Determination of Trace Elements

Determination of Trace Elements Edited by Zeev B. Alfassi

Balaban Publishers

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VCH

Weinheim . New York Base1 Cambridge Tokyo

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Determination of Trace Elements Edited by Z. B. Alfassi

Balaban Publishers

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0 VCH Verlagsgesellschafi mbH, D-69451 Weinheim (Federal Republic of Germany), 1994 ~~

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Distribution: VCH, P.O. Box 101161, D-69451 Weinheim, Federal Republik of Germany Switzerland: VCH PO.Box, CH-4020 Basel, Switzerland United Kingdom and Ireland: VCH 8 Wellington Court, Cambridge CB1 lHZ, United Kingdom USA and Canada: VCH, 220 East 23rd Street, New York, NY 10010-4606, (USA) Japan: VCH, Eikow Building, 10-9 Hongo 1-chorne, Bunkyo-ku, Tokyo 113, Japan ISBN 3-527-28424-9

Prof. Zeev B. Alfassi Department of Nuclear Engineering Ben Gurion University Beer Sheva 84 102 Israel

This book was carefully produced. Nevertheless, authors, editors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate.

Published jointly by VCH Verlagsgesellschaft, Weinheim (Federal Republic of Germany) VCH Publishers, New York, NY (USA)

Editorial Director: Miriam Balaban Production Manager: Dip].-Wirt.-Ing. (FH) Bernd Riedel

Library of Congress Card No: applied for A catalogue record for this book is available from the British Library.

Die Deutsche Bibliothek - CIP-Einheitsaufnahme

Determination of trace elements / ed. by Zeev B. Alfassi. Rehovot (Israel): Balaban Publ.; Weinheim; New York; Basel; Cambridge; Tokyo: VCH, 1994 ISBN 3-527-28424-9 NE: Alfassi, Zeev B. [Hrsg.]

0 VCH Verlagsgesellschaft mbH, D-69451 Weinheim (Federal Republic of Germany), 1994 Printed on acid-free and low chlorine paper.

All rights reserved (including those of translation in other languages). No part of this book may be reproduced in anyform - by photoprinting, microfilm, or any other means - nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printing: stmuss offsetdruck gmbh, D-69509 Morlenbach Bookbinding: IVB Heppenheim GmbH, D-64646 Heppenheim Printed in the Federal Republic of Germany

Preface Elements in low concentration, "trace elements" are very important in various fields of science and technology. In medicine the concentration of several trace elements is very crucial since some elements are essential in low concentration while a little higher concentration is hazardous. Consequently, trace element determination is essential for food analysis, water analysis, criminology, etc. In the semiconductor industry, very minute amounts of impurities can drasticall change the electrical properties of the devices. In order to understand the effect of trace elements in biology or to obtain purer materials for the semiconductor industry, it is important to be able to measure accurately and precisely the concentiation of them. Various analytical methods are sensitive enough to measure these low concentrations; however, each method exhibits its highest sensitivity for different particular elements and has its own set of interferences. Consequently, it is important to know the most important methods in order to decide which one is the most appropriate for a special problem. It is the feeling of the editor that most analytical chemists are mainly involved in only a few techniques from the large number available nowadays. Consequently, it is almost impossible for one scientist to write on all the methods, as each method should be written by an expert. The book opens with four chapters dealing with general topics concerning all methods: systematic errors, quality control, sampling and preconcentration. The following seven chapters describe the main methods for trace element analysis. The last two chapters describe the application of the various methods in two areas; one dealing with speciation and the other with biological samples. Beer Sheva, July 1994

Zeev B. Alfassi

Errata Determination of Trace Elements edited by Z. B. Alfassi By mistake, the following texts have not been included:

Dedication To Sabina with love Many adventures with the new ICP

Contents (p. VII-XIV) 1 Systematic errors in trace analysis by G. Tolg and P. Tschopel . . . . . . . . . . . . . . . . . . 2 Limits of detection and accuracy in trace elements analysis by C. J. Kirchmer . . . . . . . . . . . . . . . . . . . . . . 3 Sampling and sample preparation by J. R. W. Woittiez and J. E. Sloof . . . . . . . . . . . . . . 4 Separation and preconcentration of trace elements by K. Terada . . . . . . . . . . . . . . . . . . . . . . . . 5 Determination of trace elements by atomic absorption spectrometry by 1. Z. Pelly . . . . . . . . . . . . . . . . . . . . . . . . 6 Plasma optical emission and mass spectrometry by J. A. C. Broekaert . . . . . . . . . . . . . . . . . . . . 7 Instrumental neutron activation analysis (INAA) by Z. B. Alfassi . . . . . . . . . . . . . . . . . . . . . . . 8 Radiochemical neutron activation analysis by Z. B. Alfassi . . . . . . . . . . . . . . . . . . . . . . . 9 Determination of trace elements by electron spectroscopic methods byM. Polak . . . . . . . . . . . . . . . . . . . . . . . . 10 l h c e element determination by electrochemical methods by R. von Wandruszka . . . . . . . . . . . . . . . . . . . . 11 Determination of trace elements by chromatographic methods employing atomic plasma emission spectroscopic detection by P. C. Uden . . . . . . . . . . . . . . . . . . . . . . . 12 Speciation of trace elements with special reference to the use of radioanalytical methods by H. A. Das . . . . . . . . . . . . . . . . . . . . . . . . 13 Trace elements in environmental and health sciences by G. V. Iyengar and V. Iyengar . . . . . . . . . . . . . . . .

1 39 59 109 145 191 253 309 359 393

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543

1.1

1.2

1.3

1.4 1.5 2.1 2.2 2.3 2.4 2.5

2.6 2.7 2.8 3.1 3.2

3.3

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 General aspects of extreme trace analysis . . . . . . . . . . 1.1.2 Direct instrumental determination methods . . . . . . . . . 1.1.3 Multi-stage procedures . . . . . . . . . . . . . . . . . . . 1.1.4 Further general important statements . . . . . . . . . . . . Systematic errors and their avoidance . . . . . . . . . . . . . . . . . 1.2.1 Volatilization . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Blanks from vessels, vessel materials and working tools . . 1.2.4 Blanks from the reagents . . . . . . . . . . . . . . . . . . 1.2.5 Blanks from airborne dust . . . . . . . . . . . . . . . . . 1.2.6 Contamination by sample handling . . . . . . . . . . . . . 1.2.7 Problems due to changes of the valency state . . . . . . . . Systeinatic errors during the analytical procedure . . . . . . . . . . . 1.3.1 Sampling, sample storage and pretreatment . . . . . . . . . 1.3.2 Decomposition . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Separation . . . . . . . . . . . . . . . . . . . . . . . . . Basic rules for the recognition and elimination of systematic ei-rors . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Errors in analytical results . . . . . . . . . . . . . . . . . . . . . . Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measuring trace concentrations . . . . . . . . . . . . . . . . . . . . The problem of detection . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Random error of blank responses . . . . . . . . . . . . . . 2.5.2 Errors of the first kind- the critical level (aposteriol-i detection) 2.5.3 Errors of the second kind - the limit of detection (a priori detection) . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5.4 Limits to the use of the definitions of L, and L, . . . . . . Regression theory approaches to the problem of detection . 2.5.5 Practical applications . . . . . . . . . . . . . . . . . . . . . . . . . Reporting results at small concentrations . . . . . . . . . . . . . . . Conclusions and recommendations . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in trace element composition . . . . . . . . . . . . . . . . 3.2.1 Element specific changes . . . . . . . . . . . . . . . . . . 3.2.2 Sample specific changes . . . . . . . . . . . . . . . . . . Pre-sampling considerations . . . . . . . . . . . . . . . . . . . . . VII

2 2 3 3 4 4 6 7 9 13 15 18 19 20 20 22 27 29 31 39 40 41 42 42 42 43

46 48 50 52 53 56 59 62 63 76 77

3.4

3.5 4.1

4.2

4.3

4.4

4.5

5.1 5.2 5.3 5.4 5.5 5.6

Aspects of sampling . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Establishment of analytical control . . . . . . . . . . . . . 3.4.2 Sampling error in a test portion . . . . . . . . . . . . . . . Uniformity of laboratory samples . . . . . . . . . . . . . . 3.4.3 3.4.4 Uniformity of subsamples . . . . . . . . . . . . . . . . . 3.4.5 The gross sample . . . . . . . . . . . . . . . . . . . . . . Sample decomposition . . . . . . . . . . . . . . . . . . . . . . . . Separation and preconcentration of trace elements by coprecipitation 4.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.3 Coprecipitation with inorganic precipitants . . . . . . . . . 4.1.4 Coprecipitation with organic collectors . . . . . . . . . . . Separation and preconcentration of trace elements by flotation . . . . 4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 General procedures . . . . . . . . . . . . . . . . . . . . . Preconcentrauon and separation of trace elements by solvent extraction 4.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2 Extraction of trace elements . . . . . . . . . . . . . . . . Separation and preconcentration of trace elements by ion-exchange . 4.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.4.2 Ion-exchange resins . . . . . . . . . . . . . . . . . . . . . 4.4.3 Equilibrium and selectivity . . . . . . . . . . . . . . . . . 4.4.4 Practical column operation . . . . . . . . . . . . . . . . . 4.4.5 Preconcentration . . . . . . . . . . . . . . . . . . . . . . 4.4.6 Ion chromatography . . . . . . . . . . . . . . . . . . . . Separation and preconcentration by sorption . . . . . . . . . . . . . 4.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.5.2 Activated carbon . . . . . . . . . . . . . . . . . . . . . . 4.5.3 Porous polymers . . . . . . . . . . . . . . . . . . . . . . 4.5.4 Complex-forming adsorbents . . . . . . . . . . . . . . . . 4.5.5 Natural polymers . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality of results . . . . . . . . . . . . . . . . . . . . . . . . . . . Standards and chemicals . . . . . . . . . . . . . . . . . . . . . . . Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII

83 89 89 92 93 93 94 110 110 110 112 113 114 114 116 117 118 118 121 128 128 129 130 132 133 134 137 137 137 138 140 141 146 147 148 150 152 154

5.7 5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17 6.1

6.2

6.3

6.4

6.5

7.1 7.2

Major components of the instrument . . . . . . . . . . . . . . . . . 155 Radiation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 Wavelength selection system . . . . . . . . . . . . . . . . . . . . . 159 Atomization by flame . . . . . . . . . . . . . . . . . . . . . . . . . 164 Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Electrothermal atomization . . . . . . . . . . . . . . . . . . . . . . 173 Hydride generation . . . . . . . . . . . . . . . . . . . . . . . . . . 178 Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180 Instrumental background corrections . . . . . . . . . . . . . . . . . 183 Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 188 Atomic spectrometry with plasma sources . . . . . . . . . . . . . . 192 6.1.1 Historical development . . . . . . . . . . . . . . . . . . . 192 6.1.2 Optical emission spectrometry . . . . . . . . . . . . . . . 194 6.1.3 Plasma mass spectrometry . . . . . . . . . . . . . . . . . 199 Plasma sources and sampling . . . . . . . . . . . . . . . . . . . . . 201 6.2.1 Arc and spark sources . . . . . . . . . . . . . . . . . . . 202 6.2.2 Flames . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 6.2.3 DC plasma jets . . . . . . . . . . . . . . . . . . . . . . . 204 6.2.4 Inductively coupled plasmas . . . . . . . . . . . . . . . . 205 6.2.5 Microwave discharges . . . . . . . . . . . . . . . . . . . 207 6.2.6 Sample introduction for plasma spectrometry . . . . . . . 209 6.2.7 Discharges under reduced pressure . . . . . . . . . . . . . 215 Plasma optical emission spectrometry . . . . . . . . . . . . . . . . . 217 6.3.1 Atomic emission spectrometry . . . . . . . . . . . . . . . 217 6.3.2 ICP-Atomic emission spectrometry . . . . . . . . . . . . . 220 6.3.3 MIP-Atomic emission spectrometry . . . . . . . . . . . . 223 6.3.4 Glow discharges . . . . . . . . . . . . . . . . . . . . . . 224 6.3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 226 Plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . 226 6.4.1 ICP mass spectrometry . . . . . . . . . . . . . . . . . . . 227 6.4.2 Glow discharge mass spectrometry . . . . . . . . . . . . . 239 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 6.5.1 Power of detection . . . . . . . . . . . . . . . . . . . . . 241 6.5.2 Interferences . . . . . . . . . . . . . . . . . . . . . . . . 242 6.5.3 Economic aspects . . . . . . . . . . . . . . . . . . . . . . 243 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 Basic nuclear physics . . . . . . . . . . . . . . . . . . . . . . . . . 255 IX

7.2.1 Nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . 255 256 7.2.2 Radioactive decay . . . . . . . . . . . . . . . . . . . . . 7.2.3 Kinetics of decay of radioactive nuclides . . . . . . . . . . 260 7.2.4 Kinetics of formation of radioactive nuclides by irradiation 261 7.2.5 The chart of the nuclides . . . . . . . . . . . . . . . . . . 265 267 Gamma detection systems . . . . . . . . . . . . . . . . . . . . . . . 7.3 7.3.1 NaI(T1) - scintillation detector . . . . . . . . . . . . . . . 268 7.3.2 Solid-state ionization detector . . . . . . . . . . . . . . . 268 7.3.3 The shape.of y spectrum . . . . . . . . . . . . . . . . . . 273 278 7.4 Irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 7.4.1 Neutron sources . . . . . . . . . . . . . . . . . . . . . . 279 7.4.2 Samples introduction . . . . . . . . . . . . . . . . . . . . 7.5 Instrumental neutron activation analysis (INAA) . . . . . . . . . . . 280 7.5.1 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 280 7.5.2 Calculation . . . . . . . . . . . . . . . . . . . . . . . . . 280 284 7.5.3 Nuclear interferences . . . . . . . . . . . . . . . . . . . . A test case INAA of trace elements in silicon . . . . . . . 285 7.5.4 288 7.5.5 EpithermalINNA . . . . . . . . . . . . . . . . . . . . . . 296 7.5.6 Fast neutrons INAA . . . . . . . . . . . . . . . . . . . . 7.5.7 CyclicINAA . . . . . . . . . . . . . . . . . . . . . . . . 299 7.5.8 Prompt Gamma Neutron Activation Analysis (PGNAA) . . 301 7.5.9 Depth profiling by INAA . . . . . . . . . . . . . . . . . . 302 309 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Samples dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . 312 8.2 8.2.1 Geological samples . . . . . . . . . . . . . . . . . . . . . 312 312 8.2.2 Metalsamples . . . . . . . . . . . . . . . . . . . . . . . 8.2.3 Biological samples . . . . . . . . . . . . . . . . . . . . . 313 . . . . . . . . . . . . . . . . . . . . . . 316 Radiochemical separations 8.3 8.3.1 Radiochemicalseparationsinmaterialsciences . . . . . . 317 8.3.2 RNAA of geological and environmental samples . . . . . . 326 RNAA of biological samples . . . . . . . . . . . . . . . . 340 8.3.3 359 9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2 General review of X-ray photoelectron and auger electron spectroscopies362 363 9.2.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . . 9.2.2 Surface sensitivity and depth profiling . . . . . . . . . . . 369 9.2.3 Chemical-state information . . . . . . . . . . . . . . . . . 374 9.2.4 Quantitative analysis . . . . . . . . . . . . . . . . . . . . 375 9.3 XPSjAES applicationsintraceelementdetermination . . . . . . . . 384 X

9.4 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Anodic and cathodic stripping voltammetry . . . . . . . . . . . . . . 10.2.1 Electrodes . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Stripping waveforms . . . . . . . . . . . . . . . . . . . . 10.2.3 Film stripping . . . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Cathodic stripping . . . . . . . . . . . . . . . . . . . . . 10.2.5 Interferences . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Non-stripping methods . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Potentiometric stripping . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Adsorptive stripping voltammetry . . . . . . . . . . . . . . . . . . . 10.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Analytical information from interfaced chromatography with specific element detection . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Interelement selectivity . . . . . . . . . . . . . . . . . . . 11.2.2 Elemental sensitivity and limits of detection . . . . . . . . 11.2.3 Dynamic measurement range . . . . . . . . . . . . . . . . 11.3 Element-selective gas chromatographic detection . . . . . . . . . . . 11.3.1 Non-spectroscopicdetectors . . . . . . . . . . . . . . . . 11.3.2 The Flame Photometric Detector (FPD) . . . . . . . . . . 11.3.3 Atomic spectroscopic detectors . . . . . . . . . . . . . . . 11.3.4 Flame emission detection . . . . . . . . . . . . . . . . . . 11.3.5 Atomic absorption detection . . . . . . . . . . . . . . . . 11.3.6 Atomic plasma emission spectroscopy (APES) . . . . . . . 11.4 Classes of atomic plasma emission chromatographicdetectors . . . . 11.4.1 The microwave-introduced electrical discharge plasma (MIP) detector . . . . . . . . . . . . . . . . . . . . . . . 11.4.2 The Inductively Coupled Plasma (ICP) discharge . . . . . 11.4.3 The Direct-Current Plasma (DCP) discharge . . . . . . . . 11.4.4 The Alternating-Current Plasma (ACP) discharge . . . . . 11.4.5 The Capacitively Coupled Plasma (CCP) discharge . . . . 11.4.6 The plasma electrodeless discharge afterglow . . . . . . . 11.5 The plasma-chromatographinterface . . . . . . . . . . . . . . . . . 11.5.1 Gas chromatographs . . . . . . . . . . . . . . . . . . . . Analytical GC applications . . . . . . . . . . . . . . . . . . . . . . 11.6 11.6.1 GC-AED detection of non-metallic elements . . . . . . . . 11.6.2 GC-MIP detection . . . . . . . . . . . . . . . . . . . . .

389 393 394 395 397 399 400 400 405 405 408 416 426 427 428 428 428 429 429 431 431 432 432 433 434 434 435 435 436 436 436 436 436 438 438 444

11.6.3 GC-DCP and ICP detection . . . . . . . . . . . . . . . . . 11.7 Liquid chromatographic applications . . . . . . . . . . . . . . . . . 11.7.1 HPLC-ICP detection . . . . . . . . . . . . . . . . . . . . 11.7.2 HPLC-DCP detection . . . . . . . . . . . . . . . . . . . . 11.7.3 HPLC-MIP detection . . . . . . . . . . . . . . . . . . . . 11.8 Supercritical fluid chromatographic (SFC) applications . . . . . . . . 11.9 Chromatographic detection by plasma-mass spectrometry . . . . . . 11.9.1 HPLC-plasma mass spectrometry (MS) . . . . . . . . . . 11.9.2 GC-plasma mass spectrometry . . . . . . . . . . . . . . . 11.10 Future directions for trace analysis . . . . . . . . . . . . . . . . . . 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.2 Definition . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.3 Inventarisation . . . . . . . . . . . . . . . . . . . . . . . 12.1.4 Organization of this chapter . . . . . . . . . . . . . . . . 12.2 Principles of radiotracer methods . . . . . . . . . . . . . . . . . . . 12.2.1 Basic equations of radiotracer experiments in a closed system and their applications . . . . . . . . . . . . . . . . . . . . Isotopic exchange in one phase . . . . . . . . . . . . . . . 12.2.2 Isotopic exchange between a solid and an aqueous solution 12.2.3 in a closed system . . . . . . . . . . . . . . . . . . . . . 12.2.4 Net mass transport in a closed system . . . . . . . . . . . 12.2.5 Combination of net mass transport and isotopic exchange in a closed system . . . . . . . . . . . . . . . . . . . . . . . 12.2.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 12.3 Spatial (surface) speciation by nuclear techniques . . . . . . . . . . 12.3.1 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.3.2 Ion-beam applications . . . . . . . . . . . . . . . . . . . 12.3.3 Thermal neutron depth profiling . . . . . . . . . . . . . . 12.3.4 Depth profiling by activation analysis . . . . . . . . . . . 12.3.5 Proton induced X-ray analysis (PIXE) and proton induced ?-ray spectrometry (PIGE) . . . . . . . . . . . . . . . . . 12.3.6 Depth profiling by radiotracer methods . . . . . . . . . . . 12.4 Phase speciation and the use of radioanalysis . . . . . . . . . . . . . 12.4.1 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.4.2 Sampling and separation of sea- and surface water and the determination of trace elements in the isolated fractions . . 12.4.3 Determination of exchangeable phosphate in fresh water sediments . . . . . . . . . . . . . . . . . . . . . . . . . . XI1

448 448 448 450 451 452 453 453 454 455 462 462 463 463 471 471 471 474 474 476 477 478 479 479 479 485 488 489 490 493 493 495 499

12.4.4

12.5

13.1 13.2

13.3

13.4

13.5

13.6

Leaching experiments on granular solids by means of radiotracers and a previously radioactivated aliquot . . . . . . . 499 12.4.5 Measurement of the in situ diffusion coefficient and distribution constant in (partly) wetted soils and granular wastes . . 501 Chemical speciation of trace elements . . . . . . . . . . . . . . . . 504 12.5.1 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 504 12.5.2 Radiometric determination of conditional extraction constants 510 12.5.3 Trace element speciation in human serum . . . . . . . . . 513 12.5.4 A case in point: Arsenic speciation in aqueous samples by selective As(III)/As(V) preconcentration and hydride evap518 oration AAS . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 Need for trace element analysis of biomaterials . . . . . . . . . . . . 545 13.2.1 BTER. a multi-disciplinaryscience . . . . . . . . . . . . . 546 13.2.2 Trace element speciation and bioavailability . . . . . . . . 548 Biological standardization . . . . . . . . . . . . . . . . . . . . . . 549 13.3.1 The “Bio” sources of analytical errors . . . . . . . . . . . 549 13.3.2 Presampling factors . . . . . . . . . . . . . . . . . . . . . 550 Analytical standardization . . . . . . . . . . . . . . . . . . . . . . 556 13.4.1 Analytical quality assurance . . . . . . . . . . . . . . . . 557 13.4.2 Harmonization of measurements . . . . . . . . . . . . . . 559 13.4.3 Trace element determinations . . . . . . . . . . . . . . . . 559 13.4.4 Multianalyte determinations . . . . . . . . . . . . . . . . 559 13.4.5 Matrix related problems in sample treatment . . . . . . . . 560 13.4.6 Sample preservation and storage . . . . . . . . . . . . . . 560 13.4.7 Contamination by trace elements . . . . . . . . . . . . . . 562 13.4.8 Losses of trace elements . . . . . . . . . . . . . . . . . . 562 Clinical specimens from human subjects . . . . . . . . . . . . . . . 563 13.5.1 Special features of biofluids . . . . . . . . . . . . . . . . 563 13.5.2 Medico-legal implications . . . . . . . . . . . . . . . . . 564 13.5.3 Sampling and preparation . . . . . . . . . . . . . . . . . . 565 Environmental biomonitoring for toxicants . . . . . . . . . . . . . . 567 13.6.1 Chemicals in the environment . . . . . . . . . . . . . . . 567 13.6.2 Bioenvironmental surveillance . . . . . . . . . . . . . . . 570 13.6.3 Real time and long-term biomonitoring . . . . . . . . . . . 571 13.6.4 Human specimens for biomonitoring . . . . . . . . . . . . 571 13.6.5 Environmental Specimen Bank (ESB) . . . . . . . . . . . 572 13.6.6 Proven applications of ESB . . . . . . . . . . . . . . . . . 574 XI11

13.7 Biomineral imbalances and health effects . . . . . . . . . . . . . . . 13.7.1 Nutritional and metabolic factors . . . . . . . . . . . . . . 13.7.2 Nutritional surveillance of trace elements . . . . . . . . . . 13.7.3 Recommended dietary allowances (RDA) . . . . . . . . . 13.8 Trace elements and high altitude populations . . . . . . . . . . . . . 13.8.1 Iodine and selenium . . . . . . . . . . . . . . . . . . . . 13.9 Referencevaluesfor traceelement s i n humanspecimens . . . . . . . 13.9.1 Reference values vs normal values . . . . . . . . . . . . . 13.9.2 Referenceconcentrationsinclinicalspecimens . . . . . . . 13.9.3 Trace element content in Reference Man . . . . . . . . . . 13.10 Reference parameters for data interpretation . . . . . . . . . . . . . 13.1 1 The future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

575 575 576 576 577 579 580 580 580 581 586 586

CHAPTER 1

Systematic errors in trace analysis

G.ToLG'i2 and P.TSCHOPEL~

'I~istiturfur Spektrochcniie utid atigewatidte Spektroskopie 2 ~ ~ - P I a t i c k - l t i s t ifur t u tMetallforschung. Luboratoriumfur Reinststoflatialytik Butisen-Kirchhoff-Strasse13. 0.44139 Dortmutid I . Germany

Contents 1.1

1.2

1.3

1.4 1.5

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.1 General aspects of extreme trace analysis . . . . . . . . . . . . . . . . . . . 1.1.2 Direct instrumental determination methods . . . . . . . . . . . . . . . . . . 1.1.3 Multi-stage procedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1.4 Further general important statements . . . . . . . . . . . . . . . . . . . . . systematic errors and their avoidance . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Volatilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.3 Blanks from vessels. vessel materials and working tools . . . . . . . . . . . 1.2.4 Blanks from thereagents . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.5 Blanks from airborne dust . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.6 Contamination by sample handling . . . . . . . . . . . . . . . . . . . . . . 1.2.7 Problems due to changes of the valency state . . . . . . . . . . . . . . . . . Systematic errors during the analytical procedure . . . . . . . . . . . . . . . . . . . . 1.3.1 Sampling. sample storage and pretreatment . . . . . . . . . . . . . . . . . . 1.3.2 Decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic rules for the recognition and elimination of systematic errors . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2 2 3 3 4 4 6 7 9 13 15 18 19 20 20 22 27 29 31

2

G . TOLG AND P. TSCHOPEL

1.1 Introduction Today, modern analytical chemistry serves as a significant indicator for the optimization of the balance between the technological progress and the inevitable risks accompanying it. Therefore, steadily growing challenges are a constant driving force for improving the three most important analytical figures of merit, namely power of detection, reliability and economy [l-31. The numerous tasks of analytical chemistry of today are of an extreme variety. Many of them are focusing to trace analysis. In this contribution, only the determination of trace elements can be treated. The determination of organic trace compounds must be excluded, because organic trace analysis needs often other or additional strategies. The main problems in elemental trace analysis and particularly in extreme trace analysis, are caused by systematic errors. They are limiting factors which may occur in all steps of the whole analytical procedure, such as sampling, sample preparation, decomposition, separation and preconcentration, and the determination of the elements. At first, these problems will be discussed under more general aspects. Hence it follows that special strategies must be treated as a prerequisite for their understanding and their solutions.

1.1.1 General aspects of extreme trace analysis In principle the concentrations or absolute amounts of all the naturally occurring elements have to be determined within a very broad range and in innumerable kinds of very different organic or inorganic matrices, e.g. in technical products such as high-purity metallic or ceramic materials, glasses, plastics, as well as in solid waste, waste waters or in very complex natural occurring materials, e.g. rocks, soils, biological and medical tissues and body fluids, etc. In the past, mainly the concentrations of the elements in the bulk of the sample had to be determined, which only yields integral information about relative concentrations of elements in large sample amounts. Today, however, the scientific interest is focused far more on trace analysis in micro regions, on the distribution of the elements on the surface of the sample, in microregions or in phase boundaries (micro distributional analysis), also at trace concentration levels (micro-trace analysis) [4,

51. Furthermore, speciation is also required more and more. When only small amounts of sample are available, this challenge in ultra-trace, micro-distribution and species determination asks for methods of highest possible absolute detection power.

SYSTEMATIC ERRORS IN TRACE ANALYSIS

3

I . 1.2 Direct instrumental determination methods

In routine analysis all tasks should be solved in a simple, quick and inexpensive manner. Moreover, the procedures have to be free of errors and should enable it to determine low concentrations of the trace elements down to the pg/g level or absolute amounts in the pg-level or less in all occurring matrices. All these demands cannot be simultaneously fulfilled. For example, in routine analysis the most economic approach seems to be the application of direct instrumental multi-element methods and direct sample excitation. With these techniques, the sample to be analysed undergoes only a short pretreatment for surface cleaning, matching of the shapes, etc. and then is inserted into the instrument for signal generation. Thus, chemical sample preparation is avoided and the time required for analysis is really short. The demand for a reliable determination of absolute quantities at the ng and pg-level, however, is inconsistent with the demand for low costs, because with decreasing concentrations or absolute amounts of elements to be determined, the reliability of the results decreases enormously. Depending on the corresponding elements, systematic errors may falsify the analytical results by up to orders of magnitude, as it can be demonstrated by the results of inter-laboratory comparative analyses. In these studies even data of relatively high contents sometimes suffer from alarming discrepancies [6]. The main reason is that instrumental direct determination methods are relative physical methods requiring calibration, where considerable systematic errors may occur as a result of spectral and non-spectral interferences. The least problematic methods are neutron activation analysis (NAA), sputtered neutral mass spectrometry (SNMS), and X-ray fluorescence spectrometry (XRFA),if one applies very thin samples. Consequently, the most important requirement for the applicability of direct instrumental methods is that suitable standard reference materials must be available for calibration. Unfortunately, these materials are not yet existing at the extreme trace concentration level and for the whole variety of matrices mentioned. 1.1.3 Multi-stage procedures

The strategy to be followed for overcoming this problem in extreme trace analysis is the use of wet chemical multi-stage procedures (combined multi-step procedures), which include sampling, sample preparation, sample decomposition, separation and pre-concentration of the trace elements and finally their determination [&lo]. When the traces which are isolated accordingly are determined in smallest volumes or collected in very thin films on target areas, being as small as possible, effects of sample inhomogeniety and matrix effects are avoided or reduced to a minimum and the power of detection can be improved by orders of magnitude. However, the most important advantage is the easy calibration of a wet chemical procedure with

4

G.TOLG AND P.TSCHOPEL

aqueous standard solutions, by which the problem of the lack of reliable standard reference materials is overcome. The advantages have to be seen together with some disadvantages. Sample preparation essentially is very time consuming, laborious and expensive. The very intensive work requires much expertise and skill as well as well trained personnel. Furthermore, such procedures are hampered by risks for different systematic errors [ 11-15], which are very complex and insidious. The various causes of errors stem to a different extent from the various stages of an analytical procedure. Contamination as well as losses of elements and compounds are the main sources. In spite of these difficulties, multi-stage procedures are indispensable in extreme trace analysis and we have to trace and to eliminate their systematic errors. When the results of these procedures prove to be accurate, one can make use of them to produce reliable standard reference materials with which finally the direct instrumental methods can be calibrated and accordingly one can compensate for their systematic errors. 1.1.4 Further general important statements

In ultra-trace analysis, no generalizations or extrapolations are allowed. Many detection limits of analytical methods described in the literature were mainly obtained by extrapolation. Therewith, it very often is overlooked, that real detection limits in many cases are determined by blanks and their fluctuation. Especially for the omnipresent elements the “real” detection limits can be many times as large as the “theoretical” ones. Optimal absolute power of detection and optimal reliability can only be achieved when the trace element to be determined is available for analysis in an isolated form within the smallest possible target area or excitation volume. “A single method is no method at all.” Only when the results of two or more absolutely different methods agree, one can assume accuracy.

1.2 Systematic errors and their avoidance The causes for systematic errors can be traced back mainly to insufficient qualifications of the analysts and/or inadequate equipment in the laboratory which make any optimal analytical strategy impossible. In the first case, high-quality analytical training would be an answer and in the second case, there is a call for a growing awareness of the fact that false analytical results may finally prove far more expensive than is more advanced equipment and operation. As already mentioned, systematic errors as a rule become evident at the pg/g concentration range and increase enormously with decreasing absolute amounts or concentrations of the elements to be determined. They can exceed several orders of magnitude, depending on the omnipresence and the distribution of the elements in

SYSTEMATIC ERRORS IN TRACE ANALYSIS

5

our environment and in the laboratory. Most difficult is the determination of traces of elements, which are of high abundancy in the earth crust e.g. Si, Al, Fe, Ca, Na, K, Mg, Mn, Ti [S, 161 or of elements which are introduced into our environment as a result of anthropogene pollution (e.g. Hg, Cu, Cd,Pb, As, Ni, Zn)[6,7,9-181. There exists no chance to discern systematic errors by statistic evaluation of the analytical data, especially because the most important condition for a statistical treatment of data, which are supposed to display a normal distribution, very often does not apply. Further, no other simple means for the detection of systematic errors are available (see section 4). Systematic errors depend strongly on the element to be determined, on the matrix, on the method and procedure used, on the conditions of the laboratory and on some other parameters. The most important sources of systematic error [6] are: (a) Inadequate sampling, sample handling and storage, inhomogeneity of the sample; (b) Contamination of the sample and/or the sample solution by tools, apparatus,

vessels, reagents and airborne dust during the analytical procedure; (c) Adsorption and desorption effects at the surface of the vessels and phase boundaries (e.g. filters or precipitates); (d) Losses of elements (e.g. Hg, As, Se, Cd, Zn) and components (e.g. oxides, halides, hydrides of the elements) due to volatilization; (e) Unwanted or incomplete chemical reactions (e.g. change of the valency of ions, precipitation, ion exchange, formation of compounds and complexes); (f) Influences of the matrix on the generation of the analytical signals (e.g.

incomplete atomization, overlap of peaks); (g) Incorrect calibration and evaluation as a result of incorrect standard materials, unstable standard solutions or the use of false calibration functions or unallowed extrapolations, respectively. This contribution will mainly deal with the most serious sources of systematic errors of multi-stage procedures: element losses due to volatilizationand adsorption as well as the contamination due to the three most important blank sources: vessels, reagents and dust [ 17-24].

G . TOLG AND P. TSCHOPEL

6

Table 1-1. Elements and compoundswhichcanbe separated by volatilization(20-1,000' C) (after Bachmann and Rudolph [25]) Elements Oxides of Fluorides of

Gaseous elements, Te. Sn, Pb, T1, P, As, Sb, S, Se, Br, J, Zn,Cd, Hg As, S , Se, Te, Re, Ru, Os, Zn,Cd,Hg B, Si, Ge, Sn, P, As, Sb, Bi, S , Se, Te, Ti, Zr, Hf, V, Nb, Ta, Mo, W,Re, Ru, Os, Ir, Hg

Chlorides of Al, Ga, In, T1,Ge, Sn, Pb, P, As, Sb, Bi, S, Se. Te, Ti, Zr, Hf, Ce, V, Nb, Ta, Mo, W,Mn, Fe, Ru, Os, Au, Zn, Cd, Hg Hydrides of

Si, Ge, Sn, Pb, P, As, Sb, Bi, S , Se. Te

I .2 .I Volatilization Losses of elements by volatilization mainly occur at high temperatures. However, for very volatile elements these interferences can already be remarkably high at room temperature (Table 1-1) [25, 261. Especially Hg is well known to be extremely volatile. It can be lost during sampling, storage and sample preparation, when aqueous solutions are stored in open vessels or vessels made of organic polymers. By means of the radioactive isotope '03Hg it could be proved that at the ng/ml-concentration level within a few hours, Hg losses of up to 25% from an acidic solution out of an open quartz dish can occur 1271. In addition, Hg quickly penetrates through sample containers made of plastics such as polyethylene or polypropylene. Therefore samples in which Hg is to be determined should not be stored or transported in plastic containers so as to avoid Hg losses from the sample or contamination by the Hg present in the environment. During the dissolution of a metal sample with non-oxidizing acids the hydrides of elements such as S, P, As, Sb, Bi, As, Se or Te may escape. Also when drilling or cutting metal samples such as Al or Fe, the well known smell of H,S or PH, and other volatile hydrides often indicates the loss of these elements. The number of elements and compounds which can be lost as a result of volatilization increases with temperature. This must be considered when evaporating solutions or when performingdecomposition procedures [25] (see Table 1-1). Volatile chlorides of H P , As3+,Sb3+,Sn4+,Ge4+,Se4+,Te4-, e.g. may be lost during the evaportion of acid solutions, and @ tends to be lost as chromyl chloride when perchloric or sulfuric acid is present at temperatures above 150"C. Slight differences in the volatility of the chlorides, e.g. of As3+,Sn", Rh, and 0 s are found when samples are fumed off with perchloric or sulfuric acid. Systematic errors resulting from volatilization can be eliminated by using closed systems (see 1.3.2.2) and by working at low temperatures. One also should avoid all

SYSTEMATIC ERRORS IN TRACE ANALYSIS

7

chemical reactions, by which volatile compounds can be formed (e.g. the formation of Cr,OCl,). For the determination of elements which can be relatively easily volatilized (e.g. Hg, Se, As, Bi, Zn, Cd) from non-volatile matrices, we can make use of the very advantageous separation in the vapour phase (see section 1.3.3.4) by trapping these compounds in suitable devices.

I .22 Adsoiptioii The concentrations of the trace elements of very diluted solutions may change very quickly as a result of adsorption and desorption [6, 12, 22, 28-30]. By these processes ions or compounds of trace element are bound onto the surface of the container and may be released later on when the composition of the solution changes (Fig. 1-1). The element losses as a rule become noticeable at concentrations

Figure 1-1. Adsorptionof %o(II)-ions (adapted from [6]);c = 2 x 10-5mo~;AA glass, (7. ooquartz, AOOpH 1.5, A.OpH9.

PTFE,

below 10-6mol/l and they are of the order of 10-9-10-12mol/cm2 [31]. A general statement of the actual error in a special case or an extrapolation from the knowledge of similar analyticalproblems is not possible. However, the losses can be monitored very easily with the aid of radioactive isotopes, provided they are available [32].

G . MLG AND P. TSCHOPEL

8

The amounts of the elements adsorbed very strongly depend on numerous factors which hardly can be specified together. Therefore, an estimation of the losses and gains of trace elements as a result of adsorption and desorption is not possible. The most important factors to be taken into account are: (a) The trace element to be determined, its concentration and its valency; (b) The accompanying elements and the inorganic and organic compounds in the analyte solution (especially the major components but also minor and trace elements), their concentrations and valency, and the pH-value; (c) The composition and the purity of the vessel material, the dimensions and the constitution of the surface of the vessel, as well as its pretreatment and the cleaning procedures applied; (d) Duration of the contact and the temperature.

As a result of adsorption, strong losses of elements especially occur when the sample solution comes in contact with large surfaces. This is the case during filtration [33], ion exchange techniques etc., or even when only changing the vessel. In order to minimize element losses due to adsorption the following precautions should be taken. (a) Vessels made of quartz, PTFE or glassy carbon should be used; glass is not a suitable material in extreme trace analysis as it is linked with the highest adsorption losses; (b) The surface and the volume of the vessel as well as of the sample solution should be as small as possible in order to increase the concentration of a given sample solution; (c) The concentration of the elements to be determined should be as high as possible; (d) The time of the contact between the vessel and the solution should be as short as possible, which can be reached by working up small sample volumes; (e) Sample solutions should be acidified whenever possible, as the losses are then often lower than those occurring with neutral or alkaline solutions;

(0 Cleaning and preconditioning of the vessels should be performed by a treatment with acid vapours. This technique considerably reduces blanks as well as adsorption losses and is more effective than other techniques.

SYSTEMATIC ERRORS IN TRACE ANALYSIS

9

Trace elements can also be lost by electrochemical cementation [6] during sampling or sample preparation. This occurs when the trace elements dissolved in an electrolyte come in contact with the surface of a metal which is more elecuonegative than the trace element. Then the trace metal (e.g. Pt, Au, Ag, Hg, Cu) is precipitated onto the metal surface from where it cannot be easily removed. Element losses due to cementation may occur during sampling, milling, cutting, mixing, etc. This is the case when aqueous samples or biological fluids and tissues such as fish, muscle, blood, fruits, etc. come in contact with metal tools. In order to avoid or reduce cementation, freezing of the sample in liquid nitrogen and a treatment of the frozen sample is recommended [6].

1.2.3 Blatiksfiont vessels, vessel materials atid workittg tools No vessel material is absolutely resistant even not to water. Accordingly, as soon as a solution comes into contact with acontainer or any solid substance, each element present in this material will be found at a more or less high level in the solution [6-13,34-361 (Table 1-2). Especially glass, which contains a number of elements as major or minor components and a lot of other elements at a very high trace level, is very impure as compared to quartz, PTFE, polypropylene and polyethylene [6]. Table 1-2. Impurities (pg/g) in different materials according to the literature and investigation [6]

Element B Na

Mg A1 Si

Ca Cr Mn Fe co

cu

zn As

cd Sb Pb

Glassy carbon (sigradur G)

FTE

0.1 0.35 0.1 6.0 80-90 70-90 0.08 0.1 2.0 0.002 0.2 0.3 0.05 0.01 0.01 0.4

25

0.03

0.01 0.002

0.02 0.01 -

4 x 10-~

-

Quartz suprasi@ 0.01 0.01 0.1 0.1

main 0.1 0.003 0.01 0.2 0.001 0.01 0.1 1 x 10-4

-

0.001

Borosilicate glass main main 600 main main lo00

3 6 200 0.1 1 2-4 0.5-22 1 7-9 3-50

10

G . TOLG AND P. TSCHOPEL

In addition, the losses of elements due to adsorption are very high. Therefore, glass vessels should not be used in extreme trace analysis. Quartz is the most pure material which is commercially available in different purity classes but unfortunately quartz containers are also the most expensive ones. They definitely deliver negligeable blanks (except for Si) and should be preferred whenever it is possible. Poytetrafluoroethylene (PTFE), other plastic materials and glassy carbon are substantially less pure [21,37-391. Nevertheless, they are much more cheaper than quartz and therefore they are preferred in most of the routine laboratories. After a pretreatment by steaming, glassy carbon releases only a few impurities, because diffusion of impurities from the bulk to the surface is avoided by its structure, which has only a minimum of pores. PTFE,Polypropylene (PP) and Polyethylene (PE) however, are permeable for many substances [40], e.g. for gases or Hg. PP, PTFE and glassy carbon especially are used for solutions containing hydrofluoric acid. To avoid a diffusion of acid solutions to diffuse into the pores and tissues of PTFE, the surface layers of cleaned vessels can be molten by a gas flame and cooled down so as to provide for a dense surface [34]. Contamination by particles, enclosed in deeper layers of the material, as well as losses due to adsorption are reduced by this technique. As compared to PTFE, other fluorinated polymers such as FEP or PFA in some respect are more pure and in addition translucent,and therefore more suitable for ultra trace analysis. Especially during sampling, one has to avoid that the sample comes into contact with other materials causing severe contamination. Therefore, e.g. rubber is not a suitable material, because of its relative high contents of Sb, As, Zn, Cr, Co and even Sc [41]. Nylon contains Co and PVC Zn,Fe, Sb, Cu at the higher trace levels. In addition, we have to note that all half stuff and plastic ware are not produced and manufactured within clean rooms, but in rather dusty factory halls and thereby come into contact with different metals and materials. During the past years it appeared that the purity of plastic materials even became worse. With regard to the vessels, their bulk material is not the only source of contamination. Also the effective cleaning of the surface of the vessels is very important [22, 34, 42). The conventional cleaning technique for laboratory glassware consists of its rinsing and leaching with high-purity acids and pure water [20, 29-31, 42-53]. In addition, leaching can be supported by applying ultrasonic treatment [35]. However, leaching is very expensive and time-consuming (several days or weeks) and it requires large volumes of pure, high-purity or even ultrapure acids, which in the future also will become a waste problem because of environmental contamination. Another disadvantage is the fact that the cleaned vessels remain in contact with the acids now enriched with impurities, by which they are again contaminated. Therefore, in many cases these procedures are not effective enough so as to guarantee for residual blanks down to the lower pdml-region.

11

SYSTEMATIC ERRORS IN TRACE ANALYSIS

A very effective and much less time consuming cleaning of laboratory ware can be achieved by a steaming procedure, during which the vessels to be cleaned are brought upside down on the top of quartz tubes located inside of a glass container (Fig. 1-2) [6-13, 541. Vapours of nitric or hydrochloric acid are passing through 1 1

1

Figure 1-2. Steaming apparatus for vessel purificationwith nitric acid steam (after 161);(l)condenser, (2) steam chamber, (3) steam tubes, (4) overflow, (5) round-bottomed flask,(6) heater.

the quartz tubes by which mainly the inner surfaces of the vessels are washed continuously. During 4-6 h, acid vapour treatment is used and subsequently, water vapor is introduced for another 1-2 h. By this process, the surfaces are not only cleaned (Fig. 1-3)but also the adsorption of traces of elements during the subsequent procedure is considerably decreased. Only a few exceptions should be mentioned where the steaming technique may result in relatively high blanks, e.g. for the case of Fe-traces in PTFE. The effect of the purification process can be easily checked by TXRF (total reflection X-ray fluorescencespectrometry) [S,551. Figure 1-3b shows the impurities on a quartz plate before and Fig. 1-3a after steaming the surface with acid vapour. 1.2.3.1 Contamination by tools Especially during sampling [56] and sample preparation there is a inevitablerisk of contamination by tools for cutting, drilling, milling, sieving, crushing, grinding, pulverizing [57, 581. Metal contaminations of biological tissues and fluids (e.g. blood [48]) with Cr, Ni, Co, Fe, Mn, Cu and others due to use of scalpel blades

G . TOLG AND P.TSCHOPEL

12

0

20

10

Energy IkeV

1

Figure 1-3. TXRFA spectra of a quartz plate (adapted from [81); (a) quartz plate cleaned by steaming, (b) uncleaned.

2

L

6

lime I h l

Figure 1-4. Contamination of 150 ml of 2 M HCI with Fe by 20 micropipette tips [121.

and syringe needles are reported. Therefore, the use of forceps, knives, spatulas and needles made of plastic, titanium or quartz is recommended [20, 48, 50, 59, 601. Metallic support material (also when this is made of stainless steel), drying ovens, gas burners, electrical furnaces, washing agents and much other equipment and products in a laboratory [61] may be sources of contamination, which must be taken into account. Figure 1-4 shows the contamination of 150 m12 M HC1 caused by 20 plastic tips of usual micropipettes [12].

SYSTEhWTIC ERRORS IN TRACE ANALYSIS

13

1.2.3.2 Contamination due to man The operator himself also represents a very serious source of contamination. The number of particles emitted per minute by a person amounts up to millions. They are released by the skin, hair, clothes, jewellery, cosmetics, disinfectants, talc, etc.

161. 1.2.4 Blanbfrom. the reagents

The introduction of blanks from the reagents into the sample solution during a wet chemical procedure cannot be avoided, as no substance is absolutely pure. This is especially true when solid reagents have to be used in a large excess as is the case in decomposition by fusion. Unfortunately, the possibilities for lowering the blank contributions introduced by the reagents are very limited [6-17,20-23,62481- For most of the solid substances, the available procedures -which are always separation methods such as solvent extraction, ion exchange, chromatographic techniques, coprecipitation, electrolytic deposition in a flow through system [69] etc., are very laborous and sophisticated. In addition, they are effective as a rule for only a very few elements, whereas for others they even might be increased. Accordingly, in extreme trace analysis we often have to use only those reagents which can easily be purified, such as gases and liquids. The preparation of ultrapure water, e.g. by distillation in a quartz still or by membrane filtration and its permanent quality control is of greatest importance. Especially the sub-boiling distillation technique, described by Kuehner et al. [6, 62, 63, 65-68], is extremely efficient for most of the acids used in the laboratory (e.g. HNO,, HC1, H2S04) and for some organic solvents. The distillation still is made of quartz (see Fig. 1-5a), stills made of PTFE should only be used for the purification of hydrofluoric acid (Fig. 1-5b) [70]. In this technique a liquid is evaporated without boiling with the aid of an IR radiator, which is not allowed to dip into the liquid, and accordingly one avoids the formation of aerosols, which in the conventional distillation technique contaminate the distillate. The residual impurities for subboiled liquids are at the pdml level (Table 1-3) which is sufficient for most of the ultra trace procedures. The yield of such a subboiling still amounts to some 100 ml per day. This is sufficient for most purposes in ultra trace analysis purposes due to unavoidable contamination during storage. Therefore, only that volume of acid required for the immediate use should be prepared. Apart from this technique, no other universal (single) purification procedure is capable of removing all metallic or cationic impurities to such a low extent. Ultrapurificationof some other reagent such as H202,hydrazine, and AsCl, can be performed with the aid of sublimation at low temperature [65,66].

G.T ~ L GAND P.TSCHOPEL

14

An

6

Figure 1-5. Subboiling distillation [6,11,62], (a) quartz still, (1) IR heater, (2) cooling finger; (b) €TFE still for HF (adapted from [70]),(1) PTFE body, (2) HF, (3) PFA foil (transparent),(4) condensing area,(5) water cooling,(6) to receiver, (7) IR lamp.

Table 1-3.Residual impurities in different acids [ng/ml] [6]

Cd

Cu

0.01

Fe

A1

Pb

Mg

Zn

0.04 0.3 0.07 0.6 1 100

5 0.05 0.07 10

0.02 < 0.05 0.5

5 0.02 0.2 14

0.2

HNO3 15Msubb. 0.001 HNO3 15 Mp.a. 0.1

0.25 2

0.2 25

5: 0.005 10

5 0.002 0.5

0.15 22

HF54% subb. HF 48% p.a.

0.5 2

1.2 100

2 5

0.5 4

1.5

1

3

5

HzO subb. HCI 10M subb. HCI 12 Mp.a.

0.01 0.1

0.01 0.06

50.04

8 0.04

3

SYSTEMATIC ERRORS IN TRACE ANALYSIS

15

1.2.5 Blanksfi.oni airborne dust

The atmosphere of the laboratory is loaded with particulate matter from different sources such as the environment, the floor, the walls, the ceiling (paint), the furniture, the equipment, the clothes, the analyst himself and so on. Accordingly, various inorganic and organic compounds are present and in principle, any element can be found, depending on the environment, the laboratory itself and its history. Again the dust particles will contain relatively high concentrations of those elements which show a high abundancy in the earth’s crust (e.g. Si, Al, Fe, Ca, Na, K., Mg, P). In addition, all elements of anthropogenic pollution (e.g. Mg, Cu, Cd, Pb, Ni, Co, Zn, Mn) are always present. When the dust comes in contact with the sample, it is a severe source of contamination. Sometimes protection may be achieved with cheap and very simple means, such as closed vessels and apparatus or glove boxes. More efficient and convenient are clean rooms and clean benches [5,6,8, 13, 17,42,48,51,71,72,73] which are flushed with “dust-free’’air. A clean room (Fig. 1-6) is an area which is hermetically separated from the outside atmosphere, and which is only accessible through an air

Figure 1-6. Scheme of aclean room with a clean bench, (1) air conditioningsystem, (2) HEPA-filter, (3) clean air to clean bench, (4) clean air to clean room,(5) clean bench, (6) turbulent air flow, (7) laminar air flow, (8) clean working area,(9) air out, (10) water drain, (1 1) window, (12) door to air lock.

lock. A filter assembly provides for pure air at an overpressure against the outside atmosphere and for a circulation with a turbulent flow, as shown in the example of Fig. 1-6. The most important part of this assembly is the so-called HEPA-filter (high

G.TOLG AND P. TSCHOPEL

16

efficiency particulate air filter), which has a definite pore size of 0.3 pm. The dust removal efficiency by the deposition on the filter is about 99.97-99.95%. At the right side of Fig. 1-6 a cross section of a clean bench is shown, which is the actual working place in a clean room laboratory. Here a laminar air flow from above passes along the working space of the bench. As a result of the laminar flow, the dust content of the air in the clean bench is about one order of magnitude lower than in the room, where turbulent air takes up dust (Table 1-4). The air quality is classified according to the number of dust particles. For instance, the U.S. Federal Standard 209 specifies the cleanness of the air by the number of particles per cubic foot having a diameter between 0.5 and 5 pm and one distinguishes between the classes 100, 10,000 and 100,000. In a clean room with a turbulent air flow, e.g. a cleanness of class 1,000 to 10,000 can be maintained, whereas in a laminar flow clean bench class 100 must be reached. The use of a clean bench in a clean room illustrates only one possibility. Another more expensive alternative is to keep the whole clean room as dust-free as possible. This can be achieved with the aid of a laminar air flow which enters the room through a HEPA-filter installed over the whole area of one wall or the ceiling. For advanced technologies, however, still much higher purity demands now exist. For instance, the electronic industry claims a much lower dust content for the production of megabyte chips (class 10 or 1 according to the US-Federal Standard). For trace Table 1-4.Particle concentration in laboratory air [6] ~

Room

Normal lab

~~

Particle size

Particle concentration

[FI

[m-31

>0.5 >1

7 ~ 1 6 - 2 xi07 1 . 5 ~ 1 -62 x106 1 xio4-4 xi04 2 xld-7 x l d

2 xlo6 3.5 x 10’ 1.5~10~ 5 xld

3 . 5 ~ 1 -07~ x104 7 xld -1.5~10~ o - 1 xi03 0 - 250

5.5 x lo4 1 xi04 350 100

2 xld-3.5~10~ 350 - 700 0-70 -

0.5 x 103

>3 >5

Clean room

>0.5

(turbulent)

>1 >3

2 persons

>5

Clean bench

>0.5

(laminar)

>1 >3 >5

Mean value [m-31

3 xld

-

17

SYSTEMATIC ERRORS IN TRACE ANALYSIS

analysis, however, this enormous effort to get such a high air quality is only rarely necessary. Numerous rules have to be taken into account for working effectivelyin clean benches and clean rooms. They concern the arrangement of the bench, an ingenious way of proceeding and a highly pure working style. It is obvious that a high degree of cleanness also has to be maintained. This applies both for the bench, for the devices, and for the operator himself who spreads most of the “uncleanness” in the clean room. Also the laminar air flow may not be disturbed by tools or labour at those places, which are to be protected from dust. All openings of the vessels and devices, e.g. are such places and have to be continually flushed with pure air. For monitoring the blanks caused by the dust content of the air, dust particle counting with different types of stray-light photometers can be used. Further carefully cleaned small quartz beakers can be placed uncovered at different places of the laboratory and the clean bench for several hours during an analytical procedure. After exposure the dust is dissolved in a small volume of acid and the element content can be measured, e.g. by ICP-MS or ETAAS. Figure 1-7 exemplarly shows the contamination for Fe in an AAS laboratory at different days [ 121. Also TXRF is an efficient method to indicate the elemental composition of dust collected on small quartz plates, which are deposited in the area to be controlled. The dust content of the air is not constant but it is influenced by several factors, including the condition and the history of the laboratory, the type of air supply and ventilation, the number of persons in the laboratory, and the type of their activity. Also the weather plays an important role; when it is raining the air generally is essentially purer than when it is dry (see Fig. 1-7).

I

I

lime

lhl

L

6

Figure 1-7. Contaminationof 10 ml HCI (subb.) with Fe by airborne dust in an AAS laboratory [123; (1 and 2) rainy weather, (3 and 4) dry weather.

18

G . TOLG AND P. TSCHOPEL

Figure 1-8. Contamination of 0.1 M HCI during storage in vessels of different materials (after 111, 121); (1) vessel without cleaning, (2) vessel cleaned by rinsing with H20, (3) vessel cleaned by rinsing with 2 M HCI, (4) vessel cleaned by steaming with H N a (6 h).

