Gas chromatography in air pollution analysis

JOURNAL OF CHROMATOGRAPHY LIBRARY - volume 49

gas chromatography in air pollution analysis

This Page Intentionally Left Blank

JOURNAL OF CHROMATOGRAPHY LIBRARY- volume 49

gas chromatography in air pollution analysis Viktor G. Berezkin and Yuri S. Drugov Academy of Science of the U.S.S. R. A. V. Topchiev lnstitute of Petrochemical Synthesis, Moscow, U.S.S. R.

ELSEVlER Amsterdam

- Oxford - New York

- Tokyo 1991

This book is exclusively distributed in all non-socialist countries by ELSEVIER SCIENCE PUBLISHERS B. V. Sara Burgerhartstraat 25 P. 0. Box 211, 1000 AE Amsterdam, The Netherlands Distributors for the United States and Canada: ELSEVIER SCIENCE PUBLISHING COMPANY INC. 655 Avenue of the Americas New York, NY 10010, U.S.A.

Library of Congress Cataloging-in-Publication Data Berezkin, V. G. (Viktor Grigorkvich), 1931Gas chromatography in air pollution analysis / Viktor G. Berezkin and luri S. Drugov. 216 p. 16,5 x 24.0 cm. - (Journal of chromatography library ; v. 49) Includes bibliographical references and indexes. ISBN 0-444-98732-0 1. Air-Pollution-Measurement. 2. Air-Analysis. 3. Gas chromatography. I . Drugov, lu. S. (IuriT Stepanovich) II. Title. 111. Series. TD890.647 1991 628.5’3’0287-d~20

90-3981 CIP

ISBN 0-444-98732-0 (Vol. 49) ISBN 0-444-41616-1 (Series)

@ Akademische Verlagsgesellschaft Geest G 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

Chapter 1 Introduction

References

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

. . .

1

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

. . .

3

Chapter 2 Air as an object of analysis . . . . . . . . . . . . . . . . . . . . . . . .

2.1. Atmospheric air and sources of atmospheric pollution 2.2. General characteristics of air pollutants . . . . . 2.3. Specific features of air as an object of analysis . . . References . . . . . . . . . . . . . . . . . . .

Chapter 3 Gas chromatography in the analysis of air pollutants

4

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

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

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

10

3.1. General considerations . . . . . . . . . . . . . . . . . . . . . 3.2. Peculiarities of the gas chromatographic analysis of impurities . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

10 20 23

Chapter 4 Detectors for the gas chromatographic determination of impurities

25

4.1. 4.2. 4.3. 4.4. 4.5.

4.6. 4.1. 4.8. 4.9. References

. . . . . . . . .

Principal characteristics of detectors for gas chromatography Flame ionization detector. . . . . . . . . . . . Themionic detector . . . . . . . . . . . . . Photoionization detector . . . . . . . . . . . . Electron-capture detector . . . . . . . . . . . . Flame photometric detector . . . . . . . . . . . The thermal energie analyser (TEA) detector . . . . . Detectors for direct identification of impurities . . . . Hall electrolytic detector . . . . . . . . . . . .

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

Chapter 5 Collection and pretreatment of samples for chromatographic analysis

5.1. 5.2. 5.3. 5.4. 5.4.1. 5.4.2. 5.4.3. 5.4.4.

Sampling into containers . . . . . . . . . . Use of absorption of contaminants in sample collection . . . . Cryogenic concentration of contaminants Use of adsorption for contaminant concentration . . Activated coaland carbon adsorbents . . . . . . Porous polymer adsorbents . . . . . . . . . Sorbents used in gas-liquid chromatography . . . Silica gel . . . . . . . . . . . . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

. . . . .

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

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

. . . .

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

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

25 28 28 29 30 30 31 31 31 33

35 35 31 40 46 48 50 55 51

VI Molecular sieves . . . . . . . . . . . . . . . . . . . . . Aluminium oxide . . . . . . . . . . . . . . . . . . . . 5.5. Multilayer sorption traps . . . . . . . . . . . . . . . . . . 5.6. Sorbents for passive sampling . . . . . . . . . . . . . . . Chemisorbents . . . . . . . . . . . . . . . . . . . . . 5.7. Trapping of solid particles and aerosols . . . . . . . . . . . . 5.8. Preparation of concentration tubes containing solid sorbent . . . . . 5.9. Influence of sampling conditions on the efficiency 5.10. of sorption of pollutants . . . . . . . . . . . . . . . . . . Properties of analysed compounds . . . . . . . . . . . . . . 5.10.1. Properties of the sorbent used . . . . . . . . . . . . . . . 5.10.2. Air flow-rate . . . . . . . . . . . . . . . . . . . . . . 5.10.3. Air sample volume . . . . . . . . . . . . . . . . . . . . 5.10.4. Concentration temperature . . . . . . . . . . . . . . . . . 5.10.5. Air humidity . . . . . . . . . . . . . . . . . . . . . . 5.10.6. Coadsorption . . . . . . . . . . . . . . . . . . . . . . 5.10.7. Concentration in the flow . . . . . . . . . . . . . . . . . . 5.10.8. Desorption of pollutants from the sorbent . . . . . . . . . . . 5.11. Solvent extraction . . . . . . . . . . . . . . . . . . . . 5.11.1. Extraction in Soxhlet apparatus and ultrasonic field . . . . . . . . 5.11.2. Thermal desorption . . . . . . . . . . . . . . . . . . . . 5.11.3. Influence of experimental conditions on the completeness of 5.12. recovery of pollutants . . . . . . . . . . . . . . . . . . . 5.12.1. Temperature . . . . . . . . . . . . . . . . . . . . . . 5.12.2. Humidity . . . . . . . . . . . . . . . . . . . . . . . 5.12.3. Coadsorption . . . . . . . . . . . . . . . . . . . . . . 5.12.4. Desorption duration . . . . . . . . . . . . . . . . . . . 5.12.5. Choice of solvent . . . . . . . . . . . . . . . . . . . . 5.13. Increase in desorption efficiency . . . . . . . . . . . . . . 5.13.1. Solvent mixture . . . . . . . . . . . . . . . . . . . . . 5.13.2. Two-phase desorption systems . . . . . . . . . . . . . . . 5.13.3. Desorption methods . . . . . . . . . . . . . . . . . . . 5.14. Choice of sampling method . . . . . . . . . . . . . . . . . 5.15. Metrological aspects of air pollutant determination by gas chromatography References . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4.5. 5.4.6.

Chapter 6 The reactive-sorption method and its application for concentrating pollutans . . . . . . . .

6.1. 6.2. 6.3. 6.3.1. 6.3.2. 6.3.3. 6.3.4. 6.4. 6.4.1. 6.4.2. 6.5. 6.6. 6.7. 6.7.1.

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

.

. .

. . . . . . .

. . . . . . .

. . . .

. . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . . . .

. . . . . . . . . . .

. . . . . . . . . . .

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

70 71 72 7a 78 80 80 80 81 83 83

85 86

91 96 96 97 98 98 100 100 100 102 103 . . . . 108 113 . . .

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

Concentration of pollutants on solid adsorbents . . . . . . . . Reactive-sorption concentration . . . . . . . . . . . . . Use of the reactive-sorption method to enhance the reliability of the determination of pollutants . . . . . . . . . . . . . . . . Determination of aggressive gases . . . . . . . . . . . . . Determination of nitrogen oxides . . . . . . . . . . . . . Determination of sulphur fluorides . . . . . . . . . . . . Determination of hydrocarbons . . . . . . . . . . . . . . Diminution of coadsorption of pollutants . . . . . . . . . . Dynamics of sorption and coadsorption . . . . . . . . . . . Diminution of coadsorption . . . . . . . . . . . . . . . . Improvement of chromatographic separation . . . . . . . . . Moisture removal . . . . . . . . . . . . . . . . . . . Identification of pollutants . . . . . . . . . . . . . . . . Determination of hydrocarbons . . . . . . . . . . . . . .

58 58 59 59 62 65 67

119

. . . . . 119 . . . . . 121 . . . . . . .

. . . . . . .

. . . . . . .

. . . . . . .

. . . .

. . . . . .

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

. . . . .

123 123 124 125 126 127 127 130 133 134 135 135

VII 6.7.2. Determination of oxygen-containing compounds . . 6.7 3 . Determination of halogen-containing compounds . . 6.7.4. Iktermination of sulphur-containing compounds . . 6 7.5. Determination of pilrogen-containing compounds . 6.7.6. Determination of inorganic compounds . . . . . 6.8. Conclusion . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . 137 . . . . . . . . . . . 139 . . . . . . . . . . . 143 . . . . . . . . . . . 144 . . . . . . . . . . . 145 . . . . . . . . . . 146 . . . . . . . . . . 147

Chapter 7 Quantitative methods for the determination of impurities

150

7.1. Preparation of standard mixtures . . . . . . . . . . . . . . . . 7.1.1. Static methods . . . . . . . . . . . . . . . . . . . . . . . 7.1.2. Dynamic methods . . . . . . . . . . . . . . . . . . . . . . 7.1.2.1. Methods used to dilute (mix) streams . . . . . . . . . . . . . . . 7.1.2.2. Diffusion method (use of diffusion cells) . . . . . . . . . . . . . . 7.1.2.3. Exponential dilution flask method . . . . . . . . . . . . . . . . 7.1.2.4. Diffusion method using permeation tubes (ampoules) . . . . . . . . . 7.1.2.5. Other methods . . . . . . . . . . . . . . . . . . . . . . . 7.1.2.6. Preparation of standard aerosol mixtures . . . . . . . . . . . . . . 7.2. Detector calibration . . . . . . . . . . . . . . . . . . . . . 7.3. Calculation of impurity concentrations . . . . . . . . . . . . . . 7.4. Detection limits for air contaminants . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. .

.

150 150 151 151 152 152 153 157 159 161 161 162 163

Chapter 8 Practical application of gas chromatography to the determination of air pollutants . . . . . . . . . . . . . . . . . . . . .

165

Carbon oxides . . . . . . . . . . . . . . . . . . Halides and their derivatives . . . . . . . . . . . . . . Nitrogen-containing compounds . . . . . . . . . . . Sulphur- and phosphorus-containing compounds . . . . . . Metals and their derivatives . . . . . . . . . . . . . . Low-boiling hydrocarbons . . . . . . . . . . . . . . Aromatic hydrocarbons . . . . . . . . . . . . . . . Polyaromatic compounds . . . . . . . . . . . . . . . 8.9. Organic oxy compounds . . . . . . . . . . . . . . . 8.10. Amines and nitro compounds . . . . . . . . . . . . 8.11. Odorants . . . . . . . . . . . . . . . . . . . . 8.12. Halogenated hydrocarbons . . . . . . . . . . . . . . 8.13. Freons . . . . . . . . . . . . . . . . . . . . . 8.14. Chlorine-containing pesticides, polychlorobiphenyls and dioxins . 8.15. Bis(ch1orornethyl) ether . . . . . . . . . . . . . . . 8.16. Vinyl chloride . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . 8.1.

8.2. 8.3. 8.4. 8.5. 8.6. 8.7. 8.8.

Conclusion Subject Index

. . . .

.

.

.

. . . . . . .

. . . . . .

. . . .

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

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

. . .

. . . . . .

. . .

. . . .

.

.

. . .

. . . . . .

165 167 . . . . 170 . . . . 172 . . . 176 . . . 178 . . . 179 . . . 180 . . . 186 . . . . 190 . . . 194 . . . 195 . . . 196 . . . . 197 . . . 198 . . . 199 . . . 2Q3

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

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

208 209

This Page Intentionally Left Blank

IX

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 in 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

Chromatography of Antibiotics (see also Volume 26) by G. H. Wagman and M. J. Weinstein

Volume 2

Extraction Chromatography edited by T. Braun and G. Ghersini

Volume 3

Liquid Column Chromatography. A Survey of Modern Techniques and Applications edited by Z. Deyl, K. Macek and J. Janak

Volume 4

Detectors in Gas Chromatography by J. SevEik

Volume 5

Instrumental Liquid Chromatography. A Practical Manual on HighPerformance Liquid Chromatographic Methods (see also Volume 27) by N. A. Parris

Volume 6

Isotachophoresis. Theory, Instrumentation and Applications by F. M. Everaerts, J. L. Beckers and Th. P. E. M. Verheggen

Volume 7

Chemical Derivatization in Liquid Chromatography by J. F. Lawrence and R. W. Frei

Volume 8

Chromatography of Steroids by E. Heftmann

Volume 9

HPTLC-High Performance Thin-LayerChromatography edited by A. Zlatkis and R. E. Kaiser Volume 10 Gas Chromatographyof Polymers by V. G. Berezkin, V. R. Alishoyev and I. B. Nemirovskaya

Volume 11 Liquid ChromatographyDetectors (see also Volume 33) by R. P. W. Scott Volume 12 Affdty Chromatography by J . Turkova Volume 13 Instrumentation for High-Performance Liquid Chromatography edited by J. F. K. Huber Volume 14 Radiochromatography.The Chromatography and Electrophoresis of Radiolabelled Compounds by T. R. Roberts

X Volume 15 Antibiotics. Isolation, Separation and Purification edited by M. J. Weinstein and G. H. Wagman Volume 16 Porous Silica. Its Properties and Use as Support in Column Liquid Chromatography by K. K. Unger Volume 17 75 Years of Chromatography-A Historical Dialogue edited by L. S. Ettre and A. Zlatkis Volume 18A Electrophoresis. A Survey of Techniques and Applications. PartA: Techniques edited by 2. Deyl Volume 18B Electrophoresis. A Survey of Techniques and Applications. Part B: Applications edited by 2. 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 Modern 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

XI Volume 27 Instrumental Liquid Chromatography. A Practical Manual on HighPerformance 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 3 1 Gradient Elution in Column Liquid Chromatography. Theory and Practice by P. Jandera and J. ChuraEek Volume 32 The Science of Chromatography. Lectures Presented at the A. J. P. Martin Honorary Symposium, Urbino, May 27-31, 1985 edited by F. Bruner Volume 33 Liquid Chromatography Detectors. Second, Completely Revised Edition by R. P. W. Scott Volume 34 Polymer Characterization by Liquid Chromatography by G. Glockner Volume 35 Optimization of Chromatographic Selectivity. A Guide to Method Development by P. J. Schoenmakers Volume 36 Selective Gas Chromatographic Detectors by M. Dressler Volume 37 Chromatography of Lipids in Biomedical Research and Clinical Diagnosis edited by A. Kuksis Volume 38 Preparative Liquid Chromatography edited by B. A. Bidlingmeyer Volume 39A Selective Sample Handling and Detection in igh-Performance Liquid Chromatography. Part A by R. W. Frei and K. Zech Volume 39B Selective Sample Hndling 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

XI1 Volume 41A High-Performance Liquid Chromatography of Biopolymers and Biooligomers. Part A: Principles, Materials and Techniques by 0. Mikes Volume 41B High-Performance Liquid Chromatography of Biopolymers and Biooligomers. Part B: Separation of Individual Compound Classes 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, W and FT-IR 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 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 Applications 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

Chapter 1

Introduction

“We have reached the stage in the history of mankind where any human activity should be commensurate with nature’s potential. Man has already extinguished hundreds of species of animals and birds, destroyed up to two thirds of the forests that only recently had covered the Earth and disturbed many natural balances in the biosphere. It may seem that these facts are self-evident. However, in spite of the ever-growing concern over environmental pollution, millions of tons of various ecologically harmful substances are released annually into the atmosphere and oceans and are deposited in the outer layers of the Earth’s surface. The dustiness of the upper atmospheric layers and the contents of carbon monoxide and dioxide, organofluorine compounds and other substances are still growing; an oil film fatal to living organisms is being spread over the surface of the seas and oceans and is not shrinking. Civilization, which previously was only a blessing to mankind, today is showing its seamy side.” These concise characteristics of the vital problems facing mankind and urgently calling for Atmospheric pollution is a global solutions were given by Academician A. P. VINOGRADOV. problem. As the transfer of air and water masses is independent of the will of governments and the issuing of visas, the problems can only be solved if all governments will actively participate in their elucidation and observe international agreements. To study these problems and to develop optimal measures for environmental monitoring and control in various parts of the planet it is necessary effectively to determine the concentrations of pollutants. What is a pollutant? According to the definition adopted by the United Nations Organization, a pollutant is a substance found in an improper place at an improper time in an improper amount. Industry and transport in the form of various effluents and agriculture through fertilizers and pesticides are continuously increasing the number of actual and potential pollutants and the task of identifying pollutants therefore becomes more and more complicated each year. The definition of a harmful substance is of great importance. A harmful or noxious substance is a material which, while in contact with a human organism (under working conditions or everyday life), can cause disease or health problems that can be detected by modem methods both when in contact with the substance and in later periods of life of present and future generations [l].To characterize the permissible concentrations of harmful substances, the following values are used. The threshold limit value (TLV) is the concentration (in air) of a material to which most workers can be exposed daily without adverse effects. These values are established (and revised annually) by the American Conference of Governmental Industrial Hygienists and are time-weighted concentrations for a 7- or 8-h working day and a 40-h working week. For most materials the TLV may be exceeded to a certain extent, provided that there are compensatory periods of exposure below the value during the working day (or in some instances during the week). For a few materials (mainly those which produce a rapid response), the limit is given as the ceiling concentration (i. e., the maximum permissible concentration) that should never be exceeded, the threshold between safe and dangerous concentrations [l]. The time-weighted average (TWA) applies to the expression of permissible levels for occupational exposure. Exposure above the TWA is permitted, provided it is compensated for by equivalent excursions below the level during the working day or shift. In some national lists, the magnitude, duration and frequency of permissible excursions above these averages are specified [l]. 2 Berezkin, Gas Chrom.-BE

2

1. Introduction

In the U.S.S.R., the maximum allowable concentration (MAC) of a harmful substance in the air of the working zone is usually used as the main criterion. The MAC denotes a concentration that will not cause, in the course of work for 8 h daily or for any other period of time but not more than 41 h per week throughout the working life of an individual, any disease or deviation in the health status as detectable by the available methods of investigation, during the working life or during the subsequent life of the present or the following generations [l]. At present one of the vital problems is to protect the environment from pollutants that are the products of human enterprises. When they enter the air, water or soil, toxic substances (industrial poisons) pose a real threat to the normal existence of man, animals and plants. Industrial and transport development, the growing density of the population, the penetration of man into the stratosphere and space, the intensification of agriculture (application of pesticides), transportation of petroleum products, burial of dangerous chemical and radioactive wastes on the bottom of the oceans and the continuing nuclear arms tests-all these contribute to the constantly growing global pollution of our environment. Nowadays up to one million different chemical compounds of anthropogenic origin are constantly present in the biosphere, and the number is steadily growing. Annually almost 250000 new chemical substances are synthesized all over the world and many of them become potential pollutants. Air pollution causes grave concern because without air life on Earth would be impossible. According to the definition of the World Health Organization (WHO), air pollution takes place in those instances when a pollutant (or several pollutants) are contained in the atmosphere in an amount and during a period that harm or may harm people, animals, plants or property or can cause unaccountable damage to the health and property of people. The main sources of atmospheric pollution are discharges from industrial establishments and also processes of fuel combustion or evaporation (thermal power plants, internal combustion engines, etc.), forest fires and volcano eruptions. As a result of meteorological processes air pollutants spread over considerable distances in the atmosphere, leading to the global air pollution of our planet. At present the composition of air in rural and industrial areas is basically the same (they differ only in the quantitative composition of the pollutants). With these conditions it is a priority task (especially for industrially developed countries) to fight atmospheric pollution. The reasonable utilization of natural resources and the protection of nature, the establishment of the state sanctuaries and national parks, an increase in green plantations, a reduction in industrial discharges into the atmosphere and the development of wasteless chemical technologies are the main routes for solving the ecological problems for the benefit of mankind. However, it is impossible to solve the whole range of problems connected with the protection of the atmosphere and other parts of the environment without effective air quality control systems. The necessity to develop comprehensive methods of detecting various toxic substances in the atmosphere is universally recognized. The global pollution of the atmosphere and oceans and the significance and difficulty of tackling this problem have necessitated wide international cooperation on environmental protection. There exist numerous international programmes aimed at protecting various segments of the environment from pollution of air, water, soil and biota. Most of these programmes are being successfully carried out under the auspices of UNO, WHO, UNESCO and WMO (World Meteorological Organization). By the 1970s chemists and analysts had long realized the necessity to adopt measures for air pollution control and to develop reliable and effective analytical techniques for identifying and measuring industrial poisons in the atmosphere and in the workplace air. Rapid, sensitive and selective techniques for detecting micro-impurities of toxic organic matter, inorganic gases and aerosols of heavy metals in the air have been developed. Some countries have sanctioned standard methods (obligatory for internal application) to control the contents of the major air pollutants such as carbon monoxide, sulphur dioxide, nitrogen

1. Introduction

3

oxides, hydrocarbons, photooxidants and aerosols of heavy metals. The number of publications on the methods for air pollution analysis has also considerably increased. I n the last decade more than 25 books and about 35000 papers have been published on analytical techniques, methods of concentrating toxic micro-impurities from the air, the identification of pollutants and accurate methods for their determination. This book was written to describe systematically methods of detecting pollution of the atmosphere and the air in industrial areas. The current state of gas chromatographic methods for identifying harmful chemicals in air is described. Both general techniques for the analysis of imputities and practical applications of proposed methods in solving the concrete analytical tasks are discussed. Problems concerning the determination of impurities in air by gas chromatography are reflected in a number of books [2-141. In addition, a particularly important book devoted to applications of liquid chromatography in environmental analysis has been published [15]. It is remarkable for it general approach to the problems discussed and undoubtedly is of interest for those who are tackling similar problems using gas chromatography. However, taking into account the rapid development of this area, its complexity and versatility and the different insights on the same problem by different investigators, we considered it useful to write another book on the determination of impurities in air. We sincerely hope that the book will prove useful to readers and will prompt the further development and practical utilization of gas chromatography in the field of the air pollution control. Finally we thank Dr. H. G. STRUPPEfor valuable discussions and comments.

References (Chapter 1) [l] English-Russian Glossary of Selected Term in Preventive Toxicology. USSR State Committee for Science and Technology, Moscow, 1982. [2] WARK,K.;WARNER,C.F.: Air Pollution, its Origin and Control. New York: Harper and Row 1976. [3] THAIN, W.: Monitoring Toxic Gases in the Atmosphere for Hygiene and Pollution Control. Oxford: Pergamon Press 1980. [4] GRAEDEL,T. E.: Chemical Compounds in the Atmosphere. New York: Academic Press 1978. [S] Permissible Concentration Levels and Approximate Safe Levels of Toxicants in the Environment. Severodonetsk, VNIITBHP, 1978. [6] Technical Conditions and Methodological Instructions for Detection Methods for Air Pollutants. Annotated Index. Severodonetsk, VNIITBHP, 1981. [7] JENNINGS, W. G.; RAPP,A.:Sample Preparation for Gas Chromatographic Analysis. Heidelberg: Hiithig 1983. [8] GROB,R.L.(Ed.): Chromatographic Analysis of the Environment. New York: Marcel Dekker 1975. G. 0.: Controlled Test Atmospheres. Ann Arbor Science Publishers 1980. [9] NELSON, [lo] DRUGOV, Yu. S.; BEREZKIN, V.G.: Gas Chromatographic Analysis of Air Pollution (in Russian). Moscow: Khimya 1981. [ l l ] DRUGOV,YU. S.; BELIKOV,A.B.; DJAKOVA,G.A.; TULCHINSKYJ,V.M.: Methods of Air Pollutants Analysis (in Russian). Moscow: Khimiya 1984. [12] SCHREIER, F. (Ed.): Analysis of Volatiles. Berlin, New York: Walter de Gruyter 1984. [13] METZNER, K.: Gaschromatographische Spurenanalyse. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1977. [14] LEI THE,^.: Die Analyse der Luft und ihrer Verunreinigungen in der freien Atmosphare und am Arbeitsplatz. Stuttgart: Wissenschaftliche Verlagsgesellschaft 1974. 1151 LAWRENCE, J. F.: Liquid Chromatography in Environmental Analysis. Clifton, New Jersey: Humana Press 1984.

2'

Chapter 2

Air As an Object of Analysis

2.1.

Atmospheric air and sources of atmospheric pollution

More than 99.9%of dry atmospheric air consists of nitrogen, oxygen and argon with only about 0.1% remaining for carbon dioxide, krypton, neon, helium, xenon and hydrogen. However, carbon monoxide, ozone, nitrogen oxides and ammonia (0.003-0.25 mg/m3) and also 0.5-1.5 mg/m3 of hydrogen and methane are traceable even in pure air. The small amounts of these gases in air are accounted for by the existence of free ozone in the upper atmospheric layers, by putrefaction and decomposition processes (ammonia, methane, carbon and nitrogen oxides) and by atmospheric phenomena (nitrogen dioxide). All other compounds (solid, liquid or gaseous substances that change the natural atmospheric composition) that enter the air from the other sources (mainly of anthropogenic origin) are classified as pollutants. They include carbon, sulphur oxides, hydrocarbons, various oxidants, aerosols of metals, solid particles (dust, soot, organic aerosols) and radioactive substances. Nitrogen oxides belong to this group of compounds too and are formed in general as a result of human activities. Industry, especially in highly industrialized areas, is the main contributor to air pollution. The major sources of industrial air pollution are thermal power stations (TPS) that burn coal and release soot, ash and sulphur dioxide into the atmosphere, steel plants that release mainly soot, dust, iron oxide, sulphur dioxide and sometimes even fluorides, and cement plants, which are sources of enormous amounts of dust. The large plants producing inorganic chemicals pollute the atmosphere with diverse gases (sulphur dioxide, silicon tetrafluoride, hydrogen fluoride, nitrogen oxides, chlorine, ozone, etc., depending on the technological process. Cellulose plants and petroleum refineries emit gases with unpleasant odours (odorants) into the atmosphere. Petroleum plants (petroleum distilleries, refineries, organic synthesis) pollute the air with hydrocarbons and other organic compounds classes (amines, mercaptans, sulphides, aldehydes, ketones, alcohols, acids, chlorohydrocarbons, etc.). Atmospheric air pollution by the chemical plants may occur for the following reasons [l]: 1.Incomplete yield of the main product (incomplete reaction, losses of the main product and others). 2. Release into the atmosphere of impurities and pollutants during raw material processing (fluoride compounds, natural phosphates and ores; sulphur dioxide and hydrogen sulphide from natural gas, crude oil and coal; arsenic and selenium from the sulphur pyrite during sulphuric acid production, etc.). 3. Losses of substances used in technological processes, e. g., volatile organic solvents, carbon disulphide and hydrogen sulphide used in artificial silk and viscose manufacture, nitrogen oxides during the chamber and tower sulphuric acid production processes and fluoride compounds in the aluminium industry. 4.The escape of odorous substances and oxidation and destruction products as a result of thermo-oxidative destruction, heating or drying processes (in the food, soap, glue and furniture industries, car painting, synthesis and processing of polymers, solvent generation, etc.). In large cities, gases coming from motor vehicle exhausts and the processes of fuel evaporation add considerably to atmospheric pollution. The amount of hazardous substances in exhaust fumes depends to a large extent on the type and maintenance of the engines. A gasoline engine does not significantly increase carbon dioxide levels in the atmosphere. However,

2.2. General characteristics of air pollutants

5

it is a direct source of pollutants such as carbon monoxide (formed as a result of incomplete combustion of the fuel in the cylinders, gaseous alkanes, alkenes and unburnt fuel components, high-boiling polycyclic aromatic hydrocarbons and soot, products of incomplete fuel oxidation (e. g., aldehydes), halocarbons, heavy metals (e. g., lead in ethylated petrol) and nitrogen oxides formed in the process of fuel combustion. Exhaust fumes containing reactive alkenes and nitrogen oxides under the action of solar radiation can participate in photochemical reactions resulting in toxic smog, which is harmful to plants, building materials and living beings.

2.2.

General characteristics of air pollutants

Air pollutants are classified as primary and secondary types. Depending on the source and the mechanism of formation, the former are chemical substances that enter the atmosphere directly from stationary or mobile sources. The latter are formed as a result of the interaction of the primary pollutants in the atmosphere and their reaction with the air components (oxygen, ozone, ammonia, water) under the action of ultraviolet radiation. Frequently, the secondary pollutants, such as substances containing the peroxyacetyl nitrate (PAN) group, turn out to be far more toxic than the primary type. The major part of solid particles and aerosols in the air are secondary pollutants. Conventionally, pollutants were classified into several groups depending on their toxicity, potential danger, spread and emission sources [2]: (1) major (critical) air pollutants-carbon monoxide, sulphur dioxide, nitrogen oxides, hydrocarbons, solid particles and photochemical oxidants; (2) polycyclic aromatic hydrocarbons (PAHs); (3) traces of elements (mainly metals); (4) permanent gases (carbon dioxide, fluoromethyl chloride, etc.); (5) pesticides; (6) abrasive solid particles (quartz, asbestos, etc.); (7) various pollutants with multilateral action on the organism [nitrosamines, ozone, polychlorobiphenyls (PCBs), sulphates, nitrates, aldehydes, ketones, etc.]. All critical pollutants are primary air pollutants. Nitrogen oxides are formed predominantly by the high-temperature fixation of nitrogen and oxygen in power plants and internal combustion engines. Nitrogen oxide is formed during electrical discharges in the atmosphere and is also present in the exhaust fumes of motor vehicles. Annually up to 5. lo7t of nitrogen oxides are emitted into the atmosphere, with 53% of that amount traceable to anthropogenic sources. Ultimately in the atmosphere nitrogen oxides turn into nitrates. Sulphur dioxide is formed from burning fuels with a high sulphur content (coal, oil). This toxic gas comes from stationary combustion sources, such as thermal power stations (85-95%), industrial cites (oil refineries, fertilizers, sulphuric acid and the petroleum chemicals industry) (5-10%) and internal combustion engines (2-7%). Sulphur dioxide, which takes part in the formation of photochemical smog, is considered to be the major air pollutant, dangerous for animals and plants. The total annual emission of sulphur dioxide into the atmosphere amounts to 8 . lo7t (considerably exceeding the contributions from other toxic chemicals) and is continuously growing in proportion to the energy consumption [3]. Carbon monoxide is one of the most hazardous and extremely wide spread air pollutants. It derives its toxicity from its reaction with haemoglobin in the blood. CO is generated by the incomplete combustion of different types of fuel. Forest fires and photochemical conversions of organic substances in the atmosphere serve as natural sources of CO. Up to 25% of carbon monoxide is of anthropogenic origin. A large proportion of the CO in the atmosphere in industrial and residential regions (in the U.S.A. up to 40% of the total atmospheric pollution) is accounted for by transport exhaust fumes. The average CO atmospheric concentration (about near highways and in towns during tends to increase significantly (to up to 3 * rush hours [4].

6

2. Air as an object of analysis

It is assumed that in the future the air pollution from stationary sources by toxic substances such as sulphur, carbon and nitrogen oxides and dust will decrease. However, the gases and vapours of organic substances and heavy metals (lead, cadmium, beryllium, etc.) will still present grave dangers. The concentration of hydrocarbons emitted into the atmosphere from natural sources is slightly higher than 1 mg/m3. The annual emission of hydrocarbons reaches 3. lo8t. Of this large amount, about 50% is traceable to transport, about 15%to industrial and residential fuel burning and another 26% to coal, waste and refuse burning (up to 1 m3 of refuse per person is burned annually throughout the world) and evaporation of fuel and solvents. The “average” automobile exhaust contains up to 400 mg/m3 of alkanes, 120 mg/m3 of acetylene, 200 mg/m3 of aromatics and 300 mg/m3 of alkenes [3]. The solid particles are represented in the atmosphere by dust, sand, ash, soot, volcanic ash and aerosols of organic (high-molecular-weight compounds) and inorganic nature. Frequently the toxicity of solid particles is due to the adsorption on their surface of such dangerous compounds as PAHs and nitrosamines. Photooxidants are formed in the atmosphere when reactive hydrocarbons interact with nitrogen oxides under the action of ultraviolet radiation. As a result, highly toxic chemicals such as peroxyacetyl nitrate and peroxybenzoyl nitrate are formed. Even at a concentration of 0.2 mg/m3 these compounds have a sharp lachrymatory effect, damage plants and destroy rubber. Peroxybutyl and peroxypropyl nitrates are even more toxic. They are unstable, especially at higher temperatures, and decompose to form simpler products, e. g., methyl nitrate and carbon dioxide [4].The atmosphere over most large cities is polluted with oxidants, as their generation is proportional to the development of transport and industry. Another group of pollutants consists of polycyclic aromatic hydrocarbons (PAHs). They can belong to both the primary and secondary air pollutant categories and are usually adsorbed on solid particles. Many PAHs possess marked carcinogenic, mutagenic and teratogenic activity and present a grave threat to the population. PAHs are mainly generated by power plants fuelled by oil and coal and by the petroleum transport and industries. Of several million known chemical compounds, only about 6000 have been tested for carcinogenic activity. It has been established that 1500 compounds recognized as potential atmospheric pollutants are also characterized by marked carcinogenic properties (PAHs, nitrosamines, halohydrocarbons and others). In concentrated industrial areas the amount of PAHs and other carcinogenic substances released into the atmosphere by plants may reach 80% of the total environmental pollutants. Traces of such highly toxic pollutants as arsenic, beryllium, cadmium, lead, magnesium and chromium have been detected in the atmosphere. They are usually present in the form of inorganic salts adsorbed on solid particles. Nearly 60 metals have been identified in the combustion products of coal. Mercury, arsenic, barium, beryllium, bismuth, bromine, cadmium, chlorine, cobalt, copper, iron, lead, manganese, antimony, molybdenum, nickel, selenium, tellurium, thallium, tin, titanium, uranium, vanadium, zinc and zirconium are present in the flue gases of power plants. For most of these elements the flue gases of power plants contribute 75% of the total air pollution level from diverse sources. The major proportion of pollutants enter the atmosphere from coal burning. Of all power plants fired by coal, oil and gas, coal power plants release more than 95% of the solid particles, 85% of the sulphur oxides, 70% of the nitrogen oxides and more than 90% of the trace elements. Lead enters the atmosphere as a result of oil combustion, volcano eruptions, automobile exhausts and various technological processes. Annually nearly 2 . lo5t are emitted into the atmosphere in the form of halides. The annual increase in the mercury content in the environment of industrially developed countries is ca.5%. Metallic mercury and lead and their organometallic compounds are extremely toxic. Mercury enters the atmosphere from volcano eruptions and is released by the chemical, electronic and instrument engineering industries.

2.2. General characteristics of air pollutants

7

Organomercury halide compounds that are formed from metallic mercury and its inorganic salts under the action of microorganisms are especially hazardous to humans. I n the F.R.G. alone the burning of diverse types of fuels results in up to 40 t of mercury entering the atmosphere annually and subsequently settling on soil and water surfaces. In the atmosphere the pollutants accumulate, interact, hydrolyse and oxidize under the influence of humidity and oxygen and also change their composition under the influence of radiation. Hence, the chemical properties of toxicants determine the duration of their residence in the atmosphere. This period is 4 days for sulphur dioxide, 2 days for hydrogen sulphide, 5 days for nitrogen oxide and 7 days for ammonia, while CO and CH4, being inert, remain unchanged for 3 years [ 5 ] . The next group includes the low-activity compounds-permanent gases (Freons and carbon dioxide) with longer residence times. The ever-increasing fuel combustion and forest fires are constant sources of C 0 2 in the atmosphere. In the U.S.A. approximately 2 lo9t of carbon dioxide are emitted annually from burning mineral fuels. Refrigeration units serve as the major source of emission of Freons (fluorochloroalkanes). Permanent gases accumulate in the stratosphere, undergo chain reactions and destroy the ozone layer that protects the lower atmosphere from high-energy solar radiation. According to some workers this is the reason why C 0 2 , not being toxic in the proper sense of the word, causes global changes in the atmospheric temperature. This in its turn results in climatic changes over the planet due to the greenhouse effect. Pesticides are often sprayed from aeroplanes. Organophosphorus pesticides are particularly toxic because, when subjected to photolysis in the atmosphere, they form products that are considerably more poisonous than the initial compounds. Serious diseases such as silicosis and asbestosis may be caused by inhaling so-called “abrasive” particles (silicon dioxide and asbestos). The pollutants in this last category are the reaction products of primary atmospheric pollutants. The most noteworthy are sulphates, nitrates and nitrosamines. Nitrosamines are formed in the atmosphere as a result of the reaction of amines with nitrogen oxides. Nitrosamines have also been detected in tobacco smoke. Other widespread air pollutants, such as PCBs, are also considered to be potentially carcinogenic. They are usually added to pesticides to promote their effect. The emission sources of some of the most important atmospheric pollutants and their concentrations in industrial and rural areas are given in Table 2.1. On entering the atmosphere, most toxic compounds undergo changes under the influence of ultraviolet radiation, humidity, ozone and air oxygen. The reaction products and the initial compounds (primary pollutants) react with each other, sometimes producing more toxic and hazardous compounds (secondary pollutants) [S]. The atmospheric conversions and interactions of pollutants and processes of dilution, deposition, adsorption and absorption do not prevent toxic chemicals from accumulating in the atmosphere and spreading over wide areas [4]. Therefore, the aim of keeping the atmosphere clean necessitates thorough and effective control over the degree of air pollution, which fluctuates significantly in time and space. These fluctuations depend on the specific features of the pollution sources (type of emission source, origin and properties of pollutants, their amounts), meteorological and topographic factors (wind direction and velocity, temperature inversions, atmospheric pressure, air humidity, relief of the locality and distance from the pollution source). In the early 1970s, intensive development of methods for air pollution control and air quality standards were stimulated under the auspices of the United Nations Organization and other international organizations. The unification of units characterizing the degree of atmospheric air pollution became essential. On the recommendations of WHO and IMO (International Meteorological Organization), pg/m3 and mg/m3 were adopted as such unified units [4].

2. Air as an object of analysis

8 Table 2.1. Concentrations of atmospheric pollutants [2] Pollutant

Emission source

Concentration in urban areas (mg/m’)

Concentration in rural areas (mg/m’)

Carbon monoxide Sulphur dioxide Nitrogen oxide Nitrogen dioxide Ozone

Automobile exhaust Burning of oil Combustion (oxidation) - do Photochemical reactions in the atmosphere Natural gas. Rottening process Automobile exhaust - do Photooxidation of alkenes in the air Automobile exhaust - do -

5.0 0.2 0.2 0.1 0.3

0.1

3.0

1.4

0.05

0.07 0.03

0.001 0.001 0.001

0.02 2.0

0.001 0.005

Rottening process - do Incomplete combustion

0.01 0.004 0.05

0.01 0.002 0.001

Methane Ethylene Acetylene Peroxyacetyl nitrate Alkenes (C,-C,) Hydrocarbons (except methane) Ammonia Hydrogen sulphide Formaldehyde

0.002 0.002 0.001 0.01

To fight air pollution, air quality standards should be established (in the U.S.S.R., PCL permissible concentration level) as a basis for further measures to preserve the environment. The existing air quality standards enable atmospheric purification measures to be rationally planned in those areas where the degree of air pollution exceeds the permissible concentration level. In the U.S.S.R., national PCL standards were adopted for the most widespread air pollutants as early as 1951. At present in the U.S.S.R.the concentrations of more than 160 toxic chemicals are monitored in the air in residential areas, and in industrial zones the PCL has been adopted for more than 800 chemicals [6, 71. The pollutant concentrations are also controlled and normalized in other countries (see, for example, the Appendix in the book by JENNINCS and RAPP[S]).

2.3.

Specific features of air as an object of analysis

It must be stressed that polluted air is one of the most complicated objects for analysis, for the following reasons: 1.The atmosphere and the air in industrial zones consist of multi-component mixtures of pollutants belonging to chemical compounds of many different classes. 2. The concentrations of many pollutants are extremely low, often 10-4-10-3%or even lower. 3.The polluted air is a labile system subject to changes under the influence of meteorological conditions and chemical interactions of the pollutants. 4. Frequently, the polluted air is a heterogeneous system containing solid particles and liquid aerosols. Therefore, in order to analyse air pollutants it often becomes necessary to use all available gas chromatographic methods and instrumentation.

References

9

References (Chapter 2) LEITHE, W.: Die Analyse der Luft und ihrer Verunreinigungen in der freien Atmosphare und am Arbeitsplatz. Stuttgart: Wissenschaftliche Verlagsgesellschaft 1974. SAMSFIELD,M.: Energy Sources. 3 (1977) 111. WARK,K.; WARNER, C. F.: Air Pollution, its Origin and Control. New York: Harper and Row 1976. THAIN, W.: Monitoring Toxic Gases in the Atmosphere for Hygiene and Pollution Control. Oxford: Pergarnon Press 1980. GRAEDEL,T. E.: Chemical Compounds in the Atmosphere. New York: Academic Press 1978. Permissible Concentration Levels and Approximate Safe Levels of Toxicants in the Environment. Severodonetsk, VNIITBHP, 1978. Technical Conditions and Methodological Instructions for Detection Methods for Air Pollutants. Annotated Index. Severodonetsk, VNIITBHP, 1981. JENNINGS, W. G.; RAPP,A.: Sample Preparation for Gas Chromatographic Analysis. Heidelberg: Hiithig 1983.

Chapter 3

Gas Chromatography in the Analysis of Air Pollutants

The analysis of a complex mixture of air pollutants requires the technique employed to be highly efficient, highly selective and highly sensitive. In our opinion, gas chromatography meets these requirements most completely. Let us consider this technique in more detail.

3.1.

General considerations

Chromatography is a field of science that studies the motion of a substance (or a group of substances) in a flow of one or several phases moving relative to another one (or other ones)

111. The mobile phase employed in gas chromatography is a gas (vapour). Gas chromatography permits the separation of volatile substances and the determination of their physico-chemical parameters. Its practical application is based on the different retention and band broadening patterns exerted by the substances moving in the gas-phase flow relative to a film of the stationary phase, with the solutes distributed between the stationary (solid or liquid) and gas phases. Gas chromatography is subdivided into two types, depending on the aggregate state of the stationary phase. In “gas-solid” or “gas adsorption” chromatography the latter is a solid, whereas in “gas-liquid” or “gas-liquid-solid’’ chromatography it is a liquid applied as a thin film on a solid support. This chapter is confined to the principles of gas chromatography. For more detailed information the reader is referred to Refs. [2-71. The advantages of gas chromatography are that the separation capacity of the sorbent can easily be changed by choosing the optimum stationary liquid phase (SLP), the symmetry of solute bands and a better reproducibility of sorbent properties. Compounds of different types are used as SLPs, such as hydroxydipropionitrile, diglycerol, squalane, tricresyl phosphate and eutectics [e.g., sodium nitrate (18.2%)-potassium nitrate (54.5%)-1ithium nitrate (27.3%)]. Especially high selectivity is exhibited by complex-forming phases. Thus, silver solution in ehtylene glycol is efficient for the separation of unsaturated compounds including cis and transisomers. SLPs must be thermally and chemically stable and possess low viscosity. Thermally stable selective SLPs include polyethylene glycols (upper temperature limit 225”C), cyanoethylmethylsilicones (275”C), Apiezone (300°C),methylsilicones (350°C), methylphenylsilicones (375°C) and carboranemethylsilicones (400°C). Basic parameters used in gas chromatography to characterize retention and band broadening include the retention volume, VR, the number of theoretical plates (TP) per metre of column length ( N L ) and the height equivalent to a theoretical plate (HETP) (H). The retention volume VR is that of the carrier gas passed through a column from the moment of sample injection to the output of the maximum peak solute concentrhtion. It can be expressed as VR = F C t R ,where tR is the retention time and Fc is the carrier gas velocity at a given temperature and gas pressure in the column. A more invariant measure of retention is the adjusted retention volume, V k , representing the retention volume of a solute minus that of a non-sorbed gas: V ; = ( t R - fM) F, = t R F , (3.1) where fk = tR - fM is the adjusted retention time and tM is the retention time of a component not sorbed by the stationary phase. This parameter ranges from fractions of a millilitre to

3 . 1 . General considerations

11

several litres, depending on the experimental conditions and the column diameter. Another parameter widely used in analytical gas chromatography is the relative retention value, rlJ (relative retention volume or relative retention time),

where V R ,and VKj are the adjusted retention volumes of compounds i and j , respectively, t k , - t R , - t M , t k , = tR, - tM and tk, and fk, are the adjusted retention times of compounds i and j , respectively. The extent of solute band broadening occurring as the solute traverses the column is characterized by the number of theoretical plates defined as N = 5.545(tRlWJ2, where W , is the peak width at half-height (in time units). Other parameters widely used to characterize a column include the specific efficiency, the number of theoretical plates per metre of column length ( N J , and the height equivalent to a theoretical plate (H):

N , = NIL, H = WN, H.N=1

(3.3) (3.4) (3.5)

where L is the column length. These parameters range from 1000 to 20 000 TP/m, depending on the experimental conditions (carrier gas velocity, diameter of sorbent particles). As a chromatographic band moves in the carrier gas flow, its broadening (swamping) occurs. The three basic processes causing band broadening are eddy diffusion, diffusion in the gas phase and interphase mass transfer. Eddy diffusion is independent of the carrier gas velocity and results from the path lengths for individual solute molecules being different owing to a non-uniform sorbent packing and sorbent particles being non-uniform in size. Longitudinal diffusion occurs in the gas phase and is caused by the concentration gradient associated with each chromatographic band. The finiteness of the mass transfer rate leads to the actual solute distribution being different to the equilibrium value, which also contributes to band broadening. The relative contribution of the above factors depends on the carrier gas velocity. Normally, longitudinal diffusion and the finite interphase mass transfer rate are the major factors of broadening at low and high flow-rates, respectively. Variation of the HETP with the linear carrier gas velocity is roughly described by the Van Deemter equation:

H

=A

+ B / u + Cu

(3.6a)

or (3.6b) where A is the eddy diffusion coefficient including multi-flow-path factors, B is the longitudinal diffusion coefficient resulting from solute diffusion in the gas phase along a column, C is the mass transfer coefficient arising from the finiteness of the interphase mass transfer rate, u is the linear carrier gas velocity, 1 and y are constants, d , is the sorbent particle diameter, D, is the solute diffusion coefficient in the gas phase, k ist the capacity ratio (or partition ratio), defined as k = V',JVM, where V M is the dead volume, and dl is the effective thickness of the liquid phase film on the solid support surface. DIrepresents the solute diffusion coefficient in the liquid phase. The Van Deemter equation is helpful in optimizing the separation conditions. Figure 3.1 shows the dependence of the HETP and the role of different band broadening factors on the linear carrier gas velocity.

12

3. Gas chromatography in the analysis of air pollutants

I

_ _ _ - - - - -3 /

I

--

Fig. 3.1. Dependence of HETP on carrier gas velocity and the effect of the main band broadening factors. I Contribution of eddy diffusion; 2 contribution of longitudinal diffusion; 3 contribution of interphase mass transfer

As the solute bands are moved by the carrier gas along the column, two opposing processes occur. The distance between the concentration maxima of successive components governed by the column selectivity increases, which improves the separation, whereas the bands are broadened depending on the column efficiency, which impairs the separation. The separating ability of a column depends mainly on its efficiency and the selectivity of the sorbent used. A quantitative measure of separation of two successive components is the resolution, R , representing a dimensionless value equal to the difference in the retention volumes, AYj = VRi- VRj, divided by the sum of the peak widths at the baseline. The peak resolution is determined by the sorbent selectivity, a(= rij), the column efficiency, N , and the mass distribution ratio, D, (capacity factor, k ) .

R=-.-.-1. ( a - 1 ) 4

01

1+D,

(3.7)

Figure 3.2 gives a graphical description of the relationship between the major physical processes that occur in a capillary column and its chromatographic characteristics, such as selectivity, efficiency and capacity. It illustrates in more detail the role of the individual factors in eqn. (3.7). The carrier gases conventionally used include nitrogen, air, argon, carbon dioxide, hydrogen and helium. The carrier gas must be chemically inert with respect to the solutes and the stationary phase. It must match the detector employed, i.e., cause no decrease in the de-

t

kinetics

compound and

Fig. 3.2. Relationship between column selectivity, efficiency and capacity with the major processes occuring in the column in the course of separation

3.1. General considerations

13

tection sensitivity, and possess a low viscosity and be non-explosive (which is of particular importance for industrial use), sufficiently pure and cheap. The mobile phase in GC is compressible. A pressure drop in a column leads to the expansion of the carrier gas and increases its velocity, which affects V Rand VR.This is why in precise measurements the actual (pure) retention volume, V,, is used, which is calculated allowing for the pressure drop and is thus independent of the latter. VN = jVX (P,IPO)Z - 1 2 (Pi/P0)31

j = - 3.

(3.8) (3.9)

where j is the pressure gradient factor and Piand Po are the inlet and outlet pressures, respectively. Another parameter used in gas chromatography is the specific retention volume, V , , representing the retention volume per unit mass of the stationary phase: (3.10) where Wl is the mass of the stationary phase and T is the absolute temperature in the column. The specific retention volume is used to determine certain physico-chemical parameters such as the activity coefficient. The main purpose of the solid support in gas chromatography is to provide a thin (fractions of a micrometre or less) film of SLP to improve the mass transfer between the mobile and stationary phases. Solid supports include diatomites subjected to special treatment in order to decrease their adsorptive activity, polymeric supports based on polytetrafluoroethylene and inorganic salts. Normally, their specific surface area varies from 0.1 to 1.5 m2/g. In capillary chromatography, the inner walls of a capillary column serve as a solid support. The role played by the solid support in gas-liquid chromatography is not less important than that of the SLP. Retention in GLC is due to the absorption of solutes in the SLP and their adsorption on the SLP/solid support and SLP/carrier gas interfaces [8, 91. The actual retention volume, V N , is governed by the following parameters of the sorbent [9]: VN=K~V~+KglS~+K~K~S~

(3.11)

where K I , KgI and K , are the solute distribution constants in the systems SLP-gas, SLP surface-gas and solid support surface-SLP, respectively; V, is the column volume occupied by SLP and S, and S, are the areas of the gas/SLP and SLP/solid support interfaces, respectively. The contribution of interface adsorption to the retention value varies from 0 to 100%. Therefore, in studying solute interactions with SLPs it is essential that the fraction of the retention volume corresponding to the solvation of a component in the SLP be extracted from the overall value. Procedures have been developed that permit the quantitative characterization of all the main types of solute interactions with the stationary phase, including adsorption on the gas/SLP interface ( K g lin eqn. (3.11)) and the SLP/sblid support interface (K,) and solvation in the SLP ( K J . Gas-liquid chromatography is the basic chromatographic technique used successfully in analysing composite mixtures. Band identification is based on the retention data. The retention of a component in a mixture is compared with that of a standard. Note that given a particular sorbent and identifying a substance by means of chromatography alone one should consider the sum of retention data obtained on columns with SLPs of different types. For chromatographic identification relative retention values are used, which can be evaluated much more precisely (the error decreases by a factor of two or more). At present, the most

14

3. Gas chromatography in the analysis of air pollutants

widely accepted parameters used in analysis and identification are Kovhts retention indices. The retention index system describes the retention behaviour of a compound relative to that of n-alkanes. As the number of carbon atoms in the n-alkane molecule increases by one, the retention index, Zi, is incremented by 100 units:

(3.12) where z and z + 1 are the carbon numbers of n-alkanes eluted before and after the substance i , respectively. V k , , VX,, VR(,+ are the adjusted retention volumes of the substance i and n-alkanes of carbon number z and z + 1 , respectively. The values of retention indices depend markedly on the type of sorbent used. Thus, for example, the retention of ethyl formate on silicone SLPs is characterized by the following retention indices: 487 (OV-1), 605 (OV-17) and 766 (XE-60); the same parameters for ethyl acetate are 506 (OV-l), 632 (OV-17) and 741 (XE-60). These data indicate that the retention index is governed by the natures of both the solute and SLP employed. Parameters other than retention index and relative retention value also used in chromatography. It has been shown that the known relative values can be considered as particular cases of the following general expression [8]:

(3.13) In developing a procedure for the determination of impurities it sometimes seems desirable to calculate the required efficiency of a column when advantage is taken of the stationary phase for which the retention values of analytes are known from the literature. Such a calculation can be accomplished using eqn. (3.7). However, in the last decade, experimental data have been presented in the literature in the form of the retention index rather than separation factor (see eqn. (3.7)). In this instance the calculation should utilize the following equation [lo]: 104(i0ge)2

n = 16R2

b;(AZ)*

+I-

10Zloge b,(AZ)

(3.14)

where n is the number of effective theoretical plates, AZ = Z2- Il is the difference in the retention indices of two sorbates, R is resolution, e is the basis of natural logarithms, b, = log(t:+ l / f L ) and t : + , and t i are the adjusted retention times of two elements of a homologous series of standard compounds of carbon number z + 1 and z, respectively. The efficiency of a chromatographic column depends markedly on its type. For classical packed columns (3-5 mm I.D., 100-200 cm long, sorbent particle diameter 0.1-0.3 mm) it is normally in the range 1000-3000 theoretical plates. Capillary columns are 10-100 times more efficient, the efficiency of the open type being 30 000-100000 theoretical plates [ll-131. At present, the predominant gas chromatographic mode is capillary chromatography. In 1986 ca. 75% of the papers published on gas chromatography in leading journals such as Analytical Chemistry, Journal of High Resolution Chromatography and Chromatography Communications and Journal of Chromatography utilized capillary chromatography. The use of capillary gas chromatography is distinguished by a number of features that are responsible for its rapid development in comparison with packed column chromatography: (1) higher efficiency; (2) speed of determination; (3) high sensitivity (when using a splitless sample injection technique.and the desired components being concentrated at a reduced column temperature or using the “Grob solvent effect” [13-151; (4) better reproducibility of the thermal conditions utilizing temperature programming as a result of a smaller column diameter and weight; and (5) lower sorbent and carrier gas consumption.

15

3.1. General considerations

Table 3.1. Compounds identified in the PAH fraction of street dust from Giza Square, Egypt. Peak numbers refer to Fig.3.3 [17] Peak

Compound

Peak number

Compound

Naphthalene Dimethylnaphthalene Trimethylnaphthalene Trimethylnaphthalene Dibenzothiophene Phenanthrene Anthracene C2-9H-Fluorene C2-9H-Fluorene Methyldibenzothiophene Methyldibenzothiophene Methylphenanthrene Methylphenanthrene Methylphenanthrene Methylphenanthrene C,-Dibenzothiophene C2-Dibenzothiophene Dibutyl phthalate C2-Dibenzothiophene Dimethylphenanthrene Dimethylphenanthrene Fluoranthene Pyrene C,-Phenanthrene C,-Phenanthrene

26 27 28 29 30 31 32 33 34 3s 36

C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene C,-Phenanthrene Me thylfluoranthene Benzonaphthothiophene Benzo[ghi]fluoranthene Benzo[c]phenanthrene Benz[a]anthracene Chrysene/triphenylene Methylbenz[a]anthracene or methylchrysene Phthalate ester Phthalate ester Benzo[b]fluoranthene and benzoblfluoranthene Benzofluroanthene Benzo(e1pyrene Benzo[a]pyrene Perylene Indeno[l,2,3-cd]pyrene Dibenzanthracene Benzo[ghi]perylene Anthanthrene Dibenzopyrene

number 1

2 3 4

5 6 7 8 9 10 11

12 13 14 1s 16 17 18 19 20 21 22 23 24 25

37 38 39 40 41

42 43 44 4s 46 47 48

Capillary chromatography is very popular for air pollutant determinations and readers are referred to specialized books (11-131. Currently, capillary columns rely mainly on bonded and immobilized phases. The use of immobilized liquid stationary phases (1) enhances the lifetime of the columns because chemically immobilized phases will be incapable of curling into large drops, which usually results in a sharp decrease in column efficiency and (2) increases the detection sensitivity because the immobilized phases permit the utilization of a concentration procedure o n sample/ solvent injection, e.g., using the “Grob solvent effect”. The capillary column materials and the coating materials are equally important, for the following reasons: (1) the surface of the internal column walls which serve as a solid carrier in capillary chromatography can frequently produce a detrimental effect (especially in pollutant analysis) on the chromatographic process as a result of irreversible and catalytic conversions of the compounds treated on the internal capillary surface (see, e.g., ref. [16]); and (2) the polymeric coating of fused-silica columns has a limited temperature stability. Urban air contains a variety of harmful aromatic hydrocarbons. Figure 3.3 [17] shows a chromatogram for the fraction of PAHs in urban dust (Egypt). It is natural that such a complex mixture can only be separated by the use of high-efficiency capillary gas chromatograPhY.

16

3. Gas chromatography in the analysis of air pollutants

37

39

42 41

'

I

35f80

Fig. 3.3.

120

1

160

I

200

I

24 0

I

I

280

315

L

"C

Chromatogram of the fraction of polycyclic aromatic hydrocarbons contained in street dust

~71.

Column: 21 m x 0.32mm I.D.; SLP, SE-52; temperature programme, 35 to 80°C (temperature balistically programmed), 80 to 315°C at 5 K/min. Compounds identified as in Table 3.1

Capillary columns are equally efficient in determinations of nitroaromatic compounds, which is a difficult problem. Table 3.2 shows the concentrations of this important class of organic compounds which were detected in atmospheric dust in Tokyo [18]. The use of capillary columns allows the applicability of gas chromatographic techniques to be greatly extended for environmental control purposes. The adsorption of solutes on the interfaces results in the relative retention value in the general case depending not only on the ratio of distribution constants of the solute and standard, but also on the adsorptive properties of the SLPisolid support and gadliquid interfaces. Therefore, the relative retention time and retention index are not constants for a compound when adsorption occurs. The chromatographic constant of compound i is the invariant retention index, Ioi, corresponding to the solute interaction with the SLP only, which can be evaluated in terms of the following equation [9, 191:

I, = lor + aik,,

(3.15) (3.16)

where a is a constant defining the adsorption of the solute on the sorbent used, k,,is the capacity ratio of a standard, k,, = VX,,/V,, Vk,, is the adjusted retention volume of a standard, V , is the retention volume of a non-sorbed component, Kli is the distribution constant of

17

3.1. General considerations Table 3.2. Nitroarenes identified in airborne particulates in Tokyo air [18] Compound

1 2 3 4 5

6 7 8 9 10 11 12 13 14 15 16 17 18 19

o-Ethylnitrobenzene pEthylnitrobenzene pDinitrobenzene 2,6-Dinitrotoluene 2,4-Dinitrotoluene 1-Nitro-2methylnaphthalene 4,6-Dinitro-rn-xylene 3,s-Dinitro-o-xylene 1,5-Dinitronaphthalene 1,3-Dinitronaphthalene 5-Nitroacenaphthene 2-Nitrofluorene 9-Nitroanthracene 4,4’-Dinitrodiphenyl 2,s-Dinitrofluorene 1-Nitropyrene 2,7-Dinitrofluorene 4-Nitro-pterphenyl 6-Nitrochrysene

Relative retention.) Phenylmethylsilicone

Carbowax 20M

Reference

Sample

Reference

Sample

0.340 0.432 0.564 0.611 0.696 0.795 0.796 0.841 1.13 1.16 1.18 1.29 1.30 1.51 1.61 1.68 1.75 1.77 1.90

0.346 0.439 0.568 0.606 0.690 0.792 0.798 0.841 1.13 1.16 1.17 1.28 1.30 1.51 1.60 1.68 1.75 1.78 1.90

0.391 0.470 0.745 0.727 0.773 0.783 0.811 0.847 1.46 1.49

0.388 0.474 0.739 0.727 0.780 0.786 0.817 0.843 1.46 1.50

Concentration (ng/m3)

Mutagenicity

0.056 0.009 0.017 0.006 0.024 0.005 0.011 0.078 0.057 0.12 0.11 0.050 0.27 0.071 0.19 0.14 1.5 0.16 0.27

-

+ + + k + f + + + + + + i+ + + +

*) Internal standard: Benzo[qquinoline (retention time = 16.61 min for 5 % phenylmethylsilicone column and 27.46 min for Carbowax 20M column). Analysis of nitroarenes in airborne particulates was carried out under the following conditions. Neutral organic fraction was prepared from the particulates collected from 7 500 m3 air by a high-volume sampler. After adding 1.0 pg of benzo[flquinoline to a benzene solution of the neutral organic fraction (0.5 d ) , 3 pl auf the solution were injected on to the capillary gas chromatograph (HP 5880A) by a splitless injection mode. Identification was made by comparison of relative retentions of sample peaks with those of 40 reference substances. Quantitative determination was performed using calibration graphs for each nitroarene identified. Concentrations indicate the values obtained with the phenylmethyl silicone capillary column. The values obtained with the Carbowax 20M capillary column were as follows; 0.047, 0.009, 0.027, 0.005, 0.021, 0.006, 0.014, 0.083,0.045 and 0.083 ng/m3 for the compounds numbered 1,2,3,4,5,6, 7, 8 , 9 and 10, respectively.

compound i between the SLP and the gas and K I ,and Klc,+1) are the distribution constants of two standard compounds of carbon number z and z + 1, respectively, with K , , 5 K l i < Klc,+l). To evaluate Zoimeasurements are performed on several columns differing in SLP content on the solid support. The contribution of adsorption to the retention index value may be appreciable (several to a few tens percent). In capillary chromatography, adsorption also affects the retention values [16]. At the usual low pressures, the role of the carrier gas lies mainly in transferring solutes along the column. At higher pressures, however, the non-ideality of the gas phase begins to count, which alters the distribution of the solute between the mobile and stationary phases. The chromatographic mobility of many substances increases. 3

Berezkin, Gas Chrom-BE

18

3. Gas chromatography in the analysis of air pollutants

Fig. 3.4. Schematic diagram of a gas chromatograph. 1 High-pressure carrier gas source; 2 carrier gas preparation unit; 3 sample injection unit; 4 chromatographic column; 5 thermostat; 6 detector; 7 recorder; 8 minicomputer for instrument control and data processing

High-pressure gas chromatography (HPGC) is a technique intermediate between gas and liquid chromatography. HPGC has the following advantages over the latter: (1) the possibility of directed variation of the solute retention volumes by varying the pressure over a wide range, (2) rapidity of the analysis owing to the lower viscosity of the mobile phase and higher diffusion coefficients, (3) the possible application of universal highly sensitive detectors employed in gas chromatography. However, the HPGC instrumentation and experimental procedures remaining fairly complicated, which restricts the large scale application of this technique. A particularly interesting variant is chromatographic separation with the gaseous mobile phase in the supercritical state. Supercritical fluid chromatography (SFC) has made it possible to separate porphyrins, which could not be analysed at high temperatures owing to their thermal instability. The use of carbon dioxide and ammonia in the supercritical state proved efficient in separating compounds with molecular weights as high as 40000. The unusual characteristics of HPGC prompted the following classification of gas chromatography according to the state of the mobile phase: (1) GC at mobile phase pressures of 1-25 atm and (2) GC with the mobile phase at high pressures (100-500 atm), including the supercritical state. A schematic diagram of a gas chromatograph is shown in Fig. 3.4. The carrier gas is continuously supplied from the high-pressure cylinder I to the carrier gas preparation unit 2, where it is further purified and the required parameters of the mobile phase (pressure, velocity) are maintained throughout the experiment. The carrier gas then passes to the sample input system 3, which normally in laboratory chromatographs is an independently thermostated cylindrical flow chamber provided with a self-sealing thermostable rubber septum through which a sample (1-10 pl) is injected from, e.g., a syringe (9) into the carrier gas flow at high temperature. In the input system the liquid sample rapidly vaporises and the carrier gas flow transfers it to the chromatographic column 4 located in a thermostat. The separation is usually run at 20-400°C. Special thermostats allow the separation to be performed at temperatures as low as liquid nitrogen boiling point, which is mainly used in separating the isotopes of low-boiling gases. Analytical separations are usually performed on 0.5-5 m X 0.5-4 m m I.D. packed columns and 10-100 m x 0.2-0.5 mm I.D. open capillary columns. The columns are packed with sorbents consisting either of a non-volatile liquid (stationary phase) supported as a thin coating on a solid macroporous carrier with low specific surface area (0.5-2 m2/g) or of a solid with a developed surface (50-500 m2/g). The weight of the stationary phase is normally 2-20% of that of the solid support. The mean diameter of the sorbent particles is usually 0.1-0.4 mm, but fine fractions are used when filling the column. The separation of the components of a mixture occurs in a chromatographic column as a result of the solute bands moving at different velocities. The bands are passed by the carrier gas flow to a detector 6 whose signal, which is proportional to the analyte masses, is recorded

3.1. General considerations

19

continuously by a recorder 7. A microcomputer 8 serves for instrument control and data processing. Gas chromatography is a hybrid technique: separation of a mixture into components occurs in the chromatographic column, and quantitative analysis of the eluate is performed by the detector. Hence separation and quantitative (and, frequently, qualitative) analysis are separated in time and space. Therefore, the final result of the analysis is determined by the characteristics of both the column and the detector. The detector plays a very important role in gas chromatography. Now mostly differential detectors are used whose signal is proportional to the instantaneous value of the solute concentration or mass flow. The most useful types are the katharometer (thermal conductivity detector) and flame ionization, electron-capture and flame photometric detectors. The highly sensitive ionization and flame photometric detectors are employed in the analysis of trace impurities and very small samples and in capillary chromatography. The analytical potential of gas chromatography can be substantially expanded by employing several detectors. This makes it possible to determine quantitatively and qualitatively the composition of unresolved bands containing two or more compounds, provided that the selectivity of their detection by the detectors employed is different. The use of the mass spectrometer for detection has led to the development of highly e f i cient combined gas chromatography-mass spectrometry (GC-MS). A gas chromatograph with a packed or capillary column is connected to a mass spectrometer via a molecular separator serving to concentrate the separated bands and partially remove the carrier gas. The ion sources of a number of mass spectrometers equipped with powerful high-vacuum pumps sometimes allow the direct connection of a chromatographic column to a mass spectrometer without a molecular separator, with the solutes passing directly from the column to the ion source for ionization. The flow of charged particles is then supplied to a mass analyser where separation according to mass-to-charge ratio ( m l z ) occurs. Each substance is characterized by its specific mass spectrum, reflecting its structure. The analyte is identified by comparing its mass spectrum with standard spectra measured on pure samples or available in the literature. The mass spectrometer can be used first as a highly selective detector for taking chromatograms at a fixed mass-to-charge ratio and second for the rapid monitoring of a full mass spectrum during chromatographic analysis. Computers are widely used for data processing. The sensitivity of a mass spectrometer is l O - ” - l O - ” mg/s. GC-MS is currently one of the most universal and informative instruments and it is widely used in the analysis of air pollutants. Figure 3.5 demonstrates the relationship between the various parameters and the criteria of chromatographic separation for gas chromatography. It is made in the form of two quasicircumferences one of them includes criteria of chromatographic separation, another one units the parameters of chromatographic experiment. A similar approach was described by BERRIDGE for HPLC [20]. The inner circumference indicates the following major criteria: R,, peak resolution; t , , time of separation; AP, pressure difference on the column (the pressure drop); S, response factor (peak height) for a bulky (1) and a smaller sample (2). Intermediate parameters (dotted circumference) are k’, capacity factor; a,selectivity factor; N , column efficiency (number of theoretical plates); 7 , carrier gas viscosity; ps, volume fraction of the carrier gas component with displacing properties; d , , column diameter; T, column temperature; u , carrier gas linear velocity; d , , size of solid support (adsorbent) particles; L, column length; P,,content of SLP on the solid support; MCS, weight ratio between the mixed stationary phase components. The data in Fig. 3.5 indicate a complex relationship between the various parameters and the criteria of the chromatographic method. It is recommended that the relationships are used in the compilation of the programme for the optimization of a chromatographic separation.

20

3. Gas chromatography in the analysis of air pollutants

Fig. 3.5. Relationship between the various characteristics of chromatographic separation

Gas chromatography allows one to carry out qualitative and quantitative analyses of organic and inorganic compounds whose vapour pressures at the temperature of the column exceed 0.001-1 mm Hg and which are thermally stable at the decomposition temperature. This method is widely used for determination of compounds present in samples in trace amounts (10-4-10-8%).

3.2.

Peculiarities of the gas chromatographic analysis of impurities

The difficulties associated with the gas chromatographic determination of impurities are mainly due to the fact that when in very low concentrations the latter show a behaviour different to that exerted by macroscopic amounts of the same substances. A detailed discussion of this problem has been given elsewhere [21-231. Organic pollutants are often preconcentrated at reduced temperatures in order to analyse them in air (see, e.g., ref.24). An interesting example of determining Arctic hydrocarbon air pollutants at ppt levels was described by Norwegian researchers [25]. The detection limit was 1-5 ppt. The Arctic air was shown to contain various gases of this type (C2-C4). The 'main problem to be faced in the determination of trace impurities (10-4-10-8%)in air is potential analyte losses that may accompany any step of the gas chromatographic analysis. The disappearance of trace impurities can occur when taking a sample (sorption by container walls, incomplete consumption and chemical reactions in the concentrator, etc.), during the desorption of analytes from the trap (inefficient desorption), when injecting the samples into the chromatograph (e.g., sample decomposition in the vaporizer at high temperature) and owing to sorption on or chemical reactions with the sorbents and the inner surfaces of the

3.2. Peculiarities of the gas chromatographic analysis of impurities

21

chromatographic equipment. In the latter instance we are dealing with phenomena similar to the adsorption of traces of isotopes in radiochemistry. The concentration of the active sites in the sorbent becomes comparable to that of the analytes and therefore adsorption, which is insignificant at normal concentrations, is important at micro concentrations. Overestimated results may arise from analyte vapour adsorption on the walls of the sampling valve. There are various ways of preventing such losses, but the most useful involves continuous conditioning of the column and the whole system with analytes in order to saturate active sorbent sites and the equipment surfaces. Such conditioning may take a long time (several hours), especially when analysing reactive inorganic gases or unstable and readily hydrolysed substances (nitrogen and sulphur oxides, ozone, halogens, mixed halogen compounds, hydrides, etc.) even if all the parts of the chromatograph are made of materials resistant to corrosion (nickel, PTFE, glass, etc.), and inert sorbents (PTFE, graphite, polymers based on polytrifluoromonochloroethylene) are utilized for separation [26]. Frequently, the preliminary sorbent treatment takes about 20-30 h (e.g., for nitrogen dioxide) and must be repeated (1-2 h) before each set of analyses. It should always be borne in mind that the absence of peaks of the impurities being analysed (especially when analysing reactive, unstable and readily hydrolysed substances) by no means indicates their absence in the sample or an undetectable concentration. This may be due to irreversible sorption or chemical reactions undergone by the impurities (hydrolysis, pyrolysis, reactions with sorbents, etc.). Such interactions can be minimized by taking advantage of cryogenic gas chromatography. By analysing corrosive and reactive compounds (ozone, nitrogen, halogen and sulphur oxides, fluorides, etc.) at low temperature (below - 20°C) or with a programmed increase from - 100°C until the elution of one of the components (e.g., sulphur or nitrogen dioxide) one can successfully separate the impurities of corrosive substances in the presence of moisture and low-boiling compounds (CO, C 0 2 , N2, 0 2 , etc.) without losses. One of the problems attending the analysis of impurities is the drying of air dehumidification, which is particularly important when dealing with large samples. As the amount of moisture accumulated in traps is several orders of magnitude greater than that of the concentrated impurities, analyte losses often result which requires the solutions being analysed to be strongly diluted. A widely used means of drying air consists in passing it through a dehydration plug filled with absorbents or chemicals that selectively absorb water. One of the best absorbents of this type are molecular sieves 3A, which absorb water but allow organic and inorganic impurities except ammonia and methanol to pass through [27]. The interaction of analytes with the chromatographic equipment and sorbents results in insufficient resolution when analysing trace amounts of flavours and many other compounds of natural and biological origin. One of the means of increasing the sensitivity of such analyses is to utilize packed or capillary columns and vaporizers made of fused silica, which decreases the possibility of irreversible adsorption. Another helpful approach consists in injecting a sample of thermally unstable compounds or large volume of diluted solution directly into the chromatographic column. The purity of the solvent used for the extraction of impurities form the sorbent following sample concentration is of great importance. The solvent must not contain impurities at concentration comparable to and eluted within a short time interval of the analyte substances. It is desirable that these sorbents (e.g., chlorobenzene) should be purified of impurities by ordinary distillation. Of no less importance is the purity of the carrier and diluent gases employed for the preparation of standard mixtures and the calibration of detectors. The thorough purification of cylinder nitrogen used for sorbent vaporization and trace compound analysis by GC and GC-MS involves five traps connected in series [28]. Two of them are filled with a mixture of Carbopak, activated carbon and Chromosorb W, the latter being coated with Apiezon. The

22

3. Gas chromatography in the analysis of a h pollutants

third, fourth and fifth traps are filled with Zeolyte 3A, silica gel impregnated with 20% sulphuric acid and Carbosieve S , respectively. Neglect of the above requirements may cause appreciable errors and make the results of the analysis useless. One of the significant sources of errors can be the glass-wool used as an insert or plug in chromatographic columns and splitters. The treatment of used glass-wool with n-pentane, dichloromethane, gaseous hydrogen chloride, and carbon disulphide followed by gas chromatography recovered traces of previously analysed hydrocarbons, phthalates and organic acid salts at levels of 0.3-0.005 mg/g. It is advisable for the glass-wool to be pretreated with gaseous hydrogen chloride and then extracted for 24 h with dichloromethane in a Soxhlet apparatus. It is also recommended that its surface be silylated. Another frequently occurring source of errors is gases evolved from the septa and vaporizers of chromatographs, which give rise to ghost peaks that distorts the results. The composition and concentration of these gases have been established to depend on the septum material, temperature and the period during which the septum was used in the chromatograph. The best properties are exhibited by septa made of perfluorinated elastomers and some silicone rubbers. At any step in the analysis of impurities one should take into account the possibility of thermal and exclusive desorption of the components previously adsorbed on the column or their exclusion from subsequent samples by more polar compounds ( e g , water and acids). Competitive sorption is also observed when concentrating impurities from air, the possible exclusion of one compound by others creating conditions that are unfavourable for sorption of the former, as occurs with non-polar impurities in the presence of polar substances. When the concentration of impurities in a trap containing a sorbent increases by a factor of 102-105and the molecules are within the range of the action of the sorbent, the possibility of chemical (catalytic, heterogeneous, etc.) transformation of the sample increases sharply. Sample heating during thermal desorption (150-250°C) further increases this possibility owing to potential pyrolysis and other interactions of the concentrated compounds. This substantially distorts the results of analysis and frequently makes them useless under these conditions. Catalytic reactions lead to losses of styrene when it is being concentrated on charcoal, and the analysis of atmospheric air containing alkenes and halogens (chlorine and bromine) performed in the presence of ozone results in the formation of halogenated hydrocarbons on the column sorbent. Thus, the presence of butenes and chlorine gives rise to 2,3-dichlorobutene and such reactions can occur on both porous polymers and carbon-containing sorbents. To avoid false peaks of halogenated hydrocarbons, glass-fibre impregnated with 10% sodium thiosulphate solution is installed before the sorbent. The false peaks of halogenated hydrocarbons change with the ozone and nitrogen oxide concentrations in the atmosphere. When concentrating toxic impurities from flue gas on Tenax, the sorbent undergoes partial transformation into 2,6-biphenyl-p-quinone under the action of sulphur dioxide and nitrogen oxide. The analysis of sulphur and nitrogen oxides contained in industrial emuents is accompanied by the sulphonation and nitration of porous polymeric sorbents (Tenax and XAD 21, which affects their capacity and selectivity (especially with polar sorbents). Gas chromatography has shown ihat extremely toxic N-nitrosodimethylamines may be formed in a Tenax concentration trap during sorption from air through the interaction of dimethylamine present in the air with nitrogen oxides and ozone in the presence of traces of moisture [29]. This is a serious obstacle to the concentration and quantitation of N-nitroso compounds. The false peaks of the latter are also formed when impurities are concentrated on other sorbents (charcoal, silica gel, alumina, Florisil and the sorbents treated with aqueous KOH solution, ascorbate or phosphate-nitrate buffer). The formation of “false” N-nitrosamines is particularly intensive with activated carbon and silica gel.

