 20M-TPA Solvents: dichloromethane, chloroform, acetone The McReynolds constants are listed in Table 64.

8.4.5.

Aromatic Ethers

The aromatic moiety of these stationary phases contributes distinctly to the separation of olefins and aromatics. The relatively low proportion of ether groups and hence the accessible acceptor atoms for hydrogen bonding are adequate for the separation of polyhydric alcohols and ether-alcohols. Benzyl silyldiphenyl ether, owing to its low viscosity, gives high column efficiencies and has proved effective for the analysis of halogenated aromatics. Poly(m-phenyl ether) Structure:

c oa ) n - o ~

nzl

Min. col. temp.: 20°C Max. col. temp.: 200°C (n = 1) 375°C (n = 5 ) Commercial names: 0s 124, 0s 138, PPE-20, PPE-21 (Applied Science Lab.; Supelco) (0s124: 5 rings, 0 s 138 6 rings, PPE-20 and 21 7 rings) Polysev (7 rings) Sulphones of polyphenyl ethers have proved to belong to the most thermally stable phases. They can be used (poly-S 176, Poly-S 179) up to 400°C. Min. col. temp.: 150°C (Poly-S 176), 200°C (Poly-S 179). d:’

=

1.19-1.23 g * cm-,

Table 64. McReynolds Constants of Poly(ethy1ene oxide) Derivatives Commercial Name

FFAP SP-1000 Carbowax 2OM-TPA STAP Carbowax 400 monostearate Emulphor ON-870 Ethofat 60125 Igepal CO-630 Igepal CO-710 Igepal CO-730 Igepal CO-880 Igepal CO-990 Lutensol Polytergent B-350 Polytergent G-300 Polytergent J-300 Polytergent 1-400 Renex 678 Surfonic N-300 Tergitol NPX-728 Triton X-100 Triton X-305 Tween-80

Min. op. temp.

Max.op.

McReynolds constants

PCl

temp. PCl

X

I I I1 111 XIV X

60 60 60

260 260 260

30

125

XIV

30

125

xv xv xv

m XVI XVI

XVI XI11 XVI

xv xv

XI1

20

150

340 332 321 345 280 202 191 192 205 224 259 298 232 202 203 168 180. 223 261 197 203 262 227

Y

Z

U

S

H

I

K

L

M

580

597 393 367 400 325 251 244 253 266 279 311 345 293 260 267 227 234 278 313 258 268 314 283

602 583 573 610 512 395 380 382 401 418 482 540 438 395 401 350 366 427 484 389 402 488 438

627 546 520 627 449 344 333 344 361 379 426 475 386 353 360 308 317 381 427 351 362 430 396

423 400 387 428 350 282 277 277 289 302 334 366 315 284 250 266 270 301 334 281 290 336 310

298

228

473

161

281 301 244 179 168 172 183 198 227 261

220 235 191 140 131 136 144 157 180 205

435 484 382 289 279 288 303 321 362 406

148 163 122 080 073 078 085 095 112 133

180 180 149 159 198 228 176 181 229

142 140 119 127 156 180 139 145 183

297 303 255 265 321 364 293 304 366

084 083 061 068 095 114 081 083 113

555

537 586 486 395 382 381 397 418 461 508 425 392 398 366 375 417 462 386 399 467 430

261

8.5. Esters

4,4'-Bis(tribenzylsilyl)diphenyl ether Structure:

B.p. : 400°C (13 Pa)

Min. col. temp.: 40°C Max. col. temp.: 250°C

Poly(styrene oxide) Structure:

Q

(-;H

Max. col. temp.: >lOO"C C H ~ O- I,,

Commercial name: Dow Polyglycol 174...500 Solvent: dichloromethane. It should be mentioned that glass surfaces can be wetted only poorly by poly(pheny1 ethers), hence not permitting sufficient efficiencies to be achieved in capillaries, on Volasphers and on glass beads. Recommendable phases: poly(pheny1 ethers with n > 5 , poly(phenyl ether). Sulphone PZ-176 for GC-MS [993]. Solvent: chloroform. The McReynolds constants are shown in Table 65. Table 65. McReynolds Constants of Phenyl ethers Phenyl ether

X

Y

z

U

S

H

I

K

L

M

0s 124 0s 138 PPE-20 PPE-2 1

176 182 257 232

227 233 355 350

224 228 348 398

306 313 433 413

283 293

177 181 270 350

169 176

135 136

266 273

103 112

8.5.

Esters

Esters of carboxylic acids and of phosphoric acid possess donor oxygen atoms for hydrogen bonding, and protic solutes show high retentions. The specific retention volume of n-propanol, for example, on dioctyl sebacate is nearly as high as that on 1,2,3-tris(2-cyanoethoxy)propane, although the latter is much more polar. However, as the polar interaction of both stationary phases with such solutes is detsrmined predominantly by the hydrogen bond, which is stronger than other polar intermolecular forces, this result is not surprising [970]. This is also why the retention differences of an alcohol on dioctyl sebacate, dinonyl phthalate, dibutyl phthalate and tricresyl phosphate are small. The ester phases show an intermediate solvating power for alkanes, ethers, esters, ketones, thiols and alkyl sulphides. Owing to their electron acceptor properties, distinct interactions occur, with electron donors (olefins, aromatics, heterocyclics). The selectivity for the separa-

262

8. Liquid Stationary Phases

tion of alkanes and aromatics from alkanes increases in the order sebacateladipate < phthalate < tetrachlorophthalate < phosphate < acetate c butanediol succinate < diethylene glycolsuccinate < ethyleneglycol succinate, even if the differences between the first and the last product are not very great. Actually, the presence of both aryl and/or alkyl groups (causing the solvency for hydrocarbons) and of the carboxyl and phosphate group, respectively (effecting good solvency for oxygen-containing compounds), are the reason why the selectivity is not pronounced for other compound types. Only halogenated hydrocarbons are separated selectively, even if there are more selective liquid stationary phases, e.g. nitrilesilicones [971]. At temperatures above 120°C the ester phases may react with alcohols or amines. If the sample contains such constituents one should be conscious of the risk that the analytical result might be erroneous.

8.5.1.

Esters of Dicarboxylic Acids (Phthalates, Adipates and Sebacates)

The sorption properties of dialkyl phthalates, sebacates and adipates are very similar, so these groups can be discussed together. Their field of application results from the separation properties described above. The medium interaction of phthalates with aromatics is based on their electron affinity, while the aromatics may act as electron donors. For example, phthalic anhydride shows the (weak) tendency to form molecular complexes (donor-acceptor complexes). The electron affinity of phthalic anhydride is 0.15 eV (0.24 10 - l9 J) [972]. This behaviour is enhanced in tetrachlorophthalic anhydride and its electron affinity is increased (0.58 eV = 0.93 10 -I9 J) such that it almost reaches the value for 1,3,5-trinitrobenzene (0.7 eV = 1.12.10 - l9 J) [972], well-known for its ready ability to form organic molecular complexes. Therefore, the esters of tetrachlorophthalic acid are much more suitable for the separation of unsaturated organic compounds than the halogen-free esters, because, in addition to the increase in the solvency for donors, their sorption differences, caused by the substitution of alkyl groups in the aromatic ring, also emerge strongly. These differences result from the fact that alkyl groups tend to donate electrons to the ring and hence to increase its electron density such that the interaction with the acceptor can be increased. However, certain alkyl groups (e.g. several methyl or propyl groups) attached to the ring may be detrimental to interactions owing to steric hindrance. For example, m- and p-xylene can be well separated on di-n-propyl tetrachlorophthalate [973]. If the alkyl groups in the esters are replaced with alkoxy-alkyl groups, the selectivity can be improved too, and alkoxyakyl adipates, sebacates and phthalates exhibit higher polar interactions with all polar functional groups, evident in the greater McReynolds constants (see Table 66), the tendency to form hydrogen bonds being especially pronounced. Of the common phthalates, n-butylbenzyl phthalate is the most polar [974]. e

Dinonyl phthalate Structure: 0

II

CH3 I

d:': 0.97 g cm B.p.: 245°C (0.67 H a )

Commercial name: Narcoil 40

Min. oper. temp.: 20°C Max. oper. temp.: 130°C

263

8.5. Esters Table 66. McReynolds Constants of Phthalates, Adipates and Sebacates.) McReynolds constant

Name

Bis(2-butoxyethyl) adipate Bis(2-butoxyethyl) phthalate Bis(2-ethylhexyl) adipate (Flexol A-26) Bis(2-ethylhexyl)phthalate') Bis(2-ethylhexyl) sebacate c, Bis(2-ethylhexyl)tetrachlorophthalate Bis(2-ethoxyethyl) phthalate Bis(2-ethoxyethyl) sebacate Bis(2-ethoxyethoxyethyl) phthalate Butyloctylphthalate Dibutyl phthalate Dicyclohexyl phthalate Didecyl phthalate Di(dodecy1) phthelate Diisodecyl adipate Diisodecyl phthalate Diisononyl adipate Diisooctyl adipate Diisooctyl phthalate Dinonylphthalate Dinonyl sebacate Dioctyl phthalateb) Dioctyl sebacated) Di(tridecy1)phthalate Octyldecyl adipate *)

X

Y

Z

U

S

H

I

K

L

M

137 151 076 092 072 109 214 151 233 097 130 146 136 079 071 084 073 078 094 083 066 092 072 075 079

278 282 181 186 168 132 375 306 408 194

198 227 121 150 108 113 305 211 317 157

300 338 187 236 180 171 446 320 470 246

216 217 144 143 132 104 290 328 309 149

118 138 071 092 068 075 190 129 207 096

104 112 055 066 049 045 159 110 170 069

205 225 119 140 107 137 312 224 337 147

028 048 009 026 011 034 079 036 092 027

257 255 158 171 173 174 187 193 183 166 186 168 156 179

206 213 120 113 137 116 126 154 147 107 150 108 122 119

316 320 192 185 218 189 204 243 231 178 236 180 195 193

235 267 134 167 125 168 364 274 389 174 227 245 235 158 128 155 129 140 174 159 118 167 125 140 134

196 201 120 134 133 137 148 149 141 130 143 132 119 141

144 126 079 067 083 068 072 092 082 062 092 068 076 072

104 101 052 052 059 054 159 069 065 050 066 049 051 057

204 202 116 114 130 116 126 147 138 106 140 107 115 119

058 038 026 011 024 010 008 024 018 008 026 011 025 010

After 1324, 781. 783. 184, 824, 9691 ') Corresponds to b). and c) to d). respectively

Didecyl phthalate 0

II

CO ( CH2) gCH3

Structure:

aCO

II

(CH2)9CH3

0

Min. oper. temp.: 25°C Max. oper. temp.: 135°C

Commercial name: Perkin Elmer A Dipropyl tetrachlorophthalate Structure:

cl+f

CO(CH2)2 CH3 CO ( CH2)p CH3

CI CI

&:

1.37g.cm-'

B.p.: 174°C (133 Pa)

Min. oper. temp.: 27°C Max. oper. temp.: 75°C

264

8. Liquid Stationary Phases

Dibutyl tetrachlorophthalate Structure : $(CH2 13CH3

d:O:

1.31g

;

CI

-

Min. oper. temp.: 20°C Max. oper. Temp.: 100°C

B.p.: 178°C (67 Pa)

280°C (101 kPa) (decompos.) Dioctyl tetrachlorophthalate [bis(2-ethylhexyl) tetrachlorophthalate] COCH2CH (CH213CH3

Structure:

COCH2CH (CH2 )3CH3

I

CL

C2H5

Min. col. temp.: 20°C Max. col. temp.: 150°C Tetrachloroterephthaloyl oligomers [975] Structure:

ROOC

KJ$ 0

CI

cOO(CH2jPOC

Cl

0

COOR

CI CI

M.P.: R = butyl, n = 2 183 - 184°C Min. col. temp.: 100 - 180°C n = 3 108.3- 109.7"C Max. col. temp.: 200°C n = 4 184 - 1855°C R = propyl n = 3 127 - 128°C Mixtures tend to stay liquid on supercooling and may hence be used even below the melting point.

Dioctyl sebacate [bis(2-ethylhexyl) sebacate] C2H5

I

COOCH2CH (CH2)3CH3

Structure:

I

( CH2)8

I

COO CH2 CH ( CH2 13 CH3

I

C2H5

d;o: 0 . 9 2 g . ~ m - ~ Max. col. temp.: 130°C Commercial names: Octoil S., Perkin Elmer B

Dioctyl adipate [bis (2-ethylhexyl) adipate] PH5 COOCH2CH(CH2)3 CH3

Structure:

I

(7H2j4 COO CH2 CH ( CH2 13 CH3

I

c2 H5

Commercial name: Flexol A-26

Max. col. temp.: 120°C

265

8.5. Esters

Bis(2-ethoxyethyl) phthalate COOCH2CH20C2 H5

Structure:

aCOO

CHzCHz OC2H.j

Max. col. temp.: 150°C

Bis(ethoxyethoxyethy1)phthalate

Max. col. temp.: 150°C Recommended stationary phases in this group: For heterogeneously composed samples (polar and non-polar compounds) as all-round stationary phases for the lower and medium temperature range: didecyl phthalate, bis(2-ethoxyethoxyethyl) phthalate, bis(2-ethylhexyl) tetrachlorophthalate. Solvents: Chloroform, acetone

8.5.2.

Phosphates

The field of application of the phosphates has already been mentioned (Section 8.5). In addition, the esters of n- and iso-fatty acids can be selectively separated and also cyclohexanes and cyclopentanes. Triphenyl phosphate Structure: (CSHS)3P04 d:9: 1 . 1 8 g - ~ m - ~ B.P.: 260°C (2.7 kPa) Tricresyl phosphate (tritolyl phosphate) Structure: (CH3C6HJ3P04 d:': 1.18g.cm-' B.P.: 280°C (2.7 kPa) 430°C (101 kPa)

Min. col. temp.: 50°C Max. col. temp.: 100°C

Min. col. temp.: 20°C Max. col. temp.: 110°C

Chlorinated dialkyl phosphate (Cetamoll Q ) d:': 1.427 g . cm Min. col. temp.: 20°C Max. col. temp.: 80°C B.p.: 210-220°C (2.7 kPa) Trioctyl phosphate [tris(2-ethylhexyl) phosphate]

Structure:

C2H5 I OCH2 CH ( CHz) 3 CH3 / O=P-OCH2CH (C2H5) (CH2)3 CH3 OCH2 CH(CH2)3 CH3 Mine col. temp.: 20°C I C2H5 Max. col. temp.: 150°C

'

Recommended stationary phases: These phases owing to their technical-grade quality, should be replaced with phases with similar McReynolds constants, e.g., saccharose acetate isobutyrate (see Section 8.5.4), Emulphor ON-870, Ucon 50-HB-280 X and possibly OV-25. Moreover, tricresyl phosphate, more frequently used than the other phosphates, often contains the ortho component, which is toxic, and it should therefore be examined prior to use. Solvent: Chloroform.

266

8. Liquid Stationary Phases

Table 67. McReynolds Constants of Triorganophosphates McReynolda constant

Name

Cresyldiphenyl phosphate Tributoxyethylphosphate Tris(2-ethyl-hexyl)phosphate Tricresyl phosphate

8.5.3.

X

Y

Z

U

S

H

I

K

L

M

199 141 071 176

351 373 288 321

285 209 117 250

413 341 215 374

336 274 132 299

266 285 225 242

190 126 071 169

153 104 047 131

292 204 103 254

088 031 007 076

Stearates, Oleates

Stearates exhibit good solubilizing power for hydrocarbons, but only poor selectivity. Ethylene glycol distearate Structure: C17H3SCOOCH2CH200CC17H~~ Max. col. temp.: 200°C Diethylene glycol distearate Structure: C1,H35COO(CHJ20(CHz)200CC17H05 Max.col. temp.: 200°C Commercial name: LAC-16-R-897 Sorbitan monostearate C 17Hj5COOCH 2

Structure: HOQOH OH

Max. col. temp.: 150°C Commercial names: Span-60, Atpet-80 Sorbitan monooleate Structure:

CH3(CH2)7 CH= CH (CH2)7 COO CH2 To?

Max. col. temp.: 150°C Commercial name: Span-80

HOTOH OH

Recommended stationary phases: These phases should be replaced with specially grade poly(propy1ene glycols). Solvents: Diethyl ether, chloroform Table 68. McReynolds Constants of Stearates Name

Diethylene glycol distearate Span-60, Atpet-80 span-80 Atpet-200

McReynolds constant X

Y

Z

U

S

H

I

K

L

M

064 088 097 108

193 263 266 282

106 158 170 186

143 200 216 235

191 258 268 289

147 201 207 220

057 082 094 106

041 055 066 074

121 180 191 209

202 037 041 048

267

8.5.Esters

8.5.4.

Acetates, Citrates

Regarding the monomeric esters, the most polar interactions are shown by the acetates. Hence, they are suitable for the separation of aromatics from aliphatics and for the analysis of oxygen compounds. The analysis of samples that contain alcohols, must be performed with caution, as they might, especially at higher temperatures, react with these stationary phases. Saccharose octaacetate Structure: Cl2HI4O3(OCOCH3)a

Max. col. temp.: 100°C Max. col. temp.: 100°C

Saccharose acetate isobutyrate Structure: C12HI4O3(OCOCH3)2(OCOC3H& Min. col. temp.: 20°C Max. col. temp.: 175°C Commercial name: Saib Acetyltributyl citrate CH2 COOC4 Hg

Structure:

I I

CH3 COOC COOC4Hg CH2 COOC4 Hg

Commercial name: Citroflex A4

Min. col. temp.: 20°C Max. col. temp.: 150°C

Recommended stationary phase: Saccharose acetate-isobutyrate for the low-and mediumtemperature separation of hydrocarbons and oxygen compounds, especially ethers. Solvent: chloroform Table 69. McReynolds Constants of Acetates and Citrate McReynolds constant

Name

Acetyltributyl citrate (Citroflex A4) Saccharose acetate isobutyrate (SAIB) Saccharose octaacetate Sorbit hexaacetate

8.5.5.

X

Y

Z

U

S

H

I

K

L

M

135 172 344 335

268 330 570 553

202 251 461 449

314 318 671 652

233 295 569 543

214 264 451 446

112 147 292 273

102 128 251 247

207 270 546 521

026 054 152 131

Polyesters

The essential properties have already been discussed in Section 8.5.. In addition, it should be mentioned that they are particularly suitable for the separation of quinoline derivatives and of higher boiling olefinic, aromatic and heterocyclic compounds, of fatty acid esters and of diastereoisomeric derivatives of amino acids, provided that they have previously been thermally conditioned, as other ester-based stationary phases cannot be used owing to lack of thermal stability. The number of methylene groups between the two oxygen functions is of importance for the selective properties [976].

268

8. Liquid Stationary Phases

Polyesters were introduced into gas chromatography by Orr and Callen [977] and by Lipsky and Landowne [978] in 1958. Depending on the activity of the surface to be coated (solid support, column wall), they begin to decompose at 180°C. Especially at higher temperatures they may react with water and strongly basic or acidic compounds, and at 280°C even the polyester types that are most stable to hydrolysis decompose. The cleavage of esters by base-catalyzed hydrolysis causes the formation of additional hydroxy and carboxylic groups, which will alter the selectivity. Most stable against hydrolysis are those polyester types synthesized from hydrophobic diols and dicarboxylic acids. Amino compounds may cause the aminolysis of the ester bond, and a carboxamide and a hydroxy group will be formed. Moreover, there is the risk that residual hydroxyl and carboxylic groups in the polyester might react with reactive sample constituents, e.g., hydrohalic acids, acid halides, chlorosilanes, acid anhydrides, epoxides and isocyanates. At higher temperatures, traces of oxygen may oxidize the esters, forming hydroperoxides which subsequently decompose to free radicals from which on the one hand vinyl groups and on the other (by polymerization reactions) other polymers may originate. The thermal decomposition depends on the type and batch (impurities!) of the polyester and on the temperature. Below the decomposition temperature the esters have proved to show long-term stability. Silylating reagents should not be present in the sample. The selectivity for the separation of aromaties from aliphatics and the strength of the polar interactions increase for polymeric esters in the order neopentyl glycol sebacate < neopentyl glycol adipate < cyclohexanedimethanol succinate < ethylene glycol adipate < diethylene glycol succinate < ethylene glycol succinate. Ethylene glycol o-phthalate exhibits, in addition to a high general polarity, an enhanced selectivity in particular for N-heterocyclics. The polyesters can be synthesized, for example, according to Fkcher [979]. Their production can be carried out either by direct esterification of the acid-glycol mixture or by transesterification of the dimethyl esters (especially with terephthalic or malonic acid) with the corresponding glycol [994]. Modification by siloxanes increases their thermal stability [980]. Polyesters are predominantly used in packed columns, as they are converted into supra-polyesters above 200°C [981] and as the formation of efficient and stable filmss in open-tubular columns generally cannot be achieved. The densities range, depending on the glycols and the carboxylic acids, from about 1.2-1.3 g . cm-3. Poly(ethy1ene glycol succinate) Structure:

(--CH2CH20C CH2CH2 CO-),

II 0

II 0

Commercial names: EGS; LAC-4-R-886; HI-EFF-2B; HI-EFF-2BP Min. col. temp.: 90°C Max. col. temp.: 180-225°C Poly(diethy1ene glycol succinate)

Min. col. temp.: 20°C Max. col. temp.: 200°C (if stabilized: 225°C) Commercial names: Perkin-Elmer P; Polyester A; DEGS; HI-EFF-1B; LAC-3-R-728; DEGS-PS (caution, modified with HBP04!)

d:':

1.26g*~m-~

269

8.5. Esters

Poly(ethy1ene glycol adipate) Structure: (-CH2CH20C(H2)4CO-),

II

II

0

0

Min. col. temp.: 100°C Max. col. temp.: 190-225°C Commercial names: Reoflex 400; LAC 741; LAC-13-R-741; HI-EFF-2N2AP; EGA-P (caution, modified with H3P04!) Poly(diethy1ene glycol adipate) Structure: (-(CH2)20(CH2)20C(CH2)4CO-),

II

II

0

0

Min. col. temp.: 20-50°C Max. col. temp.: 190-210°C Commercial names: LAC-1-R-296; DEGA; HI-EFF-1A; Resoflex-R-296; DEGA-PILAC2-R-446 (with additive pentaerythritol adipate) Poly(propy1ene glycol sebacate) Structure: (-CH(CH3)CH20C(CH2)8CO-)n

II

II

0

0

Min. col. temp.: 20-50°C Max. col. temp.: 150-200°C Commercial names: PGSb; Harflex 370; Reoflex-100 Poly(butanedio1 succinate) Structure: (-(CH2)40CCH2CH2CO-)n

I1

II

0

0

Min. col. temp.: 50°C Max. col. temp.: 200-240°C Commercial names: HI-EFF 4B; 4 BP Poly(neopenty1 glycol adipate)

7% Structure:

(-CH2

C C H 2 0 C(CH2)k CO-1, I II II CH3 0 0

in. col. temp.: SO"C Max. col. temp.: 200-240°C

Commercial names: NPGA; HI-EFF-3A; LAC-769; LAC-9-R-769 Poly(ethy1ene glycol o-phthalate)

Structure:

(-CH2CH20;

. 0

. c0-1, II 0

Max. col. temp.: 190-210°C Commercial names: EGP; HI-EFF-2G; LAC-10-R-744

270

8. Liquid Stationary Phases

Poly(ethy1ene glycol tetrachlorophthalate)

Structure: Max. col. temp.: 200°C Commercial name: LAC-8-R-772 Poly(cyc1ohexane dimethanol succinate) Structure:

( - C H 2 e C H 2 0 C C H 2 C H 2 CO-I,, II II 0 0

Min. col. temp.: 50-100°C Max. col. temp.: 200-250°C Commercial names: CHDMS; HI-EFF-8B; LAC-12-R-79C

Poly(phenyldiethano1amine succinate) Structure:

(-

CH2)2 N ( C H 2 I 2 0 C (CH2)2 CO-I, II II 0

b 0

Min. col. temp.: 20°C Max. col. temp.: 200-250°C Commerial names: PDEAS; HI-EFF-1OB Recommended stationary phases: Poly(ethy1ene glycol succinate), poly(ethy1ene glycolophthalate), poly(neopenty1glycol adipate), poly(phenyldiethano1amine succinate). However, it should be endeavoured to replace these phases, because of their limited reproducibility and stability, by more suitable phases of similar selectivity, e.g., Carbowax 20M. It should be mentioned, however, that it is possible to generate a set of sixteen polyester stationary phases with a wide range of polarity from the highly polar poly(ethy1ene glycol succinate) to the virtually non-polar poly(decanedio1 dodecanedioate) and to select the optimum phase from amongst these sixteen [995]. Solvents: Chloroform, acetone The McReynolds Constants are listed in Table 70.

8.6.

Nitriles and Nitrile Ethers

The nitriles and even more the nitrile ethers confer only inferior attractive forces on non-polar, saturated compounds, but stronger forces on polar, unsaturated and protic compounds. With polar substances this is because the nitriles owing to the presence of the cyan0 groups, are strongly polar themselves (the dipole moment of alkyl-CN is p = 3.60 D and that of phenyl-CN is p = 4.05 D) and easily polarizable so that orientation forces become effective. With unsaturated, polarizable molecules an electric field can be induced by nitriles owing to their polarity, resulting in a certain attraction. Stronger, however, are the interactions of the donoracceptor type and particularly of the hydrogen bond type. The fmt type of interaction occurs as the nitriles, owing to the electronegativity of the CN group, act as electron acceptors and

271

8.6. Nitriles and Nitrile Ethers Table 10. McReynolds Constants of Polyester Stationary Phases') Name

Butanediol succinate Butanediol succinate, HI-EFF-4 BP Cyclohexanedimethanol succinate Cyclohexanedimethanol succinate, HIEFF-8 BP Diethylene glycol adipate (DEGA), HIEFF 1 AP Diethylene glycol adipate (DEGA), LAC-IR-296 Diethylene glycol adipate, with additive pentaerythrite DEGA-P, LAC-2R-446 Diethylene glycol succinate (DEGS) Diethylene glycol succinate (DEGS) DEGS-PS DEGS-PS DEGS-PSHI-EFF-1BP Ethylene glycol adipate (EGA) Ethylene glycol adipate (EGA) Ethylene glycol adipate, HI-EFF-ZAP Ethylene glycol iso-phthalate EGIP, HI-EFF2EP Ethylene glycol- o-phthalate EGP, HI-EFF-2GP Ethylene glycol succinate EGS Ethylene glycol succinate, HI-EFF 2-BP Ethylene glycol tetrachlorophthalate (EGTCP) Harflex-310 (propylene glycol sebacate) MER-2 Neopentyl glycol adipate (NPGA) Neopentyl glycol adipate, HI-EFF-3-AP Neopentyl glycol iso-phthalate (NPGIP) Neopentyl glycol sebacate (NPGSb), HI-EFF3CP Neopentyl glycol succinate (NPGS) Neopentyl glycol succinate (NPGS) Neopentyl glycol succinate, HI-EFF-3BP Paraplex G-25 (modif. alkyd) Paraplex G-40 (modif. alkyd) Phenyldiethanolamine succinate (PDEAS), HIEFF 10 BP Propylene glycol adipate (PGH), Reoplex 400 Propylene glycol sebacate (PGSb) *)

MeReynolds constant X

Y

Z

U

310 369 269 211

511 448 651 591 451 661 446 328 493 444 330 498

S

H

I

K

L

M

611 629 481 463

451 324 242 533 118 416 325 243 544 117

351 248 116 394 124 346 252 115 396 121

318 603 460 665 658 419 329 254 554 116 311 601 458 663 655 411 328 253 551 111 381 616 411 619 661 489 339 251 561 186 410 105 558 188 179 556 393 301 611 215 492 133 581 833 191 519 418 321 105 231

496 502 499 311 312 312 326 493 536 531 301 193 381 234 232 201 112 215 212 212 189 282 386

146 590 831 835 852 860 633 619 611 561

185 591 849 151 593 840 519 454 655 511 455 658 516 453 655 508 425 601

594 420 325 118 599 421 329 126 595 422 323 125 466 323 248 550 463 325 250 548 462 325 250 546 400 299 213 498

238 234 240 115 111 111 168

691 602 816 872 560 419 306 699 260 115 636 891 864 622 450 341 183 259

181 643 903 889 633 345 318 428 466 265 321 539 456 646 615 421 425 312 462 438 339 421 311 461 424 335 371 321 225 344 326 251

452 348 195 259 331 262 566 191 210 151 362 103 208 156 351 103 156 109 251 013

412 361 543 489 314 245 186 423 121 461 365 539 472 311 243 184 419 124

469 366 539 474 328 239 368 312 459 355 528 451 555 412 614 654

311 251 364 431

243 169 241 362

184 124 193 242

419 211 414 562

124 079 125 213

364 619 449 641 671 482 311 245 540 111 196 345 251 381 328 211 176 129 285 083

after 1324, 781, 783, 784, 824, 9691

retain substances that exhibit a sr-electron system of low ionization energy (aromatics) longer in the column than other compounds. Hydrogen bridge bonds come to the fore between nitrile ethers on the one hand and alcohols, phenols, carboxylic acids, primary and (less strongly) secondary amines on the other. However, it should be emphasized once more that the specific retention volumes of propanol on 1,2,3-tris(2-cyanoethoxy)propane and on the

212

8. Liquid Stationary Phases

distinctly less polar dioctyl sebacate are almost the same because in both instances hydrogen bonds are formed with high energy in comparison with the other interactions. Nitrile ethers are highly selective for the separation of unsaturated from saturated compounds. Alkylbenzenes, for example, are retained 16 times longer than n-alkanes in the column. However, this does not mean that the retentions of alkylbenzenes on these phases would be higher than on other phases; on dioctyl sebacate or tricresyl phosphate, for example, they are even higher. The retention results from the sum of all interactions, and between the two phases mentioned last and the alkylbenzenes a high dispersion contribution arises, in addition to weak donor-acceptor and induction contributions, which is responsible for the high specific retention volumes and which is smaller with the nitrile ethers. The low solubilizing power of nitrile ethers for hydrocarbons, caused by (relatively) weak dispersion forces, is indicated by the fact that the specific retention volume of n-hexane on squalane is 63.9, on dioctyl sebacate 48.3, on tricresyl phosphate 16.9, and on 1,2,3-tris(2-cyanoethoxy)propane only 1.6 ml .g - [970]. The outstanding selectivity of the nitrile ethers is based on the fact that unsaturated compounds exhibit, owing to donor-acceptor interactions and (to lesser extent) induction forces, approximately normal retention volumes, but saturated hydrocarbons exhibit extremely low retention volumes. Similar differences permit the separation of protic solutes from alkanes, the latter again showing low retentions. In addition to interactions in solution, an adsorption contribution to the retention on the gas-liquid interphase has also been measured [982]. The field of application of these phases results from the above-mentioned differences in the interactions. If a high selectivity is desirable, they are used for separations of olefins, acetylenes, cycloalkanes or aromatics from alkanes, of primary from secondary and tertiary alcohols and from acetals and ethers, of ketones and aldehydes from ethers and esters, of polar halogenated hydrocarbons from less polar ones, for the separation of cis-trans isomers, etc. Most frequently used is b,fl’-oxydipropionitrile(bis-(2-cyanoethyl) ether), first described by Tenney in 1957 [983]. Because of its high vapour pressure, it can only be used below 70°C. Therefore, Tenney [984] and later Cope and Peterson [985] and Anderson [986] replaced it with 1,2,3-tris(2-cyanoethoxy)propane which can be used up to 180°C while maintaining the good selectivity of fl$-oxydipropionitrile. Even more polar than these nitrile ethers is N,N-bis(2-cyanoethyl)formamide.The order of decreasing polarity is cyanoethyl saccharose > tetrakis(cyanoeth0xy)butane > fl,fl’-oxydipropionitrile = tris(2-cyanoethoxy)propane > hexakis (cyanoethoxy)cyclohexane > tetracyanoethyl pentaerythritol. A similary high polarity is exhibited by the stationary phases SP216-PS and SP-222-PS from Supelco, which are presumably modified by phosphoric acid. Only the McReynolds constant S is lower by 250 units (see Table 71). This fact may be utilized in cases of peak-overlapping of N-heterocyclics with other compounds. Benzyl cyanide Structure: B.p.: 233°C d:’: 1.016g.cm-’

Min. col. temp.: -24°C UCHZCN Max. col. temp.: 20°C

fl,@’-Oxydipropionitrile[fl,fl’-bis(propionitri1e)ether, bis(2-cyanoethyl) ether] Structure: NC(CH2)20(CH2)2CN Max. col. temp.: 7OoC B.P.: ca. 270°C d:’: 1.05g.cm-3 Commercial name: Perkin Elmer T

fl,fl’-Iminodipropionitrile (3,3’-imidodipropionitrile) Max. col. temp.: 60°C Structure: NC(CH2)2NH(CH2)2CN

273

8.6. Nitriles and Nitrile Ethers

y-Methyl-y-nitropimelonitrile(1,5-dicyano-3-methyl-3-nitropentane) CH3

I

Structure:

NC(CH2)z-CC- (CH2)2 CN

I NO2

Tris(cyanoethy1)nitromethane [996] Structure: OzNC(CHzCH2CN), N,N-Bis-(2-cyanoethyl)formamide 0 II

- HC N (CH2 CHZCN) 2

Structure:

Min. col. temp.: 20°C Max. col. temp.: 120°C

Structure: CHzOCHzCHzCN

I

Min. col. temp.: 20°C Max. col. temp.: 170°C

CHOCHzCHzCN

I

CH~OCH~CH~CN di0: 1 . 1 9 g . ~ m - ~ Surface tension: 49.2 mJ m Commercial name: Fractonitril I11

-

1,2,3,4,5,6-Hexakis(2 -cyanoethoxy)hexane Structure: C6H~(OCH2CH2CN)6(from sorbitol) d:': 1 . 1 8 g . ~ m - ~ Max. col. temp.: 100°C Commercial name: Fractonitril VI

1,2,3,4,5,6-Hexakis(2-cyanoethoxy)cyclohexane StrUCtUre: C~H~(OCHZCHZCN)~ (from inOSit01) Max. col. temp.: 100°C Commercial name: Cyclo-N Tetracyanoethylpentaerythritol Structure: C(CHzOCHzCHzCN)4 1,2,3,4-Tetrakis(2-cyanoethoxy)butane Structure: CHzO(CHz)zCN

I CHO(CH2)zCN I CHO(CH2)zCN I

Min. col. temp.: 30°C Max. col. temp.: 130°C Min. col. temp.: 100°C Max. col. temp.: 150°C

CHzO(CH2)zCN Commercial name: Cyano-B Some of these nitriles are generally synthesized by reacting protic compounds (e.g., OH compounds) with acrylonitrile: ROH + CHz=CHCN

---*

ROCHzCHzCN .

214

8. Liquid Stationary Phases

However, with six OH groups (mannitol, sorbitol) there is the risk that the reaction will be incomplete and moreover that, in addition to unreacted OH groups, also acrylonitrile, its oligomers or even amide and carboxylic groups (from an undesirable hydrolysis of the CN groups) may be present. Hence, the selectivity will be batch-dependent, and it is advisable to use a more uniform product. In this respect, tris(cyanoeth0xy)propane has proved its worth, and it can be purified more easily. It is not surprising that glass and fused-silica surfaces are poorly wettable by these nitrile ethers, as their surface tension (about 48-50 mJ/m2) is higher than that of most other organic compounds, preventing the formation of efficient and stable films in open tubular columns. Table 11. McReynolds constants of Nitrile and Nitrile Ether Stationary Phases.) McReynolds constant

Name

X

Y

Z

U

S

H

I

K

L

M

Bis(cyanoethy1)formamide

690 991 853 11001000113 551 311 964 219

Cyanoethylsaccharose Hexakis(cyanoeth0xy)hexane Oxydipropionitrile SP-216-PS, SP-222-PS Tetracyanoethylpentaerythritol Tetrakis(cyanoeth0xy)butane (Cyano-B) Tris(cyanoethoxy)propane

641 919 197 1043 916713 544 388 911 299

561 588 632 526 611 593

825 113 815 182 860 851

133 611 113 152

918 901620 919 1000 680 920 831621 444 333 166 231 1048 941685 1028 915612 503 315 853

*) ARer [324, 781. 183-784, 824). Recommended stationary phase: Tricl(cyanoethoxy)propane for the low and medium temperature ranges Solvents: Chloroform. acetone.

8.7.

Nitro Compounds

Nitro compounds closely resemble nitriles in their gas chromatographic properties. Remarkable again are the pronounced donor-acceptor interactions which, owing to the tendency of some nitro compounds to form molecular compounds (aromatics with 2,6-dinitrobenzoquinone, 2,4,7-trinitro-9-fluorenone, 1,3,5-trinitrobenzene1picric acid etc.) is not surprising. Further, orientation and induction forces become effective with suitable solutes. Nitrobenzene, for example, has a dipole moment of p = 4.01 D). Predominant interaction centres, however, are the oxygen atoms of the nitro groups as acceptor atoms for the hydrogen bond with protic hydrogen (e.g., in OH, SH,NH2 compounds), causing strong interaction forces with and enhanced retentions for such compounds. The nitro compounds discussed here are exclusively aromatics, and the presence of phenyl groups increases the contribution of dispersion interactions, hence increasing the solubility of hydrocarbons, the specific retention volumes of which are higher than those on comparable nitrile ethers while retaining high selectivity. This property is utilized for the separation of low-boiling hydrocarbons, which would be eluted too quickly on nitrile ethers. The outstanding selectivity permits, for example, the separation of aromatics from aliphatics, of aromatics with different substituents and of low-boiling halogenated hydrocarbons and chlorosilanes.

275

8.8. Amines

It should be pointed out that when hydrogen is used as a carrier gas it may reduce the nitro groups at high temperatures. The surface tensions of organic nitro compounds are generally ca. 40 mJ/m2, preventing the formation of uniform filsm on surfaces of low critical surface energy. m-Nitrotoluene FH3

Structure: NO2

Min. col. temp.: 17°C Max. col. temp.: 30°C

d:*: 1 . 1 6 g * ~ m - ~ p-Nitroaniline picrate

No2

1

Max. col. temp.: 110°C Hexyl ester of dinitrodiphenic acid Structure:

02N9+N02 H13C600C

COOC6H13

Max. col. temp.: 140°C 2,4,7-Trinitro-9-fluorenone yo2

Max. col. temp.: 180°C Recommended stationary phase: Trinitrofluorenone. However, these phase should be replaced with cyanoalkylsilicones. Solvents: Toluene, Chloroform

8.8.

Amines

8.8.1.

Aliphatic Amines and Imines

Aliphatic amines and hydroxyalkylamines, owing to their tendency to form -ydrogen bonds and hence to be strongly selective, have been utilized for the analysis of alcohols, glycols, pyridines, piperazines, thiols and thioethers. The additional presence of hydroxy groups in, e.g., triethanolamine, tetrahydroxyethylethylenediamineand Quadrol increases the tendency for the formation of hydrogen bridge bonds. This outstanding selectivity, indicated by the high McReynolds constants Y-H (see Table 72), permits even better separations of aromatics from oxygen compounds than on nitriles.

276

8. Liquid Stationary Phases

Owing to its strong proton-acceptor properties, polyethylene imine is extremely selective for the separation of alcohols from other compounds. Most recently, Blum [997] succeeded in coating and bonding Tetronic [N,N,N‘,N‘-tetrakis(olyethylene/propylene glycol ether)ethylene diamine] for WCOT columns.

N,N,N’,N’-Tetrakis(2-hydroxyethyl)ethylenediamine Structure:

HOCH2CH2

HOCHzCH2

>

CH~CHZOH NC H ~

N/ \

CH2CH20t-I

Min. col. temp.: 20°C Commercial name: THEED Structure:

CH3CHOHCH2 \

Max. col. temp.: 120°C

/CHzCHOHCH3 NCH CH N

CH3CHOHCH2

\CHzCHOHCH3

Min. col. temp.: 25°C Commercial name: Quadrol Polyethyleneimine Structure: H2N(CH2CH2NH),H Min. col. temp.: 20°C

Max. col. temp.: 150°C

Max. col. temp.: 170°C

N,N,N’,N’-Tetrakis(polyethylene/propyleneglycol ether)ethylenediamine

Min. col. temp.: 20°C Max. col. temp.: 260°C M.W. : 8 200 Synperonic T 904 (x :y = 2 3 ) M.W. : 26 000 Synperonic T 908 (x :y = 4:l) Commercial names: Tetronic, Synperonic Table 72. McReynolds Constants of Amine and Imine Stationary Phases*) Name

McReynolds constant X

Armeen 2 HT (aliph. m i n e from soybean oil) Armeen 2 S (aliph. amine from soybean oil) h e e n SD Ethomeen 18/25 (polyoxyethylene-soybean amine) Ethomeen S/25 (polyoxyethylene-soybean amine) Poly(ethy1ene imine) Poly(propy1ene imine)

Tetrakis(hydroxyethy1)ethylenediamine

Y

Z

U

S

H

I

K

L

M

24 36 35 103 44 18 116 382 230 353 323 215 158 118 265 012 186 395 242 310 339 285 169 121 279 019 322 800 513 524 585 122 425 168 263 224 210 463 942 626 801 a93 146 421 269 121 254

(THEED)

Tetrakis(hydroxypropy1)ethylene diamine (Quadrol) 9 After 1324. 781. 783. 784. 8241

214 511 351 412 489 431 208 142 379 111

277

8.9. Amides

Recommended stationary phases: Tetrakis(hydroxyethy1)ethylenediamine (low and medium column temperatures), poly(ethy1enimine) (medium temperatures) (caution: owing to the terminal NH2 groups, reactions with aldehydes and ketones may take place!) Solvents: Chloroform, acetone, methanol

8.8.2.

Aromatic Amines

Aromatic amines, as they have a n-electron system, can interact with the sr-electron systems of aromatic hydrocarbons and were used to effect the difficult separation of m-and p-xylene [987, 9881 in the early days of gas chromatography. The application of aromatic amines as stationary phases has declined since then. Caution is required because of their reactivity. Apart from m-phenylenediamine, a-naphthylamine and l&diaminonaphthalene, only diphenylamine has found some application. Structure:

@N H

a

Min. col. temp.: 54°C Max. col. temp.: 85°C

B.p.: 302°C

8.9.