12.6 Contamination by sample handling In spite of the fact that we have available very effective techniques for the cleaning of the vessels and equipment, for the purification of the reagents and for maintaining a clean laboratory and working place, further contamination, e.g. arising from storage of the sample solution (Fig. 1-8) [12], from sample handling and from the analytical procedure cannot be totally excluded. At the ppb- and still much more at the ppt-concentration level, even a very simple working step such as pipetting, shaking, evaporating, filtering a.s.o., already can increase the blanks considerably [6,8, 11-13, 18, 33,429 481. Alarming is not the real value of the blank of one single step but the fact that blanks occur even during very simple operations and accumulate during the whole analytical procedure to amounts in the ng/g-range. In extreme trace analysis the scattering of the blanks can exceed several orders of magnitude. In principle, the various sources of systematic errors described above are present in all steps of an analytical procedure [6] such as sampling, transport, storage [74], sample pretreatment, decomposition and separation (Table 1-5). Accordingly, the

19

SYSTEMATIC ERRORS IN TRACE ANALYSIS

Table 1-5. Frequency of sources of systematicerrors in trace analytical procedures [ 6 ] ; f positivehegativeerror, + + +/ - - - large; + + / - - medium; +/- small ~

Type of error

Contamination

Step of operation

Reagents, tools, Tools, laboratory air interfaces

Sampling Sample preparation Dissolution. decomposition Separation, preconcentration

~~~

~

Adsorption Volatilization Cross interferences by elements Chemical reactions, signal-interferences, -coincidence -background

+++

--

-

+++

-

-

f f

+++

-

---

f

++

---

-

fff

general statement can be made that with decreasing absolute amounts of the elements to be determined, systematic errors increase dramatically and that they are the main problem in extreme trace analysis. Unfortunately,this fact will often not be realized in the daily work of routine laboratories and may then be the reason of wrong results with dramatic consequences with respect to economy, safety and health.

12.7 Problems due to changes of the valency state Severe losses of elements can be caused by the change of the valency state of the trace elements during the analytical procedure. This becomes very effective when there is only a partial change and when special chemical reactions such as the formation of insoluble precipitates of organic chelates, the sorption on an ion exchange resin and others are used for the separation of the trace element from the sample matrix or when special ion sensitive techniques are used for the determination of the trace element (e.g. voltammetric methods). There are many possibilities, but only a few examples can be mentioned here. Hg-ions can be easily reduced to the volatile metallic mercury by dust particles, organic impurities in the sample solution or by the plastic container. Also Au and the metals of the platinum group are subject to similar reactions. Especially irreversible adsorption at the container materials such as PE, PP, PTFE ect. may cause severe element losses. Cementation during sampling and sample preparation also causes losses of trace elements. The oxidation of small amounts of sulphide ions in natural waters is described in section 1.3.1.

20

G . TOLG AND P.TSCHOPEL

1.3 Systematicerrors during the analytical procedure 1.3.1 Sampling, sumple storage and Pretreatment

Sampling, sample storage, transport, drying and processing as well as sample pretreatment are associated with a significant amount of very different types of systematic errors when absolute amounts of elements 5 1 pg are to be determined [28,43,46,47, 56,60,73-851. Sample inhomogeneity, adsorption and desorption effects, biological and photochemical reactions which are changing the original bonding of the trace elements, volatilization and impurities introduced by the tools as well as containers are the main keywords which should be mentioned here. These topics represent a specialized area of analytical chemistry with numerous problems, rules and techniques. The theory and the praxis of these topics must be well known by the analyst so as to avoid or minimize the inherent errors. But once again: Generalization is not allowed since all effects mentioned strongly depend on the matrix, on the respective elements, on their concentration and on the type of procedure. Therefore, also in this context the cited literature and this contribution can only present examples, which demonstrate the multiplicity and complexity of the sources of errors [56,86-891. For samples which are unstable or difficult to handle, special techniques are necessary 1901. When sampling compact solid material, inhomogeneity may cause severe errors mainly with respect to geochemical matrices (e.g. rocks and ores). But also in the case of metals and alloys, concentration gradients and clusters must be taken into consideration [91]. As the sample analysed must represent the composition of the whole material to be characterized, the sample mass and its grade of disintegration (particle size) plays an important role [92]. In the case of compact solid samples, contamination can only occur on the surface and it can be subsequently removed or greatly reduced for instance by etching. The limitations of such cleaning processes are today only investigated exemplarly as it is e.g. described for the determination of non metals in refractory metals [93]. Particularly in the case of high-purity materials (e.g. metals, semiconductors) the main impurities are enriched on the surface. Here we strictly have to distinguish between surface and bulk contamination, as it was shown for the case of carbon impurities in high-purity metals [94]. Considerable amounts of impurities may be introduced during disintegrating the sample with common tools (e.g. sawing, drilling, spark erosion). Then one has to use laser beams or other tricky sampling techniques as mentioned in the following example of brittle materials (e.g. glass, quartz or ceramics). Here one can use a simple colouring technique [95]. The material to be disintegrated will be completely coated with ink from a marking pen, subsequently the sample is wrapped in paper,

SYSTEMATIC ERRORS IN TRACE ANALYSIS

21

shattered by a hammer and only those pieces which have absolutely no colour are selected under a microscope and used for analysis. As a general conclusion it must be emphasized that in extreme trace analysis, all common techniques for sample disintegration, sieving and homogenization should be avoided whenever it is possible. For each special case, one has to find out its own way. For sample disintegration, the lowest contamination is caused by tools made of agate, monocrystals of aluminum oxide or boron nitride. Preparation tools (tweezers, needles, spatula, etc.) should be made of plastics, quartz, high-purity titanium, gold or silicon nitride instead of stainless steel or nickel. Also in the case of sampling liquids such as aquatic samples, many rules must be followed, which depend on the water type [83-861. Especially for seawater, the inherently low levels of trace metals are known to cause severe problems in sampling [43,96-981. Similar conditions exist for fresh water and rain water of which errors also resulting from contamination and adsorption during sampling and storage must be avoided [23,44,49,99, 1001. Special precaution is necessary for sampling arctic and antarctic ice or snow when extremely low concentrations of trace metals should be determined [72,1011051. Severe problems arise when storing high-purity liquids [43,101, 1021. During storage of high-purity solutions, the blank content increases with time, depending on the pretreatment of the vessel, the composition of the solution and the element Ell, 12, 31,791. Changes in element content and/or valency and in the bonding of the elements, e.g. by oxidation or by biological or photochemical reactions can be illustrated by the determination of ng/ml concentrations of S2- ions in diluted sulfide solutions. The Sz- concentration changes as a result of the oxidation by the oxygen content of the solution. A stabilization can be achieved by adding reducing agents, such as ascorbic acid @H 2 12) and storing the samples in the darkness. However, this is only true for synthetic and sterile solutions and not for samples of natural water, such as river water or mineral spring water. In these samples we have to add Zn2+ up to a concentration of 20 mg/l [pH 81 to avoid the disintegration of S2-by bacteria E1061. Similar reactions induced by bacteria (methylation process) can also convert Hgz+to elemental Hg or organic compounds. All these problems are also relevant in the preparation, storage and use of highly diluted standard solutions in the ng/ml range and lower. In the latter case it is essential to store stock solutions only with concentrations not below lo-’ molar and to renew them in short intervals. The risk of instability increases dramatically at lower concentration ranges. The dilution of stock solutions into the ng/rnl range or below with the aid of common techniques using pipettes and volumetric flasks is unreliable and it must be substituted by micro techniques. When absolute amounts of an element at the ng or pg level are to be dosed, stock solutions in the stable

22

G . TOLG AND P. TSCHOPEL

concentration range should be handled with ultra-micro burettes or pipettes, as these still allow the measurement of volumina in the sub- pl-range with relative errors below 1% [107]. Many further sources of error may occur during each treatment of the analyte solutions, e.g. when suspended matter is removed from liquid samples by filtration, traces of the dissolved elements will be lost by adsorption and blanks will be introduced. Here separation by centrifugation is an alternative [33]. The determination of components in gaseous samples [ 1081 aerosols and dust of air [109-1 111 needs much experience and special sampling strategies too. Considering environmental and biological samples [24, 1121 additional severe systematic errors are involved in the sampling step and during storage, which must be avoided from case to case. There exists a lot of information about the reliable storage of biotic samples in environmental specimen banks. It can be performed over longer periods by using liquid nitrogen temperature conditions [1131. During disintegration or homogenization of food or biological matter, losses of elements by adsorption or cementation onto the metal of the cutting tool can be well avoided by using a special mixing apparatus made of PTFE and cooled with liquid nitrogen, as already mentioned [6]. Also during the drying step of biological samples at temperatures of about 110"C losses of volatile elements (e.g. Hg, Se, Cd, Pb) may occur. Therefore, freeze drying is an alternative [ 114,1151. Many further experiences and directions are also given, e.g. for soils [116], for slurries 11171, for plants [118, 1191 and for tissues and body fluids [120, 1211. As another example, the determination of Se in hair for medical or toxicological purposes should demonstrate other severe problems. One has to differentiate between living and already mortified hair [ 1221. Also external contaminations must be removed [1231. Nevertheless it is impossible to correlate the Se-concentration found to its concentration in the blood, because by washing, the hair surface may be strongly contaminated by selenium present in all shampoos as they contain sulfur [1241. Summarizing, it should be mentioned that in this chapter we could only stress that severe errors in a trace analytical procedure may occur within the sampling and sample preparation steps. But it makes not much sense to deepen the considerations, because the sources of systematic errors are so numerous and vary from one task to another, so that no generalization is possible. We can only demonstrate that the trace analyst has to reflect very critically each step or manipulation, also when it seems to be very simple. 1.3.2 Decomposition

The dissolution, decomposition or combustion of the sample is the next step in a multi-stage procedure [ 125-1321. The aim of this step is to obtain a clear solution or

SYSTEMATIC ERRORS IN TRACE ANALYSIS

23

in some cases a gaseous sample containing all elements and compounds of interest in the sample without change of their absolute amounts. For special analytical purposes (e.g. sample preparation for XRFA) homogenisation or isoformation of the sample material can be performed by melting the sample with a flux. According to the very huge variety of different matrices with a more or less complex composition, many different decomposition principles were developed in the past, of which, however, only a few are useful in extreme trace analysis with respect to minimal systematic errors. To reduce them the following points must be considered: (a) In most cases the decomposition of the sample has to be complete. All

inorganic materials have to be transformed into easily soluble compounds, organic substances have to be totally mineralized. This demand is especially important, when an electrochemical method is used for the subsequent determination; (b) Residues of a combustion should be quantitatively soluble in a volume of a high-purity acid being as small as possible;

(c) The decomposition procedure should be as simple as possible, and should not involve large expenditure of apparatus, reagents and time; (d) The decomposition procedure must be adaptable in an optimal way to the whole analytical procedure, especially with respect to the subsequent separation or determination step;

(e) Decomposition procedures such as combustion, pyrolysis or hydropyrolysis should be preferred when easily volatile compounds are formed, which can be used for quantitative separation. Then decomposition and separation can be achieved within one step; (f) Clean vessels made of an inert and high-purity material with a possibly small

volume and minimal amounts of high-purity reagents should be used. Dust should be excluded by using clean benches and clean rooms and precautions should be taken so as to avoid losses of trace elements by adsorption or volatilization; (g) The decomposition should be checked with the aid of radioactive tracers. There are mainly two problems with decomposition: (1) The decomposition may be incomplete and parts of the sample or of the reaction products may remain as an insoluble residue. This can be the case for complex matrices, such as geological samples or organic materials containing high concentrations of inorganic compounds. Then we have to apply a second or even a third different decomposition procedure introducing additional systematic errors. (2) The amounts of trace

24

G. TOLG AND P. TSCHOPEL

elements or compounds to be determined and present in the decomposition solution do not correspond to their content in the sample as a result of systematic errors. With decreasing trace element concentrations these problems are of great weight.

1.3.2.1 Decomposition by fusion As a rule, decomposition by fusion with solid reagents is not suitable for extreme trace analysis as it requires an up to tenfold excess of flux reagents causing enormous contaminations. In addition the very impure crucible materials are attacked by the flux which introduces further contamination. Moreover, element losses can occur as a result of volatilization, adsorption or reaction with the crucible material. 1.3.2.2 Wet decomposition Acid decomposition in an open vessel or system is only suitable if losses of volatile trace elements are not to be expected. Wet decomposition in a closed system, e.g. in a pressure bomb, is performed at high temperature [133-1351. PTFE vessels allow temperatures up to 200°C which very often is not sufficient for a complete mineralization of the organic matter [136-1401. Only at temperatures above 300"C the oxidation potential of NHO, will be high enough to mineralize resistant compounds such as fats. These problems have been optimally solved in the high-pressure ashing method with nitric acid in quartz vessels as developed by Knapp et al. [126, 141-1443. A complete mineralization and a complete recovery of volatile elements as well as low blanks can be reached with temperatures up to 320°C and pressures of up to 100 bar. To avoid breaking of the quartz glass vessel as a result of the high pressure inside the vessel, a somewhat higher pressure must be applied outside the decomposition vessel. This can be achieved in a microprocessor controlled autoclave (Fig. 1-9). With this technique, even 300 mg amounts of sample materials such as PVC, polypropylene, rubber, coal and others can be decomposed without contamination within 2-4 hr and with not more than 2 ml of HNO,. Furthermore, the extreme purity of the quartz glass and its tightness in contrast to commonly used PTFE vessels lead to optimal conditions. With respect to the systematic errors introduced into the sample by the heating device, there is no essential difference between a conventional electric and the microwave heating technique. The main advantage of the microwave energy is a very fast increase of the temperature and thus a short time consumption [ 145-1491. A considerable reduction of contamination can be obtained by the use of quartz containers instead of the commonly used PTFE vessels. 1.3.2.3 Combustion with oxygen For organic samples the combustion with pure oxygen in a closed system of high-purity quartz should be preferred over wet decomposition procedures due to

SYSTEMATICERRORS IN TRACE ANALYSIS

25

Figure 1-9. Scheme of a high pressure asher (adapted from Knapp [126,141-1441); (1) autoclave, (2) furnace,(3) sample and decomposition acid, (4)quartz vessel. (5) closure. (6)compressed air; (Pop,).

the lower risk of contamination. This can be performed either in dynamic systems (e.g. Trace-0-Mat) (see 1.3.2.4; Fig. 1-10) [131, 1501 or with a low-temperature ashing technique with a microwave or hf oxygen plasma [150,1513. In order to avoid losses of trace elements the very gentle combustion at low temperatures being about 100-20O0C should be recommended when low concentrations of volatile elements or compounds are to be determined in organic samples. In both techniques the combustion residue (ash) should be dissolved in a minimal volume of diluted acid in order to reduce the blank contribution of the reagents. 1.3.2.4 Thermal volatilization When high temperatures are applied to the sample, volatile elements and compounds can be lost (see Table 1-1). One can make use of this fact very advantageously for a quantitative separation. In this way the decomposition of the sample and the separation of the trace elements can be performed within one step which might considerably reduce the systematic errors. As an example of such a technique, the Trace-0-Mat [131, 1501 should be mentioned here (Fig. 1-10). The quartz apparatus consists of a small burning chamber (ca. 75 ml) flushed with pure oxygen and a liquid nitrogen cooling unit. The sample is ignited with the aid of an IR lamp outside the vessel. The combustion can be controlled by the flow of the oxygen. The volatile components subsequently are transported into the cooling trap, are frozen out completely, and then can be taken up by refluxing in 1-2 ml of high-purity acid. Non-volatile elements remain in the sample holder from which they can be dissolved.

G.TOLG AND P. TSCHOPEL

26 5.

Figure 1-10. Scheme of a “Trace-O-Mat@)”(after [131, 1501);(1) oxygen in. (2) sample, (3) liquid nitrogen, (4) focussed IR lamp, (5) condenser.

The combined decomposition and separation via the gas phase to a very high degree meet the requirements of the extreme trace analysis. Contamination is minimal due to a closed quartz system, to high-purity oxygen and to minimal amounts of high purity acids. Losses of elements are avoided, provided the combustion and separation are complete. Examples are the determination of Hg or Se. However, for the decomposition of insoluble inorganic refractory materials [127, 128, 1521 such as high-temperature ceramics and many other technical matrices, there exists no universal method having the low blank levels required for extreme trace analysis. Here this problem has a good chance to be solved by using elementary fluorine or gaseous fluorides as decomposition agents, as these reagents possess the highest known oxidation potential at relatively low temperatures [153]. A fast and very reliable decomposition of such problematic matrices, such as silicon nitride, silicon carbide, boron carbide, zirconium oxide can be performed in a closed Niautoclave system. This approach enables an on-line determination of main and minor components such as Si, B, P and S via their volatilized fluorides by FTIRspectrometry [153]. In the case of trace elements, the volatilized fluorides can be determined by mass spectrometry in an on-line system. The elements which form non-volatile fluorides can be determined in the residue after its dissolution with special purified agents, such as ammonium fluoride. Up to now the impurities of the nickel will be the main limiting factor, which influences the power of detection for the elements to be determined [ 1541.

SYSTEMATIC ERRORS IN TRACE ANALYSIS

27

1.3.3 Separation

For chemical separation and pre-concentration, there are many useful principles available [ 155-1581 such as coprecipitation, electrolytical deposition, volatilization, liquid-liquid extraction, ion exchange, sorption, etc. [5, 127, 157-1611. The main aims of separation and pre-concentration are to reliably remove all interfering matrix compounds from the trace elements of interest and to concentrate the latter and their compounds to be determined within a possibly small volume or onto a possibly small target area and to submit them in this form to be determination procedure. Thus matrix effects are avoided, the sensitivity and the power of detection are increased and an optimal determination method can be selected. The requirements of a separation method which will be suitable for ultra trace analysis are very similar to those for sample preparation and decomposition. Therefore, they must not be repeated here. Additional demands are: (a) The yield must be 100%; (b) The procedure must be simple;

(c) Only small amounts of high-purity reagents should be required; (d) Quartz vessels or in the case of HF-containing solutions PTFE, PP or glassy carbon vessels must be used. Generally, separation should be performed in closed micro-systems using the on-line flow through or flow injection principle, as it has been recently demonstrated in the literature [144,162-1721. In extreme trace analysis these techniques must permit it to transfer dropwise the micro-volumes of solutions containing the preconcentrated elements onto a possibly small target area in order to drastically reduce systematic errors in the whole multi-stage procedure as well as to achieve very intense analytical signals in e.g. voltammetry, GDMS or TXRF. The working time can be reduced significantly by automation. For instance by combining HPLC or micro adsorption columns for separation and preconcentration with direct high pressure injection of the solution fractions into flames or HF-plasmas for atomic spectrometrical determination of the isolated elements to be determined (e.g. FAAS, ICP-AES, ICP-MS) [173, 1741 blanks and time may be furthermore reduced considerably. 1.3.3.1 Precipitation, co-precipitation, electrolysis In trace analysis the separation of the trace elements by precipitation is suitable only in a few cases because the formation of the precipitates very often is incomplete. If matrix components are separated as insoluble compounds [175], interesting trace elements may also be lost due to adsorption or co-precipitation.

G . TOLG AND P. TSCHOPEL

28

In this respect only a co-precipitation [ 176, 1771 by which the trace amounts are fixed in the precipitate of a small amount of a well-defined inorganic or organic compound as a result of co-crystallization, sorption or occlusion will not introduce severe systematic errors [ 155, 1571. Precipitation exchange [178] in *in layers of a insoluble compound such as ZnS should be briefly mentioned as a special separation technique which shows less errors than precipitation techniques. The sample solution is filtered through a membrane filter carrying a freshly prepared layer of about 300 pg ZnS. Trace elements (e.g. Ag, Cu, Pb, Bi, Cd, As, Sb, Sn, Se and Te) of which sulfides have a solubility product below the one of ZnS, will be exchanged very quickly against Zn during filtering and then can be reliably determined in the now very well defined new matrix. Electrolysis is a special case of precipitation by which special groups of elements can be separated from each other r160.179-1851. Trace elements with a concentration of less than a few pg/ml can only be deposited completely if either a Hg-cathode or a hydrodynamic flux system [160, 1791 is used. In the latter apparatus, made of PTFE, the sample solution is rapidly moved by a PTFE pump through a small graphite tube, which serves as cathode. The Pt/Ir-anode is situated along the axis of the cylinder. Up to -5 V can be applied without evolution of hydrogen, ng- and pg-amounts of elements such as Cu, Co, Fe, Zn and Bi, can be deposited inside the graphite tube and directly determined by e.g. ETAAS. Another innovative electrochemical preconcentration method [ 162, 186-1 891 should be mentioned here, because of very low blanks introduced by the method, because of the excellent separation yield and because of its on-line character (Fig. 111). The flow-through cell is made of PTFE and contains a small crushed vitreous 5

8

2

10 rnm

I

Figure 1-1 1. Scheme of a flow-through electrochemicalcell (after ([162]);(1) working electrode, (2) polyethylene frits, (3) graphite counter elecrode, (4) leads, (5) reference electrode, (6) body (plexiglas), (7)graphitecontact electrode, (8) sample solution.

SYSTEMATICERRORS IN TRACE ANALYSIS

29

carbon tube as cathode. Depending on the volume of the porous working electrode many elements behave according to the Nerstian law and display electrochemical yields of 100%. Also here the elements separated by electrolysis can be leached into a small volume of an acid or directly determined by ETAAS using the carbon tube as an atomizer. 1.3.3.2 Liquid-liquid extraction Extraction methods are available in a large variety. Different organic solvents and an enormous variety of inorganic and organic complexing reagents are at our disposal. The possibilities of separation procedures are extended by the variation of the pH and by using additional masking agents. For ultra trace determinations, the extraction methods can be optimized and systematic errors can be minimized by their miniaturization, by their adaptation to the single vessel principle 161 and by using quartz containers whenever possible. After an acid decomposition, e.g. the extraction step should be carried out directly in the quartz decomposition vessel which has a volume of a few ml instead of in a separation funnel. Then pipettes can be employed to separate the two liquid phases after centrifugation. 1.3.3.3 Separation by volatilization Due to the fact that every step of an analytical procedure may introduce systematic errors, such procedures which enclose two steps in one have enormous advantages. This is the case when volatile elements or compounds (see section 1.2.1) are released during decomposition or combustion, provided the formation of the volatile compound is quantitative [6, 11-13, 25, 1561. Such a procedure is possible, e.g. with the “Trace-0-Mat,” described in section 1.3.2.4. 1.3.3.4 Other separation procedures A wide variety of other separation and pre-concentration procedures are at our disposal. They meet to a different extent the requirements of ultra trace analysis of which a comprehensivereview is not possible in the frame of this chapter [103,1901. Many of the procedures are based on ion exchange, adsorption or sorption techniques using common ion exchangers, modified cellulose, charcoal or membranes. The latter can be impregnated, for example, with immobilized chelating agents or metal hydroxides [170, 191-1971. As all these techniques make use of large surfaces of the separation media, great care has to be taken in order to avoid losses of elements as a result of irreversible adsorption or contamination due to impure exchangers. 1.4 Basic rules for the recognition and elimination of systematic errors Especially because of the lack of standard materials at the extreme trace concentration level, there are no absolutely sure methods to recognize and avoid systematic errors. However, at least some basic rules can be summarized so as to minimize these problems [6-131:

G. TOLG AND P. TSCHOPEL

30

(a) The reproducibility of the results of an analytical procedure gives absolutely no information on its accuracy. A high standard deviation only might give a hint that systematic errors might be present. However, a low standard deviation does not at all prove that the results are reliable; (b) The accuracy of a result must be confirmed by at least one or two independent analytical procedures, which differ in all steps such as decomposition, separation, preconcentration and determination. A further way to confirm analytical results is to make use of interlaboratory comparison; (c) Storage of samples should be short as possible and should be performed at temperatures below - 15"C; (d) The number of manipulations and working steps has to be restricted to a minimum; (e) Monitoring of the different steps of a combined procedure can be well done with radioactive tracers; (f) Microchemical techniques, which use small apparatus and vessels should be

preferred. It would be the optimum when all steps of the procedure could be performed in one vessel ("single vessel principle"); (g) To avoid losses of elements by volatilization, closed systems should be used and the temperatures applied should be as low as possible; (h) To reduce blanks and losses of elements by adsorption, all apparatus, vessels and tools should be made of materials which are as pure and inert as possible. These requirements are met to a high degree by quartz and to a lesser extent by glassy carbon, PTFE and PP; (i) For the cleaning of apparatus and vessels, a treatment with vapours of nitric acid and water is most effective; (i) Reagents have to be very pure. If possible, liquids which can be purified by subboiling distillation such as acids or organic solvents are to be preferred; (k) Contamination by the dust of the air can be excluded to a high degree by using clean benches and clean rooms; (1) All kinds of generalizations and extrapolations are not allowed;

(m) Perseverance and self-criticism of the analyst are most important prerequisites for obtaining reliable analytical data.

SYSTEMATIC ERRORS IN TRACE ANALYSIS

31

Systematic errors also may occur during the classical or instrumental determination of the isolated elements. These are the last step of a multi-step procedure and cannot be considered here. Finally only one important source may be mentioned here and is due to calibration. The standard addition technique often seems to be favoured and to be very sure. However, one should be aware that the use of this technique requires careful critical treatment too, regarding e.g. nonlinear calibration functions, unknown background and blank levels and different binding forms of the element to be determined in samples and standard solutions.

1.5 Conclusion With this contribution only some of the problems and possibilities of elemental extreme trace analysis (near to the detection limits which can be reached today) could be reviewed. They cannot be generalized. Regarding the determination of higher trace levels the difficulties are decreasing and the extreme claims, which are demonstrated here, can be often reduced. But to do this, each analyst should be aware that no sources of systematic errors arise due to the simplifications he is performing. Also the role of the multi-stage procedures in modem elemental trace analysis treated here in some detail cannot be generalized. The importance of these procedures for analyses at the pg-concentration level has diminished in favour of direct instrumental methods. But up to now the multi-stage procedures are indispensable for the determination of ndg- and even more so for pdg-contents as long as standard reference materials at this concentration level are not available for the calibration of the more economical and comfortable instrumental methods. Therefore, further progress in sensitivity and reliability requires an enormous effort of the analysts. The careful elaboration of such procedures proves to be very time consuming and can only be performed by laboratory teams having extended knowledge and experience in this field. Furthermore, high instrument and operation costs are involved in the analytical strategy discussed. However, when considering the economical aspects we have to keep in mind that wrong analytical data, which result of poor knowledge and lack of analytical criticism are not only useless, but moreover, they often lead to wrong conclusions and to unforeseeable consequences. References 1 TOlg, G. and Garten, R.P.H., Angew. Chemie, 97 (1985) 439.

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CHAPTER 2

Limits of detection and accuracy in trace elements analysis

CLIFF J. KIRCHMER

Manager, Quality Asswatice Section, Washirigtoil State Departmetlt of Ecology, Munchester, WA 983534488. USA

Contents 2.1 2.2 2.3 2.4 2.5

2.6 2.7 2.8

Introduction.. . . . . . . , . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . 39 Errors in analytical results . . . . . . . . . . . . . . . . . . . , . . . . . . . . . . . . 40 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Measuring trace concentrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 The problem of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Random error of blank responses . . . . . . . . . . . . . . . . . . . . . . . 42 2.5.1 2.5.2 Errorsof thefirst kind-thecriticallevel (aposrerioridetection) . . . . . . . 43 2.5.3 Errors of the second kind - the limit of detection (a priori detection) . . . . . 46 2.5.4 Limits to the use of the definitions of LC and LD . . , . . . . . . . . . . . . 48 2.5.5 Regression theory approaches to the problem of detection . . . . . . . . . . 50 Practicalapplications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Reporting results at small concentrations . . . . . . . . . . . . . . . . . . . . . . . . 53 Conclusions and recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

2.1 Introduction A survey of the literature reveals that different meanings have been given to the expression “limit of detection.” However, some of the differences have been semantic rather than substantial. In order to avoid adding to this historical situation, it is appropriate to start by defining some relevant terms that will be used in this chapter to explain the concept of “detection” and its relation to the “accuracy” of measurement.

CLIFF J. KIRCHMER

40

The following definitions are those proposed by D.T.E. Hunt and A.L. Wilson [13. A determiitand is that which one determines, and is synonymous with the term

arialyte when referring to chemical analyses. The term arzalyticafniethod denotes a set of written instructions specifying an analytical procedure to be followed by an analyst in order to obtain a numerical estimate of the concentration of a determinand in each of one or more samples. The term urzulytical systeni denotes a combination of analyst, analytical method, equipment, reagents, standards, laboratory facilities, and any other components involved in carrying out an analytical procedure. The analytical system does not include the sample(s). The term analytical result denotes a numerical estimate of the concentration of a determinand in a sample, and is obtained by carrying out once the procedure specified in an analytical method. Note that a method may specify analysis of more than one portion of a sample in order to produce one analytical result. All analytical systems include a measurement sub-system (e.g. spectrophotometric measurement of the absorbance of a solution)whose function - on presentation to it of a portion of a sample or treated sample - is to produce a numerical observation whose magnitude is related to the amount or concentration of the determinand in that portion. This numerical observation is termed an analytical respowse. One or more analytical responses (as specified by a method) are used, in conjunction with a calibration curve or factor, to produce an analytical result. The phrase “to have detected the determinand” is understood as meaning “to have obtained experimental evidence that the determinand concentration is greater than zero.” 2.2 Errors in analytical results The absolute error, E of an analytical result, R has been defined as the difference between that result and the true value, T :

E=R-T This error may thus be positive or negative, and its units are the same as those used for T and R. Currie has stated that the basic question facing the analyst is: What is the relationship of R to T [21? The structure of the errors, which provides an answer to this question, is given by Eqn. (2.1).

R = T + A +a+h+f(t) where

A

= bias or constant portion of the systematic error

(2.1)

LIMITS OF DETECTION

41

6

= random error, that part of the variability which can be described by the laws

b

= an erratic blunder or mistake (e.g. in the recording of a final result)

of probability

f ( t ) = the “lack of control” term, that is a variation which depends on some external variable t [the notation .f(t) is not meant to imply an analytic relationship, but rather a “variation with”; time, for example, often plays the role of the surrogate for other causative factors.] Systematic error encompasses all contributions that are not “random,” and is therefore A + b + .f(t).Effective quality control should be able to eliminate blunders and nonrandom variation [b, f ( t ) ] ,in which case the systematic error is constant and equal to the bias, A . The chemical measurement process (CMP) is used to determine the result, R, which Currie described as follows:

E * R=:r+4 -1-6 11

Note that for the CMP the errors are completely characterized by constant (bias) plus random components. As indicated in Eqn. (2.2), the total error E of an individual result equals A + 5, and the limiting mean p of the measurement population equals T + A. If the bias A can be eliminated, then the population mean p equals the true value T.

2.3 Accuracy From the above discussion, it is clear that accuracy is the absence of the two components of error, one fixed (bias) and one random. Some authors have used the term accuracy to denote only the bias component. For example, the commonly used expression “accuracy and precision” implies that precision (i.e. random error) is separate from accuracy. However, for individual analytical results, accuracy is affected by both bias and random error. Only if results are calculated as the average of a large number of individual determinations would random error be negligible and bias be a measure of accuracy.

42

CLIFF J. KIRCHMER

2.4 Measuring trace concentrations With this background on the nature of errors, their effects on analytical results, and their relation to accuracy,the effects of eirors on measurements at low concentrations can be considered. Low is used here in a relative rather than absolute sense, since there are significant differences in sensitivity among methods. Errors in measurements at low concentrations are no different in principle from errors at higher concentration levels. However, at low concentrations, errors can also make it difficult to decide whether or not the determinand is present, which is usually not the case at higher concentrations. Random errors increase significantly as concentration is decreased. The effect of random errors on blank determinations, in particular, is an important consideration in the measurement of low concentrations. Concentrations can be considered low when they are comparable to the standard deviation of the blank.

2.5 The problem of detection 2.5.1 Random error of blarik responses

By definition, the blank is a sample with zero concentration of determinand. Heinrich Kaiser recognized the importance of the blank in determining the lowest concentration that can be distinguished from zero. He compared the problem of distinguishing a sample containing the determinand from a blank, with that of “searching for a ship in a stormy sea.” He asked “Is the higher prominence glimpsed really a ship or a larger wave than usual?’ The higher the waves, the more difficult it is to detect the presence of a ship. Conversely, it is easy to detect a ship when the sea is calm. Kaiser also argued that just as it is not the depth of the sea, but rather the height of the waves that determines whether we can see the ship, so “the cause of the uncertainty in the analytical value is not due to the size itself of the measure, but to the size of the fluctuations in it. A constant blank measure of whatever size can always be compensated’’ [3]. Others have also argued for an approach to the problem of detection based on the variability of blank responses. Possibly the first authors to introduce statistical concepts to the problem of determining the smallest concentration significantly different from zero were Sillars and Silver 141. Kaiser was apparently the first to use statistical concepts for a general definition of the limit of detection of spectrographic methods [5]. A later publication by Kaiser and Specker recommended the same approach for all analytical methods 161. A comprehensive review of the historical development of the concepts of detection is beyond the scope of this chapter. Reviews on the subject have been published by Kaiser 17, 81 and Currie [13]. A detailed mathematical treatment of the subject is given in a book by Liteanu and Rica [lo].

LIMITS OF DETECTION

43

There are two problems regarding detection decisions. The first is whether, after measurement, the result can be considered to indicate that the concentration of the sample is greater than zero. Currie has referred to this as a posteriori (i.e. after the fact) detection [9]. The second is what the concentration of the sample must be in order that the determinand will likely be detected. Currie referred to this as a priori (i.e. before the fact) detection [lo]. To understand these a priori and a posteriori decisions, it is necessary to understand the concepts of errors of the first and second kinds in statistics.

2.5.2 Errors of thefirst kind - the critical level (a posteriori detection) Kaiser also recognized that decisions regarding detection could only be solved through the use of statistics. Indeed, at low concentrations one may not be able to give a definitive answer to the question of whether or not the determinand is present. When performing trace analyses, an analytical result ( B )is obtained by subtracting the apparent concentration of the blank ( B ) from that of the sample (S) (i.e. R = S - B). If we compare individual sample and blank responses, it corresponds to the “paired comparison” situation. O’Haver stated that “In analytical procedures involving sample preparation, separation, or preconcetitratioti steps, it is almost always essential that a blank be carried through the entire procediire” [ 111. In fact, several blanks should be analyzed by the complete procedure in order to obtain the necessary degrees of freedom for a satisfactory estimate of within-batch variability. This needs to be emphasized because it is often ignored. In general, blanks and samples ntust be analyzed by the same procedure. Failure to do so can result in a biased blank correction. Of course, situations arise where it is impractical or not essential to analyze blanks and samples identically, but such situations generally require experimental confirmation” [ 121. In water analyses, the blank is pure water that is analyzed identically to samples. Reagents added during analysis may contain the determinand and give rise to an analytical response, or the response may arise from some indeterminate cause. Blank correction is needed so that the response present at zero concentration of determinand is not included in the result. In some procedures, blank determinations are made but their responses are instrumentally adjusted to zero, and sample responses are then read with respect to this adjusted zero. Provided that these blank determinations are made by exactly the same procedure as that used for samples, this is equivalent to the explicit subtraction of blank response from sample response described above [l]. However, O’Haver has emphasized that it is desireable that the blank signals be measured and recorded individually, rather than simply setting the instrument response to zero on the blank [ 1 11. In this way, excessively high blank values can

CLIFF J . KIRCHMER

44

be recognized. Currie has emphasized that insti.umenta1 zero adjustment, as for the null level or baseline, does not eliminate the need for this blank or baseline estimate error propagation [ 131. The limit of detection must refer to the entire analytical process. Instrumental detection limits may be useful in evaluating and comparing the detection capabilities of instruments, but will not be considered in this chapter, since they do not include effects of sample preparation and cleanup (e.g. digestion) on detection capabilities. Consider the case in which the sample has zero concentration of determinand, and has a response comparable to that of a blank. The analytical result is then equal to the difference between two blank responses, R = B, - B, and, assuming a normal distribution, the distribution of results corresponds to that represented in Fig. 2-1. It is clear that a claim of detection is not justified solely by a positive result since,

Critical Level = 2.330~ I (Shaded Area Denotes 5%

I x

-30:

-20'

-

0'

0

0'

DifferenceBetween Two Blanks

of All Results)

20'

30' c

Figure- 2-1. Illustration of the distribution of the differences between analvtical resuoiises for two blanks and the definition of the critical level -0-

when the sample does not contain the determinand, a positive result will be obtained 50% of the time. What must be the result to conclude that the sample concentration is greater than zero? It is apparent that there is no definitive answer to this question. Instead, the question must be stated in terms of probability, and specifically the probability of wrongly concluding that the determinand has been detected (i.e. the concentration is greater than zero) when in fact none is present. In statistics, this is refened to as an error of the first kind, and the probability of this error is equal to the portion of

LIMITS OF DETECTION

45

the total area under the distribution curve to the right of the analytical result chosen as corresponding to detection. Each point on the distribution curve corresponds to a probability (or percentage) point, with value of a (or 100 a). The acceptable probability of an error of the first kind should be selected according to the seriousness of the risk involved, which may involve a difficultjudgement call. Currie suggested a default value of 0.05 for a, corresponding to a 5% probability of a false positive, which is the risk level illustrated in Fig. 2-1. When n = 0.05, the result is equal to 1.645(&)oB or 2 . 3 3 0 ~(where oB is the within-batch standard deviation of the blank response) meaning that values greater than 2.330, are considered to indicate that the determinand has been detected, with 1 chance in 20 of being wrong. Note that this corresponds to a one-sided significance test. It assumes that oB is known (the degrees of freedom used to calculate standard deviation is large) and the blank responses are normally distributed. Figure 2.1 conveys visually what is meant by detection. In statistical terms, we are comparing a null hypothesis (Ho: The net sample concentration is equal to zero) with an alternative hypothesis ( H A : The net sample concentration is greater than zero). Hypothesis testing is equivalent to determining whether a sample result exceeds the positive upper 90% confidence limit for the expected results of a sample with zero concentration of determinand (i.e. a blank). Following the definition by Currie. this value of 2.33 oBis called the critical level. Three important characteristics of this definition are: (1) that a decision regarding a posteriori detection be based on experimental evidence of the magnitude of the variability of the blank response, (2) that blank correction of sample responses are used to obtain results, and (3) that one chooses the risk (5% for Currie’s definition) of concluding that the determinand has been detected when in fact the concentration of the determinand is zero. Due to the variability of the blank responses, a general conclusion of this analysis is that zero detection limits are unattainable. One may argue that for some analyses there is no variability of the blank responses, since the response of the blank is always zero. This is the case in which the discrinzination (defined as the smallest interval by which differences in analytical responses can be reliably distinguished) of an analytical system is coarse in relation to the size of the random errors of results. This does not mean that any concentration above zero can be detected or that the variability of the blank is not the key factor in determining the limit of detection, but rather that valid estimates of random error cannot be obtained if the discrimination is not sufficiently fine. Whenever the detection of very small concentrations is of interest, it is necessary to adjust the sensitivity so that the discrimination has negligible effect on the apparent size of random errors. Note that the decisioti regarding detection can be made based on a paired comparison of raw data responses for blank and sample. There is no need to convert response to concentration using a calibration curve or factor.

46

CLIFF J. KIRCHMER

2.5.3 El-i-0l-sof the second kind - the liniit of detection (a priori detection) To answer the question regarding the lowest concentration of sample that one can a yl-iol-ibe certain of detecting with an acceptable level of confidence, we need to consider errors of the second kind, i.e. false negatives, or falsely accepting the null hypothesis that the sample concentration is equal to zero. From Fig. 2- 1, it can be seen that the probability of a false negative is 50% if the concentration of the

F

Distribution of Difference Between

Mean, p = LD = Limit of Detection = 4.65 0 g Distribution of Difference Between a Sample and a Blank When the Sample Concentration is Equal lo the limit of Detectton

1p z

t

s

P

1

a

0

-b’

-20’

-0.

Figure 2-2. Illustration of the relation between the critical level and the limit of detection

sample is equal to the critical level. This means that the probability of detecting the determinand is also only 50%, equivalent to flipping a coin to decide if the determinand is present. Clearly, this is not an acceptable level of confidence for a priori detection. The acceptable probability should be selected according to the seriousness of the risk involved. Following the default definition by Cuirie, the probability of a false negative, 0, can be set at 5%. which, for normal symmetric distributions, makes the limit of detection L, equal to 4.65 uB,twice as large as the critical level [9]*The detection power is defined as 1-,B and coi-responds to the probability of detecting a sample of concentration L,, in this case 95%. The limit of detection and its relationship to the critical level is illustrated in Fig. 2-2. Table 2- 1 presents a Truth Table which describes the relationship between hypothesis testing ‘Currie used the expression “detection limit” rather than “limit of deteclion” for u priuri detection. These expressions are considered to be synonymous, but for uniformity “limit of detection” is preferentiallyused in this chapter.

LIMITS OF DETECTION

47

Table 2-1. Truth table - detection Possible true situations Analyte not

Analyte

present Ho: T=O

present HA: T=LD

Incorrect decision

Correct decision

False positive (Type I error)

1-P

Possible decisions Detected R >Lc

a:

Not detected R 5 LC

Correct decision 1-a

Incorrect decision False negative (Type I1 error)

P and component detection. Note that the probability of making the correct decision is 1 - a if the determinand is absent (H,,), and 1 - /3 if the determinand is present at a concentration equal to L, (HA), Finally, it should be emphasized that the limit of detection is basically a statement on the inherent detection capability, but is not used for making detection decisions. For detection decisions, the sample result is always compared directly with the critical level to decide if the determinand has been detected. This means that if the determinand concentration is greater than the critical level, it is reported as detected. But if the result of analysis is less than the critical level, it is reported as less than the limit of detection to take into account the possibility of an error of the second kind. For example, in the analysis of lead in water by ICP-AES, if the critical level, L,, is estimated to be 5 pg/l and the limit of detection, L, is estimated to be 10 p a , then a result of 7 p a would be reported as such (i.e. 7 p a ) . But if a result of 4 pg/l were obtained, it should be reported as less than 10 &I.This can be confusing if one is not familiar with the concept of errors of the second kind. It should be emphasized that other values for a and /3 could have been selected. Kaiser, for example chose a value of 0.0014 for both a and ,4 (rather than the 0.05 value chosen by Currie), corresponding to values of 3 oB and 6 mB, rather than the 2.33 m B and 4.65 bB values, for the critical level and limit of detection. He chose a value with a high degree of confidence for two reasons. First, it includes a reserve necessary to offset the uncertainty resulting from the practical determination

CLIFF J. KIRCHMER

48

of the standard deviation. Second, it provides some allowance in case the frequency distribution of measures departs from the normal [3]. Terms other than critical level and limit of detection have also been used to describe the same concepts. For example, Kaiser used the German expression “nachweisgrenze” (which has been translated as “limit of detection”) to include only a consideration of errors of the first kind (corresponding to Currie’s critical level) and the expression “garantiegrenze” (which has been translated as “limit of guarantee of purity”) to include consideration of both errors of the first and second kinds (coiresponding to Cunie’s detection limit). Roos [ 141 and later Wilson [ 151 used the expression criterion of detection with the same meaning as Cuirie’s critical level. Keith has recently proposed that the expression method detection limit (MDL) should be used when only errors of the 1st kind are considered, and the expression reliable detection limit be used when both errors of the 1st and 2nd kinds are considered [16]. It is important not to attach much importance to the different names used to describe detection. What is important is to know the probabilities for errors of the 1st kind (a)and 2nd kind (p). However, standardized nomenclature is crucial for facilitating communication among the international scientific community. Some authors have not recognized the existence of errors of the second kind in defining the limit of detection [17-181. This is basically a failure to distinguish between detection decisions and detection capabilities, as Currie has pointed out [ 131. False negatives occur whether their existerrce is recognized or not.

2 5 . 4 Limits to the use of the definitions of L, atid L, The definitions for L, and LD presented in Sections 2.5.2 and 2.5.3 are based on “paired comparisons,” in which each sample response is considered to be individually blank corrected. Currie has also described the case of a “well known blank,” in which the blank response has been estimated from a large number of observations and therefore its variability in calculating a sample result is negligible [9]. The variability of the difference between sample and blank response is then reduced by a factor of fi,giving values of 1.64 gB and 3.29 uB,for L, and L,, respectively. Hunt and Wilson [ 11have presented a general expression for L, based on n replicate blank responses and m replicate sample responses, in which the standard deviation of the difference between sample and blank responses ( o ~ -is~given ) by Eqn. (2.3): Cs-B = gB[(l/nz) -k (1/?%)]‘

(2-3)

This equation is valid only for very low concentrations, where standard deviation is independent of concentration. The equations derived for Lc and LD assumed that the population within-batch standard deviation of the blank is known and that standard deviation is independent of concentration (up to L,). However, in practice, only estimates of the standard

49

LIMITS OF DETECTION

deviation will be available. Currie has emphasized that the critical level, used to make detection decisions, can be estimated directly from the empirical estimate of sBand Students t as L, = ts(a = 0.05). The limit of detection, however, must include a confidence interval for the uncertainty in the estimate of sBand Cui-rie states that this can be derived from the x2 distribution as L, = 2 t s ( ( ~ ~ / sValues )~. fort and aUL (UL is the upper limit of 0)are given in Table 2-2 for a selected number of replicates. (Note that this expression represents a conservative upper limit for LD).

The theoretical definitions of critical level and limit of detection have been based on the following assumptions, which may not always hold: 1. The within-batch standard deviations of both the blank and samples containing

very small concentrations of the determinand are the same; 2. The expected value of the analytical response is not zero for finite concentrations of the determinand; 3. The sample and blank are not biased with respect to each other (that is, there are not interfering substances in the sample or the blank); 4. The blank (i.e. pure water in the case of water analyses) does not contain the determinand. If the blank does contain the determinand, then a special analysis must be done to determine the concentration in the blank, which must be included in the calculations of L, and L,.

If any one of the above assumptions is not true, then the critical level and limit of detection cannot be calculated using the equations given previously. However, adjustments can sometimes be made to the equations. Currie has presented an analysis which allows for corrections when assumptions (1) and (3) above are not met [2]. For example, as illustrated in Fig. 2-3, adjustments can be made to allow for differences in the standard deviation for blank and sample responses ( ( T ~ # ( and T~) for different values for errors of the 1st and 2nd kinds (i.e. (Y and /3 values). Table 2-2. LD estimation by replication: Student’s t and ( a / s ) - bounds vs. number of observations [ 131 ~~~~~

No. of replicates

~

5

10

13

20

120

00

Students’s t:

2.13

1.83

1.78

1.73

1.66

1.645

auL/s(a)

2.37

1.65

1.51

1.37

1.12

1.000

(a)

u n / s is at the 95th percentile (i.e.

1-sided 95% confidence interval)

50

CLIFF J . KIRCHMER

Figure 2-3. Illustration of the case in which the standard deviation for sample and blank responses differ and in which the values chosen for errors of the first kind (a)and the second kind (p) also differ. (Adapted with permission from Ref. 2. Copyright 1978 Wiley).

When systematic error cannot be assumed negligible, the critical level must be increased by amount A, and the limit of detection must be increased by an amount 2 Am, where An, is the assumed upper bound for the bias, i.e. L,, = L, + A ,,, and L,, = 2L,,

2 5.5 Regression theoiy approaches to the yrobleni of detection Several different approaches have been taken to account for the effect of uncertainty in the calibration curve or factor on the critical level and limit of detection. These approaches involve the application of regression theory to the responses from a set of calibration standards, to calculate values for the intercept, i, the slope, m, and the within-batch standard deviation of the blank, sB [19-221. The simplest approach is to estimate the uncertainty in the slope of the calibration curve, m. A graphical approach has been described by Long and Winefordner [21]. The uncertainty in m can be expressed as a confidence interval rn f tsm, where s, is the standard deviation of the slope and t is a t distribution value chosen for the desired confidence level, a, and the degrees of freedom, d f . The effect of the confidence interval can best be seen by referring to Fig. 2-4, which shows the graphical relationshipbetween the blank corrected analyticalresponse and concentration. To convert the estimate of the critical level, L,, in response units to concentration units, a calibration curve is needed. Figure 2-4 shows that uncertainty in the slope of the calibration curve leads to uncertainty in the estimate of the critical level in concentration units. If the uncertainty in the slope of the calibration curve

51

LIMITS OF DETECTION

Concentration

Upper estimate of LD (in concentration untis)

Figure 2-4. Illustration of the fact that uncertainty in the slope of the calibration curve leads to uncertainty in the estimate of the critical level, Lc. ( B is the mean blank response, A. is a constant, and SB is an estimate of the within-batchstandard deviation of the blank.)

is sufficiently large, the lower bound for m can be used to calculate and report an upper estimate of L, (or L,) in concentration units. Other approaches which are intended to take into account the uncertainty in the calibration curve or factor have been proposed. A “Propagation of Errors Approach,” which takes into account the error in the intercept, i, as well as the slope has been described by Long and Winefordner [21]. While there are instances when errors due to uncertainty in the calibration curve should be included in the estimate of the critical level and limit of detection, there are some problems with the regression theory approach as well. First, the estimates of the y-intercept and the standard deviation of the blank responses, which refer to zero concentration, are derived from calculations using some or all of the calibration standards, whose concentrations are greater than zero. Thus, the accuracy of these estimates depends on the accuracy with which the true relation between response and the determinand concentration is known, and on the accuracy with which the true relation between the standard deviations of responses and concentration is known [13. Second, the assumption in this approach is that the uncertainty in the calibration curve contributes to the uncertainty in determining whether or not the determinand

52

CLIFF J. KIRCHMER

has been detected. However, if the relation between response and concentration can be considered linear, the decision regarding detection can be made independently of the calibration curve, based on a comparison between the critical level and the difference between the sample response and the blank response, before the calibration curve is used to convert a result into concentration units. Of course, the calibration relation is necessary to estimate the limit of detection, which must be expressed in concentration units. The slope of the calibration curve does not affect the detection decision, only the accuracy of the concentration reported. Indeed, O’Haver [l 11 and others have argued that, because random error is so large, a concentration should not be reported at or near the critical level or the limit of detection, and that detection should just be a qualitative yes or no decision. However, this would result in loss of information, since a detection decision can always be made and the estimated concentration and its uncertainty can also be stated. The third problem with the emphasis on the uncertainty in calibration curves as being the determining factor in detection decisions is that, as R.V. Cheeseman and A.L. Wilson have demonstrated, the sample response, and not the calibration curve, is usually the primary source of random error in determining the result of analysis [ 121. This is because several standards can be used to reduce the uncertainty in the slope of the calibration curve to an acceptable level, while sample results are usually only based on a single analysis. In summary, the approach based on a comparison of sample and blank responses is more directly related to the problem of detection decisioiis than the approach based on regression theory, h d to require fewer assumptions for its validity, since the standard deviation used in the calculation of L, is estimated only from blanks.

2.6 Practical applications There have, unfortunately, been relatively few instances in which the theoiy of limit of detection based on the variability of blank responses has been applied in practice. Some methods have even been written to prohibit blank correction of sample responses, which of course makes a comparison of sample results to the critical level meaningless for deciding on whether the determinand has been detected. However, in the United Kingdom, several of the “Methods for the Examination of Waters and Associated Materials” prepared by the Standing Committee of Analysts of the Department of the Environment, have been evaluated by individual laboratories to determine the limit of detection based on the variability of the blank and using “paired comparisons” for blank correction. Published values for the limit of detection (a = /3 = 0.05) for several of these methods are listed in Table 2-3. The Standing Committee of Analysts has adopted a policy of including an estimate of the limit of detection (or the within-batch standard deviation of the blank, which is

LIMITS OF DETECTION

53

Table 2-3. Estimated limits of detection taken from “Methods for the Examination of Waters and Associated Materials” [22-261 Element

Estimated limit of detectioda)

Copper

Degrees of freedom

used in estimate(s) 5

Chromium

7-9

Phosphorus

35

Silicon

10

Aluminum

10

(’)

Note: Range for some elements is due to results in different laboratories

used to calculate the limit of detection) as one of the “Performance Characteristics of the Method” [23-271.

2.7 Reporting results at small concentrations Parallel to the problem of making decisions regarding a yriori or u posteriori detection, there is the problem of how to report results at small concentrations. An indication of this problem was given in Section 2.5.3, where it was concluded that results above the critical level should be reported as such, while results below the critical level should be reported as less than the limit of detection. Hunt and Wilson [ 11have suggested that all results at low concentrations simply be reported as the result plus or minus the confidence limits at a stated level of confidence (R f t,s), in order to avoid bias and information loss, especially when averages of low concentration results are calculated. Implementation of this recommendation would completely eliminate the use of the critical level and the limit of detection in reporting results. However, they could be provided separately as reference information for the user to aid in the interpretation of the reported results and associated confidence limits. In this regard, it should be remembered that negative results are possible, as is clear from Fig. 2-1. Sample and blank responses should always be greater than or equal to zero, but results, which are calculated as the difference between the sample and blank responses, can be negative. Indeed, for a sample of zero concentration of determinand, the results would be expected to be negative approximately 50% of the time. Investigators in water quality monitoring also have proposed that the limit of detection or the limit of quantitation, L,, not be used to censor data [28]. Like Hunt and Wilson, they believe that L, and L, aid in the interpretation of individual

CLIFF J. KIRCHMER

54

measurements, but they hinder statistical analysis of water quality data. Instead, one should report the results of all analyses plus an estimate of observation error. The concept of “not detected” is not altered because a confidence interval that overlaps zero is a valid statistical definition of “not detected.” lhere are also some who have proposed that numerical values not be reported at or near the limit of detection, since random error, as measured by the 95% confidence limits on a result whose concentration is equal to the limit of detection, is equal to approximately 60% at the 95% confidence level. With this level of eiror, one can argue that it is only necessary to report qualitative detecthion-detect decisions at or near the limit of detection. O’Haver has stated that “It should also be kept in mind that a concentration at the detection limit can only be detected, as the term “detection limit” implies and is not measured quantitatively” [ l l ] . Currie has defined a “determination limit” (other authors have referred to this as the limit of quantitation), at which a given procedure will be sufficiently precise to yield a satisfactory quantitative estimate [9]. He defined the determination limit as that concentration at which the relative standard deviation of measurement is 10%. Assuming that the standard deviation at this concentration is the same as the within-batch standard deviation of the blank, the determination limit, LQ,is equal to 14.1 uB in the case of paired observations and 10 gB in the case of a “well-known” blank. Thus, according to Cui-rie, the “working”expressions for L,, L,, and LQ can be summarized in Table 2-4, and Fig. 2-5 illustrates the three principal analytical regions [9]. An assumption is that the method of analysis is sufficiently precise so that relative standard deviations of 10% are achievable at the concentrations indicated. This is true for most spectroscopic methods of analysis, but for some analyses, a precision of 10%RSD may not be achievable at any concentration. Table 2-4. “Working” expressions for Lc, LD, L Q ( ~ ) LC

Paired observations

2.33 U B

4.65 CTB

14.1 U B

“Well-known”blank

1.64 U B

3.29 U B

10

UB

One can also describe an external or required limit of detection, L,, which determines the selection of the method to be used. This required limit of detection can be based on a regulatory requirement. Currie described L, as a regulatory limit, coiresponding to the external limit which drives the design of our measurement process [ 131. Wilson has recommended as a rule of thumb that the required limit of detection be at least lox less than the regulatory standard or criteria to be enforced. This is because, as explained previously, the random error at the limit of detection

55

LIMITS OF DETECTION

REQlON 111 D E T E R W I N A T I ONt UNRELIABLE DETECTION

DETECTION: OUALITATIVE ANALIBIS

a U A N T I T A T I V E ANALYSIS

I

I

I I I I

2.330~ 4

:

h'

I 1 1 1 0

Blank-Correcled

8 I

Response

I

:4.660~ 4

I

I

(Paired comparisons of sample and blank responses)

b

I I

4

I I

14.1

uB

I

, I

I

Lo 0

b

Figure 2-5. Illustrationof the three principal analytical regions (assumptionsare that the error of the first kind (a)and the error of the second kind (p) are both equal to 0.05 and that the standard deviation of results is independentof concentration in the range 0 - LQ

is very large relative to the concentration measured and, if the limit of detection is too close to the regulatory standard or criteria to be enforced, the results will not be suitable for making decisions as to whether a regulatory limit has been met. It is worthwhile noting that the critical level and limit of detection can be lowered by calculating results based on the analysis of more than one portion of a sample, since the standard error of the mean is equal to s/,/Z where n is the number of measurements used to calculate a result. In the absence of bias, therefore, repeated analyses of portions of a sample can be an effective way to lower the detection capability. The International Atomic Energy Agency recommendation is to make detection decisions by comparing the averages of at least three or four (but preferably nine or more) paired comparisons of sample and blank responses [29]. Repeated analyses can also make the assumption of a normal distributionof results more likely, because the Central Limit .Theorem states that averages derived from a sequence of mutually independent random variables having a common distribution tend toward normality, often rather quickly (by the time la = 3 or 4).

CLIFF J. KIRCHMER

56

2.8 Conclusions and recommendations

Decisions regarding detection must start with a recognition of the importance of the blank. The within-batch variability of the blank, as measured by standard deviation, is the most important factor in developing a theory of detection. It is important that blank responses be subtracted from sample responses before deciding whether sample results indicate that the determinand has been detected. Both errors of the first kind (false positives) and errors of the second kind (false negatives) must be taken into account when defining terms related to detection. For simple detection based on paired comparisons of sample and blank responses and an assumption of a normal, symmetrical distribution of within-batch blank responses and standard deviations of low level samples equal to those of the blank, Cui-rie defined the critical level as equal to 2.33 gB,correspondingto a 5% probability of false positives [9]. Decisions regarding detection are always made by comparing net (i.e. blankcorrected) sample response with the critical level. Similarly, Cui-rie defined the limit of detection as equal to 4.65 uB.coi-responding to 5% probability of false negatives as well as 5% probability of false positives 191. There is a 95% probability that the analysis of a sample with a concentration equal to the limit of detection will have a net sample response greater than the critical level. Different values for ei-rors of the first and second kinds can be selected. For example, 1% probabilities for both of these types of errors were recommended by Kaiser 131. Adjustments can be made to this basic theory to account for bias due to interference or calibration, random error in calibration, and increases in random error with concentration. When reporting data, particularly monitoring data, the critical level, limit of detection, or limit of quantitation should not be used to censor data. To avoid information loss and biased calculations of mean sample concentrations, all data should be reported, together with an estimate of the uncertainty in the results. The critical level should be provided separately as an aid in interpreting the reported results. References 1

Hunt, D.T.E.and Wilson, A.L.. The Chemical Analysis of Water. 2d ed.. The Royal Society of Chemistry,London, 1986.

2

Currie,L.A., in: TreatiseonAnalylicalChemislry,I.M. KolthoffandP.J. Elving, (Eds.), Vol. 1, 2nd ed., Wiley, New York, 1978, pp. 95-242.

3

Kaiser, H., lbo Papers on the Limit of Detection of a Complete Analytical Procedurc, A.C. Menzies, ed. Adam Hilger, Lld., London. (confains translations of Refs. 7.8).

4

Sillars, I.M. and Silver, R.S.,J. Soc. Chem. Ind., 63 (1944) 177.

5

Kaiser, H., Spectrochim. Acta. 3 (1947) 40.

6

Kaiser. H. and Specker, H., Z. Anal. Chem., 149 (1956) 46.

LIMITS OF DETECTION 7

Kaiser, H., Z. Anal. Chem., 209 (1965) 1.

8

Kaiser, H.. Z. Anal. Chem.. 216 (1966) 80.

9

Currie, L.A.. Anal. Chcm., 40 (1968) 586-593.

57

10 Liteanu, C. and Rica. I., StatisticalThcory and Mclhodologyof Trace Analysis.Ellis Horwood, Ltd., Chichester. 1980. 11 O’Haver. T.C., in: Trace Analysis - Spectroscopic Methods for Elements. J.D. Winefordner (Ed.), Wiley, New York, 1976, pp. 15-62. 12 Cheeseman, R.F. and Wilson, A.L., Manual on Analytical Quality Control for the Water Industry, Technical Report TR66. Water Research Centre. England, 1978. 13 Currie, L.A.. Chapter 1 in: Detection in Analytical Chemistry - Importance, Theory and Practice, ACS Symposium Series 361. L.A. Currie (Ed.). American Chemical Society,Washington, D.C., 1988,pp. 1-62. 14 Roos, J.B.. Analyst, 87 (1962) 832. 15

Wilson, A.L., Talanta, 20 (1973) 725-732.

16 .Keith. L.H.. Environmental Sampling and Analysis: A Practical Guide, Michigan, Lewis Publishers. Inc., 1991. 17

Glaser. J.A., Focrst, D.L., McKee. G.D.. Quave. S.A.. and Budde. W.L.. Env. Sci. & Tcchnol.. 15 (1981) 1426-1435.

18 “Nomenclature. symbols, units and their usage in spcclrochemical analysis - 11” International Union of Pure and Applied Chemistry. Analytical Chemistry Division, Commission on Spectrochemical and Other Optical Procedures for Analysis. Spectrochim. Acta B.. 33B (1978) 242. 19

Hubaux. A. and Vos, G.. Anal. Chem., 42 (1970) 849-855.

20

Gibbons. R.D., Jarke, EH., and Stoub. K.P., in: Waste Testing and Quality Assurance. D. Friedman (Ed.). Vol. 3, ASTM STP 1075, Philadelphia, 1991, pp. 377-390.

21

Long. G.L. and Winefordner, J.D.. Anal. Chem.. 55 (1983) 712A-724A.

22

Currie, L.A., Chapter 5 in: Trace Rcsidue Analysis - Chemometric Estimations of Sampling, Amount, and Error. ACS Symposium Series 284: D.A. Kurtz (Ed.). American Chemical Society. 1985. pp. 49-81.

23

Standing Committee of Analysts, Copper in Potable Waters by Atomic Absorption Spectrophotometry. 1980, Her Majesty’s Stationery Office, London. 1981.

24

Idem. Chromium in Raw and Polable Waters and Sewage Effluents, 1980, H.M.S.O.. London. 1981.

25

Idem, Phosphorus in Waters. Effluents and Sewages, 1980, H.M.S.O., London, 1981.

26

Idem. Silicon in Waters and EMuents, 1980, H.M.S.O., London, 1981.

27

Idem, Acid-Soluble Aluminum in Raw and Potable Waters by Spectrophotometry, 1979,

H.M.S.O.. London, 1980. 28

Porter, R.S.. Ward, R.C., and Bell, H.F., Env. Sci. & Techno]., 22 (1988) 856-861.

29

Currie, L.A. and Pam, R.M., Chapter 9 in: Deteclion in Analylical Chcrnislry - Imporlance, Theory. and Practice, ACS Symposiuin series 361. L.A. Curric (Ed.). American Chemical Socicty, Washington. D.C., 1988, pp. 171-193.

This Page Intentionally Left Blank

CHAPTER 3

Sampling and sample preparation

J.R.W. WOI?TEZ and J.E. SLOOF

Iitterfaculty Reactor Institute, Delft University of Techiiology,Mekelweg 15, 2629 JB Delft, The Netherlarids

Contents 3.1 3.2

3.3 3.4

3.5

Introduction.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Changes in trace element composition . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Element specific changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Sample specific changes . . . . . . . . . . . . . . . . . . . . . . . . . . . Pre-sampling considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Aspects of sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Establishment of analytical control . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Sampling error in a test portion . . . . . . . . . . . . . . . . . . . . . . . . 3.4.3 Uniformity of laboratory samples . . . . . . . . . . . . . . . . . . . . . . . 3.4.4 Uniformity of subsamples . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.5 Thegrosssample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

59 62 63 76 77

83 89 89 92 93 93 94

3.1 Introduction “Unless the complete history of any sample is known with certainty, the analyst is well advised not to spend his time in analyzing it.” This statement was made over 35 years ago [ 1301 and since then quoted by analysts who are concerned about the quality of a sample [40,137]. Nearly 20 years after Thiers made his remarks, a paper on sampling for chemical analysis concludes: “It is the capability of understanding and executing all phases of analysis that ultimately characterizes the true analytical chemist, even though he or she may possess special expertise in a particular separation or measurement technique” [72]. At present, this statement is still a basic principle for the trace element analyst, since the evolution in detection capability combined with the growing awareness of the effects of trace levels of pollutants

J.R.W. WOI’ITIEZ AND J.E.SLOOF

GO

on a national or global scale, asks for more analyses of more elements on a lower concentration level. Also, the questions asked to the analyst become more and more complicated. Interdisciplinary research in the medical, biochemical, environmental, geological or industrial field no longer demands data on just trace element contents, but needs information on interaction with other components, availability, transport with accessory kinetics and distinction between possibly occurring chemical forms. Unfortunately, some of the reactions of trace element analytical chemistry to this development (e.g. automation, computerization, modelling, lab-accreditation) tend to take away the view of the analyst from understanding and executing all phases of analysis and force him or her to “desk-top analytical chemistry.” It is the explicit task of the analyst to provide an interdisciplinary project with accurate, quantitative data. This starts with awareness of the possibility of appropriate sampling and sample preparation. Sampling and sample preparation strategies which parantee the chemical integrity of a trace element compound are extremely difficult and sometimes not (yet) possible to design. Especially when multi-element analysis in different but interlinked parts of an environmental or biological system is required, the knowledge of adequate sampling is mandatory. This statement, how superfluous it may sound, may be well illustrated by considering the complexity of biochemical and physical processes in the ocean and chemical forms of trace elements involved [109]. They are enumerated in Table 3-1. It is clear that simply analyzing seawater for its total elemental content is only partially meaningful in such complicated systems. Often, demands set by appropriate sampling and sample preparation directly contradict the demand of chemical integrity of a trace element compound. This may be illustrated by a practical example, concerning multi-trace element analysis, among which different inorganic iodine compounds, in seawater [152]. Here, preservation of the sample asks for acidification so as to prevent absorption of trace elements to the container wall. Addition of H+ however definitely disturbs the equilibrium between IO;, I- and I, [79], which is given by

51-

+ 10; + 3H,O

;t

31,

+ 60H-, K

= 1.0 x

The solution for this type of dilemma is to acknowledge that there is not just one sampling strategy for both analytical problems. Recognition, understanding and control of parameters which threaten good sampling, of course, is not beyond reach. Expert laboratories have demonstrated that high quality sampling and sample preparation is possible. Manufacturers of environmental and biological certified and standard reference materials (CRM’s and SRM’s) spend years of research on such items as homogeneity, preventing contamination, minimal sample size and tenability of their products. The attention which is paid to the understanding of all phases of the analysis here, is visualized by the contents of the International Symposium on Biological and Environmental Reference Materials (BERM), which has been held every two years since 1984 [e.g. 9, 101.

SAMPLING AND SAMPLE PREPARATION

61

Table 3-1. Physicocheniical fomis of trace elementsand major biogeochemical and physical processes in the ocean [ 1091

Physicocheniicalfornts of trace elentents in natural waters Tme solution (< 0.001 pm):

Collodial(O.OOl-O.1 pm):

Particulate (0.1-50 pm):

Simply hydrated ions Inorganic complex ions Organic chelates Molecules and polymeric species Ion pairs Mineral substances Hydrolysis and precipitation products Biopolymers and detritus Organic particles: Plankton Detritus Bacteria and microorganisms Inorganic particles: Mineral substances Precipitated and coagulated particles

Major biogeochemical and physical processes in the ocean Physical processes:

Influx by rivers and ice Transport by wind Influx by dissolved gases Wave action on sediments Transport by animals Efflux by animals and man Diffusion Turbulent mixing Volcanic action Upwelling of deep seawater Sedimentation

Chemical processes:

Chemical precipitation Sorption by sediment surface Redissolving from sediment

Biological processes:

Decompositionand respiration by marine animals Photosynthesis Sedimentation and decomposition by bacteria

Combined processes:

Halmirolysis Dissolution Scavenging

62

J.R.W. WOITTIEZ AND J.E. SLOOF

The items sampling, sample preparation, stability, quality control, application of appropriate trace element analytical techniques and statistical evaluation techniques are all addressed. One project deserves special mentioning in this context. The US EPA/NBS National Environmental Specimen Bank was founded in 1973 [98] with, among other things, the aims to “develop and evaluate protocols for contamination-free sampling of environmental specimens” and “develop and evaluate storage conditions which would permit the long-term storage of samples without change in pollutant concentrations” [99]. In this project, expert analysts protocolize, verify and control their trace element analytical techniques to a level that inadequacy in sampling and sample preparation can be detected. Subsequently, sampling and sample preparation protocols are improved until adequacy can be proven. Since long-term storage of samples and trend monitoring are prime goals of the project, this level of understanding of all phases of analytical chemistry has to be maintained during years [ 1611. In the specimen bank project trace element analytical chemistry is well incorporated in interdisciplinary research. In this chapter, theoretical and practical aspects of sampling and sample preparation, mainly for the biomedical and environmental field, are discussed. This includes causes (and precautions) of changes in trace elemental composition due to sample preparation (contamination, losses, inhomogeneity), sample pre-treatment (washing, drying, etc.) and sample decomposition (wet and dry ashing). Although preconcentration of trace elements may be considered as a part of sample preparation, it is treated separately in Chapter 4 of this book. Most of the knowledge on sampling and sample preparation presented here comes from research from a selected number of analytical laboratories and analysts. Without pretending to be even close to completeness, some names should be mentioned: Pioneer work in the biomedical field has been done by Versieck and Cornelis, Heydorn and Iyengar. Their work has been documented in a series of monographs and topical papers [39,41,51,61,139]. In the environmental field, the analytical group of NIST (formerly NBS), the US and German Specimen Bank Projects, the Max-Planck Institute (FRG) and those cooperating with reference material manufacturers (BCR, Belgium; NIST, USA; NRCC, Canada) have provided expert work. The importance of sampling and sample preparation of food items has been documented by the US Food and Drug Administration [46]. Important individual contributions have been made by Thiers in 1957 [130] and Zief and Mitchell in 1976 [162]. Sampling and sample preparation, focused on isotopic and radionuclide analysis, has been covered by the laboratories connected to the IAEA and Euratom [48].

3.2 Changes in trace element composition The very process of sampling and sample preparation bares the risk to change the trace element composition of the sample, resulting in systematic errors. The cause

SAMPLING AND SAMPLE PREPARATION

63

can be a trace element specific change, such as contamination or losses and a sample specific change, such as segregation or change of moisture content. Since a sample inevitably contacts some container material and remains in the container for some time, all the processes are likely to happen during sampling and sample preparation. For the preparation of a uniform sample, suitable for elemental analysis, a sequence of actions with appropriate tools is obligatory (washing, crushing, milling, etc.) and thus contamination and losses again are bound to happen. In this paragraph some of the main causes of changes in trace element composition are discussed.

3.2.1 Elenierit specijic churiges 3.2.1.1 Contamination Contamination may arise from various distinctive sources. Air particulates, the analytical laboratory and the analyst, chemicals and equipment can be distinguished. Air-particulate matter-always plays a role during sampling outside the laboratory, unless the environment at the sampling spot is conditioned. Air particulates may carry major components of the earth’s crust (Si, Al, Fe, Ca, Mg, Mn, Ba, Sr) and the presence of seawater will be reflected in an aerosol contribution (Na, C1, Br, I). Heavy industrialization (V, Sn, Cd, Ni, Cr, Ag) and traffic (Pb, Pt) may also contribute. When deposited on a sample, elemental contamination will be a fact. The problem is manifest for the sampling of atmospheric deposition. Bulk precipitation consists of dry and wet deposition. Particulate matter may be separated by filtration, but leaching of trace elements into the solution cannot be prevented this way. Wet deposition can be obtained separately by so-called “wet-only samplers,” where the polymer sampling bucket is covered with a polymer cap that only opens duringrainfall[66]. Air particulate matter can be sampled separately by air filtration using impactors or filterpacks [25]. Similar problems may occur for sampling snow, hail, dew or fog. In biomonitoring, the dilemma whether to consider air-particulate matter and soil dust as an inherent part of the sample or not, always exists. An often used approach is applying some washing procedure, such as developed for human hair [l lo]. An obvious disadvantage of washing is the risk of both leaching of trace elements from the sample and contaminating it with the chemicals used for washing. In case of moss and lichens, often used for biomonitoring purposes, the biomonitor function is based on accumulation of trace elements into the cryptogams rather than on filtration of dust particles. Since this accumulation is more likely to occur from the aqueous or gaseous phase than directly from a solid phase (air dust), the washing step seems justified form a sampling point of view. Much attention has been paid to prevent contamination with air particulate matter in the laboratory. Several sources, apart from air from the outside world, have been identified: Particulates from the laboratory air supply (filters, vents), a concrete

J.R.W. WOITI'IEZ AND J.E. SLOOF

64

or painted wall, heating, furniture, ceiling, doors, sinks, etc. Also, the analyst is a source of air particulates through its clothes, shoes, hair, cosmetics, etc. The need for a clean laboratory environment has long been recognized. Key publications on the topic have been produced by Zief and Mitchell in 1976 [162], who systematically deal with all aspects of contamination control, and Moody in 1982 [92], who described the NBS Clean Laboratories. More recent communication in 1988, 1990 and 1991 [ 12,21,27,85] illustrate that the clean room concept, developed throughout the seventies and early eighties, is still state of the art. A clean environment can be classified according to the maximum number of particulates of a certain size per ft3, class 100 being the most stringent. Class 100 environments should be free of particles larger than 5 pm, not contain more than 10 particles larger than 2 pm per fe and not more than 100 particles larger than 0.5 pm per ft3 [92]. For a class 1000 environment these numbers are 10, 100 and 1000 respectively. This division in classes not only refers to particle sizes, but intrinsically also to the total mass of airborn particulates. Class 1000 environments are acceptable for non-laminar-flow clean labs, while class 100 is the requirement for laminar flow environments. It should be noted, that not only the number of particles is important with respect to contamination, but the composition is just as vital. This may be illustrated by Fig. 3- 1, on the relative size of common air contaminants [96]. The basic principles of the air-household for a clean laboratory are four-fold: (1) The inner laboratory

10 :

8 % E v)

F

5

Qm

v)

c

2

v)

0.01

-

I

I

I

I

a' I

I

Data from Murphy, 1 9 7 6

Figure 3-1. Particle size distribution of some common air contaminants

I

I

I

SAMPLING AND SAMPLE PREPARATION

65

atmosphere is in overpressure compared to the outer environment. This usually demands an air supply system which is independent of the main laboratory air supply and may thus be costly. (2) The air which enters the laboratory has to be purified from airparticulates. To this aim, the high efficiencyparticulate air filter (HEPA) can be used, which has a minimal scavenging capacity of 99.99% for 0.3 pm particles. (3) In order to minimize air turbulence within the laboratory, vertical laminar-flow air inlet is the best solution. (4) The total laboratory volume of air should be changed sufficiently frequently so as to allow the removal of particles generated in the lab. Again, since a frequency of several times per minute will be needed, the principle asks for a separate, costly air supply system. The NBS Clean Laboratories described by Moody [92] meet these criteria. Many laboratories, however, compromise on the laminar laboratory air flow. Instead, a class > 1000 non-laminar laboratory environment is created, in which class 100 laminar flow clean boxes are situated. Open sample preparation work is performed in these boxes. This situation has been described for the Max-Planck Institute Clean Laboratory by Tschopel et al. [ 1321. In their recent book on quantitative trace analysis of biological materials, McKenzie and Smythe [85] warn against too high an expectation of class 100 clean laboratories. They state: “It in no way overcomes contamination by gasses, poor quality reagents, faulty sampling, careless techniques, problems of sample preparation, digestion methods, etc.” This is true of course, when all these parts of an analytical procedure are not considered in their coherent context. Since this coherence is the leading thread running through this chapter,we feel another threat for a well equipped, clean laboratory: To realize the needed time (and money) for the challenging task to construct a clean lab is one thing, to bring in the required devotion and discipline for the unthankful task to preserve its cleanness is another.

As already pointed out, the analytical laboratoiy and the analyst should be considered as important sources of contamination. This refers mainly, but not exclusively, to airborne contamination. Boyer and Horwitz [141presented a practical checklist of necessary actions to change an ordinary laboratory into a trace element laboratory. This list, with some additions, is reproduced in Table 3-2. In general, poor hygienic conditions such as rusty surfaces, peeling off paint, remnants of metal spatula, tweezers, etc. and chemicals in glass or metallic containers, dirty lab coats are uncontrolled sources of contamination. The most important parameter in the successful exploitation of a trace element laboratory is the discipline of the analyst in charge. c The analyst itself may be an unexpected source of contamination. Human-fingers are a notorious source of Na, C1 but surprisingly also of pg amounts of lead [96]. Human sweat, hair, skin flakes, and the analysts’ clothing are established sources of contamination. Often overlooked, the main pathway of contamination originating from the analyst is by cross-contaminating, i.e. transfer of contamination from

J.R.W. WOITI'IEZ AND J.E. SLOOF

66

Table 3-2. Necessary actions to change an ordinary laboratory into a trace element laboratory [ 141

Replace all rusted and corroded hot plates, heating units, ashing ovens, etc. with ceramic-top hot plates and ceramic-lined ovens. Replace rusting ring stands, clamps, racks, hood and window panels. metal cabinets, etc. or strip and paint with epoxy paint. Remove all unnecessary shelving, partitions, furniture and other dustcollection items from the laboratory. Install filters on all incoming air sources (air conditioning, heating, etc.) Class loo00is advisable. Install a laminar-flow hood, provided with HEPA filters, for cleaning of equipment by boiling in acids or for open dry or wet digestion. Install a laminar-flow box where cntical work has to be done. Coat ordinary bench tops with epoxy

paint and cover them with adhesive backed Teflon or polyethylene sheeting.

Remove all metal glass cylindersfrom the laboratory lo an adjacent lab or separate cylinder room. Run a Teflon or polyethylenedistribution line from each gas source to the laboratory requiring the gas. Paint walls in the analysis room with two component epoxy paint or cover the walls with plastic (e.g. PVC) insertions. Cover floors with one-piece vinyl flooring. Place sticky mats near the entrance. Limit the accessibility of personnel into the lab. Construct a barrier in the entrance of the lab which discourages quickly slipping in and out. Oblige the wearing of special dustpoor clothes and change of shoes. Preferably, hair should be covered. Leave the trace element lab as empty as possible. The more equipment present, the more people have to be in the lab and the more difficult the clean conditions are to be maintained.

Replace all metal parts of lab furniture or equipment with plastics when possible, or cover with epoxy paint.

source to source via the analyst. In radiochemical laboratories, this phenomenon is well known since for health physics reasons, uncontrolled appearance of a few Bequerel, corresponding to picogram amounts or less of radionuclides, are considered to be radioactive contamination. A radiochemical analyst, working with an open radioactive source thus has to avoid the transfer of picogram amounts of a certain radionuclide from the source to his working environment or to himself. Inadequate manual operation of micropipettes and tips, spilling drops of radioactive solution, contaminating wiping tissues and touching contaminated glassware,acid hoods, etc. have been found to be some of the main routes of radioactive contamination of the analyst. Trace element contamination via the analyst, sometimes considered to be

SAMPLING AND SAMPLE PREPARATION

67

cross-contamination, follows exactly the same pathways as radioactive contamination. It goes without saying that the prevention of the effects of both manifestations of the same contamination principle are identical. Some are explicited here: (a) Two sources of different concentration should never be brought close together. Standards and sample should be handled separated, and stored separated; (b) Re-use of chemical equipment such as pipettes, beakers, spatula, etc. should be avoided. Only disposables have to be used and to be disposed after use; (c) Non-chalked disposable gloves, preferably polyethylene, have to be used and they should be disposed after each manipulation; (d) Disposable protective clothing, preferably of a non-fluffing polymer, should be used and should be disposed after use. Of most importance, again, is the attitude of the analyst towards contamination. Murphy states: “And, finally, the analyst must ‘think blank’ that is he must be aware as to the effect on the blank of every step of the procedure. He must ask himself “If I do this, what is the effect on the blank?” and avoid those operations which tend to increase the blank or whose effect is not known, whenever possible” [96].

Chemicals arid equipnzent are the third main source of contamination. In sampling, chemicals are used for preservation or washing. Mineral acids, often in high excess, are used in the decomposition of geological and biological material. Acids are also used for the cleaning of polymer laboratory equipment and for stabilization of aqueous samples. Water is consumed in enormous quantities in every trace element laboratory. It is used for rinsing, cleaning and dilution. Organic solvents in trace element analytical chemistry are used for cleaning, solvent extraction or, nowadays, HPLC separation of metal-ligand compounds. The need for the highest obtainable degree of purity of these chemicals has long been perceived. Organic solvents are commonly purified by fractional distillation [162]. Already in 1955, Thiers describes the need of purification of water, hydrochloric acid, nitric acid, sulfuric acid, perchloric acid and ammonia. His conclusions are that ion exchange produces far better quality water than just distillation, but the purest water is obtained by ion exchange followed by distillation from fused quartz. For HCl and NH, Thiers recommends the synthesis of the chemicals from gaseous products and pure water rather than purification of the aqueous solutions. Such prepared HC1 was reported to contain a few ngmkg-’ of transition and heavy metals, compared to pg.kg-’ amounts for reagent grade material. Considering the state of the art of trace element analysis of nearly 40 years ago, this may be considered an extraordinary achievement. Nitric acid, used widely in wet ashing procedures, was identified as the major chemical contamination problem by Thiers. He recommends repeated distillation of the azeotrope (65% HNO,, 35% H,O) from a quartz still.

68

J.R.W. WOITTIEZ AND J.E. SLOOF

A few years later Irving and Cox [49] describe the simple procedure of isopiestic (or isothermal) distillation to prepare upto 10 N HC1 or NH,OH within 3 days, starting from double distilled water and reagent grade chemicals. This technique combines the volatility of gaseous acids and their solubility in water at room temperature. In his 1972 review, Tcilg [131] also emphasizes the advantages of the use of the gaseous phase at low temperature, so as to minimize contamination of chemical reagents. In 1972 Hamilton et al. [37] mention the need for a continuous recycling system of pure water, based on filtration, distillation, ion exchange and again, filtration. It is the better alternative for storage in PVC tanks, which leads to leaching. For ultra trace element analysis, Hamilton et al. find nitric, sulfuric and perchloric acid to lack sufficient purity, while HC1 (obtained by isopiestic distillation) and HF (triple distillation in Teflon) are qualified as acceptably pure. In the early seventies, the first papers describing the purification of acids by sub-boiling appear. Pure quartz sub-boiling units are described for purification of HCI, HNO,, HClO,, HZSO, and H 2 0 and all-Teflon units for the purification of HE The two bottle Teflon still, designed by Mattinson [82], resulted in Pb blank values in 48% HF down to the 2-5 ng.kg-' levels. Kuehner [74] reported a combined impurity for 16-18 elements of 6.2 pg-kg-' in sub-boiled HC1, of 16 pg-kg-' in HClO,, of 2.3 pgakg-' in HNO,, of 28 pgekg-' in H2S0,, of 17 pug-kg-' in HF and of 0.5 pg-kg-' in H20, while reagent grade and commercial pure acids displayed orders of magnitude more impurities. This was achieved at the NBS laboratories by sub-boiling purification in Quartz and Silica stills and NBS home-design Teflon units, under clean conditions. In their 1976 monograph, Zief and Mitchell [ 1621 summarize the existing techniques of purification of acids and water. They mention sub-boiling distillation, isopiestic distillation and absorption of volatile gases in purified water as the main mechanisms. They follow Hamilton's suggestion to use ion-exchange of distilled water for the productionof pure water. In 1982, Moody and Beary reviewed 10 years of experience at NBS in the production of pure acids [91]. No principle changes are reported as compared to the pilot work at NBS of Kuehner et al. in 1972 [74], with the exception that the need for a specialized class 100 laboratory for distilling acids has been recognized. The NBS clean acid lab has been completed in 1981. The main feature of the lab is that corrosion of the environment by acidic fumes is accepted, understood and controlled; the complete laboratory is constructed from polymer or polyurethane-painted aluminum, the environment is class 100 and the air flow balanced subtly to minimize the action of acidic fumes in corroding the laboratory. Figure 3-2 shows the schematic diagram of the quartz double sub-boiling distillation unit, used for the purification of H2S04 at the NBS laboratories since 1976. In 1982, Michell reviews the past decade as far as purification techniques for trace element analysis are concerned [88,89]. He discusses a modification by Oehme and Lund [102], of the nowadays widely used Millipore Milli-Q system for

69

SAMPLING AND SAMPLE PREPARATION FEED

BomE

COLD FINGER

8 . 1.. ~I.~ ...1.

COLD FINGER

.I.l+....~~ I...

I.

I..I.I..I.I..III(I(..~I..I.

Figure 3-2. Schematic diagram of apparatus for sub-boiling distillation1911

water purification. The data for Pb, Cd and Cu presented by Oehme and Lund in the water from the modified Milli-Q system are yet one order of magnitude higher than the levels obtained one decade earlier by sub-boiling distillation [74]. The results given by Moody and Beary [91] probably still represent the state of the art in clean acids and water. Figure 3-3 visualizes the results. Recent developments in the purification of chemicals, relevant for trace element analysis, are the use of preparative HPLC for organic solvents [106], improvements in the commercially available water purification systems down to < 0.5 pg.kg-' total dissolved metals [24] and the interest in anionic impurities in mineral acids [95]. Sampling and sample preparation with minimal contamination demands proper equipment. The history of precautions against contamination via equipment is more or less similar to that of chemicals. The general trend since the sixties has been the change from glassware, porcelain and stainless steel to wherever possible the use of polymers, quartz and pure metals. Again, pioneer research has been done in a few laboratories, mostly the same who experimented with clean environments and purification of chemicals (NIST, Max Planck Institute). Already in 1955, Thiers strongly recommended the use of polyethylene as a container material for aqueous solutions of trace elements, since it is orders of magnitude purer than Pyrex glass, polystyrene, methacrylate and Tygon. In 1968, Robertson [ 1081 produces an impressive amount of data for 10 trace elements in a wide variety of container materials and structural materials. Robertson concludes, that polythylene, Teflon and Plexiglas tubing and synthetic quartz are the purest materials. Rubber, structural nylon, polyvinylchloride, borosilicate glass, wiping tissue, adhesive tape and membrane filters all contain unacceptable high

J.R.W. WOITTIEZ AND J.E. SLOOF

70

=

HCI

. I WO,

HCIO,

H,SO,

I+

500

4

Y

.B.

300

20

200

+.

u

3

U

100

0

Pb

TI

Ba

Te

Sn

Cd Ap Element

Sr

Se

Zn

CP Ni

Figure 3-3a. Average impurity concentration in doubly distilled acids (ng/kg)

6000

4800

3600

2400

1200

0

Fe

Ct

C8

K

Mg

Al

Element

Figure 3-3b. Average impurity concentration in doubly distilled acids (nglkg)

N8

SAMPLING AND SAMPLE PREPARATION

71

amounts of trace elements. By the early and mid seventies, the exclusive use of Teflon, polyethylene and synthetic quartz were established to be the materials of choice for sample containment [37,38,65,96,107,131]. In 1976, Zief and Mitchell review the state of the art in sample containment material [162]. Polytetrafluoroethylene (Teflon PTFE and Tefzel ETFE), fluorinated ethylenepropylene (Teflon FEP), polychlorotriRuoroethylene (Kel-F), conventional polyethylene (CPE), synthetic quartz, pyrolytic carbon, high purity platinum and aluminium are mentioned as clean materials from a trace element point of view. The choice of these materials does not appear to have changed drastically in the last decades. Tschdpel et al. [132,133] draw some attention on the use of glassy carbon for simple laboratory ware such as beakers, while Hoffmann [44] et al. report on contamination by different polymer foils and quartz in the trace element analysis of purified water. They conclude that cleaning procedures for foils and quartz can be appropriate, but airborne contamination can only be prevented by experimenting in a clean box. In an extensive study on cleaning of polymer containers, Moody and Lindstrom [90] conclude that the various available polyfluorocarbons (PTFE, ETFE, FEP, PFA [perfluoroalkoxy]) and conventional polyethylene, have been found to be the least contaminating materials for storage, provided they have been cleaned properly. These authors recommend a combined HCl/HNO,/H,O cleaning procedure, which is reproduced in Table 3-3. A comparison of cleaning methods for polyethylene, prior to the determination of trace elements in freshwater samples, leads to a much simpler procedure 1751. These authors recommend a 48 h soak with 10% HNO, for both preliminary and routine cleaning to allow the collection and storage of freshwater prior to analysis of Zn, Cd, Pb and Cu. The knowledge on clean sampling-, storage- and sample preparation materials has led to many specific applications. Iyengar et al. [50] developed the brittle fracture technique for biological material, nowadays part of the well-known cryogenic homogenization procedure. A suprasil quartz knife and nylon tweezers and forceps were used to cut liver samples, which were then milled in an all Teflon mill with Teflon and quartz balls. The technique was later sophisticated by Zeisler et al. [170], who exclusively used titanium knives and Teflon PTFE, PFA and FEP containers, sheets and cryogenic homogenization equipment for preparing human liver samples for analysis. In a series of contributions, Iyengar and colleagues addressed the topic of contamination during sampling of several selected clinical specimens [52,54,56,58,111,112]. The success of the use of polyfluorocarboncompounds for containers and equipment and pure metals like Al, Ti or Pt for sampling and sample preparation can be deduced from the successful operation of the NBS EPA National Environmental Specimen Bank for human liver [ 1461, the successful preparation of a mixed human diet reference material [59,166], a codfish candidate reference material [71,103] and a bovine serum reference material [ 1671. All these materials are prepared in kg

J.R.W. WOITTIEZ AND J.E. SLOOF

12

Table 3-3. Suggested method for cleaning plastic containers [901

(a) Fill the container with a 1 + 1 H20/HCI (AR grade) mixture. (b) Allow to stand for one week at room temperature. Teflon should be heated to 80" C. (c) Enipty the container and rinse with distilled water. (d) Fill the container with a 1 + 1 mixture of HzO/HNO3 (AR grade).

(e) Allow to stand for one week at room temperature. Teflon should be heated to 80"C. (f) Empty the container and rinse with distilled water. (g) Fill with the purest available water.

(h) Allow to stand for several weeks. Change the water periodically to ensure continued

cleaning. (i) Rinse with the purest available water and allow to dry in a particle and fume-poor

environment.

amounts from fresh gross samples under clean conditions (air, laboratory, chemicals and equipment) and successfully certified for trace elements on or below the pg.kg-' level. An even more illustrative demonstration of the success of polymers in trace element analytical chemistry is the application of polyfluorocarbon materials in all types of pressurized wet ashing, including modern micro-wave digestion. The classical stainless steel housing is usually combined with PTFE digestion vessels, while microwave digestion is peiformed in PFA [67]. Critical reviews of Maienthal [Sl], and more recently Gillain [33], learn that the situation is identical for the analysis of trace elements in seawater and its components. Glassware, stainless steel, coating with silicone oil, tygon tubing in pumps, membrane filtration setups, etc. have all shown to generate serious contamination problems and are replaced whenever possible, by polyethylene, polyfluorocarbon and synthetic quartz. A rather new development in the large scale use of polymers is in High Performance Liquid Chromatography (HPLC). The need for less oxygen permeability, higher temperature resistance and improved thread integrity compared to polyfluorocarbon tubing and fittings has caused the introduction of polyetheretherketone, PEEK, into HPLC devices. A comparison of different polymers with regards to solvent compatibility and thread integrity suggests better characteristics for PEEK with the exception of chemical resistance to concentrated inorganic acids [ 1641.

SAMPLING AND SAMPLE PREPARATION

73

It should be noted that, although polyfluorocarbons are relatively clean towards contamination by leaching, the material is not inert to mechanical decomposition. Large amounts of F, stemming from PTFE particles, were found in human liver and total human diet samples which were cryogenically crushed using Teflon PTFE plungers [lsl]. Here, not just the (lack of) transfer of trace element from the polymer to the sample is important for contamination control, but also the trace element content of the plastic itself. In 1987, Versieck et al. [ 1411 published a paper which took away the confusion on natural levels of trace elements in human serum or plasma. In this paper, he describes the preparation of a “second generation reference material,”being human serum to be certified for some of the so-called “difficult” trace elements such as Cr, Co, Mo, Mn, Al, Ni, Hg and As, present in serum around or below the pgskg-’ level. Figure 34 gives the trace element composition of Versieck’s serum compared to updated

=

I I1yeng8r.r

Versiook’r

mrnm

ref oroooo

A1 Cr

ME U

E!

Ei

co Ni A8

Mo C1

cs HS 0.0 0

0.40

0.s 0

1.20

1.60

2.00

Content In ng/mL

Figure 3-4. Comparison of Versieck’s serum to Iyengar’s reference values for serum

reference values for trace elements in human serum [60,142]. Versieck describes the successful large-scale application of sampling human serum by clean techniques, developed throughout the years. The critical part is the use of polyethylene or Teflon intraveneous catheters for venepunctures and Spectrosil, Suprasil or Teflon containers to collect blood. Some of the more recent publications [ 135-1401 amply illustrate, that only this type of utmost care in sampling and sample preparation under extremely clean conditions generates unbiased results for trace elements in serum. It also means, that numerous data published in the literature on normal or pathological values of trace elements in human serum can be considered as analytically inadequate. This statement was confirmed by an attempt to establish

74

J.R.W. WOITIlEZ AND J.E. SLOOF

trace element reference values from literature data for six different human clinical samples [60]. For the elements F, I, Ni, Al, B, Br, Cs, Li, Rb, U and V, it was not possible to reach meaningful median values. 3.2.1.2 Losses Element specific losses during sampling and sample preparation can be divided into losses due to evupoi*utioii,ubsorptioii und (co)precipitutioa. It is obvious that a major source of element specific losses in the analytical procedure takes place during chemical manipulation in the preparation step directly prior to detection (preconcentration, extraction, electroplating and stripping, chromatographic separation, hydride generation, etc.). Only those techniques using a yield determination for every element in each sample (IDMS and RNAA) can escape this source of negative systematic enor. Further discussion of this item is beyond the scope of this chapter. It is disputed in detail elsewhere [ 150,1561. Losses of trace elements via evuporutiori is most likely to occur during the ashing step. A discussion on methods of ashing is given in Section 3.5. Elements may volatilize as oxides, hydrides, halides, organometal compounds, etc. Those techniques most vulnerable to losses are low pressure wet ashing, such as variations of the H,SO,/H,O, (Kjeldahl) and the HNO,/HClO, (Bethge) procedures and open high temperature dry ashing. Losses of Hg, As, Ge, Sn, Se and Sb as halides, Cr, Os, Ru as oxides or halogenated oxides have been reported during wet ashing. In dry ashing, at elevated temperatures, also Pb, Cd, Cu, Co, Ni, V, Fe and Zn may be lost, mainly due to volatilization of the halides. High pressure wet ashing, including microwave destruction and recently developed dry ashing techniques, such as Trace-0-Mat and Low Temperature Asher, appear to overcome this problem [ 1531. Another part of the analytical procedure where the risk for losses by evaporation is obvious, is drying of a sample or evaporation to dryness of an aqueous solution or wet-ashed residue. Volatile chlorides of selected elements are bound to be lost from chloride rich environments (HC1, HCIO,) at elevated temperatures [ 1331. Iyengar [51,53] reports on tissue specific losses between 2 and 15% for Hg, Se, I, Sb, Sn, Ce, Mn and Sc during oven-drying at temperatures of 80, 105 and 120"C. Lyophilization, or freeze-drying appears to be the better approach to prevent losses during dehydration. However, results are strongly dependent on the sample matrix studied and the chemical form(s) in which the element is present in the sample. Iyengar et al. [51,53,55] report no losses for Sb, Co, I, Hg, Se, Zn, Hg, Cs, Ce, Mn, Sc, Ag, Cr(V1) and Sn after freeze-drying of 10 different rat tissues. Only Cr(II1) is lost for about 7% from skin tissue. Hg is lost for upto 50% through evaporation from acidified aqueous solutions simply by storing the solution sealed in polyethylene at room temperature for 5 weeks [41]. Hg has been reported to be lost during freeze-drying for upto 10% as methylmercury and inorganic mercury from blood [76], for 8-71% as methyl mercury and inorganic mercury from biological

SAMPLING AND SAMPLE PREPARATION

75

material and sediments [77] and for 100% from aqueous solution [16]. Also Se is reported to be lost during lyophilization, depending on the sample matrix and chemical form [20,31,32,41]. There is consensus in literature, that there is not one selected drying procedure that a priori can be considered to be free of losses. Furthermore, the extent of loss is definitely matrix, element and chemical form-dependent. The best way to check on possible losses due to drying, is to analyze the trace element of interest before and after the drying procedure. The use of stable or radioactive tracers is only allowed when the tracer has been metabolized by the sample under study, i.e. all the chemical forms in which the trace element occurs in the sample should be proportionally represented by the added tracer. The extent of absorption of a trace element by a container wall is dependent on pH, composition and ionic strength of salts, trace element under study, its chemical form(s), its concentration, contact time, ratio surface of container over volume of solution, temperature, type of container material and cleaning procedure. Again, there is not one specific procedure which can claim to prevent losses through absorption, but there are guidelines to be followed to minimize the effect. These guidelines are constructed and summarizedhere, based on several review and topical papers [20,26,41,81,83,93,112,119,126,127,131-133,162,163]: (a) The shorter the contact time, the better. This means fresh diluted solutions such as animal blood, urine, milk or seawater, fresh water and rainwater should preferably not be stored, but analyzed on the spot. When required, the trace element(s) of interest should be separated and transferred to a solid chemical form (pre-separated immediately);

(b) If storage is inevitable, than in thoroughly cleaned (cf. cquipmertt) polyfluorocarbons or conventional polyethylene. Laboratory glassware should be avoided for contamination reasons. The ratio inner container surface over sample volume should be as low as possible; (c) If a sample solution is collected in the storage container, it should (if possible) be stored frozen or at least refrigerated. Kinetics of absorption equilibria are slowed down by decreasing temperature; (d) If refrigeration is not possible, the solution has to be acidified to at least pH 1. This has to be done with either sub-boiling distilled HC1 or HNO,. Some researchers advise pre-equilibration of the container with the sample matrix prior to real sample storage, so as to occupy absorption sites on the container wall; (e) The time span between sample storage and analysis has to be minimized.

76

J.R.W. WOITTIEZ AND J.E. SLOOF

There are many comments thinkable against this type of procedure. Freezing and defreezing may change the sample composition of biological material (denaturation of proteins) or seawater (changing salinity). Cleaning procedures may etch the container’s surface and increase the number of absorption sites. Acidification may transform a trace elemental compound into an insoluble chloride or volatile hydride. It may also disturb the equilibrium between the amount of trace element bound to suspended matter and that in true solution or act as a denaturating agent. The laboratory may not have the capacity in containers, personnel or detection devices to avoid storage of samples. Finally, it may simply not be possible to avoid sample containment, transport and storage, e.g. as for in hospitalized patients or a midoceanic monitoring campaign. Basically, these limitations should all be considered in the pre-sampling plan.

(Co)precipitation can be considered as a special form of absorption. Colloidal elemental hydroxides, phosphates, sulfates or organic material or suspended particles, etc. present in the sample or formed after the addition of a preservative or acid or after defrosting, may (co)precipitate or sedimentate during sample storage. The only way out for a quantitative trace element analysis for such a sample is complete mineralization of the total sample.

3.22 Saniple speciJic changes Some of the sample speciJic changes are trivial. One could consider spillage, sputtering during ashing, formation of a precipitate during storage, etc. as trivial changes; it needs no further comments. Segregation of particles of different particle size often occurs in geological or freeze-dried and homogenized biological samples during long-term storage. Re-homogenization by mechanical shaking is mandatory here [141. The problem has been elegantly handled by BCR, which provides some of its certified reference materials in bottles containing a Teflon ball. The bottle has to be shaken manually for 2 min before it is allowed to be opened [7].Disintegration of biological samples via enzymatic or bacteriological action can be avoided by immediate (freeze)drying and sterilization [93]. Iyengar [56] has made a detailed study on sample changes for human or animal clinical specimens and mentions, e.g. cell-swelling, haemolysis, medication, sub-clinical conditions, humidity, freezing (and defrosting) as threatening for the integrity of the sample. Drying and mineralization are by far the most drastic actions which cause sample specific changes. Although it may seem a trivial discussion, since drying and ashing are meant to change the sample, a few notes can be made. In a solid, but humid material it is best to determine the moisture content of a sample on a separate aliquot. When this is not possible, as in the case of a small, unique sample, the method of preference for drying is lyophilization. A critical point here is that the freeze-dried material may be extraordinary hygroscopic. Pauwels et al. [ 1031 states that only a

SAMPLING AND SAMPLE PREPARATION

77

conditioned atmospheric environment with low residual humidity allowed control of the moisture content of lyophilized codfish powder. Earlier, a rapid uptake of moisture from humid air by lyophilized human serum and NBS SRM 1577 “Bovine Liver” has been reported [ 1471. Within 100 min, 90 mg of dried serum gained 20% of its weight by moisture uptake from air with a relative humidity of 82%. The very contact of the sample with laboratory air at the moment the underpressure in the freeze-dryer is annulled means absorption of water and can only be controlled and standardized by air, which is conditioned in humidity. Another change in sample matrix caused by oven-drying at higher temperatures, may be via chemical reaction. Decomposition,oxidation or volatilisation of organic compounds such as ethers can be detected by the analyst’seye and nose. Sometimes sample changes are less easy to discover. In an attempt to convert a slurry of CaS04.2H20(dihydrate) in water to dry gypsum and subsequently determine natural radioactivity, a polyethylene bottle containing the slurry was dried in an oven at 70”C for 48 h. The Ca determination in the dry gypsum, however, revealed the presence of CaS04-1/2H,O (hemihydrate) rather than the dihydrate, inducing a systematic error in the determination of the water content of the slurry [157]. The conversion of calcium sulfate dihydrate to hemihydrate is known to occur at 125’ C, but apparently also happens, though slower, at lower temperature. As already mentioned, addition of chemicals for preservation purposes is to be considered a drastic change of the sample composition. It may lead to unexpected consequences. Addition of acid to a sample induces a higher degree of protonation of inorganic and organic anions, leading to changes in volatility (e.g. HI, HE HCl compared to the corresponding sodium salts) or solubility of organic complexing agents. Adjustment of the pH of a solution by addition of HC1 or NH40H may lead to precipitation of chlorides or hydroxides. Leaching of suspended material and destruction of the quartinary structure of proteins in biomedical fluids has already been mentioned.

3.3 Pre-sampling considerations Most laboratories active in the field of trace element analysis and operating on a routine base, agree to analyze samples “as received.” If the analyst is not involved (or consulted) in the sampling procedure, it means that the responsibility for the interpretation of the analytical result is denied by the analyst. If sampling has taken place in cooperation with and in accordance to the guidelines of the analyst, “as received” analysis may be an acceptable practical procedure. An alternative approach is, that the analyst, or well trained coworkers, perform the sampling themselves. In this case, the sampling step should be part of a sampling protocol, based on knowledge of and experience in the required trace element analysis. The need to design a protocol is explicitly stressed by several authors, [26,40,72,1123.

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J.R.W. WOITTIEZ AND J.E. SLOOF

In theory, the construction of a sampling protocol should be done prior to execution of any part of the analytical procedure. This implies that a pre-sampling plan, being a critical a priori evaluation of the sense of the analytical contribution in a multidisciplinaryresearch project, has to be assembled. In practice, a pre-sampling plan may consist of a checklist of questions as suggested in Table 3-4. This list asks for the active participation of the analyst in a project before samples actually reach the analytical laboratory. The need for an accurate definition of the actual research topic (question 1) is vital. It may look like an obvious statement, but research aims, clearly and explicitly explained in advance, are rare. An elucidating paper by Cornelis [ 181 entitled: “Trace elements in hair; failure of a mission” illustrates the limited significance of elemental analysis of human hair and clearly demonstrates how much analytical effort is based on insufficient definition of the research goals. The exclusive derivation of the analytical task in the project (question 2) is of no less importance. An example here may be the distinction between wet and dry atmospheric deposition for a proper interpretation of trace element patterns in rain water [66]. Sometimes, the analytical task cannot be performed because analytical science is not yet ready for this type of problem (question 3). This has long been the case for the analysis of trace elements in human serum [60] and is now the case for the analysis of metal-ligand complexes in environmental and biomedical systems 11551. Elemental analysis on trace (< 100 pgakg-’) and ultra-trace level (< 1 ,ug.kg-’) requires special expertise, experience and equipment (question 4). Clean room facilities, for example, are slowly being introduced in trace element laboratories for a number of reasons [85], while their need in obtaining accurate results in environmental and biomedical analysis is firmly established [93]. The question of sample definition (question 5) is often recognized but seldom appreciated. Current terms in literature like liver biopsy (with or without arteries, veins and blood?), ecosystem, litterlayer (including or excluding animal life), atmospheric deposition (dry, wet, gaseous?) ask for a detailed, specific definition. Many studies on the kinetics of metal-uptake by microorganisms lack comparability (and thus verifiability) by the nature of the equipment, since the results for uptake-rate constants are dependent on substrate, microorganism and experimental conditions [94,122]. The possibility of drawing this well defined sample (question 5) is an extension of the sample definition problem. Often, practical circumstances or ethical considerations force the analyst to compromise, as in the case of human biopsy samples. The topic of sample representativity (questions 6 and 7) has been addressed in the literature 147,723. It will be discussed in further detail in the next paragraph. Knowledge about the distribution of the element in the sample (uniform or homogeneous) and thus on the necessary sample size is requisite. Question 8 refers to the well-known physical principle that no measurement on a system can be performed without disturbing the system. h trace element analysis, the risk of contamination and losses due to sampling and sample transport has been fully recognized and thoroughly

SAMPLING AND SAMPLE PREPARATION

79

Table 3-4. Pre-sanipling plan in checklist format N

Question

Remarks

Has the interdisciplinary research item been adequately fomiulated?

If not, sampling and analysis will be a waste of time. Refomiulate.

Has the derived analytical task been exclusively defined?

If not, sampling and analysis will be a waste of time.

Can this task be executed considering the "state of the an" of trace element analytical chemistry?

If not, sampling and analysis will be a waste of time, unless pioneer analytical work is budgeted.

Does the laboratory have the necessary expertise. experience and infrastructure to execute the analytical task?

If not, the analyst should be assisted by expert laboratories. If time and money for this assistance are not properly budgeted, sampling and analysis will be a waste of time.

Can the object of sampling be exclusively defined and is it possible to draw this sample?

If not, sampling and analysis will be a waste of time. An alternative is to redefine the analytical task (question 2).

Is it necessary to consider the saniple to be representative for a bulk

If so, infomiation about the uniformity of the analyte in the sample and of the sample in the population has to be known or gained.

sample or a population

Is is possible to take a representative sample?

If not, conclusions on the final analysis will be restricted to the specific single sample drawn. Refomiulate the analytical task (question 2).

Can the integrity of the sample be guaranteed during sampling and sample transport?

If not, analysis will be a waste of time. Reformulate the analytical task (question 2).

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J.R.W. WOlTTIEZ AND J.E. SLOOF

documented [ 1621. In the earlier mentioned specimen bank project, all samples are frozen to LN, temperature immediately after sampling to block all chemical and biological activity in the sample (proteolysis, cell division, infection) [29]. In the field of trace element speciation research, not much is known yet on the effects of sampling on vulnerable chemical equilibria. The pre-sampling plan can also be used to evaluate the sense of published analytical results a posteriori. Table 3-4a illustrates the results of application of the checklist to an investigation on the occurrence of protein-bound trace elements in human serum. The work was done by one of the authors [ 1471. The table illustrates that in an early phase of the project, some essential questions on the design of the experiments were not sufficiently taken into account. The definition of protein fractions and the qualitative and quantitative analysis of (at least some characteristic) proteins in these fractions has not adequately taken place. Ultimately, this has led to a very modest impact of the interpretation of the results. The essential weakness has been that the interdisciplinary character of this work has been severely underestimated. Table 3-4b shows the results of the application of the checklist to a study on the preparation of codfish candidate reference material to be certified for selected trace elements [103]. The aims of the study were explicitly made clear: to prepare a reference material with a trace element composition as close to the original natural situation as possible. Materials (seafish muscle), equipment (only titanium, Teflon and KEL-F), elements (Pb, Cd, Hg,Fe, Zn) and analytical techniques (solid sampling Zeeman AAS) were selected and tested on their suitability in advance. The factors which might jeopardize a successful completion of the work (moisture content, contamination, losses, minimal sample size, bacterial infection, etc.) were recognized, analyzed and their influence brought under control. The study was successful, since the codfish material has been subjected to a BCR certification campaign [ 1651. In speciation chemistry, the need of a sample protocol has to be even further stressed. Mutatis mutandis, the considerations on total elemental analysis are equally valid for inorganic or organic metal compounds. To understand the behaviour of trace elements in biomedical, environmental or geological compartments, it is necessary to gain insight on the chemical form(s) in which they exist in the compartment. An early example of speciation (formerly called “chemistry”) is the understanding of the behaviour of N in soils by dividing it into a nitrateand an ammonia problem. This was successful, since NH; and NO; are not easily chemically converted into each other and are thus both stable with respect to sampling [ 1231. In general, stability of the compound is the extra complicating factor in speciation analysis. Consider a compartment containing the simple elemental compound ExLywhere E is the element. The compouiid may be inorganic, as in 10, or organic, e.g. CH,HgCI. The type of chemical bonding may be ionic (Sr3(POJ2 in

81

SAMPLING AND SAMPLE PREPARATION

Table 3-4a. Evaluation of a pre-sanipling plan in checklist fomiat, applied to the detemiination of protein-bound trace elements in human serum [147]. The project comprised serum sampling, protein separation by gel-filtration and neutron activation analysis (NAA) of the separated fractions N

Question

Remarks

Has the interdisciplinary research item been adequately formulated?

The approach was purely analytical. The choice to analyze serum was for practical reasons. No serious consideration of other clinical specimen has taken place.

Has the derived analytical task been exclusively defined?

No. Quantitative protein analysis, study of the influence of buffers, etc. have been ignored.

Can this task be executed considering the No. The applicabilityof the combination of protein separation and “state of the art” of trace element analytical chemistry? trace element analysis for human serum had to be investigated. NAA was sufficiently under control Does the laboratory have the necessary expertise, experience and infrastructure to Clean-room facilities were available. execute the analytical task? Contaniination-free sampling of serum was developed. For gel-filtration assistance was obtained.

Can the object of sampling be exclusively Whole blood and serum are well defined and is it possible to draw this defined. Serum protein fractions are sample? defined according to fractionation technique.

Is it necessary to consider the sample to be representative for a bulk sample or a population.

Yes. A blood sample is supposed to be representative for the individual. The representativity of the individuals for a population was not the item.

ISit possible to take a uniform sample?

Yes. Venepuncture with a Teflon needle had proven to generate reproducible results.

Can the integrity of the saniple be guaranteed during sampling and sample transport?

No. The effects of Lt v i m chemistry on the nietal-protein equilibrium have been subjected to limited study.

Conclusion: Results cannot be interpreted quantitatively. Very modest impact.

J.R.W. WOITTIEZ AND J.E. SLOOF

82

Table 3-4b. Evaluation of a pre-sampling plan in checklist format, applied to the preparation of codfish candidate reference material to be certified for Pb, Cd, Hg,Fe and Zn [ 1031. The project comprised collection of codfish base material and transforniation into a candidate reference material. Homogeneity studies were done by solid sanipling Zeeman AAS. ~~

N