References

23

Account should also be taken of the direct interaction of the sorbent in the trap with the impurities being analysed, which leads to losses of the latter. This can be exemplified by the absorption of nitrogen dioxide impurities by Porapak Q (accompanied by sorbent nitration) or irreversible interaction between sulphur-containing compounds and activated carbon..The concentration of ozone and chlorine dioxide impurities is not safe because when concentrated these compounds are explosive [26]. When employing the thermal desorption of impurities from sorbent-containing traps it is essential that the temperature applied is not too high (150°Cis the optimal value), as certain compounds undergo isomerization at 250°C to yield “false” compounds of higher molecular mass. Thus, C1-C3 hydrocarbons give cis-2-butene and cis-2-pentene under these conditions 1301. It should be noted that the adsorptive activity of the inner walls of capillary columns, including the quartz type, may cause irreversible adsorption of polar analytes [31, 321. The above peculiarities in the determination of impurities should be taken into account when analysing air pollutants.

References (Chapter 3) BEREZKIN, V. G. ; GAVRICHEV, V. S.; KOLOMNETS, L. N.; KOROLEV, A. A,; LIPAVSKII, V. N.; NIKITINA, N. S.; TATARINSKII, V. S.: Gazovaya Khromatografiya v Neftekhimii (Gas Chromatography in Petrochemistry), Moscow: Nauka 1975, p. 17. GROB,R. L. (Ed.): Modem Practice of Gas Chromatography. New York: John Wiley 1985. KATZ,E. (Ed.): Quantitative Analysis Using Chromatographic Techniques. Chichester: John Wiley 1987. SUPINA,W. R.: The Packed Column in Gas Chromatography. Bellefonte, Pennsylvania: Supelco 1974. S. A,: Contemporary Practice of Chromatography. Amsterdam: Elsevier POOLE,C. F.; SCHUETI’E, 1984. LEIBNITZ, E.; STRUPPE, H. G. (Ed.): Handbuch der Gaschromatographie. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1984. KISELEV,A. V.; YASHIN,Y. I.: Gas-Adsorption Chromatography. New York: Plenum Press 1969. BEREZKIN, V. G.: J. Chromatogr. 98 (1974) 477. BEREZKIN,V. G. In: GIDDINGS, J. C.; GRUSHKA, E.; BROWN,P. R. (Eds.): Advances in Chromatography, vol. 27. New York: M. Dekker 1987, pp. 1-35. BEREZKIN,V. G.; RETUNSKY, V.N.: J. Chromatography 330 (1985) 71. JENNINGS, W.: Gas Chromatography with Glass Capillary Columns. New York: Academic Press 1980. JENNINGS, W.: Comparisons of Fused Silica and Other Glass Columns in Gas Chromatography. Heidelberg: Huthig 1981. LEE,M.; YANG,F.; BARTLE, K.: Open Tubular Column Gas Chromatography. New York: Wiley-Interscience 1984. GROB,K.; GROB,G.: J.Chromatogr. Sci. 7 (1969) 584. GROB,K.; GROB,G.: J. Chromatogr. Sci. 7 (1969) 587. BEREZKIN, V. G.: Gas-Liquid-Solid Chromatography (in Russian). Moscow: Khimiya 1985. MASHALY, M.; SANDRA, P. In: VIIIth Intern. Symp. on Capillary Chromatography. Vol. 1. Riva del 0. (Ed.): Heidelberg: Huthig 1987, p. 476. Garda, May 19-21, 1987. SANDRA, MATSUSHITA, H.; IIDA,Y.: J. HRC and CC. 9 (1986) 710. BEREZKIN, V. G.: J. Chromatogr. 159 (1978) 359. BERRIDGE, J. C.: Techniques for the Aromated Optimization of HPLC Separations. Chichester: John Wiley 1986. BEREZKIN, V. G.; TATARINSKII, V. S.: Gas-Chromatographic Analysis of Trace Impurities. New York: Consultants Bureau, 1973.

24

3. Gas chromatography in the analysis of air pollutants

Yu.S.; BEREZKIN, V. G.: Gas Chromatographic Analysis of Pollution Air. (in Russian). DRUGOV, Moscow: Khirniya 1981. JENNINGS, W.G.; UP, A,: Sample Preparation for Gas Chromatographic Analysis. Heidelberg: HUthig 1983. BRETIZLL, T.A.; GROB,R. L.: Intern. Lab., Apr. 1985, p. 30. SCHIDBAUER, N. S.; OEHME,M.: Journal HRC and CC. 8 (1985) 404. ANVAER,B. I.; DRUGOV,Yu.S.: Gazovaya Khromatografiya Neorganicheskikh Veshchestv (Gas chromatography of inorganic compounds). Moscow: Khimiya 1976. BEREZKIN, V. G.; DRUGOV,Yu.S.; GORYACHEV, N. S.: Zhurnal analyticheskoi khirnii. 37 (1981) 319. WESTRICK, T. J.; LAMPARSKI, L. L.: Anal. Chem. 53 (1981) 22. BERKLEY, R.E.; PELLIZZARI, E.D.: Anal. Lett. A l l (1978) 327. CRIP, S.: Ann. Occup. Hyg. 23 (1980) 47. PURCELL, J . E.: Chrornatographia 15 (1982) 546. BLOMBERG, L.G.: Journal HRC and CC 7 (1984) 232.

Chapter 4

Detectors for the Gas Chromatographic Determination of Impurities Gas chromatography is a hybrid method (11 in which a column serves for separation and a detector for the quantitative determination of the separated components. The determination is effected in flow of carrier gas to which, as a rule, the detector is insensitive. In this way, the chromatographic separation enables one to simplify the problem of quantitative determination; instead of the initial problem of determining the composition of a multi-component mixture, the detector after chromatographic separation solves a number of simpler problems such as the determination of the concentration of one of the constituents of the mixture to be analysed as a binary mixture with the carrier gas. The characteristics of the detector essentially determine the most important overall paand rameters of the chromatographic technique. Therefore, as pointed out by ZHUKHOWTSKI TURKEL’TAUB. “The history of progress in gas chromatography is to a certain degree the history of detector development” [2]. The problems of detection have been considered in a number of excellent books [3-51. Moreover, this problem has been discussed in detail in some books covering general aspects of gas chromatography [5-81. The selectivity of determination is a function of the properties of both the column (sorbent) and the detector. In practice, it is usually advantageous to use a highly sensitive detector that selectively reveals contaminants of interest (hazardous or otherwise important for industrial hygiene) and producing no response to the major substance. In some instances it is expedient to use directed chemical conversions to change the sensitivity to contaminants of interest and to the major component. A book has been devoted to the use of chemical methods in the analysis of impurities [9].

4.1.

Principal characteristics of detectors for gas chromatography

The most important detector characteristics are sensitivity and detection limit together with the linearity and response time of the detection system. Sensitivity is one of the most significant parameters of a chromatograph. Increasing the sensitivity allows one to lower the detection limit for admixtures and to improve the separation in some instances. Under otherwise equal conditions (the degree of back-diffusion, decrease in concentration of components due to dilution with the carrier gas, etc.), the sensitivity of a gas chromatograph is determined by the sensitivity of the detection system employed. Therefore, we shall consider below the sensitivity of detection systems. It should be noted that sometimes high sensitivity of a detection system cannot be effectively used owing to unfavourable characteristics of the chromatograph. These may be insufficient, e.g., to meet the requirements of precision of thermostating, stability of gas flow-rate or thermal stability of the sorbent, which causes a significant noise signal. At present only differential detectors are employed in gas chromatography. They measure an instantaneous concentration (concentration-dependent detectors) or a mass flow-rate (mass flow-rate-dependent detectors) of a substance of interest in the carrier gas flow. The chromatographic peak area of each component of a mixture is proportional to its concentration in the sample and to the corresponding sensitivity of the detector.

4. Detectors for the gas chromatographic determination of impurities

26

[lo]and later by HAThe above classification of detectors was proposed by KALMANOVSKY LASZ [ 111.

For a concentration-dependent detector, the relationship of the output signal E , to concentration is

E, = S,C

(4.1)

where S, is the sensitivity of the detector and C is the instantaneous concentration of the substance in the volume of the detector chamber. For mass flow-rate-dependent detectors, the relationship of the mass flow-rate of the substance to the output signal Ej is

Ej = Bj

(4.2)

where B is the sensitivity of the detector and j is the mass flow-rate of the substance through the detector: j = dq/dt

(4.3)

where q is the amount of the substance reaching the sensor and r is time. The sensitivity of a gas chromatographic detection system is the ratio of the change in its output signal to the corresponding change in concentration or mass flow-rate of the substance being analysed in the detector input. The sensitivity of a concentration-dependent detector is expressed as

where Sfis the arithmetic mean of the peak areas found in five independent measurements (cm*), A , the sensitivity of the recording unit (mV/cm), Q the carrier gas flow-rate (cm3/min), M the ratio of the detector signal to the input signal of the recorder, q the volume of a control substance (cm3), e the density of the control substance under conditions of detection (mg/cm3)and fi the chart speed of the recorder (cm/min). The sensitivity of a mass flow-rate-dependent detector is determined (in C/mg) by

where A l is the sensitivity of the measuring circuit, i.e., the ratio of the signal of an electrometer amplifier to the length of the recorder scale (Ncm) and qm is the mass of the control substance loaded into the column (mg). In chromatographic measurements, the minimum measurable signal Eminis usually taken as twice the fluctuation noise signal 6, that is, Emin= 26.

(4.6)

To estimate the value of weak signals that can still be detected by a particular chromatograph, the term “detection limit” is used. The detection limit of a gas chromatographic detection system is actually the minimal concentration or mass flow-rate of the desired substance that can be reliably measured over the noise. In chromatography the detection limit is defined as the concentration or a mass flow-rate that induces a chromatographic peak twice as high as the fluctuation of the noise level. The detection limits for a gas chromatograph and the corresponding detection system usually coincide, although the former may sometimes exceed the latter if the chromatograph is of insufficient quality.

27

4.1. Principal characteristics of detectors for gas chromatography Table 4.1. Detector response as a function of amount of the substance to be analysed (taken from ref. 10)

Type of detector

Kind of signal peak area

Concentrationdependent

s-

Mass flow-ratedependent

S. J = A 14 .

S,'q

f-Q

peak height (for Gaussian peaks) h, = 2.51 -.

hj =

2.51 A j f i Q ' 4 VR

For a concentration-dependent detector the detection limit (%) is

Cmin= 26 lOO/S,e.

(4.7)

For a mass flow-rate-dependent detector the detection limit (mg/s) is j,, = 26/B. (4.8) As follows from eqns. (4.7) and (4.8), among detectors with identical noise levels, that having the highest sensitivity will always have the lowest detection lilhit. If the noise levels are different, a detector of lower sensitivity can nevertheless have a better detection limit. Within the linear range of a detector its sensitivity corresponds to slopes of straight lines E, = ScC or Ej = Bj plotted as E, versus C or Ej versus j,respectively, the minimal measurable signal being Emin= 26. The detection limit can be decreased either by increasing the sensitivity or by decreasing the noise level. When investigating the possibility of detecting low concentrations of a substance, one should bear in mind that its concentration in a sample should exceed the detection limit in proportion with the dilution of the substance during separation caused by back-diffusion. The detector response as a function of the amount of the substance to be analysed is given in Table4.1 [lo]. The detection limits of mass flow-rate- and concentration-dependent detectors are interrelated as follows: jmIn = QCmin.

(4.9)

Detection systems usually deviate from linearity with increasing signal. The detector signal is considered to be linear if the deviation of the true signal from ideal linearity does not exceed 5%. The maximum signal corresponding to the linear range of measurement can therefore be determined as (4.10) where El is the detector signal corresponding to the linear range limit at ideal linearity and Em is an actually measured signal. As the minimal measurable concentration is equal to the detection limit, the linear range of measurement (also called the linear dynamic range) can be expressed as the ratio of the input signal causing a 5% deviation of the output signal from linearity to the lowest detectable signal. Errors due to the lack of linearity for detection extending beyond the linear range can be markedly reduced by using correction of the results based on a previously performed calibra-

28 ThI5

4. Detectors for the gas chromatographic determination of impurities

Error of peak-height measurement

Error of peak-area measurement

(%I

(%)

0.085

0 1.5

0.170 0.255 0.340

6.1 11.4 16.7

0.428

21.8

0

0 0 0.5

Table 4.2. Experimental error as a function of the ratio of the detector time constant to the peak width

1 .o 3.0 4.8

tion. However, even in this instance the experimental error will be higher than that for measurements made in the linear range owing to the decrease in detector sensitivity. The dynamic properties of a detection system are determined by its response time (inertia). The response time results from contributions of various elements of the detector and the measuring circuit, depending on the type of the detection system. To a first approximation, a detection system can be considered as a first-order aperiodic unit that is characterized by a single time constant T. For a stepwise variation of the input signal, the output signal should in this instance change as E=Ek(l-e-'?

(4.11)

where E is the instantaneous amplitude of the output signal and Ek is the final amplitude of this signal at the new amplitude of the input signal. The time constant T is the time period during which the amplitude of the output signal reaches 0.632 of its final value with stepwise variation of the input signal. Distortions due to the inertia of the detection system are increased when the rate of variation of the input signal increases. Table4.2 gives values of errors in the results of analyses due to inertia of the detection system as a function of the ratio between the rate of peak alterand the detector time constant T (121. ation (represented by peak width

4.2.

Flame ionization detector

The operation of this detector is principally based on the phenomenon of the appearance of charged particles in a hydrogen burner flame if traces of organic compounds are present. This ionization process results in a sharp increase in the electric current between the burner jet playing the role of one electrode and a second electrode located above the flame. The strength of the induced current is proportional to the flow-rate of organic material through the hydrogen burner flame of the detector. The flame ionization detector was proposed and DEWAR[13]. It is highly sensitive and practically universal, 30 years ago by MCWILLIAM but its sensitivity decreases in the series hydrocarbons > esters > alcohols > acids. The detector is suitable for detecting organic substances at concentrations of lo-'- lo-"%.

4.3.

Thermionic detector

The thermionic detector is a flame ionization detector in which the flame is continuously fed with traces of an alkali metal salt (potassium, rubidium, caesium), and is characterized by high selectivity and sensitivity. Various modifications of the thermionic detector have been

29

4.4. Photoionization detector

Table 4.3. Selectivity of the photoionization detector (solvents producing no positive response for UV lamps having different radiation energies) (taken from ref. 17)

w lamp radiation energy (em 9.5

10.2

11.7

H20 CH3OH CZHSOH CH,Cl CHzC12 CHClS CCI4 Cl2HCCHClz Freons CSHl2

HZO CHSOH CHjCl CH3Cl CHzClz CHCI, CCI4 ClzHCCHClz Freons

HZO Freons (some)

C6H14

ClH15

described, mostly differing in the method of delivering the alkali metal ions into the hydrogen flame. Thermionic detectors for the detection of phosphorus- and nitrogen-containing com[14] and by WELLS[15], respectively. In pounds were proposed by KARMENand GUIFFRIDA one of the widely used detector modifications the source of metal ions is an electrically heated ceramic cylinder with the walls covered with a rubidium salt. Inside this cylinder a low-temperature plasma is formed, the conductivity of which is determined by the concentration of nitrogen- or phosphorus-containing compounds entering the detector [16]. In comparison with the flame ionization detector, the sensitivity of the thermionic type is approximately 50 times (and more) and 500 times higher for nitrogen- and phosphorus-containing compounds, respectively. The selectivity of this detector for these compounds with respect to organic substances (e.g., hydrocarbons) is as high as ca. 1 X lo4.

4.4.

Photoionization detector

The operating principle of the photoionization detector [17-191 is based on absorption of ultraviolet radiation by the sample substance followed by its dissociation into an ion and an electron (ionization): A + hv+A++ewhere A is a molecule of the substance to be analysed, A+ the ionized molecule, e - an electron and h v a photon with energy above the ionization potential of the molecule. The ionization results in a large increase in the conductivity of gas in the detector chamber, proportional to the concentration of the substance therein. A photoionization detector consists of an ultraviolet lamp (the source of ionizing radiation) located adjacent to an ionization chamber containing a pair of electrodes. Application of an electric potential to the electrodes enables one to quantitate the ions produced by absorption of the ultraviolet light by the molecules of a sample compound. To ionize molecules of a sample substance, UV lamps with a radiation energy of 9.5-11.7 eV are available. Using a lamp with an energy of 11.7 eV makes it possible to analyse compounds of various classes including aliphatic hydrocarbons. It is also possible to use lamps of lower radiation energy but sufficient to ionize compounds of analytical interest. In this interest compounds having

30

4. Detectors for the gas chromatographic determination of impurities

higher ionization potentials will not be detected, and the photoionization detector will show selective properties [20]. As the photoionization detector is concentration dependent and non-destructive, it is advisable to use it connected in series with other gas chromatographic detectors, e.g., a flame ionization detector. Taking into account the features of the photoionization detector, it is expedient to use it in capillary chromatography. The detection limit of the photoionization detector for organic species is 10-100 times that of the flame ionization detector. The detector has a sufficiently wide dynamic range of 1 X lo’. Table 4.3 [17] shows some solvents used with photoionization detectors that produce no positive detector response for photoionization sources having different radiation energies. These solvents often represent the main substance (carrier) in sample mixtures.

4.5.

Electron-capture detector

The electron-capture detector consists of a chamber with a pair of electrodes to measure an ionic current and an electron-emitting radioactive source. Owing to collisions with molecules of the carrier gas which flows through the detector chamber after the chromatographic column, the emitted electrons are retarded and turn into a number of slow thermal electrons. Application of an electric potential to the electrodes of the chamber will thus induce an appreciable background current. On passing through the detector, the “electron-trapping” compounds react with electrons to form anions and the electric current across the chamber sharply decreases, the decrease being proportional to concentration of the compound in the chamber. The electron-capture detector is widely used to detect polyhalogen-containing compounds, including many pesticides and polychlorinated biphenyls, polyaromatic compounds, nitriles, organo metallic and sulphur-containing compounds. The detector is also used for other compounds that are quantitatively converted into derivatives capable of being detected by the electron-capture detector with high sensitivity (e.g., see ref. 9). The electron-capture detector was proposed nearly 30 years ago by LOVELOCK and LIPSKY [21]. The bases of the theory and practical application of this detector, especially in the field of environmental protection and biomedicine, were covered in a book [22]. It has been shown that addition of a “dopant” (e.g., nitrogen oxide) to the carrier gas could dramatically increase the sensitivity of the electron-capture detector to those compounds which usually produce very low signals [23, 241.

4.6.

Flame photometric detector

The flame photometric detector can be considered as a flame emission photometer. The compounds to be analysed are burnt in a hydrogen-enriched flame which is similar to a low-temperature plasma. Under these conditions, atoms and simple molecules are undergoing a transition to a state of higher energy. During the reverse transition to the ground state the excited particles emit characteristic radiation. Optical filters used in the detector enable one to select a characteristic optical line, the radiation of which is amplified by a photomultiplier. The flame photometric detector is used for the determination of sulphur- and phosphoruscontaining compounds. The selective sulphur radiation is due to S2 emitting at a wavelength of 394 nm. Phosphorus is detected at 526 nm, the wavelength determined by HPO. The selectivity of phosphorus and sulphur detection with respect to hydrocarbons is 10s:l and 104:1, respectively. The sensitivity of a flame photometric detector is 1 X lO-”g/s for sulphur and

31

4.9. Hall electrolytic detector

1 X lo-’’ gls for phosphorus. The flame photometric detector was proposed by BRODYand CHANEY (251. The so-called double detector modification is a frequently used variant [26].

4.7.

The Thermal Energy Analyser (TEA) Detector

The TEA detector is a chemical emission-based, selective and highly sensitive device [27], used for the specific determination of N-nitroso compounds. A stream of carrier gas from the chromatographic column flows into a catalytic pyrolyser where the N-nitroso compounds are thermally decomposed to produce two radicals, nitrosyl and an organic radical. The two radicals are transported by the carrier gas flow into a reactor fed with ozone. Ozone reacts with the nitroso radicals, giving rise to electron-excited molecules of nitrogen oxide. The excited molecules return to the ground state, emitting near-infrared radiation, which is detected by a photomultiplier. The selectivity and sensitivity of the detector are sufficiently high, e.g., the detection limit for N,N-dimethylnitrosoamine is 100 pg.

4.8.

Detectors for direct identification of impurities

Detectors of this type include the highly sensitive and superselective Fourier transform infrared and mass spectrometric detectors. Gas chromatographs equipped with these detectors are frequently considered as special hybrid instruments (abbreviated to GC-FTIR and GCMS, respectively) and function mostly as micropreparative rather than analytical devices. In contrast, the analytical function in most instances is fulfilled by the detectors. The detection limit for the GC-FTIR system is 20-40 ng, some modifications of the instruments being even more sensitive [28-301. The GC-MS system is generally characterized by higher sensitivity and selectivity [31-341.

4.9.

Hall electrolytic detector

Various electrochemical detectors used in gas chromatography have been described. However, the simplest and most reliable is, to our knowledge, the Hall electrolytic conductivity detector [35-371. The electrolytic detector was first introduced in gas chromatography by COULSON[38]. The operation of this type of detector includes, first, oxidation (reduction) of nitrogen-, sulphur- or halogen-containing compounds to give stable inorganic compounds and, second, their dissolution in a stream of water followed by measurement of the electrolytic conductivity of the resulting aqueous solution.

Elemental composition

Generated products

of analysed compounds

oxidative method

reductive method

COZ HX NO2 (low yield) SOz and SO3

CH4 HX NH3

~

Carbon Halogen (X) Nitrogen Table 4.4. Products of catalytic conversion of organic substances prior to electrolytic detection

Sulphur Phosphorus

p4010

HzS PH3

32

4. Detectors for the gas chromatographic determination of impurities

Table 4.5. Major detectors for gas chromatography Detector

Field of application

Sensitivity (g/s)

Selectivity

Linear dyPrice namic range

1

2

3

4

5

6

Flame ionization

broad (CH-containing organic compounds) limited (selective detection of nitrogen- and phosphorus-containing organic compounds)

2 x 10-12

low

1 x 10'

medium

10-13

high 1 x 105 (N/C = lo4, PIC = 104)

medium

Thermionic flame ionization

(nitrogen) 5 x 10-14 (phospho-

mS) Electron-capture

Photoionization

Flame photometric

Electrolytic

limited (selective detection of organic molecules capable of capturing electrons, e.g., pesticides, polychlorinated biphenyls, polyaromatic hydrocarbons, nitriles, sulphur-containing compounds) broad (selective detection of the compounds whose ionization potential is lower than the radiation energy of the UV lamp employed

5X high (e.g., for carbon tetrachloride)

1 x lo2to 1 x 105

medium

2 x 10-13

1 x 10'

medium

limited (selective detection of sulphur- and phosphorus-containing organic compounds and some other elements) limited (selective detection of halogen-, sulphur- and nitrogen-containing organic compounds)

1 x 10-11 (sulphur) 1 x 10-12 (phosphorus) 5 x 10-13 (chlorine) 3 x 10-12 (sulphur) 3 x 10-12 (nitrogen) detecton limit 100 pg for dimethylnitrosamine)

103 (sulphur) 105 (phospho-

medium

Thermoenergetic

limited (selective detection of nitrosamines)

GC-FTIR

universal, selective

GS-MS

universal, selective

detection limit 200 pg4 0 ng detection limit 1 pg1 ng

high (determined by the ratio of radiation energy to ionization potential) high (SIC 104 P/C 105)

-

NS)

high (Cl/C 106 N/C lo6 SIC 105)

-

high

106

(chlorine) lo4 (nitrogen) 104 (sulphur) 106

medium

much higher than the medium price

high, changeable

104

very high

high, changeable

105-106

very high

References

33

The products generated during the catalytic conversion of organic substances are presented in Table 4.4. The data indicate the limits of practical use for this detector. The detector is equipped with a closed water circulation system which includes a column of an ion-exchange resin to remove dissolved ions. The selectivity of the Hall detector for nitrogen and chlorine with respect to carbon is better than 1 X lo6, and for sulphur being 5 x lo4.The sensitivity of this detector to nitrogen, chlorine and sulphur is 2 X 5 x and 2 x g/s, respectively. The Hall detector is used for the determinations of pesticides, herbicides, polychlorinated biphenyls and trichloromethane. Finally, it should be noted that the detectors described above may be successfully applied to the problem of air pollution control. For the trace analysis of complex mixtures it is advisable to employ a set of detectors that differing in selectivity and to modify chemically the compounds to be analysed. The principal specifications of gas chromatographic detectors are summarized in Table 4.5.

References (Chapter 4) 111 ZOLOTOV, Yu.A.: Essays on Analytical Chemistry (in Russian). Moscow: Khimiya 1977. A. A.; TURKEL’TAUB, N. M.: Gas Chromatography (in Russian). Moscow: Gostop[2] ZHUKHOVITSKI, techizdat 1962. [3] DAVID,D. J.: Gas Chromatographic Detectors. New York: Interscience 1974. J.: Detectors in Gas Chromatography. Amsterdam: Elsevier 1976. [4] SEVCIK, [S] KATz, E. (Ed.): Quantitative Analysis Using Chromatographic Techniques. Chichester: John Wiley 1987. V. G.; TATARINSKII, V. S.: Gas-Chromatographic Analysis of Trace Impurities. New York: [6] BEREZKIN, Consultants Bureau 1973. [7] LEIBNITZ,E.; STRUPPE,H. G. (Eds.): Handbuch der Gaschromatographie. Leipzig: Akademische Verlagsgesellschaft Geest & Portig K.-G. 1984. (81 POOLE,C. F.; SCHUE-ITE, S. A,: Contemporary Practice of Chromatography. Amsterdam: Elsevier 1984. V. G.: Chemical Methods in Gas Chromatography. Amsterdam: Elsevier 1983. 191 BEREZKIN, V. 1.: Works on Chemistry and Chemical Technology. (in Russian). Gorky: NII of [lo] KALMANOVSKY, Chemistry, GSU 3 (1960) 545. I . : Anal. Chem. 36 (1964) 1428. [ l l ] HALASZ, V.I.; ZHUKHOVITSKI, A.A.: J. Chromatogr. 18 (1965) 243. [12] KALMANOVSKY, [13] MCWILLIAM, I. G.; DEWAR,R. A,: Nature 181 (1958) 760. A,; GUIFFRIDA, L.: Nature 201 (1964) 1204. [14] KARMEN, [15] WELLS,C.: USFDA Pesticide Workshop, Kansas City, MO: May, 1966. P.: J. Chromatogr. Sci. 15 (1977) 53. [I61 KOLB,B.; AYER,M.; POSPISIL, J . N.; FORD,J.; JARAMILLO, L.; BECKER, J. H.; HEWITT,G.; MARSHALL, J. K.; ORISHUK, F.: [17] DRISCOLL, American Laboratory 11 (1978) 137. J . ; KRYSL,S.: Chromatographia 6 (1973) 375. [18] SEVCIK, J.N.: J. Chromatogr. 134 (1977) 49. [19] DRISCOLL, J. N . : J . Chromatogr. Sci. 6 (1982) 375. [20] DRISCOLL, [21] LOVELOCK, J . F.; LIPSKY,S. R.: J . Amer. Chem. SOC.82 (1960) 431. A,; POOLE,C. F.: Electron-Capture Detector. Theory and Practice in Chromatography. Am[22] ZLATKIS, sterdam: Elsevier 1981. [23] SIMMONDS, P.G.: J. Chromatogr. 166 (1978) 593. [24] PHILLIPS, M. P.; SIEVERS, R. E.; GOLDEN, P. D.; KUSTER,W. C.; FELSENFELD, F. C.: Anal. Chem. 5 1 (1979) 1819. S. S.; CHANEY, J. E.: J. Gas Chromatogr. 4 (1966) 42. [25] BRODY, P.L.; HOWE,R.L.; SHUMAYS, A.A.: Anal. Chem. 5 0 (1978) 345. [26] PAITERSON, 4

Berezkin. Gas Chrom -BE

34

4. Detectors for the gas chromatographic determination of impurities

[27] FINE,D. H.; ROUNBEHLER, D.P.: J. Chromatogr. 109 (1975) 271. [28] ERICKSON, M.D.: Appl. Spectrosc. Rev. 15 (1979) 261. G.; HUSMANN, H.; PODMANICZKY, L.; WEEKE,F.; RAPP,A. In: SCHREIER, P. (Ed.): Anal[29] SCHOMBURG, ysis of Volatiles. Berlin, New York: Walter de Gmyter 1984, p. 121. K.-H.; HERRES,W.: GIT-Supplement 5/84 Chromatographie, p. 33. (301 KUBECZKY, A.M.; SIMPSON, C.F.: J. Phys. 13 (1980) 1131. [31] GREENWAY, [32] TENNOEVERDE BRAUW, M. C.: J. Chromatogr. 165 (1979) 207. W. H.: J. Chromatogr. Sci. 17 (1979) 2. [33] MCFADDEN, [34] JENSEN,T. E.; KAMINSKY, R.; VEETY,B. D.; WOZNIAK,T. Y.; H m s , R. A.: Anal. Chem. 54 (1982) 2388. [35] HALL,R. C.: Crit. Revs. Anal. Chem. 8 (1978) 323. S O . ; GAGE,D.R.; KAGEL,R.A.: J. Chromatogr. Sci. 19 (1981) 358. [36] FARWELL, [37] HALL,R. C.: J. Chromatogr. Sci. 12 (1974) 152. D. M.: Adv. Chromatogr. 3 (1966) 197. [38] COULSON,

Chapter 5

Collection and Pretreatment of Samples for Chromatographic Analysis The sampling and identification of contaminants in complex mixtures is one of the most important aspects of modern analytical chemistry of air pollutants [l]. The recovery of trace amounts (up to lo-' mg/m3) of various chemical compounds from complex physico-chemical systems such as air is a difficult analytical problem [l,31. The collection and pretreatment of samples may involve some losses or chemical transformations of the sample, resulting in changes in the initial sample composition [l,2, 6-91, which may significantly affect the end results of the determination. Undesirable changes in the sample composition may also occur during concentration of the sample and its recovery from the concentrator (e. g., by thermal desorption), pretreatment of the sample, etc. [8-121. Qualitative changes in the composition of a processed sample may result from the use of high temperatures, the presence of oxygen and other oxidants, a considerable (1000-fold and more) increase in the concentration of all the components of a sample during the enrichment process, the presence of highly reactive, readily polymerizable or unstable compounds, etc. [ l , 8, 121. The collection of a sample of air and its concentration always involve a certain risk of changes in the sample composition. Irreversible processes that occur early in the analytical procedures should receive particular attention [8, 11, 121. The use of sophisticated instrumentation, inert sorbents and highly effective columns in subsequent stages of the analysis cannot give accurate information if the sample was pretreated incorrectly [8]. Sample pretreatment problems include collection of a representative sample, concentration of target components, transport of concentrated contaminants from the sampler, chemical modification of the sample and introduction of target components on to a chromatographic column. The problem of sample pretreatment for chromatographic separation and determination has been considered in great detail in reviews [2, 7, 111, books [8, 9, 121 and in separate chapters of books devoted to general chromatography (see, e.g., refs. 1 and 10). Determination of contaminants in workplace atmospheres usually requires a sample collection stage which lasts 4-8 h. This is necessitated by the requirements of industrial hygiene, specific technological aspects and the cyclic nature of operations [l, 121. The extensive employment of protracted collection procedures [l,10, 121 and the increase in the duration of air flow through a sorbent from 10-30 min to.severa1 hours have necessitated the reconsideration of the requirements of sample collection systems [l,8, 9, 121. Monitoring increases the number of parameters such as temperature, humidity, air flow-rate and the presence of contaminants that need to be considered [ll, 121 and enhances their importance. At present, methods of collection of various types of harzardous substances from air are largely confined to the collection of polluted air into samplers, absorption of contaminants by a solvent, their freezing out, sorption in tubes packed with a sorbent and trapping of solid particles and aerosols by filters [l, 11, 121.

5.1.

Sampling into containers

Contaminated air samples are collected in containers for monitoring gaseous (at normal temperatures) substances [l-51. Usually this procedure does not involve sample pre-enrichment.

36

5. Collection and pretreatment of samples for chromatographic analysis

Fig. 5.1. Schematic diagram of the equipment for the detection of peroxyacetyl nitrate (PAN) and its homologues in air [24]

The containers are composed of various reservoirs (vessels) made of stainless steel, glass or plastics. The air sample is passed through the container at a low rate (0.1-0.2 l/min) and sampled into a preevacuated vessel; the container can be also filled using a nipple device. An aliquot of the air in the container is then introduced into the chromatographic evaporator by means of a gas syringe [ l , 3, 51. This, the simplest sampling technique, allows the direct analysis of air pollutants with small relative errors (5-10%) as it does not require pollutant preconcentration [13]. The limitations of the container technique are as follows: restricted range of analytes; the risk of adsorption (chemisorption) of the compounds on the walls of the container and gas syringe; restricted range of concentrations to be determined, depending on the detector sensitivity; possible occurrence of chemical reactions in the container during sample storage (in the presence of air containing moisture and oxygen), especially with reactive gases; impossibility of obtaining a representative sample owing to the presence of compounds of low volatilitv in air. Adsorption on the internal walls of the container can be minimized by preconditioning the surface with suitable chemical reagents [l]. The detection limit can be lowered by subsequent concentration of the pollutants in the container, e. g., on solid sorbents or an inert material [14]. In the latter instance the air containing, e. g., vapours of solvents used in industry of lacker and paint) is displaced from the heated container (125°C) by a current of nitrogen (20 ml/min) into a liquid nitrogen-cooled trap (10 cm X 2 mm) filled with glass beads (30-50 mesh). Following trapping of the pollutants, the trap is heated to 200°C and the compounds are displaced by the carrier gas and analysed in a chromatograph [14]. However, this procedure is rarely used. A more popular technique is the direct analysis of gases in a container using highly sensitive detectors. It has been applied in the analysis of very low concentrations of Freons (10-4-10-5mg/m3) in air using an electron-capture detector [15, 161, formaldehyde using a photoionization detector [17], hydrocarbon gases (C,-C,) using a flame ionization detector [18], carbon monoxide and dioxide and other low-boiling gases (10-100 mg/m3) using a katharometer [18, 191, gaseous odorants such as hydrogen sulphide, ammonia, amines and mercaptans [20], vinyl chloride, vinyl acetate and other gases [ 19-23]. This type of analytical procedure can be readily represented schematically. The equipment for analysing peroxyacetyl nitrate and its homologues in air is shown in Fig. 5.1. This compound can be detected in contaminated air accumulated in a 3 ml miniature loop-like container using an ECD electron-capture detector at a 13 ppb by volume concentration [24].

37

5.2. Use of absorption of contaminants in sample collection

r

I

.

I I

I I

I L

‘ 4

5

3

.4

2

.6

bl

2 Fig. 5.2. Device for atmospheric air sampling [25]. 1,2 Flexible plastic panels; 3 sampling chamber; 4 elastic helical helico-conical spring; 5 self-sealing plug stopper for air inlet; 6 safety plate (against inadvertent piercing of the lower panel)

For the analysis of gaseous air pollutant mixtures, the most frequently used containers are bags made of laminated plastics such as PTFE, Lavsan, nylon, polyethylene, polypropylene, PVC, polyamide, etc., and fitted with a valve or nipple for filling [19-231. One of these devices is depicted in Fig. 5.2. It is intended for the rapid sampling of contaminated air and is composed of a small rectangular package with sealed edges and is made from a thin and durable inert material (polyethylene terephthalate) [25]. Inside the package there is a tension spring 4 made of stainless steel, which is compressed prior to sampling. The package has a small opening for intake of air, with a self-sealing stopper 5 made of PTFE (Fig. 5.2). During sampling the stopper 5 is removed and a moderate negative pressure (vacuum)) is generated inside the package due to extension of the spring, the ambient air is admitted into the package and the stopper is replaced. Plastic containers are light, very durable, and the polymeric film is almost insensitive to many reactive gases such as chlorine, ozone, nitrogen, sulphur oxides, and ammonia [20-231. This allows storage of the sample gases in such containers after completion of the sampling for as long as 10-14 days without any significant changes [20,23]. In bags made of laminated aluminium-coated plastics, polyethylene, nylon or PVC the stored gases decompose by no more than 1% per day [23]. Container-type sampling method is optimal for the extraction of pollutants from air provided that the sample is composed of gaseous compounds only and that a highly sensitive chromatographic detector of the electron-capture, flame photometric, photoionization or thermionic type is used. In these instances the detection limit may be as low as 10-4-10-s mg/m3.

5.2.

Use of absorption of contaminants in sample collection

Absorption of toxic contaminants by a solvent is often used in air sampling [l, 51. The advantages of the method lie in the possibility of the simultaneous concentration of contaminants in a wide range of sample substances (except aerosols and solid particles) and a high selectivity of determination associated if an appropriate solvent is chosen. In addition, the method of

38

5. Collection and pretreatment of samples for chromatographic analysis

determination is simplified as the sample is analysed in the liquid form irrespective of the analytical method. The drawbacks of the method involve the impossibility of obtaining a representative sample in the presence of aerosols and solid particles and, frequently, a low sample enrichment in the analysis of low concentrations. The latter circumstance is due to the high dilution of the sample, as 5-10 ml of the solvent solution are used for sample collection. This difficulty may be partially overcome by repeated concentration of contaminants by evaporation of the solvent under vacuum, selective extraction of contaminants from the solution or a chemical reaction. One should bear in mind, however, the possibility of losses of sample or changes in the sample composition arising from side-reactions. Sample collection is usually performed by passing the polluted air through a glass vessel (absorber) with several millilitres of an organic or inorganic solvent, depending on the sample composition. The flow-rate may vary over a wide range, from 0.1 to 30 l/min [26]. The most effective method of absorption of contaminants from air is the use of an absorber with a porous plate, which increases the absorption of contaminants owing to the large surface area (small air bubbles) compared with the conventional design of an absorber. A frittedglass filter should not have excessively large pores. The most efficient absorbers with respect to the volume of the solvent (the smaller the amount of the liquid, the greater is the sample concentration, all other conditions being equal) are those with a fritted-glass plate welded along the entire diameter. The capacity of such vessels reaches 30 l/min [26]. When selecting a solvent and a flow-rate, one should remember that the use of volatile organic solvents and a flow-rate above 1 l/min leads to considerable losses of absorbent owing to evaporation. As a result, changes in the collection efficiency occur early in the collection procedure and the errors in quantitative determinations arising from unaccounted for changes in the solvent volume may reach 50% or more [27]. This effect may be reduced by cooling the absorber, but in general highly volatile solvents (ethanol, acetone, chloroform, etc.) are not recommended. The extent of absorption depends on the contaminants being determined and on the absorbent, their concentrations, the air flow-rate, the temperature of the absorber, its design and some other factors. A proper choice of the solvent may permit selective sample collection, for instance, by absorbing in distilled water compounds that are soluble in it. In this way it is possible to separate inorganic from organic substances, C1-CI alcohols from hydrocarbons, aldehydes from organic sulphur compounds, water-soluble amines from phenols, etc., in the process of collection [l]. The effect of selective absorption may be achieved by the use of other polar solvents, e. g., dimethylformamide, n-butyl acetate or polyethylene glycol. Particularly effective is absorption based on chemical reactions of the absorbed substances with the solvent. Chemical absorption allows an extremely selective sampling (depending on the nature of the solvents, acting as reagents). Thus, trapping of small concentrations of organosulphur compounds from air in the form of a mercury complex, decomposition of this complex and determination of the recovered compounds by gas chromatography proved effective [28]. The air was pumped for 24 h at a volume flow-rate of 600 l/h through an absorber containing HgClz solution, which was then placed on a water-bath (60°C), concentrated HCl was added and then a flow of helium was passed through the absorber. The sulphur compounds recovered were collected in a trap cooled with liquid nitrogen and, after subsequent warming, they were analysed on a Durapak OPN/Porasil C (80-100 mesh) column (2 m X 2 mm I. D.) using a flame photometric detector ( t = 200°C). For the rapid determination of hydrazine, methylhydrazine and 1,l-dimethylhydrazine, used as high-energy propellants, in the working area atmosphere, an air sample was absorbed in acetone, the hydrazines being quantitatively trapped and subsequently derivatized to stable compounds [29]. The solution obtained was analysed on a chromatograph using a thermionic detector. The detection limit was 4 ppb.

5.2. Use of absorption of contaminants in sample collection

39

Concentrate

* I

Sample Collection

Sample Preparation

Fig. 5.3. Sampling and treatment for headspace analysis of aromatic hydrocarbons in urban air [46] Derivatization of a sample on collection in order to convert compounds of interest into a form convenient for chromatographic determination and concentration in order to ensure adequate selectivity and precision is common in air sampling [30].The absorption of trace amounts of hydrogen fluoride by 0.1 N alkali solution [31],hydrogen chloride by an aqueous solution of 7-oxabicyclo[4.l.0]heptane [32],ozone by a 1% solution of stilbene in ethyl acetate at -20°C [33],nitrogen dioxide by aniline [34]and nitrogen oxide by a solution of copper bromide and p-chloroaniline in acetonitrile [35],considerably decreases the detection limit for these reactive toxic substances. A similar method is used in the selective absorption of organic contaminants [30].Small concentrations of CI-C4 carbonic acids are absorbed by water [36],an alkali solution [37],sodium carbonate [38]or methanol [39].Trace amounts of aldehydes and ketones were collected from air in a saturated solution of 2,4-dinitrophenylhydrazinein 0.1 N sulphuric acid [40],nitrosoamines were absorbed by a 1 N solution of KOH [41],dimethylformamide by distilled water [42]and, in the determination of lower aliphatic amines by gas chromatography the compounds were derivatized with dilute HC1 [43].Dilute sulphuric acid is a good absorbent for the collection of small concentrations of ethylene oxide [44]and a mixture of hydrochloric and acetic acids was used for the concentration of diisocyanates and chlorine derivatives of diaminophenylmethane [45].The derivatization of contaminants is usually to 96-98% complete [45],and the analysis of the reaction products by gas chromatography using plasma ionization or thermionic detection allows the determination of mg/m3 concentration [41,451.Traces of chlorophenols may be similarly detected after their acetylation or fluorination using electron-capture detection. The absorption of contaminants by organic solvents may increase their concentration in the sample about 10-fold. The enrichment of the sample may be even higher if not the liquid solvent but the equilibrium gas phase above the solution is analysed. A 20-25 1 air sample is passed through the absorber with several millilitres of acetic acid until complete saturation occurs (Fig. 5.3). The solution obtained is neutralized by an alkali and, on achieving interphase equilibrium, the gas phase is analysed by gas chromatography using a plasma ionization detector. The addition of the alkali to the solvent increases the concentration of the pol-

40

5 . Collection and pretreatment of samples for chromatographic analysis

lutants (aromatic hydrocarbons) in the gas phase. The method permits the determination of alkylbenzenes in urban atmospheres (a 9 1 sample) after absorptive (vapour-phase) concentration at levels of 0.01-10mg/m3 with an accuracy of 7-14% and a detection limit of 0.008-0.002 mg/m3 [46]. Vapour phase concentration of contaminants is used to enrich the sample for gas chromatography of various samples, including air pollutants. Absorption of the contaminants and the analysis of the gas phase have been used to determine trace amounts of acetic acid [38], C2-Cs carbonic acids [47], formaldehyde, chloroprene, benzene and acrylonitrile in air [48]. Afther the vapour phase concentration of SO, it was determined on a Teflon column containing Chromosorb 108. An electrolytic conductimetric detector gave a detection limit of several ng/ml with a standard deviation of 0.05 ng/ml [49]. The direct determination of small amounts (from ppb to ppm) of formaldehyde in air proved possible after the absorption of its vapour by a 1%solution of Na,CO, [50]. The chromatographic separation was carried out on a Porapak T (80-100 mesh) column (2 m x 3 mm I. D.) at 95°C using a helium ionization detector. To detect trace amounts of hexafluoro-2-propanol in air the sample was collected in two or three absorbers each containing 20 ml of distilled water [51]. The absorbers were cooled with ice. The chromatographic analysis of 5 pl of the solution obtained on a 3 m column containing 10%Sylar 1OC on Chromosorb with temperature programming from 40 to 210°C using a plasma ionization detector gave a detection limit of 0.03 ppm, the volume of the sample being 120 1. In the analysis of the C2-Cs aliphatic alcohols [52] and phenol [53] mixed with other waterinsoluble organic compounds the pollutants were absorbed by distilled water. To decrease the volatility of the liquid phase and increase its selectivity, it is expedient to use not the pure water in the absorber, but solutions of crystal hydrates or melts of crystal hydrates. In the latter instance the absorber temperature should not be higher than the melting point of the crystal hydrate. High selectivity with aqueous solutions and crystal hydrate melts has been reported [54].

5.3.

Cryogenic concentration of contaminants

This method of concentration involves freezing out toxic species from polluted air at temperatures significantly lower than the boiling points of the sample substances [l]. The cryogenic concentration of contaminants consists in pumping air through a cooled trap (condenser) with a sufficiently large surface area. Metal capillaries or steel and glass tubes packed with an inert material (glass-wool, metal shavings or spirals, copper shot, glass beads, etc.) to increase the condensing (cooling) surface are used as cooling traps. Various coolants and mixtures (Table 5.1) are used, depending on the required temperature of the trap. The freezing-out technique is indispensable for the analysis of unstable or reactive com-

Cryogenic system ice-water ice-sodium chloride dry ice-acetone liquid oxygen liquid air liquid nitrogen

Temperature obtained ("C) 0 - 16 - 80 -183 - 192 - 195

Table 5.1. Coolants used for cryogenic trapping of air contaminants

5.3. Cryogenic concentration of contaminants

41

pounds. The cryogenic concentration of C102 in a glass column packed with glass-wool at -80°C can be used to separate this toxic and highly unstable substance from ozone, chlorine, nitrogen oxides and hydrogen chloride [55]. A similar technique was used to freeze out CO and COz from the complex mixture of toxic components of cigarette smoke and to concentrate small amounts of hydrogen sulphide in traps cooled by liquid argon [56]. The same coolant was used [57] for trapping odorants such as sulphur-containing compounds, lower aliphatic amines, carbonyl compounds, hydrocarbons, lower aliphatic monoalcohols and phenols from air in the vicinity of cellular concrete production facilities. With the use of a flame photometric detector the method can be applied at ppb levels. A concentrator containing glass shot may efficiently sorb small amounts of HIS, SOz, COS and CS2 and also volatile mercaptans, sulphides, disulphides and sulphoxides after being cooled by liquid oxygen [58]. After thermal desorption these compounds were analysed on a capillary column 30 m long containing SE-30 silicone with temperature programming from 70 to 100°C using a flame photometric detector. Ultramicroscopic concentrations of Freons and nitrous oxide were trapped from the stratosphere at liquid nitrogen temperature [59]; at - 130°C tetraalkyllead compounds are effectively frozen out from ambient air [60]. The use of a cryogenic trap has been used in studies of the distribution of halohydrocarbons (fluorochloromethanes and -ethanes) in the stratosphere after concentration of the contaminants by freezing out and application of a column (3 m x 2 mm I. D.) packed with OV-101 on Chromosorb W HP with temperature programming from -50 to +60"C using an electron-capture detector [61]. Concentration of contaminants is even more effective in cooled traps packed with a sorbent [62]. In this instance deep freezing of a trap is not needed and dry-ice suffices, except for low-boiling gases (e. g., CO and, occasionally, C,-C3 compounds). On concentration in a trap containing Silochrome C-80 at -78"C, up to mg/m3 of hydrogen sulphide can be determined in ambient air. Using a plasma ionization detector, hydrocyanic acid was concentrated from exhaust gases in a trap containing Porapak Q cooled by dry-ice [63] and organolead compounds were completely trapped at the same temperature using a short column packed with silicone stationary phase on Chromosorb W [64]. To concentrate and determine CO by gas chromatography at the level of lo-' mg/m3 it is necessary to cool the concentrator containing molecular sieves 5A by liquid nitrogen [65]. The cryogenic concentration of small amounts of highly volatile organic compounds (hydrocarbons and chlorohydrocarbons) has been discussed [66]. A trap containing active charcoal cooled by liquid nitrogen to - 120°C absorbed C2-C6 organic compounds from exhaust gases. Dry-ice sufficiently cooled a packed trap to concentrate very small amounts of vinyl chloride [l], highly carcinogenic N-nitroso compounds [l] and traces of Freons in a concentration tube containing Carbopak B [67]. Deeper cooling was employed for the concentration of C1-C4hydrocarbons, which were then determined at the ppb level using plasma ionization detection [68]. Sometimes low temperatures are used for the intermediate concentration of Contaminants giving a 1000-fold or greater enrichment of the sample [I, 681. This technique is especially frequently used in the gas chromatographic-mass spectrometric determination of pollutants [ l , 691. It consists in trapping the sample substances on a sorbent, thermal desorption of the concentrated substances on to a liquid nitrogen-cooled metal capillary (or the primary section of the analytical capillary column) and subsequent capillary column analysis. An effective method of concentrating toxic organic compounds from an air sample has been described [70]. The air is passed through a trap (6 cm X 1.5 cm) containing Tenax GC (35-60 mesh) and the concentrated contaminants are desorbed at 270°C and collected in a nickel trap (75 cm X 0.5 mm) cooled by liquid nitrogen. The sample is desorbed at 250°C and the recovered substances are analysed on a capillary column containing SE-30 on Carbowax 20M with temperature programming from 25 to 240°C at a rate of 3-4 K/min.

42

5. Collection and pretreatment of samples for chromatographic analysis Adsorption rube

Detect Recorder

4'-I;;

"V

I,

I I

I

I

Cold Trap

I1 I

1I

integrator

I

I

I

I

Cold F O P

Fig. 5.4. System for two-stage concentration, thermal desorption and gas chromatography of air pollutants [71]

Repeated cryogenic concentration of pollutants desorbed from a sorbent trap allows a more compact sample to be injected into a chromatographic column as thermal desorption from such a cryogenic trap takes much less time than with a sorbent-filled trap (5-10 and 20-100 s, respectively). This results in improved pollutant desorption kinetics and chromatographic separation of the sample components [l].The process of such a two-stage pollutant concentration followed by gas chromatographic determination of the isolated components is illustrated in Fig. 5.4. This technique has been applied to the determination of CI-C4 organohalogen compounds in sea and continental air at ppb levels with the use of an ECD [71], the detection of trace amounts of volatile organic species in the atmosphere [72] and the determinationp'f low concentrations (ca. 3 ng per component in a sample) of c6-cI5 aliphatic and aromatic hydrocarbons and c]-c6chlorohydrocarbons [73]. In the latter instance, in the analysis of air (70 1) at 100% humidity, the analytical results were more reproducible for the C1-CIscompounds and worse for the lighter (
Concentration (mdm3)

Compound

Concentration (mg/m3)

n-hexane n-heptane n-octane n-nonane ethanol n-butanol acetone methyl ethyl ketone ethyl acetate diethyl ether

100 100 100 100 22 26

cyclohexane benzene toluene ethylbenzene chloroform dichloroethane carbon tetrachloride triethylamine propylmercaptan dipropyl sulphide

110 10 52

*)

20

27 19 12

Methanol vapour is an exception. It is adsorbed by 3A in an amount of 30 rng/g.

54

16 16 14 10 5 5

43

5.3. Cryogenic concentration of contaminants

0 1 2 0 1 2 0 1 2 3 b O Time [min]

1 2

Fig. 5.5. Direct chromatographic determination of sulphur dioxide (100 ppbv) in dry and moist air. All the chromatograms were obtained on the same scale [77]. A Dry air; B dew point + 15"C, sampling and injection temperature 23°C; C dew point + 15"C, sampling temperature 4"C, injection temperature 23°C; D dew point injection temperature 45OC

+ lS°C, sampling temperature 4"C,

genic concentration of contaminants from large volumes of air. It considerably decreases the sorption efficiency of sorbents and may result in the displacement of contaminants from the concentrator, alteration of the sample composition and a decrease in the desorption of the contaminants by the solvent. A similar effect is produced by carbon dioxide which accumulates on sorbents at low temperatures [l]. The most effective means of removing moisture from sampled air is to use various drying agents. They should sorb water without entering into the reactions with the substances being analysed. However, this requirement is rarely observed [55]. Calcium chloride, often used for drying inorganic gases, and potassium carbonate, used to remove moisture in the analysis of organic compounds, are not very effective. Stronger reagents (phosphorus pentoxide and magnesium perchlorate) have limited application, as they react with many organic and inorganic compounds [l,551. Such effective drying agents as molecular sieves SA and 13X, which adsorb many other substances, are also inapplicable in this instance. Molecular sieve 3A activated in a nitrogen flow at 250°C is a good drying agent [74]. After a special treatment [75], molecular sieve 3A adsorbs molecules of small dimensions (water, ammonia) and virtually do not adsorb organic contaminants, except methanol (Table 5.2). To deep-dry an air sample, 100 ml of adsorbent are required per 1 1 of air, and the equilibrium concentration of water vapour sorbed on the sieves is 1.94.10 - g at 90% relative humidity (751. This adsorbent, which is uniquely selective for water vapour, may be used effectively to dry air polluted by vapours of organic compounds and inorganic gases [76]. The use of molecular sieve 3A as a drying agent considerably extends the application of the technique of contaminant concentration from ambient air and ensures a very efficient adsorption of moisture. This adsorbent is extremely selective for water vapour compared with other drying agents. The described method of concentration is economic because the drying agents may be regenerated for repeated use over a virtually unlimited period of time. Molecular sieve 3A does not change its aggregate state on adsorption of moisture and retains its properties for a long period. Unfortunately, the sorption characteristics of molecular sieve 3A may vary considerably from batch to batch. However, its is very promising drying agent for the concentration of toxic agents from polluted air. The humidity of air samples has a very strong effect on the results of the determination of reacting inorganic compounds [55]. It can be seen from Fig. 5.5 that the results of the direct gas chromatographic determination of sulphur dioxide in dry (Fig. 5.5a) and moist (Fig. 5.5.c) air [77] differ markedly. The reproducibility of these results depends on the sampling and in-

5. Collection and pretreatment of samples for chromatographic analysis

44

Recovery

Compound

(%I 92 k 16

H2S

cos

91

+ 10

Table 5.3 Efficiency recovery of volatile sulphurcontaining compounds after storage for 1 week at - 196°C in a Tenax GC trap (sampling tube) [78]

93* 7 98 f 11 100f 8

CH3SH CH3CH2SH CH3SCH3

jector temperature, the volume of the sample and the mode of its concentration [77]. The reason lies in the chemisorption of sulphur dioxide on the walls of the chromatograph lines, which is strongly dependent on the presence of water molecules reacting with sulphur dioxide to yield sulphuric acid [55]. This effect cannot always be eliminated by predrying the air sample. Thus, the cryogenic trapping of sulphur-containing compounds such as H2S, COS, CS2, thiols, sulphides and disulphides in a small glass tube containing Tenax GC at liquid nitrogen temperature turned out to be a very good method for the concentration and subsequent storage of toxic samples (Table 5.3). However, despite the use of a drying column containing CaC1, [78], the low-temperature concentration of sulphur-containing compounds (Fig. 5.6a and b) does not eliminate a risk of sulphur dioxide losses (Fig.5.7a and b) resulting from the interaction of the trace amounts of water that are not absorbed in the CaC12 column with sulphur dioxide in the Tenax tube and the chromatographic column, which interfers with SO, detection. TANGERMAN [78] suggested that the sol problem can be solved by using, for example, more selective and inert sorbents. Among such sorbents one can possibly use a porous polymer with grafted functional groups [79] composed of a copper polycomplexes based on a copolymer of 2-methyl-S-vinylpyridine, divinylbenzene and glycidyl methacrylate (Fig. 5.8). This sorbent can be used to advantage not only for the concentration of hydrogen sulphide and sulphur dioxide. As is evident from Table 5.4, the retention indices of hydrocarbons,

8 1

2

5

4

5

Fig. 5.6. Preconcentration [78] of air samples on Tenax GC at room temperature (a) and at - 196 "C (b). 1 Gas-tight polypropylene syringe (100 ml); 2 polypropylene connecting tube; 3 glass sampling tube; 4 Tenax GC; 5 silanized glass-wool; 6 glass tube; 7 CaC12.2H20;8 aluminium foil; 9 liquid nitrogen; 10 Dewar flask

45

5.3. Cryogenic concentration of contaminants

2

2

‘I

a, u)

C

0

9

jA

a, QT

c 0

t e a2

+I

a

i

, J

2

bli

0 2 4 0 2 4 Time [min]

Fig. 5.7

4-

6 min

Fig. 5.8

Fig. 5.7. Gas chromatogram of a standard mixture [78] of hydrogen sulphide (l), carbon oxysulphide (2) and methanethiol (3) (0.2 nmol of each component) and sulphur dioxide (1.2 nmol). Column, 20% silicone SE-30 on Chromosorb P, 2 m, temperature 35°C; flame-photometric detector (at 355 nm). a Direct sample injection; b injection after preconcentration of Tenax GC at - 196°C Fig. 5.8. Chromatogram of a mixture of air (l), sulphur dioxide (2) and hydrogen sulphide (3). Sorbent: copper polycomplexonate based on a copolymer of 2-methyl-5-vinylpyridine, divinylbenzene and glycidyl methacrylate [79]. Column, 1 m x 3 mm I. D., temeprature 140°C; carrier gas, helium (30 ml/min)

Table 5.4. Retention indices for some helogen-, sulphur- and nitrogen-containing compounds on polymeric sorbents [79] Sorbate

Dichloromethane Chloroform Carbon tetrachloride Chlorobenzene Ethyl benzene Thiophene Dimethyl sulphide Diethyl sulphide Tributylamine Dibutylamine Acetonitrile Propionitrile Butyronitrile Nitromethane

Base copolymer 778 857 71 1061 1041 865 664 783 1203 1289 792 857 1019 897

Polycomplexonate Copper

Mercury

Silver

1112 1000 96 1 1181 1036 1016 785 896 1600 1600 1040 1112 1151 1066

952 917 659 1197 993 1118 816 938 1132 1456 1065 1103 1152 1176

90 1 814 776 1121 1009 946 739 849 1358 1476 843 886 992 93 1

46

5. Collection and pretreatment of samples for chromatographic analysis

chlorohydrocarbons, sulphur-containing compounds, amines, nitriles and nitro compounds differ markedly from one another for similar complexes of coordinatively unsaturated metals such as copper, mercury and silver. Therefore, a trap filled with such a porous polymeric sorbent is promising for the selective extraction of HzS, SOz,NH3, amines and other toxic compounds from air [79]. Another approach to the solution of this problem consists in the selection of optimal drying systems. Thus, the cryogenic concentration [77]and determination of sulphur dioxide at the 13 ppt level made it possible to eliminate completely the effect of moisture by passing contaminated air at a flow-rate of 0.25-1.0I/min through a tube containing a Naflon perfluorinated membrane. In coficlusion the cryogenic trapping of toxic species from contaminated air is efficient for a wide variety of compounds, especially low-boiling gases and low-molecular-weight compounds. A serious drawback is undoubtedly the adverse effect of moisture, which should be removed in the course of sampling.

5.4.

Use of adsorption for contaminant concentration

Sorption on solid sorbents is the principal method for sample collection and the concentration of toxic agents from polluted air. Large volumes of air are passed through a sorbent bed with a large surface area and the concentrated contaminants are recovered by heating the trap containing the adsorbent or by using extraction solvents (71. Table 5.5 Properties of some solid sorbents [l, 5, 11-13] Sorbent

Active charcoal Coconut-based charcoal Petroleum-based charcoal Synthetic charcoals: Carbosieve B Carbopack C Spherocarb Silica gel Porasil Spherosil Porous polymers: Tenax GC Chromosorb 101 Chromosorb 102/XAD-2 Chromosorb 103 Chromosorb 104 Chromosorb 106 Chromosorb 108 Porapak Q Polysorb-I Thermosorb*)

Specific surface area

Pore diameter

(mZ/g)

(nm)

800- 1000 800- 1000

2.0 1.8-2.2

1000 13.6 1000- 1200 300-800 300-480 5-500

1 .o- 1.2

19 50

300-400 15-25 100-200 700-800 100- 200 600-650 200-250 30-50

1.3-1.5 2-4 8-10 8-300

140 300-400 8.5 300-400 60-80 5 23.5 7.5 13

*) An inorganic adsorbent for trace organic contaminants; it can withstand temperatures above 500°C.

47

5.4. Use of adsorption for contaminant concentration Table 5.6. Standard sorption tubes [80]

Dimensions, Number Sorbent Length of weight x 0. D. sections (mg) (mm) Alumina Ambersorb XE-340 Coconut-based charcoal Petroleum-basedcharcoal Carbosieve B Chromosorb 102 Florisil Molecular sieve 5 A Porapak Q Porapak N Amberlite XAD-2

1oox 8 70x 6 11ox 8 70x 6 70X 6 70x 8 lox 6 11ox 10 70x 6 70x 6 100 x 10

2 2 2 2 1 2 2 2 2 2 1

200/400 701140 200/400 50/100 100 50/100 50/100 29011160 39/18 44/88 600

The application of solid sorbents for the sampling and analytical concentration of toxic substances in industrial hygiene has increased considerably in the last 10 years. The use of solid sorbents is advantageous not only for industrial hygienists (solid sorbent sampling tubes are easy to use and transport) but also for analytical chemists as the technique provides simple and reliable means for the determination of many toxic compounds [12]. To determine toxic vapour exposures in the workplace atmosphere, continuous sampling (monitoring) in the breathing zone of workers has been increasingly employed. The samples are then transported to a laboratory. Such a procedure requires suitable pumps for sampling and effective means for the concentration of contaminants. Light and portable but sufficiently powerful battery-operated pumps may be employed at different air flow-rates (particularly at flow-rates, in the range commensurate with the breathing rate) and may be used for monitoring. Various sorbents with a well developed surface area (charcoal, silica gel, molecular sieves, etc.) are especially effective as the media for contaminant desorption [l, 7, 111. Concentration tubes containing active charcoal, which are the most efficient and cheapest samplers, are widely used in sampling industrial atmospheres and ambient air. These are glass tubes (5-6 cm x 4 mm) containing about 100 mg of solid sorbent in the primary section and 50mg in the rear section, separated by a polyurethane foam plug. Other types of sampling tubes are used, depending on the purpose, and differ in the sorbent employed. The amount of sorbent may be increased to improve the sorption efficiency; an increase in the internal diameter may reduce the back-pressure (resistance to the air flow) and increase the flow-rates. The sorbent in the rear section is often placed in a separate tube when recovery is effected through thermal desorption or in order to detect the migration of the concentrated analyte during storage. A sampler connected to a small pump is placed directly in the breathing zone of a worker and is fixed on his chest. A certain volume of air is passed through the tube for a specified period. Passive samplers, which operate without pumps, are also used. A wide range of substances such as charcoals, silica gels and numerous porous polymers are used as sorbents (Table 5.5). They adsorb air contaminants and are desorbed sufficiently completely (not less than 75-80%) with a solvent or by thermal desorption. Some standard (commercially available) sorption tubes for the collection of various substances are listed in Table 5.6. The major problems concerning the concentration of volatile contaminants on sorbents have been discussed in reviews [7, 12, 80, 811 and in books [ l , 9, 681, and in a detailed survey

48

5. Collection and pretreatment of samples for chromatographic analysis

Type of sorbent

Number of compounds reported adsorbed

Active charcoal and carbon sorbents Porous polymers Packed chromatographic columns Silica gel Molecular sieves Alumina Other sorbents

160 168 62 96 16 12 8

Table 5.7. Use of solid sorbents in air sampling [ l , 5-13,24, 77-79]

by CRISP[ll].The principal types of sorbents used for sampling toxic agents and their further determination at the TLV level specified in the U.S.S.R.and the U.S.A. are listed in the recommendations formulated by the American Industrial Hygiene Association [82] and given in several publications [7, 9, 11, 12, 801. Sorbents used as concentrators of air contaminants must fulfil several requirements: (1) they must efficiently trap small concentrations of contaminants and retain them until the analysis can be performed; (2) their capacity must be sufficiently high; (3) they must not react with the contaminants while the sample is stored; (4) they should sorb the contaminant in the presence of other contaminants; and (5) they should not give rise to spurious contaminants. Convenient quantitative methods of desorption must be available [9, 11, 681. Table 5.7 lists solid sorbents used for the adsorption of vapours and gases in air sampling. The data were obtained from literature published during the period 1960-1986 and illustrate the number of compounds concentrated in solid sorbent tubes.

5.4.1.

Activated coal and carbon adsorbents

In the U.S.A. air sampling is generally performed with the use of charcoal obtained from coconut shells or petroleum, whereas in the U.S.S.R. active charcoals of AG, SKT and BAU grades are used. The analogue of the first grade is recommended by NIOSH for air sampling. It has a pore size and structure which ensure a high adsorption efficiency. Micropores are very important for adsorption [83]. However, if an adsorbent has very long pores, adsorption is slow because the path of the adsorbed molecules travelling through pores is very long. If the micropores are transversed by macropores the path length is reduced and the adsorption rate increases. As the adsorption rate is largely dependent of the structure of the pores along which the adsorbed molecules travel to the micropores, it may be considerably increased by crushing the charcoal, which considerably reduces the molecular path. The adsorption capacity is an exclusive function of the micropore structure [ll, 81, 831. Therefore, crushing of charcoal may increase its sorption efficiency only insignificantly. According to the IUPAC classification, the pore size in charcoal may be defined as follows: macropores, r > 25 nrn ; mesopores, 25 nm > r > 1 nm ; micropores, 1 mm > r > 0.4 nm ; subrnicropores, 0.4 nm > r (where r is the pore radius). The best grades of charcoal used for adsorption of air contaminants have a specific surface area of ca. 1000 m2/g with 70-75% of the surface area containing pores smaller than 2 nm in diameter. Active charcoal obtained from coconut shells is considered to be an almost all-purpose sorbent. Petroleum charcoal is less active but is also often used in sampling. Charcoal is a very efficient sorbent and is usually used to trap non-polar organic vapours from air. It may also concentrate polar substances but the latter compounds are difficult to desorb. However, many organic substances that are reactive, polar or volatile, e. g., chloroprene, acetic acid or

49

5.4. Use of adsorption for contaminant concentration Table 5.8. Compounds collected from air on active charcoal [12, 841 Compound

Boiling point ("C)

Acrylonitrile allyl alcohol allyl chloride n-amyl acetate arsine acetone acetonitrile benzene 1,2-butadiene 2-butanone sec.-butyl acetate tert.-butyl acetate n-butyl acetate sec.-butyl alcohol tert.-butyl alcohol n-butyl alcohol n-tert.-butyltoluene vinyl phenyl ether vinyl chloride hexane 2-hexanone hexene tellurium hexafluoride heptane hexachloroethane glycidol diacetone alcohol diisobutyl ketone dimethylamine dioxane carbon disulphide o-dichlorobenzene p-dichlorobenzene dichlorodifluoromethane dichloromonofluoromethane 1,2-dichloroethylene dichloroethyl ether 1,l -dichloroethane toluene trichlorofluoromethane 1,1,2-trichloroethane trichloroethylene diphenyl ether fluorotrichloromethane chlorobenzene chloroprene chloroform cyclohexane cyclohexanol cvclohexanone

73-79 97.08 45.1 149.2 -62.5 56.2 81.6 80.1 10.3 79.5 112.3 116.0 126.1 99.5 82.4 117.7 198.0 156.0 - 13.9 68.7 127.2 115.6 -35.5 98.4 162.O 162.O 166.0 165.0 6.9 100-106 46.3 180.5 174.1 -28 8.9

5

Berezkin, Gas Chrom.-BE

48-60 178.7 83.4 110.6 24.1 113.5 88-90 259 24.1 131.7 59.43 61.1 80.7 161.1 155.6

Compound epichlorohydrin isoamyl acetate isoamyl alcohol isobutyl acetate isobutyl alcohol isopropyl acetate isopropyl alcohol diisopropyl ether isophorone xylenes cumene methyl acrylate methyl acetate methyl bromide dichloromethane methyl isobutylcarbinol methyl iodide a-methylstyrene methyl chloride 1,l, 1-trichloroethane methyl Cellosolve methylcyclohexane 2-methylcyclohexanol naphthalene mesityl oxide propylene oxide ethylene oxide octane pentane 2-pentene pyridine n-propyl acetate propylene dichloride n-propyl nitrate propyl alcohol styrene tetrahydrofuran carbon tetrachloride

1,1,2,2-tetrachloroethane ethyl acrylate ethyl acetate ethylbenzene ethyl bromide ethylene bromide ethylene dichloride ethyl alcohol diethyl ether ethyl formate ethyl chloride 2-ethoxyethanol acetic acid

Boiling point ("C) 117 142.0 132.0 118.0 107.9 82.2 83.4 68.3 214.0 138-144 152.4 80.0 56.3 3.6-4.5 41.6 131.8 42.5 160.5 -24.2 74.1 124.4 100.9 162.0 218.0 131.4 36.5-38 10.7 125.6 36.0 36.4 115.6 101.5 87 46-48 97.1 145.2 64-66 76.8 146.2 99.5 77.1 136.2 38.4 131.7 83.4 78.3 34.4 54.2 12.7 124.4 117.7

50

5 . Collection and pretreatment of samples for chromatographic analysis

acetone, may be efficiently collected in a sorption tube and are sufficiently completely recovered [l, 9-12]. Organic substances commonly concentrated in active charcoal tubes are listed in Table 5.8. Sorption tubes containing charcoal have certain disadvantages. Air humidity may reduce the sorption efficiency of these concentrators and the attendant contaminants may displace the target substances on the sorbent. Sometimes long exposures (sampling times) exceed the breakthrough time for very volatile compounds [l]. Other carbon sorbents used for sampling large volumes of atmospheric air include graphitized carbon black [85] and its modifications, Graphon and the Carbopaks. Thermal graphitized carbon black is obtained by pretreatment of the common black under vacuum, in an inert gas or in a reductive atmosphere at 3000°C. This non-specific adsorbent does not contain sites of irreversible adsorption on the surface and does not retain compounds of low molecular weight such as methane, carbon oxides and water vapour. To desorb the contaminants temperatures of up to 400°C are used. Carbochrome, the product of thermal treatment of graphitized carbon black coated with methylsiloxane, is a modification of this sorbent [69]. Mention should be also made of carbon molecular sieves (carbosieves) obtained by thermal decomposition of polymers, e. g., polyvinyl chloride. These adsorbents are recommended for the adsorption of low-boiling C,-C4 hydrocarbons [ll]. Preliminary concentration of traces of CS2 in a tube (20 cm x 2 mm) containing carbosieve B at 25°C (99% desorption efficiency) permits the determination of this toxic compound in the atmosphere at ppt levels [86]. This adsorbent traps methyl formate and alkylmercury compounds [87]. A study of the degree of sorptionldesorption of natural and anthropogenic hydrocarbons in tubes (25 cm X 4 mm) containing Tenax, Carbopak C and B, Porapak T and active charcoal showed [88] that Carbopak B is the most suitable for this purpose. Synthetic charcoals and graphitized carbon blacks such as Carbosieve B are similar to natural charcoals in their physical characteristics. However, desorption from synthetic charcoals occurs more readily. Thus methyl formate cannot be recovered from coconut and petroleum charcoals whereas the efficiency of its recovery from Carbosieve B is reasonably high (not less than 80%) [12].

5.4.2.