Amides

Dimethylformamide has frequently been used for the analysis of Cl-C5 hydrocarbons, and Keulemans, Kwantes and Zaal[989] in 1955 succeeded in separating even cis from trans-2-butene and isobutene from 1-butene. Because of its high vapour pressure, however, such colu m n packings have only a short lifetime. Formamide, proposed by Noucik and Jandk [990], is less volatile, affects the FID response less than other volatile organic phases, can act both as an electron donor and electron acceptor and prevents tailing even on relatively active supports. Nevertheless, its applicability is limited, as it can only be used at room temperature. Longchain substitution, e.g., as in stearamide, certainly increases the temperature range, but impairs the selectivity considerably. By the condensation of long-chain dicarboxylic acids with piperidine derivatives, Matthews et al. [991] succeeded in preparing polyamides that can be used up to 250°C. Owing to the beneficial McReynolds constants Y and Z (see Table 73), alcohols can be selectively separated from aldehydes and ketones, and polyamides are also suitable for the analysis of amino acid derivatives, fatty acids, saccharides and steroids. Formamide Structure:

0 II - HCNH2

B.p.: 105°C (1.5 kPa) d:’ : 1.128 g . cm-3

Min. col. temp.: 3°C Max. col. temp.: 20°C

N,N-Dimethylstearamide Structure:

CI~H~~CN(CH~)~

II

0

Commercial name: Hallcomid M-18

Min. col. temp.: 40°C Max. col. temp.: 130°C

278

8. Liquid Stationary Phases

Polyamide Poly-A-103

Min. col. temp.: 70°C Max. col. temp.: 250°C Commercial name: Poly-A-103 (Applied Science Labs.) Recommended stationary Phase: Poly-A 103 Solvents: Chloroform,mixtures of chloroform and methanol Table 73. McReynolds Constants of Amides.) McRcynolds constant

Name

X

Y

Z

U

S

H

I

K

L

M

Flexol8N8 [bis(ethylhexanoyloxyethyl)ethyl-

096 254 164 260 179 197 098 064 147 023

hexaneamide] Hallcomid M-18 (dimethylstearamide) Hallcomid M-18 OL (dimethyloleylamide) Poly-A 101 A (polyamide) POly-A 103 Poly-A 135 Poly-I 110 Versamid 930 (linear polyamide) Venamid 940

079 089 115 115 163 115 108 109

*)

268 280 357 331 389 194 309 314

130 143 151 149 168 122 137 145

222 239 262 263 340 204 208 212

146 165 214 214 269 202 207 209

202 211 233 221 282 152 222 225

082 048 106 016 093 058 211 021 064 062 055 110 057 148 077 112 057 150 079

AAcr [324.781,7S3-784,824]

8.10.

Heterocyclics

In the same way as aromatic amines, heterocyclics also show strong dispersion interactions with aromatics, which have been utilized for the separation of xylenes [987]. The tetrazole derivative has been applied to the analysis of C1-C7hydrocarbons. l-Methyl-S-(2-methoxyethyl)tetrazole N-N

Structure:

CH,WH,CH,J,>

I CH3

Max.col. temp.: 80°C 7,8-Benzoquinoline Min. col. temp.: 53°C Max. col. temp.: 70°C B.p.: 338°C (96 kPa)

279

8.11. Sulphur Compounds

1-Hydroxyethyl-2-heptadecenylimidazoline Structure:

LL

I C17H33 CHzCH20H

Min. col. temp.: 20°C Max. col. temp.: 170°C

Commercial name: Amine 220 McReynold constants ofAmine 220: X = 117, Y = 380, Z = 181, U = 293, S= 133, H = 274 Solvents: Chloroform, dichloromethane

8.1 1.

Sulphur Compounds

Temporarily dimethylsulpholane (3,4-dimethyltetrahydrothiophene-S-dioxide) had a certain significance, as it retains olefins selectively. In an 1:4 adimixture with di-n-propyl sulphone it separates all C2-C5hydrocarbons in a sufficienctly long column [992]. It must be noted, however, that cupriferous column material must not be used if acetylenes are to be investigated, as these compounds may react with copper in the presence of such sulphur compounds. Alkylaryl sulphonates were used for the separation of phenols, aldehydes and ketones and of cycloalkanes from olefins. Owing to its pronounced property to act as a hydrogen bond acceptor, sodium dodecylbenzenesulphonate is especially appropriate for the selective separation of hydrogen donors from hydrocarbons (also aromatic hydrocarbons), as the McReynolds constants ( Y = 569 and X = 100) indicate. (see also Section 5.5.6). Di-n-propyl sulphone Structure: (CH3CH2CH2)2S02

Max. col. temp.: 60°C

Dimethylsulpholane (3,4-dimethyltetrahydrothiophene S-oxide) Structure:

7Y3 o”

\\

0

Max. col. temp.: 35°C

Commercial name: Perkin-Elmer E Sodium dodecylbenzene sulphonate CH3(CHzJ,, --@so,N~ Structure:

Min.col. temp.: 20°C Max. col. temp.: 200°C

Commercial name: Siponate DS-10 McReynolds constants of Siponate DS-10: X = 99, Y = 569, Z = 320, U = 344, S = 388, H = 466, I = 114, K = 61, L = 437, M = 63. McReynolds constants of another alkali metal alkylbenzenesulphonate, Stepan DS-60: X = 97, Y = 550, Z = 303, U = 338, S = 402, H = 440, I = 111, K = 60, L = 418, M = 61. Tide (mixture of sodium alkylarylsulphonate and sodium laurylsulphate) Structure:

R-@S03Na

Commercial name: Tide (type E and F) Max. col. temp.: 200°C Recommended stationary phase: Siponate DS-10 Solvents: Chloroform, n-heptane, ethers

8. Liquid Stationary Phases

280

8.12.

Fluorine Compounds

Partially fluorinated compounds can be separated on hydrocarbon, silicone or ester stationary phases; perfluorinated compounds, however, are eluted from these phases much quicker than expected. Their solubility is distinctly increased on replacing the CH-bonds with C-F or C - C l bonds in the stationary phase. Such peffluoro- or fluorochloroalkyl phases are suitable for the separation of peffluoroalkanes, -alkenes and -cycloalkenes. The functional organic groups of the stationary phase, e.g., COOR, influence the selectivity as usual. Conversely, hydrocarbons, especially cyclic compounds, are extremely poorly soluble in perfluorinated phases and elute rapidly, owing to the weak intermolecular interactions. The oleaginous polymers of trifluorochloroethylene are chemically very stable and have therefore been used for the separation of aggressive compounds such as HC1, HF, HBr, Cl,, Br, , ClF, CIFJ, BCIJ, PC13, POCl,, SF, and volatile metal fluorides. For such problems, the stationary phase is coated on polytetrafluoroethylene or polychlorotriiluoroethylene support material. It should be noted that these fluorinated stationary phases may decompose, in particular above 25OoC, to form extremely toxic gaseous compounds. Two fluorinated esters, Zonyl E-7 and E-91, in comparison with other stationary phases of similar polarity, exhibit a relatively high Z and low Y McReynolds constant, especially Zonyl E-7. Hence it is possible to reverse the elution order of hydroxy and carbonyl compounds on Zonyl E-7 compared to the elution on poly(ethy1ene glycols) or polyesters. In addition to the stationary phases such as poly(trifluorochloroethylene), poly(perfluoroa1kyl ethers) are also chemically inert and show only weak intermolecular interactions with non-fluorinated compounds, which are therefore eluted more rapidly than would be expected from their boiling points. Hence high-boiling compounds, especially aromatics and thermally lable substances that would decompose at the usual column temperatures, can be analysed at lower temperatures. The perfluoropolyethers can be coated on Chromosorb P, and good efficiencies have ben achieved [998], unless the support is too inactive (Gas Chrom Q) or has been deactivated by silylation; in such instances the frlm of the stationary phase is not uniform enough. The thermal stability of the column packing (impregnation rate 10%)is 225"C, while the peffluoropolyether Formblin YR (3M Company) (-OCFCF,),,(OCF,), (M.W. 6000-7000)

I

CF3 can be thermally stressed up to >3OO0C [998]. A similar product, Fluorcol (DuPont), coated on Carbopack B, has been shown to be unaffected even by large amounts of HF, HCl or other reactive gases and to resolve a wide range of fluorocarbons, including positional isomers that are difficult-to-separate (e.g., R 113 and R 113a, CFzCICFClz and CF3CCl,, R 114 and R 114a, CCIFzCCIFzand CClzF-CF3)[999]. Whereas generally deactivated surfaces (column walls of capillaries, inactive support materials) can be wetted only poorly by the fluorocarbon stationary phases discussed so far, this does not apply to fluorinated surfactants. For example, Fluorad FC-430 [lOOO] and Fluorad FC-431 [1001] deactivate diatomaceous supports on their own to such an extent that even low-level aldehyde and carboxylic acid impurities in underivatized acids (e.g., acetic and propionic acid), and both polar and non-polar compounds in aqueous samples can be determined. These Fluorads (3M Company) are peffluorinated, partially ethoxylated octadecanols. Owing to their behaviour also in f m l y adhering to glass surfaces, they can be used for coating open-tubular columns.

281

8.12. Fluorine Compounds

Peffluorotributy lamine Structure: (C4F&N Commercial name: MMM FC-43

Max. col. temp.: 30°C

Ethyl ester of tetrachloropeffluorocaprylic acid Structure: C1(CF2CFC1),CF2COOCzHs Max. col. temp.: 80°C Commercial name: Ethyl ester of Kel-F acid 8114 Poly (trifluorochloroe thylene) Structure: (-CF2CFCl-).

Min. col. temp.: 20°C Max. col. temp.: depends on n Max. col. temp. PC] 50 100

Commercial names: Kel-F oil 3 Kel-F oil 10 Kel-F wax (grease), Kel-F 90, Kel-F wax 550 Fluorolube GR 362, oil Fluorolube S 30 Fluorolube oil 2000 Fluorolube HG 1200 Halocarbon 10 to 25 Halocarbon 14 to 25 Halocarbon 25 to 55 Halocarbon K-352 Halocarbon wax

200 75 100 175 200 100 150 200 250 150

Zonyl E-7, ester of pyromellitic acid and a trihydropolyfluoroalcohol Structure:

COOCH,( CF21n H

H(CF2ln CH200C

H ( C F 2 I n CHzOOC =COOCH2

(CF2)n H

Max. col. temp.: 200°C Commercial name: Zonyl E-7 (DuPont) Zonyl E-91, ester of camphoric acid and a trihydropolyfluoroalcohol CH3

I

CCOOCHz(CFz),, H

Structure:

HzC’ I H C

I

C(CH312

I

‘CCOOCHz(CF2)n

H

I

Max. col. temp.: 200°C

H

Fluorad-FC-430, 431 Supposed structure: H(CF2),,CH2OR

R = H, OC2H5 Min. col. temp.: 20°C Max. col. temp.: 200°C Commercially available from 3M Co. as a solution of 50% active material in ethyl acetate (batch-to batch variations of quality have been observed [lool]) Perfluoropolyether CF3

Structure:

I

[ - 0 C F C F 2 -I,,

[- OC F2 Irn

282

8. Liquid Stationary Phases

Min. col. temp.: 30°C Max. col. temp.: >225"C Pour point-20°C M. W.6000-7000 Commercial names: Fomblin YR (DuPont) [998], Fluorcol [999] Recommended phases: Fluorad (polar organic compounds) Fluorcol, Fomblin (reactive compounds) Zonyl (separation of alcohols/ketones) It should be emphasized, however, that the possibilities of synthesizing partially fluorinated compounds, tailor-made for gas chromatography, are far from being exhausted and that special selectivities should not be a surprise as a consequence of the outstanding decrease in dispersion interactions on replacing the hydrogens atoms in CH bonds with fluorine. Solvents: R 113 (trifluorotrichloroethane), chloroform, acetone, ethyl acetate. The McReynolds constants are shown in the following Table.

Table 74. McReynold Constants of Fluorinated Stationary Phases.) McReynolds constant

Name

Fluorad FC-430 Fluorad FC-431 Fluorolube HG 1200 Fomblin-YR Halocarbon 10-25 Halocarbon K-352 Halocarbon wax Kel-F wax Zonyl E-7 Zonyl E-91 *)

X

Y

Z

U

S

H

I

K

178 281 051 015 047 047 055 055 223 130

466 423 068 138 070 070 071 067 359 250

381 297 114 088 108 073 116 114 468 320

462 509 144 141 133 238 143 143 549 377

460 399 117 094 360 326 223 183 118 068 012 053 051 066 015 111 140 123 070 016 057 116 073 016 057 465 338 146 137 293 235 081 095

L

M

349 -42 294 078 104 003

110 109 469 295

004 004 062 010

Mer I324.781, 783-784,824.1OO0, 10011

8.13.

Fatty Acids and their Salts

Higher fatty acids and their alkali metal salts have been used as additives (10%)to low- or non-polar stationary phases (hydrocarbons, methylsilicones) in order to prevent tailing of hydrogen donor compounds (free carboxylic acids, alcohols, amino acid esters). It appears to be necessary for the carrier gas not to be completely dry [1002]. Trimer acid, a C,,-tricarboxylic acid, which is only weakly polar, but has a relatively high McReynolds S-constant and hence retains heterocyclics selectively has been directly applied as a stationary phase. Heavy metal salts (Mn2+,Co2+,NiZ+,Cu2+, Znz+) of the higher fatty acids are, in the molten state, superselective for the separation of amines and N-heterocyclics, owing to the strong coordination forces that act between these solutes and the metal atoms. These forces depend on steric factors. For example, a-,0-and y-picolines and 2,6-lutidine could easily be separated [1003]. Whereas the McReynolds constants X,Z and U are below 100, Y is 231 and S as high as 544 (zinc-stearate) (see Table 75).

283

8.14.Salts

Trimer acid Structure: Cu-tricarboxylic acid

Min. col. temp.: 20°C Max. col. temp.: 150°C

Stearic acid Structure: CH3(CHz)16COOH

Max.col. temp.: 100°C

Behenic acid Structure: CH3(CH2)zoCOOH

Max. col. temp.: 120°C

Zinc stearate Structure: (C17H3SCOO),Zn

Min. col. temp.: 50°C Max. col. temp.: 150°C

Copper stearate Structure: (C17H3sC00)zCu

Max. col. temp.: 160°C

Table 75. McReynolds Constants of Fatty Acids and Zinc Stearate.) McRcynolds constant

Name

X

SP 1200 Trimer acid Zinc stearate ~~

Y

Z

U

S

H

I

K

L

M

067 170 103 203 166 166 145 094 271 163 182 378 234 094 057 216 060 061 231 059 098 544 098 050 029 078 033 ~

9 After 1324, 781, 783-784. 8241

Recommended Phase: Zinc stearate Solvent: Chloroform, acetone

8.14. 8.14.1.

Salts Silver Nitrate

Being an electron acceptor, Ag+ is able to interact with suitable donor molecules (olefins, aromatics) and to retain them selectively in the column. In the early days of gas chromatography, silver nitrate was added to benzyl cyanide or oligomeric glycols and used for the separation of branched from unbranched and cis- from trans-olefins. The solutions of silver nitrate in these nitrile or glycol media become unstable above about 65"C, and the desired labile adducts with the olefins can no longer be formed. The formation of these &NO,-olefm complexes was investigated by Schnecko [1004].

8.14.2.

Dissolved Alkali Metal Halides

Some polar compounds, such as poly(ethy1ene glycol), may partially dissolve alkali metal halides so that they can then specifically interact with certain compounds, e.g., acid amides. The specific retention contribution of alkali metal halides depends on the solubility of the halide in the stationary phase and on the dipole moment of the amide. The amides to be investigated undergo a salting-in solting-out effect [1005], als also the hydrogen atoms of the amide contribute to the interaction. It was shown [1006-10071 that alcohols, esters and ketones are also retained selectively on LiC1-poly(ethy1ene glycol) by the LiCl, although less strongly than amides. The stationary

284

8. Liquid Stationary Phases

phase consists of 20% PEG, containing 10-20%of its weight of the halide, coated on a deactivated support. The interactions increase in the order LiCl< LiBr < KI < NaI < LiI. Liquid stationary phases: PEG 400, Carbowax 20M Alkali metal halides: LiC1, LiBr, LiI, KBr, KI, NaC1, NaBr, NaI Recommended: LiCl or LiI in Carbowax 20M

8.14.3. Eutectics For the gas chromatographic investigation of high-boiling compounds there is a maximum operating temperature that limits the application of organic and even of organosilicon stationary phases. Temperatures higher than 350-400°C over long periods can be tolerated only for inorganic salts. They become effective as stationary phases above their melting points or mixed melting points, and often show, corresponding to their chemical composition, excellent selectivity. On molten chlorides, coated on the usual supports, numerous volatile metal halides can be separated, because they can form more or less stable chloro complexes in the melt with the accessible chloride ions of the eutectic. As the stability ratios of such complexes are generally different from the relative volatilities of the corresponding metal halides, also equally or similarly boiling mixtures are separable. The melting points and the maximum column temperatures are shown in Table 76. Table 76. Eutectics of Metal Halides Composition [mol XI

AlCl, (59), NaCl(41) NaAlCI, (100) NaFeCI, (100) ZnClz (52), TlCl(48) RbCl(40), AgCl(60) InCl, (50), TlCl(50) KCl(49), LiCl(51) CdClz (67), KCl(33) NdCI, (41,2), NaCl(58,8) LiCl(lO0) cscl(loo)

M.P. Wl (min. col. temp.)

126 152 158 213 253 262 352 383 430 614 642

Max.col. temp. I'Cl

450 300 365 680 290 680 750 680

Caution is required if inorganic samples containing anions other than that of the eutectic are to be analysed, because there is the risk of undesirable chemical reactions in the column. A precondition for the investigation of organic or inorganic substances is, of course, that they do not decompose or react with the packing at high column temperatures. Moreover, these high temperatures and the corrosive influence of the molten salts place great demands upon the gas chromatographic apparatus. It should be noted that the column walls and connections must be especially inactive, because active sites will not be deactivated by decomposition or volatilization products, as would be the case with organic stationary phases. Polyphenyls could be analysed on an alkali metal nitrate eutectic [1008]. Even alloys can be separated into their constituents. De Boer (10091 separated zinc from cadmium at 620°C on lithium chloride (coated on sea-shore sand). Especially well separated are metal halides on chloride eutectics, e.g., ZrCWHfC1, or TaClS/NbCl5.

285

8.14. Salts

The separation of organic compounds on molten eutectics is based on adsorption processes at the liquid-gas interface. For metal halide samples, however, gas-liquid partition might prevail [loll]. Eutectic of alkali metal nitrates Composition 27.3% (w/w) LiN03, 18.2% (w/w) NaN03, 54.5% (w/w) KNO,. M.p. : 150°C Recommended stationary phase: Owing to their great expense, one should try to find a less expensive means of separation. E.g., metals can often be separated more easily in the form of their complexes with (3-diketones, fluoroacetylacetones, (3-thioketones, (3-ketoamines, salicylaldimines or dialkyldithiocarbamates (e.g., I1012-10151) on poly(dimethylsi1oxane) stationary phases). Solvent: water.

8.14.4. Molten Organic Salts Owing to the presence of charge-bearing groups in molten organic salts, strong orientation and hydrogen bond interactions occur, accomponied by strong induced dipole interactions. On the other hand, dispersion interactions are weak. This selectivity is very useful for the separation of polar solutes. However, adsorption contributions cannot be excluded. The molten organic salts tetra-n-butylammonium tetrafluoroborate, tetra-n-butylammonium 4-toluenesulphonate, tetra-n-hexylammonium benzoate, tetra-n-heptylammonium chloride, l-methyl3-ethylimidazolium chloride, ethylpyridinium bromide, tri-n-butylbenzylphosphonium chloride and sodium isovalerate have been used, as they have proved to behave as stable isotropic liquids over a temperature range of B 50°C, for the separation of alcohols, halocarbon substituted aromatics and essential oils [1016, 10171. In an excellent, detailed survey, Poole et al. [lo171 evaluated these molten salts as stationary phases for gas chromatography. Their McReynolds constants are shown in Table 77. Table 77. Gas chromatographic Characteristics of Molten Organic Salts*) Name

Solvent

Min. COl.

temp.

Max. col. McReynolds constants temp.

[TI r c ] x

Tetra-n-hexylammonium benzoate Ethylammonium nitrate Tetra-n-heptylammonium chloride**) (a) 1-Methyl-3-ethylimidazolium chloride**) (b) Ethylpyridinium bromide**) (a) Tri-n-butylbenzylphosphonium chloride Tetra-n-butylammonium tetrafluoroborate Tetra-n-butylammonium 4-toluenesulphonate Sodium isovalerate

Y

z

u

s

H

r

K

-

L

M

CH2C12

20

110 154 799 384 427 330 649

98 236

55

CH30H CHzC12

20 20

120 130 161 799 222 312 326 597 -42 101 237

101

CHzCl2

40

170

CH30H

110

160

CH30H

165

240

CH2C12

170

290

CHzClz

55

200

188

280

H2O

*) after Poole et al. [1016. 10171.

")

87 566 156 236 273 511 -73

-

678 613 580 485 565

-

McReynolds constants measured at (a) 120°C and (b) 55°C.

80 187 -179

-

-

-

286

8. Liquid Stationary Phases

Recommended stationary phases: In spite of interesting effects (Y:X and Y:Z ratios), which permit a very selective separation of alcohols from aromatics or ketones, respectively, it cannot be overlooked that the simple sodium dodecylbenzenesulphonate (Siponate DS-10, Section 8.11.) shows a similar behaviour and can be used over a wider temperature range. Nevertheless, for special purposes tetra-n-hexylammonium benzoate and the tetra-n-butylammonium 4-toluenesulphonate appear to have unusual selectivities for oxygen compounds and are to be recommended.

8.14.5. Complexes of Some Transition Metals with N-Dodecylsalicylaldie and Methyl n-octylglyoxime Similarly to heavy metal salts of fatty acids (Section 8.13.), the N-dodecylsalicylaldiminesof nickel, palladium, copper and platinum and the methyl n-octylglyoximes of nickel, palladium and platinum also retain superselectively in the column those solutes which can be bonded as ligands 111231. This is especially relevant to mines, but also to ketones and alcohols. Similar unique properties are shown by the p-diketone complexes of Be, Al,Ni, Zn and Cu [1124]. Nickel bis(N-dodecylsalicylaldimine)

I c1ZH25

Min. col. temp.: 54°C Max. col. temp.: 180°C Platinum bis(methy1-n-octylglyoxime)

Structure:

Min. col. temp.: 120°C Max. col. temp.: 190°C

8.15.

Chiral Stationary Phases*)

The resolution of enantiomers can be achieved on the basis of two principal methods. The first, separation on a conventional (achiral) stationary phase, requires the conversion of the enantiomers into their diastereomeric derivatives with appropriate derivatization reagents *)

The author is indebted to Dr. G. Fabian who supplied some usefUl data.

8.15. Chiral Stationary Phases

287

prior to analysis on an optically inactive stationary phase. Chiral reagents for the preparation of diastereoisomers have been reviewed by Liu and Ku [1019]. They are of only limited applicability, because the presence of an active group to be derivatized is assumed and chemical and thermal stability of the diastereoisomeric mixture is necessary [1020- 10211. Nevertheless, such a stereodifferentiation of enantiomers has recently been reported by Mosandl et al. [1164]. The second method requires a kind of chiral discriminator or selector in the column. It was shown more than 20 years ago that optically active substances such as d-[Co(en)JC13 [1022], ureides and N-trifluoroacetyl-a-aminoacid esters [1023-10271 permit the direct gas chromatographic resolution of enantiomers. Gil-Au and co-workers especially carried out pioneering and comprehensive work on the development of chiral phases, on the separation of enantiomeric amino acids, amino alcohols, amines, hydroxy acids, etc. [1023-1025, 1028-10301, and on the assignment of the corresponding configurations. Generally, five classes of chiral stationary phases have been used for the gas chromatographic separation of enantiomers, namely cyclodextrin derivatives, peptides, diamides, ureides, and metal complexes. Other compounds have also been studied, e.g., tartaric esters [1031]. The state-of-the-art was reviewed by Liu and Ku [1021]. Most of the stationary phases were wall-coated on open-tubular columns. As cydodextrins have been already discussed in 8.4.3., they are not dealt with here once more.

8.15.1. Peptide Stationary Phases N-Trifluoroacetylamino acid esters, first introduced by Gil-Av (N-trifluoroacetyl-L-isoleucine lauryl ester) [1023, 10251, have proved successful for the separation of N-trifluoroacetylu-amino acid ester enantiomers on the basis of interactions being different of the D- and Lconfigurations of the solute esters with asymmetric (chiral) phase molecules. For the interaction, only one hydrogen donor is present in both the solute and solvent molecules (from the respective NH group) which can form hydrogen bonds with C=O bonds [1028]. The assumption that a second hydrogen donor function, placed in a suitable position in the stationary phase molecule, would increase the selectivity of the phase through the possibility of forming two further hydrogen bonds per solute molecule, proved correct and has become the basis for the development of dipeptide ester phases [1028]. Numerous N-trifluoroacetyldipeptide and N-pentafluoropropionyldipeptide esters of CI-C6 aliphatic alcohols, especially isopropanol and cyclohexanol, have since been synthesized [1020, 1024, 1028, 1032- 10411, and exhibit both an improved enantioselectivity and better thermal stability. A typical representative is N-trifluoroacetyl-L-valyl-L-valinecyclohexylester [1028, 10371: CH3 CH3 CH3 CH3 \ / \/ CH CH

0

I

I l l

F3CCNH CHCNHCHCO - CH

II

0

II

0

It can be used between 90 and 110°C and has been applied to the investigation of N-trifluoroacetyl-D,L-amino acid ethyl and isopropyl esters and of N-pentafluoropropionyl-D,L-amino acid ethyl esters. In order to improve the thermal stability of these stationary phases, one or both valine moieties in the above formula were replaced with phenylalanine as dipeptide constituents [1035, 1037-1040, 10421, and column temperatures between 130 and 165°C were shown to be applicable without serious bleeding.

288

8. Liquid Stationary Phases

The selectivity of a dipeptide and the order of elution of the enantiomers has been found to depend distinctly on the order of the original amino acids in the dipeptide, the N-terminal amino acid predominantly determining the enantiospecific behaviour. These results led to the development of stationary phased based on diamides, in which in a way the unnecessary moiety was removed. The separation on a dipeptide, composed e.g., of two L-amino acids, occurs because the L-enantiomers in the sample are more retained in the column than the D-enantiomers as a consequence of the stronger specific hydrogen bond interactions with the former, and the lower the column temperature, the better is the resolution of the enantiomers.

8.15.2. Diamide Stationary Phases We have just discussed the essential role of the NHCOC*H(R)NHCO group in the dipeptide ester stationary phases, which became the basis of experimental work for improving the selectivity and thermal stability. It was found that the separation is favoured by branched alkyl groups, N-acyl derivatives of L-valyl-tert.-butylamideshowing the best resolution factors:

With increasing length of R1the thermal stability can be increased within certain limits. Thus, N-lauroyl- or N-dodecenoyl-L-valine-tert-butylamides have proved to be very efficient and relatively stable chiral stationary phases. With R1= n-CZ1Hd3 and Rz= -C(CH3)2(CHZ)14CH3, the thermal stability could be increased to 200°C while maintaining the excellent stereoselectivity for enantiomeric amides derived from a-and y-amino acid esters, amines and amino alcohols [1030]. An essential step for improving the thermal and spreading behaviour further was to attach the diamide structure from L-valine-tert.-butylamidevia a spacer to a polysiloxane backbone by coupling of the diamide to the carboxylic groups of a copolymer of dimethylsiloxane and carboxyalkylmethylsiloxane, as already discussed in detail in Section 8.3.10. In addition to the foregoing modifications, a further advantage became evident, namely that the great flexibility of the siloxane chain leads to cooperative effects which for enzyme structures have been discussed as induced fit effects [1045]. Other polysiloxanes with which diamides have been combined were considered in Section 8.3.10. Specially modified diamide-type stationary phases were synthesized by Oi et al. [1046, 10471, namely N-(1R,3R)-trans-chrysanthemoyl-(R)-l-(a-naphthyl)ethylaineand N-lauroyl-6)-proline-(S)-1-(a-naphthyl)ethylamide,exhibiting excellent enantioselectivities for enantiomeric amines, amino acid isopropyl esters, carboxylic acid tert.-butylamides (e.g., of cis and trans-chrysanthemic acid), carboxylic acid ethyl esters, alcohols and even enantiomeric nitriles [e.g., 2-phenyl propionitrile and 2-(2-fluorophenyl)isovaleronitrile]. Further stationary phases and separations were reviewed by Liu and Ku [1021].

8.15.3. Uteide Stationary Phases The structure of these carbonyl bis(amino acid esters) exhibits two asymmetric centres, similarly to the dipeptides, but differs from them and from diamides by the central CO group being immediately adjacent to two NH-groups:

289

8.15. Chiral Stationary Phases R'

0 R' II IU ROCCHNHCNHCHCOR II II la

0

0

(after Lochrniiller and Sourer [1048])

These stationary phases have been used to separate enantiomeric amine derivatives in liquid [1024, 10491, solid [lo491 and mesophase forms [lOSO, 10511 depending on the column temperature, with high resolution factors, hence permitting also the use of packed columns.

8.15.4. Metal Complexes Optically active metal complexes were first shown to be able to separate enantiomers by Schurig in 1977 [1052], who used an optically active rhodium complex. Compounds applied since then include optically active nickel(II), europium(III), and copper(I1) complexes for the separation of the enantiomers of olefins [1052, 10531, epoxy compounds [1054, 10561, ahydroxy carboxylic acid esters [1057,10581, amino alcohols, amino acid esters, amines and alcohols [1059]. The metal complexes, e.g., of optically active Schiffs bases [1057, 10581 or of N-salicyliden-(R)-2-amino- 1,l-bis(5-tert.-butyl-2-0ctyloxypheny1)propan-1-01 and of N-salicyliden-(S)-2-amino-l,l-diphenylpropan-l-01, are suspended in poly(dimethylsi1oxane) (1:1) or in poly(ethy1ene oxide/propylene oxide) (1:1) and wall-coated on glasdfused silica glass capillaries or, for packed columns, coated on Chromosorb W AW DMCS (6%) as a mixture of the complex and poly(dimethylsi1oxane) (5 :1) [1059]. This type of selective complexation gas chromatography, in which there are both gas-liquid and gas-solid contributions to the retention, has been utilized for the separation of structural, configurational and optical isomers in glass and fused-silica open-tubular columns [1055]. Finally, it should be mentioned that grafted chromatographic packings have also been developed for the analysis of enantiomer mixtures. They contain optically active a-arylalkylamines and aminoalkylsilanes bonded by basic carboxylic acids [ 10601.

8.1 5.5. Recommendable Chiral Stationary Phases and Some Applications Of the following stationary phases, A-C and M are commercially available A. N-tert.-Butyl-L-valinamide polysiloxane Min. col. temp.: 90°C Structure: see Section 8.3.10 Max. col. temp.: 200°C Commercial name: Chirasil-Val (Applied Science Labs.) (wall-coated capillaries); Heliflex Chirasil-Val (15% phenyl substitution in the polysiloxane)

B. L-Valine-tert.-butylamide-modifiedpoly(cyanopropylmethylsi1oxanes) Min. col. temp.: 90°C tion 8.3.10) Max. col. temp.: 200°C Commercial name: RSL-007(Alltech) (wall-coated capillaries)

(see

Sec-

290

8. Liquid Stationary Phases

C. L-Valine-(S or R)-a-phenylethylamide-modified poly(cyanoethylmethyl/carboxymethylsiloxane) Min. Col. temp.: 100°C Max. col. temp.: 190°C Commercially available from Chrompack cyclohexyl ester D. N-trifluoroacetyl-L-valyl-L-valine Structure: see above in this section Min. col. temp.: 90°C Max. col. temp.: 110°C

E. N-Trifluoroacetyl-L-phenylalanyl-L-phenylalaninecyclohexyl ester Min. col. temp.: 130°C Max. col. temp.: 165°C

F. N-Trifluoroacetyl-L-phenylalanyl-L-aspartic acid bis(cyclohexy1) ester Y Structure:

Y

G H ~ I O C C H ~ ~CkNHCCF3 NH

II

0

I

'1 I

C=O°CH,

I1

0

I

bC6 H11

Min. col. temp.: 95°C Max. col. temp.: 160°C

G.N-Docosanoyl-L-valine-2-(2-methyl)-n-heptadecylamide Structure:

1 FH3

CH3 (CH2120 C NH C$NHC(CH2)1,CH,

lo

11

CH

I

CH3

/ \CH3

CH3

Min. col. temp.: 65°C Max. col. temp.: 200°C

H. N-(1R,3R)-trans-Chrysanthemoyl-(R)-l-(a-naphthyl)ethylamine Min. col. temp.: 80°C Max. col. temp.: 120°C I. Carbonyl-bis(L-valine-tert.-butylester)

Min. col. temp.: 90°C Max.col. temp.: 150°C

K. (+) Dodecyltartrate HOH

Structure:

C12H250C L'h'COC12H25 1 I I I 1 I OOHHO

L. Binuclear copper(I1) complex of N-salicyliden-(R)-2-amino-l,1-bis(5-tert.-butyl-2-octyloxypheny1)-propan-1-01

291

8.16. Liquid Crystals

M. Hydroxypropyl and Dialkyl Cyclodextrins (Chiraldex) Some Applications: Separation of the enantiomers of acylated amino acid esters:

Stationary phase

References

A

[896, 10191 [9031 [lo281 [lo371 [1037, 10401 [lo301 [lo461 [1052-1055,1059] [936a -c]

B,c

D E F G H L M C G L M A D I L

acylated amino alcohols:

amine derivatives:

K

alcohols:

L a-hydroxycarboxylic acid esters: alkylcarboxylic esters: epoxides nitriles ephedrine derivatives amphetamine derivatives, carbohydrate derivatives

I

chrysanthemic acid derivatives

8.16.

M H L H L H

B A C

M H

[lo301 [lo501 [936a] [896, 10191 [lo281 [lo511 (1052-55,10591 [lo311 [1052-55,1059] [936a-c] [lo461 [lo591 [lo461 [1052-10551 (10461 [g031 [896, 10191 [936a-c] [lo461

Liquid Crystals*)

Liquid crystals or, more correctly, mesomorphic compounds (meso phases), have structures with a state of order that is between an isotropic liquid and a solid crystal. If by input of thermal energy the three-dimensional crystal structure is destroyed, a phase is formed at the melting point which shows anisotropic properties. On melting, a viscous opaque liquid develops, in which a two-dimensional state of order predominates, called a smectic structure. Here the molecules are constrained parallel and oriented in layers; some compounds are claimed to exist even in two or more distinct smectic phases. Further heating leads to another state, in which the molecules are still oriented parallel but no longer in layers. This meso phase with its monodimensional orientation is called nematic. A variant of the nematic structure, restricted to optically active compounds, is the cholesteric meso phase, the align*)

The author is indebted to Dr. G. Kraus for valuable advice.

2 92

8. Liquid Stationary Phases

ment of which is helically oriented and can be characterized by a gradual change of molecular order in the twist-axis direction [lo611and, as it occurs in chiral compounds, the twist direction depending on the existence of the D-or L-enantiomer. The pitch of the helix is temperature dependent [1062].The term "cholesteric" is connected with the existence of this meso phase in many cholesterol derivatives. With a further increase in temperature the nematic state, i.e., the orientiation, is removed at a certain point and a true isotropic liquid is formed. If these meso phases exist within certain temperature intervals they are denoted thermotropic. Each pure meso phase is stable over a certain temperature range, inside which it goes through one or more discrete phase transformations. The fmt is the solid-smectic transition, the corresponding temperature being T,,or solidnematic, with temperature TN. Provided that a smectic phase exists, there may be further transitions: smectic I-scmectic 11, smectic I-nematic or smectic II-nematic, with the correTS1-N and TS1I-N. At the transition temperature TN-1(nematic-issponding temperatures Tsl-sll, otropic transition) the substance passes to the normal or isotropic liquid state. An example is shown in Table 78. It should be noted that certain nematic crystals can be supercooled below TN, the normal solid-nematic transition temperature, while remaining in the ordered liquid state, in some cases down to 60°C below T N [1063]. The gas chromatographic behaviour of mesogenic stationary phases is characterized by a high solubilizing power for stretched molecules. For example, in the nematic phase the liquid crystal molecules are oriented with the long axis parallel and, owing to their rod-like molecular structure in the nematic temperature range, longer (or more rod-like) isomers can interact more strongly with the aligned nematic crystal phase than isomers with rounder molecules. This enthalpy gain, making a stretched molecule more soluble than a less rod-like molecule, overcompensates the entropy loss caused by the sacrifice of translation and rotational freedom for the longer molecules [1063].Especially positional isomers, owing to differences in their molecule shapes, show retention differences. KeZker was the fmt to use a liquid crystal as a stationary phase [1064].He studied p,p'azoxyphenetole, which has a nematic range between 138 and 168°C and dissolves para-substituted benzene derivatives better than meta- or ortho-substituted derivatives. The selectivity of such stationary phases is highest immediately above the melting point (i.e., solid-nematic transition, TN) and decreases in the direction of the clear point (TN-3. For isomeric compounds, the elution order will depend more on the shape of the molecules than on the vapour pressures of the compounds being analysed at the column temperature, which necessarily has to be chosen such that it is within the nematic (or smectic) range, Kelker succeeded in completely separating the xylene isomers on the nematic phase of 4,4'azoxyanisole [1064,10651. Mesogenic compounds are coated as usual on an inert support or on the column wall of a capillary and used within the region between the melting point and nematic-isotropic transition temperature. They are especially suitable for the separation of disubstituted benzene derivatives, polycyclic aromatic hydrocarbons, n-alkene isomers, isomeric terpenes, steroids and polychlorinated biphenyls. The properties and application of these phases have been reviewed by Kruus and Wnterfeld [1067],KeZker and Hutz [lo681and Wtkiewicz [1069]. Of the many more than 5000 reported liquid crystals, about 100 have been utilized as stationary phases. As it is not possible to deal with all types and all special compounds, only some characteristic mesogenic stationary phases will be mentioned here. It should be noted that the macrostructure of liquid crystals, unlike isotropic phases, depends strongly on the surface on which the meso phase is deposited. Even support materials, such as Chromosorb W HP should be loaded with >2,5%, as with lower loadings the properties of the meso phase would be affected (V, decrease with increasing loading, alteration of the smectic-nematic range, supercooling of the nematic melt) [1076-10781.

293

8.16.Liquid Crystals Table 78. Transition temperatures of substituted toluidine deriva-

tives of 1,2-diphenylethane R

T* PCI

Tsi-sn

TS.N

TN

PCl

PCl

Wl

OCHS') OCzH5 OC3H7 OC4H9 OCSHII

OC6H13 OC7H1S

OCaH1,

TN-I PCl

181 173 169 159 139 127 119 118

337 341 311 303 283 274 262 255 403 >370

176 188 208 229 238 244

201 203 203 202

257 253

C6HS

CH,OC6Hd

T, = solid-smectic; TsI-sIl= smectic I-smectic 11; T9.N = smectic-nematic; T N = solid-nematic; TN.I= nematic-isotropic transition *)

Roduct of Eastman Kodak Co.

References: [1063,1073-10751 It has been shown that glass capillaries, after an efficient pre-treatment [1079], can be uniformly coated, and high efficiencies can be achieved [1062]. However, great difficulties may arise when coating these monomeric compounds on well deactivated glass or fused-silica walls [1080, 10811. Therefore, liquid crastalline moieties have been attached to better spreading silicone backbones. We have already discussed this topic in Section 8.3.11. Another possibility consists in bonding liquid crystalline moieties via spacers to polyacrylates [1081]. Such a phase will be dealt with below. Benzilideneanilines Structure:

e

R,+CH=N

Meso phase: nematic except R1= C2HS0 R1= C9H110 R1= ClzH19O References: [1066, 1070-10721

~

z

R2 = -CH=CHCOOCzHS smectic-nematic Rz = -CH==CHCOOCsHll smectic Rz = -CH=CHCOOCSHll smectic

N,N'-Bis[(R)-benzylidenel-a,a'-bi-p-toluidine Structure:

R +CH

=N

C H C~

H

~ = CH~

N R

References: [1063, 1073-10751 Application: In WCOT columus and packed columns for the separation of polycyclic aromatic hydrocarbon isomers, steroid epimers, pesticide isomers, phenols. Solvent: Benzene, toluene or ethanol Fig. 33 is intended to illustrate the use of liquid crystals for the separation of structurally isomeric high-boiling derivatives of polycyclic aromatic hydrocarbons.

p,p'-Bis@-methoxybenzylidene amino)-3,3'-dichlorobiphenyl

TN:154°C; TN-,: 334°C; Reference: [lo711

294

8. Liquid Stationary Phases

+ c

"i r 1 m

I

al

w -L 0

I m

v)

*

7

@@

z

m I V

N

I V

0 v) I

r !-

c, V

-5

I -

I

0

10 20 30 Fig. 33. Separation of the trimethylsilylated highly carcinogenic hydroxymethyl isomers of 7,12-dimethylbenz[a]anthracene. Stainless steel WCOT column (0.75 mm I.D.)coated with an N,N'-bis(Rbenzylidene-aa'-bi-p-toluidinederivative (see text) (after Zielinski and Janini [lo631

p-(p-Ethoxypheny1azo)phenylcrotonate

TN.1:

197°C

Manufacturer: Eastman Organic Chemicals Co. Reference: [lo611

2-R1-4'-R-4-(4-n-Alkoxybenzoyloxy)azobenzene Structure:

e N =N

R

-&- ; 0-

-n -olkyl

+o

0

OCH, OCH, OCH, CH3 n-C4H9

H CH3 CH3 CHI H

Solvents: Benzene, ethanol Reference: [lo821

C4H9 CZHS CH, C2HS n-C4H9

116 125 160 125 94

280 244 253 220 234

295

8.16. Liquid Crystals

p,p'-Azoxyphenol ethers Structure:

RO +N

= N +OR

d

~

CH3 CZHS n-C,HI3 n-C7HIS

~~

~~

121 132 71 75

135 168 130 127

80 95

Reference: [lo831 p-Phenylene bis-4-n-heptyloxybenzoate Structure:

n-C7~150 -Q-;O+oc

II

0

Ts:83°C TS-N:125°C Reference: [lo711

e

0

-

n -c7H15

0

TN.1:204°C

p-Biphenylene bis-4-n-alkoxybenzoate Structure:

RO -@CO-OC

I1 0

R = n-C4H9 Ts:171°C TN.1:358°C (decomposition) R = n-C7HI5 Ts:150°C TN.1:316°C References: [1071, 10831

II

+OR

0

TS.N:184°C TS.N:211°C

Cholesteryl Derivatives

Structure:

R

CbHSCHdHCOOCbHSCOOCgH17C00CH3COOC13H&OO-

161.6 149 78 95 70

216 180 91 117 123

TS.Ch= smectic-cholesteric transition temperature TCh.,= cholesteric-isotropic transitions temperature References: [1061, 10711

296

8. Liquid Stationary Phases

Cholesteryl butyrate

References: [1062, 10851 Manufacturer: Merck Liquid-crystalline polyacrylates Structure:

...C H p CH CH2 CH CH2 CH I

M:

I

I

(u:

I

I

o=c I

0

0

0

R

R

k

R= +COO+N=N-@R'

R' = OCH3, OCzHs, CzHS,n-C4H9(the central phenylene group can also be further substituted) Glass-nematic transition temperatures: 69-95°C Nematic-isotropic transition temperatures: 2 10-291°C This stationary phase is reported to coat the capillary columns wall uniformly and to be usable over a wide temperature range (at least over 115°C) with neglegible bleeding. The spacers (-COOCHzCHz-) are relatively flexible. Application: Separation of isomeric aromatic hydrocarbons. Reference: [lo811 Mesogenic polysiloxanes: See Section 8.3.11. Further liquid crystal phases: See [1067-10691, [1085-10891

8.17.

Mixed Stationary Phases

The use of mixed stationary phases, well established practice in gas chromatography since its beginning, is intended both to achieve difficult separations of multi-component samples and to reduce the number of stationary phases necessary. This topic has decreased in significance owing to the increasing application of open-tubular columns, and it may be concluded that the latter development might result in the utilization of mixed stationary phases declining further, apart from, two exceptions, however: the application of copolymers, with the possibility of precisely adjusting the desired selectivity by choosing the corresponding ratio of the monomer moieties, and the use of open-tubular columns coupled in series. There are three ways of combining the separation ability of two or more stationary phases: a) coupled columns; b) mixed bed columns (heterogeneous); c) homogeneous mixed (blended) stationary phases. We shall discuss these three methods in detail.