~~~

~

Question

Remarks

Has the interdisciplinary research item has adequately formulated?

Yes. A fresh seafish muscle material had to be prepared and certified for Pb,Cd, Hg, Fe and Zn.

Has the derived analytical task been exclusively defined?

Yes. A priori,the parameters particle size distribution, moisture content, stability, contamination. losses, heterogeneity and microorganisms were perceived.

Can this task be executed considering the “state of the art” of trace element analytical chemistry?

Partially. Special cryogenic grinding equipment in Teflon had to be developed.

Does the laboratory have the necessary expertise, experience and infrastructure to execute the analytical task?

Yes. The study was executed at CBNM, Belgium. This laboratory is experienced in clean room work and accurate trace element analysis.

Can the object of sampling be exclusively defined and is it possible to draw this sample?

Yes. The sample is codfish muscle, to be obtained by filleting and removal of bones. Sampling is done by fishing.

Is it necessary to consider the sample to representative for a bulk saniple or a population.

No. Some 200 kg of codfish was caught. After preparation, this defines the entire population of candidate reference material codfish.

Is it possible to take a unifomi saniple?

Yes. The study focused on saniple preparation techniques to allow representative subsanipling of nigs.

Can the integrity of the sample be guaranteed during sampling and sample transport?

Yes, as far as trace element concentrations are concerned. No information available on vulnerable chemical fornis of trace elements.

Conclusion: Results can be interpreted quantitatively. Adequacy of sanipling and saniple preparation procedure has been proven. Material has been subjected to certificationanalyses.

SAMPLING AND SAMPLE PREPARATION

83

bone) or covalent (e.g. tetraalkyltin derivatives), but many compounds in nature are some metal-ligand complex form. If the elemental compound decomposes slowly and reacts slowly with chemicals from the environment (including the laboratory equipment), compared to the time needed for sampling and analysis, the speciation analysis may be meaningful. This situation may be valid for covalently bonded organic-metal compounds. In case of ionic species or metal-ligand complexes, the compound can be expected to be in equilibrium with its environment. In a simplified dissociation equilibrium E,L, F! xE + yL, either inorganic or organic, the knowledge of the equilibrium constant li,the formation rate constant b, and the dissociation rate constant b2 are requisite. Also, not only contamination and losses of the element, but also of the complexing agent has to be taken into account. If other ligands are present, or (un)deliberately added in the laboratory, complexation of the element with this ligand has to be anticipated. Also, competition with other elements are likely. It is clear that the utmost precautions have to be taken during the sampling and the sample preparation step. Accuracy of the sampling procedure should be checked by analysis in different phases of this procedure. However, the determination of ExL, demands some kind of separation, which by its nature is a distortion of the equilibrium between E, L and ExLy.If the reaction rate constants are high compared to the time needed for analysis, the problem ends in a vicious circle. If not, a structured sampling protocol can only be designed when all interactions during the analysis are fully understood and controlled. 3.4 Aspects of sampling

This paragraph discusses aspects of sampling and subsampling. It is again based on the sampling plan concept, described by Kratochvil[72] and Taylor. The need for a proper pre-sampling plan, a basic part of the sampling plan, has been pointed out extensively. The sampling plan basically deals with the technical aspects of sampling and visualizes the expected fate of the trace element(s) during sampling and sample preparation steps. It should show the rational set of decisions, taken from the moment samples are taken until the actual trace element analysis starts. Table 35 gives a list of possible questions to be asked in order to construct a thorough sampling plan, in check-list format. The application of the list is illustrated by its application to trace element analysis of human serum by Neutron Activation Analysis. Grosso modo, a sampling plan, covers activities outside the laboratory, sampling, and activities inside the laboratory to prepare the sample for analysis, sample prepamion. The leading thread running through the sampling plan is size reduction of the sample, while maintaining its original composition. Table 3-6a shows a generalized process of size reduction of the sample population to the test portion actually analyzed and gives definitions of sample terms [72]. To illustrate the practical meaning of the terms used, Table 3-6b gives the application of the transformation scheme for the example of the preparation of a codfish candidate

J.R.W. WOIlTIEZ AND J.E. SLOOF

84

Table 3-5. Sampling plan in check-list format Stage of sampling plan

Example: trace elements in human serum [ 1471

1. Sampling What is sampled?

Human blood.