Porous polymer adsorbents

Porous polymers, introduced into analytical practice about 20 years ago [89], are used no less extensively than active charcoal. In instances when charcoal and silica gel are impracticable as contaminant concentrators because of poor recoveries, a low breakthrough volume or insufficient stability when stored, an alternative for trapping polar organic substances, sensitive to hydrolysis, is porous polymers, e.g., Chromosorbs, Polysorbs, Porapaks, Tenax and Amberlites of the XAD series [l, 121. Porous polymer sorbents are relatively inert, hydrophobic and normally have large surface areas (see Table 5.5). Most porous polymers poorly retain volatile compounds and wate.r and solvent vapours, but this is turned into an advantage if a sample is collected in an atmosphere with a high content of water and solvent vapours. Table 5.9 lists compounds for which the sampling properties of porous polymers have been carefully studied, as these contaminants pose greater problems than the trapping of other compounds on charcoal or silica1 gel. Porous polymers are most successfully used to trap toxic agents of high molecular weight and non-volatile substances such as pesticides. They may be used for sampling many compounds sorbed by charcoal and silica gel. Many of the limitations associated with the applications of porous polymers are conditioned by their batch-to-batch variations. Careful selection of the sorbent and the sampling

5.4. Use of adsorption for contaminant concentration

Table 5.9 Porous polymers and their application in sampling [I21

51 sorbent

Tenax GC

Amberlite XAD-2

Chromosorb 101 Chromosorb 104 Chromosorb 108 Chromosorb 103 Porapak Q Chromosorb 105

Contaminant adsorbed ally1 glycidyl ether 2-aminopyridine diphenyl nitroglycerine yellow phosphorus anisidine (ortho- and para-) methyl methacrylate nicotine nitroethane quinone alkyllead compounds ph osdrin heptachlor n-butylmercaptan 1-chloro-1-nitropropene formic acid furfuryl alcohol methylchlorohexanone nitromethane

conditions requires tests of the variations in recovery efficiency and sorption capacity resulting from batch variations, and several batches should be tested. The choice of a porous polymer is exemplified by a study [12] devoted to the conditions of n-butylmercaptan sorption. When collected on active charcoal the substance was readily oxidized to dibutyl disulphide. However, the mercaptan cannot be analysed in the dilsulphide form as the latter substance may also be present in the workplace atmosphere. Silica gel is able to trap mercaptans from dry air but at a relative humidity of 80% silica gel predominantly adsorbed water and its sorption efficiency decreased relative to the target contaminant. Charcoal was unsuitable as it reacted with many sulphur compounds. XAD-2, Chromosorb and Porapak porous polymers were studied with the same aim in view. These sorbents exhibited a high sorption capacity and gave a satisfactory mercaptan recovery with short thermal desorption times. However, on prolonged storage major losses of the substance occurred with most of the sorbents. Chromosorb 104, a copolymer of acrylonitrile and divinylbenzene, showed the best performance, the losses were insignificant after a 7-day storage. Chromosorb 104, which proved to be the most suitable for concentration of the mercaptan, is likely to have fewer adsorption sites reactive to mercaptans than other sorbents of the same class. Various aspects of the preparation and application of polymer sorbents in gas chromatography and, in particular, for the concentration of toxic substances from air have been considered in reviews [89, 901 and a book [91]. Certain disadvantages of these sorbents create a number of problems [ l l , 121: (1) displacement of more volatile compounds especially by C02; (2) irreversible adsorption of some compounds, e. g., amines and glycols; (3) oxidation, hydrolysis and polymerization of the sample; (4) contamination of the sorbent due to chemical reations in the presence of reactive gases and vapours; e. g., oxides of nitrogen and sulphur, inorganic acids; (5) formation of new compounds arising from reactions and thermal desorption; (6) limited retention capacity; (7) thermal instability; and (8) limitations of sampling volume, rate and time. However, their pronounced selectivity towards particular classes of organic compounds, hydrophobicity, the possibility of obtaining a representative sample and ready and complete

52

5. Collection and pretreatment of samples for chromatographic analysis

thermal desorption of the concentrated contaminants made these sorbents very useful in the collection of diverse compounds from air [90]. In the U.S.A. porous polymers such as Tenax GC, Porapaks, Chromosorbs, XAD resins and polyurethane foam, used for sampling vapours and aerosols of pesticides, are employed. In the U.S.S.R. ,Polysorbs are more frequently used for these purposes [92]. In addition, polymer resins (polyimides, polyamides, polyacrylates, polyphosphonates and halogenated resins) have been synthesized. Many of these substances exhibit high polarity and have a specific affinity towards aldehydes, alcohols, organic acids and nitriles [89]. These polyfunctional polymers, however, require further study. Tenax GC is a polymer sorbent based on 2,6-diphenyl-p-phenyleneoxide. Other porous polymers are copolymers in which one moiety is styrene or ethylvinylbenzene and the other monomer is a polar vinyl compound. The proportion of monomers governs the polymer’s polarity, thermal stability, surface area, pore size and retention characteristics. The physical properties and the chemical compositions of porous polymers have been surveyed [91]. Many analytical chemists prefer Tenax GC because of its high thermal limit (350-4OO0C), which facilitates thermal desorption. This sorbent can be used to collect organic bases, neutral compounds and high-boiling compounds. High-boiling alcohols, diols, phenols, amines, diamines, ethanolamine, amides, aldehydes and ketones can be collected on Tenax. It can also be used to collect aromatic hydrocarbons [93], hydrocarbon chlorides of various molecular masses ranging from vinyl chloride to pesticides [93-951, N-nitroso compounds and complex mixtures of polluted air compositions [96]. Tenax is selective to small concentrations of white phosphorus in air and retains up to 90% of the sample for 2 weeks [97]. The suitability of Tenax and its ability to concentrate diverse organic compounds with a wide range of boiling temperatures made it indispensable in the analysis of complex compositions of toxic agents, particularly with the use of gas chromatography -mass spectrometry [l]. Tenax is one of the few sorbents that can effectively sample polychlorobiphenyls, polyaromatic hydrocarbons, pesticides, alkanes (up to eicosane), phthalates, etc. [98]. A Tenax sampler containing 10 g of the sorbent at temperatures from 0 to 40°C and relative humidities of 30-loo%, flow-rates of 0.35-0.5 m3/min and a general volume of 300-1600 m3 effectively traps (up to 95%) low- and high-molecular-weight hydrocarbon chlorides (Arochlor, Chlordane, DDT, Toxaphene, etc.) [95]. The breakthrough of polychlorobiphenyls and chlorinecontaining pesticides in a tube containing 10-20 g of adsorbent is insignificant in the collection of 500-700m3 of air at rates of 0.35-0.5m3 [99]. A high sorption efficiency for chlorohydrocarbons permits very low detection limits, provided that the samples are enriched on Tenax. Thus the detection limit of a hexachlorobutadiene sample from 11 of air passed at a rate of 50 ml/min through a short tube (15 cm X 3 mm) containing Tenax using plasma ionization detection was 1-2 ppb [94]. Owing to its high thermal stability and its ability to trap compounds of various molecular weight and polarity, Tenax is used more often than other sorbents for the recovery of pollutants from contaminated air. However, it should be borne in mind that Tenax, while efficiently trapping non-polar high-molecular-weight compounds, is less efficient with respect to volatile polar compounds such as alcohols, ketones, ethers and chlorohydrocarbons. The sorption efficiency for these compounds, even with a small volume of the aspirated air (0.5-5.0 I), is low with the rapid occurrence of breakthrough [loo]. Porapaks consist of a group of porous polymer sorbents with a wide range of polarity. The least polar, Porapak P, facilitates the separation fo compounds with a common functional group such as ketones, aldehydes, alcohols and glycols. The most polar, Porapak T, is sufficiently polar to separate water and formaldehyde. Like non-polar Porapak P, polar Porapaks are rarely used to trap organic contaminants (amines, insecticides) [loll. The disadvantages of polar Porapaks arise from their strong water retention and the greater energy needed to remove sorbates for analysis. The efficient sorption of low-boiling hydrocarbon chlorides (e. g., methyl chloride) is achieved on Porapak Q at -50°C [102]. The more polar Porapak N is ef-

5.4. Use of adsorption for contaminant concentration

53

fectively used to collect small concentrations of aldehydes, alcohols, ketones, acids, esters and olefin oxides from air 193, 1011 and C1-CI2chlorohydrocarbons [101, 1031. Polysorb-I proved to be an effective sorbent for nitrogen-containing organic substances [104]. Polymer chromosorbs are similar to Porapaks in their ability to retain small concentrations of toxic organic substances. Chromosorb 106 is the least polar and, with respect to water retention, Chromosorb 104 is the most polar. Chromosorb 102 is the most frequently used sorbent in this class. It has the largest specific surface area and traps 78-100% of chlorinecontaining pesticides at concentrations in air of ca. 0.004 mg/m3 [105]. The polarity classification of chromatographic columns containing polymer sorbents based and on retention indices for a number of polar compounds proposed by ROHRSCHNEIDER MCREYNOLDS (ethanol, butanol, pyridine, nitromethane, nitropropane, benzene, methyl ethyl ketone and methylpropyl ketone) showed [lo61 that with the exception of Porapak T and Chromosorb 107 and 108, the mean sorbent polarity remains unaltered on prolonged operation (30 days) of a column at 200°C. At the same time, the polarity of Chromosorb 107 and 108 and Porapak T increases with duration of operation. The sorbents may be arranged as follows according to their polarity: Chromosorb 106 < Porapak Q < Chromosorb 102 < Porapak R = Chromosorb 105 < Porapak N < Chromosorb 101 < Porapak P < Chromosorb 103 < Chromosorb 104 [106]. These data account for the efficient trapping of amines on Chromosorb 103, which is one of the best sorbents for the concentration (and gas chromatographic separation) of amines. Thus, the breakthrough volume in the concentration of N-methylrnorpholine (0.25-7 mg/m3) in a glass tube (10 cm X 4 mm) containing 0.2 g of Chromosorb 103 (60-80 mesh) at an air flow-rate of 80-90 ml/min is 100 l/g of the adsorbent, the relative humidity being 70%[107]. The results obtained [lo71 are in a good agreement with those obtained using the normal amine sampling procedure (absorption in 0.05 M H2S04).Trapping of amines on Chromosorb 103 with subsequent thermal desorption and gas chromatographic determination of the recovered substances gave a relative standard deviation of less than 3% [107]. The presence of water does not impede the concentration and determination of amines. Chromosorbs are also used for the concentration of inorganic contaminants. A Chromosorb 101 column traps organomercury compounds from air with the 95-96% efficiency [log]. The direct elution of organometallic compounds from Chromosorb 101 on to a chromatographic column with a microwave plasma detector permits the detection of the organomercury compounds in air at the level of 0.1 ng/m3. Polymer resins of the XAD series are no less popular sorbents than Porapaks and Chromosorbs [109]. XAD-2 (Amberlite), similar in its properties to Chromosorb 102, is most often used. This sorbent is used in the sampling of nitroso compounds, while polychlorinated biphenyls [110-1121 are trapped (83-97%) by Amberlite XAD-2 [110] or XAD-7 [ill] with breakthrough occurring only after the passage of 100-400 m3 of air [112]. XAD-2 has been used for the collection of a large number of toxic compounds including reactive organothiophosphates [12]. The reactive compounds, such as 0,O-diethylphosphorochloridothioate, 0,O-dimethylphosphorochloridothioate and monoethylphosphorochloridothioate, were concentrated on XAD-2 and no decrease in recovery was observed after humid air (95%relative humidity) had been passed through. Hydrolysis of these toxic phosphorus compounds is observed in atmospheres with high humidity, but once collection has occurred the resin tends to stabilize the trapped compounds. Even after storage for 7 days reproducible recoveries were obtained after gas chromatographic determination on a 2-m glass column containing OV-17 silicone on Chromosorb 750 with temperature programming from 70 to 150°C at a rate of 4 K/min using a flame photometric detector (Fig. 5.9). XAD-4 resin has been used to collect other organophosphorus pesticides [113]. A comparison of XAD resins and other sorbents [lo91 for the collection of many volatile compounds showed that XAD resins performed better than other sorbents owing to their greater surface

5 . Collection and pretreatment of samples for chromatographic analysis

54 3

I

I

18

16

1

14

I

12

I

I

I

I0 8 6 Time [rnin]

I

I

I

4

2

0

Fig. 5.9. Chromatogram of organophosphates after concentration on XAD-2 resin and sample desorption with a solvent [12]. 1 solvent yield; 2 methyl ethyl phosphorochloridothioate (5.4 ng); 3 dimethyl phosphorochloridothioate (4.7 ng); 4 diethyl phosphorochloridothioate (4.3 ng) area and smaller pore size. XAD-7, -8, -9 and -12 are more suitable for polar compounds and XAD-1, -2 and -4 for non-polar compounds. The capacities of various sorbents were found to follow the sequence XAD-4 > XAD-7 > Porapak Q > XAD-2 z Porapak P > XAD-1 = Tenax GC 11091, although XAD-7 offered advantages over XAD-4 with respect to collection e f i ciency [ 1111. Active charcoal cannot compete with Amberlite in collecting aliphatic (nitromethane, nitroethene, 2-nitropropane) and aromatic nitro compounds (nitrobenzene, dinitrobenzene, a mixture of 2,4,6-trinitrotoluene isomers) [114]. A sampling tube (5 cm x 4 mm) containing 150 g of Amberlite collected these compounds almost completely from air on passing 5 1 at a flow-rate of 0.15 Vmin and subsequent desorption with 3 ml fo diethyl ether for 30 min. Amberlite XAD-2 collected 80-100% of aromatic and XAD-7 collected 85-100% of aliphatic nitro compounds. Prolonged storage after the sorption and elution of collected contaminants for several days after sample collection are possible. A similar trial using charcoal tubes gave unsatisfactory results. Similar results were obtained for the recovery of such toxic pollutants as 2,2', 3,3', 4,4'-hexachlorobiphenyl, hexachlorobenzene and 1,2,3,4-tetrachlorodibenzo-n-dioxin, which were concentrated from large volumes of air (22-256 mm3)on Amberlite XAD-2 and activated charcoal at room temperature for 4-48 h [115]. The major portion (88.7%) of hexachlorobenzene was detected in the tube containing XAD-2, whereas on the activated charcoal the sorption of this compound was less than 1%.When contaminated air (256m3) was passed through a concentrator containing HAD-2, up to 95% of hexachlorobenzene was trapped. Polyimides are polycondensation products of an acid anhydride (e. g., pyromellitic acid) and 4,4-diaminodiphenyl ether. The surface area of these adsorbents varies from 40 to 70 m2/g and the pore size from 200 to 2 000 nm. Their adsorption of saturated hydrocarbons is like that of Porapak but stronger towards unsaturated hydrocarbons. The interaction with polar compounds depends on the polarity of the adsorbate and the possibility of hydrogen bond formation. Polysorbimides are specific sorbents for compounds with boiling points between 200 and 300°C [ l l , 691. Polyurethane foam with a density of 0.021 g/ml has been used in traps to concentrate chlorine- and phosphorus-containing pesticides [116]. This adsorbent is cheap, easy to prepare, can be produced in any shape and size, is portable, and is used in rapid sampling. Low-

55

5.4. Use of adsorption for contaminant concentration

volatility chlorine-containing pesticides are almost completely retained by foam but more volatile compounds (e. g., aldrin) are trapped to the extent of 50%. Phosphorus-containing pesticides are adsorbed with 65-85% efficiency, and for polychlorobiphenyls the collection efficiency is 70-85%. The collection efficiency for polychloronaphthalenes is lower (ca. 60 %). Field tests showed the high efficiency of this method for the collection of airborne toxic pesticides. Small concentrations of polychlorobiphenyls are virtually completely collected by a 15-cm sorbent bed from volumes of air up to 2700 m3. The sensitivity of the gas chromatographic determination of pesticide vapours in air using electron-capture detection after preliminary enrichment of the sample in a trap containing polyurethane foam is at least 0.1 ng/m3 and the detectability of polychlorobiphenyl is, in this instance, over 99%[117]. The residual volatile compounds are removed from the foam by prolonged solvent washing in a Soxhlet extractor before use. Pesticide vapours are likewise removed from the foam. The study of the efficiency of collection of high-boiling hydrocarbons and polyaromatic hydrocarbons on filters made of glass-wool and polyurethane foam (with further extraction with organic solvents and gas chromatography of the analyte) showed [118] that the greater part of polyaromatic hydrocarbons with 3-4 rings settles on polyurethane foam while polyaromatic hydrocarbons with a higher molecular weight are collected on the filter. The organic substance migrates through the polyurethane foam plug depending on the temperature and the volume of air passed through the filter. At 20°C and an air volume of 600 m3, the breakthrough of phenanthrene and anthracene is 15%, whereas the breakthrough of heavy hydrocarbons (CI9and over) is 25%. At 25°C this value increases to 40%.

5.4.3.

Sorbents used in Gas-Liquid Chromatography

Short packed columns for gas-liquid chromatography have been successfully used for the concentration of air contaminants (Table 5.10). All these columns are characterized by a high content of a stationary liquid phase which facilitates the retention of analytes on the sorbent. To collect low-boiling compounds completely on these sorbents it is necessary to cool the trap, and thermal desorption is used to recover the contaminants, which is a drawback. Table 5.10. Packed gas chromatographic columns used for air sampling [ l l ] Stationary phase

support

Loading

(%I Silicone E301 PEG 400 Carbowax 1540 Dimethyl sulpholane Silicone oil Apiezon L Apiezon K SE-30 or OV-17 Cottonseed oil Carbbwax 600 Didecyl phthalate Tricresyl phosphate Carbowax 400 Oxypropionitrile Phenyl isocyanate Durapak n-octane

Celite 545 or silica gel Celite 545 or silica gel Firebrick Firebrick C-22 Celite C-33 Chromosorb W Sterchamol Chromosorb W Pyrex glass beads Chromosorb W Chromosorb P Chromosorb W Porasil C Porasil C Porasil C Porasil C

3.7 3.1 10 10

20 20 25 10 2.2 20 25 20 Chemically bonded phase bonded bonded bonded

56

5. Collection and pretreatment of samples for chromatographic analysis

Table 5.11. Properties of volatile components of gas emissions from polystyrene and rubber materials Compound

Boiling point

(“0 Propane 1,3-Butadiene Methylmercaptan Acetaldehyde 1-Pentene Isopentane Isoprene Carbon disulphide Acetone 1-Hexene Methanol 2,3-dimethyl-2-butene Ethyl acetate Acrylonitrile Hexane Benzene n-Propanol *)

- 45 4.5 7.8

20.8 30

Retention volume*) (cm7 0.7 3.5 4.8 8.3

10 13.5 14.5

27.9 34 46 56 63.5

43.7

64.6 73.2

50 69

77.2 77

80.4

68.3 80.1 97

24

35.5

83 86.9 96

Compound

Vinylcyclohexene Toluene 1-0ctene 1,2-Heptadiene 3-Octene 4-Octene Ethylbenzene m-Xylene o-Xylene Styrene Cumene Propylbenzene a-methylstyrene n-decane Benzaldehyde p-Divinylbenzene Heptane

Boiling point (“C)

Retention volume (cm’)

108 110.6 121 107 127 123 136 139 144 145 152.4 159.5 163 174 180 200 98.4

289 317 374 502 563 651 725 891 1020 1096 1779 2345 2631 2755 3091 7080 230.4

162

Packed column (30 % SE-30 on Chezasorb) at 20°C.

The possibility of a wide variation in the selectivity of these column sorbents, determined by the nature of the stationary liquid phase, is an advantage of this method. Chromatographic columns packed with an adsorbent with a highly developed surface which is modified by a small amount of the stationary phase or a polymer can also be included in this class. Good results were obtained, for instance, in the concentration of gaseous hydrocarbons from coal mine atmospheres using a short column (20 cm x 4 mm I. D.) containing 1.5% of Apiezon N on SKT charcoal [119]. The analytes were thermally desorbed at 80-100°C. A number of toxic agents (Table 5.11) were identified in gas emissions from various samples of rubber linoleums, polystyrene facing tiles and polystyrene foam [ 1201. The determination of the breakthrough volumes of the compounds using a molybdenum glass column (18 cm X 4 mm I. D.) containing of 30% SE-30 silicone on Chemosorb at 20°C helped in establishing the optimal conditions for collecting organic air contaminants at the phase equilibrium concentration. The last decade has seen the emergence of new sorbents in which the stationary phase is chemically bonded with a solid support. These packings are sometimes called “molecular brushes” and are used for sampling high-boiling contaminants such as pesticides, herbicides and polyaromatic hydrocarbons [81]. Porasil C with bonded phenyl isocyanate may serve as an example. The sorption efficiency of these sorbents in the collection of air contaminants was studied by PELLIZZARI et al. [121]. The sorption of contaminants on these sorbents, which largely have special applications, occurs in the process of solution and orientation of the organic moiety in the thin layer of the stationary phase on the support. A small amount of the bonded liquid phase on the deactivated solid support facilitates desorption of high-boiling compounds by large amounts of the solvent. The procedure is often used for the sampling and analysis of certain types of pesticides. Highly toxic chlorine- and phosphorus-containing pesticides are collected in concen-

51

5.4. Use of adsorption for contaminant concentration

tration tubes containing Durapak, which is Porasil C bonded with various organic compounds. The average recovery efficiency for contaminants was about 88-96% [12]. The recovery efficiency for organic chlorine-, phosphorus- and sulphur-containing compounds on common sorbents (charcoal, silica gel, alumina) is very low. However, even short tubes (5 cm x 3 mm) containing 0.1 g of chemically bonded sorbent can be used for continuous 8-h sampling with recoveries reaching 95%. Very low retention of water vapour and volatile solvents is another advantage of these packings. Solvent vapours pass through the bonded sorbent without affecting the retention volume of the pesticides. This property reduces interferences and allows the collection of the solvent vapours on a charcoal tube connected in series to a concentrator containing a bonded packing for collection of pesticide vapours and other high-boiling air Contaminants. To adsorb amines and nitriles at ppm concentrations, alkaline Porasil A is more suitable than Tenax, Porapak T, Chromosorb T and Chromosorb 103 [122]. A capillary column with an immobilized stationary phase has been used as a trap in the determination of pentachlorophenol. The concentrated contaminants are converted into the corresponding derivatives with CHzNzor (CH3C00)20and detected by FID [123]. SE-54 silicone was used as the stationary phase in the trap (2 m X 0.3 mm) and the analytical column (20 m x 0.33 mm I. D.) with temperature programming from 30 to 235°C.

5.4.4.

Silica gel

Silica gel and alumina are usually used in sampling as a complement to active charcoal t'o concentrate polar compounds from air. Silica gel is a polar adsorbent as its surface contains hydroxyl groups. Water vapour is strongly adsorbed on silica gel, which results in deactivation of the sorbent and breakthrough of the compounds by frontal elution. Affinity to moisture is a limitation but in dry air silica gel is an excellent sorbent. Difficulties may also arise with compounds that hydrolyse easily. Like charcoal, various grades of silica gel (anhydrous silicic acid) collect toxic agents effectively from air. This adsorbent (specific surface area 100-800mz/g) is, however, less frequently used than charcoal or polymer sorbents in air sampling. The reason lies in the hydrophilicity of silica gel, resulting in a considerable decrease in its sorption capacity in the analysis of humid air. In addition, thermal desorption of contaminants from silica gel presents difficulties, and it is necessary to cool the trap to concentrate light contaminants (e.g., C,-C2 hydrocarbons) [124]. On the other hand, the chemical properties of the sorbent surface and its structure favour selective adsorption of polar compounds, particularly amines, halogen and oxygen derivatives, hydrocarbons and other compounds listed in Table 5.12. This list includes some water-soluble aliphatic and aromatic amines. In this instance humidity does not affect the collection efficiency of the sorbent. In certain instances, e. g., the collection of chloroacetaldehyde, adsorption of water vapour even improves the collection ef-

Acetylene tetrabromide Aniline

Table 5.12. Contaminants collected from air on silica gel [12, 1251

Chloroacetaldehyde Diethylaminoethanol Diethylamine Dimethylacetamide Dimethylformamide Dimethylamine Ethylamhe N-ethylmorpholine

Methylamine Methanol Morpholine p-Nitrochlorobenzene Nitrotoluene Mixture of phenyl and biphenyl ether vapours o-Toluidine Xylidine

5 . Collection and pretreatment of samples for chromatographic analysis

58

ficiency. Chloroacetaldehyde forms a very stable hydrate in the presence of humid air. It is also effectively sorbed from dry air. Microporous silica gel is more efficient than the wide-pore material. The most suitable grade has a range of pore diameters from 0.3 to 2 nm, a specific surface area from 100 to 200 m2/g and a density of 0.7-0.8 g/cm3 [126]. Traps containing silica gel are mainly used in the analysis of air pollutants to collect amines. To desorb the trapped contaminants polar solvents (water, methanol, dimethyl sulphoxide, etc.) are used, which may considerably increase the selectivity of determination, as, for example, in the separation of alcohols collected on silica gel from hydrocarbons by water extraction. Like charcoal, silica gel is used extensively. Concentration tubes containing this adsorbent have been used to collect small concentrations of amines [ 1251, nitro compounds [127], vapours of organic solvents and complex mixtures of hydrocarbons [127], organometallic compounds [128], highly carcinogenic dimethyl sulphate [129], aldehydes [130], pesticide vapours, carbon dioxide and many other organic and inorganic compounds [7, 111. The use of polar solvents helps to achieve virtually complete desorption of contaminants from silica gel traps. A high efficiency in the recovery of polar compounds (methanol, formaldehyde and dimethylformamide) from air is exhibited by a sorbent patented in the G.D.R. [131]. It is produced by compressing 50 g of silica gel (0.6-0.8 mm) mixed with 20 g of polytetrafluoroethylene (fibre length 0.5-2.0 mm). The tablets obtained are placed in a polystyrene substrate. Such a sorbent permits the extraction of methanol at 10-1000 mg/m3 concentrations during 8 h or formaldehyde pollutants to be trapped in 0.1-10.0 mg/m3 concentrations during 30 min.

5.4.5.

Molecular sieves

Zeolites are largely used to collect toxic inorganic compounds, as most organic compounds are irreversibly adsorbed by them. Only such highly volatile compounds as acrolein [132], formaldehyde [ 1331 and some sulphur-containing compounds [134] are exceptions. These contaminants are collected in tubes containing molecular sieve 13X activated under vacuum at 400°C [132]. The analytes are recovered by thermal desorption at 240°C [133] or extraction with ice-water [132]. Molecular sieves are among the few sorbents that are suitable for the efficient collection of small concentrations of gaseous inorganic compounds. Zeolites 5A and 13X are used to sample nitrogen oxides. Zeolite 13X coated with triethanolamine was found to be even more effective for this purpose [135]. Zeolite 5A efficiently collects trace amounts of hydrogen sulphide and sulphur dioxide [136]. This sorbent was found to collect hydrogen sulphide better than 13X. Molecular sieves 5A have been used in a liquid nitrogen-cooled trap to concentrate carbon monoxide, which is one of the major air contaminants. CO may be completely trapped on this sorbent at room temperature using zeolite of Y grade, in which silver cations are substituted for sodium cations. This technique for the concentration of carbon monoxide with subsequent gas chromatography of the desorbed substances has already found wide application in industrial hygiene [137]. Zeolite 3A has been used to collect methanol and ammonia selectively for subsequent chromatographic determination [l, 761. Molecular sieve containing cadmium ions ( 2 7 is an excellent adsorbent for the collection of very small concentrations of hydrogen sulphide [138].

5.4.6.

Aluminium Oxide

Like molecular sieves, alumina is seldom used in air sampling. It is employed in samplers to collect polar compounds (determination of ethanolamines in exhaled air) [139], benzene or

5.6. Sorbents for passive sampling

59

gaseous hydrocarbons [ 1401. In the latter instance C2-C4hydrocarbons are efficiently trapped only on cooling the sorbent to - 80°C. The same effect is achieved with a cooled capillary column with walls coated inside with aluminium oxide. To recover low-boiling hydrocarbons thermal desorption at 120-150°C is used [140]. Polar compounds are extracted by polar solvents [139]. A method for the concentration of formaldehyde in a tube containing 30 mg of aluminium was described. After desorption the substance was determined by gas chromatography in the vapour phase [48].

5.5.

Multilayer sorption traps

To analyse air contaminants of unknown compositions, traps containing sorbents with good sorption (e. g., charcoal) or sorption/desorption (e. g., Tenax) properties may be used for preconcentration. Multicomponent traps containing sorbents with different properties are the most suitable for the recovery of compounds of different chemical natures, the concentration of which on one sorbent is inadequate. One of these traps designed for the collection of trace organic compounds [141] consists of four sections packed with Tenax (to collect high-boiling organic compounds), Spherisorb (to collect oxygen-containing substances and compounds with “medium-range” boiling points), silica gel (to adsorb water) and molecular sieves (to concentrate light hydrocarbons). Desorption occurs in the reverse order, and after reconcentration in the cryogenic trap the contaminants are analysed by gas chromatography or by gas chromatography-mass spectrometry. To collect mercury compounds selectively from air [87], successive adsorption of contaminants in tubes containing Chromosorb W (HgClJ, Tenax (CH3C1Hg), Carbosieve B [(CH,),Hg] and gold wire (mercury vapour) has been used. Desorption was conducted at 2.50-400°C. A trap designed to collect volatile compounds of unknown composition [142] contained Tenax GC, Chromosorb 106 and Ambersorb XE-340, a polymeric carbon-containing sorbent. A three-section tube with 5-cm sections packed with these sorbents efficiently collected all volatile organic compounds, as Chromosorb 106 and Ambersorb retain low-molecular-weight compounds and light hydrocarbons, which are poorly collected by Tenax. Carbonized (carbon-containing) polymeric sorbents are obtained by temperature-programmed pyrolysis of polyvinylidene chloride. They are 3-5 times more efficient than commonly used charcoal in collecting highly volatile compounds such as vinyl chloride or methyl chloride [12]. Carbosieve B and S have similar adsorptive properties. They are suitable as concentration tube packings in the same form in which they are used in gas chromatography for partitioning of contaminants, although special-purpose Carbosieve preparation for sampling is possible. Carbonized adsorbents of the Ambersorb XE-340 type represent a new class of synthetic sorbents [80]. In chemical composition they are intermediate between charcoal and polymer sorbents. A concentration tube containing Ambersorb retains highly volatile organic compounds of low molecular weight better than other polymer sorbents.

5.6.

Sorbents for passive sampling

The use of sorbents (active charcoal, silica gel, etc.) in other sampling techniques, such as the concentration of light contaminants in preevacuated columns, passive samplers [143] and the concentration of contaminants on a thin layer of sorbent [144-1461 should be also mentioned.

5. Collection and pretreatment of samples for chromatographic analysis

60

Fig. 5.10. Passive sampler with a permeable membrane for monitoring air pollutants [147]. 1 permeable membrane; 2 upper lid with an opening for air; 3 adsorption tube; 4 plastic tube; 5 gas space; 6 wire net; 7 activated carbon; 8 lower lid; 9 porous stopper Most passive samplers (Fig. 5.10) are based on molecular diffusion [147]. In the ideal case, the sampling rate is determined by Fick's first law of diffusion [148]:

m

A (C, - Co) 1

=D-

(5.1)

where m is the rate of the vapour flow through membranes, D ist the diffusion coefficient, A is the membrane area, 1 is the membrane thickness, C, is the vapour concentration in atmosphere and C, ist the vapour concentration in the gas phase on the sorbent surface. Under these conditions, the flow of contaminants through a semipermeable membrane will be sustained because the sorbent continuously adsorbs new portions of vapours of the substance and the concentration C, will continuously decrease, i. e., Co+O. Then the vapour mass passing through a membrane will be determined by the equation [147]

Q = C, tlKo

(5.2)

where Q is the amount of vapours collected by the sorbent, t is the sampling time and KOis a constant determined by the nature of the contaminants and the membrane size. The theoretical aspects and mechanism of passive sampling with subsequent thermal desorption of pollutants concentrated on a thin layer of sorbent have been discussed elsewhere [149]. The features of such a sampling process are controlled by the reversible-type sorption whose inclusion makes it possible to drecrease significantly the errors in measurements of concentrations of harmful compound. This is exemplified by an experiment on the detection of seventeen organic species in air. At present passive samplers containing Tenax GC are preferred [150] because, compared with standard passive samplers filled with activated charcoal, they allow the detection limit to be reduced by a factor of ca. 200. The body of such a sampler is made of stainless steel and has external dimensions of 3.8 cm diameter x 1.2 cm height with a total weight no greater than 36 g. The pollutants concentrated in the sampler are desorbed at 200°C and analysed by gas chromatography. It is completely decontaminated at 225°C. This operation takes 8-9 h. Such a system can not only be applied in environmental monitoring but can also be used as an individual sampler. Passive samplers are convenient and economic as they do not require air pumps. The air passes through a polymer membrane at a constant rate on its own accord and is trapped by the sorbent. These samplers reliably determine small concentrations of toxic substances over long periods of time at a wide range of concentrations [143]. The method ensures high selectivity. It is achieved by choosing a suitable membrane which is permeable only to the target contaminants, which then pass into a tube containing the adsorbent. Thus, to collect sulphur dioxide a semipermeable membrane of silicone rubber is used. Such a sampler does not re-

5.6. Sorbents for passive sampling

61

Table 5.13. Comparison of collection efficiencies of passive samplers with Tenax and dynamic Tenax samplers (collection time 1 h) [148] Compound

Concentration in air (ppb) Chamber

Tenax concentration

Passive sampler

Tenax concentrator/ passive sampler

12.9 13.8 9.7 12.1 12.5 13.7 12.4

1.7 1.o 0.91 1.9 1.13 1.59 1.13

tube’)

Chloroform 1,2-dichloroethane Carbontetrachloride Trichloroethylene 1,1,2-trichloroethane Tetrachloroethylene Chlorobenzene *) Air

11.6 13.7 8.8 10.4 10.2 8.2

12.1

11.1 13.7 10.6 10.2 11.1 8.6 11.0

flow-rate in Tenax tube, 28 cm’lmin.

quire precalibration and determines of chlorine, sulphur dioxide, vinyl chloride, lead alkyls, benzene, ammonia, hydrogen sulphide and hydrocyanic acid vapours. I n principle it is also possible to determine hydrogen fluoride, formaldehyde and other pollutants in workplace atmospheres [143j. A passive sampler designed by POPLER and SKUTILOVA [151] was used to determine toluene concentrations by gas chromatography in the range 100-400ppm with a relative error of +20%. A passive sampler for Lindane vapour in the atmosphere [152] consisted of a glass tube (900 x 10.5 mm) containing Chromosorb 102. The trapped substance was analysed by gas chromatography using electron-capture detection (275°C) after separation on a column (2 m x 4 mm I. D.) containing 6% of OV-2 10 silicone on Chromosorb W HP (80-100 mesh) at 165°C. A comparison of this technique with dynamic sampling using a tube 10 mm in diameter with two sections (40 and 20 mm) containing Chromosorb 102 (1 g and 0.5 g, respectively) showed good agreement with the results with air passing for 0.5-1 h at a rate of 200 ml/min and recovery of the collected compounds with a 50% solution of acetone in hexane ( 2 x 10 ml). However, it was noted [152] that the passive sampler was less accurate. The method has a number of other advantages and the collection eficiency of passive samplers is similar to that of dynamic sampling (Table 5.13). Passive samplers have also been used to determine small concentrations of nitrogen dioxide, carbon disulphide, carbon monoxide, mercury vapour, carbon tetrachloride, formaldehyde, toluene, styrene, ethylene oxide, anaesthetics (halothane and enflurane), n-hexane, acrylonitrile, acetone, l,l,l-trichloroethane, methyl isobutyl ketone and carcinogenic tri- and tetrachloroethylene [ 1531. WEST[143] considered these methods to be promising as they are simple, reliable and may be used in industrial hygiene to determine a large number of contaminants over a wide range of concentrations. Similar membranes are used in another sampling method patented in Japan [144]. A membrane lets air into a chamber with walls coated with an adsorbent layer. After exposure, the adsorbent is recovered from the chamber and after desorption the contaminants are analysed by gas chromatography. An original method for sampling airborne solid particles was developed in the U.S.A. [145]. An air sample is passed through a glass-wool filter inserted in a slot cut in an adsorbent plate. The sample is then analysed by thin-layer chromatography directly on the same plate. A similar method was used in the determination of heavy contaminants in air [146]. The air

62

5 . Collection and pretreatment of samples for chromatographic analysis

is passed through a chamber containing an adsorbent plate (or the plate is simply left in air). The contaminants are collected on the part of the plate open for contact with the air and coated with a layer of active charcoal. The concentrated contaminants are analysed by thinlayer chromatography, the separation occurring on the part of the adsorbent layer that was covered during the exposure. Separated components (or groups of compounds) can be analysed by gas chromatography to identify the substances present. The method has been used to determine the concentrations of high-boiling organic substances, e. g., phthalates [146, 1541. A unique procedure for the concentration (with subsequent thermal desorption) of sulphur-containing compounds has been described elsewhere [155]. Air is aspirated through a concentrator made of PTFE and equipped with an internal metal foil (30 mm X 7 mm X 0.025mm) made of silver, nickel, palladium, platinum, rhodium or tungsten. The film is intended for trapping air-contaminating compounds such as H2S, CS2, COS, SO2, CH,SH, CH3SCH3 and CH,SSCH, at mg/m3 concentrations. The most efficient recovery (ca. 45%) is provided by palladium or platinum foil. On concentration the foil is rapidly heated electrically and the desorbed pollutants in the form of a compact “plug” enter the chromatograph fitted with a flame ionization detector. With increasing aspiration rate between 0.15 and 4.0 Vmin the efficiency of concentration of sulphur-containing compounds decreases. However, variations in the humidity of the analysed air and its temperature (in the range 25-50°C) and also the presence of nitrogen oxides and ozone (at 250 mg/m3 concentrations) do not have any significant effect on the efficiency of trapping. As the time of the film storage increases to 14 days, one can observe a slow increase in the gaseous sulphur dioxide concentration, which is probably associated with the desorption of the pollutants that were previously trapped on the PTFE chamber walls.

5.7.

Chemisorbents

Methods based on the chemisorptive interaction between a sample substance and a sorbent have certain advantages over the methods considered above and based on the physical sorption of contaminants. The main advantage of the chemisorption method lies in the very high selectivity determined by the specificity of the chemical reactions used, for instance, in the method of the reactive gas chromatography [156]. Reagents are coated on a solid support, e. g., inorganic salts or sorbents with a well developed surface. Thus, to determine small concentrations of phosphine it is adsorbed on silica gel impregnated with silver nitrate solution. Phosphine and silver nitrate form an unstable compound that decomposes on heating to give PH3, which is determined by gas chromatography [1571 PH,

+ 6 AgN03 = Ag,P. 3 AgN03 + 3 HN03

(5.3) Such a concentrator may adsorb 1-2OOpg of phosphine and the breakthrough volume is 127 1. A sampling tube containing glass-wool and Chromosorb P impregnated with a 5% solution of sodium carbonate can efficiently collect vapours of hydrochloric, nitric, acetic and other acids at concentrations of 1-20 mg/m3 after the sorbent has dried [lSS]. These tubes are cheap, easy to prepare and stable on storage. Desorption of the collected contaminants is efficiently performed using water or acetone. An unstable compound is also formed in the interaction of nitrogen oxides with triethanolamine coated on molecular sieve 5A [135]. The sorbent efficiency for a nitrogen oxide concentration of 10 mg/m3 is 96-97%. A concentrator containing triethanolamine (TEA) on glass beads is also effective [159]. About 1 g of TEA is dissolved in 30 ml of dichloroethane, 10 g of glass beads are added to the

63

5.7. Chemisorbents

3

(I, (r,

Fig. 5.11. Chromatogram of acetyl derivatives obtained after collecting NOz in a concentrator with triethanolamine for 3-5 days [159]. Conditions: column (2 m X 3 mm I. D.), Chromosorb W containing 1.5%OV-17; temperature, 3 triethanolamine

c

0

2 2

0

10

20 30 Time [min]

40

solution and the solvent is removed at low pressure. Then 5 g of beads (3-5 mm in diameter) impregnated with TEA are placed in an adsorption tube (9 cm X 35 mm) through which air is passed at the rate of 1 llmin for 3 h (the NOZconcentration is at the ppm level). The air passing through tube is previously stripped of some organic contaminants and traces of water by a column with charcoal and CaClZ.Nitrogen dioxide is converted into nitrosodiethanolamine by the reaction N

/ CH2CH20H CH2CH20H

\

CHzCH2 OH

/ CH2CH20H CH2CH20H

N-

I

(5.4)

N=O

and is subsequently analysed as acetyl derivative using electron-capture detection. A chromatogram is shown in Fig. 5.11. Similarly, for the collection of NO, from air a catridge containing florisil coated with diphenylamine is used. The sampled air is passed through the trap at a rate of 1-4 ml/min. The trap capacity is about 100 ppb of NOz. The products of the interaction of NOz and diphenylamine (N-nitro-DPA, 4-nitro-DPA and 2-nitro-DPA) are eluted with 2 ml of CH30H [160]. SO2 is stabilized on the surface of sorbents (XAD-2, XAD-7, molecular sieve SA, etc.) using derivatization by small amounts of polar organic compounds, e. g., aldehydes [161]. The possibility of using coordination compounds for the collection and preconcentration of contaminants has been indicated. In particular, polyurethane foam is employed for this purpose [162]. Hydrophobic chelating agents coated on a porous polymer can be used to collect toxic aerosols of metals. Sorbents impregnated with chemical reagents are often used when sample collection and the recovery of contaminants from the sorbent (active charcoal, silica gel or porous polymers) are inefficient owing to a high reaction ability of the analytes or their instability on collection and storage. The most convenient procedure in this instance is to obtain stable derivatives or products with the original characteristics of the main compounds. The formation of amine salts during the collection of ammonia from air or organic amines collected on acid-impregnated silica gel may serve as an example. The same result is achieved in the formation of an amalgam of silver and mercury if mercury vapour is collected on silver-coated Chromosorb P. The formation of adducts by the Diels-Alder reaction (diene synthesis) facilitates the com-

5. Collection and pretreatment of samples for chromatographic analysis

64 Sorbent

Contaminant

Acid-treated silica gel (e. g., sulphuric acid) Silver-coatedChromosorb P TEA on molecular sieves Mercury cyanide on silica gel Mercury chloride on silica gel Maleic anhydride on Chromosorb P 2-(hydroxymethy1)piperidine on Amberlite XAD-2 2,4-dinitrophenylhydrazine on silica gel HBr on Amberlite XAD-2 K2CrZ07and Cu on Charcoal

ammonia, n-butylamine

Table 5.14. Some chemisorbents used for sampling contaminants [5,12,131

mercury vapour nitrogen dioxide phosphine stibine cyclopentadiene aldehydes aldehydes, ketones ethylene oxide HCN, (CNh

plete and selective recovery of butadiene from polluted air when it is passed through Chromosorb 104, trated with maleic anhydride: //CH2 HC

I

HC -CO,

+

HC 1 I -CO’ 0

“ (i CH2

-

(5.5)

o&: H

Stability on storage is an important criterion in the applicability of such sorbents. Some chemisorbents are listed in Table 5.14. Chemisorbents are most frequently used to concentrate a single one substance. Their high selectivity is a very important advantage. For example, consider the sorbent for the collection of carcinogenic bischloromethyl ether. The sorbent is prepared by pretreatment of glass beads (0.10-0.12 mm) with a 1.5% solution of potassium 2,4,6-trichlorophenolate.When air polluted with bischloromethyl ether is passed through the sorbent, a derivative is formed that is analysed by gas chromatography using electron-capture detection: ClCH2 -0-CH2CI

+ CI CI

CI

The derivative is stable and is recovered from the sampler with methanol. The excess of the reagent is removed by dilution of the sample with 1 M NaOH solution and hexane extraction. In addition to the sorbent, the last section of the tube contains silica gel to concentrate the bischloromethyl ether derivative, which may “break through” the front section. In the process of sample collection the ether is stabilized. It is retained on the solid sorbent, thus counterbalancing the drawbacks inherent in other sorption methods. In addition, the derivative obtained improves the selectivity of determination, which amounts to a few picograms of the substance 111, 121. To collect toxic agents from air, use is made of aerosol filters impregnated with chemical reagents that simultaneously trap gases, vapours, aerosols and solid particles. This method permits the collection of a representative sample of pesticides (a filter and a net impregnated with polyethylene glycol), sulphur dioxide and sulphates, nitrogen oxides and ammonium nitrate and ammonium salts [l].

5.8. Trapping of solid particles and aerosols

5.8.

65

Trapping of solid particles and aerosols

Airborne solid particles of industrial and natural origin (dust, soot, pesticides, polyaromatic hydrocarbons, etc.) or aerosol particles (high-molecular-weight compounds, metals, inorganic salts, etc.) considerably exceed atomic or small molecular dimensions and are not trapped by commonly used sorbents. Various devices (e. g., electrostatic traps, cascade impactors, cyclone separators) are used for this purpose [163-1651. They allow the rapid collection of aerosol particles of different sizes. Various filters are more often used for the chemical analysis of air. These filters are made of glass-fibre [166], ceramics, PTFE (for collection or reactive compounds), polyamide, polysulphone, polyacrylonitrile, poly(viny1 chloride) [167] and other materials [168] which virtually completely trap aerosols with particle sizes of 0.1-0.2 pm. In the U.S.S.R. the domestically produced fine-fibre perchlorovinyl Petryanov filters are used. Their mass is small, they are resistant to aggressive media and they are soluble in organic solvents [169]. Filters through which several cubic metres of air are passed at a flow-rate of 1 m3/h collect solid particles of dust and aerosols containing various organic compounds [166, 1671, polyaromatic hydrocarbons [169, 1701, polyaromatic heterocyclic compounds [170], pesticides [171], polychlorobiphenyls, inorganic salts [172] and metals [173]. Filters made of glass-fibre which are more often used to collect metals, polyaromatic hydrocarbons, pesticides, polychlorobiphenyls and other aerosols of organic nature are unsuitable for trapping dusts of elements such as arsenic, lead, cadmium, chromium, nickel, selenium, vanadium and tantalum. For these purposes membrane filters made of cellulose nitrate or acetate are used [12]. A cellulose acetate membrane filter retains tri- and tetracyclic polyaromatic hydrocarbons more efficiently than a glass-fibre filter (1741. Air is passed at 8-15°C at a volume flow-rate of 50 m3/h through a series of membrane filters (257 mm in diameter). The filters are then dried over silica gel for 5 h and extracted with toluene. The extract is purified using an XAD-2 column containing Dexyl 300 using plasma ionization detection with temperature programming from 100 to 280°C. To trap aerosols of lead and its compounds, solid sorbents are preferred. Of alumina, silica gel, Chromosorb 102 and Tenax (80-200 mesh) tested for this purpose, Tenax was shown to be the most suitable [175]. With 200 mg of Tenax in a tube and taking into account elution of lead from the tubes, the collection efficiency ranged between 95.8 and 99.9%. Using electron-capture detection, gas chromatography permits the collection of about mg/m3 of aerosols of the toxic metals beryllium, chromium and aluminium after several cubic metres of air have been passed through a perchlorovinyl filter [173]. The contaminants collected on filters are desorbed by strong acids or alkalis (for inorganic compounds or organic solvents, for organic compounds). The sample extraction, however, is time consuming, which is a serious disadvantage [l]. When the sampled air contains gases, vapours and aerosols of toxic substances (pesticides, polychlorobiphenyls, polyaromatic hydrocarbons, etc.), the impregnated filters mentioned earlier [176, 1771 and combinations of a filter and a trap containing the adsorbent, a filter and a polyurethane foam plug (for the analysis of polyaromatic hydrocarbons, phthalates, polychlorinated naphthalenes) [ 1781, glass-fibre filter containing a polyurethane foam packing (for the analysis of polyaromatic hydrocarbons) or glass-fibre [ 1791 or perchlorovinyl fibre impregnated with active charcoal [l] are used to obtain a representative sample. These sorbents [l], however, are not widely used owing to the low sorption efficiency for light contaminants; they are generally used in the analysis of solid particles. A system consisting of a paper filter and polyurethane foam may be used to concentrate aerosols in the analysis of pesticide mixtures [180]. A typical device for trapping PAH-containing dust and scot from air is presented in Fig. 5.12. It is composed of a glass-fibre filter (47 mm in diameter) and a polyurethane foam 6

Rerezkin, Gas Chrom -BE

66

5. Collection and pretreatment of samples for chromatographic analysis

~

T

Glass Tube

1

T*h:;;:rFih Filterholder

PU Plugs

Fig. 5.12. Device for small-volume air sampling w11

block (200mm x 18 mm). After the air (80-90 mm3) has been passed through such a system for 24 h, the concentrated pollutants are extracted with cyclohexane and the PAH fraction isolated by liquid chromatography is analysed by gas chromatography and gas chromatography-mass spectrometry [181]. During the protracted collection of polyaromatic hydrocarbons on glass-fibre filters, losses of volatile polyaromatic hydrocarbons, e. g., fluoranthene, take place. To prevent this, the filters are impregnated with glyceryl tricaprylate [176]. To trap at the same time both sulphur dioxide and sulphates at low concentrations, cellulose filters are impregnated with tetrachloromercurate solution [177]. Such sampling systems collect over 90% of toxic substances in different aggregate states. The sensitivity of determination for the desorbed contaminants ranges from 1 ng to 1 pg using gas chromatography. The vapour/particle phase ratio is very important in determining filter-trapped metals (dust, aerosols), organic and inorganic salts and organic substances with a very low vapour pressure, particularly when large concentrations of the analysed contaminant are present in the sampled air (e.g., in the analysis of pesticides). This facilitates the correct choice of filters and adsorbents and minimizes the error of determination. It has been suggested [12] that if the ratio of the equilibrium concentration of the analysed substance to the TLV at 25°C is 0.05-50, it is necessary to take into account the presence of vapours and aerosol particles. If the ratio is lower than 0.05 only the vapour content need be accounted for. Such a determination may be carried out in the analysis of a model atmosphere of toxic substances at the TLV level. It should be borne in mind that the vapour/particle ratio may be dependent on the concentration and volume of the sample. The collection efficiency of the sorbent and the sorption capacity of the filter should be determined individually [ l , 11, 121. Organic compounds adsorbed on solid particles of fumes (soot) and those which are present in the atmosphere in vapour form are sampled in the following manner [182]: (1) combined sampling (simultaneous collection of solid particles and vapours); (2) consecutive sampling (solid particles are collected on a filter and vapours are collected in a trap containing the sorbent); (3) parallel sampling (simultaneous and independent collection of particles and vapours); (4) hybrid sampling (a combination of the consecutive and the parallel methods).

5.9. Preparation of concentration tubes containing solid sorbent

5.9.

67

Preparation of concentration tubes containing solid sorbent

Before a solid sorbent can be used to collect a contaminant it must be activated, i. e., treated to remove any compounds occupying adsorption sites. These compounds reduce the sorption capacity and introduce new compounds (artifacts) into the analysis. No standard practice for sorbent pretreatment has been evolved so far, but the most common procedure is to heat the solid in a flow of inert gas for a period of time [l]. Charcoal has been conditioned at temperatures as low as 120°C and as high as 600”C, the heating times ranging in various techniques from 10 min to 16 h [147]. Acid leaching and impregnation of the adsorbent with hydroquinone in order to prevent polymerization of sample substances have been found useful as part of the pretreatment. In acid pretreatment, 12 g of charcoal are boiled with 50 ml of 1 N HCl for 2 h, washed with water several times, boiled in 250 ml of water for 1 h, filtered, washed with water and kept for 16 h at 250°C in a helium flow. Unlike commonly prepared charcoal, the pretreated charcoal does not decompose hydrocarbon chlorides concentrated on its surface [183]. A good way to pretreat charcoal of BAC grade is to wash it with acetone and heat it at 200°C in a nitrogen flow for 5 h [184]. Removal of background contaminants arising from charcoal during chromatography should be mentioned. Consideration of the possible sources of contamination in a closed system designed to blow out contaminants from the sample showed the necessity to clean charcoal filters. To clean filters [185], they are placed in a test-tube and 30% HzOZis poured over them, maturing at 20°C for a few hours. After that test-tube is heated up to 60°C and is matured at this temperature overnight. Then washed with 5% NH, solution, water, CH30H, CHzClzand CS2, in that order. As a result of the treatment the organic substances adsorbed on the charcoal decompose. A comparison of chromatograms (Fig. 5.13) of extracts from treated an untreated filters obtained on a capillary column (14 m X 0.32 mm I.D.) containing SE-54 with temperature programming from 25 to 250°C at 8 K/min shows the efficiency of this method of filter cleaning, which may also be used for sorbent regeneration and for the pretreatment of charcoal in crucial cases. Pretreatment of porous polymers is different only with regard to temperature and time. Porapaks are normally heated for 1-3 h at 180-190°C, Tenax GC (and carbonized sorbents of the Carbopak type) is conditioned for 24 h at 360°C and Ambersorb XE-340 is heated at 450°C. Chromosorbs are pretreated for 12-24 h in a nitrogen flow at 225°C. All porous polymers, charcoal, silica gel and alumina adsorbents may also be treated by keeping them for 24 h in a pure helium flow at a rate of 50 l/min at 150-200°C [109]. The heat treatment is often preceded by extraction of organic compounds with solvents (most frequently these contaminants arise during synthesis of the polymer). These compounds may distort the analytical results in the subsequent use of the sorbent in sample collection and desorption. To avoid this, Porapaks, polymeric resins of the XAD-2 type and Ambersorbs are treated with organic solvents (acetone, light petroleum, etc.) for several hours. Tenax is treated in a Soxhlet extractor with methanol, ethyl acetate or n-pentane or washed with light petroleum [95]. Polyurethane foams, before they are used to collect pesticide vapours from air, are washed with acetone, dried and washed with light petroleum [95]; they may be treated with acetone or n-hexane in a Soxhlet extractor [ll]. The polymeric resin XAD2, which is one of the most commonly used polymers for the collection of toxic contaminants, should be treated [112] by washing it with water for 15 min, acetone for 24 h and light petroleum for 48 h and drying it at 60°C for 24 h. Contamination arising from the use of synthetic adsorbents is, however, possible even after the pretreatment (e. g., the appearance of “false” peaks). By using GC with glass capillary columns it was shown that there is contamination from alkylbenzenes, styrene, indole, naphthalene and biphenyls in Amberlite XAD-2 and XAD-4 and Ambersorb XE-340 and XE-348. It is recommended that these adsorbents are pretreated with water, methanol and dichloromethane for prolonged periods [186].

68

5 . Collection and pretreatment of samples for chromatographic analysis

before cleaning

-

b)

60' prog. 6 "C lmin

-----

Fig. 5.13 Chromatograms of carbon disulphide extract of a filter containing activated carbon (a) before and (b) after treating the carbon with 30% HzOzand subsequent extraction with hot 5 % ammonia solution (1851. Conditions: capillary column (14 m x 0.32 mm I.D.), SE-54(0.4 pm); temperature, programmed from 60 to 250°C at 8 Wmin; flame ionization detector To collect ethylene glycol on XAD-2 or 2-methoxyethanol, 2-methoxyethyl acetate, 2-ethoxyethanol and 2-butoxyethanol in a trap containing XAD-7,the sorbents should be pretreated with water, methanol and diethyl ether [187]. The collection efficiency of these pretreated sorbents is significantly higher than that of active charcoal and for XAD-7 it amounts to 90- 100%.Adsorption of acetone, ethanol, 2-propanol, benzene, toluene, mono-, di- and trichlorobenzens, dichloromethane and chloroform on porous polymers varies over a wide range depending on preheating [ 1881. With higher temperatures of preheating (generally from 150 to 300"C), the polarity of Porapak decreases and the polarity of Chromosorb 101 and 102 increases. An increase in the preheating temperatures renders the VR vs. 1 / T dependence linear for all the adsorbent in the range 50-150°C. VR for Tenax does not depend on preheating at 200-300°C and is a linear function of 1/T. These results may be explained by alterations in the chemical properties with increase in the conditioning temperature, which gives rise to decomposition and loss of weight. This does not apply to Tenax, which ist thermally stable up to 400°C. Silica gel is heated at temperatures ranging from 120 to 350°C at a low flow-rate of nitrogen or under vacuum; the heating time may vary from 15 min to 24 h. CRISP[ l l ] washed the silica gel with water before the heat treatment. In industrial hygiene, pretreatment of silica gel in air sampling is performed by washing it with hydrochloric acid (1:l) and distilled water, followed by drying and calcination at 350°C for 2.5 h [189]. Sometimes silica gel used to concentrate organic solvents from air is kept at 110°C for only 3 h [l].

5.9. Preparation of concentration tubes containing solid sorbent

69

Molecular sieves 13X are pretreated by heating under vacuum at 350-400°C for 24 h [132]. Zeolite 5A ist treated at 300-350°C and for pretreatment the sample tube is blown out with nitrogen at 250°C [74]. The dimensions and design of the sample tube and the amount of adsorbent vary according to the adsorbent, the problem to be solved and the concentration to be studied (i. e., whether workplace or ambient atmospheres are being sampled). A very important requirement is to avoid a back-pressure at the operating flow-rates, particularly if sampling is continued for a long period. The tube must have a sufficient volume to contain an adequate amount of adsorbent. The material of construction must be chemically inert and the inner surface of the tube should not adsorb the sampled contaminant in significant amounts and react with it; after collection of the sample the tube (usually made of glass) should be easy to seal in order to prevent losses of the sample during storage and transportation [l,11, 121. Tubes of standard design are normally used, e.g., those recommended by NIOSH [12]. The tube should be filled with charcoal and is made of glass 7 cm long and 4 mm I. D., both ends being sealed before the analysis. There are two sections, the first containing 100 mg of charcoal and the second 50 mg of charcoal to indicate breakthrough. In front of the primary section there is a plug of silylated glass-wool. A 2 mm spacer of polyurethane foam separates the two charcogl sections. Another 3 mm polyurethane foam spacer ist placed behind the second section. Various tube designs have been used, including those with a single bed of charcoal whose weight may be up to 400-700 mg [190, 1911. Such tubes are used to concentrate contaminants of multicomponent compositions [190]. The inner diameters of the tubes rarely exceed 5 mm and the length varies from 7 to 10 cm. Porous polymers are usually packed as a single bed in tubes made of glass, stainless steel or aluminium. The weight of adsorbent is from 100 mg for workplace air samples to 5 g (the maximum amount) for collecting small concentrations of contaminants from ambient air. Polyurethane used to sample pesticides in ambient air is cut into cylindrical plugs 2-10 cm in diameter and 0.5-5.0 in length [ l l ] . Tubes packed with silica gel contain more adsorbent than those packed with charcoal of polymer. They are 15-20 cm in length with inner diameter not less than 6-7 mm. The weight of adsorbent is normally about 3-5 g, sometimes up to 20 g are used. Molecular sieves require longer tubes but with a smaller inner diameter. Columns 15-43 cm long and 3 mm I. D. are used to concentrate carbon monoxide, dinitrogen oxide (N,O), hydrogen sulphide and sulphur dioxide [136]. Columns containing zeolite 13X of length of 5 and 15 cm were used to trap acrolein and formaldehyde, respectively [132, 1331. Similar columns containing alumina [140] were used, the amount of molecular sieves 5A and alumina being 800-1000mg [136, 1401 The mesh size of the adsorbent is chosen to achieve the optimal belance between adsorption efficiency and low resistance to air flow. As all adsorbents have large surface areas, good efficiency may be achieved with fairly coarse size ranges. Particles with dimensions of 0.84-0.42 mm are most often used in the case of charcoal and silica gel [ll]. The study of the effect of the charcoal particle size on the adsorption efficiency showed that all the sizes studied (from 3.4-4.8 to 0.4-0.6 mm) adequately sorb contaminants, but 0.85-0.42 m m is the most convenient size for packing and give reproducible flow-rates of 1 l/min. Charcoal with particle sizes of 0.5-0.25 mm or 0.8-1.0 mm is commonly used in the U.S.S.R. Silica gel is efficient with a grain size of 0.2 to 0.15 mm but, like charcoal, the most convenient fraction for concentrating contaminants is 0.84-0.42 mm. Molecular sieves used to concentrate CO have a grain size of 0.42-0.25 mm. The most frequently used, 5A and 13X, have grain sizes from 1.68 to 0.21 mm [132, 1361. Porous polymers and chromatographic compositions are used in fine sizes from 0.25 to 1.25 mm, the 0.15-1.25 mm fraction being the most common; we have no knowledge of special studies of the effect of particle size on sampling efficiency, but it was found [lo91 that variations within the range 0.3-0.074 mm for XAD-2 do not affect sampling efficiency.

70

5. Collection and pretreatment of samples for chromatographic analysis

The pressure drop across the sample tube should not exceed 3.4 kPa within an air flow-rate of 1 l/min. This criterion, recommended by NIOSH, has been confirmed by other workers who studied the concentration of contaminants on sorbents [ l l ] . Higher pressure differentials have been reported for porous polymers that have finer grain sizes than charcoal or silica gel. In a column with coarser Tenax GC (0.6-0.25 mm) the pressure drop did not exceed 0.1 kPa. With fine-grained chromatographic packings the pressure differential may reach 53.4 kPa. Sorbent tubes retain contaminants if they are effectively sealed after sampling. The efficiency of storage depends on the volatility and reactivity of the contaminant sampled. Storage conditions improve at low temperatures. Thus, low concentrations of vinyl chloride (0.1-1.0 mg/m3) can be retained for 2 weeks in a small tube containing charcoal BAC if the concentrator is cooled by ice (1841. A small decrease in the vinyl chloride concentration was found after 2 weeks; this occured owing to migration of vinyl chloride to the back-up section of the tube [ l l , 121. Fairly often, samples are not analysed on the day of sampling and some compounds may desorb by 10-15% or more after 1-2 weeks of storage [81]. To prevent losses on storage, cooling is recommended, otherwise contaminants should be immediately desorbed from the sorbent after collection, provided that the contaminant extracts are stable. Sometimes it is possible to retain a cooled dissolved sample for a long period of time. It has been found [192] that time, temperature, light and atmospheric pressure affect the storage of compounds adsorbed on charcoal. Although many compounds can be efficiently stored for up to 3 months, some of them become redistributed between the primary and back-up sections of the tube. Dichloromethane attained an equilibrium redistribution in 22 days whereas trichloroethylene showed no transfer after 20 days and only 15%of hydrocarbon chloride had desorbed from the primary section after 92 days. Toluene and styrene showed no redistribution after 5 weeks. Most organic vapours showed no losses from tubes containing charcoal when the storage period was moderate. Only methyl ethyl ketone showed a continuous loss, amounting to 24% after 78 days, presumably owing to chemical decomposition [192]. The storage efficiency of Tenax has been studied for various compounds, including acrolein, propylene oxide, diethyl sulphate, nitromethane and glycidyl aldehyde [193]. Only insignificant losses occurred on storage of some of the compounds for 3 weeks. Microconcentrations of white phosphorus after Tenax column concentration altered by 10% after 12 days of storage [97]. The storage of humid air samples (over 80% R. H.) proved less satisfactory. The concentration of bischloromethyl ether was reduced after 3 days. Sulphur dioxide on molecular sieves was retained for 2 weeks only at -2°C and a similar sample of hydrogen sulphide showed a loss after only 2 days. A sealed column of 13X retained microamounts of NO2 unaltered for over 1 week. Reactive trimethylamine was retained on a chromatographic packing (glass beads treated with tartaric acid) for only 4 days [194].

5.10.

Influence of sampling conditions on the efficiency of sorption of pollutants

In developing methods of air analysis, theoretical calculations for choosing the sorbents are seldom applied. Most investigators restrict themselves only to verifying the fact of the presence or absence of a breakthrough in the sorption system used. Sometimes the sampling conditions are chosen based on measuring chromatographically the retention volumes of concentrated pollutants and plotting the chromatographic characteristics against column temperature (log V, - l/q.The V, values necessary for practical work are obtained by extrapolating to the values of the retention volumes for normal temperature (18-20°C). This ap-

71

5.10. Influence of sampling conditions on the efficiency of sorption of pollutants

proach to evaluating the sorbents is, however, insufficiently accurate: it does not take into account the impact of numerous external factors (air humidity, flow-rate, coadsorption of pollutants, etc.), and when the data are extrapolated the accuracy of determinations of the sorption capacity of the adsorbent may be low. The use of sampling data taken from the literature is also insuffienciently reliable in choosing a selective and efficient sorbent, as the influence exerted by different factors on the concentration of untested substances and their mixtures is highly diversified and complicated. When concentrating air pollutants, one has to take into account all the factors that affect the sampling efficiency, especially in long-term sampling. In this connection, we should examine the model of a sampling procedure on a solid sorbent [81] in a collection tube with two sorbent beds, most frequently used to collect pollutants on activated carbon. The first section of the column (two thirds of its length) usually contains 100 mg of carbon, and the second (one third of the length), containing 50 mg of carbon, is used to detect the breakthrough. Breakthrough is defined as the detection of the sample compound at the outlet from the collection tube (as a percentage of the input concentration). In sampling practice, the breakthrough, according to NIOSH data, usually amounts to about 5%. The breakthrough volume (in litres) is the product of the sampling rate w and sampling time z,and is used for a more accurate evaluation of the efficiency of pollutant sorption. There is an approximate rule according to which there is no significant loss of the sample substance if the one-third backup section of the tube contains less than 10% of the total amount of collected substance. If this amounts reaches 25%, loss has probably occurred [81]. We discuss below the factors that affect the efficiency of concentration of pollutants on solid sorbents. The contribution of each factor depends on the specificity of the sorbent and the sorbate.

5.10.1. Properties of analysed compounds The influence of the properties of toxic pollutants on sorption efficiency has been discussed (11, 1951. However, it has been suggested [88] that it is sometimes easier to obtain experimental data on sorbed pollutants than to find unambiguous data in the published literature. 160 . 140 120 -

80 60 -

.

-

Arornotic *Halogenoted 0 Alcohol 'I Acelate Ketones A Alkone

w,

4.

/P #*

A"

*

t / e/

100

W -

Breakthrough time of organic compounds on activated carbon vs. boiling point Fig. 5.14

PI1

20

/ * /*

0-

0

72

5. Collection and pretreatment of samples for chromatographic analysis

Some idea of the influence exerted by the nature of analysed pollutants on their sorption can be derived from a graph of breakthrough volume (or time) against the physical properties of the compounds, obtained experimentally or taken from published data (Fig. 5.14). Some investigators have tried to obtain data o n the mechanism and magnitude of sorption of pollutants from chromatographic determination of the specific retention volume of the substances in question. These data, however, are not reliable, sometimes differing for different workers by 1-2 orders of magnitude [ll]. Measurement of the efficiency of pollutant sorption in collectors containing polymer sorbents has shown that, when compounds of a similar type (e. g., of one homologous series) are being concentrated, the most volatile components (e. g., dichloromethane, chloroform, carbon tetrachloride) have the smallest retention volumes [102].

5.10.2. Properties of the sorbent used Charcoal is a non-specific sorbent, adsorbing most organic and inorganic pollutants. Perrna, H2, Co) and CH4, however, are not adsorbed on charcoal. Ethylene, formnent gases (0 2NZ, aldehyde and other gases, whose boiling points lie between - 100 and O'C, are only partially adsorbed, whereas gases with boiling points above 0°C are readily adsorbed [147]. Mercury vapour is effectively absorbed by charcoal, but water vapour only slightly, although moisture reduces the sorption of other substances by charcoal. The physical properties of this popular adsorbent depend to a great extent on its source and subsequent treatment. For instance, for the recovery of vinyl chloride from air, all other sampling conditions being equal, the most effective sorbent is activated carbon produced by petroleum processing, and the efficiency of recovery of vinyl chloride and other organic pollutants from it with the help of CS2 depends significantly on the batch of carbon used. This follows from the data in Table 5.15,obtained for two batches (A und B) of coconut-based carbon (20-40 mesh) under the standard conditions recommended by NIOSH. The mechanism of sorption of pollutants on silica gel has been comprehensively studied [195]. The polarity of this adsorbent, resulting from the presence of silanol groups (SiOH) on its surface, causes a strong interaction of silica gel with polar compounds, e. g., amines. The degree of absorption of a compound on silica gel is a function of polarity, and there is a correlation between the heat of adsorption of a substance and the dipole moment (or dielectric constant) of its molecule, as illustrated by the data in Table 5.16. In the opinion of CRISP [ l l ] , some factors are especially favourable for the adsorption of pollutants on silica gel: (1) a strong interaction between polar groups and strong electron donors, such as oxygen- and nitrogen-containing compounds, ethers and amines; (2) the presence of multipolar groups, making possible multiple interactions in the adsorbate-adsorbent system; (3) a low temperature of adsorption; and (4) a high molecular weight of adsorbed pollutants.

Compound

Toluene Methyl ethyl ketone 2-Ethoxyethyl acetate 1-Butanol Vinyl chloride

Recovery (%) Carbon A

Carbon B

98 70

100 91 90 60 71

72 46

89

Table 5.15. Recovery of organic substances from carbon with hydrogen sulphide for two batches of coconut-based carbon [147]

73

5.10. Influence of sampling conditions on the efficiency of sorption of pollutants Table 5.16. Heats of adsorption for substances of different polarity [ l l ]

Compound

n-Octane Cyclohexene Benzene Methanol Water

Heat of adsorption (J/crnZ.lo-') 5

11 31 140 143

Dielectric constant at 20°C

Dipole moment D

1.95 2.02 2.28 33.62 80.40

0 0 0 1.70 1.85

The presence of factors opposite to the above reduces adsorption on silica gel to the minimum. Adsorption on silica gel decreases in the presence of alkali metal ions, but the decrease in adsorption efficiency can be especially drastic with the concurrent sorption of polar adsorbates, such as water vapour. When considering the sorbent selectivity and its effect on concentration efficiency one should simultaneously take into account the degree of desorption of pollutants after concentration. It is often necessary to make a compromise in order to obtain an acceptable sorption system. Some of these systems are listed in Table 5.17; the overlap between the sorption regions of the different sorbents makes it possible to choose the most appropriate sampling system to recover mixtures of substances. To a certain approximation, r for organic pollutants on charcoal depends to a considerable extent on the boiling point of a compound, and on polar sorbents (silica gel and alumina) on the polarity of the compound collected. When sorption is conducted on porous polymers, such as Tenax and Amberlite XAD-2, the most important factors are the boiling points of pollutants and the polarizability of their molecules. It should be remembered that the choice of the sorbent constitutes a compromise between the sorptive and the desorptive properties of a given compound. For instance, charcoal is an excellent sorbent for numerous compounds, but their desorption is difficult for most of them. A difficult problem is choosing a sorbent to recover from air traces of toxic substances such as methanol. Those used for this purpose are silica gel, collection tubes with a large amount of activated carbon (200-300 mg) or zeolite ZA, which proved to be selective towards methanol. None of these sorbents, however, is ideal, which is obviously explained by the structure and properties of the sample compound. It is of interest that, of all porous polymer sorbents (PPS), methanol is retained best by polar Porapak T and Chromosorb 107 [196]. The breakthrough volumes of methanol on these sorbents are 3.60 and 3.25 l/g, respectively, whereas for other compounds (hydrocarbons, ethers, aldehydes, amines and chlorinated hydrocarbons) they are 4-5 times higher. The new carbon-containing PTFE-based adsorbent [197] will probably find use for this purpose. Having a high specific surface area (about 800 m2/g) and considerable thermal stability (the usable upper temperature limit is 290"C), this adsorbent is suitable for the recovery of traces of polar compounds (methanol and ethanol) and hydrocarbons from air. The adsorption capacity of the PTFE-based adsorbent with respect to methanol is 1.184 mg/g, and for ethanol it is only slightly less, 1.012 mg/g. A trap containing such a filling will preferentially sorb alcohols, whereas for n-alkanes its sorptive capacity is no higher than 0.6-0.69 mg/g. The alcohols collected on this sorbent can be displaced from the collection tube by heating at 277°C. This adsorbent has been used successfully to determine traces of toxic organics (including methanol) in the atmosphere of experimental biological containers. A reasonable choice of an appropriate sorbent for collecting traces of toxic compounds from polluted air is thus determined by a large number of diverse factors, the most important of which are the properties of the compound in question, the nature of the sorbent and the

74

5 . Collection and pretreatment of samples for chromatographic analysis

Table 5.17. Sorption-desorption systems for analysis of pollutants [5, 12, 13, 811 Sorbent

Desorption solvent

Type of sample compound

Sorbent

Desorption solvent

Type of sample compound

Activated carbon

Carbon disulphide, dichloromethane, diethyl ether (1 % methanol or 5 % isopropanol sometimes added)

Methyl chloride, vinyl chloride, aliphatics and aromatics, acetates, ketones, alcohols, chlorinated aliphatics, etc.

Silica gel

Methanol, ethanol, diethyl ether, water

High-boiling and polar compounds: alcohols, phenols, chlorinated phenols, chlorinated benzenes, aliphatic and aromatic amines, acids, etc.

Activated alumina

Water, diethyl ether, methanol

High-boiling and polar compounds difficult to recover from silica gel: alcohols, glycols, ketones, aldehydes, amino alcohols, phenoxyacetic acids, etc.

Porous polymers Diethyl ether, (Amberlite XAD-2, Tenax, carbon disulphide, Chromosorb 101, 102, 106, alcohols Porapak N and Q , Polysorbs)

Wide range of compounds: phenols, acidic and basic organics, multi-functional organics, etc.

Carbon-containing sorbents: Saran carbon, Carbosieves B and S, Ambersorbs XE-340, XE-347, XE-348

Highly volatile compounds (often better than activated carbon): methyl chloride, vinyl chloride, chloroform, dimethyl ether, C,-C3 hydrocarbons, etc.

Diethyl ether, hexane

Chromatographic sorbents Diethyl ether, with grafted phase hexane, methanol

High-boiling compounds, pesticides, herbicides, polynuclear aromatics, etc.

Impregnated sorbents (covered with chemical reagents)

Compounds containing functional groups and compounds with specific chemical properties

~~

Thermal desorption

~

~~

possibility of the pollutants being completely recovered from the sorbent after concentration. Factors such as pretreatment (activation) and amount of the sorbent should also be taken into account. In the general case, doubling the amount of sorbent doubles the breakthrough volume [81, 1901. This technique has been used successfully to collect poorly sorbing compounds, e.g., methanol, on activated carbon. Sorbent activation affects very strongly the efficiency of absorption of pollutants. For instance, not only different activated carbons but individual batches of the same carbon differ i n sorption efficiency [196, 1981. With the exception of chemosorbents, the selectivity of solid sorbents in general is not high. As a rule, each of them is capable of sorbing from air a large number of pollutants, in the form of organic compound vapours, and some inorganic gases [68]. Also, while there

5.10. Influence of sampling conditions on the efficiency of sorption of pollutants

I5

exists an acceptable theory explaining the affinity of silica gel for polar compounds (polar silanol groups on the adsorbent surface) [195], the mechanism of the selective sorption of some compounds on carbon-containing and polymer sorbents is still not clear. It is difficult to speak of the specificity of sorbents when analysing multicomponent mixtures of toxic compounds in polluted air, as all of them, to a greater or lesser extent, sorb organic compounds of different classes. However, proceeding from experimental data, one can form some opinion of the affinity of sorbents for some specific compounds. For instance, Tenax collects chlorinated hydrocarbons, phenols and aromatics better than do other porous polymers [93, 1991, Porapak N retains more efficiently oxygen-containing organic substances and halogenated aliphatic hydrocarbons [93, 1011, and polar Porapaks P are used to collect the vapours of amines and insecticides [lo]. Chromosorb 103 is indispensable in recovering odorants of different nature from air [200]. Interesting data on the influence of the nature of the adsorbent on the sorption of pollutants have been obtained [201], a correlation being found between the volume of micropores in the carbon and the amount of styrene adsorbed. The adsorption efficiency proved to be independent of the surface area and structure of the adsorbent but to be caused by the steric conformity of styrene molecules with the carbon pores. Obviously, this kind of steric effect influences the anomalously high capacity with respect to methanol manifested by activated carbon obtained as a result of PTFE carbonization [197]. This adsorbent, with a specific surface area of 800 m2/g, has a sorption capacity for methanol many times greater than those of all the other types of activated carbons. On the other hand, the mechanism by means of which zeolite ZA entraps toxic methanol vapour, whereas other organic pollutants are not retained at all by this adsorbent, remains unclear [76]. With a decrease in pore size of the sorbent (at a constant mass in the collecting tube) the sorption efficiency increases, because of the increase in surface area [202]. The recovery of toxic pollutants from air, based on the results obtained by different investigators, is illustrated by the data presented in Tables 5.18 and 5.19. Carbon molecular sieves (carbosieves) efficiently retain hydrocarbons (including the lowboiling compounds) and chlorohydrocarbons, but they are inefficient for the adsorption of more polar compounds, e.g., ethanol and acetone, the extent of adsorption of which does not exceed 3-5% [203]. For porous polymers such data are much more scarce. Tenax GC absorbs benzene, toluene, ethyl acetate and n-butanol quantitatively, but retains only 68% of acetone, 80% of diethylamine and as little as 1%of ethanol [203], although for acetone the absorption can reach 97%, and the amounts of o-cresol and diphenylamine retained reach 90-93%. The Compound

Sorption efficiency (%)

Table 5.18. Sorption efficiency of sampling organic vapours with activated carbon (111

Benzene Toluene m-Xylene Ethanol n-Butanol Acetone Ethyl methyl ketone Dioxane Dichloromethane Chloroform Carbon tetrachloride Dichlorethylene Trichloroethylene

89-102 74-105 95- 96 90 66-100 80 61- 82 99 89- 91

100-103 95-109 96- 98 96-106

76 Compound Benzene Toluene m-Xylene n-Butanol Ethyl methyl ketone Ethyl acetate Dichloromethane Chloroform Carbon tetrachloride Dichloroethylene Trichloroethylene Aniline Nitrobenzene

5. Collection and pretreatment of samples for chromatographic analysis

Sorption efficiency

(%I

Table 5.19. Sorption efficiency of sampling organic vapours with activated silica gel [ll]

95-100 96-100 95-104 91 91 92- 93 100 82 85-100 100 94-100 91 93

sorption efficiency with respect to the vapours of epoxides, 0-lactones, sulphonates, nitrosamines, aldehydes, nitro compounds and chloroalkyl ethers on Tenax and Chromosorb is higher than 90% and remains invariable even with an air aspiration rate of 9 l h i n [121]. Amberlite XAD-2 recovers 83-97% of polychlorobiphenyls from air [110], and the sorption of pesticides by Chromosorb 102, Tenax and polyurethane foam reaches 78-100% [95, 1051. The limiting sorption capacity of porous polymer sorbents increases in the order XAD-1 = Tenax GC < Porapak P 5 XAD-2 < Porapak Q < XAD-7 < XAD-4 [109]. Almost all Table 5.20. Efficiency of the sorption of organic vapours on porous polymers [93] ~~

Sorbate

Concentration (mg/m’)

Sorbent

Sorption efficiency

(%I Acetic acid Benzene Benzyl chloride Bis(chloroethy1) ether n-Butanol Butylbenzyl phthalate Dimethylacetamide Dichloropropane Dichloropropene m-Xylene Dichloromethane Monochlorobenzene N-Nitroaniline PCB (Aroclor 1016) Styrene Toluene Trichloroethylene Triethylamine Phenol

10 10 1 5 50 5 10 15 15

100 200 15

1 5 100 100 100 25 5

Porapak N Porapak N Porapak N Porapak N Porapak N Tenax GC Tenax GC Porapak N Porapak N Porapak N Porapak N Porapak N Tenax GC Tenax GC Tenax GC Porapak N Porapak N Porapak N Tenax GC

95-102 95-105 83- 93 98-106 100-102 91- 96 102-104 94-101 88- 89 101-102 91- 98 102- 104 93- 96 91-114 92-101 100-101 93 98-101 93- 94

77

5.10. Influence of sampling conditions on the efficiency of sorption of pollutants Table 5.21. Breakthrough volumes for different compounds (1/100 mg) [192, 1931

Activated carbon Toluene Styrene Ethyl methyl ketone l,l,l-Trichloroethane Trichloroethylene Tetrachloroethylene Chlorobenzene Aniline Nitromethane

23.1 29.1 8.2 5.8 11.9 33.2

Tenax GC

2 5 30 98 4

solid sorbents, including the polymeric type, have the drawback of not fully recovering lowboiling compounds. A sample can be made more representative if the collector is injected with Porapak or Tenax in combination with the carbon-containing polymer Ambersorb XE-340, which effectively retains highly volatile pollutants. A collector for recovering mixtures ot toxic compounds of known composition from air was proposed by AVERILLand PURCELL[142]. They succeeded in achieving virtually complete absorption of organic pollutants of different molecular weight in a concentrator containing Tenax GC, Chromosorb 106 and Ambersorb XE340. Such a collector retaines minimal amounts of water vapour, nitrogen, oxygen and carbon dioxide. The efficiency of the recovery of traces of pollutants with porous polymers is demonstrated by the data in Table 5.20. As can be seen, porous polymer sorbents provide almost complete absorption of the airborne vapours of oxygen-, chlorine- and nitrogen-containing toxic substances, and also hydrocarbons and some high-boiling compounds. The accuracy of m-xylene and phenol sampling is higher for porous polymers than for activated carbon [93]. In the first instance it amounts to f 2 5 % of the true concentration at the 95% confidence level. There are certain correlations between the breakthrough volume and the sorption capacity of a collection tube containing an adsorbent [204]. Table 5.21 shows the results of measuring breakthrough volumes on activated carbon and Tenax GC. For some compounds the most frequently taken 10-1 volumen of air is too large for a collector containing only 100 mg of adsorbent. Also, the breakthrough volume of different chlorine-containing organics has been noted to vary when activated carbon is used [205]. Instead of the breakthrough volume, the sorption capacity of a sampler (sorbent) ist sometimes used. This can serve as an indicator showing whether the breakthrough occurs if the amount of concentrated compound approaches the collector tube capacity with respect to a given substance. The NIOSH data presented in Table 5.22. confirm this possibility.