297

8.17. Mixed Stationary Phases

(a) Coupled columns (in part together with D. Glindemann [1092]) The behaviour of coupled columns is determined by the pressure gradient, and is affected by the sequence in which the columns are coupled. Even the elution order may be influenced on reversing the sequence in which two columns containing different stationary phases are coupled [1090, 10911. However, the retentions can be calculated if the pressure drop m/po & = pressure at the column inlet of the second column) is taken into account. Equations have been derived to correct for the effects of carrier gas compressibility within both open-tubular [1093, 10941 and packed column section [1095, 10961. Moreover, it should be noted that for samples the main component of which is strongly polar, the first column should contain the more polar stationary phase, as otherwise the undesirable tailing of this component would be more severe. Further, it will be shown by the following model that the total number of theoretical plates of the coupled columns must be lower than the sum of the individual columns. From the well known equation

n = ti/d

(34) the resulting total number of theoretical plates can be calculated, provided that the peak variances (d),for columns with properties (stationary phase, temperature, column cross section, etc.) varying steadily with respect to time, along the separation coordinate x behave additively:

with w(x) =

4x1 1+ k ( x )

where w(x) = migration rate of the “peak”, h(x) = height equivalent to a theoretical plate, L = length of the (coupled) column, u = carrier gas velocity, k = capacity ratio. The resulting number of theoretical plates is equal to the sum of the n-values of the individual segments (or columns): L

only when

that is, experimental precautions have to be taken to keep h(x) constant. W(X)

Once again, the model gives evidence that the total number of theoretical plates is in principle smaller than the sum of the individual segments (columns). This effect is a basic

298

8. Liquid Stationary Phases

property and not a problem of coupling techniques to arrange turbulence-free conditions, and also not a problem of the packing procedure, sample injection or sample size. This model has been experimentally tested and the following conclusions can be drawn. If the first (or pre-) column has a larger cross-section, the efficiency of the total system is reduced dramatically. The loss of efficiency can, however, unexpectedly be stopped if the following measures are applied simultaneously: (i) packing of the first (or pre-) column with more efficient particles; (ii) operating the fmt (or pre-) column at a higher temperature than the following column. Summarizing, method (a) cannot be classified as very advantageous. Its only preferences are first, the stability of the packing over a long period, as it is not likely, in contrast to methods (b) and (c), that the stationary phases may mix with each other in the course of time (b) or may interact with each other (c), causing deviations from the linear dependence described below. Second, it was shown [lo961that for certain sets of conditions a coupled column configuration is likely to achieve a more rapid analysis than an equivalent mixed bed column [1096].Third, coupling columns in series is, from the practical point of view, most convenient for combining the retention characteristics of more than one stationary phase, especially in open-tubular column chromatography, where different surface treatments may be necessary for each of the phases [1096].It has been shown, however, that this technique allows a greater reliability of chromatographic identification by the use of the retention index concept [1104al. (b) Mixed bed columns

This method only requires that packings coated with different stationary phases are packed in a single column. Purnell and co-workers [1093-11071 have shown that the gas chromatographic behaviour of mixed stationary phases can be described by the simple equation K~(mix)= PAKLA + PBKLB

(187)

where KLAand KLBare the partition coefficients of the pure liquids A and B, respectively, and p, is the corresponding volume fraction. With more than two stationary phases, the retention of any solute can be calculated from the general equation KL(mix) =

c P&LP

Hence, the solvent properties of mixed stationary phases can simply be attributed to those of the pure components of the mixture. Klein and widdecke [ll08]found that this linear relationship is also valid for polymeric blends, statistical and block copolymers and graft polymers, and it makes no difference whether two different homopolymers are coated from one solution [method (c)] or applied in form of a mixed bed column [method (b)]or whether a copolymer with the corresponding ratio of the monomers is coated on the support. This result permits copolymers to be used more generally as non-volatile mixed stationary phases in gas chromatography than hitherto, and possibilities of varying the comonomer ratio for adjusting the selectivity, as has been dexcribed for nitrile silicones [872,8741. Even copolymers with different sequence length distributions (i.e., block copolymers and statistical copolymers) obey to the above equation (hence no information on sequence structure can be obtained from gas chromatographic partition measurements!). When comparing mixed bed columns, consisting on the one hand of poly(dimethylsi1oxane) and poly(methylpheny1siloxane) homopolymers separately coated, with dimethylsiloxane - methylphenylsiloxanecopolymers with a corresponding phenyl content directly coated as copolymers on the other, the retention characteristics proved to be approximately equally, provided that the degrees of polymerization are similar 111091. However, whether the copolymer is composed of dimethylsiloxane and methylphenylsiloxane units or of dimethylsilox-

299

8.17. Mixed Stationary Phases

ane and diphenylsiloxane units has an effect, even if the total phenyl content is the same [1110].This behaviour has structural causes (see Section 8.3.5). An example can be given to demonstrate the possibilities of mixed bed columns for the analysis of multi-component mixtures. As pA+ pB= 1, equ. (187)can be written in the form

K~(mix)= KLB+ - KLB). (187s) Hence, from this linear relationship, after the determination of Ku and Km,any KL(mk) value between pA= 0 (pB= 1) and pA= 1 (pB = 0) can be determined graphically. The following solutes were investigated by h u b and Purnell [1101]on di-n-nonyl phthalate (A) and squalane (B): Component

No. Name

KU

KLB

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

24.5 58.6 49.0 97.1 254 202 745 1906 414 980 847 138 276 884 462

33.7 41.2 67.2 140 200 301 416 618 630 830 1315 108 22 1 281 372

n-hexane benzene n-heptane n-octane ethylbenzene n-nonane ethoxybenzene acetophenon n-decane n-butylbenzene n-undecane toluene p-xylene benzaldehyde n-propylbenzene

From equ. (187a) Fig. 34 is obtained where KLcmk)is plotted against pA.The value of pAfor which the separation of all components appears to be the most favourable (qA= 0.075) can be read from the diagram. For pA= 0.41, 0.6 and 0.82, however, given by the intersecting straight lines, superimposition or overlapping of several compounds will occur. In Fig. 35, the chromatograms of the 15-component mixture are illustrated, obtained on pure squalane (1) (pA= 0), pure di-n-nonyl phthalate (2) (pA= l), squalane + di-n-nonyl phthalate coated together [method (c)] at a ratio pa = 0.075 (3) and on a mixed bed column corresponding to pA= 0.075 (4). All columns were identical with respect to length and volume of stationary phase and were operated with the same carrier gas velocity and temperature. In (l), components 6 and 14 coincide and 8 and 9 overlap, and in (2)compounds 11 and 14 coincide. For pA= 0.075 however, a complete separation can be achieved both with the mixed bed technique (4) and with homogeneous mixed stationary phases (3). As the precise determination of partition coefficients is a laborious procedure, the specific retention volumes should be determined instead. This is possible because of the relationship

300

8. Liquid Stationary Phases

1750

-

1500

-

5 I I

I7,

Fig. 34. Diagrams of KL(mix) vs. pAfor 15 solutes. Column temperature, 100°C; stationary phases, (A) di-n-nonylphthalate and (B) squalane (after h u b and Pumell [llol])

d) J

260

I

210

180

150

I

I20 Time in min

90

L

60

30

0

Fig. 35. Chromatograms of a 15-component mixture. Column temperature, 100°C; column inlet pressure, 15 p.s.i.g.; column 118 in x 12 A stainless steel; column packing, respective liquid stationary phase on Chromosorb G AW DMCS (60-80 mesh); liquid stationary phases: a) squalane (pA= 0); b) di-n-nonyl phthalate (pA= 1); c) mixture of squalane + di-n-nonyl phthalate (pA= 0.075) coated from their common solution; d) mixed-bed packing of squalaneKhromosorb + di-n-nonyl phthalate/Chromosorb, corresponding to pA= 0.075 (after h u b and Pumell [llol])

8.17. Mixed Stationary Phases

301

where T, = column temperature and hnC, = density of the stationary phase at the column temperature. Further “practical” quantities such as rz,l and K have been employed, in addition to V,, rather than KL 11102-11051 and adapted to computation [1106-11071. As could be shown, also with copolymers the corresponding absolute retention data of the polymeric blends (or copolymers) can be calculated from the specific retention volumes of the pure polymers according to &(mix)= WA vgA+ 56 (188) where W, and WB are the weight fractions of A and B, respectively [ll08]. In investigations on the dependence of the retention index on the composition of the mixed phase, non-linear relationships where found [1108, 11111. This is to be expected, as the retention index does not depend linearly, but logarithmically, on the retention. An equation for the approximation of the retention index as a function of the composition of binary stationary phase mixtures was developed by Gdbler and Bcflizs [1111, 11121 and extended to a wide variety of solutes and stationary phases [1113]. The proposed procedure allows the stationary phase composition to be calculated for a predetermined retention index using the retention indices of the solute in question and of benzene, both on the pure stationary phases [1113]. The possibilities included in eqn. (187) have been further utilized by Purnell and co-workers [1097-1101, 11141 by using the relative adjusted retentions rz,l related to the plate requirement for substance pairs that are drnicult to separate. A so-called window diagram shows the variation of rz,l with the composition of the mixed phase. As the variation of this parameter is invariably discontinuous, but valuable for calculations intended to minimize the overall plate requirement, computer programs had to be developed and have been described for implementing the original window diagram technique [1106, 11071, and the approach has been extended such that even readily measured retention parameters such as capacity factors are sufficient for optimization of the mixed phase composition. Hence, relatively simple measurements permit the calculation of the optimal cpvalues (i.e., the mixed phase composition) for separating complex mixtures with optimized column lengths and analysis times [1102-1107, 1114, 11151.

(c) Homogeneous mixed stationary phases

In this method, the stationary phases are dissolved in a suitable solvent and then deposited on a single packing. It offers the advantage that the more polar of the two liquids will block the active sites of the support particles and hence act as a tail reducer [1116]. Eqns. (187), (187a) and (188) and their possiblities discussed above for @) also apply to method (c). However, there is, as already mentioned, the danger of non-ideal miscibility effects causing a special type of selectivity which is not present with the pure liquid stationary phases [1117]. Depending on the degree of mutual interaction of the two stationary phases, the retention behaviour may be affected [1118], deviations from linear behaviour [eqn. (187)] may occur [1119] and /or phase segregations on the support may lead to retention changes [1120]. Summarizing, we can state that workers have preferred the mixed bed technique @), as it seems to be the “least of the evils” [1091].

9.

Selection of Stationary Phases

9.1.

General Recommendations for Choosing a Suitable Stationary Phase

In introducing this topic, it must be emphasized that the selection of the most advantageous stationary phase requires now, as ever, great practical experience. Therefore, a universal recipe cannot be given, because the theories of solution and adsorption have not yet reached such a stage of development that all possible and occuring interactions can be mathematically taken into consideration. The interactions between the stationary phase and/or the adsorbent on the one hand and the solute (sorbate) on the other are too complex to be able to predict the retention behaviour of a gas chromatographic sample component with the required exactness. Owing to these complex relationships, there does not (and can not) exist a simple order of stationary phases that would comprise a uniform model for the retention of all possible analytical samples. Before searching for the optimal stationary phase, with very complex mixtures, it should be considered whether a simplification might be achieved by appropriate sample preparations (distillation, chemical reactions, derivatization, etc.). If these possibilities have been exhausted, and if the sample contains constituents belonging to different chemical types, a stationary phase should be chosen that should not be or is only slightly selective for all sample compounds such that generally the components will be eluted in order of decreasing vapour pressures. However if only two types of compound are present in the sample and if the boiling range is not too large, a strongly selective stationary phase would be more suitable, because the constituents of one type of compound would be eluted first, distinctly separated from the other, and possibly from each other within each types. If an unknown mixture has to be separated and if there is no background information concerning the desired gas chromatographic separation, a poly(dimethylsi1oxane)and a poly(methylphenylsiloxane) (50% phenyl) should be used as the first stationary phases, especially when coated on open-tubular column walls. Such a general-purpose capillary system is extremely useful for screening complex mixtures. For separations within a homologous series, generally a stationary phase is used that is chemically similar to the sample components, because the partition coefficients are high in such solute-solvent systems, and the quality of the separation is almost exclusively determined by the quality and length of the column, i.e., by the h value and the number of theoretical plates. If the result is inadequate, the packing (or column) must be improved or a longer column must be used. Members of a homologous series that differ from each other in the number of methylene groups, show greater retention differences on alkane stationary phase (Section 8.2.1) and on poly(dialkylsi1oxanes) (Section 8.3.3) than on other stationary phases. Chemically bonded stationary phases can be used for the analysis of wide-boiling compounds, especially if they are non-polar or only slightly polar, if short analysis times are desired, if coupling with a mass spectrometer is provided or if trace organic compounds have to be determined in water or air. For certain applications gas-solid chromatography may be superior to gas liquid chromatography. This applies not only to the separation of gases and low-boiling liquids (common practice since the early days of gas chromatography), but also to, e.g., the separation of many types of positional isomers and even of stereoisomers and of deuterated compounds.

303

9.2. Stationary Phases for Special Separation Problems

9.2.

Choosing Stationary Phases for Special Separation Problems with Regard to the Desired Selectivity

Now, it would be desirable quickly to elicit which stationary phase would first be, for a certain separation problem (e.g, separation fo alcohols from hydrocarbons) or for a certain compound type (e.g., xylenes, where the isomers must be separated from each other), particularly selective (in order to avoid superimposition or overlapping), or second show a low selectivity (in order to separate constituents of a homologous series). The McReynolds constants and numerous indications of selectivities have been given and fully discussed, especially in Chapters 4, 5 , 6 and 8, and phases with similar selective propertties have been collected in separate subdivisions (e.g., Section 5.2.1, 5.5.2,8.3.1, 8.3.7, 8.4.4.1 and 8.12, to mention only a few). Nevertheless, it appeared useful to facilitate searching for a suitable stationary phase by giving a further list. In Table 79, which is not claimed to be complete, separation problems or the types of compounds that are to be separated are listed alphabetically, and under the headings Strong, Medium or Weak (selectivity) the groups of stationary phases are listed, designated with Section numbers. The indications strong, medium and weak alone indicate the qualitative nature of these recommendations, and certain deviations may occur for the reasons mentioned already. According to the analytical problem it is known (see the comments above) whether a strongly, intermediate or weakly selective phase is required, and the group of stationary phases listed against the separation problem can be selected. From this group a stationary phas is chosen which fits the required temperature range by referring to the corresponding subdivision in Chapters 5, 6 and 8. The phases specially recommended in these sections are intended to make the decision easier. Before starting with the table, we shall recall how important selectivity is. The resolution, R,,depends, according to eqn.(100), on three terms, of which two, the selectivity and the capacity term, can be influenced by the choice of the most advantageous stationary phase. We have seen in Section 2.6 that the smaller the capacity ratio k, the worse is R,; we should therefore search for a stationary phase that exhibits a good solubilizing power in order to avoid low k values, especially in complicated separations. In particular, however, the selectivity, expressed by rz,l in eqn. (loo), must be selected carefully, because greater benefits can be achieved by changing r for poorly resolved peaks than by increasing h,, even in the era of open-tubular coumns with their high efficiencies. When R2,1, approaches 1, no practical increase in qeq will improve R, such that the components are resolved, and changing the only choice, can only (and fortunately easily) be changed by selecting another stationary phase with a different selectivity. The compound types (families) in Table 79 are arranged alphabetically. For the separation of two groups from each other one generally has to look for the groups which occur first alphabetically; the same separation problem is not listed a second time under the initial letter of the second group. For example, the separation of ketones from aldehydes is to be found under aldehydes from ketones, and not under ketones from aldehydes. Table 79. Separation Problems and Suitable Stationary Phases of Different Selectivity Separation Problem

Stationary Phase Oroup Selectivity Strong

Medium

Weak

8.6; 8.3.6; 8.5.1 8.6 8.6

8.4.4.1; 8.4.4.3 8.4.4.1 8.4.4.1; 8.4.4.3 8.4.4.1; 8.4.4.3

~

Acetals Acetals from ethers Acetals from aldehydes Acetals from alcohols

8.14.5 8.5.1

304

9. Selection of Stationary Phases

Table 1 9 (continued) Separation Problem

Stationary Phase Oroup Selectivity

Medium

weak

8.6; 8.3.6 8.3.6 8.5.1; 8.5Sb); 8.4.1

8.4.4.1; 8.4.4.3 8.4.4.1; 8.4.4.3 5.5.1; 8.5.2; 8.4.4.1

8.2.1; 8.3.1; 8.2.2; 8.5.1; 8.5.2; 8.5Sb) 8.5.1

8.4.4.1; 8.4.4.3; 8.3.4

8.3.1; 8.3.4; 8.3.6; 8.3.7; 8.2.1; 8.5.1; 8.5.2; 8.6; 8.9; 8.12

8.4.3; 8.4.4.1; 8.4.4.3

5.5.1; 8.4.4.1 8.2.1; 8.3.3 8.6; 8.4.4.1

8.8.2”;

8.3.1; 8.3.2 8.5.2 8.5.1; 8.5.2; 8.3.7; 8.9; 8.12 8.5.2

8.8.2’);

8.5.2 8.6; 8.2.1; 8.3.3

strong

Acetals from fatty acid esters Acetals from ketones Alcoholsc)

Alcohols from minesc) Alcohols from carboxylic acid esters Alcohols from hydrocarbons9 Alcohols from ketonesc)

Alcohols from waterc) Aldehydes Aldehydes from alkanes Aldehydes from alcohols Aldehydes from fatty acid esters Aldehydes from hydrocarbons Aldehydes from ketones

8.6 5.5.2; 5.6.6; 8.8.1; 8.3.10; 8.4.1; 8.4.2; 8.14.5; 8.13; 8.4.3; 8.4.5 5.5.1; 8.14.5 8.3.4; 8.8.1; 5.5.1 8.4.4.1; 8.4.4.3; 8.6; 8.1 8.14.5; 8.12; 8.8.1

5.5.1; 8.4.4.1; 8.4.4.3; 8.3.4 5.5.2 8.6 8.14.5; 8.9; 8.12 8.8.1.); 8.12 8.6 8.8.1’); 8.4.3

Alkaloids Alkanes Alkanes from alcohols Alkanes from alkenes Alkanes from aromatics Alkanes from cycloalkanes Alkanes from esters, ethers, ketones Alkenes Alkenes from alkynes Alkenes from aromatics Alkenes from cycloalkanes Alkenes from cycloalkenes Alkylchlorosilanes Alkylchlorosilanesfrom hydrocarbons Alkylcyclohexanols Alkynes Alkynes from aromatics Alkynes from cycloalkanes

5.2.1; 5.6.1; 5.3.6; 5.6.1 8.2.1; 8.6 8.6; 8.1; 8.3.1; 8.11 8.4.2 + 8.14.1 8.6; 8.1; 8.5.5; 8.3.1; 8.13 5.6.1 5.6.1; 8.6 8.3.1; 8.6; 8.1 8.6; 8.3.1 5.6.1 5.6.1 8.3.1; 8.1 8.3.7; 8.1 8.4.2; 8.4.3 8.4.4.1 8.6; 8.1

8.3.6; 8.4.4.1; 8.4.4.3 8.4.4.1; 8.4.4.3 8.5.2 5.5.2; 8.4.4.1; 8.5.1; 8.3.6; 8.3.4; 8.5.5 8.3.6; 8.4.4.1; 8.5.1; 8.5.4 8.10; 8.2.1; 8.6; 8.9 8.5.2 5.2.6; 6.2.3; 8.6 8.5.2 8.5.2; 8.2.2 8.6; 8.7; 8.12; 8.9 8.4.4.1; 8.2.2; 8.6 8.3.6; 8.5.1 8.3.6; 8.5.1 8.6; 8.4.4.1 8.5.2 8.5.2

+ 8.14.1

8.2.1; 8.3.3; 8.4.4.1; 8.5.1 8.4.4.1 8.4.4.1; 8.2.2; 8.3.4 8.3.1; 8.3.2 8.2.1; 8.3.3; 8.3.1

8.2.1; 8.3.1; 8.3.3 8.2.1 8.3.1 8.2.1; 8.5.1 8.2.1; 8.3.3 8.2.1; 8.3.1; 8.3.3 8.2.1; 8.12 8.4.4.1; 8.2.1 8.2.1 8.3.1; 8.2.1 8.2.1; 8.3.1 8.3.1 8.3.1

9.2. Stationary Phases for Special Separation Problems

305

Table 79 (continued) Separation Problem

Stationary Phase Group Selectivity ~~

Amides Amines? Amino acids (as trimethylsilyl esters or as N-trifluoroacetylamino acid trimethylsilyl esters) (see also optically active compounds) Aromatics Aromatics from alcohols Aromatics from cycloalkanes Aromatics from cycloalkenes Aromatics from heterocyclics Aromatics from hydroxyl compounds Aromatics from oxygen containing organic compounds Benzene derivatives (positional isomers) Biphenyl derivatives Boron hydrides Carbohydrates (as methyl- or trimethylsilyl esters) Carboxylic acids see monocarboxylic acids, dicarboxylic acids, fatty acids, hydroxycarboxylic acids Catecholamines Chinoline see quinoline Chiral compounds see optically active compounds Chlorinated hydrocarbons Chlorosilanes (see also alkyl- and phenylchlorosilanes) cis-trans Isomers Cycloalkanes Cycloalkanes from cycloalkenes Cycloalkenes Cyclohexanols see Alkylcyclohexanols Dicarboxylic acids (as esters)

Strong

Medium

8.14.2 8.14.5; 8.3.10; 8.13; 8.4.4.1; 8.4.2 8.3.10

8.4.4.3 5.5.1; 8.4.1; 8.3.7; 8.3.5 8.4.4.2 8.3.1; 8.3.4; 8.9

Weak

6.2.3

5.6.2; 8.8.2; 8.10; 8.5.4; 8.4.3; 8.3.4; 8.2.1; 8.3.1; 8.4.5 8.16; 8.5.1.c); 8.2.2; 8.3.8; 6.2.3 8.3.11 8.14.4; 8.11 8.6; 8.3.7 8.4.4.1; 8.5.1; 8.2.1 8.5.2 8.6; 8.3.7 8.4.4.1; 8.9 8.2.1 8.4.4.3 8.11; 6.2.1 8.3.6 8.8.1 8.3.6 8.16 8.3.10; 8.16 8.2.1; 8.3.3; 8.3.4 8.4.3; 8.5.4; 8.3.9; 8.2.1; 8.3.1 8.9

8.3.9

8.6; 8.7; 8.5.1; 8.5.2; 8.3.7 8.3.7; 8.6; 8.7

8.3.4; 8.3.6 8.3.6; 8.5.1

8.2.1; 8.3.1

8.3.7; 8.3.11; 8.16 8.6; 8.7 8.2.1; 8.3.1 5.2.1 8.2.1; 8.3.1; 8.6 8.6; 8.4.4.1; 8.5.4; 8.2.1; 8.3.1 8.11; 8.10 8.6; 8.10

Esters

5.5.2; 6.2.1

Esters from ketones Esters from wateP) Ethers Ethers from alcohols Ethers from aldehydes Ethers from alkanes

8.8.1; 6.2.1 8.3.4 5.5.2; 8.3.1; 8.5.4 8.14.4; 8.6; 8.2.1 8.6 8.14.4; 8.4.2; 8.5.4

8.8.2; 8.3.4; 8.5.4b) 8.5.1; 8.3.4; 8.5.2; 8.5.4; 8.4.4.3 8.3.7; 8.6; 8.3.8 5.6.1; 8.4.5 8.3.1; 8.5.1 8.2.1; 8.9 8.2.1

8.4.4.1; 8.4.4.3; 8.2.1; 8.3.1 8.5.1; 8.5.3 8.2.1 8.4.4.1 8.4.4.1; 8.3.1

306

9. Selection of Stationary Phases

Table 79 (continued) Separation Problem

Stationary Phase Oroup Selectivity Strong

Medium

Weak

Ethers from fatty acid esters

8.14.4; 8.6

8.3.1

Ethers from ketones Fatty acids (free) (see also monocarboxylic acids) Fatty acids (as esters, e.g. FAME)

8.6 8.3.7

Fluorinated compounds (see also perfluoroalkanes and -alkenes) Fluorinated compounds from hydrocarbons

8.3.4; 8.3.7; 8.6; 8.5.5; 5.4.1 8.2.1; 8.12

Gasesh)

5.4.1; 5.4.2; 5.2.3; 5.6.1 5.5.2; 8.8.1 8.5.2; 8.6; 8.7; 8.3.7; 8.14.4 8.4.2; 8.4.3; 8.5.5; 8.13 8.13; 8.3.4; 8.4.2; 8.4.3; 8.8.1; 8.8.2 8.6; 8.7; 8.3.7

8.2.1; 8.3.7; 8.4.4.1 8.2.1; 8.3.7 8.4.4.3; 8.5.2 + 8.13 8.5.2; 8.3.4; 8.5.59 8.7; 8.3.6; 8.2.1; 8.5.1; 8.5.2 8.5.1; 8.5.2; 8.3.6; 8.4.5 8.5.1; 8.11

Glycols Halogenated compounds Heterocyclics (generally) N-Heterocyclics (see also pyridine derivatives) Hydrocarbons (generally;see also alkanes, alkenes, alkynes, aromatics, cycloalkanes, cycloalkenes) Hydrocarbons from nitriles Hydrocarbons from nitro compounds Hydrocarbons from oxygen containing organic compounds Hydrocarbons from water Hydrosilanes and hydrosiloxanes Hydroxycarboxylic acids (as trifluoroacetyl derivatives) Ketones Metal halides

8.6; 8.7; 8.3.7 8.6; 8.7; 8.3.7; 8.4.4.1 8.2.1; 8.6; 8.4.4.3

5.5.1 8.3.4; 8.3.6; 8.5.1

8.3.1 8.3.1

+ 8.13;

5.5.1

8.5.1; 8.5.3; 8.3.5 8.12

8.5.1; 8.5.4; 8.5.5

8.12; 8.3.1; 8.2.1; 8.4.5 8.3.1

8.3.8; 6.2.1

8.3.1

8.3.4; 8.3.6; 8.5.1; 8.5.2; 6.2.1

8.2.1; 6.2.3

8.5.4 8.3.4 8.4.2; 8.4.4.1; 8.9; 8.11

8.5.1; 6.2.2

8.2.2; 8.3.4 8.4.4.1: 8.3.4

8.3.1 8.3.1

8.3.4 8.14.3; 8.14.4; 8.2.1

8.3.1

8.5.1 + 8.13; 8.3.4 + 8.13; 8.5.4; 5.5.2; 8.5.5 + H,PO,? 8.5.2; 8.4.4.1; 8.4.4.3.; 8.5.4 8.4.4.1; 8.4.4.3; 8.5.4

8.3.1

8.4.4.1; 8.3.4 8.3.10 8.14.5; 8.13; 6.2.1

Methylchlorosilanes see alkylchlorosilanes Monocarboxylicacids (see also fatty acids and hydroxycarboxylic acids) Monocarboxylic acids (as esters)

8.3.7

Nitriles

5.5.2

Nitriles from nitro compounds Nitroalkanes Nitroaromatics Optically active compounds alcohols m i n e derivatives amino acid derivatives biphenyl derivatives

8.3.6; 8.3.7; 8.2.1 8.3.6 8.3.6 8.3.10; 8.15.2 8.3.10; 8.15.3 8.3.10; 8.15.1 8.3.10

8.3.4 8.2.1; 8.3.4

8.3.1

8.2.1 8.3.1

+ 8.13; 5.5.1

9.2. Stationary Phases for Special Separation Problems

307

Table 79 (continued) Separation Problem

epoxide compounds hydroxycarboxylicacid esters nitriles stereoisomers unsaturated compounds Organometallic compounds Oxygen-containing organic compounds

Stationary Phase Oroup Selectivity Strong

Medium

Weak

8.15.3 8.3.10; 8.15.2 8.15.2 8.15.2 8.15.3 8.14.3; 8.14.4 5.5.2; 8.2.1; 8.8.1; 8.14.4 5.4.1; 8.5.5 8.5.5 8.2.1; 8.3.1; 8.2.2; 8.3.4; 8.6; 8.7

8.2.1; 8.3.1; 8.12 5.5.1; 8.5.1

8.4.4.1

Peffluoroalkanes Peffluoroalkanes from peffluoroalkenes Peffluoroalkanes (and -alkenes) from partially fluorinated hydrocarbons or chloroand bromo-, hydrocarbons Perhalogenated (Cl, Br) compounds from hy- 8.3.7; 8.6 drogen-containing organic halo compounds Peroxides Pesticides Phenols? 5.6.6 Phenol ether@) Phenol ethers from phenols Phenylchlorosilanes Phosphorus compounds Polychlorinated biphenyls (PCBs) Polycyclic aromatic hydrocarbons (PAHs) Polyphenyl ethers Pyridine homologues Quinoline bases Organosiloxanes Stereoisomers Steroids

8.3.7; 8.4.2; 8.4.3 8.3.7 8.16 8.3.11; 8.16 8.8.1; 8.13 8.8.1: 8.13

8.2.1; 8.3.1; 8.12

8.2.1; 8.5.2

8.3.4; 8.5.1

8.5.1 8.3.6 8.4.2; 8.5.1; 8.4.2; 8.3.4 8.3.6 8.2.1 8.3.1;

8.4.4.1; 8.5.4; 6.2.3 8.4.3; 8.3.4

8.16; 8.3.11 8.16; 8.7; 8.3.11 8.8.1 5.5.1; 8.4.4.1

8.3.1; 6.2.3 8.3.1; 8.2.1 8.3.1; 8.5.1; 8.5.4 8.4.4.1 8.3.1; 8.4.5 8.3.1

8.3.4

8.3.4; 8.4.2; 8.3.8 8.5.4 8.3.4; 8.3.6; 8.3.7

8.3.11; 8.16; 8.15.2; 5.2.1 8.16; 8.3.11

Sugar derivatives Terpene alcohols Terpene hydrocarbons Thioethers Thioethers from thiols Thiols Water from organic compounds

8.5.1 8.3.6; 8.3.1; 8.5.1 8.12

8.4.4.3; 8.9; 8.3.1; 8.3.9; 6.2.3 8.9; 8.3.9; 8.4.3; 8.3.1 8.4.3; 8.4.4.1 8.4.4.1; 8.6 8.8.1; 8.8.2; 8.2.1 8.4.2 8.3.4

8.3.1 8.3.5 8.2.1; 8.4.4.1 8.2.1; 8.3.1; 8.4.4.1 8.3.1; 8.3.5 8.5.4; 8.3.1 8.3.1 8.3.1 8.2.1; 8.3.1; 8.5.4 8.11 8.4.4.1; 8.5.4 8.11

a) Caution, aldehydes may react with certain stationary phases of groups 8.8.1 and 8.8.2 (azomethine formation with primary amines or H-bonding with the enolic form of the aldehyde, resp., in the case of secondary and tertiary amines, respectively). b, Especially polyesters with keto groups. 3 For the investigation of free acids, phenols, alcohols and strongly basic compounds an inactive support or column wall is nccessary ! d, The support must be completely inert (e.g., OTCB..PTFE). e, Especially tetrachlorophthalates. 9 For example 111251 in earlier days; this type should be substituted by other more suitable phases ') Methyl- or trimethylsilyl ethers b, A review on the analysis of gases and light hydrocarbons was published by Mindrup [1126].

9. Selection of Stationary Phases

308

9.3.

Preferred Stationary Phases

With the large number of liquid stationary phases available (more than 1000) there is obviously a need for rationalization. This problem was first considered at the 1956 London Symposium on Gas Chromatography and has been the subject of controversial discussions since then. The opinions of gas chromatographic practitioners range from fan-shaped selection of the most suitable from all available stationary phases (and even mixing them) to a restriction to only three standardized all-purpose phases. This is not surprising, as in many research laboratories the number of possible combinations of compounds to be separated may be almost infinite, whereas in laboratories for the quality control of a few products the separation problems may well be resolvable with only two stationary phases. It has been emphasized many times in this book that in order to achieve the necessary reproducibility, only phases manufactured specifically for gas chromatography should be used, and that obsolete, questionable trade materials, possibly taken from drums in the plant, should be replaced by specialty-grade products. This limitation to products that are described in terms of physical properties (including spectroscopic and size-exclusion chromatographic specifications!), purity and gas chromatographic specifications (KovBts retention indices and McReynolds constants) is, however, only one, although important, reason for the reduction of the large number of phases. Some hrther reasons are that many liquid phases are similar to each other, recognizable by similar McReynolds constants, that almost 80% of 1472 gas chromatographic separations published in 1969 were achieved with only five liquid phases of different chemical structure [1127]and that the exchange of analytical specifications between different laboratories suffers from inadequate reproducibility and additional expenditure (as the column in question must be specially prepared, which would be superfluous if a few standardized columns would suffice for most gas chromatographic tasks). Some systems of preferred liquid phases have therefore been proposed by several committees or authors, and most of them are listed in Table 80. However, there are many factors that contradict such a restriction of the possibility of selecting from a greater number of liquid phases: a) Not all separation problems can be solved with one of these sets; special mixtures of substances may require a certain stationary phase. b) The analysis time, being a decisive criterion, especially for routine analyses which have to be carried out frequently, may be decreased, while maintaining or even improving the resolution, by using a special phase not included in the general list in Table 80. c) The presence of reactive constituents in the sample may possibly require the use of other phases. d) For highly specific separations of positional and geometric isomers or of optically active compounds, the use of special phases, e.g., liquid crystal or chiral phases, is necessary. e) Separations above 300-350°C can only be achieved if thermally stable products are applied. f) For gas chromatographic separations below 50°C the k-values cannot be optimum for the phases in this list, and other phases which would fail at higher temperatures owing to their high vapour pressures would be more appropriate. An expanded list was suggested by the Hawkes Committee [1128]for special cases where a stationary phase is needed that is intermediate in properties between those in the primary list (fourth column in Table 80). The classification in terms of a single sample (benzene) for the polarity scale of 24 proposed phases is unsatisfactory however, and does not permit the desired gradation of polarity against solutes other than aromatic hydrocarbons. The Committee’s proposals therefore did not meet with the approval of practising analysts and, as they did not achieve any importance, they have not been listed here (for details see refs. [1128]and [1136]).A third list by Huwkes et al. [1128]includes phases with retention properties that

309

9.3.Preferred Stationary Phases

differ substantially from those in Table 80. At the time (1975) they could not be recommended owing to lack of experience or thermal stability. They are given in Table 81. Owing to their outstanding selectivities, these phases have been recommended in the corresponding Sections in this book, with the exception of Nos. 12, 16 and 17, which should be replaced by phases of similar selectivity but with better thermal behaviour. Table 80. List of Preferred Liquid Stationary Phases No.

1

2 3

4 5 6

7 8 9 10

Stationary Phase (in Parentheses: Section in this book)

Poly(dimethylsi1oxane) (8.3.1.) Poly(methy1phenylsiloxane) (8.3.4.) 50%phenyl 75% phenyl Poly[cy anoalkyl(methyl)phenylsiloxane] (8.3.7.) 25% CN 50% CN 90-100% CN Poly(trifluoropropy1methylsiloxane) (8.3.6.) 50% CF, Poly(ethy1ene oxide) (8.4.4.1.) Poly(diethy1eneglycol succinate) (8.5.5) Poly(butanedio1 succinate) (8.5.5.)

Max. Hawkes Biskm Col. Commit- and Temp. tee Souter

Ben$

Hawka

111341

111351

X

x

x

X

x

x

X

x

x

kibmnd Commit- 111311 tee

DeuCy British and PharmaFriedrich copoeia I11321

111331

AOAC

PCl

111281

[11291

111301

350

X

X

X

X

X

X

X

X

X

X

X X

X

X

X

350 X

250270 X

X

X

250 X

250 x

X

X

X

X

X

200 x

200

Number of preferred phases

X X

5

6

7

4

4

4

3

5

Table 81. Special Liquid Stationary Phases (after Huwkes et al. [1128]) No.

Name (in parentheses: Trade Name)

Section in this book

11 12 13 14 15 16 17 18

Zinc stearate Trimer acid Fluoroalkyl camphorate (Zonyl E 91) Tetra(fluoroalkyl)-1,2,4,5-tetracarboxybenzene(Zonyl E 7) Octakis(2-hydroxypropy1)sucrose (Hyprose SP 80) Tri(ethylhexy1)phosphate Tri(butoxyethy1)phosphate N,N,N',N'-Tetrakis(2-hydroxypropy1)-ethylenediamine (Quadrol) N,N,N,N'-Tetrakis(2-hydroxyethy1)-ethylenediamine VHEED) Digylcerol Sodium dodecylbenzene sulphonate (Siponate DS-10) Di-2-ethylhexyl tetrachlorophthalate Phenyldiethanolamine succinate (HI-EFF-1OB)

8.13. 8.13. 8.12. 8.12. 8.4.3. 8.5.2. 8.5.2. 8.8.1. 8.8.1. 8.4.2. 8.11. 8.5.1. 8.5.5.

19

20 21 22 23

3 10

9. Selection of Stationary Phases

Not covered so far in our discussion are liquid stationary phases for wall coating of opentubular glass and fused-silica columns. Owing to the possibilities for cross-linking and bonding, all types of silicones predominate at present and are expected to continue to predominate in the future. The chemical nature of the polysiloxane backbone, uniquely meeting the requirements for the formation of uniform films with their high separation efficiency on the one hand, combined with the possibility of attaching most functional groups of widely differing selectivity on the other, offers the potential for synthesizing almost any desired selectivity in the future. We have seen that already today there are more than thirteen types of silicones that exhibit dBerent functional groups (Sections 8.3.1-8.3.13) and hence different selectivities (Sections 8.3.1 and 8.3.3-8.3.13) which can be modified even further in most types by adjusting the ratios of methyl groups to functional groups, which can range from ethyl (non-polar) through phenyl, trifluoropropyl, cyanoalkyl and pyrimidone groups to mesogenic and chiral groups, to mention only a few. The chances are that high efficiency and the required selectivity will in the future be considered jointly and tailormade columns will be prepared for many specifk and critical separations. This does not preclude each laboratory from having stock packings and WCOT columns impregnated with certain preferred liquid stationary phases. This will open up the following prospects for the performance of analyses: routine analyses with low demands can be easily performed; screening analyses of unknown mixtures can be carried out with low expenditure; the interchange of results between different laboratories is facilitated; and the elaboration of analytical specifications is rationalized. Naturally, the type of samples that generally have to be investigated in a particular laboratory mainly determines the stock liquid phases or corresponding WCOT columns. Nevertheless, the following stationary phases or corresponding WCOT columns, respectively, are to be preferred, (for reasons of interchangeability of results) and were compiled on the basis of Table 80 [1128-11351,the work of Haken I11371 and Bums and Hawkes [1138]and the experiences of the author (all phases of specialty grade): Section (1) Poly(dimethylsi1oxane) 8.3.1. 8.3.4. (2) Poly(methylphenylsi1oxane) (50% phenyl) (3) Poly(cyanoalkylsi1oxane) (50% cyanoalkyl) 8.3.7. 8.3.7. (4) Poly(cyanoalkylsi1oxane) (100% cyanoalkyl) (5) Poly(ethy1ene oxide) (M. W. = 20 000) 8.4.4.1. One or more of the following liquid stationary phases or WCOT columns, respectively, should, if necessary, additionally be included in the stock of a GC laboratory to cope with different sample types which may require the use of these phases or of their outstanding specific interactions (which have been indicated in the respective Sections in Chapter 8): (6) Poly(trifluoropropylmethylsi1oxane) (50%CFJ 8.3.6. 8.5.5. (7) Poly(diethy1ene glycol succinate) 8.6. (8) Tris(cyanoethoxypr0pane) (9) Sodium dodecylbenzene sulphonate 8.11. (10) Tetrahydroxyethylethylenediamine (THEED) or polyethyleneimine 8.8.1. (1 1) Zinc stearate 8.13. 8.12. (12) Highly fluorinated phase (13) Mesogenic phase 8.16. 8.15. (14) Chiral phase 8.2.1. or (15) Reference stationary phase 8.3.3. In addition, the following solid stationary phases or PLOT columns, respectively, are recommended for special separation problems dealt with in Chapters 5 and 6: (16) Graphitized thermal carbon black 5.2.1.

9.4. Approaches to Stationary Phase Selection

311

(17) Carbon molecular sieve

5.2.3. (18) Silica gel 5.4.1. (19) Porous polyaromatic beads without additional functional groups 5.5.1. (20) Polar porous polymers 5.5.2. (21) Zeolitic molecular sieves 5.6.1. (22) Chemically bonded stationary phases 6.2.3. In conclusion, it is emphasized once again that these selected stationary phases are not intended to restrict in any way the many possibilities that the numerous other stationary phases may offer for solving particular problems. Exhausting all the possibilities which the large number of more than 1000 available liquid stationary phases bring about is one factor to be considered. Another, however, is the fact that even now too many similar or almost identical phases are in use. This number could and should be reduced to a certain extent by eliminating closely similar phases. This could be done without any loss of any desired selectivity provided that only such phases are eliminated which behave chromatographically very similarly. We shall discuss this topic in the next section.

9.4.

Approaches to Stationary Phase Selection

The enormous number of stationary phases obviously requires some rationalization. An important approach to a systematic selection is the classification of stationary phases, which must precede the selection. We have dealt with this topic in detail in Section 4.2.There are various means of classifying liquid stationary phases and in this book we have subdivided them according to their chemical nature, each section within Chapter 8 representing a certain type of chemical compound. Generally, the phases within each section also exhibit similar liquid phase-solute interactions. However, many phases show distinct quantitative differences in selectivity caused, for example, by the concentration and/or type of terminal groups and by differences in the ratio of comonomers in a copolymer, in the concentrations of the respective functional group, in the type of backbone to which the functional group is attached, etc. To circumvent such effects, Rohrschneider [317, 322-3231 classified liquid stationary phases on the basis of their ability specifically to retain sample solutes by polar interactions with these solutes, which were selected so as to represent most of the organic compound types and the interactions to be expected. This system was further developed by McReynolds [324]who published Koviits retention index differences, AI, for five or ten sample solutes, also being representative of the different interactions of fimctional groups on each of 226 Iiquid stationary phases. Rohrschneider constants (for details see Section 4.2.4.) are given in Tables 82 and 83 and McReynolds constants of 304 liquid stationary phases in Table 84. Further classifications were developed by Keller et al. [1139]and Kurger et al. [1140],who used four test solutes, and by Snyder [1141],Klee et al. [1142],Shah et al. [1143]and Beta [1134],who selected only three test compounds which were believed to classify sufficiently the stationary phases in order to provide a systematic approach to their selection. Other approaches to gas chromatographic phase selection and classification were given by Semenchenko and Vigderguuz [1144],Sidorou [1145]and SevEik and Lowentap [358].Methods based on thermodynamic parameters were reported by Noutik et al. [333,341,3461,Risby et al. 13481 and Poole et al. [406a, 406b], the latter approach being of great promise. Classification schemes can also be developed from solubility parameters, as proposed by LsfJbrt and Putte [1146]and by Kurger et al. (11471.Patte et al. [1148]derived solubility factors for stationary phases from McReynolds data, and solubility parameters were published by Price et al. [1149].