When is sampled?

Patients should be fasting.

Where is sampled?

Hospital.

How is sampled?

Venepuncture in the left ami. Patient in upright position.

Special equipment needed?

Yes. Teflon intraveneous catheter.

Who samples?

Qualified instructed nurse. Analyst present.

How many samples?

Two samples per patient. The first used for clinical analysis, the second for trace elements.

Which size per sample?

The first 3 ml, the second 10 nil.

How are samples coded?

Patients’ registration number with extension.

Is a composite saniple needed?

No.

Are samples directly contained for transport?

Yes.

Which containers are used?

25 ml Teflon tubes with PE snap-cap.

Can contamination and losses at containment be avoided?

The use of just Teflon minimizes contaniination.

Iti situ analysis necessary?

No.

111situ sample cleaning needed?

No.

Geographical and meteorological data needed?

No.

Which data are recorded and how?

Regular personal hospitalizationdata.

2. Subsampling and (re)packing

III siru subsampling needed?

Yes. Serum needs to be separated from red- and white blood cells.

85

SAMPLING AND SAMPLE PREPARATION

Table 3-5.(Continued) Stage of sampling plan

Iii

situ sub(samp1ing) repacking needed?

Example: trace elements in human serum [ 1471 Yes. Separated serum needs to be contained.

Which containers are used?

15 nil Teflon containers with cap.

Are containers clean?

Yes. Cleaned by HNO3 and H20 cleaning procedure.

Can contamination and losses be avoided?

Using just Teflon materials minimizes contamination.

Are containers coded?

Yes. Both test-tubes and containers are re-used.

In sitic chemical preservation needed?

No.

Can contamination and losses be avoided?

Not applicable.

1ii sitic physical

No.

preservation needed?

Special equipment needed?

No.

Which data are recorded and how?

For details see [ 1471.

3. 'Ransport, and sample preparation Dead-line for sample transport?

Yes. Serum needs to be transported to the lab as soon as possible.

Storage mom available?

Yes, but not applicable,

Immediate preparation needed?

Yes. Weighing and lyophilization.

Special conditions in lab needed?

Yes. Clean room absolutely necessary.

Is sample cleaning needed?

No.

Is sample division needed?

No.

Is sample reduction needed?

Lyophilization.

Is drying needed?

Ibid.

Is crushing needed?

No.

Is milling needed?

No.

Is sieving needed?

No.

J.R.W. WOITIIEZ AND J.E. SLOOF

86

Table 3-5. (Continued) Stage of sampling plan

Example: trace elements in human serum [ I471

Is mixing needed?

Yes. The dried powder (around 1 g) is mixed with a Teflon bar in the Teflon

container.

Can contamination.losses and change in saniple composition be avoided in all these steps?

All materials are Teflon. The critical step

is lyophilization. Losses during evacuation and contamination during aeration may occur.

Is immediate analysis needed?

No.

Is the laboratory sample stable?

If stored locked at -25 C for several weeks.

Which data are recorded and how?

For details see [ 1471.

O

reference material. In this example, 200 kg codfish is gradually transformed to hundreds of bottles, each containing 7 g of fish powder with known moisture content and particle size distribution. The sampling plan begins with the understanding of sampling errors. Following Heydorn’s definition, a sample is homogeneoics when the trace element is evenly distributed in the sample and thus the analytical result is independent of the sample size. The term represenfufivssample refers to homogeneity [72]. Homogeneity is usually only the case for the matrix components of a sample and only to be expected for minor or trace elements in a well-defined solution. A sample is uniform for some trace element when it is randomly distributed through the sample and thus the analytical result is a function of the sample size. Providers of certified reference materials (CRM)have to indicate the users of their material that drawing a sample of prescribed minimal size from a bottle induces a known inherent analytical ei-ror due to non-uniformity. They have to guarantee that this non-uniformity error is negligible on the sample size level of a bottle, i.e. the uniform distribution has approached the even distribution for bottle sample sizes. This guarantee is vital, since it means that all bottles can be regarded identical with respect to the trace elements certified. From an analytical point of view, the discussion on sampling errors can best be constructed starting from the analytical measurement up, ending with the interpretation of results for the target population. The sequence of actions the analyst has to undertake is the establishment of analytical control, estimation of the sampling error by analyzing a test portion from the laboratory sample (test portion error),

87

SAMPLING AND SAMPLE PREPARATION

Table 3-6a. Transfoniiationof a population to a sample to be analyzed through sample size reduction Sample size

Definition

Assumptions and remarks

Actions

Target population

Population to which the interpretation of the analytical result is intended to be extended to.

To be addressed when restricted in size and characteristics. Otherwise hard or impossible to define.

Restraint in definition.

Part of the target population which is sampled.

Parent population is representative for or identical to target population.

Careful sampling obliged to relate to target population.

May be representative, random1y selected, composite or systematic.

Size reduction required with maintenance of chemical integrity of trace elements of interest.

Aliquots or aliquands of the gross sample.

The homogenized gross sample is uniform for the elements of interest. No contamination or losses occurred.

Random sampling and analysis has to take place in all stages of sample preparation.

Aliquots or aliquands of the subsaniple.

The homogenized subsaniple is uniform for the elements of interest. No contaniination or losses occurred.

Random sampling has taken place to confirm uniformity.

Aliquots or aliquands of the laboratory sample, used for elemental analysis.

The homogenized laboratory sample is uniform for the elements of interest on the weight level of the test portion.

Sampling error has to be established as well as the number of samples to be analyzed.

1

Parent population

1 Gross samples Actual samples in some kind of relation with the parent population. 1.

Subsamples

J.

Laboratory samples

1

Test portions

J.R.W. WOITTIEZ AND J.E. SLOOF

88

Table 3-6b. Sample size reduction for BCR candidate reference material codfish Sample size

Remarks

Assumptions

Actions

Muscle of the population of codfish in the North Sea.

None. The final material does not pretend to be representative for the target population.

None.

Muscle of 200 kg of codfish, caught in the North Sea.

None. Parent population and target population are identical.

None.

No Pb, Cd, Hg. Fe and Zn has entered or left the material other than with loss of sample.

Jaw crushing, ball milling, freeze drying, sieving and mixing with minimal losses or contamination.

1232 bottles of each 7 g codfish muscle powder.

The unifomi and known distributionof Pb, Cd, Hg, Fe and 2% in the 9.3 kg powder remains unchanged by bottling.

Random sampling and analysis has taken place in all stages of saniple preparation.

Identical to subsample.

cf. subsample.

cf. subsample.

At least 100nig froni

Sampling error inherent to unifomiity in 100 mg sample is < 5% for each element.

Sampling error has been established and found identical for one bottle and a composite of twenty bottles.

~~

~~~

Target population

1 Parent population

1 Gross samples 72 batches of 1 kg of codfish flesh, gradually J. transformed to 9.3 kg of dry powder.

Subsamples

1

Laboratory samples

.1 Test portions

a bottle to be used

for elemental analysis.

SAMPLING AND SAMPLE PREPARATION

89

estimation of the sampling error by preparing different laboratory samples from a subsample (laboratory sampling error), estimation of the sampling error caused by taking different subsamples from a gross sample (subsampling error) and defining the meaning of the gross sample with respect to the target population (gross sampling error). 3.4.1 Establishment of analytical control The first task of the analyst is to establish the sampling error by analyzing a test portion from a laboratory sample. To determine this error, his analytical technique has to be in statistical control. For most trace element analytical techniques, statistical control depends not only on elemental content or elemental mass (sample size x content), but especially on the type of sample. This means statistical control has to be proven by analysis of a certified reference material with an identical or similar matrix. This is usually referred to as matrix matching. In practice, it is often impossible to apply perfect matrix matching, and thus proof of statistical control has to be based on a compromise. One technique, neutron activation analysis (in its instrumental and radiochemical mode) is less sensitive to matrix effects and can be brought to statistical control by considering an a yriori analyticalerror [17,40,147]. Table 3-7 outlines the statistical considerations and an application to demonstrate control of precision, based on the concept of predicted or internal and observed or external variances (s; and sz). If a test quantity F, defined as the ratio of the external variance over the internal variance exceeds a critical value, and the material analyzed is guaranteed to be homogeneous,the analytical technique is out of control. It is shown in Table 3-7 that statistical control is difficult to achieve for multi-trace elements problems.

3.4.2 Sampling error iit a test yortiori Once statistical control has been demonstrated for some element in some sample matrix, the sampling error caused by drawing a test portion from the laboratory sample can be estimated. There is no use in taking a portion of the laboratory sample, if the distribution of the element is not at least expected to be random. This means that a reasonable amount of homogenization (mixing) must have been performed. Consider the analysis of some trace element in a laboratory sample, from which a number of test portions have to be drawn. The first questions to be answered are: How many test portions have to be drawn to be able to establish a mean value with a certain level of confidence and what is the minimal sample size? A practical approach is the undertaking of a pilot experiment [3,72]. From this pilot experiment a provisional average (zavg)and external standard deviation (s,) are determined. If a certain acceptable standard deviation in zavgdue to sampling analysis, savg, is set, the number of analyses necessary to obtain zaVg f savg with a certain level of confidence (usually Q = 0.05) is given by N = (t - S , / S , ~ ) ~ .

90

J.R.W. W O I T I E Z AND J.E. SLOOF

Table 3-7. Statistical considerations on analytical control of elemental analysis by instrumental neutron activation [ 1471

In principle, the sources of statistical errors in a neutron activation analysis of a sample can be quantified. Thus, an interiiul relative variance s:(T) in the final counting result can be defined as: s

m = s:o7,T) + s:w

4-

s:
where s:(p, T) accounts for the variance in the peak area determination of the nuclide of interest. sf(p) accounts for the variance in the pulser-peak area determination. s:(f) accounts for the variance in neutron flux corrections. $(s) accounts for the variance in assaying a certain sample weight.

The highest contribution comes mostly from sf(p,T). Consider an individual analytical result ziwith internal variance s;(s; sf is proportional to a weighing factor w i= si/s:. The weighted mean of a set of analytical results xican be given by:

= s:(T). x:).

The expression of the predicted or internal variance of this weighted mean is given by

The appropriate measured weighted variance in the weighted mean or external variaiice (similar to the ordinary standard deviation calculated from N replicate measurements), is calculated from

The ratio of the external and internal variance in the weighted mean is used to establish whether or not the observed deviation in the results is to be explained by the known contribution from the various sources of statistical errors. If this holds, the results are consistent. The test quantity 2

91

SAMPLING AND SAMPLE PREPARATION

follows the distribution of Fisher-Snedecor. The experimental value of F is compared to a critical F-value for the 95% confidence level (a = 0.05) with the number of degrees of freedom being N - 1 and m. Significant large values of F (inconsistent results) indicate the influence of parameters not taken into account in the estimation of the variance in each individual result. The technique is out of statistical control. For homogeneous samples such as standards or certified reference materials, consistency of data cannot be accepted. It indicates lack of analytical control, or, when this is proven, erroneously assumed homogeneity of the sampled material. The following table illustrates the use of internal and external variance for instrumental neutron activation analysis of Br, Zn and Se in reference solutions, NBS SRM 1577A “Bovine Liver” and a pooled serum sample. The sensitivity of F for the decision “in or out of statistical control” can be seen from the table. For Br ~~

Element Sample

sr,e

(%I

sr,i

(%I

AT

F

Fv,m (a = 0.05)

Br

Reference solution Bovine liver Pooled serum

0.50 0.53 0.50

0.52 0.84 0.59

8 16 4

0.96 0.39 0.70

2.01 1.67 2.60

Zn

Reference solution Bovine liver Pooled serum

1.53 0.78 0.88

0.74 0.45 1.36

6 16 4

4.27 2.94 0.42

2.21 1.67 2.60

se

Reference solution Bovine liver Pooled serum

0.73 2.7 3.3

0.84 4.2 4.5

6 16 4

0.76 0.42 0.53

2.21 1.67 2.60

and Se, the values of F are not far from the theoretical peifect value F = 1. None of the F-values for Br or Se exceeds the critical F-value, meaning that the analytical technique is in statistical control and the samples can be considered homogeneous for Br and Se on the level of sample intake (250 mg). For Zn, the situation is different. The external variance exceeds the internal one, even for reference solutions. This clearly indicates lack of analytical control, caused by inadequate peak area determination of the 1115 keV gamma-ray energy of 65Zn [ 1471. The technique as applied cannot be used to establish sampling errors in bovine liner or pooled serum.

92

J.R.W. WOIIITIEZ AND J.E. SLOOF

An initial value oft of 1.96, belonging to N = 00, is inserted, which results in a value for N . This leads to insertion of a new value of t(t,,,), which gives again a value of N . This process of iteration can be continued until N does not change anymore. If, for example, for the determination of Zn in a laboratory sample xavgf s, have been provisionally determined to 126 f7 mgakg-' and saVg is set of 6 mgakg-', the iteration produces N = 5 , 9 , 7 , 8 and 7, respectively. The second question, concerning the minimal sample size to exclude an extra sampling error with a certain level of confidence, has been studied in detail by several authors [3,4,42,47,72]. The general accepted approach is the use of Ingamells sampling constant K,.This quantity is defined as Ii', = 112 . lo4 . ( s , / T ~ , , ~ where )~, m represents the sample mass and s, the standard deviation caused by random sampling. The term lo4 - ( ~ ~ / a . ~ stands , , ~ ) ~for the relative sampling variance in square percentages, referred to by Ingamells as R'. The meaning of the sampling constant is that it produces the sample mass at which the contribution of sampling to the external standard deviation is 1%with 68% confidence. For a set of analytical results ri, each with their expected or internal standard deviation s+ the observed or external variance :s = C(xi- xaVg)*/(N - 1) is thought to be composed of the total internal variance s: = N / ( ~ ( ~ / and s ~ the , ~sampling ) variance s: [4]. Thus, sz = sz +,:s or I(, = 972 - lo4 . (s: - S~)/Z:,,~. In the earlier mentioned example for the laboratory sample containing 126 f 7 mgakg-' Zn and yielding an internal standard deviation of 1.8% (or 2.2 mg.kg-') and provided the analytical technique is under statistical control, Ii, can be calculated from 0.25 . lo4 . (49 - 5)/15129 to be 7.3 g. Once I<, is known, the test portion sample size which limits s, to a specific value can be calculated. In the example, with s, set to 5%, n?.points to 0.3 g.

3.4.3 Unifornzity of luborutoiy saniples Once a laboratory sample has been found sufficiently uniform to allow the drawing of test portions, possible differences in uniformity between laboratory samples, drawn from a subsample, have to be considered. Inter-laboratory sample variations can be due to either random or non-random causes. The latter may be phase separation, segregation, different moisture content, etc. and if non-random variations are detected, li', can no longer be used. A simple method to distinguish between the two cases is to analyze samples of different size from all laboratory samples. When the cause of the variations is random, I(, remains constant and when m increases, s, is expected to decrease. If s, remains identical while n2 increases, the cause is a non-random one. This approach has been successfully used to check interbottle variations for several BCR candidate reference materials [42]. It is important to establish whether or not different laboratory samples can be regarded uniform. It is on the laboratory sample level, that different divisions of a laboratory peiforming multi-component analysis or different laboratories in

SAMPLING AND SAMPLE PREPARATION

93

intercomparisons are requested to analyze. For such a case, the laboratory sample and the subsample may be identical. 3.4.4 Un~orntityof subsuniples Once a set of laboratory samples has been measured to be uniform, the subsample from which the laboratory samples have been taken can be considered uniform. A similar set of tests as described above can be performed to control the uniformity of different subsamples. Once this is achieved, the sample preparation procedure on the gross sample has been successful. 3.4.5 The gross saniple The discussion of the gross sample covers two separate items, being the relation of the gross sample towards the parent population and the treatment of the gross sample so as to enable analysis. According to Kratochvil, the relation of parent population and gross sample can be classified with a decreasing order of uniformity into representative, random or systematic samples. Represerttutive samples can only be drawn from a homogeneous parent population and thus suggest even distribution of the determinand in the matrix. This may be the case for trace elements in academic solutions. Sea- and fresh water cannot a yriori be expected to be homogeneous: there is always a change in trace element content in solution with salinity, depth, etc. Another example of a representative sample may be an aliquot of human blood. It is reasonable to consider this sample to be representative for the sampled individual at the moment of sampling. It is not a yriori allowed to extend this representativity to other points in time (e.g. change due to food intake). Kratochvil strongly suggests not to encourage the term representative, since the advantages compared to random sampling are not clear and the very concept demands much work for proof. A special case of representative samples are composite samples. Here, several samples from a batch (preferably taken at random) are combined. The composite sample is prepared for analysis and the analytical result is supposed to represent the average of the batch. Composite samples are often made for monitoring purposes, e.g. industrial waste. The procedure is only valid when sufficient knowledge is gained about the distribution of the analyte between samples so as to be able to estimate how many samples of which sample size have to be combined to make an estimate of the average content of the batch with a certain level of confidence. The information obtained this way refers to the averages of batches, while the information of the individual samples is lost.

94

J.R.W. WOITTIEZ AND J.E. SLOOF

Ratidonz samples are taken when the parent (and the target) population are considered to have a uniform distribution of the trace element of interest. For many environmental analytical problems, random distribution is often assumed. Nevertheless, the sampling approach in these cases is rarely random. fiatochvil states: “Random sampling is difficult. A sample selected haphazardly is not a random sample. On the other hand, samples selected by a defined protocol are likely to reflect the biases of the protocol. . . . Whenever possible, the use of a table of random numbers is recommended as an aid to sample selection.” The parent population is to be divided into numbered segments or grids and the segments which are to be sampled are identified by random selection. The randomness has to be dictated by a random number generator or table. In practice, this may be done to identify lichens which have to be sampled from trees in a particular area [121,168], spots in a rock formation where cores are to be taken 1841 or how in a region, devices should be situated to collect atmospheric deposition [120, 1691. Random sampling of a continuous flow (e.g. industrial waste water) may be done by sampling at evenly spaced time intervals and selecting random samples from tliis collection, or by collecting samples at randomly selected points in time. Random sampling is always laborious, but the information obtained on the distribution of the individual samples is vital for the correct interpretation of results. If the population to which one wants to extend the conclusions is limited to the samples drawn, these are named systeniatic samples. Thus, systematic samples are by definition, representative of the parent population. Very often, the complete parent population is collected in one or more samples, which makes the systematic sample(s) identical to the parent population. Basically, one pretends no generalization of the analytical results beyond the parent population (Table 3-6a). Systematic samples are drawn, for example, to investigate the influence of varying parameters with time in an industrial process. The status of such samples is not to be representative for the complete process, but rather, to reflect a well-defined segment of the process. Systematic samples (either or not composite) may have the characteristic to contain the target population they refer to. Here, systematic sample(s), parent population and target population are all the same. In this case, the analyst only has to deal with proper sample size reduction.

3.5 Sample decomposition Most trace element analytical techniques are based on measurement of the analyte in solution. Thus, standard procedures to convert solid (or solid containing) samples to solutions prior to detection is required. The terms decomposition, destruction, digestion, dissolution, ashing and mineralization all refer to this process. In this contribution, the general expression will be decomposition, which is specified to dry or wet ashing. Mineralization refers to those procedures which result in inorganic chemical forms of the analyte only.

SAMPLING AND SAMPLE PREPARATION

9s

Numerous papers have been published on decomposition procedures for trace elements. Comprehensive books and reviews have been published [ 11,34,67, 69,113,1281. Decomposition techniques have been discussed in topical reviews or papers [1,2,5, 14, 19,23,35,36,40,54,63,64,70,80, 111, 114, 116, 118, 125, 131,133,1621. Within the scope of this chapter, a comprehensive discussion on decomposition techniques is not feasible. A review on sample decomposition of biological materials, based on over 200 references, has recently been produced by one of the authors [ 1531, while methods of decomposition in inorganic analysis have been comprehensively treated by Sulcek and Povondra [ 1281. In this paragraph some aspects of decomposition of biological material, based on most of the above cited literature are presented. In general, it is required from any decomposition procedure, to alter the original chemical environment of the sample into a digest, i.e. a solution in which the analyte is distributed homogeneously. This may sound obvious, but especially for samples of botanical origin containing dust particles and silicates, a homogeneous digest is very hard to obtain 1361. More specific conditions set to a decomposition technique

are: (a) Guarantee of the integrity of the procedure with respect to the analyte. The necessity of the use of clean equipment and chemicals as well as appropriate cleaning procedures such as sub-boiling distillation need no further elucidation, after which has been discussed in the paragraph on Changes iti trace elenierit coniposition in this chapter; (b) Removal of residual matrix components which interfere in the detection. The first step here is to obtain a complete decomposition; (c) Possibility of adjustment of the oxidation state of the analyte and, consequently, compatibility with post-decomposition chemistry; (d) Execution is not hazardous or dangerous for lab personnel. Straightforward techniques such as Flame or Graphite Furnace Atomic Absorption Spectrometry and Inductively Coupled Plasma Atomic Emission- or Mass Spectrometry may perhaps not need complete mineralization and the demand for the analyte to be in solution might do. However, the risk for matrix effects is obvious. All those techniques, based on pre-detection chemistry, such as Isotopic Dilution Mass Spectrometry, need complete mineralization. Furthermore, a equilibration step, which adjusts the oxidation state is often needed. To this aim, HC10, is a reliable agent, when applied adequately [ 1051. The existing ashing techniques can be divided into wet ashing and dry ashing. Sansoni and Panday [113] distinguish some 30 different techniques in decomposition of biological material, while Sulcek and Povondra [ 1281 easily reach this number for trace element analysis in inorganic material.

J.R.W. WOITTIEZ AND J.E. SLOOF

96

Wet ashing is the hydrolysis and oxidation of the sample using one or more mineral acids. It comprises the decomposition of the matrix by oxygen, released at elevated temperatures by the acid(s). The primary chemicals reported to have been used in wet ashing are HNO,, H2S04,HClO,, HF and H202. Besides, many so-called ashing aids have been used, such as Mg(NO,),, KNO,, NaNO,, HBr, HC1, HCOOH and H3P0,. Table 3-8 displays some physical constants of these chemicals [22]. Wet ashing is applied in two modes: open vesselhow pressure and closed vesselhigh pressure techniques. Dry ashing uses the combustion of the sample with oxygen at high temperatures or excited oxygen/oxygen radicals at low temperatures. The source of oxygen here is from the air or from flasks and not from mineral acids. Dry ashing techniques can be divided in high temperature and low temperature and is applied both open and closed. Table 3-9 summarizes advantages and disadvantages of 12 often used applications of wet and dry ashing with respect to losses, blanks, sample size, ashing time, degree of decomposition and economical aspects. Techniques like fusion [45], Fenton's reagent [ 1131 or Wickbold combustion [ 1171 may be useful for specific analytical problems for biological materials, but have gained limited application and are not discussed here. For the use of fusion for inorganic materials, reference no. [ 1281 is recommended reading. There is probably not one decomposition technique which suits all trace element analytical techniques. With respect to requirements specific Table 3-8. Physical properties of selected acids, oxidizing agents and ashing-aids used in wet-ashing Compound

Molecular weight

Density (kg-I-')

Boiling point C)

Concentration (M)

(% WIW)

100.46

1.67

200

11.6

70

98.08

1.84

338

18.0

96

20.01

1.16

120

27.8

48

36.46

1.12

110

7.7

25

80.92

1S O

126

8.7

47

98.00

1.71

213

14.8

85

60.05

1.06

101

17.0

96

63.01

1.40

121

14.4

65

34.01

1.12

106

9.9

30

17.03

0.91

13.4

25

(O

Content

97

SAMPLING AND SAMPLE PREPARATION Table 3-9. Advantages and disadvantages of decomposition techniques Ashhg technique

Possible way Source of of losses blank

Sample size

Ashing time

Degree ofashing

Economic aspects

Cheap Needs supervision

Low pressure wet ashing HzSO4

+

Volatization

Acids, vessels,air

Up to seve- Up to 6 hours ralgrams

Highwith HClO,

HZSO4 HZOZ

+

Volatization

Acids, vessels, air

Up to seve- u p to 1 hour ral grams

High

None

Acids, vessels

Up to seve- u p to 1 hour ral g-

High

Acids (low)

250 mg

LOW

HN03 + HClO,

HN03 + HClO, with reflux

I

"

High pressure wet ashing --Teflon lined pressure vessel

Retention

Quaxtz-lined pressure vessel

Retention

Acids (very low)

500 mg

Several hours

High

pwave assisted retention

Retention

Acids (low)

250 mg

< 1 hour

Incomplete

2 hours

Needs no supervision I

Expensive

Needs no supervision

High tempemture dry ashing Muffleiinnace ad heated tube

Volatization and retention

Vesselsand Urmallyup air (high) to 1 gram

Several hours

High

Schoniger flask

Retention

Vesselsad

<10 min

High

<100mg

ChP Needs no supervision I)

reagents

(low for I) Oxygen bomb

Retention

Vessels and Several reagents grams (high for metals)

<20 min

High

Needs supervision

Modified Schiiniger flask

Retention

Vesselsad reagents

<5OOmg

10 min

High

Cheap Needs supervision

Trace-0-Mat

None

Acids (very low)

250 mg

< 1 hour

High

Expensive Needs supervision

Acds (very low)

upto2 grams

Several hours

Dependson sample size

Lav temperature dry mhing LTA

volatization (Hg)

"

98

J.R.W. WOITTIEZ AND J.E. SLOOF

to trace element analytical chemistry (no contamination or losses through volatilization or retention), low temperature dry ashing, microwave assisted wet digestion, quartz-lined high pressure wet ashing and low pressure dry ashing with reflux (Trace-0-Mat) seem to be the best choice. However, all of these techniques require considerable apparatus investments. Apart from the microwave, these techniques also demand supervision from well trained laboratory personnel. As far as economical aspects are concerned (low procurement, short decomposition time, high sample throughput), microwave assisted wet digestion and Schaniger combustion appear to be high ranking. The latter technique has its application mainly limited to the determination of the halogens. Conventional dry ashing in muffle furnaces or heated tubes is cheap and easy to perform, but is not compatible with elemental analysis on the & k g level, unless an extremely well equipped laboratory applies the technique. A similar remark is true for the different modes of conventional low pressure wet ashing. Frequently, the need for complete decomposition is stated in papers, but only a few researchers actually studied the degree of mineralization. Stoeppler [ 1241 et al. use the carbon mass balance to conclude that high pressure decomposition using only nitric acid just yields 6 0 4 5 % . Yang and coworkers [158] have combined a radiotracer technique and paper electrophoresis to investigate the optimal decomposition conditions for zinc in liver material. They used an open H,SO,-HNO, digestion with reflux and dry-ashing using a low temperature asher. They concluded that dry-ashing resulted in 45% of 65Zn still in an organic form, while the reflux cycle in the wet ashing procedure had to be passed three times to obtain complete mineralization. In a later study, they extended their research to Zn, Cu, Se and Co using adequate radionuclides and concluded that Co and Se compounds in liver are more easily decomposed than Zn and Cu compounds [159]. Pratt et al. [lo41 used voltammetry and liquid chromatography to investigate the result of a microwave digestion of bovine liver. He concluded that the microwave decomposition using just HNO, is not capable of destroying the major decomposition compounds 0-,m- and p-niaobenzoic acid. Also, other organic compounds which irreversibly complex Cu remain existing. Complete mineralization is obtained by refluxing with HCIO,. As already mentioned, not all detection techniques demand the same degree of mineralization. Analytical techniques may thus be classified according to the amount of mineralization they need. (a) No miner-ulizution.Ideally, a purely instrumental approach is the only way to prevent losses and Contamination due to decomposition. Of the instrumental techniques, NAA is by far the most powerful. Some X-ray techniques, including PIXE, suffer from insufficient sensitivity to be applied in trace element analysis below the 1 m a g level. A rather recent development is Solid Sampling Zeeman Graphite Furnace Atomic Absorption Spectrometry

SAMPLING AND SAMPLE PREPARATION

99

(SSZGFAAS), in which slurries of mg amounts of solid materials are directly analyzed. Recent papers give results on Pb, Cd,Cu, Ni in different reference materials [6] and Mn, Zn, Fe, Cu, Pb and A1 in NBS Citrus Leaves [87] and Pb, Cd, Hg, Fe and Zn in fish muscle [103]. The low sample intake (upto 10 mg) sets high demands on the uniformity of the distribution of the elements of interest in the sample to be analyzed. ICPMS has been demonstrated to generate accurate results for human body fluids with a minimum of sample treatment [ 1341. For the analysis of solid samples, many users of ICPMS rely on simply dissolving the sample, as the plasma has ashing capabilities itself. Greenberg et al. [36], however, state that the danger of losing non decomposed particulate matter because of non-uniform distributions or sticking to container walls, is obvious. (b) Partial mineralisution. In general, the accuracy of techniques such as GFAAS, ICPMS, ICPAES and FAAS is not strictly dependent on complete decomposition. There is always the risk of matrix effects due to remaining organic materials and incomplete sample recovery from the decomposition vessel. For these techniques, teflon-lined pressure vessel decomposition gained much popularity. Basically, all decomposition techniques which meet the demands of losses and contamination can be applied. (c) Complete minerulizutioic. The electrochemical techniques, DPASV, DPCSV and ISE, demand complete mineralization. The same is true for all methods which require some chemical separation step, such as HGAAS, IC, RNAA, IDMS and all spectroscopy techniques depending on extraction, precipitation, flow injection or ion-exchange. RNAA aid IDMS are techniques which use a spike, either a stable- or radioactive isotope, to quantify or intrinsically correct for chemical yields of the separation procedures. For these techniques, not only complete mineralization, but also equilibration of the chemical form of analyte and spike is obligatory. Though often neglected, standard addition experiments also need careful consideration in this respect. The safest way of operation to obtain total mineralization is to complete any decomposition technique with the addition of perchloric acid and heating until perchloric acid has evaporated to dryness. All single acid wet decomposition procedures, including that in the microwave, result in insufficient decomposition. Combustion in a dynamic system (SchOniger, Trace-0-Mat), high temperature dry ashing and possibly quartz-lined pressure wet ashing, are recommended alternative ways to achieve thorough mineralization. Choosing a decomposition mode only to be able to meet the requirements of the detection technique is an incomplete approach. The choice of decomposition should primarily be directed by both the matrix and the element of interest.

J.R.W. WOITTIEZ AND J.E. SLOOF

100

(a) As to investigate matrix specific analytical problems in multi element analysis, some authors compared different decomposition techniques. Reference materials [36,43,63,87] and body tissues [78,102] are the prime target materials. The common conclusion is to achieve as much a complete mineralization as possible; (b) Ordinary elements of interest to the nutritionist being on the mgkg level, such

as Na, K, P, Cu, Zn, Fe, Ca, Mg usually can undergo conventional dry- or wet ashing for accurate determination. The same is true for elements of interest to the toxicologist when occurring on the mg/kg level, such as Cd, Pb, Ni, Pt; (c) For those elements present on a level < 100 m e g , it is recommended to use the contamination poor decomposition techniques, i.e. either micro-wave assisted and quartz or Teflon-lined high pressure wet ashing or dynamic dry ashing (Trace-0-Mat). This restriction is valid for most elements in body tissues: (d) For those elements occurring on a level < 100 p g k g and sensitive to contamination such as Al, Cr, Mn, Hg, V, Pb and Sn, contamination-poor decomposition techniques should be executed in dust-poor laboratories:

(e) For elements featuring losses through volatilization, such as Hg, Se, As, S b, Sn, I, F, Ge, In and Te, several options are possible. The halogens can conveniently be made accessible from the matrix by the Schoniger combustion. Especially selenium has been subjected to much research as far as adequate decomposition is concerned [28,100,144]. The safest techniques are the all closed systems, such as microwave assisted wet digestion or the Trace-0-Mat apparatus.

References 1 Analytical Methods Committee, Analyst, 84 (1959) 214-216.

2 Analytical Methods Committee, Analyst, 92 (1967) 403-407. 3 ACS Committee on EnvironmentalImprovement,Analytical Chemistry,52 (1980) 2242-2249. 4 ACS Committeeon EnvironmentalImprovement,Analytical Chemistry, 55 (1983)22 10-2218. 5 Association of Official Analytical Chemists, Official Methods of Analysis, K. Helrich (Ed.), 1990.

6 Bagschik, U., Quack. D., and Stoeppler, M., Fresenius J. Anal. Chem., 338 (1990) 386-389. 7 BCR, Community Bureau of Reference Certificateof Analysis of CRM 278 ‘Trace Elements in Mussel Tissue,” Commission of the European Communities, Brussels, 1988. 8 Behne, D., J. Clin. Chem. Clin. Biochem, 19 (1981) 115-120.

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9 BERM 4, Biological and Environmental Reference Materials, Proceedings of the Third Symposium,Fresenius Z. Anal. Chem., 332 (1988). 10 BERM 5. Biological and Environmcntal Reference Materials, Proceedings of the Fourth

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59 Iyengar, G.V.. Tanner, J.T., Wolf, W.R., and Zcislcr. R.. The Science of the Total Environment. 61 (1987) 235-252. GO Iyengar, G.V. and WoiltieL. J.R.W., Clinical Chemistry. 34 (19883) 474481.

61 Iyengar, G.V. and Iyengar, V.. in: Quantitative Trace Analysis of Biological Materials, H.A. McKenzie and L.E. Smythe (Eds.), Elsevier, Amsterdam, 1988b, pp. 401419.

62 Iyengar, G.V., The Science of the Total Environment, 100 (1991) 1-15. 63 Jones, J.W. and O'Haver, T.C., Spectrochimica Acta, 40B (1985) 263-277.

64 Jones, J.W., in: Quantitative Trace Analysis of Biological Materials. H.A. McKenzie and L.E. Smythe (Eds.), Elsevier, Amsterdam, 1988, pp. 353-367. 65 Karin. R.W., Buono, J.A.. and Fasching. J.L.. Analytical Chemistry. 47 (1975) 2296-2299.

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68 Kingston, H.M. and Jassie, L.B., Introduction to Microwave Sample Preparation: Theory and Practise, American Chemical Society. 1990.

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77 Litman, R.. Finston, H.L., and Williams, E.T., Analytical Chemistry, 47 (1975) 2364-2369.

78 a k e , J., Anal. Chim. Acta. 104 (1979) 225-231. 79 Luten, J.B., Thesis, State University of Utrecht, 1977.

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82 Mattinson, J.M., Analytical Chemistry, 44 (1972) 17 15-1716. 83 May. T.W. and Kane, D.A., Analytica Chimica Acta, 161 (1984) 387-391. 84 Maxwell, J.A., in: Accuracy in Trace Analysis: Sampling Sample Handling, Analysis, P.D. LaFleur (Ed.), National Bureau of Standards Special Publication 422,1976, pp. 285-299.

85 McKenzie, H.A. and Smythe, L.E., in: Quantitative Trace Analysis of Biological Material, H.A. McKenzie and L.E. Smythe (Eds.). Elsevier. Amsterdam, 1988, pp. 19-39.

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86 Ibid. pp. 3949. 87 Miller-Ihni. N.J.. J. Res. National Bureau of Standuds, 93 (1988) 350-354.

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93 Moody. J.R., Trends in Analytical Chemistry, 2 (1983) 116-1 18. 94 Morris. J.G., A Biologist’s Physical Chemistry, 2d ed., Edward Arnold, London. 1978.

95 Murayama, M., Suzuki. M.,and Talritani. S., J. Chromatography, 466 (1989) 355-363. 96 Murphy, TJ.. in: Accuracy in Trace Analysis: Sampling, Sample Handling, Analysis, P.D. LaFleur (Ed.). National Bureau of Standards Special Publication422,1976, pp. 509-541. 97 Murphy, TJ.,National Bureau of Standards Special Publication 492, (1977) 5-8.

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Biology4, P. Bratter and P. Schramel (Eds.), Waller de Gruytcr, Berlin. 1987, pp. 349-358. 101 Nieboer, E. and Jusys, A.A., in: Chemical Toxicology and Clinical Chemistry of Metals, S.S. Brown and J. Savory (Eds.), Academic Press, New York, 1983. pp. 3-17. 102 Oehme, M.and Lund, W., Fresenius 2.Anal. Chem., 298 (1979) 260-268. 103 Pauwels. J., Kramer, G.N., De Angelis, L., and Grobecker, K.H.. Fresenius J. Anal. Chem., 338 (1990) 515-520. 104 h t t , K.W.,Kingston, H.M., MacGrehan, W.A., and Koch, W.F., Anal. Chem., 60 (1988) 2024-2027.

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110 Ryabukhin,

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of Biological Systems, Vol. 2. G.V. Iyengar, K.S. Subramanian and J.R.W. Woittiez (Eds.), CRC, Boca Raton, FL, 1994a (in press). 154 Woittiez, J.R.W., Changes in Trace Element Composition due to Sample Preparation: Causes and Precautions. in: The Elemental Analysis of Biological Systems, Vol. 2, G.V. Iyengar, K.S.Subramanian and J.R.W. Woittiez (Eds.), CRC, B O CRaton, ~ FL,1994b (in press). 155 Woittiez, J.R.W., On the use of Separation Techniques for Elemental Analysis of Protein Fractions, in: Trace Element Analytical Chemistry in Medicine and Biology, Vol. 6, B. Ribas (Ed.), Walter de Gruyter, Berlin, 1994c (in press). 156 Woittiez, J.R.W., Subramanian, S., Volkers, KJ., and de Goeij, J.J.M., J. Radioanal. Nucl. Chem., 168 (1993d) 5-14. 157 Woittiez, J.R.W., Chemical Behaviour of 210Po,""Pb and 226Raduring Production of Phosphoric Acid via the "Wet Process," IRI report, Interfaculty Reactor Instituteof the Delft University of Technology, Delft, The Netherlands (in Dutch only). 1993. 158 Yang, J.Y., Yang, M.H.. and Lin, S.M., Analytical Chemistry, 57 (1985) 472474.

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159 Yang, J.Y., Yang. M.H., and Lin, S.M., Andylical Chemistry, 62 (1990) 146-150. 160 Zeisler. R., in: The Pilot National Environmental Specimen Bank, National Bureau of Standards Special Publica!ion 656, R. Zeisler. S.H. Harrison and S.A. Wise (Eds.), 1983. pp. 35-39. 161 Zeisler, R., Greenberg. R.R., Stone, S.F., and Sullivan, T.M.. Fresenius Z. Anal. Chem., 332 (1988)612-616. 162 Zief, M. and Mitchell, J.W., Contamination Control in Trace Element Analysis, Wiley, New York, 1976. 163 Zief, M. and Horvath. J., in: Accuracy inTrace Analysis: Sampling Sample Handling Analysis, ED. LaFleur (Ed.). National Bureau of Standards Special Publication, 1976,422,pp. 363-377. 164 Upchurch Scientific Catalogue. 1992. 165 Quevauviller, Ph., Kramer. G.N., and Griepink, B., Marine Pollution Bulletin, 24 (1992) 601-4506.

166 Wolf, W.R., Iyengar, G.V.,and Tanner, J.T.. Fresenius J. Anal. Chem., 338 (1990) 473476. 167 Greenberg, R.R., Zeisler, R., Kingston, H.M., and Sullivan, T.M., Fresenius Z. Anal. Chem., 332 (1988) 652-657. 168 Sloff. J.E. and Wolterbeek, H.T., Environmental Monitoring and Assessment. 25 (1993~) 225-234. 169 Sloof, J.E. and Wolterbeek, H.T., Lichenologist, 23(2) (1991) 139-165. 170 Zeisler, R.. Harrison, S.H., and Wise, S.A., Biological Trace Element Research, 6 (1984) 31 4 9 .

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CHAPTER 4

Separation and preconcentration of trace elements

KIKUO TERADA Departmetit of Chemistry.Faculty of Scietice. Katiazawa University. Kakumamachi. Katiazawa 920.11. Japan

Contents 4.1

Separation and preconcentration of trace elements by coprecipitation . . . . . . . . . . 110 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Coprecipitation with inorganic precipitants . . . . . . . . . . . . . . . . . . 112 Coprecipitation with organic collectors . . . . . . . . . . . . . . . . . . . . 113 Separation and preconcentration of trace elements by flotation . . . . . . . . . . . . . 114 4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 4.2.2 hinciple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116 4.2.3 Generalprocedures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 Preconcentration and separation of trace elements by solvent extraction . . . . . . . . 118 4.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118 Extractionof trace elements . . . . . . . . . . . . . . . . . . . . . . . . . . 121 4.3.2 Separation and preconcentration of trace elements by ion-exchange . . . . . . . . . . . 128 4.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128 4.4.2 Ion-exchange resins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 4.4.3 Equilibrium and selectivity . . . . . . . . . . . . . . . . . . . . . . . . . . 130 Practical column operation . . . . . . . . . . . . . . . . . . . . . . . . . . 132 4.4.4 4.4.5 heconcentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 Ion chromatography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134 4.4.6 Separation and preconcentration by sorption . . . . . . . . . . . . . . . . . . . . . . 137 4.5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 4.5.2 Activatedcarbon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Porous polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138 4.5.3 4.5.4 Complex-forming adsorbents . . . . . . . . . . . . . . . . . . . . . . . . . 140 Natural polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 4.5.5 4.1.1 4.1.2 4.1.3 4.1.4

4.2

4.3

4.4

4.5

KIKUO TERADA

110

4.1 Separation and preconcentration of trace elements by coprecipitation 4.1.I Introduction

In separation chemistry, it has been an undesirable problem that precipitates were usually contaminated with other elements which are normally soluble under the conditions of the precipitation. Later, this phenomenon has proven to be of great value for an effective preconcentration process of trace elements. First, in 1951 Bonner and Kahn published a summary on the separation of carrier-free radioactive tracer by coprecipitation method [ 13. In recent years, this technique has found wider application to separation and preconcentration of trace elements in various kinds of samples, such as natural waters, high-purity materials, and so on. In modern books, the reviews on this subject are given for the purpose of preconcentration of trace elements [2-51. In general, coprecipitation of trace elements are carried out with inorganic and organic collectors attaining high degrees of concentration, especially when subsequent determination can be performed by using solid samples, e.g. X-ray fluorescence or atomic emission spectrometry. 4.1.2 Mechanism

Depending on the nature of the solid phase produced in a solution and the experimental conditions, coprecipitation occurs by different mechanisms. Although the distinction between the various types of coprecipitation is not sharp, they may be classified according to the following mechanisms: the formation of mixed crystals and mixed chemical compounds, the surface adsorption, and the occlusion and mechanical inclusion of trace components into the other compounds during crystal formation. However, these processes often proceed concurrently making the precipitation process quite complicated.

4.1.2.1 Isomorphous mixed-crystal formation The processes of coprecipitation by isomorphous mixed-crystal formation have been studied relatively well, and the distribution of trace elements were found to be governed by the following two laws: 1. Berthelot-Nernst Distribution Law (Homogeneous distribution)

If digestion is continued throughout the precipitation process, equilibrium will be established between the trace elements in the interior of the crystals and the solution, resulting in homogeneous distribution of the trace element in the precipitate. Then, the following equation is applicable. (Trace element)+, - (Trace element),,, -D (Carrier)g, (Carrier)sol,, The larger the value of D, the higher is the enrichment of trace elements.

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

111

2. Doerner-HoskinsLaw (Logarithmic distribution) When the ions cannot reach the interior of the crystals, equilibration will be established between the trace elements in the solution and those on the extremely thin surface layer of the crystals. This results in a logarithmic distribution of the impurities, and the followingequation should be applicable. log-=A*logTo Co

Ts

cs

where T and C represent trace element and carrier, and and denote the concentration in the solution before and after the precipitation, respectively. In practice, coprecipitation of the trace elements may occur between the above two limiting distribution laws.

4.1.2.2 Surface adsorption The surface of a precipitate is particularly reactive. Ions at the surface of a crystal are incompletely coordinated and, hence, are free to attract other ions of opposite charge from the solution. The suiface adsorption of ions on ionic precipitates has been described by Paneth-Fajans-Hahnrule [6] which describes that adsorption generally increases with the growing surface area of the crystal and with the decrease in the solubility of the compounds of the trace elements which the elements form with oppositely charged ions of the crystal. However, there are many exceptions in this expression, e.g. in spite of the slight solubility of PbCl, or PbI,, it does not coprecipitate with HgCl, or HgI,. Two other main factors, the dissociation constant of the adsorbed electrolyte and the deformation ability of the ions, are effective for the adsorption. The smaller the dissociation of the adsorbed electrolyte, the larger is the adsorptivity. The adsorptivity also increases with the increase of deformability of the adsorbed electrolyte. The deformability usually increases with size of the ion. Another mechanism of adsorption presented by Pauling is an ion-exchange process. When the radius of an ion in the solution is similar to an ion on the surface of the crystal, they are exchangeable with each other. This is more effective when the ion in the solution forms slightly soluble compounds with the ion of opposite charge in the crystals. Thus, lead (11) ion can be adsorbed on the surface of barium sulfate precipitate even in the absence of excess of sulfate ion in the solution according to the following exchange reactions: BaSO,(surface)

+ Pb2’

e PbSO,(surface)

+ Ba”

KIKUO T E W A

112

4.1.2.3 Occlusion and mechanical inclusion When an ion adsorbed on the crystal surface from the solution is trapped by subsequent crystal layers, the ion will be occluded interior of the precipitate. It is more prevalent with colloidal precipitates than with crystalline ones, especially under a rapid precipitation process. For example, freshly precipitated hydroxides or sulfides contain a certain amount of impurities, most of which are released upon aging of the precipitates. Thus, coprecipitation by occlusion generally gives a poorly reproducible yield of the trace elements to be coprecipitated.

4.1.3 Coprecipitation with inorganic precipitants Coprecipitation of trace elements with inorganic precipitants is usually carried out by using amorphous precipitates with large surfaces, e.g. metal hydrated oxides, sulfides, oxynates and others. The mechanism and applications of such coprecipitation processes have been well studied. Among various hydrated oxides, coprecipitation with those of iron (111) and manganese (IV) were mainly used, although many other hydroxides, such as Al(OH),, La(OH),, Zr(OH),, Th(OH),, Mg(OH),, and Ni(OH), were also employed. The coprecipitation techniques are commonly used for separating and concentrating trace elements from very dilute solutions, e-g. natural waters or treated waste waters, and high-purity metals. Since the solubility of hydroxides and sulfides is mainly governed by pH value of the solution, pH control should be essential for the effective collection of trace metals. Figure 4-1 shows coprecipitation yields of 100’

8 u

=50 a I

6

.

0 6

7

8

9

PH

10

I1

6

7

9

8

10

II

PH

Figure 4-1. Coprecipitation of metal ions with aluminum hydroxide precipitates

some metal ions with aluminium hydroxide. It can be seen from the figure that a removal of many metal ions from a solution may be possible at pH 9-10. By the use of iron (111) hydroxide, similar results are obtained, in addition, a quantitative

SEPARATION AND PRECONCENTRATIONOF TRACE ELEMENTS

113

collection of metal ions is also attained at pH > 10, because of insolubility of the Fe(OH), precipitates. Since most metals form sparingly soluble hydroxides, coprecipitation by hydrous oxides is usually low selective, so that many trace metals are likely to be simultaneously coprecipitated. Table 4- 1 shows the examples of preconcentration of trace elements by coprecipitation with inorganic collectors. 4.1.4 Coprecipitation with organic collectors

Organic collectors usually have good selectivity and effectiveness for concentrating trace elements even when it is present in a solution in the ratio of l:1015. Table 4- 1, Co-precipitationof trace elements with inorganic collectors ~

Collector

~~~

Precipitated elements

Substance analyzed

As, Bi, Cr, Ge, In, Mn, Sb, Sn Te, 'Ii, V Bi, Sb, Se, Te, 14 REE

Cu, Ni, Pb, Th, V. Zn Be, Eu, Mn, etc. As, Bi, Ga, In, Ni, Pb,Sb, Sn,

Te,?I Cr, Cu,Mg, Mn, Zn

Method of determination AES

High-purity Cu rocks, minerals, meteorites Seawater

AAS NAA NAA ICP-AES

AES

High-purityA1 Seawater

flame A A S ICP-AES

Co, Cr, Cu, Fe, Mg, Mn, Ni, Si, 'Ii,Zn

High-purityA1

ICP-AES

Pt metals

-

AES

As, Bi, Cd, Co, Cu. Hg, Se, Sn, Zn

various substances

XRF

Ba

Alkali-metalhalide Tap water Tap water Natural waters

AES

Al. Cu, Fe, Ni, 'Ii, Y, Zn

so;-

c1I-

~

XRF

XRF photometry

KIKUO ‘IERADA

114

Other advantages of the organic collectors are that the collectors can be removed by simple combustion and be dissolved in an organic solvent. The mechanism of coprecipitation of trace elements with organic collector may be classified into the following six types [4]: 1. The sparingly soluble organic compound is a bulky organic cation which forms an ion pair with the anionic complexes, [6] e.g. antipyrine dye salts for some metal thiocyanates;

2. An insoluble salt of organic anion was formed with the metal cations, as was done with many inorganic precipitants. The insoluble salt is coprecipitated together with excess of the reagent in the acid form, e.g. 8-quinolinol complexes of metals on excess of the reagent [7];

3. A chelate complex is formed between the metal and acidic reagent. This chelate is coprecipitated with the precipitate formed by the excess of acid reagent and a bulky cationic organic reagent [8]; 4. The coprecipitation of an inner-complex compound is of the ion to be separated

with an organic reagent, with the large excess of the organic reagent, such as PAN [9];

5. A chelate of the metal ion with an organic reagent is absorbed-coprecipitated on a water-insoluble organic compound. Several metal ions form a chelate with dithizone which are coprecipitated on phenolphthalein [ 101. In this case it is a solid version of solvent extraction [ 111;

6. “Colloidal-chemical” sorption of the metal ions on a mixture of insoluble organic reagents. Typical examples of the coprecipitation of trace metals with organic collectors are described in Table 4-2.

4.2 Separation and preconcentration of trace elements by flotation

4.2.1 Introduction Among the preconcentration techniques developed to date, coprecipitation method may be widely used according to its versatility and simplicity. However, the settling of the precipitate and the filtration are generally time-consuming and tedious, especially for the treatment of large volumes of a sample. On the contrary, the flotation techniques which have been introduced to separation chemistry by Sebba [ 121 and first applied to concentrating trace elements by Fukuda and Mizuike [ 131, have

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

115

Table 4-2. Co-precipitationof trace elements with organic precipitants Precipitants

Elements to be precipitated

Conditions and samples

Method of detemiination

Alizaline blue

Ag, Bi, Co, Cu, Fe, In, Pd, Th, Ti, V, W

pH < 2

XRF

APCD

Ag, As, Cr, Cu,Fe, Ga, Ni, Pb, V, Zn

Microporous filter

XRF

Cupferon (Cu)

Bi, Fe, Hf, Mo, Nb, Sn, Ta. Ti, V, W. Zr

In metal

AES

As, Ga. Mn, Mo, Zn, 19 other elements

MF filter

XRF

DDTC DDTC, S-mercaptoquinoline

Co, Cr, Cu, Fe, Mn. Mo, Nb, Pb, Ta, Ti, V, W

PH 6

AES

DDTC, Cupferon, 8-quinolinol

Co, Cu, Fe, Mo, Bi, Ti, V

pH 4.5

AES

DDTC, DMG, I-nitroso-2-naphtol

Ag, Au, Bi, Co, Cr, Cu, Fe, In, Mo, Ni. Pb, Sn. Ti, V

Diphenylthiourea

Ag, Au, Ir, Pd, R, Rh, Ru

Industrial waste

AES-ICP

Dithizone

Au, Pd

MF filter

AAS

DMABR

Hg

Seawater High-purity Cu

AAS Photom.

Thionalide

As, Ag

Seawater

Photom.

AES

APCD: ammonium pyrrolidinecarbodithioate,DMG: dimethylglyoxime,DMABR: p-diniethylaminobenzilidenerhodanine

great advantages: the coprecipitate can be readily separated from the mother solution in a few minutes by the use of adequate surfactants and with the aid of a rising stream of inert gas. Flotation techniques also eliminate the difficulties associated with separation of colloidal precipitates and allow treatment of very large volumes of samples, thus increasing the preconcentration factor. Special reviews [ 14,151are devoted to analytical preconcentration.

116

KIKUO TERADA

4.2.2 Principle

Flotation is widely employed as an effective method of enriching mineral resources from raw ground ore materials. According to the differences in surface activity, the mineral resources are selectively adsorbed on an ionic or non-ionic surfactant which is located at the surface of air bubbles rising through the suspended solution, and are separated from the raw materials. The preconcentration techniques by flotation are classified into the following two methods. 4.2.2.1 Ion flotation A trace metal ion or its complex ion in a solution forms a hydrophobic bulky ion-pair compound with a surfactant of the opposite charges at the surface of inert gas bubbles, and ascends to the top of the solution forming a thin film. Ion flotation techniques are a type of foam separation method. With ion flotation, although a high recovery can be obtained for the trace elements, the separation may be poor between the trace elements and the matrices which enter into the foam layer. It is preferable to use a surfactant which selectively reacts with the desired ions or their complex ions. The quantity of the surfactant should be a little greater than the stoichiometric amount, but too excessive amounts are undesirable. The optimum pH range for the reactions between the desired ions and the complexing agent and surfactant should be selected. Moreover, the bubbling time and gas flow rate should be carefully optimized in each case. 4.2.2.2 Flotation after coprecipitation A more effective flotation technique is a combination of the coprecipitation and the flotation processes. Trace metal ions are coprecipitated with the collector precipitates of cationic or anionic charges, which are then floated by bubbling with or without the aid of surfactant ions of oppositecharge. This technique is extensively used for preconcentration purposes. The flotation processes are improved by the following factors: (1) Bulky flocculent precipitates larger than gas bubbles are desirable as collector precipitates, because numerous tiny gas bubbles are easily trapped in the interstitial spaces and on the surfaces of the precipitates to give sufficient buoyancy. (2) Addition of small amounts (ca. 1%) of ethanol (or other solvents such as methanol and acetone) is essential to generate numerous tiny bubbles, which will cause a good separation during flotation. (3) Since collector precipitation and flotation are generally carried out at the same pH, the optimum pH range should be chosen on the basis of recoveries of the trace elements in collector precipitates. (4) Generally, surfactant ions of opposite charge to precipitate surface are necessary for the flotation of the inorganic collector precipitates. Since the surface charge of common hydroxide precipitates depends on the pH of the solution and changes its sign at the isoelectric

SEPARATION AND PRECONCENTRATIONOF TRACE ELEMENTS

1 01

I

4

6

,; 8

PH

117

I

10

I2

Figure4-2. Flotation of iron (111) hydroxide precipitates. In the presence (-) ethanol. (a) Sodium oleate, (b) Benzalkoniumchloride.

or absence (- - -) of

point, either cationic or anionic surfactants are effective over a relatively wide pH range as shown in Fig. 4-2 [16]. The important functions of the surfactants are that they turn the hydrophilic surfaces of the collector to hydrophobic, and form a stable foam layer to support the floated precipitates on the solution surface. Flotation of inorganic collectors without surfactants is possible in some cases. For example, bulky flocculent iron (111) hydroxide precipitates are floated by bubbling with small amounts (ca. 1S)of mehtyl cellosolve. 4.2.3 General pmcedures

The general procedures for separation and preconcentration of trace elements by flotation were reviewed by Mizuike and Hiraide [14,15]. Typical flotation cells are shown in Fig. 4-3. The flotation setups generally include several common basic devices such as a gas-generating source, a regulator system, a gas-washing flask, a flow meter, a flotation column, a sintered-glass disk, and devices for collection of the foam and precipitate. Because of the simplicity of the apparatus, difficulties are eliminated for handling them. The other advantages of flotation techniques are (1) a large volume of a water sample can he rapidly treated, resulting in a high concentration factor; (2) the apparatus is inexpensive; (3) relatively small amounts of the reagents are necessary for complete flotation and separation of the trace elements. As a typical example, a multi-elements preconcentration of heavy trace metals in water by coprecipitation and flotation with indium hydroxide will be described [ 171. To a 1,200 ml of water or seawater in a flotation cell, 1 ml of indium nitrate solution

KIKUO TERADA

118

(4

(b)

Figure4-3. Vpical flotation cells. (I) Nitrogen or air introduction, (2) sintered glass disc, (3) air bubbles, (4) sample solution, (5) foam layer. (6) sampling bottle or tube, (7)suction, (8) foam, (9) vitreous silica wool,(10) magnetic stirrer, (1 1) polyethylene insert.