Table 5.22 Sorption capacity of a sampler containing activated carbon for the recovery of organic vapours [ l l ]

Compound

Sorbent capacity (mgM

Benzene Toluene Styrene Ethyl methyl ketone Dichloromethane Tetrachloroethane 1,1,l-Trichloroethane Trichloroethylene

60 280 145 90 45 2 10 110 100

78

5 . Collection and pretreatment of samples for chromatographic analysis

Having established, based on the chromatographic characteristics of the retention of pollutants, the breakthrough volumes of organic compounds (alcohols, ketones, chlorohydrocarbons, etc.) at 20°C on porous polymers and Carbosphere, NAMIESNIK and KOZLOWSKI [206] proposed subdividing the sorbents into three gropus: (1) sorbents with a high adsorption capacity (Carbosphere, XAD-7, Chromosorb 102), (2) sorbents with a medium adsorption capacity (XAD-2, Chromosorb 106, Porapak R and S) and (3) sorbents with a low adsorption capacity (Tenax, Chromosorb 104 and 105).

5.10.3. Air flow-rate The rate of the flow of an air sample through a tube containing a sorbent should make the air volume necessary for quantitative determination pass through the sorbent as rapidly as possible. If the flow-rate is too high, however, the sorbent resistance increases, and the time of passage of pollutants through the tube can be too short for effective adsorption. The sampling rate usually varies from 100ml/min to several litres per minute. Very high rates, e. g., 800 l/min, are used for the aspiration of air through polyurethane foam blocks to determine very low concentrations of pesticides in ambient air. The influence of flow-rate on the efficiency of the sorption of pollutants varies with the sorbent. One of the most important characteristics of a concentration column, viz., its ineffective height (the length of the working layer), increases with increasing flow-rate of air through the sorbent [75]. Sometimes, on reaching the optimal sampling rate, no increase in the breakthrough volume is observed with a decrease in flow-rate [81]. In other instances, the sorption efficiency continuously increases. The maximum efficiency of adsorption of pollutants on coconut-based carbon is reached at a flow-rate of 100 ml/min, whereas for Saran carbon it increases continuously (Fig. 5.15). A very important condition for the comparison of the results for the sorption of pollutants obtained with tubes of different size is the linearity of the air flow-rate under the optimal sampling conditions. In the general case, the adsorption capacity of a tube containing carbon increases with decreasing linear air flow-rate [202].

5.10.4. Air sample volume An adsorption column acts as a chromatographic column, and under the influence of an air flow the pollutants will move along the column. The volume of air that has passed through the column when the sorbed pollutants start to leave it corresponds to the breakthrough vol-

5

10

15

20

Breakthrough Volume [Liters]

25

Fig. 5.15. Sorption efficiency of vinyl chloride pollutants on activated carbon vs. air aspiration rate [81]. 1 Coconut-based carbon; 2 Saran carbon

19

5.10. Influence of sampling conditions on the efficiency of sorption of pollutants Table 5.23 Air sample volumes for organic solvent vapours when tubes containing actived carbon as recommended by NIOSH are used [I471

Compound

Acetone Benzene Carbon tetrachloride Chloroform Dichloromethane Ethylene dichloride Methyl ethyl ketone Tetrachloroethylene 1,1,2-Trichloroethane Toluene

Recommended air sample volume (I) minimum

maximum

0.5 0.5 10 0.5 0.5 1

7.7 55 60 13 3.8 12 13 25 97 22

0.5 1 10 0.5

ume. This volume is a function of the concentration and of the nature of the adsorbed compound and the adsorbent, and volatile compounds usually have a very small breakthrough volume. In other words, the maximum sample volume, limited by the breakthrough, depends on the concentration of the substance recovered from air and the sorption capacity of the tube containing the sorbent. In the NIOSH standard procedures for analysing organic solvent vapours, a system is assumed to be capable of measuring concentrations exceeding five times the NIOSH standards. Proceeding from this condition, air sample volumes were determined for deleterious substances, shown in Tables 5.23 and 5.24. Humidity can reduce the sorption efficiency (the sorption capacity of the sorbent can be halved). Knowing the sampling time, one can calculate the rate of air aspiration through a concentrator, using the data in Tables 5.23 and 5.24, but it cannot be higher than 1 Vmin [147]. Ideally, the air sample volume must be smaller than the breakthrough volume of the most volatile compounds in the sample. When this is impossible, one can introduce a correction based on the previously determined pollutants sorption efficiency. The air sample volume is usually several litres, and most often it is approximately 10 1. When determining very low pollutant concentrations in the atmosphere, much greater air volumes are usually taken so as to accumulate in the tube an amount of substance sufficient for reliable determination.

Compound

Table 5.24. Air sample volumes necessary for determining aromatic amines with the help of NIOSH tubes') [147]

Aniline N,N-Dimethylaniline o-Toluidine 2,4-Xylidine 0-Anisidine p-Anisidine p-Nitroaniline

Recommended air sample volume (I) minimum

maximum

5 5 5 5 250 250 80

15 190 300 430 35 800 33 000 12 100

*) NIOSH tubes contain three silica gel layers; two of 150 mg each and one of 700 mg.

80

5 . Collection and pretreatment of samples for chromatographic analysis

5.10.5. Concentration temperature Although adsorption increases with decreasing temperature, it is not always convenient in practice to use low temperatures directly at the sampling site, and often there is also a danger of possible freezing of water vapour and even carbon dioxide, which will render the sampler inoperative. Therefore, the principal (most toxic) sample components, with the exception of highly volatile compounds, have to be sorbed with a high efficiency at ambient temperature. Cooled samplers containing a sorbent have been used by numerous investigators, the degree of cooling varying widely, depending on the composition and boiling point of the pollutants. To recover low hydrocarbon concentrations from the atmosphere the temperature can vary from -60 to -120°C. With an increase in temperature the breakthrough volume decreases. For activated carbon every 10°C rise in temperature in samplers containing a sorbent will result in a decrease in the breakthrough volume of approximately 1- 10%[81]. Methods of determining the reliability of sorbents, and data on the dependence of sample storage time on air temperature and humidity, have been published [ l l , 121. During sample storage, the compounds sorbed in the front section of the tube migrate to the “back-up” section. For highly volatile compounds, after equilibrium has been established, the latter section may contain up to 33% of the sampled matter [196]. This amount can be decreased by cooling the sample or separating the “back-up” and the front sections [Ill.

5.10.6. Air humidity An increase in air humidity decreases the breakthrough volume [109, 2021. This effect is caused by the properties of the sorbent and the sorbate. Reducing the volume and flow-rate of the air passing through the sampler reduces this influence to some extent [192]. Although only a small amount of water is sorbed on carbon, a high percentage of water molecules in the sample strongly affects the sorption-desorption equilibrium process of pollutants and sometimes increases the breakthrough by as much as 50%.An even greater effect is observed for very low sorbate concentrations when the humidity of the air sample is high. Polar sorbents, such as silica gel and alumina, absorb water vapour much more intensively than most organic compounds. This not only reduces continuously the effective length of the sorption tube, but also leads to desorption of the already sorbed compounds. The latter effect should be considered in the light of coadsorption of the pollutants.

5.10.7.

Coadsorption

The regularities characterizing the dynamic sorption of pollutants on solid sorbents have been established for individual air pollutants. However, if two or more compounds are being sorbed, the substances held more strongly on the sorbent surface will displace those held less strongly, which will lead to a decrease in the effective length of the sorbent bed, i. e., reduce the sorption capacity of the tube (Fig. 5.16). Variations in the breakthrough volume caused by et al. [207]. The effect the presence of other pollutants in the mixture was studied by BERTONI of a reduction of sorbent activity is manifested most strongly for a compound with an “intermediate” breakthrough volume (e. g., n-pentane), caused by the concurrent sorption of diethyl ether and n-hexane. The first of these compounds changes the sorption of n-pentane to a lesser extent, as it is eluted before n-hexane. For given compounds at a given concentration one can calculate the content of a certain compound on the sorbent surface, knowing the breakthrough volumes of the individual substances and those observed in the sorption of the mixture.

5.10. Influence of sampling conditions on the efficiency of sorption of pollutants

Fig. 5.16. Effective sorbent bed length vs. coadsorption of accompanying impurities [81]

81

213 I/ 3 Length of Sorbent Bed

On polar sorbents (silica gel, alumina), compounds with the highest dielectric constant and dipole moment are held most strongly. Of non-polar compounds, the more strongly absorbed are those with a high boiling point or a large molecular volume.

5.10.8. Concentration in the flow Breakthrough occurs sooner at higher concentrations of substances. To measure the breakadthrough time when using charcoal, one can use an equation similar to the FREUNDLICH sorption isotherm equation [63]: log re = log a + b log C

(5.7)

where tB is the breakthrough time, C is the sorbate concentration and a and b are temperature-dependent constants. A plot of breakthrough time against pollutant concentration is a straight line (Fig. 5.17), from which it follows that, when the pollutants are concentrated from humid air, the breakthrough volume decreases and the sorption efficiency with respect to the target components is reduced.

2: 0, 0 3

c

f -x 01 0

Log & = l o g o + b l o g C

& Slope = b 0,

-4 0

Fig. 5.17. Logarithms of breakthrough time vs. Dollutant concentration for different air humidiiies [Sl]. Relative humidity: (I) 20%; (2) 80% 7

Berezkin, Gas Chrom.-BE

Intercept = Log a

Log Concentration c (Specified Flow] or Log MOSS Rote [ m o l l m i n ]

82

5. Collection and pretreatment of samples for chromatographic analysis

, , 20 b.,

Fig. 5.18. Breakthrough volume on a 50-cm column containing Carbopak B vs. toxic pollutants concentration in the atmosphere at 20 "C [207]

Some information on the mechanism of the sorption of pollutants can be obtained from the dependences of breakthrough volumes on pollutant concentration (Fig. 5.18) on Carbopak B. As can be seen, Carbopak B retains better compounds with a high boiling point and a high molecular weight, e. g., n-heptane; polar substances, e. g., diethyl ether, are retained worst. A large breakthrough volume of CS2 vapour on Carbosieve B has made it possible to use carbon molecular sieves to concentrate very small amounts of carbon disulphide and to determine it at the ppt level [86]. In the opinion of BERTONIet al. [207], air humidity does not affect the efficiency of sorption of pollutants on hydrophobic sorbents, and the sorption capacity of carbon-containing sorbents is much higher than that of porous polymers. For instance, the breakthrough volume of n-hexane on Tenax GC is approximately 3.5 times lower than that on carbopak B. At the same time, the linear dependence between n-hexane breakthrough volume and its concentration on Carbopak B does not change with increase in the air humidity from 20 to 80%. A polar sorbent, e. g., acetone, is most strongly held on polar porous polymers (Chromosorb 107,

5.11. Desorption of pollutants from the sorbent

83

Porapak N and T), and the smallest breakthrough volume is observed for acetone when it is concentrated on non-polar porous polymers (Porapak Q and Chromosorb 101 and 103) [196, 2061. A study of breakthrough volumes in a concentrator containing Tenax of 90 organic compounds (halogenated hydrocarbons, aliphatic amines, sulphur- and oxygen-containing comet al. pounds, C1-Cl0 hydrocarbons) present in air at ppm concentrations allowed KAWATA [208] to subdivide all the sample substances, proceeding from their structural features, into 1 1 groups (e.g., aliphatic amines, sulphur compounds, aldehydes, ketones, branched alkanes, etc.). For all the compounds and individual groups the linearity of the relationships between the breakthrough volume and the boiling point ( i = 0.833), 1/T ( i = 0.846), molecular weight ( F = 0.637) and n the number of carbon atoms in the molecule of a compound (7 = 0.527), where F is the mean correlation coefficient for all the groups, was established. Tabular data of this kind serve as a basis for establishing the efficiency of the concentration of pollutants in the analysis of pollutants of different composition.

5.11.

Desorption of pollutants from the sorbent

Desorption is the most important aspect of the sorption-desorption process of recovering pollutants from air, determining the quality of sampling. The most widely used methods of desorption of pollutants from the collection tube are as follows: (1) solvent extraction, (2) Soxhlet extraction, (3) thermal desorption, (4) vacuum desorption and (5) steam desorption.

5.1 1.1. Solvent extraction This method is very simple to perform: the adsorbent is poured out of the tube into a measured volume of the solvent and left in a stoppered vessel for the time necessary for the pollutants to be completely extracted, followed by analysis of the extract chromatographically, spectroscopically or by any other suitable method. The disadvantage of solvent extraction is dilution of the sample. In addition, the theoretical desorption efficiency cannot be higher than loo%, although it often approaches this value [209]. Solvent extraction is widely used to recover organic and inorganic pollutants from sorbents. In particular, this method of extraction is recommended for charcoal in the gas chromatographic analysis of pollutants. A 100-mg amount of adsorbent is placed in a vessel containing 0.5 ml of carbon disulphide and left there for 30 min [192]. When using a larger mass of charcoal it is recommended to shake the mixture for 30 min or to leave it for 3 h so as to achieve a more complete recovery of pollutants, although the bulk of the concentrated substance is extracted within the first 15 min after addition of the solvent. Carbon disulphide is used more frequently than other extractants extracting almost 100% of organic pollutants from charcoal and not being recorded by a flame ionization detector. A serious shortcoming of this solvent is its high toxicity. Even more efficient for the extraction of pollutants from solvents is a mixture of carbon disulphide and methanol (95:5), although it has been found [81], after gas chromatographic analysis of the extract, that desorption for 4 h results in the appearance of new compounds (sulphides, mercaptans, polythioethers) in the extract, contaminating the sample. Benzyl alcohol, which extracts practically 100%of numerous organic pollutants [210], has also proved to be an efficient extractant. It is difficult to obtain this solvent free from impurities, however, even after distillation on high-efficiency columns. It is not always possible to solve successfully the problem of purifying the solvents used for the extraction of pollutants. One of the principal requirements for a desorbent is the absence of impurities that interfere in the analysis. Sometimes, even after thorough purification, solvents (cyclohexane, dichloro7'

84 Acrylonitrile Ally1 glycidyl ether n-Amy1 acetate Benzene n-Butanol n-Butyl acetate n-Butyl glycidyl ether Chlorobenzene Cyclohexane Cyclohexanone o-Dichlorobenzene p-Dichlorobenzene Diethyl ether N.N-Dimethylaniline Epichlorohydrin 2-Ethoxyethyl acetate Ethyl acetate Ethyl benzene Ethyl butyl ketone Furfurol

Heptane Hexane Isoamyl acetate Isobutanol

5. Collection and pretreatment of samples for chromatographic analysis

Isooctane Isophorone Isopropyl acetate Isopropyl glycidyl ether 2,6-Lutidine Methyl acetate Methyl acrylate Dichloromethane Methyl ethyl ketone Methyl isobutyl ketone Methyl methacry late Methyl n-butyl ketone a-Methylstyrene p-Methylstyrene n-Octane 3-Octanone Pentane 2-Pentanone a-Pinene n-Propyl acetate 1,1,2,2-Tetrachloroethane Tetrahydrofuran Toluene

Table 5.25. Extraction solvents for the recovery of organic pollutants from sorbents [212]

Trichlorotrifluoroethane

(Freon 113) Isobutyl acetate methane, methanol, etc.) contain scores of impurities (phthalates, alkanes, chlorohydrocarbons, etc.) whose traces interfere in the analysis, e. g., in the gas chromatographic determination of pesticides [211]. However, some of the fairly efficient solvents, e. g., chlorobenzene, can be purified by simple distillation with a dephlegmator. The U.S. National Institute of Occupational Safety and Health (NIOSH) has published a list of solvent desorbents that extract more than 80% of organic pollutants from charcoal (Table 5.25). When choosing the solvent, one should take into account all the above-mentioned factors, in addition to the selectivity of the solvent and its conformity to a certain sorption-desorption system (see Table 5.18). Thus, acetone extracts pollutants from Gas-Chrom G, methyl chloride, hexane, light petroleum, diethyl ether and KOH solution from XAD-2 resin [112], xylene from Tenax, and diethyl ether and carbon disulphide, with the use of ultrasound [lll], extract polychlorobiphenyls, naphthalenes and phenols from Amberlite XAD-2. Water is often used to extract water-soluble compounds from silica gel or zeolites [132, 2131 with a possible degree of extraction reaching 97% [213]. Other polar solvents or their mixtures with water, e.g., dimethyl sulphoxide and acetone [127, 1291, are also used for this purpose, in addition to carbon disulphide and cumene mixtures with water, KOH solution, methanol, chloroform and other organic solvents (hydrocarbons, chlorohydrocarbons and aromatics) [68]. Extraction with water is also used to recover pollutants concentrated on alumina [l]. In the detection of volatile nitrocellulose lacquer components in air (the foreitry, wood working and furniture-making industries) the pollutants are desorbed with superheated vapour [214]. The air sample is aspirated at a flow-rate of 0.5-1.0 Vmin through a tube (0.7 m x 1.5 mm I.D.) filled with CKT-6A activated charcoal (1-1.5 mm fraction), then the concentrated pollutants are desorbed with superheated vapour. A single-stage vapour treatment al-

5.11. Desorption of pollutants from the sorbent

85

Fig. 5.19. Stages of air sample conditioning for headspace analysis [214]. (a) Injection of pure water into the sorbent tube; (b) desorption of the analysed impurities using water vapour and accumulation of condensate in a bottle. 1 Gas syringe containing pure water (5-ml capacity); 2 coupling nut with soldered-in steel needle; 3 demountable electric furnace

lows the recovery from a concentrator of 96-100% of n-butanol, ethyl and butyl acetate and 80-85% of toluene and m- and p-xylene at 1 : l O TLV concentration. The condensate is accumulated in a presealed and preevacuated bottle (Fig. 5.19) for further headspace analysis. After the pollutants have been chromatographed on a column (4 m x 3 mm I.D.) containing a mixed stationary liquid phase (silicone-polyethylene glycol adipate, 3 : l ) at 100°C using a flame ionization detector. The nitrocellulose components can be analysed at the 200 pg level with a relative standard deviation of 0.20-0.25. A chromatographic system with an attachment for headspace analysis is shown in Fig.5.20. In conclusion, it should be noted that in order to increase the efficiency of extraction of pollutants it is expedient in a number of instances to pass the solvent flow directly through the concentration tube with a sorbent in the direction opposite to the flow of the air sample and to use as the solvent substances that displace target pollutants on a given sorbent.

5.11.2. Extraction in Soxhlet apparatus and ultrasonic field This process is a variation of solvent extraction, most frequently used to recover pollutants from sorbents and filters designed to trap aerosols, solid particles and vapours of high-molecular-weight and high-boiling toxic substances, such as polycyclic aromatic hydrocarbons (PAHs), polychlorobiphenyls (PCBs), pesticides, etc. This method was used to extract the

Fig. 5.20. System and attachment for headspace analysis with pneumatic dispensation of an equilibrated gas phase [214]. f Circulating liquid ultrathermostat; 2 six-way heated cock; 3 line for bottle gas pumping; 4 standard pressure gauge; 5 chromatograph injector; 6 carrier gas line; 7 FID; 8 chromatographic column; 9 jacketed sample-containing bottle; 10 metal

86

5 . Collection and pretreatment of samples for chromatographic analysis

sample from Chromosorb 102, polyurethane foam and a combination of a filter and a tube containing activated carbon [ l , 11, 121. A disadvantage of the method is the long duration of the extraction process necessary for the complete recovery of PAHs from solid carbon black particles or PCBs from polyurethane foam, etc. The process of Soxhlet extraction can require 10-15 h and more [215]. The minimum extraction time for the most efficient solvents (benzene, methanol and carbon disulphide), necessary for the subsequent determination of most organics, is about 8 h. Methanol makes it possible to desorb phthalic acid esters and acidic compounds eficiently from collectors and filters; cyclohexane and cyclopentane are used to extract aliphatic hydrocarbons; benzene, methanol and carbon disulphide are especially eficient in the extraction of PAHs. Herbicides and PCBs are extracted completely (recovery efficiency 85- 100%)from concentrators containing polyurethane foam or XAD-2 resin using light petroleum (extraction for 24 h) or a mixture of acetone with n-hexane. In the former instance the degree of sample extraction can reach 99% [112, 1171. The sample extraction process is accelerated considerably when ultrasound is used, e.g., in recovering PCBs, phenols and naphthalenes from a trap containing 150mg of XAD-2 or XAD-7. When 3 ml of carbon disulphide is used as the solvent the whole extraction operation takes 30 min [ l l l ] . This method is even more efficient for the extraction of PAHs from atmospheric dust [216]. The sample (a Petryanov filter cut into small particles) is placed in a 100-150-ml steel cylinder and benzene is added in an amount of 25 ml per perchlorovinyl filters. The cylinder is placed on an electrodynamic shaker operating in the 8-12 Hz frequency range. Almost complete extraction of PAHs (95-100%) is achieved in 15-30 min. A similar device can be also used to speed up the extraction of pesticides and PCBs from polyurethane foam, Amberlite or a combined trap consisting of a filter and polyurethane.

5.11.3.

Thermal desorption

The advantage of thermal desorption over the extraction method for the recovery of pollutants from a sorbent is the absence of sample dilution. This allows the detection limit of pollutants in air to be improved almost 200-fold compared with that using sample extraction with carbon disulphide, where out of the whole extract volume of 1 ml an aliquot of 5 pl is taken for analysis [l]. The method is frequently used to recover pollutants from collectors containing chromatographic sorbents and porous polymers, but very seldom to extract pollutants from charcoal and other adsorbents with a highly developed surface. In the latter instance thermal desorption is greatly hindered (the temperature needed is 100-450°C), and its efficiency is low. A shortcoming of adsorbents of the charcoal type is the poor desorption kinetics [109], and all carbon-containing adsorbents (activated carbons, graphitized black and carbon molecular sieves) are characterized by a high reactivity, especially at elevated temperatures. Moreover, a high desorption temperature increases to a considerable extent the probability of desorbed pollutants interacting not only with the sorbent but also with each other (decomposition, polymerization, condensation and other reactions), with the formation of new substances that were absent in the initial sample, which distorts the results of analysis. There is no such disadvantage in the method of very fast sample desorption from charcoal with the help of a pyrolyser rapidly heated to the Curie point [217]. This sampling technique consists in collecting the pollutants (hydrocarbons, alcohols, ethers, ketones, chlorohydrocarbons) in a concentration trap made of metal wire (pyrolyser) in the form of a loop with a small recess for the sorbent. Several activated carbon pellets are placed in the recess, and the wire is kept for a few minutes in the atmosphere of the air sample. The vapours concentrated on the charcoal are desorbed for 1-10s at a temperature of up to 480°C. It has been established that the degree of thermal desorption in pyrolysers at the Curie point is not less than

5.11. Desorption of pollutants from the sorbent

To

87

Time

Fig. 5.21. Typical concentration profiles after thermal desorption [147]. To time of the beginning of carrier gas passage (after preheating the concentration tube). a) Variation in concentration of desorbed pollutants in the gas flow leaving the tube, where a small amount of sample substance is collected in the initial portion of the tube (direction of air flow during concentration and of the carrier gas during desorption coincide); b) variation of pollutant concentration in the gas flow during desorption, when the concentrated pollutants are distributed along the whole lenghth of the tube (direction of air flow during concentration and of the carrier gas during desorption coincide); c) variation of pollutant concentration in the gas flow during desorption (direction of air flow during concentration is the opposite of that of the carrier gas during desorption)

90% for all organics, with no pyrolysis products being detected. The method is recommended for the analysis of substances with a high vapour pressure. Some workers used thermal desorption from silica gel and zeolite 5A. The temperature limit of desorption in this instance is from 100 to 450°C. It is possible that desorption from a trap containing 5A molecular sieves at reduced pressure and a temperature of 250-300°C will not lead to sample decomposition, but even brief heating of zeolite up to 250°C or of a trap containing silica gel up to 450°C at normal pressure is highly undesirable. Thermal desorption from silica gel can only be recommended for recovering from the adsorbent pollutants consisting of C,-C, hydrocarbons at a temperature not higher than 1OO"C, when the probability of side-reactions is low. With the help of thermal desorption one can achieve a good recovery of light pollutants from charcoal, e. g., 93-100% for vinyl chloride, 70-99% for methyl chloride and 95-100% for vinylidene chloride [205]. The recovery of pollutants from a trap containing Tenax is achieved by heating it to 250-300°C at a low carrier gas (nitrogen or helium) flow-rate or by using programmed heating to displace the pollutants. Thermal desorption is also used to recover compounds sorbed on Porapak P and Q, Chromosorb 101 and other polymer sorbents. The efficiency of desorption of organic compounds from Chromosorb 101 and activated carbon can reach 85%, and for Tenax it exceeds 90%. The duration of heating has been found to depend on the type of sorbent and to increase in the order activated carbon < Porasil with a chemically bonded stationary phase < Chromosorb 104 < Tenax GC < Chromosorb 101 [121]. The upper temperature limit of the use of these sorbents should be taken into account in this instance; for Carbopak C, Tenax and Porapak Q and T it is 500, 375, 250 and 190"C, respectively [102]. The mechanism of thermal desorption resembles that of elution from a chromatographic column. Typical profiles of desorbed pollutants (Fig. 5.21) are determined by the overall amount of sorbate and its distribution in the adsorbent (inlet or outlet sections of the concentration tube). It is important to provide conditions for the thermal desorption of pollutants from a collector containing a sorbent that would make it possible for the concentrated pollutants to leave the tube as soon as possible. However, as thermal desorption does not take place instantaneously, the sample compounds enter the column in very wide zones, which reduces consider-

88

5 . Collection and pretreatment of samples for chromatographic analysis

ably the quality of the separation of components and can even lead to the loss of the peaks of some substances on the chromatograms. This explains the fact that the list of organic compounds detected in the air of Paris [218] starts with toluene and octane; the peaks of lighter hydrocarbons, with boiling points below lOO"C, were strongly diffused and blended almost completely with the background signal. The negative effects accompanying thermal desorption can be avoided by intermediate concentration of the substances eluted with a gas flow in a cooled precolumn or in the initial section of the chromatographic column [68-701. Thermal desorption is technically feasible with the use of a gas chromatograph evaporator [219] or special attachments [69,220]. In the former instance the concentration tube is placed inside the evaporator instead of a removable insert serving to reduce the evaporation volume. The organics eluted during 15 min with a carrier gas flow (20 ml1min) from a 110-cm sorption tube of O.D. 10 mm, placed in an evaporator heated to 290"C, were retained in a coiled metal capillary (1.8 m x 0.5 mm I.D.), cooled with solid carbon dioxide or liquid nitrogen, whose internal surface was covered with a layer of stationary phase. Instead of a capillary tube one can apply a cryogenic method to recover pollutants directly in the initial section of the chromatographic (analytical) column. To avoid the small amounts of water vapour sorbed by Tenax freezing in the capillary and forming an ice plug, it is recommended [69, 2201 that several loops of the coil are passed outside the Dewar flask containing the cooling agent, so as to create a temperature gradient and condense water in this portion of the capillary without being frozen. After the desorption of pollutants from the trap, the capillary is joined to an analytical column of the same diameter and containing the same stationary phase for subsequent desorption of the pollutants captured in the capillary in a chromatographic column. Also convenient in operation is the desorption attachment used to determine atmospheric pollutants [69]. The principal unit of this attachment is an oven heated by an electric coil and placed in a protective jacket. The sorption tube is placed inside the oven in such a way that the carrier gas passes through it in the opposite direction to the air flow during the sampling. The attachment includes a two-position six-way heated gas-cock, making it possible to direct the carrier gas flow through a capillary designed to recover the pollutants eluted from the sorbent, or by-passing it. This design allows the duration of the analytical cycle to be reduced. When an inert gas flow is passed through a sorption tube heated to the necessary temperature, the minimum desorption duration is defined by the relationship [69] where VBfis the specific retention volume of the sample compound sorbed most strongly at the adsorption temperature t, m is the sorbent mass (9) and F is the carrier gas volume velocity (ml1min). The V,, values are calculated from the equation vgf

=

V, 273 m (273 + t )

(5.9)

where V, is the true retention volume (ml) determined experimentally by the chromatography of known compounds on a given sorbent at temperature t ("C). Thermal desorption is especially efficient when techniques of additional concentration of pollutants are used to improve the detection limit [219]. The air containing organic pollutants is first passed through a trap (10 cm X 4 mm) containing Tenax (80-100 mesh); the concentrated pollutants are desorbed at 230°C and carrier gas is blown through the tube for 7 min; the pollutants are collected in an intermediate concentrator (85 mm x 6 mm) containing 5% OV-101 silicone on Chromosorb W, located in a chromatograph evaporator, and cooled to - 150°C. After this the temperature in the evaporator is rapidly raised to the necessary level, and the organic pollutants are separated on a quartz capillary column (30m x 0.33 mm I.D.) of DB-1, with the column temperature programmed from 0 to 250°C at 2 K1 min. This technique (with the use of a flame ionization detector) makes it possible to im-

5.11. Desorption of pollutants from the sorbent

89

Fig. 5.22. Chromatogram of air pollutant (hydrocarbons) separation obtained after recovery of the pollutants from a concentrator by thermal desorption (a) and carbon sulphide extraction (b) [222] prove the detection limit 500- 1000-fold in comparison with the method of extractive desorption of pollutants. Comparison of the results of liquid extraction and thermal desorption for recovering organics from atmospheric aerosols trapped on PTFE filters has shown [221] that the direct gas chromatographic determination of thermal desorption products provides a 100-fold higher sensitivity. In liquid extraction numerous organic pollutants may be absent on the chromatogram. In desorption, a ,piece of the filter is placed for 5 min in a desorption tube (5 cm x 3 mm) heated at 350°C and blown through with a current of helium (30 mllmin). The organic compounds desorbed by heating enter a capillary column (50 m X 0.25 mm I.D.) containing SE-54, where they are separated with temperature programming from -30 to 70°C at 20 K/min and from 70 to 250°C (30 mm) at 4 K/min. With the use of a flame ionization detector the detection limit is 0.01-1 pg/m3.

90

5. Collection and pretreatment of samples for chromatographic analysis

Compounds identified

GC-MS GC-MS (thermal (extraction) destruction)

Ethane Dichloromethane Diethyl ether Ethyl chloride 2-Butene Acrolein Methyl acetate b- Propiolactone Acetone Furan Methyl vinyl ether tert.-Butylamine Ethyl acetate Ethanol Acetic acid 1-Chlor-2-methylpropane Benzene Butyl chloride Methyl propyl ether Crotonaldehyde 1-Methylbutanol Methylacrolein Propyl acetate Toluene Isoamyl acetate Isobutanoic acid 3-Chloro-2-butanol 2,3-Butanediol Heptanal Chlorobenzene Valeric acid Dimethyl phthalate

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

Table 5.26. Identification of volatile compounds resulting from thermal destruction of PVC ~ 5 1

+ + + + + + + +

+ + + + + + + + + + +

Similar results are obtained by comparing two methods [222], viz., hydrocarbon desorption from a sorbent (headspace analysis) and liquid extraction using carbon disulphide (Fig. 5.22). It can be seen that the thermal desorption variant (a) is to be preferred, especially for light hydrocarbons, which are not so readily extracted from a trap as heavy hydrocarbons. At the same time, the recovery efficiencies for the heavier impurities [right-hand sides of chromatograms (a) and (b) in Fig. 5.221 are almost identical. Similar results are obtained with different sorbents. Thus it was shown [223] that in the concentration of trichloroethylene on activated charcoal and Porapak Q , subsequent desorption of the species from the Porapak sorbent (at 180°C for 5 min) yields a better recovery of trichloroethylene than its extraction from activated charcoal with carbon disulphide. The above considerations favour the selection of thermal desorption, which is widely used in the analysis of air pollutants, involving their concentration in adsorbent-filled traps [ 12 11 or column sorbents [l,2241. However, a risk of sample decomposition with high-temperature desorption of pollutants should be kept in mind.

5.12. Influence of experimental conditions

91

This can be exemplified [225] by the results of the identification of volatile compounds arising from thermal destruction of PVC (artifical leather production) by gas chromatography-mass spectrometry and an reactive-sorption concentration with gas chromatography combination (see Chapter 6). It is evident from Table 5.26 that the thermal desorption of the impurities concentrated on Tenax at a desorption temperature of 200°C and analysis of the concentrate on a capillary column containing OV-101 with temperature programming between 50 and 250°C by gas chromatography, GC-MS permits the identification of over 30 compounds. It is remarkable that many of these compounds are absent from the chromatographic spectrum produced by the same method but following their extraction with toluene. This is obviously associated with the thermal destruction of the concentrated sample compounds at a high desorption temperature. If the desorption temperature is fairly high (15O-25O0C), no methods for the desorption of impurities, e. g., using a high-frequency generator [226], microwave desorber [227], pulsed heated carrier gas [228] or discrete gas extraction [229], can fully ensure the integrity of the sample composition unless special control tests are conducted for detecting such a probable decomposition. Moreover, thermal desorption is a time-consuming process (requiring between 10-15 s and 1-5 min), which generally reduces the efficiency of the subsequent chromatographic decomposition of impurities. To eliminate this drawback, during thermal desorption the abovementioned two-stage impurity concentration technique is employed. In so doing, a repeated concentration of the impurites recovered from the sorbent in a cryogenic trap allows a sample to be injected into the chromatographic column in the form of a more compact “plug”, compared with the thermal desorption technique, and narrower chromatographic zones to be obtained.

5.12.

Influence of experimental conditions on the completeness of recovery of pollutants

The main reason for a low recovery of pollutants often lies in a low efficiency of the desorption of concentrated pollutants from the sorbent. The efficiency of adsorption and recovery can be approximately predicted from data published in the literature of from the results of testing compounds similar to those being analysed. However, until the mechanism of desorption of pollutants and the factors that affect it have been studied in sufficient detail, certain difficulties can arise in sampling. The recovery of naphthalene and diphenyl from charcoal proves to be much lower (Table 5.27) than predicted on the basis of the data for the recovery of other aromatics. A stronger retention of naphthalene and diphenyl molecules by the sorbent surface can obviously be explained by favourable steric effects. Desorption in the order 0- (84%) > m(81%) > p - methyldiphenyl (76%)confirms this mechanism. On the other hand, in the light of the above data, the recovery of benzene should be very low, which is not observed in this instance. The most general technique for determining the efficiency of the desorption of pollutants from a sorbent consists in injecting the investigated compound or its solution directly into the solid sorbent [12]. The mixture is allowed to remain overnight, then the pollutants are desorbed and analysed. This is not an ideal method. More accurate results can be obtained by aspirating the air sample with an exactly known concentration of toxic substances through a sorbent, followed by desorption and analysis of the pollutants. Gases and highly volatile substances are usually inserted as a mixture in air or in nitrogen from a plastic bag (Saran film) or some other container. The percentage recovery is determined in the same way, introducing

92

5. Collection and pretreatment of samples for chromatographic analysis

Compound

Desorption efficiency (%)

Benzene Toluene Naphthalene Diphenyl Diphenyl oxide Diphenylmethane p-Methyldiphenyl rn-Methyldiphenyl o-Methyldiphenyl

after 1 h

after 16 h

98 98 43 55 93 93 16 81 84

98 98 40 53 98 91 80 86 91

Table 5.21. Efficiency of desorption of organic compounds [81]. Coconut-based carbon, 0.5 g; carbon disulphide extractant, 0.5 ml; pollutants concentration, 0.2-0.3 mg per 5 ml

a correction for the flow-rate of air aspirated through the sorbent. Even more accurate data are obtained when all these effects in the workplace or atmospheric air are taken into account. The desorption efficiency is the percentage of substance desorbed with respect to the overall amount of the concentrated pollutants in the absence of breakthrough. Desorption efficiency is the most important characteristic of a sorption-desorption system. Some of the most popular systems are listed in Table 5.17. It has been shown [209] that for some types of compounds sorbed on charcoal the system used can be regarded as in equilibrium. Most chemical compounds in a closed system consisting of a solvent and a solid sorbent, at a constant temperature, are in dynamic equilibrium between the sorbent and the solvent. Assuming that such an equilibrium exists, one can derive the relationship between the desorption efficiency, D, and the equilibrium constant, K . Dividing the concentration of the toxic compound found in the desorbing solvent, c, by D, we can determine the total mass of the investigated toxic compound, M , in the system:

M

= c MrlD

(5.10)

M can also be expressed as the sum of the mass of a compound in solution and its mass in an adsorbent, Mads: (5.11) M = c Msol + Mads Msorb where Msol is the mass of solution (solvent-extractant), Msorb is the mass of sorbent. From eqns. (5.10) and (5.11) one can obtain the following relationships: CMso~ ID = C Msol

Mads Msorb

Mads =

c Msol ID Msorb - c MSOI lMsorb

Mads =

c Msol lMsorb [(l/D)

(5.12)

- l1.

The equilibrium constant for the distribution of a compound between the two phases (liquid and solid) is expressed as the ratio of the concentrations in the two phases at a constant temperature:

K

=

Mads/c Or

Mads =

KC.

(5.13)

From eqns. (5.10) and (5.11), we obtain

KC=C[(l/D)-l] MsollMsorb;

K = [ ( l / D ) - I ] MsollMsorb

(5.14)

93

5.12. Influence of experimental conditions

(5.15) Tentative data of DOMMER and MELCHER [209] show that the same regularities apply to silica gel and porous polymers. Organic compounds are distributed between the solid adsorbent and the desorbing solvent in accordance with eqn. (5.15). This equation relates the efficiency of desorption of pollutants to the volume of solvent and the amount of sorbent. The system is assumed to be in equilibrium, and this can be used to calculate desorption (recovery) efficiencies for most organic pollutants in the concentration ranges of interest in industrial hygiene. With the help of this equation one can choose an optimum ratio between the amounts of sorbent and solvent used in a given analytical procedure. The value of D can be determined from D=- S" - s x SS

(5.16)

where S,, S, and Ssare the peak areas of the sample, the blank sample and the standard, respectively. For the adsorption of pollutants on charcoal the experimentally found K values should be lower than those calculated from eqn. (5.15) if the sample is irreversibly retained on the sorbent surface, or if the kinetic rate values of the pollutants are small. If K has been determined, one can calculate the efficiency of desorption of pollutants for any ratio of liquid and solid phases. Another equation [230], characterizing phase equilibrium in a solvent-sorbent system, can be obtained from eqa(5.15) by substituting into it the K values which are the same for two different solvent volumes: D,

=

n D1

1 + ( n - 1) D1

(5.17)

or _=_

Dn

n

(l/D1 - 1)+ 1

(5.18)

where n is the ratio of solvent masses (volumes)- new volume/initial volume, D, is the initial desorption efficiency and D,, is the efficiency of multiple desorption, obtained with the use of n times the initial solvent volume (Fig. 5.23).

D,

= 0,8

D,= 0,9 Fig. 5.23. Variation of desorption efficiency (DJ with the number of repeated extraction

stages (n) [230]

94

5 . Collection and pretreatment of samples for chromatographic analysis

0

1

2

3

mg Siyrene Monomer/Sml CS,/O.5g

5

G Carbon

Fig. 5.24. Efficiency of desorption of styrene from activated carbon with the help of carbon disulphide vs. desorption duration: (1) 1; (2) 2; (3) 16 h [el]

Eqn. (5.18) can also be used to determine the n, ratio, necessary to obtain the desired desorption efficiency, z:

n, = [(l/Dl) - 11 z/(l - z)

(5.19)

A nomogram [231] can be used to calculate n,. As seen from eqn. (5.15), a change in the solvent-sorbent ratio directly affects the desorption efficiency. This important property of sorption-desorption systems can be used to increase the recovery of pollutants from sorbents. For instance, the desorption efficiency of 100 mg of charcoal with the use of 0.5 ml of carbon disulphide is 50%, whereas an increase in the volume of extractant to 5 ml raises the desorption efficiency to 91% [ll]. However, this approach is not always justified because simultaneously the sample is being diluted. I n an optimal desorption system, both the necessary dilution of the sample and the sensitivity of the analytical method should be taken into account [231]. The theory of desorption, according to which its efficiency depends only on the distribution coefficient and the ratio of sorbent and solvent masses, and not on the adsorbate concentration, was confirmed experimentally on compounds such as pentane, ethyl methyl ketone and 1,1,2-trichloroethane [81, 2091. Similar conclusions were arrived by other investigators, but later it turned out that the desorption efficiency for more polar compounds becomes dependent on concentration [ll]. In the general case, the phase equilibrium model, on which the theory of desorption of pollutants is based, should not vary with concentration for a linear adsorption isotherm for pollutants. For most methods of desorption of organics from charcoal this concept holds true, within the limits of +5%, for the concentrations of the deleterious substances of interest in industrial hygiene [81, 2311. Nevertheless, some compounds tend to show lower desorption, as illustrated by styrene monomer (Fig. 5.24). From Fig. 5.24, it follows that with decreasing monomer concentration the desorption efficiency of this compound decreases. Also of interest is the fact that the factors affecting the desorption of pollutants (temperature, humidity, duration of desorption, etc.) are of greater significance, i. e., exert a stronger effect on desorption, at low sorbent concentrations [l, 811. Of interest from the point of view of the efficiency of desorption of pollutants is the work of JOHANSEN and WENDELBOC [232], who used N,N-dimethylformamide (DMF) to desorb polar compounds. Apart from its high toxicity, carbon disulphide, proposed by NIOSH for extraction of pollutants, has a high vapour pressure (37 kPa at 20°C) and a low efficiency in extracting polar substances from solid sorbents. In addition, because of its short retention time, on most chromatographic columns (except for analysis with the use of a flame ionization detector), carbon disulphide interferes with the analysis of the most important volatile compounds in the sample. Although DMF is also toxic, it has a vapour pressure two orders of

95

5.12. Influence of experimental conditions

Table 5.28. Efficiency of desorption of organic compounds with N,N-dimethylformamide and carbon disulphide (%) 12321 Compound

N,N-Dimethylformamide

Carbon disulphide

liquidsolid

liquidsolid

solidliquid

difference

solidliquid

(%I 1 Alcohols 2-methyl-2-propanol 2-propanol 2-methylpropanol 2-ethoxyethanol 2-butoxyethanol Esters ethyl acetate methyl methacrylate butyl acetate hexyl acetate 2-ethoxyethyl acetate Ketones acetone butanone 4-methyl-2-pentanone 4-methylpent-3-ene-2-one cyclohexanone Hydrocarbons benzene toluene ethylbenzene styrene indene Chlorine- and nitrocontaining compounds dichloromethane 1,2-dichloropropane 1,2-dichloroethane 2-nitropropane 1,2,3- trichloropropane

2

3

102.37 101.55 101.14 100.00 99.66

100.30 91.63 93.82 92.67 97.51

102.71 97.33 99.61 96.25 101.12

107.76 93.69 90.23 93.52 98.05

100.91 101.50 100.57 95.00 99.04

4

difference

(%I 5

6

7

86.13 82.04 82.54 60.87 37.82

87.20 75.54 77.74 53.32 41.94

-1.1 6.5 4.8 7.6 -4.1

-5.0 3.6 9.4 2.7 3.1

99.26 92.93 98.34 100.31 98.31

103.92 91.43 90.66 95.98 91.82

-4.7 1.5 1.1 4.3 6.6

94.42 87.31 94.84 19.87 67.11

6.5 14.2 5.1 15.1 31.3

94.51 98.12 93.49 81.34 86.58

93.15 80.19 88.02 72.00 58.22

1.0 9.0 5.5 9.3 28.4

84.13 76.15 97.98 52.45 0.04

81.83 74.59 85.44 49.28 00.00

2.3 1.6 2.5 3.2 -

101.01 99.60 97.84 84.87 81.27

101.77 95.20 93.69 82.13 32.05

-0.7 4.4 4.2 2.7

99.78 100.49 97.05 11.66 73.04

91.26 98.15 93.78 0.01 63.26

8.5 2.3 3.3 9.8

100.74 103.95 100.78 23.19 101.25

102.25 101.89 97.03 0.02 96.97

- 1.5 2.1 3.8

2.07 9.9 1.3 7.3 2.2

4.2

magnitude lower (0.35 kPa at 20°C) and a longer retention time on chromatographic columns, i. e., DMF does not hinder the determination of the most important components of the investigated air pollutants. To confirm the advantages of DMF over carbon disulphide experimentally, 25 compounds used in industry were subjected to gas chromatographic analysis. Five samples were analysed, each containing five compounds belonging to one of the classes alcohols, ethers, ketones, aromatic hydrocarbons and chlorine- and nitrogen-containing organics. With phase equilibrium in liquid-solid and solid-liquid systems taken into account, the efficiency of desorption of pollutants was calculated for DMF and carbon disulphide. Table 5.28 indicates that the efficiency of desorption of polar compounds using DMF is close to loo%, whereas carbon di-

5 . Collection and pretreatment of samples for chromatographic analysis

96

sulphide recovers non-polar compounds from charcoal better. The samples are preserved much better in DMF than in carbon disulphide. A technique for the efficient desorption of traces of toxic styrene from a tube containing activated carbon has been described [233]. After Cz-C4 fatty acids have been collected on silica gel they can be recovered from the tube with an efficiency of 85-98%, for subsequent gas chromatographic analysis with flame ionization detection, by extraction with water, acetone or 1%formic acid solution [129].

5.12.1.

Temperature

At high temperatures, decomposition, polymerization and other chemical reactions of the sample components can take place on the active surface of a sorbent. The standard means of preventing this is to cool the solvent prior to sample desorption, which reduces the temperature effect. In this instance, however, the equilibrium constant will change with temperature, and the ultimate desorption temperature can become critical. For instance, the desorption efficiency of methyl ethyl ketone changed from 89.5 to 66.8% with a temperature decrease from 25 to 0°C [209]. This effect depends on the nature of the sample compound. Thus, vinyl chloride does not show this effect when the desorption temperature changes within the above range. The latter fact is extremely important, as vinyl chloride is very volatile, and its analysis, in addition to all the operations of sampling, desorption and sample processing, should be conducted at a low temperature [184]. To increase the desorption efficiency, DOMMERand MELCHER[209] recommend cooling the extractant to ambient air temperature. For the same purpose, to recover some polar compounds (e.g., alcohols) firmly retained by the sorbent, a mixture of solvents should be used.

5.12.2.

Humidity

A high humidity of an air sample in the course of concentrating traces of pollutants may re-

sult in a low degree of desorption for samples of easily hydrolysable and reactive compounds. If a large amount of water has accumulated on the sorbent, this can hinder good contact with a non-polar desorbing solvent or change the sorption equilibrium if water dissolves in the desorbing solvent. This effect manifests itself to a greater extent when the pollutants are recovered from polar sorbents, but it should also be taken into account when recovering pollutants from cryogenic traps containing activated carbon [234]. In the latter instance, with a weight fraction of water in the carbon of 0.02, the error of determination is negligibly small, but it increases to 45% for ethanol and to 6-12% for butanol and acetone if the water fraction is increased to 0.15. This can be attributed not only to the cryogenic condensation of moisture on the sorbent surface but also to the adsorption of water molecules on the activated charcoal (Fig. 5.25). Thus, for example, the preadsorbed moisture on an activated charcoal surface with a specific surface area of 1120-1150m2/g and a specific volume of 0.37-0.43 cm3/g renders ineffective the extraction therefrom of non-polar compounds, viz., toluene and l,l,l-trichloroethane with carbon disulphide [235]. To increase the recovery of these compounds a polar solvent (a few microlitres) is added to the extraction solvent. I n this instance the desorption efficiency increases in the order ethyl Cellosolve > n-butanol > dioxane, i. e., with increasing the polarity of the solvents and hence the affinity of such additives for the water (Fig. 5.26). The reason for the reduced efficiency of the recovery of pollutants from sorbents due to the presence of preadsorbed water lies in the proportional distribution of these pollutants in the water-carbon disulphide system according to the distribution coefficients. An addition of a small amount of a polar solvent significantly changes this distribution pattern, thereby promoting the transfer of a definite portion of pollutants from the water layer to the organic

97

5.12. Influence of experimental conditions

2 40 0 0

u c

$

30

z

g 20 --. 4

& 4

10

c .o ?d

F *1 0

q o

20

40

80

60

Relative humidity

100

PA,

Fig. 5.25. H20adsorption isotherm Merck and SKC for activated charcoal [235] 100

I

.honol

I

-2

-5

1

2

5

10

The g u a n t i t y o f odding solvent[@]

Fig. 5.26. Desorption efficiency as a function of the amount of solvent (dioxane, n-butanol and ethyl Cellosolve) added to carbon disulphide used as a desorbent in the recovery of organic compounds from SKC activated charcoal with preadsorbed water [235]. , 2., _ ._ ._ ._ ._ * 20 Volume of water: , 0>

phase (carbon disulphide). If readily water-miscible dimethylformamide is used as the additive, the efficiency of desorption of pollutants from charcoal with the aid of carbon disulphide in the presence of water will not decrease and may even become higher [235].

5.12.3.

Coadsorption

The efficiency of desorption of the main components in a sample can also be affected by the coadsorption of other compounds present in an air sample. When choosing a method for the recovery of pollutants from a sorbent, one should take into account the chemical properties of all the sorbed compounds and their possible influence on one another and on the sorption conditions. These effects should be checked in the laboratory before conducting air analyses. 8

Berezkin, Gas Chrom.-BE

98

5. Collection and pretreatment of samples for chromatographic analysis

5.12.4. Desorption duration The usual time sufficient for complete desorption of pollutants is 30 min with shaking of the desorption vessel. Reduction of this time can lead to a loss of the sample matter [231]. Under optimal desorption conditions the sorbent-solvent-pollutants system is usually stable. Exceptions, however, are also possible. Thus, the degree of styrene desorption was observed to decrease within 16 h [231]. This effect was more prominent at low monomer concentrations. To obtain appropriate results in this instance, the best way is to remove the extract from the sorbent, if it cannot be analysed after an optimal (short) desorption time. This type of reduction in desorption has been observed for naphthalenes, biphenyls and some other aromatics [81]. The solvents and reagents used for the recovery of pollutants must be stable in order to avoid catalytic effects on the active surface of the sorbent. In some analytical methods in which charcoal is used as the sorbent, 1%methanol is added to carbon disulphide to increase the desorption efficiency of polar compounds. If such a system is left at rest for more than 4 h, new compounds (sulphides, mercaptanes, thioethers) are formed, which was revealed after the chromatography of the extract. When isopropanol was used instead of methanol these compounds were not formed [12].

5.12.5. Choice of solvent Different solvents provide different recoveries of pollutants. As a rule, before choosing an optimal solvent and sorbent it is necessary to test different sorbent-solvent combinations with the specific compounds in the sample. The mechanism of sample desorption with a solvent is reduced to selective transport (displacement) of the pollutants from the sorbent. This process can be accomplished both with the help of a more polar (than the displaced pollutants) solvent, e.g., from charcoal, and with the help of a more active ion replacing less active ions, e. g., in the case of ion-exchange forces. A suitable solvent is usually chosen with the sample polarity and solubility taken into account. For instance, carbon disulphide, which is used more often than other substances to recover organic compounds from activated carbon, displaces simple alcohols from this adsorbent very poorly. To improve desorption in this instance it is necessary to add 1-5% of some other alcohol to carbon disulphide [ll,121. The general requirements for extraction solvents used to recover toxic pollutants from solid sorbents can be formulated as follows [l]: 1.The solvent should conform to the analytical procedure. Thus, the frequent use of carbon disulphide in gas chromatographic procedures is justified as it gives a very weak signal when a flame ionization detector is used. 2.The solvent should not react with the sample. For instance, carbon disulphide cannot be used to desorb amines as a chemical reaction takes place. 3. The solvent should be compatible with the sorbent used. Some bases can form gel-like substances in the recovery of pollutants from silica gel, which is to be avoided. It is also necessary to take into account the possibility of dissolution of porous polymers in some frequently used solvents, e.g., dichloromethane or carbon disulphide. In the latter instance good results are obtained by using non-polar hydrocarbon solvents, e. g., hexane or diethyl ether. 4. The solvent, as far as possible, should not be toxic. It is undesirable, therefore, to use toxic extractants such as benzene, carbon disulphide, carbon tetrachloride and some hydrocarbon halides. Before choosing the solvent it is necessary to test the sorption-desorption system, including the determination of the desorption efficiency for the sample pollutants. The extraction of matter from the sorbent must reach at least 75%, although 80-90% is to be preferred. To check the efficiency of recovery the toxic chemical pollutants from air and subsequent verifi-

-

99

5.12. Influence of experimental conditions Table 5.29. Efficiency of desorption of organic compounds from activated carbon [l, 1901 Compound

Extraction solvent carbon disulphide

Cyclohexane Benzene Toluene Xylene Styrene Methanol Ethanol n-Butanol Acetone Ethyl methyl ketone Ethyl acetate Dioxane Dichloromethane Dichloroethane Chloroform Carbon tetrachloride Trichloroethylene Tetrachloroethylene Vinyl chloride

100 96-100 92-98 95-98 80-87 21 54-67 70-91 86 62-96 89 88-91 95 96-100 97-101 96-100 96 90

chlorobenzene

90 90 86 86 80 86

n-butyl acetate

93 95 93 90 83 93 95 95

chloroform

80 low efficiency

95 90 88 88 86 88 86 87-88

93 68

cation of the completeness of their desorption from the sorbent, one can use the NIOSH data [236]. It should be borne in mind that when air sampling is performed on solid sorbents the breakthrough volume should be at least 1.5 times smaller than the recommended air volume necessary for determining the MPC at 80% relative humidity (121. This is necessary because a sorption tube containing the sorbent must have a sufficient reserve capacity to eliminate the effect of the accompanying impurities present in the sample air interfering with the target components. The stability of the sample during storage must not change by more than 10% in a period of 7 days, which gives the necessary time for the sample to be delivered from the sampling site to the laboratory. Sometimes a sample is immediately recovered from the sorbent and kept in solution prior to analysis. The choice of the method of sample preservation and safe keeping depends on the specific composition of the air sample. There are numerous data on the efficiency of extraction of pollutants from activated carbon with the help of carbon dioxide. The data in Table 5.29 demonstrate the high recovery achieved with this extractant for organics, except for some polar substances, expecially methanol. To recover methanol one can use chloroform (desorption efficiency 80%) or chlorobenzene (desorption efficiency 86%). The selectivity and efficiency of the recovery of alcohols can be increased by adding about 1%isobutanol or isopropanol to carbon disulphide or by extracting with water, which makes possible the simultaneous separation of C1-C4 alcohols from the hydrocarbons accompanying them. This method was used to determine low methanol concentrations in the products of gasoline-metal fuel evaporation [237]. It has been suggested that it is better to perform extraction with carbon disulphide using a cooled sorbent and solvent [205].

100

5 . Collection and pretreatment of samples for chromatographic analysis

An almost complete (93-100%) recovery of toxic pollutants from silica gel can be achieved by extracting them with polar solvents, e. g., dimethyl sulphoxide, alcohols, water or aqueous solutions. From 83 to 93% of amines are recovered from silica gel with sulphuric acid [ll], and very toxic dimethyl sulphate after being collected on silica gel is 90-95% desorbed with acetone [129]. About 90% of very volatile vinyl chloride is recovered from charcoal with the help of carbon disulphide. However, one can extract this carcinogen with almost the same efficiency using chlorobenzene and cooling the sorbent with ice [184].

5.13.

Increase in desorption efficiency

One of the ways of raising the desorption efficiency is to increase the solvent to sorbent ratio, as already mentioned when the phase equilibrium in a sorption-desorption system was considered. Two other approaches consist in using a solvent mixture or a two-phase solvent system.

5.13.1. Solvent mixture Polar compounds are recovered from charcoal less than non-polar compounds. By adding a polar solvent (a few percent) to carbon disulphide, one can improve the recovery of pollutants concentrated on charcoal by 10-20% [12]. This technique is effective if it does not interfere with the gas chromatographic determination of pollutants that may also be present in the added solvent. If the methanol-carbon disulphide system is used, the sorbent is processed not later than 4 h after desorption, because in the presence of charcoal methanol reacts with carbon disulphide [81]. In some instances, to increase the desorption efficiency, ethanol, butanol, isopropanol or acetone is added to carbon disulphide. To increase the recovery of acetonitrile from charcoal, 1-2% of acetone is added to carbon disulphide. The amount of acetone can also be much larger if it is needed to extract concentrated samples of other compounds, as acetone mixes with carbon disulphide in any ratio. A good desorbent for recovering hydrocarbons of various classes from charcoal is n-decane - 1,3,5-trimethylbenzene (4:1) [238]. The former solvent provides a good recovery of alkanes, olefins and naphthenes and the latter is efficient in extracting the aromatics contained in the sample, with an overall recovery of hydrocarbons of at least 80%. A technique for eluting chlorine-containing pesticide pollutants and PCBs, adsorbed in a trap containing graphitized thermal black, has been reported [239]. Desorption of these compounds with 2-4 ml of methanol and then with 2-4 ml of diethyl ether, makes it possible to recover residual water from the collector with the first fraction of the eluate (methanol), after which the efficiency of desorption of pollutants with diethyl ether increases considerably. This allows the solvent volume to be reduced to 1-2 ml, instead of the previously used 50 ml, which increases the sensitivity of the subsequent gas chromatographic determination of pesticides and polychlorobiphenyls on glass capillary columns with electron-capture detection.

5.1 3.2. Two-phase desorption systems The use of mixed solvents is limited, especially when one takes and subsequently desorbs samples containing numerous pollutants of different chemical nature. The interfering effect of solvents is strongly manifested in gas chromatographic analysis (addition of polar solvents, addition of the pollutants already present in the sample, etc.) with a very high sensitivity. At the same time, with the use of a two-phase solvent system it becomes possible to investigate both the polar and the non-polar fraction (components) of the organic pollutants present si-

101

5.13. Increase in desorption efficiency

Table 5.30. Efficiency of recovery for some solvents with a two-phase system (water-carbon disulphide) used for desorption [12, 2401 Compound

Ethanol Acetone 2-Propanol 1-Propanol Methyl ethyl ketone Ethyl acetate 1-Ethoxyethanol 1-Butanol Isopropyl acetate 2-Ethoxyethanol n-Hexane n-Propyl acetate Methyl isobutyl ketone n-Heptane Toluene

Molecular weight

46 58

60 60 72 88

70 74 102 90 86 102 100 100 92

Density

Retention time

Distribution coefficient in aqueous phase

Recovery

(g/cm3)

(min)

(%I

(%I

1.83

100

97

2.41

74

100

96

95 94 83 92 95 91 97 95 99 92 91 100 102

0.801 0.791 0.782 0.804 0.805 0.902 0.965 0.810 0.872 0.980 0.660 0.888 0.801 0.684 0.865

3.20 4.06 5.21 6.10 6.33 7.70 8.65 9.23 9.33 10.10 11.90 13.60 13.70

85 47

16 100 71

5 100 0 4

3 0 0

multaneously in workplace air. In this instance, for example, the pollutants concentrated in sampler tubes containing charcoal are desorbed using of carbon disulphide-water (1:l).After desorption the aqueous and organic extractant layers are separated and subsequently analysed. A high recovery of polar compounds results from a high distribution coefficient in the carbon disulphide-water system, which results in the aqueous phase being considerably enriched with polar pollutant compounds after desorption of the sample from charcoal with carbon disulphide. Table 5.30 gives data on the efficiency of recovery of pollutants for fifteen solvents widely used in air analysis. The distribution of pollutants between poorly miscible solvents eliminates the interfering effect of some polar and non-polar combinations of the investigated compounds, and the distribution coefficients provide additional quantitative data that have been used successfully for the reliable identification of the components of complex mixtures of air pollutants [l]. Other modifications of the method employ acidic or basic solvents, or immiscible solvents such as carbon disulphide-methanol. If necessary, after the compounds have been desorbed, a more complex extractive separation scheme [12] can be used before the sample analysis. In practice a desorbing solvent is initially chosen on the basis of the literature data, taking into account the nature of the pollutants and the efficiency of the contemplated sorption-desorption system. For correct determinations, however, it is necessary subsequently to perform a thorough study of the conditions for the recovery of target components, e. g., using extraction curves [241], with all the above-mentioned factors being-taken into account. In choosing the extraction methods, it is essential that the solvent contains no impurities that interfere with the subsequent analysis and it should be readily available and purifiable by conventional means directly in the laboratory.

102

5. Collection and pretreatment of samples for chromatographic analysis

5.13.3. Desorption methods An increased efficiency of desorption of impurities using extraction solvents in many instances requires the use of special equipment. For example, in the trapping of organic pollutants from atmospheric air with the aid of a special sampler [lo91 composed of a pair of cigarette filters each containing 25 g of activated charcoal connected in series (Fig. 5.27), the impurities from the filters were extracted in a microextractor (Fig. 5.28) using 0.3-0.5 ml portions of carbon disulphide for 30 min. It should be noted that such a desorption technique does not provide complete recovery of the most volatile components as they evaporate together with the solvent and are then retrapped by the charcoal. Therefore, the extraction is sometimes replaced by the frontal displacement of carbon disulphide (0.5-1.0ml) [l]. A simplier technique is more often used. The sorbent from the concentration tube (Fig. S.29a) is poured into a small test-tube fitted with a ground-glass stopper, previously filled with a desorbing solvent (0.5-1.0 ml) such as carbon disulphide, benzyl alcohol, dichloromethane or methanol. Subsequently the sorbent is kept in the test-tube (with periodic shaking of its contents) for 30 min, then an aliquot of the extract is analysed by gas chromatography. The recovery, depending on the sorbent used, varies between 75 and 92% [l,681. In rapid determinations, especially when making a qualitative analysis only, it is possible to use a simpler technique [68] consisting in the study of the first drop of the concentrate displaced

t

t

Fig. 5.27

Fig. 5.28

Fig. 5.27. Sampler for concentration of microimpurities on activated charcoal filter [log]. I Aerosol filter; 2 glass tube with first charcoal filter; 3 PTFE collar; 4 tube with second charcoal filter; 5 silicone-rubber sealing ring Fig. 5.28. Microextractor for the recovery of organic compounds trapped by charcoal filters [109]. 1 5-ml flask; 2 tube with charcoal filter; 3 PTFE tube with openings for vapours; 4 cooler

103

5.14. Choice of sampling method

Fig. 5.29. Methods for impurity extraction from a sorbent concentrator [68]. (a) Desorption in test-tube. 1 5-ml glass test-tube; 2 ground stopper; 3 concentration tube with sorbent; 4 solvent/desorbent. b) Desorption directly from a concentration tube. 1 Sorbent tube; 2 glass-wool wads; 3 syringe containing solvent; 4 extract drops

V

from a vertically positioned concentrating tube containing a sorbent, the solvent being added to the sorbent (1-3 ml) using a syringe or a dropper (Fig. 5.29b).

5.14.

Choice of sampling method

The development of an appropriate analytical method for determining traces of toxic chemicals in workplace air in industrial plants (or in the atmosphere) normal starts with the collection of data on the physical and chemical properties of investigated substances and on the methods of sampling and analysis, and of toxicological data regarding the sources of emission and the occurrence of pollutants. From the point of view of sorbent selectivity in collecting the toxic pollutants to be analysed, the choice of the sorbent-solvent system for a given sorbate and the choice of a highly sensitive and selective detector, which determines the detection limits for the pollutants, should be regarded as the most important factors in developing the analytical method. Choosing the sampling method is usually the longest process in elaborating the method of analysis. The first to be investigated are various combinations of sorption-desorption systems. Of all the available solvents charcoal is investigated first, as it is readily available and possesses high sorption characteristics. If charcoal or synthetic carbon prove to be unacceptable, silica gel or porous polymer sorbents should be tested. If it happens that polymer sorbents do not yield satisfactory results in recovering the pollutants from air, one can consider the possibility of using impregnated sorbents that make it possible to analyse the derivatives of the sample compounds. When a sorbent shows promise for pollutants analysis, it is necessary to check the stability of the concentrated sample on storage before the analysis and to establish the breakthrough volume of air pollutants in the sample atmosphere. After elaborating the sampling and analytical techniques, test (control) determinations should be performed with the help of a model atmosphere containing known concentrations of the investigated compounds. Detailed criteria of the acceptability of sampling procedures are discussed below and have been described elsewhere [12]. Several series of samples are then taken from the model atmosphere to determine the accuracy and reproducibility of the

104 Solvent Carbon disulphide Benzene Dichloromethane

5 . Collection and pretreatment of samples for chromatographic analysis

Amount taken

Extraction

(I%)

(%)

60.2 30.1 15.1 60.2 60.2

35.1 32.8 29.1 52.0 < 2.0

Table 5.31. Recovery of biphenyl pollutants from charcoal [I21

method and to ascertain the stability of the sample compounds on storage. If unsatisfactory results are obtained at a certain stage of the method, testing and controlling the elaboration of the method should be started at earlier stages. An example has been given [242] of developing the technique of pollutant sampling and analysis based on biphenyl. The physical and chemical properties of this toxic substance are as follows: molecular weight, 154.20; melting point, 69.2"C; boiling point, 255.2"C; and saturated vapour pressure, 0.14 kPa at 70.6"C, 0.70 kPa at 101.8"C and 1.40 kPa at 117.O"C. At 25"C, i. e., at the optimal sampling temperature, the pressure of saturated biphenyl vapour is 0.007 kPa, which corresponds to an approximate concentration in air of 70 mg/m3. Biphenyl is soluble in ethanol, diethyl ether, carbon tetrachloride, benzene, carbon disulphide and methanol and is insoluble in water. It is used as a heat carrier. Its maximum permissible concentration (U.S.A.) is about 0.2 mg/m3. Chemically, biphenyl is a relatively unreactive compound, not subject to hydrolysis, and relatively involatile. These characteristics, however, do not exclude the use of certain special sorbents, e.g., for its selective recovery. As regards the possible interference with the analysis of biphenyl and other toxic pollutants accompanying it in workplace air nothing definite can be said in the general case, because biphenyl occurs widely and the qualitative and quantitative compositions of the accompanying pollutants may vary over fairly wide ranges. The impurities interfering with the analysis may obviously include aromatics with different molecular mass. As the MPC of biphenyl is low, it is obvious that the interfering impurities can accompany it in much higher concentrations. From this, it follows that the accompanying impurities can affect the sorbent capacity to a considerable extent, which must be taken into account when evaluating the sampling method. It was previously established that biphenyl should not be concentrated on coconut-based carbon because it retains this compound very strongly (Table 5.31). It has been recommended [242] that a less active carbon should be used for concentrating biphenyl from air. A review of the literature showed that there was no sorbent for the successful sampling of biphenyl pollutants. From the few data available, have been chosing petroleum carbon and three polymer sorbents for biphenyl recovery from air. The carbon chosen was much less active than coconut-based carbon, and the other sorbents were polymers based on aromatic compounds and could potentially serve as good collectors for non-polar aromatics. Small portions of solutions containing 113 pg of biphenyl were injected directly into a tube containing sorbents, after which the samples were desorbed with various solvents (Table 5.32). From the data in Table 5.32, it follows that some sorbent-solvent combinations yield recoveries of more than 90%. Based on these tentative results, a Tenax GC-carbon tetrachloride combination was chosen for the concentration of biphenyl pollutants, because it provides a good recovery, and the preliminary experiments with Tenax GC revealed the reproducibility of batch-to-batch desorption results for this polymer to be better than those for all other sorbents. Carbon tetrachloride was chosen because it produces a very weak signal on a flame ionization detector and has good characteristics for use in gas chromatographic analysis. The sorption capacity of Tenax with respect to biphenyl vapour, the stability of the con-

105

5.14. Choice of sampling method Table 5.32. Efficiency of biphenyl desorption [12]

Sorbent

Extraction Solvent

Carbon 104 (petroleumbased), 100 mg

Methanol

Porapak Q, 50 mg

Tenax GC, 35 mg

XAD-2,30 mg

Carbon disulphide Benzene Carbon tetrachloride Diethyl ether Acetone Methanol Carbon disulphide Benzene Carbon tetrachloride Diethyl ether Acetone Methanol Carbon disulphide Benzene Carbon tetrachloride Diethyl ether Acetone Acetone Carbon tetrachloride

Recovery (%) 0

52 65 2 0 2 53 lb4 108 93 92 93 83 64 19 99 93 92 89 94

centrated samples on storage and the recovery, accuracy and reproducibility were detemined and proved to be acceptable. An example of choosing a sorbent for concentrating methanol from air in the presence of and MURAWOVA [237]. Taking other alcohols and various hydrocarbons was given by DRUGOV into account all the aspects of the determination, they recommended recovering methanol vapour from air by using concentration tubes containing a large amount of BAC activated carbon, although, in principle, activated carbon is not the best adsorbent for such a purpose. Table 5.33 shows the results of choosing optimal sorption-desorption systems for the gas chromatographic determination of pollutants of different nature in the workplace air. An attempt to develop a universal sampling device for pesticides [12] showed that the most acceptable system was a combination of a filter based on cellulose methyl ether and a tube containing chromosorb 102 or XAD-2 polymer sorbent. After recovering the pesticides aerosol and vaponr, they were extracted with toluene and analysed by gas chromatography with electron-capture flame photometric detection. The method produced good results for endrin, chlordane, ronnel, phosdrine, heptachlor, demetone and some other chlorine- and phosphorus-containing toxicants. Table 5.33. Adsorbents for standard procedures for the recovery of toxic pollutants from air [147] Compound

Adsorbent

Eluent or elution method

Acrylonitrile Aromatic amines Formaldehyde Bis(chloromethy1)ether Sulphur dioxide Nitroglycerine

Carbosieve B Silica gel A1203 Chromosorb 101 Molecular sieves 5A Tenax GC

Methanol Ethanol 1%CH,OH solution in water Thermal desorption Thermal desorption Ethanol

106

5 . Collection and pretreatment of samples for chromatographic analysis

Table 5.34. Sorbents for the concentration of organic and inorganic pollutants from air [l,5,7,2-13,81, 109,1391

1 Hydrocarbons saturated methane ethane unsaturated ethylene acetylene alicyclic aromatic polyaromatic Alcohols saturated unsaturated aromatic amino alcohols phenols Aldehydes saturated unsaturated aromatic Ketones Carboxylic acids Esters Ethers saturated unsaturated aromatic Organic oxides Organohalides alkyl halides aryl halides chlorine-containing pesticides Organic nitrogen compounds alkylamines alicyclic amines aromatic amines azo compounds hydrazines nitrates nitriles

2

3

4

5

+ + + + + + + + + + +

+ +

+ + + + + +

+

+ + + + + + + + + +

+ + +

+ + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + +

6

I

8

+

+ + + +

+ + + + + + +

+

+ t

+

+ + + +

+ + +

+ + +

+

+ + +

107

5.14. Choice of sampling method

1 nitro compounds (aliphatic) nitro compounds (aromatic) nitrosamines Organic sulphur compounds mercaptans alkyl sulphides alkyl sulphonates alkyl sulphates Organophosphorus compounds Phosphorus-containing pesticides Organosilicates Heterocyclics nitrogen-containing oxygen-containing nitrogen- and sulphurcontaining Organometallic compounds organolead organomercury organotin Inorganic compounds ammonia carbon monoxide carbon dioxide carbon disulphide carbonyl sulphide cyanogen chloride hydrocyanic acid hydrofluoric acid and fluorides hydrogen sulphide iodine chlorine bromine mercury(I1) chloride mercury nitrogen monoxide nitrogen dioxide dinitrogen oxide phosgene phosphine phosphorus (white) sulphur dioxide hydrazine arsenic and its compounds metal aerosols

2

3

+ +

+

4

+

5

6

I

8

-t

+

+ +

+

+ + + +

+

+

+ +

+ +

+ +

+ +

+

+ + + + + + + + + + + + + + + + + +

+

+ + t

+ t

+

+ + + + +

+ + + +

+

+ +

+ +

+

+

t

108

5. Collection and pretreatment of samples for chromatographic analysis

In choosing the methods for the recovery and subsequent analysis of aldehydes (acetaldehyde, chloroacetaldehyde, formaldehyde, furfural, crotonaldehyde and acrolein), the instability of these compounds was taken into account, which makes it desirable to analyse them in the form of derivatives, already accurring in the course of the sampling process [l]. Good results in determining traces of tetraethyl- and tetramethyllead in air were obtained by collecting them in a tube containing XAD-2 polymer, extracting them with n-pentane and subjecting the solutian obtained to gas chromatographic analysis with photoionization detection (PID). A large amount of sorbent is needed in this instance for the complete recovery of the more volatile tetramethyllead and of tetraethyllead pollutants. Considerable difficulties are encountered in developing methods used to determine traces of toxic substances such as mercaptans, amines, amino alcohols and nitroalkanes in air. Thus, it has proved impossible to apply a single absorbing medium for amines of different structures and amino alcohols. Silica gel, silica gel treated with sulphufic acid and porous polymer sorbents with subsequent thermal desorption of the concentrated amines were used for this purpose. Preference was given to sampling on silica gel followed by treating the sample with dilute HC1 immediately after the recovery of amines. This method has been used successfully to recover ammonia and most aliphatic amines. Another method suitable for amines consists in concentrating the pollutants on porous polymer sorbents, followed by gas chromatographic analysis after their thermal desorption. This kind of method, based on using a trap containing Chromosorb 104 and final gas chromatographic determination, was developed and tested for n-butyl mercaptan, with the sorbent not having to possess a high capacity with respect to methyl and ethyl mercaptan. At the same time, nitroalkanes, after trace amounts have been collected on solid sorbents, do not yield reproducible stability results on storage [l, 11, 121. Table 5.34 lists the main sorbents used in the determination of air pollutants.

5.1 5.

Metrological aspects of air pollutant determination by gas chromatography')

Investigations on the metrological assessment of the results of the sorption and subsequent recovery of toxic pollutants from a collector containing a sorbent [81] made it possible to establish seven criteria for adequate sampling. 1.The accuracy and reproducibility of the combinations of analytical and sampling procedures for toxic substances with concentrations from 0.1 to 2 TLV must amount to +16% (relative) at the 95% confidence level. 2. The efficiency of recovery of pollutants from air must be at least 75%. 3. The difference between the efficiency of sorption of pollutants from air and their recovery from the sorbent must not exceed 10% (relative). 4 . A representative sample must be obtained within 10-15 min. 5.To ascertain the mean concentrations for a work shift, the sampling time must be not less than 1 h, preferably [4-81 h. 6.The degree of sample preservation after 14 days must be at least f10%(relative) with respect to the initial amount of material. 7. The sampling must be known with an accuracy of f5%. The last condition is very important, as an incorrectly measured aspiration rate results in substantial determination errors because it is impossible to account exactly for the amount of air passed. ') This part was prepared by doctor NEYMAN, E. Ya.