312

9. Selection of Stationary Phases

Table 82. Phase Specific Rohrschneider Constants of Liquid Stationary Phases (except silicones) (after Supina and Rose [329]) Stationary Phase

Chemical Composition

X

Y

Alkaterge T Amin 220

Oxazole derivative

0.89

3.18 3.88

1.63

2.11 2.30 2.50 2.14

0.39

0.25

0.48 0.55

0.96 0.45

0.29 0.37

0.51 0.36 0.57 1.03

1.06

0.79

1.36 0.78

1.59 5.13

1.83 4.30

2.19 2.04

-

-

4.07 5.46 5.33 2.72 1.76

3.16 3.86 3.81 1.80 1.28

5.41 7.15 7.02 2.66 2.36

3.67 5.17 5.04 2.56 1.46

2.66 8.71 2.53 7.58 1.65

2.11 7.34 2.18 6.14 1.15

3.70 10.78 3.57 9.50 2.20

2.33 8.69 2.27 8.37 1.24

0.83 0.84 2.04

1.65 1.76 4.08

1.43 1.48 2.72

2.53 1.54 2.70 1.53 5.12 3.51

1.78 3.43 3.00 4.51 3.21

3.79 5.46 4.76 7.06 5.79

2.48 4.52 4.10 5.67 4.11

4.74 7.11 6.51 9.04 7.17

3.20 6.00 5.17 7.69 6.45

0.51 0.78 0.96 0.51 5.89

3.16 2.71 2.93 4.17 8.65

2.48 1.36 1.59 3.44 7.65

2.51 2.69 2.99 2.61 11.09

4.00 1.47 1.77 0.85 8.74

2.96 2.94

5.57 5.12

4.22 3.59

6.53 5.91 6.64 4.67

4.93

7.58

6.14

9.50 8.37

4.51

7.06

5.67

8.24 7.69

3,67 3.16

5,27 5.28

434 3.78

7.10 5.88 1.04 5.07

Apiezon L Armeen 2 HT b e e n2S Armeen SD Aroclor 1254 Bentone 34

Bis(2-methoxyethyl)-adipate Carbowax 4000 Carbowax 20M Castorwax Celanese Ester Nr.9 Citroflex A4 Cyanoethylsucrose Dibutyl phthalate Diethylene glycol succinate Di(2-ethylhexyl) sebacate (Octoil S) Diisodecyl phthalate Dinonyl phthalate Emulphor ON-870 Ethofat 60f25 Ethylene glycol adipate Ethylene glycol phthalate Ethylene glycol succinate

FFAP Fluorolube GR 362 Hallcomid M-18 Hallcomid M-18 OL Halocarbon K-352

1-Hydroxy-2-heptadecenylimid- 1.07 azoline 0.32 Hydrocarbons + polym. Methyl phenylether Amine? 0.24 sec. aliphatic Amine from Soy0.35 bean Oil prim. aliphatic Amiie from Soy- 0.44 bean Oil Chlorinated Di- and Polyphenyls 1.27 Montmorillonite treated with 2.41 Dimethyldioctadecylaoniumchloride 2.21 Polyethylene glycol 3.22 Polyethylene glycol 3.18 Cz8-Hydroxycarboxylicacid 1.05 Trimethylolpropane tripelargo0.83 nate Acetyltributyl citrate 1.36 5.40 1.30 4.85 0.65

Polyethylene glycol, end groups partially etherified with longchain alcohols Polyethylene glycol stearate

Carbowax ZOM/Z-Nitroterephthalic acid Polychlorotrifluoroethylene Dimethylstearylamide Dimethyloleylamide Polychlorotrifluoroethylene

1,2,3,4,5,6-Hexakis(2-cyanoethoxy)cyclohexane Hyprose SP 80 Igepal CO-990 LAC-2-R-446 LAC-4-R-886 MER-2 MER-2 1

Octakis(2-hydroxypropy1)sucrose Nonylphenoxypoly(ethy1ene oxy)ethanol Diethylene glycol adipate polyester Ethylene glycol succinate polyester Polyethylene dvcol Polyethylene glycol

z

S

U

313

9.4.Approaches to Stationary Phase Selection Stationary Phase

Chemical Composition

X

MER-35 Neopentylglycol succinate &P’-Oxydipropionitrile Phenyldiethanolamine succinate Polyethylene imine Polyphenylether 0s 124 (5 rings) Polypropylene glycol sebacate (Harflex 310) Quadrol Squalane STAF’ (Steroid Analysis Phase)

Y

2

U

S

1.56 2.64 2.68 4.88 5.88 8.48 3.61 6.24

2.26 3.22 3.10 3.81 3.81 5.21 8.14 12.58 9.19 4.10 1.24 6.32

2.91 1.15

1.12 2.21

2.34

6.35 4.19 3.26 2.84

1.93

3.38

2.58

4.36 3.21

N,N,N,N’-Tetrakis(2-hydroxypro-2.00

5.62

3.41

5.24 4.65

0 0 0.89 2,56

0 1,64

0 0 2,44 2,35

1.13

2.36

2.65

4.39 2.96

1.94 3.96

2.68

4.82 3.49

1.65

6.19

9.93 8.11

1.14 3.22 0,89 2.13

2.58 1.64

4.14 2.95 2.06 3.68

6.00 8.11

1.94

11.53 9.40

2.01 4.09 2.80 2,14 4.20 2.18

4.99 3.61 5.20 3.65

1.06 2.16 2.31 4.31

1.68 5.16

3.12 2.08 5.84 -

1.34 2.59

3.38

4.03 2.98

Bis(phenoxyphen0xy)-benzene

py1)ethylenediamine Hexamethyltetracosane Carbowax 20M,modified with succinic acid

Sucrose acetate isobutyrate

-

(SAW

Tergitol NPX Tetracyanopentaerythritol Tricresyl phosphate Trimer acid

Nonylphenoxypoly(ethy1ene0xy)ethanol

5.11 CS4-Tricarboxylicacid C,,-Dicarboxylic acid

+ 10%

1,2,3-Tris(2-cyanoethoxy)proPane Triton X 100 Tween 80 Ucon LB-550-X Zonyl E-I Zonyl E-91

Octylphenoxypolyethoxyethanol Polyoxyethylene sorbitolmonooleate Polypropylene glycol monoether Fluoroalkyl ester of pyromellitic acid Fluoroalkyl ester of camphoric acid

9. Selection of Stationary Phases

314

Table 83. Phase Specific Rohrschneider Constants of Poly(organosi1oxane) Stationary Phases (after Haken [330]) Stationary Phase

Chemical Composition

X

Y

2

U

5

100%Poly(dimethylsi1oxane) = 100%Methyl

0.16

0.20

0.50

0.85

0.48

100%Methyl 100%Methyl, 10%Aerosil (SiOz) as fier 30% Stearoyl 10%Phenyl 20% Phenyl 33%Phenyl 35%Phenyl

0.15 1.05

0.20 1.50

0.48 0.82 1.61 2.51

0.47 1.90

0.23 0.42 0.70 0.98 1.13

0.50 0.81 1.12 1.30 1.57

0.48 0.75 0.85 1.19 1.57 1.69

1.52 1.98 2.38 2.70

0.56 0.89 1.34 1.85 1.95

50%Phenyl

1.30

1.60

1.79

2.83

2.47

60%Phenyl 75%Phenyl 50%Phenyl 55%Phenyl 11%Chlorophenyl 11%p-Chlorophenyl

1.58 1.76 1.05 1.37 0.33 0.31

1.80 2.00 1.50 1.73 0.49 0.49

2.04 2.15 1.61 1.87 0.82 0.82

3.27 3.34 2.51 2.85 1.08 1.08

2.59 2.81 1.90 2.23 0.83 0.83

11%pChloropheny1

0.33

0.49

0.82

1.08

0.83

50%Trifluoropropyl

1.41 2.13

3.55

4.73

3.04

50%Trifluoropropyl

1.41

2.13

3.55

4.73

3.01

25%Cyanopropyl, 25%Phenyl Higher Cyanoethyl content

2.17 3.96

3.20 6.01

3.33 5.39

5.16

-

3.69 6.50

25 % Cyanoethyl

2.08

3.85

3.62

5.33

3.45

50%Cyanoethyl Carboaranesiloxane (Dimethylsiloxane units together with carborane)

3.18 0.43

5.33 0.64

3.81 7.02 1.11 1.51

5.04 1.01

ov 101 ov 1 SE-30 GC

OD-1 JXR DC 200 MS 200 Perkin Elmer Column C (DC 200) Perkin Elmer Column Z (SE 30) E 301 Hi Vac grease extract SF 96 Hi Vac Grease S 2116 OV-3 OV-7 OV-61 ov-11 OV-17 SP-2250 ASI-50 Methyl ov-22 OV-25 DC 710 SP 392 SP 400

DC 560 SP 2401 FS 1265 QF 1-10065 OV 225 XF 1125 XF 1150 Dexsil300 GC

Table 84. McReynolds-Constants of Important Liquid Stationary Phases (after McReynolds [257, 3241, H a k n [782, 7851, and [1163]*) Stationary Phase

Hydrocarbons Squalane Apiezon H Apiezon I Apiezon L Apiezon M Apiezon N Apiezon W Bitumen castorwax Convoil20 Hexatriacontane Nujol Oronite Polybutene 32 Oronite Polybutene 128 P a r a E i Oil SP 525 Squalene Halogenated Hydroclubons Aroclor 1254 Fluorolube HG 1200 Halocarbon 10-25 Halocarbon K-352

Halocarbon Wax Kel-F Wax

X (Benzene)

r

z

(1-Butanol) (2-Pentanone)

S

H

I

K

(F’yridine)

(2-Methylpentanol)

(I-Iodobutane)

(2-Octyne)

0 046 023 013 016 012 025 090 02 1 202 010 0 002 018 014 002 190 248

0 053 042 035 036 032 041 093 016 105 015 010 009 024 029 012

0 023 015 011 011 010 015 042 005 073 005 002 002 008 008 002

0 081 042 031 033 028 043 109 02 1 196 014

040 043 009

0 037 035 033 033 029 035 059 010 049 010 008 006 024 033 009

140

101

265

064

0

0

151 049 032 037 030 052

029 026 006 255 341

081 027 015 019 015 028 099 014 175 008 -003 002 024 025 002 253 238

02 1 229 017 001 006 042 041 007 368 329

0 129 057 042 047 040 058 154 047 246 02 1 011 011 040 042 013 320 344

127 05 1 047 047 055 055

068 070 070 071 067

114 108 073 116 114

144 133 238 143 143

204 118 111 146 123 116

070 120 070 073

017 016

055 057

044 045

067 066

043 043

032 033

0 059 038 032 035 031 038 082 019 108 014 012 009 021 025 011 225 152

0 086 030 022 028 022 040 135 058 265 014 002

U (1-Nitropropane)

005

155

M L (1.4(cisDioxane) Hydrindane)

005 006

pw-1

(X+ r + z

+ U + S)

0 506 201 143 166 138 216 625 159 1023 074 023 033 156 159 039 1421 1404

495 469 574 016 016

057 057

110 109

004 004

508

495

Silicones Methylsilicones AS1 100 Methyl Bayer M (Elastomer)

226 227

Table 84. (continued) Stationary Phase

r

X

U

Z

( B ~ I I Z ~ (~I -) B u t ~ o l ) (2-Penta-

none)

DC 200/MS 200 DC 330 (of very low viscosity) DC 400 (Elastomer) DC 401 (Elastomer) DC 410 (Elastomer) DC 430 (Elastomer, 1%vinyl) DC 11 (10%Si02-Filler) E 300 (Elastomer) E 301 (Elastomer) Embaphase Oil F 111 Gensil S 2116 (Stearoylsilicone) Hi Vac Grease extract (extracted silicone grease, liberated from Si02 filler) JXR (Elastomer) G45 (UcL-45) G46 (UC L-46) MS 2211 (Elastomer) OD-1 (Elastomer) ov-1 OV-1 (Elastomer) ov-101 Perkin-Elmer Column C @C 200) Perkin-Elmer Column 0 (10%Si02-Filler) Perkin-Elmer Column Z (Elastomer SA-30) PMS 100 SE-30(Elastomer) SE-30GC (Elastomer) SE-30Ultraphase (Elastomer)

016 013 015 017 018 016 017 015 016 014 016 023 016

H

I

K

(2-Methylpentanol)

(1-lodobutane)

( 2 - 0 ~ t y 1 ~ )(1.4(CisDioxane) Hydrin-

L

M

pwed (X+ Y + Z

+ U + S)

043 036 040 046 044 043 056 040 044 043 043 056 043

033 031 032 034 034 032 033 032 032 033 033

227 203 221 236 234 223 276 221 224 225 227

033

227

042 043 041 040 042 043 042 043 043 056

032 033 033 032 032 033 032 033 033 033

222 226 222 221 222 229 222 229 227 276

065

042

032

222

065 065 065 065

043 042 042 042

033 032 032 032

224 222 220 222

056 058 057 054 086 056 055 057 057

045 042 044 047 047 045 048 044 044 045 045

066 06 1 066 068 068 065 069 066 065 066 066

057

045

066

016 016 016 015 016 017 016 017 016 017

055 057 056 056 053 057 055 057 057 086

044 045 044 044 045 045 044 045 045 048

065 065 065 066 066 067 065 067 066 069

016

055

044

016 016 016 016

056 055 053 055

044 044 044 044

057 05 1

S

(1-Nitro- (Puridine) propane)

004

023

046

-002

a

Table 84. (continued) Stationary Phase

X (Benzene)

Y Z (1-Butanol) (I-Pentanone)

SF-96 SF-96-200 SF-96-2000 Silastic 401 (Elastomer) SP 2100 UC W 982 (O,lS%Vinyl, Elastomer) Viscasil Methylphenylsilicones AS1 50 Methyl (50%Phenyl) DC 510 (50%Phenyl) DC 550 (25%Phenyl) DC 556 (10%Phenyl) DC 702 (25%Phenyl) DC 703 (25%Phenyl) DC 710 (50%Phenyl) E 350 (5%Phenyl, Elastomer) E 351 (5%Phenyl, 1%Vinyl) OV-3 (10%Phenyl) OV-7 (20%Phenyl) OV-l1(35% Phenyl) OV-17 (50%Phenyl) OV-22 (60%Phenyl) OV-25 (75%Phenyl) OV-61 (33%Phenyl) SE-52 (5%Phenyl, Elastomer) SE-54 (5% Phenyl, 1%V i y l , Elastomer) SP 392 (55%Phenyl) SP 2250 (50%Phenyl) SR 119 (Harz) XE-61 (33%Phenyl)

U

S

H

I

K

(1-Nitropropane)

(Pyridine)

(2-Methylpentanol)

(1-Iodobutane)

(2-Octync)

L (1.4-

M

prm-1

(cis. (X+ Y + Z Dioxanc) Hydrin- + U + S) dane)

014 014 014 017 017 016 016

053 053 053 058 057 055 057

042 042 042 047 045 045 045

061 061 06 1 068 067 066 066

037 037 037 046 043 042 043

031 03 1 031 034 033 033 033

119 025 074 037 077 076 107 032 033 044 069 102 119 160 178 101 032 033

158 065 116 077 124 123 149 072 072 086 113 142 158 188 204 143 072 012

162 060 117 080 126 126 153 065 066 081 111 145 162 191 208 142 065 066

243 089 178 118 189 189 228 098 098 124 171 219 243 283 305 213 098 098

202 057 135 079 142 140 190 067 067 088 128 178 202 253 280 174 067 067

112 042 081 053 090 089 170 044 046 055 077 103 112 133 144 099 044 046

884 296 620 391 658 654 827 334 336 423 592 786 884 1075 1175 773 334 336

133 119 166 098

169 158 238

176 162 221

258 243 3 14

219 202 299 185

123 112 175

955 884 1238

004

023

046

-001

207 207 207 236 229 224 227

Table 84. (continued) Stationary Phase

X (Benzene)

Y 2 (1-Butanol) (2-Pentanone)

U

S

H

(1-Nitropropane)

(Pyridinc)

(2-Methylpentanol)

I (1-Iodobutane)

K

L

(2-Octyne)

(1.4(cisDioxane) Hydrh-

ps-'"'

M

(X+ Y + 2

+ U + S)

dane)

Halogenated Silicones AS1 50 Methyl (50%TrifluoroProPYl) DC 560 (11%p-Chlorophenyl) F-60 (ditto) F-61 (ditto) FS-1265 (50%T ~ u o ~ o pr~pyl)= QF-1 LSX-3-0295 (50%Trifluoropropyl) OV-210 (50%Tri!luoropropyl) = SP 2401 Nitrile Silicones AN-600 (25%Cyanwthyl) AS1 50 Methyl (25%Cyanopropyl, 25%Phenyl) AS1 50 Phenyl (50%Cyanopropyl) OV-105 (5%Cyanoethyl) OV-25 (25%Cyanop~~pyl, 25%Phenyl) OV 275 (100%Cyanoethyl)

silar 5 CP (50%Cyanopropyl, 50% Phenyl) = SP 2300 silar 7 CP (75%Cyanopropyl 25% = SP 2310 Silar 9 CP (90%Cyanopropyl, 10% Phenyl) = SP 2330 silar 10 c (100%cyanopropyl) = SP 2340 XE-60 (25%Cyanoethyl, Elastomer) XF-1125 (25%Cyanoethyl)

1520

146

238

358

468

310

206

032 032 032 144

072 072 072 233

070 070 070 355

100 100 100 463

068 068 068 305

049 049 049 203

024

035

069

007

136

053

280

059

342 342 342 1500

152 146

241 238

366 358

479 468

319 310

208 206

144 139

055 056

291 283

064 060

1557 1520

202 228

369 369

332 338

482 492

408 386

282

319 036 228

495 108 369

446 093 338

637 139 492

531 086 386

379 074 282

781 629 319 319 440

1006 872 495 495 638 638 725 725 755 757 381 381

885 763 446 446 605 605 631 630 659 659 340 340

1177 1106 637 637 844 844 913 913 942 942 493 493

1089 849 530 530 673 673 778 778 801 800 367 367

440

489 490 523 520 204 204

379 379 492 492 566 566 584 584 289 289

1793 1813 320

216

470

175

401

268

603

225

459

292

696

256

480

298

722

261

2428 462 1813 4938 4219 2427 2427 3200 3200 3536 3536 3680 3678 1785 1785

Table 84. (continued) Stationary Phase

XF-1150 (50%Cyanoethyl) Carboranesiloxane polymers Dexsil300 GC (Methyl) Dexsil400 GC (Phenyl) Dexsil400 GC (2-Cyanoethyl) Pentasil350 Ethylene glycol succinate Silicones ECNSS-M (Cyanoethylsilicone moieties) ECNSS-S (Cyanoethylsilicone moieties, Silicone portion lower) EGSP-A (Phenylsilicone moieties) EGSP-Z (Phenylsilicone moieties, Silicone portion higher) EGSS-X (Methylsilicone moieties) EGSS-Y (Methylsilicone moieties, Silicone portion higher)

W

z

X

Y

(Benzene)

(1-Butanol) (2-Pentanone)

U (1-Nitropropane)

S (Pyridme)

H

I

K

L

@-Methylpentanol)

(1-lodobutane)

(2-Octyne)

(1.4-

M (cis-

Dioxane) Hydrindane)

prmml

(X+Y + 2 + U + S)

308

520

470

669

528

037 047 072 071 016

078 080 107 286 003

113 103 118 174 121

154 148 168 249 131

117 096 123 171 162

42 1

690

581

803

732

548

438

659

566

820

722

530

397 308

629 474

519 399

727 548

700 549

496 373

279

278 220

469

167

2972 2278

484 391

710 597

585 493

831 693

778 66 1

566 469

412 335

316 261

713 591

237 190

3388 2835

401

2495 499 474 588 951 433

055

074

025 383

259

644

211

286

3205

Aminoalkylsilane and -siloxane Compounds (only restricted applicability as stationary phases) N-0-Aminoethyl-y-aminopropyl247 700 393 454 433 trimethoxysilane y-Amiiopropyltriethoxysilane 145 426 226 313 297 555 N-Allyl-y-aminopropyl323 653 441 593 trimethoxysilane y-Morphinylpropyltrimethoxysilane 072 539 129 511 469 N,N-Dimethyl-y-aminoethoxy- -008 143 076 069 082 methylpolysiloxane Diglycerol, Polyethylene glycols and Polyethylene glycol derivatives Carbowax 600 350 631 428 632 carbowax 1000 347 607 418 626

605 589

472 449

3227

2227 1407 2565 1720 362

308 306

240 240

503 493

162 161

2646 2587

Table 84. (continued) Stationary Phase

Carbowax 1500 Carbowax 1540 carbowax 4000 carbowax 4000 Carbowax 6000 Carbowax 6000, MER-21 Carbowax 20 M Carbowax 400 Monostearate Carbowax 20 M-TPA(Terephthalic acid end groups) Diglycerol Emulphor ON-870 (Polyethylene glycol octadecyl ether) Emulphor ON-870 (Polyethylene glycol octadecyl ether) Ethofat 60125 (Polyethylene glycol monostearate) FFAP (Carbowax 20M with 2-Nitroterephthalic acid end groups) Igepal CO-630 (Polyethylene glycol monotetramethylbutylphenylether) Igepal CO-710 Igepal CO-730 Igepal CO-880 (Nonylphenoxypoly(ethy1eneoxy)ethanol) Igepal CO-990 (Nonylphenoxypoly(ethy1eneoxy)ethanol) Lutensol (Nonylphenoxypoly(ethy1eneoxy)ethanol) Polytergent B-350 Polytergent G-300 Polytergent 5-300

z

X

Y

(Benzene)

(1-Butanol) (2-Pentanone)

U

S

H

(1-Nitropropane)

(Fyidme)

(2-Methyl- (1-Iodopentanol)

I

K

L

(2-Octyne)

(1.4(cisDioxane) HydrinU e )

butane)

M

Pa-

Y+ Z + U + S)

(X+

347 371 325 317 322 322 322 280 321

607 639 551 545 540 541 536 486 531

418 453 375 378 369 370 368 325 367

626 666 582 578 577 575 572 512 573

589 641 520 521 512 512 5 10 449 520

449 479 399 400 390 392 387 350 387

325 285

255 224

534 443

172 148

282 283 282 244 281

222 222 221 191 220

437 438 434 382 435

147 149 148 122 148

2587 2770 2353 2339 2320 2320 2308 2052 2318

371 202

826 395

560 251

676 395

854 344

608 282

245 179

141 140

724 289

036 080

3287 1587

202

396

251

395

345

283

179

139

289

080

1589

191

382

244

380

333

277

168

131

279

073

1530

340

580

397

602

627

423

298

228

473

161

2546

192

381

253

382

344

277

172

136

288

078

1552

205 224 259

397 418 461

266 279 311

401 428 482

361 379 426

289 302 334

183 198 227

144 157 180

303 321 362

085 095 112

1630 1728 1939

298

508

345

540

475

366

261

205

406

133

2166

232

42 5

293

43 8

386

315

202 203 168

392 398 366

260 267 221

395 40 1 350

353 360 308

284 290 266

1774 180 180 149

142 145 119

297 303 255

084 083 061

1602 1629 1419

Table 84. (continued) Stationary Phase

X (Benzene)

Y 2 (I-Butanol) (2-Pentanone)

U

S

H

L

M

pgcncra1

pyridine)

(2-Methylpentanol)

I (1-Iodobutane)

K

(]-Nitropropane)

(2-Octyne)

(1.4-

(cis-

( X +Y + 2 + U + S)

159 198

127 156

Dioxane) Hydrindane)

180 223

375 417

234 278

366 427

317 381

270 301

332

555

393

583

546

400

345 261

586 462

400 313

610 484

627 427

428 334

301 228

235 180

484 364

163 114

2568 1947

197

386

258

389

351

281

176

139

293

081

1581

203

399

268

402

362

2 90

181

145

304

083

1634

262

467

314

488

430

336

229

183

366

113

1961

227

430

283

438

396

310

131 128

314 294

185 173

277 264

243 226

214 196

110 106

101 098

205 194

046 045

1150 1085

Poly(oxyethyleneoxypropy1ene)Products Pluracol P-2010 129 Pluronic F-68 264 Pluronic F-88 262 Pluronic L-35 206 Pluronic L-81 144 Pluronic P-65 203 Pluronic P-85 201 POlyglyCOll5-200 207 Ucon 50-HB-280 X 177

295 465 461 406 314 394 390 410 362

194 309 306 257 187 251 247 262 227

266 488 483 398 289 393 388 401 351

227 423 419 349 249 340 335 354 302

197 331 327 286 211 276 271 289 252

106 229 227 177 120 174 172 179 151

099 184 183 148 108 146 145 150 130

195 363 359 296 212 289 285 301 256

046 115 114 085 055 083 082 086 065

1091 1949 1931 1616 1183 1581 1561 1634 1419

Polytergent J-400 Renex 678 (Nonylphenoxypoly(ethy1eneoxy)ethanol) SP 1000 (Polyethylene glycol with 2-Nitroterephthalic acid end PUPS) S T M (modified Carbowax 20M) Surfonic N-300 (Nonylphenoxypoly(ethy1eneoxy)ethanol) Tergitol NPX 728 (Nonylphenoxypoly(ethy1eneoxy)ethanol) Triton X-100 (Polyethylene glycolmono(tetramethylbuty1)phenylether) W.6001 Triton X-305 (Polyethylene glycol-

265 321

068 095

1472 1726 2409

w.

mono(tetramethylbuty1)phenyl-

w.

ether) W.15001 Tween 80 (Polyoxyethylenesorbitanmonostearate) Polyprcipylene glycols PPG 1000 (Thanol) PPG 2000 (Jefferson)

1774

Table 84. (continued) Stationary Phase

Ucon 50-HB-660 Ucon 50-HB-2000 Ucon 50-HB-3520 Ucon 50-ED-5100 Ucon 50-HB-1800 X Ucon 75-H-90 000 Ucon LB-550 X Ucon LB-1715

X

Y

(Benzene)

(I-Butanol) (2-Pentanone)

Z

U

S

(1-Nitro-

(mdine)

propane)

H (2-Methylpentanol)

I

K

L

(I-lodobutane)

(2-Octyne)

(1.4-

(cisDioxane) Hydrindanc)

(X+ Y+ 2 + U + S)

P-d

M

193 202 198 214 123 255 118 132

380 394 381 418 275 452 271 297

241 253 241 278 161 299 158 180

376 392 379 42 1 249 470 243 175

321 341 323 375 212 406 206 235

265 277 264 301 179 32 1 177 201

166 173 169 185 101 220 096 109

141 147 144 155 095 180 091 100

274 289 278 316 181 348 177 199

075 080 080 086 045 110 040 046

1511 1582 1522 1706 1020 1882 996 1119

176

227

224

306

283

177

169

135

266

103

1216

182

233

228

313

293

181

176

136

273

112

1249

257 232

355 350

348 398

433 413

137 151 157 076

278 282 292 181

198 227 233 121

300 338 348 197

235 267 272 134

216 217 272 144

118 138 143 071

104 112 117 055

205 225 233 119

028 048 050 009

1148 1265 1302 709

092 072 072 112 109 214 151 233 097 130

186 168 167 150 132 375 306 408 194

150 108 107 123 113 305 211 317 157

236 180 179 168 171 446 320 470 246

167 125 123 181 168 364 274 389 174 227

143 132 132 110 104 290 238 309 149

092 068 068

066 049 049

140 107 106

026 011 011

831 653

075 190 129 207 096

045 159 110 170 069

137 312 224 337 147

034 079 036 092 027

Ethers 0 s 124 Bis(phenoxyphenoxybenzene)] 0 s 138 Bis(phen0xyphenoxy)phenyl ether] PPE-20 (Poly-m-phenylether) PPE-21 (Poly-m-phenylether of higher molecular weight)

Esters of Mcarboxylic Acids Bis(2-butoxyethyl)adipate Bis(2-butoxyethyl)phthalate Bis(2-ethylhexyl) adipate (Flexoi A-26) Bis(2-ethylhexyl)phthalate Bis(2-ethylhexyl) sebacate ditto (Octoil S) Bis(2-ethylhexyl) tetrachlorophthalate Bis(2-ethoxyethyl)phthalate Bis(2-ethoxyethyl)sebacate Bis(ethoxyethoxyethy1) phthalate Butyloctyl phthalate Dibutyl phthalate

270 350

734 693 1704 1262 1817 868

Table 84. (continued) Stationary Phase

Dicyclohexyl phthalate Didecyl phthalate Didodecyl phthalate Diisodecyl adipate Diisodecyl phthalate Diisononyl adipate Diisooctyl adipate Dikooctyl phthalate Dinonyl phthalate Dinonyl sebacate Dioctyl phthalate Dioctyl sebacate Ditridecyl phthalate Octyldecyl adipate

X (Benzene)

Z

Y

(1-Butanol) (2-Pentanone)

M

pamud

U

S

H

L

(Pyridine)

(2-Methylpentanol)

I (1-lodobutane)

K

(1-Nitropropane)

(2-Octyne)

(1.4-

(cisDioxane) Hydrindane)

( X CY + Z + U + S)

146 136 079 07 1 084 073 078 094 083 066 092 072 075 079

257 255 158 171 173 174 187 193 183 166 186 168 156 179

206 213 120 113 137 116 126 154 147 107 150 108 122 119

316 320 192 185 218 189 204 243 231 178 236 180 195 193

245 235 158 128 155 129 140 174 159 118 167 123 140 134

1% 201 120 134 133 137 148 149 141 130 143 132 119 141

144 126 079 067 083 068 072 092 082 062 092 068 076 072

104 101 052 052 059 054 159 069 065 050 066 049 05 1 057

204 202 116 114 130 116 126 147 138 106 140 106 115 119

058 038 026 011 024 010 008 024 018 008 025 010 025 010

1170 1159 707 668 767 681 735 858 803 635 831 651 688 704

135 108 043 056 041 084

268 282 110 135 109 182

202 186 061 083 065 122

314 235 088 136 112 197

233 289 122 097 071 143

214 220 086 102 085 143

112 106 041 049 037 077

102 074 024 040 029 055

207 209 073 081 06 1 127

026 048 018 005 -001 018

1152 1100 424 507 398 728

136 199 064 136

351 193 257

285 106 182

413 143 285

233 336 191 227

266 147 202

190 057 130

153 041 086

292 121 194

088 020 052

1584 697 1087

172

330

251

378

295

264

147

128

276

054

1426

344 099

570 569

46 1 320

67 1 344

569 388

457 466

292 114

251 061

546 437

152 063

2615 1720

Other Esters, Acids, Salts Acetyltributyl citrate Atpet 200 (sorbitan monooleate) Beeswax Butoxyethyl stearate Butyl stearate Celanese Ester No.9 (trimethylolpropane tripelargonate) Citroflex A4 (Acetyl tributyl citrate) Cresyldiphenylphosphate Diethylene glycol distearate Estynox (Acetoxy derivative of Butyl oleate) Saccharose acetate isobutyrate (SAW Saccharose octaacetate Siponate DS-10(Dodecyl benzenesulphonate)

Table 84. (continued) Stationaty Phase

X (Benzene)

Y

U

Z

(1-Butanol) (2-Pentn-

none)

Sorbitolhexaacetate SP 1200 Span 60 (Sorbitol monostearate) Span 80 (Sorbitol monooleate) Stepan DS-60 Tributoxyethyl phosphate Triethylhexylphosphate Tricresyl phosphate Trimer acid (CS 4 Tricarboxylic Acid) Zinc Stearate Zonyl E-7 (fluorinated Ester of Pyromellitic Acid) Zonyl E-91 (fluorinated Ester of Camphoric Acid)

(1-Nitropropane)

S (pvridinc)

H

I

(2-Methyl- (1-Iodopentanol)

butane)

K

L

(2-Octyne)

(1.4(CisDioxane) Hydrin-

M

P-=d

(X+ Y +

335 067 088 097 097 141 071 176 094

553 170 263 266 550 373 288 321 271

449 103 158 170 303 209 117 250 163

652 203 200 216 338 341 215 374 182

543 166 258 268 402 274 132 299 378

446 166 201 207 440 285 225 242 234

273 145 082 094 111 126 07 1 169 094

247

521

131

055 066 060 104 047 131 057

180 191 418 204 103 254 216

037 041 061 031 007 076 060

2532 709 967 1017 1690 1338 823 1420 1088

061 223

23 1 359

059 468

098 549

544 465

098 338

050 146

029 137

078 469

033 062

993 2064

130

250

320

377

293

235

08 1

095

295

010

1370

370 369 269 271

571 591 446 444

448 457 328 330

657 661 493 498

611 629 48 1 463

457 476 351 346

324 325 248 252

242 243 176 175

533 544 394 396

178 177 124 127

2657 2707 2017 2006

378

603

460

665

658

479

329

254

554

176

2764

377

601

458

663

655

417

328

253

55 1

177

2754

387

616

471

679

667

489

339

257

567

186

2820

470

705

558

788

779

556

393

301

617

215

3300

492

733

581

833

791

579

418

321

705

237

3430

Polyesters Butanediol succinate Butanediol succinate HI-EFF-4 BP Cyclohexanedimethanol succinate Cyclohexanedimethanol succinate HI-EFF 8 BP Diethylene glycol adipate (DEGA), HI-EFF 1AP Diethylene glycol adipate (DEGA) LAC-IR-296 Diethylene glycol adipate with additive Pentaerythritol (DEGA-P) LAC-2R-446 Diethylene glycol succinate (DEW Diethylene glycol succinate @EW

Z

+ U + S)

Table 84. (continued) Stationary Phase

z

X

Y

(Benzene)

(1-Butanol) (2-Pentanone)

U

S

H

(1-Nitropropane)

(F’yridine)

(2-Methylpentanol)

I (1-lodobutane)

K

L

(2-Octyne)

(1.4-

M

pmml

(cisDioxane) Hydrin-

(X

+ Y+ 2

+ U + S)

dane)

Diethylene glycol succinate (DEGS-PS) Diethylene glycol succinate Diethylene glycol succinate, HIEFF-1 BP Epon 1001 (Epoxide Resin) Ethylene glycol adipate (EGA) Ethylene glycol adipate (EGA) Ethylene glycol adipate (EGA), HIEFF2AP Ethylene glycol isophthalate (EGIP), HI-EFF 2 EP Ethylene glycol o-phthalate (EGP), HI-EFF 2 GP Ethylene glycol succinate (EGS) Ethylene glycol succinate (EGS), HI-EFF 2 BP Ethylene glycol tetrachlorophthalate (EGTCP) Harflex 370 (Propylene glycol sebacate) MER-2 Neopentyl glycol adipate (NPGA) Neopentyl glycol adipate (NPGA), HI-EFF 3 AP Neopentyl glycol isophthalate (NPGIP) Neopentyl glycol sebacate (NPGSb), HI-EFF 3 CP Neopentyl glycol succinate (NPGS) Neopentyl glycol succinate (NPGS) Neopentyl glycol succinate HIEFF 3 BP

496

746

5 90

837

835

594

420

325

718

238

3504

502 499

755 751

597 593

849 840

852 860

599 595

421 422

329 323

726 725

234 240

3555 3543

284 371 372 372

489 579 577 576

406 454 455 453

539 655 658 655

601 633 619 617

378 466 463 462

291 323 325 325

207 248 250 250

502 550 548 546

187 175 177 177

2317 2692 2681 2613

326

508

425

607

561

400

299

213

498

168

2427

453

697

602

816

872

560

419

306

699

260

3440

536 531

775 187

636 643

897 903

864 889

622 633

450 452

347 348

783 795

259 259

3708 3759

307

345

318

428

466

265

327

193 381 234 232

1864

539 425 42 1

456 312 311

646 462 461

615 438 424

42 1 339 335

337 210 208

262 157 156

566 362 357

197 103 103

2637 1871 1849

371

207 172

327

225

344

326

257

156

109

251

073

1394

275 272 272

472 467 469

367 365 366

543 539 539

489 472 474

374 371 371

245 243 243

186 184 184

423 419 419

127 124 124

2146 2115 2120

Table 84 (continued) Stationary Phase

Paraplex G-25 (modified Alkyd) Paraplex G-40 (modified Alkyd) Phenyldiethanolamine succinate (PDEAS), HI-EFF 10 BP Propylene glycol adipate (PGA), Reoplex 400 Propylene glycol sebacate (PGSb)

X

r

(Benzene)

(1-Butanol) (2-Pentanone)

z

U (1-Nitropropane)

S (Pyridine)

H

K

L

M

(2-Octyne)

(1.4-

(cis-

pentanol)

I (1-Iodobutane)

(2-Methyl-

Pa(X+ Y + Z

Dioxanc) Hydrindant)

+ U+S)

189 282 386

328 459 555

239 355 472

368 528 674

312 457 654

257 364 437

169 247 362

124 193 242

271 414 562

079 125 213

1436 2081 2741

364

619

449

647

671

482

317

245

540

171

2750

196

345

251

381

328

271

176

129

285

083

1501

991 919 825

a53 797 713

1110 1043 978

lo00

4644 4382 3984

Tetracyanoethylpentaerythritol Tetrakis(cyanoeth0xy)butane

690 647 567 588 632 526 617

875 782 860

733 677 773

(Cym0-B) Tris(cymoethoxy)propane

593

857

Amines, N-Heterocyclics Alkaterge T (substituted Oxazoline) Amine 220 (substituted

89 117

380

Nitriles

Bis(cyanoethy1)fonnamide Cyanoethylsaccharose Hexakis(cyanoeth0xy) hexane Oxydipropionitrile SP-216-PS, SP-222-PS

Imidazoline) h e e n 2 HT Armeen 2-S (aliphatic Amine from Soybean Oil) Armeen SD (aliphatic Amine from Soybean Oil) Bentone 34 (Dimethyl dioctadecylammonium derivative of Montmorillonite) Ethomeen 18/25 (Polyoxyethylene Soybean Amine)

773 713 620

557 544

371 388

964 917

279 299

lo00 920 1048

976 901 919 680 837 941

621 685

444

333

766

237

752

1028

915

672

503

375

853

181

293

230 133

274

24 35

36 103

44

78

3920 3742 4239 4145

1104

241 176

382

230

353

323

275

158

118

265

072

1464

Table 84 (continued) Stationary Phase

X

(Benzene)

r Z (1-Butanol) (2-Pentanone)

Ethomeen S/25 (Polyoxyethylene Soybean Amine) Polyethyleneimine Polypropyleneimine Tetrakis(hydroxyethy1)ethylenediamine (THEED) Tetrakis(hydroxypropy1)ethylenediamine (Quadrol)

Amides Flexol8N8 (Bis[(ethylhexanoyloxy)ethyl]ethylhexaneamide} Hallcomid M-18 (Dimethylstearamide) Hallwmid M-18 OL (Dimethyloley lamide) Poly-A 101 A (Polyamide) Poly-A 103 Poly-A 135 Poly-I 110 Versamid 930 (linear Polyamide) Versamid 940

U (1-Nitro-

S (F’yridine)

propane)

H I (2-Methyl- (I-lodopentanol) butane)

K

L

(2-0ctyne)

(1.4-

M

prcnd

(cisDioxane) Hydrindane)

(X+ Y + Z + U+S)

186

395

242

370

339

285

169

127

279

079

1532

322 122 463

800 425 942

168 626

573 263 801

524 224 893

585 270 746

427

269

72 1

254

1202 3725

214

571

357

472

489

431

208

142

379

111

2103

096

254

164

260

179

197

098

064

147

023

953

079

268

130

222

146

202

082

048

106

016

845

089

280

143

239

165

211

093

058

211

021

916

115 115 163 115 108 109 109

357 331 389 194 309 313 314

151 149 168 122 137 144 145

262 263 340 204 208 211 212

214 214 269 202 207 209 209

233 221 282 152 222 225 225

110 112 112

055 057 057 057

148 150 150

077 079 078

1099 1072 1329 837 969 986 989

140 121 093 281

255 284 210 423

209 169 140 297

318 259 224 509

239 217 162 360

198 191 166 326

134 100 090 223

103 095 065 183

202 186 146 294

047 039 020 078

1161 1050 829 1870

112 162 185 222

234 200 370 265

168 178 242 230

261 268 370 469

194 256 327 612

187 140 267 229

102

077

176

027

969

165 197

130 161

275 285

075 115

1064 1798

064 062

Other Phases Elastex 50-B Flexol B-400 Flexol GPE Fluorad FC-431 (surface-active fluoroorganic compound) Hercoflex 600 MER-35 OroniteNI-W Poly(ester-acetal) (cross-linked)

Table 84 (continued) Stationary Phase

X

Y

(Benzene)

(1-Butanol) (2-Pmtanone)

U

2

S cpyridine)

(1-Nitro-

propane)

If I (2-Methyl- (1-Iodopentanol) butane)

L K M (cis(2-Octyne) (1.4Dioune) Hydrin-

P-d

(X+ Y + 2

+U+S)

h e )

Triton X-200 Triton X-400

117 068

289 334

172 097

266 176

237 131

180 218

105

081 036

192 095

048 023

1081 806

3 In m e instances two or even more sets of McReynoldaconstants of the 'same" stationm phase an given. They originate from ditferrnt experimentalrrsults,possibly caused by batch-tobatch dflmnces of the phsscs concerned.

(Am

Table 85. Infrared Spectroscopie Frequency Shifts of the Stretching Vibration of Solute Functional Groups under the Influence of Liquid Phases (&r Echig, Kriegsmann. Rotzsche and Kkinert [362, 362% 3641) Stationmy Phase

Functional Q m u p s of the gtntionmy phase (simplitid)

A*Acetone

Squalane ov-101 OV-17 ov-25 OV-61 OV-225')

ov-210 OE 4178 Dioctylphthalate Tricresylphosphate Ethylene glycol bis(propionitrile) ether Triethanolamine

A~c-a

Methylethylketone

Ropion- BumMethylaldehyde aldehyde acetate

0

CH2 Si-CH, 50% Si-Ph 75% Si-Ph 33% Si-Ph 25% Si(CHJ3CN 25% Si-Ph 50% Si-(CHJ2CF3 35% Si(CHJ3CN CsHSCOOC CH,--CsH,&P--O CN, C - 0 - C

10 15

9 14

12

12

174

71

12

11

-

-

296

9

-

18

') The frequency shifts caused by both nitrile and phenyl groups are listed.

-

-

7

-

0 0

9

6 6 6 9 6 6,s 9

-

-

0 0

Methanol

14

3 7

7 7 7 10 7 7,5 11

0 0

AVOH

Butylchloride

7 10

2

-

2 2 2 7

13

Ropylchloride

0 0 40 40 40 111 40 16 111 83 140 240

0

0

0

0 0

Ethylacetate

-

6

8 8

-

Ethanol

0 0

-

100 70 140 245 300

329

9.4.Approaches to Stationary Phase Selection

Classification based on spectroscopic measurements was proposed by Ecknig et al. [362, 362a, 3641 and Chong et al. [1150](see also Section 4.2.8). The infrared spectroscopic frequency shifts of the functional groups

>C=O,

-!!--OH

and L - C l

I

I

under the influence of some liquid stationary phases are listed in Table 85. Based on relative retentions and on refractive indices of stationary phases and of test solutes, stationary phases were classified according to indices of dispersion, polarity, basicity and acidity by Burns and Huwkes [365](see also Section 4.2.9). Such indices of 26 phases are listed in Table 86. Several mathematical procedures have been reported for the reduction of the large number of stationary phases. Leury et al. [1151]and Huken et al. [I1521proposed the so-called "nearest neighbour technique", and Mussurt et al. [1153]developed a "numerical taxonomy approach" using McReynolds' compilation to obtain 30 groups within which the liquid phases resemble each other. Factor analysis was applied by Weiner and Purcher [1154],and the ten-dimensional data from McReynolds were reduced to two dimensions by projecting along the two primary eigenTable 86. Indices of Dispersion, Polarity, Basicity and Acidity (after B u m and Hawkes [365])**) Stationary Phase

Squalane Poly(dimethylsi1oxane) Apiezon Grease

Poly(methylphenylsi1oxane) Polyphenyl ether

Di(3,5,5-trimethylhexyphthalate l) Poly(methyltrifluoropropylsi1oxane) Poly(propy1ene glycol) Poly(ethy1eneglycol) Poly(methylcyanopropylsi1oxane) Poly(diethy1eneglycol succinate) Poly(cyclohexanedimethano1 succinate) Poly(butanedio1succinate) Poly(phenyldiethano1aminesuccinate) Di(2-ethylhexyltetrachlorophthalate) 1,2,3-Tris(2-cyanoethoxy)propane N,N-Bis(2-cyanoethy1)formamide Zinc stearate Trimer acid, CJ1Hloo(COOH)3 Fluoroalkyl Camphorate (Zonyl E-91) Tetrafluoroalkyltetracarboxybenzene(Zonyl E-7) Octakis(2-hydroxypropy1)sucrose Tetrakis(hydroxypropy1)ethylenediamine (Quadrol) Tetrakis(hydroxyethy1)ethylenediamine (THEED) Sodium dodecylbenzenesulphonate Diglycerol *) **)

Values by analogy For the interpretation of the indices see section 4.2.9.