(100 mg In/ml) is added. The pH is adjusted to 9.5 with 0.3 M sodium hydroxide solution while stirring with a magnetic stirrer to form a floc of hydrated indium oxide. A bubbler (porosity 4) is placed in the cell, and 2 ml of sodium oleate solution (1 mg/ml in 70% ethanol) and 1 ml of sodium dodecyl sulfate solution (4 mg/ml in 70% ethanol) are added. While gently stirring, nitrogen gas is passed for 3 to 5 min to obtain complete mixing and flotation of the precipitate with the aid of numerous tiny bubbles (diameter below 0.5 mm). The precipitates which have become mixed with foam are sucked into a sampling tube, as shown in Fig. 4-3(b). Another 1 ml of sodium dodecyl sulfate solution is added to the water sample, nitrogen gas is passed and the precipitates are sucked once more as described above. The foam in the sampling tube is destroyed with two 1 ml portions of ethanol, and the solution is removed by filtration under suction. The precipitates are washed with three 2 ml portions of water. A 2 ml of 14 M nitric acid was added to dissolve the precipitates, the filtrate is collected in a 5 ml volumetric flask, and then amounts of heavy metals, such as Cr(II1). Mn(II), Cu(II), Co, Ni, Cd and Pb are determined by AAS or ICP-AES. The typical examples of the preconcentration of trace elements by flotation are listed in Table 4-3.

4.3 Preconcentrationand separation of trace elements by solvent extraction 4.3.1 Introduction Solvent extraction is probably the most extensively used method for separation and preconcentration of trace elements according to its simplicity, rapidity, and wide scope [2,18]. The theory and the extraction systems have been almost established. Numerous investigations have been done to ascertain the composition, structure and

119

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

Table 4-3. Concentration of trace elements by flotation Collector precipitates

Trace elements

PH

Method of determination

Precipitatioii Flotatioir after Co-ppttt.

AKOHh

Cd, Co, Cr(II1)

9.5

AAS

Fe(OHh

As(V), Bi, Mo P. Sb(II1, V) Se(IV), Sn(I1, IV)

4

AAS, Photoni.

V(V) UWI)

5 5.7

AAS Photom.

(MC)

Cu, Zn As(II1, V)

7.6 8-9

AAS, Photom. AAS

MOW3 (Na-oleate)

Cd, Co, Cr(II1) Cu, Mn(I1)

9.5

ICP-OES

'WOW4 MBT Iorr potation DPC + Na-DDS

U(W

5.7

Huor.

Ag

Seawater

AAS

CrtIII)

Water

Photom.

PDTZ + Na-DDS

Cu. Fe(II1)

Seawater

Photom.

But-xanthate

CU,HgC13CI

Water

AAS

UWI)

Seawater

Photom.

Fe(OH13

+ (TMA Arsenazo I11

+ TDDMA

MBT: 2-mercaptobenzothiazole, DDS: dodecylsulfate, DPC: diphenylcarbazide. CI'MA: cetyltrimethylammonium,TDDMA: tertradecyldimethylammonium, PDTZ: 3-(2-pyridyl)5,6-diphenyl-l,2.4-triazine,MC: methyl cellosolve properties of the compounds to be separated, the dependency of extractability of metals on the composition of the aqueous phase, the type of extractants and their concentrations in the organic phase, the concentration of the element to be separated, and temperature. Solvent extraction is based on the distribution of a solute between two immiscible liquid phases, in general, water and organic solvent. In equilibrium conditions the ratio of concentrations of the solute distributed in both phases is a constant value at a given temperature. This ratio is designated as partition coefficient, A', and described by

KIKUO TERADA

120

CO I(, = Ca

Here, Goand C, are the equilibrium concentrations of the solute in one and the same form distributed in the organic phase and the aqueous phase, respectively. This is, however, an ideal and rarely occurring case in inorganic analysis. In real cases, a solute usually exists in more than one form in an aqueous and an organic phase; therefore, a more useful parameter to describe its distribution during solvent extraction is the distribution ratio, D.

D=-c C Ca

C O

(4.2)

where C C, and C Ca are the total concentrations of the solute in the organic and in the aqueous phase, respectively. The extraction efficiency, E, is related to the distribution ratio by the following equation:

where Vo and V, are the volumes of the organic phase and the aqueous phase, respectively. The extraction efficiency depends on the value of D and the volume ratio of the organic phase and the aqueous phase. For preconcentration purposes, a solute should be extracted from an aqueous phase into a small volume of an organic phase because the ratio of Va/V, represents the preconcentration factor. Then, to achieve a high extraction efficiency (E > 99%) in one extraction process, the D value must be greater than lo3 for the volume ratio of 10. If the volume ratio is 100,the D value must be greater than 104 for the same extraction efficiency. In the case of insufficient extraction efficiency, repetition of an extraction process may be necessary in order to achieve complete recovery of the solute. However, this will greatly reduce the preconcentration factor. Separation of two trace elements A and B which form extractable species with a reagent is provided by the separation factor which is presented by the following equation:

P,”,= D,lD,

(4.4)

where D, and D, represent the distribution ratio of the species A and the species B, respectively. In general, the D values for the extraction of metal chelates depends on the pH value of the aqueous phase. Hence, with proper pH control, selective extraction of trace metals may be achieved with a common chelating reagent. The possibility of a complete separation of trace metals can be estimated by a comparison of the pH values of 50% extraction (pH,,,) of each metal of interest.

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

121

4.3.2 Extraction of truce elements Many natural samples, such as seawater, sediments and biological samples, contain certain amounts of alkali and alkaline-earth metals which are often desired to separate from the trace elements for an accurate determination. In such cases, the separation and preconcentration of trace elements by solvent extraction is possible by using a reagent which does not react with the matrix elements.

4.3.2.1 Extraction of metal chelates In an aqueous solution, most metals generally exist in ionic species which are not extractable by organic solvents because of the high electrostatic energy required to transfer them from aqueous phase into an organic phase. Therefore, charge neutralization is necessary for the extraction of ionic species. In addition, a hydrophobic property is essentially desired to the compounds to be extracted into an organic solvent. The metal chelates which are formed by the reaction of metal ions with chelating agents seem to be most adequate for these purposes. Chelating agents are usually organic reagents containing anionic groups and electron donor atoms which can effectively replace solvated molecules associates with metal ions, leading to the formation of neutral chelate compounds. Commonly used chelating agents for the extraction of trace metals are: dithizone, ,8-diketones, 8-quinolinol, cupferon, and dithiocarbamates, etc. Recently, macrocyclic polyethers (crown ethers) have been extensively studied for selective extraction of alkali metals, alkaline earth metals, and the rare earth elements [ 19-2 11. The extraction of a metal M”+ after chelation with a ligand H L can be expressed by the following equation:

HL(aq) = H + + L (aq)

I(, M”(aq)+nL-

= [H+IrLl/[HLl = ML, (aq)

(4.5)

P = [M~,l/[M”+I[Ll”

(4.6)

(aq)

= ML, (erg) I., = [M~,l,/[ML,l, H L (aq) = HL(org)

ML,(aq)

I(,

= [HLl,/[HLl,

(4.7)

(4.8)

Combining Eqns. (5-8), the distribution ratio D can be expressed by the following equation D = pIC& [H L I y I(Ln (4.9) Here by putting I(, = p I<, I.; / Icfl (4.10)

KIKUO TERADA

122

we have

D = Ii-,,[HL]y[H+]&

(4.1 1)

where K,, represents the extraction constant. The distribution ratio D,therefore, depends on the ligand concentration in the organic phase and the pH of the aqueous phase at the equilibrium. Then,

log D = log K,,

+ nlog[HL],, + 72pH

(4.12)

This is the main equation for extraction of the metal chelates. According to Eqn. (lo), the extraction constant depends on the stability constant, p, of the chelate being extracted, reagent dissociation constant, KA, distribution constants of chelates, KD, and distribution constant of reagent, KL. Thus, the better the extraction, the higher is the stability of the metal chelate and the greater is its distribution constant. In addition, the greater the acidity of the reagent and the less the solubility of it in the organic solvent, the more efficient is the extraction. According to Eqn. (12),at constant pH of the aqueous phase log D should increase linearly with the logarithms of reagent's equilibrium concentration and the slope should correspond to n as shown in Fig. 4-4(a). The equilibrium concentration of a

Figure 4-4. Relationships between distributionratio of meld chelate and equilibrium concentration of reagent in organic phase at constant pH (a), and pH in aqueous phase at constant equilibrium reagent concentration in organic phase (b).

chelate-forming reagent can be taken to be equal to initial concentration, provided very small amounts of metal are extracted with a reagent having a high distribution constant. It also can be seen from Eqn. (12),at constant equilibrium concentration of a reagent in an organic phase, log D depends linearly on pH, the slope being equal to n as shown in Fig. 4-4(b). These relationships are very important for choosing

SEPARATION AND PRECONCENTRATIONOF TRACE ELEMENTS

123

preconcentration conditions. Moreover, the ratio of metal to chelating reagent of the extracted species can be estimated as 1 : n from the slopes in Fig. 4-4. In analytical practice the value of distribution ratio is not often used. The extraction efficiency expressed in percent is of greater significance. Its dependence on pH give a S-shaped curve with different slopes, depending on the metal ion charge, as shown in Fig. 4-5. By studying such a dependence, the pH control 100

----

--z5 0 w

N

N

N

0

Figure 4-5. Relationshipbetween extraction efficiency and pH of aqueous phase for metal ions with different charges (Z)

0

2

4

6

PH

8

1

0

1

2

Figure 4-6. Typical extraction curves for metal-dithizonechelates

may be possible to in performing mutual separation or group separation of different metals with one reagent. A typical example is shown in Fig. 4-6. Several extraction systems widely used for metal chelates are shown in Table 4-4.

4.3.2.2 Extraction of ionic associates The alternative approach to the extraction of metal ions is to use an ionic associates extraction system which includes a very important group of extracted compounds applicable to preconcentration of trace elements. The ionic associate complexes extensively used are formed between metallo acids, e.g. metal halides, and a bulky organic ion of opposite charge. The ionic species containing the metal, as well as the large ion may be cationic or anionic. These complexes are held together by relatively

KlKUO TERADA

124

Table 4-4. Preconcentration of trace elements by solvent extraction Chelating agent

Trace elements

Solvents

Method of dctemiination

8-quinolinol

Mn, Mo Ga

CHC13 MIBK

AAS, GFA AS Fluor

APCD

Se Ag, Cd, Co. Cr Cu, Fe, Mn, Mo, Ni, Pb, V, Zn

MIBK MIBK, DIBK, CCl4

Fluor AAS

APCD + DDTC

Cd, Co, Cu,Fe, Ni, Pb, Zn

MIBK

AAS

DDTC

Sb, Se(IV) Bi Au, Cd, Co, Cu, Fe, Hg, Mn, Pb, U, Zn

MIBK

AAS GFAAS AAS, NAA, Fluor

cc14 DIBK, 3-nlethyll-butanol, CHCl3

ATMDC, DBADBDC

As, Cd.Co,Cu, Cr, Fe, Mo. Ni. Pb, Se. Sn. V,Zn

2-ethyl-hex yl acetic acid

ICP-AES

Dithizone

Ag. Cd, Co. Cu Ni. Pb. Zn

CHC13

AAS

Dilhizone + TBPO

Co

cc14

GFAAS

STTA + TOPO

Cd. Co, Cu, Mn, Pb

Cyclohexane

GFAAS

APCD: ammonium pyrrolidinecarbodithioate, ATMDC: animoniuni tetraniethylenedithiocarbamate, DBADBDC: dibenzylaninionium dibenzyldithiocarbamate, TOPO: trioctylphosphine oxide, TBPO: tributylphosphineoxide, STTA: monothiothenoyltrifluoroacetone

weak electrostatic forces, but are sufficiently stable to be efficiently extracted into organic solvents under appropriate conditions. The stability of these ionic associate complexes is generally higher in the organic phase than in the aqueous phase. The theoretical treatment of extraction equilibria in ion-association systems is

more difficult than that of chelate extraction systems. The first reason is the existence of relatively large numbers of chemical species and equilibria which may participate in the extraction. Secondly, there are great changes in the activity coefficient, dielectric constant and volumes of the aqueous

SEPARATION AND PRECONCENTRATlONOF TRACE ELEMENTS

125

and organic phases, due to high electrolyte concentrations generally used in ionassociation extraction systems and the resulting enhanced mutual solubility of the two phases.

4.3.2.3 Extraction of complex metallo-acids Here a very important group of systems in which the compounds of the type H,MXn,+,, are extracted from acidic media only with highly basic oxygen-containing extractants, e.g. ketones, ethers, esters, etc. Where M and X represent metal and single-charged electronegative ligand (for example, fluoride, chloride, cyanide, nitrate), respectively, and n signifies the metal ion charge. These compounds include HFeCl,, HNbF, and HInI,, etc. The extraction equilibrium is described by the following equation: mHf + M ! : + (911. + n)Xaq- H, = H,,,MX,,,+,,,

(4.13) (4.14)

log D = log Kex+ n i log[H']

+ (m + n) log[X-]

(4.15)

A typical example of this extraction system is the extraction of iron (111) from > 6A4hydrochloric acid into diethyl ether, in which the extractable species is estimated as [(GH,),O],H+. H,O-FeCl;. In Fig. 4-7shows the distribution of FeCl; from hydrochloric acid media into various kinds of extractants. A decrease of extraction at higher acid concentration is attributed to competition between the HFeCl, and hydrochloric acid. 106

10'

-

10

-

10

-

10

-

D

10

*

2

4

8

6

10

PH

Figure 4-7. Distributionof FeCI; from hydrochloricacid media into various organic solvents. TAP Tri-n-amyl phosphate, TBP: tri-n-bulyl phosphate, DNPK di-n-propyl ketone, MIBK methyl-i-butyl ketone

KIKUO TERADA

126

In practice, extraction of metal-containing acids is employed for separation of Nb and Ta from fluoride solutions; Fe, Ga,Au and Sb from floride solutions; Au and In from bromide solutions; Bi and Cd from iodide solutions; and Fe and Co from thiocyanate solutions.

4.3.2.4 Extraction involving solvation/coordinationby organic compounds Certain organic compounds such as 4-methyl-pentane-2-one, tributyl phosphate and tri-octylphosphine oxide can bring about the extraction of metal ions by direct coordination and/or solvation of the metal ion or of the proton. Uranium (VI) can be extracted in this manner from a nitric acid medium with tributyl phosphate (TBP), and the technique has been used in concentrating uranium from nuclear fuel materials. The extraction equilibrium is described. M"+

+ mNO; + pTBP = [M(NO,),

*

pTBP],

(4.16) (4.17)

log D = log Kex+ nz log[NO;],,

+ plog[TBP],

(4.18)

Here the value of p represents the solvation number of TBP. With respect to Y,Ce, Eu, Tb,Tm, Lu and Am, the extracted species may be M(II1) (NO,),(TBP),, for Zr, Th, Np and Pu, the species included may be M(1V) (NO,), (TBP),. Usually, these reagents are used after diluting to appropriate concentration with an inert solvent, such as kerosene. 4.3.2.5 Extraction with amines

High molecular weight amines (mol wt in the range of 250 to 600 g/mol) are often used in the extraction and purification of transuranic elements. Usually, metals are extracted from acidic solutions as complex anions into a solution of alkylamines diluted with an inert solvent. A tri-n-octylamine (TOA) is miscible with many organic solvents, but is insoluble in water. By shaking TOA with an acid solution, it is readily protonated, forming a bulky cation and is associated with metal-containing acid anion, and then the ionic associatedcomplex is extracted into an organic solvent. The extraction equilibrium is described

By using TOA, FeCl;, SbCL, TlCl;, AuCl; and GaCl,, etc. are extracted into toluene. The distribution ratio has a maximum value at certain hydrochloric acid concentrations because of incomplete formation of TOAH' cation in low acidity, and competition between the metal-containing acid anion and chloride ion in high

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

127

acidity. Another example is the extraction of Cr in seawater with Aliquot-336 which is a mixture of methyltri-n-alkylammonium chloride with alkyl groups that consist mainly of C8-Cl0chains and a mean in01 wt of 475 [22]. In this report, extraction was peiformed with 500 mi of seawater and 15 ml of a 0.1 M solution of Aliquot-336 in toluene. Cr(V1) was quantitativelyextracted at pH 2. 4.3.2.6 Synergistic effect The addition of a second reagent sometimes enhances the extraction efficiency. For example, extraction of UO, from nitric acid as a thenoyltrifluoroacetone (TTA) complex into cyclohexane is enhanced lo3 times by adding TBP. This effect is called synergistic effect or synergism. The extent of the synergism in this organophosphorus system depends on the donor ability of the oxygen in the compound used. Thus the distribution ratio is increased when a more polar phosphine oxide, e.g. tributylphosphine oxide (TBPO), is employed instead of TBJ?.In this case, enhancement of the distribution ratio is more than lo4 times that for TTA alone. The formulae of the extracted species is found to be UO, (TI'A), TBP and UO, W A ) , 3TBPO. The similar effects were also observed for the extraction of divalent (Ca),trivalent (Pm, Dy, Ho, Tm, Am, and Cm), and tetravalent (Th and Pu) metal ions [23-251. Synergistic effects were also found in the extractions of lanthanides with mixtures of ionizable macrocyclic compounds and lTA [26,27] and in the extractions of Cs and Ra with mixtures of crown ethers and lipophilic acids [28,29]. In general, synergistic effect leads to a decrease in the selectivity as a result of a significant increase of the extraction efficiency for most metal ions co-existing in a solution. This may be preferable to a simultaneous separation and preconcentration of multi-elements. Recently, however, a synergistic extraction system has been found in the extraction of lanthanides with mixture of 1,lO-phenanthrolineand TTA [30]. Both the adduct formation constant and the synergistic extraction constant increased with an increase of atomic number of lanthanides. This new trend shows that the synergistic extraction will be an improvement in the separation efficiency together with the extraction efficiency. 4.3.2.7 Masking Masking techniques are frequently used in solvent extraction of trace elements. Most commonly used masking agents include cyanide, tartrate, citrate, fluoride and EDTA. The presence of high concentrations of iron in many natural or industrial samples, such as waste waters, sediments, coal, rocks, and biological samples, restricts the determination of trace metals. In such cases, for the application of APCD-MIBK extraction of trace metals, by addition of EDTA or citrate to the sample solution, iron can be effectively masked and, for example, Cd(II), Co(II), Cu(II), Hg(II), Ni(II), Pb(II), Tl(I), and Zn(I1) are efficiently extracted by the APCD-MIBK system.

128

KIKUO TERADA

4.3.2.8 Extraction concentration techniques

As can be seen from Eqn. (3), to achieve a high extraction efficiency, we have to use a system of high distribution ratio or to reduce the volume ratio of However, the latter choice should greatly decrease the preconcentration factor. Therefore, for preconcentration of trace elements by solvent extraction, various efforts have been done to increase a concentration factor. The employment of the synergistic extraction technique may be the first choice because of an efficient increase of distribution ratio of the extraction systems. Three-phase extraction system which consisted of two organic phases and one aqueous phase, is also applicable to the concentration process. For example, when diantipyrilmethane in a 7:3 mixture of benzene and chloroform is equilibrated with halide and thiocyanate media containing various metal ions, the aqueous phase, the main organic phase which is diluent containing a small amount of DAPM thiocyanate, and DAPM thiocyanate with a small volume of diluent dissolved in it appears as the top layer, as middle layer and as bottom layer, respectively [311. Up to 95% of extractable elements concentrate in the third phase, e.g. Ag, Au, Co, Fe, Ge, Mo, Sb, Sn, Ti,Th, W., Zr and others, and can be transfeired directly to a carbon electrode for the emission spectrographic determination. In this case the concentration factor of trace metals can be increased by one to two orders [32]. Some organic solvents are immiscible with water at room temperature, but become miscible at higher temperatures. This property has been applied to the so-called “homogeneousextraction.” A propylene carboxylic acid forms a homogeneous system with the solution of extractableelements, e.g. Mo(V1) at temperatures exceeding 70’ C [33]. A two-phase system is formed on cooling the mixture to room temperature. A single extraction of metal was 97% complete. An isopropyl alcohol mixes with water in all proportions under ordinary conditions, but saturatingit with sodium chloride, two phases appear. This fact has been applied to separatingAu(III), Ti(IV), V(IV), and other elements. The enhancement of extraction by adding surfactants has been reported on the extraction of Fe(II1) with l T A [34]. 4.4 Separation and preconcentrationof trace elements by ion-exchange 4.4.1 Introduction

From the early 1950s the theory and practice of ion-exchange rapidly developed and further development seemed to be reduced, because of increasing progress of the other methods for separation and preconcentration, such as solvent extraction, and excellent instrumental techniques, such as atomic absorption spectrometry (AAS), inductively coupled plasma atomic emission (ICPAES) and mass spectrometry (ICPMS), which reduced the requirement for separation and/or preconcentration

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

129

of trace elements to a certain extent. However, in the early 1970s a new concept was introduced to establish the ion-exchange chromatogaphy (ion-chromatography, IC) for sensitive and rapid analysis of traces of cations and anions, and recently, the method has been applied to not only inorganic chemistry but life sciences. Many monographs have been published on ion-exchange and its applications [35-391. 4.4.2 Ion-exchange resins

Ion exchange resins are usually synthetic copolymers of polystylene and divinylbenzene containing ionizable functional groups. These are internally homogeneous and are known as microporous or conventional resins. Figure 4-8 shows a strongly acidic exchanger produced by sulfonation of the resins. As the degree of crosslink-

-CH-CH,-

0 - CH-

CH--CH,-CH-CH~-

'

CH-CH~- C H - C H ~ -

SO;Ht

QHt

Figure 4-8. Strong acidic cation-exchangeresin

- CH-

age, i.e. percentage of divinylbenzene increases, the wet volume capacity and the resistance to shrinking and swelling increase, while the equilibrium rate and the permeability to large molecules decrease. For common purposes, 8-12921 crosslinked resins with relatively small pores (microreticular resins) in the shape of spherical beads of 50 to 400 mesh (297 to 37 pm) are most useful. The ion-exchangers are classified into the following five types according to the functional groups introduced in the aromatic ring of the resins:

1. Strongly acidic cation exchanger is a typical exchanger having sulfonic acid as a functional group, and dissociates in an aqueous solution as follows: RSO,H

+ H,O

e RSO;

+ H,O'

2. Weakly acidic cation exchangers are copolymers of metacrylic acid and DVP containing carboxylic acid, phosphonic acid or phenol as a functional group and only dissociates in basic solution;

130

KlKUO TERADA

3. Strongly basic anion exchanger is a typical anion exchanger having quaternary ammonium as a functional group, and dissociates in an aqueous solution as follows: +/RI

+R,

+ OH-

4. Weakly basic exchangers are the resins introduced with primary, secondry or tertiary amine as a functional group, and exhibit the exchangeability in acidic to neutral media; 5. Chelating resins contain special functional groups, such as the iminodiacetate group, CH,N (CH,COOH),, and can be produced from chloromethylated polystylene. These have special affinity for heavy metal ions and are applied to concentrate trace heavy metal ions from seawater samples.

In Table 4-5, various kinds of ion-exchangers are tabulated. Among them a strongly acidic cation-exchanger, a strongly basic anion-exchanger and a chelating exchanger are most frequently used for inorganic trace analysis.

4.43 Eqrrilibriirw arid selectivity The exchange of an ion between an aqueous and resin phases can be represented by an equation n(H+), + M1'++(M1'+),+ 12H' where r stands for the resin phase. This reversible reaction has a selectivity coefficient, A'!, defined as:

The selectivity coefficient is a measure to compare the exchangeability of various ions under certain experimental conditions. The true equilibrium constant, Ii', is obtained from the selectivity coefficient by introducing activity coefficients. In Table 4-6, the selectivity coefficients for several ions are tabulated. In the case of dilute solution, as the measure of exchangeability, the distribution ratio, Ii'd is often used. Amount of M"+ adsorbed (mmol g-'), - Amount of M"+remained (mmol cm-3)s

Ii' -

If the ion concentration is sufficiently low compared with the amount of resin, K , gives a constant value for the certain resin at a given temperature. Extensive measurements of these ratios have been reported in the literature [40]. With a strongly acidic or strongly basic ion-exchanger, the following tendency is observed in the exchangeabilities for cations and anions.

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

131

Table 4-5. Properties of various ion exchange resins Type

Functional groups

pH range

Exchange capacity (meq g-'1

Trade names

Strongacid

-SO3H

0-14

4.3-5.2

Amberlite IR-112, 120, Dowex 50,50W, Zeokarb 2 1 5,225, Diaion SK 1,

Weakacid

-COOH

6-14

10-10.3

Aniberlite IRC-50, Dowex CCR- 1, Zeokarb 226, Diaion WK10,ll

-CHzN(CH3)3CI

0-14

3.W.2

CH2CH20H -CH2 N( CH3)2CI

0-14

3.0-3.3

Amberlile IRA-400, 401,402, Dowex 1, AG-1, Zeokarb FF, Duolite A-42, Diaion SA 10. 11 Amberlite IRA 410, Dowex 2, AG 2, Diaion SA 20

Catioii exchaiigers

Aitioii exchangers

Strongbase

Weakbase

Chelating resin -CH2

-CH2 NH((CH3)2OH 0-7 -CH2H2NCH30H

N/CH2CooNa \

CH2COONa

5.0-6.0

Amberlite IR-45, Dowex 3, AG 3, Zeokarb H Amberlite XE-318,

6-14

> 0.5

Dowex A-1, Chelex

100, Diaion CR-10, Uniselec UR-l0,40

132

KIKUO TERADA

1. The higher the charge of the ion, the higher is its adsorptivity on the exchanger. Then, the order of adsorptivity is

Th4+ > A13+ > Ca2' > Na', and SO:-

> C1-;

2. Among the ions of equal charge, the smaller the ionic radii of hydrated ions, the higher is its adsorptivity, then, (monovalent) Tl+ > Ag' > Cs' > Rb+ > K+ e HNf > Na+ > H30+ > Li', (divalent) Ra2+ > Ba2' > Pb2+ > Sr2+> Ca2+ > Ni2+ 2 Cd2+ 2 Cu2+2 Co2+2 Zn2' 2 Mg2+ 2 U02+ 2 Be2+,and (trivalent) Ac3+ > La3+ 2 Ce3' > RE (light to heavy) 2 Y3+ 2 Lu3+2 Sc3+ 1 A13+. With monovalent anion, I- > NO; > Br- > C1- > OH- > F-. On the other hand, weakly acidic cation exchanger and weakly basic anion exchanger strongly adsorb H30+and OH-, respectively. Formation of metal complexes, especially negatively charged complex anion, e.g. halides complexes or 8-quinolinol-5-sulfonicacid chelates of transitional metals, is much favorable to ion-exchange separation of metal ions by the use of strongly basic anion-exchange resins. Table 4-6. Selectivity coefficient of ions Resin

K#

Dowex 50 Monovalent cation H3O Na Li 0.6

K$, Dowex 1

1.0

1.2

NH4

K

Rb

Cs

Ag

TI

1.2

1.5

2.2

2.0

8.7

8.6

Monovalent anion OH H2PO4 HCO3 C1

CN Br

NO3

HS04

I

0.09 0.09

1.6

3.8

4.1

8.7

F

0.25

0.32

1.0

2.8

4.4.4 Practical colunin operation In the usual method, the solution is percolated through a column packed with an ion-exchange resin at a definite flow rate. During column operation, the solution is continuously exposed to new exchange resins, thus displacing the equilibrium in the required direction. This continuous exchange process will ensure an increase in selectivity between the different ions of relatively low selectivity coefficients. For simple separations, it is usual to choose conditions of the ions so that selectivity coefficients are high, but the weight distribution coefficients are less 30. This minimizes the volume of eluate for an ion, which is directly proportional to the distribution coefficient.

SEPARATION AND PRECONCENTRATIONOF TRACE ELEMENTS

133

Since commercial-grade resins contain various kinds of impurities, such as soluble organic compounds, iron and calcium derived from manufacturing processes, they should be purified by washing with acid, alkali, and methanol before use. On the other hand, prepurified resins are also available from Bio-Rad (Dowex resins) or Malinkrodt Chem. Works (Amberlite resins). These resins need only washing in distilled water before use for sufficient time to swell. These treatments are called “conditioning of resins.” For analytical purposes, resin particle sizes of 100 to 200 mesh are generally recommended. In ion-exchange separation of trace metals, the column length used is usually 10 to 20 times the diameter which is in the range of 6 to 10 mm. A resin bed 10 mm in diameter and 150 mm long is a good compromise. The purified resins are packed in a column by pouring as a water slurry. Packing should be as uniform as possible so a narrow sieve range of particle size is important. The resin must be in the column wet, and air does not enter in the resin since this would cause channeling which considerably reduces the exchange efficiency. 4.4.5 Preconcentration

Preconcentration techniques have been mainly applied to water samples such as seawater, river and lake waters, and other natural water samples. In this field, the chelating resins are much preferable than ordinary ion-exchange resins, because the former reveals higher selectivity to transition metals, but not to alkali and alkaline earth metals. A variety of chelating resins have been synthesized and used for separation and preconcentration of trace heavy metals from natural waters. A typical chelating resin, Chelex- 100 in which the iminodiacetate chelating group is incorporated to a PS-DVB resin, exhibits the ability of selective retention for many transition metals from saline media [40]. However, arguments about the quantitative preconcentration of trace metals with the resin from seawater have continued for years. Recently, it was reported that the lower metal chelating efficiency of the resin in seawater than in fresh water should be caused by the complicated speciation of heavy metals in seawater media and by the high concentration of magnesium and calcium present which act as competitors to heavy metals, and a column containing 2 g of resin in the magnesium form with a flow rate of 4 ml min-’ was suitable for preconcentration of Cd, Co, Cu, Mn, Ni, Pb and Zn in seawater after adjusting to pH 6.5 [41,42]. High exchange velocity and capacity, high selectivity, physical and chemical stability, and reusability are usually required for the chelating resins. To date, many efforts have been made to improve the specificity of the resin and techniques of the application. The functional groups capable of forming complexes and introducing into the polymer by chemical modification of the matrix have been reviewed by Suzuki et al. [43] and Kantipuli et al. [MI.

134

KIKUO TERADA

4.4.6 Ion chromatography Ion chromatography (IC) was first introduced in 1975 and since that time, the technique has grown in usage at a phenomenal rate. The reason for this is clear: IC offers the only simple, reliable and sensitive means for the simultaneous separation and determination of inorganic (and organic) ions in complex mixtures. The IC now embraces a very wide range of separation and detection methods, many of which bear little resemblance to the initial concept of ion-exchange separation coupled with conductivity detection [45,46]. The sample to be separated is introduced into the flowing eluent stream by means of an injection device inserted into the flow-path prior to the column. The detector usually contains a low volume (e.g. 4 pl) cell through which the eluent flows. The components of an ion chromatograph are shown schematically in Fig. 4-9.

Figure4-9. Components of ion chromatograph. (1) Eluent, (2) pump, (3) sample injection, (4) separation column, (5) suppressor column. (6) detector, (7) data system

The main factor which differentiates the ion-exchange materials used in IC from the conventional ion-exchangers is their ion-exchange capacity. Ion-exchange separations in IC are generally performed on ion-exchangers with low ion-exchange capacity, typically in the range 10-100 peq g-I. This characteristic can be attributed chiefly to the fact that IC was developed originally for use with conductivity detection, which introduces a preference for eluents of low background conductance. Though the detection methods currently available make it possible to use columns of much higher ion-exchange capacity, the majority of separations continue to be performed on low capacity materials. Agglomerated ion-exchange resins contain an internal core particle, to which is attached a monolayer of small-diameter particles which carry the functional groups comprising the fixed ions of the ion-exchanger. Provided the outer layer of functionalized particles is very thin, the agglomerated resin exhibits excellent

135

SEPARATION AND PRECONCENTRATIONOF TRACE ELEMENTS

chromatographic performance due to the very short diffusion paths available to solute ions during the ion-exchange process. The central core particle is generally PS-DVB of moderate cross-linking, with a particle size in the range 10-30 pm. Schematic illustrations of agglomerated anion-exchange resin and analogous cationexchange resin using electrostatic binding are in Fig. 4-10(a) and (b), respectively.

Aminaled latex particles

Sulfonrted latex particles

i

(b)

(a)

Figure 4-10. Schematic representation of agglomerated(a) anion- and (b) cation-exchangers

fl (Li-Csl

Li' I

0

r L

8

12 16 20

Time (min)

(4

0

I

I

L

8

1'2

Time (rnin)

6;

-

0

2

L

6

Time (min) (b)

Figure4-11. npical chromatograms obtained with (a) surface sulfonated resins, and (b) PS-DVB based surface aminated exchanger

136

KIKUO TERADA

The suppressor is a device inserted between the chromatographic coluinn and a conductivity detector. The function of the suppressor is to reduce the background conductance of the eluent. Suppressors operate through one of the following mechanisms:

1. The most common mode of suppressor operation is one in which the eluent cations are replaced by hydrogen ions. This mode of suppression is used for anion-exchange eluents and the pertinent a reaction is represented in the following equation for an eluent made up of the sodium salt of the weak acid, HE. Suppressor - H ' + Na'E-e Suppressor - Na+ + HE The background conductance of the eluent is therefore reduced because the relatively strongly conducting ions Na' and E- are replaced by the weakly conducting species HE; 2. The opposite mechanism to that described above involves the replacement of eluent anions with hydroxide ions from the suppressor. The hydroxide introduced into the eluent may then be used to neutralize acidic eluents, or to precipitate metal ions from the eluent represented in the following equations: Suppressor - OH2Suppressor - OH-

+ H' A-= Suppressor - A- + H,O + Mz++ 2NO; e 2Suppressor - NO; + M(OH),,,,

In some cases, it is possible for both the eluent anion and cation to be removed, usually with the aid of a precipitation reaction. For example, a cation-exchange eluent formed from AgNO, can be suppressed by exchange of the eluent NO; ions for C1- using a suppressor in the C1- form. Alternatively, an anion-exchange eluent formed from NaI can be suppressed by exchange of the eluent Na' ions by Ag' using a suppressor in the Ag+ form. The conductance of an eluent can be suppressed through the use of a suitable complexation reaction which converts the eluent components into non-conducting or weakly conducting components represented as follows: Suppressor - CU'+

2 ~ + +EDTA~ e Suppressor - (I!?)'

+

+ Cu-EDTA

Typical chromatograms obtained with surface sulfonated resins and PS-DVPbased surface-aminated anion exchanger are illustrated in Figs. 4-1 1(a) and (b), respectively.

SEPARATION AND PRECONCENTRATION OF TRACE ELEMENTS

137

4.5 Separation and preconcentration by sorption 4.5.1 Introduction

The preconcentration techniques based on sorption seem to be convenient, rapid and capable of attaining high concentration factors. The sorption phenomena used for preconcentration generally include adsorption, absorption, chemical adsorption, and capillary condensation of gaseous components or dissolved substances on solid or liquid adsorbents. The efficiency of sorption at equilibrium is usually described by the distribution coefficient which is represented as a ratio of the total amount of an element in unit quantity of the sorbent (g or ml) and the total amount of the element in unit quantity of the solution or carrier gas (ml). The selectivity of sorption is given by a ratio of the distribution coefficients of two elements interested and is expressed as the selectivity coefficient. The recent development of sorption techniques for preconcentration of trace elements have been summarized in several books [3-51. 4-52 Activated carbon

It has been found that various trace metals are effectively retained on activated carbon in the presence of some complexing agents such as ethylxanthate, diethyldithiocarbamate, ammonium pyrrolidinecarbodithioate,dithizone, 8-quinolinol, and xylenol orange. Moreover, single metal ions such as mercury (11),methyl mercury, and iron (111) are also adsorbed from hydrochloric acid solution. Table 4-7 shows some examples of preconcentration with activated carbon. Generally, preconcentration of trace metals with activated carbon is carried out by one of the following two procedures:

1. The sample solution added with a suitable complexing agent is passed through a thin layer of the sorbent (50 to 150 mg) supported on a filter paper; 2. After shaking the sample solution containing a certain amount of the sorbent (50 to 150 mg) and complexing agent, the solution is filtered through a filter paper.

The metal collected on the sorbent is readily leached out with hot nitric acid for atomic absorption or induced plasma emission spectrometry, while the sorbentloaded filter can be directly submitted to the X-ray fluorescence method or neutron activation analysis.

138

KIKUO TERADA

Table 4-7. Sorption of trace metals on activated carbon Matrices

Trace metals

Complexing agents

Determination method

Water

Ag. As, Ca, Cd, Ce, Co, Cu, Dy, Fe, La, Mg, Mn, Nb, Nd, Ni, Pb, Pr, Sb, Sc, Sn, U, V, Y,Zn

8-quinolinol

SSMS, XRF

Water

Ba, Co. Cs,Eu, Mn, Zn

APCD, DDTC. PAN, 8-quinolinol

XRF

Water

Hg, Methyl mercury

-

AAS

Water

Hg (halide)

-

AAS

Water

U

L-ascorbic acid

INAA

HN03, water Ag, Bi, Cd. Cu,Hg, Pb. Zn

Dithizone

AAS

Mn, MnO2, Bi, Cd, Co, Cu, Fe, In, Ni, Mn salts Pb, TI, Zn

Ethyl xanthate

AAS

Al

Cd, Co, Cu, Ni, Pb

Thioacetamide

AAS

Ag, TIN03

Bi, Co, Cu,Fe, In, Pb

Xylenol orange

AAS

Cr salts

Ag, Bi, Cd.Co,Cu.In, Ni,

HAHDTC

AAS

DDTC

AAS

Pb, TI, Zn Se

Cd, Co, Cu,Fe, Ni, Pb, Zn

APCD: ammonium pyrmlidinecarbodithioate, DDTC diethyldithiocarbamate,HAHDTC: hexamethyleneammonium hexaethylenedithiocarbaniate, PAN: 1-(2-pyridylazo)-2naphtol, INAA: instrumental neutron activation analysis, SSMS: spark-source inass spectrometry, XRF: X-ray fluorescence analysis

45.3 Poroirs polymers 4.5.3.1 Styrene-divinylbenzene copolymers

Macroreticular polystyrene-divinylbenzene copolymers (Amberlite XAD- 1, XAD-2, and XAD-4) and more hydrophobic methyl metacrylate-based copolymers (Amberlite XAD-7 and XAD-8) have been studied for the preconcentration of trace metals as their complexes from sea- and surface-waters. Chromium (VI) ions formed a Cr (111)-diphenylcarbazone complex in aqueous solution and the complex was effectively adsorbed on XAD-2 resin in the presence of sodium chloride [47]. A XAD-4 resin was employed as a collector of silver, cadmium, cobalt, copper,

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iron, mercury, nickel, lead, and zinc in seawater after complexation with sodium bis(2-hydroxyethy1)dithiocarbamate [48]. On the other hand, the resins impregnated with liquid extractants or complexing agents are extensively used for concentration of trace metals. Gold was concentrated on XAD-4 resin impregnated with p-dimethylaminobenzilidenerhodanine (DMABR) [49], and In(II1) and Pb(I1) were collected on XAD-2 resin impregnated with pyrocatechol violet [50]. 4.5.3.2 Polyurethane foams Braun and co-workers have extensively investigated polyurethane (PU) foam sorbents regarding the preconcentration and separation of trace metals [51]. The selective sorption and separation of Fe, Co, Hg, Zn, and In, Rh, and Ir, and Pd by unloaded PU foam were successful in acidic aqueous thiocyanate media. The foam columns are prepared by placing the foam cylinders in glass columns and applying gentle pressure with a glass rod. The columns are then filled with water under vacuum. A sample solution is allowed to percolate through the foam column at different flow rates. Various reagent-loaded open-cell PU foams have been prepared and used for preconcentration and separation of traces of inorganic and organic species of metals under static and chromatographic conditions. Since according to neutron activation analysis, ether-type PU foams contain relatively high amounts of metal contaminants, great care will be needed to use them for preconcentrating trace metals. 4.5.3.3 Poly(chlorotrifluoroethy1ene)(PCTFE) resin PCTFE resin which has been widely used as a reversed-phase chromatography column packing, was found to efficiently adsorb some soluble metal complexes in trace concentration [52]. A commercially available PCTFE, Neoflon molding powder was used for preconcentration of Cd, Cu, Fe(III), Mn(II) and Zn as their 8-quinolinol (Ox) and 8-quinolinol sulfonate (SOX)complexes, and Bismuthiol I1 (Bis-11)complexes of Cd,Cu and Ag [53]. In the case of Cu(I1)-Sox complex anion, quantitative adsorption was achieved in the presence of tetrabutylammonium cation (TBA+) as a counter ion. In addition, adsorption increased with increases in TBA+ concentration. Therefore, it is seen that an ion-associated complex can be sorbed on the resin by a hydrophobic interaction. Preconcentration of Cu(I1)-Ox complex on the resin has been successfully applied to the determination of oxine-Cu which is spread on golf links as a fungicide, in environmental water sample [54].

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45.4 Conipfex-forniirtgadsorbeti rs 4.5.4.1 Reagent-immobilized silica gel or glass beads Hill [ 5 5 ] has synthesized 8-quinolinol immobilized silica via silylation, amidization, diazotization and coupling reactions. The major use of this new chelating adsorbent has been in the preconcentration of trace metal ions from dilute aqueous samples or relatively simple separations of pairs of metal ions [56,57]. SG-8HOQ was used for the preconcentration of Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn from seawater prior to their determination by GFAAS. Some other kinds of complex-formingfunctional groups such as P-diketone [ 5 8 ] , 2,3-dihydroxy-benzoyI and 3,4,5-trihydroxy-benzoyl-amide [59] and Chelex-100, [60] have been immobilized on silica gel and studied for their sorption ability and application to preconcentration of traces of metals. 4.5.4.2 Reagent-loaded silica gel or glass beads

Silica gel seems to be excellent support material for sorbent of trace metals owing to its porosity and large surface, hydrophilic property, resistance to acids, resistance to heat, lack of swelling in various solvents, and mechanical strength. In preparation of reagent-immobilized silica gel, the silylation of the surface of the gel results in loss of its hydrophilic nature so that fast retention of metal ions cannot be attained. Therefore, several complex-formingsorbents have been prepared by simple loading water-insoluble complexing agents on silica gel [6 11. 2-Mercaptobenzothiazole-loadedsilica gel (MBT/SG) is prepared by impregnating SG with tetrahydrofurane (THF) containing MBT,drying the impregnated gel under reduced pressure, washing with pure water to remove free reagent, and drying again under reduced pressure [62]. The amount of MBT retained on SG was found to be 114 f 6 mg and retention capacity for silver was 8 1.5 pmol g-'. By the use of a MBT/SG column (10 mm i.d.), Ag, Cd,Cu(II), Hg(II), Pb, and Zn are quantitatively retained at flow rates 15, 17, 21, 23, and 21 ml min-', respectively. The metals retained on the loaded SG column are readily eluted with suitable eluents, and the metals were determined by AAS or ICP-AES. The thionalide/SG shows high selectivity for Pd(II), Sb(II1) and Bi(II1) ions at pH below 1.0. Arsenic (111) is retained at pH 2 6.5, but As(V) and organic arsenic compounds are not adsorbed; hence sequentialconcentration of As(II1) and the other arsenic species is capable. The sorption behavior of several metal ions by p-dimethylaminobenzilidenerhodanine loaded SG (DMABWSG) revealed selective adsorbability for Ag, Au (111), and Pd (11) ions at pH below 1.0, but no adsorption occurred for Cd, Cu, Fe (111), Pb and Zn at this pH. Therefore, the separation of noble metals from the latter metals is possible.

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455 Natitrnl polymers

4.5.5.1 Celluloses The modified celluloses which contained 1-(2-hydroxyphenylazo)-2-naphthol (Hyphan) or 4-(2-pyridylazo)-resorcin (PAR) was used for concentrating Cd, Co, Cr, Cu, Fe, Hg, Mo, Ni, Pb, U, V, and Zn in natural waters [63]. The exchange capacity of these resins equals 0.5 mmol g-'. After passing 1 to 5 liters of water sample through a column of the sorbent, the retained trace elements were eluted with 1 M hydrochloric acid varying concentration factor between 20 and 100. Four types of dithiocarbamate-celluloses were prepared [64] by treating cellulose with p-toluene-sulfonyl chloride in pyridine with subsequent reaction of tosyl cellulose and amines (aniline, benzylamine, n-butylamine, and piperazine) in dimethylformamide and then by treating the aminocelluloses obtained with carbon disulfide in a NH3-CH,OH medium. The last cellulose showed good adsorption characteristics with relatively large capacity for Ag, Cr (VI), Cu, Hg (11). Pb, Se (IV) ranging from 9.5 to 70 mg g-' of the modified cellulose.

4.5.5.2 Chitin and chitosan Since the natural polymer chitosan, which is readily obtained by deacetylation of chitin, has been utilized as an adsorbent for collection of metal ions [65], this polymer has received attention regarding its use for the removal of toxic heavy metals or radionuclides, as well as for the recovery of useful metals from industrial sewage. On the other hand, only a few investigations on chitin regarding concentration heavy metals have been reported. Chitin has been used for the rapid concentration of Ni, Cu and Cd ions as maleonitriledithiolateanionic complexes, and also of Fe, Cu and Cr (VI)as their colored complexes with 1,lO-phenanthroline, neocuproine, and diphenylcarbazone,respectively. Recently, dithiocarbamatefunction has been introduced into an aminosaccharide chain of chitosan (DTC-chitosan) [66]. The adsorbability of DTC-chitosan has been studied for several metal ions in the pH range from 1 to 12. However, the DTC-chitosan has a tendency to gelatinize in an acidic medium so that it can hardly be used for column operation. Hence, dithiocarbamate-chitin has been prepared by mixing chitin powder with carbon disulfide, 2-propanol, and tetramethylammonium hydroxide in benzene. DTC-chitin obtained does not gelatinize even in 6 M nitric acid and shows effective adsorbability for transition metal ions from an acidic solution. Silver, copper and cadmium ions are quantitatively retained on DTCchitin from 1 M or higher acidic solution, nickel and cobalt at pH 2-3,and iron at pH 4 1671. Gold and palladium are quantitatively retained on DTC-chitin at 6 M hydrochloric acid medium [68]. The metals retained on the column are eluted with

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dilute nitric acid containing 2-395 thiourea except for palladium which is strongly retained on the polymer and eluted with 1.6-fold diluted inverse aqua regia.

References 1 Bonner, M.A. and Kahn, M., Behavior of carrier-free tracers, in: Radioactivity Applied to Chemistry, A.C. Wahl and N.A. Bonner (Eds.),Wiley, New York, 1951,p. 102.

2 Minzewski, J., Chwastowska, J. and Dybczynski, R., Separation and Preconcentration Methods in Inorganic Trace Analysis, Ellis Horwood, Chichester, 1982, 3 Mizuike, A., Enrichment Techniques for Inorganic Trace Analysis, Springer-Verlag, Berlin, 1983. 4 Zolotov, Yu. A. and Kuz’min, N.M., Preconcentration of Trace Elements, Elsevier, Amsterdam, 1990.

5 Alfassi, Z.B., Preconcentration by Coprecipitation of Trace ELements, in: Preconcentration Techniques for Trace Elements, Z.B. Alfassi and C.M. Wai (Eds.), CRC Press, Boca Raton, FL, 1992,p.33. 6 Hahn. 0.. Applied Radiochemistry, Cornell University Press, Ithaca, NY, 1936.

7 Caramella Crepsi, V., Genova, N., Meloni, S. and Oddone, M., J. Radioanal. Nucl. Chem., Art., 114 (1987)303. 8 Goshkov, V.V.. Org. Reagenty Anal. Khim., Tezisy Dokl. Vses., 2 (1976)63.

9 Vanderstappen, M.G. and Van Ccieken, R.E., Talanta, 25 (1978)653. 10 Genova, M., Caramella Crepsi, V. and Meloni, S., Radiochem. Radioanal. Lett., 58 (1983)271.

11 Tappnieyer, W.P.and Pickett, EE.. Anal. Chem., 34 (1962)1709. 12 Sebba, F.. Ion Flotation, Elsevier, Amsterdam, 1962. 13 Fukuda, K. and Mizuike, A., Bunseki Kagaku, 17 (1966)319. 14 Mizuike, A. and Hiraide, M., Pure and Applied Chem., 54 (1982)1556.

15 Hiraide, M. and Mizuike, A., Kagaku, 31 (1976)57. 16 Hiraide, M.and Mizuike, A., Bunseki Kagaku. 26 (1977)47. 17 Hiraide, M.,Ito, T., Baba, M., Kawaguchi, H. and Miuzuike, A., Anal. Chem., 52 (1980)804. 18 Zolotov, Yu. A., Boodnya, V.A. and Zagmzina, A.N.. CRC Crit. Rev. Anal. Chem., 14 (1982) 93. 19 Federson, C.J., Science, 241 (1988)536. 20 Bunzli, J.G. and Wassnef, D.. Coord. Chem. Rev., 60 (1984)191. 21 Bartsch, R.A., Solv. Extr. Ion Exch., 7 (1989)829. 22 De Jong, G.J. and Brinkman, U.A.T., Anal. Chim. Acta, 98 (1978)243. 23 Healy, T.V., J. Inorg. Nucl. Chem., 19 (1961)314. 24 Cox, E.C. and Davis, M.W., Sep. Sci. Technol., 8 (1973)205. 25 Patil, S.K., Ramasrishna, V.V. and Prakas, B.H., Sep. Sci. Technol., 15 (1980)131.

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26 Manchanda, V.K. and Chang, C.A., Anal. Chem., 58 (1986)2269. 27 Manchanda. V.K. and Chang. C.A.. Anal. Chem., 59 (813) 1987. 28 McDowell, W.J.. Scp. Sci.Technol.. 23 (1988) 1251. 29 Tang, J. and Wai. C.M., Anal. Chem.. 58 (1986)3233. 30 Naliamura, S. and Suzuki, N.. Polyhedron, 5 (1986) 1805. 31 Zhivopistsev, V.P., Ponosov, I.N., and Selezneva. I.A., Zh. Anal. Khim., 18 (1963) 1432. 32 Petrov, B.I. and Galinova, K.G., Zh. Anal. Khim., 33 (1978) 1481. 33 Murata, K. and Ikeda, S., Bunseki Kagaku, 18 (1969) 1137. 34 Kawamoto, H. and U w a , H., Chem. Lett.. No. 3 (1973) 259. 35 Calmon, J.E. and Hale, D.K., Ion Exchange, A Laboratory Manual, ButterwoRhs.London, 1957. 36 Samuelson, O., Ion Exchange Separations in Analytical Chemistry, Wiley, New York, 1963. 37 Rieman, W.,111 and Walton, H.F.. Ion Exchange in Analytical Chemistry, Pergamon Press, Oxford, 1970. 38 Marinsky, J.A. and Marcus. Y.. Ion Exchange and Solvent Extraction. Vol. 5, Deliker, New York, 1973. 39 Marcus, Y. and Kertes, AS.. Ion Exchange of Metal Complexes, Wiley, New York, 1969. 40 Riley, J.P. and Taylor. D., Anal. Chim. Acta, 40 (1968) 479. 41 Pai, S.-C., Whung, P.-Y., and Pai, R.-L., Anal. Chim. Acta, 211 (1988) 257. 42 Pai, S.-C.. Anal. Chim. Acta. 211 (1988b) 271. 43 Suzuki, T.M. and Yokoyama, T., Bunselii, (1984) 736. 44 Kantipuli, C., Katragadda, S.,Chow, A., and Gesser, H.D., Talanta, 37 (1990) 491. 45 Walton, HP.. Ionexchange chromatography. in: Chromatography, Fundamentals and Applications of Chromatographic and Electrophoretic Methods, Part A: Fundamentals and Techniques, E. Heftman (Ed.), Elsevier, Amsterdam, 1983. 46 Haddad, P.A. and Jackson, RE., Ion chromatography, principles and applications, J. Chromatogr. Library, Vo1.46, Elsevier, Amsterdam, 1990. 47 Osaki, S., Osaki, T., and Talrashima. T., Talanta, 30 (1983) 683. 48 King, J.. and Fritz. J.S., Anal. Chem., 57 (1985) 1016. 49 Chien, Chung-C., and Chang. Fu-C.. J. Chin. Chem. SOC.,Ser. 11.30 (1983) 243. 50 Brajter, K., Olbrych-Sleszynska, E., and Staskiewucz,M., manta, 35 (1988) 65. 51 Braun, T., Navratil, J.D., and Farag, A.B., Polyurethan Foam Sorbents in Separation Science

and Technology, CRC Press Boca Raton, FL, 1986. 52 Akita, C., Matsumoto, K., and Terada, K., Anal. Sci., 3 (1987) 473.

53 Yamaguchi, T., Zhang Liping, Matsumoto, K., and Terada, K., Anal. Sci., 8 (1992) 851. 54 Yamada. N., Tomita, B., Chaya, K., and Mumkami, N., Eisei Kagaku, 38 (1992) 188. 55 Hill, J.M.. J. Chromatogr.. 76 (1973) 455.

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56 Sugawafa, K.F., Weetall, H.H. and Schucker, G.D., Anal. Chem., 48 (1974)489. 57 Jezorek, J.R. and Freiser, H., Anal. Chem., 51 (1979)366.

58 Seshadri, T. and Kettrup, A., Fresenius Z. Anal. Chem., 296 (1979)247. 59 Seshadri, T.,Dietz, G. and Haupt, H.-J., Fresenius Z.Anal. Chem., 319 (1984)403. 60 Ryan, D.K. and Weber, J.H., Talanta, 32 (1985)859. 61 Terada, K., Anal. Sci., 7 (1991)187. 62 Kubota, M.. Matsumoto, K. and Terada, K., Anal. Sci., 3 (1987)45. 63 Lieser, K.H. and Gleitsmann, B.. Fresenius Z. Anal. Chem., 313 (1982)203. 64 Imai, S.,Muroi, M. and Hamaguchi, A., Anal. Chim. Acta, 113 (1980)139. 65 Muzzarelli, R.A.A., Chitin, Pergamon Press, Oxford, 1978,p. 139. 66 Muzzarelli, R.A.A., Tanfani, F., Mariotti, S., and Emanuelli, M., Carbohydr. Res., 104 (1982) 235. 67 Hase, A., Kawabata, T. and Terada, K., Anal. Sci., 6 (1990)747. 68 Terada, K. and Kawamiira, H., Anal. Sci., 7,Suppl. (1992)71.