(Moscow).

5.1 5. Metrological aspects of air pollutant determination by gas chromatography

109

Fig. 5.30. Dependence of random error (SJon concentration of trace component (X)in air. c, was determined for S,= 0.33 and S,= 0.25 It can be easily imagined that the contribution of sampling errors to the overall error in the analytical results will play a dominant role when using the described method for the analysis of air pollutants. It should be noted that in this instance the term “sampling” is used more widely than in common analytical practice because it includes a number of succesive steps (i. e., extraction of toxic impurities, desorption, sample pretreatment, etc.), each being characterized by a certain error. The overall sampling error is the sum of the errors in the separate steps. The situation may arise that one of the sampling steps (for example, desorption) is characterized by such a large error that the overall analytical determination is meaningless. Hence the determination of the overall error depends in many respects on the determination of the sampling error. According to the described approach, the determination of the maximum permissible error for every stage of sampling that guarantees satisfactory results being obtained in the determination of impurities is of considerable interest. An attempt consisting in the stepwise consideration of sampling stage errors and the ascertainment of maximum permissible limits for its variance has been reported [81]. It was stated that the relative error of both analysis and sampling procedures is about 0.1-2.0 times their TLV and not higher than k16% relative for a confidence level P of 0.95. This statement seems to be debatable. First, the described characteristic is not valid from the metrological point of view. Generally, the deduction of accuracy standards (and the described characteristic is such a standard) should be based on the estimation of reproducibility parameters and the correctness of the separate stages of an analytical procedure. Reproducibility, which characterizes the random error of the analytical results, can be simply calculated [243]. A series of experiments must be carried out using air samples with different contents of toxic trace substances. Based on n parallel determinations, for every concentration standard deviation S for every experiment and a relative standard deviation S, = S/Xcan be calculated, where X i s the content of toxic component for the set of samples. As a result the set of S, values for fixed Xis obtained. The curve shown in Fig.5.30 is a plot of S, against X. Using the standardized TLV values and a curve similar to shown in Fig. 5.30, the reproducibility errors can be established in accordance with the TLV value and any desirable concentration of a toxic trace substance, for example 0.1 or 2.0 times the TLV value [81]. When USing such a procedure, the situation can arise when the lower limit of the monitored component content (c,) (i. e., the maximum content found with a fixed reproducibility error S,) is higher than the lower standardized limit (for example, c, > 0.1 TLV). Such a procedure cannot be used for solving practical problems because its random error for 0.1 TLV is so high that the results become qualitative rather than quantitative. The c, value is defined by the fixed S, value. In trace analysis, c, is usually assumed to correspond to S, = 0.33, which fact makes the reported [81] value of the total error doubtful. The relative random error in our case can be written in simplified form as

5 . Collection and pretreatment of samples for chromatographic analysis

110

where S,,c , S,.ds, S,, and S , are the relative standard deviations for the concentration stage, desorption stage, rate of aspiration of the polluted air and the analytical determination. As it is very difficult to determine the random error for each of the above stages it becomes reasonable to use the total value which can be obtained for overall procedure, and to use it as the function S,= The limit of random error can be calculated as the boundary of the confidence limit for from the equation 6 = *st,,& (n<20) (5.21)

s,,

f(r).

where tp,, is tabulated Student’s grees of freedom: 6 = k2S/J;;

(n 2 20)

x

t

value for the confidence probability P and f = n - 1 de-

(5.22)

Hence the relative bounderies of the confidence limit can be obtained as k SIT. A more complicated problem consists in establishing and determining the systematic error levels characterising the bias of the results of the determination of toxic trace substances from the actual content of such a substance in air; this is the accuracy of the procedure. In contrast with the random error, the systematic error cannot be reduced by increasing the number of parallel determinations. Elucidation of the sources of systematic error is almost always connected with the stepwise investigation of an analytical process and the determination of the maximum possible bias of the analytical results from the real content of a toxic impurity in polluted air for every stage of an analytical gas chromatographic determination. One of the possible sources of systematic error is incomplete extraction from the air sample, i.e., the content of an extracted component is smaller than its content in the polluted air (systematically reduced results). Naturally, the question arises of what the permissible difference between the found and the real content is that allows the experimental results to be considered real. To answer this question it is necessary to apply the relationship between the random and systematic errors. If the difference A = g - X, (where F is the experimental result and X, is the real content) lies within the confidence level x k 6, it is impossible to elucidate the systematic error from the background of random variations. It such instances it is customary to consider the systematic error to be statistically insignificant. Hence the results of analysis are considered to be correct if

IF-X,lSS

(5.23)

If this relationship is not valid then the systematic error is considered to be significant and it estimate, A, should be taken into account either during the analysis or by reducing it to a level 6 affecting the process by some means. Based on the above results, it seems difficult to explain an extraction efficiency (from polluted air) of as low as 75% [81], because this estimate is not connected with the random error. For a 75% extraction the bias A = 25% could be insignificant only if the reproducibility confidence level is x k 0.25 F. Otherwise, it is necessary to calculate the bias precisely and take it into account when calculating the final results. The second possible source of systematic error is connected with incomplete desorption of concentrated toxic trace components from the sorbent. It has been stated [81] that the difference between the efficiency of sorption from polluted air and the degree of extraction from the sorbent should be not greater than k 10% (relative). It seems to us that this criterion is not correct. Let us consider the situation when the previously mentioned condition is fulfilled, i. e., only 75% of trace components are adsorbed by the sorbent. Therefore, with regard to the mentioned error (* lo%),it is permissible during the desorption stage to extract only about 65% (of the initial content) from the trap. On the basis of this requirement, the analysis becames virtually qualitative.

5.15. Metrological aspects of air pollutant determination by gas chromatography

111

Fig. 5.31. Correctness control for results of analysis by means of air volume variation. Verification based on the found mass of trace component. 1 without systematic error; I' systematic increase in results; 1" systematic decrease in results; 2 verification based on the found trace concentration. V,, V,, V,, etc. - folded drawn volumes of the air

The most succcessful means of verifying the correctness of separate analytical stages applied in polluted air control can be carried out with the help of standard gas mixtures, in which the contents of the components are known precisely. In this instance the results are considered to be correct if the relationship

IX- xi,I 6 6

(5.24)

(a

is fulfilled, which means that the difference between the found trace concentration and the introduced trace concentration (Xi,) is not higher than the random error limits of the conIn spite of this, the preparation error Xi, for the standard gas mixture (&,) fidence level for should be not higher than 1/3 S. In this instance the error is considered to be statistically insignificant and Xi, is a precise value. Because preparation and certification of the standard gas mixture are often very complex, especially for case of multi-component mixtures of actual polluted air containing components of different natures, it was proposed to use the so-called sample variation method (or "the method of double sample" in chromatography) as an alternative to the method mentioned above. Different volumes of analysed air are drawn through the trap with a sorbent and, after desorption, the concentrated trace components are determined by gas chromatography. It is convenient to use 2-fold, 3-fold, etc., volumes of air (relative to the initial volume). As a result, different amounts of toxic traces (rn) are found. A relationship between the analytical response (y) and the found value ($ is obtained (Fig. 5.31); this dependence Ly =f(m3] shows a monotonous increase. If this dependence is approximated by a straight line passing through the origin, one may accept the insignificant systematic error [243]. Indeed, iii should be equal to zero with a zero volume of analysed air. In contrast, when y = k K + b , the b value indicates the systematic bias of the results of the determination and the reason for this bias should be established. Fig. 5.31 shows the modification of the proposed method for correctness control; graphs of found mass (line 1) and found concentration (line 2) are shown. The latter represents the dependence of y on the concentration of the controlled trace component (g.If the value of X remains constant irrespective of the volume of air that was drawn through the sorbent, provided that there is no significant systematic error, the dependence y = f( should be a straight line parallel to the X axis (at the boundaries of the confidence level for the random error.

r.

x>

112

5. Collection and pretreatment of samples for chromatographic analysis

Recently similar approach was described by WOLTERS and UTEMAN [244], where objective criteria for the evaluation of precision and accuracy for analytical results have been postulated. The relative error of the determination of pollutants according to different investigators, et al. [81] concentrated and analysed of more than 50 comvaries from 6 to 25%. MELCHER pounds with sampling periods from 4 to 8 h and obtained a relative precision was k6.5%with a range from k 3 to k 10%at the 95%confidence level, not including the sampling pump precision. The overall precision of the analytical and sampling procedure was f16%. The most accurate method for the determination of pollutants is the gas chromatographic method recommended by NIOSH, using charcoal and carbon disulphide as a desorbent. The error in this method varies from + 6 to +17% for different compounds. It has been reported that it is possible to perform such analyses with a precision of 10.5% [192], and for very low bis(chromomethy1) ether concentrations a precision of 10% was obtained. SHADOFF[245] found that the attainable total relative error is 12.7%,of which 9.5%is being accounted for by the analytical error and 8.3% by the sampling error. In a study carried out by NIOSH (2461, fifteen investigators took for 10 min samples of polluted air containing seven different components, each in the 0.6-2 TLV range (each investigator took 300 samples). The relative error was 12.7%,and at 0.05 TLV it doubled to about +25%. Taking and preparing samples for gas chromatographic analysis is a major stage in determining air pollutants. The extensive experience gained by many investigators in different countries gives rise to the hope that most of the practically important problems encountered in present-day environment control will eventually be successfully solved. Although the diversity of problems to be solved in the analysis of pollutants in ambient air fully justifies the diversity of the sampling and concentrating methods that have been used, we think that it is most expedient to develop further the following methods: 1. Concentrating the pollutants on sampling columns containing hydrophobic adsorbents or sorbents with a stationary phase used in gas-liquid chromatography. 2. Using chemically selective sorbents for high-efficiency absorption of specific pollutants from air. 3. Developing efficient gas chromatographic concentration and desorption methods, making it possible to combine the stages of pollutant desorption and concentration, to perform intermediate concentration and injection of the concentrated pollutants into the chromatographic system in one stage (see, e. g., refs. [247] and [248]). 4. Using various chromatographic displacement methods to desorb pollutants from the sorbent. 5. Using hybrid reaction-sorption (chemical sorption) methods, making it possible at the sampling stage to eliminate the interferences from accompanying impurities by absorbing them in precolumns containing sorbents and chemical reagents, which will radically facilitate the subsequent reliable identification of the target components.

References

113

References (Chapter 5 ) DRUGOV,YU. S.; BELIKOV,A.B.; DYAKOVA, G.A.; TULCHINSKII,V.M.: Methods of Air Pollutants Analysis. Moscow: Khimiya 1984 (in Russian). LIBERTI, A.; CICCIOLI, P.: Sci. Chromatogr. Lect. A. J. P. Martin Honor Symp. F. BRIJNER(Ed.): Urbino 1985. Amsterdam: Elsevier 1985, p. 219. SCHREIER, P. (Ed.): Analysis of Volatiles. Berlin/New York: W. de Gruyter 1984. VANLOON,J.: Selected Methods of Trace Analysis: Biological and Environmental Samples. New York: John Wiley and Sons 1985, p.357. LANGHORST, M. L.; COYNE, L. B.: Anal. Chem. 59 (1987) 1. WALLING, J. F.: Atmos. Environ. 18 (1984) 855. DRUGOV,YU. S.: Zavod. Labor. 48 (1982) 3. JENNINGS, W. G., RAPP,A.: Sample Preparation for Gas Chromatographic Analysis. Heidelberg: Hiithig Verlag 1983. BERTSCH, W.: Sampling of Organic Volatiles. Heidelberg: Hiithig Verlag 1983. POOL,C. F.; SCHUETE, S. A.: Contemporary Practice of Chromatography. Amsterdam: Elsevier 1984, p. 473. CRISP,S.: Ann. occup. Hyg. 23 (1980) 47. CHOUDHARY, G. (Ed.): Chemical Hazards in the Workplace. Measurement and Control. (ASS Symp. Ser. 149), Anal. Chem. SOC.Washington D.C.: (1981) 5, 155, 179. Fox, D. L.: Anal. Chem. 59 (1987) 280. S.; NILSSON,B.: J. Chromatogr. Sci. 18 (1980) 171. BERG,S.; JACOBSON, DIETZ,E.A.;HOFFMA",~.J.: Am. Ind. Hyg. Assoc. J. 45 (1984) 382. RASMUSSEN, R. A.; KHALIL, M. A. K.: J. Geophys. Res. D89 (1984) 11 599. BECKER, J. H.: Abstr. Pap. Pittsburgh. Conf. and Exp. Anal. Chem. and Appl. Sepctrosc. Atlantic City/New York 1986, p. 194. MCNIDER,T. E.: Mine Vent. Proc. 2nd Mine Vent. Symp. Reno, Nev. 1985, MOLJSSET-JONES, P. (Ed.): Rotterdam/ Boston/ Balkema 1985, p. 759. POLACEK, J.C.; BULLIN, J.A.: Environ. Sci. 8cTechnol. 12 (1978) 708. HENRY,J.G.;GEHR,R.:J. Water Pollut. Contr. Fed. 52 (1980) 2523. SAVELYEV,V.A.; L I B E R M A N , ~SHAPOSHNIKOVA,E.YU.; .~.; KHANINA,G.D.; MEDELYAN,G. G . :Zav. laboratoriya 46 (1980) 600. JANAGI, I.; AKAHOSI, S.: Japan patent No. 55-7173 (1980). S. J.; MEEKS,S. A.: AIChE Symp. Ser. 73 (1977) 84. GORDON, LIBERTI, A,; CICCIOLI, P.: Chromatographia 9 (1986) 492. GILL,B.;YERGEAN, J.: Patent (Canada) No. 1196800 (1985). PEREGUD, Ye. A.: Chemical Analysis of Air. Leningrad: Khimiya 1976 (in Russian). WRZESKI,L.:Ochr. powietrza (PRL) 8 (1974), 86. LUCE,C.; CARLIER, P.; GIRARD,R.; HANNACHI, H.; FRESNET, P.; L O ~ E R G.:, Analusis 12 (1984) 350. HOLIZCLAW, J. R.; ROSE,S. L.; WYAIT,J. R.; ROUNBEHLER, D. P.; FINE,D.: Anal. Chem. 56 (1984) 2952. S.; GORIACHEV,N. S.: Zhurn. analit. khimii 36 (1981) 371. DRUGOV,YU. DRUGOV,YU. S.; BEREZKIN,V.G.: Uspekhi khimii 48 (1979) 1884. K.: J. Chromatogr. 186 (1979) 219. VIERCORN, R. B.; SAVELSBERG, M.; BACHMANN, LEPSI,~.: Chem. prum. 27 (1977) 411. MORTIMORE, J. C.; ZIEGLER, J. M.; MULLER, J. F.: J. Chromatogr. 172 (1979) 249. FUNAZO,K.;TANAKA,M.; SHONO,T.:Anal. Lett. All (1978) 661. GRAVING, J. E.; JONKMAN, J. H. J.; DE ZEEUW,R.A.:J. Chromatogr. 148 (1978) 389. SMALLWOOD,A. W.: Am. Ind. Hyg. Assoc. J. 39 (1978) 151. K. E.; MAZUR, J. F.: Am. Ind. Hyg. Assoc. J. 41 (1980) 1. WILLIAMS, BENTE,WATHNE,M.:Analyst. 105 (1980) 400. HOSHIKA,~.; KOZIMA,~.; KoIKE,K.;YOSHIMOTO,K.: Gas L Liquid Chem. Abstracts 24 (1981) 34. FISHER,R. L.; REISER,R. W.: Anal. Chem. 49 (1977) 1821. BoHM,G.; WLISZAR, W.: Mikrochim. acta 1 (1980) 495. 9

Berezkin, Gas Chrom.-BE

114

5 . Collection and pretreatment of samples for chromatographic analysis

[43] [44] [45] [46]

HEBERER, H.; BI-ITERSOHL, G.: Z. Chem. 20 (1980) 361. ROMANO, S. J.; RENNER, J. A,: Am. Ind. Hyg. Assoc. J. 40 (1979) 742. EBELL, G. F.; FLEMING, D. E.; GENOVESE, J. H.; TAYLOR,G. A.: Ann. occup. Hyg. 23 (1980) 185. T~BULSKAIA, I. A.; VITENGERG, A. G.; IOFFE,B. V.: Perkin-Elmer Bulletin. Applications of gas chromatographic head space analysis 1981, No. 29. SIROTKINA,N. N.; MARUNYAK, M. N.; KLIMUK,Z. A.: Gigiyena i sanitariya 1 (1984) 64. KOLB,B. (Ed.): Application of Headspace Gas Chromatography. London: Heyden 1980, p. 41. BARNETT, D.; DAVIS, E. G.: J. Chromatogr. Sci. 2 1 (1983) 205. ANDRAWES, F. F.: J. Chromatogr. Sci. 22 (1984) 506. KISSA,E.: J. Chromatogr. 319 (1985) 407. KRYNSKA,A.; POSNIAK,M.: Pr. Cent. Inst. Ochr. Pr. 34 (1984) 215. POSNIAK,M.: Pr. Cent. Inst. Ochr. Pr. 34 (1984) 229. BEREZKIN,V.G.;VIKmRovA,Ye.N.: Doklady Akad. Nauk SSSR 271 (1983), No.6, p. 1412. ANVAER, B. I.; DRUGOV, Yu. S.: Gas Chromatography of Inorganic Substances. Moscow: Khimiya 1976 (in Russian). GOLDBERG,A.B.; MARGULIS,~. J.; WILNER,R.A.;BANDY,A.R.:Atmos. Environ. 15 (1981) 11. HOSHIKA,Y.; MURAYAMA,N.: J. Jap. SOC.Air. Pollut. 20 (1985) 89. FARWELL, S. 0.; GLUCK, S. J.; BAMESBERGER, W. L.; SCHWITE, T. M.;ADAMS,D. F.; Anal. Chem. 51 (1979) 609. TYSON, B. J.; VEDDER, J. F.; ARVESEN, J. C.; BREFER, R. B.: Geophys. Res. Lett. 5 (1978) 369. DE JONGHE, W. R. A,; CHAKRABORTI, D.; ADAMS,F. C.: Anal. Chem. 52 (1980) 1974. FABIAN,^.; GoMER,D.:Fresenius Z. anal. Chem. 319 (1984) 890. VAGIN,Ye. V.: In: “Progress in Gas Chromatography”. CHMUTOV, K. V.; SAKODYNSKII, K. I. (Ed.): Moscow: Nauka 1972, p. 262 (in Russian). FUKUDO, N.; ITOH,H.; TSUKAMOM, A.; TAMARI, H.: Japan. Anal. 28 (1979) 569. RADZIUK, B.; TOM ASS EN,^.; VANLOON,J. C.: Anal. chim. acta 105 (1979) 255. POPP,P.; OPPERMAN, G.: J. Chromatogr. 148 (1978) 265. SEIFERT, B.; ULRICH, D.: Atmos. Pollut. 1978. Amsterdam: Elsevier Publ. 1978, p. 69. VIDAL-MADIAR, C.; PAREY, F.; EXCOFFIER, J. L.; BEKASSY, S.: J. Chromatogr. 203 (1981) 247. DRUGOV, Yu. S.; BEREZKIN, V. G.: Gas Chromatographic Analysis of Polluted Air. Moscow: Khimiya 1981 (in Russian). ISIDOROV, V. A.; ZENKEVICH, I. G.: Chromato-mass-spectrometric Determination of Trace Organics in the Atmosphere. Leningrad: Khimiya 1982 (in Russian). KROST, K. J.; PELLIZZARI, E. D.; WALBURN, S. G.; HUBBARD, S. A.: Anal. Chem. 54 (1982) 810. KIRSCHMER,~.; BALLSCHM~TER,K.: Intern. J. Environ. Anal. Chem. 14 (1983) 275. A m s , R . R . : J. Chromatogr. 329 (1985) 399. TERMONIA,M.; ALAERTS,G.: J. Chromatogr. 328 (1985) 367. ROY,S. S.; SINGH,K.:Z. anal. Chem. 292 (1978) 240. VANECEK, M.; WALDMAN, M.: Coll. Czech. Chem. Commun. 44 (1979) 519. BEREZKIN,~. G.; DRUGOV,YU. S.; GORIACHEV,N. S.: Zhurn. analit. khimii 37 (1982) 319. THORNTON, D. C.; DRIEDGER,A. R.; BANDY,A. R.: Anal. Chem. 58 (1986) 2688. TANGERMAN,A.: J. Chromatogr. 366 (1986) 205. SAKODYNSKI,K. I.; PANINA,L.I.; REZNIKOVA,Z.A.; KARGMAN,~. B.; GALITSKAYA,N.B.: J. Chromatogr. 364 (1986) 455. The CHROMPACK Guide to Chromatography. General Catalogue. Chrompack, Midelburg. The Netherlands 1984, p. 208. MELCHER,R.G.; LANGNER,R. P.; KEGEL,R.0.:Am. Ind. Hyg. Assoc. J. 39 (1978) 349. Anon: Am. Ind. Hyg. Assoc. J. 32 (1971) 404. SERPIONOVA, Ye. N.: Industrial Adsorption of Gases and Vapours. Moscow: Vysshaya shkola 1969 (in Russian). WEBER,L.;KNECHT,U.:Fresenius Z. anal. Chem. 321 (1985) 544. KISELEV, A. V.; YASHIN, Ya. I.: Gas-adsorption and Liquid Chromatography. Moscow: Khimiya 1979 (in Russian). BANDY,A. R.; TUCKER, B. J.; MAROULIS, P. J.: Anal. Chem. 57 (1985) 1310. SCHROEDER,W.H.; JACKSON,R.A.: Heavy Metals Environ. Int. Conf. Heidelberg 1983, Vol. 1, Edinburgh: CEP Consultants Ltd. 1983, p. 94.

[47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] (641 [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87]

References

115

[88] CICCIOLI, P.; BRANCALEONI, E.; POSSANZINI, M.; BRACHETTI, A.; PAOL,C.: Sci. Total Environ. 36 (1984) 255. K. I.: Zhurn. Vsesoyuzn. h i m . ob-va im. D. I. Mendeleeva 28 (1983) 34. 1891 SAKODYNSKII, [90] TANAKA,T.:J.Chromatogr. 153 (1978) 7. K. I.; PANINA, L. I.: Polymer Sorbents for Molecular Chromatography. Moscow: [91] SAKODYNSKII, Nauka 1977 (in Russian). [92] DMITRIYEV, M. T.; MISHCHIKHIN, V. A.: Gigiyena i sanitariya 9 (1980) 66. M. W.; CHAPMAN,L.M.; MIEURE,J. P.: Am. Ind. Hyg. ASSOC,J. 39 (1978) 385. [93] DIETRICH, R. G.; CALDECOURT,~. J.; Anal. Chem. 52 (1980) 875. [94] MELCHER, W. N.; BILDEMAN, T. F.: Environ. Sci. & Techno]. 14 (1980) 679. 1951 SILLINGS, VIRELIZIER, H.; GAUDIN, D.: Analusis 6 (1978) 401. [96] HAGEMANN,R.; [97] DILLON,K. H.; BARE=, W.; ELLER,P. M.: Am. Ind. Hyg. Assoc. J. 39 (1978) 608. M. P.; PAUKOW, J. P.: Anal. Chem. 57 (1985) 1138. [98] LIGOCKI, W.N.; BILDEMAN,T.F.: Atmos. Environ. 17 (1983) 383. [99] SILLINGS, H. In: Sci. Chromatogr. Lect. A. S. P. Martin [loo] ZLATKIS,A,; WEISNER,S.; GHAONI,L.; SHANFIELD, Honor. Symp., BRUNER, F. (Ed.). Amsterdam: Elsevier, p. 449. W.: Environ. Sci. &Technol. 9 (1975) 1175. [loll RUSSEL,J. C.; GONNORD, M. F.; BENCHAH, F.; GUIOCHON, G.: J. Chromatogr. Sci. 16 (1978) [lo21 VIDAL-MMAR, 190. J. W.; SHADOFF, L. A,: 1.Chromatogr. 134 (1977) 375. [lo31 RUSSEL, R. S.; IVANOVA, N. P.: Zhum. analit. khimii 35 (1980) 1813. I1041 SHEFIER,V. Ye.; KRUPENINA, NISHIOKA,Y.A.: Am. Ind. Hyg. Assoc. J. 41 (1980) 599. [lo51 THOMAS,T.C.;JACKSON,J.W.; [lo61 CASTELLO,C.; D’AMATo,G.:J. Chromatogr. 254 (1983) 69. P.; JONSCON, J. A.: J. Chromatogr. 286 (1984) 279. [lo71 LOVKVIST, D. S.; ZOLLER, W.H.: Anal. Chem. 56 (1984) 1288. [lo81 BALLANTINE, R.; PIETRZYK, D. J.: Anal. Chem. 50 (1978) 1842. [lo91 SYDOR, [110] DosKEY,P.V.;ANDREN,A.W.:Anal. Chim. acta 110 (1979) 129. LEVIN,J. D.; NILSSEN,C.A.:Chemosphere 10 (1981) 137, 143. [ l l l ] ANDERSSON,K.; J.; EISENREICH, S. J.: Anal. chim. acta 124 (1981) 31. [112] HOLLOD,G. SELBER,J.N.:Anal. Chem. 50 (1978) 1229. [113] WOODROW,J.F.; K.; LEVIN,J. D.; NILSSON, C. A.: Chemosphere 12 (1983) 377. (1141 ANDERSSON, N. S.; LINDGREN, L. J.; JENKES,J. M.: Abstr. Pap. Pittsburgh Conf. Anal. Chem. and Appl. [115] ZARR, Spectros. Atlantic City/New York 1986, p. 522. C. G.; BILDEMAN, T. F.: Anal. Chem. 51 (1979) 1110. [116] SIMON, BERLINCIONI,M.: Am. Ind. Hyg. Assoc. J. 41 (1980) 352. [117] VANNUCCHI,C.; C. D.; BIDLEMAN,T. F.: Atmos. Environ. 18 (1984) 837. [118] KELLER, V. Ya.; KOSHOVSKII, B. I.; ZABORSKAYA, A. N.: Gigiyena truda i profzabolevaniya 6 [119] ALPEROVICH, (1980) 5 5 . (1201 RUDENKO, G. I.; MALTSEV, V. V.; STUDENICHNIK, V. N.;UsnNov, Ye. P.: Zhurn. analit. khimii 40 (1985) 1119. E. D.; BUNCH, J. E.; CAwemR, B. H.; SAWICKI, E.: Environ. Sci. & Techno]. 9 (1975) [121] PELLIZZARI, 552. H.: J. Japan. SOC.Air Pollut. 18 (1983) 425. 11221 KASHIHIRA, H. P.: J. Chromatogr. 295 (1984) 49. I1231 GROII,K.; NEUKOM, J.; MOHANRAO,A.M.: Chromatographia 22 (1986) 183. [124] NETRAVALKER,A. YAMAZAKI,~.; UEBORI,M.:Japan. Anal. 29 (1980) 170. [125] KUWATA,K.; TuNEK,A.:Ann. occup. Hyg. 20 (1977) 127. (1261 GAGE,J.C.;LAGESSON,~.; S. A.; DRUGOV, Yu. S.; LULOVA, N. I.; KOROLEVA, N. M.: Zh. Anal. Khim. 32 (1977) [127] LEONIYEVA, 1638. [128] RIGGLE,C. J.; SGONTR,D. L.; GRAFFEO, A. P.: 4th Joint Conf. Environ. Pollutants, New Orleans, La 1977, Washington, D. C. Amer. chem. SOC. 1978, p. 761. J.C.; BRIGHT,A.P.: Am. Ind. Hyg. Assoc. J. 41 (1980) 459. [129] GILLAND, , ~ SLUZCOVSKAYA, . L. P.: Gigiyena i sanitariya 5 (1985) 53 (in Russian). [130] S H E ~ R Ye.; [131] LENSCHKE, W.; FAHNERT, R.; WIRTH,M.; MUNZBERG, B.: Patent (DDR) No. 232764 (1986). . PERNI,R. B.: Anal. Chem. 50 (1978) 1839. [I321 GoLD,A.; D u B E , ~E.; Y.; FuJII,T.; A M I I E , ~FUWA, .; K.: J. Chromatogr. 180 (1979) 133. [133] YOKOUCHI,

116

5 . Collection and pretreatment of samples for chromatographic analysis

[134] WARDENCKI, W.; STASSZEWSKI,R.: Chem. anal (PRL) 25 (1980)841. [135] WILLEY,M.A.;Mc C A M M O NS.; , ~ .DOEMENY, L. J.: Am. Ind. Hyg. Assoc. J. 38 (1977)358. (1361 BLACK,M.S.;HERBST,R.P.; HITCHCOCK,D.R.: Anal. Chem. 50 (1978)848. [137] BANAKH, 0.S.;FEDOROVICH, I. P.; PLASTUNOV, B. A.; DATSENKO, I. A,; ANDRONIKASHVILI, T. G . ; TIUNOVA, L. A.; SHLEMKEVICH, M. P.; GLADYSHEVSKAYA,T. N.: Gigiyena i sanitariya 11 (1978)75. [138] VASIREDDY, S.; STREETH,K. W.; MARK,H.B.J.: Anal. Chem. 53 (1981)868. [139] LANGVARDT,~. W.; MELCHER,R.G.:Anal. Chem. 52 (1980)669. [140] NEUBUR, E.; MWLLER,J.; SARTORIUS,R.: Mikrochim. acta 2 (1979)131. [141] Cox, R. D.; OGLE,L. D.; LEE,K. W.: Abstr. Pap. presented at Pittsburgh Conf. and Expo. Anal. Chem. and Appl. Spectrosc. Atlantic City/New York 1982,p. 1, p. 545. [142] AVERILL, W.; PURCELL, J. E.:Chromatogr. Newslett. 6 (1978)30. [143] WEST,P. W.: Amer. Lab. 12 (1980)35. [144] AMATANI, K.: Japan patent 52-106502 (1979). [145] SILVESTRI,A.: USA Patent 4033720 (1977). V. G.; DRUGOV, Yu. S.; MURAWOVA, G .V.: Auth. cert. USSR No. 693254 (1979).Bull. [146] BEREZKIN, izobr. No. 39 (1979). [147] THAIN,W.: Monitoring Toxic Gases in the Atmosphere for Hygiene and Pollution Control. Oxford: Pergamon Press 1980. [148] LEWIS,R.G.;MuLIK,J.D.; Comvl,R.W.; WOOTEN,G.W.;McMILLIN,C.R.:Anal. Chem. 57 (1985) 214. J. D.: Anal. Chem. 57 (1985)219. [149] COUTANT,R.W.;LEWIS,R.G.; MULIK, [150] LEWIS,R.G.;MuLIK,J.D.; COUTANT,R.W.; WOOTEN,G.W.;McMILLIN,C.R.:Anal. Chem. 57 (1985) 214. [151] POPLER,A.;SKWIILOVA,~.: Pr. lek. (PRL) 35 (1983)404. [152] BLAND, M. A.; CRISP,S.; HOWLGATE, P. R.; LLEWELLYN, J. W.: Analyst. 109 (1984) 1317,1523. [153] RosE,V.E.; PERKINS,J. L.: Am. Ind. Hyg. Assoc. J. 43 (1982)605. [154] BEREZKIN,V.G.;DRUGOV,YU. S.;MURAWOVA,G.V.: Zav. laboratoriya 47 (1981),No. 3, p. 18. [155] KAGEL, R. A.; FARWELL, S. 0.: Anal. Chem. 58 (1986) 1197. V. G.: Chemical Methods in Gas Chromatography. Moscow: Khimiya 1980 (in Rus[156] BEREZKIN, sian).

11571 GREIFER,B.; TAYLOR, J. K.: In: Nat. Bur. Stand. USA, NBSIR. 75-977,1976. E.; Eswsm,G.G.: Am. Ind. Hyg. Assoc. J. 42 (1981)476. [158] WILLIAMS,K. [159] AOYAMA,T.; YASHIRO,T.: J.Chromatogr. 265 (1983)69. 11601 LIPARI,F.:Anal. Chem. 56 (1984) 1820. [161] IRGUM,K.; LINDGREN, M.: Anal. Chem. 57 (1985) 1330. [162] BRAUN,T.; FARAG,A. B.: Anal. chim. acta 99 (1978) 1. [163] ADAMs,F.: Pure and Appl. Chem. 55 (1983) 1925. [164] Fox,D.L.: Anal. Chem. 57 (1985) 223R. [165] MELCHER, R. G.; LANGHORST, M. L.: Anal. Chem. 57 (1985)238. [166] KARASEK,F. W.; DENNY, D. W.; CHANK,K. W.; CLEMENT, R. E.: Anal. Chem. 50 (1978)82. J.; MEHTA,D.V.; MELTZER,T. H.: J.Air Pollut. Control Assoc. 30 (1980)261. [167] ROBERTSON,D. [168] CELMAN, C.; MEHTA,D.V.; MELTZER,T. H.: Am. Ind. Hyg. Assoc. J. 40 (1979)926. [169] PROKHOROVA, Ye. K.; PERWKHINA, I. V.; GAYEVAYA, T. Ya.; OSIPOVA, A. G.; MIZGAREVA, V. V.: Nauchn. raboty in-tov okhrany truda VTsSPS 1977, iss. 107 (3)42. [170] LEE,M. L.; WRIGHT,R. W.:J. Chromatogr. Sci. 18 (1980) 345. D. R.; LIOUBIN, M.; CAP^, R. E.: Abstr. Pittsburgh Conf. and Expo. Anal. Chem. and [171] KOLBOW, Appl. Spectrosc. Atlantic City/New York 1980,p. 140. [172] APPELL,B.R.;WALL,S.M.;TOKIWA,~.; HAIK,M.:Atmos. Environ. 13 (1979)319. S.;MURAVYOVA,G.V.; GRINBERO,K.M.; NESTERENKO, K.M.; SOKOLOV,D.N.: Zav. la[173] DRUGOV,YU. boratoriya, 38 (1972)1305. [174] SPIIZER,T.;DANNECKER, W.: Anal. Chem. 55 (1983)2226. [175] BETZ,L. R.; GROB,R.L.: Anal. Lett. A17 (1984)701. W.; BALFANZ, E.; GROSCH,B.;Pom,F.: Atmos. Environ. 14 (1980)609. [176] KONIG,J.; FUNCKE, [177] KLOCKOW,D.; TECKENTRUP,A.: Int. J. Environ. Anal. Chem. 8 (1980) 137. [178] ERICKSON,M. D.; MICHAEL,L. C.; ZVEIDINGER,R.A.; PELLIZZARI,E. D.: Environ. Sci. & Technol. 12 (1978)927.

References

117

[ 1791 BAMBERGER, R. L.;ESPOSITO,G. G.; JACOBS, B. W.; PODOLAK,G. E.; MAZUR,T. F.: Am. Ind. Hyg. As-

SOC.J. 39 (1978) 701. [180] KLISENKO, M.A.; BE LASH OVA,^. G.; KHOKHOL’KOVA, G.A.: Gigiyena i sanitariya 8 (1984) 58. [181] JAKLIN,J.; KRENMAYR,~.: Intern. J. Environ. Anal. Chem. 21 (1985) 33. [182] JuNK,G.A.;JEROME,B.A.:Int. Lab. 14 (1984) 10. [183] YASUDA, S . K.; LOUGHRAN, E. D.: J. Chromatogr. 137 (1977)283. [184] DRUGOV,YU. S.; MURAWOVA,G.V.: Zhurn. analit. khimii 35 (1980) 1319. [l8S] GROB,K.;GROB,G.;HABICH,A.:J.High Resolut. Chromatogr. and Chromatogr. Commun. 7 (1984) 340. N.: Anal. Chem. 54 (1982) 369. [186] HUNT,G.; PANGARO, (1871 ANDERSSON, K.; LEVIN,J. 0.; LINDACH, R.; NILSSON, K.A.: Chemosphere 11 (1982) 1115. [188] COPPI,S.; BEITI,A.; BLO,G.;BIG HI,^.: J.Chromatogr. 267 (1983) 91. S.; MURAWOVA,G.V.: Zhurn. analit. khimii 34 (1979) 2252. [1891 DRUGOV,YU. F. X.; MILLER, J. A.: Am. Ind. Hyg. Assoc. J. 40 (1979) 380. (1901 MUELLER, S.: Gigiyena i sanitariya 2 (1979) 50-52. [191] KOMALOV,R. A. T.; MCCAMMON, C. S . ; ROPER,I. C.; CARLBERG, K. C.; Am. Ind. Hyg. Assoc. J. 38 [192] SAALWAECHTER, (1977)476. (1931 PELLIZZARI, E. D.; BUNCH, J. E.; BERKLEY, R. E.; MCRAE,J.: Anal. Lett. 9 (1976) 45. [194] NAGASEMAKOTO:Japan Anal. 29 (1980)293. Am. Ind. Hyg. Assoc. J. 36 (1975) 538. (1951 WOOD,G.O.;ANDERSON,R.G.: J.; TORRES,L.;KOZLOWSKI, E.; MATHIEV, J.: J. Chromatogr. 208 (1981) 239. [196] NAMIESNIK, V.; WURST,M.;CATSKA,~.: J. Chromatogr. 193 (1980) 137. [I971 PATZELOVA, C. S.; ROPER,C. P.; SAALWAECHTER, A. T.: 2nd NIOSH Solid Solvent (1981 TEASS,A. W.; MCCAMMON, Roundtable. Cincinatti, Ohio 1973,HEW Publication No. (NIOSH) 76-193(1976) 5 5 . [199] EICEMAN,G. A,; KARASEK, P. W.: J. Chromatogr. 200 (1980)115. [200] BARNES, R. D.; LAW,L. M.; McLEoD,A.J.: Analyst. 106 (1981) 412. S.; KITA,T.:Bull. Environ. Contam. and Toxicol. 24 (1980) 185. [201] BOKI,K.; TANADA, Ind. Health 18 (1980) 61. [202] MATSUMURA,J.: P.; BERTONI, G.; BRANCALEONI, E.; BRUNEN, F.: J. Chromatogr. 126 (1976) 757. [203] CICCIOLI, A,; MAKINO, K.; KIRITA,K.; YOCHIHIKA, W.: J. Chromatogr. 239 (1982) 617. [204] KASHIHIRA, (2051 SEVERS, L. W.; SKORY, L. K.: Am. Ind. Hyg. Assoc. J. 36 (1975) 669. KOZLOWSKI,E.: Z. anal. Chem. 311 (1982) 581. [206] NAMIESNIK,J.; (2071 BERTONI,G.; BRUNER,F.;LIBERTI,A.;PERRINO, C.: J. Chromatogr. 203 (1981) 263. K.; UEMURA,T.; KIFUNE,~.; T O M I N A G AOCKAWA, ,~.; K.: Japan Anal. 31 (1982) 453. [208] KAWATA, (2091 DOMMER, R. A,; MELCHER, R. G.: Am. Ind. Hyg. Assoc. J. 39 (1978) 240. [210] KOLW,B.; POSPISIL, P.: Chromatogr. Newslett. 8 (1980) 25. [21 I] BOWERT, W. D.; PARSONS, M. L.; CLEMENT, R. E.; EISEMAN, G. A.; KARASEK,F. W.: J. Chromatogr. 206 (1981)279. [212] NIOSH Manual of Analytical Methods. U.S. Department of Health, Education and Welfare, Cincinnati, Ohio 1974,p. 127/1. [213] V I N C E N T J.; , ~ . HAHN,K. J.; KETCHAM,N. H.: Am. Ind. Hyg. Assoc. J. 40 (1978) 512. [214] STOLYAROV, B. V.; NAGIMULLINA, A. G.; GROGOEWA, T. A,; VITENBERG, A. G.: Zh. Anal. Khim. 42 (1987) 132. [215] LAMPARSKI, L.L.; NESTRICK,T. J.: Anal. Chem. 52 (1980)2045. [216] MILUKAITE, A. P.: Candidate’s thesis. V. Kapsukas Vilnius State University, Vilnius 1979. V. A,; THORNBURN, S.: Chromatographia 12 (1979)519. [217] COLENUIT, (2181 RAYMOND,A.; GUIOSHON,G.: Environ. Sci. & Technol. 8 (1974) 143. F.; JULICH, E.: Chromatographia 18 (1984) 313. [219] NITZ,S.; DRAWER, [220] BERTSCH, W.; CHANG, R. C.; ZLATKIS,A.:J. Chromatogr. Sci. 12 (1974) 175. R. L.; HAWLEY-PEDDER, R. A,; PARSONS, M. L.; KARASEK, F. W.: J. Chromatogr. 303 (1984) [221] CROUCH, 53. [222] KRZYMIEN, M. E.: Intern. J. Environ. Anal. Chem. 21 (1985)43. (2231 THOMPSON, J. M.; STEP HEN,^. I.: Anal. Chim. Acta 182 (1986) 229. I.; PAN’KINA, M.A.; FEDOTOV,YU.I.: Neftepererabotka i neftekhimiya 11 (1984)23. [224] KULAGINA,~. [225] DRUGOV,YU. S.:Abstr. Papers Vth All-Union Conf. Anal. Chem. Org. Comp. Moscow 1984.Moskau: Nauka 1984,p.291.

118

5. Collection and pretreatment of samples for chromatographic analysis

[226] NEU,H.-J.; MERZ,W.; PANCEL, H.: J. High Resolut. Chromatogr. and Chromatogr. Commun. 5 (1982) 382. J.: Lab. Prax. 6 (1982) 1100. [227] SEVCIK, N. M.; NOLDE,T. V,; SUKHORUKOV, 0.A.: Auth. Cort. USSR, No. 968748 (1982), Bull, [228] VATULYA, izobr. 39 (1982). G. I.; ISIDOROV, V. A.; IOFPE,B. V.: Vestnik LGU 22 (1983) 107. [229] BYSTROVA, J. C.: Am. Ind. Hyg. Assoc. J. 41 (1980) 63. [230] POSNER, J.C.; OKANFUSS, J.R.: Am. Ind. Hyg. Assoc. J. 42 (1981) 643. [231] POSNER, I.; WENDELBOC, J. F.: In: Advances in Chromatography 1981. Proc. 16th Int. Symp., [232] JOHANSEN, Barcelona 1981. ZLATKIS,A.(Ed.): Dep. Chemistry, Houston, USA 1981, p. 307. S.N.: Am. Ind. Hyg. Assoc. J. 42 (1981) 403. 12331 EvANs,P.R.; HORSTMAN, Coll. Czech. Chem. Commun. 47 (1982) 2491. [234] KALAB,~.: BJORKHOLM,E.: Am. Ind. Hyg. Assoc. J. 47 (1986) 615. 12351 RUDLING,J.; E. C.; ANDERSON, C. C.: Development and Validation of Methods for Sampling and [236] GUNDERSON, Analysis of Workplace Toxic Substances. Contract No. 210-76-0123, Cincinnati, Ohio, NIOSH 1980. S.; MURAVYOVA,G.V.: Zhurn. analit. khimii 37 (1982) 1302. [237] DRUGOV,YU. S.; GORIACHEV,N. S.: Zhurn. analit. khimii 37 (1982) 691. [238] DRUGOV,YU. I2391 P E ~R.,L.: Anal. Chem. 53 (1981) 1548. P. W.; MELCHER,R. G.: Am. Ind. Hyg. Assoc. J. 40 (1979) 1006. [240] LANGWARDT, Ann. Inst. super. sanita 14 (1978) 425. [241] CARELLLG.;RIMATORI,v.; SPERDUM,B.;ZOCCOLILLO,L.: D. C. (Ed.) Vol. 2-5, Cincinnati, Ohio 1979. [2421 NIOSH Manual of Analytical Methods. TAYLOR, 12431 N E w , E . Y a . : Lakokrasochniye materiali i ih primeneniye 5 (1988) 26 (in Russian). [244] WOLTERS,R.;KATEMAN,G.:Anal. chim. acta. 207 (1988) 111. L. A.: Anal. Chem. 45 (1973) 425. [245] SHADOFF, N O S H Contract HSM-99-72-98. Scott Research Laboratories. Collaborative Teasting of Acti"61 vated Charcoal Sampling Tubes for Seven Organic Compounds. HEW Publication (NIOSH) 76-173, 1916. 12471 BEREZKIN,V.G.; TATARINSKII,~. S.: Gas Chromatographic Analysis of Trace Impurities. New York: Consultants Bureau 1973. , Applications of Mass Spectrometry to Trace Analysis. Amsterdam: Elsevier [248] F A C C H ES.~ (Ed.): 1982.

Chapter 6

The Reactive-Sorption Method and its Application for Concentrating Pollutants 6.1.

Concentration of pollutants on solid adsorbents

The recent literature [l-131 dealing with the analysis of air pollutants has devoted much space to the problems of sample collection and the concentration of micropollutants. It is not accidental that researchers all over the world have paid considerable attention to these problems. Sampling is a most difficult stage in analytical procedures for determining air pollutants. The results of an accurate and thorough analysis become meaningless if the sample preparation or sampling has been wrongly conducted [3]. This is particularly true in the analysis of pollutants in ambient air, which is a complex labile chemical system containing numerous chemically active compounds in low concentrations, which is affected by factors such as moisture, oxidants and radiation. An important problem is the selection and introduction into an analyser of a representative sample of air pollutants whose components differ greatly in their physico-chemical properties and concentration (from 10 to mg/m3) [3, 9-11]. This problem is especially important because, when in a concentrator the sample compounds are converted into an entirely different (from the initial) state [3, 5 , 7, 141, (1) the concentration of the pollutants increases considerably (100- 1000-fold or more); (2) the trap containing the sorbent captures substances that differ in their chemical properties and concentrations (differing 10-109-fold), most of which are reactive, unstable and liable to oxidation, hydrolysis and other reactions; (3) an increased concentration of pollutants in the trap increases the probability of chemical interactions and irreversible transformations of the sample components; (4) the active centres of the sorbent surface (and sometimes possible impurities in the sorbent, e.g., metals) act as catalysts that accelerate these transformations; and (5) during sample concentration coadsorption (competitive sorption) of the accompanying impurities takes place on the sorbent, which leads to anomalous filling of the sorbent surface and distortion of the initial sample composition. These processes result in tangible changes in the pollutant concentrations observed in the sorbent trap. The pollutants are adsorbed non-uniformly, and the composition of the concentrated pollutants may differ greatly from their composition in ambient air [3, 15, 161. The chemical transformations of the sample in the trap lead to changes in its qualitative composition and distorted analytical results [3, 13, 141. Thus, for example, amines are liable to undergo specific interactions with free vinyl groups on the surface of Chromosorb 102 and Porapack Q [17]. Traces of oxygen in the carrier gas (nitrogen) during the concentration of amine pollutants from air and elevation of the sorbent temperature to 100°C during thermal sorption led to the depolymerization and oxidation of the sorbent [18]. The resulting chromatograms displayed false peaks of aromatic ketones and other carbonyl compounds. This distorts the qualitative and quantitative determination of the pollutant concentrations. The same sorbents vigorously interact with NO2 to form NO at 25"C, and NO plus HzO at 125°C with simultaneous nitration of polymer sorbents [19]. The sorption capacity of Tenax and Amberlite XAD-2 for n-pentane, benzene, acetone, 2-propanol and acetonitrile changes considerably as a result of nitration and sulphation of the sorbent with chemically active components of industrial wastes (nitric and sulphur OXides), the maximum change in the sorbent capacity being noted for polar sorbates [20]. All reactive gases (C12,HCl, HBr, ClF,, F2,HF, CIOz, 0 3 ,SOz, SO3,NOz, etc.) behave similarly,

120

6. The reactive-somtion method and its amlication for concentrating pollutants

Table 6.1. Generation of dichlorocyclohexane during concentration of cyclohexane and chlorine on Tenax (241 Content of product generated during sample collection

Concentration of pollutants contained in ambient air Standard inorganic pollutants [added bpb)l

ClZ OPb)

Cyclohexane OPb)

0 100 0 0 160 180 0 0 100 720

104 0 104 104 104 104 104 104 0 104

90 90 90 818 818 818 818 818 818 818

0 0 300 0

0 0 0 100 0 0

0 0 350 0 0 0 0 0 0

0

1,2-Dichlorohexane (a)

0

0 0 100 110 120 110 100 0 120

interacting between each other and almost all known sorbents and parts of the chromatographic system [21]. In detecting smoke fumes in ambient air, Tenax gradually decomposes under the action of NO2 and SO2 to form 2,6-diphenyl-p-quinone [22]. When ambient air containing amines and nitrogen oxides is aspired through sorbents such as active carbon, A1203,silica gel, Florisil and Tenax, nitrosamines initially unobserved in the air are generated [23]. During the concentration of pollutants from ambient air containing olefin hydrocarbons and free halogens, chemical reactions (e.g., interaction of chlorine with 2-butene or cyclohexene) take place on the surface of Tenax, Amberlite and carbon sorbents, which result in the appearance of halogenated hydrocarbons previously unobserved in the sample mixture [24]. The concentration of gaseous detonation products in a Tenax GC trap is also accompanied by the appearance of artefacts (with the emergence of new peaks in the chromatogram) as a result of the cracking of the polymeric sorbent as the air sample is passed through it [25]. The possible appearance of false peaks belonging to new compounds during sample collection is confirmed by the data in Table 6.1. Concentration of ethylbenzene, l-hydroxyethylbenzene, acetophenone, n-octanol and n-octanal from ozonized air (1-1.3 mg/m3) on Tenax showed [26] that at room temperature the content of these compounds did not change in the concentration tube containing Tenax. However, the presence in air of both oxidants ( 0 3 , NO2, SO2, C12) and water vapour led to the decomposition of Tenax and changed the contents of micropollutants (toluene, styrene and cyclohexane) concentrated on it [27]. Gas chromatography-mass spectrometry established the generation of several deuterated products of oxidation, halogenation and nitrosylation (three chlorostyrene isomers, benzaldehyde and dimethylnitrosamine) [28], in addition to acetophenone, phenol, a-hydroxyacetophenone and ethylene oxide [27]. In similar experiments (with pre-drawing of the air through a glass filter treated with 1%or 10% Na2S2O3solution), the content of halogenation and oxidation products decreased sharply [27, 281. In the sorption concentration of styrene in a tube containing active carbon, the initial concentration of styrene may actually change owing to catalytic reactions [29]. CRISP[7] showed that during thermal desorption some of the C4-Cs hydrocarbons initially concentrated on ac-

121

6.2. Reactive-sorption concentration

Fig. 6.1. Acetone retention as a function of benzene concentration in the vapour phase on Carbopack C at 10°C (161

lo2

103

104

Benzene [ppm]

tive carbon isomerize at 150-250°C to generate new compounds unobserved in the air sample, namely cis-2-butene and cis-2-pentene. Coadsorption of the pollutants in the sorbent trap also changes the sample composition. Studies of the quantitative laws of benzene sorption on graphitized soot and its effect on the relative retention volumes of n-pentane, acetone, nitromethane, propanol and tetrahydrofuran showed that, depending on the degree of filling of the soot surface, benzene coadsorption may either increase (at a degree of filling of 0.3-0.5) or decrease (at a degree of filling lower than 0.3) the retention of pollutants [16]. For example, it follows from Fig. 6.1 that the sorption of acetone by Carbopack C first decreases as the free adsorbent surface is filled by benzene molecules. Frequently water vapour is a specifically strong competitor in the sorption of organic pollutants [15]. According to modem concepts [30], the main reason for the strong adsorption of water molecules on hydrocarbon sorbents is the presence of adsorption centres capable of forming hydrogen bonds. The strong sorption of water and carbon dioxide [15] changes the sorption of other pollutants and leads to their anomalous concentration [3]. Polar water molecules can displace preconcentrated organic pollutants from the sorbent, dissolve them and change the dynamic and kinetics of subsequent compounds in the adsorbent trap. When the weight ratio of water on active carbon is 0.02, the error of determination is small [15], but when it is 0.15 the error is 45% for ethanol and increases by 6-12% for butanol and acetone. Sulphates and nitrates can also be generated during sample preparation owing to the reaction of certain concentrated compounds with sulphur- and nitrogen-containing gaseous anhydrides and acid vapours (SOz, NOz, H N 0 3 , etc.) present in the sample matrix. The shortcomings of the method related to side-reactions of the pollutants on the sorbent surface can result in errors in the determination of pollutants of 8-11 mg/m3 [8]. These difficulties can be surmounted by using the reactive-sorption concentration method.

6.2.

Reactive-sorption concentration

The reactive-sorption concentration (RSC) method’) makes it possible to prevent chemical interaction of the sample components during concentration and minimizes the effect of coadsorption [3, 31, 32). The RSC method [33] consists in the preremoval of pollutants during sample collection with the aid of a column (reactor) containing sorbents or chemical reagents positioned in front of the concentrator (see Fig. 6.2 b). Hence, in this instance the sorbent trap captures only the main components. This hybrid method has an increased selectivity that can be varied within a broad range as the sorbent reagent changes in the reactor [3, 311. Fractionation (separation) of some components prior to their concentration improves both

I)

The Term RSC was suggested by the authors of this book.

122

6. The reactive-sorption method and its application for concentrating pollutants

4

Fig. 6.2. Schemes of (a) conventional gas chromatographic determination of air pollutants and (b) RSC-assisted concentration. 1 Concentrator containing sorbent; 2 analytical column; 3 detector; 4 monitoring device; 5 precolu r n reactor

the kinetics and dynamics of the sorption of target components and creates conditions for their more reliable identification, as most of the sample components (in some instances up to 90%)remain in the precolumn reactor. It should be noted that preadsorption of interfering compounds has previously been used to increase the selectivity of determining toxic pollutants in the ambient workplace air in industrial plants by a colour method [31]. In this instance analysis was conducted by a twostage technique involving first chemical separation and then detection. Colour reactions of the pollutants under study with appropriate colorimetric reagents were used for detection. This method has been used for the gas chromatographic determination of pollutants. The following analysis scheme should be used for the chemisorption variant of the concentration and gas chromatographic determination of pollutants [3 1, 321: chemical (or physical) separation-concentration-chromatographic separation-detection. In this variant the selectivity of pollutant separation (the total selectivity of separation is determined by both the pre-column and the chromatographic column) is much higher than in the conventional variant. The limit of detection with this scheme is usually 3-4 orders of magnitude lower than with the colour method [31]. The reactive-sorption method of concentrating pollutants greatly differs from the substraction method used in gas chromatography [32]. The former involves selective adsorption of miTable 6.2. Some parameters of RSC variants for determining air pollutants Compounds under analysis

Sorbent in precolumn

Adsorbed pollutants

Main components

Ref.

Glass-fibre at -78°C

CIOz

C12, NO2

1211

NzO, NOz, Nz04 COz, CH4, light hydrocarbons

NO CO

135, 361 133,371

NOz, Clz

Hydrocarbons

1381

Acids

Ketones, aldehydes

1381

Molecular sieves 13X Molecular sieves 13X, ascarite, active carbon, 0°C Alkenes, NOz, alkanes, NazSz03 Clz Organic and inNaZCO3 organic acids, aldehydes, ketones

123

6.3. Use of the reactive-sorption method

nor components in combination with subsequent concentration and chromatographic determination of the target components [3, 311. The reactive-sorption method is a hybrid technique, and it is free of many of the shortcomings inherent in the conventional techniques of detecting pollutants in ambient air [33]. Let us consider the possibilities of the reactive-sorption method as applied to the solution of certain practical tasks.

6.3.

Use of the reactive-sorption method to enhance the reliability of the determination of pollutants

The reliability of the determination of pollutants can be enhanced by decreasing the chemical interaction of the sample components [31-331. The broad possibilities of varying the composition of sorbents and chemical reagents in the precolumn-reactor packing ensure an RSCassisted solution of some important problems of determining low concentrations of reactive compounds contained in ambient air (Table 6.2).

6.3.1.

Determination of aggressive gases

Apart from interacting with the column packing and being adsorbed in the chromatograph components, reactive gases often interact with one another (especially during concentration on sorbents), thus hampering their chromatographic determination or rendering it impossible [21]. A typical example involves reactions in the C12-HC1-C10z-03-N0z system characterizing air pollution around inorganic synthesis plants [39]. During concentration on solid sorbents (silica gel, graphitized carbon black, porous FTFE and others) the components interact at ambient temperature to generate more toxic but not less reactive substances [39,40]. The major reaction is the interaction of chlorine dioxide and nitric oxide: C102 + 3 NOz

+

N03Cl + NzOS.

The kinetics of the process can be described by a plot reflecting the degree of consumption of NO2 depending on the initial concentration of nitric oxide in ambient air (Fig. 6.3). Air aspired through a tube containing a sorbent (e.g., graphitized carbon black) at 20-35°C induces at least a 50% reaction between C102 and NO2. If we take into account numerous sidereactions [39] and decomposition of the concentrated sample during thermal desorption at 150°C [39,40], it becomes evident that the chromatographic column may receive a mixture of aggressive gases whose composition differs from that of the initial ambient air (Fig.6.4). Various transformations of the sample result in a decreased concentration of NOz plus CIOz and an increased concentration of Clz. A dry-ice cooled precolumn containing glass-fibre pack-

.oc 50

IN02

L

2 c,

Fig. 6 . 3 . Kinetics of chemical interaction between chlorine dioxide and nitrogen dioxide [40]

:

IClO, I

25 I I

0

10

I I

20 30 40 Time [mmin]

50

124

6. The reactive-sorption method and its application for concentrating pollutants

n

1

1

5 a)

4 3 2 Timejmin]

1

0

5 6)

4

3

2

1

0

Time [min]

Fig. 6.4. Chromatographic of ClZ,CIOz and NOz [40]. (a) Conventional variant; (b) method with a precolumn (RSC). 1 Chlorine; 2 nitrogen dioxide; 3 chlorine dioxide

ing removes C102 pollutant from the mixture to be analysed prior to chromatography (at - 78°C). The subsequent gas chromatographic determination becomes much more accurate, the standard deviations for C12, NO2 and CIOz decreasing 2-3-fold [40]. The removal of ozone from air samples by passage through an FeS04 filter allowed the accuracy and selectivity of the chromatographic determination of five tetraalkyl lead compounds to be increased [41]. Air (ca. 80 1) is passed through a PTFE tube (5 cm x 5 mm I.D.) connected in series with a steel adsorption tube (20 cm X 5 mm I.D.), the former being filled with FeS04 and the latter with Porapak Q . The Porapak trap is kept for 5 min at 150°C in a stream of helium and the desorbed lead compounds are collected in a cryogenic trap (15 cm x 2.5 mm) containing 4% Apiezon on Chromosorb P (60-80 mesh) at - 190°C. The concentrated pollutants are then displaced from the trap into the chromatograph at 90°C and separated on a short packed column of 3% OV-101 on Gas-Chrom Q (100-200 mesh) with temperature programming between 35 and 135°C at 20 K/min. When an atomic-absorption spectrometer is used as detector, the detection limit is 0.25-0.37 ng/m3 [41].

6.3.2.

Determination of nitrogen oxides

The direct determination of NO, NO2, N2O4 and N 2 0 is often complicated by the high reactivity of these gases [21]. The reactive gas chromatographic (RGC) method consists in transforming nitric oxide into organic derivatives [36]. It requires preoxidation of NO to NO2 [42] with subsequent measurement of the NO content by the difference of two determinations (NO2 plus the sum of NO and NO2).The individual determination of NO in a mixture of nitrogen oxides'), which have similar properties (and hence are difficult to differentiate), and with a low limit of detection is possible only by using the reactive-sorption method assisted for the concentration of pollutants [35, 431. The determination of NO by the reactive gas chromatographic method assisted by the reactive-sorption method consists in aspiring the air sample through a 16 cm X 5 mm I.D. precolumn containing molecular sieves 13X (0.5-0.25 mm fraction) precalcinated for 4 h in a flow of dry nitrogen at 350°C. Such a prechromatographic reactor adsorbs 95-97% of N20, NO2 and N2O4, but lets NO pass through. The loss of NO is less than 0.5-1.2 vol% after preconditioning of the precolumn for 30 min with a mixture of nitrogen oxides with an NO concentration of 10 mg/m3. The analytical scheme involves the determination of the NO content in the mixture of nitrogen oxides not by the difference of two measurements but directly (following the necessary procedure for oxidizing NO to NO2 by H202)as a derivative (nitrobenzene) obtained by the reaction of NO2 with benzene in the presence of concentrated H2SO4 [42]. The limit of detection of nitric oxide by this method is about mg/m3 with a relative standard deviation of 0.30%. I)

Individual determination of NO and NOz is of significant value for industrial labour hygiene.

125

6.3. Use of the reactive-sorption method

1

I1

Fig. 6.5. Reactive chromatographic determination of NO2 in exhaust fumes from a diesel engine (after NO2 has been converted into nitrobenzene) [35]. Glass column (1.5 m X 4 mm I.D.) of 15% Apiezon L on Chromaton N; electron-capture detector; column temperature 120 "C; detector temperature 275 "C. 1 Nitrobenzene; NO2 concentration in air, 8 mg/m3

4

'

7

0

I j

I

1

1

0

Time [min]

Figure 6.5 shows a chromatogram for the determination of NO (in the form of NO2) in exhaust fumes from a diesel engine, obtained by using the above procedure and the RSC [35, 431. HNOJ, HNOz, NO2 and NO impurities can be individually extracted from air by chemisorption by passing the air stream through a series of tubes with their inside surfaces covered with W 0 3 , a mixture of potassium and iodine oxide, copper iodide and Co203at a rate of 1 l/min. On thermal desorption the chemisorbed impurities can be analysed by any suitable method [44].

6.3.3.

Determination of sulphur fluorides

The analysis of a complex mixture of aggressive sulphur fluorides (SF6, SF,, S2F2,SOFl and S02F2,plus HF and F,) is greatly hampered by the active interaction of the exceedingly reactive compounds (especially HF and F2) with virtually all sorbents and materials of construction of the chromatograph [21,431. This activity results in a low reproducibility of determining the main component, namely sulphur tetrafluoride, whose toxicity is higher than that of phosgene [34]. The magnitude of the determined SF4 content depends on the sample volume, the presence of moisture, plus the ratio between the components of the fluoride mixture and the amount of HF and F2. The relative standard deviation varies within the wide range 0.4-0.9%.

126

6. The reactive-sorption method and its application for concentrating pollutants

1

I

1

20

10

0

lime [min]

Fig. 6.6. Chromatogram of sulphur hexafluoride and traces of lower sulphur fluorides on a stainlesssteel column (3 m x 4 m I.D.)of Porapak Q at 50 "C aAer RSC-assisted removal of F2 and HF from the mixture [34]. 1 carbon tetrafluoride; 2 sulphur hexafluoride; 3 sulphur monofluoride; 4 sulphur tetrafluoride; 5 sulphuwl fluoride; 6 thionyl fluoride

RSC-assisted concentration of sulphur fluoride pollutants helps to eliminate the effect of reactive Fz and HF. The air under analysis js drawn through a 4 cm x 4 mm I.D. precolumn containing sodium fluoride 0.5-0.25 mm fraction, following which the pollutants are concentrated in a trap containing Porapak Q [43]. The precolumn retains almost 100%of the aggressive Fzand HF, but lets low concentrations of SF4 (0.1-1.0mg/m3) pass through virtually intact (the adsorption of sulphur tetrafluoride by the column is less than 0.4%[34]). Figure 6.6 shows a chromatogram of sulphur fluorides generated in the ambient air of a foundry during smelting in an atmosphere of sulphur hexafluoride. RSC-assisted concentration of fluoride pollutants helps in the determination of a low content of SF4 (the limit of detection is 0.05 pg) with a relative standard deviation of 0.22% [33, 341.

6.3.4.

Determination of hydrocarbons

When determining organic compounds it is much more difficult to find the reasons for changes in the composition of air pollutants under analysis (chemical interactions with the sorbent, coadsorption, polymerization reaction, etc.) than in the determination of reactive inorganic gases, where the mixtures under analysis are simpler and usually only chemical interactions takes place [3, 431. In studying the sorption (concentration) of organic pollutants on solid sorbents with a well developed surface, the changes in the composition of the sample are greated during thermal desorption of the concentrated sample from the sorbent. An increase in temperature accelerates the chemical reactions in the concentrated mixtures including, for instance, thermal destruction [3, 7, 321. To emphasize the high accuracy of determining pollutants by the RSC-assisted gas chromatographic method as opposed to the conventional concentration technique, we can cite the results of comparative measurements of low concentrations of isopentane mixed with other hydrocarbons (Cs-C7alkanes and 1-hexene). It follows from the data in Table 6.3 that RSCassisted gas chromatography gives more accurate values for isopentane concentration (after preadsorption of the accompanying normal hydrocarbon pollutants in a precolumn containing zeolite SA). This results in a much lower probability of isomerization side-reactions leading to the generation of cis-2-butene and cis-2-pentene during thermodesorption (200°C) [71.

127

6.4.Diminution of coadsorption of pollutants

Table 6.3. Results of determining isopentane by the conventional method (concentration-gas chromatography) and the RSC-assisted GC method [38] Isopentane concentration in samples

Concentration-GC

Cf €if)

(mg/m3)

(mg/m3)

c k a*) (P= 95%)

S

(mg/m3)

(mg/m3)

(mg/m3)

( p = 95%)

52 64 80 91 102

70 82 93 110 120

26 34 38 40 56

70 f 27 82 f 36 93 f 40 110 f 42 120f59

55 70 75 88 104

12 15 20 22 25

55 k 13 70 k 16 75 f 21 88 k 23 104 f 26

*)

Isopentane detected

c

S

RSC-GC

c

n=6

6.4.

Diminution of coadsorption of pollutants

The composition of organic pollutants that are being concentrated often becomes distorted owing to coadsorption of the pollutants in a trap containing a sorbent [3, 7, 451. This process, which affects the correctness of analysis, is inevitable during the extraction of the components of any pollutant mixture from ambient air [3]. To explain the nature and mechanism of coadsorption and its effect on the parameters of sorption (concentration) of pollutants on sorbents, we should consider the behaviour of pollutant mixtures in the sorption-desorption system.

6.4.1.

Dynamics of sorption and coadsorption

When the pollutants to be analysed enter a trap containing a sorbent they are frontally concentrated and adsorbed in the sorbent layer. During this process their concentration in the air flowsharply decreases as it passes through the sorbent layer. The termination of concentration (sorbent “expiration”) is the moment when the substance under analysis appears behind the sorbent layer, namely the breakthrough. The pollutants are distributed in the sorbent layer according to their adsorptional isotherms [46]. The maximum service life of a trap containing a sorbent (the period before breakthrough) can be calculated by the Shilov equation [46, 471:

t=KL-t,

(6.2)

where L is the length of the sorbent layer, tois the time of the layer-protecting action and €? is a coefficient of the layer-protecting action (Shilov’s constant):

K = Ao/wCo

(6.3)

where A . is the equilibrium static activity, w is the flow velocity and Cois the initial concentration of pollutants in ambient air. The coefficient K is a magnitude inverse to the rate of

(

the stationary adsorption front K = L,(t - to) . l ) The utilization of a precolumn in the RSC method is handicapped by the associated impurities, which hamper the determination of the desired components by decreasing the Co va-

128

6 . The reactive-sorption method and its application for concentratingpollutants

lue. This increases I f and hence 7 (eqns.6.2 and 6.3). As this takes place, the increased t value “improves” the sorption behaviour of the desired components (in the absence of associated impurites), enhances the trap sorption capacity and allows errors in pollutant determinations due to abnormal sorption to be avoided. A more rigorous consideration of the role of RSC in decreasing the coadsorption is possible by using some equations of the classical theory of adsorption [48, 491. Using the lattice adsorbent model as applied, in particular, to the surface of crystalline adsorbents (graphitized carbon black, zeolites, etc.), one can obtain the Langmuir sorption isotherm: biP Qi = ____ (6.4) 1 + biP where Qi is the surface coverage; bi is the adsorption coefficient; P is the adsorbate pressure in the gaseous phase. To derive the Langmuir equation, the following conditions should be observed [48]: surface homogeneity (equivalence of the lattice sites); the energy of interaction of the adsorbent molecules should be neglected; one molecule is adsorbed on an adsorption (lattice) site (monomolecular adsorption); and the adsorption is localized in nature. A real surface of carbon adsorbents is inhomogeneous but it can be subdivided into homogeneous portions whose adsorption can be described by the Langmuir equation. If this condition is observed and the portions are exponentially distributed over the adsorbent surface, the heats of adsorption can give the known empirical Freundlich equation for an inhomogeneous surface [48, 491:

0 = mPn (6.5) where m and n are temperature-dependent coefficients. Let us consider the behaviour of the pollutant sorption mechanism when using the RSC method. For this purpose we shall write the Langmuir equation for a mixture of pollutants being concentrated in the sorbent trap:

where Ci is the concentration of the associated impurities; C1 is the concentration of the desired (controlled) component; k is the number of pollutants in the mixture to be analysed. Owing to the previous trapping of the associated components in the precolumn, the sorbentk

containing concentrator receives only a desired component so that the term

1 biCi in i=2

eqn. 6.6 becomes negligible. Therefore, the adsorption of the desired component on a homogeneous surface can be described by the Langmuir adsorption isotherm equation:

(6.7) If the adsorption sites are distributed exponentially, then it can be demonstrated that the desired component adsorption can be described by Freundlich’s equation (eqn. 6.5). At a constant flow (Co*W) of the aspirated air, C, (the initial concentration of the pollutants in the air) acts as an equilibrium absorbate concentration in the gaseous phase. According to Shilov’s equation (eqn. 6.2), the time to breakthrough (z)is linearly related to the sorbent layer protective action coefficient (K)(eqn.6.3) and therefore it is inversely proportional to Co.

129

6.4. Diminution of coadsorption of pollutants

Fig. 6.7. log t - log C dependence (for the desired components) at low concentrations (1-10 mg/m3) of vinyl chloride (.), acetone (01 and methanol (A) in a tube filled with charcoal (0.8-1.0 mrn fraction) at 20°C and ah air aspiration rate of 0.4 Vmin (after removal of the associated impurities by the RSC method)

C

w

0 710 log concentration [ m g l r n j ]

Hence, increasing Co gives rise to a decrease in t,i.e., the breakthrough occurs faster for higher concentrations (see eqns.6.2 and 6.3). Because the amount of adsorbent is an increasing function of Co, the surface coverage is proportional to l / t . Then Freundlich's equation can be written as follows'):

rnC" (6.8) where n is negative. Hence the use of the RSC method, permitting irreversible retention of the identification (determination)-impeding impurities in the precolumn, so that only the desired component finds its way into the concentrator trap, is favourable for creating conditions for the sorption of the controllable components that are identical with those typical of the equilibrated adsorption of the individual components. Making the dynamic conditions of the pollutant sorption come nearer to the static conditions, which is possible by using the RSC method, allows the sorption of the desired pollutants in the case of competing sorption of the associated impurities to be described with the aid of Freundlich's static sorption isotherm for an inhomogeneous surface.2) The possibility of representing the experimental data on log t - log C coordinates in the form of a straight line is a criterion for the applicability of Freundlich's equation. The logarithmic form of this equation: t=

(6.9) l o g z = l o g r n + n logC is shown graphically in Fig. 6.7. It is evident that log rn is the intercept on the y-axis (AB) whereas the other constant, n = log a = AB/AC, defines the slope of the straight line [45]. By determining the values of the constants rn and n from the plot, it is possible to draw practically important conclusions about the optimal concentrator parameters and nature (suitability) of a sorbent for the effective sorption of the desired components in the case of competitive sorption of the associated impurities. Thus for vinyl chloride and acetone (Fig. 6.7) log shows a slight variation with the concentration of these compounds between 1 and 10mg/m3 so that activated birchwood charcoal I ) The same conclusion can be made with the use of an equation describing the sorption characteristics of a compound being extracted from a flow with an adsorbent, which is known in chromatography. ') This reason is responsible for the fact that using Eqn. 5.7 in Chapter 5 as described elsewhere [45] is not adequately correct.

10 Berezkin. Gas Chrom.-BE

130

6. The reactive-sorptionmethod and its application for concentrating pollutants

(ABC) can be used to advantage for concentrating these species as they are extracted from contaminated air in the maximum permissible concentrations. On the other hand, for methanol in the same concentration range one can observe a significant decrease in log with increasing concentration. This suggests that activated charcoal is unsuitable for concentrating polar CH,OH molecules; traps containing silica gel or polymeric sorbents are preferred. However, such a selection is not always an absolute requirement. For example, when there are a number of the desired components and not all of them can be extracted with the same sorbent, one has to adopt a compromise solution from consideration of the maximum extraction of all the desired pollutants. Thus, in the solution of a practical problem such as the determination in air of the toxic products resulting from the evaporation of methanol-containing fuel, i.e., those incorporating methanol and isobutanol (the products containing alcohols, alkylbenzenes, alkanes and naphthenes), it is important to detect not only CH30H but also another, even more harmful, target product, namely benzene. We have shown [78] that when the problem consists in extracting methanol and benzene impurities from air, activated charcoal is more suitable than sorbents such as ASM silica gel, zeolite 3A and carboncontaining polymeric Ambersorb XE-340, exhibiting a n affinity for CH30H. This can be attributed to the fact that activated charcoal is almost an ideal adsorbent for trapping hydrocarbon pollutants so that for the complete adsorption of low methanol concentrations it would be sufficient to increase the amount of sorbent in the tubular concentrator to 300 mg. This allows the efficient extraction (87-90%) from air of such dissimilar compounds as benzene and methanol.

6.4.2.

Diminution of coadsorption

The effect of coadsorption can be so strong as to cause 20-50% losses of target components during the concentration of pollutants. This is 7-8 times higher than the losses due to incomplete desorption of pollutants from the sorbent during the extraction of individual compounds from ambient air [3]. An example is the conventional concentration from air of low concentrations of vinyl chloride in the presence of dichloromethane (with a concentration of 2.5 mg/m3 in ambient air), 1,2-dichloroethylene (4.1 mg/m3), chloroform (5.1 mg/m3), dichloroethane (3.4 mg/m3), butyl chloride (3.0 mg/m3), trichloroethylene (26.8 mg/m3) and tetrachloroethylene (66.7 mg/m’). The data in Table 6.4 show the effect of coadsorption of chlorinated hydrocarbon pollutants on the detection of vinyl chloride. In this instance the presence of chlorinated hydrocarbon pollutants in ambient air leads to a considerable loss of vinyl chloride (44%). It should be noted that the coadsorption of other organic pollutants that often accompany vinyl chloride in ambient workplace air during the treatment of poly(viny1 chloride)-based Table 6.4. Effect produced by coadsorption of chlorinated hydrocarbons on the detection of vinyl chloride in a concentrator containing active carbon [51] Vinyl chloride concentration in air sample (mg/m’)

10.2 10.0

Concentration of accompanying pollutants (chlorinated hydrocarbons) (mg/m’)

Period before breakthrough

Vinyl chloride losses.) during concentration and desorption extraction

(min)

(%I

0 110

40 24

6.2 44.0

*) Concentration temperature, 20°C; aspiration rate, 400 ml/min.