Dispersion Index d

Polarity Index

Basicity Index

P

H

0 0 0 0 0 1 0 3 4 3*) 3

3*I 2 *I 1 3*I 5 *I

4? 6 0 0

Acidity Index AC

Section inthis book

0 0 0 1 0 0 1 0 0

8.2.1 8.3.1 8.2.1 8.3.4 8.4.5 8.5.1 8.3.6 8.4.4.2 8.4.4.1 8.3.7 8.5.5 8.5.5 8.5.5 8.5.5 8.5.1 8.6 8.6 8.13 8.13 8.12 8.12 8.4.3 8.8.1

1 1

1 0 0 0 6 3 1

I I

1 2 2

?

2

9*I 9

4

-

8.8.1 8.11 8.4.2

330

9. Selection of Stationary Phases

vectors by Lowry et al. [1155],hence condensing McReynolds’ 226 phases to 7 clusters of phases with roughly similar polarity. Thirty-five phases were not included in any of the arbitrarily drawn clusters (outliers). Statistical and pattern recognition techniques, also based on McReynolds’ data, were applied by Wold and Anderson [1156],Lowry et al. [1157],Howery et al. [1158],McCZoskey and Hawkes [1159],Wold [1160]and Hu6er and Reich [llal]. In a similar approach, Kliiden, Schlapa and Rotzsche [1162]analysed a data matrix consisting of the first five McReynolds constants (X,Y, Z, U and s)of 280 liquid stationary phases by a cluster analysis method. The critical distance was chosen to be 25.It was found, that the restriction to the first five constants, which have been reported more frequently, especially for newly developed phases, in lieu of the complete set of ten constants ( X - M ) , is justifiable and that involving all ten constants would improve the result only negligibly. The results are presented in Table 87. The clusters are arranged in the order of increasing overall polarity. Similar phases are condensed in 36 clusters, and the average value of each cluster is given fmt. The phases within each cluster show such a resemblence that they are interchangeable, with respect to the selectivity, with each other without sacrificing selective preferences, and that it therefore suffices, as a rule, to use only one phase from a cluster. Seventy-two phases could not be included in any cluster because of a greater distance between one or more constants and the corresponding average value than defined (25 units). These outliers are listed subsequently after the 36 clusters in Table 87, also in order of increasing overall polarity. Those of them which can be recommended, owing to their unique selectivities, provided that their other gas chromatographic properties permit a recommendation, are marked with an asterisk. Such noticeable selectivity differences of these outliers, compared with the classifiable phases, are discernible by one or several outstanding McReynolds constants. By this approach, the 280 phases evaluated can be reduced to 50 phases without a selectivity loss. It should be emphasized that, when a phase is selected from a cluster or from the set of the outliers, the specialty grade type should be used, which can be taken from the respective group number (last column in Table 87) dealt with in Chapter 8. These results are intended to eliminate especially odd, obsolete and dubious phases, and the method is intended to stimulate fitrther investigations on the reduction of the large number of stationary phases to a justifiable minimum. We have seen in Section 4.2.7.that the selectivity index proposed by Evans, Haken and 7‘0th [691]reveals the contribution of polar interactions, moderated by steric factors, and may be expected to provide a means for the classification of stationary phases that is independent of a reference phase such as squalane. In Table 88 some stationary phases are characterized by means of this selectivity index. It is hoped that the dispersion index and selectivity index (see Section 4.2.7) will be of value for the study of molecular structure-retention relationships, for the prediction of retention data and for the characterization of stationary phases, as indicated by the authors. In future, however, the direct utilization of thermodynamic data (free standard enthalpies) for the characterization and classification of stationary phases is assumed to come to the fore (see Sections 4.25,4.2.9.,8.1.6.). A promising approach in this direction was developed by Poole et al. [406a,406bl. Concluding this chapter and concluding this book, it is hoped that it has been shown distinctly enough that it is necessary to use only specially and reproducibly manufactured stationary phases and columns in order to rationalize the large number of phases while utilizing fully the potential of selectivity differences, and to develop new phases, provided that they *) Note the asterisk of Table 84.

331

9.4. ADDroaches to Stationarv Phase Selection

exhibit unique selectivities and/or better physical properties and hence give reproducible and solute-characteristic retentions that would help in solving difficult identification problems. In addition to Table 84*), in Tables 89-93,287 liquid stationary phases are arranged [1163] in order of increasing McReynolds constants X, Y,2, U and S. In Table 89, X values are listed [solute = benzene, selected for induction forces (acceptor) and donor-acceptor interactions and intended to characterize aromatics and olefins]. Table 90 lists stationary phases in order of increasing Y values [solute = 1-butanol, selected for hydrogen bonds (H-donor) and intended to characterize alcohols, acids and other compounds with hydroxy groups]. Table 87. Liquid Stationary Phases, Condensed in Clusters of Similar Selectivities (after Kloden, Schlapo and Rorzsche [1162]) Cluster No.

1

2

Average value of McReynolds constant

Stationary Phase (wmmercial name)

Section in this book

X9.20 Y 5.40 Z 1.80 U 6.20 s 11.20 X 18.08 Y 51.48 Z 41.13 U 61.75 S 42.63

Squalane Convoil20 Hexatriacontane Nujol Paraffin Oil Apiezon I Apiezon L Apiezon M Apiezon N Oronite Polybutene 32 Oronite Polybutene 128

8.2.1 8.2.1 8.2.1 8.2.1 8.2.1 8.2.1 8.2.1 8.2.1 8.2.1 8.2.1 8.2.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1 8.3.1

ov-1

OV-1 elastomer ov-101 VC W 982 AS I 100 Methyl Bayer M (Elastomer) DC-200/MS-200 DC-330 DC-400 (Elastomer) DC-401 (Elastomer) DC-410 DC-430 (Elastomer) E-300 E-301 (Elastomer) Embaphase Oil F 111 Silicone grease (extracted) JXR (Elastomer) L-45 (uc L-45) L-46 (VC L-46) M S-2211 (Elastomer) OD-1 (Elastomer) Perkin Elmer Z PMS-100 SE-30 (Elastomer) SE-30 GC SE-30 Ultraphase SF-96 SF-96-200

332

9. Selection of Stationary Phases

Table 87 (continued) Cluster No.

Average value of McReynolds cono1pnt

X 28.50 Y 74.10 Z 62.80 V 91.90 S 64.10

StationaryPhase (commemial name)

Section in this book

SF-96-2000 Silastic 401 SP 2100 Viscasil DC 560 DC 11 Perkin Elmer 0 DC 510 E 350 E 351 F 60 F 61 DC 556 OV-3

8.3.1 8.3.1 8.3.1 8.3.1 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4

Halocarbon Wax Kel-F Wax Fluorolube HG 1200 Halocarbon 10-25 Dexsil300 GC Apiezon W DC 550 DC 102 DC 703 OV-7 Dexsil400 GC (Phenyl) Bis-Adipate (Flexol) Bis-Sebacate Octoil S Didodecyl phthalate Diisodecyl adipate Diisodecyl phthalate Diisononyl adipate Diisooctyl adipate Dinonyl sebacate Dioctyl sebacate Ditridecyl phthalate Octyldecyl adipate Celanese Loter No.9 Bis-2-ethylhexyl phthalate Butyloctyl phthalate Diisooctyl phthalate Dinonyl phthalate Dioctyl phthalate Flexol GPE Triethylhexyl phosphate Hallamid M-18 Hallcomid M-18 OL

8.12 8.12 8.12 8.12 8.3.5 8.2.1 8.3.4 8.3.4 8.3.4 8.3.4 8.3.5 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.3 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.1 8.5.2 8.9 8.9

SE-52 SE-54 X 40.50 Y81.50 2 80.50 v 121.00 S 83.50 X 49.00 Y 70.80 Z 113.00 V 143.40 S 117.00 X 75.00 Y 119.67 Z 116.17 V 175.00 S 137.00

X 75.46 Y 171.54 Z 117.38

U 191.31 S 134.62

8

X91.83 Y 192.00 Z 149.67 V 236.00 S 167.17

9

X 79.67 Y 278.67 2 130.00 V 225.33 S 141.67

333

9.4. Approaches to Stationary Phase Selection Table 87 (continued) Cluster No.

10

Average value of McReynolds constant

Stationaty Phase (commercial name)

Section in this book

X 116.50 Y 155.67

AS1 50 (50%Phenyl) DC 710 ov-11 OV-17 SP 392 SP 2250 Flexol8N8 Hercoflex

8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.3.4 8.9 8.9

Versamid 930 Versamid 940 Polyamide (linear)

8.9 8.9 8.9

Castorwax Atpet 200 Span 60 Span 80

8.5.3 8.5.3 8.5.3 8.5.3

ov-22 Mer-35

8.3.4

PPG 1000 PPG 2000 Pluracol P-2010 Pluronic L-81 Ucon 50-HB-1800 X Ucon LB-550 X Flexol B-400 Triton X-200 Bis(2-butoxyethyl) adipate Dicyclohexyl phthalate Didecyl phthalate Acetyl tributyl citrate Estynox Elastex 50-B 0s 124 0s 138 OV-25 SR 119

8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.5.1 8.5.1 8.5.1 8.5.4 8.5.4

Polytergent J-300 Polytergent J-400 Ucon 50-HB-280 X Ethomeen 18/25

8.4.4.3 8.4.4.3 8.4.4.2 8.4.418.8

Saccharose acetate isobutyrate

8.4.3

Z 160.00

U 239.00 S 198.83

11

X 104.00 Y 244.00 2 166.00 U 260.50

12

X 108.67 Y 312.00

S 186.50 Z 142.00

U 210.33 S 208.33

13

X 100.25 Y 269.00 Z 172.25

u 220.00

S 265.25

14

15

16

X 161.00 Y 194.00 2 184.50 U 275.50 S 254.50 X 126.38 Y 292.00 Z 172.38 U 264.13 S 227.13

X 138.33 Y 261.69 Z 201.67

U 308.83 S 235.67

17

X 175.50 Y 225.50 2 220.25 U 309.50

18

X 175.25 Y 371.25

8.4.5 8.4.5 8.3.4 8.3.4

S 288.75

Z 229.50

U 355.00 S 312.50

19

X 181.00 Y 330.20

334

9. Selection of Stationary Phases ~~~

~

~~

~

Table 87 (continued) Cluster No.

20

21

22

23

Stationary Phase (commercial name)

Z 243.20 U 369.00 S 312.00

Tricresyl phosphate Neopentyl glycol sebacate HI-EFF 3 CP Paraplex G-25 Propyleneglycolsebacate FS-1265 LSX-3-0295 ov-210 AS1 50 (50%trifluoropropyl)

8.5.2 8.5.5 8.5.5 8.5.5 8.5.5 8.3.6 8.3.6 8.3.6 8.3.6

Emulphor ON-870 Ethofat 60125 Igepal CO-630 Igepal CO-710 Polytergent B-350 Polytergent G-300 Tergitol NPX 728 Triton X-100 Pluronic L-35 Pluronic P-65 Plutonic P-85 POlygly~ll5-200 Ucon 50-HB-660 Ucon 50-HB-2000 Ucon 50-HB-3520 Ethomeen S-25 Oronite NI-W Igepal CO-730 Renex 678 Ucon 50-HB-5100 Lutensol Tween 80 AS1 50 (25%Phenyl, 25%cyanopropyl)

8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.4.2 8.4.418.8 8.4.4 8.4.4.3 8.4.4.3 8.4.4.2 8.4.4.3 8.4.4.3 8.3.418.3.7

OV-25 XE 60 XF 1125 Carbowax 400 monostearate Igepal CO-880 Surfonic N-300 Triton X-305 Pluronic F-68 Pluronic F-88 Ucon 75-H-90000 Neopentylglycol succinate HI-EFF 3 BP Paraplex G-40

8.3.4 8.3.7 8.3.7 8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.3 8.4.4.2 8.4.4.2 8.4.4.2 8.5.5 8.5.5 8.5.5

Carbowax 4000 Carbowax 6000

8.4.4.1 8.4.4.1

X147.00 Y237.50 2 359.25 U 469.50 S 311.00 X198.59 Y 391.18 2 253.12 U 388.94 s 343.35

X224.00 Y 421.60 2 282.20 U 430.40 S 383.40 X216.00 Y 375.00 2 339.00 U 492.50 S 376.50

24

X263.29 Y 464.86 Z 311.00 U 486.71 S 425.71

25

X275.00 Y 466.75 2 363.25 U 537.25 S 473.00 X323.00 Y 543.57

26

section in

Average value of McRoynoldsconstant

this book

335

9.4. Approaches to Stationary Phase Selection

Table 87 (continued) Cluster No.

27

28

29

30

31

32

33

34

35

Average value of MeReynolds constant

Stationary Phase (commercialname)

Section in this book

Z 374.29 U 577.14 S 520.14

MER-2 1 Carbowax 20M Carbowax 2OM-TPA SP 1000 AS1 50 (50%cyanopropyl) Silar 5 CP SP 2300

8.4.4.1 8.4.4.1 8.4.4.3 8.4.4.1 8.3.7 8.3.1 8.3.7

FFAP STAP

8.4.4.3 8.4.4.3

Carbowax 600 Carbowax 1000

8.4.4.1 8.4.4.1

Butanediol succinate HI-EFF 4 BP Diethylene glycol adipate HI-EFF 1AP UC-IR-296 Ethylene glycol adipate HI-EFF 2 AP Phenyldiethanolamine succinate HI-EFF 10 BP Diethylene glycol adipate + pentaerythritol EGSS-Y

8.5.5 8.5.5 8.5.5 8.5.5 8.5.5 8.5.5 8.5.5 8.5.5 8.5.5 8.538.4.3

Diethylene glycol succinate EOSS-X

8.5.5 8.5.5

Diethylene glycol succinate-PS HI-EFF-1 BP

8.5.5

Silar 9 CP SP 2330

8.3.1 8.3.7

Silar 1OC SP 2340

8.3.418.3.7 8.3.418.3.7

X 319.00 Y 495.00 Z 446.00 U 637.00 S 530.33 X 342.50 Y 583.00 2 398.50 U 606.00 S 627.00 X 348.50 Y 619.00 Z 423.00 U 629.00 S 597.00 X 374.38 Y 581.63 Z 457.13 U 661.00 S 634.50

X 389.00 Y 606.50 Z 482.00 U 683.00 S 664.00 X 488.00 Y 721.00 Z 583.00 U 832.00 S 784.50 X 499.00 Y 750.67 z 593.33 U 842.00 S 849.00 X 489.50 Y 725.00 Z 630.50 U 913.00 S 778.00 X 521.50 Y 756.00 Z 659.00 U 942.00 S 800.50

8.5.5

8.5.5

336

9. Selection of Stationary Phases

Table 87 (continued) Cluster No.

Average value of McReynolds conrtant

Stationary Phase (commercial nme)

Section in this book

36

X536.50 Y 781.00 2 639.50 U 900.00 S 876.50

Ethylene glycol succinate HI-EFF-2 BP

8.5.5 8.5.5

Appendix to Table 87. Liquid Stationary Phases which cannot be placed in one of the 36 clusters (arranged according to the overall polarity) Liquid Stationary Phase

McReynolds constant X

Bitumen N,N-Dimethyl-y-aminoethoxymethylpoly siloxane Butyl stearate Beeswax Pentasil350 ov-105 Apiezon H Butoxyethyl stearate Halocarbon K-352 Bis(2-ethylhexyl) tetrac Aorophthalate Diethyleneglycol distearate SP 1200 OV-61*) Triton X-400 Poly-1110 Dexsil400 GC (2-cyanoethyl)*) Zinc stearate.) Ucon LB-1715 POly-A 103 Trimer acid*) Poly-A 101 A Amine 220 Polypropyleneimine*) Bis(2-ethoxyethyl) sebacate Bis(2-butoxyethyl) phthalate Poly-A 135 Tributoxyethyl phosphate Zonyl E-91 Squalane y-Aminopropyltriethoxysilane SP 525 Cresyldiphenyl phosphate

Bis(2-ethoxyethy1)phthalate Siponate DS-10.) y-Morphinylpropyltrimethoxysilane AN-600 Poly(ester acetal) Bis(ethoxyethoxyethy1)phthalate Neopentyl glycol adipate polyester (HI-EFF 3 AP)

19 -8 41 43 16 36 59 56 47 112 64 67 101 68 115 71 61 132 115 94 115 117 122 151 157 163 141 130 152 145 225 199 214 99 72 202 222 233 234

Y

58 143 109 110 3 108 86 135 70 150 193 170 143 334 194 286 231 297 331 271 357 380 425 306 292 389 873 250 341 426 255 351 375 569 539 369 265 408 425

Z

14 76 65 61 121 83 81 83 73 123 106 103 142 97 122 174 59 180 149 163 151 181 168 211 233 168 209 320 238 226 253 285 305 320 129 332 230 319 312

Section

U

21 69 112 88 131 139 151 136 238 168 143 203 213 176 204 249 98 175 263 182 262 293 263 320 348 340 344 377 329 313 368 413 446 344 511 482 469 470 462

S

47 82 71 122 162 86 129 97 146 181 191 166 174 131 202 171 544 235 214 378 214 133 224 274 272 269 274 293 344 297 320 336 364 388 469 408 612 389 438

in this book

8.2.1 8.3.12 8.5.3 8.2.1 8.3.5 8.3.7 8.2.1 8.5.3 8.12 8.5.1 8.5.3 8.5 8.3.4 8.4.4.3 8.9 8.3.5 8.13 8.4.4.2 8.9 8.13 8.9 8.8.2 8.8.1 8.5.1 8.5.1 8.9 8.5.2 8.12 8.2.1 8.3.12 8.2.2 8.5.2 8.5.1 8.11 8.3.12 8.3.7 8.4.4.3 8.5.1 8.5.5

331

9.4. Approaches to Stationary Phase Selection ~~

Liquid Stationary Phase

McReynolds constant

Y

X

Ethylene glycol tetrachlorophthalate Fluorad FC-431') Cyclohexanedimethanol succinate polyester Zonyl E-I*) N-Allyl-y-aminopropyltrimethoxysilane Quadrol') Igepal CO-990 N-0-aminoethyl-y-aminopropyltrimethoxy silane Ethylene glycol succinate silicone EGSP-Z Epon 1001 Ethylene glycol isophthalate polyester XF- 1150 Sorbite hexaacetate Saccharose octaacetate Mer-2 Carbowax 1000 Propylene glycol adipate polyester PGA-400 (Reoplex-400) Carbowax 1540 EGSP-A (polyester silicone) Silar I CP*) SP 2310.) ECNSS-S (polyester silicone with cyanoethyl groups) ECNSS-M (ditto) Diglycerol Diethylene glycol succinate polyester Ethylene glycol o-phthalate polyester (EGP), HIEFF 2 GP THEED.) Tetracyanoethyl pentaerythritol SP-216-PS, SP-222-PS

Hexakis(cyanoeth0xy)hexane Tetrakis(cyanoeth0xy)butane (Cyano-B) Cyanoethylsaccharose Bis(cyanoethy1)formamide OV-275.) *)

~

Z

301 281 269 228 323 214 298 241 308 284 326 308 335 344 381 341 364

414 489 508 520 553 510 539 601 619

318 291 328 468 441 351 345 393 399 406 425 410 449 461 456 418 449

311 391 440 438 42 1 371 410 453

639 629 638 659 690 826 105 691

453 519 605 566 581 560 558 602

463 526 632 561 617 641 690 I81

942 182 815 825 860 919 991 1006

626 617 133 113 113 191 853 885

345 423 446 359 653 511 508

I00

U

Section in this book

S

428 509 493 549 593 412 540 454 548 539 601

669 652 611 646 626 641 666 121 844 820 803 616

I88 816

466 360 481 465 555 489 415 433 549 601 561 528 543 569 615 689 611

8.5.5 8.12 8.5.5 8.12 8.3.12 8.8.1 8.4.4.3 8.3.12 8.3.9 8.5.5 8.5.5 8.3.1 8.5.4 8.5.4 8.4.4.1 8.4.4.1 8.5.5

641 I00 613 122 132 854 119 812

8.4.4.1 8.3.9 8.3.1 8.3.9 8.3.9 8.4.2 8.5.5 8.5.5

801 893 8.8.1 820 831 8.6 1000 680 8.6 918 901 8.6 1048 941 8.6 1043 916 8.6 1110 1000 8.6 1111 1089 8.3.7

Liquid stationary phases that are interesting and should be included in a short list because of particular selectivities.

Table 88 Characterization of some Stationary Phases by means of Selectivity Index (afier Euans, Haken and Toth 16911) Stationary Phase

Selectivity Index I* Benzene

Squalane Apiezon Dinonyl sebacate Dinonyl phthalate Neopentyl glycol sebacate Neopentyl glycol succinate Carbowax 20M Carbowax 1540

I: I' 5

110 141 116 193 282 382 432 481

is the sum of the selectivity indices for solutes 1-5.

n-Butanol

76 98 242 259 403 545 612 115

2-Pentanone Nitropropane

21 42 134 114 252 393 395 480

31 61 209 262 315 510

603 691

Pyridine

I. 5

149 189 267 308 415 623 659 190

393 531 1028 1196 1187 2513 2701 3163

338

9. Selection of Stationary Phases

Table 89. Stationary Phases Arranged in Order of Increasing X Values, i.e., Increasing Induction and Donor-Acceptor Interactions with Olefms and Aromatics N,N-Dimethylaminoethoxymethylpolysiloxane Squalane Nujol Paraffin Oil Hexatriacontane DC 330 Convoil20 Embaphase Oil SF-96 SF-96-200 SF-96-2000 DC 400 (Elastomer) E 300 MS 2211 (Elastomer) UC W 982 Bayer M (Elastomer) DC 2OO/MS 200 DC 430 (Elastomer) E 301 (Elastomer) F 111 Silicone Grease (without filler) JXR (Elastomer) L-45 (uc L-45) L-46 (VC L-46) OD-1 (Elastomer) OV-1 (Elastomer) Perkin-Elmer Column C Perkm-Elmer Column Z PMS 100 SE-30 (Elastomer) SE-30 GC SE-30 Ultraphase Viscasil Pentasil350 ov-101 AS1 100 Methyl DC 401 (Elastomer) DC 11

ov-1

Perkin-Elmer Column 0 Silastic 401 SP 2100 DC 410 (Elastomer) Bitumen Oronite Polybutene 32 Oronite Polybutene 128 DC 510 Apiezon M Apiezon L DC 560 E 350 SE-52 F 60

-8

0 9 11 12 13 14 14 14 14 14 15 15 15 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 16 17 17 17 17 17 17 17 17 18 19 21 25 25 31 32 32 32 32 32

F 61 E 351 SE-54 ov-105 DC 556 Dexsil300 GC Apiezon I Apiezon N Butylstearate Beeswax OV-3 Halocarbon 10-25 Halocarbon K-352 Fluorolube HG 1200 Halocarbon Wax Kel-F Wax Butoxyethyl stearate Apiezon H Zinc Stearate Diethylene glycol distearate Dinonyl sebacate SP 1200 Triton X-400 OV-7 Diisodecyl adipate Triethylhexyl phosphate Dexsil400 GC (2-Cyano) Bis(2-ethylhexyl) sebacate Ditto (Octoil S) Dioctyl sebacate Dexsil400 GC (Phenyl) y-Morphinylpropyl trimethoxysilane Diisononyl adipate DC 550 Di(tridecy1)phthalate Bis-(2-ethylhexyl) adipate (Flexol) DC 703 DC 702 Diisooctyl adipate Didodecyl phthalate Octyldecyl adipate Hallcornid M-18 Apiezon W Dinonyl phthalate Diisodecyl phthalate Celanese Ester No.9 Span 60 Hallcomid M-18 OL Bis-2-ethylhexyl phthalate Dioctyl phthalate Flexol OPE Diisooctyl phthalate Trimer Acid Flexol8N8

32 33 33 36 37 37 38 38 41

43 44 47 47 51 55 55

56 59 61 64 66 67 68 69 71 71 71 72 72 72 72 72 73 74 75 76 76 77 78 79 79 79 82 83 84 84 88 89 92 92 93 94 94 96

339

9.4. Approaches to Stationary Phase Selection Table 89 (continued) Butyloctyl phthalate Span 80 Stepan DS-60 Siponate DS-10 OV-61 ov 11 DC 710 Castorwax Atpet 200 Versamid 930 Bis(2-ethylhexyl) tetrachlorophthalate Linear Polyamide Versamid 940 Hercoflex 600 Bis(2-ethylhexyl) tetrachlorophthalate Poly-A 101 A Poly-A 103 Poly-I 110 Triton X-200 Amine 220 Ucon LB-550 X AS1 50 Methyl OV-17 SP 2250 Flex01 B-400 Polypropyleneimine Ucon 50-HB-1800 X PPG 2000 Pluracol P-2010 Zonyl E-91 PPG 1000 Ucon LB-1715 SP 392 Acetyltributyl citrate Didecyl phthalate Estynox Bis(2-butoxyethyl) adipate Elastex 50-B Tributoxyethyl phosphate FS-1265 Pluronic L-81 y- Aminopropy ltriethoxysilane ov-210 Dicyclohexyl phthalate AS1 50 Methyl Bis(2-ethoxyethyl) sebacate Squalene LSX-3-0295 Bis(2-butoxyethyl) phthalate ov-22 MER-35 Poly-A 135 SR 119 Polytergent J-300 Saccharose acetate Isobutyrate

97 97 97 99 101 102 107 108 108 108 109 109 109 112 112 115 115 115 117 117 118 119 119 119 121 122 123 128 129 130 131 132 133 135 136 136 137 140 141 144 144 145 146 146 146 151 152 152 157 160 162 163 166 168 172

0s 124 Tricresyl Phosphate Ethomeen 18/25 Ucon 50-HB-280 X ov-25 Polytergent J-400 0s 138 Oronite NI-W Ethomeen St25 Paraplex 6/25 Ethofat 60/25 Igepal CO-630 Ucon 50-HB-660 PGSB Tergitol NPX 728 Ucon 50-HB-3520 Cresyl diphenyl phosphate Pluronic P-85 Emulphor ON-870 Polytergent B-350 Ucon 50-HB-2000 AN-600 Polytergent G-300 Triton X-100 Polyronic P-65 XE 60 (Elastomer) XF-1125 Igepal OO-710 Pluronic L-35 POlyglyCOi 15-200 Ucon 50-HB-5100 Bis(2-ethoxyethyl) phthalate Quadrol Poly(Ester Acetal) Renex 678 Zonyl E-7 Igepal OO-730

SP 525 Tween 80 ASI-50 Methyl OV-25 NPGA HI-EFF 3 AP Lutensol Bis(ethoxyethoxyethy1) phthalate NPGA N-p-Aminoethyl-y- Amiopropyltrimethoxysilane Ucon 75-H-90 000 Igepal CO-880 Surfonic N-300 Triton X-305 Pluronic F-88 Pluronic F-68 Cyclohexanedimethanol succinate Ditto, HI-EFF 8BP

176 176 176 177 178 180 182 185 186 189 191 192 193 196 197 198 199 201 202 202 202 202 203 203 203 204 204 205 206 214 214 2 14 214 222 223 223 224 225 227 228 228 232 232 233 234 247 255 259 261 262 262 264 269 271

340

9. Selection of Stationary Phases

Table 89 (continued) NPGS NPGS HI-EFF 3 BP NPGS Carbowax 400 Monostearate Paraplex G-40 Fluorad FC-43 1 Epon 1001 Igepal CO-990 EGTOP EGSP-Z XF-1150 Carbowax 4000 AS1 50 Phenyl Silar 5 CP Phenyl SP 2300 Carbowax 2OM-TPA Carbowax 6000 Carbowax 6000 MER-21 Carbowax 20M N- Ally 1-y-Aminopropyltrimethoxysilane Carbowax 4000 Ethylene glycol Isophthalate SP 1000 Sorbitol hexaacetate FFAP Saccharose octaacetate STAP Carbowax 1000 Carbowax 1500 Carbowax 600 PGA Reoplex 400 Butanediol succinate HI-EFF-4B Butanediolsuccinate Carbowax 1540 Diglycerol Ethylene glycol adipate

272 272 215 280 281 281 284 298 307 308 308 317 319 319 319 321 322 322 322 323 325 326 332 335 340 344 345 341 341 350 364 369 370 371 371 371

Ethylene glycol adipate Ethylene glycol adipate HI-EFF Diethylene glycol adipate LAC-IR 296 Diethylene glycol adipate HI-EFF 1AP MER-2

372 372 371 378 381

0

PDEAS HI-EFF 10 BP Diethylene glycol adipate DEGA-P EGGS-Y EGSP-A ECNSS-M ECNSS- S Silar 7 CP SP 2310 EGP HI-EFF 2 GP THEED Diethylene glycol succinate EGGS-X Silar 9 CP (10%Phenyl) SP 2330 Diethylene glycol succinate Diethylene glycol succinate DEGS-PS Diethylene glycol succinate HI-EFF-1 Diethylene glycol succinate HI-EFF-1 SP 2340 Silar 10 C Tetracyanoethylpentaerythritol Ethylene glycol succinate (EGS) EGS, HI-EFF 2 BP Hexakis(cyanoeth0xy)hexane Tetrakis(cyanoeth0xy)butane SP-216-PS, SP-222-PS Cyanoethylsaccharose

Bis(cyanoethy1)fonnamide OV-275

386 387 391 397 42 1 438 440 440 453 463 470 484 489 490 492 496 499 502 520 523 526 536 537 567 617 632 647 690 781

Table 90. Stationary Phases Arranged in Order of Increasing Y Values, i.e., Hydrogen Bond Interactions (Solute 1-Butanol acting as H-Donor and the Stationary Phase acting as H-Acceptor) Squalane Hexatriacontane Pentasil350 Nujol Paraffin oil Convoil20 Apiezon L Apiezon M Oronite Polybutene 128 Oronite Polybutene 32 Apiezon I Apiezon N DC 330

0 2 3 5

6 14 22 22 26 29 30 40 51

OD-1 (Elastomer) SE-30 GC SF-96 SF-96-200 SF-96-2000 DC 430 (Elastomer) UC W 982 AS1 100 Methyl E 301 (Elastomer) JXR (Elastomer) OV-1 (Elastomer) Perkin-Elmer Column Z SE-30 (Elastomer)

53 53 53 53 53 54 55 55 55 55 55

55 55

341

9.4. Approaches to Stationary Phase Selection Table 90 (continued)

-

SE-30 Ultraphase 55 DC 400 (Elastomer) 56 E 300 56 56 L-46 (UC L-46) MS 2211 (Elastomer) 56 PMS 100 56 ov-101 57 Bayer M (Elastomer) 57 DC 200/MS 200 57 DC 410 (Elastomer) 57 Embaphase Oil 57 F 111 57 Silicone Grease (without filler) 57 L-45 (UC L-45) 57 ov-1 57 Perkin-Elmer Column C 57 SP 2100 57 Viscasil 57 Bitumen 58 DC 401 (Elastomer) 58 Silastic 401 58 DC 510 65 KEL-F WAX 67 Fluorolube HG 1200 68 Halocarbon 10-25 70 Halocarbon K-352 70 Halocarbon WAX 71 DC 560 72 E 350 72 E 351 72 SE-52 72 SE-54 72 F 60 72 F 61 72 DC 556 77 Dexsil300 GC 78 Apiezon H 86 DC 11 86 Perkin-Elmer Column 0 86 OV-3 86 Dexsil400 GC (Phenyl) 107 ov-105 108 Butyl stearate 109 Beeswax 110 OV-7 113 DC 550 116 DC 703 123 DC 702 124 Bis-Z(ethylhexy1)tetrachlorophthalate 132 Apiezon W 135 Butoxyethyl stearate 135 ov-11 142 OV-61 143 N,N-Dimethyl-y-aminoethoxymethylpolysi- 143 loxane

DC 710 Bis(2-ethylhexyl) tetrachlorphthalate Ditridecyl phthalate Didodecyl phthalate AS1 50 Methyl OV- 17 SP 2250 Dinonyl sebacate Bis(2-ethylhexyl) sebacate (Octoil S) Bis(2-ethylhexyl) sebacate Dioctyl sebacate SP 392 SP 1200 Diisodecyl adipate Diisodecyl phthalate Diisononyl adipate Octyldecyl adipate Bis(2-ethylhexyl) adipate (Flexol) Celanese Ester No.9 Dinonylphthalate Bis(2-ethylhexyl) phthalate Dioctyl phthalate Diisooctyl adipate ov-22 Diisooctyl phthalate Diethylene glycol distearate Butyloctyl phthalate Poly-I 110 MER-35 ov-25 Flexol GPE 0s 124 Zinc stearate FS-1265 0s 138 Hercoflex 600 o v - 2 10 SR 119 AS1 50 Methyl LSX-3-029s Zonyl E-91 Flexol8N8 Didecyl phthalate Elastex SO-B SP 525 Dicyclohexyl phthalate Estynox Span 60 Castorwax Poly(ester acetal) Span 80 Acetyltributyl citrate Hallcomid M-18 Ucon LB-550 X Trimer Acid

149 150 156 158 158 158 158 166 167 168 168 169 170 171 173 174 179 181 182 183 186 186 187 188 193 193 194 194 200 204 2 10 227 231 233 233 234 238 238 238 241 250 254 255 255 255 257 257 263 265 265 266 268 268 271 271

342

9. Selection of Stationary Phases

Table 90 (continued) Ucon 50-HB-1800 X Bis(2-butoxyethyl) adipate Hallcomid M-18 OL Atpet Flexol B-400 Dexsil400 GC (2-Cyano) Triethylhexyl phosphate Triton X-200 Bis(2-butoxyethyl) phthalate PPG 2000 Pluroacol P-2010 Ucon LB-1715 Bis(2-ethoxyethyl) sebacate Versamid 930 Linear Polyamide PPG 1000 Pluronic L-81 Versamid 940 Tricresyl phosphate NPGSB HI-EFF 3 CP Paraplex G-25 Saccharose acetate isobutyrate POly-A 103 Triton X-400 Squalene PGSB EGTCP Cresyldiphenylphosphate POly-A 101 A Zonyl E-7 Ucon 50-HB-280 X Polytergent 5-300 AN-600 ASI-50 Methyl ov-25 Oronite NI-W Tributoxyethyl phosphate Polytergent J-400 Bis(2-ethoxyethyl) phthalate Ucon 50-HB-660 Amine 220 Igepal OO-630

Ucon 50-HB-3520 XE 60 (Elastomer) XF-1125 Ethofat 60125 Ethomeen 18/25 Tergitol NPX 728 Poly-A 135 Pluronic P-85 Polytergent B-350 Pluronic P-65 Ucon 50-HB-2000 Emulphor ON-870 Ethomeen S/25

275 278 280 282 284 286 288 289 292 2 94 295 297 306 309 313 314 314 314 321 327 328 330 331 334 341 345 345 351 357 359 362 366 369 369 369 370 373 375 375 380 380 381 381 381 381 382 382 386 389 390 392 394 394 395 395

Igepal CO-710 Polytergent G-300 Triton X-100 Pluronic L-35 Bis(ethoxyethoxyethy1)phthalate POlyglyCOll5-200 Renex 678 Igepal CO-730 Ucon 50-HB-5100 NPGA HI-EFF 3 AP Fluorad FC-431 NPGA Lutensol Polypropy leneimine y-Aminopropyltriethoxysilane Tween 80 Cyclohexane dimethanolsuccinate HI-EFF-8 BP Cyclohexanedimethanol succinate Ucon 75-H-90000 Paraplex G-40 Igepal-CO-880 Pluronic F-88 Surfonic N-300 Pluronic F-68 Triton (2-305 NPGS NPGS HI-EFF 3 BP NPHS EGSP-Z Carbowax 400 Monostearate Epon 1001 AS1 50 Phenyl Silar 5 CP Phenyl SP 2300 Igepal CO-990 Ethylene glycol isophthalate (EGIP) XF-1150 Carbowax 20M Carbowax 2OM-TPA MER-2 y-Morphinylpropyltrimethoxysilane Carbowax 6000 Carbowax 6000 MER-21 Carbowax 4000 Stepan DS-60 Carbowax 4000 Sorbitolhexaacetate PDEAS HI-EFF 10 BP SP 1000 Siponate DS-10 Saccharose octaacetate Butanediol succinate Quadrol Ethylene glycol adipate HI-EFF 2 AF'

397 398 399 406 408 410 417 418 418 421 423 425 425 425 426 430 444 446 452 459 461 461 462 465 467 467 469 472 474 486 489 495 495 495 508 508 520 536 537 539 539 540 541 545 550 551 553 555 555 569 570 571 571 576

343

9.4. Approaches to Stationary Phase Selection Table 90 (continued) Ethylene glycol adipate Ethylene glycol adipate

577 579 FFAP 580 586 STAP Butanediol succinate HI-EFF-4BP 591 597 EGSS-Y 601 Diethylene glycol adipate LAC-IR-296 603 Diethylene glycol adipate HI-EFF 1AP Carbowax 1000 607 Carbowax 1500 607 Diethylene glycol adipate DEGA-P LAC-2R- 616 446 619 PGA Reoplex 400 629 EGSP-A 631 Carbowax 600 Silar 7 CP 638 SP 2310 638 Carbowax 1540 639 653 N-Ally laminopropyltrirnethoxysilane 659 ECNSS-S 690 ECNSS-M 697 EGP HI-EFF 2 GP N-0-aminoethylaminopropyltrimethoxysi700 lane

Diethylene glycol succinate EGSS-X Silar 9 CP (10%Phenyl) SP 2330 Diethylene glycol succinate Diethylene glycol succinate PS Diethylene glycol succinate HI-EFF-1 BP Silar 1OC Diethylene glycol succinate SP 2340 Ethylene glycol succinate (EGS) Tetracyanoethylpentaerythritol EGS, HI-EFF 2 BP

Hexakis(cyanoethoxy)hexane Diglycerol Tetrakis(cyanoeth0xy)butane SP-216-PS, SP-222-PS Cyanoethylsaccharose THEED (283) Bis(cyanoethy1)formamide OV-275

705 710 725 725 733 746 750 755 755 757 775 782 787 825 826 860 875 919 942 991 1006

I

In Table 91, the 287 liquid stationary phases are arranged in order of increasing 2 values [solute = 2-pentanone, selected for hydrogen bonding (H-acceptor) and representing ketones, ethers, aldehydes, esters, epoxides and dimethylamino derivatives],in Table 92 in order of increasing U values [solute = 1-nitropropane,selected for induction forces (donor), orientation forces and donor-acceptor forces (donor) and representing nitro and nitrile compounds], and finally in Table 93 in order of increasing S values [solute = pyridine, selected for orientation forces and representing N-heterocyclic compounds). Even though many obsolete stationary phases which cannot be recommended, are included it is hoped that these compilations will be an aid for practising analysts attempting to tune the selectivity, if substance pairs that are difficult to separate must be resolved or if the separation must be optimized, especially when the critical distances in Table 87 (25 units of McReynolds constants) are too large.