CHAPTER 5

Determination of trace elements by atomic absorption spectrometry

ITHAMAR Z . PELLY

Department of Geology and Mineralogy. Ben Gurion University of the Negev. Beer.Sheva. Israel

Contents 5.1 5.2

5.3 5.4 5.5

5.6 5.7

5.8 5.9 5.10 5.11 5.12 5.13 5.14 5.15 5.16 5.17

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quality of results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Standards and chemicals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Samplepreparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Major components of the instrument . . . . . . . . . . . . . . . . . . . . . . . . . . Radiationsources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wavelength selection system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atomization by flame . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrothermal atomization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydride generation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumental background corrections . . . . . . . . . . . . . . . . . . . . . . . . . . ModiEers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

146 147 148 150 152 154 155 156 159 164 172 173 178 180 183 187 188

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5.1 Introduction

Atomic absorption spectrometry is a method for elemental analysis in solution (mainly). It is very sensitive, can detect different elements (67) and can detect elements in the range of a few ppm or less (atomization by flame) or in the range of a few ppb or less (electrothermal atomization). In most cases it is not important in what molecular form the metal exists (measuring the total concentration, in all the molecular forms in the sample). The analysis of the metal can be carried out in the presence of many other elements without, usually, having to separate the analyzed element from other elements in the sample, an advantage which makes the process simpler and saves a lot of time and errors. Atomic Absorption Spectrometer (AAS) is one of the first modem commercial instruments for trace element analysis. Since its appearance it became the “working horse” of analytical laboratories. Later, other methods, namely X-Ray Fluorescence (XRF) and Inductively Coupled Plasma (ICP) spectrometry threatened its position and became relatively widespread. Still, it seems that to this day atomic absorption spectrometry in all its forms (flame, electrothermal, hydride generation and Zeeman) remains the most commonly used methods. When one scans the early literature dealing with atomic absorption analysis (as well as that dealing with the introduction of almost all other new major analytical instruments) one gets the impression that it was considered to be the magic that will solve all elemental analysis problems. From now on all one has to do is to reduce the sample to the form needed for introduction to the instrument, press the botton and collect the results. With the passage of time, when experience was gained and the problems were identified, the limitations of the method were realized and its usefulness, compared with other major methods, was determined. As a result, when one wants to buy a major instrument for elemental analysis, one has to consider (in addition to the price of the instrument) what he needs the instrument for (mostly) i.e. analysis of major elements, analysis of trace elements (and at what level - ppm or ppb), samples with a generally constant matrix (for instance water or alloys) or not, number of samples per unit time, operational expenses, etc. A word of caution. It is a common practice to talk about the detection limit and the sensitivity of the instrument, hut one should realize that these are not the only factors determining the quality of the results (precision and accuracy). This is determined equally, or even to a higher extent, by the treatment the sample received before it was analyzed by the instrument, i.e. by sampling, homogenization or splitting, weighing, dissolution or melting, etc. Extreme care should be taken in executing these operations in order to fully utilize the capabilities of the instrument. This chapter was written aiming at a reader who has only a basic knowledge of the technique (some basic undergraduate course) and no real experience in using the method. Theory, the instrument and methods will be described mainly paying

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attention to the implications to experimental work, so that a reader who wants to use AAS for analytical work would save a part of the necessary trial and error. If one is interested in enlarging his knowledge of the physics and theory relevant to the various sections, one should turn to the appropriate textbooks or journal articles (see Section 5.2). There are hundreds of specific applications of atomic absorption analysis and several thousands of references to articles in the literature concerning these applications. When one wants to carry out an analysis (or to develop a new method), unless one has specific instructions for the analysis, the best advice is to start by a thorough scan of the literature (or compiled references, see Section 5.2). This should be done either to find an existing method or to find similar cases and based on this to decide whether to use the method or to develop a new method (knowing now what the specific problems are and how they were solved by others) without wasting time to reinvent the wheel. One should remember that many analytical methods are subject to strict analytical procedures resulting from legislation (implemented by governments or the European Common Market, etc.) or recommended by various organizations. If an analytical method is required for compliance with regulations, then the relevant guidelines and procedures should be obtained and followed.

5.2 Literature Atomic absorption spectrometry is a well established analytical method, consequently, one can find a vast amount of literature dealing with various aspects of AAS. The theory of the method and a description of the various parts of the spectrometer can be found in textbooks dealing with instrumental analysis methods, for instance [1-4], or books wholly dedicated to AAS [5-71. A short description of the theory, in addition to description of important experimental aspects, can be found in manufacturers’ books [8-101 obtained with the instrument. These books, usually referred to as “cookbooks,” describe the detailed experimental conditions for the analysis of each element that can be determined by AAS in water or other materials, e.g. geological materials, biological materials, blood and serum, urine, beer, cement, alloys, food products, plastic materials, etc. Perkin-Elmer Corp. used to include a bibliography of articles dealing with AAS that were published in various scientific journals and update them from time to time. In recent years the updating of bibliography is published every 6 months in the journal Atomic Spectroscopy (published up to 1980 under the name Atomic Absorption Newsletter) together with new original articles concerning AAS methods. Sometimes, bibliographies concerned with a special subject are published in this journal, for example, a bibliography for the years 1973-1989 on the subject of “Chemical Modification in Electrothermal Atomization Atomic Absorption Spectrometry,” published in 1991.

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Varian Co. publishes articles dealing with new AAS instrumental techniques and new analytical methods for analysis of elements in different environments or i n different matrices, in a series called Varian Instruments at Work-Atomic Absorption. Analytical Chemistry has a section called Analytical Chemistry Reviews or R pages (the letter R follows the page number) which contains a chapter in which progress in AAS in the preceding two years is reviewed (there also are chapters with reviews of other analytical methods). Every alternate year the application reviews are published according to instrumentation methods (theories, instrumentation and applications). In the years in between, they are reviewed according to research fields, i.e. different methods used in the discussed field, reported in the preceding two years. Articles dealing with AAS (both theoretical and experimental aspects) can also be found in several journals, for instance, Applied Spectroscopy, Arialytica Chiniica Acta, Talanta, Fresenius’ Journal of Analytical Chemist!y (formerly Fresertius’ Zeitschrift fur Artalytische Chemie, including articles also in English), Journal of Analytical Atontic Spectrometry, Progress in Analytical Spectrontetiy whose name was changed to Spectrochintica Acta R and Aiiulytical Chemistry. In addition, paragraphs dealing with AAS methods can be found in the experimental sections of articles in other journals dedicated to other branches of science. 5.3 Quality of results

One can often see that an unexperienced analyst who wants to buy an analytical instrument is concerned with one thing only - what is the lowest concentration of the determined element the instrument can detect. One should realize that there are many factors that determine the quality of the results. One should consider factors such as the intensity of radiation from the radiation source, the fuel-oxidant composition of the flame, the matrix of the sample, the treatment the sample received before the determination by the instrument, etc. All these things determine the sensitivity, the detection limit and the reliability of the results. Detection limit- a term that shows the lowest concentration that can be measured, under favorable conditions, with a certain degree of reliability. This depends on the signal to noise ratio, the higher the noise the higher should be the signal in order to be distinguished as something above the noise - an analytical signal. One can find different definitions in the literature. Here we refer to it as the concentration in solution of an element (in pg/ml) which can be detected with a 95% certainty, i.e. a reading equal to twice the standard deviation of a series of at least ten determinations near or at blank level. When one compares numbers published by different manufacturers, one should check what their definition is and how was the experiment conducted. One can measure the signal of a solution concentrated 5-fold and divide the result by 5. This means that the experiment was

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carried out under much more favorable conditions and had it been carried out with the right concentration, maybe nothing above noise level would have been detected. Sensitivity - a term that shows the change in absorption due to a change in concentration or, in other words, the slope of the calibration curve (absorbance against concentration). It is defined as the concentration of an element in aqueous solution (in pg./ml) which absorbs 1% (absorption) of the incident radiation. In absorbance units this means 0.0044. The sensitivity is useful for checking if the determination conditions are optimal and result in the same sensitivity described in the instrument book. One can measure the sensitivity as the concentration that results in 0.44 absorbance and divide the result by 100 (provided all this is in the linear range). For instance if for an element a concentration of 1 mg/ml is needed for 0.44 absorbance, the sensitivity is 0.01 mg/ml (or 10 pg/ml). One can use the sensitivity to determine the needed working range. If the sensitivity is 10 pg/ml than the absorbance of 1 mg/ml is 0.44 and of 2 mg/ml is 0.88 (again, provided all this is in the linear range). If one analyzes a complex material and does not obtain the absorbance expected from the sensitivity, one should prepare a pure solution of the analyzed element and check to see if the operating conditions are optimal and the published sensitivity is obtained. If the answer is positive, than the reason for the low sensitivity is probably interferencedue to the matrix and this should be dealt with. The values of the elements’ detection limits and sensitivities for the specific instrument one uses are printed in the instrument’s book or in the analytical methods book (“cookbook”) of the instrument. Usually one does not measure the sensitivity as defined. The analytical methods books or the operation programs of computerized instruments inform the user what is the concentration needed to give an absorbance of 0.2. One can calculate the expected absorbance for the concentration range of the analytes (or of the standards for the calibration curve) and check if it is really obtained. Some “cookbooks” do not state the sensitivity but the optimum working range (the concentration range that will yield 0.2-1.0 absorbance). The relevant terms for graphite furnace work are listed in Section 5.12 Detection limits and sensitivities are the results of the instrument design, the quality of its parts, and of the operational parameters. All these do not ensure a “true” result. This also depends on other factors such as how representative the sample is, grinding, splitting, weighing, dissolution, transfer by pipets, etc. The “exactness” of the results is measured by precision and accuracy. Precision - a term measuring the reproducibility of the measurements. The precision really measures how much the results differ from each other. There is a possibility that the precision is good, i.e. all the measurements of the same sample are almost identical, but, the result can be totally wrong (for instance, if the samples were weighed using a balance that has a constant error of 10%). How can one

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check the precision of the method to be used (or rather of the analytical system including how much the sample is representative, the effect of the matrix of the sample, handling of the sample, the instrument, calibration curves, the operator and his personal errors, etc.?) In atomic absorption-analysis the precision may (and should) be checked in several ways. A method to estimate the precision of a whole analytical procedure is running one unknown (after grinding and mixing) 5 times, including weighing and dissolution or melting and calculating the standard deviation. Another method is analyzing duplicates of some samples (if possible one should assign sample names and order in such a way that the operator will not know that he is running the duplicate, to avoid bias). Another method (helping in checking the change in instrument readings with time) is reading one of the solutions every few samples. Accurucy - a term that expresses the “correctness” of the measurement, or, how close a measurement is to the “true” value. Well, what is the true value and how can one determine it? After all, to know the true value is the purpose of the analysis, which means it is not known. What can the obtained measurements be compared with? The mean of several measurements of the same sample is considered to be the true result. In atomic absorption analysis, one should always check the results obtainable by the analytical procedure to be used, by analyzing certified standards exactly the way he intends to analyze his samples and comparing the results with those stated for the standards (for the types of relevant standards see Section 5.4). When a complex material is analyzed and good results are not obtained, one should prepare pure solutions of the analyzed element and check if the instrument operates under optimal conditions and the desired sensitivity and detection limit are obtained. If not, something is wrong with the instrument or the operation mode. If everything is right, then the complex material is responsible for the bad results and one should look for ways to lower the material’s interferences.

5.4 Standards and chemicals One type of standards used in atomic absorption spectrometry is chemicals (solids or solutions) used for calibration curves. It cannot be overstated that atomic absorption is a comparative technique. Consequently, the accuracy of quantitative measurements depends on the accuracy of the standards employed. If (in addition to having the right concentration) standard solutions and samples are not matched with respect to physical and chemical properties, the comparative analytical response in the flame may be different (see Section 5.14) destroying the basis for comparison. If matching cannot be obtained, the standard additions method should be used. These calibration standards used to be very pure chemical compounds (e.g. CaCO,, KCl, etc.) or metals (e.g. Ni, Cu as wires or powder) which were weighed, dissolved and diluted to obtain (master) standard solutions of known concentrations.

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One has to be careful with metals (powders, wires or foils) as once many of them are exposed to air (by opening their containers) their surface is oxidized quite easily. The next time they are to be used, if they can still be used at all, they have to be etched or “purified” in another way before use. The various “cookbooks” describe, for each element, in addition to the operational analytical parameters, also a method to prepare a standard solution. The master standard solutions, which are prepared for long time use, have to contain a high concentration of the element (usually the solutions are made to contain 1000 mg/L metal) and these master solutions are diluted to prepare the low concentration solutions needed for the calibration curves. These dilute solutions can be used the same day or for a maximum of a few days, and then fresh solutions must be prepared. Nowadays prepared elemental master standard solutions (usually 1000 mgA, or, an ampule containing a precisely defined quantity of the element, to be diluted to a specified volume) spectroscopically pure for atomic absorption analysis, can be bought from various commercial sources. There are standards for aqueous matrices or oil dissolved standards to match samples in an oil matrix, etc. In addition to being easier to use, they prevent oxidation problems and “technician errors” of weighing and dilutions. These may cause severe problems as they may be discovered (if at all) after aresearch program or a set of analyses was finished (not to say reported). Using certified standards (see the following) to check the method may help in catching such errors. Quite often such prepared standards should be preferred for calibrations. Special attention must be paid to the blank solution which is a most important standard (if not the most). This solution should contain the appropriate amounts of all the components of the sample solution, except for the analyte itself. These components can include, for example, the same water used for dilutions, acids used for dissolution, flux used for melting, organic material added to the sample (also to match physical properties), ionization suppressor, etc. All these materials may contain trace amounts of the analyzed element which should be subtracted from each measurement. Another type of standards includes materials of known composition, for checking analytical procedures. There are several kinds of this type of standards to fit the various fields using AAS, e.g. biological standards, geochemical standards. The concentrations of the elements in some standards can be determined beforehand, by preparing mixtures with the desired concentrations. For example, standardsof serum containing Cu ions, Norm - in the range of normal serum Cu concentrations and Path - in the range of pathological concentrations, or, standards containing desired concentrations of elements (organometallic compounds) in an organic oil matrix, for the analysis of metal wear in engines’ lubrication oils. A different method had to be used for geochemical standards, e.g. those prepared by the U.S. Geological Survey. These are rocks (such as granites, basalts, syanites) of which very large amounts were taken, ground very finely, mixed very thoroughly and sent to many

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laboratories for analysis by different methods. The mean of all the results (for the same element) was taken as the true result (which was changed as time passed and new results kept coming from additional laboratories or by using more sophisticated methods and instruments). Samples of the first batches are not available any longer, however, new standards of rocks or ores (e.g. different phosphates) are available from many sources, in different countries, from both institutional and commercial sources. The compiled results - working values - could be found in different sources (for instance, [111 for values of some U.S. Geological Survey standards or 1121 for a discussion of subjects such as the determination of usable values, verification of values, statistical problems and values for about 75 various rock and mineral standards obtainable from different sources). Since 1977 a special journal, Geostundurds Newsletter, is dedicated to reference samples, values and analysis methods, in addition to which a new compilation of working values is published every few years (for the last one, 272 geostandards, see [ 131). These “international” standards are quite expensive, so that “home” standards are frequently used, in addition to the certified commercial standards. A rock sample is finely ground, thoroughly mixed and analyzed (the method is checked with a certified standard). This “home” standard is then used whenever a standard is needed when a new method is developed. Only for the last check, after debugging of the method, is the certified standard used. The purity of reagents is one of the things that determine the accuracy of the analysis, especially in the case of trace element analysis and all reagents (and distilled water!) should be of the highest purity availahle (though, if one analyzes elements not in trace levels, there is no gain in using expensive spectroscopically pure acids for dissolution). These include acids for dissolution, fluxes for melting, chemicals used as ionization suppressors, releasing agents or chemical modifiers, etc. One should not forget the level of cleaning needed for labware and the laboratory itself (e.g. no dust). Special care must he attached to the quality of the water used, for dilutions and preparation of reagents, especially when analyzing trace levels. In most cases, distilled water or ordinary deionized water can be used, but sometimes the element to be analyzed has to be removed from the water by a better procedure. 5.5 Sample preparation

Sample preparation is one of the critical factors determining the quality of the analysis results. Any error in preparation will be expressed in the analysis results. Most samples cannot be analyzed as obtained and need some treatment before analysis. Samples have to be reduced to solution in order to be presented to the spectrometer and aspirated into the flame. Even if the sample is originally a solution, pretreatment may be needed, e.g. dilution, preconcentration, addition of ionization

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suppressor, extraction. In graphite furnace, sometimes analysis of solid samples is possible (e.g. parts of plants, animal tissue) in graphite cups, as drying and ashing stages may take care of the matrix. Some geological materials can be analyzed this way if the analyzed element can easily be volatilized. The problem in such cases (and that is why solid state analysis is almost not used) is that the part of the sample actually used for analysis cannot be considered to be representative of the whole sample. If in order for a rock sample to be representative, one thousand grams are taken and ground very finely, how representative will a few milligrams used in the spectrometer be. When one wants to analyze geological samples, the first step is to collect fresh (uneroded) representative samples in the field. The next step is grinding the rock sample or minerals separated from the rock using one of the several mineral separating processes. There are several grinding techniques and the grinders are made of different materials. During grinding of the rocks, a small amount of contamination comprising several elements will be transferred to the sample, depending on the material the grinder is made of and the hardness of the sample. In the analysis of trace elements this can be a problem and the grinder should be chosen also with this aspect in mind. The following is a brief listing of the more common sample preparation techniques. The reader should turn to other sources for experimental details and for other techniques. The sample may be reduced to solution in several ways (depending on the sample), such as dissolution (in organic or inorganic solvents, in an acid or in a combination of acids - including HF to decompose silicates and volatize silicon as a fluoride) or melting (with sodium carbonate, sodium hydroxide, sodium peroxide or lithium metaborate, etc. as a flux, and then acidifing). Dissolution can be carried out in a beaker (glass or teflon), in a teflon bomb (inside a metallic jacket), using a flame, an electric oven or a microwave oven (in a closed teflon bomb without a metallic jacket). Melting is usually carried out in Pt crucibles, Pt - 5 % Au crucibles (not wetted by silicate melts) or Zr crucibles (with NaOH flux silicates can be melted at relatively low temperatures without losing relatively volatile trace elements). For geological samples, sometimes two “openings” are needed, one for major elements and one for trace elements or, one in which silicon is volatilized (getting rid of an interfering matrix) and one for silicon analysis. Pretreatment of samples may include the use of chelating resins (ion exchange) and/or extraction, either to remove interfering elements or to separate the wanted elements. These methods can also be used for preconcentration of the analyzed elements from dilute solutions. Flow injection analysis system (FIAS) can be used for analysis in a closed system to diminish contamination, for on-line dilution, concentration, introducing reagents and for working with microliter samples.

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5.6 Theory

Only the bare minimum of theory will be given in this section. Readers interested in more thorough theoretical treatment of the theory and of some subjects discussed in the following sections should refer to the proper textbooks (Section 5.2). Under normal conditions, atoms are in their lowest possible energy state, the ground state. The technique is based on the absorption of energy by valence electrons of ground state atoms, by which the electrons are raised to a higher energy level (excited state). Transitions that start of end in ground state levels are resonant transitions. There are allowed transitions and forbidden transitions. In the process of absorption, atoms change from a low energy state to a higher energy state. There are also transitions between excited states (non resonant) but these are not used in atomic absorption, thus restricting the number of transitions (wavelengths in the ultraviolet and visible range) that can be used for each element. Different atoms need different energies to be excited. This energy can be supplied in many forms like flame and heating by electric current. In atomic absorption spectrometry, the energy used is radiation from a specially built light source, emitting radiation in wavelengths that the analyzed atom can absorb. The total amount of radiation absorbed depends (among other factors) on how many atoms there are to absorb it. In atomic absorption analysis the analyzed solution is aspirated into the spectrometer, turned into an aerosol and passed into a flame where the sample is dissociated into ground state atoms. Radiation from a lamp, at a wavelength that can excite the analyzed atom, is passed through the flame where it is absorbed by the analyte atoms. The radiation is measured before absorption and after absorption and the amount absorbed is proportional to the concentration of the analyte. Beer-Lambert law (a combination of Beer's law and Lambert's law) mathematically describes the absorbance of light passing through a sample solution (in atomic absorption - the atom population in the flame) as a function of the length of the optical path through the sample (length of the flame) and the concentration of the absorbing species (ground state atoms).

It = 1,e-e'c A = logIo/It = d c where A is the absorbance (or optical density), I, the incident radiation power, It the transmitted radiation power, c is absorptivity (absorption coefficient at the wavelength used for the analysis), 1 the length of the absorption path, and c the concentration of absorbing atoms. This equation means that when analyzing the same type of atom (e.g. Cu) in samples of unknown concentrations and standard solutions of known concentrations, where the absorptivity remains the same and the absorption path (length of flame) remains the same, the absorbance will be a linear function of the Cu concentration. It should be linear but it is not always

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linear throughout all the range of concentrations. Hence, one cannot rely upon mathematical calculations and analytical measurements are made using calibration curves (though work is essentially limited to the range where the calibration lines are not too curved). There is no need to know the values of E and 1 as atomic absorption is a comparative technique. One measures a set of standards of known concentrations, prepares an absorbance vs. concentration line and for each absorbance reading of samples with unknown concentrations, finds the respective concentration (in modern spectrometers this is done by the instrument’s computer). Atomic absorption spectrometers read the amount of incident light without the sample and with the sample (after absorption), compare them, and show the results in absorbance units. 5.7 Major components of the instrument

The atomic absorption spectrometer includes many components and may contain parts which are unique to a special model of a certain manufacturer, but all spectrometers include some major basic components (whose shape, mode of operation, efficiency, etc. may change from one manufacturer to another):

I. Rdiution source; Usually a hollow cathode lamp which emits light of the special wavelength needed to excite the analyzed atoms; 2. An opticd system to direct the light from the radiation source through the ground state atoms in the atomizer to the monochromator and on to the detector;

3. Groundstute atom reservoir. There are two types: (a) a device for atomization by flame, including a nebulizer which aspirates the analyzed solution and passes it to the spray chamber where it is turned into a fine aerosol and passed onto the burner where the solvent is evaporated and the analyzed compound is dissociated into ground state atoms; and (b) a device for electrothermal atomization (known also as graphite furnace, graphite tube atomizer, carbon rod atomizer or flameless atomization) which includes a specially coated graphite tube (or cup) which acts as a sample holder and is heated by an electric current passing through two carbon electrodes, thus evaporating the solvent and dissociating the analywd compound into ground state atoms;

4. Monochromator. A grating that selects the desired wavelength and passes it onto the detector;

5. Detector. A photomultiplier that translates the residual intensity of the light from the hollow cathode lamp, before and after absorption by the analyzed atoms in the flame, into an electric current and amplifies the current;

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6. Signal mrriipulation device. In modern instruments - a computer that translates the response of the detector into analytical results, calculates concentrations (compared to a calibration curve) and standard deviations (in addition to acting as a system control - selecting the desired wavelength, controlling fuel-oxidant ratios and flow rates, etc.);

7. Reudout device. In older instrument, meters, recorders, or digital readout devices were used. In modern instruments, the computer screen and printer are used. In addition to differences in the components, instruments and computers of different manufacturers differ in software - the amount of automation, special “tricks,” data handling and report managing. 5.8 Radiation sources In order to measure the absorption under conditions that will guarantee a high sensitivity, a radiation with a very narrow wavelength band, at the absorption peak, is needed. Atomic ahsorption lines are very narrow. If a relatively wide bandwidth reaches the reading device (photomultiplier), the narrow absorption line will only be a small fraction of it, resulting in a reduced sensitivity and Beer-Lambert’s law will not be obeyed. This will result in a reduced correlation with concentration, i.e. a non linear calibration curve. One could wonder why for a special single wavelength (or several individual wavelengths) a radiation source (a line source) is needed, as the simplest way would seem to be the isolation of a single wavelength form a continuum source (emitting light over a wide wavelength range) by using a monochromator. The problem is that the width of the band separated by the monochromator is wider than the width of the atomic absorption peaks (which in the flame is determined by temperature and pressure). Contrary to this, emission from a line source (operating at much lower temperatures and pressures) has a bandwidth considerably narrower than the matching absorption peak. A monochromator and a slit are used, nevertheless, in addition to the line source, in order to select the needed wavelength region from all the wavelengths that the radiation line source emits. The best source for a single sharp wavelength would be a laser source. No commercial instrument is built with a laser. The most commonly used source is the hollow cathode discharge tube or the hollow cathode lamp (HCL) as it is usually called. The HCL emits the resonative wavelength which will be absorbed by the analyte, so the HCL is specific to the metal to be analyzed (i.e. a Fe lamp for analyzing iron, a Ni lamp for analyzing nickel, etc.). The HCL is formed of a glass cylinder, one end of which is a quartz (or Pyrex, i.e. a material which does not absorb the needed emission line) window and to the other (closed) end an anode

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GLASS SHIELD 1’

/

HOLLOW CATHODE /‘ QUARTg WINDOW

, J

I

I

\

\

ANODE

Figure 5-1. A scheinatic diagram of n hollow cathode lamp

and a cathode are sealed (see Fig. 5-1). The tube is filled with a noble gas (argon or neon) at a pressure of a few torr. The anode is made of tungsten. The narrow hollow cylindrical cathode is made of, or lined with, the metal (or alloy) of the element to be analyzed and whose spectrum the lamp is to emit. For convenience and in order to save the cost of lamps, some cathodes are made of a combination of a few metals (usually 2 or 3), a possibility only where the lines used for analysis do not overlap. These multi-elemental lamps may result in the need for a better monochromator (more expensive) and may shorten the lamp’s life. Nevertheless, some multi-element lamps are very common. A potential difference is applied between the electrodes causing a current between them. Electrons leave the cathode, collide with atoms of the inert gas and ionize them. These positive ions hit the cathode and as they have enough kinetic energy, they eject atoms from the surface (sputtering). Some of these metallic atoms are in excited states (probably by collisions with the inert gas atoms or ions) and when they return to the ground state they emit the characteristic radiation. The cylindrical shape of the cathode causes the radiation to be located within the tube. When one looks at an HCL, one sees a narrow black ring around the glass cylinder. This is a very thin metal layer, evaporated onto the walls during the manufacturing process, to serve as a getter (to trap gas remains and create a better vacuum). In addition, while most of the metallic atoms return to the cathode (because of the cylindrical shape of the cathode) a part is deposited on the surface of the glass walls. A warm-up period of a few minutes is needed to stabilize lamp intensity before use. What lamp current should be used? There are several factors to be considered. If the lamp current is too small, the analytical signal will be low and require excessive amplification. In addition, the amplification will cause higher noise. Noise is inversely proportional to the square root of the current. This means that increasing current 10-fold (e.g. from 10 to 100 mA) will decrease the noise 3-fold. One may think that one could obtain better results by increasing lamp current. However, by increasing lamp current too much, the lamp’s life is shortened considerably (proportional to the square of current change, i.e. an increase from 6 to 12 mA will

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shorten lamp life 4-fold). A second factor to be considered is that above a certain current, in most cases, increasing lamp intensity will actually lower sensitivity (lower absorbance for the same current). This means that there is a reason to increase lamp current only near the detection limit, where noise may be too high. Increasing lamp current can lower sensitivity for two reasons. One is Doppler broadening of emission lines width with increasing currents. Atoms emitting the radiation in the lamp, move randomly relative to the observer (in this case the monochromator); thus the apparent emitted wavelength will be higher or lower according to the direction, and instead of a single line, a wider band will be obtained. The other affect is self absorption or self reversal. A higher current will form more ground state atoms in the cloud and these will absorb a part of the radiation from the emitting atoms. The absorption is greater at the wavelengths in the center of the broadened emission line (which, as a result, will look like a saddle) and hence a lowering of the sensitivity. The amount of this effect depends on the element and the lamp’s construction. The right current to be used for a specific lamp depends on the above-mentioned considerations and on the specifics of the lamp’s construction. Unless there is a special need to change lamp current, it is best to use the current recommended by the manufacturer in the “cookbook”, in the certificate arriving with the lamp, or the default value in the computer controlled instruments. For Varian Co. lamps, this is usually 5-10 mA and for Perkin-Elmer Corp. lamps, 10-20 mA, respectively. Modern instruments have a turret for 4 or 8 lamps. The turret can be rotated manually or automatically (in computer controlled instruments). This has two benefits: (1) one can warm-up the next lamps to be used while still using the current lamp and save time; and (2) in computer-controlled instruments, this enables automatic consecutive analysis of many elements according to a prearranged program. The instrument recognizes the lamps, turns them on for warm-up (with the recommended or other pre-determined current), turns the turret to bring the needed lamp to the right position in the optical system, and turns off the lamps. A word of caution - this automatic analysis can be run without the operator being present, for instance, at night, gaining working hours, or during the day, saving operator time. While this can work well with a graphite furnace, it may be dangerous to leave a flame burning without supervision! A different type of radiation source, sometimes used in atomic absorption analysis, is the electrodeless discharge lamp (EDL). These lamps are energized by a microwave or radio frequency radiation. The lamp is a quartz tube containing a small amount of the metal (or its salt) of interest and filled with inert gas (at low pressure) which is ionized and accelerated by the field until it has enough energy to excite the metal atoms. These lamps emit a much more intense radiation than the HCLs. They were claimed to be more sensitive than the HCLs but less reliable.

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EDLs for several metals are commercially available. These EDLs need a special control unit. A different type of lamp used in atomic absorption analysis is the deuterium lamp, which emits a continuous radiation in the ultraviolet region. This lamp is used for background correction - correction of interferences caused by absorption (or scattering) of particles other than the analyzed element. As the output of radiation in the visible zone is low, this lamp is used for elements whose analytical liens have wavelengths below 350 nm. The use of this lamp for background correction will be described in Section 5.14. The construction of a deuterium lamp is different from that of the HCL. It is filled with deuterium, has a metal anode, the cathode is an electron emitting themionic cathode and there is a restrictive aperture between them, causing an area of high excitation and high light emission. This lamp is used simultaneously with the element’s HCL. In former instruments, a very careful tedious alignment of the two was needed, otherwise, an erroneous correction resulted as the gaseous system (flame or arc) is very unhomogeneous. In modern instruments, this alignment is very simpel. In computer-controlled instruments, the current for the deuterium lamp and the ratio of intensities of the two lamps are automatically controlled.

5.9 Wavelength selection system The first step of isolating a specific wavelength is taken by using a line source, i.e. a hollow cathode lamp emitting only a small number of wavelengths. A wavelength selection system (a monochromator) is needed to isolate a very narrow emission line from all the emitted wavelengths and produce a beam of radiation of high spectral purity, the wavelength of which can be varied at will, over a wide range. The main components of such a system are: an entrance slit, a collimating mirror (causing the beam to travel as parallel rays), a dispersive device (separating the polychromatic light into the monochromatic components), a focusing mirror and an exit slit (Fig. 5-2). There are several types of monochromator system arrangements (referred to in the literature as mounts, e.g. [23: the Littrow, Ebert and Czerny-Turner. The schematic diagram of the last one is shown in Fig. 5-2. They differ from one another in compactness, number of mirrors used, the use of prisms and focus arrangements, GtC. The light emitted from the exit slit contains unwanted radiation scattered from different internal parts of the instrument (to avoid this, some parts are painted black) and from dust particles inside the instrument. In order to avoid this (and damage), the whole monochromator is a sealed unit with quartz windows for entrance and exit of light.

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SLIT

1 MIRROR

GRATING

ENTRANCE SLIT

T

Figure 5-2. A schematic diagram of a monochromator (Czerny-Turner type)

A slit consists of two exactly parallel metal jaws, carefully machined to have very sharp edges. In some instruments there are several slits with a fixed aperture and the operator has to use the one which is best for the analyzed element. In other instruments the distance between jaws can be varied as needed. There are two types: one in which one jaw is fixed and the other moves (which means that the center of the slit moves with the change of width). In the second type (more common) both jaws move relative to each other and the center remains fixed. The physical distance between the jaws is called the mechanical slit width. Another term used is the spectral slit width (or spectral bandwidth) which is the range of wavelengths (polychromatic light) that passes through the slit for a given monochromator setting (or, the span of monochromator settings needed to move the image of the entrance slit across the exit slit [seethe following]). The entrance slit acts virtually as a light source to the system (passing on the light arriving from the hollow cathode lamp). If a lens or a mirror (which is usually used in the commercial instruments) is placed after the slit in such a way that its distance from the entrance slit equals its focal length, the light arriving from the slit will be collimated - passed on as parallel rays. If a second lens or mirror is placed before the exit slit in such a way that its distance from the slit equals its focal length, the parallel rays arriving from the first mirror will be focused on the focal plane the surface containing the exit slit. The rectangular image of the first slit - a bright line, hence the term “emission line,” will be formed on the surface containing the second slit. The system is arranged in such a way that the image will be formed on the exit slit and the light passed on from the slit to the detector. Usually the exit slit is adjusted to the dimensions of the entrance slit. The dispersive unit (in former instruments, a prism and in modern instruments a grating) is located between these two slit-mirror systems. The grating resolves the light into separate wavelengths so that several rectangular images of the entrance slit, one for each wavelength, will be

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formed on the focal surface of the second mirror, the surface which contains the exit slit. By rotating the grating, the image formed by the desired wavelength is brought to overlap the exit slit and will be passed on to the detector. By rotating the grating, any one of the separated wavelengths can be selected, by moving the slit image it forms, so that it overlaps the exit slit. In computerized instruments, the grating is rotated automatically so that the prescribed wavelength reaches the slit and after that, a careful scan (called peaking), in small increments, is carried out to locate the exact point of the emission line’s peak. One of the reasons a grating is preferred to a prism is that the spectrum produced by a grating is linear (i.e. it has a linear dispersion, an equal length for the same range of wavelengths, i.e. and equal length for a 100nm range throughout the whole uv and visible spectrum) while that of a prism is not. As a result, in order to obtain a linear change of wavelengths reaching the detector, a grating has to be rotated in a linear way while the prism should be rotated in a non linear way, which is much more complicated and expensive. If the monochromator passes on the desired wavelength A, the image should exactly fill the exit slit. If the slit is wide, part of the adjacent images will also pass the slit. The choice of the slit width is acompromise. A wide slit is needed to increase the light throughput to the monochromator, to increase signal-to-noise ratio. On the other hand, good separation of wavelengths will be obtained with narrow slits. If the separation is not good, the calibration line will become curved (Section 5.8). The optimal width of the slit (for each element) will be determined by the existence of close lines and their distance from the desired line. In the “cookbooks” the width for each element is recommended, usually between 0.1-1.0 nm. In computerized instruments, a default value of a recommended slit width is automatically used (it can be changed by the operator if desired and should be changed if warranted). If monochromatic light passes through a narrow slit, the slit acts as a light source and the light, after passing through the slit, instead of forming the image of the slit, will form a diffraction pattern which consists of a central bright band bordered by alternating dark and bright bands of decreasing intensity and quite diffused borders. As the number of slits is increased, interference occurs and the central and subsidiary bright bands are split into several sharp bright lines with most of the intensity in the central lines. The greater the number of slits, the sharper, narrower and brighter are the illuminated lines. A grating is essentially an array of a great number of slits. The first gratings consisted of fine wires stretched across a frame, acting as a series of slits. Today a grating is a series of parallel equidistant straight lines (grooves; for the uv and visible range, usually 1200-2000/mm, but the number may be lower or higher) cut into a plane surface (coated with highly reflective aluminum) each of which acts as a slit a light source. Each groove in the grating gives rise to a diffracted beam and these

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diffracted beams then interfere with one another to produce the final pattern. There are two types of gratings, transmission gratings (where light falls on one side of the grating and is emitted as separated wavelengths on the other side) and reflecting grating (where incidence and dispersion occur at the same side of the grating, the lines are actually ruled on a mirror). In modern instruments, reflection gratings are used. The dependence of grating dispersion on interference means that there are angles in which the diffracted rays are destroyed (dark lines) and angles in which the rays are reinforced. The condition for reinforcement is described by the grating equation IZA = &in i f sin 8) where A is the wavelength of the radiation, d is the distance between grooves which is the reciprocal of the number of grooves per unit distance, a the angle between the normal to the surface and the line of incidence of the light beam, 8 is the angle between the normal to the surface and the line of dispersion of wavelength A, and n.is a whole number called the order. Incident light may be diffracted on either side of the normal. When i and 0 are on the same side of the normal, the + sign applies and when they are on opposite sides, the - sign applies. Zero order, 11 = 0, corresponds to the direct reflection ray (reflection from a mirror) which will occur at an angle of -i, i.e. an angle equal to i on the other side of the normal to the surface. It does not depend on the wavelength and all wavelengths in the incident beam will be included in the zero order ray. For different wavelengths, the angle of dispersion 8 is different. This means that a beam of polychromatic light will be dispersed into the monochromatic components because each wavelength will be dispersed at a different angle. A beam of polychromatic light falling upon the grating will be dispersed into a series of spectra located symmetrically on each side of the zero order position, the closest spectrum is the first order, the next the second order, etc. Each order consists of a whole spectrum, in which the shorter wavelengths are at the smaller angles. The equation shows that there will be overlapping lines, for instance, wavelengths 4000 A of the first order, 2000 A of the second order and 1000 A of the fourth order will be dispersed at the same angle, as they have the same value of n X . Master gratings are prepared by ruling the lines on a hard polished surface with the aid of a properly shaped diamond tool (having elaborate controls to ensure the necessary precision). From this master replica, gratings are prepared by applying a layer of parting agent to the master, a layer on which a film of aluminum is deposited. To this film a glass or quartz base is attached with a suitable cement and the replica is separated from the master. In modern instruments the so-called holographic grating is used. The grating is obtained by using laser technology to form the grooved surface. Much better line shapes, line dimensions, a much higher groove density and bigger gratings can be formed, thus causing purer spectral lines, at a relatively low cost. The quality of a monochromator is measured by the purity of the radiation it

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passes on, in other words, by the ability to resolve adjacent wavelengths and by the intensity of the radiation passed on. Resolution is the smallest wavelength interval that a grating is capable of isolating. Resolving power is a measure of the ability to separate one wavelength from another, nearly identical, wavelength, or, the ability to distinguish between two adjacent slit images having a small wavelength difference. Some books do not distinguish between these two terms. The resolving power R of a grating can be determined in two ways: (1) by the equation R = n N where n is the order and N is the total number of grooves. It is not the density of lines which determines R but their total number, i.e. a 5 cm, 1200 lines/cm grating will have in the 3rd order the same resolving power as a 10 cm, 1800 lines/cm grating in the first order. One should note that it is not the total numbers of grooves on the grating which counts, but rather, the total number of grooves upon which light falls by the optic system, a difference which may give room to false claims of high R. The dependence upon N enables working with bigger, coarse gratings having a lower groove density (easier to manufacture and cheaper). (2) By the equation R = A A where is the mean wavelength of the two lines to be resolved and A A is the difference in wavelength of the two lines that can just be distinguished as two lines. These two equations enable calculating the resolving power, and hence the combination of order - number of grooves needed to separate two lines (e.g. to just separate the sodium doublet lines 5890 and 5896 A - the resolving power needed is 5893/6 = 982 which is only about 1% of obtainable values) or, to calculate if a given n. and N can resolve two given wavelengths. Dispersion is the separation of polychromatic light into the monochromatic components. The effectiveness can be described in different terms: (1) angular dispersion - the change in angle of dispersion from the grating - 8 - per unit wavelength, i.e. do/ dA (radians/A). The greater it is the better the separation; (2) linear dispersion - the lateral separation of the slit images per unit wavelength, i.e. dZ/ dX (mm/&. The greater it is, the better the separation; (3) reciprocal linear dispersion dA/ d l (hmrn). The smaller it is (less images/mm), the better the separation. The last two definitions depend on the focal length (the distance between the mirror and the exit slit) of the monochromator, the longer it is, the better the separation. The reciprocal linear dispersion of a grating is almost constant throughout the uv visible range. Attention should be paid to the fact that there is an overlap of wavelengths so that 1000 A in the second order falls exactly in the same place as 2000 A of the first order and 1000A of the third order falls where 3000 A of the first order falls. If for resolving power considerations one has to use third order 1000 A,one should take care to eliminate first order 3000 A, second order 1500 fourth order 750 A, etc. This can be accomplished by putting, in front of the grating, a filter that will pass on

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to the grating, only wavelengths close to the desired one, or, a prism that will pass on only the desired (or a part of it) order (order sorter). One of the measures of a monochromator’s quality is the energy throughput or the amount of radiation passed on to the detector. For the same grating, the higher the order the better the dispersion, but the lower the intensity of the radiation, with the greatest intensity being in the first order. This can be changed by changing the shape of the grooves in such a way that the intensity is concentrated mainly in a predetermined angle. Instead of the grooves being symmetric, they are grooved or blazed in such a way that they have the form of steps having broad faces from which reflection occurs and narrow faces which are unused (Fig. 5-3). There is an GRATING

I NORMAL

Figure 5-3. A schematic diagram of an echelette grating

angle p between the broad faces and the surface of the grating, or, in other words, between the normal to the surface and the normal to the broad face. This type of grating is called an echelette grating and p is the blaze angle. This angle will determine where the intensity will be concentrated. The diffracted energy will be concentrated around the diffraction angle for which the ordinary law of reflection from the individual faces is most nearly obeyed. By optimization of the blaze angle, as much as 90%of the available energy of a particular wavelength can be made to fall in a single order. The wavelength for which ordinary reflection and first order diffraction coincide is called the blaze wavelength A,. Most light is concentrated in the first few orders and centered about A, in the first order, &/2 in the second, etc. The monochromator can be blazed at two different wavelengths to provide high energy at both the uv and visible ranges.

5.10 Atomization by flame One of the methods of atomization used in atomic absorption analysis is atomization by flame (the other being electrothermal atomization). The purpose of the

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atomization system is to generate uncombined analyte atoms, which in turn will be excited by radiation having the characteristic wavelength A. The system includes (Fig. 5-4) a nebulizer, a spray chamber (and a liquid trap), a burner, and a flame.

CAPILLARY

(

//A

GLASS BEAD

IDANT WASTE

Figure 5-4. A schematic diagram of a nehulizer-spray chamber-burner system (with a flaw spoiler and a glass bead)

This system is very important in determining the sensitivity, detection limit, precision and accuracy of the determinations. There are momentary variations in flame characteristics so that conditions are not exactly the same during the analysis and the stability of the excitation system (nebulizer-burner) is one of the major parameters determining the quality of results. At the beginning of the analysis, several adjustments should he done (see the following) to raise sensitivity, but the aim is not to obtain maximum signal or maximum sensitivity as such, but to obtain the highest possible sensitivity while remaining under stable conditions (and acceptable signal-to-noise ratio). The nebulizer sucks up the sample solution to the spray chamber where it is broken into a very fine aerosol which is passed on to the flame (ignition by a pushbutton) where it is heated to a temperature high enough to evaporate the solvent and then to dissociate the compound into its constituent atoms. The light beam from the HCL excites the atoms in the flame and the amount of the absorbed light is measured. A delay time (aspirating the solution to the flame without reading the absorbance) of a few seconds before measuring each sample is necessary. First, to clean the system from the remains of the former solution, and second, it takes time from the beginning of aspiration to obtain the stabilization of the highest part of the

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absorption peak. The instrument reads a great number of readings per second. Two or three second-long measurements are usually taken to obtain the mean and the relative standard deviation of the sample’s concentration. Ordinary nebulizers are fixed nebulizers (fixed uptake rate) but there are also ad-justable nebulizers (adjustable uptake rate). They are recommended for work with organic solvents, as these also act as fueIs. With a fixed nebulizer, there will be access fuel during aspiration and the acetylene flow rate has to be lowered. The result is that when aspiration stops there will be too little fuel and the flame may go off. The adjustable nebulizer solves this problem. The oxidant flows through the nebulizer, passes through the venturi and draws solution (from the sample vessel, through a plastic capillary tubing) into the venturi as a mist of fine droplets which is then mixed with the incoming fuel and passed on to the burner. There is an optimum size for an analyte droplet which will allow it to be evaporated completely and atomized during the short time the droplet is in the flame. This is considered to be in the range of 2-10 p. Small droplets will be carried to the flame while the big drops will be drained away to the liquid trap. The higher the part of the solution used, the higher the sensitivity and the detection limit. The amount utilized will depend on the distribution of drop size, i.e. the percentage of droplets having the right size. The aim is to increase the percentage of droplets having the right size and this is done by breaking the bigger droplets into smaller ones. In some instruments the spray from the nebulizer hits flaw spoilers (or baffles) in the form of a propeller (not rotating) in the spray chamber, and are broken into smaller droplets. A better way seems to he the glass bead (or impact bead) used in other instruments. This is a few cm long, bent, thin glass rod with a bead at its end (with a special coating), in the spray chamber opposite the nebulizer. The spray hits the head and is shattered into fine droplets. The absorbance critically depends on the distance of the bead from the nebulizer venturi (reaches a maximum and then drops) and this depends upon the physical properties of the solution. The distance of the glass bead has to be adjusted at the beginning of the analysis, while aspirating a standard solution of the analyzed element, until maximum stable signal is obtained. Nowadays, there are instruments that use both types of spoilers. Though the amount of droplets of the right size in the spray can almost be doubled, only about 10%of the solution is utilized while the rest is drained. There were total consumption burners in which most of the sample was passed on to the flame, but the energy in the flame was not high enough to evaporate and dissociate this amount of solution during the short residence time in the flame, resulting in low sensitivity and numerous interferences. In modern pneumatic nebulizers only about 10% of the solution reaches the flame, but this is done under conditions which favor stability. Burners are made of special metals that can resist corrosion by acidic or basic solutions reaching the burner at high temperatures. There used to be a plastic material film on the inner part of the burner, to avoid corrosion, but this was

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discontinued as it was found to cause a memory effect. Solution droplets adhered to the film and later renebulized and passed on to the flame while the next solution was aspirated. There are special burners for air - acetylene and for nitrous oxide - acetylene flames and they should not he mixed as explosions may occur. The burner's design depends on parameters such as gas flow rates, burning rate, fueloxidant type, efficient heat dissipation so that the burner will not become red hot (some burners have cooling fins). The burner's slot dimensions depend on the fueloxidant for which it is intended. The burning velocity is the speed of propagation of the flame front in the gas mixture. A certain correlation must be maintained between burning velocity and the combustible mixture flow velocity. Higher flow velocities can cause the flame front to blow away while lower ones can cause a flashback (the flame propagates within the burner) which can lead to explosion. Safety precautions in the instrument's manual should be followed. In modern instruments, several safety precautions are built in the system, for example, a burner interlock inhibits ignition if a burner is not fitted to the spray chamber or a burner is fitted which is not suitable for the gas mixture selected; a pressure relief bung at the rear of the spray chamber is designed to minimize the effects of a flashback; an interlock designed to inhibit ignition if the pressure relief bung is not correctly fitted and to shut down the flame if the bung is ejected as a result of a flashback; a liquid trap provides a gas-tight seal between the spray chamber and the atmosphere and allows excess solution to be drained from the spray chamber, fitted with a lever sensor which is designed to inhibit ignition if the liquid trap is not filled to the correct level and to shut down the flame if during operation the level falls below the required minimum. Several gases were used as oxidants and fuels. The most common were:

1. Air-propane; relatively low temperature. It was used mainly for excitation of alkali metals (which can be ionized in hotter flames). Severe chemical interference can occur and noise level is relatively high. Used today only for flame emission analysis (flame photometer) of the alkali metals; 2. Air-hydrogen; relatively low temperature. A reducing medium fit for arsenic, selenium and tin, for which this flame is mainly used. Severe chemical interference can occur;

3. Air-acetylene; compressed air is used (from a lab central supply, a cylinder or a lab compressor). A relatively hot flame (about 2300" C); causes partial ionization of alkali and alkaline-earth elements; used for most elements; 4. Nitrous oxide-acetylene: the hottest flame used (about 3000OC); used for refractory elements and to reduce chemical interference; may cause severe ionization and an ionization suppressor may have to be added.

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Because of safety, cost, usefulness and convenience, only the last two flames are used in modern commercial instruments. The quality of the gases to be used depends upon the purity of the gases available locally. In some places commercial acetylene is good enough and in some places extra pure acetylene should be used. The compressed air may contain water and oil and the system should contain a trap to remove them from the gas. A compressor which does not need lubrication and does not emit oil vapors should be used. In all flames, temperature profiles and flame characteristics are determined by fuel-oxidant ratios. There are fuel lean or oxidizing flame (the hottest), fuel-rich or reducing flame (the coolest) and chemically balanced or stoichiometric flame (about midway between the first two). They can be identified by the height and color of the inner part of the flame (not a cone as in ordinary lab burners but a long inner part parallel to the length of the burner). One can find in the "cookbook" (in the analysis instructions for each element) what type of lame should be used (according to the element, the matrix, etc.) what interferences can be expected and what to do in order to overcome them. The properties of the analyzed atom, its molecular association and the final products it may form in the flame will determine what type of flame has to be used (as recommended in the "cookbook" or by the default parameters of computerized instruments). There are several types of atoms according to their behaviour in flame:

1. Easily atomized atoms (such as potassium, sodium, lead) that can be analyzed in air-acetylene flame. There are only a few interferences and the fuel-oxidant ratios have no great influence;

2. Atoms that can be determined in both air-acetylene and nitrous oxideacetylene flames, but not as effectively. For instance, if a solution of Ca nitrate is analyzed, it will decompose in both flames intoCaO, but a much higher proportion of this oxide will dissociate into atoms in the nitrous oxide flame and a higher signal will be obtained. The analysis can be carried out also in air-acetylene flame but many interferences may occur and the fuel-oxidant ratio is important;

3. Refractory elements which almost will not be atomized by a cool flame and have to be atomized in a nitrous oxide flame; 4. Elements for which not only temperature but also the chemical nature of the flame determines the dissociation so that a specific fuel-oxidant ratio is needed. For instance, Mo in a stoichiometric air-acetylene flame will remain as an oxide but, in a fuel-rich flame, though it is cooler, it will be reduced to the free atom. Similarly, a strongly reducing nitrous oxide-acetylene flame is needed to effectively atomize silicon;

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5 . Atoms that react with other species in the sample, In the presence of aluminum, silicon and phosphate, atoms like magnesium, calcium, strontium and barium may form the respective compounds and will only slightly dissociate into atoms. The use of the hot nitrous oxide-acetylene flame will cause higher dissociation and better results will be obtained. Another common method to overcome such a problem is the use of releasing agents. If a large excess of strontium is added to a silicate containing calcium (to be analyzed) nitrate solution, most of the silicate will combine with the strontium. Lanthanum is a very common releasing agent. Another common method is matching the calibration standard solutions with the samples, i.e. adding the interfering element to the standards (in about the same concentrations), thus avoid high results for the standards and low for the samples.

For atomic absorption, ground state atoms which can be excited by radiation are needed. There is an optimum flame temperature for vaporization and dissociation. If the flame temperature is too high, the atoms may receive enough energy for ionization to occur (which, depending upon the atom, temperature and concentration, may reach the range of 80%). The electron will leave the atom completely and will not take part in the excitation by radiation process. This may be a serious problem for elements with low ionization energies. The method to overcome this problem is to add to the analyzed solution another element, such as Na, K or Cs, which is ionized more easily (or about the same) at a very high concentration compared with that of the analyzed element. For instance, for the analysis of trace amounts of Ca (in the range 1-4 pLg/ml) in the nitrous oxide-acetylene flame, the solution is made to contain 2000 &ml potassium, which will cause signal enhancement due to suppression of ionization. In the analysis of traces of strontium, the sensitivity may be highly increased by the addition of an ionization suppressor-potassium ions (with the same amount of potassium the sensitivity may be highly increased by raising the temperature, using nitrous oxide-acetylene instead of air acetylene flame). The ionization suppressor has to be added to the unknown solutions as well as to the calibration standard solutions. The effectiveness of the suppressor may change with its concentration and the right amount has to be determined experimentally (absorption vs. concentration, till a plateau is found) or taken from a “cookbook”. One has to be careful not to cause harm by this addition. The added potassium chloride contains, for example, some Na (depends on the purity of the material used, let’s say 0.01%). If one analyzes a very low concentration of Na, this may cause an error. In this case a very pure potassium chloride powder or solution has to be used. Raising the temperature should (due to increased evaporation and dissociation) increase the total number of atoms in the flame and hence the signal. On the other hand, the intensity may be lowered because of increased excitation (not by HCL but by flame), ionization and Doppler broadening. Because of these opposed

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effects, each element has an optimal absorption as a function of location in the flame (e.g. absorption increasing with height, decreasing with height or increasing to a maximum and then decreasing with height). Maximum signal will be obtained if the light beam from the HCL passes in the flame, through the maximum population zone which is not identical for all elements. For any given combination of element, matrix, sample aspiration rate, flame type and burner, the population density of absorbingatoms will vary throughout the flame in a manner characteristic of that combination. At the beginning of the analysis, several adjustments are made while aspirating a standard solution of the analyzed element. The height of the burner relative to the light beam has to be adjusted (care should be taken not to raise the burner into the light beam) for each element until maximum stable signal is obtained. The burner position has to be adjusted rotationally, so that the length of the burner is parallel to the light beam (so that the whole length of the flame will be utilized) until maximum stable signal is obtained. The burner position has to be adjusted horizontally (backward and forward) to align the center of the burner slot with respect to the light beam, until maximum stable signal is obtained. For each element there are several optimum working ranges (a linear calibration curve) at different wavelengths, e.g. for Cu, 2-8 &ml at 324.7 nm, 6-24 pg/ml at 327.4 nm, 70-280 pg/ml at 222.6 nm. One has to dilute the Cu solutions to obtain concentrations in the working range to be used. In many cases, when the concentration is higher than the optimum range, it is worthwhile (instead of diluting the samples) to rotate the burner at a certain angle. The distance that the light beam travels in the flame is shortened, the beam meets less atoms, thus lowering the sensitivity, resulting in the same absorbance as if the concentration is lower. The angle of rotation can be adjusted to reach absorbance values in one of the optimum working ranges for lower concentrationsand the analysis is carried out at that angle. In order to be able to do this, one has to know the true range of concentrations (e.g. by diluting and measuring the lowest and highest concentrations) and to prepare calibration standards that will fit that range when they are measured at the same rotation angle. Attention must be paid to the fact that when the whole length of the burner slot is used, the interferences at the two edges are small compared to the optical path length in the flame. Using a rotated burner, the optical path length is shortened and the interferences at the two edges are comparatively high. Changes in sample flow rate can change the number of atoms in the flame and can change the sensitivity. One may think that if more solution reaches the flame the more dissociated atoms there are, however, the additional solution is not effectively evaporated and atomized because the solution cools the flame, thus lowering the signal. The system tends to be saturated and the curve describing absorption as a function of intake rate rises and then flattens at rates higher than about 6 ml/min. At the beginning of the analysis the nebulizer has to be adjusted (while aspirating a

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standard solution of the analyzed element) to an optimum flow rate until maximum stable signal is obtained. Changing oxidant-fuel ratios will change the flame temperature profiles (hence the location of maximum population) and characteristics (oxidizing, reducing or stoichiometric). At the beginning of the analysis of each element, the ratio has to be adjusted (while aspirating a standard solution of the analyzed element) so as to obtain maximum stable signal. In computerized instruments there are (for each element) default values for flow rates of air, acetylene and N,O so as to obtain a high sensitivity and the right amounts are delivered by a control unit. One can change these values when needed, for example to compensate for matrix effects, simply by typing the new values to be used. A difference in physical properties (such as surface tension or viscosity) between unknown solutions and calibration standards will result in different flow rates (for the same instrument setting) and hence, in different signals for equal concentrations. Thus one has to match (as much as possible) these properties. If because of the dissolving procedure the solutions have a relatively high acid concentration, the calibration standards should contain the same amount of the, same acid. If the aqueous solution contains a certain amount of inorganic salts or of an organic material, the same amount should be added to the standards. For instance, for the analysis of trace amounts of Cu in blood serum, the Cu is leached into a 12.5% trichloroacetic acid. The standards should also contain 12.5% TCA [ 141. The analyte atoms in the flame do not know that they are in an atomic absorption spectrometer and in addition to the absorptionof energy from the HCL, they also emit their characteristic radiation at the resonance line (flame emission) and this (together with radiation from other atoms and flame luminosity, at this same wavelength so that separation by a monochromator cannot be achieved) may compensate for a part of the absorbance at this same wavelength. The problem is solved by modulation of the system. One way is to modulate the output of the source (the power to the source is modulated) so that its intensity fluctuates at a constant frequency. Another way is by use of a chopper - a disk of which alternate quadrants are removed. Rotation of the disc causes the beam to be chopped at the desired frequency. In both cases the HCL radiation is modulated but the flame emitted radiation is not. Only the modulated light is amplified and read. Caution! Some safety precautions should be strictly enforced. A flame should never be left unattended. Keep all compressed gas cylinders outside the laboratory in a cool, ventilated area. The fumes and vapors produced in the analysis can be toxic and should be extracted from the instrument by an efficient exhaust system. Always use a liquid trap filled to the correct level to avoid explosions. A special type of burner is needed for each type of flame. An air-acetylene burner should never be used for a nitrous oxide-acetylene flame. Acetylene can react with copper,

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or copper alloys, to produce acetylides which may explode. Hence, no copper (or brass) pipes should be used with this fuel and special pressure regulators, not containing copper, should be used. Do not use oxygen-acetylene flames. Acetylene cylinders should not be used completely as the acetylene is dissolved in acetone and when the acetylene pressure in the cylinder is low, acetone will also be evaporated, will enter the burner and may explode. Always stop using the cylinder when some pressure still remains (for the exact amount check the manufacturer’s instructions). Nitrous oxide can cause spontaneous combustion of oil, and care must be taken to clean connections and the pressure regulator from oil. When N,O is drawn from the cylinder the cooling effect can freeze the gas and a regulator with a built-in heater should be used. The operator should familiarize himself with these and many other safety precautions specified in the instrument’s manual and follow them.