131

6.4. Diminution of coadsorption of pollutants

5 I0 15 20 25 30 35 40 c-

Fig. 6.8. Effect of accompanying pollutants (propylene, methanol, acetone, acetaldehyde, tetrachloroethylene) on the isotherm of vinyl chloride sorption by active carbon [33]. a is the amount of vinyl chloride adsorbed by acitve carbon (mg/g); c is pollutant concentration (mg/ m3); I isotherm of vinyl chloride adsorption; 2 isotherm of vinyl chloride adsorption in the presence of accompanying pollutants (with a 1:l ratio of vinyl chloride to pollutant concentration); 3 isotherm of vinyl chloride adsorption in the presence of pollutants (with a 1:8 ratio of vinyl chloride to pollutant concentrations)

polymers [3, 501 can lower the adsorption of monomeric pollutants to a greater extent than halogenated compounds (Fig. 6.8). The use of a precolumn in the above instances has a positive effect in terms of a prolonged period (volume) before the breakthrough of the main components, an increased real sorption capacity of the sorbent concentrator and an improved limit of detection of pollutants. This is Table 6.5. Changes in volumes before the breakthrough of organic pollutants in a column containing active carbon during RSC-assisted concentration [38, 511 Target component

Benzene Toluene n-Hexane Ethyl methyl ketone Ethanol Tetrachloroethylene Triethylamine Dipropylsulphide

Concentration

100 50 100 160 1000 160 160 100

TLV

5 50 100 200 1000 10 10 50

Volume before breakthrough of active carbon (MOO mg)

No precolumn

Precolumn

Literature data*) [52 - 561

30 12 14 3 4.5 8 4 12

53 20 35 8.8 11 24 8

42 23.1 33 8.2 3-25 33.2 6.6

20

22

*) Literature data are cited for the values obtained under conditions closest to those of the given experiment [38, 5 11 (concentration, temperature, aspiration rate, type of sorbent, etc.). 10'

132

6. The reactive-sorption method and its application for concentrating pollutants

Target components

Sorption capacity (mg/g of sorbent)

Benzene Toluene n-Hexane Ethyl methyl ketone Ethanol Tetrachloroethylene Triethylamine Dipropyl sulphide

No precolumn

Precolumn

Literature data [3,5,7,13, 54, 551

30 80 18 36 15 26 20 64

53 210 35 75 35 78 38 170

60 280 42 90 42 100 35.5 156

Table 6.6. Sorption capacity of a concentrator containing active carbon during RSC-assisted concentration of pollutants from ambient air [38, 511

ascertained by experimental data obtained during the sorption of various organic pollutants (see Table 6.5) when use was made of RSC-assisted concentration and a trap containing active carbon. In Table 6.5 the period before the breakthrough was determined for individual compounds in the presence of accompanying pollutants. A prolonged period before the breakthrough of the studied substances in trace concentrations resulted in a corresponding 2-3-fold increase in the real sorption capacity of the column containing a sorbent with respect to the main pollutant components (Table6.6). The efficiency of sorption in the concentrator (active carbon) -precolumn system was comparable to that of individual compounds on the same adsorbent as calculated from the literature data [3, 5, 7, 13, 52-56]. The sorption capacity was calculated from the results of chromatography of the studied mixtures with account being taken of the moment of breakthrough, or else was determined by the direct method, viz., by weighing the concentrator containing carbon prior to air aspiration and after the breakthrough. It can be seen from Tables 6.5 and 6.6 that the removal of accompanying impurities (pollutants) from the precolumn (in calculating the sorption characteristics for each compound listed in the tables the assumption was made that all the other compounds in the mixture were accompanying impurities) makes it possible to reduce coadsorption considerably and increase the efficiency of detecting the target components. The greater capacity of the sorbent for certain pollutants, as evidenced by literature data, is due to the fact that those values were obtained for pure individual substances whereas when a precolumn is used the accompanying pollutants are not always completely removed. This affects the residual sorption

Pollutants under analysis

Aromatic hydrocarbons Alkanes Halogenated hydrocarbons Oxygen-containing compounds Sulphur-containing compounds Aromatic amines

Determination limit.) (mg/m3) Conventional concentration

RSC-assisted concentration

0.10 0.10 0.50 0.80 0.60 0.20

0.03 0.06 0.25 0.40 0.40 0.10

*) With a relative standard deviation of 0.25-0.35%.

Table 6.7. Reduced limit of determination of pollutants using RSC-assisted concentration [38]

133

6.5. ImDrovement of chromatographicseparation

Table 6.8. Comparison of some metrological characteristics of two methods of determining vinyl chloride: concentration-gas chromatography (method I) and RSC-assisted gas chromatography (method 11) [57] Method I

c

Method I1

S

s,

c+s

c

S

sr

6 (AC) ( P = 95%)

0.98 0.56 0.10

0.17 0.14 0.03

0.18 0.25 0.30

0.98 k 0.12 0.56 k 0.10 0.10 k 0.02

( P = 95%) 0.70 0.44 0.11

0.28 0.16 0.05

0.40 0.36 0.45

+

0.70 0.20 0.44 f 0.12 0.11 k 0.04

c+

Experimental conditions: rn = 10 p1; n = 10 ; 6 and S are given in mg/m3.

(coadsorption), reducing the sorption of target components by approximately 15-25%). An increased efficiency of sorption of target components during RSC-assisted concentration helps to increase the sensitivity of determination. It can be seen from Table 6.7 that reactivesorption concentration ensures an approximately 2 -3-fold improvement in the limit of detection of the main organic pollutants. Another important advantage of the RSC related to reduced coadsorption is the improvement of virtually all the metrological characteristics of the determination of pollutants. This is evident from the data in Table 6.8 for the determination of vinyl chloride micropollutants. Comparison of the results obtained by determining this compound by two different methods (concentration-GC and RSC-GC) demonstrates vividly the advantages of the RSC.

6.5.

Improvement of chromatographic separation

An important aspect of using the RSC is the improvement of the chromatographic separation of pollutants. Precolumn adsorption of the accompanying pollutants increases the efficiency of separation of the target components from other compounds in the sample. Apart from the possibility of obtaining more reliable information regarding the sample composition, this approach also helps to increase the accuracy of determining the target components [3, 36, 381. An example is the direct (without preconcentration of pollutants) chromatographic determination of low concentrations of carbon monoxide, a major pollutant in ambient workplace COz and water vapour, which air [37]. The adsorption of CI-C3 volatile hydrocarbons, Oz, Nz, prevent the determination of CO, with the aid of a precolumn containing Askarit, molecular sieves 13X and active carbon at 0°C ensures a good separation of CO from CHI and other pollutants on an analytical column containing zeolite 5A, and increases the accuracy of the determination of CO. When concentrating CO in the range 20-50 mg/m3 the RSC helps to reduce the relative error in determining this compound from 18-20% to 12%. This can also be achieved by using a precolumn containing a layer of Chromosorb W impregnated with 0.1 M SnClZsolution (to absorb H2S) and a layer of active carbon (210 mg) to adsorb hydrocarbons capable of oxidation [58]. A unique method for the isolation of CO, NOz, CzH4,Oz and other compounds from a gaseous mixture has been proposed [59]. The method is based on the formation of the compound MnXz (phosphine), where X is an inorganic ion (chlorine, iodine, bromine or thiocyanate) and phosphines are represented by compounds with the general formula P(Ph)3, P(Ph)zR, PPhR, or PR3, where R is the alkyl radical. Such a complex is capable of retaining

134

6. The reactive-sorutionmethod and its application for concentratingpollutants

these gases reversible. Selection of a substituent X and a suitable phosphine can result in a situation when the extraction of a mixture of gases from air can give selectively (with subsequent complete desorption) any of the components. Thus, to extract COz from a mixture with other gases, MnI,[P(C,H,),] in tetrahydrofuran can be used as the trapping solution as the precolumn is cooled to -69°C. On completion of COz trapping the precolumn is heated to O"C, and the resultant carbon monoxide is determined by a suitable method. To differentiate mixtures of reactive gases in determining SOz, nitrogen oxides are absorbed in 0.6%sulphamic acid, and H2S in Pb(CH3C00)z [60]. Different variants of RSC-assisted concentration have been employed to improve the chromatographic separation of hydrocarbons [511, halogenated hydrocarbons and oxygen-containing organic compounds [38], and also reactive inorganic gases [36] and various inorganic compounds [43]. To reduce the error of determining low concentrations of hydrazine aerosol and asymmetric dimethylhydrazine during their extraction from air, the interfering pollutants (ethyl acetate, 2-furfuralde. hyde and their derivatives) were first removed from the mixture using a precolumn [61].

6.6.

Moisture removal

An important condition for preventing losses of pollutants due to coadsorption is the removal of moisture during extraction of the pollutants from ambient air [3,21, 361. This problem has several solutions [36, 431 the best of which is, in our opinien, the utilization of reactive-sorption systems [31, 321. RSC-assisted concentration, involving the removal of water in precolumns containing zeolites, does not affect other components of the sample or change the sample composition 131. When CO ultramicro-concentrations are determined automatically in the troposphere with the aid of a helium ionization detector, an additional column containing zeolite SA, which is periodically regenerated, is attached in front of the analytical column to separate major interfering pollutants (H,, 02,N z , COz and HzO)[62]. Another RSC variant is based on using a short (7 cm X 4 mm I.D.) column containing zeolite 3A activated for several hours in a flow of dry nitrogen at 300°C [63]. This precolumn effectively removes HzO but leaves organic pollutants unsorbed except CH30H. Therefore, molecular sieves 3A can be regarded as promising sorbents for absorbing moisture when determining micropollutants in ambient air [3, 36, 43, 631. The precolumn can be packed with lithium chloride, which does not trap organic impurities. It is applied to the glycerine-modified surface of porcelain powder [64]. To concentrate amphetamine vapour from air, a layer of polymeric sorbent is placed ahead of the polymeric sorbent layer; a mixture of Ca(OH),, NaOH and H20is placed ahead of the polymeric sorbent layer in a Tenax GC trap to prevent the competitive sorption of COz [65]. The air sample is passed through a tube (7.5 cm x 4 mm I.D.) filled with adsorbent (50 g of natroncalk') and 30 g of Tenax) at a flow-rate of 1 l/min for 60 min, then the tube is connected to the chromatograph's injector and the volatile impurities and moisture are blown out with the carrier gas (nitrogen) for 40 s at 250°C. Subsequently the nitrogen stream is connected to a chromatographic column packed with OV-1 on Ultrabond 2 0 M and the concentrate obtained is analysed at 160°C using a nitrogen-phosphorus detector. The detection limit is 0.03 pg/m3.

I)

Natroncalk is a mixture of NaOH, CaO and asbestos-(Mg,CaSi,O,,).

13s

6.7. Identification of pollutants

6.7.

Identification of pollutants

A major task in environmental analytical chemistry is the reliable identification of pollutants of very complex compositions [3, 331. Conventional gas chromatographic analysis of pollutants involves the sorption of all compounds contained in air on solid sorbents, extraction of the concentrated pollutants with a solvent and gas chromatographic separation and identification of the sample components [2, 31. All the compounds extracted from air, without, must be fixed on the chromatogram. This fact considerably hampers the identification of pollutants, as generally we are dealing with complex mixtures containing compounds of various classes [4, 5-71. The difficulties related to obtaining reliable information concerning the composition of complex mixtures of organic pollutants’) are due to insignificant differences in the values of the retention characteristics of the pollutants (e.g., Kovhts retention indices) [3, 311. The use of a chemical filter (which, in fact, the RSC is) that consecutively removes certain compounds from the mixture greatly facilitates the chromatographic identification of pollutants and makes it more reliable [3].

6.7.1.

Determination of hydrocarbons

The simplest analytical task solved with the assistance of the RSC is the identification and subsequent gas chromatographic determination of hydrocarbons in [3, 331. This is explained by the fact that hydrocarbons are the most inert of all organic compounds, and the accompanying organic pollutants with various functional groups can be removed from the air sample without affecting the hydrocarbons themselves. This can be achieved by using a prechromatographic reactor containing a strong reducer (e.g., LiAlH4) or zeolites (SA, lox, 13X) that differentiate and “filter” hydrocarbons from the accompanying organic pollutants of different classes [66, 671. A precolumn containing H3P04,Versamide 900, L M H 4and Pb(CH3C00)2would be effective for this purpose [66]. This reactor absorbs all organic pollutants well (75-95%), except for the hydrocarbons themselves (2-4%) [67]. These compounds are listed in Table 6.9. They belong to harmful pollutants of various classes.

Table 6.9. Organic pollutants retained by a precolumn during RSC-assisted determination of hydrocarbons [66,67]

Oxygencontaining compounds

Sulphurcontaining compounds

Nitrogencontaining compounds

Halogencontaining hydrocarbons

Alcohols Aldehydes Ketones Acids Acid anhydrides Esters Lactones Epoxides

Mercaptans Dithiols Thioesters Disulphides Sulphides Thiocyanates Isothiocyanates

Amines Amides Lactams Imides Nitriles Isonitriles Aliphatic nitro compounds Aromatic nitro compounds Isocyanates

Akyl halides Aryl halides

I ) The identification of inorganic pollutants often differing by their chemical and physical properties is generally a simpler task as compared to the determination of the qualitative composition of organic mixtures.

136

6 . The reactive-sorption method and its application for concentrating pollutants

I

I

-50

-50

bl

I

- 10

I

I

I

30 70 Column temperature

Pc]

110

Fig. 6.9. Chromatograms of compounds (a) unretained and (b) retained by the precolumn obtained on analysing gasoline vapour during thermal oxidative cracking [70]

To absorb reactive organic pollutants selectively from ambient air (e.g., alkenes participating in the generation of photochemical smog) and separate them from inert alkanes, use is made of a precolumn containing Chromosorb P impregnated with a solution of chromium anhydride in concentrated sulphuric acid [68].To achieve the reliable identification of traces of styrene in air containing various organic pollutants, HOSHIKA [69]used a reactor-precolumn containing NaOH to absorb esters and 2,4-dinitrophenylhydrazineto remove carbonyl compounds from the air. Traces of styrene passed through the system unchanged and were concentrated in a Tenax column. Another application of RSC-assisted concentration involves complex-forming sorbents based on lanthanoid chelates [70].Thus, to identify components of various organic pollutants contained in tobacco smoke, use was made of a reactor-precolumn filled with a europium This reactor seleccomplex with p-di(4,4,5,5,6,6,6,-hepta-fluoro-1,3-hexanedionyl)benzene. tively retains nucleophilic compounds (alcohols, aldehydes, ketones and others) but allows

137

6.7. Identification of pollutants Table 6.10. RSC-assisted identification of hydrocarbon pollutants (72 -751 Main components

Sorption of main components

Sorbent in precolumn

Retained in precolumn

(%I

(%)

Aromatic hydrocarbons

2-3

Zeolite 5A

Alkanes alkenes and naphthenes*) Alkanes, alkenes naphthenes, aromatic hydrocarbons Alkanes, naphthenes, aromatic hydrocarbons**)

5-6

Poly(viny1 chloride) Maleic anhydride***)

1-2 2-3

Alkanes, cycloalkanes'.)

2-4

Sum of hydrocarbons*)

3-8

Sorption completeness

1:l mixture of HgC104 and HC104 Mixture of 20% HzSO4 and 20% HgSO,***) Concentrated H2S04***)or mixture of 4% AgNO, and 95%H2S04 Molecular sieves 1OX

n-Alkanes n-Alkenes n-Alkynes Aromatic hydrocarbons Dienes

89- 97 80- 86 98-100

Alkenes Alkynes

86- 93

Alkylbenzenes, alkanes, alkynes

90

Alkylbenzenes

93

*) Verified **) Can be

and used by the authors to identify pollutants. used for analysing hydrocarbon pollutants. ***) On a diatomite carrier.

alkanes, alkenes, aromatic hydrocarbons and chlorinated organic compounds to pass intact. The compounds unretained by the precolumn were then analysed on a capillary column containing SE-54 silicone with temperature programming from -50 to 180°C. Figure 6.9 illustrates the application of the above method to the determination of the products of thermal oxidative cracking. It should be noted that chelate formation reactions can be used successfully for the selective concentration of many heavy metals, e.g., in a trap containing polyurethane sponge impregnated with ligand solution [71]. Table 6.10 lists some examples of the application of RSC to the chromatographic analysis of pollutants.

6.7.2.

Determination of oxygen-containing compounds

A reactor containing AgN03, H3P04,Versamide 900 and lead acetate [31,761 for the separation of oxygen-containing compounds from air pollutants cannot be regarded as completely reliable for their selective separation. The reactor allows alkanes and naphthenes to pass in addition to the main components (oxygen-containing compounds). To remove their interfering effect, subsequent gas chromatographic analysis of the pollutants concentrated in a sorbent trap was carried out on a column containing superpolar 1,2,3-tris-l3-(cyanoethoxypropane). This approach ensures the unequivocal identification of the harmful pollutants

138

6. The reactive-sorption method and its application for concentrating pollutants

Table 6.11. RSC-assisted concentration and identification of oxygen-containing organic pollutants [3, 31,72-751 Target components

Sorbent in precolumn

Retained in the column

Oxygen-containingcompounds

AgNO,, HJ’04, H,POJ, Versamide 900 and Pb(CH,C00)2 Versamide 900, AgNO, Poly(viny1 chloride), AgNO, , Versamide 900

All organic compounds except alkanes and naphthenes

Oxygen-containingcompounds Oxygen-containingcompounds Aldehydes, ketones

Boric acid

Alcohols Ketones Esters Lower carboxylic acids Aldehydes, ketones, esters

2,4-Dinitrophenylhydrazine o-Dianisidine, zeolite SA Molecular sieves SA Benzidine KOH

Halogenated hydrocarbons Halogenated hydrocarbons, alkylbenzenes, alkenes, alkynes Primary and secondary alcohols Aldehydes, ketones Aldehydes Carboxylic acids Aldehydes, ketones Carboxylic acids

pertaining to polar compounds (e.g., alcohols or aldehydes), which are readily separated from non-polar compounds in such column [3, 31, 771. When 15%stationary liquid phase (SLP)is used in the chromatographic column, CH30H is eluted after n-decane, and with 30% SLP, after tridecane. This technique of combining RSC-assisted concentration with a subsequent highly selective separation of the main components from the accompanying impurities, in which in the first stage (RSC) oxygen-containing pollutants are separated from non-polar hydrocarbons and in the second stage (chrornatography) additional selective separation of polar and non-polar pollutants is carried out, ensures the unequivocal identification of polar target components (e.g., methanol or benzene) [78]. To determine trace amounts of methanol in aldehyde presence when analysing air of automobile cab [3,38] use was made of a reactor containing NaHSO,, an Chromaton N [38]. This precolumn retains law-molecular-weight aldehydes but allows methanol and hydrocarbons to pass to a trap containing a sorbent, after which they are separated on a column containing 0.2%PEG 1500 on graphitized soot and unequivocally identified using the characteristics of pollutant retention [3]. In simpler instances, when the air sample does not contain all possible pollutants but only certain classes of compounds (e.g., only hydrocarbons, only halogenated hydrocarbons, only alcohols and aldehydes, etc.), the identification of pollutants becomes easier (see Table 6.1 1). Several workers [79-831 used RSC to identify and determine oxygen-containing organic pollutants. When the air sample is drawn through a short precolumn containing 2% NaOH on a solid carrier [81], a mixture of 2,4-dinitrophenylhydrazineand H3P04on glass pellets [82] or Na2C03[79], correct identification of lower aliphatic acids and carbonyl compounds is possible after the pollutants have been concentrated in a trap containing silica gel or in a concentrator containing a mixture of 0.1%FFAP and H3P04on Carbopack C [79, 821. A similar precolumn containing 20% KOH and glass pellets [83] was used to identify phenols reliably in the presence of carboxylic acids. Another application of RSC [38] is the determination of ethanol in expired air. A good approach to separating the accompanying aldehydes (especially acetaldehyde) and ketones is to use several prechromatographic reactors containing H$03, o-dianisidine, 2,4-dinitrophenylhydrazine, benzidine or hydroxylamine (see Table 6.11).The use of precolumns with varied

139

6.7. Identification of pollutants

combinations of these reagents ensures the consecutive removal (and identification) of ketones, aldehydes and alcohols according to the following reactions [84]:

(6.10) RCHO + NH20H a HCI

3 C z H 5 0 H t H3B03

6.7.3.

-

do2 RCH =NOH t HCI

+ H20

(6.11)

(6.12)

(CzH5O)j B + 3 H 2 0

Determination of halogen-containingcompounds

RSC-assisted concentration of halogen-containing substances with the aid of a precolumn (zeolite 5A, Pb(CH3C00)2,H,P04 or H2S04][31] proved sufficiently reliable for determining the composition of harmful volatiles generated during the production of poly(viny1 chloride) and the manufacture of synthetic leather, and for the selective determination of toxic compounds such as vinyl chloride (TLV = 0.1 mg/m3) and tri- and tetrachloroethylene [85-871. RSC can help to solve the problems of identifying not only chlorinated hydrocarbons in complex mixtures of air pollutants [31, 871 but, of course, also simpler compounds when, for instance, chlorinated hydrocarbons must be separated from compounds belonging to a certain class, e.g., hydrocarbons or oxygen-containing compounds (Table 6.12). It should be noted that the use of molecular sieves can lead to adsorption (8-12%) of the concentrated pollutants on the external facets of zeolite crystals, i.e., additional (“random”) sorption [32]. This fact should be taken into account when interpreting the results of pollutant identification obtained with the aid of reactors filled with zeolites SA, 1OX and 13X [3, 32, 721. Another interesting and promising application of RSC is the identification of traces of vinyl chloride in complex compositions of various pollutants. There are many chromatographic techniques for the determination of this toxic gas, which has a pronounced carcinogenic effect [31, 501. However, few of them are applicable to the reliable determination of vinyl chloTable 6.12. RSC-assisted identification of chlorinated hydrocarbons [3, 72-75, 871 Target components

Sorbent in the precolumn

Retained in the precolumn

Chlorinated hydrocarbons

Molecular sieves 5A concentrated H2S04 Molecular sieves 5A concentrated H2S04 Metallic hydrides

Oxygen-containing compounds

Chlorinated hydrocarbons Chlorinated hydrocarbons Vinyl chloride tri- and tetrachloroethylene

Molecular sieves 5A, H2S04and Versamide 900

Hydrocarbons, oxygen-containing compounds Oxygen-, nitrogen- and sulphur-containing compounds Hydrocarbons, oxygen-containing compounds

140

6. The reactive-sorption method and its application for concentrating pollutants

Table 6.13. Retention volumes of pollutants accompanying vinyl chloride during its determination (detection) in ambient air [31, 501 Pollutant

Methane Ethane Ethylene 1,l-Difluoroethylene Propene Propane Methylacetylene Methyl chloride 1,l-Difluoroethane Chlorodifluoromethane Cyclopropane Formaldehyde 1-Chloro-1 ,1-difluoroethane Acetaldehyde Vinyl chloride Freon 114 Isobutane Isobutene Methanol 1,3-Butadiene 1-Butene Vinyl methyl ether trans-2-Butene Ethyl chloride cis-2-Butene Vinyl bromide 1,l-Dichloroethylene

Chromosorb 102 Chromosorb 102 Porapak Q (100°C)

(145°C)

(100°C)

0.15 0.21 0.21 -

0.33 0.33 0.62

0.05 -

-

0.54 -

0.63 -

-

0.93 1.o 1.22 1.37 1.57 1.43 1.57 1.70 1.73 2.00

-

0.51 0.53 0.92 1.o 1.21 1.25 1.27 1.30 1.36 1.38 1.43 1.85 -

-

0.46 0.52 0.56 0.57 0.59 0.62 0.95 1.0 -

-

-

-

-

-

0.4% PEG 1500 on Carbopak A (20°C) 0.20 0.29 0.26 0.63 0.63 0.63 0.45 -

-

0.77 1.o

-

1.38 2.92 1.54

-

ride in the presence of accompanying impurities (see Table 6.13), such as chromatography on columns of Carbowax 1500 on graphitized soot, a mixture of Porapak S and T or picric acid on Carbopack C [3, 311. It can be seen from Table 6.13 that vinyl chloride is eluted almost simultaneously with C1-C3hydrocarbons, Freons, chlorinated hydrocarbons, acetaldehyde (the most frequent pollutant) and many other volatiles, whose number may reach 65 [31, 501. RSC helps to overcome the difficulties of separating vinylchloride from accompanying pollutants and reliably identifying it in various mixtures of toxic pollutants after preaspiration of an air sample through a precolumn containing zeolite 5A, H2S04 and K2C03 (or Versamide 900) [86,87]. This approach makes it possible to remove about 70-80% of the accompanying pollutants and to determine vinyl chloride selectively at a level of lo-' mg/m3 [86] using conventional chromatographic techniques [87]. Another example of the effective application of RSC to the determination of low concentrations of vinyl chloride in workplace air is the separation of volatiles in the air of a shop manufacturing synthetic leather (Fig. 6.10) and in a plant forming poly(viny1 chloride) into articles (Fig. 6.11). Fig. 6.11 shows that after the air sample has been drawn through a precolumn

141

6.7. Identification of pollutants

a)

bl

I

1

I

12

9

6

12

9

6 Time [min]

k. I

1

3

0

3

0

Fig. 6.10. Identification of vinyl chloride in a complex mixture of volatiles entering air during the heating of synthetic leather. (a) Chromatogram obtained without a precolumn; (b) chromatogram obtained with a precolurnn containing zeolite 5A and sulphuric acid [86]. 1 Vinyl chloride; chromatographic peaks 2-10 were not identified; 11 toluene (solvent)

containing zeolite 5A and HzSO4, all alcohols, aldehydes and aromatic hydrocarbons have been removed from the sample. Subsequently, after the air has been aspired through a precolumn containing Versamide 900 (ethylenediaminetetracetate amide), certain chlorinated hydrocarbons are removed, except for vinyl chloride [87],whose reliable identification is the major objective of this technique. A similar method was used to advantage in determinations of residual amounts of monomer (vinyl chloride) in a variety of PVC samples on heating the polymer to SO-140°C 1881. An interesting example illustrating the potential of RSC is the comparison of results obtained in identifying volatile pollutants generated during the thermal destruction of poly(vinyl chloride) (manufacture of synthetic leather) by the GC-MS -RSC-GC methods (see Table 6.14). It can be seen that during thermal desorption of the concentrated pollutants (at 200°C) over 30 compounds were identified by GC-MS, most of which are absent from the mass spectrum obtained by GC-MS after extraction of sample pollutants. This is apparently due to sample decomposition in the former instance, which leads to the generation of new accompanying pollutants.

142

6. The reactive-sorption method and its application for concentrating pollutants

I

22

0

0

5

10

n

14

23

I

I

40 Cl

"

0

I

I

15

?/me [min]

Fig. 6.11. Chromatograms of hydrocarbons generated during poly (vinyl chloride) treatment [87]. (a) Without a precolumn; (b) after aspiring the air through a column containing zeolite SA plus sulphuric acid; and (c) after aspirating the air through a precolumn containing zeolite 5A, sulphuric acid and Versamide 900. I Methane; 4 vinyl chloride; 7 dichloromethane; 9 1,2-dichloroethylene; I 1 1,2-dichloroethane; 15 butyl chloride; I7 trichloroethylene; 22 tetrachloroethylene; 23 chlorobenzene (solvent); peaks 2, 3, 5, 6, 8, 10, 12-14, 16 and 18-21 were not identified

143

6.7. Identification of pollutants Table 6.14. Identification of volatiles generated during thermal destruction of poly(viny1 chloride) Compound identified

GC-MS*) (thermal desorption)

GC-MS**) (extraction)

RSC-GC**) (extraction)

Ethane Dichloromethane Diethyl ether Ethyl chloride 2-Butene Acrolein Methyl acetate PPropiolactone Acetone Furane Methyl vinyl ester tert.-Butylamine Ethyl acetate Ethanol Acetic acid

+ + + + + + + + t + + + + + + + + + + + + + + + + + + + + + + +

+ + + + + + +

+ +

1-Chloro-2-methylpropane Benzene Butyl chloride Methyl propyl ester Crotonaldehyde 1-Methylbutanol Methylacrolein Propyl acetate Toluene Isoamyl acetate Isobutyric acid 3-Chloro-2-butanol 2,3-Butanediol Heptanal Chlorobenzene Valeric acid Dimethyl phthalate

+ + + + + + + + + + +

+ + +

+ + + + + +

*) Sampling in a Tenax column and desorption at 200°C. **) Extraction of pollutants form the sorbent with chlorobenzene or toluene.

On the other hand, the compounds identified by the RSC-GC method were also detected by GC-MS when “mild” conditions were used to remove the concentrated pollutants from the trap. It should be noted that in addition to the pollutants cited in Table 6.14, some of which

are false, the RSC-GC method helped to identify 1,2-dichloroethylene, chloroform and triand tetrachloroethylene in the analysed mixture.

6.7.4.

Determination of sulphur-containing compounds

Sulphur-containing organic compounds, most of which have strong odours, need to be detected in certain branches of petroleum chemical production. They actively pollute the ambient air of oil refining plants [3].

6. The reactive-sorptionmethod and its application for concentrating pollutants

144 Main components

Sorbent in precolumn

Retained by precolumn

Mercaptans

Zeolite 4A

Mercaptans, sulphides, thiopenes Sulphides, mercaptans, thiophenes Sulphur dioxide

Versamide 900

C1-C3hydrocarbons, C1-C3alcohols Alkyl halides, Aryl halides

Poly(viny1chloride) (55 - 75°C) NaHCO,

Table 6.15. ~~~~~l~~ of identification of sulphur-containing organic pollutants with the aid of RSC 172, 751

Alkylbenzenes, primary alcohols, amines Sulphides, mercaptans, HIS

RSC-assisted concentration is, in our opinion, as effective a method of identifying organosulphur pollutants as the use of sulphur-selective flame photometric and photoionization detectors [3, 381. The flame photometric detector gives no noticeable signal for hydrocarbons but responds to many compounds with functional groups containing sulphur, phosphorus, etc. [89]. The photoionization detector allows the determination of microgram amounts of sulphur compounds [3, 431 but gives similar signals for alkylbenzenes and some other hydrocarbons [90]. In some practical GC analyses, especially when it is desired to separate thioalcohols and thioesters from hydrocarbons, the use of the precolumns cited in Table 6.15 yielded good results. RSC-assisted concentration of pollutants proved effective for identifying traces of sulphur compounds in air containing vapours of BR-2 gasoline [91] and white spirit [31], in volatile components of rubbers derived from butyl rubber [92] and in the workplace atmosphere of oil plants [38, 511. A precolumn containing NaHCO, makes ist possible to separate traces of sulphur dioxide from H2S, mercaptans, sulphides and disulphides [93].

6.7.5.

Determination of nitrogen-containing compounds

The application of RSC for this purpose is the most difficult (particular applications excluded), as nitrogen-containing compounds are often reactive, which hampers their chromatographic determination. For this reason, it is difficult to choose reagents (or sorbents) that would effectively sorb the accompanying pollutants (generally less reactive than the nitrogenated compounds) but would not react with nitrogen-containing compounds (amines, amides, nitriles, nitro compounds, etc.) [3, 311. A reactor containing zeolite 5A and Versamide 900 [31] used for this purpose retains normal hydrocarbons, some oxygen-containing compounds and active halogenated compounds, but allows alkylbenzenes, naphthenes, iso-hydrocarbons and some oxygen-and sulphur-containing substances to pass through the trap. Nonetheless, by removing about 40% of the accompanying pollutants this reactor proved useful for identifying amines in a complex hydrocarbon mixture after the pollutants had been concentrated in a trap containing silica gel [3, 381. Good results were also obtained [94] in identifying diethanolamine in a mixture of volatile organic compounds generated during thermal oxidative destruction of cooling lubricants (Carbamol 11-1) used in machine building (KAMAZ engine plant). The use of a precolumn containing lithium aluminium hydride (1:l mixture with Chromaton N) permitted the indirect identification of diethanolamine (boiling point 270°C), which under the usual conditions is not eluted from the chromatographic column. The presence of diethanolamine in the gas

145

6.7. Identification of pollutants

Table 6.16. RSC-assisted identification of nitrogen-containing organic compounds [3, 31, 731

Target components Sorbent in precolumn Amines

HXh

Amines Amines

2,4-Dinitrophenylhydrazine Zinc oxide

Amines

KOH

Nitrogencontaining compounds

Versamide 900, silver nitrate

Retained by precolumn Alcohols, phenols Aldehydes, ketones Carboxylic acids, alcohols, phenols Carboxylic acids, phenols, cresols, esters Halogenated hydrocarbons, halogenated aromatic compounds

fumes of cooling lubricants was also confirmed by a colorimetric reaction with p-nitrophenyldiazonium chloride [3]. The RSC method has made it possible to detect alcohols, ketones, ethers, alkylbenzenes and chlorinated hydrocarbons in the same environment [94]. RSC yields good results in determining pollutants containing nitrogenated compounds, e.g., amines, nitriles and amides. Table 6.16 lists several possible variants for identifying amines. The precolumns listed were used in practice for determining air pollutants in order to detect and identify amines in gas discharges from rubber articles [38, 511, and for establishing the composition of volatiles entering the workplace air during the production and treatment of polymers [3, 31, 381. A precolumn containing glass-wool or a glass filter impregnated with N-[(4-nitrophenyl)methyl]propylamide was used to identify and determine traces of toluene aminocarbonyl derivatives [95]. To determine N-nitrosocompounds generated from anti-corrosion additives in cooling lubricants, nitrite was first removed by ion exchange and denitrosation with the aid of sodium iodide or sulphanilamide [96]. To make a correct identification of traces of N-nitrosocompounds which would preclude the effect of amines (precursors of N-nitrosocompounds), use was made of a two-section precolumn containing sulphaminic acid to absorb amines in the first section and a mixture of sulphaminic acid plus magnesium silicate in the second section to detect N-nitrosocompounds [97]. After sampling, the contents of the second section were desorbed with acetone and analysed by GC.

6.7.6. Determination of inorganic compounds RSC-assisted concentration has been used successfully to determine traces of toxic inorganic pollutants [3, 21, 31, 36, 431. To detect nitrogen monoxide the air sample is prepurified from accompanying HzO and CO, by drawing it through a precolumn containing anhydrous CaSO, , Mg(ClO,), and ascarite [98]. A column containing zeolite 13X irreversibly retains NOz and NzO but it separates O,, Nz and NO [35, 991, which ensures the preseparation of reactive nitrogen oxides, whose individual separation is difficult [21, 36, 431. To preclude the interaction of inorganic volatile pollutants, use is made of a fine-fibred polystyrene filter containing iron chelate compounds. The concentration of NO,, NH3, H2S and SO, in the air aspirated through such a filter at a rate of 0.2 I/min decreases approximately 50 fold [loo]. To separate highly reactive sulphur trioxide, which is prone to polymerization, 11 Berezkin, Gas Chrom

-BE

146

6. The reactive-sorptionmethod and its application for concentrating pollutants

from a major air pollutant, sulphur dioxide, the air sample is aspirated through an absorber containing 97% [loll or 100% [lo21 sulphuric acid, which effectively absorbs SOz. The effect of SO2 in preventing the gas chromatographic determination of traces of HzS can be avoided if the air sample is first drawn through a tube containing glass pellets treated with NazS03. Further chromatography of the sample on a Poropak Q column at 100-200°C was used to detect mg/m3 of H2S in air over different regions of the Atlantic Ocean [103].

6.8.

Conclusion

RSC-assisted concentration of pollutants greatly increases the potential of analytical gas chromatography, including reactive gas chromatography, in identifying air pollutants and makes the results obtained in this important stage of analysis more reliable. RSC improves all the metrological characteristics of the method, and helps to obtain more correct analytical results. Future applications of RSC-assisted concentration in the analysis of air pollutants are related to the possibilities of changing the composition of chromatographic precolumns and the high selectivity of chemical methods. This new hybrid method is universal and can be used for determining pollutants in gas and liquid media, e.g., in determinations of 2-mercaptobenzylimidazole in natural and sewage waters by HPLC using a precolumn containing mercury(I1) 8-hydroxyquinolinate, purification of vegetable extracts to determine residual amounts of pesticides by gas chromatography (precolumn packed with Sep-Pak and Florisil) [lo51 or the purification and fractionation of the same extracts on a similar column in gas chromatographic determinations of 1-nitropyrene in diesel exhaust solid particles [106].

References

147

References (Chapter 6) [l] NATUSCH, D. F. S.; HOPKE,P. K. (Eds): Analytical aspects of environmental chemistry (Chemical Analysis Series, vol. 64). New York: Wiley Interscience 1983. [2] GROB,R. L.; KAISER,M. A.: Environmental problems solving using gas and liquid chromatography. Amsterdam: Elsevier 1982. Yu. S.; BELIKOV, A. B.; D'AKOVA, G.A.; TULCHINSKIY, V. M.: Methods of analysing air pol(31 DRUGOV, lutants. Mosow: Khimiya 1984 (in Russian). [4] ISIDOROV, V. A.; ZENKEVICH, I. G.: Chromatography-mass spectrometry of organic pollutant traces in the atmosphere. Leningrad: Khimiya 1982 (in Russian). [5] CHOUDHARY, G. (Ed.): Chemial hazards in workplace (ACS Symp. Ser. 149), h e r . Chem. SOC. Washington, D.C.: 1981, p.5, 155, 179. V. G.: Chemical methods in gas chromatography. Moscow: Khimiya 1980. [6] BEREZKIN, [7] CRISP,S.: Ann. ouup. Hyg. 23 (1980) 47. (81 JENNINGS, W. G.; U P , A,: Sample preparation for gas chromatographic analysis. Heidelberg: Huthig-Verlag 1983. J. F. (Eds.): Chemical derivatization in analytical chemistry. Vol.2, Separ[9] FREI,R. W.; LAURENCE, ation and continuous flow techniques. Modem analytical chemistry. New York: Plenum Press 1982. [lo] Fox, D.L.: Anal. Chem. 57 (1985) 223. R. G.; LANGHORST, M. L.: Anal. Chem. 57 (1985) 238. [ l l ) MELCHER, Yu. S.: Zavod. labor. 48 (1982) 3. [12] DRUGOV, [I31 BERTSCH, W.: Sampling of organic volatiles. Heidelberg: Huthig-Verlag 1983. K.V.; SAKODYNSKY, K. I.), Moscow: [14] KAISER,R.: In: Successes of chromatography (Ed. CHMUTOV, Nauka 1972, p. 193 (in Russian). [lS] KALAB,P.: Coll. Czech. Chem. Commun. 47 (1982) 2491. J. F.: Anal. Chem. 56 (1984) 274. [16] HYVER,K. J.; PARCHER, M.G.: J. Chromatogr. 60 (1971) 319. [17] HERTLE,W.; NEUMANN, M.G.; MORALES, S.T.: J. Chromatogr. 74 (1972) 332. [18] NEUMANN, J.M.: J. Chromatogr. Sci. 9 (1971) 253. 1191 TROWELL, C. H.; EWALT, M. W.; JENSEN,E.C.: Int. J. Environ. Anal. Chem. 8 (1980) 37. [20J LOCHMULLER, Yu. S.: Gas Chromatography of inorganic substances. Moscow: Khimiya [21] ANVAER,B. I.; DRUGOV, 1976 (in Russian). [22] NEHER,M. B.; JONES,P. W.: Anal. Chem. 49 (1977) 512. D. P.; RELSCH,J.N.; COOMBS, J. R.; FINE,D. H.: Anal. Chem. 52 (1980) 273. [23] ROUNBEHLER, E. D.: J. Chromatogr. 186 (1979) 811. [24) BUNCH,J. E.; PELLIZZARI, [25] JOHNSON, J.; ERICKSON, E. D.; SMITH,S. R.: Anal. Lett. 19 (1986) 316. A,; KAMPSTRA, M.: J. Chromatogr. 269 (1983) 179. [26] VENEMA, E.D.; DEMIAN,B.; KROST,K.J.: Anal. Chem. 56 (1984) 793. [27] PELLIZZARI, E. D.; KROST,K. J.: Anal. Chem. 56 (1984) 1813. [28] PELLIZZARI, S.N.: Am. Ind. Hyg. Assoc. J. 42 (1981) 403. [29] EVANS,P. R.; HORSTMANN, M.M.; NIKOLAEV, K.M.; PETUKHOVA, G.A.; POLYAKOV, N.S.: Izv. Akad. Nauk SSSR, ser. [30] DUBININ, khim. 4 (1984) 743. (in Russian). [31] DRUGOV, Yu. s.;BEREZKIN, V.G.: Gas chromatographic analysis of polluted air. Moscow: Khimiya 1981 (in Russian). [32] DRUGOV, Yu. S.; GORYACHEV, N. S.: Zh. Anal. Khim. 36 (1981) 371. Yu.S.; BEREZKIN, V.G.: Usp. Khim. 55 (1986) 999. [33] DRUGOV, [34] BEREZKIN, V. G.; DRUGOV, Yu. S.: Zh. Anal. Khim. 39 (1984) 1249. E.A.; DRUGOV, Yu.S.; TULCHINSKY, V.M.: Zavod. labor. 51 (1985) 16 (in Russian). [35] VSEMIRNOVA, [36] DRUGOV, Yu. S.: Gas chromatographic analysis of inorganic gases. Coll. transcripts of the First USSR Conference on analysis of inorganic gases. Leningrad 1983. Leningrad: Nauka 1983, p. 111 (in Russian). [37] DRUGOV, Yu.S.; MURAWEVA, G.V.: Zavod. labor. 49 (1983) 6 (in Russian). [38] DRUGOV, Yu. S.: In: Transcripts of the Fifth USSR Conference on analysis of organic compounds. Moscow: 1984. Moscow Nauka 1984, p. 291 (in Russian).

148

6. The reactive-sorption method and its application for concentrating pollutants

[39] DRUGOV,Yu. S.; YAVOROVSKAYA, S. F.: In: Gas chromatography. Moscow: Nauchno-issledovatelskii fiziko-khimicheskii institut imeni L. Ya. Karpova 14 (1971)65. [40] DRUGOV, Yu. S.: Analaysis of a mixture of aggressive gases C12, CIOz and NO2 in air using gas chromatography. Cand. thesis. Nauchno-issledovatelskiiinstitut gigieni truda i profzabolevanii Akademii meditsinskih nauk SSSR. MOSCOW 1969. ROYM.: Anal. chirn. acta 167 (1985)277. [41] HEVITT,C. N.; HARRISON, [42] TESCH,J. W.;REHG,W. R.; SIEVERS,R. E.: J. Chromatogr. 126 (1976)743. [43] DRUGOV, Yu,S.: Zh. Anal. Khim. 40 (1986)585. [44] BRAMAN,R. S.; CANTERA, M.A.; QING,X.H.: Anal. Chem. 58 (1986)1537. [45] MELCHER, R.G.; LANCER,R.R.; KAGEL, R.O.: Am. Ind. Hyg. Assoc. J. 39 (1978)349. [46] RACHINSKY, V. V. : Introduction into the general theory of sorption dynamics and chromatography. Moscow: Nauka 1964 (in Russian). [47] SERPIONOVA, E.N.: Industrial adsorption of gases and vapours. Moscow: Vys. Shkola 1969 (in Russian).

[48] ROGINSKY, S. Z.: Adsorption and Catalysis on Inhomogeneous Surfaces. Moscow: Akad. Nauk. S.S.S.R. 1948,p.643. [49] LOPATKIN, A. A.: Theoretical Foundations of Physical Adsorption. Moscow: State University, Moscow 1983,p. 239. [50] LANDE,S. S.: Am. Ind. Hyg. Assoc. J. 40 (1979)96. Yu.S.: Coll. reports of the First International Conference of Council of economic mutual [Sl] DRUGOV, aid countries “Chromatographic methods and their applications in petroleum and oil refining industries‘‘. Bratislava: Slovak chemical society 1982,p. 81. [52] NAMIESNIK, J.; KOZLOWSKI, E.: Z. anal. Chem. 311 (1982)581. I531 BERTONI, G.; BRUNER,F.; LIBERTI,A.; PERRINO,C.: J. Chromatogr. 203 (1981)263. [54] SAALWAECHTER, A.T.; MCCAMMON, C. S.; HOPER,I. C.; CARLBERG, K. C.: Am. Ind. Hyg. Assoc. J. 38

(1977)476. (551 PELLIZZARI, E. D.;BUNCH,J. E.; BERKLEY, R. E.; MCRAE,J.: Anal. Lett. 9 (1976)45. [56] CICCIOLI,P.: J. Chromatogr. 126 (1976)757. [57] DRUGOV, Yu.S.; MURAVYEVA, G.V.; LETUNOVSKAYA, G.A.: In: Reports of the Second USSR Conference “Chromatographic Processes and automation of measurements”. Tartu 1979. Moscow: NIITEKhIM 1979, P. 50 (in Russian). [58] GONZALES, L.A.; SEFTON, M.V.: Am. Ind. Hyg. Assoc. J. 44 (1983)514. [59] MCAULIFFE,C. A.: Oxygen and the conversion of future feedstocks: The proc. of the Third BOC Priestly conf.; spons. by the BOC gases div. trust a. organized by the Roy SOC.of Chemistry in conjunction with the Imperial college of science a. technology, London, Sept., 12-15,1983,London: Roy SOC.of Chemistry 1984,p. 263. [60] BHAIT,A.; GUFTA,V.K.: Analyst. 108 (1983)374. [61] MAZUR,J. F.; PODOLACK, J. F.; HEITKE,B.T.: Am. Ind. Hyg. Assoc. J. 41 (1980)66. [62] MARENCO, A.; DELAUNY, J.C.: 3. Geophys. Res. C85 (1980) 5599. V.G.; DRUGOV,Yu. S.; GORYACHEV, N. S.: Zh. Anal. Khim. 37 (1982)319. [63] BEREZKIN, [64] SIMONOV, V.A.: Zh. Anal. .Khim. 40 (1985)489. [65] LAWRENCE, A. H.;ELIAS,L.:Anal. Chem. 57 (1985) 1485. [66] DRUGOV, Yu. S.; GORYACHEV, N. S.: Zh. Anal. Khim. 37 (1982)691. [67] DRUGOV,Yu.S.; GORYACHEV, N. S.: Patent S.S.S.R. No 851259 (1981). Bull. Izobr. 28 (1981) 160. [68] PIKKOV,V.;LUIGA,P.: In: Proceedings of Symposium “Modern methods of sanitary and hygienic investigations and their applications for sanitary control”. Tartu: Tartu University Press 1978, p. 116 (in Russian). [69] HOSHIKA, Y.: Japan Anal. 28 (1979)629. R. E.: J. Chromatogr. 217 (1981)265. [70] PICKER,J. E.; SIEVERS, [71] IRVING,H.: Proc. 18th Int. Conf. on Coordination Chemistry, Sao Paolo, Brazil, 18-23 July, 1977, (Ed. S. MATHIAS), Oxford: Pergamon Press 1979,p. 6. [72] PEYRON,L.: Chim. Anal. 52 (1970) 1384. [73] BEREZKIN, V.G.; DRUGOV, Yu.S.; GORYACHEV, N.S.: Zavod. Labor. 45 (1979)1075 (in Russian). 1741 GRIPPEN,R. C.: Identification of organic compounds with the aid of gas chromatography. New York: MacGraw-Hill 1973.

References

149

BEREZKIN,V. G. : Zhurnal Vsesojuznogo khimicheskogo obstchestva imeni Mendeleeva. 28 (1983) 73 (in Russian). Yu. S.; BELIKOV, A. B.; GORYACHEV, N. S.: In: Proc. of the IVth U.S.S.R. Conf. on analytiDRUGOV, cal chemistry of organic compounds. Moscow: Nauka 1979, p. 179 (in Russian). DRUGOV, Yu. S.; GORYACHEV, N. S.: Zh. Anal. Khim. 38 (1983) 323. G.V.: Zh. Anal. Khim. 37 (1982) 1302. DRUGOV, Yu. S.; MURAWEVA, VOTINOV, Yu. I.; ISAEVA, V. S.; SMIRNOVA, V. I.: Patent of the U.S.S.R. No 834506 (1981). Bull. izobr. 20 (1981) 82. WILLIAMS, K. E.; ESPOSITO,G. G.; RINEHART, D. S.: Am. Ind. Hyg. Assoc. J. 42 (1981) 476. HOSHIKA, Y.: Analyst. 106 (1981) 166. HOSHIKA, Y.: Analyst. 106 (1981) 686. HOSHIKA, Y.; MUTO,G.: J. Chromatogr. 157 (1978) 277. SHRINER, R. L.; FUSON,R. C.; CURTIN, D. Y.:MORRILL, T. C.: The systematic identification of organic compounds. N. Y.: John Wiley 1980. KRISHEN, A,; TUCKER, R. G.: Anal. Chem. 48 (1976) 455. G. V.; LETUNOVSKAYA, G. A.: Patent of the U.S.S.R. No 728081 DRUGOV, Yu. S.; MURAVYEVA, (1980), Bull. izobr. 14 (1980) 44. DRUGOV, Yu. S.; MURAWEVA, G.V.: Zh. Anal. Khim. 35 (1980) 1319. BEREZKIN, V. G.; DRUGOV, Yu. S.: Zavod. Labor. 52 (1986) 6. HOVERMANN, W.; MULLER, E.: Technisches-Messen TM (Munchen) 47 (1980) 131. TANAKA, T.; SHINOZAKI, M.: Pollut. Contr. 18 (1983) 157. Yu.$4.; NAUMOVA, A. P.: In: Problems of identifying benzene hydrocarGORYACHEV, N. S.; DRUGOV, bons. Moscow: Nauchno-issledovatelskii institut gigieni truda i profzabolevanii Akademii meditsinskih nauk SSSR 1979, p. 46. DRUGOV, Yu.S.; MURAWEVA, G.V.; BELIKOVA, A.B.: In: Proc. of Symp. “Modem methods of sanitary and hygienic investigations and their applications for sanitary control”, Tartu: Tartu University Press 1978, p. 34 (in Russian). SOUZA, T. L.; BHATIA,S. P.: Anal. Chem. 48 (1976) 2234. BEREZKIN,V.G.; DRUGOV, Yu.S.: Zavod. Labor. 52 (1986) 16. S. P.; ARNOLD, J.F.: Anal. Chem. 54 (1982) 1137. TUCKER, Cox, R. D.; FRANK,C.W.: Anal. Chem. 54 (1982) 557. ROUNBEHLER, D. P.; REISCH,J. W.: U.S. Patent 4249904 (1981) U.S. Patent 4381408 (1983). PUNPENG, T.; FROHLIGER, J.O.; ESMEN,N.A.: Anal. Chem. 51 (1979) 151. A.: Analusis 5 (1977) 372. AMOUROUX, J.; RAPAKANLIAS, D.; SAINT-YRIEX, SUDZUKI, H.; HAMAMOTE, SUMITANI, H.; TOGAVA, H.: Japan Patent 56-46897 (1981). BRIGGS,J. P.; HUDGINS, R. R.; SILVESTON, P. L.: J. Chromatogr. Sci. 14 (1976) 335. D. W.: Chrornatographia 10 (1977) 665. WAINWRIGHT, M. S.; WESTERMAN, BRAMAN, R. S.; AMMONS, J. M.; BRICKER,J. L.: Anal. Chem. 50 (1978) 992. NIELEN,M.W.; BLEEKER, R.; FREI,R.W.; BRINKMAN, U.A.Th.: J. Chromatogr. 358 (1986) 393. BICCHI,C.; AMATO,A. D.; T o m , I.; TACCHEO, B. M.: Chromatrographia 20 (1985) 219. [lo61 LACOURSE, D. L.; JENSEN, T. E.: Anal. Chem. 58 (1986) 1894.

Chapter 7

Quantitative Methods for the Determination of Impurities Quantitative determinations of impurities rely heavily on various modifications of comparative methods with which the results obtained in studies of mixtures of unknown concentration are correlated with those relating to a known mixture concentration. Therefore, the development of an analytical method for the determinations of impurities requires a mixture of known qualitative and quantitative composition, i. e., a reference mixture, to be produced. Calibration mixtures containing microconcentrations of gaseous and vaporous substances in the range 10-3-10-7%are used as standards [l]. This chapter is devoted mainly to methods of obtaining gas mixtures containing added trace components. The general problems of quantitative chromatographic analysis were reviewed recently [2]. The precision of the preparation of standard mixtures governs the final result of quantitative determinations of impurities. There are a few methods that can be used to obtain microconcentrations of organic and inorganic substances, and these are usually divided into static and dynamic techniques. The former are based on the preparation and storage of standard mixtures in enclosed containers and the latter on the production of a flow of such a mixture of known concentration. There is no multi-purpose method that is equally applicable to all substances. It is up to the analyst to select a particular method for generating calibration mixtures based on the features of the substances under study (type of impurities, their concentrations and the analytical problems to be solved) (31.

7.1.

Preparation of standardamixtures

7.1.1.

Static methods

When using static methods, a known amount of a compound (or mixture of compounds) is placed in a container of specified volume, usually fitted with a means for rapidly stirring its contents. These methods, as applied to the preparation of calibration mixtures of gaseous and vaporous toxic compounds, are simple and can be used without any special equipment. However, in most instances they are suitable only for the production of mixtures of low concenThis is attributed to the appreciable sorption effects of the glass or metrations (10-5-10-7%). tal containers employed (see, e. g., refs. 4 and 5 ) . These effects are not so pronounced on the surfaces of polymeric materials, so that bags made of synthetic films are sometimes used for these purposes; however, they should be tested independently. For the preparation of calibration mixtures of hydrocarbons and other unreactive organic compounds, glass containers are used. A sample of the substance to be studied is injected with a microsyringe into a clean, dry glass bottle evacuated with a water-jet pump. After the contents of the bottle have been thoroughly stirred with a perforated PTFE disk, the bottle is allowed to stand for 6-8 h. Following the establishment of equilibrium (sorption on the bottle walls can be as great as 20-30%), the concentration of the mixture in the bottle is measured by a standard method. Such mixtures are regarded as “secondary”. This method ist useful for the preparations of mixtures with concentrations for 10-100mg/m3 within 6-8% [l].A PTFE container can be used for the preparation of mixtures of reactive inorganic gases such as chlorine, chlorine dioxide, hydrogen chloride and sulphur tetrafluoride for gas chromatographic detector calibration purposes.

7.1. Preparation of standard mixtures

151

Fig. 7.1. System for the preparation of PH,-air mixtures in containers (61 1 Absorption tube; 2 tap for air flow-rate control; 3 dropping funnel; 4 micro-flask; 5, 6 absorbents; 7 needle valve; 8 container

To prepare large amounts of gaseous calibration mixtures, it is possible to use steel oxygen containers whose interior is chrome- or nickel-plated or has an inert polymeric coating. Thus, for example, in the production of low concentrations of phosphine (Fig.7.1) used for the calibration of indicator tubes the reaction cell is connected to a 12-dm3 pre-evacuated steel container whose inner walls are covered with Bakelite lacquer [6]. Then the container is filled with clean air up to 10 100 kPa (100 atm). The resulting mixture with a PH, concentration between 2 and 10 mg/m3 is stable for several months and that with a concentration between 0.1 and 1.0mg/m3 for 1 month. Low concentrations of sulphur and nitrogen oxides (below are stable for 2 months when stored in aluminium containers [7] and those of hydrogen sulphides carbonyl sulphide and methylmercaptan for 2 years. 7.1.2.

Dynamic methods

Dynamic methods are advantageous when a continuous gas stream with known concentrations of the components is essential, especially when the gases under study are unstable. The methods are simple, and microconcentrations of the compounds can be calculated from the liquid vapour pressure or outflow rate of the calibration gases used. Adverse adsorption effects with these methods are normally insignificant, as in this instance the molecules adsorbed and those in the gas phase are dynamically equilibrated.

7.1.2.1. Methods used to dilute (mix) streams The simplest method consists in diluting pure container gases; however, to obtain fairly low concentrations (10-4-10-6%)it is necessary that the process be composed of two or three dilution steps. This restricts severely the use of the method as the flow-rate limiters and metering devices employed should be very precise. To purify gases to be used for dilution, one can rely on a system composed of five traps filled with Carbopack, activated carbon, molecular sieves 3A, sulphuric acid on silica gel and a Carbosieve [8].Special dilution systems allow over 230 compounds to be obtained, including gases, vapours, fumes and aerosols at concentrations of 0.5-2000 ppm [9, lo]. Another modificaton of the dynamic method is the slow addition of a dosed impurity to the diluting gas stream. However, such a system attains equilibrium very slowly, especially at low concentrations. Moreover, when changing from one sample to another, much time is taken in cleaning the system. This method ist used for generating mixtures of 10-4-50% concentrations, and extremely rarely for substances that are liquids above 0°C. The required con-

7. Quantitative methods for the determination of impurities

152

centration of each gas can be obtained by changing its rate in the diluting gas stream and calculated with the following equation:

c, = 22.4 (F, + F d ) . 106 FCM

where C, is the concentration of the gas under study (mg/m3), M ist the molecular mass of the gas, Fc and Fd are the flow-rates of the gas to be calibrated and the diluting gas, respectively (ml/min), and 22.4 is the molar volume (1). 7.1.2.2.

Diffusion method (use of diffusion cells)

In this method the vapour of the liquid in the reservoir slowly diffuses through the capillary into the diluting gas stream. If the rate of vapour diffusion and that of the gaseous mixture are known, it would be possible to calculate the concentration of the desired component. The method is one of the most convenient techniques. The cell (chamber) consists of two bottles (flasks) joined in a bar-bell design with a diffusion tube. The lower flask contains the liquid under study and saturated vapour which diffuses into the upper flask to mix there with a flowing diluting gas. The rate of diffusion, p, can then be calculated as follows [ll]: y = 2.303

DPMA

RTL

P log P-Pi

(7.2)

where D is the coefficient of vapour diffusion (cm2/s), P is the total pressure in the chamber (kPa), M is the molecular mass of the diffusing vapour of the liquid, A is the cross-sectional area of the connecting diffusion tube (cm2), R is the gas constant, T is the absolute temperature (K), L is the length of the diffusion tube (cm) and P, ist the partial pressure of the diffusion vapour of the liquid at T (kPa). As D and P,depend on temperature, the cell should be thermostated. Experimental diffusion rates are found by mass loss. A disadvantage of the cell is that it must be calibrated each time it is used. The same drawback relates to a single-chamber diffusion cell, although it gives more accurate results for concentrations between lo-) and 1.0% 7.1.2.3.

Exponential dilution flask method

The method proposed by LOVELOCK is suitable for generating gaseous mixtures with a wide range of concentrations [ll].It is based on the dilution of mixtures of standard concentrations with a carrier gas. A typical system used for this purpose is shown in Fig. 7.2. The starting mixture of known composition is passed into a flask either by flushing it with the prepared mixture or injecting a specified amount of a liquid component into the known gas

Fig. 7.2. Device for generating standard gaseous mixtures by the exponential dilution method 1111 1 Dilution gas inlet; 2 magnetic stirrer; 3 gaseous mixture outlet

7.1. Preparation of standard mixtures

153

volume with a syringe. The gases are quickly mixed by means of a magnetic stirrer and a paddle wheel assembly. In this instance one must know the volume of the mixing chamber, which can be found by a simple technique, e. g., by measuring the volume of water in the chamber. When the chamber is ready for use, the inlet and outlet taps are opened. The diluting gas enters the chamber at a constant rate and quickly mixes with the gaseous mixture within the chamber. The concentration of the desired component, C (mg/m3), in the gas leaving the chamber varies exponentially with time. It can be determined as follows [ll]: c = col-rF/Vm (7.3) where C, is the concentration of the desired component in the starting mixture (pglml), z is time (min), F is the flow-rate of the diluting gas and that of the gas leaving the chamber (ml/ min) and V , is the volume of the mixing chamber (ml). This method is well suited to cover a wide concentration range. Thus, if the mixing chamber volume is 300 ml and the diluting gas flow-rate is 500 ml/min, in 60 min the output mixture concentration will change by 4.5 orders of magnitude. Therefore, the method allows the entire linear detector dynamic range to be covered in a single experiment. Because the gas can usually be diluted with adequate accuracy and reproducibility, all the calibration graphs can also be obtained in a single experiment [2,111. In theory, the method is suitable for preparing infinitely low concentrations and can be applied to the calibration of detectors up to their sensitivity limit. In practice, however, one should take into account the volatility and adsorption capacity of the compound under study. Therefore, the exponential dilution method is most suitable for high-pressure gases and compounds. When using glass mixing chambers for generating low concentrations of the compounds to be calibrated, a major portion of the impurities is sorbed on the inner chamber walls, which detracts from the precision of the method. The precision is also adversely affected by metering the gas with a syringe, so a special device would be more appropriate for this purpose. The best results with this method are obtained with moderate concentrations of unreactive gases, and it is not frequently used for detector calibration [ll]. Routes to more extensive applications of the method were proposed elsewhere [12]. To increase the accuracy of preparing low concentrations of the desired compound, it is fed to the diluting gas stream by bubbling an additional stream of this gas through a non-volatile liquid containing a diluted starting compound. If the components in the vessel are mixed perfectly, then at any given instant the phases are equilibrated, the equilibrium being characterized by the distribution coefficient, Kd . The concentration in the stream decreases exponentially according to the following equation: (7.4)

where V, is the volume of the non-volatile liquid phase. This method has the following advantages: (1) using various non-volatile liquids, i.e., varying the Kd values, it is possible to control the effective vessel volume, V,,= (v+ b K d ) , without any variation in its geometry; (2) the role of adsorption on the vessel walls decreases; (3) production of the gaseous mixtures of the compounds which are liquid at experimental temperature becomes more convenient. 7.1.2.4.

Diffusion method using permeation tubes (ampoules)

Calibration mixtures are most frequently prepared by use of a method proposed by A. E. O’KEEFEand G.C.ORTMAN[13]. It is based on the diffusion of the gas under study through a tube made of polymer, e. g., a fluorinated ethylene-propylene copolymer (FEP-Teflon). The method was designed for generating mixtures with low concentrations of sulphur, nitrogen and carbon oxides, benzene, fluorinated hydrocarbons and C3-C4 hydrocarbons. The

154

7. Quantitative methods for the determination of impurities

Fig. 7.3. Rate of mass loss for permeation tubes containing hydrocarbons halides [14] 1 C2Cb; 2 CCl,CHI; 3 C2C13F3(Freon-113); 4 C2C12F4(Freon-114); 5 CHClF2 (Freon-22); 6 CHC12F(Freon-21); 7 CC12Fz(Freon-12); 8 CCll; 9 CH2Clz; 10 CC1,F (Freon-1 lh 11 CHCl, ; 12 CzHClp

inner diameter of the permeation tube is ca. 1.5-4.5mm and the wall thickness is 0.4-0.8 mm. The diffusion rate of the gases and vapours through the walls of the tube is proportional to its area and wall thickness, and is an exponential function of absolute temperature, which should be closely controlled. Also, the diffusion rate depends on the molecular mass of the diluting gas flowing along the outside of the tube and on its humidity if the compound under study interacts with water. The rate of diffusion of gases from the permeation tube is approximately described by Fick's law of diffusion:

Pi - Po

Vd= D A S -

W

(7.5)

where Vd is the volume of the diffusing gas (ml), D is the diffusion constant of the gas (ng/cm. min), A is the area of the tube surface (cm2), S is the solubility of the gas in the tube material (pl/g), Pi is the inside pressure of the tube (gPa), Po is the outside pressure of the tube containing the compound to be calibrated (gPa) and W is the thickness of the tube walls (cm). As S and D depend on temperature, the tube should be thermostated throughout the calibration process. The temperature should be constant to at least fO.l"C as a temperature variation of 1°C changes the diffusion rate by almost 10%[l].

7.1. Preparation of standard mixtures

155

Fig. 7.4. Calibration plots for determining hydrocarbon halides with electron-capture detection using permeation tubes [14] 1 CC4 ; 2 CC13F (Freon-1 1); 3 C,C&; 4 CCI2F2(Freon-12); 5 CH3CC13; 6 C2CI3F3(Freon-113); 7 C2HC13

Experimentally, the diffusion rate can be calculated by measuring the tube mass loss over a long period, which is usally expressed in ng/cm * min. The mass loss rate for polymeric permeation tubes filled with Freons is shown in Fig.7.3. One disadvantage of the method is that it requires regular and very precise weighing of the tube containing a liquefied gas by means of an automatic microbalance. The permeation tubes are inconvenient because establishing a constant rate of diffusion through the tube takes a fairly long time, at least a few days. On the other hand, gases and vapours are capable of diffusing through polymeric membranes during a long period, sometimes several years, at a constant rate. The total error of such a method arises from weighing the tubes, controlling their temperature and consumption of the carrier gas, it being responsible for 1-2% at most [l]. A typical curve for calibrating a chromatograph equipped with an electron-capture detector by use of hydrocarbon halide microconcentrations is depicted in Fig. 7.4 [14]. Knowing the diffusion rate for a given gas at a specified temperature, it is possible to determine the length of the permeation tube required for a particular calibration step [13]. Thus, for example, to calibrate the detector with respect to hydrogen sulphide, it would be necessary to find the diffusion rate of this gase, which at 40°C is equal to 280 ng/cm * min. If a hydrogen sulphide monitor is operated at 100ml/min and supplies to the atmosphere ca. 0.1 mg/m3 of hydrogen sulphide, the tube length can be calculated by dividing the desired diffusion rate, V, (ng/min), by the rate of diffusion through a tube made of a polymeric material, V , (ng/cm. min), i. e., L = V,/V,. Converting ppm to ng/ml gives 0.10 ppm. 34.04 ( M H,S)/2.404 = 3.404/2.404 = 1.42 ng/ml; 1.42 ng/ml. 100 ml/min = 142 ng/min; L = 142 ng/min/280 ng/cm. min = 0.51 cm. In addition to PTFE and FEP-Teflon tubes, it is possible to use permeation tubes made of silicone rubber, polypropylene, polyester, polyamide or poly(viny1 fluoride). Such systems alto low gaseous mixtures to be produced with concentrations from without additional dilution of the mixture. Permeation tubes are used for the calibration and preparation of standard mixtures with very low concentrations of hydrocarbon halides [ 141, nitrogen and carbon oxides, sulphur dioxide and nitrogen sulphide [15], methane and light hydrocarbons, vinyl chloride, hydrogen chloride, 1,2-dichloroethane and methyl mercaptan [16], nitrogen dioxide, ammonia and phosgene [17], the useful lifetime of the tubes being filled with such

156

7. Quantitative methods for the determination of impurities

an aggressive gas as hydrogen fluoride being at least 6 months. Heating a PTFE tube containing a low-volatile compound such as toluene 2,4-diisocyanate permits the generation of low concentrations of this toxic compound [18]. Diffusion of a compound through polymeric walls underlies the construction of the Microgas dynamic system [19]. The substance to be studied is injected into an ampoule made of fluorinated plastic with the aid of a gas syringe, which is then hermetically sealed in a special electric device at 350-400 "C and placed in a thermostat at an operating temperature of 45-169°C. Pure, dry nitrogen is allowed to pass through the thermostat, and then the substances diffusing through the ampoule walls flow into the nitrogen. The amount of a substance diffusing through the fluorinated plastic is found from the mass loss of the ampoule during a specified time. Knowing the diffusion rate and the flowrate of the carrier gas flowing around the ampoule, it is possible to calculate the concentration C (mg/dm3) of the substance at the system's outlet:

where A m is the mass loss of the ampoule per unit time at the thermostat operating temperature (mg/h), Kdivis the division coefficient of the carrier gas stream and Fdilis the flow-rate of the diluting gas (dm3/h). To fill the polymeric ampoules with gases, especially in the absence of liquefied gases or appropriate equipment, use can be made of a simple technique [19]. It consists in the gas saturation of a sorbent having a highly developed surface, which is placed in the ampoule. This technique is particularly useful in generating mixtures with a low concentration of vinyl chloride. For this purpose the standard PTFE ampoule is filled with ground activated carbon (35-60 mesh) l), then a long, hollow steel needle with an opening at its tip is dipped nearly to the bottom of the ampoule whose interior is filled to two thirds of its volume with an adsorbent. Then vinyl chloride gas is blown through the needle into the ampoule from the container at -20 to 0°C to the saturation point of the activated carbon (for ca. 5-7 min at a flow rate of 80-100ml/min). The needle is then removed from the sorbent and the ampoule is sealed with an electrical solderer. This technique permits the preparation of low concentrations of vinyl chloride (down to 10-4-10-5%by mass) with a relative error of +5%. In so doing, the filling of the ampoules with the sorbent whose surface contains the adsorbed vinyl chloride allows their lifetime to be considerably enhanced. As can be seen from Table 7.1, the lifetimes of the sorbent-filled ampoules are 20-100 times longer than those filled with a gas. Sometimes when using permeation membranes one can observe variations in the concentrations of the compounds to be calibrated as a result of their instability or pressure changes in the tube. To prepare stable mixtures in this instance [20], a complex composed of unstable compounds and organic ligands is prepared in the tube. Thus, to obtain time-invariant concentrations of NO, the PTFE tube (116 X 6 mm I. D.) is filled with a solution of this gas in 0.7 M Fez+-EDTAsolution and is plugged at both ends with glass stoppers. At 32°C (a flowrate of 0.5 l/min) the NO concentrations obtained (6-6.2 ppm) in the nitrogen stream are stable for 5 days. Similar complexes can be obtained from NO with Fe-aminocarbonyl acids or Fe-porphyrins and from CO, O2 and N,O and then used for the production of standard mixtures [20]. Methods of sealing (plugging) polymeric ampoules (tubes) have been described elsewhere [14, 17, 191. To obtain even lower concentrations of the compounds to be calibrated I ) The carbon is preliminarily washed with chloric acid, distilled water, acetone, methylene chloride, and then dried for 2 h in a stream of dry nitrogen at 200°C.

7.1. Preparation of standard mixtures

Table 7.1. Comparison of lifetimes of PTFE ampoules containing vinyl chloride with and without sorbent I191

157 Temperature ("'1

Ampoule lifetime (h) without sorbent

with sorbent

20 30 50

6 2 0.5

126 85 44

(10-5-10-7%by mass), it is possible to use glass or metal vessels with a PTFE stopper or a polymeric tubular plug [14]. In this instance the compound diffuses only through a small polymer surface. Such a method is of great utility, for example, in the absolute calibration of electron-capture detectors by use of microconcentrations of Freons during their detection in the atmosphere [21]. As the diffusion rate remains almost constant throughout the entire lifetime of the permeation tube, the method seems to be the best of the available dynamic techniques for preparing calibration gaseous mixtures. Sometimes it is reasonable to combine various methods of generating mixtures with a known low concentration of the desired components, in particular the permeation tube and exponential dilution techniques [22]. 7.1.2.5.

Other methods

Among the other methods of preparing standard mixtures for the calibration of gas chromatographic detectors, mention should be made of numerous chemical reactions such as electrochemical [23], decomposition [24], pyrolysis [25], hydration [26], exchange [2] and redox reactions [ l , 2, 91. Some of them are listed in Table 7.2. Standard mixtures obtained by chemical methods used especially widely in Japan are secondary, as the resulting concentrations of the mixtures are controlled with standard analytical (chromatographic) techniques. However, their preparative accuracy is high. Thus, in the production of standard gaseous mixtures (SO2,NO,, H2S, HCN and NH,) by heating the corresponding salt solutions (pH 5-6 for SOz, 6-7 for NO, and 11-12 for H2S) with a temperature change of 0.3"C the accuracy changes do not exceed 1%[19], and the total relative error is 2-3% at most. An interesting method is the ultraviolet radiation-induced generation of low concentrations of very unstable and reactive ozone from air at 50 0.1"C [15]. Its content is monitored spectrophotometrically. Another, complicated, problem consists in the production of metrologically certified mixtures') of low hydrogen sulphide concentrations [l,21. For this purpose it is possible to use catalytic hydration of sulphur-containing compounds at concentrations in the gas around 1-100mg/m3 [26]. The method is applied in the hydration of ethyl mercaptan and sulphur dioxide on a platinum catalyst as follows: m C2H5SH + H2 H2S + CzH6 (7.7)

*

-

SO2 + 3H2-

R

H2S + H2O

(7.8) Chemical methods used to generate micro-amounts of toxic compounds were decribed in a book [27]. I ) These are commercially available mixtures which composition is determined by means of the commonly used technique.

158

7. Quantitative methods for the determination of impurities

Reactants C2HjOH CaHjOH Fe(COA 2Hz+02 CloHB + 12 02 Clz + Hz RNH2 + H2 02 + 2 c H2 + AgzS H2 + NiCl, Hz + CoF4 6 H2O + Mg,N2 6 H20 + AlZS3 3 HzO + A1(OR)3 HCI + Na2S203 2 CF3COOH + CaC2 H 2 0 + CaC2 2 C12 + N2H4.H2S04 C12+ 2 NaC102 SO2+ 3 NaCIOz 3 NO + 2 Cr0, 3 NO + 2 HMnO, (as.) H2O + CHjMgI HzO + PClj 2HCl+2NaHC03 H2O + 2 RCOCl 5 H2 + Ag2SO4 NaNO, NazS03 NaHSO, KCN NazS NH,CI [WNHNllC12 [Co(NH,),NO,lCI2 PbC204

Gaseous components obtained C2H4+ HzO(in N2) CO + 3 H2 + 5 C (in N2) -+ 5 C O + F e ( i n N z ) 2H2O 1OCO2 + 4H2O + 2HCI RH + NH, (in N2) 2CO (in Nz) H2S + Ag (in N2) 2 HCI + Ni (in N2) 2 H F + 2 CoF2 (in N2) 2 NH3 + 3 Mg(OH)2 + 3 H2S + 2 AI(OH)3 --., 3 ROH + Al(OH), SO2 + NaHSO, + NaCl CzH2+ (CFICOO)zCa C2H2+ CaO 4 HCI + H2S04+ Nz 2 C102 + 2 NaCl 2 C102+ NaCl + Na2S04 3 NOz + Cr203 3 NOz + 2 MnOZ+ H 2 0 CH4 + MgOHI + 2HCl + C02 2 HCl + (RC0)202 H,S+4H,O+Ag NO,

-+ -+

Table 7.2. Utilization of chemical reactions for generation of components of gaseous mixtures 12, 9, 24, 251

-+

-+

-+

-+

-+

-+

+

-+

-+

-+

+

-+

+

+

-+

-+

-+

-+

-+

-+

-+

+

+ + -+

-+

-+

so2

HCN H2S NH3 NH3 NO

co, coz

Mention also should be made of the other, less popular, physical methods for preparing standard mixtures. In the saturation technique a diluting gas stream is passed through a thermostated liquid-containing chamber. Here, on complete saturation of the gas stream with the vapour of the liquid, one obtains a diluting gas stream with a specified concentration of the volatile liquid-phase vapour. At the chamber outlet the gas stream contains vapour of a known concentration corresponding to the pressure of the liquid-phase saturated vapour at the thermostating temperature (see, e. g., refs. 27-30). Of interest is a modification of this method in which the desired compound is deposited on a solid support, and the resulting sorbent is used for filling the column through which a carrier gas is allowed to flow [31]. To lower the concentration obtained, the diluting gas stream is saturated not with pure desired compounds but rather their solutions in a non-volatile solvent (see, e. g., ref. 32). This method is very popular, not unduly complicated and requires simple instrumentation. However, the concentration of the volatile compound in the desired gas-vapour mixture varies with time, which is a serious drawback of the method. This problem is obviated in a method (see Fig. 7.9,in which, in order to obtain a time-invariant composition of a gas-vapour mix-

159

7.1. Preparation of standard mixtures

4

3

7

Fig. 7.5. System for generating volatile impurities in a gaseous stream by the saturation method with a continuous supply of the desired component 1 Gas source; 2 feeding vessel for a solution of the volatile component in the,non-volatile liquid; 3 mixedsaturator; 4 gaseous stream containing a volatile impurity; 5 container for depleted volatile substance solution in the non-volatile liquid

ture stream, after the contact of the gas stream with a solution of the volatile substance in the non-volatile substance, the volatile component-depleted solution is continuously separated from the gas-vapour stream and the depleted solution is replaced with a fresh one (see, e. g., refs. 33 and 34). In this method of gas stream saturation with a volatile substance (as a result of its contact with a non-volatile liquid in which the volatile substance is dissolved), continuous renewal of the liquid in contact with the gas stream with a fresh solution and removal of the depleted solution allow a steady-state stream of volatile substance in the gas phase to be obtained. The concentration of the substance can be calculated as follows:

(7.9) where C& is the concentration of the volatile substance in the gas after its contact with the liquid containing this substance, all is the concentration of the volatile substance in the nonvolatile liquid, K is the distribution constant of the volatile substance between the liquid and gas phases and F, and F,, are the flow-rates of the gas and liquid streams, respectively. The saturation method is fairly general and effective for the production of a mixture of volatile substances in a gas stream. 7.1.2.6.