Hexatriacontane Squalane Nujol Paraffin oil Convoil20 Bitumen Apiezon L Apiezon M Oronite Polybutene 32 Oronite Polybutene 128

-3 0 2 2 8 14 15 15 24 25

ApiezonI ApiezonN DC330 SF-96 SF-96-200 SF-96-2000 AS1 100 Methyl DC 400 (Elastomer) E300 E 301 (Elastomer)

27 28

42 42 42 42 44 44 44 44

344

9. Selection of Stationary Phases

Table 91 (continued)

JXR (Elastomer) L-46 (UC L-46) MS 2211 (Elastomer) OV-1 (Elastomer) Perkin-Elmer Column Z PMS 100 SE-30 (Elastomer) SE-30 GC SE-30 UL Traphase ov-101 UC W 982 Bayer M (Elastomer) DC 2OO/MS 200 DC 430 (Elastomer) Embaphase Oil F 111 Silicone grease (without filler) L-45 (Uc L-45) OD-1 (Elastomer)

ov-1

Perkin-Elmer Column C SP 2100 Viscasil DC 401 (Elastomer) DC 410 (Elastomer) Silastic 401 DC 11 Perkin-Elmer Column 0 Zinc stearate DC 510 Beeswax Butyl stearate E 350 SE-52 E 351 SE-54 DC 560 F 60 F 61 Halocarbon K-352 N,N-Dimethyl-y-aminoethoxymethylpolysiloxane DC 556 Apiezon H OV-3 Butoxyethyl stearate ov-105 Triton X-400 Apiezon W SP 1200 Diethylene glycol distearate Bis(2-ethylhexyl)sebacate-Octoil S Dinonyl sebacate Bis(2-ethylhexyl)sebacate Dioctyl sebacate

44 44 44 44 44 44 44 44 44 45 45 45 45 45 45 45 45 45 45 45 45 45 45 47 47 47 48 48 59

60 61 65 65 65 66 66

I0 70 70 73 76 80 81 81 83 93 97 99 103 106 107 107 108 108

Halocarbon 10-25 OV-7 Bis(2-ethylhexyl) tetrachlorphthalate Diiiodecyl adipate Dexsil300 GC KEL-F Wax Fluorolube HG 1200 Halocarbon Wax Dihnonyl adipate Triethylhexyl phosphate DC 550 Dexsil400 GC (Phenyl) Octyldecyl adipate Didodecyl phthalate Bis(2-ethylhexyl) adipate (Flexol) Pentasil350 Ditridecyl phthalate Celanese Ester No.9 Poly-I 110 Bis(2-ethylhexyl) tetrachlorophthalate Diisooctyl adipate DC 702 DC 703 y-Morphinylpropyltrimethoxysilane Hallcornid M-18 Diisodecyl phthalate Versamid 930 Flexol GPE OV-61 Hallcornid M-18 OL Linear Polyamide Versamid 940

ov-11

Dinonyl phthalate Poly-A 103 Bis(2-ethylhexyl) phthalate Dioctyl phthalate POly-A 101 A DC 710 Diisooctyl phthalate Butyloctyl phthalate Ucon LB-550 X Span 60 Ucon 50-HB-1800 X AS1 50 Methyl OV-17 SP 2250 Trimer acid Flexol EN8 Herwflex 600 Polypropyleneimine Poly-A 135 Flexol B-400 Span 80 Triton X-200

108 111 113 113 113 114 114 116 116 117 117 118 119 120 121 121 122 122 122 123 126 126 126 129 130 137 137 140 142 143 144 145 145 147 149 150 150 151 153 154 157 158 158 161 162 162 162 163 164 168 168 168 169 170 172

345

9.4. Approaches to Stationary Phase Selection Table 91 (continued) PPG 2000 Pluracol P-2010 Dexsil400 GC (2-Cyano) Castorwax SP 392 MER-35 Ucon LB-1715 Arnine 220 Estynox PPG 1000 Atpet 200 Pluronic L-81 ov-22 Bis(2-butoxyethyl) adipate Acetyltributyl citrate Dicyclohexyl phthalate OV-25 Tributoxyethyl phosphate Elastex 50-B Bis(2-ethoxyethyl) sebacate Didecyl phthalate SR 119 0s 124 NPGSB HI-EFF 3 CP y-Aminopropyltriethoxysilane Polytergent J-300 Ucon 50-HB-280 X 0s 138 Ethomeen 18/25 Poly(ester acetal) Bis(2-butoxyethyl) phthalate Polytergent J-400 Squalene Paraplex 6-25 Ucon 50-HB-660 Ucon 50-HB-3520 Ethomeen S/25 Oronite NI-W Ethofat 60125 Pluronic P-85 Tricresyl phosphate Emulphor ON-870 Pluronic P-65 Saccharose acetate Isobutyrate PGSB Igepal CO-630 Ucon 50-HB-2000 SP 52.5 Pluronic L-35 Tergitol NPX 728 Polytergent B-350 Polyglycol 15-200 Igepal CO-710 Polytergent G-300 Triton X-100

173 174 174 175 176 178 180 181 182 185 186 187 191 198 202 206 208 209 209 211 213 22 1 224 225 226 227 227 228 230 230 233 234 238 239 241 241 242 242 244 247 250 251 251 251 251 253 253 253 257 258 260 262 266 267 268

Renex 678 Ucon 50-HB-5100 Igepal CO-730 Tween 80 Cresyldiphenyl phosphate Lutensol Fluorad FC-431 Ucon 75-H-90 000 Stepan DS-60 Bis(2-ethoxyethyl) phthalate Pluronic F-88 Pluronic F-68 Igepal CO-880 NPGA HI-EFF 3 AF' NPGA Surfonic N-300 Triton X-305 Bis(ethoxyethoxyethy1) phthalate EGTCP Siponate DS-10 Zonyl E-91 Carbowax 400 Monostearate Cyclohexanedimethanol succinate Cyclohexanedimethanol succinate HI-EFF 8BP AN-600 ASI-50 Methyl OV-25 XE 60 (Elastomer) XF-1125 Igepal CO-990 FS-1265 Paraplex G-40 Quadrol(284) o v - 2 10 AS1 50 Methyl NPGS LSX-3-0295 NPGS HI-EFF 3 BP Carbowax 2OM-TPA NPGS Carbowax 20M Carbowax 6000 Carbowax 6000 MER-2 1 Carbowax 4000 Carbowax 4000 N-0-Aminoethyl-y-aminopropyltrimethoxysilane SP 1000 FFAF' EGSP-Z STAP Epon 1001 Carbowax 1000 Carbowax 1500

278 278 279 283 285 293 297 299 303 305 306 309 311 311 312 313 3 14 317 318 320 320 325 328 330 332 338 338 340 340 345 355 355 357 358 358 365 366 366 367 367 368 369 370 375 318 393 393 397 399 400 406 418 418

346

9. Selection of Stationary Phases

Table 91 (continued) Diglycerol ECNSS-S ECNSS-M Diethylene glycol succinate

42 5 42 8 441 N-Allyl-y-aminopropyltrimethoxysilane 446 AS1 50 Phenyl Silar 5 CP (F'henyl) 446 446 SP 2300 448 Butanediol succinate 449 Sorbitol hexaacetate PGA Reoplex 400 449 Carbowax 1540 453 Ethylene glycol adipa HI-EFF 2 AP 453 Ethylene glycol adipate 454 Ethylene glycol adipate 455 MER-2 456 Butanediol succinate HI-EFF 4BP 457 Diethylene glycol adipate LAC-IR-296 458 Diethylene glycol adipate HI-EFF 1AP 460 Saccharoseoctaacetate 46 1 468 Zonyl E-7 470 XF-1150 Diethylene glycol adipate DEGA-P LAC-2R- 47 1 446 472 PDEAS HI-EFF 10 BP EGSS-Y 493 EGSP-A 519 558 Diethylene glycol succinate Ethylene glycol isophthalate Carbowax 600

EGSS-X Diethylene glycol succinate DEGS-PS Diethylene glycol succinate HI-EFF-1 BP Diethylene glycol succinate EGP HI-EFF 2 GP Silar 7 CP SP 2310 THEED SP 2330 Silar 9 CP (10%Phenyl) Ethylene glycol succinate (EGS) EGS, HI-EFF 2 BP silar 1oc SP 2340 Tetracyanoethylpentaerythritol

Hexakis(cyanoethoxy)hexane SP-216-PS, SP-222-PS

Tetrakis(cyanoethoxy)butane Cyanoethylsaccharose

Bis(cyanoethy1)fonnamide OV-275

560 566 581 581 585 590 593 597 602 605 605 626 630 63 1 636 643 659 659 677 713 733 773 797 853 885

Table 92. Stationary Phases Arranged in Order of Increasing U Values, i.e., Increasing Induction, Orientation and Donor-Acceptor Interactions with Strongly Polar Solutes Squalane Hexatriacontane Nujol Paraffin Oil Convoil20 Bitumen Apiezon M Apiezon L Oronite Polybutene 128 Oronite Polybutene 32 Apiezon I Apiezon N DC 330 SF-96 SF-96-200 SF-96-2000 DC 430 (Elastomer) E 301 (Elastomer) JXR (Elastomer) L-46 (VC L-46) OV-1 (Elastomer) Perkin-Elmer Column Z PMS 100

0 1 6 7 17 21 30 32 41 42 49 52 61 61 61 61 65 65 65 65 65 65 65

,

SE-30 (Elastomer) SE-30 GC SE-30 Ultraphase UC W 982 Bayer M (Elastomer) DC 2OO/MS 200 DC 400 (Elastomer) E 300 Embaphase Oil F 111. Silicone grease (without filler) L-45 (uc L-45) MS 2211 (Elastomer) OD-1 (Elastomer) Perkin-Elmer Column C Viscasil ov-101 AS1 100 Methyl ov-1 SP 2100 DC 401 (Elastomer) DC 410 (Elastomer) Silastic 401

65 65 65 66 66 66 66 66 66 66 66 66 66 66 66 66 67 67 67 67 68 68 68

341

9.4. Approaches to Stationary Phase Selection Table 92 (continued) DC 11 Perkin-Elmer Column 0 N,N-Dimethyl-y-aminoethoxymethylpolysiloxane Beeswax DC 510 Zinc Stearate E 350 E 351 SE-52 SE-54 DC 560 F 60 F 61 Butyl stearate DC 556 OV-3 Pentad350 Halocarbon 10-25 Butoxyethyl stearate ov-105 Halocarbon Wax Kel-F Wax Diethylene glycol distearate Fluorolube HG 1200 Apiezon H Dexsil300 GC Apiezon W Dexsil400 GC (Phenyl) Bis(2-ethylhexyl)tetrachloro phthalate Bis(2-ethylhexyl)tetrachloro phthalate OV-7 Ucon LB-1715 Triton X-400 Dinonylsebacate DC 550 Bis(2-ethylhexyl) sebacate (Octoil S ) Bis(2-ethylhexyl sebacate) Dioctyl sebacate Trimer Acid Diisodecyl adipate Diisononyl adipate DC 702 DC 703 Didodecyl phthalate Octyldecyl adipate Ditridecylphthalate Bis(2-ethylhexy1)adipate (Flexol) Celanese Ester No.9 Span 60 SP 1200 Diisooctyl adipate Poly-I 110 Venamid 930 Linear Polyamide

69 69 69 88 89 98 98 98 98 98 100 100 100 112 118 124 131 133 136 139 143 143 143 144 151 1.54 155

166 168 171 171 175 176 178 178 179 180 180 182 185 189 189 189 192 193 195 197 197 200 203 204 204 208 211

Venamid 940 OV-61 Triethylhexyl phosphate Span 80 Diisodecyl phthalate

ov-11 Hallcomid M-18 Flexol GPE DC 710 Castorwax Dinonyl phthalate Atpet 200 Bis(2-ethylhexyl) phthalate Dioctyl phthalate Halocarbon K-352 Hallcomid M-18 OL Ucon LB-550 X Diisooctyl phthalate AS1 50 Methyl OV- 17 SP 2250 Butyloctyl phthalate Ucon 50-HB-1800 X Dexsil400 GC (2-Cyano) SP 392 Flexol B-400 Flexol8N8 Hercoflex 600 Poly-A 101 A Polypropyleneimine Poly-A 103 PPG 2000 Pluracol P-2010 Triton X-200 Mer-35 PPG 1000 ov-22 Estynox Pluronic L-8 1 Amine 220

Bis(2-Butoxyethy1)adipate OV-25

0s 124 0s 138 y- Aminopropy ltriethoxysilane Acetyltributyl citrate SR 119 Dicyclohexyl phthalate Elastex 50-B Bis(2-ethoxyethyl) sebacate Didecylphthalate Squalene Stepan DS-60 Poly-A 135 Tributoxyethyl phosphate

212 213 215 216 218 219 222 224 228 229 231 235 236 236 238 239 243 243 243 243 243 246 249 249 258 259 260 261 262 263 263 264 266 266 268 277 283 285 289 293 300 305 306 313 313 314 3 14 316 318 320 320 329 338 340 341

348

9. Selection of Stationary Phases

Table 92 (continued) 344 344 348 350 351 353 366 368 368 370 370 374 376 377 378 379 380 381 382 388 389 392 393 395 395 398 40 1 40 1 401 402 413 42 1 427 428 428 438 438 446 N-6-Aminoethyl-y-aminopropyltrimethoxy 454 silane NPGA Hi-EFF 3 AP 461 NPGA 462 FS-1265 463 ov-210 468 AS1 50 Methyl 468 Poly(ester-acetal) 469 Ucon 75-H-90 000 470 Bis(ethoxyethoxybthy1)phthalate 470 Quadrol 472 LSX-3-0295 479 Igepal CO-880 482 AN-600 482 Pluronic F-88 483 Surfonic N-300 484 Triton X-305 488 Siponate DS-10 NPGSb HI-EFF 3 CP Bis(2-Butoxyethyl) phthalate Polytergent J-300 Ucon 50-HB-280 X Ethomeen 18/25 Polytergent J-400 Paraplex G-25 SP 525 Ethomeen S 25 Oronite NI-W Tricresyl phosphate Ucon 50-HB-660 Zonyl E-91 Saccharose acetate isobutyrate Ucon 50-HB-3520 Ethofat 60/25 PGSB Igepal CO-630 Pluronic P-85 Tergitol NPX 728 Ucon 50-HB-2000 Pluronic P-65 Emulphor ON470 Polytergent B-3 50 Pluronic L-35 Igepal CO-710 Polytergent G-300 POlyglyCOl 15-200 Triton X-100 Cresyldiphenyl phosphate Ucon 50-HB-5100 Renex 678 Igepal CO-730 EGTCP Lutensol Tween 80 Bis(2-ethoxyethyl) phthalate

Pluronic F-68 ASI-50 Methyl OV-25 Cyclohexanedimethanol succinate XE-60 (Elastomer) XF-1125 Cyclohexanedimethanol succinate HI-EFF 8 BP Fluorad FC-431 y-Morphiny lpropyltrimethoxysilane Carbowax 400-Monostearate Paraplax G-40 Epon 1001 NPGS NPGS HI-EFF 3 BP Igepal CO-990 NPGS EGSP-Z Zonyl E-7 Carbowax 20M Carbowax 20-M-TPA Carbowax 6000 Mer-21 Carbowax 6000 Carbowax 4000 Carbowax 4000 SP 1000 N-Allyl-y-aminopropyl trimethoxysilane FFAP Ethylene glycol isophthalate STAP Carbowax 1000 Carbowax 1500 Carbowax 600 AS1 50 Phenyl Silar 5 CP (Phenyl) SP 2300 Mer-2 PGA Reoplex 400 Sorbitehexa acetate Ethylene glycol adipate Ethylene glycol adipate HI-EFF 2 AP Butanediol succinate Ethylene glycol adipate Butanediol succinate Hi-EFF-4BP Diethylene glycol adipate LAC-IR-296 Diethylene glycol adipate HI-EFF 1AP Carbowax 1540 XF-1150 Saccharose octaacetate PDEAS HI-EFF 10 BP Diglycerol Diethylene glycol adipate DEGA-P EGSS-Y EGSP-A Diethylene glycol succinate

488 492 492 493 493 493 498 509 511 5 12 528 539 539 539 540 543 548 549 572 513 575 577 578 582 583 593 602 607 610 626 626 632 637 637 637 646 647 652 655 655 657 658 661 663 665 666 669 671 674 676 679 693 721 788

349

9.4. Approaches to Stationary Phase Selection Table 92 (continued) THEED ECNSS-M EGP HI-EFF 2 GP ECNSS-S EGSS-X Diethylene glycol succinate Diethylene glycol succinate DEGS-PS Diethylene glycol succinate HI-EFF-1 BP Silar 7 CP SP 2310 Diethylene glycol succinate Ethylene glycol succinate (EGS)

801 803 816 820 831 833 837 840 844 844 849 897

EGS, HI-EFF 2 BP Silar 9 CP (10%Phenyl) SP 2330 Te tracyanoethylpentaerythitol Silar 1OC SP 2340

Hexakis(cyanoethoxy)hexane SP-216-PS, SP-222-PS Cyanoethylsaccharose

Tetrakis(cyanoethoxy)butane Bis(cyanoethy1)formamide OV-275

903 913 913 920 942 942 978 1000 1043 1048 1110 1177

-

Table 93. Stationary Phases Arranged in Order of Increasing S Values, i.e., Increasing Orientation Interactions, especially with N-Heterocyclics Squalane Hexatriacontane Nujol Paraffin Oil Convoil20 DC 330 SF-96 SF-96-200 SF-96-2000 Apiezon M Oronite Polybutene 32 DC 400 (Elastomer) E 300 MS 2211 (Elastomer) L-46 (LJCL-46) Apiezon L Oronite Polybutene 128 UC W 982 JXR (Elastomer) OD-1 (Elastomer) OV-1 (Elastomer) Perkin-Elmer Column Z SE-30 (Elastomer) SE-30 GC SE-30 Ultraphase ov-101 AS1 100 Methyl Bayer M (Elastomer) DC 2OO/MS 200 DC 430 (Elastomer) Embaphase Oil F 111 Silicone grease (without filler) L-45 (uc L-45) ov-1 Perkin-Elmer Column C

0 11 11 13 21 36

31 31

31

40 40 40

40 40 41 42 42 42 42 42 42 42 42 42 42 43 43 43 43 43 43 43 43 43 43 43

PMS 100

SP 2 100 Viscasil DC 410 (Elastomer) E 301 (Elastomer) DC 401 (Elastomer) Silastic 401 Bitumen DC 11 Perkin-Elmer Column 0 Apiezon I DC 510 Apiezon N E 350 E 351 SE-52 SE-54 DC 560 F 60 F 61 Butyl stearate DC 556 N,N-Dimethyl-y- Aminoethoxymethylpoly-

siloxane OV-105 OV-3 Butoxyethyl stearate Halocarbon 10-25 Kel-F Wax Dexsil300 GC Dinonyl sebacate Fluorolube HG 1200 Beeswax Halocarbon Wax Bis(2-ethylhexyl) sebacate (Octoil S ) Dioctyl sebacate

43 43 43 44 44 46 46 47 56 56 57 57 58 67 67 67 61 68 68 68 71 79 82 86 88 97 111 116 117 118 118 122 123 123 123

350

9. Selection of Stationary Phases

Table 93 (continued) Dexsil400 GC (Phenyl) Bis(2-ethylhexyl) sebacate Diisodecyl adipate OV-7 Apiezon H Diisononyl adipate Triton X-400 Triethylhexyl phosphate Amine 220 Bis(2-ethylhexyl) adipate (Flexol) Octyldecyl adipate DC 550 Diisooctyl adipate Ditridecyl phthalate DC 703 DC 702 Celanese Ester Nr. 9 Hallcomid M-18 Halocarbon K-352 Apiezon W Diisodecyl phthalate Didodecyl phthalate Dinonyl phthalate Flexol GPE Pentasil350 Hallcomid M-18 DL SP 1200 Bis(2-ethylhexy1)phthalate Dioctyl phthalate Bis(2-ethylhexyl) phthalate Dexsil400 GC (2-Cyano) Butyloctyl phthalate Diisooctyl phthalate OV-61

ov-11

Flexol8N8 Bis(2-ethylhexyl) tetrachlorophthalate DC 710 Diethylene glycol distearate Hercoflex 600 AS1 50 Methyl OV-17 SP 2250 Poly-I 110 Ucon LB-550-X Versamid 930 Linear Polyamide Versamid 940 Ucon 50-HB-1800 X Poly-A 101 A Poly-A 103 Flexol B-400 SP 392 Polypropyleneimine PPG 2000

123 125 128 128 129 129 131 132 133 134 134 135 134 140 140 142 143 146 146 154 155 158 159 162 162 165 166 167 167 168 171 174 ,174 174 178 179 181 190 191 194 202 202 202 202 206 207 209 209 212 214 214 217 219 224 226

Pluracol P-2010 Estynox Acetyltributyl citrate Ucon LB-1715 Bis(2-butoxyethyl) adipate Didecyl phthalate Triton X-200 Elastex 50-B PPG 1000 Dicyclohexyl phthalate Castorwax Pluronic L-8 1 ov-22 Mer-35 Span 60 Span 80 Poly-A 135, Bis(2-butoxyethyl) phthalate Bis(2-ethoxyethyl) sebacate Tributoxyefhyl phosphate OV-25 0s 124 Atpet 200 0s 138 Zonyl E-91 Saccharose acetate isobutyrate y-Aminopropyltriethoxysilane Tricresyl phosphate SR 119 Ucon 50-HB-280 X FS-1265 Polytergent J-300 ov-210 AS1 50 Methyl Paraplex G-25 Polytergent J-400 LSX-3-0295 SP 525 Ucon 50-HB-660 Ucon 50-HB-3520 Ethomeen 18/25 NPGSB Hi-EFF 3 CP Oronite NI-W PGSB Ethofat 60125 Pluronic P-85 Cresyldiphenyl phosphate Ethomeen Sl25 Pluronic P-65 Ucon 50-HB-2000 Squalene Emulphor ON-870 Igepal CO-630 Pluronic L-35 Tergitol NPX 728

227 227 233 235 235 235 237 239 243 245 246 249 253 256 258 268 269 272 274 274 280 283 289 293 293 295 297 299 299 302 305 308 310 310 312 317 319 320 321 322 323 326 327 328 333 335 336 339 340 341 344 344 344 349 351

351

9.4. Approaches to Stationary Phase Selection Table 93 (continued) Polytergent B-350 POlyglyCOll5-200 Polytergent G-300 Fluorad FC-431 Igepal CO-710 Triton X-100 Bis(2-ethoxyethyl) phthalate XE 60 (Elastomer) XF-1125 Ucon 50-HB-5100 Trimer Acid Igepal CO-730 Renex 678 ASI-50 Methyl ov-25 Lutensol Siponate DS-10 Bis(ethoxyethoxyethy1) phthalate Tween 80 Stepan DS-60 Ucon 75-H-90000 AN-600 Pluronic F-88 Pluronic F-68 NPGA HI-EFF-3 AP Igepal CO-880 Surfonic N-300 Triton X-305

353 354 360 360 361 362 364 367 367 375 378 379 381 386 386 386 388 389 396 402 406 408 419 423 424 426 427 430 N-0-Aminoethyl-y-amiopropyltrimethoxy- 433 silane 438 NPGA 449 Carbowax 400 Monostearate 457 Paraplex G-40 463 Cyclohexanedimethanol succinate HI-EFF 8 BP Zonyl E-7 465 466 EGTCP 469 y-Morphiny lpropyltrimethoxysilane 472 NPGS 414 NPGS HI-EFF 3 BP 475 Igepal CO-990 481 Cyclohexanedimethanol succinate 489 NPGS 489 Quadrol 510 Carbowax 20M 512 Carbowax 6000 512 Carbowax 6000 Mer-21 520 Carbowax 4000 520 Carbowax 20-M-TPA 521 Carbowax 4000 528 XF-1150 530 Silar 5 CP (Phenyl) 530 SP 2300 531 AS1 50 Phenyl 543 Sorbitol hexaacetate

-

Zinc Stearate SP 1000 EGSP-Z N-Ally l-y-Aminopropyltrimethoxysilane Ethylene glycol isophthalate Saccharose octaacetate Carbowax 1500 Epon 1001 Carbowax 600 Butanediol succinate Poly(ester acetal) Mer-2 Ethylene glycol adipate HI-EFF 2 AP Ethylene glycol adipate FFAP STAP Butanediol succinate HI-EFF 4 BP Ethylene glycol adipate Carbowax 1540 PDEAS HI-EFF 10 BP Diethylene glycol adipate LAC-IR-296 Diethylene glycol adipate HI-EFF 1 AP EGSS-Y Diethylene glycol adipate DEGA-P LAC-2R-446 PGA Reoplex 400 Silar 7 CP SP 2310 SP-216-PS, SP-222-PS Carbowax 1000 EGSP-A ECNSS-S ECNSS-M Silar 9 CP (10%Phenyl) EGSS-X SP 2330 Diethylene glycol succinate Diethylene glycol succinate SP 2340 Silar 1OC Diethylene glycol succinate DEGS-PS Tetracyanoethylpentaerythritol Diethylene glycol succinate Diglycerol Diethylene glycol succinate HI-EFF-1 BP Ethylene glycol succinate (EGS) EGP HI-EFF 2 GP EGS, HI-EFF 2 BP THEED

Hexakis(cyanoethoxy)hexane Tetrakis(cyanoeth0xy)butane Cyanoethylsaccharose

Bis(cyanoethy1)formamide OV-275

544 546 549 555 561 569 589 601 605 611 612 615 617 619 627 627 629 633 641 654 655 658 66 1 667 671 673 673 680 689 700 722 732 778 778 778 179 791 800 801 835 837 852 854 860 864 872 889 893 901 94 1 976 1000 1089

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Author Index

The author index includes the names of all authors whose work is cited including those cited as “et al.”. Numbers without parentheses indicate the page on which the author is cited in the text. Numbers in parentheses are reference numbers (List of references see “Literature”, pp. 352-377). Abadie, A. (800) Abe, I. (902), (909), (1020) Abel, E. W. (595) Adams, N. W.(237), (760), (847), (1121) Adlard, E. R. (941) Aguilera, C.(920) Aizhu, X.(656) Al Taiar, A. H. (597) Al-Bayati, R. I. (840) Al-Saigh, Z. Y. (678) Al-Thmir, W. K. (1104) Albaiges, J. (303) Albert, K. (627) Alessi, P. (974) Alexander, G. (99), (189) Alisoev, V. R. (752) Allegrini, I. (542) Allen, R. R. (968) Allmendinger, H. (913) Alt, S. G. A. H. (1114) Altenau, A. G. (750) Anders, G. (193) Anderson, B. C. 272, 330, (986) Anderson, D. G. (994) Anderson, J. H. (458) Anderson, K. (1156) Andreazza, D. (248) Andrews, F. (1042) Andms, M. B. (847) Andms, R. S. (760) Anokayev, V. I. (201) Ansel, R. E. (994) Anthony, V. (682) Appleyard, J. (929) Aries, R. E. (655) Armistaed, C. (443) Armstrong, D. W. (934b), (934c) Ashes, J. R. (928) Ashton, W. T. (967) Aue, W.A. 151, 152, 171, 177, 251, (589), (594), (6791, (683), (684), (6941, (736), (948), (949) Augl, J. M. (679) Averill, W. (151) Avgul, N. N. (381)

Babashkina, Ye. S. (832) Badings, H. T. (169) Baiulescu, G. E. (279) Bajgubekova, A. A. (463) Balizs, G. 301, ( l l l l ) , (1112), (1115) Ballantine, J. A. (993) Baney, R. H. (753, (802) Bapat, B. V. (976), (1082) Barber, W. E. (24) Barcelo, D. (730) Barkamova, T. V. (392), (393) Barney, J. E. (1130) Barr, J. K. (102) Barry, E. F. (2391, (571) Barth, E. (880) Bartha, A. (746) Bartle, K. D. (80), (81), (174), (251), (297) Basila, M. R. (441) Bass, J. L. (638) Basu, A. N. (531) Bate, R. (943) Batich, C. D. (676) Bauer, U. (926) Baumann, F. (763) Bayer, E. (285), (627), (635), (895), (896), (897), (899), (1035), (1038), (1039) Bayer, F. L. (129) Baylay, A. (417) Bebris, N.K. (466) Behlau, H. (178) Behrensmeyer, P. (624) Beitler, U. (898), (1029) Belokleytseva, G. M. (668) Belova, N. S. (857), (858), (931) Belyakova, L. D. (424), (565) Bendre, S. B. (954) Benecke, I. (905), (906), (907), (908), (911), (912) Benesi, H. A. (453) Berek, D. (636) Berendsen, G. E. 155, 157, (615) Berezkin, V. G. 166,208,226, (49, (136), (677), (7521, (7851, (860) Berg, R. H. (657), (658) Berg, W. T. (801)

380 Berger, P. (403) Beroza, M. (967) Berrod, G. (1031) Bertsch, W. (238) Best, F. W. (159) Betti, A. (1006), (1007) Betts, T. J. 309, 311, (338), (1134) Bhatia, Q. S. (769) Bhattacharjee, A. (528), (531), (532) Bhattacharya, A. C. (528) Bhattacharya, S. C. (976) Bhaumik, A. (532) Bierl, B. A. (967) Bighi, C. (1006), (1007) Billeb, K. A. (76), (263) Birrell, P. (1026) Bishara, R. H. 309, (1129) Blaser, W. W. (1001) Blaszo, M. (813) Blau, K. (172) Blinder, J. S. (1158) Blomberg, L. 66,69, (80), (83), (175), (180), (205), (206), (2071, (2091, (2121, (2201, (221), (2221, ( W , (223a), (224, (2301, (2401, (241), (2421, (738), (8451, (877), (879), (955) Blum, W. 276, (588), (878b), (997), (1165) Bocek, B. (133) Boeva, V. L. (5 11) Bohemen, J. 20, (36) Bojkova, A. S. (399) Bollmann, D. H.(422) Bombaugh, K. J. (577) Bonastre, J. (343), (344), (345) Boneva, S. (788) Bonner, D. C. (799) Bontoyan, W. R. (1130) Bonvitsky, H. (177), (735) Bossart, C. A. (593) Bossart, C. I. 66,75, (203) Bouche, J. (192) Bourdon, J. (1031) Bradshaw, J. S. 245, (2111, (232), (233), (234), (2351, (2371, (7601, (8331, (8411, (8461, (8471, (8781, (9211, (9221, (927), (1121), (1122) Brassal, B. (934a) Bravina, N. N. (857) Brazell, R. (1042) Brazhnikov, V.V. 182, 183, (714), (728), (729) Brenner, N.(986) Bretting, H. (907) Briegleb, G. (972) Brookman, D. I. (567) Brooks, F. R. (992) Brunauer, S. (662) BNner, F. (2701, (414), (538), (542) Brush, T. P. (430) Budahegy, M. V. 87, (304)

Author Index Budantseva, M. N. (677) Buijten, J. 69, 222, (138a), (180), (205), (220), (221), (222), (2231, (223a), ~ 2 4 (2301, ) ~ (240)~ (2411, (2421, (51O), (7381, (8451, (8771, (879, (955) Buker, J. (86) Burgraff, L. W. (604) Burke, M. F. (438), (508), (632), (687) Burkle, W. (1054) Bums, W. 101,329, (365), (835), (1138) Buryan, P. (353) Bush, S.G. 117, (440), (633) Buszewski, B. (636) Butler, H. T. (1017) Butler, L. D. (508) Caddy, B. (932) Cadogan, D. F. (39), (120), (450), (569) Calas, R. (800) Callait, R. (429) Callen, J. E. 268, (977) Callis, J. B. (605) Canellas, J. (298) Capponi, G. (552) Carbin, J. A. (1044), (1049) Card, T. W.(678) Carlstrom, A. A. (1130) Carpenter, D. J. (753) Carr, P. W.(24) Cartoni, C. (283), (1123), (1124) Cartoni, G. P. (270) Case, L. C. (988) Castello, G. 122, 126, 128, 197, (354), (488), (489), (4901, (491), (4921, (4931, (4941, (793) Castle, L. W. (211), (232), (234) Castle, R. N. (918), (919) Caswell, R. L. (1130) Chabot, E. (239) Chambaz, E. M. (184), (204) Chang, C. D. (934c) Chang, H.C. K.(921) Charitonov, N. P. 207, (818), (819) Charles, R. (898), (1029), (1030) Charles-Sigler, R. (1023), (1025) Chaubey, U. D. (350) Chauhan, J. 77, (271) Chen, C. (869) Chen, J. K.(769) Chi FUxiao (834) Chiavari, G. (1087) Chinghai, W. (656) Choe, E. W. (797) Chong, E. 329, (1150) Chovin, P. (319), (320) Chow, L. C. (1083) Chow, W. M. L. (932) Chretien, J. R. (404)

Author Index Christian, S. D. (666) Chuiko, A. A. 175, (693) Ciccioli, P. (538), (554) Cirendini, S. (464) Claessens, H. (634a) Claque, D. H. (628) Clarson, S.J. 203, (805) Clauss, H. (812), (868) Claver, R. F. (763) Clement, R. E. (482), (483) Coddens, M. E. (1017) Coenen, G. (460) Colber, C. (934) Colbom, A. S. (618) Coleman, A. E. 233, (866), (876) Collier, R. H. (1130) Comius, N.R. (158) Conaway, J. E. (838) Conder, J. R. (39), (120), (732), (944) Condon, R. D. 57, (144) Cook, G. T. (674) Cook, L. E. (573) Cooke, N.H. C. (571) Cooke, W. D. (94), (705) Cooper, W. J. (882) Cope, A. C. 272, (985) Costa, B. (943) Cowan, J. C. (585) Cram, S. P. (478), (479), (480), (481) Cramers, C. A. 72,73,189, (123), (133), (245), (253), (634a). (749) Cremer, E. (3), (14) Crescentini, G. (538) Cronin, D. A. (773) Crowley, S.J. (233), (846) Crowther, J. B. (649) Cumbers, M. (860a), (888) Curthoys, G. (456) D’Arnato, G. 122, 126, 128,197, (354), (488), (4891, (490), (491), (492), (493), (4941, (793) Dabrowski, R. (1077) Daemen, J. M. H. 185, (505), (727) Dagliesch, C. E. (672) Dahlmann, G. (290), (1117) Damaeva, A. D. (933) Damascens, L. (878 b) Dandeneau, R. (79, (179), (837) Daniels, J. E. (578) Daniewski, M. M. 177, (683), (684). (694) Dankelman, W. (505), (727) Darbre, A. 77, (271) Dark, W. A. (577) Datar, A. G. (574) Dave, S. B. (498) David, F. (848), (878c) Davis, C. M. (88)

381 Davison, V.L. (582) Dawes, P. (860a), (888) De Bellow, M. (604) De Boer, F. E. (1009), 284 De Briceno, B. (1150) De Galan, L. (615) De Goey, J. 57, (140) De Haan, J. (630), (631), (634a) De Nijs, R. C. M. (167), (170), (245), (SlO), (749), (952) De Ruwe, J. J. M.(170) De Stefano, J. J. (596), (599), (646) De Zeeuw, J. (SlO), (514a), (952) Debye, P. 83, (282) Deger, W. (1164) Deleuil, M. (464) Delly, R. 309, (1132) Demus, D. (1085) Denson, K. B. (554) Deshpande, D. D. (1120) Desty, D. H. 57, (13), (74), (139), (141), (941), (987), (1003) Dewar, M. J. S. (522), (1071) Deya, P. M.(294) Dhanesar, S. C. (998), (lOOO), (1017) Di Corcia, A. 108, 138, (388), (413), (537), (541), (5441, (54% (5461, (5471, W ) , (5511, (5521, (5531, (5551, (5561, (557) Di Mauro, P. (239) Diedrich, H. (695) Dielmann, G. (308), (735) Dijkstra, G. 57,63, (140) Dimov, N.(788) Diricks, G. (181), (878c) Dittmar, K. (884) Dmokhovskaya, Ye. B. (855), (856) Do Phuong, N.(1152) Dobashi, Y. (914c) Doi, T. (1046), (1047), (1057), (1058), (1059) Dolova, N. A. (411) Dominguez, J. A. G. (1116) Dondi, F. (1006), (1007) Dong-Peng, X.(588a) Donike, M. (115), (675) Donnet, J. B. (583), (640) Dooper, R. P. M. (167), (170) Doring, C. E. (lo), (ll), (468) Dorofeyenko, L. P. (856) Dousek, F. P. (423) Dreux, J. (1031) Dua, R. K. (507) Dufek, P. (889) Duffy, M. L. 209,214, (831) Diirbeck, H. W. (86) Durbin, D. E. (421) Duszyk, K. (530) Duvekot, J. (138a)

382 Dwyer, J. (517), (519) Dyer, A. (517) Ecknig, W. 328, 329, (362), (362a), (3641, (839) Eek, L. (730) Eggertsen, F. T.(663) Eiceman, G. A. (483) Eisenbeiss, F. 163, (661) Elkington, P. A. (456) Emerson, J. A. (769) Emmet, P. H. (662) Endo, M. (469) Engelhardt, G. (842) Engewald, W. 103, (85), (137), (156), (187), (252), (396), (400), (403), (405), (406c), (5001, (1104a) Enqvist, J. (829)

Eon,C. (367), (1140), (1147) Eppert, G. (1) Erard, J. F. (650) Erkelens, J. (459) Erlton, J. (429) Ersova, S. V. (752) Estel, D. (468) Etcheto, M. (777), (1148) Ettre, L. S. 43,57, 75, 87, (9,(28), (53), (56), (64), (771, (1491, (2551, (2621, (2631, (2681, (2891, (2911, (3021, (3061, (32% (326), (3271, (990) Evans, M. B. 98, 99, (271, (349), (359), (361), (691), (7791, (8111, (870). (9371, (9381, (946) Everett, D. H. (417)

Fabian, G. 286, (914a) Fairbrother, F. 72, (250) Farkas, P. (1062), (1088) Farlinger, P. W. (577) Farre-Rius, F. (150) Farroha, S. M. (840) Farwell, S. 0. (78) Fazio, S. D. (649) Fedyanin, A. A. (1080) Feibush, B. (898), (900), (1023), (1024), (1025), (1028), (1029)

Felice, L. J. (25) Feltl, L. 118, (432), (465) Fenimore, D. C. (88) Field, L. R. (481), (605) Fields, S.M. (236), (766) Figgins, C. E. (370) Filbert, A.M. (153), (704) Filchev, P. I. (116) Filonenko, G. V. (668) Finckelmann, H. 240, (915), (916), (917) Finucane, J. (338) Firpo, G. (303) Fischer, J. (31) Fischer, R. F. 268, (979) Floyd, T. R. (649)

Author Index Ford, D. G. (387) Forman, P.(943) Fraissard, J. (429) Francey, K. F. (741), (742), (743) Frank, H. 152, 157,173,238, (446), (608), (611), (612), (893, (8961,

W),(8991, (587)

Franken, J. J. (167), (170) Frederick, D. H. (705) Fredericks, E.M.(992) Freeman, R. R. (882) Friedrich, K. (1132) Fripiat, I. J. (428) Fritz, D. (496), (791) Fruitwala, N. A. (732), (944) Fujimoto, T. (965) Fuochard, R. C. (957) Furton, K. G. (1016) Gabel, D. (648) Gacke, D. (869) Galceran, M. T. (730) Galin, M. (756) Gallei, E. (445) Ganansia, G. (386) Ganansia, J. (264) Gangoda, M. E. (619), (623) Garcia-Raso, A. (294), (298) Garcia-Raso, J. (298) Garrido, L. (754) Garzo, G. (813) Gasparini, G. M. (702) Gassiot, M. (303) Gavrilova, T. B. (380), (425) Gawdzik, J. 152, (138), (205), (590) Geiseler, G. (278) Geiss, F. (1011) Gellerman, J. L. (936) Genieser, H. G. (648), (651), (652) Genkin, A. N. (104), (670) Gent, P. L. (523) Gerber, J. N. (570) Gerrard, W. 204, (807) Ghatge, B. B. (9541, (976), (1082) Giddings, J. C. ( 3 3 , (95) Gil-Av, E. 287, (898), (900), (1023), (1024), (1025), (1028), (1029), (1030), (1035), (1043)

Gilbert, A. R. (244) Gillham, V. K.(864) Gilpin, R. K. (438), (607), (619), (620), (623), (6321, (6421, (687)

Givand, S. H. (573) Glaijch, J. L. (999) Glazunova, L. D. (511), (512) Glindemann, D. 297, (130a), (1092) Glockner, G. 194, (776) Gluck, S. J. (78) Gnauck, G. (31)

Author Index Godefroot, M. (157) Godovsky, Y.K.(924) Golay, M.J. E. 54,57,64,181,(20), (21),(72), (706) Golding, B. T. (1056) Golovnya, R.V. 95,96,(200),(359,(356), (362b) Golubchikova, L. A. (857) Gonnord, M. F.(404) Goodwin, B.L. (198) Goodwin, T. A. (107) Goretti, G. C.(273),(956),(1089) Goryayev, V.M.(201) Gossner, M.(1164) Graham, J. A. 147,(581) Grand, D.W.(849) Grant, D.W.(272),(961) Grassie, N.(741),(742),(743),(814),(815) Green, C.R.(159) Grenier, P. (343),(344),(345) Gridina, B. F. (856) Grigoryeva, D.N.(359, (356) Grinevich, K.P. (857) Grob, G. (154),(155), (176),(216L (217),(219), (225),(2261,(259),(737L (765),(844) Grob, K. 57,58,62,66,74,(147),(154), (155), (165),(1681,(176),(W, (216),(217), (2191, (2251,(2261,(2591,(5881,(7371,(7651,(844) Grob, K., Jr. (lSS), (216),(259),(771), (772) Grobler,A. 301,(llll), (1112), (1115) Gross, D.(996) Grossmann, D.(1128) Grumadas, A. Y.(408) Grushka, E.46,47,(92),(93),(107) Guang-Liang, L. (588a) Guanghua, W.(656) Guardino, X.87,(303) Guenther, J. R.(168) Guillemin, C.L. 118, 163,(464), (559), (660) Guillet, J. E.(1149) Guiochon, G. 57,72,108,140,141,182,(291, (33), (150),(2641,(386),(4041,(558), (5631, (710) Gunther, C. (1164) Guthrie, G. R.(801) Gutteridge, C.S.(655) Gvosdovich, T.N.113,(4181,(484),(483, (486) Haachte, E. 0.A. (867) Habboush, A. E.(840) Hair, L. M.(704) Hair, M.L. (153), (163) Haken, J. K.87,91,122,195,197,200,208,233, 242,244,314,315, 329,(4),(296),(300),(301), (3301,(349). (3521,(497),(6911,(7791,(7801, (8211,(822), (782),(785),(7891,(7% W), (893),(928),(929),(930),(1137), (1152)

383 Halasz, I. 75,108,145, 164,181,(73),(261),(383), (3841,(467),(5791,(6221,(665),(701), (707) Halkiewicz, J. (1014) Hallgren, K.C.(725) Hamaguchi, N.(722),(1005) Hambleton, F.(443) Hambleton, L. G. (1130) Hammers, W. E.(641) Hanneman, W.W.(1008) Hansbrough, J. R. (836),(1110) Hanson, J. F.(763) Hansson, L. (654) Hara, S. (914c) Haraldson, L. (106), (786) Harbison, M.W.P.(40) Haresnape, J. N. (74), (139) Hargrove, G. L. (12) Harper, C. W.(846) Harris, J. M.(617),(618) Hartkopf, A. (1128) Hartwig, R.A. (649) Hastings, C.R. 151, 152,(589),(594),(679),(948) Hatano, H. (1081) Hatz, R. 292,(1068) Haupke, K.(502) Hause, J. A. (111) Hawkes, E. C.(939) Hawkes, S.J. 101,308,309,329,330, (365),(747), (7481,(7531,(8071,(835),(8431,(939),(11281, (1135), (1138),(1150), (1159) Hawkins, B. L. (634) Hayes, G. R. (628) Hayes, J. M.(554) Hayes, W.V. (724) Hazen, G. G. (1 11) Heckers, H. (884) Heckman, R. A. (159) Heeg, F. J. 74,(260), (305) Heine, E.(467) Heintz, A. (826) Heldt, U.(339) Hemetsberger, H.(624) Henderson, D.E. (862), (1013) Hendriks, M.E.(727) Hendriks, W.E.(505) Henke, M. (502) Henley, R.S. (112), (981) Hennetsberger, H.(602) Hennig, J. (624) Henniker, J. (150) Hensley, J. L. (774) Herington, E. F.G. 23,(38) Hernandez, M.(76) Herraiz, M.(227) Hertl, W.(163) Hesse, G. (90) Hetem, M. (634a)

384 Hevesi, T. (787) Heyer, W. 120, (472), (473) Hill Jr., H. H. (483) Hill, T. L. (47) Hishta, C. (705) Hlavay, J. (746) Ho, D. K. M. (792), (930) Hockey, J. A. (443), (454) Hofberg, A. H. (1130) Hoffmann, H. (502) Hoffmann, S. (241), (242), (738), (877), (879) Hofmann, M. (299), (311) Holik, M. (449) Hollis, 0. L. 120, 122, 123, (474), (479, (476), (7231, (724)

Hoppe, M. J. (601) Horiba, M. (1057), (1058) Homing, E. C. 182, (71), (110), (267), (672), (682), (711), (867)

Homing, M. G. (672) Horvath, C. 75,181, (73), (261), (384), (707) Hose, W. (516) Hoshika, Y.(543), (549) Hossenlopp, I. A. (801) Howard, P. Y.(1032), (1033), (1034), (1036) Howery, D. G. 330, (1158) Hoyos, M. (1015) Hrivnac, M.(795) Huball, J. A. (239) Huber, J. F. K. 47, 163, 330, (61), (96), (134), (135), (661), (1161)

Huber, L. (249) Hubitzki, J. A. (111) Huis, R. (628) Hunnicutt, M. L. (618) Hurd, D. T. (243) Hurrel, R. A. (55) Husmann, H. 63, (821, (1601, (1771, (178), (188), (1971, (22’0, (735)

Iler, R. K. (442), (570a) Ilie, U. A. (279) Imelik, B. (429) Inda, Y.(1046), (1047) Isbeil, A. F. (568) Isenhour, T. L. (1128), (1151), (1155), (1157) Issaq, H. J. (914 b) Itabashi, Y.(886) Itsikson, L. B. 195,208, (785) Iwao, M. (918), (919) Jackson, R. B. (1002) Jackson, W. P. (236). (833) Jadhav, A. L. (1081) Jain, S. K. (507) James, A. T. 19, 216, (6), (69) Jamieson, G. R. (885)

Author Index Janhk, J. 182,277, (342), (346), (712), (799, (990), (1088)

Jancke, H. (842) Janini, G. M. (692), (914b), (1063), (1074), (1076) Janowski, F. 120, (472), (473) Jansson, B. 0. (725) Jansta, J. (423) Jaroniec, M. 152, (590) Jastorff, B. (648), (651), (652) Jenkins, K. J. (957) Jennings, W. G . 59, (161), (162), (199), (734), (816), (8821, (958)