5.11 Instruments The major necessary components of an atomic absorption spectrometer were mentioned in Section 5.7 and described in detail in the previous sections. Different manufacturers use different forms of these components but they all should be included in the instrument. Different manufacturers have different software to control the instrument’s operation and data handling. All this (including the rank or upgrading of the instrument which (depends whether the instrument is a single or double beam type, flame only or including graphite furnace, with or without hydride generator, a data station or an advanced computer, etc.) may run in the range of $20,000-$70,000. Contrary to emission spectrographs or ICP spectrometerswhich are simultaneous, i.e. can measure the signals of many elements at the same time, atomic absorption spectrometers are sequential, measuring one element at a time. One has to measure, for instance, copper in all the samples, then nickel in all the samples, otherwise one has to change the monochromator settings and recalibrate separately for each element in each sample. This sequential system makes the analyses much longer (operator time), a factor that has to be taken into account in efficiency or price/element calculations. Generally atomic absorption spectrometers are divided into two types, i.e. single beam and double beam instruments (each containing all the major components mentioned above). In a single beam instrument there is only one beam of light which passes from the source into the flame and optical system and then to the photomultiplier. Any change in the light source intensity or in flame characteristics will influence the final signal.

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In a double beam instrument, the light from the source passes through a beam splitter and is split into two beams of about equal intensity. One passes through the flame (I,)as in a single beam instrument and the other passes along the same optical system, but not through the flame, i.e. it is not influenced by the analyte (Io). This second beam serves as a reference beam to which the intensity of the first beam (the sample beam) is compared (Beer-Lambert). The intensity of each beam is read in turn (the beams become intermittent) by the photomultiplier and their ratio is amplified and displayed. Thus, any change in the source intensity, the detector sensitivity or the electronic amplification will affect both beams and the effect will be cancelled. The measurements of a double beam instrument are much more stable - less noise - the result of which is that higher amplifications are possible so that lower concentrations can be measured. On the other hand, this is not a real double beam compensation, as the reference beam does not pass through the flame and changes in the flame (for instance absorption by molecules formed from the solvent at high temperatures) or momentary changes in the nebulizer uptake will not be compensated for. In addition, the double beam system means more parts (the splitter, mirrors, half silvered mirrors) and a bigger, complicated instrument, hence its high price compared to that of a single beam instrument (but if one can afford it it's more than worth the difference). One system of a beam splitter is a rotating disc with alternate quadrants removed. The remaining quadrants which have mirrored surface reflect the light into the reference path, where through an array of mirrors, the beam avoids the flame and then a half silvered mirror recombines it with the sample beam along the optical path. The spaces (missing quadrants) pass the radiation into the sample path. This action of the beam splitter also modulates the beams into intermittent beams, so that only they will be read and not the continuous emission from the flame (see Section 5.10). In another system the splitter consists of a partially aluminized quartz plate. A part of the light is reflected to the flame and the other part passes through the splitter, through an array of mirrors, directly to the monochromator. A rotating reflecting chopper is located in front of the monochromator entrance slit, where it alternately interrupts the sample beam to direct the reference beam into the monochromator. This way a two-beam system with modulation is achieved.

5.12 Electrothermal atomization This technique is known by many names such as electrothermal atomization, graphite furnace, carbon rod, graphite tube atomization, etc. The function of both flame and electrothermal (electrically heated graphite furnace) atomization is the generation of free analyte atoms ready to be excited by the right wavelength radiation. Flame atomization is simple, reproducible and good results are obtained, but

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the method has a sensitivity limitation. For a large number of elements, furnace techniques are more sensitive than flame methods and can determine concentrations typically a hundred times smaller than those possible by flame, but the analysis is much longer (may reach 2-3 min per elemendsample). This is a technique which is complementary to flame absorption, rather than a technique which replaces it. Sample volumes in the microliter range can be analyzed. As combustible gases are not used, a furnace system can safely be left overnight unattended (provided no other trouble develops). Sampling efficiency of the flame is low as almost 90% of the sample are drained, residence time in the flame is very short (a very small fraction of a second) and dissociation depends both on temperature and the chemical nature (reducing, oxidizing or stoichiometric) of the flame. In electrothermal atomization the sample is very small but all of it is utilized and residence time of the atoms in the optical path is about a second. The concentration of analyte atoms is high, resulting in a high absorption, or, the proper absorption working range (about 0.1-0.8 absorbance) can be reached with lower concentrations. The dissociation depends on the final temperature, the rate of approaching it and the reducing nature of the graphite. The small sample volume (usually 10-40 p1) requires a very precise measurement and introduction into the furnace and this may become the limiting factor for reproducibility. At first micropipets with interchangeable plastic tips (to prevent contamination from sample to sample) were used for manual injection, but this proved not good enough. For reproducible results an automatic injector is a must. In the “cookbook,” relevant data for the instrument’s performance may be given in different forms such as: the typical characteristic concentration - the weight in grams of an element that would typically yield an absorbance equal to 0.0044 absorbance- 1% absorption - (in the peak height mode, under specified conditions), which is in the few pg range, based on a 20 p1 aliquot (which is the range of a few ng/ml-ppb) or/and as the typical response, for instance, a 10 pl aliquot of 5.0 ndml is expected to yield about 0.3 absorbance, and/or as the analytical working range, for instance (for a 20 p1 aliquot) 0.5-12 ng/ml (expected to yield 0.3-0.8 absorbance). In older instruments it was quite a complicated task to install the graphite furnace into the spectrometer. In modern instruments the furnace is interchangeable with the nebulizer-burner unit and only small adjustments (for correlation with the optical path) are needed. The sample holder (Fig. 5-5) is a replaceable graphite tube which is contained in a water cooled cell fitted at each end with quartz windows. A stream of inert gas (argon or nitrogen) flows through the cell protecting the graphite from oxidation. The graphite tube is a cylindrical tube, open on both sides (the optical path) with a hole in the center of the upper part of the tube for sample injection and through which the inert gas emerges with the products of drying and ashing. The tube fits into a pair of graphite electrodes (the form of which differs according to the manufacturer). A toggle mechanism locks the graphite tube in place, so that there

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ATOMIC ABSORPTION SPECTROMETRY

GRAPHITE TUBE

\

INJECTION HOLE

'CARBON ELECTRODES APHITE TUBE

"'.EXTERNAL

CARBON ELECTRODES

QAS

FLOW

FLOW

Figure 5-5. A schematic diagram of two types of graphite furnaces. Toggle mechanism, cooling water, springs, connections, etc., are not shown.

is no gap between the tube and the electrodes (due to graphite corrosion) so that the same voltage will result in the same temperature. The tube is electrically heated in programmed steps by a high current (low voltage) passing along its length. The temperature is measured by an external sensor (a pyrometer) or by voltage-current measurements. Coating of the graphite tube with pyrolitic carbon (obtained by passing through the cell an inert gas and a hydrocarbon, at a high temperature) seals pores in the tube. This prevents soaking of the sample into the graphite, which would affect reproducibility and will lower sensitivity. This hard surface also prolongs tube life and minimizes carbide formation which can prevent analysis of carbide forming elements such as Mo, Ti, U, V. The autosampler (automatic injector) can inject into the furnace a programmed volume of a blank solution, a chemical modifier, a sample solution and can prepare (and inject) the programmed amounts of standard solutions by injecting the right amounts of diluent and of the analyzed element stock solution (up to a constant volume). It also can rinse the injector plastic tip between samples. Standard editions analysis is often used for the furnace analysis of samples. This involves the

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preparation of several solutions for each sample, Automatic preparation of these solutions saves time and nerves (and errors). A small volume of sample is place on the (inner) bottom of a graphite tube located in the place of the conventional flame of AA instrument. The process consists of 3 stages. In the first stage (drying) the solvent is evaporated. The sample is gently diied by programmed heating of the graphite tube. If the temperature is raised too fast or to temperatures above the boiling point, the sample will sputter, loosing material (which also may stain the quartz window). Because of this, if by mistake a wrong sample was introduced, one should be careful not to use the “clean” option as this raises the temperature immediately to very high temperatures (close to those of the atomization) and sputtering will occur. This option should be used for cleaning only before analysis or after atomization. The best results may be obtained, for example, by raising the temperature slowly to a temperature below the boiling point and hold it there for a time, then raise it to the boiling point and hold it there and then raise it to a temperature a few degrees above the boiling point and hold it there (in this example, a drying stage consisting of 6 programmed steps). There is a trade-off between temperature and time but one must remember that the longer the steps, the longer the analysis. The second stage is ashing, which is a further heating to a much higher temperature, to vaporize the major constituents (pyrolysis, combustion and distillation) leaving the trace elements. There is a danger that at high enough temperatures a part of the analyzed element will vaporize also and this is a serious danger when analyzing volatile or relatively volatile elements. Sometimes ashing at a relatively low temperature may solve the problem, causing volatilization of the interfering component and not of the analyte. On the other hand, when using low temperature ashing, sometimes not all the matrix is destroyed and at the atomization state this may result in non specific background absorption which needs correction (see Section 5.14 and Section 5.15). For refractory elements, ashing at high temperatures may be carried out without analyte. loss. With volatile samples there may be a problem and sometimes (for instance if the matrix is an organic matter) the use of a modifier can help, as it may stabilize the analyte at higher temperatures and this facilitates ashing at higher temperatures without analyte loss (see Section 5.16). In the case of an organic matrix, sometimes the use of oxygen as an auxiliary gas in the ash stage will help in getting rid of the matrix more easily. In planning the analysis, a temperature which is high enough to get rid of as much undesired elements as possible, without loosing any of the analyte, is desired. This may cost in a lot of trial and enor, as the suggestions in the “cookbook” are guidelines only, and can’t fit all matrices. Here again, several heating steps may be used. Finally, in the third (atomization) state, the tube is very quickly (at rates of up to 2000” C/sec) heated to between 2000-3000” C to atomize the remaining material and produce a transient cloud of atoms (in the optical path) to be analyzed by atomic

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absorption and quantified using peak height or peak area measurements. This stage is very short and may last less than a second, up to a second or two. During atomization, the inert gas flow is stopped so that all the atom population will remain in the tube and not be swept out. A small amount of inert gas still remains in the tube and it takes time for oxygen to enter and oxidize the graphite. By repeatedly injecting and drying (or drying and ashing) aliquots of sample, an effective concentration of the sample is obtained prior to atomization, thus increasing the analytical signal. This is equivalent to a better detection limit. With many injections the sample may be smeared and not concentrated in one place in the tube, causing a wider and lower signal. In such cases (can be checked on the graphic screen) one must measure the peak area. In Computerized instruments, programming of the different steps becomes a lot easier by using the graphic screen. The background and atomic signals are graphically displayed together with the furnace temperature program (T vs. t). Peaks are obtained also in the drying and ashing stages as the volatiles and solid particles block a part of the radiation. The operator can see the changes during the various steps. The need for higher temperatures or longer heating times is immediately obvious. Complex samples or samples close to the detection limit may require an investment of a day or two to prepare the right analysis method, by trial or error, finding the right time durations, the right temperatures, the right rates, to decide whether a modifier is needed and what modifier, etc. During the atomization stage, absorbance rises very quickly and then falls, so that the peak exists very shortly (even less than a second). A high speed recorder and later an electronic peak measuring device (built into the spectrometer) were used. Nowadays, with computerized instruments, this is done easily. Peak area or peak height are measured and compared to those of standards. For some elements, better results are obtained by using a L'vov platform. This is a pyrolitic graphite platform (with a central depression to contain the sample) which is placed on the tube's inner bottom, under the injection opening, touching the tube by lobes at the ends of the platform. The sample is injected and deposited on the platform instead of the furnace wall. During atomization the platform temperature lags behind the furnace wall temperature and reaches the designated temperature only after the tube has reached a stable higher temperature. The sample is vaporized into a stable higher temperature environment. Recombination of the vapor components is avoided, thus reducing interferences. For instance, in the determination of lead in a salt matrix, the lead vapors may reach a lower temperature zone, recombine with the salt anion (even redeposited) and not take part in the atomization and excitation. With the platform, the lead atoms will find a stable high temperature and recombination will not take place. The use of the platform may also cause a great reduction of background absorption due to the higher temperatures

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experienced by the sample after vaporization. A better reproducibility is obtained. The L‘vov platform is useful for the determination of volatile elements. Refractive elements which require high atomization temperature are better determined without the platform. Many times samples that are near the detection limit and can hardly be analyzed by flame atomization and have a matrix which causes interferences, can still be analyzed by furnace atomization. One can dilute the sample, for example, one hundred times, get completely (or partially) rid of the matrix effect and still, a high signal may be obtained. Even if the signal is small, it is still worth diluting, as the matrix effect is lowered much more. There may be a light scattering problem, as small solid particles (smoke) may be in the tube, scatter the light beam, thus increasing absorbance. In short wavelengths this is a problem and background correction has to be applied. If one has no idea of the concentration and the related signal size, the way to find it is to inject a small sample and check, increase the sample and check and so on, till a peak is seen. If one starts with a big sample, the tube may become totally contaminated and many cleaning cycles or a change of tube may be needed. The sample has a matrix and this may cause a problem as to what to use as a blank solution. If, as usual, a solution containing only the solvents is used, this may not fit the matrix and it may be critical. One can prepare a standard additions calibration line of the sample, dilute it by two and prepare another standard additions line. If the lines are parallel, then the blank can be used, if not, it means that this blank cannot be used for this analysis. One can try adding to the blank solution the proper concentrations of the major constituents of the matrix and determine what causes the problem. There are situations of chemical interferences where the standard additions method has to be used. This has to be done in the linear part of the calibration curve otherwise one does not know how to do the extrapolation, as, a small change in the curved part will yield a big change (or infinity) in the extrapolation. One can check in the “cookbook” what is the absorbance for the pure analyte, let’s say 0.0044 absorbance for 5 ppm analyte, which means 0.44 absorbance for 500 ppm (provided it is still in the linear range of the calibration curve). Let’s say that the sample yielded 0.200 absorbance. If one uses additions of 100 and 200 ppm the readings will be in the rnage of 0.3 and 0.4 absorbance and there will be 3 points for the standard additions line.

5.13 Hydride generation Hydride generation is used for the analysis (especially traces) of arsenic, antimony, tin, selenium, bismuth (and lead). The method is used to separate and preconcentrate analytes from sample matrices by a reaction that turns them into

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their hydride vapors (ASH,, SbH,, SnH,, SeH,, BiH,, TeH, and PbH,). The analyte is separated from the complex sample and also is concentrated. Such a procedure improves the detection limits by a factor of 10 to 100 (for lead there seems to be a small change only) and reduces chemical and spectral interferences. Determinations of traces of these elements in air particulates, sewage, alloys, foods, biological and geological materials, etc. were reported. Automated hydride generator systems (continuous flow with a peristaltic pump or a pressurized reagent pumping system) which are attached as an accessory to the spectrometer, are manufactured by a few spectrometer manufacturers. The elements mentioned above were determined by atomic absorption spectrometry, the sample being introduced by direct solution nebulization. There were problems with elements such as arsenic and selenium, whose optimal absorption lines were far into the ultraviolet region. One of the major problems was a very high background absorption in air-acetylene flame. The cooler argon-hydrogen flame reduced the background absorption (which still remained high) but caused other problems (such as chemical reactions in the fame). Turning the elements into hydrides and then passing them into the atomizer resulted in a recommended sensitive method. At first the hydrides were generated by a metal-acid reaction where the metal (usually Zn) liberated hydrogen atoms which combined with the analyte to form the hydride. This was a very slow reaction which resulted in broad peaks, not all of the elements formed the hydrides and some elements had to be prereduced. The common method today is NaBH,-acid reduction. An acidified aqueous solution of the analyte is combined with an aqueous solution of sodium borohydride. Sodium borohydride in acid liberates hydrogen atoms which combine with the analyte to form the hydride. After a short period the volatile hydride is swept (by an inert gas) into the atomizer where the hydride is dissociated. Pentavalent arsenic has to be reduced (by Kl in the borohydride solution) to trivalent arsenic prior to the borohydride reduction (this can be done, preferably separately, before the reaction, as it takes time). Copper, nickel, cobalt, oxidizing agents, etc. interfere with hydride formation and have to be dealt with. A precondensation (liquid nitrogen) is used to concentrate the analyte before the atomization, thus sharpening the response. The borohydride reduction is quite fast (several seconds) and all the above-mentioned elements form the hydrides. The atomizer can be the flame (usually argon-hydrogen) a graphite furnace or a heated quartz device which is a quartz tube heated to several hundred degrees. The radiation passes from the source through the tube to the monochromator. The vapor generator can be used for mercury analysis, without forming the hydride. Mercury compounds in acid are reduced to the free element with stannous chloride and the mercury vapor is swept by an inert gas into the quartz tube (no need of heating for dissociation). There are instruments in which the mercury is collected

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on a gold gauze and then all the collected amount is heated and swept into the quartz cell, thus improving the detection limit to the ppt level (good for ultratrace analysis of mercury in drinking water). 5.14 Interferences

A few of the interferences were mentioned in previous sections and will briefly be mentioned here again, to see the whole picture. One type of interference is instability of the hollow cathode lamp output (see Section 5.11). This interference can be corrected by the use of double beam instruments. The light from the radiation source is split into two beams, one which passes through the fame and sample, the other serving as a reference to which the first beam is compared. Changes in lamp output will affect both beams. A part of the factors that may cause the analytical signal of the samples to be different from those of standard solutions with the same concentrations are physical (or transport) interferences such as viscosity, caused for instance, by temperature or composition difference between sample and standards (sample taken out of a refrigerator and standards at room temperature, acids or organic materials in the sample and not in the standards) thus changing the rate of solution uptake or the aerosol droplets size. Cu in aqueous solution will have a calibration curve with a slope very different from that of Cu in the organic solvent MIBK. In furnace analysis (micro quantities) small variations in physical properties can have a large influence on the result. For example, differences in viscosity can cause a difference in spreading on the inside surface of the graphite tube and this may change the absorbance, because residence time of the atoms in the optical path will be dependent on how far from the center atomization takes place (because of this spreading, organic solvents should be injected deep into the tube, very close to the bottom, and for some, preheating is preferred). These types of interferences should be corrected by physical means (T), or careful matching of sample and standards (see Section 5.10), or by using the standard additions method. In many cases, dilution of the sample solves the problem. The analyte concentration is lowered but the interference may be lowered much more, or even cancelled. In atomic absorption spectrometry it is assumed that the absorbance is caused by the analyzed element only and is proportional to its concentration. An interference occurs when proportionality between the absorbance and the concentration varies. In practice, the analytical signal is changed hy factors other than concentration, due to spectral interferences or chemical interferences. The measured absorbance may be higher than expected by the atomic absorbance and will consist of the atomic absorbance and absorbance by other sources - background absorption (mostly in the ultraviolet region and very little in the visible). A background correction will be needed to obtain the true absorbance due to the analyte only. Sometimes the total

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absorbance will he lower than expected due to chemical reactions of the analyte and other types of corrections will he needed. Spectral interferences which cause hackground absorption occur when, during atomization, there is another species whose absorption lines are very close to the analyte absorption so that resolution by the monochromator is impossible and the result is as if there is a higher analyte concentration. This type of interference depends very much on the interfering element/analyte concentrations ratio. Emission lines of hollow cathode lamps are very narrow so that overlapping because of absorption of close lines of other elements is rare (emission lines of the elements in the sample, caused by flame temperature, are not modulated and are not read). In such a case usually absorption at another wavelength of the analyte should be used. Sometimes working with a narrower slit may solve the problem, or sometimes standard solutions with a matching matrix may be used (if the amount of the interfering element is known), or, sometimes the standard editions method may be used, or, sometimes instrumental background correction may solve the problem. In furnace analysis there is a real prohlem in the analysis of Se in blood where iron is present and has about 15 peaks (structured background). The analysis of 0.5% Cu,for example, in steel causes a problem. The iron has to be taken out (extraction), or if the amount of iron is known, standard solutions with a matching matrix are prepared, or the standard additions method may be used, or sometimes an instrumental background correction may be applied (depending also on the method of correction). The spectral interference in flames is usually caused not by another element, but by a broad absorption band (non-specific absorbance) caused by other molecules, combustion products, which originate in the flame or in the matrix. This may occur in flames when the flame is not hot enough to decompose all molecular compounds in the sample or formed in the flame. These molecules will absorb light and if the spectrum overlaps the analytical wavelength, this will be measured, increasing the signal. The interfering hroad absorption bands may be caused by molecules which originate in the matrix (e.g. by metals combining with OH radicals from the flame) which may overlap absorption lines of the analyte. Sometimes their dissociation by a nitrous oxide-acetylene flame will eliminate the interfering band. Non-specific absorbance can also result by radiation scattering particles in the flame (by the formation of metal oxides of refractory elements), thus lowering the beam intensity. Working with a fuel-rich flame may eliminate the formation of the scattering oxide. In flame atomization, background absorption is usually important when working near the detection limit or at low wavelengths. Sometimes the problem can be avoided by a change of operational parameters like flame temperature or fuel-oxidant ratio (to decompose oxides), measuring at a higher wavelength where molecular absorption is not high, or by the use of an instrumental background corrector.

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The most common commercial instrumental methods are the continuous source correction (deuterium background correction) and correction based on the Zeeman effect (see Section 5.15). The flame itself (without a sample) produces absorption, especially at low wavelengths. To correct for absorbance of absorbing products originating in the flame, i.e. fuel-oxidant-solvent system (e.g. the formation of OH by water dissociation), measurements with a blank solution should be made and subtracted (in modern instruments by the computer) from each analyte reading. Spectral interferences by matrix products are a much greater problem in graphite furnace than in flame atomization. In the ashing stage, one sometimes has to use a low temperature, not high enough to decompose all the matrix, to avoid loss of a relatively volatile analyte. Leftover molecular species will be present during atomization, and an incomplete breakdown during the brief atomization in the graphite furnace will result in broad molecular absorption bands. In samples containing halogens, stable halides with wide absorption bands can be formed, incompletely decomposed organic material can form absorbing molecules or particles which can scatter the light (the same as carbon particles from a much used graphite tube). In these (and other) cases, the use of a background corrector may be necessary. For some cases of furnace analysis the use of a chemical modifier can solve the problem (see Section 5.16). Chemical interferences are caused by various chemical processes, occurring during atomization, that alter the absorption of the analyte. Chemical interferences are more common than spectral interferences. The effect usually can be minimized by use of proper operating conditions. One type of chemical interference is ionization of analyte atoms due to high flame temperatures (mostly alkali and alkaline earth elements). While the effect is small in air-acetylene flame, it is very high in acetylene-nitrous oxide flame (may reach ranges of 8096, depending on the element) thus removing them from the ground state atom population (the ions will ahsorh at other wavelengths). One (not much used) solution is to work with a colder flame, but this may cause other problems. The usual remedy is the addition (to samples and standards) of high concentrations of an ionization suppressor, an easily ionized element which will saturate the flame with electrons and inhibit ionization (see Section 5.10). Another type of chemical interference is caused by the fact that in the hot environment during atomization, many dissociations and associations take place, among them, reactions leading to the formation of molecular species, for instance, metal oxides and hydroxides. Some of these are stable and their molecular bands are wide, bright and prominent in the spectra of the sample’s components (e.g. alkaline earth oxides). Changing the analytical conditions, for instance, working with a fuel-rich flame, may cause the dissociation of the oxide (see Section 5.10). Na in the presence of HCl can form NaC1, decreasing sodium atom concentration. The absorption of V

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may be enhanced by the presence of A1 or Ti in a fuel-rich flame. The concentration of oxygen and OH is relatively small, the other metals use a part of it and a part of the vanadium oxide is decomposed, increasing its atomic absorbance. In fuel-lean flames, there is enough oxygen so that the addition of A1 and Ti will not make much of a difference. In another type of chemical interference, low results will be obtained because of reactions (in the flame) of the analyte with anions forming low volatility compounds. For instance, calcium in the presence of phosphate will form a compound which is difficult to break down. Calcium chloride is volatilized and dissociated easily. Two solutions with equal concentrations of calcium will absorb different amounts of radiation, depending on the anion. Calcium nitrate in solution together with aluminium nitrate may form calcium aluminium oxides in the flame, which may dissociate only at very high temperatures. A good knowledge of chemistry is sometimes needed to understand the nature of the problem. In some cases the use of higher temperatures may cause dissociation of the compound, thus solving the problem. In most cases the addition of a releasing agent is needed. Lanthanum will bind the phosphate, when added in high concentrations, thus releasing the calcium (see Section 5.10). Sometimes the addition of the interfering ion (in about the same amounts as in the sample) or the addition of the main matrix components (in about the same amounts as in the sample, e.g. silicon and/or aluminium in the case of some geological samples) to the calibration standards, or the use of the standard additions method may solve the problem. Sometimes extraction of the analyte element or the interfering element may be needed. Usually the ion is extracted into an organic solvent. If the interfering ion is extracted, a quantitative extraction is not always necessary, just to remove most of the interfering species. In the determination of trace elements in geological samples, it is best to avoid interferences by getting rid of the silicon (evaporating with HF) but unless a specific compound of an analyzed element with silicon can be formed, the separation does not have to be quantitative,just removing the bulk of the silicon. The same may be applied to the extraction of iron into isobutyl acetate as the chloride complex, in the determination of trace metals in iron ores. If the analyte has to be extracted, care must be taken to ensure complete removal (for instance, an exact pH, a specific organic extractor) and the method should be checked with solutions of known analyte concentrations.

5.15 Instrumental background corrections For a good background correction, one must measure the background in the same wavelength as that of the analyte and at the same time. All commercial instruments measure the analtye and the background one after the other (a very short time difference) and not simultaneously. The time difference between the two

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measurements is very important, especially in furnace analysis and should be as small as possible (ideally none) as the backbround can change very fast (a fraction of a second). All background corrections are carried out by making one measurement of total absorbance (analyte and background) and one measurement of background only and then subtracting it from the total (one can read the total, the background and the analyte absorptions). As hackground can change with time, one could measure the background before the absorbance peak and after the peak and use the average. This may work only if the change is linear and no one can guarantee that. Four background correction methods were used in commercial instruments, of which one is not used anymore, one is rare, and only two are widespread, the continuum (deuterium) correction and a correction based on the Zeeman effect. The oldest, cheapest (and not used anymore) is the two line correction method. At one wavelength the total absorption (analyte and background) is measured and at another wavelength, as close as possible to the analyte line (but not absorbed by the analyte) the background absorption is measured, assuming that the background is the same for both wavelengths. The second line may be another line from the same hollow cathode lamp (A1 HCL, first line 3093 A, second line 3070 A), from the filler gas of the same HCL (Ba - 5536 A, second line Ne - 5409 or from another HCL (NA - 5890 A, second line Mo HCL - 5883 A). There is a big time difference between the two measurements, depending on how fast one can change wavelengths (may reach a few seconds) and the hackground may change). This means that the method can he used for flame atomization and not for furnace analysis, as two atomization heatings will he needed and the backgrounds may differ. Two monochromators can be used simultaneously but this will complicate the instrument structure and will be expensive. Another method, the newest, (Smith-Hieftje), rare with commercial instruments, is based on self reversal [15]. An HCL needs a certain small amount of current to emit radiation at precise, narrow wavelengths. At higher currents the emission lines are broadened and at still higher ones there is a high concentration of the lamp’s element unexited atoms, which absorb a great part of the desired wavelengths (see Section 5.8). The emission peak will look like a saddle and the result is that the source emits light at wavelengths other than expected (i-e. those absorbed by the analyte) close to and on both sides of the desired wavelengths. This liability is used for hackground correction. The hollow cathode lamp is cycled through periods (each precisely timed) of low and high currents. At low currents the spectrum (background and analyte) is determined, while at high currents the background only is determined and then subtracted from the total. The method requires special HCLs (to prevent arcing at high currents) and an electronic system. The most widespread method is the deuterium background correction. This needs a deuterium lamp to emit continuous radiation (see Section 5.8). The output of the

A)

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deuterium lamp is quite low at wavelengths above about 350 nm and this type of correction is not used above the ultraviolet region. The principle of this method is that the slit width is kept sufficiently wide. Absorption line of the analyte is typically very narrow in relation to the total spectral bandpass of the monochromator so that the background absorption of the sample may be measured using a suitable continuum source, but the fraction of the continuous source that is absorbed by the atoms of the sample (the analyte absorption line) is negligible. The radiation from the continuous source and the HCL are passed alternately through the analyte vapor (in the flame or the graphite tube). Measurements of the total absorption are made with the analyte HCL while measurements with the deuterium lamp give the background absorption only (which is subtracted by the instrument’s computer). The HCL and the deuterium lamp must be aimed at exactly the same line along the flame, as the sample vapor may be very inhomogeneous. This alignment used to be a hard task but in modem instruments the lamp is installed at the factory and only small adjustments are needed before use. When the background is not a continuum but consists of a fine structure molecular spectra (structural background), errors may occur with a continuum source, as a narrow line may overlap the analyte resonance line which may then contribute a big part of the measurement. There are only a few such cases. If this happens, one should make sequential measurements (and subtraction) of the absorption of the sample and of a standard solution whose composition with respect to the interfering species is very carefully matched to that of the sample (but does not contain the malyte). Another type of background correction [16-1 81 is based on the Zeeman effect (the splitting of spectral lines in a strong magnetic field). The advantages are: correction for spectral interferences, correction over the complete wavelength range, correction for structural background, correction for high background absorbances (2 compared to 0.7 absorbance with deuterium correction) and the use of a single line source with no possibility of misalignment. Less sample preparation (to reduce the concentration of materials which can cause interferences) is needed. This type of correction is also used particularly with graphite furnaces. This correction requires a strong magnet, a polarization device, etc. and these cannot be bought as accessories and attached on an existing commercial instrument (unless one wants to build his own instrument). One has to buy an electrothermal atomic absorption spectrometer manufactured for this type of correction. This is a very expensive instrument which is justified only in special cases, like the determination of ultra trace metals in drinking water (water standards), or the determination of trace metals in a difficult, problem causing matrix like seawater (high background because of reflecting salt particles) or blood, urine, milk, etc. (where the decomposition of the organic matrix may lead to the formation of absorbing molecules and

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very high background). For most analyses, deuterium background correction (alone or combined with the use of a chemical modifier or with extraction) is enough. There is a normal Zeeman effect (where both terms involved in the transition are singlets, or the values for both Lande g values are identical - some transitions of Ca, Cd, Ba, Be, Mg, Sr and Zn) and an anomalous Zeeman effect (where the terms are not singlets and the g values are not equal, for instance, other Zn transitions) where more splitting occur, either a relatively simple (Ge, Pb, Sn) or a very complicated (Cr, Mo) splitting. We shall deal with the simplest case of a singlet transition, a case of a normal effect. In this case the spectral line is split into three, one central (n)at the original wavelength, with half the intensity of the original line and polarized parallel to the direction of the magnetic field, and on each side a line (+a and -u,not at the analyte wavelength) each with a quarter of the intensity of the original line and polarized perpendicular to the magnetic field. The background is insensitive to the magnetic field. There are several methods to use this splitting for background correction. In all commercial instruments the magnetic field is perpendicular to the optical path (transverse). In most instruments the magnet surrounds the graphite tube. In this method, correction is performed at the resonance line, not at a slightly different wavelength, an important advantage. In another type, the magnet surrounds the hollow cathode lamp and the correction is at a wavelength slightly different from that of the analyte. In the case of a corrector based on the Zeeman effect where the magnet surrounds the HCL, the light emitted from the HCL (and not from the analyte atoms) is split into n and a components. A rotating disc transmits alternately the n and u components of the radiation. With the n component, the total absorbance and with the a component, the hackground absorbances are measured. In this method, the u component does not read the background at the same place that the n component reads the analyte (hut at the edges of the analyte peak, depending on the amount of separation caused by the magnetic field). In one method the graphite atomizer is surrounded by a permanent magnet that splits the analyte line (again, the simple case of splitting into a 7r and two u components). Unpolarized light from the HCL is passed through a rotating polarizer which separates the beam into two plane polarized waves, perpendicular to one another. These waves pass into the graphite tube. During that part of the cycle in which the radiation is polarized parallel to the field (n),the total absorbance is measured. When the radiation is polarized at 90"to the field (a),absorbance of the background (caused by molecular absorption and by scattering) is measured and subtracted from the total. In another method where the magnet surrounds the graphite atomizer, a polarizer is inserted in the optical path to remove the 7r component. The measurements are

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very fast (100 measurements per second). The total absorbance is measured with the magnet off (no splitting). The background absorbance is measured with the magnet on (acomponents only and the K component removed). A number of background measurements before and after total absorbance measurement and a polynomial interpolation are used to find the background at the time the correction should take place. In Zeeman atomic absorption there may be a curve rollover, where the curve of absorption vs. concentration rises, reaches a peak and falls again. One absorbance reading will fit two concentrations. To avoid the problem, the concentration working range is limited, depending on field strength (the maximum absorbance is predetermined by the manufacturer and can be changed by the user after investigation of high concentration standards) and the computer will indicate when the maximum permissible absorbance is acceded. 5.16 Modifiers When an analtye presents a problem due to its high volatility, or because the analyte and matrix volatilize at similar temperatures, the use of chemical modification should be considered. In some cases chemical modification will allow ashing at higher (or atomization at lower) furnace temperatures, getting rid of the matrix without loosing a relatively volatile analyte (or atomizing the analyte without the matrix). A good knowledge of chemistry or a study of a chemical handbook will help to determine if the analyte forms a compound that has a high melting point, its decomposition temperature and intermediate compounds and to decide if the addition of the proper atom or anion may help solve the problem (i.e. shift the atomization temperature to prevent atomic and background absorption at the same time, reduce the volatility of the analyte to allow a higher ashing temperature to remove all materials that produce background absorption, form a single compound of the analyte to ensure a single large atomic peak and not several small peaks due to the decomposition of several compounds). The modifier can be premixed with the sample or added to the sample in the furnace (a must if it will form a precipitate). A few examples will illustrate the modifiers effect. In the analysis of metal traces in seawater, NaCl has a high background absorption. One can add ammonium nitrate which will react with sodium chloride and get rid of the chloride as the volatile ammonium chloride. Addition of a large excess of phosphoric acid to Pb, Cd and Zn will raise the temperature corresponding to the maximum of the atomic absorption peak (peak atomization temperature). The addition of EDTA to lead will result in a complex that will atomize at lower temperatures than a nitrate or chloride matrix. The peak atomization temperature for lead in 1% NaCl is 9 10 C, with 0.5% EDTA 560" C and with 5% phosphoric acid 1180"C. This may become very useful O

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when one must shift the atomic peak from a coincident non atomic absorption. Nickel is typically used to stabilize arsenic and selenium to higher temperatures (by forming metal selenides). A relatively new matrix modifier is palladium (introduced to the furnace as Pd solutions, 50-1000 mg/l, preferably as nitrate). If the solution contains oxidizers such as nitric acid, sodium sulfate and sulfuric acid, Pd becomes a poor matrix modifier. With the addition of a reducing agent such as hydroxylamine hydrochloride or, better, ascorbic acid, Pd can raise ash temperatures by 400-800" C for several semimetallic elements and by 200-500" C for transition metals [ 19-21]. The reducing agent guarantees that the Pd will be reduced to the metal form early in the temperature program, before the analyte is volatilized (before this method was used, the Pd was preinjected and heated to 1000°Cprior to the injection of the sample). The metal retains the analyte element (believed to result from the formation of an intermetallic species) until a higher gas phase temperature is achieved. Scanning electron microscopy showed that the size and distribution of the Pd on the graphite surface (hence the effectiveness as a modifier) depend on the reduction method used. The development of a successful analytical method requires the analyst to carefully optimize the palladium concentration and temperature program. Increasing the palladium concentration shifts the atomization signal to higher temperatures, up to a limit (depends on the matrix) where broad irregular signals, with reduced sensitivity, are obtained. For clean water samples, 50-200 mg/l may be sufficient while for more complex samples, 200-1000 mg/l may be necessary. Ascorbic acid precipitates Pd so rapidly that palladium and ascorbic acid must be separately introduced into the furnace, so that the use of 5% hydrogen in 95% argon (a standard mixture that can be bought from gas suppliers) was recommended to be a better reducing agent [21].

5.17 Automation The different atomic absorption spectrometers on the market differ from each other in the amount and sophistication of automation, though several properties are common nowadays (in one form or another) in all modern instruments. Automation can relate to both analytical procedure (and instrumental parameters) and data processing (and reporting). Several examples of automation were described in former sections. The following is a partial list of examples of automation that can be found in AA spectrometers (by no means in all of them). Some operations need operator's choice and input to the computer and some are controlled by default values which can be overridden by the operator. Since the spectrometer performs sequential elemental analysis, unique analytical conditions can be established for each element. This can be done in a develop program mode (for completely new procedures) or modify program mode (for programs

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already stored in the computer). Automation may include provision for permanent storage and retrieval of complete analytical procedures. The computer commands the instrument to retrieve an analytical program, set up the proper physical (optical and atomizer) parameters, perform calibrations (ordinary or standard additions method, with automatic blank subtraction) and draw calibration graphs, analyze unknowns, draw absorption peaks, calculate means and standard deviations and print the results. Statistical data processing programs can be added or carried out through an external computer. The computer may inform the operator of various operational errors. Automation of spectrometer physical parameters may include monochromator wavelength, spectral bandwidth, controlling lamp turret position to select the desired lamp, lamp current, selection of the atomic line (and peaking), selection optimization and control of source intensity, selection of the continuum background source and its intensity, correction of background absorption throughout the ultraviolet and visible spectrum, using a double beam technique or Zeeman effect. Automation of atomizer systems may include selection of oxidant, determining and controlling gas flow rates, ignition and signal optimization (for flame atomizers), heating cycle program (for the graphite furnace) and operation of an autosampler (together with options for automatic analysis with hand held samples or non-automatic analysis with hand held samples). In electrothermal analysis, in addition to the possibility of programming the dry, ash and atomization stages, the computer can draw the appropriate absorbances together with the temperature curve and the atomization peak, making it easy to check the effectiveness of the cycle. There may be many options the operator may choose from. One can choose between measuring absorbance, flame emission or sometimes lamp emission (to check the lamp spectrum or work with a specific wavelength not for absorption purposes). One can choose an ordinary calibration curve, a standard additions calibration (including measurement and calculation of sample concentrations). One can choose the ordinary scale or scale expansion, remembering that scale expansion does not make the measurement more precise, as the noise is also amplified, just easier to read a very small absorbance. This is important if one wants to dilute the sample to overcome interferences. In these cases, where the noise is relatively high, the use of a higher HCL current may be justified, as the noise is inversely proportional to the square root of the current. The measurement can be peak height, peak area, integration (reading during a specified duration, e.g. 3 s, at 0.1 s increments and giving the average), or precision optimized measurement time, where the operator asks for a specified precision (for instance 1% relative standard deviation) and the computer reads only long enough to reach the specified precision. One can use sequence control to determine the order in which the elements will be analyzed automatically (each with calibration curve) and determine the last sample after which one can program an automatic turning off of the flame. Printing of results

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can be done after each reading (during run) or after all measurements (sequential or multielement). Automatic labeling - naming of samples can be used. Automation may include the possibility of editing analytical results, e.g. to disregard specific data points from the data processing procedures, or the possibility of calculating final concentrations in the sample, considering sample weight or dilutions. Automation makes life easier, the analysis simpler to carry out (also reducing operator’s errors), helps in sustaining precision and (maybe the most important) allows repeating analysis of samples at different times under almost identical conditions. A very important feature is automatic control of many safety precautions.

References 1 Skoog, D.A., Principles of Instrumental Analysis, 3d ed, Saunders, Philadelphia, 1985. 2 Mann, C.K., Vickers, T.J. and Gulick, W.M., Instrumental Analysis, Harper & Row, New York, 1974. 3 Strobel, H.A., Chemical Instrumentation, Addison-Wesley, Reading, MA, 1973. 4 Ewing, G.W., Instrument Methods of Chemical Analysis, McGraw-Hill Kogakusha, Ltd, Tokyo, 1969. 5 Slavin, W., Atoic Absorption Spectroscopy, Interscience, New York, 1968. 6 Kirkbright,G.E and Sargent, M., Atomic Absorptionand Fluorescence Spectroscopy, Academic Press, London, 1974. 7 Alkemade, C.ThJ. and Herrmann, R., Fundamentals of Analytical Flame Spectroscopy (translated from German), Adam Hilger, Bristol, 1979. 8 Analytical Methods for Flame Spectroscopy, Varian Techtron Pty, Ltd. 9 Analytical Methods for Graphite Tube Atomizers, Varian Techtron Pty. Ltd. 10 Analytical Methods for Atomic Absorption Spectrophotomelry, Perkin Elmer Corp. 11 Flanagan, F.J., Geo. Cosmo. Acta, 37 (1973) 1189-1200. 12 Abbey, S., 1977 Edition of “Usable” Values, Geological Survey of Canada, Paper 77-34. 13 Govindaraju, K., Geostandards Newsletter, 13 (July, special issue) (1989). 14 Pelly, I., Clin. Chim. Acta, 213 (1992) 51. 15 Maugh 11, T.H., Science, 220 (1983) 183. 16 Maugh 11, T.H., Science, 198 (1977) 3940. 17 Brown, S.D., Anal. Chem., 49 (1977) 1269A-1281A. 18 Fernandez, F.J., Myers, S.A. and Slavin, W.,Anal. Chem., 52 (1980) 741-746. 19 Voth-Beach, L.M. and Sharder, DE., Spectroscopy, 1 (1986) 49. 20 Voth-Beach. L.M. and Shrader, D.E., J. Anal. Atomic. Spectrometry, 2 (1987) 45.

21 Beach, L.M., Spectroscopy, 2 (1987) 21.

CHAPTER 6

Plasma optical emission and mass spectrometry

J.A.C. BROEKAERT University of Dorimund. Departmen1 of Chemistry. 0-44221 Dortmund. Germany

Contents 6.1

Atomic spectrometry with plasma sources . . . . . . . . . . . . . . . . . . . . . . . . 192 Historical development . . . . . . . . . . . . . . . . . . . . . . . . . . . . 192 Optical emission spectrometry . . . . . . . . . . . . . . . . . . . . . . . . 194 Plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . 199 Plasma sources and sampling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 6.2.1 Arc and spark sources [7] . . . . . . . . . . . . . . . . . . . . . . . . . . . 202 Flames . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 6.2.2 DCplasmajets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 6.2.3 6.2.4 Inductivelycoupled plasmas . . . . . . . . . . . . . . . . . . . . . . . . . 205 6.2.5 Microwave discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 6.2.6 Sample introduction for plasma spectrometry . . . . . . . . . . . . . . . . .209 6.2.7 Discharges under reduced pressure . . . . . . . . . . . . . . . . . . . . . . 215 Plasma optical emission spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . 217 6.3.1 Atomic emission spectrometry . . . . . . . . . . . . . . . . . . . . . . . . 217 6.3.2 ICP-Atomicemission spectrometry . . . . . . . . . . . . . . . . . . . . . . 220 MIP-Atomic emission spectrometry . . . . . . . . . . . . . . . . . . . . . . 223 6.3.3 Glow discharges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 6.3.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 6.3.5 Plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 6.4.1 ICP mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Glow discharge mass spectrometry . . . . . . . . . . . . . . . . . . . . . . 239 6.4.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 Power of detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 241 6.5.1 6.5.2 Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 6.5.3 Economic aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 6.1.1 6.1.2 6.1.3

6.2

6.3

6.4

6.5

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6.1 Atomic spectrometry with plasma sources 6.1.1 Historical devdopnrertt

Atomic spectrometry is the oldest instrumental method of inorganic analysis and in its origins dates back to the mid 19th century. In that time Bunsen and Kirchhoff established the spectroscopic characterization of elements by optical observation of the occurrence of typical atomic lines of the elements when spraying solutions in a flame. Apart from the observation of the emission, also absorption of elementspecific lines by an atomic vapour of the respective element was mentioned. These experiments are the basis of modern atomic emission and absorption as standard methods in optical atomic spectrometry. However, the experience gathered with the introduction of the solutions also gave rise to the overwhelming number of techniques we now have for sample introduction. Optical atomic spectrometry led to the discovery of several elements and became really important for elemental analysis when new sources were brought into use. Indeed the geometry and the stability of flames were excellent for analytical work but the low temperature (below 2500"K) was a serious drawback, as it hampered the sensitivity for a high number of elements and also led to the occurrence of a number of serious interferences. The availability of electrically generated so-called plasma sources changed this situation. With these sources a high temperature is achieved (above 5000" K) in a chemically inert environment, such as with one of the noble gases, and also the ionization is considerable. The development of arcs and sparks enlarged the applicability of optical atomic spectrometry to the analysis of solids. Both the survey analysis of non-conducting as well as electrically-conducting solids by DC arc methods and the analysis of metallic samples by spark analysis started to become important and remained of interest up to now. New impulses nowadays are coming from the electrically-generated plasmas treated in this chapter. They comprise the inductively coupled and the microwave plasmas as well as the low-pressure discharges and the laser sources. The first are mainly of use for chemical analysis, whereas the latter are applicable to electrically-conducting and non-conducting samples and became of interest both for bulk and for microdistributional analysis of solids. The development in atomic spectrometry not only was initiated by the research done on radiation sources but also profited enormously from the development in spectral isolation. Indeed, in the early decades of atomic spectrometry, prisms were the only way of producting optical spectra. The availability of high-quality diffraction gratings opened the way to highest linear reciprocal dispersion and a linear resolution, as they were asked for by new problems in the analytical chemistry of lanthanides and actinides, for instance. As a result of this development, we now have powerful spectrometric systems allowing sequential as well as simultaneous determinations. Also the developments on the detector side were of great importance. Indeed, at the beginning of optical atomic spectrometry only spectroscopic

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techniques and later on, photographic detection were available for the detection of radiation. The photographic plate, which is difficult and laborious to process and, moreover, of low precision, still has the broadest coverage of information. The spectrometers with photomultipliers as radiation detectors, in this respect, are of low flexibility and as single-channel detectors, are certainly very limited, however, they allow very precise radiation density measurements and deliver signals which can be directly processed in interfacing with computers. Therefore, they now are standard devices in commercially-available sequential and simultaneous spectrometers. With the development of the low-intensity radiation detection systems stemming from astronomy and space research, the use of multichannel detection came up and is still going on. It resulted in charge injection and charged coupled devices, which offer sensitive techniques for multiwavelength detection and are already available in commercial plasma spectrometers. Apart from optical emission spectrometry, atomic absorption appeared on the horizon in the beginning of the fifties. Flame atomic absorption spectrometry is still a standard technique in the routine analytical laboratory. Especially, however, with the use of electrothermal atomization, as it has been introduced by the work of L'vov and of MaBmann at the end of the sixties, atomic absorption developed to the most sensitive method in optical atomic spectrometry. The same level of power of detection also was obtained with atomic fluorescence spectrometry. This was especially the case when the furnace was used as atom reservoir and when saturation of the excited level was obtained, as it became possible through the use of tuneable dye lasers. This method might gain interest for the analytical chemistry with the rapid expanding availability of diode lasers. Both atomic absorption and atomic fluorescence spectrometry, though very sensitive, remained from their principle monoelement methods and from this side there was continuous motivation to develop other spectrometric methods with multielementcapacity but with a power of detection being similar to that of electrothermal atomization atomic absorption or laser atomic fluorescence. In the early eighties, plasma mass spectrometry was introduced and in one decade it developed to a mature method. This was possible as a result of the availability of relatively inexpensive but high-quality quadrupole mass spectrometers, on one side, and thesnormous experience available with classical mass spectrometric techniques such as spark source mass spectrometry and thermionic mass spectrometry, on the other side. Plasma optical emission spectrometry and plasma mass spectrometry are two fully developed and powerful atomic spectrometric methods for multielement determinations. Through the varieties of sources which can be used as radiation or ion source, and by the wide range of devices available for the introduction of the sample today, suitable analytical procedures for very diverse problems can be elaborated on the basis of these methods.

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194

6.1.2 Optical emission spectrontetiy 6.1.2.1 Optical spectra Optical emission spectrometry is based on the excitation of atomic spectra having their wavelengths in the range of 150 to 800 nm. Its principal is based on the measurement of the electromagnetic radiation emitted during transitions between the outer electronic energy levels of the atom. The electronic energy levels are characterized by the different quantum numbers, and thus the energy difference between these levels can be expressed in terms of them. In the case of the main quantum numbers, the relation is

A E = (27rmZ2e4/hz)(l/n; - 1/12;)

(6.1)

with m the mass of the electron (9 x 10-28g), 2 the nucleus charge, e the elementary charge ( 1 . 6 ~ erg s) and n the main Clb), h Planck’s constant (6.67 x quantum numbers ( q = 1,2,3,. . . and n2 = 2,3,4,5,. . .). When excitation from the ground level to an excited level and a subsequent decay of the excited level takes place, electromagnetic radiation is emitted with a wavelength given by Planck’s Law:

E, = E2 = hc/X or hu

(6.2)

cis the velocity of light (3 x 101Ocms-l), X is the wavelength and v is the frequency (in s-I). A complication of the atomic transitions arises from the fact that in Bohr’s atomic model further quantum numbers were required to account for different interactions. The 1 quantum number reflects the quantization of the angular momentum component along the electron orbit, the magnetic quantum number rnl accounts for the quantization of the momentum component along the direction of an external magnetic field and the spin quantum number s for the electron spin. In the case of atoms with more than one electron, the L-S coupling further increases the number of atomic terms and finally, both for the neutral atoms as well as for the ions of different charge, a complex term scheme exists. The complexity of such a scheme is shown for the sodium atom (Fig. 6-1). For most atomic and ionic species the term schemes are available in atlases (see tables published by Grotrian [I] and by the NIST or the former NBS [2]). The atomic and ionic spectra of the elements thus consist of a high number of lines with wavelengths from the VUV to the infrared region. These line spectra are the finger prints of the respective chemical elements. The spectral lines not only have well-defined wavelengths but also have defined physical widths. They are the result of several broadening processes:

1. Nutirrul broudening: resulting from the Heisenberg uncertainty principle. This contribution relates to the lifetime of the excited level and is normally at the 0.01 pm level;

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

195

2. Doppler broadening: stemming from the velocity component of the radiating particles in the observation direction. This contribution is important especially in sources with high kinetic temperatures for atoms and ions; halfwidths of several pm may result of it;

3. Teniperaturebroadening: being due to the interaction of the radiating particles with other particles. It increases with pressure and accordingly in sources at atmospheric pressure, broadening up to several pm may occur, whereas in low-pressure discharges this effect is much smaller; 4. Other sources of line broadening such as isotope broadening, giving rise to more or less resolved hyperfine structures, resonance broadening resulting

Figure 6-1. Energy level diagram for sodium (reprinted with permission from Ref. 111

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196

from interaction of radiating and non-radiating species of the same element and Stark broadening resulting from the interaction of the electrons with the magnetic field in the plasma. The importance of these different components for the so-called physical line widths is well-reflected by high-resolution records of some rare earth lines obtained for the inductively coupled plasma (ICP) (Fig. 6-2) [3]. It should be emphasized

I11

I21

(3)

I&)

I51

16)

17)

Figure 6-2. Line profile of some rare earth atomic emission lines [3]. Photographic measurements. Theoretical resolving power Ro: 460 OOO, (1): Ho I1 345.6 nm, (2): Er I1 369.2 nm, ( 3 ) Yb I1 369.4 nm, (4): Y I1 371.0 nm, (5): Tm I1 376.1 nm, (6): Eu I1 381.9 nm, (7): La I1 398.8 nm

that these line widths differ from one radiation source to another, as a result of differences in temperature and pressure, as well as in the predominant excitation processes. The intensities of the spectral lines are related both to the properties of the discharges as well as to the number densities for the different atomic species, as written in the notations used in Ref. [4]:

Iqp= nqhvAq, Asp is the Einstein transition probabilityfor spontaneousemission, being the inverse of the lifetime of the excited state. In the case of an allowed transition, the latter is of the order of lO-'s, whereas when no allowed transitions start from the energy level, values may be up to some lo-' s. Provided so-called thermal equilibrium exists, the number density of excited atoms in a certain state is given by Boltzmann's law, namely: ng = ng,/Z(T)exp(-E,/kT) (6.4) g4 is the degeneration of the excited level ((2j + l), with j being the total quantum number defined as 1 f s), Z ( T )is the partition function for the species of a certain

level of ionization, Eq is the excitation energy of the excited level q (in eV) and

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

197

k is the Boltzmann constant (1.38 x 10-l6erg.s). When taking the possibility of ionization into account the ion and atom densities should be expressed as: ?a+ = Q

. 12 or 11 = (1 - a). 11

with Q the degree of ionization. The lifetimes of the excited levels are considerably dependent on the oscillator strengths of the lines themselves but also on the energy exchange processes possible. They may comprise:

1. Excitation by electron impact;

2. Excitation by collisions of the 1st kind with neutrals; 3. Excitation by collisions of the 2nd kind with excited species; 4. Excitation by absorption of radiation; 5. De-excitation by collisions with electrons;

6. De-excitation by collisions with neutrals;

7. De-excitation by spontaneous and stimulated emission of radiation. Further, the temperature of the plasma is included in Eqn. (6.4). Finger-print spectra of each of the elements will include the sequence of the characteristic line wavelengths. The line widths and relative intensities are also related to the atomic terms, but in addition, include influences of the radiation source. Therefore, the availability of atlases displaying the elemental spectra for particular atomic emission radiation sources is of prime importance for analytical atomic emission spectrometry. Such atlases have been made available for arc and spark sources and, however, still largely incomplete, are being made for modern sources such as glow discharges and plasma discharges. These complete finger-prints of the elements are the basis for an unambiguous qualitative analysis. Indeed, the presence of the most sensitive lines in the emission spectrum allows it to confirm the presence of an element in a given sample. 6.1.2.2 Optical emission spectral analysis As the emitted intensities are related to the element number densities Eqn. (6.3), the measurement of the intensities of the atomic spectral lines also allows quantitative determinations. Provided all dissociation steps required to generate the atomic vapour in the radiation source, the excitation processes and the ionization could be quantitatively described an absolute measurement of the atomic line intensities, would enable an estimate of the elemental number densities. However, the knowledge on these processes in the analytical radiation sources as well as of

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atomic constants and spectrometers are too poor for allowing a such absolute atomic emission spectral analysis. Accordingly, atomic emission spectrometry is a relative method of analysis, where a quantization is only possible after a calibration procedure with samples of known elemental composition and structure. The calibration procedure is an essential part of each optical atomic spectrometric procedure of analysis. It includes measurements of the intensities of the element-specific spectral lines for a number of well-characterizedcalibration samples with analyte concentrations covering the analytical range within which the concentrations of the unknown samples are likely to be, and with a matrix as similar as possible to the one of the unknown samples. The latter requirement depends on the interferences, which the matrix might induce in the case of a particular atomic spectrometric method and which are connected with the analytical accuracy of the method. In the case of wellcharacterized calibration samples with a matrix composition similar to the one of the samples to be analyzed are not available, combined analytical procedures including a separation of the analytes from the matrix and their matrix-free determination using synthetic standards, has to be used. Also, under well-defined conditions, mathematical procedures can be helpful for correcting some well-known differences in matrix composition between the calibration samples and the unknown samples. This may include the determination of the net analytical signals by stripping brutto signals from contributions of matrix line intensities determined by a separate procedure. Atomic emission spectrometry involves consequently a radiation source (for the development of this field, see Ref. [ 5 ] ) , into which a representative part of the sample is entered and in a first step atomized. In a second step, excitation of the atomic and ionic lines takes place. The first step mentioned requires the use of high energies, so as to atomize the sample material as complete as possible, the second step should be as selective as possible so as to obtain high power of detection and low matrix influences. Therefore, it can often be very useful to use so-called tandem sources, where both steps are performed in a separate and independently optimizable source. This concept which has been used throughout the development of atomic spectrometry has e.g. been described for modern sources in [6]. The radiation emitted by the atomic spectrometric source thus contains the element-characteristic lines for many of the elements introduced into the source but also the radiation emitted by the gas atmosphere. The radiation is conducted into the spectrometer with the aid of a suitable optical arrangement. One therefore can make use of lenses and minors and in addition, optical fibres. In spectrally resolving spectrometers, the radiation is brought as a parallel beam on a dispersive element, namely, a prism or a grating, and the spectrally resolved radiation including this at the analytical wavelength is measured with a multichannel multiwavelength covering detector (photoplates, diode arrays, etc.), or the radiation at the analytical wavelength only is isolated with the aid of an exit slit and measured with a photomultiplier. Accordingly, both flexible sequential determinations of several elements with a single-channel instrument

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

199

(monochromator) but also simultaneous multielement determinations with the aid of a multichannel spectrometer with exit slits prepositioned at a number of analytical wavelengths are possible. After suitable amplification of the photocuirents they are digitized and led to a data processing unit. Here calibration functions are calculated and for analysis the analyte concentrations are calculated from the measured intensities with the aid of the calibration functions eventually after matrix correction. Atomic emission spectrometry with spark excitation is now a standard method for production and product control in the metal industry [7].Glow discharge emission spectrometry for routine applications also is of great interest as it enables depthprofiling in solids, due to its layer-by-layer ablation. On the other hand, inductively coupled plasma emission spectrometry is now a routine tool besides atomic absorption spectrometry which is now present in almost any larger analytical laboratory. Apart from high-frequency discharges, microwave discharges and especially the microwave induced plasmas have also come under investigation as they proved to be low-cost sources which in some cases enable it to really achieve low absolute detection limits for the non-metals also. As from this assembly of methods (for a discussion see Ref. [S]) especially ICP-OES is of use for trace analysis in liquid samples or in solids, after dissolution it will be treated in detail in this chapter. Brief reference will also be made to the developments in MIP atomic spectrometry, when it comes to its coupling with special sampling techniques.