Preparation of standard aerosol mixtures

Test atmospheres containing aerosol particles (solid or liquid) in low concentrations and required for calibration and control of analytical apparatus and detectors can be generated by the following techniques [9, 101: (1) spray drying; (2) atomization; (3) condensation; (4) thermal decomposition and (5) dust dispersion. Spray drying is applicable to substances that are soluble in inorganic solvents. The technique involves spraying a solution to form a mist (an aerosol). Larger particles are removed by impaction whereas the remaining small particles are mixed with an impurity-free solvent, which is then evaporated. As a result of evaporation the particles remaining in the solvent escape from it to form a stream of aerosol particles of a specified concentration. Liquid aerosols can be also obtained by direct atomization, e. g., by a pneumatic method. When using a pneumatic atomizer, the concentration of the aerosol particles is varied by changing the pressure of the atomizer air supply. The diameter of the aerosol particles obtained by means of a small pneumatic atomizer is several micrometres. The direct atomization technique has been applied to the production of a standard aerosol atmosphere of dimethyl phthalate (liquid) and dibutyl phosphate (a low-melting-point solid) [9].

160

7. Quantitative methods for the determination of impurities

The condensation technique requires the formation of a vapour-air mixture which is completely saturated (supersaturated with the corresponding vapour). The supersaturated vapour is capable of condensing on either the particles of the solid substance present in the gas (heterogeneous nucleation) or on groups of molecules of the gas under study, emerging spontaneously as a result of fluctuating condensation (homogenization). Condensation can be also centred on the ions. The technique can be useful in generating an aerosol with particles of size, ca 0.01 pm. Although condensation is the best means of generating monodisperse aerosols, it is equally suitable for the production of polydisperse aerosol particles. The technique has been used, for example, for generating an aerosol containing dinitro compounds [9, 101. Condensation techniques give highly and uniformly dispersed aerosols. One way of generating a test atmosphere containing aerosol particles is to use chemical reactions that yield new liquid or solid phases (e.g., ammonium chloride from the interaction of hydrogen chloride gas with ammonia) with a low pressure of the saturated vapour. This process can be exemplified by the formation of ammonium chloride or the evaporation of sulphur trioxide in moist air: SO3 gas

+ HzO vapour

HC1+ NH3 gas

gas

= HzS04

(7.10)

new liquid phase = NHdC1

(7.11)

new solid phase

The thermal decomposition technique is used to prepare aeroemulsions (mists) whose aerosol particles constitute spherical droplets which, as opposed to aerosuspensions (fumes and dust), contain suspended crystals, their fragments, variously shaped amorphous formations or aggregated flocs composed of individual fine particles. This technique is applicable to the generation of aerosols of metal oxides such as iron and vanadium, which readily decompose on heating, and some aromatic amines [35]. Mineral and some other aerosols can be obtained by dispersion. The technique produces polydisperse and comparatively coarse-disperse aerosols such as dusts and powders. To effect an accurate determination of concentrations of substances in a mixture of different aerosol paticles, gases or vapours, irrespective of the method used to obtain a standard aerosol atmosphere (or standard mixtures of air with toxic gases or vapours), one can rely on the sampling of a gas-, vapour- or aerosol-containing air (see the chapter 5 ) with a subsequent analysis of the concentrated impurities by the method selected. This can be exemplified by the generation of standard mixtures of aromatic amines with air by the condensation technique [9, 351. To do so, two procedures can be used. In one, eleven aromatic amies were mixed with air at concentrations from 1 to 1000 ng/l by allowing the air to flow through an amine-sand mixture, and in the other air was used to dilute the amine vapour diffusing into the air stream from an amine-containing container. The finished mixtures remained stable for several months. To study the resulting compositions of the standard amine-air mixtures, 11 of a mixture was passed through a hydrochloric acid-containing liquid absorbent at a flow-rate of 250 ml/ min, then to the solution obtained were added small amounts of sodium tetraborate, heptafluorobutyryl chloride, and cyclohexane. After the solution had separated into layers, an aliquot of the cyclohexane layer containing heptafluorobutyrylamine derivatives was studies by gas-liquid chromatography on a glass column (1.5 X 6 m m I. D.)with a 2% silicone SE52-stationary phase or 5% neopentyl glycol succinate on Chromosorb W with Ni-63 electroncapture detection.

7.3. Calculation of impurity concentrations

7.2.

161

Detector calibration

To effect the calibration, it is usually essential to determine quantitatively the relationship between the concentration of the substance under study (e.g., toxic impurities in air) and the signal of the detector (flame ionization, electron-capture, thermionic, photoionization, etc.). Assuming that the signal depends linearly on the sample size, on can construct calibration graphs by use of a least-squares technique [ll]. Micro-impurities of toxic substances in air are most frequently measured via absolute calibration of a detector by use of these substances themselves, each measurement being repeated five or six times. The method is disadvantageous in that it requires a long operating time, especially with multi-component contaminant mixtures, and the presence of pure sample components, which are frequently difficult to obtain. Therefore, the absolute calibration method is only suitable for dealing with highly toxic sample components whose determination is the most critical problem within a given study. In all other instances, especially in studies of different homogeneous compounds entering the sample, e. g., homologous hydrocarbons, an internal standard method is preferred.

Calculation of impurity concentrations

7.3.

A particular method for the quantitative study of the composition of a mixture of air contaminants is selected on the basis of the nature of the toxic substances, their concentration in air and the problems to be solved, such as the analysis of the background impurities, sources of contamination, individual components, elaboration of standard specifications, etc. Concentrations of the contaminants are usually expressed in mg/m3 or pg/m3, or else in units of volume, i.e., one part per million or billion air volumes @pm or ppb). In the U.S.S.R. concentrations are normally expressed in mass units, and in other European countries by both methods; in the U.S.A. on ppm and ppb are used.’) The latter method is advantageous in that it is independent of air pressure and temperature whereas the mg/m3 unit obeys the gas laws. One system of units can be converted to the other as follows: molar volume ppm = mg/m3. (7.12) molar mass mg/m3 = ppm .

molar mass molar volume

(7.13)

The molar volume is 22.4 1. This value is more suitable for individual toxic substances at very low concentrations compared with those obtained by calculation from the gas density, as the latter is usually determined for a pure gas at normal pressure. The quantitative analysis of air contaminants covers a wide range of concentrations (ca. six orders of magnitude), considering the greatly differing limits of toxicity, frequently identical with threshold limit values (TLV), and the actual concentrations of harmful substances in the atmosphere and industrial environments. If for carbon dioxide a concentration range between and 10-lo%is encountered, which is of practical interest, a lo-’% concentration for hydrogen fluoride is dangerous and therefore should be considered. At the same time, in studies of unreactive C5-C12alkanes in air (MPC of 100mg/m3 in the industrial environ-

I)

12

1ppm

=

lo-‘% ; 1 ppb = lo-’%.

Berezkin, Gas Chrom.-BE

162

7. Quantitative methods for the determination of impurities

ment), there is no sense even in differentiating these low-toxicity compounds, so that frequently they are dealt with as a total, e.g., using a thermionic detector without a column. The concentration of a toxic substance in a sample (usually in mg or pg) found by one of the above methods is recalculated to that of the same substance in air (mg/m3) using the following equation [36]: (7.14) where a is the total concentration of the substance in the volume of the liquid under study (pg), V is the total volume of the liquid, e.g., of the absorptive or extractive solution (ml), V, ist the volume of the liquid taken for study (ml) and V,, is the volume of the air taken for study and adjusted to normal conditions, i. e., 20°C, 101 kPa [l]. Note that in aspiration sampling V,, is determined by the equation 'st=

V, 273P (273 + 7') 101

(7.15)

where V; is the volume of the air at temperature t in the sampling region (1) and P is atmospheric pressure (kPa).

7.4.

Detection limits for air contaminants

This parameter is not strictly fixed and depends on the detector used, the method of concentrating the impurities and the physico-chemical properties of the system under study. At present, at a mean TLV (threshold limit value) for hamful substances in the environment of 10 ppm, such problems can be solved by gas chromatographic techniques using ionization detectors. However, in the analysis of atmospheric air (ppb levels) and determination of especially toxic compounds in industrial environments (organic phosphorus, sulphur and nitrogen compounds, polycyclic aromatic compounds, polychlorobiphenyls, benzodioxins, etc.) the detection limits should be considerably lower, sometimes at the ppt (part per trillion) level.

Method Impurity concentration - static sorption - dynamic sorption - condensation Matrix separation (impurity isolation) - chemical methods - physico-chemical methods Enhancement of detector sensitivity - use of sensitive detectors - conversion of a microcomponent to more analytically convenient derivatives (derivatization) - use of a large sample

Application

Detection limit ( P . lo-'%)

General General Special

1-10 0.001-0.1 100

Special General

1000-10000 10-100

Special Special

0.01-1 0.01-1

Special

5

Table 7.3. Major methods for chromatographic studies of impurities (flame ionization detector) [37]

References

163

Some methods for ensuring a high sensitivity of gas chromatography in studies of impurities are listed in Table 7.3. Gas chromatography is one of the simplest and most reliable instrumental techniques for determining volatile impurities. It is suitable for studies of a variety of organic and inorganic substances in air. Thus, in the U.S.A. this method is used for regular determinations of ca. 120 chemical compounds in the environment [38]. In the U.S.S.R. gas chromatography is responsible for at least 30-40% of such determinations [39]. Generally, the method is finding ever increasing application. To obtain reliable results, it is essential to have standard mixtures both for calibrating the detectors and verifying the method as a whole, including the sampling procedure. Equally important is the further development of highly selective and sensitive detectors and the extended application of chemical methods (see, e. g., ref. 27). In conclusion, we express the hope that the efforts of specialists w o r d g on chromatographic studies of air contamination will not only provide solutions to the numerous quantitative problems but will also enrich analytical chemistry with novel and elegant approaches.

References (Chapter 7) [l] POPOV,V.A.; PECHENNIKOVA, E.V.: Zav. Lab. 40 (1974) 1. [2] KATZ,E. (Ed.): Quantitative analysis using chromatographic techniques. Chichester: John Wiley 1987. [3] NAMIESNIK, J.: J. Chromatogr. 300 (1984) 79. [4] KIECKBUSCH, T.G.; KING,C. J.: J. Chromatogr. Sci. 17 (1979) 273. [5] RASMUSSEN, R.A.: Atmos. Environ. 12 (1978) 2505. [6] KOKK,H. J.; ORAV,I. P.; MUST,M.A.; KAART,K. S.: in Materials of Symposium “Modem Methods of Sanitary-Hygienic Studies and their Application to Sanitary Control Practice”, Tartu University, Tartu 1978, p. 80. F. J.; WECHTER,S. G.: J. Chromatogr. Sci 18 (1980) 674. [7] KRAMER, L. L.: Anal. Chem. 53 (1981) 122. [8) NESTRICK,T. J.; LAMPARSKI, C. C.; GENDERSON E. C.; COULSON, D. M.: ACS Symp. Ser. 120 (1980) 1. [9] ANDERSON, C. C.; GENDERSON E. C.; COULSON, D. M.; in: Choudhary, G. (Ed.): Chemical Hazards [lo] ANDERSON, in the Workplace. Measurement and Control, ACS Symp. Ser., No. 149 American Chem. SOC. Washington: D.C. 1981, pp.5, 155, 179. J.E.: Anal. Chem. 33 (1961) 162. I l l ] LOVELOCK, [12] PANKOV, A. G.; TRUBIN, A. M.; BEREZKIN, V. G.; TRUBINA, I. V.; BUDANTSEVA, M. N.: U.S.S.R. Auth. Cert. 603898, Byull. Izobr. 1978, No. 15. G. C.: Anal. Chem. 38 (1966) 760. [13] O’KEEFE,A. E.; ORTMAN, [14] CRESCENTINI, G.; MANGANI,F.; MACTRAGIACOMO, A. R.; BRUNER,F.: J. Chromatogr. 204 (1981) 445. N. H.; HABOOSHEN, H.; BEST,W.: Amer. Lab. 13 (1981) 105. [15] MCQUACKER, C.: Anal. Chim. Acta 96 (1978) 222. [16] GODIN,J.; BOUDENE, I171 DE MAIO,L.: Instrum. Technol. 19 (1972) 37. [18] BURG,W. R.; SHAU-NONG CHANG:Am. Ind. Hyg. Assoc. J. 42 (1981) 426. V. G.; DRUGOV, Yu. S.: Zav. Lab. 52 (1986) 12. [19] BEREZKIN, T.; KANEKO, X.:Japan Patent No. 56-37494 (1981). [20] HODZAKI, C.; PAREY, F.; EXCOFFIER, J. L.; BEKASSY, S.: J. Chromatogr. 203 (1981) 247. [21] VIDAL-MADJAR, [22] BRUNNER, F.; CANULLI, C.; POSSANZINI, M.: Anal. Chem. 45 (1973) 1790. [23] PUNING,K.H.: Zav. Lab. 46 (1980) 403. [24] HASHIMOTO, Y.; TANAKA, S.: Environ. Sci. Technol. 14 (1980) 413. K.; ABE,N.; MASUDA, I.: Bull. Chem. SOC.Japan 55 (1982) 3647. [25] MIYOKAWA, [26] BESKOVA, G. S.; BUTUSOVA, A. I.; KORABELNIKOVA, I. M.: Zh. Anal. Khim. 40 (1985) 1098. [27] BEREZKIN, V. G.: Chemical Methods in Gas Chromatography. Amsterdam: Elsevier 1983. 12‘

164

7. Quantitative methods for the determination of impurities

[28] NOVAK, J.; VISKA,V.; JANAK, J.: Anal. Chem. 37 (1965)660. [29] KING, W.H.; DUPRE,G.D.: Anal. Chem. 41 (1969)1936. [30] KOZLOV, S.T.; POVOLOTSKAJA, M.I.; TANTSYREV, G.D.: Zh. Anal. Khim. 30 (1975) 30. 1311 VWROSTA, J.; NOVAK, J: J. Chromatogr. 175 (1979)261. [32] FOWLIS,I.A.; SCOIT,R.P.W.: J. Chromatogr. 11 (1963) 1. [33] BEREZKIN, V.G.; BUDMSEVA,M.N.:U.S.S.R.Auth. Cert. 697922 Byull. Izobr. No. 42 (1979). [34] BEREZKIN, V. G.; BUDMSEVA,M. N.; VORONOV, G. N.: U.S.S.R.Auth. Cert. 759953, Byull. Izobr. No. 32 (1980). D. W.;SMITH,A. F.: Analyst. 106 (1981) 1082. [35] MEDDLE, [36] GOST 12.1.016-79"System of Standards for Labour Safety. Working Zone Air. Requirements for Methods of Measuring the Concentrations of Harmful Substances", Standarty, Moscow 1979. [37] YANAK,Ya. L.: Zh. Vsesoyuzn. Khim. Obshch. Im. D. I. Mendeleyeva 28 (1983)1. S. M.: J. Chromatogr. Sci. 20 (1982)402. [38] SONCHIK, Yu.S.;BELIKOV, A. N.; DYAKOVA, G. A,; TULCHINSKY, V. M.: Methods for Studies of Air [39] DRUGOV, Contaminations. Moscow: Khimiya 1984.

Chapter 8

Practical Application of Gas Chromatography to the Determination of Air Pollutants The renaissance in analytical gas chromatography has been bound up with the introduction of capillary column techniques [l] and the wide use of chemical reactions in gas chromatography [2].Such innovations as small-diameter columns, quartz capillary columns, the use of stationary liquid phases of new types (bonded and cross-linked) and the development of effective techniques for injecting samples into capillary columns have substantially increased the analytical potential of gas chromatography. An important role in the development of the analytical chemistry of air pollutants [l, 21 is played by sample derivatization, i. e., the conversion of sample components into forms that can conveniently be determined. Together with direct chromatography and chromatography preceded by the concentration of contaminants, reaction gas chromatographic (RGC)techniques are frequently applied to the determination of volatile inorganic and hydrocarbon compounds. The latter are based on converting reactive inorganic gases (C12, F2, HF, 03, NO,, SOz, etc.) [6]and polar or high-boiling organic substances (acids, aldehydes, nitrates, amines, phenols, etc.) [3]into compounds convenient for chromatography. Their advantages are a substantial increase in selectivity and reliability of determining contaminants, a decrease in detection limits, and simplification of a number of other metrological and methodological problems [7]. New developments in the gas chromatographic analysis of air pollutants (means for inserting large samp.les into capillary columns, rapid heating of the injector, production of fusedsilica capillary columns with chemically grafted stationary phases, utilization of combinations of selective chromatographic detectors for pollutant identification, etc.) have been reviewed elsewhere [8-111. Gas chromatography permits the determination of the major air pollutants such as sulphur and nitrogen oxides, hydrocarbons, carbon monoxide, photooxidants and aerosols [ 11, 121 produced by industrial enterprises and transport facilities, and numerous harmful substances in the industrial and office environments whose composition is governed by the technology used in these areas [I,31.

8.1.

Carbon oxides

There are two principal techniques for the gas chromatographic determination of CO. The first involves concentrating CO from air at a low temperature, followed by thermal desorption of the concentrated contaminants and their chromatography. Air is passed through a steel column (12 cm x 4 mm I. D.) packed with molecular sieves 5A and cooled by liquid nitrogen. The column is then heated to 200-300°Cfor several seconds and the sample is displaced into a chromatographic column (1m X 3 mm I. D.) containing zeolite 5A, where CO is separated from accompanying contaminants (H,, 0,, N2,COz and NH4) at 28°C.Micro-amounts of CO are then detected using an argon ionization detector at a level of 0.1mg/m3 [7].Similar sensitivities to CO are obtained with a thermochemical pyroelectric detector and a neon ionization detector, where signals arise from the ionization of CO molecules by metastable neon atoms [13].The neon detector is less feakish and more realiable than the most sensitive and frequently used helium ionization detector, the sensitivity of which to CO and other permanent gases is not less than 0.001mg/m3.

166

8. Practical application of gas chromatography

30 20 0 0,

L 0

x)

-Y 0 QJ

Q-

'

2

4

6

8

1

0

Fig. 8.1. Calibration graph for subpicogram amounts of CO [16]

The second technique enables carbon monoxide to be determined at a level of 0.1 mg/m3 after first converting it catalytically to CH4. Trace amounts of methane are then determined by FID. Carbon monoxide is separated from 02,N2 and light hydrocarbons on a cooled precolumn of zeolite 5A, and hydrogenated in a flow of hydrogen over a Ni catalyst at 250-300°C. The sensitivity of the technique is not lower than 0.1 mg/m3 [14]. Hydrogenation (methanation) over 10% nickel nitrate on Chromosorb P as catalyst can be carried out directly within a chromatographic column, which makes the CO peaks more symmetrical and allows measurements to be taken with the help of commonly used gas chromatographs [15]. A study of various solid carriers for nickel and ruthenium catalysts (Chromosorbs, Sil-0-Cel, Celite C-22, glass beads) has shown that the highest sensitivity (Fig. 8.1) can -be attained by depositing a catalyst on shimalite. The detection limit is then mg/m3 or lower [16]. The same or even higher sensitivity can be achieved by combining the two chromatographic techniques for determining CO, i. e., preconcentration and conversion to methane. Using helium ionization detectors of the most recent designs enables very low concentrations of many inorganic gases in air to be detected (Table 8.1). The best technique for the direct determination of micro-concentrations of carbon monoxide is the use of an ECD sensitized by nitrous oxide. The procedure involves injecting small amounts of N 2 0 (about 20 ppm) into the carrier gas (nitrogen) line between the chromatographic column and the detector. The CO detection limit is 16 pg. The increase in sensitivity is explained by the catalytic conversion of CO to C 0 2 in the presence of N 2 0 on the hot detector walls. The effect is stable and reproducible [17].

Table 8.1. Detection limits (% v/v) for inorganic gases [6] Detector FID TID Helium ionization Aerosol detector ECD*) FPD COULD**) PID***)

CO

C02

H2

O2

N2

HCI

HF

O3

H2S

SO2

10-5 10-4

10-5

10-7

10-7

10-5

10-5

lo-?

10-6 10-6

10-8

10-5

lo-'

10-6

10-6

10-6

HCN

10-6

10-3

10-5

10-3

10-2

10-4 10-7

10-7

10-8 10-7 10-9

10-7

167

8.2. Halides and their derivatives

Table 8.2. Sorbents for analysis of inorganic gases [6] Sample mixture

Sorbent

Porapak Q,carbon molecular sieves Zeolite 5A, carbon molecular sieves Surface-layer sorbents, carbon molecular sieves Zeolite 5A, surface-layer sorbents Zeolite 13X, surface-layer sorbents Porapak R Teflon-4, Kel-F oil on Anaport, Kel-F 300') Teflon-4 Teflon-4 Porapak Q, Polysorb-1, Chromosorb 101 and 102, Porasil A, Chromosil310 Chromosorb 104 so2, so3 Chromosorb 103 and 104 N20, NO, NO,, NzO4 Air, C02,NH3, HzO Porapak N NH,, H20, amines Chromosorb 103 and 104, Tepasorbs, Polysorbonitrile**) Air, C02, CHI, Low-molecular-weight hy- Silica gel, Porapak N and Q, Polysorb-1 drocarbons Porapak T, Chromosorb 105 CHZO, H20, CH30H COCl, Chromosil3 10 HCN Porapak Q PH3, ASH, Porapak Q

h. 0 2 . Nz

He, Ne, H2 H2,Oz, CO, COz H2, 0 2 , N2, CO Hz, 0 2 , CO, CH4 Clz, HCI F,, HF, ClF3, CIFS,CI2 Br,, BrF, BrFS, BrF,, BrF7 0 2 , CF4, NF3, HF, M o F ~WF6 , C02, H2S, CS,, COS

*) Polymers from chlorotrifluoroethylene with various degrees of polymerization. **) Copolymer of acrylonitrile with p-divinylbenzene.

Halides and their derivatives

8.2.

The principal condition for determining aggressive inorganic compounds in air by gas chromatography is the use of inert column packings (Table 8.2) and corrosion-resistant materials for the communication lines. Halides cannot be analysed successfuly without first conditioning the sorbents and the whole chromatographic system with these contaminants for a long period [18]. However, even the use of PTFE and polymers based on fluorochlorohydro-

NHS

NO

NO2

NZO

PH3

10-5

10-5

10-5

10-6

10-5

10-5

10-4

10-6

10-4

10-7

10-6 10-7

10-6

10-7 10-7

10-8

10-6

10-7

of ECD to permanent gases is explained by its sensitization on addition of small amounts of nitrous oxide to the carrier gas. **) Coulometric detector. ***) Photoionization detector.

*) The high sensitivity

168

8. Practical application of gas chromatography

"919 H a Fig. 8.2. Calibration graph for determining HC1 in the form of 2-chlorocyclohexanol I231

carbons does not ensure the absence of losses caused by irreversible adsorption of materials during chromatography. Also, detectors that are sufficiently sensitive and at the same time stable to aggressive gases are lacking [19].For these reasons, highly reactive gases such as fluorine, hydrogen fluoride and chlorine trifluoride are analysed using reaction gas chromatographic techniques, by converting them into inert derivatives with high ionization detection sensitivities [5, 6, 18, 191. A technique for quantitatively determining micro-concentrations of HF in air after converting it into trimethylfluorosilane by reaction with trimethylchlorosilane has been developed. The product is chromatographed on a column (2m X 4 mm I. D.) of 12% NG 100 on Porolite S using FID for quantification [20]. The direct determination of HF in the air is also possible. The contaminant was first concentrated on a column (35 cm x 3.5 mm I. D.)containing granulated NaF (0.5-10 mm fraction) with 5-10% KF [21].After thermal desorption at 450-5OO0C, HF was determined using a katharometer, but with a far lower sensitivity than with flame ionization detection. Very low concentrations of hydrogen chloride in the stratosphere and troposphere were measured after reacting HCl with epibromohydrin to produce l-chloro-3-bromo-2-propanol. Accompanying contaminants were removed on an OV-17 column with temperature programming from 100 to 180°C and the product was determined by ECD [22],which is most sensi-

Fig. 8.3. Chromatogram of acetone solution of 7-oxabicyclo[4.l.0]heptane [23]. Conditions: column (6 m X 4 mm I.D.)packed with Chromosorb W + 3-5% Carbowax 20M; temperature, programmed from 140 to 190°C; FID. (a) Initial compound; (b) compound after treatment with HCl. The 2-chlorocyclohexanol peak is indicated by an arrow

169

8.2. Halides and their derivatives 3

Fig. 8.4. Chromatogram of products separated after reaction between C12 and dimethylphenol on a quartz capillary colmn [26]. Conditions: column ( 2 5 m X 0.3 mm I.D.) packed with

L

4

-

The interaction of the same reagent with chloroformates gives di-n-butyl carbamates, thiol carbamates or amides. After sampling, the carbamates and urea are desorbed with hexane, washed with HC1 and chromatographed on a fused-silica capillary column with DB-5 using FID (Fig. 8.5). The recovery efficiency is at least 98% and the detection limit is 0.7 pg in 1.5 1 of air. Data for the recovery and determination of phosgene derivatives and chloroformates are given in Table 8.3. Gas chromatography of the thermal destruction products from sulphur hexafluoride (sulphur tetrafluoride, thionyl fluoride, sulphuryl fluoride, etc.) used as a cover gas in casting in-

170

8. Practical application of gas chromatography

0

2

4

6

8

10

12

16 [min]

Fig. 8.5. Chromatogram for separation of chloroformates and phosgene after their conversion into the corresponding di-n-butylcarbamates [27]. Numbers of peaks on the chromatogram correspond to those of compounds in Table 8.3

volves serious difficulties. Traces of toxid SF, and other fluorine compounds can be well separated from SF6 on a column of Porapak Q or fluorosilicone as stationary phase on Chromatone and detected with a FPD. This technique was applied to the identification of sulphur tetrafluoride, which is more toxic than phosgene, in the atmosphere of a foundry where smelting was carried out under sulphur hexafluoride [28]. Sulphur hexafluoride in mixtures with Freons can be determined using gas adsorption chromatography on columns of zeolite 5A or A1203[29]. Employing ECD enables SF6 to be detected in the atmosphere at the ppt level. Table 8.3. Summary of recoveries in hexane [27] Number of Compound peak on chromatogram

9

Methyl chloroformate Isopropyl chloroformate Ethyl chloroformate sec.-Butyl chloroformate n-Propyl chloroformate Isobutyl chloroformate S-Ethyl chlorothioformate 2-Ethylhexyl chloroformate Pivalyl chloride 3-Chloropivalyl chloride Phosgene*)

Average recovery

(%I 98.2 100.8 98.0 100.8 98.6 100.0 103.7 101.8 69.2 68.6 105.9

Standard deviation (%)

Number of trials

4.3 4.9

21 21 21 24 21 21 21 21 6 4 45

5.1 3.1 4.5 7.4 6.9 6.8 3.5 6.7 6.9

Range of loading (Me)

4.03-147 2.91-120 3.17-108 3.16-107 3.07-104 3.60-157 16.3-318 4.52-185 19.6-40.5 24.3-50.2 5.37-390

*) Eluted for 26.6 min as a tetra-n-butylurea derivative.

8.3.

Nitrogen-containing compounds

Reaction gas chromatography solved the problem of determining very low nitrogen oxide concentrations i n the atmosphere. Conversion of nitrogen dioxide to nitrobenzene made it possible to attain a mg/m3 sensitivity for this compound [7].A very high ECD sensitivity towards nitrogen oxides can be increased by increasing the detector chamber temperature to 300°C.

171

8.3. Nitrogen-containing compounds

Fig. 8.6. Chromatogram of products separated after reaction between NO2 and n-butylamine [31]. Conditions: Column (3 m X 3 mm I. D.) packed with Chromosorb W + 5% Se-30; 60°C; FID

I

I

l

l

I

Time[mln]

To determine nitrogen dioxide in industrial exhaust gases air was drawn through an absorber containing 5 ml of aniline at 40 l/h. About 1 pl of diazoaminobenzene formed in the reaction was chromatographed on a column (5 m x 4 mm I. D.) of 5% Apiezon L on silanized Chromosorb W with FID for quantification. Not less than 0.1 mg/m3 nitrogen oxide was detected in air using ECD with 63Nias the radioactive source after converting NO into p-chlorobromobenzene [30].The same group was able to determine 10-’rng/m3 of NO and NOz in exhaust gases under similar conditions. The oxides were reacted with aniline chloro derivatives, accompanying contaminants were removed on a column of 10% OV-17 on shimalite

172

8. Practical application of gas chromatography

and ECD was employed. ECD enables NO to be detected in amounts of not less than 0.01 ng after its separation from COz, CO, Oz, HzO, CH4 and light hydrocarbons on a column of carbon SCT impregnated with nickel sulphate [7]. About 10 ppm of NO2 can be detected in 1ml of air after its reaction with n-butylamine [31]. A chromatogram for the separation of the resulting hydrocarbons and C4 alcohols is shown in Fig. 8.6. Nitrogen oxides and other contaminants (Nz,NzO, NH,, HCN, Hz, CH4, CO, C02 and H20)were identified gas chromatographically in combustion products and the atmosphere [32]. Hydrogen cyanide together with phosgene are the most dangerous components resulting from the combustion of nitrogen-containing polymer materials. Rapid analyses for HCN in fire areas (burning of polyurethane foam, carbamine resins, etc.) can be made using a thermionic detector capable of detecting 5-4000 mg/m3 concentrations of HCN. A flame ionization detector can be employed to detect as little as 0.1 mg/m3 on HCN in exhaust gases after concentrating a sample in a trap containing Porapak Q cooled by solid COz. Chromatography is then carried out in a 2-3 m column of Porapak Q programmed from 30 to 200°C. The HCN peak appears between those of ethane and propylene under these conditions [33]. Low concentrations of HCN and (CN), were trapped from air using activated charcoal impregnated with NazCr20,, and after desorption in a current of humidified air were separated on a Porapak Q column (2 m x 4 mm I. D.) at 100°C using FID [34]. For the two substances the detection limit was less than 1pg/l. Calibration mixtures were prepared through a chemical method when HCN was obtained by reacting KCN with HzS04,and micro-concentrations of (CN), through the reaction of a Cu2+diamine complex with HCN. Micro-amounts of ammonia, nitrogen oxides, nitriles and nitroalkanes can be effectively separated on a column packed by a high-polarity polymeric sorbent such as Chromosorb 104 or its analogue, an acrylonitrile copolymer with p-divinylbenzene called polysorbonitrile [7]. In determining highly unstable and reactive hydrazine and its derivatives in air in the vicinity of rocket fuelling, an air sample containing hydrazine and 1,l-dimethylhydrazine is passed through a tube of silica gel impregnated with sulphuric acid. Concentrated contaminants are desorbed with water and the resulting solution is treated with furfural dissolved in aqueous sodium acetate. The reaction products are extracted with ethyl acetate and the extract is gas chromatographed with FID [34].

8.4.

Sulphur- and phosphorus-containing compounds

Gas chromatography is one of the principal techniques for determining low concentrations of sulphur dioxide, one of the major air pollutants, and also other inorganic (H2S, COS and CS2) and organic (sulphides, mercaptans, thiophenes, etc.) sulphur compounds in air. Among various solid sorbents (coal, Porapaks, molecular sieves, porous glasses, gas chromatographic packings and metal oxides), molecular sieves 5A were found to be the best for trapping micro-amounts of sulphur dioxide from the air, the SOz absorption efficiency being 94% [7]. Trace amounts of low-molecular-weightsulphur compounds can be detected in air using a selective detector based on measuring the emission at the 182.04-nm sulphur emission line. Analyses are made on a PTFE column of Chromosil310 at 60°C. The detection limit for carbon sulphoxide, hydrogen sulphide and sulphur dioxide is mg/m3 [35]. A good technique for determining micro-amounts of hydrogen sulphide in air involves gas preconcentration in a trap 16 cm long with Silochrom S-80 (washed with HC1 and water and heated at 400°C for 4 h prior to use) at -78°C [36]. Using a chromatograph with glass columns and FPD enables hydrogen sulphide to be detected at concentrations of 10-4mg/m3

173

8.4. Sulphur- and phosphorus-containing compounds

Table 8.4. Boiling points, retention times and corrected detector responses for various sulphur compounds [41] Compound

Formula

Hydrogen sulphide Carbonyl sulphide Sulphur dioxide Methanethiol Ethanethiol Dimethyl sulphide Carbon disulphide 2-Propanethiol 2-Methyl-2-propanethiol 1-Propanethiol Ethyl methyl sulphide 2-Butanethiol Thiophene 2-Methyl-1-propanethiol Diethyl sulphide 1-Butanethiol Dimethyl disulphide

Peak number 1 2

Boiling point Retention ["C (1 atm)] time*) @in)

Corrected response, (-39**)

Jhw

3 4 5 6 7 8

-61 - 50 - 10 6.2 35 37.3 46.3 52.6 64.2

0.96 1.10 1.30 2.00 3.40 3.90 4.50 5.02 6.70

5.9 6.0 6.1 12.0 6.1 6.3

9 10

67.5 66.6

7.65 7.95

6.2 6.3

11

11.55 11.55 12.65

6.1

12

85 84.2 88.7

13 14 15

92.1 98.5 109.7

15.30 16.85 21.30

6.6 5.5 11.7

6.3 6.2

6.3

*) At a column temperature of 50°C.

**) Calculated for 200 pmol of each compound at a detector attenuation of 10 X 64 and an initial column temperature of 50°C. increased at a rate of 35'C/min to 100°C after 8 min; h peak height response; w peak width at half peak height.

with an accuracy of k26%. Similar results were obtained after trapping H2S at liquid argon temperature and removing accompanying contaminants on a column of Chromosil310 [37]. With complex mixtures of sulphur compounds containing inorganic gases (SO2,H2S, COS and CS2) and organic contaminants (twelve mercaptans, five sulphides, dimethyl disulphide and dimethyl sulphoxide), the most effective separation technique was cryogenic gas chromatography [38, 391. An air sample was passed through a U-shaped trap with glass beads cooled by liquid oxygen. After desorption, the concentrated contaminants were chromatographed on a 30-m capillary column of SE-30 with temperature programming from -70 to 100°C and FPD [37]. The same procedure can be applied to the separation of mixtures of SO2 with other reactive and low-boiling gases (N2, O2N20, NO, CO, C02, water vapour) on a single 4-m column of Porapak QS with temperature programming (the temperature is first increased from -50 t o 0°C and then at a higher rate from 0 to 220°C) [7]. Determinations of low SO2 contents are associated with the high reactivity of this gas. Therefore, for SO2concentrations below 10 mg/m3 use should be made of glass, plastic or aluminium columns [6, 7, 191. The reproducibility of SO2 determinations is especially dependent on the humidity of the air under study. To avoid losses in the analysis of moist air [40], the gas is first dried, e. g., by passing it through a Naflon perfluorinated membrane. Following the sorption of sulphur dioxide in the sorbent trap, it was identified on a column (6 m x 4 mm I. D.) of 4% polyphenyl ether and 0.5% H3P04on Chromosorb T (60-80 mesh). With 3 1 of air sampled for 3 min, the detection limit was 13 ppt [40].

8. Practical application of gas chromatography

174

- 0

0

8

Time[mh]

72

Fig. 8.7. Chromatogram of air contaminated with sulphur-containing compounds [411

In the analysis of the composite sulphur-containing species [41] listed in Table 8.4, sulphur dioxide determinations are dubious because during low-temperature trapping from air (at - 196°C) the Tenax GC trap accumulates a significant amount of moisture, which reacts with SOz to give sulphuric acid. As a result of this reaction the SOz micro-impurities are irreversibly trapped by the chromatographic system. This problem cannot be solved even by predrying the air over CaClZ,as reported elsewhere [41]. Determinations of the other compounds (see Table 8.4) present no problems. On desorption of the concentrated pollutants at 200°C from the Tenax tube, they were detected on a glass column ( 2 m x 4 m m I. D.) containing 20% silicone SE-30 on Chromosorb P (60-80 mesh) at 120°C with FID (at 355 mm), which is selective with respect to sulphur-containing species (Fig. 8.7). A calibration graph for determining sulphur-containing compounds is shown in Fig. 8.8. For these compounds the detection limit is at the ppt level. It was shown [41] that the logarithm of the retention time of a sulphur-containing compound is linearly dependent on the boiling point of these compounds (Fig. 8.9). This fact can be exploited for the identification of the compounds in question. Gas chromatography is one of the best techniques for determining micro-concentrations of phosphine in air. The pollutant can be analysed directly using a thermionic detector. Air was collected in a polyester bag and injected by means of a syringe; PH3 was isolated on a column (3.5 m X 4 mm I. D.) of 30% squalane on Chromosorb P at 50°C and detected by TID at the 3 . l o - 5 mg/m3 level [42]. An even higher sensitivity was attained after concentrating this toxic gas in a tube (20 cm x 3 mm I. D.) of Chromosorb 102 at ambient temperature. At a sampling rate of 30-70 ml/min the sampling error was 1-2% for 0.1-1.0 ng of PH,.Further steps involved thermal desorption and gas chromatography on a nickel column (2 m X 3 mm I. D.) of Chromosorb 102 at 60°C; TID is employed for quantification [43]. An original reaction chromatographic procedure for determining PH3 was developed in the U.S.A. [44]. Micro-amounts of phosphine were trapped from air on silica gel impregnated with silver ni-

175

8.4. Sulphur- and phosphorus-containingcompounds

Fig. 8.8. Calibration graph for analysis of sulphur-containing compounds [41]. Numbers of compounds correspond to those in Table 8.4

Fig. 8.9. Graphical dependence of log (retention time) of sulphur-containing compounds on their boiling temperatures [41]. Numbers of compounds correspond to those in Table 8.4

1-

0

1

2

3

4

-60 -40 -20 0 20 40 60 80 100 b o i l i n g point

'c

8. Practical application of gas chromatography

176

11111111111 1 0 8

6

4

2

Time [min]

Fig. 8.10. Chromatogram of the absorption solution after air (10 1) containing 0.12 mg of ozone had been passed through it. Sample volume, 2 pl [45]. 1 ethyl acetate; 2 benzaldehyde (corresponds to ozone); 3 acetophenone (internal standard)

trate. The resulting mixed salt decomposed on heating the concentrator, evolving phosphine. The procedure is exceedingly selective towards phosphiue. Mention should be also made of gas chromatographic determinations of toxic and extremely reactive ozone. Among a few methods for the direct determination of O3[6, 18, 191, the reaction chromatographic technique proposed by LEPSIis the most interesting [45]. It is based on the ozone-stilbene reaction, with detection of the resultant benzaldehyde by FID:

%+

R-CH=CH-R

2R-CHO

-R-CH

tH202 -R-CHO

/O\

I 0-0

CH-R

HO

I

tR-CCOOH+H20

Air (2-10 1) was passed at a rate of 0.2 l/min through an absorber with a glass porous plate, containing 10 ml of absorption solution (1 g of trans-stilbene in 100 ml of ethyl acetate) and cooled to -20°C with a mixture of ice and common salt [45]. Then the concentrate (1-2 pl) was inserted into the injector of the chromatograph equipped with a column (1.5 m X 3 mm I. D.) filled with Chromosorb W containing 10% of Carbowax 20M and thermostated at 130°C. The internal standard was acetophenone. The chromatogram is shown in Fig. 8.10. Using FID, this method permits the determinations of ozone in the range 40-400 mg/m3. An electron-capture detector, which is very sensitive to benzaldehyde, would undoubtedly be suitable for reducing the detection limit by two or three orders of magnitude [18, 191.

8.5.

Metals and their derivatives

At present, gas chromatography can hardly compete with spectroscopic techniques for determining metal aerosols in air. However, together with atomic-absorption spectrometry and neutron activation analysis it has been successfully employed to determine ultramicro-concentrations of certain especially toxic metals and organometallics [6, 7, 461. pg/m3 is trapped Very toxic beryllium metal present in aerosols at a concentration of on a glass-fibre filter, which is then treated with hydrochloric or sulphuric acid. After extracting beryllium in the form of Be(TFA),, it can be quantified at the 10-4mg/m3level in the presence of substantial amounts of other metals (Fig. 8.11).

177

8.5. Metals and their derivatives

r

3

Fig. 8.11. Chromatogram for the separation of volatile beryllium and aluminium trifluoroacetylacetonates after extraction of metal aerosols from air (400 I) on a perchlorovinyl filter [48]. Column: PTFE (1 m x 4 mm I.D.) packed with nitrile silicone XE-54 on Chromosorb G; column temperature, 150°C; evaporator temperature, 170"C, detector, ECD (with a tritium source). 1 Benzene (solvent); 2 Be(TFA)*; 3 AI(TFA)3

5

4

3 2 firne Lmin]

2

1

1

0

Gas chromatography has been employed to determine toxic mercury and its compounds and other elements in air [49]. An air sample containing traces of mercury, selenium, arsenic and their compounds is enriched by passing it through a trap containing activated carbon, hopcalite, silver wadding and various solvents. Concentrated micro-contaminants are analysed by GLC or atomic-absorption techniques. A trap with silver wadding, which is more effective than silver wire, foil or net, enables mercury vapour at concentrations of 15 ng/m3 to 10 mg/m3 to be collected from air at a rate of 100 ml/min. Its efficiency is about 3-4 pg of Hg per 1g of Ag. Mercury is recovered by thermal desorption at 400°C for 30 s, and the trap is then cleaned for repeated use by heating it at 800°C for 2 h. A carbon trap absorbs 97-99% of mercury and its compounds from air at an air aspiration rate of 200-1000 ml/min and samples can be stored for 120 days or more before measurement. At a rate of 200 ml/min mercury is effectively trapped at concentrations of 6-180 ng/l. Active carbon is, however, far less stable in operation than silver fibre [7, SO]. Chromosorb 101 has been found to be an effective sorbent for extracting micro-amounts of organomercury compounds from air [51]. Air samples of 0.3-5.5 m3 were drawn at a rate of 1.3-4.2 l/min through a 1 m x 6 mm trap containing 60-80-mesh sorbent. Methylmercury chloride was 95 ic 3% absorbed at room temperature and dimethylmercury 98 k 2%at -80°C. After thermal desorption, methylmercury cloride (200°C) was chromatographed on a column' of 5% FFAP on Gas-Chrom Q and dimethylmercury (desorption temperature 90°C) on Chromosorb 101 with FID for quantification and argon as carrier gas. Combined organomercury contaminants were trapped using a column containing gold-coated glass beads and desorbed at 350°C. The detection limit was 0.1 ng/m3 [51]. Gas chromatography is a particularly effective means for detecting and quantifying microconcentrations of organometallics in air [6, 18, 191. Lead alkyl derivatives were collected in a trap containing glass beads or 3% OV-101 on Chromosorb W cooled by solid C 0 2 or liquid nitrogen. The efficiency of trapping was 94- 100%.The concentrated contaminants were transferred to a chromatograph by heating the trap on a water-bath and analysed on a glass column with a silicone stationary phase and an atomic-absorption spectrometer as detector. The trap was sometimes packed with Porapak Q, which quantitatively extracts lead compounds from samples 11 in volume. The breakthrough volume was larger than 10 I, and samples could be stored for at least 5 days at room temperature before analysis [6, 71. The use of atomic-absorption spectrometry for detection (A = 283.3 nm) enabled lead alkyl derivatives t o be determined at the 0.03-0.1 pg/m3 level after preconcentration [52-541. An overview of work concerning the utilization of gas chromatography and atomic-absorption spectrometry for the determination of low concentrations of alkyl lead compounds in air suggests that cryogenic traps filled with various sorbents are to be preferred [53]. The detection limit of individual organolead compounds is 40-90 pg with atomic-absorption spectrometry using a graphite furnace and 23-53 pg with a quartz cuvette spectrophotometer. 13

Berezkin, Gas Chrom.-BE

178

8. Practical application of gas chromatography

In the extraction of organolead compounds from air, the problem is to eliminate the detrimental effect of ozone. Therefore, ozone was trapped by passing the air through a PTFE tube containing FeSO, [54]. On sampling the adsorption tube with Tenax GC was heated for 5 min at 150°C and the concentrated impurities in the tetraalkyllead compounds were displaced into a cryogenic trap (15 cm X 2.5 mm I. D.) containing 4% Apiezone M on Chrornosorb P (60-80 mesh) at - 190"C, and then the impurities were displaced into the chromatograph by heating. The separation was conducted on a column (1m x 2 mm I. D.) of 3% silicone OV101 on Gas-Chrom Q (100-120mesh) with temperature programming from 35 to 135°C at 20 K/min. The detection limit in the sampling of air (80 1) was 0.25-0.37 mg/m3 [54]. Organotin derivatives were collected from air at room temperature using a trap containing Porapak N or QS or Chromosorb 102 [55]. After extracting them from the trap with hydrochloric acid - diethyl ether they were converted into the corresponding methyl derivatives by reaction with MgClCH3 and detected at the 0.1 mg/m3 eve1 by FPD. Reaction gas chromatographic techniques proved capable of determining very toxic nickel tetracarbonyl in a 1mg/ m3concentration [6]. An even lower detection limit for iron and nickel carbonyls was attained by LASOSKI[%I, who employed an electron-capture detector and a glass column (1.8 m X 4 mm I. D.) of 10% squalane on Chromosorb WHP (80-100 mesh) to determine ppb concentrations of these compounds. Low concentrations of alkyl selenides can be determined using a cooled trap containing glass beads, thermal desorption and retrapping in a trap tube with chromatographic column packing at liquid nitrogen temperature prior to atomic-absorption measurement [57]. A similar technique was successfully employed to detect methylcyclopentadienylmanganese tricarbonyl as air pollutant [58].

8.6.

Low-boiling hydrocarbons

Methane was effectively concentrated from air on Porapak N [7] and analysed on a column (1.5 m x 3 mm I. D.) of Porapak Q at 65°C. The low-volatile C1-C3 hydrocarbons were well separated on a short column (ca. 1 m) of Spherocarb (80-100 mesh) at 140°C. Spherocarb is a carbon adsorbent similar to molecular sieves with the pores of size 1-5 nm and a specific surface area of 1000 m2/g. Among the recently introduced adsorbents, this species is the most promising for the analysis of light organic pollutants [59]. Active carbon is of only low utility for trapping low-molecular-weight C1-C5 hydrocarbons and silica gel is applicable at very low temperatures. Active alumina is used for concentrating ethane and ethylene. Silica gel cooled by dry-ice adsorbs acetylene effectively, and Carbopak A traps 95-100% of Cz-C4 hydrocarbons present in air in low concentrations at liquid nitrogen temperature. Less severe cooling, to -20 to - 120"C, is required to concentrate CI-Cs hydrocarbons on a mixture of Spherosil and Carbosieve B [60]. Micro-amounts of low-boiling hydrocarbons were displaced from a trap by a flow of carrier gas at 150-250°C and chromatographed at room temperature using various packings, mostly porous polymer sorbents and silica gel [60]. Micro-amounts of C,-C5 hydrocarbons were analysed and separated on Porapak Q [61], Porapak Q S , Chromosorb 102 or 104 or molecular sieves 13X [7]. The chromatographic packings employed most frequently are high-polarity stationary phases such as 0,P'-oxydipropionitrile or 1,2,3-tris(cyanoethoxy)propane on diatomite or alumina [7]. Using FID, low-molecular-weight hydrocarbons can be detected at very low concentrations of 0.002 mg/m3 [61] and methane at the 0.6 ppm level [62]. Together with chromatographic retention characteristics [7], chromatography with several detectors has been employed to identify hydrocarbons in their mixtures or in the presence of

8.7. Aromatic hydrocarbons

179

other organic materials [63]. Thus combinations of three (FID, PID and ECD) [64] or two (FID and PID) [65] detection modes were used to distinguish C2-C8 hydrocarbons from lowmolecular-weight chlorinated hydrocarbons. PID signals [66] depend strongly on the air humidity [65].

8.7.

Aromatic hydrocarbons

A very important problem is the separation of micro-amounts of toxic aromatic hydrocarbons from far less dangerous aliphatic hydrocarbons whose MPCs in the workplace air are one to two orders of magnitude higher than those of the former. This can be achieved using a simple reaction - sorption technique, viz., by passing a sample through a precolumn of zeolite SA (see Chapter 6). This procedure removes all C,-C2o normal hydrocarbons (alkanes, alkenes, alkynes and dialkynes) and considerably facilitates further gas chromatographic identification of aromatic contaminants concentrated within a trap. Similar results can be obtained by chromatographing hydrocarbon mixtures on a column containing superpolar SLP, and the combined use of the two techniques mentioned enables a very high selectivity in determining aromatic hydrocarbons to be attained [7]. Trapping C6-Clnaromatic contaminants is most frequently performed using a concentrator with active carbon, which effectively adsorbs these compounds at room temperature, and carbon disulphide or organic solvents are employed to desorb the concentrated pollutants (see Chapter 5 ) [67]. An aliquot of an extract is chromatographed using a column with a polar SLP (e.g., cyanoethyl esters for low-boiling aromatic hydrocarbons or FFAP or Dexil 300 for higher boiling compounds) and FID. Most of these packings provide a good separation of aromatic hydrocarbons from non-polar naphthenes, alkanes and alkenes. Thus Cs-C1, aromatic hydrocarbons were almost completely separated from hydrocarbons of other classes, and benzene was eluted only after C,o-C11 and sometimes also C,, alkanes when a column (2 m X 3 mm I. D.) of 30% N,N-bis(2-cyanoethyl)formamideon Chromosorb W heated to 100°C was used [68]. This technique was in particular employed to determine toxic alkylbenzenes, methanol and isobutanol in the presence of non-polar gasoline constituents to analyse the vaporization products from gasoline-methanol fuel in cars [69] and also very low concentrations of aromatic hydrocarbons in the atmosphere [70]. If the detailed identification of CI-C12hydrocarbons of various classes is required, e. g., in studying volatiles from rubber, gasolines, and various solvents based on hydrocarbons, toxic contaminants are trapped on active carbon or silica gel, desorbed with an organic solvent and chromatographed on a capillary column with squalane at 100°C. Contaminants are identified by measuring their retention indices with the help of reaction gas chromatography (the subtraction technique) [71] or by gas chromatography-mass spectrometry (GC-MS) [72]. Thus more than 100 compounds at concentrations of 25-27 000 pg/m3 (alkanes, cycloalkanes, cycloalkenes, aromatic hydrocarbons, chlorinated hydrocarbons, phenols, ethers, etc.) were detected in shoe-factory air after trapping vulcanization gases on active carbon and desorbing them with trichlorofluoromethane prior to GC-MS analysis [72]. C6-C9 aromatic and aliphatic hydrocarbons, Freons and chlorinated hydrocarbons were determined in town air after concentrating the contaminants on Tenax and chromatographing them on a capillary column [73]. Alkylbenzenes and n-butyl acetate were identified in stack gases from the manufacture of glass-reinforced plastic rolls using a portable gas chromatograph with FID [74]. Reaction gas chromatography is an effective technique for selectively determining certain aromatic hydrocarbons. It can, in particular, be employed to determine 0.005 mg/m3 styrene concentrations with a relative error of 10% in a 5-1 air sample after converting the hydrocarbon into 1,2-dibromostyrene and ECD [75] (Fig. 8.12).

180

8. Practical application of gas chromatography 1

3

2

1

0

Fig. 8.12. Determination of styrene in the form of 1,2-dibromostyrene after reaction with bromine in n-hexane [75]. Conditions: column (1.5 m x 4 m m I.D.)packed with Chromaton N + 5% OV-17; 160°C; ECD. 1 n-Hexane; 2 dibromostyrene

Time [m in]

8.8.

Polyaromatic compounds

The analytical chemistry of polyaromatic compounds (PACs), of which many are known to be carcinogens, was described in a detailed review [76] and a book [77]. Combining GC with TLC or LC is one of the principal and most effective techniques for determining PACs and PAHs, and a detailed study of the latter is only possible after effectively separating complex PAC mixtures (200-300 constituents) on glass or quartz capillary columns. Difficulties in analysing PACs, particularly PAHs, by GC mostly arise from their high boiling points. Such analyses require the use of thermally stable SLP because the b.p.s of PACs range from 218°C (naphthalene) to 525°C (coronene). In this respect, liquid chromatography is the better technique for analysing volatile PACs, however, it fails to provide as high an efficiency of separating PACs as gas chromatography on glass capillary columns [7]. PACs from industrial sources do not occur in air as pure compounds but are adsorbed on solid particles (dust, soot, ashes, etc.) less than 10 pm in diameter on average. If the particles are smaller than 5 pm, ultrafine glass-fibre filters are employed to trap particles larger than 0.05 pm with an efficiency of ca. 100%;polymer filters, e.g., made of perchlorovinyl fibre, can also be used [7]. The contaminants are extracted from the filter with benzene in a Soxhlet apparatus for 8-16 h, or with methanol, cyclohexane [78], dichloromethane [76] or liquid C 0 2 [79]. The duration of extraction can be reduced substantially by vibrating the system [80]. Two-step samplers are employed to analyse large air volumes; these consist of a filter and a polyurethane block [8 11. After sampling the air (80-90 m3) for 24 h the filter and the polyurethane block were extracted in a Soxhlet apparatus with in purified cyclohexane (200 ml) and the extract was concentrated to a volume of 10 ml by boiling in a rotary evaporator under a flow of nitrogen [82]. A small portion of the extract (1-5 ml) was shaken with 0.8 ml of dimethylformamide. As this took place, all the aromatic and most of the polar pollutants passed from the cyclohexane into the DMF, whereas all the aliphatic hydrocarbons remained in the cyclohexane. The layers were separated on a centrifuge and then, to isolate a narrower fraction of PAH, the extract

I81

&0

Q

( I N TERUAL

(P

m

E

E 6'

P

i.

(P

4 6

w

Po c

182

8. Practical application of gas chromatography

Table 8.5. List of PAHs determined at Getreidemarkt [82] Compound

Abbreviation

PAH concentration (ng/m3) Winter half-year

Naphthalene*) Acenaphthylene') Acenaphthene') Fluorme') Phenanthrene Anthracene 2-Methylanthracene Fluoranthene Pyrene Benzo[b]naphtho[2,1-d] thiophene Benzo[ght]fluoranthene Benzo[c]phenanthrene Cyclopenta[cd]pyrene Benz[ alanthracene Chrysene + triphenylene B,F-Binaphthyl Benzo[b]fluoranthene Benzolilfluoranthene Benzo[k]fluoranthene Benzo[a]fluoranthene Benzo[e]pyrene Benzo[a]pyrene Perylene Indeno[l,2,3-cd]pyrene Dibenz[a,c]anthracene Dibenz[a,h]anthracene Benzo[ghi]perylene Anthanthrene Coronene Tetradecane Tricosane Dotriacontane

}

I

Summer half-year

Annual average value

NAPH ACNYLEN ACNEN FLU0 PHEN ANTH 2MEANTH FLUA PYR BNT

(68) (78) (21) (92) 197 47 18 94 95 4.4

BGHIFLUA BCPHEN CPCDPYR BAANTH CHRYTRI

17 6.3 19 16 22

BBJKFLUA

30

9.5

BAFLUA BEPYR BAPYR PER INDPYR

6.8 13 15 3.8 13

1.3 4.6 3.9 1.0 4.5

2.2

0.6

1.4

20 4.0 15

8.5 1.o 6.3

14 2.5 10

DBACAHANTH BGHIPER ANTHAN CORO

8.1 1.8 3.3 5.1 8.7 Internal standard

12 4.0 11 10 15 19 3.9 8.4 9.3 2.3 8.6

Standards CW 32

*) PAH concentrations determined on a semi-quantitative basis.

was subjected to liquid chromatography on a LiChrosorb Si 100 column (2.5 mm x 4 mm I. D.) water-saturated cyclohexane being as the mobile phase (flow-rate 1 ml/min) with UV detection. It is interesting that the losses of PAHs in these stages (extraction from air, sample treatment, purification of the PAH fraction by HPLC, etc.) increased with decreasing molecular mass, viz., from 10%for coronene to 43% for naphthalene [82]. After the PAH fraction had been isolated by HPLC, the individual hydrocarbons were separated on a fused-silica capillary column (25 m X 0.32 mm I. D.) with silicon SE-52 (a film 0.15 pm thick). The column was programmed from 130 to 300°C at a rate of 5 K/min and FID was used (Fig. 8.13.). The individual PAHs were identified by GC-MS (Table 8.5).

183

8.8. Polyaromatic compounds

Summer- Half-Year

I

Fig. 8.14. Concentration profiles for PAHs in urban air [82]. For hydrocarbon designations, see Table 8.5

Variations in PAH concentration determined by this method [82] in the central part of Vienna in 1983-84, which are similar to those in large European cities like Berlin and Essen, are shown schematically in Fig. 8.14. The extract was first resolved into several fractions (PAHs, aliphatic hydrocarbons, organic compounds with functional groups, etc.) by LC or TLC techniques, and then subjected to gas chromatographing. Extracts containing PAHs and other high-boiling organic compounds (containing nitrogen, oxygen and sulphur) were chromatographed on a column (10 cm x 1 cm I. D.) of silica gel with dichloromethane as the eluent to isolate PAHs. The eluate thus obtained was evaporated to dryness under vacuum and the residue was dissolved in dichloromethane and chromatographed again on a column (10 cm X 1 cm I. D.) of alumina using the same eluent [83]. PAHs can also be isolated by TLC. The eluate obtained by either of the two techniques (LC or TLC) was evaporated to dryness, dissolved in cyclohexane and chromatographed on a capillary column with a silicone SLP and temperature programming from 50 to 300°C [83] using FID [83] or ECD [84]. As polyaromatic mixtures contain a large number of isomers, their separation is not a simple analytical task [85].Stationary phases used to analyse PACs are listed in Table 8.6. Most frequently, a universal flame ionization detector whose sensitivity to PACs is in general only insignificantly below l ng is employed to determine micro-amounts of PACs in air [86]. Relative FID sensitivities to various PAHs are given in Table 8.7. Table 8.7 shows that the FID sensitivity to PAHs increases with increasing complexity of the molecular structures and molecular mass. Also, detectors that respond selectively to individual PACs or groups of PAC are used [87], such as electron-capture detectors specific to certain heteroatoms and spectral detectors. The electron capture detector sensitivity is about lo-' g; with certain compounds it can, however, amount to g [77]. Valuable information can be obtained with the help of detectors that respond specifically to certain heteroatoms or functional groups, such as a thermionic detector sensitive to ni14'

184

8. Practical application of gas chromatography

Table 8.6. Stationary liquid phases for analysing polyaromatic compounds (771 Upper temperature limit ("C*))

Stationary phase

~

Methylsilicone fluid OV-101 or SF-2100 Methylsilicone resin SE-30 or OV-1 Methylphenylsiliconefluids: SE-52 (5% of phenyl-substituted) SE-54 (1%of vinyl and 5% phenyl-substituted) Methylphenylsiliconefluids: OV-3 (10%phenyl-substituted) OV-17 (50%phenyl-substituted), SP-2250 (50%phenyl-substituted) Carborane silicone polymer Dexil3QO(methyl-substituted), Dexil400 (methylphenyl-substituted) Polyphenyl ether (Polysieve) Polymethaphenoxylene(Poly-MPE) Polyphenyl ether sulphone (Poly-S- 179)

320 350 320 320 290 300 400 250 350 400

*) Approximate values. Actual values depend on preprocessing, solid carrier modification, gas carrier purity and other factors.

trogen compounds and capable of detecting PACs containing nitrogen at the picogram level [87,88]. Thus, simultaneous detection of PAHs and azaarenes by FID and TID enabled these toxic compounds to be identified reliably in a complex mixture of air contaminants. A highsensitivity flame photometric detector was employed in a similar way in analysing PACs containing sulphur. A novel promising detector for determining polyaromatic compounds in composite samples of contaminated air (after separation on capillary column) has been proposed elsewhere [89]. It is composed of a fluorescence detector using a pulsed supersonic jet interface. A combination of such a detector with capillary chromatography allows, in a number of instances, even geometric PAH isomers, e. g., monomethyl anthracenes, to be separated. Polycyclic azaarenes and PAHs were determined in the atmosphere after fractionating samples by liquid chromatography on a column (10 X 1.4 cm I. D.) of XAD-2 and with methanol, ethanol and n-pentane as eluents [86]. The azaarene fraction was analysed on a capillary column (15 X 0.3 mm I. D.) of OV-215 and the PAH fraction on a capillary column (25 m X 0.3 mm I. D.) of OV-225; the temperature was programmed from 110 to 280°C and a plasma ionization detector was employed. Isomers of nitrocyclopenta[deflphenanthrenewere determined after the compounds concentrated from air were separated on a capillary column '(30m x 0.25 mm I. D.) of DV-5 with temperature programming from 50 to 300°C [90]. One of the most important tasks in the analytical chemistry of air contaminants is the determination of PAHs [91, 921 and also their nitrogen derivatives [87,88, 93, 941 in extracts obtained from solid particles occurring in exhausts from diesel and car engines, which make substantial contributions to air pollution in towns. The technique includes trapping solid particles on a glass filter [88, 921 followed by fractionation of the concentrated sample by HPLC on a column (25-30 cm x 8-10 mm I.D.) of alumina [92, 931, silica gel [88] or Spherisorb [74] with gradient elution with n-hexane - CH2C12-CH3CN[91, 941, CH30H-CH2Cl2[88]or benzene - CH30H mixtures [72]. Fractions of PAHs and their derivatives obtained by LC or

185

8.8. Polyaromatic compounds

Compound

Relative PID signal

Biphenyl FIuorene 9,lO-Dihydrophenanthrene 9,lO-Dihydroanthracene Phenanthrene Anthracene Dihydropyrene Pyrene Fluoranthene 2-Methylfluoranthene 3-Methylpyrene 1-Methylpyrene Benz[a]anthracene Chrysene 1-Methylchrysene 6-Methylchrysene BenzoMfluoranthene Benzo[k]fluoranthene Benzo[ blfluoranthene Benzo[a]pyrene Benzo[e]pyrene Perylene 3-Methylcholanthrene Benzo[b]chrysene Picene Benzo[ghi]perylene Anthracene Coronene

0.751 0.864 0.827 0.803 0.920 0.880 0.962 1.067 1.000 1.070 1.142 1.138 1.245 1.239 1.334 1.321 1.293 1.426 1.330 1.322 1.331 1.337 1.337 1.348 1.354 1.356 1.350 1.483

Table 8.7. Relative sensitivity of plasma ionization detector (PID) towards certain polyaromatic hydrocarbons (PAHs) (fluoranthene signal taken as reference) [77]

HPLC techniques were analysed on quartz capillary columns (25-30 m x 0.25-0.32 mm I.D.) of SE-54 187, 88, 931 or DV-5 [91, 921 with temperature programming from 80-100°C to 280-300°C [88,911. PACs are most frequently identified by GC-MS techniques [87, 91, 921 in combination with the selectively detection of PACs and PAHs by ECD or TID [87, 881. With a chemiluminescence detector, the detection limit for PAHs containing N is 10-25 pg [93]; using ECD it varies from picograms to nanograms depending on the fraction being analysed (PAHs, PACs dibenzo-p-dioxins, azaarenes, etc.) [79]. Similar combined techniques (GC and LC) have been applied to the determination of PAHs and PACs in combustion products from coal [93], aerosols in the atmosphere [86, 921, tobacco and drug smoke condensates [95], extracts from metallurgical works dust [77] and air in industrial regions [79, 901. Volatiles from car exhausts containing various organic compounds can be analysed without preliminary fractionation of the samples. After trapping contaminants on Tenax and thermal desorption at 250°C they were separated on a quartz capillary with a silicone stationary phase and with temperature programming from -60 to 250°C [73, 961. About 180 PAHs were identified in atmospheric aerosols by HPLC with a fluorescent detector, gas chromatography using FID and chromatographic-spectrometric techniques [97].

186

8.9.

8. Practical application of gas chromatography

Organic oxy compounds

One of the major photochemical smog components, peroxyacetyl nitrate, is an unstable compound which decomposes at elevated temperatures. Gas chromatdgraphic determinations of peroxyacetyl nitrate are therefore carried out at temperatures not exceeding 4O-5O0C, and short (30-60 cm) columns with polyethylene glycol 400 on Chromosorb G and 63NiECD are employed to separate peroxyacetyl nitrate from accompanying contaminants. About mg/m3 of peroxyacetyl nitrate can be determined when 5-ml air samples are analysed under these conditions [98]. Still lower detection limits for peroxyacetyl nitrate and their homologues (ca. 0.13 ppb) have been obtained [99] (Fig. 8.15a) with the use of a semiautomatic analyser designed for ambient air monitoring. A direct peroxyacetyl nitrate determination was effected on a column (2 m x 2 mm I. D.) of 10% Carbowax X at 35-40°C using 63NiECD (Fig. 8.15b). Separating micro-amounts of CH30H from hydrocarbons can be achieved effectively using a column of 8%, 1,2,3-tris(cyanoethoxy)propane on Chromaton N, C5-C9alkanes and cycloalkanes being eluted soon after sample injection, and benzene and C7-C9 alkylbenzenes being well separated from each other and also from methanol and isobutanol. This technique enables micro-amounts of methanol to be determined in the presence of gasoline, white spirit and other solvent hydrocarbons [69]. Air contaminated by methanol can be sampled using a trap containing coal or silica gel, but zeolite 3A is the best packing for this purpose, as it is highly selective towards methanol [loo]. Furfuryl alcohol and furfurol are concentrated on active coal, and samples remain unchanged when kept in a refrigerator for more than 2 weeks. The efficiency of desorption of furfuryl alcohol admixtures with dichloromethane is about 76%, and that of furfurol is 80%. Using a mixture of carbon disulphide and 2-propanol as the extractant increases the furfurol desorption efficiency to 85%, and the sensitivity towards reactive furfurol amounts to 0.1 mg/ m3 [loll. An interesting example of chromatographic analysis was described by PFAFFLIet al. [102]. I n establishing MPC values for a workplace atmosphere, furfuryl alcohol was identified by

8 6 4 2 0 [n bl

Fig. 8.15. (a) Calibration graph for the analysis of peroxyacetyl nitrate in air 1991; (b) chromatogram for real air (3 ml), obtained at the time of photochemical smog formation. Column: glass (2 m X 2 mm I.D.) packed with 10% Carbowax 600 on Chromosorb W; temperature, 35-40°C; detector, ECD (63Ni). I CH3ONO2;2 peroxyacetyl nitrate (2 ppb); 3 peroxypropionyl nitrate

8.9. Organic oxy compounds

187

gas chromatography and its metabolite furilic acid, by HPLC. For determining the alcohol, air (6-15 1) was passed at a rate of 0.2 l/min through a tube containing Porapak Q , then the concentrated pollutants were desorbed from the tube using absolute ethanol (1 ml). An aliquot of the extract was chromatographed on a capillary column (25 m x 0.2 mm I. D.) of SE-54 with temperature programming from 60 to 145°C at 20 K/min and with ECD. The detection limits of furfuryl alcohol and furilic acid were 0.017 mg/m3 and 1 pg/ml, respectively. 2-Ethylhexanol was detected in air of offices after trapping it on a active carbon filter and desorption with dichloromethane; GC-MS was employed [103]. The compound was determined at the 10 pg/m3 level using a quartz capillary column (50 m x 0.2 mm I. D.) of OV-101 and with temperature programming from 130 to 200°C and FID. Ethylene glycol vapour and aerosols were trapped in a sampler consisting of a glass filter and a two-section tube containing silica gel (20-40mesh); 520-260mg of the adsorbent were contained in each section [104]. Micro-amounts of ethylene glycol were desorbed with 1 ml of 2% aqueous 2-propanol and analysed on a column (1.9 m x 2 mm I.D.) of 3% Carbowax 2 0 M on Chromosorb 101 at 165°C using FID. In the analysis of aliphatic alcohols in industrial effluents, their vapours were subjected to equilibrated concentration [lo51 in a thermostated saturator with n-butanol (1.5 ml). Air was aspirated at a rate of 0.2 Vmin within k l % . Organic and inorganic acid vapours can conveniently be concentrated selectively on Chromosorb P impregnated with Na2C03[7] or glass beads treated with Sr(OH)2 [lo61 or 1% aqueous NaOH [107]. Concentrated contaminants were desorbed with water acetone (the desorption efficiency i s above 98%) [108]. Elution of acetic, propionic and butyric acids from a tube containing silica gel using 1% aqueous formic acid gave an efficiency of 85-95% [108]. C2-C4acids were then analysed directly by FID after sampling. A direct technique was also employed to detect low benzoic acid concentrations in air [109]. Air was pumped through a tube (15 cm X 6 mm I. D.) with a 25-mm layer of Porapak Q. After thermal desorption at 240°C, benzoic acid was chromatographed on a quartz capillary column ( 5 m x 0.19 mm I. D.) of Superox with temperature programming from 50 to 240°C. FID provided a detection limit of not lower than 0.01 ppm. Reaction gas chromatographic techniques are, however, the best for detecting low concentrations of polar carboxylic acids in air. For this purpose, low-molecular-weight C1-C7 fatty acids are converted into trimethylsilyl [107], p-bromophenacyl [110] or pentafluorobenzyl [lo61 esters, detectable by FID [110] or ECD [lo61 at the picogram level. Thus, carboxylic acids can be determined at the 0.5 ppb level in 200 1 of air aft& converting them into trimethylsilyl esters by reaction with trimethylsilylimidazole [107]. A 10-pg detection limit with a standard deviation of 18% was obtained in the determination of C1-C3 volatile organic acids after converting them into p-bromophenacyl esters and analysing these derivatives on a quartz capillary column (30 m x 0.25 mm I. D.) of DB-5 with temperature programming from 40 to 290°C and FID [110]. The techniques described above were employed to determine low-molecular-weight carboxylic acids which are odorants in air in the working areas of plants [108], atmospheric precipitation and moist air [llo], and in analysing dust from a pig farm [ l l l ] . In the last instance, contaminants were trapped on a filter and extracted with ethanol (extraction efficiency 90-100%). p-Cresol was identified in the phenol-indole fraction and acetic acid was the major component in the fatty acid fraction [ l l l ] . Carbonyl compounds are not analysed by direct techniques. Thus, micro-amounts of very polar and apt to polymerize formaldehyde can only be determined using polar sorbents of the Porapak T type, which can be employed to separate formaldehyde from formic acid and water [71. Other aldehydes and most ketones, which are reactive compounds, are also inconvenient for chromatography. For this reason, reaction gas chromatographic techniques are employed to determine these compounds in air.

188

8. Practical application of gas chromatography

1

I

0

10

tR

Lrnin1

Fig. 8.16. (a) Typical chromatogram for air in a road tunnel and (b) chromatogram obtained after addition of acrolein (0.05 pg) to the air sample. Peak 1 acrolein derivative (1201

After trapping in a bubbler containing water or adsorption on molecular sieve 13X,'formaldehyde was converted into bis(ehty1thio)methane by reaction with ethyl mercaptan [7] or into the corresponding derivatives by reaction with 0-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine [112],3,3,3-trifluoropropyl(methyl)cyclotrisiloxane [113]or dimetone [114].The derivatives formed were analysed on capillary columns of OV-101[112,1141 with temperature programming with ECD [114],FID or photoionization detection [113].These techniques are capable of detecting 0.1pg/m3 of formaldehyde in a 3-1 air sample [112-1141. A technique for determining traces of formaldehyde [115] and acrolein [116]in air based on chemisorbing the former on Chromosorb 102 impregnated with N-benzylethanolamine and trapping the latter in a tube of Amberlite XAD-2 treated with 2-(hydroxymethy1)piperidine was described. After extraction with isooctane or toluene, aldehyde contaminants were separated on a packed column or a quartz capillary containing Carbowax 20M. The temperature was programmed from 70 to 250°C and FID [115] or TID [116]was employed. With the latter detector mode, the detection limit was 6 ng for a 48-1air sample.

189

8.9. Organic oxy compounds

1

Fig. 8.17. Typical chromatogram for automobile exhaust [120]. Peak 1 acrolein derivative

0

I

10 tR[min]

Aldehydes in air, however, are most frequently determined using the well known reaction with 2,4-dinitrophenylhydrazine[117-1191. The latter is deposited on silica gel [117, 1181 or Amberlite XAD-2 [119]. After the dinitrophenylhydrazones formed in the reaction have been extracted with acetonitrile, toluene or ethanolamine, they are analysed on packed or capillary columns using ECD or FID. The technique is simple, fast and accurate. i n analysing 50-100 1 air, the detection limit for acetaldehyde is in the range 0.05-0.25 ppb, with a 4% variation [118, 1191. A unique method for reaction gas chromatography was applied by NISHIKAWA et al. to the detection of micro-concentrations of acrolein, one of the major components of automobile, including diesel, exhaust [120]. Air (3-40 1) was passed through a pair of adsorbers containing 10 ml of ethanol each at a rate of 0.5-1.0 l/min. The resulting solutions were sequentially treated with sodium acetate, hydrochloric acid, methoxylamine, sulphuric acid and KBr0,-KBr, the excess of bromine being removed with sodium thiosulphate. The bromo derivative obtained (92% conversion) was extracted with diethyl ether (1.5 ml), and the concentrate (4 ml) was chromatographed on a glass column (2 m x 3 mm I. D.) of 3 % silicone XE-60 on Chromosorb W at 90°C using ECD (Fig. 8.16). The detection limit was 1 ppb. The method has the disadvantage that uncontrollable ethanol evaporation in the adsorber can give significant errors in the detection of the concentrated pollutants [7, 711. Another method [120] is highly selective and can be applied to the detection of acrolein in composite air pollutants (Fig. 8.17). Low concentrations of toxic phenols and cresols are trapped from air using chromatographic sorbents [121], and impregnated glass-fibre filter, or silica gel [122]. Silica gel can be employed to detect up to 1 ng of cresols after desorption with 5% methanol in carbon disulphide and using FID; the detection limit for phenol is not lower than 5 mg/m3, the relative standard deviation being 0.05% [121]. To reduce the detection limit and increase the selectivity of the determination of phenols they were converted into trimethylsilyl [ 1231 or 2,4-dinitrophenyl ethers, which were detected by 63NiECD after separation on a 50-m capillary column of OV-210 or SP-2100 with temperature programming from 60 to 220°C [124].

8. Practical application of gas chromatography

190

On concentration of impurities and phenol and its alkyl-substituted compounds on silica gel and their desorption with ethanolamine, it was possible to lower the detection limit to 0.5 mg/m3 [125]. The chromatographic separation of phenols was effected on a capillary column (60 m x 0.32 mm I. D.) of DB-1701 with temperature programming from 150 and 200°C. Aliphatic carboxylic acids, aldehydes, alcohols, esters, phthalates and phosphites were identified in air of offices. These contaminants come from tobacco smoke, detergents, plasticizers and cosmetics [7]. Several monocarbonyl compounds were extracted from cigar smoke concentrated in a trap containing silica gel; they were determined quantitatively after conversion into benzyloxime derivatives and separation on a FFAP capillary column [126]. Gas chromatography was applied to determine acetic anhydride and ethylene oxide in air from the amount of ethylene glycol produced on hydrolysis of ethylene oxide [7]. Trace amounts of phthalic anhydride (0.2 mg/m3) were detected in air by FID after conversion into dimethyl-phthalate, and trimellitic anhydride was methylated with diazomethane and analysed in the form of the methyl ester obtained [127]. About 0.5 ng of ethylene oxide can be detected in volatiles from linoleum based on PVC using a direct technique, after concentrating contaminants in a trap containing Polysorb-1 [128]. Concentrations of ethylene oxide in air of 0.5 ppm were determined in the form of 2-bromoethanol produced when ethylene oxide underwent chemisorption on a sorbent (24% HBr on Amberlite XE-347) in a glass tube (50 X 5 mm I. D.). 2-Bromoethanol was extracted with acetonitrile - toluene (1:l) and determined on a column packed with diethylene glycol succinate on Chromosorb W at 155°C using ECD [129]. A similar reaction can be used to lower the detection limit of ethylene oxide by two or three orders of magnitude if first its impurities (from 201 of air) are concentrated on activated charcoal treated with HBr [130]. Quantitative determination was achieved on a fusedsilica capillary column (50 m x 0.22 mm I. D.) of CP Wax-57CB with temperature programming from 120 to 135°C and with FID. Gas chromatography was applied to detect toxic phenol and carbonyl contaminants in smoke from smokeries [123] and tobacco [123, 1261, to determine traces of methyl tert.-butyl ether in air [131] and ethyl formate [132], to analyse gases evolved in heating panels with printed circuits (alcohols, cresols, aldehydes, ethers, etc.) [133] and to study the composition of toxic compounds in stack gases from processes involving 0x0 snytheses (aldehydes, ethers, hydrocarbons) [ 1341.