Jequir, W. (410) Jin, H. L. (934b) Johnson, J. F. (1008) Johnson, R. S. (760), (847), (1121) Johnston. K. (1073) Jones, A. C. (453) Jones, B. A. 240, (234), (2351, (2371, (841), (878), (922), (1122)

Jones, W.L.(34) Jonsson, J. A. (106), (786) Jorgensen, J. W. (633), (440) Jowitt, H. (126) Jung, G. (1043) Jiintgen, H. (415) Jurczyk, H. U. (615 a), (645) Jurs, P. C. (348), (368) Justic, J. B. (1151) Juutilainen, T. (829) Juvet, R. S. (478), (1010) Kadam, A. N. (954) Kaiser, M.A. (336), (676), (1142) Kaiser, R. E. 57, 113, (521, (541, (911, (108), (131), (1421, (254), (419), (420)

Kaizuma, H. (383 a) Kalashnikova, E. V. (316), (401), (406), (412) Kalinowski, H. 0. (884) Kalpakjan, A. M. (565) Kanazashi, M. (214) Kantor, S. W. (243), (244) Kapila, S. (948) Kaplar, L. (331) Karabanov, N. T. (1080) Karagounis, G. (1022) Karasek, F. W. (482) Karger, B. L. 102,311, (42), (94), (117), (366), (367), (708), (1139), (1140), (1147)

Karmanova, L. I. (855) Katayama, T. (41) Kaufman, H. R. (421) Kavabanov, N.T. (1078) Keesom, W. H. 81, (277) Kelker, H. 292, (1064), (1065), (1066), (1068) Keller, R.A. 311, (1031, (3661, (9431, (1091), (1139)

Author Index Kemula, W. (529),(530) Kendall, D.S. (604) Kersten, B.R.(406a), (406b) Kessaissa, Z.(583),(640) Keulemans, A. I. M. 277,(989) Keutmann, E.H.(690) Khasinevich, S.S. (832) Khripach, V.A. (201) Kieser, M. E.(942) Kikic, I. (974) Kim, Y.S. (638) King,G.(172) Kirichenko, E.A. (933) Kirkland, J. J. 182,(596),(598),(599), (646), (7091,(713) Kirsh, S. I. (1080) Kiselev, A. V. 105,108,109,112,113, 148,157, (461,(481,(491,(3151,(377), (3781,(3791,(380), (381), WO), (393),(3941,(397), (402),(4061,(4071,(408),(411),(4181,(4241, (4% (433),(434),(457),(4621,(46% (466), (470),(471),(484),(4851,(486),(5211,(5351, (5391,(540),(561),(5651,(5861,(6001,(606), (6101,(6141,(700) Kissinger, P. T. (25) Kitahara, H.(1046),(1047),(1057),(1058),(1059) Klaucke, C.(85) Klee, M.S. 311, (336),(1142) Klein, J. 298,(671),(1108) Kleinert, T.328,(364),(839),(842) Kleinovskaya, M.A. (817) Klinkenberg, A. 19,(19) Klinskaya, N.S. (509) Kloden, W.330,331,(1162) Knoll, H.(516) Knollmueller, K. 0.(853) Knox, J. H.(58) Knox, K. L. (672) Kobayashi, T. (965) Koberstein, J. T.(769) Koch, H.J. (915) Kochetov, V.A. (933) Koenig, W. A. (934),(934a) Kohno, T.(1020) Kolarovic, L. (1166) Komers, R. 130,(379,(513) Kong, J. M.(748),(843) Kong, R.C.222,240,(202),(236),(766),(918), (919) Konig, W.A. (905),(906),(907),(908),(1037), (1039),(1040),(1041) Konovalova, L. V. (931) Kopecka, H.(375), (513),(514) Kopecni, M.M.(566) Koppenhoefer, B. (913) Korol, A. N.101,165,195,197,200,208,(276), (3631,(6681,(781), (790),(794)

385 Koroleva, T. V. (857),(858) Kortum, G.(50) Koser, H. J. K. (339), (290),(1117) Kostylev, I. M.(933) Kotsev, N.K. (116) Koury, A. M.(836),(1 110) Kovaleva, N.V. (392), (393),(540) Kovfits, E.85,89,(287),(288),(292),(293),(321), (4961,(6501,(7151,(791) Kozeny, J. (30) Kracht, W.R.(1001) Kralik, D.(307),(332) Kralovicova, E. (1062), (1079) Kramer, J. K. G. (957) Kramer, R.E. (750) Kratz, P.D.(314) Kraus, G.292,(787),(l062), (1067),(1072), (1079) Kraus, M. (373,(513), (514) Krawiec, Z.(404) Kriegsmann, H.328,(362),(362a), (364),(839) Krishen, A. E.(619) Kritzler, S.233,(893) Kriicke, B. (925) Krupak, J. (295) Krupcik, J. (787),(1079) Kruppa, R.F.233,(112),(876),(981) Krysl, S. (515) Ku, W.W.287,288,(1019), (1021) Kubin, M.(372),(373) Kucera, E.(371) Kucera, P.(427) Kuei, J. C.(211),(833),(878) Kuey, J. C. (232),(233),(2341,(235),(237) Kuhne, J. (37) Kuramoto, S. (902),(909) Kurita, R.(469) Kwantes, A. 277, (989) Kwasnik, H.R. (853) Laffort, P. 311, (777), (1146), (1148) Landault, C.182,(710) Landowne, R.A. 186,268,(731),(978) Lange, E.(468) Lange, K. R. (426) Langer, S. (615a),(686),(699a) Langer, S. H.(351), (973),(975) Langmuir 27 Lanza, E. (887) Larsen, J. V. (679) Larson, P.A. (179),(246),(667),(761),(837) Laub, R.J. 299,300,(40), (881), (916),(1099), (llOO), (1101), (1102), (1103), (1104),(1105), (1106), (1107) Laub, R. L. (566),(914b) Lauer, H.H. (96),(134),(135) Laufer, S. (461)

386 Laughlin, K. B. (336), (1142) Lautamo, R. (882) Lauwereys, M. (1153) Lawton, D. (524) Leary, J. J. 329, (1109),(1128),(1151),(1157) Lebbe, J. (320) Lee, M. L. 68,240,(152), (174),(202),(208),

(2101,(2111,(2131,(2321,(2331,(2341,(2351, (236),(237),(2971,(7601,(7661,(7671,(768), (841),(8461,(8471,(878). (9181, (830),W), (919),(921), (922),(927),(1121), (1122) Leibnitz, E. (7) Leibrand, R. J. 309,(1131) Lemperle, E. (1022) Lenders, P.(1153) Levina, N.S. (104),(670) Lewis, D. A. (117),(708) Leyden, D. E.(604) Leymann, W.(86) Li, W.Y.(934c),(934b) Liberti, A. 108, 138,(273),(385),(388), (537), (542),(545), (551), (553),(556),(956), (1124) Lichtenstein, H. A. (1039) Lichtenthaler, R. N.(826) Linsen, B.G. (459) Linton, R. W.(633),(634) Lipsky, S.R. 57,186,268,(76), (164),(182), (183), (185),(189h (731),(978) Little, J. N. (577) Littlewood, A. B. (Pi), (970),(lolo), (1025) Litvinov, I. A. (677) Liu, R. H.287,288,(1019),(1021) Lloyd, F. P. (862) Lo, F.B. (93) Loche, D. C.(120) Lochmuller, C. H.152,289,(591),(616), (617), (618),(621), (1048),(1050) Locke, D. C. (39) Lodi, G. (1007) Lombosi, E.R. (304) Lombosi, T.S. (304) London, F. 80,(275) Longeray, R. (1031) Lopes, S. (888) Lovelock, J. E. 57,(145) Liiwentap, M. S.H. 96,311,(358) Lowrie, R. S. (283),(1123) Lowry, S.R. 330,(1151), (1155), (1157) Lu, X.-R. (934d) Liillmann, C. (651),(652) Luie, A. A. 195,208,(7841,(823) Luskina, B. M. 226,242,(817),(857),(858),(859), (931) Lutovsky, P. (465) Lutz, S. (934),(934a) Lygin, V.I. (462) Lynch, D. F. (1109)

Author Index Maasfeld, W. (602) Macak, J. (353) Macfarlane, I. G. (7411,(743), (814) Machata, G. (119) Maciel, G. E.(447),(448),(625), (626),(634) Macrae, R.(655) Maczek, A.0.S. (525),(526) Madani, C.66,(184), (204) Madden, B.G. (300) Maddock, K.C.(682) Mailen, C. H.(534a) Majors, R. E.(601) Malina, M.(1130) Mangani, F.(414) Marik, K.137,(533), (534) Mark, J. E.(754) Markides, K.E.62,69,240,(175), (180), (209), (W, (2201,(221), (2221,(223,(223a), (2241,

(23O),(2401,(2411,(2421,(7381,(7671,(768), (7691,(8331,(8451,(8471,(8771,(8791,(9211, (927),(959,(1121) Markov, B. A. (933) Marshall, D. B. 152,(591),(616),(617) Martin, A. J. P. 19,94,164,216,(6),(as), (340) Martin, K. (383),(665) Martin, R. L. (43), (982) Martinez de la Gandara, V.(953) Martinez, F. (559) Martinez-Castro, I. (953) Martire, D.E. (40), (441,(121), (1083) Mason, W.B. (690) Massart, D.L. 329,(1153) Matejkova, B. (449) Mathews, R. G. (266), (721) Mathiasson, L. (106), (786) Mathur, D. S. (350) Matsuo, M. (914c) Matthews, R. G. 277,(991) Maurer, T.(1104a) Mauret, P.(800) McAdoo, D. I. (750) McCloskey, D. H.330,(1159) McCullough, J. P.(801) McCullough, P.R.(684) McDonald, R. S. (452) McDonell, H.L. (113), (703) McFadyen, P. (659) McIlwrick, C.R. (733) McMinn, A. L. (885) McMurray, W.C. (164) McMurray, W.J. (76), (182), (183),(185) McNair, H.M. (764), (774) McReynolds, W.0.75, 90,92,93, 195, 208, 311, 315, (257), (3241,(828) Mehran, F.F. (882) Meinschein, W.G. (554)

387

Author Index Melcher, F. W. (884) Melda, K.(231) Mentele, J. W.(759,(802) Merker, R.L. (757) Merrit, C. (750) Messerly, J. (705),(801) Meszaros, S. Y. (304) Metcalfe, L. D.(1125) Metzner, K.(7a), (313) Meyer, E.F.(778) Millen, W.(747) Miller, G.(1150) Miller, M. L.(633),(634) Milonjic, S. K.(566) Mindrup, R.307,(1126) Miner, D.J. (25) Mischnick-Luebbecker, T. (934a) Mistryukov, E.A. (200) Mitchell, S.(443) Miyahara, N. (965) Moad, G. (888) Mocak, J. (787) Mohammadi-Tabrizi, F.(861) Mohnke, M. (265),(269),(510) Mokeev, B.Ya. (466) Molera, M. J. (1116) Molnar, E.B. (332) Mooney, E.F.(807) Moore, D.J. (582) Moore, R. T. (801) Morales, S. (501) Morandi, F.(248) Moreau, M. (1031) Morel, D. (643),(644) Mori, S. (584) Moriguchi, S. (469), (572) Morinaga, K.(41) Mortimer, J. V. (523) Mortimore, D.M. (422) Mosandl, A. 287 Mosands, A. Q.(1164) Moscatelli, E.A. 182,(110),(711) Moseman, R.F.(950) Moseva, L. I. (7281,(729) Motta, L. (248) Mueller, M. D.(878a) Muller, D.(627) Muller, K.D. 203,205, (804) Muller, R. (156) Munari, S. (354) Munk, P. (678) Munoz, J.G. (1116) Murthy, A. S.N. (281) Muschik, G. M. (914b), (1073),(1074) Musha, S. (909),(1020) Muska, S. (902)

Mussche, Ph. (138a), (634a), (647), (653) Muto, G.(543),(549)

Na,H.(337),(1143) Naikeadi, K.P. (1082),(1081) Naito, K.(469),(572) Nakagawa, T.(722),(1005) Nakamura, K.(914c) Nakaparksin, S. (1027) Nasarova, V.I. (402) Nawrocki, J. (579,(637) Nazarova, D.V. (931) Neff, B. (826) Neff, W.E.(585) Nelsen, F.M. (663) Nestrick, T. J. (739) Neukom, H. P. (771), (772) Neumann, M. G. (501) Nicholson, G. J. (895),(896),(899),(1037) Nickless, G.(595) Nieder, M. (627) Nieman, J. (550) Nikitin, Yu. S. (463), (466) Nikolaev, G. A. (850) Nikolov, R. N. (122) Nikulina, L. S. (865) Nishioka, M. (237), (921), (922), (927), (1122) Nitta, T. (41) Noebels, H. J. (986) Noll, W. (215),(798),(803) Nondek, L.(444),(613),(636) Norem, S.D.(262) Norr, M. K.(679) Nota, G.(273) Novak, J. 94,95,277,311,(8), (9),(333),(341), (3421,(3461,(3471,(990) Novakova, N. (8) Novotny, M. 57,72, (79),(80),(81), (148),(186), (251) Nyiredy, S.Y. (304)

w),

Oathout, J. M. (674) Obst, D. (31) Oelert, H.H.(290),(1117) Ogden, M. W.(764) Oi, N. 288,(1046), (1047), (1057),(1058), (1059) Oina, S. (1060) Oleshima, M. (1081) Olsson,A.M. (106),(786) Onuska, F.I. (482), (483) Oro, J. (1039) On,C. H.268,(977) Osborn,M. J. 100,(361) Osbome, A. (801) Ostaszynski, A. (716) Osthoff,R. C.(243) Ostrovskij, V.V.207,(818),(819)

388 Ostrovsky, I. (l062), (1079),(1088) Othman, M.Y.B. (1113) Ottenstein, D.M.167,(673),(681) Pacakova, V. (889) Pacholec, F. (1017) Palocsay, F.A. (1109) Palombari, R. (1 124) Panina, L.I. (509),(511), (512) Panse, D.G.(1082) Pantazoplos, G.(973) Papirer, E.(583),(640) Papo, A. (974) Paramasigamani, V.(736) Parcher, J. F.329,(717), (836),(1110),(1128), (1154) Park, N. J. (233) Parr, W. (1032),(1035),(1036), (1038), (1039), (1042) Pastorino, A. M. (124) Patnode, W.70,(223b) Patte, F.311, (777),(1146),(1148) Patterson, D.(1 120) Patzelova, V. (423) Pauling, L. (796) Pavlov, V.V. (693) Peaden, P. A. 68,(2081,(210),(213), (830) Pearce, M.(238) Pedro, N.A. (721) Peene, J.A. (138a), (510) Pehle, W.(468) Peichang, L.(656) Peri, J. B. (451) Pem, F.J. (604) Perry, S.G.(55) Persinger, H.E.(940) Pesek, J. J. 147,(578),(581) Peters, E.N.(854) Peterson, P. E. 272,(985) Petho, A.(37) Petsev, N.D.(116),(122) Pettitt, B. C. (991) Pfaffenberger, C.D.(267) Pfeffer, B. (962) Phillips, C.S.G. (283),(525),(526), (733),(751), (1123) Piccola, W. A. (758) Picker, J. E.(564) Pickett, E.E. (684) Pierce, A.E.(171) Pikaart, K.A. (615) Pilgrim, G.W.(1091) Pistolesi, M.(538) Pleterski, J. (1038) Podyacheva, G.M.(391) Polanuer, B. M.(362b) Polard, Z.H. (595)

Author Index Pollock, G.E. (486a),(863) Polmanteer, K.E. (759) Pommier, C.(33) Pondor, L.H.(506) Poole, C.F.(406a),(406b), (998),(lOOO), (1016), (1017) Poole, S. K. 102,285,311,(406a),(406b) Poppe, H.(96),(1341,(1351, (439) Porcaro, P. J. (1061) Porschmann, J. 103,(1371,(393,(3961,(4001, (406c) Portmann, A. (168) Porzel, A. (868) Poshkus, D. P. (381), (407),(408) Possanzini, M.(542) Powell, H.M.(524) Pratab, R.(918),(919) Preddy, C.R. (25) Preston, S.T.(1127) Pretorius, V. (238),(158) Prevot, A. (688) Price, G.I. 311, (1149) Prietz, U.(695) Prochazka, M.(560) Proot, M.(848) Proske, A. (699a) Pryda, E.H.(585) Przybyciel, M.(1167) Pulsipher, M.A. (1121) Purcell, J. E. (76),(262),(263), (268) Purnell, J. H.20,138,298,299,300,301,(17), (361,(39), (401,(571, (63,(1201,(3511,(536), (881),(1093),(1094),(1095),(1097), (1098), (1099),(llOO), (1101),(1102),(1103),(1104), (11051,(11061,(1113), (1114), (1149),(1107) Purnell, S. H.(1096)

Quinn, C.P. (67) Radecki, A. (1014) Raglione, T. V. (649) Ramanathan, P. S. (574) Ramstad, T.(739) Rao, C.N.R. (281) Rayanakorn, M. (669), (720) Rayss, J. (105),(1077) Redant, G.(191),(218) Reed, T.M.(534a) Rees, L.V.C. (520) Rehage, G.(915) Reich, G.330,(1161) Reichsfeld, W.0.(806) Reid, E.H.(885) Reinbold, B. (348), (368),(369),(370) Reiners, J. (627) Repka, D.(787) Reschke, R. F.(705)

389

Author Index Resing, H. A. (629) Reznikov, S.A. (1119) Rhoad, J. E.(1044) Richter, B.E.(211),(232),(2331,(234) Ricken, H.(602),(624) Riedl, P.(44),(121) Riedo, F. 122,197,(496),(791) Riekolla, M.L. (771) Rijks, J. A. (1231,(133),(1671,(170),(194),(245),

Sakakibara, T. (1057) Sakharov, V. M.(357) Sakodynski, K.I. 124,182,183,(509),(511),

(5121,(714),(7281,(72% (8501,(8511,(8521, (8551, (856) Sambucini, C. (553) Samperi, R. (413),(541),(544),(543,(546),(547), (5511,(W, (553). (5551, (556) Samuel, U. (468) Samusenko, A.L.(200) (6301,(6311,(749) (2531,W), Rinaldi, G. (554) Sanchez, E.F.(1116) Rindt, J. (460) Sand, D.M.(936) Ringsdorf, W.178,180,(680), (696),(920) Sanders, L.C.(605) Risby, T.H.102,311, (348),(368),(369),(370), Sandorfy, C. (280) Sandra, P. (157),(181),(191),(196), (218),(228), (4791,(480).(481) Ritter, E. (405) (229),(647),(848),(8784,(903),(9041,(951) Ritter, G.L.(1155) Sands, B.W.(638) Ritter, J. 218,(827a) Santangelo, M.A. (1167) Roberg, H.(962) Sanz, J. (953) Roberts, W.L.(916) Satterfield, C. N.(376) Robin, J. (410) Saura-Calixto, F. (294), (298) Rodewald, D.(193) Sawyer, D.T.(12), (102), (450), (567), (568),(569), Rodriguez-Vinals, R. (303) (570) Rodzevich, N. Ye. (857) Schacht, E. (218) Schelfaut, M.(229) Rogers, L. B. (337),(838),(1044),(1049), (1143) Scheurle, B. (1066) Rohrschneider, L.74, 88, 89,311, (256),(317), Schieke, J. D.(158) (322),(323) Schiffer, P. (1031) Rokushika, S. (1081) Schindel, W.G.(999) Roller, M.B. (864) Schlapa, J. 330,331,(1162) Ronchetti, M.(1089) Schlenk, H.248,(936) Roper, F.G.(89) Schlitt, H.(1011) Rose, L.R. 91,208,312,(329),(499),(783) Schmid, P. (878a) Roselius, L. (14) Schmidt, N.(934) Rosenberg, H.(797) Schnautz, N.(238) Roth, M.94,95,(333) Schnecko, H.283,(1004) Rotzsche, H.182,183,204,205,208,212,217, 225,250,328,330,331, (97),(101).(109),(125), Schneider, P. (374) Schneller, A. (920) (1281,(17% (286).(2991,(3111, ( 3 W , (3281, (360),(362),(362a).(615a),(645),(686),(699, Schollner, R. 132,(518a) Schomburg, G.57,62,63,74, (82),(146),(177), (697),(7151,(7161,(718),(7191,WO), (7441, (178),(188), (1971,(2271,(258),(3081,(733, (7621,(799~11, (8081,(8091,(8121,(8241,(8251, (9111,(9121,(160) (8271,(8681,(8721,(8731,(874),(880),(893, Schou, 0. (667) (949,(971),(1086), (1136),(1162),(1163) Schregenberger, C. M.(760),(847),(921) Russo, M. V. (956) Schreiber, H.P. (1120) Ruthe, S.(227) Schroeder, J. P.(522),(1071) Rutten, G.A. (100). (167),(194),(245),(630), Schubert, H.(1072) (6311,(749) Schuchmann, H.(87),(118), (127),(130),(698), Ruzickova, J. (341),(346) Rybkina, T.I. (933) (699) Schulte, E. (947) Ryzhov, V. N. (860) Schulting, F.L.(170) Schurig, K.(914) Saeed, T. (903),(904) Schurig, V. 289,(1052). (1053),(1054),(1055) Saeki, T.(914c) Schuster, P. (280) Saffert, W.(269) Schutjes, C.P.M. (245), (749) Safronova, 0.A. (817) Schwachula, G.(502) Saha, N. C.(507) Schwartz, R. D. (266),(721),(991) Said, A. S. (59)

390 Schwedt, G. (1012) Schweer, H. (890) Schwenke, W. (697),(809),(812) Scott, C.G.(562) Scott, D.W.(801) Scott, R.P.W. 54,57,141,(22), (65),(70),(143),

(427),(440a),(576),(603) Sebastiani, E. (555) Sebestian, I. 145,(579),(580),(622),(701) Seewald, H.(415) Seidel, H.(278) Seifert, K. H. (1070),(1072) Selenkina, M.S. (1090) Selke, E. (585) Sellars, P. (1056) Semenchenko, L.V.311,(1144) Semina, G.N.(852),(856) Semlyen, J. A.203,(805) Serpinet, J. (643),(644),(664) Sevcik, J. 96,311,(358) Severin, G.(912) Severini, C.(413),(541),(544),(555) Shah, P. 311, (337),(1143) Shaligin, G.F.(677) Shank, J. T.(940) Shapatin, A.S. (860) Shcherbakova, K. D.148,(394),(398), (399), (400),( 4 W ,(4061,(5861,(700) Shelton, J. I. (211),(232) Shi-Hsien, H. (649) Shiba, K. (1059) Shields, R. (550) Shingari, M. K. (732),(944) Shkolina, L.A.(136) Shoup, R.E. (25) Shubiak, P. (1061) Shukhovitskii, A.A. (1090) Shulyateva,T.I. (860) Sidorov, R.I. 311,(1145) Sieckhaus, J. F.(853) Sievers, R.E. (564) Sievers, S.(906),(908) Sindorf, D.W.(447),(448),(625),(626) Singer, G.(1164) Singh, S. (281) Sinha, A. (350) Sissons, D.J. (942) Slaver, H. T.(887) Slotfeldt-Ellingsen,D.(629) Small, P.A. (775) Smead, D.(112) Smith, C.A. (916) Smith, E. D.(114),(674) Smith, J. F. (26),(27),(937),(946) Smith, J. M.(374) Smith, J. R.L. (487),(499,(597) Smith, R.J. 87,126,130,171,175,(41,(300),(301)

Author Index Smith, W. R. (387) Smolkova, E. 118, 137,(432),(4651,(533),(5341,

(935) Smolkova-Keulemansova,E. (5 15), (560) Snyder, L. R. 93,311,(334),(339, (366),(367),

(431),(439), (709), (1139),(1140), (1141), (1147) Sobolevski,N.V. (857) Sohn,J. E. (769) Sojak, L. (295),(1062),(1079),(1088) Solomon, I. (429) Sorge, H.(527) Sorio, A.(1166) Sorokina, Ye. Yu. (677),(860) Sosnova, L.(465) Souter, R.W.289, (1048),(1050), (1051),(1129) Sparagana, M.175, (690) Spencer, C.F. (1008) Springston, S.R.(231) Sprouse, J. F.(312) Staab, H. A. (1084) Stark, F. 0.(758) Stark, T.J. (179),(213),(246),(761),(830),(837) Staszewski, R. 182,(712) Stefanova, A.D.(122) Stegalkina, V.V. (931) Steimann, H. 212,217,225,250,(740),(762), (8041,(8271,(8681,(945) Stewart, G. H. (103) Stoelting, K. (1041) Stolyar, Z.M.(832) Storoshenko, T.M.(380),(425) Stouffer, J. E. (991) Street, H. V. (119) Strisung, K. D.(352) Struppe, H. G.54,57,(7),(161,(181,(231,(311, (321,(511, (601,(621,(631,(66),(681,(132), (142), (187),(2521,(274,(718) Stubbs, A. E. 72,(250) Stutler, K.A. 152,(591) Suffolk, B.R.(607) Summers, D.M.(1105) Supina, S.B. (499) Supina, W.R.91,195,208,312,(329),(783), (894,(980) Suprynowicz, Z.152,(138),(591),(1118) Svyatishenko,A.T.(136) Swanton, W.T.(987) Sweeley, C.C.182,(71), (110),(711) Syavtsillo, S.V. (857) Sybilska, D.(529), (530) Szafranek, J. (267) Szczepaniak, W.153, (573,(592) Szekely, T.(813) Szentirmai, Z.S. (332) Szita, C. (331)

Author Index Ta-Chung Lo Chang. (416) Tacacs, J. M. (304),(307),(331), (332) Takagi, T. (886) Takayama, Y.172,(685) Takei, S. (572) Takeoka, G. (958) Taker, S. (469) Takeuchi, T. (965) Tameesh, A. H. H. (495) Tani, T. (1057),(1058),(1059) Tarbet, B.J. (237),(760),(833),(847),(921),(927), (1 122) Tarjan, G. (304),(310),(331), (496),(791) Teller, E. (662) Temmerman, I. (228) Tenney, H. M. 272,(284),(983),(984) Tertykh, V. A.(693) Tesafik, D. 57 Tesafik, K. (79) Tesafik, T. (148) Tetu, T. (862) Thica, P. A.(454) Thompson, B. (477) Thompson, J. C. (238) Timar, I. (304) Todd, S.S. (801) Tomellini, S. A.(649) Tominaga, Y.(918),(919) Tomita, H. (166),(84) Torbina, Ya. I. (540) Torres, J. (266) Torri, G.(702) Toth, T. (349),(691),(779) Tou, J. C. (739) Traiman, S. (603) Traitler, H.(1166) Trash, C. R. (745) Troitskaya, N. N.(817),(858), (859),(931) Trojer, L.(654) Tsuchiya, I. (609) Tsuge, S. (965),(1151), (1157) Tucker, E.E.(666) Turkel’taub, N.M. (1090) Turro, N. J. 133,(518) Tutorskij, I. A.(752) Tweedie, H. A.(338) Tyler, A. (443) Ubeit, M. T. (692),(1076) Uden, P.C. (599,(862),(1013) UltT, D.W. (901) Unger, K. (382) Uno, T. (722),(1005) Urone, P.(717) Uytterhoeven, J. (428) Valade, J. (800) Van Cruchten, H. (631)

391 Van de Veen, A. (630),(631) Van de Veen, L. (630),(631) Van de Venne, J. (460) Van Deemter, J. J. 19,(19) Van den Dool, H. (314) Van den Heuvel, W. J. A. 227,(71). (682),(867) Van den Ven, L. G. J. (190) Van der Pool, J. G. (169) Van Hout, P. 77,(267) Van Lenten, F.J. (838) Van Miltenburg, J. V. (641) Van Roelenbosch, M. (157),(181), (228), (951) Van Swaay, M. (971) Van Tilburg, C. E. (245),(749) Van Wijk, R. 123,185,(504),(726) Vanheertum, R. (309) Varano, A. (312) Vargas de Andrade, J. H. (1097). (1098) Velden, G.V. D. (628) Velev, V. N.(116) Veltl, L.(935) Venanzi, L. M. (283),(1123) Venema, A.(190), (247) Vermont, J. (464) Vernon, F. 122,200,(497),(669),(720),(789), (995) Versino, B. (1011) Verstappe, M. (157) Verzele, M. 64,69,(157),(181), (191),(192), (1961,( 2 W ,(228),(2291,(647),(65% (903), (904) Vetrova, Z.P. (1078) Vidal-Madjar, C. (264),(386) Vidal-Madjar, S. 140,141,(558),(563) Vigdergauz, M. S. 311,(391),(1144) Vigh, Gy. (746) Voigt, C. E.(753,(802) Volchinskaya, N.I. (832) Volkov, S.A.(851) Volkov, S.M. (201) Von der Bey (934) Voskov, V. S. (357) Vouros, P.(117),(708) Vsetecka, J. (935) Vycudilik, W. (119) Vyskocil, V. (444), (613) Waddington, D. J. (499,(597) Wainwright, M.S. (4),(300), (301),(352), (1152) Wainwright, P.(1113) Waksmundski, A.(105),(1077),(1118) Wall, R. F.(986) Walla, M. D. (1167) Wan, P. 133,(518) Wang, C.-M. (9344) Wannman, T. 69,(175). (180).(209,(206),(207), (2091,(2121,(220),(221),(2221,(2231,(223a),

392 (2241,(2301,(240),(HI), (242), (738),(845), (87'0, (879),(955) Ward, J. P.(431) Warrick, E.L.(758) Wasiak, W. 153,(579,(592),(639) Wassink, J. G.(169) Watanabe, C. (84), (166) Wayne, R.S. (1130) Weber, R. (1055) Weeke, F. (82). (160) Wehrli, A.(292) Weigel, H.(996) Weiner, P.H.329,(1154),(1158) Weinstein, S.(1043) Welsch, K.(156) Welsch, T.152,157,173,(85), (137),(193),(396), (4001,(4031,(406~1, (4461,(SOO), (5871,(6111, (612) Wennrich, L. (405),(500) Wenz, G.(934),(934a) Wenzel, B. (500) Wenzel, R. N.(100) Werner, G.(156) Whitford, J. H.(88) Whyman, B. H. F. (74),(139) Wicar, S. (346) Wickersheim, K.A.(458) Widdecke, H.298,(671),(1108) Widmark, G. (725) Wieland, H. (527) Wiesner, K.(1018) Wijnheijmer, F.A. (253) Wilcock, D. F. 70,(223b) Wilder, D.R. (616),(621) Williams, K.(993) Williams, P.S. (40),(1093),(1094), (1095),(1096), (1105),(1106),(1107),(1113), (1114) Winterfeld, A. 292,(1067) Winterscheidt, H.(1066) Wirzing, G. (455) Wise, S.A.(297),(919) Withers, M.K.(966) Witkiewicz, L. (1077) Witkiewicz, Z. 292,(1069) Witte, U.(695) Wohler 202 Wojcik, J. (138) Wold, S. 330,(1156),(1160) Wong, A.K.(1056) Woodruff, H.B. (1155) Woolley, C.L.(766),(7671,(768)

Author Index Wright, B. W. 68,74,(152), (174),(208), (210),

(2131,(830) WU, C.-Y. (934d) Wu, S. (770) Wurtz 250 Xong. Xia, X. (588a) Yancey, J. A. (871) Yang, C. (1035), (1038) Yang, F. J. (481),(482),(483) Yarger, K. (672) Yashin, Ya. I. 107, 113,(48),(315), (377),(378),

(379),(3941,(3971,(4111,( 4 W ,(4661,(4701, (4711,(4841,(4851,(4861,(521), (5351,(539), (56l), (1078) Yates, P. C. (598) Yong, J. A. (534a) Young, G. J. (430) Younker, D.R.(949) Yu. Fu, L.(588a) Yudina, I. P. 222,224,(512), @SO), (851),(852), (855), (856)

Yushelevski, Yu. A. @SO), (851),(855), (856),

(512) Zaal, P.277,(989) Zado, F. M.(1010) Zahn, C. (973) Zaizeva, G. E.(466) Zakharova, T.K.(856) Zapremetov, A.Yu. (409) Zaschke, H.(1085) Zelvenskii, V.Yu. (851) Zeng, Z. R.(934d) Zerenner, E.H.(75) Zhao Guoliang (834) Zhdanov, S.P.(470),(471) Zhigalin, G.Ya. (860) Zhuravlev, L. T. (434) Ziegler, E.(146) Zielinski Jr., W. L. (1063) Zielinski, W. L. (1073),(10741,(1075) Zima, J. (432) Zisman, W. A. (98) Zivny, K. (889) Zlatkis, A. 57,(88), (145),(421),(990),(1042) Zoccolillo, L. (1089) Zollner, G. (342) Zugenmaier, P. (923) Zuidemeg, F.J. 19,(19) Zundel, G.(280)

General Subject Index

In this index, general subjects predominate, and stationary phases are listed here only as groups. For particular, especially commercial products see “Index of Stationary Phases”. Acetates 267 Acid washing 171 Acidity index 103,329 Acids 246 Acrylonitrile 273 Activated charcoal 113 Activation energy of viscous flow 203 Active catalytic sites 164 Activity of solid supports 164,166 Activity, indication of 166 Adamantyl siloxanes 224 Adipates 262 Adjusted retention time 4 Adjusted retention volume 5 Adsorbed monolayers 138 Adsorbent, specific area of 7 Adsorbent-adsorbate interactions 116 Adsorbents for bonding reactions 143 Adsorbents of type I 104 Adsorbents of type I1 105 Adsorbents of type 111 105 Adsorbents, classification of 103 Adsorbents, modified 137 Adsorbents, non-specific 104 Adsorbents, porous 107 Adsorbents, salt-modified 141 Adsorption coefficient 8,26 Adsorption isotherm 165 Adsorption on the liquid surface 48 Adsorption sites 60 Adsorption, partial molar enthalpy of 10,26 Aerosil 11 8 Aggressive compounds 280 Air peak time 4 Alcohols 112,184,246 Alcohols, monohydric 246 Alcohols, polyhydric 246 Aldehydes 112,246,250 Aliphatic hydrocarbons 196 Alkali metal halides 283 Alkali metals 184 Alkali treatment 171,172 Alkanes 112 Alkenes 112 Alkoxysilanes 150 Alkylaromatics 112

Alkylnaphthalenes 112,201 Alumina 119 Aluminium 118 Aluminium oxide 119 Aluminosilicates 132 Amides 277 Amine, WCOT column 276 Amines 184,246,249,275 Amines, aliphatic 246,275 Amines, aromatic 277 Aminoalkyl-substituted polysiloxanes 242 Aminoalkylalkoxysilanes 244 Aminosilanes 184 Analysis time 33 Antidepressants 255 Application of chemically bonded phases 153 Aromatic amines 277 Aromatic ethers 259 Aromatic hydrocarbons 200 Aromatic siloxanes 222Aromatics 153 Asymmetry factor 74 Autoxydation 250 Azo-isobutyronitrile 67 Azo-tert.-butane 67 Azo-tea.-dodecane 67 Barbiturate cross-linked polymer 243 Basicity 102 Basicity index 239,329 Bentonites 134 Benzenesulphonates 137 Benzoyl peroxide 67 Benzyldiphenyl201

Bis(cyanopropy1)cyclotetrasiloxane 62 Bis(cyanopropy1)dichlorosilane 69 Bis(dichlorobenzoy1)peroxide 67 Bis(trimethy1silyl)acetamide 150 Bleeding 187,206 Boiling point 86 Bond energy 203 Bonded phases 155 Bonding 62,64,66,70,206 Bonding reaction 144 Boron 118 Boron nitride 114

394 Bristle model 155 Bristle type phases 155 Brush type phases 145 Bulky layer 142 C-C bond 203 Calcit 135 Capacity ratio 9,46 Capacity ratio required 46 Carbohydrates 246 Carbon adsorbents 108 Carbon molecular sieve 113 Carbopack 139 Carborane siloxanes 224,226 Carbowax 177 Carbowax degradation 250 Carboxamide groups 195 Carboxylic acids 184 Carrier gas velocity 13 Carrier gas velocity, optimal 13 Carrier gas, oxygen content of 186 Catalytic sites 164 Charcoals 113 Chemical inertness 186 Chemically bonded phases with Si-CH2-R bonds 147 Chemically bonded phases with S i - 0 4 bonds 144 Chemically bonded phases with Si-0-Si bonds 148 Chemically bonded phases, application 153 Chemically bonded phases, behaviour of 157 Chemically bonded phases, characterization 155 Chemically bonded phenyl groups 156 Chemically bonded poly(ethy1ene oxide) phases 251 Chemically bonded stationary phases 142, 144 Chemically bonded/cross-linked polyglycols 254 Chiral polysiloxanes 236 Chiral recognition 248 Chiral stationary phases 286 Chlorosilanes 150 Choosing suitable stationary phases 302,303 Citrates 267 Classification of adsorbents 103 Clausius-Clapeyronequation 28 Cluster analysis 330 Coating efficiency 73 Coating of the support 177 Coating procedures 49,63 Cohesive energy density 189 Column activity 74 Column bleeding 187,206 Column conditioning 51 Column diameter 56 Column dimensions 45,56 Column inner diameter 47

General Subject Index Column length 57 Column length, required 45 Column materials 43 Column outlet pressure 5 Column permeability 18 Column radius 56 Column testing 52 Column types 77 Column types, comparison of 77 Complexation gas chromatography 153 Condensation 66 Contact angle 47, 192 Corasill44 Corrected mobile volume 6 Corrected retention volume 5 Corrosive compounds 184 Coupled columns 296 Covalent bonding 62 Coverage, degree of 26 Critical surface energy 191,192 Critical surface tension 47, 191 Cross-linked liquid stationary phases 189 Cross-linked poly(dimethylsi1oxanes) 189, 190, 209,213 Cross-linked poly(methylphenylsi1oxanes) 223 Cross-linking 64,66,70,189,206,214 Cross-linking, in situ- 66 Cross-polarization MAS-NMRspectroscopy 192 Crown ether 245 Curing 66 Cyanoakyl-substituted polysiloxanes 229 Cyanoakylsilicones 229 Cyanoethylpolysiloxanes229 Cyanopropylpolysiloxanes229 Cyclodextrin 248 Cyclopropane246 Cyclosiloxane 62, 151 D-unit 202 Deactivation 44,60,62 Deactivation treatments 60 Decomposition of silicone phases 206 Decomposition reactions 204 Definitions 40 Degradation 206,250 Degradation of Carbowax 250 Degradation studies of silicones 206 Dehydroxylation 115 Depolymerization 250 Desoxycholic acid 136 Di-tert.-butyl peroxide 67 Diamide stationary phases 288 Diatomite support, deactivation of 177 Diatomite supports 167 Diatomite supports, properties of 176 Dicumyl peroxide 67 Didecyl phthalate 195

395

General Subject Index

Didecyltetramethyldisilazane152 Differential scanning calorimetry 190 Diffusion coefficient 189 Diffusion of organic compounds 219 Difunctional chlorosilane 61 Dimethyldichlorosilane 61,173 Dimethylsiloxane-diphenylsiloxanecopolymer 222

Dimethylsiloxane-m-carborane copolymer 225 Dimethylzinc 155 Dioxane 116 Diphenylphosphinic group 150 Diphenylsiloxanes 216,218

Diphenyltetramethyldisilazane61 Dipole moment 270 Dipole-dipole interactions 102 Dispersion forces 80,96, 102,121 Dispersion index 216,329 Dispersive interactions 96,102 Dissolution 193 Distillation theory 11 Distribution coefficient 7 Donor-acceptor forces 83,91 Donor-acceptor interactions 83,91 Durapak 145,146 Effective peak number 30 Electron affinity 262 Electron spectroscopy for chemical analysis 166 Electrostatic charging 182 Electrozone method 161 Enantiomers 287 End-capping reaction 150 Enthalpy of vaporization 10 EPN 30 ESCA 166 Essential relationships of gas chromatography 35ff. Esters 112,246,251,261 Etching 58 Ethereal oils 251 Ethers, aromatic 259 Eutectics 284 Evaporation technique 49 Excess enthalpy 10 Excess molar free energy 94 Factor analysis 329 Fatty acid methyl esters 233 Fatty acid salts 282 Fatty acids 282 Film of liquid phases 190 Film thickness 71 Films, formation of 57, 190 Filtration technique 49 Flow rate 5 Fluorine 184

Fluorine compounds 280 Fluoroalkyl silicones 227 Fluorocarbon supports 182 Folded alkyl blanket model 156 Free radical cross-linking 66 Freezing point 208 Frontal technique 49 Fronting 188 Fused-silica glass 55 Fused-silica glass capillaries 55 Gamma radiation 67 Gas hold-up time 4

Gas-liquid-solid chromatography 138 Gas-solid chromatography 103 Gases, reactive 280 General polarity 93 Geometric homogeneity 118 Geometric isomers 241 Geometrical structure 106 Gibbs-Helmholtz equation 23 Glass capillaries 55 Glass columns 44 Glass surface 58 Glass surfaces, deactivation of 44 Glass transition 190 Glass transition temperature 191,208,218 Golay equation 14 Graphite 108 Graphitized thermal carbon black 108,113,122, 124

Graphitized thermal carbon blacks, modified 139 Grignard reagent 155 Gross retention time 4 Guest molecules 131 Gummification 66 Halides 184 Halogen derivatives 112,251 Halogenated compounds 251 Halogens 184 Heat of sorption 134 Heating cycles 190 Hedgehog flask 180 Height equivalent to a theoretical plate 11 Height equivalent to an effective plate 12 Henry equation 27 Heterocyclics 278 Hexadecyltrichlorosilane 152

Hexamethylcyclotrisiloxane62 Hexamethyldisilazane 60, 61, 150 High performance liquid chromatography 154, 155

High-temperature silylation 61 Homogeneity 194 Homogeneous mixed stationary phases 301 Host structures 131

396 HPLC 154,155 Hydrazine 184 Hydrocarbons 153,154,196 Hydrocarbons, aliphatic 196 Hydrocarbons, aromatic 200 Hydrocarbons, unsaturated 112 Hydrogen bonding 102,155 Hydrogen bridge bonds 81,91,116,249 Hydrothermal treatment 115,156 Hydroxy end groups 250 Hydroxyl groups 59,117,250 Hydroxylation 115 Hylon P 169 Hypersil 154 Imines 275 Immobilization 66,209 Impregnation rate 48,49,50,167 Impregnation, degree of 50 Inclusion compounds 13 1 Indicators of residual activity 166 Indices of activity 329 Indices of basicity 329 Indices of dispersion 329 Indices of polarity 329 Induction 91, 102, 121 Induction forces 91, 102, 121 Inert metal column 55 Inertness 167 Infrared spectroscopicfrequency shift 100,329 Infrared spectroscopy 155,193 Interhalogen compounds 184 Intermolecular forces 80 Interparticle porosity 17 Ion-dipole interactions 83 Ionic adsorbents, non-porous 140 James and Martin factor 4.19 Ketones 112,251 Kinetic effect of zeolites 134 Kovats retention index 84,86, 87, 91, 194 Ladder structure 224 Langmuir adsorption isotherm 27 Leaching 58 Lewis acid centres 118 Lewis acidity 102 Lewis base centres 119 Lewis centres 134,164 Lewis sites 134, 164 Ligand mobility 156 Liqua-Chrom 182 Liquid crystal 240 Liquid crystalline polysiloxanes 241 Liquid distribution 166 Liquid phases with special selectivities 282

General Subject Index Liquid stationary phases, selectivity of 331 Liquid-crystalline polysiloxane phases 240 Liquid-solid interface 25 List of essential relationships 40 Load 47 Loading 73 Luminescence 156 M-unit 202 Macropores 163 Maximum operating temperature 186,187 McReynolds constants 89,92,198,315 Mean pore diameter 107 Melting phenomena 190 Melting point 189 Mesh sizes 162 Mesh widths 162 Mesomorphic moiety 241 Mesophases 292 Mesopores 163 Metal column, inert 55 Metal columns 44 Metal complexes 141,289 Metal halides 284 Methane 155 Methylcyclohexanols 247 Methylene group 97 Methyllithium 155 Methylphenylsiloxane 2 16 Methyltrichlorosilane 152 Micro glass beads 152,181 Micro-packed columns 53,78,110 Minimum analysis time 34 Minimum column length 45 Minimum operating temperature 189 Mixed bed columns 298 Mixed phase composition 301 Mixed stationary phases 296,301 Mobile time 4 Mobile volume 4 Modified adsorbents 137 Modified graphitized thermal carbon blacks 139 Molecular retention index 98,189 Molecular sieves 131 Molecular volume 189 Molecular weight 188 Molecules of type A 105 Molecules of type B 105 Molecules of type C 105 Molecules of type D 105 Molten organic salts 285 Molybdenum disulphide 114 Monofunctional chlorosilanes 61 Monolayer 142,150 Monomolecular layer 142,150 N-Heterocyclics 246 Nematic liquid crystals 241

397

General Subject Index Net retention volume 6 Nitrile ethers 270 Nitrile silicones 231 Nitriles 270 Nitro compounds 274 NMR spectroscopy 156 Non-polar porous polymers 126 Non-porous ionic adsorbents 140 Non-porous solids 107 Non-specific adsorbents 104 Number of effective plates 12 Number of theoretical plates 11 Numerical taxonomy approach 329 0-Heterocyclics 246 Octadienes 112

Octamethylcyclotetrasiloxane62 Octyl-modified silica gel 145 Octynes 112 Oleates 266 Olefines 153 Open-tubular columns 54 OPGV 34 Optimal carrier gas velocity 13 Optimum capacity ratio 46 Optimum practical gas velocity 34 Organosiloxane 62,179 Organosiloxanes, mechanical properties 190 Organosiloxanes, thermal properties 190 Orientation forces 81, 91,103 Outlet velocity 18 Oxidation catalyst 156 Oxidation resistance 225 Oxidative stability 203 Oxygen content of carrier gas 186 Oxygen-insensitivecarborane siloxanes 226 Ozone 62 Packed columns 43,78 Packing procedures 47, 51 Packing, preparing of 47 Partial molar enthalpy of adsorption 10,26 Particle diameter 47 Particle shape 161, 167 Particle size 160, 167 Particle size analysis 161 Partition coefficient 4,25,194 Peak broadening 19,20 Peak resolution 17 Peak width 4 Pellicular supports 181 Pentasil 135 Peptide stationary phases 287 Perfluorinated compounds 242 Perfluorocarbons 196 Peffluoroisobutene 184 Pefluoropolyether 281

Perisorb 182 Peroxides 25 1 Pesticides 153,251 Phase ratio 9 Phase specific Rohrschneider constants 312,314 Phenols 246 Phenyl content 219 Phenyl silicone 216 Phenyltrichlorosilane 153 Phosphates 265 Phthalates 262 Phthalocyanine 140 PLOT column 75,76 Polar porous polymers 128 Polar porous polymers, application 130 Polarity 87, 88,272 Polarity index 102,329 Polarity, general 93 Polarizability 2 16,2 18-22 1 Polyalkylene oxides 249,251,252,255

Polycarboranylenesiloxanes224 Polychloroarylsiloxanes23 3 Polycondensation 66, 153 Polycyanoalkylsiloxaes 230 Polycyanoethylmethacrylate 129 Polydialkylsiloxanes215,216 Polydimethylsiloxanes 62, 188, 189 Polydimethylsiloxanes, properties 208,210 Polydimethylsiloxanes, stability of 179 Polydimethylsiloxanes, vinyl-modified 215 Polydimethylsiloxanes, viscosity of 190 Polydiorganosiloxanes, mechanical properties 190 Polydiorganosiloxanes, thermal properties 190 Polydispersity 188 Polyester silicones 236,237 Polyesters 267 Polyethers, surface tension of 25 1 Polyethylene glycols 189 Polyethylene oxides, cross-linking of 25 1 Polyethylmethylsiloxanes190 Polyglycols 180,249 Polyhydroxyethylmethacrylate 129 Polyimide 129 Polymerization 66

Polymetalorganosiloxanes245 Polymethylphenylsiloxanes216,220 Polymethyltolylsiloxanes222 Polymethyltrifluoropropylsiloxanes227 Polyorganosiloxane liquid phases 201,3 14 Polyorganosiloxane phases, synthesis of 202 Polyorganosiloxane stationary phases 201,314 Polyorganosiloxanes 188 Polyoxyalkylene derivatives 256 Polyphenylene oxide 121 Polyphenylquinoxaline 129 Polypropylene glycols 255 Polysiloxanes, adamantyl substitution 224

398 Polysiloxanes, aminoalkyl substitution 242 Polytetdluoroethylene 114,280 Polytetrafluoroethylenesupports 184 Polytrifluorochloroethylene184 Polyvinylpyridine 129 Polyvinylpyrrolidone 128,129 Porasil, chlorinated 147 Porasil, octyl-modified 147 Pore diameter 156 Pore diameter, mean 107 Pore size 163, 164, 167 Pore size distribution 107 Pore structure 161 Porous adsorbents 107 Porous glasses 120 Porous layer beads 181,182 Porous layer open-tubular column 75,76 Porous organic polymers 120,127 Porous polyaromatic beads 185 Porous polymers, non-polar Porous polymers, polar 128 Pour point 189,191 Pre-columns 53,298 Preferred stationary phases 308 Preparing of the packing 47 Prepolymers 66,70 Proton acceptor 82 Proton donor 82 PTFE supports 184 Purification 204 Purity 194 Pyrimidone moieties 242 Pyrocarbon 177 Quality test 73 Quartz 177 Quartz powder 180 Raoults law 22 Reactive gases 280 Relationships, list of 40 Relative retention 15 Required minimum column length 45 Required temperature 46 Residual silanols 156 Resistance to oxidation 225 Resolution 17 Retention capacity 9 Retention index 83,86, 87, 91, 194 Retention time 4 Retention volume 4 Rohrschneider constants 89,312,314 Rough surface 47 Roughening factor 192 Saccharides 248 Salt-modified adsorbents 141

General Subject Index Salts 283 Scanning electron microscopy 193 SCOT column 75,76,79 Sebacates 262 Selection of stationary phases 302,311 Selectivities, special 282 Selectivity 88,189, 193,195,207,262,272,331 Selectivity coefficient 84 Selectivity index 99 Selectivity, influence of terminal groups on 189 Separation 29 Separation enthalpy 25 Separation factor 16 Separation number 30 Separation power 32 Separation problems 303 Shape of padcles 161 S i - 0 bond 203 Si-0-Si bond angle 203 Sieve effect of zeolites 133 Sieves 162 Silanediols 59 Silanol clusters 143 Silanol groups 59,115-118,150,155,164 Silanol, geminal59 Silanol, lone 59 Silanol, vicinal59 Silanols, free 59 Silanols, residual 156 Silanols,sterically hindered 150 Silanox 76 Silica gel, octyl-modified 145 Silicagels 107,114,117,152,180 Silica gels, commercial 119 Silica surface 118 Silica-based supports 177 Silicone grease 213 Silicone phases 71 Silicones 201 Siloxane bridges 59,150 Siloxanes, adamantyl substitution 224 Siloxanes, aromatic substitution 222 Silsesquioxane units 224 Silver nitrate 283 Silylation 61, 148, 151, 172 Silylation at high temperatures 61 Silylation procedures 175 Silylation reaction 60 Size distribution 160 Skewing 188 Smectic liquid crystallime polysiloxanes 241 Sodium chloride 184 Solid core beads 181 Solid modifiers 140 Solid stationary phases 103 Solid support 160 Solid support, activity 164, 166

399

General Subject Index Solid-liquid-phase transitions 190 Solid-nematic transition 292 Solid-smectic transition 292 Solubility 193 Solubility factors 193 Solubility parameters 194 Solubilizing power 193 Special selectivities 282 Specialty grade polydimethylsiloxanes 208 Specialty methylsilicones 208 Specific area of adsorbent 7 Specific molecular adsorption 118 Specific pore volume 107 Specific retention volume 6, 194 Specific surface 107 Specific surface area 161, 163 Specific surface energy 103 Spreading behaviour 203 Stability 167 Stability of polydimethylsiloxanes 179 Stainless steel 55 Standard reference phases 196 Standard test mixtures 74 Stationary phase selection 302,311 Stearates 266 Styrene 124 Sub-ambient temperatures 190,191 Sulphur compounds 251,279 Superficially porous beads 181 Superficially porous sorbents 143 Support, activity of 166 Support, coating of 177 Support-coated open-tubular column 75 76,79 Supports, silica-based 177 Surface 167 Surface area 104,161 Surface concentration 26 Surface concentration of organic groups 55 Surface coverage 157 Surface energy 155,184 Surface energy, critical 191, 192 Surface pretreatments 59 Surface tension 191,218,222,251 Surface tension, critical 47, 191 Surface wettability 47,60, 167, 192 Symbols 35 Synthesis of polyorganosiloxane phases 202 T-unit 61 Temperature 86 Temperature limit 188 Temperature, required 46 Tenax-GC 121,123,185 Terminal hydroxy groups 189

Test mixture 74 Thermal behaviour 206 Thermal degradation 62,231 Thermal resistance 184 Thermal stability 186,203,207,226 Thermodynamic effect of zeolites 133 Thick-layer column 77 Time of analysis 33 Total retention time 4 Transition temperature 292,293 Transition, solid-nematic 292 Transition, solid-smectic 292 Transitions, solid-liquid phase 190 Trennzahl30

Trifluoropropylmethylsiloxanes 227 Trifunctional chlorosilane 61 Trimethylchlorosilane 150 Tube material 55 Type I support 169 Type I1 support 169 TZ 30 Ultrabond146 Unsaturated hydrocarbons 112 Ureide stationary phases 288 Vacuum vibration technique 50 Van de Graaffgenerator 67 Van Deemter curves 14 Van Deemter equation 13 Vapour pressure 186 Vinyl groups 2 14 Vinyl-modified polydimethylsiloxanes 215 Viscosity 189 Viscosity of polydimethylsiloxanes 190 Viscosity-temperature coefficient 190, 191 Viscous flow, activation energy 203 Vulcanization 66 Vulcanization. in situ- 66 Wall-coated open-tubular columns 48 WCOT columns 48 Weight loss 187 Weight of the stationary phase 48 Werner complexes 136 Wettability of a surface 47,60,167, 192 Window diagrams 232

2 value 32 Zeolites, application Zeolites, selectivity of 134 Zeolites, sieve effect of 133 Zeolites, thermodynamic effect of 133 Zeolitic molecular sieves 131-135

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Stationary Phase Index

This index lists both liquid and solid stationary phases discussed or mentioned in this book, including solid supports. A-1120,1100/88542 244 Acetates 267 Acetyltributyl citrate 267,323 Active charcoal 107, 113 Adipates 262 ff. AD-264 252 Aerosilll8 Alcohols 246ff. Alkali metal halides 283 Alkali metal nitrates 285 Alkanes 302 Alkaterge T 312,326 Alkoxybenzoyloxy azobenzene 294 Alkylnaphthalenes 201 Alumina 107,119 Aluminium oxide 119 Amides 277,278 Amine 220 279,312,326 Aminoalkylsubstituted polysiloxanes 242, 3 19 Aminoethyl-aminopropyltrimethoxysilane319 Aminopropyltrimethoxysilane319 Ana Prep A, ABS, P, Q 169,170 Anakrom P, Q, A, ABS, U, C22, C22A 169,175,176 Anaport glass beads 182 Anaport Kel F 184 Anaport Tee Six 185 Antarox CO-990 259 AN-600 234,318 Apiezon H, I, L, M, N, W 198,199,312,315,329 Apiezon MH 122,197,200 Apolane-87 122, 126,197, 198,200 Armeen 2HT, 2S, SD 276,312,326 Aroclor 312, 315 AR-20 221 Asphalt 198,199 ASI-100 208,209,315 ASI-25Ph-25Cyanopropyl234,318 ASI-50 220,314 ASI-5OMe-5OPh 317 ASI-5OMe-5OTFP 228,318 ASI-50Ph-5OCyanopropyl3 18 Atpet-200, -80 266, 323 Azoxyphenol ethers 295 Barium sulphate 105 Bayer M 211,315

Beeswax 323 Behenic acid 283 Bentone 34 312,326 Bentonite 134 Benzenesulphonate 137 Benzyl cyanide 272 Benzyldiphenyl201 Benzylideneanilines 293

Biphenylene-bis-4-alkoxybenzoate295 Bis((R)-benzy1idene)-bis-p-toluidine 293, 294 Bis(2-butoxyethy1)adipate 263,322 Bis(2-butoxyethy1)phthalate 263, 322 Bis(2-cyanoethy1)ether 272 Bis(2-cyanoethy1)formarnide 93,272,273,274, 324,329 Bis(2-ethoxyethoxyethy1)phthalate 263,265,322 Bis(2-ethoxyethy1)phthalate 263,264,322 Bis(2-ethoxyethy1)sebacate 263,322 Bis(2-ethylhexy1)tetrachlorophthalate 263,264, 265,322 Bis(2-ethylhexanoyloxyethy1)ethylhexylamide 278 Bis(2-ethylhexy1)adipate 263,264, 322 Bis(2-ethylhexy1)phthalate 263, 322 Bis(2-ethylhexy1)sebacate 263,264, 322 Bis(2-methoxyethy1)adipate3 12 Bis(p-methoxybenzy1idenearnino)dichlorobiphenyl293 Bis(tribenzylsily1)diphenyl ether 261 Bitumen 315 Bondapak 153,154 Boron nitride 105,107,114 BP-1 213 Butanediol succinate 271,324 Butoxyethyl stearate 323 Butyl-L-valinamide polysiloxane 289,291 Butyloctylphthalate 263,322 Butylstearate 323 C-22 Super Support 168,169 Calcit 5 135 Carbochrorn 1,2 112 Carbochrom A 109,112 Carbohydrates 246 Carbon molecular sieve 107, 113,311 Carbonyl-bis(aminoacid esters) 288 Carbonyl-bis(L-valine-tert.buty1ester) 290

402 Carbopack A 112 Carbopack B, B-HT 112,139 Carbopack C 112,139 Carbopack F 112 Carborane siloxanes 224 Carbosieve B, C 114 Carbosphere 114 Carbowax 1000 250,252,319 Carbowax 1500,1540 319 Carbowax 20000 250,251,252,270,312,320 Carbowax 20M 250,251,252,270,312,320 Carbowax 20M-TPA 256,257,259,260,320 Carbowax 40000 252,254 Carbowax 400 dioleate 258 Carbowax 400 monostearate 260,320 Carbowax 4000 252,312,320 Carbowax 4000 dioleate 258 Carbowax 4000-TPA 257 Carbowax 40M 252,254 Carbowax 600 93,252,319 Carbowax 6000 252,320 Carbowax-TPA257 Castorwax 312,315 Cekachrom 122 Cekachrom 1,2.3 125,126 Cekachrom 4,6 129,130 CekachromS 129 Celanese ester 312,323 Celite 144, 153, 169, 170, 173, 176 Cera Beads 182 Cetamoll Q 265 CEM 129,130 Chemically bonded stationary phases 142ff., 311 Chezasorb 169,176 Chiral polysiloxanes 236ff., 310 Chiral stationary phases 286ff., 310 Chiraldex-A-DH, -PH 248 Chiraldex-G-DA, -PH 248 ChirasiLVal238,239,289 Chlorinated dialkyl phosphate 265 Chlorphenyl 0 237 Cholesteryl butyrate 296 Cholesteryl derivatives 295 Chromaton N 167,170,176 Chromosorb 101 125,126 Chromosorb 102 124,125,126 Chromosorb 103 124,125,126 Chromosorb 104,107,108 128,129,130 Chromosorb 105 125,126 Chromosorb 106 124,126 Chromosorb 750 175,176 Chromosorb A, R 169,170,176 Chromosorb G 167,175,176,179 Chromosorb P 144,168,169,171,175,176 Chromosorb T 185 Chromosorb W 144,146,153,165,169,171,175, 176,181

Stationary Phase Index Chrysanthemoyl-naphthyl-ethylamine288,290, 291 CHDMS 270,271 Citrates 267 Citroflex A4 267,312,323 CN-1, -2, -3, -5, -6 235 Columpack-T 185 Convoil20 198,315 Copper phthalocyanine 140 Copper stearate 283 Corasill44 CP-Sil SCB 213 CP-SiI 8 CB 223 CP-Sil19CB 223,235 CP-Sil88 235 CP-Wax 40M 254 CP-Wax 52CB 254 CPG650 144 Cresyldiphenyl phosphate 266,323 Crown ether siloxanes 245,246 Cyan0-B 273,274,326 Cyanoalkylsiloxanes l95,229ff., 242 Cyanoethyl saccharose 272,274,312,326 Cyclodextrin propionate 248 Cyclodextrins 131,287,29 Cyclohexanedimethanol succinate 270,271 Cynoalkylsilicones 229 DB-1 213 DB-1301 235 DB-1701 223,235 DB-210 229 DB-225 235 DB-5, -17, -608 223 DB-560 233,314,317 DB-624 235 DB-FFAP 257 DC11 316 DC123 210 DC 200 210,314,316 DC 220,330,400,401,410 210,316 DC 430 215,316 DC 510,556,702,703 221,317 DC 530,531,535 244 DC 550,710 221,314,317 DC560 237 DC730 215 Desoxycholic acid 136 Dexsil300GC, 400,410 225,314,317 DEGA, -P 269,271,324 DEGS, -PS 268,271,324,325 DEGS-PS HI EFF-1 BP 271,325 DFOK 221,222 Di-o-thymotide 136 Dialkylcyclodextrins 291 Diamide stationary phases 288 Diaphorit 170

Stationary Phase Index Diatomaceous brick dust 169 Diatomite C, CQ, M 169,170 Diatoport P, W 169,170 Dibutyl phthalate 263,312,322 Dibutyltetrachloro phthalate 264 Dicalite 170 Dicyano-3-methyl-3-nitropentane273 Dicyclohexyl phthalate 263,323 Didecyl phthalate 195,263,265,323 Didodecyl phthalate 263, 323 Diethylene glycol adipate 271,324 Diethylene glycol distearate 266,323 Diethylene glycol succinate 93,271,312,324 Diglycerol247,309, 320,329 Diisodecyl adipate 263, 323 Diisodecyl phthalate 263,312,323 Diisononyl adipate 263,323 Diisooctyl adipate 263, 323 Diisooctyl phthalate 263, 323 Dimethylaminoethoxymethylpolysiloxane 319 Dimethylformamide 277 Dimethyloleylamide 278,327 Dimethylstearamide 277,278,327 Dimethylsulpholane 279 Dimethyltetrahydrothiophene S-oxide 279 Dinitrodiphenic acid hexyl ester 275 Dinochrom N 169,170 Dinonyl phthalate 262,263,300, 312, 323,329 Dinonyl sebacate 263,323 Dioctyl adipate 264 Dioctyl phthalate 263, 323,328 Dioctyl sebacate 263, 264,312,323 Dioctyltetrachloro phthalate Diphenylamine 277 Dipropyltetrachloro phthalate 262,263 Ditridecyl phthalate 263,323 Divinylbenzene 105, 108 DMF 05,1,12,30,100 211 Docosanoyl-L-valine-methylheptadecylamide 290, 291 Dodecenoyl-L-valine-tertbutylamide 288 Dodecylsalicylaldimie 286 Dodecyltartrate 290 Dow Polyglycols 261 Dowfax 9N9,9N15,9N40 259 Durabond 213 Durabond DB-210 229 Durabond DB-5, DB-17, DB-608 223 Durabond-Wax 254 Durapak 107,145 Durawax-1, -2, -3, -4 254 E 300,301 210,314,316 E302,303 215 E 350,351 221,317 ECNSS-M 237,314,319 ECNSS-S 237,.319

403 ED-NO2 130 EGA 271,325 EGA-P 269 EGIP 271,325 EGP 269,271,325 EGS 268,271,312,325 EGSP-A, -2237,319 EGSS-X, -Y237,319 EGTCP 271,325 Elastex 50HB 327 Embacel Kieselguhr 170,176 Embaphase Oil 211,316 Emulphor ON-870 258,260,265,312,320 Epon 1001 325 Erythritol247 Esters 261 Estynox 323 Ethers 246ff. Ethofat 60125 258,260,312,320 Ethomeen 18/25, S/25 276,326,327 Ethoxyphenylazophenyl crotonate 294 Ethylammonium nitrate 285 Ethylene glycol adipate 271,312,325 Ethylene glycol distearate 266 Ethylene glycol isophthalate 271, 325 Ethylene glycol o-phthalate 271,312, 325 Ethylene glycol succinate 271,312,325 Ethylene glycol tetrachlorophthalate 271,325 Ethylene glycol-bis(propionitri1e)ether 328 Ethylester of Kel-F acid 8114 281 Ethylester of tetrachloropertluorocaprylicacid 281 Ethylpyridinium bromide 285 Ethylvinylbenzene 105 Eutectic of alkali metal nitrates 285 Eutectics 284,285 F111 210,316 F 50 Versilube 237 F 50,60 233,237,314,318 F 61 237,314,318 F-4050 237 Fatty acid salts 282,283 Fatty acids 282,283 FFAP 256,257,259,260,312,320 Flexol8N8 278,327 Flexol A-26 264,322 Flexol B 400, GPE 327 Fluorad FC-430 280,281,282 Fluorad FC-431 280,281,282,327 Fluorcol280,282 Fluorinated esters 280,310 Fluorine compounds 280,310 Fluoroalkyl camphorate 309,329 Fluorolube GR 362, S 30, HG 1200 281,282,312 Fluorolube oil 2000 281 Fluoropack 80 185 Fluoroplast 4 powder 18s

404 Fomblin YR 280,282 Fonnamide 277 FS-1265 228,314,318 FS-16 228 FS-169 228 FS-303, -328 228 Gas Chrom A, R,Z 169,176 Gas Chrom Q 153,175,176 Gas Pak FS 170 Gas-Pak F 185 GB-5 223 GC-Bondapak 152 Gensil316 Glaskugeln 182 Glass beads 182 Glycerol 247 GLC 100,110 182 Graphitized carbon 112 Graphitized thermal carbon black (GTCB) 104, 107,108,122,310 Graphon 112 Graphpac-GC 112 GTCB Sterling MT 110 Halides 283 Hallcomid M-18 277,278,312,327 Hallcomid M-18 OL 278,312,327 Halocarbon 10-25 281,282,315 Halocarbon 14-25,25-55 281 Halocarbon K-352 281,282,312,315 Halocarbon wax 281,282,315 Haloport F 185 Haloport K 184 Harilex 370 269,271,313,325 Heliflex Chirasil-Val289 Hercoflex 600 327 Heterocyclics 278,279 Hexadecane 198 Hexadecanol246 Hexakis(2-cyanoethoxy)hexane 273,274 Hexakis(cyanoethoxy)cyclohexane 272,273,3 12 Hexatriacontane 198,315 HEM 129 Hi vac grease 314,316 HI-EF-1BP 325 HI-EFF-1A 269 HI-EFF-1AP 271,324 HI-EFF-1B 268 HI-EFF-2A, -2AP 269,271,325 HI-EFF-2B, -2BP 268,271,326 HI-EFF-2EP 271,325 HI-EFF-2GP 271,325 HI-EFF-3A, -2G 269 HI-EFF-3AP, -3BP, -3CP 271 HI-EFF-4B, -4BP 269,271,324 HI-EFF-8B, -10B, -10BP 270,271,309,326

Stationary Phase Index HI-EFF-8BP 324 Hostaflon TF 185 Hostaflon-C2 184 HP-1 213 HP-101 213 HP-17 223 HP-20M 254 HP-5 223 HT-5 226 Hydroxyethyl-2-heptadecenyl-imidazoline279 Hydroxypropyl cyclodextrins 291 Hylon P 169 Hypersil-WP 154 Hyprose SP-80 309,312 Igepal CO-630 259,320 Igepal CO-710,730 260,320 Igepal CO-880, -990 259,260,312,320 Iminodipropionitrile 272 Inerton, -Super 170,176 Inositol247 INS-600 169,176 Jado 114 Jascosill82 Johns Manville C22 fiebrick 176 JXR 208,209,314,316 KBM-603 244 KBS-2,-3 226 KBS-2F, -3F 225,226 Kel-F 184, 280 Kel-F90 281 Kel-F oil 3, 10 281 Kel-F wax (grease) 281,315 Kel-F wax 550 281,282 Khs-2-1 237 Khs-2-1 W 237 Kieselguhr 167,168,170 Kieselguhr Merck 170 Kromat FB, CE 169,170 KS-1014 210 L-45, -46 210,316 L-Valine-(S or R)-phenylethylamide-siloxane 290, 291 L-Valine-tert.-butylamide-cyanosiloxane 289,291 Lauroyl-(S)-1-maphthylethylamide 2 88 Lauroyl-L-valine-tert.butylamide 288 LAC-10-R-744 269 LAC-12-R-79C 270 LAC-16-R-897 266 LAC-2-R-446 269,271,312,324 LAC-3-R-728 268 LAC-4-R-886 268,312 LAC-769 269 LAC-9-R-769 269

405

Stationary Phase Index LAC-IR-296 271,324 LC-430 215 Lestosil221 Liqua-Chrom 182 Liquid crystals 291 ff. Liquid-crystalline polyacrylates 296 Lithium bromide 284 Lithium chloride 283,284 Lithium iodide 284 LSX-3-0295 228,318 Lubrol MO 253 Lukooil M 100,200,500 211 Lukooil X 100,200,600 236,237 Lukopren G 1000 215 Lukopren M 50 211 Lukosil MF, DF 221 Lutensol259,260,320 Mer-2, -21, -35 252, 313, 320, 325, 327 Mesogenic phases 299ff., 310 Mesogenic polysiloxanes 240ff., 296, 310 Metal chlorides 141 Metal complexes 141,289 Metal fluorides 141 Methyl-n-octylglyoxime 286 Methyl-nitropimelonitrile 273 Methylethyl-imidazolium chloride 285 MER-2 271 MH 1 , 2 , 3 169 Micro glass beads 181 Micro-Porasill44 Mikroglaskugeln 182 Mixed bed columns 298 Mixed stationary phases 296 MMMFC-43 281 Molecular sieve PLOT column 134 Molecular sieves 107 Molten organic salts 285 Molybdenum disulphide 105,107,114 Morphinylpropyl-trimethoxyailane3 19 MS-200, -2211 210,314,316 MS-2560 244 Mulgofon ON-870 258 Naloit 4,13 135 Narcoil 40 262 NB-1,-30 213 NB-1701 223 NB-54 223 Neopentyl glycol adipate 271,325 Neopentyl glycol isophthalate 271,325 Neopentyl glycol sebacate 271, 325 Neopentyl glycol succinate 271,313,325 NFS-100 228 NG100 211 NG300 215 Nickel-bis-(N-dodecylsalicylaldimine)286

Nitrile ethers 270ff. Nitrile silicones 229ff. Nitriles 270ff. Nitro compounds 274K p-Nitroaniline picrate 275 m-Nitrotoluene 275 NM 1-200, -100000, -300000 211 NM 4146,4180 221 Nonex76 258 NPGA 269,271,325 NPGIP 271 NPGSb 271 NPS-25, -50, -100 234 NSKT-100 234 NSKT-25, -33, -50 234 Nucleosil 154 Nujol198,199,315 NVG700 208 Octadecane 198 Octadecanol246

Octakis-O-(2-hydroxypropyl)saccharose248,309, 329 Octoil S 264,312, 322 OD-1 208,209,314,316 OE 4178 234,328 Oleates 266 Oronite NI-W 327 Oronite Polybutene 198,315 0s-124, -138 259,261,313,322 OV-1 208,209,213,314,316 OV- 1 Vinyl 2 15 OV-101 93,208,209,212,314,316,328 OV-105 234,318 OV-17 93,220,314,317,328 OV-17 Vinyl 221 OV-1701 234 OV-202, -215, -215vi 228 OV-210 228,314,318,328 OV-22 220,265,314,317,318 OV-225 93,234,239,314,328 OV-25 93,220,265,314,317,318,328 OV-275 93,234,318 OV-3, -7, -11, -61 220,222,314,317,328 OV-330 259 OV-73 220 OV-73 Vinyl 221 Oxidwax 253 Oxydipropionitrile 94,272,274,313,326

P 425,1025,2025 255,257 Paraffin oil 198,199,315 Paraffin wax 199 Paraplex G-25, G-40 271,326 PDEAS 270,271,326 Pellosill44 Pentasil ZSM-11 135

406 Pentasil ZSM-5 135 Pentasil-350 226,319 Peptide stationary phases 287 Peffluoropolyether281,282 Perfluorotributylamine 281 Perisorb A, B 144,182 Perkin Elmer A 263 Perkin Elmer B 264 Perkin Elmer C 211,314.316 Perkin Elmer E 279 Perkin Elmer K 253 Perkin Elmer 0 316 Perkin Elmer P 268 Perkin Elmer Z 211,314,316 Permaphase 154 Permaphase CPMS/1701 235 Permaphase CPMS/225 235 Permaphase DMS 213 Permaphase PEG 254 Permaphase PVMSI17 223 Permaphase PVMS/54 223 PES-l,-S 216 PES-V-2 216 PFMS-4, -6 220 PFMS-5 221 PGA 271 PGSb 269,271 Phase Chrom Q 170 Phase Prep A 170 Phase Sep HC, N,P, W 169,170,176 Phase Sep T6 185 Phenyldiethanolsmine succinate 271,309,313, 326,329 Phenylene-bis-4-n-heptyloxybenzoate295 Phenylsilicone oil 217 Phenylsiloxanes 219,245 Phosphates 265 Phthalates 262 ff. Phthalocyanine 140 Plascon 184 Platinum-bis(methy1-n-octylglyoxime)286 Pluracol P-2010 256,257,321 Pluronic F-68, F-88 256,257,321 Pluronic L-35, L-61, L-64, L-81 256,321 Pluronic P-65, P-84, P-85 256,257,321 PMS-100, -1000, -100T, -lMio 208,209,210,316 Poly(propy1ene imine) 276, 327 Poly-A-2103, -135, -101A 278,327 POly-1-110 278, 327 POly-S 176,179 259 Polyacrylates 296 Poly(acry1onitrile)105 Poly(aky1ene oxides) 249ff., 309 Polyamide Poly-A-103 278 Polyamides 277 Poly(butanedio1succinate) 269,309,329 Poly(carborany1enesiloxanes) 224,225

Stationary Phase Index

Poly(chloroalkylsi1oxanes) 233 Poly(chloroarylsi1oxanes) 233 Polychrom-1 185

Poly(cyanoalkylsi1oxanes)229ff., 309,310,329 Poly(cyc1ohexanedimethanol succinate) 270,324, 329 Poly(decanedio1dodecanedioate) 270 Poly(dialky1siloxanes)302 Poly(diethy1eneglycol adipate) 269, 324 Poly(diethy1ene glycol succinate) 268,309,310, 324,325,329 Poly(diethylsi1oxanes) 191,215,216 Poly(dimethy1diphenylsiloxanes)191,212,2 16, 217,218 Poly(dimethylsiloxane/ethylene oxides) 259 Poly(dimethy1siloxanes) 188-191,202,204ff., 207ff., 309,310,329 Poly(dimethylsiloxanes), vinyl modified 2 15 Polyester A 268 Polyester silicones 236 Poly(ester-acetal) 327 Polyesters 195,267ff. Polyethylene 199 Poly(ethy1eneglycol) 98,189, 195,25lff., 329 Poly(ethy1eneglycol adipate) 269,325 Poly(ethy1eneglycol succinate) 268,270,325 Poly(ethy1eneglycol tetrachlorophthalate) 270, 325 Poly(ethy1eneglyco1)bis-dinitrobenzoate257 Poly(ethy1eneglycol)di(octadecyl)ether25 8 Poly(ethy1eneglyco1)diglutarate 257 Poly(ethy1eneglyco1)dimaleate 257 Poly(ethy1eneglyco1)dinitrobenzoatestearate 258 Poly(ethy1eneglyco1)distearate 257 Poly(ethy1eneglyco1)disuccinate 257 Poly(ethy1eneglyco1)diterephthalate257 Poly(ethy1eneglyco1)ditosylate25 7 Poly(ethy1eneglycol)mono(tridecyl)ether258 Poly(ethy1eneglyco1)mono-nonylphenylether 259 Poly(ethy1eneglyco1)mono-octylphenylether 2 59 Poly(ethy1eneglyco1)sorbitane monooleate 258 Poly(ethy1eneglyco1)sorbitane monostearate 258 Poly(ethy1eneglycol-o-phthalate) 269,270,325 Poly(ethy1eneglycols) 251ff., 329 Poly(ethy1eneimine) 276,277,310,313,327 Poly(ethy1eneoxide) 100T, 200T, 300T 252 Poly(ethy1eneoxide) 600T, 4Mi0, SMio 252 Poly(ethy1eneoxidelpropylene oxide) 257 Poly(ethy1eneoxides) 251ff., 310 Poly(ethylmethylsi1oxanes) 190, 191 Polyglycerol247 POlyglyCOll5-200 256,321 Polyglycols 249ff. Polyimide 129,130 Poly(m-phenyl ether) 259,261,329 Poly(metalorganosi1oxanes)245

407

Stationary Phase Index

Poly(methy1-trifluoropropylsiloxanes) 191,227ff., 309,310,329 Poly(methylphenylsi1oxanes) 191,212, 216, 218, 219,309 Poly(methylphenylsiloxanes) 329 Poly(methyltolylsiloxane)222 Poly(neopenty1glycol adipate) 269,270 Polyoctylmethylsiloxanes 215 Polyox 253 POlyOX-100 253 Poly(phenyldiethano1amine succinate) 270, 313, 329 Poly(pheny1ene oxide) 121,313,322 Polyphenylquinoxaline 129 Poly(propy1eneglycol sebacate) 269,325, 326 Poly(propy1eneglycols) 255,257,266, 321, 329 Polysev 259 Polysorb-1 125 Polytergent B-350, G-300,1-300,1-400 260, 320 Poly(tetrafluoroethy1ene)280 Poly(trifluoroch1oroethylene)280,281 Poly(trimethy1eneoxide) 257 Pora PLOT Q 127 Porapak N, R, S, T 128,129,130 Porapak P 124,125,126 Porapak P-S 127 Porapak Q 124,125,126 Porapak Q-S127 Porasil B, C, F, S 144, 146,147, 153,154 Porasils 107,118, 119 Porochrom I, 11,111 170, 176 Porolith 169, 176 Porous glasses 120 Porous layer beads 181 Porous polymers 105,120,311 Porovina 169,176 Potassium halides 284 PON 129,130 PPE 20,21 259,261,322 PPG 255 PPG 1000,2000 256,257,321 Propylene glycol adipate 271,326 Propylene glycol sebacate 271,313,326 PS 041,045,050 211 PS 060,060.5,062,063 221 PS 094,095 222 PS 097,098 226 PS 140 216 PS 160,162,264 221 PS 181,182,183,286 228 PS 422,426,428 215 PS812 244 PS 902,906 234 PS 908,910 235 PS914 216 PTFE 182,183,184,280

Pyrimidone poly(diorganosiloxanes) 242 PYR 129,130 QF-1 227,228,314 QF-1-0065 227,228,314 Quadrex 007-1 213 Quadrol275,276,309,313,327,329 Quartz 177,180 Renex 678 259,260,321 Reoflex 100 269 Reoflex 400 269 Reoplex 400 271 Resoflex-R-296 269 RSiL spherical 144 RSL-150, -160 213 RSL-400 229 R t x - 1 213 Rt x -5, -20, -35, -170 223 Rysorb BLK 169, 176 S2116 314 Saccharides 248 Saccharose acetate isobutyrate 265,267,323 Saccharose octaacetate 267,323 Salicyliden-derivative 289,290 SAIB 267,313,323 SD-NO2 129,130 Sebacates 262ff. Separon AE 129,130 Separon SE 129,130 SE-30 210,213 SE-30 GC 208,209,314,316 SE-31, -33 215 SE-52 217,221,317 SE-54 221,317 SF-96 210,314,317 Shimalite A, B, C, D, W 169,170 Shimalite F 185 SiB-1, -1.5 226 Sil-0-Cel C22 firebrick 169 SiI-X-2 144 Silar 5 CP 234,318 Silar 7 CP 234,318 Silar 9 CP 234,318 Silar 10 C 234,239,318 Silastic 132, 152 213 Silastic 132, 152,401 210, 317 Silastic DC 430 215 Silastic LS-24 228 Silbor-1 226 Silica gel 105,107, 114,144, 180,181,311 Silica V 144 Silica, GC grade 119 Silicalite 131,133, 135 Silicone grease 213 Silicone M-430 210

408 Silicone Oil 81705 211 Silicone-Carbowaxcopolymers 259 Silicones 195,201ff. Silikagel, narrow-pore 119 Silikagel, wide-pore 119 Silipors 119 Silochroms 118,119 Silsesquioxane 224 Silver nitrate 283 Siponate DS-10 279,309,323 SI A, B, C, D 153 SKIIT-25, -50, -5OX, -100 228 SKT 208,209 SKTA-1 222 SKTE type A 216 SKTF-100 221 SKTFV-803 221 SKTN 210 SKTV,-l 215 Sodium chloride 107 Sodium dodecylbenzene sulphonate 279,310,329 Sodium halides 284 Sodium isovalerate 285 Sodium mordenite 133, 135 Sorbitan monooleate 266,323 Sorbitan monostearate 266 Sorbitol247 Sorbitol hexaacetate 267,324 Sorptophase 114 Span-60, -80 266,324 Spherisorb 144,154 Spherocarb 114 Spherochrom 1 , 2 , 3 169,170 Spheron 112 Spheron MD 129 Spheron SE 129,130 Spherosils 107,118,119, 144,146,153,181 SP 1000 259,321 SP 1200 283, 324 SP 2100 208,209,213,317 SP 216-PS 272,326 SP 222-PS 272.274.326 SP 2250 220,314,3 7 SP 2300 234,318 SP 2310 234,318 SP 2330 234,318 SP 2340 234,318 SP2401 314,318 SP 392,400 314,31 SP400 233 SP525 201 SP70 211 SPB-1 213 SPB-2250 223 SPB-5, -20, -35 223 Squalane 91,94,122,196,300,313,315,328,329 Squalene 315

Stationary Phase Index SR119 317 Stabilwax 254 Stabilwax DA 254 Stabilwax DB 254 STAP 257,313,312 Stearates 266 Stearic acid 283 Stepan DS-60 279,324 Sterchamol169,176 Sterling FT 112,139 Sterling MT 112 Steroid Analysis Phase (STAP) 313,321 Stilbene trimer 201,202 Styrene 105,108 Sucrose acetate isobutyrate 313 Sulphone PZ-176 261 Sulphones 259 Sulphur compounds 279 Supasorb 170 Supelcon A W 176 Supelcoport HD 170,175,176 Supelcosill44, 154, 155 Superox 0.1,0.6,4 252 Superox 20M 251,255 Surfonic N-300 259,260 Surfonic TD-300 258 Synachrom 125 Synperonic T 904,908 276 Tartramide 239 Teflon powder 185 Teflon-1, -6 182, 185 Tenax-GC 121,123,125,126,185 Tergitol NP-35 259 Tergitol NPX-728 259,260,313,321

Tetra(fluoroalky1)tetracarboxybenzene 309,329 Tetra-n-butylammonium tetrafluoroborate 285 Tetra-n-butylammonium-4toluenesulphonate 285,286 Tetra-n-heptylammonium chloride 285 Tetra-n-hexylammonium benzoate 285,286 Tetrachloroperfluorocaprylic acid ethyl ester 281 Tetrachloroterephthaloyloligomers 264 Tetracosane 198 Tetracyanoethyl pentaerythritol272-274,313,326 Tetrakis(2-hydroxyethy1)ethylene diamine 275, 277,309,310,327,329 Tetrakis(2-hydroxypropy1)ethylene diamine 276. 309,310,321,329 Tetrakis(cyanoeth0xy)butane 212-274,326 Tetrakis(polyethylene/propylene glycol) ethylene diamine 276 Tetronic 276,279 THEED 276,309,310,327,329 Thiourea 137 Tide 171 Tide E, F 279

409

Stationary Phase Index Toluidine deriv. of 1,2-diphenylethane 293 Tri-n-butylbenzylphosphoniumchloride 285 Tri-o-thymotide 136 Tributoxyethyl phosphate 266,309,324 Tricresyl phosphate 265,266,313,328 Tridox 258 Triethanolamine 328 Trifluoroacety I-L-phenylalanine-cyclohexylester 290,291 Trifluoroacetyl-L-isoleucine laurylester 287,290 Trifluoroacetyl-L-valyl-L-valine cyclohexylester 287,290,291 Trifluoropropylsiloxanes227 ff., 242 Triisobutylene 198 Trimer acid 282,283,309,313,324,329 Trinitrofluorenone 275 Trioctyl phosphate 265 Triphenyl phosphate 265 Tris(2-cyanoethoxy)propane 93,271-274,310, 313,326,329 Tris(2-cyanoethy1)nitromethane 273 Tris(2-ethylhexy1)phosphate 265,266, 309,324 Tritolyl phosphate 265 Triton QS-15 259 Triton X-100 259,260,313,321 Triton X-200, -400 328 Triton X-305 259,260, 321 Tween61 258 Tween 80 258,260,313,321 Ucon 50 HB 1800X, 2000,3520,5100 256,257, 322 Ucon 50 HB 280X, 660 256,257,265,321,322 Ucon 50 HB 5100 256 Ucon 50 HB-2000 93,256,321 Ucon 75 H 90000 256,322 Ucon LB 55OX 91,93,256,257,313,322 UC-W 96,98,960,982 215,317 Urea 137

Ureide stationary phases 288,289 Versamid 930,940 278,327 Vinyl modified poly(dimethylsiloxanes) 215 Vinylpyridine 105 Viscasil210, 317 VKZh-94 216 Volaspher 161, 167, 177, 178,179 Voltalef 184 Vydac SC 144 Werner complexes 136

WSRN 10 253 X-12-563, -570 244 X-22-857 244 XE-60 93,234,239,314,318 XE-61 317 XF-1105 234 XF-1112, -1125 234,314,318 XF-1150 234,314,318 YWG Silica 144 Z 6020 244

Zeolite A, 3A, 4A, 5A 131,132,135 Zeolite LZ-MS, -Y52 135 Zeolite S-115 135 Zeolite Silicalite 131 Zeolite X, 13X 131,132,135 Zeolite Y 131, 132 Zeolite ZSM 131 Zeolites 105, 107, 131,133-135, 311 Zeolith CaA, KA,NaA 135 Zeolith CaX,NaX 135 Zeosorb 4A, SA, SAZ, lox, 13X 135 Zinc stearate 283, 309,310, 324, 329 Zipax 144,154,181,182 Zonyl E-7, E-91 280-282,309,313,324,329

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