8.10.

Amines and nitro compounds

Organic compounds containing nitrogen, especially amines and N-nitroso compounds, are highly reactive and interact with many sorbents, which makes them difficult to analyse by gas chromatographic techniques. Low concentrations of aliphatic amines can be trapped from air on silica gel, alumina or Porasil A (porous silica gel treated with 5% KOH in methanol) [135]. 2-Butanol [136] and methanol [ 1371 have been employed as effective desorbents. Trimethylamine is collected with high efficiency in a tube (18 cm x 6 mm I. D.) of Tenax and N,N’-dimethylaminopropionitrile in a Polysorb-1 trap [7]. Micro-amounts of trimethylamine can be stored unchanged for 4 days in a concentration tube containing glass beads treated with 0.2% tartaric acid. Microconcentrations of aliphatic amines, nicotine and N-nitrosodimethylamines have been found to be firmly held on asbestos fibre [138]. Enriched samples were analysed by gas chromatography with FID after thermal desorption at 120-280°C and extraction with chloroform or aqueous copper chloride. The extraction efficiency is 97% and the sensitivity is at the nanogram level [139].

8.10. Arnines and nitro compounds

191

Chromosorb 103 [ 1401 and amino polysorb prepared by nitrating a styrene-divinylbenzene copolymer and reducing the nitration product [71 are among the best sorbents for separating micro-amounts of amines and amides chromatographically. Arnines can be separated effectively on Chromosorb 102 and 104 treated with a dilute solution of KOH and on a column containing a mixture of Carbowax 20M and KOH solution. Reactive amines and their derivatives can also be analysed by first converting them into less active compounds that are more suitable for detection, such as amides of heptafluorobutyric acid [141, 1421. Analysing these derivatives on a glass capillary column (13m x 0.32 mm I.D.) of OV-73 with temperature programming from 114 to 214°C and using electron-capture and thermionic detectors allowed the detection limits of these compounds to be reduced to the picogram level [142], which is about ten times lower than with that achieved in the analysis of the unchanged amines collected on sorbents [135, 1371. In determining very toxic nicotine, an alkaloid of the pyridine series, it is important to decrease its adsorption on the sorbents and chromatograph lines to the minimum. For this purpose, a mixture of ammonium hydroxide, diisopropyl ether, heptane and Antifoam B was added to the solution to be analysed prior to chromatography. The solution was then stirred, and the organic layer was analysed by GLC on a 1.8-m glass column containing 10% Apiezon L [143] and 3% KOH on Chromosorb W at 190°C with TID. Gas chromatography is frequently used to analyse condensates from tobacco smoke (cigarettes and cigars) and marihuana smoke [7]. A GC procedure for determining the six most important cigarette smoke constituents, viz., tar, nicotine, CO, HCN, acrolein and aldehydes, has been developed [144]. Volatiles from cigarette smoke were separated by HPLC on a Sephadex column using isopropanol as the mobile phase into 35 fractions which were studied further by GC-MS [145]; 405 organic compounds were identified, including aromatic hydrocarbons, alkanes, alkenes, terpenes, N- and 0-heterocyclic, alcohols, aldehydes and nitriles. A procedure suggested for the gas chromatographic determination of amphetamines, cocaine and heroin [146, 1471 included preconcentration of drugs in a short tube (75 x 6.3 mm I. D.) of Tenax (for amphetamines) and platinum powder treated with silicone OV-17 (for cocaine and heroin) [147]. Thermal desorption was performed in a flow of helium, and durg vapours were analysed on columns (2 m x 3.2 mm I. D.) packed with 3% OV-101 on Ultrabond 20M at 160°C (amphetamines) and with Ultrabond 20M (cocaine and heroin); TID was employed for quantification. A capillary column (25 m x 0.2 mm I.D.) with an Ultra A 2 (phenyl methyl silicone) chemically immobilized stationary phase was applied to the determination of N-methylmorpholine in workplace atmospheres [148]. The compound was preconcentrated from air (5 ml) in a trap containing Amberlite HAD-2 at a flow-rate of 0.1 I/min and the concentrated pollutants were desorbed with ethanolamine (1 ml) for 30 min. The detection limit with the use of a thermionic detector was 0.05 ng. The method was applied to N-methylmorpholine determinations in the range between 2 and 100 mg/m3. Gas chromatographic techniques enable micro-concentrations of very toxic N-nitrosoamines to be determined in air with high sensitivity and selectivity. Among 130 nitroso compounds most abundant in the environment, 100 are potential carcinogens. N-Nitrosoamines can be trapped very effectively in a tube containing thoroughly dried active powder of calcium and magnesium silicates; magnesium trisilicate is especially effective [149]. An interesting procedure for detecting micro-amounts of N-nitrosoamines was described by CASTEGNARO and WALKER [150]. Samples were subjected to pyrolysis at 450°C to produce NO, which was further converted to excited NO2 under the action of ozone. Detection was based on monitoring the luminescence intensity in the gas phase. Reaction gas chromatography has also been applied to the determination of highly toxic cyanates. The quantitative determination of toluene diisocyanate in working area air is based

8. Practical application of gas chromatography

192

260 "C

1moc

Fig. 8.18. Chromatogram for the separation of aromatic amine pollutants (products of thermal destruction of foamed polyurethane) after their derivatization via reaction with pentafluoropropionic acid [153]. I Aniline; 2 2-methylaniline; 3 4-methylaniline; 4 methylendianiline; Column, glass capillary (20 m x 0.3 mm I. D.) packed with PS-225; temperature, programmed from 100 to 260°C at 10 Wmin; detector, TID (aniline peak corresponds to 250 pmol)

on its hydrolysis in an acidic medium, treating the resulting 2,4-toluenediamine with dimethylformamide-dimethylacetal and analysing the corresponding 2,4-toluenediamine monoand di-derivatives by GC-MS [1511. A low detection limit for 1,6-hexamethylene diisocyanate was achieved by applying ECD after converting the compound into a fluoro derivative by reaction with heptafluorobutyric anhydride and removing other contaminants on a column (1.8 m X 4 m I. D.) of SE-30 on Gas-Chrom Q at 145°C [152]. The direct determination of cyanates with sufficiently high sensitivity is also possible if TID is employed for quantification. The thermal decomposition of foamed polyurethane, e.g., in fires, leads to the evolution of very toxic products. The decomposition of the foamed polyurethane base, i. e., 4,4'-methylenediphenyl isocanate (MDI), gives anilines and phenyl isocyanates, although the formation of diamines, phenyl isocyanates and 4,4'-methylenedianiline is also possible. The isocyanates are detected by HPLC whereas a mixture of anilines is analysed by gas chromatography upon their conversion to fluoro derivatives via reaction with pentafluoropropionic acid [153]. The chromatographic separation of the aniline pollutants is shown in Fig. 8.18. With the use of a TID the detection limit is 0.001 mg/m3. Traces of isocyanic acid were determined by analysing air on a PTFE column of Porapak Q at 110°C and TID [154]. The detection limit was 0.7ppm and the standard deviation 0.045 ppm. FID was employed to determine 0-diethylaminoethyl chloride and 0-diethylaminoethyl acetoacetate in workplace air [155]. Acrylonitrile present in air was determined by concentrating it in a trap containing active carbon, extracting the contaminants with carbon disulphide-acetone and recording them by FID after separation on a column of Porapak Q or 1,2,3-tris(cyanoethoxy)propaneon Chromosorb W. At a 0.5 mg/m3 acrylonitrile concentration the efficiency of desorption was 93-96% (1561. Higher selectivity and sensitivity were provided by a reaction chromatographic method of acrylonitrile analysis, which has been patented in the U.S.S.R. [157]. This compound was trapped from air in alkaline KMn04solution, with subsequent .formation of bromocyanogen by reaction with bromine. After extraction of the bromocyanogen with toluene it was de-

193

8.10. Amines and nitro compounds

5 7

2

Fig. 8.19. Chromatogram of micro-amounts of alkanolamines (amino alcohols) in the form of heptafluorobutyryl derivatives (160). Conditions: column, 15% phenyl diethanolaminosuccinate on diatomite carrier, solid carrier deactivated with polyglycol. 1 Monoisopropanolamine; 2 monoethanolamine; 3 and 4 isomers of diisopropanolamine; 5 diethanolamine; 6 triisopropanolamine; 7 triethanolamine; 8 impurities in alkanolamines; 9 false peak (artifact) arising from the reagent used to prepare fluoro derivatives of amino alcohols

8

1 0

2

4

6

L 8

Time [min]

tected on a glass column (2 m x 4 m m I.D.) of 15%Carbowax 1500 on Chromosorb N at 60°C using ECD. The detection limit was 0.02 mg/m3 with a relative error of no more than 5%. Micro-concentrations of dinitrotoluene concentrated from air on silica gel can be detected with 0.1 mg/m3 sensitivity using FID; samples were desorbed with chloroform. A trap containing Chromosorb 106 showed the highest adsorption capacity towards 2-nitropropane, which could be stored there for several days. Concentrated contaminants were then extracted with ethyl acetate and an aliquot of the solution was analysed on a 6-m column packed with 10% FFAP on Chromosorb at 90°C with FID [158]. For the detection of nitromethane and nitroethane (4-40 mg/m3) in workplace air, 5-10 1 of air were passed through a concentration tube filled with activated charcoal, the concentrated pollutants were desorbed with chloroform for 30 min and then the extract (2 ml) was chromatographed on a column (2 m x 2 mm I. D.) of Carbowax 20M on Chromosorb W at 80°C using FID 11591. Alkanolamines, which are highly polar basic compounds, are the most difficult to determine among all the compounds containing nitrogen. Although these inconvenient compounds can in principle be determined by gas chromatography, direct techniques are inapplicable to trace amounts of amino alcohols, as low concentrations of these compounds are sorbed irreversibly by the packings and chromatograph lines. For this reason, a procedure for analysing these toxic compounds in air at concentrations below 0.1 mg/m3 in the form of fluoro derivatives has been developed [160]. A sample is collected on alumina and eluted with aqueous octane-1-sulphonic acid. The acid not only facilitates effective extraction but also forms non-volatile salts and thus almost completely removes water through lyophilization. The dry salts thus obtained are then converted into fluoro derivatives by reaction with heptafluorobutyrylirnidazole and analysed by gas chromatography. Chromatograms for the separation of heptafluorobutyryl derivatives of

194

8. Practical application of gas chromatography

alkanolamines are shown in Fig. 8.19. A good separation of six amino alcohols in the form of heptafluorobutyryl derivatives was obtained on a column of 1%phenyldiethanolamine succinate on a diatomite carrier (0.15-0.18 mm fraction) deactivated by polyglycol chemically attached to the carrier for 8 min (the concentration of contaminants was 20-50 pg/ml).

8.11.

Odorants

Almost all organic and inorganic sulphur derivatives are odorants (compounds with a disagreeable odour) and have a toxic action on man. Gas chromatography with FID or PID is the best technique for determining low concentrations of organosulphur compounds in air. As sulphur compounds interact with active carbon, porous polymer sorbents such as XAD-4 and Tenax are employed to concentrate them from air. The efficiency of desorbing the concentrated products with acetone or diethyl ether is approximately 95% [161]. Dimethyl sulphate, which is carcinogenic, was collected from air in a trap containing silica gel (sample volume 10-15 l), desorbed with 0.5 ml of water using ultrasonics, and the extract thus obtained was chromatographed on a 3-m column of 10%Apiezon L on Chromosorb P at 110°C with FID. The sensitivity of the determination of diethyl and dimethyl sulphates in 20-96 1 of air with FID was 0.05 mg/m3 [162], and with FPD the detection limit was 0.04 mg/ m3 [163]. The very weak FPD response to hydrocarbons allows this type of detection to be used for the selective determination of compounds containing sulphur in complex mixtures of air pollutants in the presence of large concentrations of hydrocarbons that often accompany sulphur compounds in the atmosphere and workplace air [161]. Mercaptans in air can be selectively determined by collecting them in a trap or on a glassfibre filter impregnated with aqueous mercury(I1) acetate. Mercaptans are trapped in the form of mercaptides and recovered from the latter under the action of HCl [164]. The sensitivity of the determination of sulphides and mercaptans with FPD ist about 1 ng/ m3 [161]. The direct determination of mercaptans with the use of a photoionization detector sensitive to these substances is possible at pg/m3 concentrations [165]. A very low detection limit (8.8-20 pg) for sulphur compounds [COS, H2S, CH3SH, CS2, (CH3)2S,(CH3),S2]was attained after repeatedly concentrating these contaminants, first in a combined trap with Tenax and zeolite 5A, and then in a cryogenic trap at liquid nitrogen temperature [166]. After desorption, the contaminants were separated on a 3 m X 3 mm I. D. column of Chromosorb 300 with temperature programming and with FPD. The detection limit for mercaptans present in a similar mixture of sulphur compounds in cellulose plant flue gases was 1 ppb [167]. Isothiocyanates, which are produced in the decomposition of herbicides, were desorbed with carbon disulphide after concentrating them on silica gel (extraction efficiency 92%) and determined using FID in amounts of 0.02 pg after separation from other sample constituents on a column of polyethylene glycol 1540 on Carbopak C [168]. Sulphur compounds can be effectively separated on a column of Porapak Q or Chromosorb 104, especially if the sorbents are silanized [169]. Pesticides containing phosphorus were analysed on glass or nickel columns containing a small amount of a silicone of the OV-1 type on Chromosorb W; TID was employed [161]. After passing 700 1 of air through a concentrator, pesticides can be determined at the pg/l level with an accuracy of +lo%. Carbovos, dichlorvos, phosphamide and other pesticides containing phosphorus were collected from air in a combined trap consisting of a filter and an organic absorber or a filter and a tube containing carbon or silica gel. Pesticides were detected using ECD or TID. Bazudine vapours were well absorbed by silica gel and extracted with diethyl ether. The extract was concentrated and

195

8.12. Halogenated hydrocarbons Table 8.8. Conditions for gas chromatography of odorants [171] Group of odorants

Concentration technique

RSR

Cooled trap')

R,N

0.5% aqueous

Air volume

1 20

H3B0J

RCOOR

Cooled trap

1

Hydrocarbons

Tenax GC

1

ROH

Cooled trap

1

PhOH

Tenax GC

1

RCOOH

SP-1000 and H,P04 on Shimalite Tenax GC

1

In do1e s

Analysis conditions

Detector

Detection limit (C . mg/ mJ)

25% tricresyl phosphate on

FPD

0.1

PID

0.5

PID

0.5

PID

1.0

PID

2.0

PID

1.0

PID

1.0

PID

0.05

(1)

SO

Shimalite, length 3 m, 75°C 4% polyethylene glycol, 20 m, and 0.8% KOH on Carbopack B, length 1.5 m, 105°C 5% tricresyl phosphate and 0.4% Triton X-100 on Carbopack B, length 1.5 m, 70°C 5% SP-1200 and 1.75% Bentone-34 on Chromosorb W, length 3 m, 80°C 0.5% polyethylene glycol 1500 and 0.2% KOH on Carbopack B, length 1.5 m, 135°C 0.1% SP-1000 on Carbopack C, length 1.7 m, 220°C 0.3% FFAP and 0.3% HjP04 on Carbopack B, length 1.5 m, 200°C 5% silicone XE-60 on Chromosorb W, length 3 m, 170°C

*) Liquid oxygen.

analysed first by TLC on Silufol and then by GLC on a glass column with a mixture of silicone and nitrilesilicone stationary phases on Chromaton at 180°C using TID [170]. HOSHIKAstudied [147] odorants of different natures [171], including sulphur compounds, low-boiling aliphatic amines, carbonyl compounds, hydrocarbons, low-molecular-weight aliphatic monoalcohols, phenols, fatty acids and indoles. The classification of odorants and the conditions for their chromatographic determination in air are given in Table 8.8. This table shows that odorants can be determined in air at ppb concentrations with a relative error of 10%after concentrating them in a cooled Tenax trap [171].

8.12.

Halogenated hydrocarbons

The principal technique for analysing these toxic industrial poisons is gas chromatography. Many chlorinated organic compounds, such as vinyl chloride, chloroprene, vinylidene chloride, tri- and perchloroethylenes, trans-2,4-dichlorobutene,hexachlorobutadiene and ally1 chloride, are believed to be carcinogenic.

196

8. Practical application of gas chromatography

Low concentrations of halogenated hydrocarbons are effectively collected from air by passing samples through a tub containing active carbon. The most effective traps, however, are those containing Tenax or Amberlite XAD-2. The concentrated contaminants are desorbed by heating and also by extraction with carbon disulphide, carbon tetrachloride and chlorobenzene. The resulting solutions are analysed by GLC on columns with Carbowax 20M or silicones as stationary phases using PID, ECD or FID [7]. For a long time, the electron-capture detector was considered to be the perfect instrument for detecting micro-concentrations of halogens and their compounds. However, a photoionization detector is more stable in 0peration;and it can be used to detect about 0.001 mg/m3 of chlorobenzene [172] and 19 ppb of dibromodiethylene concentrations in air after concentrating the contaminants on Amberlite XAD-2. This detector, the selectivity of which depends on the source of UV radiation employed, has a 10-100-fold lower detection limit compared with a flame ionization detector, and its linear dynamic range exceeds lo7 at the highest detector operating temperature of 300°C [66]. Picomol amounts of methyl chloride can be determined in air using a 3 m X 4 mm I. D. column of Porasil A at 90°C and ECD. The detection limit for chlorinated hydrocarbons can be decreased by converting them into iodo derivatives by reaction with NaI at 240°C [173]. Reaction gas chromatography with ECD has also been used to determine chlorophenols as fluoro derivatives in workplace air [71]. Gas chromatography is the best technique for measuring low concentrations of organic solvents (chlorinated hydrocarbons, alcohols, ketones, hydrocarbons) in air, and has been widely applied to the analysis of gases evolved in the paint and varnish industry and in drying painted materials (chlorinated hydrocarbons, aromatic hydrocarbons), and to determine halogenated hydrocarbons with different molecular masses in air [174-1771. KRECHKOVSKII [177] studied the gas chromatographic determination of the in operating anaesthetic gas fluothane (halothane; 2-bromo-2-chloro-l,l,l-trifluoroethane) theatre air. It is trapped on active carbon, desorbed with carbon disulphide and analysed on column of 15%FFAP on a diatomite carrier. Nanogram amounts of this compound can be determined using ECD or FID. Not less than 0.2 pg of epichlorohydrin can be detected in workplace air after concentration in a standard tube containing active carbon (100 and 50 mg) and desorption with carbon disulphide by chromatography of the extract on a column (1.5 m X 2 mm I.D.) of Porapak Q with FID [178]. The reaction chromatographic method for the detection of sodium trichloroacetate in air (ca. 0.02 mg/m3) consists in the decarboxylation of this compound to chloroform, which is then detected by ECD [179]. HPLC and gas chromatography (headspace analysis) have been used for the detection of a variety of lachrimators such as bromoacetone, chloroacetophenone, dinitrile (NC-CN), o-chlorobenzylidenemalonic acid and bromoacetates in workplace atmosphere in the production of toys and fireworks [180].

8.13.

Freons

The determination of very low concentrations mg/m3) of Freons (fluorochlorohydrocarbons), whose main source refrigerator coolants, is a problem pertaining to the chemistry of the atmosphere. Freons occur in the atmosphere at ppt concentrations, but they destroy the ozone layer even when present in ultramicro-concentrations [181]. Several litres of air are drawn through a trap containing an inert adsorbent of the Carbopak B type at -78°C [182]. The concentrated contaminants are desorbed thermally at 200°C and analysed on a Carbopak column using ECD. KISELEVet al. [181] determined Freons using a Gazokhrom-1106-E chromatograph equipped with a electron-capture detector and after separation on a Carbochrom B column. Gas chromatographic techniques can be used to

6pi

197

8.14. Chlorine-containing pesticides, polychlorobiphenyls and dioxins

determine very low concentrations of fluorochlorohydrocarbons in the atmosphere over the surfaces of seas and oceans and in the stratosphere and troposphere. A portable gas chromatograph with an electron-capture detector has been used for rapid analyses on board airplane [183]. The problem of Freons in workplace air is not as important as in the general atmosphere, as the TLV for, e. g., trichlorofluoromethane is 1000 ppm. Freons were therefore also able to be determined using a katharometer after trapping them in a standard tube containing carbon (100 and 50 mg), desorption with n-hexane and separation from other contaminants on a column (3 m x 4 mm I. D.) of silicone DS-550 on Chromosorb W at 75°C [184].

8.14.

Chlorine-containing pesticides, polychlorobiphenyls and dioxins

Gas chromatography is the only reliable technique for the rapid monitoring of the presence in the atmosphere, other environmental areas and foods of toxic pesticides containing chlorine and polychlorobiphenyls added to chemical poisons to increase their action [ 1851. Polychlorobiphenyls (PCBs) include a total of 209 compounds, of which 100 are used in industry (cooling agents, insulation media in transformers, hydraulic fluids, etc.). They are high-boiling (300-380°C), chemically stable, toxic compounds which remain unchanged in the environment and can be biologically accumulated to cause serious poisoning and occupational diseases [186]. Micro-concentrations of high-boiling chlorinated hydrocarbons (pesticides) are trapped from air using a combined concentrator consisting of a filter and a tube containing active carbon or silica gel [185] or with the help of porous polymer sorbents such as Tenax, Amberlite XAD-2 or polyurethane foam. The adsorption efficiency is about 95% and the contaminants can be determined in ng/m3 concentrations in 1200-3500-m3 air samples using ECD [187]. One of the best type of sorbent for trapping pesticides and related compounds from air is those with chemically bonded stationary phases [160], e.g., those listed in Table 8.9 for lindane and chloropyrofos.

Table 8.9. Efficiency of collection of pesticides from air on chemically bound packings [160] Pesticide

Lindane

Chloropyrofos

15

Sorbent

Durapak-Carbowax 400 on Porasil F Durapak-phenyl isocyanate on Porasil C Durapak-H-octane on Porasil C Durapak OPN on Porasil C Durapak-Carbowax 400 on Porasil F Durapak-Carbowax 400 on Porasil F

Berezkin. Gas Chrom.-BE

Introduced (Pg)

Found (pg)

Recovery (%)

Front section

Back section

5.93

5.68

0.002

96

5.2 1

4.56

0.05

88

5.97

5.74

0.004

96

5.90 0.083

5.53 0.072

0.02 94 not detected 87

0.19

0.17

0.008

93

198

8. Practical application of gas chromatography

Concentrated pesticide and PCB contaminants are extracted with light petroleum ether or hexane [188]. The use of ultrasonics substantially accelerates the process. Micro-amounts of these toxic compounds are separated on glass packed or capillary columns with a silicone SLP using temperature programming and 63NiECD [188, 1891. Gas chromatography has been employed to monitor residues of pesticides in soil, water, plants, animal feed, food, etc., and also in air. Very low concentrations of toxic PCBs and polybrominated biphenyls were determined in laboratory air [188], workplace air and in the general atmosphere [187, 1901. To determine PCBs in workplace air at TLV levels they were trapped in a tube containing zeolite SA, desorbed with CS2 and analysed on a column (2 m X 2.5 mm I. D.) of 5% silicone OV-101 on Chromosorb W at 200°C using FID [191]. Very low concentrations of PCBs in the atmosphere were detected by passing 600-900 m3 of air for 2-3 days at 40 m3/h through a polyurethane foam filter [192, 1931. The filter was extracted with hexane - diethyl ether (9:1), the extract was fractionated by HPLC [193] on a silica gel column with a 0.4% solution of acetonitrile in hexane, and the major eluate fraction was analysed by GC-MS on a glass capillary column (25 m x 0.3 mm I.D.) of silicone OV-1. This technique, which is capable of detecting PCBs at 1 pg/m3 concentrations, was used to prove that PCBs were transported to Arctic regions from Western Europe, Asia and North America [193]. ZELINSKI et al. [194] studied the chromatographic behaviour and retention characteristics of 54 PCBs containing from one to ten chlorine atoms per molecule. To a first approximation, the retention of PCBs on a pneumatic liquid crystal [N,N’-bis(p-methoxybenzylidene)a,a’-bi-p-toluidine]appears to increase with increasing molecular lengthlwidth ratio for similarly structured PCBs containing the same number of chlorines per molecule. Similar techniques were used to determine picogram amounts of chlorinated dibenzo-p-dioxins [195, 1961 and dibenzofurans [197], e.g., very toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin. After extracting contaminants in a Soxhlet apparatus for 16 h, they were separated from accompanying compounds by HPLC and studied by GC or GC-MS [195]. Dioxins include a large group of polychlorinated dibenzodioxin and dibenzofuran derivatives. These compounds are formed as side-products in the production and processing of chlorophenols, in the synthesis of chlorinated biphenyls, naphthalene and chlorobenzenes and also in the oxidation of aromatic hydrocarbons and during their further transformations. Dioxins are formed in large amounts in burning refuse. Polychlorinated dibenzodioxins and dibenzofurans from the last source [195] were determined by extracting soot with benzene, concentrating the extract, and fractionating by HPLC on a column (30 cm X 8 mm I. D.) of Micropak MCH-10. The fractions were analysed on a capillary column (30 m X 0.32 mm I. D.) of DB-5 with temperature programming from 80 to 300°C and using FID and ECD. Individual constituents were identified by GC-MS [198]. A similar column was used for determining the content of octa- and heptachlorodibenzop-dioxins [199] in industrial dust. The resulting extracts were purified by HPLC and analysed by gas chromatography for dibenzo-p-dioxins.

8.15.

Bis(chloromethy1)ether

The very toxic bis(chloromethy1)ether (BCME) is a well known carcinogen. It is evolved in flue gases in the chemical industry and it can be formed in the atmosphere by the Mannich reaction. Ultramicro-concentrations of this compound mg/m3) were extracted from the atmosphere in concentration tubes containing porous polymers and, after processing of the samples, were analysed on a Tenax column with ECD [200]. A comparison of the sorption properties of polymer sorbents (Porapak Q, Chromosorb 101 and 104 and Tenax) showed that

(w-

8.16. Vinyl chloride

199

none of them is perfect for trapping micro-amounts of BCME; Tenax GC gave the most satisfactory results [201]. The concentration of BCME on Carbopak B allowed the analysis of trapped contaminants at the 6.3 pg/m3 level with a measurement error of f20-30% using GLC with ECD. About mg/m3 of BCME can be determined by reaction gas chromatography [202] after sampling on Tenax and conversion into bis-p-phenylphenoxymethylether followed by ECD detection, or by FID after analysing the products of the interaction of BCME with a b l a t e s or sodium chlorophenolate. The reaction leading to bis-p-phenylphenoxymethylether is recommended for monitoring the amount of BCME evolved into the atmosphere by chemical plants employing this compound as a starting material or organic synthesis intermediate [202]. A gas chromatographic system for monitoring BCME in production area air was developed. This allowed the analysis of 20-24 contaminated air samples simultaneously with a detection limit of mg/m3.The detection limit for ECD determinations of BCME can be decreased 40-fold by using nitrogen containing micro-amounts of oxygen as the carrier gas [200].

8.16.

Vinyl chloride

It is 50 years since poly(viny1 chloride) was first synthesized and prepared on an industrial scale, and since then this polymer has found wide applications in many fields. However, only in 1971 was it recognized that vinyl chloride is carcinogenic [203]. In the 1970s, hundreds of studies were carried out [54] on procedures for determining vinyl chloride in the atmosphere and production area air [204]. Intense interest in detecting vinyl chloride resulted in improvements in sampling techniques and methods for measuring low concentrations of toxic substances [7, 711. The National Institute of Industrial Hygiene (U.S.A.) developed a standard procedure for determining vinyl chloride in workplace air [205], and this was followed by several dozen similar publications in various countries [71, 2041. The principal features of the technique are as follows. Air (several litres) is passed through a tube containing active charcoal and consisting of two sections with 100-500 mg of the sorbent. After adsorption, charcoal is placed in a flask with 1 ml of carbon disulphide for 30 min then 50 pl of the extract are chromatographed using FID for quantification. If air is pumped at a flow-rate of 100 ml/min and vinyl chloride is present at concentrations of 0.5-50 mg/m3; the breakthrough volumes are 9.8 1 (5 mg/m3 vinyl chloride concentration), 22.8 1 (25 mg/m3) and 18.1 1 (50 mg/m3). Vinyl chloride losses after storing samples on charcoal amount to 12.5%in 2 h (under ambient conditions), 20%in 1 week and 29%in 2 weeks. The losses can be decreased to 10%in 2 weeks by reducing the temperature. Carbon disulphide extracts 80-90% of the vinyl chloride present in samples [201]. A comparison of the adsorption properties of Carbopacks, Carbosieves, various porous polymer sorbents, silica gel and molecular sieves 13X showed that vinyl chloride can most conveniently be trapped on Porapak N and Carbosieves (carbon molecular sieves) as they adsorb water more weakly than other sorbents [201]. Tenax [206] has been found to adsorb vinyl chloride only weakly, whereas Spherocarb (a recently synthesized sorbent) traps almost 100% of vinyl chloride. In addition to carbon disulphide, benzyl alcohol and chlorinated hydrocarbons, when sufficiently pure not to interfere in the analysis of the major component, can be employed for the effective extraction (more than 80%)of vinyl chloride from sorbents. Chlorobenzene subjected to rectification on a column with an efficiency of 16 theoretical plates is suitable for the gas chromatography of vinyl chloride and other halogenated hydrocarbons [207]. IS'

200

8. Practical application of gas chromatography

Table 8.10. Sensitivity of chromatographic detectors to vinyl chloride [204, 208, 2091 Detector

Specificity of detection

Flame ionization Electron-capture Electron-capture (sensitized by adding nitrous oxide to carrier gas) Microcoulometric Chemiluminescence Photoionization Mass spectrometric

none Halogens Halogens Halogens Olefins none Ions m l z 62 and 64

Detection limit (approx.) (g) 0.1.10-9 2.0.10-9

lo-'? 70. 20.10-9 5.10-10 10-20~10-'*

Micro-concentrations of vinyl chloride are most frequently determined using a flame ionization detector, the sensitivity of which to this gas is approximately 20 times higher than that of an electron-capture detector (Table 8.10). The sensitivity of the latter towards vinyl chloride increases 1000-fold, however, if 0.002%of N20 is added to the carrier gas (nitrogen) [208]. Vinyl chloride peaks can only be identified reliably after removing accompanying contaminants eluted before and after vinyl chloride. One of the best chromatographic columns for this purpose is Carbopak C impregnated with 0.2%polyethylene glycol 1500 [210]. Separation columns containing a mixture of Porapak S and T enabled vinyl chloride to be separated from 21 organic compounds [211], and such a superselective packing as 0.19%picric acid on Carbopack C was found to be capable of separating vinyl chloride from 65 organic compounds eluted almost simultaneously with it [2 121. Reaction chromatographic (reaction sorption) techniques (RSC) are even more reliable for the identification of micro-amounts of vinyl chloride [207, 213-2151 (see Chapter 6). Most gas chromatographic techniques for determing micro-concentrations of vinyl chloride in air are based on extracting contaminants concentrated in a trap with carbon disulphide [201] or other organic solvents [71]. Thermal desorption is an alternative procedure. This is employed less frequently, although it allows a lower detection limit to be attained, other things being equal. Typical chromatographic profiles of vinyl chloride and certain ac-

C

2

-0 OI

.s 0

G

L

0

.

I

,

I

I

I

I

I

l

l

\

10

5 [min]

-+

.

.

l

.

Fig. 8.20. Chromatogram of carbon disulphide extract from the front section of a tube containing active charcoal (100 mg) obtained in determining vinyl chloride (2161

201

8.16. Vinyl chloride

CS2 Peak

Fig. 8.21. Typical chromatogram obtained after thermally desorbing vinyl chloride from a tube containing active charcoal. Conditions: column, Chromosorb 102; temperature, programmed from 25 to 150°C; detection, FID [216]

5

0 140°C

10 [min] 190 O C

32 K/m in

companying substances are shown in Figs. 8.20 and 8.21. Also, it has been shown [216] that thermal desorption gives a better reproducibility than extracting vinyl chloride from traps (usually containing charcoal) with solvents (Table 8.11). One should bear in mind, however, the possibility of sample decomposition during thermal desorption at high temperatures, which gives rise to false peaks (artifacts). Thus, a comparison of chromatograms obtained in the analysis of gases evolved from poly(viny1 chloride) leather by GC (adsorption on charcoal and desorption with chlorobenzene) and GC-MS (ad-

Fig. 8.22. Chromatogramm for the latex vapour phase based on a copolymer of vinyl chloride and vinylidene chloride [224]. 1 Vinyl chloride; 2, 5 unidentified; 3 vinylidene chloride; 4 ethanol. Conditions: Column, steel (3 m x 3 mrn I.D.) packed with 10% tricresyl phosphate on Chromosorb N; temperature, 70°C; detector, FID

S

L

3 2 1 Time [min]

0

202

8. Practical application of gas chromatography

Thermal desorption.')

Extraction by carbon disulphide*) Vinyl chloride peak area

Relative standard

Vinyl chloride peak area

~~~~

263.5 243.0 232.5 200.6 214.5 207.2

10.6

107.0 104.5 91.9 92.8 97.1 99.3 99.7 96.8 103.7 103.4 100.5

Relative standard

Table 8.11. Reproducibility of gas chromatographic determinations of vinyl chloride microconcentrations [216]

~

4.1

*) 1.29 ng of vinyl chloride in 5 yl. **) 717 ng of vinyl chloride (278 ml = 1 ppm vinyl chloride).

sorption on Tenax and thermal desorption at 200-250°C) showed that the latter procedure led to at least 10- 14 new compounds (mostly chlorinated hydrocarbons) absent from the initial air samples [217]. Gas chromatographic techniques have been employed to detect TLV amounts of vinyl chloride in workplace air [205, 207, 2131, the atmosphere [218-2201, stack gases from various industrial processes [22 11, gases evolved by poly(viny1 chloride) materials (artificial leather, linoleums, wallpaper, films, wire insulation, etc.) [214, 2161, after using aerosols and other materials and compositions based on PVC, vinyl chloride and its copolymers in household materials [204], and in the analysis of residual vinyl chloride and vinylidene chloride in PVC samples [222, 2231 and latexes used as protective coverings for cheese and food packaging [224]. In the last instance, a headspace analysis technique was employed for latex samples heated to 90°C (Fig. 8.22). Low concentrations (7 pg per sample) of carcinogenic vinylidene chloride in air were determined by gas chromatography [225]. For this purpose, contaminated air was passed through a tube containing active carbon and contaminants were desorbed with carbon disulphide (extraction efficiency 80%) and analysed on a chromatograph with FID. Samples of vinylidene chloride can be stored unchanged for 3 weeks at 2°C. The material described in this Chapter shows that gas chromatography is a widely and frequently employed technique for determining toxic contaminants in air. This important field of gas chromatographic applications is now rapidly developing.

References

203

References (Chapter 8) [l] JENNINGS, W.: Amer. Lab. 16 (1984) 18. V. G.: Khimicheskie reaktsii v gazovoi khromatografii (Chemical reactions in gas [2] BEREZKIN, chromatography). Moscow: Khimiya 1980. R. G.; LANGHORST, M. L.: Anal. Chem. 57 (1985) 238R. [3] MELCHER, [4] Fox, D. L.: Anal. Chem. 57 (1985) 223R. [5] Chemical Derivatization in Analytical Chemistry. Vol. 2, Separation and Continuous Flow Techniques. FREI, R. W.; LAWRENCE, J. F. (Ed.). Modem Analytical Chemistry. New York: Plenum 1982. Yu. S.: Zh. analit. Khim. 40 (1985) 585. [6) DRUGOV, Yu.S.; BELIKOV, A.B.; D'YAKOVA, G.A.; TULCHINSKII, V.M.: Metody analiza zagryaznenii [7] DRUGOV, vozdukha (Techniques for analysis of air pollutions). Moscow: Khimiya 1984. G.; MOHR,P.; TRUMMER, I.; WALF,A.: Osterr. Chem. Z. 87 (1986) 110. [8] MIKSCHE, [9] BOWEN,H. J. M. (Ed.): Environmental Chemistry. Vol. 3, A specialist Periodical Report. London: The Royal Society of Chemistry, 1985, p. 144. J.: Fresenius Z. anal. Chem. 320 (1985) 689. [lo] ASSHAUER, A,; CICCIOLI, P.: Sci. Chromatogr. Lect. A. J. P. Martin honor Symp. Urbino, BRUNER,F. [ l 11 LIBERTY, (Ed.), 1985. Amsterdam: Elsevier 1985, p. 219. K. E.: Schriftenr. Ver. Wasser, Boden und Lufthyg. 65 (1985) 19. [12] PRESCHER, B.P.; ROTIN,V.A.; KHOKHLOV, V.N.; YUSFIN,V.S.: In: Khromatografiya. Itogi nauki [13] OKHOTNIKOV, i tekhniki (Chromatography. Results of science and technology). Zhukhovitskii, A. A. (Ed.). Moscow: VINITI 2 (1978) 28. L.Ya.; AKCHURIN, F.G.; CHEREZOVA, V.V.: Khim. i Tekhnol. Topliv i Masel 11 (1983) [14] RUVINSKII, 39. M. T.; KOLESNIKOV, G. M.: Gigiyena i Sanitariya 3 (1980) 53. (151 DMITRIEV, , K.; KASAHARA, Y.; SHIRAI,T.; YANAGISAWA, S.: Jap. Anal. 29 (1980) 152. [16] SUZUKI,K.; HORIUCHI, P. D.; FEHSENFELD, F. C.; PHILLIPS,M. P.: J. Chromatogr. 239 (1982) 115. (171 GOLDAN, Yu. S.: Gazovaya khromatografiya neorganicheskikh veshchestv (Gas [18] ANVAER,B. I.; DRUGOV, chromatography of inorganic substances). Moscow: Khimiya 1976. Yu. S.: Zh. analit. Khim. 35 (1980) 559. [19] DRUGOV, G.: Z. gesamte Hyg. 26 (1980) 218. (201 HANIKA, V.I.; RYBALCHENKO, Yu. P.; BASKIN, Z.L.: Author's certificate U.S.S.R. No. 920523, [21] KALMANOVSKII, Byull. izobr. No. 14 (1982). K.; MATUSKA, P.; VIERKORN-RUDOLPH, R.; KONTOS, M.: Z. anal. Chem. 310 (1982) 89. [22] BACHMANN, R.; BACHMANN, K.: J. Chromatogr. 217 (1981) 302. (231 VIERKORN-RUDOLPH, K.; REMEKE,F.; SCHWARZ, B.: Z. anal. Chem. 320 (1985) 706. [24] FUCHS,G.R.; BACHMANN, L. J.; PRESCOIT,B. E.: J. Chromatogr. 264 (1983) 405. [25] ANTHONY, J. M.; BARROW, C.; WHITE,E. L.: Anal. Chem. 56 (1984) 1194. [26] CHEPLEN, J. P.: Amer. Ind. Hyg. Assoc. J. 47 (1986) 742. [27] HENDERSHOIT, V. G.; DRUGOV, Yu. S.: Zh. analit. Khim. 39 (1984) 1249. [28] BEREZKIN, R.; OEHME,M.: J. Chromatogr. 243 (1982) 168. [29] HEGGEN, M.; SHONON, T.: Anal. chim. acta 119 (1980) 291. [30] FUNAZO,K.; TANAKA, [31] MITRA,G. D.; GHOSH,S. K.: J. High Resolut. Chromatogr. and Chromatogr. Commun. 8 (1985) 150. [32] WEISS,R. F.: J. Chromatogr. Sci. 19 (1981) 611. N.: Jap. Anal. 28 (1979) 569. [33] FUKUDO, P. R.; SMITH,S. J.; ALDER,J. F.: Analyst. 111 (1986) 695. [34] FIELDEN, W. D.: J. Chromatogr. 238 (1982) 103. [35] GENNA,J. L.; MCANINCH, L. B.; BESKOVA, G. S.; BUTUSOVA, A. I.: Zavodskaya Lab. 46 (1980) 797. [36] PREOBRAZHENSKAYA, A. B.; MAROULIS, P. J.; WILNER,R.A.; BANDY, A. R.: Atmos. Environ. 15 (1981) 11. [37] GOLDBERG, S. 0.; GLUCK,S. J.; BAMESBERGER, W. L.; SCHUITE,T. M.; ADAMS,D. F.: Anal. Chem. 51 [38] FARWELL, (1979) 609. [39] FUJII,T.: Jap. Anal. 28 (1979) 388. D. C.; DRIEDGER, A. R.;BANDY,A. R.: Anal. Chem. 58 (1986) 2688. [40] THORNTON, A.: J. Chromatogr. 366 (1986) 205. [41] TANGERMAN,

204 [42] [43] [44] [45] [46] 1471 [48] I491 [SO] [Sl] [52] [53] [54] [SS] [56] [57] IS81 [59] [60] [61] 1621 [63] [64] [65] [66] [67] [68] (691 [70] [71] [72] [73] [74] [75] [76] [77] [78] [79]

[80] [81] [82] [83] [84] [85] 1861

8. Practical application of gas chromatography VINSIANSEN, A,; THRANE,K.E.: Analyst. 103 (1978) 1195. DUMAS,T.; BOND,E. J.: J. Chromatogr. 206 (1981) 384. GREIFER, R.; TAYLOR, J.K.: Nat. Bur. Stands. U.S.A. NBSIR 75-977, 1976. LEPSI,P.: Chem. prum. 27 (1977) 411. SOKOLOV, D. N.: Gas chromatography of volatile complex of metalls. Moscow: Nauka 1981 (in Russian), p. 75. ENSOR,P.C.: J. Assoc. Publ. Anal. 16 (1978) 129. DRUGOV, Yu. S.; MURAVYEVA, G.V.; GRINBERG, K.M.; NESTERENKO, K.M.; SOKOLOV, D.N.: Zavod. labor. 38 (1972) 1305. DORNER,W.G.: Lab. Prax. 5 (1981) 332. BZEZINSKA A,; VANLOON,J.; WILLIAMS, D.; OGUMA,K.; FAWA, K.; HARAGUCHI, I. H.: Spectrochim. Acta B38 (1983) 1339. BALLANTINE, D. S.; ZOLLER,W. H.: Anal. Chem. 56 (1984) 1228. DMITRIEV, M.T.; BRAUDE, A. Yu.; BYKHOVSKII, M. Ya.; EMEL’YANOV, B. V.; PAUTOVA, L. F.; ROTIN, V. A.: Gigiyena i Sanitariya 9 (1984) 55. DE MORA,S. J.; HEWIIT, C. N.; HARRISON, R.M.: Anal. Proc. 21 (1984) 415. HEWIIT,C.N.; HARRISON, R.M.: Anal. chim. acta 167 (1985) 227. NIELSEN,T.; EGSGAARD, H.; LARSEN, E.: Anal. chim. acta 124 (1981) 1. LASOSKI, B.A.: Patent U.S.A. No. 4426452 (1984). JIANG,S.; DE JONGHE, W., ADAMS,F.: Anal. chim. acta 136 (1982) 183. COE,M.; CRUZ,R.; VANLOON,J.C.: Anal. chim. acta 120 (1980) 171. SAKODYNSKII, K. I.: Zhurn. Vsesouzn. chim. obzch. of D. I. Mendeleeva 28 (1983) 34. RUDOLPH, J.; ENHALT, D.H.; KHEDIM,A.; JEBSEN,C.: J. Chromatogr. 217 (1981) 301. STEPHENS, E. R.; HERLICH,0. P.: Environ. Sci. & Technol. 14 (1980) 836. VALENTIN, J. R.; CARLE,J. C.; PHILLIPS,J. B.: Anal. Chem. 57 (1985) 1035. BOUCHERTALL, F.: Mar. Chem. 19 (1986) 153. REINEKE,F. J.; BACHMA”,K.: J. Chromatogr. 323 (1985) 323. BARSKY, J. B.; QUEHEE,S. S.; CLARK,C. S.: Am. Ind. Hyg. Assoc. J. 46 (1985) 9. VAZCHUN, S. S.; VOIITENKO, L. I.; GOLDSHTEJN, Zh. I.; MELIWAN,A. A,; SAJFI,R. I.; YAKOVLEV, S.A.: Zavod. labor. 52 (1987) 5. Esmsrro, G.G.; JACOBS,B. W.: Am. Ind. Hyg. Assoc. J. 38 (1977) 401. BAXTER, H.G.; BLAKEMORE, R.; MOORE,J.P.; COKER,D.T.; MCCAMBLEY, W. H.: Ann. occup. Hyg. 23 (1980) 117. DRUGOV, Yu. S.; MURAV’EVA, G.V.: Zh. analit. Khim. 37 (1982) 1302. DMITRIEV, M.T.; KOLESNIKOV, G.M.: Gigiyena i Sanitariya 2 (1982) 59. DRUGOV,Yu. S.; BEREZKIN, V. G.: Gazokhromatograficheskii analiz zagryaznennogo vozdukha (Gas Chromatographic analysis of contaminated air). Moscow: Khimiya 1981. COCHEO,V.; BELLOMO, M.L.; BOMI,G.G.: Am. Ind. Hyg. Assoc. J. 44 (1983) 521. HUITE,R. S.; WILLIAMS, E. J.; STAEHELIN, J.; HAWIIIORNE, S. B.; BARKLEY, R. M.; SIEVERS, R. E.: J. Chromatogr. 302 (1984) 173. BENDETSKAYA, 1.D.;KRUSHEL’NITSKAYA, L.N.; MAKSIMOVA, V.T.: Zavodskaya Lab. 51 (1985), No. 5, p. 15. NOVITSKII, V. F.; PERTSOVSKII, A. L.: Gigiyena i Sanitariya 6 (1985) 45. BARTLE, K.D.; LEE, M.L.; WISE,S.A.: Chem. SOC.Rev. 10 (1981) 113. LEE, M. L.; NOVOTNY, M.; BARTLE,K. D.: Analytical Chemistry of Polycyclic Aromatic Compounds. New York: Acad. Press 1981. CHOUDHARY, D. R.; BUSH,B.: Anal. Chem. 53 (1981) 1351. STRAY, H.; MANO,S.; MIKALSEN, A.; OEHME,M.: J. High Resolut. Chromatogr. and Chromatogr. Commun. 7 (1984) 74. MILUKAITE, A. R.: Candidate’s Dissertation, Vilnius State University, Vilnius 1979. THRANE,K. E.; MIKALSEN, A,: Atmos. Environ. 15 (1981) 909. JAKLIN, J.; KENMAYR, P.: Intern. J. Environ. Anal. Chem. 21 (1985) 33. SZEPESY, L.: J. Chromatogr. 206 (1981) 611. MILLER,D.A.; SKOGERBOE, K.; GRIMSRUD, E.P.: Anal. Chem. 53 (1981) 464. FUJIWARA, Y.; TOSAKA, S.; FUKAZAWA, S.: Hokkaido Inst. Technol. 1986, No. 14, p. 19. SPITZER,T.; DANNECKER, W.: J. High Resolut. Chromatogr. and Chromatogr. Commun. 57 (1984) 301.

References 1871 [88] [89] [90] [91] [92] [93] [94] 1951 [96] [97] [98] [99] [loo] [loll (1021 [lo31 11041 [lo51 [lo61 (1071 [lo81 [lo91 [llO] [lll] (1121 11131 [114] Ills] [116] [117] [I181 [119] [120] (1211 11221 [123] (1241 [125] [126] I1271 I1281 [129] [130] [131] [132] [133] [134] [135]

205

CAMPBELL, R. M.;LEE,M. L.: Anal. Chem. 56 (1984) 1026. HARTUNG, A,; KRAFT,J.; SCHULZE, J.; KIESS,H.; LIES,K. H.: Chromatographia 19 (1984) 269. PEPICH,B. V.; CALLIUS, Y. B.; BURNS,D. H.; GOUTERMAN, M.: Anal. Chem. 58 (1986) 2825. RAMDAHL,Th.: J. High Resolut. Chromatogr. and Chromatogr. Commun. 8 (1985) 82. TONG,H.Y.; KARASEK, F.W.: Anal. Chem. 56 (1984) 2129. LIBERTI,A,; CICCIOLI,P.; CECINATO, A.; BRANCALEONI, E.; DI PALO,C.: J. High Resolut. Chromatogr. and Chromatogr. Commun. 7 (1984) 389. Yu WING,CHIUKINS;BIEMANN, K.; FINE,D.H.: Anal. Chem. 56 (1984) 1158. TONG,H.Y.; SWEETMAN, J.A.; KARASEK, F.W.; JELLUM EGIL;THORSRUD, A.K.: J. Chromatogr. 312 (1984) 183. KRISHNAN, S.; HITE, R.A.: Anal. Chem. 53 (1981) 342. YERGEY, J.A.; RISBY,T. H.; LESTZ,S. S.: Anal. Chem. 54 (1982) 354. WISE,S. A,; BENNER,B. A.; CHESLER, S.N.; HILPERT,L. R.; VOGAT,C. R.; MAY, W. R.: Anal. Chem. 58 (1986) 3067. GROSJEAN, D.; FUNG,K.; COLLIN,J.; HARRISON, J.; BREITUNG, E.: Anal. Chem. 56 (1984) 569. LIBERTI,A,; CICCIOLI, P.: Chromatographia 9 (1986) 492. BEREZKIN, V.G.; DRUGOV, Yu. S.; GORYACHEV, N. S.: Zh. analit. Khim. 37 (1982) 319. SIDHU,K. S.: Bull. Environ. Contarn. and Toxicol. 28 (1982) 250. PFAFFLI,P.; TOSSAVAINEN, A,; SAVOLAINEN, H.: Analyst. 110 (1985) 377. ANDERSON, B.; ANDERSON, K.; NILSSON, C.A.: J. Chromatogr. 291 (1984) 257. TUCKER, S. P.; DEYE,G. J.: Anal. Lett. A14 (1981) 959. FILIMONOV, V.N.; MILIJEV, Yu.F.; BALIIATINSKAYA, L.N.; ANASHKINA, L.A.; DAVIDOV, V.D.: Zhurn. anal. khirn. 41 (1986) 430. KIMARA, K.; SAWADA, M.; SHONO,T.: J. Chromatogr. 240 (1982) 361. AOYAMA, T.; YASHIRO, T.: J. Chromatogr. 265 (1983) 57. GILLAND, G.C.; JOHNSON, G.T.; MCGEE,W.A.: Anal. Chem. 42 (1981) 630. HALVORSON, D. 0.:Am. Ind. Hyg. Assoc. J. 45 (1984) 727. KUWAMURA, K.; KAPLAN,I.R.: Anal. Chem. 56 (1984) 1616. HARTUNG, J.: Environ. Techno]. Lett. 6 (1985) 21. NISHIKAWA, H.; TAKAHARA, H. Y.; MORI,H.; HAYAKAWA, T.: J. Jap. SOC.Air. Pollut. 19 (1984) 387. JONSSON, A.; BERG,S.: J. Chromatogr. 279 (1984) 307. 5 Int. Symp. Capillary chromatogr. Riva del Garda 1983. PELTONEN, K.; PFLFFLI,P.; ITKONEN,A.: J. Chromatogr. 315 (1984) 412. KENNEDY, E.R.; HILL,R.H.: Anal. Chem. 54 (1982) 1739. KENNEDY, E. R.; O’CONNOR, P. F.; CAGNON, Y.T.: Anal. Chem. 56 (1984) 2120. GUENIER, J.P.; SIMon, P.; DELCOURT, J.; DIDIERJEAN, M.F.; LEFEVRE, C.; MALLER, J.: Chromatographia 18 (1984) 137. AOYAMA, T.; YASHIRO, T.: J. Chromatogr. 265 (1983) 45. NEITZERT, V.; SEILER, W.: Geophys. Res. Lett. 8 (1981) 79. NISHIKAWA, H.; HAYAKAWA, T.; SAKAI,T,: J. Chromatogr. 370 (1986) 327. KULAGINA, V. I.; PAN’KINA, M.A.; FEDOTOV, Yu. I.: Neftepererabotka i Neftekhimiya (Moscow) 11 (1984) 23. GROLE,A.A.: Am. Ind. Hyg. Assoc. J. 39 (1978) 78. DOWER,V.G.: EmahrJNutr. 8 (1984) 395. LEHTONEN, M.: J. Chrornatogr. 202 (1980) 413. KNECHT,U.; NITSCH,H. J.: Z. anal. Chem. 324 (1986) 142. MAGIN,D. F.: J. Chromatogr. 202 (1980) 255. POPLER,A,: PI. lek. 32 (1980) 216. DMITRIEV, M. T.; MISHCHIKHIN, V. A.: Gigiyena i Sanitariya 4 (1982) 65. ESPOSITO, G . G.; WILLIAMS, K.; BONGIOVANNI, R.: Anal. Chem. 56 (1984) 1950. LEFEVRE, C.; FERRARI, P.; DELCOURT, J.; GUENIER, J. P.; MULLER, J.: Chromatographia 21 (1986) 269. PRICE,J. A,; SAUNDERS, K. J.: Analyst. 109 (1984) 829. KASPURA, B.: Pr. Cent. inst. ochr. pr. 34 (1984) 243. NAGASE, M.: Jazaki Techn. Rep. 7 (1982) 93. DMITRIEV, M.T.; KULESH, T.A.; RASTYANNIKOV, E.G.: Gigiyena i Sanitariya 8 (1985) 51. KASHIHIRA, M.; MAKINO, K.; KIRITA,K.; WATANABE, Y.: Jap. Anal. 33 (1984) 402.

206

8. Practical application of gas chromatography

[136] BECHER,G.: J. Chromatogr. 211 (1981) 103. K.; AKIYAMA, E.; YAMASAKI, Y.; YAMASAKI, H.; KUGE,Y.; b s o , Y.: Anal. Chem. 55 [137] KUWATA, (1983) 2199. [138] CONTOUR,J.P.; GUERIN,1.; MOWIER,G.: Environ, Pollut. B1 (1980) 243. K.: Jap. Anal. 29 (1980) 170. [139] KUWATA, T.; OKUNO,T.; SHINTANI, Y.: J. Jap. SOC.Air Pollut. 17 (1982) 58. [140] TSUJ,M.; YAMASAKI, C.: Analyst. 109 (1984) 859. [141] ROSENBERG, G.; SMITH,B. E.; DALENE,M.: J. Chromatogr. 303 (1984) 89. [142] SKARPING, 0.; ELRST,M. W.; HUBER,G. L.: Anal. Chem. 52 (1980) 1755. [143] GRUBNER, W. S.; ROBINSON, J. C.; YOUNG,J. C.: J. Toxicol. and Environ. Health 6 (1980) 351. [144] RICKERT, Y.; MOREE-TESTS, P.; TESTA,A.: Analusis 1 1 (1983) 12. [145] SAINT-JALM, A. H.; LORNE,E.: Anal. Chem. 57 (1985) 1485. [146] LAWRENCE, A. H.; ELIAS,L.; AUTHIER-MARTIN, M.: Can. J. Chem. 62 (1984) 1886. [147] LAWRENCE, B.; ANDERSSON, K.: Anal. Chem. 58 (1986) 1527. [148] ANDERSSON, D.; TINE,D.: Patent U.S.A. No. 4194884 (1980). [149] ROUNBEHLER, M.; WALKER,E.A.: Analusis 8 (1980) 125. [150] CASTEGNARO, A.: COBELLI, L.; PALADINO, R.; PASTORELLO, L.; FRIGERIO,A.; SALA,C.: J. Chromatogr. [151] DE PASCALE, 256 (1983) 352. [152] Esposr~o,G. G.; DOLZINE,T. W.: Anal. Chem. 54 (1982) 1572. L.; SANGO,C.; SKARPING, G.: Am. Ind. Hyg. Assoc. J. 47 (1986) 621. 11531 RE”, F. F.: J. Chromatogr. 284 (1984) 487. [154] ANDRAWES, [155] BAKE,M.Ya.: Gigiena Truda i Profzabolevaniya 2 (1985) 54. J.T.; POSNER,J.C.: Am. Ind. Hyg. Assoc. J. 40 (1979) 923. [156] GAGNON, A. L.; NEMEZKU, A. Y.: Patent U.S.S.R. No. 1154612 (1985), Byull. izobr. No. 17 [157] PERZOVSKU, (198 5 ) . [158] GLASER,R.A.; WOODFIN, W.J.: Am. Ind. Hyg. Assoc. J. 42 (1981) 18. M.: Pr. Cent. inst. ochr. pr. 35 (1985) 3. [159] MIAZEK-KULA, G. (Ed.), American [160] Chemical Hazards in the workplace (ACS Symp. Ser. 149). CHOUDHARY, Chem. SOC.Washington: D. C. 1981, p. 5, 155, 179. B. W.; SEIBER, J.N.: Anal. Chem. 53 (1981) 1077. [161] HERMANN, [162] SIDHU,K. S.: J. Chromatogr. 206 (1981) 381. [163] KELLER,S.: In: 8. Internationales Kolloquium fur die Verhiitung von Arbeitsunfallen und Berufskrankheiten in der chemischen Industrie. (Frankfurt am Main 7.-9.6. 1982) Internationale Vereinigung fur soziale Sicherheit. Heidelberg 1982, S.179. S. M.: Anal. Chem. 52 (1980) 733. [164] KNARR,R.; RAPPAFQRT, [165] STEIN,V. B.; NARENG,R. S.:Anal. Chem. 54 (1982) 991. P.; KIJOWSKI, W.: Anal. Chem. 56 (1984) 1432. [166] STEUDLER, Y.; COMI,E.; MURAYAMA, N.: J. Environ. Pollut. Contr. 20 (1984) 801. [167] HOSHIKA, A.; MAINI,P.: Bull. Environ. Contam. and Toxicol. 22 (1979) 400. [168] COLLINA, M. S.: J. Chromatogr. 258 (1983) 228. [169] LATIF,S.;HAKEN,J. K.; WAINWRIGHT, M.A.; GIRENKO,D.B.: Gigiyena i Sanitariya 9 (1980) 57. [170] KLISENKO, G.: J. Jap. SOC.Air Pollut. 14 (1979) 210. [171] HOSHIKA, [172] WERNER,P.: J. Chromatogr. 300 (1984) 249. A.C.; NOTEN,L.G.: J. High Resolut. Chromatogr. and Chromatogr. Commun. 7 (1984) [173] OOMENS, 280. N.; NARANG,R. S.: Anal. Chem. 53 (1981) 2130. [174] VANTASSEL,S.; AMALFITANO, T.: J. Assoc. Ofic. Anal. Chem. 65 (1982) 913. [175] ,DUMAS, R. D.; HARDY,J. K.: Anal. Chem. 56 (1984) 1621. [176] BLANCHARD, E.A.; ANISIMOVA, I.G.: Gigiena i Sanitariya 2 (1979) 58. [177] KRECHKOVSKII, [178] LUGARI,M.T.: Ateneo parm. Acta natur. 20 (1984) 35. I. S.; KEDDIK,L. M.: Zhurn. anal. chim. 40 (1985) 937. [179] NOVIKOVA, [l80] HILD,J.: Dtsch. Lebensm.-Rdsch. 82 (1986) 116. A.V.; KOVALEVA, N.V.; KOROPALOV, V.M.; FILATOVA, G.N.:Tezisy dokladov na I1 Vaes[181] KISELEV, oyuzn. konferentsii “Khromatograficheskie protsessy i avtomatizatsiya izrnerenii” (The IIndAllUnion Conference on Chromatographic Processes and Automatization of Measurements), Tartu 1979. Moscow: NIITEKhIM 1979, p. 59. T.: Chemistry 36 (1981) 573. [182] TOMINAGA,

References [183] [184] [185] [186] [187] [188] [189] [190] [191] 11921 [193] [194] [195] 11961 [197] [198] [199] [200] [201] [202] [203] [204] [205] [206] [207] [208] [209] [2 101 [21 I] [212] [213] [2 141 [215] [216] [217] [218] [219] [220] [221] [222] [223] [224] [225]

207

KUSTER,W.C.; GOLDAN,P.D.; FEHSENFELD, F.C.: J. Chromatogr. 205 (1981) 271. TYRAS,H.; BLOCKWICZ, A,; STUFKA-OLCZYK, J.: Analyst. 107 (1982) 824. KLISENKO, M.A.: Zh. analit. Khim. 34 (1979) 1382. MILLER,S.: Environ. Sci. & Technol. 17 (1983) A l l . BIDLEMAN, T. F.; CHRISTENSEN, E. J.; BILLINGS,W. N.; Ross, L.: J. Mar. Res. 39 (1981) 443. WILLIAMS, D.T.; LE BELGUY,L.; FURMANCZYK, T.: Chemosphere 9 (1980) 45. PIS’MENNAYA, M. V.; KLISENKO, M. A,: Gigiena i Sanitariya 5 (1977) 57. CHMIL,V. D.: Zh. analit. Khim. 35 (1980) 2049. KOLLAR, V.; KEMKA,R.: Pr. lek. 36 (1984) 321. OEHME,M.; STRAY, H.: Z. anal. Chem. 311 (1982) 665. OEHME,M.; MANE,S.: Z. anal. Chem. 319 (1984) 141. ZELINKSKI, W. L.; MILLER,M. M.; ULMA,G.; WASIK,S.P.: Anal. Chem. 58 (1986) 2692. NESTRICK, T.J.; LAMPARSKI, L.L.; GRUMMETT, W.B.; SHADOFF, L.A.: Anal. Chem. 54 (1982) 823. MITCHUM,K.; KEWMACHER, W.A.; MOLER,G.F.; STALLING, D.L.: Anal. Chem. 54 (1982) 719. RAPPE,C.: Nature 292 (1981) 524. TONG,H.Y.; SHORE,D.L.; KARASEK, F.W.: Anal. Chem. 56 (1984) 244. KORFMACHER, W. A.; RUSHING, L.G.; NESTAARICK, D.M.; THOMPSON, H. C.; MITCHUM, R. K.: Chemosphere 14 (1985) 847. KALLOS,G. J.: Anal. Chem. 53 (1981) 963. CRISP,S.: Ann. occup. Hyg. 23 (1980) 47. LANGHORST, M. L.; MELCHER, R. G.; KALLOS, G. J.: Am. Ind. Hyg. Assoc. J. 42 (1981) 47. VIOLA,P.L.; BEGOW,A,; CAPUTO, A.: Cancer. Res. 31 (1971) 516. LANDE,S . S.: Am. Ind. Hyg. Assoc. J. 40 (1979) 96. NIOSH Manual of Analytical Methods, 2nd Edn. U. S. Dept. of Health, Education and Welfare, Cincinnati, Ohio 1977, p. 178. LA REGINA, J.; BOZZELLI, J.; KEBBEKUS, B.: Abstr. Pap. Pittsburgh Conf. Anal. Chem. and Appl. Spectrosc. Atlantic City, NJ 1980, p. 786. DRUGOV, Yu. S.; MURAV’EVA, G.V.: Zh. analit. Khim. 35 (1980) 1319. GOLDAN, P. D.; FEHSENFELD, F.C.; KUSTER,W. C.; PHILLIPS,M. P.; SIEVERS,R. E.: Anal. Chem. 52 (1980) 1751. ZEPECK,R.: Chem.-Techn. (BRD) 9 (1980) 177. Supelco Inc. Chromatography, Pensylvania, Bulletin 744 B 1976. KROCKENBERGER, D.; LORKOWSKI, H.; ROHRSCHNEIDER, L.: Chromatographia 12 (1979) 787. BURGHARDT, E.; JELTES,R.; VANDE WIELL,N.J.: ORANGE,E.J.: Atmos. Environ. 13 (1979) 1057. DRUGOV,Yu. S.; MURAV’EVA, G. V.; LETUNOVSKAYA, G. A,: Author’s Certificate U.S.S.R. No. 728081. Bull. izobr. No. 14 (1980). DRUGOV, Yu. S.; MURAV’EVA, G. V.; LETUNOVSKAYA, G. A; Proizvodstvo Shin, rezinotekhnicheskikh i asbestotekhnicheskikh Izdelii 9 (1978) 25. DRUGOV, Yu.S.; BEREZKIN, V.G.: Usp. Khimii 55 (1986) 999. PURCELL, J. E.; GIORDANO, D.O.: Perkin-Elmer Inc. Gas Chromatography Applications. No. GCD43, 1975. DRUGOV, Yu. S.: Tezisy dokladov V Vsesoyuzn. konf. PO analitecheskoi khimii organicheskikh soedinenii (The Vih All-Union Conference on Analytical Chemistry of Organic Substances. Abstracts of Papers). Moscow 1984. Moscow: Nauka 1984, p.291. DMITRIIEV, M.T.; MIZCHIHIN, V.A.: Gigiena i sanitarija 4 (1981) 46. MOONS,S.; BOYD,D.W.; LICHTMAN, A.; PORTER,R.A.: Amer. Lab. 17 (1985) 98. MOONS,S . ; BOYD,D.W.; LICHTMAN, A.; PORTER,R.A.: Int. Lab. 15 (1985) 56. ZABAIROVA, R. A,; BORTNIKOV, G. N.; GORINA, F.A,; SAMARIN, K. M.; YASHIN,Ya. I.: Zh. analit. Khim. 35 (1980) 187. KALMIKOVA, T. A,; LAZARIS, A. Y.; KONSTANTINOVA, E. I.: Zhurn. anal. Khim. 40 (1985) 915. BEREZKIN, V.G.; DRUGOV, Yu.S.: Zavod. labor. 52 (1986) 6. KALMIKOVA, T.A.; LAZARIS, A.Y.: Zhurn. anal. chim. 41 (1986) 2080. FOERST,D.: Am. Ind. Hyg. Assoc. J. 40 (1979) 888.

Conclusion

Nowadays gas chromatographic methods are widely used to detect air pollutants and the field of their practical application is constantly widening for a number of objective reasons. It should particularly noted that most pollutants are volatile substances and consequently gas chromatography is an effective and relatively simple method for their identification. There are numerous examples in which gas chromatography has been applied to solve diverse tasks in environmental control, confirming the promising prospects of gas chromatography as an analytical method for identifying air pollutants. In conclusion, we can summerize the most promising directions in the development of gas chromatography for environmental applications. 1. In spite of recent advances in sampling techniques, they still remain one of the least developed stages in analytical methods for environmental control. It is expedient to continue physico-chemical research on conventional sampling techniques and to develop essentially new procedures in such sampling methods as “on-line mode” and “off-line mode”. Another important direction is the elaboration of automatic and remote control systems for environmental analysis. 2. The development of new and the improvement of existing methods for the direct quantitative analysis of the analytical system as a whole. In solving this problem, one of the important aspects will be to develop techniques for producing mixtures of pollutants of the required concentrations (in the form of both vapours and aerosols) and methods for controlling the analytical instruments, and not only their chromatographic and detection components. 3. The development of high-resolution chromatography with both open-capillary and capillary micro-packed columns is of considerable interest. These columns are characterized by higher capacities and specific efficiencies than classical open columns. 4. Higher selectivity and sensitivity of detectors greatly simplify the detection and identification of impurities. Recent work in this area has demonstrated that even the main characteristics of well studied classical detectors such as the electron-capture detector can be improved, for example by introducing an appropriate sensitizer into the carrier gas. 5. Chemical methods are characterized by an extremely wide range of selectivity and can be used effectively in all stages of identification from sampling to detection. Their further development in the field of environmental control is positively expedient. The development of any of the above gas chromatographic methods for air pollution identification is a task of extreme complexity. Doubtless it will require the combined efforts of experts with different specializations: chromatographers, working in the field of experimental planning; analysts, in the field of sampling techniques; physical chemists, in the field of detection instruments; and engineers, in the field of computerization and automation of analytical measurements; and many others. In the course of the development of any important applied discipline (including chromatography), it may be possible to single out several crucial phases. This may hold true both for the development of the discipline as a whole and for its subdivisions. In this connection we one of the would like to draw attention to a metaphorical statement by K. E. TSIOLKOVSKY, founders of space flights: “First inevitably follow thought, fantasy, fairy-tale. Then comes scientific calculation, and towards the end implementation crowns the thought”.

Subject Index

A Acetic acid 187 Acetic anhydride 190 Acids 187 Acrolein 188 Acrylonitrile 192, 193 Adsorbents for sampling 47 - activated coal 48 - aluminium oxide 57 - carbon adsorbents 48 - chemisorbents 62 - gas-liquid chromatography sorbents . .. 55 - silica gel 57 Alcohols 187 Aldehydes 187 - detection 28, 30, 32 - determination 139, 187 - identification 139 - reaction gas chromatography 187 Aliphatic alcohols 187 Aliphatic amines 190 Aliphatic hydrocarbons 178 - detection 28, 32 - determination 126, 135, 178 - identification 135 - recovery from mixture 135 Alkyl benzenes 179 Alkyl selenides 178 Amides 190 Amines 190, 193 - detection 29, 32 - determination 144, 145, 190 - identification 144, 145 Amino alcohols 193 Ammonia 172 - detection 29, 31, 169 - determination 172 Amphetamines 191 Anaesthetic gas (halothane) 196 Aromatic hydrocarbons 179 - detection 28, 32 - determination 135, 179 - identification 135 - recovery from mixture 135

B Benzene 179 Benzoic acid 187

Beryllium aerosol 176 Bis(chloromethy1)ether 198 Breakthrough of pollutants 81 - Freundlich equation 81 Bromacetates 196 Bromacetone 196 Butane 178 Butyric acid 187

C Calculation of pollutants concentrations 161 Calibration of detectors 161 Carbon disulphide 172 Carbonyl sulphide 172 Carboxylic acids 187 Chloracetophenone 196 Chlorine 123, 168 Chlorine-containing pesticides 197 Chlorine dioxide 123 Chlorobenzene 196 o-Chlorobenzylidenemalonic acid 196 Coadsorption of pollutants 127 - diminution of coadsorption 130 Cocaine 191 Concentration of pollutants 35, 119 - cryogenic 40 - in solvent 36 - on solid adsorbents 46, 119 - reactive-sorption concentration 121 Concentration tubes 67 - preparation 67 - with solid sorbents 67 Cresols 187

D Desorption of pollutants 83 - choice of solvent 98 - desorption methods 102 - effectivity recovery of pollutants 91 - - influence of coadsorption 98 - - influence of humidity 96 - - influence of temperature 96 - solvent extraction 83 - solvent mixture 100 - thermal desorption 86 - two-phase desorption systems 100 Detectors 25, 166 - aerosol 167

2 10

-

atomic-absorption spectrometer 177 - calibration 161 - characteristics 25 - coulometric 167 - detection limit 26, 162 - electron-capture 30, 166 - flame ionization 28, 168 - flame photometric 30, 166 - GUFTIR system 31 - G U M S system 31 - hall electrolytic 3 1 - helium ionization 167 - photoionization 29, 168 - plasma ionization 184 - sensitivity 26 - thermal energy analyser 31 - thermoionic 28, 168 Dibenzo-p-dioxins 198 Dibenzofurans 198 Dibromodiethylene 196 Dicyan 172 Dimethylmercury 177 Dimethyl sulphate 194 Dinitrile 196 Dinitrotoluene 193 Dioxins 197

Subject Index

H Heroin 191 1,6-Hexamethylene diisocyanate 192 Hydrogen bromide 169 Hydrogen chloride 169 Hydrogen cyanide 172 Hydrogen fluoride 168 Hydrogen sulphide 172

I Identification of pollutants 31, 135 - GC/FTIR 31 - G U M S 31 - reactive-sorption concentration 135 Improvement of chromatographic separation 133 Indoles 195 Isocyanates 191 Isocyanic acid 191 Isothiocyanates 194

K Ketones 137-139, 187 - determination 137, 187 - identification 137-139 - recovery from mixtures 137-139

E

L

Epichlorohydrin 196 Esters 190 Ethanol 187 Ethers 190 Ethylene glycol 187 Ethylene oxide 190 Ethyl formate 190 2-Ethylhexanol 187

Lachrimators 196 Lead alkyl derivatives 177

F Fatty acids 187 Formaldehyde 187 Freons 196 Furfuryl alcohols 186 Furilic acid metabolite 187

G Gas chromatography - general consideration 10 - peculiarities analysis of impurities 20 - relative retention value 16 - theory 10 Gasoline-methanol fuel 179 Gasolines 179

M Mercaptans 172 Methane 178 Methanol 186 Methods of concentration of air pollutants - absorption 37 - adsorption 46 - chemisorption 62 - cryogenic 40 - reactive-sorption 121 Methods of determination of air pollutants 165 - aggressive gases 123 - amines and nitro compounds 144-146, 190 - ammonia 172 - aromatic hydrocarbons 179 - bis(chloromethy1)ether 198 - carbon oxides 133, 134 - chlorine-containing pesticides 197 - dioxine 198 - freons 196 - halogenated hydrocarbons 139-143, 195 - hydrogen cyanide 172 - inorganic halides 123, 167 - inorganic sulphur compounds 143, 145, 172

211

Subject Index

-

low-boiling hydrocarbons 126, 178 metallo-organic compounds 123, 176, 178 metals 176 metrological aspects 108 nitrogen oxides 124, 145, 170 organic oxy compounds 137, 186 organic sulphur compounds 143, 194 ozone 176 phosphine 174 polyaromatic compounds 180 polychlorobiphenyls 198 sulphur fluorides 125 vinyl chloride 199 Methyl chloride 196 Methylcyclopentadienylmanganese tricarbonyl 178 Methylmercury chloride 177 Methyl tert.-butyl ether 190

- permeation tubes 153 - static methods 150 Propane 178 Propionic acid 187

R Recovery of pollutants 35 - completness of recovery 91 - from air 35, 37, 39, 48, 5 5 , 57-62, 65 - from adsorbents 81, 86 - increase in desorption efficiency 100

S

Organomercury compounds 177 Ozone 176

Sampling - absorption 37 - adsorption 46 - air flow-rate 78 - air humidity 80 - choise of method 103 - coadsorption 80 - cryogenic concentration 40 - efficiency of sorption 70 - into containers 35 - passive 59 - properties of the pollutants 71 - properties of the sorbents 72 - temperature 80 Sorption and coadsorption 127 Styrene 180 Sulphides 172 Sulphur dioxide 172 Sulphur hexafluoride 125, 170 Sulphur tetrafluoride 125, 170 Sulphur fluoride 125, 169

P

T

Peroxyacetyl nitrate 186 Pesticides 194 Phenols 189 Phosgen 169 Phosphine 174 Phosphites 190 Phosphorus containing pesticides 194 Phthalates 190 Phthalic anhydride 190 Polychlorobiphenyls 197 Polycyclic - azaarenes 184 - compounds 180, 184 - hydrocarbons 180 - nitrogen derivatives 184 Preparation of standard mixture 150 - aerosol mixtures 159 - chemical reactions 157 - dynamic methods 151

Thionyl fluoride 168 Thiophenes 172 2,4-Toluene diisocyanate 192 Trapping of contaminants - gas and liquid 46 - multilayer sorption traps 59 - solid particles and aerosols 65 Trichloracetate sodium 196 Trimethylamine 190

N Nickel tetracarbonyl 178 Nicotine 190, 191 N-methylmorpholine 191 Nitroethane 193 Nitrogen dioxide 124 Nitrogen oxides 123, 124, 172 Nitromethane 193 2-Nitropropane 193 N-nitrosodimethylamines 190 Nitric acid 124

0

V Vinyl chloride 139, 199 - concentration 199, 200 - detection 199 - identification 139

X Xylenes 179

This Page Intentionally Left Blank