6.1.3 Plasma mass spectrometry The sources used for atomic emission spectrometry also enable the generation of ions for most elements. This is known from optical emission where, apart from the atom lines, sensitive ion lines may also be used in many cases. The ionization of an element in a high-temperature plasma, which is in local thermal equilibrium, is given by the so-called Saha-Eggert equation. With the symbols used in [4],the latter can be written as: logni . n,/?~,= 3/210gT - 5040/T Vi - log Zj/Z, - 15.56

(6.6)

where ni is the ion number density, n, is the electron number density, n, is the atom number density, T is the temperature in the plasma, V i is the ionization energy of the analyte (in eV), and Zi and 2, are the partition functions for the ions and the atoms, respectively. In elemental mass spectrometry, which in all its forms has been extensively treated in [9], the ions generated in a plasma are extracted from the source and lead into the high vacuum of the mass spectrometer via a suitable ion optics. After a separation or isolation of ions with different mass-to-charge ratio, signals for all ionic species can be obtained and detected.

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Ion sources used for elemental mass spectrometry were originally preferred to be operated at high vacuum so as to cope with the requirement of having a vacuum in the mass spectrometer below mbar. For such aims, high-vacuum sparks and arcs were very useful [lo]. This led to the development of spark source mass spectrometry as a technique for direct solids analysis or also for the analysis of dry solution residues. These techniques became commercially available with large double-focussing mass spectrometers. Originally one used photographic plate detection. Later on, however, photoelectric detection with electron multipliers and vidicon detection, including phosphors also became available. The method was very valuable for solids analysis especially in the case of high-purity metals and of semi-conductor materials, as the detection limits were in the sub-ng/g range. Apart from sparks, laser ablation has also been used and could be applied to the analysis of non-conductors even with a fair lateral resolution. With a Nd-YAG laser for evaporation and ionization, extremely sensitive microanalyses became possible in the LAMMA technique, where the absolute detection limits are down to the g level [ll]. The other very successful line in mass spectrometry for inorganic analysis made use of thermionic sources, in which the sample, which was generally a dry solution residue, was volatilized by heating from a refractory metal filament. This approach could be used for most non-refractory elements, as it has been shown in many applications, e.g. as described Heumann et al. [ 121, but also for refractory elements after volatile compound formation. The approach thus is extremely valuable for combination with extensive sample pretreatment involving matrix removal by chemical procedures,especially for micro- and ultratrace analysis. Whereas in spark mass spectrometry the analytical precision was rather limited (of 10 to 30%), thermionic mass spectrometry enabled high-precision work. This was especially true when applying isotope dilution techniques, which in the case of mass spectrometry, can be applied with stable isotopes, and has been impressively shown by high-accuracy work for u235/U238 ratio measurements in nuclear fuel, e.g. [ 131. Inorganic mass spectrometry went through a revival, as during the last ten years, high-quality quadrupole mass spectrometer instruments became available. Further reliable vacuum equipment now enables ion sampling from sources operating at higher working pressures. Thus it became feasible to use more sources known from atomic emission spectrometry as ion sources for mass spectrometry. The inductively coupled plasma was successfully introduced by Houk et al. [14] in 1981 and studies on the use of different types of glow discharges as ion sources started [15,16]. In these investigations much information known from diagnostic as well as from optimization studies performed for atomic emission spectrometry could be used and led to a rapid development of these techniques. To date, both inductively coupled mass spectrometry (ICP-MS) [ 171 and glow discharge mass spectrometry (GD-MS) [181 are important tools for extreme trace analysis, mostly for the analysis of liquid samples or for direct solid analysis, respectively. They will be treated in

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

20 1

detail in this work. Plasma mass spectrometry, due to its rather recent introduction, still will go through substantial further development. Here it can be expected that other types of mass spectrometers, in addition to other types of discharges, will gain interest in overcoming limitations of the method in its present form or to create possibilities for special application.

6.2 Plasma sources and sampling The term plasma in its physical meaning is a partially ionized gas. In analytical plasma spectrometry one mostly refers to those cases where the plasma is generated by the dissipation of electrical energy. The relevant sources (Fig. 6-3) can be divided into sources operated under reduced pressure and sources operated at atmospheric pressure [19]. In both cases, DC and also AC as well as high-frequency and microwave sources are used. As for their analytical use, the techniques applied for

-ARC

-+-

Q

-FLAME

,PLASMA

,LOW

SOURCES

PRESSURE DISCHARGES

-

ga,

GDL -GRAPHITE FURNACE

c ; L D ,LASER

PLUME

Figure 6-3. Sources for atomic spectrometry [8]

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J.A.C. BROEKAERT

introducing the samples are very important, a treatment of these techniques will be included in this chapter as well.

6.2.1 Arc arid spark sources [7] These sources have been used extensively at atmosphereic pressure and applied to the direct analysis of solids. Indeed, DC arcs have been used since the 1930s in combination with spectrography and since the 1950s with photoelectric registration. DC arc spectrometry thus became a very sensitive method for direct solids analysis. The method can be applied to metals, which then have to be brought in the form of chips and must be transferred into a cup. However, a rod can also be used directly as electrode or a powder can be analyzed after briqueting pellets. In the arc, currents of up to 20 A are applied and the burning voltage is below 60 V. The sample is volatilized by thermal evaporation and the analyte vapour is excited in the positive column of the arc located between the electrodes. The temperatures in this source are of the order of 5000"K, by which most of the elements vaporize. Reactions of the analyte with the electrode material (often graphite) or with the arc atmosphere may hamper the volatilization as then refractory carbides or oxides are formed. This can be avoided, however, by special precautions, such as halogenation or the use of noble gas atmospheres. Arc emission spectromery enables it to reach for many element detection limits down to the p/g level. Because of the volatilization of the elements by distillation, anions may cause matrix interferences and the elaboration of a reliable calibration may be very time-consuming. Therefore, DC arc spectrometry nowadays still has use for only semi-quantitative survey analysis. Spark sources are especially important for metal analysis. Here, the sample often is positioned in point-to-plane geometry and a tungsten or carbon rod is used as counter electrode. Sparks are generated in an R-L-C circuit, where the voltage over the capacitor is led to the electrodes. The R-L-C circuit is powered with AC or interrupted DC voltage. To date, medium voltage sparks (with voltage of 0.5-1 kV) often at high frequencies (1 kHz and more) are used under argon atmosphere. In order to keep the sparking times low, as it is required in on-line production control for steel, e.g. the preburn times are kept low by applying high-energy pre-sparking. Accordingly, spark analyses can be performed in as less as 30 s. For the simultaneous determination of a high number of elements including C, P, and S spark emission spectrometers are now available as large instruments, but also in portable form. They enable the direct determination also of trace constituents in electrically-conductive samples. For elements such as Mg, B, Cu a.o., the detection limits are at the pg/g level. By using intensity ratios of the analyte lines and a reference line (often a matrix line) as a function of concentration ratios, a high precision (better than 1%) can be obtained. For accurate analyses extensive sets of calibration samples must be used and mathematical procedures may be helpful so as to perform corrections for matrix interferences. Both in arc as well as in spark emission spectrometry

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PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

the spectral lines used are situated in the ultraviolet (200-400 nm), in the visible (400-550 nm) and even in the vacuum ultraviolet (below 200 nm) region.

6.2.2 Flames Since the work of Bunsen and Kirchhoff in the mid 19th century, flames were used as emission spectroscopic sources [20] and they are currently being used as atom reservoirs for atomic absorption work [21]. Their possibilities for atomic emission spectrometry, however, are limited by their low temperatures. Indeed, the temperatures for the adpropane (2000' K), the airhydrogen (2300' K) and the aidacetylene (2400 K) flame, for the nitrous oxide/propane (2900' K), the nitrous oxide/hydrogen (2900 K) and the nitrous oxide/acetylene (3200 K) flame are well known [22]. Except for the latter case, only for elements with low excitation potentials such as the alkali elements, a sufficient excitation can be expected. In flame emission spectrometry the sample, which as a rule is a liquid, is brought into the flame by pneumatic nebulization. Here, both concentric and cross-flow nebulizers are used, which are now standard devices also for plasma spectrometry. Their origin goes back to work in the 19th century, where Gouy [23] first described a concentric nebulizer. Advanced methods for sample introduction are e.g. the use of the Delves cup [24], as a technique for dry solution residue analysis in atomic absorption spectrometry. A further possibility is explored by Berndt et al. [25], who used a combustion of organic material in an oxidant gas stream by heating with focussed radiation from a tungsten lamp and lead the vapors are led into a flame atomic absorption system. Both techniques show how innovative sample introduction still leads to improvements of mature methods. Despite its age, flame emission spectrometry is still one of the most sensitive methods for the determination of the alkali elements and is still widely used, e.g. for the determination of sodium and potassium in body fluids. Due to the flow temperature, the detection limits for other elements are insufficient for trace analysis work (much above the &ml level). Moreover, the low temperature is also responsiblefor matrix effects due to the formation of thermally stable compounds. In this respect the Ca-phosphate interference is well known. The latter hampers the determination of calcium and is based on the following reactions: O

O

O

Ca3(P04), e +P205 e CaO e + C e Ca+CO Here the formation of the stable Ca,(PO,), hampers the formation of atomic Ca vapors and thus causes signal depressions. The spatial and temporal stability of flames as well as the diversity of techniques which could be used for sample introduction, in fact, were an impetus for the rapid development of the plasma spectrometric analysis of liquid samples later on. The only point to overcome in flame spectrometry was the establishment of sources operating in a chemically inert environment and at higher temperatures.

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J.A.C. BROEKAERT

6.2.3 DC yiusniu jets Electrical DC discharges can be operated in argon or helium at atmospheric pressures and with currents of up to some A at a burning voltage of about 50 V. In such a discharge we have a so-called normal characteristic, as at increasing current the material evaporation increases and the discharge thus can be sustained at decreasing voltage. Most of the voltage drop occurs in the immediate vicinity of the cathode (cathode region) and the voltage drop in the other zones (positive column and anode region) is low. Such argon plasma discharges have temperatures of the order of 5000"K and are in local thermal equilibrium, as the kinetic energy distribution of all plasma constituents (electrons, ions, neutrals) are equal and thus the electron temperatures, the gas temperatures, the ionic temperatures and the excitation temperatures, as well. Two types of such plasma discharges exist, which have substantially different properties. In the current-carrying type the analyte material is introduced in the current-carrying region of the discharge. Here the excitation and the ionization of analyte constituents may greatly change as a result of small changes in the concentration of easily ionized elements. This is due to the fact that as the electron and ion number densities change, the burning voltage and the energy delivered to the plasma greatly change. When the aerosol loading of the plasma changes, its temperature and thus its conductivity would also change. However, the burning voltage would then immediately increase by which the temperature is adjusted automatically back to its original value. Thus, current-carrying plasmas are strongly influenced by changes of the alkali element concentrations in the samples, but are rather insensitive to changes in sample supply to the plasma. Such plasma discharges were widely used for the analysis of liquids and described by Korolev and Vain'shtein [27] and by Owen [28]. A second type of DC plasma jet is the transferred plasmas type. Here the sample is introduced into the plasma plume which lies outside the current-carrying region. In this type of plasma, easily ionized elements will hardly influence the electrical conductivity of the plasma and thus only cause minor matrix effects. Their only influence may lie in an ambipolar diffusion which causes changes in the plasma volume. Indeed, due to the analyte dissociation there is an overpressure in the central part of the plasma. As now the electrons diffuse with a higher velocity to the outer zones than the heavier positive ions, there is an additional field directed from the inner to the outer part of the plasma, by which the latter tends to increase its volume. A cooling of the plasma resulting from an increased sample supply will have a drastic influence as changes in plasma temperature do not couple back to the burning voltage and thus cannot be compensated for. Such a plasma has been described by Kranz [29] and has been used successfully for the spectrochemical analysis of solutions. Both types of plasma sources have been studied extensively. They have their own approach to cope with the problem of entering a cold aiialyte vapour into a hot plasma, which is always hampered by the high viscosity of the latter. These

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

205

studies led to a commercially available three-electrode plasma [30], which is now a standard device for atomic emission spectrometry(Fig. 6-4). This source operates at

Figure 6-4. Electrode arrangement for three-electrodedirect current plasma jet

2 x 10 A current and a total argon consumption of below 8 I/min. Two gas-shielded carbon anodes with separated electrical supply and a common well-shielded cathode are used. The sample aerosol is introduced just below the point where both positive columns come together and is prevented from entering the cathodefall. Accordingly, the excitation in this source is very efficient. 6.2.4 Inductively coupled plasmas Due to the temperature restrictions imposed by flames and the possible contamination from electrode material encountered with arc sources and plasma jets, efforts were made to realize an electrodeless source with high temperature and high sampling efficiency for atomic spectrometric analysis. Such a source was realized with the indictively coupled high-frequency plasma (ICP), which was described for crystal growth studies by Reed 1311. This source was first used for atomic emission spectrometric analysis by Greenfield et al. [32] and by Wendt and Fassel[33], who both applied it for solution analysis and obtained detection limits in the ng/ml range. The ICP is operated in an assembly of three concentric quartz tubes (torch), in which argon flows (Fig. 6-5). The high-frequency energy is coupled into the argon with the aid of a working coil having 2-3 turns around the outer quartz tube. The argon is seeded with electrons with the aid of a Tesla discharge and at a frequency of 2-40 MHz and at suitable gas flows, a toroidal plasma is formed. In a tube with ca. 22 mm diameter, an outer gas flow of 6-10 ymin and a power of about 1-2 kW are required. A wet aerosol can be introduced with an argon carrier gas

J.A.C. BROEKAERT

206

data acquisition and processing

-

r.f. generator

ICP-torch

L

nebulization chamber

Figure 6-5. Instrumentation for ICP-AESusing convenlional pneumatic nebulization

flow into the ICP so that the analyte flow pierces the plasma along its central axis. This is the result of the skin effect, by which electrical currents only flow at the outer surface of the plasma while the internal part of the plasma is current-free. ICPs are operated normally with argon, however, at higher power, also nitrogen, oxygen or air can be used as outer gas flow, as shown by Greenfield et al. [32] and studied by Ohls et al. [34] and Broekaert et al. [35]. The high-frequency generator normally consists of an R-L-C circuit using triode tubes for power amplification, however, solid-state generators have also come into use. They became especially of interest since so-called low consumption torches, as studied by Rezaaijaan et al. [36], became available and the working frequency was increased from 27 to 40 MHz. The ICP is not completely in local thermal equilibrium. Indeed, the excitation temperatures determined from intensity ratios of lines originating from the same ionization level of an element are about 6000°K [37], whereas the rotational temperatures as determined, for example, from OH bands or N; bands are about 4000°K [38]. It was also found that the electron number density determined from the broadening of the H, line is 10'6cm-3 and the one from intensity ratios of an ion and an atom line only lOI4 ~ m - ~Moreover, . calculated ion to atom line intensity ratios were a factor of 100 smaller than the measured ones, which would demonstrate an over-ionization in the ICP. This discrepancy vanishes when ionization temperatures are taken for the calculation, as shown in a state-of-the-art paper by De Galan et al. [39]. Some of these facts could be explained by considering the relevant excitation and ionization mechanisms:

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

207

-

(a) Electron impact Ar + e

Ar*, Ar'and Aim (metastable levels of argon at 11 eV) M + e +M*,M*+

(b) Radiation trapping

Ar + hu

-

Ar'

(c) Radiative recombination M++e-M+hu (d) Charge transfer Ar'+M (e) Penning effect Af'+M

-

Ar+M+*

-

Ar+Mt*

The latter mechanism could explain the over-ionization of the analyte whereas the fact that metastable argon is easily ionized (ionization potential of argon is 15.9 eV, i.e. only 4 eV above the metastable levels) could explain the high electron number densities [40]. This would explain why ionization interferences caused by alkali elements are moderate [41] as the plasma is buffered with electrons. However, in more recent work where eIectron number densities and electron temperatures are determined with high spatial resolution and with the aid of Thomson scattering, it was shown that many of the effects described might also be due to the predominance of different mechanisms at different locations in the ICP [42,43]. Atomic emission spectrometry with the ICP now is a mature method in trace element analytical chemistry. Due to its analytical figures of merit and also because of the wide range of possibilities for sample introduction, which are treated in Section 6.2.6, it can be used for a wide variety of analytical problems and will be treated in detail. 6.23 Microwuve discharges

Microwave discharges are operated in the GHz region and have since long been applied in sealed tubes at low pressure. Here they served as line sources. They really became of interest when they also could be operated at atmospheric pressure as then sample introduction became more feasible. Microwaves are generated with the aid of a klystron (at a power below 100 W)or in a magnetron. Here a ultra high frequency field is applied perpendicular to a circular current which flows in a series of cavities so that at suitable frequencies, amplification by interference is obtained

208

J.A.C. BROEKAERT

and a high-power microwave can be transported with the aid of a coaxial cable and fed to a resonant cavity or to a wave guide. The part of microwave energy which is reflected to the magnetron can be minimized by realizing resonance conditions in the wave guide or in the cavity. Mavrodineanu and Boiteux [44] first realized a plasma at the top of an electrode through microwaves. An outer metal cylinder hereby served as grounded electrode. This capacitively coupled microwave plasma (CMP) (Fig. 6-3) was obtained with argon, helium, nitrogen or even air flows of as low as 2 l/min at a power between 200 and 600 W, it is not in local thermal equilibrium. This can be understood as the frequency is higher than in the case of the ICP. Therefore, the electrons follow fairly well such frequencies, however, not the much heavier ions. Major drawbacks of the CMP lie in its less favorable geometry with respect to analyte introduction and in the high matrix effects caused by alkali elements [45]. Electrodeless microwave induced plasmas can also be obtained. They became attractive for spectrochemical analysis, since Beenakker [46] described a TMol0 resonator in which, at a power below 100 W and with less than 1 Vmin of argon or helium, an MIP could be stabily operated at atmospheric pressure (Fig. 6-6). In

swfatron

Figure 6-6. T h f o l 0 resonator according to Beenakker [46] and surfatron [51] sources for MIPatomic spectrometry

this cavity the microwave power is introduced with the aid of a loop or an antenna. MIPS operated in a resonator according to Beenakker are very weak plasmas with respect to their atomization capacity. This hangs together with the low power

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

209

dissipated in these sources and the low gas temperatures. Indeed, when taking the rotational temperatures as an approximation, one should consider that values of only 2000'K are obtained [47]. When a capillary with a diameter of 1 mm is used as discharge tube, a filament-type plasma is obtained. However, with wider tubes also multi-filament and even toroidal plasmas can be obtained, as described by Kollotzek et al. [48]. The latter could only be obtained, however, with a wet aerosol. Also by the use of multitube torches with threaded inserts a toroidal plasma could be obtained [49]. With higher power, also MIPs operated in nitrogen have been described [50]. Apart from the cavity described by Beenakker, other devices were also useful for sustaining MIPS with good analytical properties. Hubert et al. [5l] described a so-called sui-fatron (Fig. 6-6). Here the microwave energy is coupled into the discharge with an antenna, which causes a surface wave to propagate through a slit between the quartz discharge tube and the front plate of the device along the plasma surface. This type of MIP easily forms multifilament and quasi-toroidally shaped discharges, which are less influenced in their stability by the entrance of an analyte in the plasma. They can be operated in argon and also with helium, the latter, however, with more difficulties than the cavity according to Beenakker. The spectroscopic properties of the surfatron have been investigated by Moussanda et al. [52], who performed rotational temperature measurements and found values on the order of 2000' K. Further types of microwave discharges were produced in the so-called strip-line cavity [53] and in the microwave plasma torch (MPT) described by Jin et al. [54]. In the latter, both argon and helium discharges can be obtained. The device can be made wholly of brass and the microwave power is coupled into the plasma by an antenna. The latter has a ring at its end, which glides over the central tube through which the analyte is introduced. Microwave discharges can also be obtained at high power and with a torch similar to the one used in ICP-spectrometry. For power coupling, Leis and Broekaert [55] described the use of a rectangular wave-guide with a hole, through which a threetube quartz torch was positioned. With such a device, MIPs could be sustained with argon and nitrogen at powers of up to 500 W. The field of microwave discharges itself is an actual field of research; many different plasma arrangements may be obtained through widely varying shape and volume, within a wide range of power and with different gases, Therefore, MIPs together with a suitable sample introduction can be tailored to solve special analytical needs rather than be a general purpose work horse for routine analysis. 6.2.6 Saniple introductionfor ylusniu syectsonietry Sample introduction techniques are a very important link in an analytical plasma spectrometric method, as they enable it to optimally use the potential of the method,

J.A.C. BROEKAERT

210

in view of the nature of the sample to be analyzed, i.e. its state of aggregate, its volume, its homogeneity, etc. Accordingly, the choice and optimization of the sample introduction techniques to be used must be made very carefully. For sample introduction in atomic spectrometry, a wide variety of techniques exists (Fig. 6-7) and they have been the subject of many studies (for its treatment see Ref. [56,57]). Here a brief review and discussion of the techniques used in combination with plasma discharges operating at atmospheric pressure will be elaborated. For the analysis of solutions, pneumatic nebulization and introduction of the aerosol into the plasma is well-known from flame emission and flame atomic absoiption spectrometry. For plasma spectrometry with an ICP, DCP or MIP, the pneumatic nebulizers that can be used include the concentric nebulizer, the cross-flow nebulizer, pneurnotic nebulization

7 t

-Thermal spmy

- Ultmsonic

nebulizotion

- Electrothermol

U

evaporation

Furnoce

L

Rloment

-Hydride

w-

- Electroerodon

Loser ablation . .

- Direct sample insertion Y

m-tt Figure 6-7. Sample introduction techniques for ICP atomic spectrometry

21 1

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

1.

SAMPLE

A

F

t

G

ARGON I

Y

h

II

SAMPLE

3.

I

MPLE

Figure 6-8. Pneumatic nebulizers for plasma atomic spectrometry [56]. (1) concentric glass nebulizer; (2) cross flownebulizer;(3) Babington-typenebulizer: (4) fritted-disc nebulizer

the Babington nebulizer and special types such as the fritted-disc nebulizer (Fig. 68) [56]. The concentric nebulizer, which in the case of ICP-spectrometry, often is made of glass or of quartz and is known as Meinhard nebulizer, is self-aspirating. All other types make use of forced sample feeding, as it is possible with a peristaltic pump. The liquid is nebulized by a high-velocity gas flow through a nozzle which splits off small volumes of liquid from a liquid flow as a result of viscosity drag forces. Pneumatic nebulization results in the formation of droplets of which the particle size can be described with the Nukuyama-Tanasawaequation [58]. where vG is the gas velocity, QG is the gas flow rate, QLis the liquid flow rate, 7 is the viscosity, Q is the surface tension of the liquid and c', c" and c"' are constants. With an increase of the gas pressure and the gas flow the Sauter diameter, 4,i.e. the diameter of the droplets of which the surface to volume ratio equals that of the entire aerosol, decreases. The aerosol then becomes finer and its loading also increases. Both improve the sampling capacity and the power of detection obtained but also make the sampling less dependent on the physical properties of the solutions, by which the nebulization effects decrease. Most of the nebulizers consume about 1 ml/min of analyte solution and are operated with a nebulizer gas flow of 1-2 l/min, by which residence times in the plasmas in the ms range can be obtained. At these gas flows the nebulizers already obtain their maximum efficiency, which is in the 1-3% range. The large droplets are removed by placing the nebulizer in a nebulization chamber, of which the form and

212

J.A.C. BROEKAERT

eventual impact surfaces must be optimized for the type of nebulizer used. Then the mean droplet size is in the pm-range. The cross-flow nebulizer is less sensitive to clogging, in the case of solutions with high salt contents. With the Babington nebulizer this also applies and even suspensions and slurries can be nebulized, as first described by Ebdon and Cave for atomic absorption spectrometry 1591. Nebulizers made of PTFE are now available for work with solutions containing hydrofluoric acid. For the case of organic solutions, special attention must be paid to the tubings used, but also to the clean-up times. For aqueous solutions and glass spray chambers they are in the 30 s range, for organics and especially in the case of PTFE, which is difficult to wetten, they may be considerably longer. From the early beginning of plasma spectrometry ultrasonic nebulization has also been used for aerosol generation in the case of solutions (for a report on the state-of-the-art, see Ref. 1601). Here the sample liquid is brought on the transmitter of an ultrasonic vibrator operated at up to 1 MHz, by which for the case of aqueous solutions, an aerosol with a mean particle size around 5 pm is formed. The latter occurs as a result of geisers built as droplets split off from the vibrating soIution surface. Due to the small droplet size, aerosol generation efficiencies of up to 30% are possible and thus increased sensitivities as compared to pneumatic nebulization can be expected. However, in the case of solutions containing high salt contents, deposits may be formed which shorten the lifetime of the transducer. Further, memory effects occur and accordingly, the clean-up times a e long. The overall aerosol generation constancy is inferior to the one of pneumatic nebulization. Very promising is the use of high-pressure nebulization, as described by Berndt [6 11. Here the liquid is supplied by a high-pressure pump as used in high-performance liquid chromatography and nebulized directly by the pressure drop at a nozzle and without use of a nebulizer gas. The aerosol particle size is low and the aerosol production efficiency high [62], also for viscous solutions. The system, because of its operation at high pressure, is ideally suited for use in speciation work, when ICP-spectrometry is used as an element-specificdetector in HPLC. Partially similar features can also be expected from thermal spray systems, as described for use in ICP spectrometry [63]. In all three systems and for the analysis of solutions, a desolvation of the serosol may be required, so as to bring the solvent loading of the aerosol to a reasonable level and this in turn may introduce interferences and memory effects. For the elements which form volatile hydrides such as arsenic selenium, etc., hydride generation, as known from atomic absorption spectrometry, may be used to bring the analyte in an isolated form and to deliver it with a high efficiency to the plasma. In the case of the ICP, flow-cell hydride generation (Fig. 69) [64] may be used to generate the hydrides, as the excess of hydrogen can be tolerated by the plasma. Here detection limits for As, Se, Sn, Ge, etc. are in the 0.5-2 ng/ml range [65]. In the case of weak plasmas such as the MIP, the excess of hydrogen must mostly be removed and an isolation of the hydrides by freezing

213

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY to

solution

If I waste Figure 6-9. Flow-cell hydride generator for ICP spectrometry [641

them out and a subsequent sweeping into the plasma may solve the problem, as it was already described by Fry et al. [66] for the case of the ICP. In the case of microsamples, electrothermal evaporation, as known from AAS, may be of use. The samples may be evaporated from a graphite rod [67], from a graphite furnace [68], or from a tungsten filament [69]. Here, the consumption of the whole sample volume and the high sample introduction efficiency generally allow it to obtain lower detection limits than in pneumatic and in ultrasonic nebulization (Table 6- 1). Moreover, as a dry aerosol is generated, the approach is very valuable also for the low power MIPS [70]. In the case of direct solids sampling, a number of approaches have been shown to be useful for plasma spectrometry. One aimed at the same power of detection and ease of calibration as in the case of solution analysis is described. For the direct analysis of powders, slurry nebulization can be used, however, limitations arise from the particle size which may cause difficulties in the nebulization process, but also in the subsequent thermochemicaldecomposition processes in a plasma, as discussed in the case of ceramic powders in Ref. [71]. Powders as well as metal chips can also be directly analysed by insertingthem directly into a high-temperature plasma (Fig. 6-10) [73]. As then, the sampling efficiency is very high, the power of detection can be expected to be the largest of all sampling techniques. This can be expected both for dry solution residues [72] and for impurities in powders [73]. In the case of electrically conductive samples, spark ablation may be very useful, and was proposed already in early work on ICP spectrometry [74]. Here, an aerosol having a low particle size (in the pm range) may be produced and for a wide variety of samples, selective effects are low [75]. In the case of electrically non-conductive samples especially, laser ablation is very useful. Both when using the laser plume

2 14

J.A.C. BROEKAERT

Table 6-1. Detection limits in ICP-AES ~

Element

~~

Pneumatic nebulization [88]

Electrothermal evaporation [68]

(Pglnll)

(Cldml)

(Pg/g>

Hydride generation [65] (ng)

at lOg/l Ag A1 As Au Ba B Cd Ce

co Cr cu cs Ge Fe Hg In La Li Mg Mn Mo Ni Pb Rb Se Sn Te

Th Ti TI U

V W

zn Zr

0.007 0.02 0.05 0.02 0.00 1 0.005 0.003 0.05 0.006 0.006 0.005 42 0.05 0.005 0.02 0.06 0.01 0.8 0.0002 0.001

0.008 0.01 0.04 37 0.07 0.025 0.04 0.06

0.004 0.004

0.25 0.005 0.03 0.002 0.001

0.7 2

5 2 0.1 0.5 0.3 5 0.6 0.6 0.5 4200 5 0.5 2 6 1 80 0.02 0.1 0.8 1 4 3700 7 2.5 4 6 0.4 0.4 25 0.5 3 0.2 0.1

0.3

0.4 0.2 0.5

0.1

0.005 0.02 4 6

4

0.3

(Pg/llN

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

215

dc motor adjustable

Figure 6-10. Direct sample insertion device for high-power ICP [731j. (1) switches, (2) lead screw with 2 guidances, (3) carriage gliding on 2 guidance stubs

itself as emission source [76] or as atom reservoir for fluorescence [77], a high power of detection was obtained (below the pg/g level) and the matrix interferences were low as well [78]. The technique, moreover, enables microdistributional analysis and has become a strong tool for multielement determinations, especially when the sample vapor produced is led into ICP mass spectrometry [79].

62.7 Discharges under reduced pressure In a discharge under reduced pressure, one can position the sample as one of the electrodes. The ions and energetic neutrals produced in the discharge and directed onto the sample can volatilize the analytes and bring them into a suitable region of the discharge where they can be excited and ionized. When a DC voltage is across two electrodes, ionization takes place and positive ions impact with high energy on

216

J.A.C. BROEKAERT

the cathode from which material is ablated and at the same time the electrons flow to the anode. In this way, a high positive space charge is built up in the neighbourhood of the cathode. The electrons are found back at high number density in the luminous negative glow further away from the cathode. The cathode thus will be volatilized partly by ion impact, during which material is ablated by momentum transfer from the impacting particles to atoms or ions on grating locations. When the impulse transferred exceeds the grating energy, the particle is released. The fraction of the energy of the impacting particles which is uansferred is a function of the masses of the impacting (772) and of the grating (Ad) particles and is given by

Em = E[4mM/(m+ M y ]

(6.8)

As a result of the bombardment of the cathode, however, its temperature increases and thermal volatilization also can occur. The ablated material then enters a lowpressure plasma, which is not in thermal equilibrium. Excitation occurs through electron impact where the high-energy electrons excite high-energy terms and low energy electrons are available for radiative recombination, by the Penning effect, and also by radiation trapping. As a result of the absence of local thermal equilibrium, the background continuum is also abnormally low. The kinetic temperatures in a low pressure plasma are also low by which line broadening in the optical spectra will be small. As analytically important low pressure discharges (Fig. 6-1 l), hollow cathode

Hollow cathode

Glow discharge lamp

FANES

Figure6-11. Glow discharge sources for atomic spectrometry

discharges are known since the work of Paschen in the 1930s. They are interesting as atomic spectrometric sources as the analyte species have a long residence time in the negative glow of the plasma, which is totally contained within the electrode. Therefore, they are used as atomic emission sources for microanalysis (see Ref. [80]). The most important glow discharge sources for atomic spectrometry are the discharges with flat and pin cathodes. The first one was introduced by Grimm in 1968 [81]. The Griinm lainp is an obstructed glow discharge where the cathode is cooled and thus volatilization is uniquely the result of cathodic sputtering. The

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

217

discharge is confined to the free surface of the cathode as the distance between the anode or the floating restrictor and the cathode is below the free path length of the electrons at the working pressure of 1-3 mbar. Due to the high burning voltage in argon (up to 1-2 kV), an intensive layer-by-layer ablation of the cathode occurs (some mdmin in the case of an 8 mm burning spot diameter) and the material is excited and eventually ionized mainly in the negative glow. Accordingly, the source is very useful both for atomic emission and for mass spectrometry, as will be discussed.

6.3 Plasma optical emission spectrometry 6.3.1 Atomic eniissiori spectrometry

In optical emission spectrometry, the radiation emitted by the source is led with the aid of an illumination system into the spectrometer, where the radiation is spectrally resolved. The monochromatic radiation at the analytical wavelength is then measured with the aid of a suitable detector and the signal obtained fed into a computer, which has often also the role of controlling the operation of the plasma and the spectrometer. As spectrometers both sequential and simultaneous dispersive spectrometers are used. As optical emission spectra are very linerich and the line widths are of the order of 1-3 pm, the spectrometers used should have both a high resolving power (of the order of A/ A A > 40000), determined by their dispersive element, as well as a large reciprocal linear dispersion (expressed in nm/mm in the focal plane), determined by the focal length of the spectrometer as well. For sequential spectrometers, computer-controlled monochromators are used. They mostly have an Ebert or a Czerny-Turner set-up (Fig. 6-12) [ 5 ] , of which the latter has less chromatic aberrations. As dispersive element, a grating is used. In the case of a holographic grating, the response over the whole range is rather uniform, whereas in the case of a blazed grating, a maximum response at the blaze angle and the respective wavelengths in the different orders is obtained. The latter type of grating can be used for working in higher orders, which enables it to obtain a high resolving power even with gratings having a low grating constant, i.e. a low number of lines per mm. The slit widths used in the spectrometers may be down to the 10 pm level, which improves the practical resolution but reduces the optical conductance and finally may lead to detector noise limitations. For wavelength selection, a computer-controlled stepping of the grating across the profile and the immediate environment of the analytical lines is very useful. In this way both accurate peaking of the line as well as a measurement of the spectral background intensities are feasible.

218

J.A.C. BROEKAERT

s.3

Figure 6-12. Mountings for optical emission spectrometers. (3) Ebert monochromator; (b) CzemyTurner monochromater; (c) Paschen-Runge spectrometer. sc: entrance slit, sa: exit slit, g,: plane grating, &: concave grating, m: mirror.

In simultaneous spectrometers, a Paschen-Runge set-up (Fig. 6-12) with a concave grating is often used. Here, often more than 30 exit slits with detectors can be provided in the focal plane. The practical resolution because of thermal stability reasons mostly is low as wider slits are used. A measurement of the spectral background can be achieved by a computer-controlled displacement of the entrance slit by which a scan over all analyticallines simultaneouslyis possible. Apart from these conventionaltypes of spectrometers, also Eschelle systems with crossed and parallel dispersion are used. Here an Eschelle grating is used as first dispersive element. It normally has a low grating constant but is used at a high order (30-80) where its reflection qualities still are good. In addition, a second dispersive element (prism or rating) is used as order sorter. In a crossed arrangement, the spectrum then appears in different rows above each other, each containing the wavelength-dispersed spectra for a given order with a height equalling the entrance slit height. Accordingly, a very high resolution can be obtained with a compact spectral apparatus [82]. Radiation detector photomultiplier tubes are used in most cases. They have a photocathode at which a photoelectron flow is produced with a given quantum efficiency and it is amplified with the aid of up to 15 dynodes, across each of which up to 100 V is provided. Accordingly, an amplification of up to lo5 is possible. The lower working limit is determined by the dark current, which is in

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

219

the nA range. Different types of photocathodes are available, enabling it to work in different spectral ranges, among which is the S 20 type for the UV-VIS region and the bialkali type for the VIS-IR. One can also use filters so as to remove radiation from unwanted orders, or so as to isolate low wavelength radiation (with solar blind tubes). Apart from the single wavelength detector consisting of an exit slit and a photomultiplier, a multichannel detection for different wavelengths simultaneously, now is possible with the aid of advanced photodiode arrays and more recently also with charge coupled and charge injection devices [83]. They provide for the electronic registration of small spectral areas, however, with limited signal to noise ratios. In the case of image intensification with the aid of microchannel plates, this disadvantage no longer occurs. In optical emission spectrometry, the net spectral line intensities are used as analytical signals and are proportional to the analyte concentrations. The detection limits of the methods are determined by the spectral background and its fluctuations and can be calculated as:

Here the line-to-background intensity ratio determines the background equivalent concentration and a(Iu) is the absolute standard deviation of the spectral background. When measurable blank contributions are present, the noise level is determined by their fluctuations and the detection limit then will be given as:

CI-= F[IB + 3&a(lB)]

(6.10)

when F is the functional relation for the calibration. Thus, a reliable calibration is only possible with the true net signals. In order to measure these signals an accurate evaluation of contributions from the spectral background, which includes stray radiation in the spectrometer, continuum radiation, band spectra and wings of matrix lines, must be performed. This must be done very carefully, especially in trace analysis. Indeed, when accepting for (~(1,) the value of 1%at a concentration of only 10 times the detection limit, we have an error in line intensity and in concentration which is twice as high. In plasma emission spectrometry spectral interferences therefore will be the main source of systematic errors. They cannot be avoided by standard addition. In plasma emission spectrometry further systematic errors may arise from ionization interferences or from influences of concomitants on the sampling rate, which then can be corrected for by standard addition or by the use of an internal standard. The latter approach is very well known from classical arc and spark work and is very worthwhile for the new plasma sources as well [84,85]. The selection of the reference element as well as the selection of the suitable reference line are an important point of the working procedure. In direct solids analysis one often can select a matrix constituent but in solution analysis, any element, which is not present in the sample to be analyzed, can be used.

J.A.C. BROEKAERT

220

6.3.2 ICP-Atomic eniissiori syectronzetiy 186,871 6.3.2.1 Figures of merit ICP-atomic emission spectrometry using pneumatic nebulization is a very powerful method both for the sequential and for the simultaneous determination of trace elements in solutions. Especially for the elements which form refractory oxides (such as Al, Nb, Zr, rare earths) the detection limits are in the ng/ml range (Table 6-1) [88,89]. This especially applies when sensitive ion lines are available and when the ionization potentials are not too high, as is the case for Be, Mg, the rare earth elements, etc. However, also for elements such as B, P and S, the detection limits are in the 0.1 pdml range. The optimization of ICP-AES with respect to the power of detection necessitates a careful optimization of the nebulizer gas flow, of the power and of the observation height, for each element. The parameters are interOBSERVATION HEIGHT

p:E:i-'

TEMPERATURE atoms, ions are excilet

molecules dissociote

material evapomtes c,droplets

I

dry

0.

.*

*:

X

-

.'.' .. .. 7 7 .i

X

droplet size

-

biscosity,.I

GAS FLOW

Figure 6-13. Importance of optimizationparameters in ICP spectrometry

related (Fig. 6-13). Indeed, the nebulizer gas flow determines both the nebulization efficiency and the droplet size but also the temperature and the residence time for the analyte in the plasma. All three are important for the free atom concentration in the ICP, which determines the line intensities. Further, the observation height is important as the kinetics of the evaporation and dissociation steps depend on the time travelled through a plasma region of a given temperature, which again depends

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

221

on the observation height. Finally, the power of the ICP again influences the plasma volume and therefore the temperature distribution. Especially for the optimization in the case of multielement determinations, simplex procedures are very useful [90]. The analytical precision obtainable with ICP-AES is high. Already at integration times in the s-range and concentration levels of 10-20 times the detection limits, relative standard deviations in the 1% range are achievable. The most important noise sources are nebulizer fluctuations, resulting mainly in white noise, but also gas supply fluctuations causing plasma pulsations [9 1J and frequency dependent noise [92]. Also at high concentrations standard deviations of 0.1% can be obtained when using a reference line. This could be shown for the analysis of binary alloys with ICP-AES at a concentration range of 500 to 900 mg/g [93]. The linear dynamic range in ICP-AES extends from the detection limits to over 5 orders of magnitude from the side of the radiation source. This is a result of the absence of analyte material in the cooler plasma zones and therefore of rather low self-absorption. Matrix effects in ICP-AES may arise from: 1. Nebulization effects, which can be minimized by acid matching in samples and standards;

2. Effects in the plasma resulting from easily ionized elements, such as ionization shifts, geometry changes by ambipolar diffusion, etc. These effects are low as, for example, in concentrations of up to 500 pg/ml Ca, Na or Mg were found to cause no matrix effects [3]. At higher concentrations these matrix effects occur but can be corrected for by matrix matching or by standard addition;

3. Spectral background interferences, which are the most important source of systematic errors. Here line selection with respect to freedom from interferences by matrix lines should be aimed at, and for this task the spectral atlas published by Winge et al. [94] and the tables of Boumans [95] including critical concentration ratios are very helpful. Further, instrumental background correction is of prime importance. 6.3.2.2 Applications and special techniques ICP-AES is of use for a wide range of applications. A number of applications are in the field of geological samples. Here the sample dissolution used is of prime importance both with respect to possible nebulization effects and the longterm stability, as it is known already since the early applications of ICP-emission spectrometry [96], as well as with respect to ionization interferences. It has been shown in the case of determination of the rare earths in bastnaesite and monazite that minor components of the samples almost have no influence from the side of

222

J.A.C. BROEKAERT

the excitation [3]. The concentrations of the fluxes used, such as Na,B,O,, cause matrix effects because of different reasons [97] and have to be matched in the standard solutions and i n the samples. In the case of particular minerals, further work has to be done on the optimization of wet chemical dissolution procedures, as shown, in the case of phosphate minerals [98]. This particularly is true as microwave heating may have some impetus especially for routine work. Extensive work on the analysis of minerals subsequent to sample dissolution has been published regularly (see for example, Ref. [99]. The analysis of enviroonmentully-reele~~u~it samples is a main field of application for ICP-AES. Based on the early work of Garbarino and Taylor for water analysis [ 1001, a method was proposed for waste water analysis by EPA [loll and later on by DIN [102]. In the latter, the sample decomposition, the analytical range for 22 elements and possible spectral interferences are treated. For the analysis of natural waters, the use of various preconcentration techniques is of prime importance. They may be based on liquid-liquid extraction of complexes (see Ref. [ 103,1041 for the case of dithiocarbamates), but also on coprecipitation, e.g. with In(OH), [ 1051 or sorption on acetylated cellulose [ 1061. Special impulses came from the use of flow-injection, by which concoinitants can be removed and analytes preconcentrated, as discussed by Ruzicka and Hansen [107]. The use of microcolumns allows it to shorten the preenrichment-release cycle considerably, as shown by Hartenstein et al. [ 1081. For environmental analysis, speciation now becomes very important [ 1091. Here, a coupling of ICP-AES with column chromatography is especially powerful. This has already been shown by early works on the speciation of Cr(II1) and Cr(V1) compounds with the aid of A1,0, columns which were eluted with acid or basic solutions alternatively [ 1101. For water analysis, attempts were made to make ultrasonic nebulization [ 1113 and thermal spray [1121procedures more reliable, e.g. by concomitant removal during sample preparation. However, this did not lead to a general use of these techniques. They are in principle capable of bringing an improvement in power of detection as they increase the sampling efficiency. From this point of view, hydride generation in the case of the elements which form volatile hydrides is also interesting. It has been introduced as flow-cell technique by Thompson et al. [ 1131 already and proved to be useful for the determination of As in waste water down to the &l-level. However, it should be remembered that the As compounds, which often are organic substances, must be mineralized. This can be done by treatment with H,SO, and H,O, but must be performed carefully so as to avoid analyte losses. Moreover, hydride techniques are especially prone to interferences of transition metals, which must be masked, e.g. by complexing with tartaric acid, must be eliminated by coprecipitation with La(OH), or of which the interferences can be avoided in some cases by calibration by standard addition [64]. In soil analysis, the development of suitable complete dissolution procedures, which can be used in routine, is particularly difficult.

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

223

A further series of applications of ICP-OES is in the field of biologicul suniyles. It has already been shown very early that Ca, Fe, Cu, Mg, Na and K can be simultaneously determined in human serum. This even applies for the case of microamounts of serum, for which after a 1 5 dilution, a direct analysis of 50 pl samples is possible [ 1141, similar to earlier flame atomic absorption work [115]. In the case of microamounts of biological samples, also evaporation from a carbon rod [67], from a graphite furnace [68] and even direct insertion is very useful. The latter from its principle certainly is the most sensitive ICP-AES technique and allows it to obtain detection limits in the sub-ng level [72]. All approaches, however, suffer from anion and matrix interferences as a result of the thermal evaporation. In the case of plant tissues, ICP-AES has also been used extensively (for an early review see Ref. [ 1161. However, it suffers from the lack of power of detection for toxic elements such' as Cd, Pb, TI, etc. For the analysis of industrial materials ICPAES nowadays finds wide application in all fields, ranging from ore analysis to the analysis of steel and slags as well as to the analysis of high-purity chemicals. In the actual field of high-purity powders for advanced ceramics, both analysis subsequent to a suitable sample dissolution as well as direct analyses of powders by slurry nebulization are possible. The limiting factors are the sample nature and the powder granulometry which influence the nebulization and vaporization in the ICP [71]. It has been shown that particles larger than 15 pm cannot be kept in the aerosol at the gas flows used. However, for very refractory substances such as ZrO,, volatilization already becomes incomplete above the 8 pm level. The approach could, however, be shown to be very valuable within these limits for the analysis of A1,0, [117], S i c [118], and Zro, powders [119,120]. Apart from slurry nebulization also direct insertion of powders [73] has been shown to be very useful for the determination of volatile elements in A1,0, powders. This technique can also be applied to the analysis of used oils, which even may contain particulate material [ 1211. In the case of metal analysis, ICP-OES finds wide application, as it has been demonstrated by a collaborative study [ 1221. Especially for this field, direct methods such as spark ablation El231 are very useful. As shown for the case of A1 alloys [75], as well as in the case of steels [123], a wide variety of alloys can be analysed with a high precision with the same calibration curve. The same advantage also applies in the case of laser ablation, as shown by Uwamino et al. [ 1241 in the case of ICP-OES and with more advanced Nd-YAG lasers in general, recently [125]. The laser ablation technique allows it to perform also laterally resolved measurements and in addition, also is suitable for non-conducting samples, such as advanced ceramics.

6.3.3 MIP-Atomic emissiotl spectrometiy MIP-AES in the case of a TM 010 cavity according to Beenakker is ideally suited for combination with graphite furnace evaporation and allows the multielement analysis of microsamples. The detection limits in the case of this approach

224

J.A.C. BROEKAERT

are at the sub-ng level [126]. Analyses of real samples require, however, a very careful calibration, as shown for serum samples. It also has been found that with improved plasma forms such as the toroidal MIP, not only wet aerosols can be taken up with limited power of detection [127], but also the sampling from a graphite furnace can be much improved [128]. The possibility of entering wet aerosols in the case of MIP-OES made them in principle also useful for element-specific detection in liquid chromatography [129]. The helium MIP is very suitable for the excitation of non-metals and thus is of use for element-specific detection in gas chromatography. For this aim, instrumentation including a gas chromatograph, a MIP and a diode away emission spectrometer is commercially available. This is not only suitable for pesticide residue analysis, but also for the speciation of metals such as Pb and Sn [131]. Considerable improvements of the stability of MIPS are obtained with the surfatron. This has been shown by comparative studies of a surfatron and a TM 010 cavity both in combination with evaporation from a tungsten filament [132]. Indeed, for Cu and Cd,both the power of detection, the linear dynamic range and the interferences of Na, in the case of a surfatron, were more favorable. Also the microwave plasma torch described by Jin et al. [54] constitutes a progress in terms of discharge robustness. This approach enables it to operate a stable argon and helium plasma. In the case of an argon discharge, samples can be introduced as desolvated aerosols, enabling detection limits in the sub-pg/ml range, but also as volatile hydrides, vapours generated by electrothermal atomization, etc. Also the CMP (capacitively coupled plasma) remains an interesting source for atomic emission spectrometry. Despite it is highly prone to interferences from concomitants such as Na, which is well-known [133], it remains of interest [134], also because high-power nitrogen discharges could be operated. This is also possible with electrodeless discharges, as published by Leis and Broekaert [55]. In all cases, however, the detection limits were found to be considerably higher than in the case of ICP-AES. The use of helium, which is possible in ICP-AES, as shown by Montaser et al. [ 1351,again might be easily possible with the aid of the high-power microwave discharges. Therefore they also remain an interesting area of research. 63.4 Glow discharges Glow discharges have been used as spectroscopic plasma sources since the early work of Paschen [136]. They especially have the advantage that through the use of noble gases with high ionization energy and by the absence of local thermal equilibrium, very highly energetic terms of also 0, N, S, and the halogens can be excited. In the case of the hollow cathode, the analyte is held for a long time in the excitation region. As a result of these facts, Mandelstam and Nedler 11371 and Falk [138] showed that this plasma source is the most sensitive atomic emission source. This has been proven by microanalyses performed by Russian groups [ 1391.

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

225

In hot hollow cathodes the analyte may even selectively vaporize from the matrix, which in a number of cases, is a supplementary advantage. A hollow cathode lamp for routine use has been constructed by Thornton et al. [ 1401 and shown to be useful for the direct determination of volatile elements with high excitation energies such as As, Se, Bi, etc. in high-temperature alloys [ 1411. However, the source is difficult to handle and moreover, a general technique for the sampling of solids is not available. The main advantage of the hollow cathode sources is that the sample composition largely influences the material volatilization, especially in the case of the hot hollow cathode. Therefore, a separation of the evaporation by external heating of a graphite furnace under vacuum was combined with excitation in the negative glow of a DC discharge. This approach, known as furnace atomic non-thermal emission spectrometry (FANES) was described by Falk et al. 11421 and could be shown to enable detection limits in the pg-range. As in every low-pressure discharge, easily ionized elements disturb the favorable excitation conditions. Therefore, the approach is very valuable as a detection principle in combined analytical procedures, where the elements to be determined are separated from the matrix and are present as a trace element concentrate in a microvolume. The method, however, is more difficult to handle than the MIP,as it is operated at low pressure and its multielement capacity may suffer from selective volatilization. A further interesting type of discharge under reduced pressure is the Grimm type glow discharge lamp, which was introduced as source for atomic emission spectrometry in 1968 [81].Due to its time-stable operation, this glow discharge lamp can be used both for simultaneous as well as for sequential emission spectrometric determinations in electrically-conductive samples. The latter should be available in the form of flat discs and are used as cathode [143,144]. At a power dissipation of up to 100 W (currents of up to 100 mA at 1 kV), ablation rates of some mg/min can be obtained. Detection limits in the case of metals can lay in the pg/g range or even lower [145]. Due to the optically-thin character of the plasma, calibration curves were found to be linear over a wide range of concentration, excepted in the case of resonance lines where self-reversal seems to be considerable. Glow discharges are very stable sources and display no flickering noise [146]. Therefore Fourier Transform spectrometry can be used in glow discharge atomic emission spectrometry. This easily enables it to perform line profile measurements. The line widths determined accordingly were found to be in the pm range. In the case of resonant lines of major constituents, severe self-reversalmay take place, as shown for the case of Cu I 324.7 nm line. The material ablation in the case of Grimm type glow discharges, uniquely takes place as a result of cathodic sputtering. Therefore matrix effects related with selective thermal evaporation do not occur. The Grimm type glow discharge source is mainly used for the determination of minor components in metals rather than for trace analysis. However, as the samples are ablated layer-bylayer, glow dischages are now also important sources for depth-profiling of metal

226

J.A.C. BROEKAERT

samples [ 1471. Glow discharges may also now become important for bulk analysis and for depth-profiling in electrically non-conducting samples, as r.f. discharges with sufficient ablating capacity now become available [ 1481. Also gas sampling glow discharges have been described. They consist of a cathode plate having an opening of below 0.1 mm for sampling and being attached to a discharge chamber which is grounded. The assembly is connected to a highdisplacement rotary pump. These devices were introduced as soft ion sources in organic mass spectrometry, and go back to the work of McLuckey et al. [149]. These sources have been shown to be extremely sensitive also for molecular ions of large organic substances. They can be operated at currents of up to 200 mA and voltages of up to 700 V in argon, but also in helium and in neon at pressures of 1 to up to several mbar. It recently has been shown that such discharges in helium are capable of exciting the optical spectra of halogens and S [150]. In the case of each of the three gases, the device could be coupled also with hydiide generation even when allowing the excess of hydrogen produced to enter the discharge [ 1.511 and the detection limits for As in the case of atomic emission spectrometry are at the 10-30 ng/ml level. The capabilities of the gas sampling glow discharge for trace analysis, however, certainly will only be fully explored when applying mass spectrometry.

6.3.5 Coriclusion Especially as a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multielement trace determinations. As summarized in Table 6-2, absolute detection limits with several techniques are below the ng level. This applies especially when the total sample really can be brought into the radiation source, as it is possible with direct sample insertion, for example. In this case, however, volatilization interferences are very high, so that the techniques are only useful when the elements to be determined are present in matrix-isolated form. Atomic emission spectrometry only in a restricted number of cases can be used for the trace determination of elements such as the halogens, P and S. This requires the use of radiation sources where also helium can be used.

6.4 Plasma mass spectrometry Plasma mass spectrometry, with ion sources at moderate vacuum (mbar range) and atmospheric pressure, was started in the sixties when ion sampling at various sources such as flames was realized mainly for diagnostic purposes. Its breakthrough occurred with the work of Houk et al. [ 141, who made use of the inductively coupled plasma as ion source (ICP-MS). Already in their first work, they succeeded in showing that the power of detection of ICP-MS was superior to and that the spectral interferences, especially at higher masses, were low as compared to atomic emission

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

227

Table 6-2. Power of detection of plasma optical atomic emission spectrometric methods Method

Detection limits Re1ative

Absolute

Flame AES

1 ng/nil (alkali)

-

DC arc AES

0.1-2 pLg/g

1-20 ng

Spark AES

1-5 Pg/g

10-100ng

Glow discharge AES

1-5 Pg/g

1-5 ng

Hollow cathode AES

0.05-1 /lg/g

lo-loopg

FANES

0.05-1 pg/g

10-100 pg

Laser AES

1-10 pg

ICP-AES

1-10 PLg/lg 1-20 ng/ml

0.1-1 ng (ETV)

DCP-AES

1-20 ng/nil

-

MIP-AES

2-40 ng/nil

0.05-0.5 ng

spectrometry. This development was only possible because in the seventies mass spectrometers went through a thorough development both with respect to the price performance ratio as well as their resolution. In particular, the development of quadrupole mass spectrometers of high quality was of prime importance. ICPmass spectrometry in the meanwhile developed to a strong tool for trace analysis; developments in the case of other plasmas and especially in the case of the microwave plasmas and of the glow discharges are also to be discussed. 6.4.1 ICP mass spectsontetiy

6.4.1.1 Instrumentation (Fig. 6- 14)

In ICP-MS, ions are extracted from the analytical zone with the aid of a conical metal aperture (sampler). The diameter of the aperture is between 0.3 and 1 mm. Towards smaller values the diameter is restricted by the critical thickness of the cold boundary layer. Under this limit value, a sampling of analyte ions from the ICP is not possible. Towards higher values, the diameter is limited by the pressure in the intermediate stage, which should not be higher than some mbar. At these dimensions of the aperture, a powerful oil rotation pump or a turbomolecular pump must be used, so as to provide for the vacuum mentioned. The sampler can be made of different metals. Both copper and nickel can be used. When aggressive acids such as HF or HNO, are present in the analyte solutions, as it is often the case in

J.A.C. BROEKAERT

228

,

IOIFFERENTIALLY PUMPED REGION AORUPOLE MASS

R

Figure 6- 14. Instrumentationfor ICP-MS

the analysis of geological samples, samplers made of Pt may be useful. In that case, it was also found useful to nebulize a solution containing Ti for a longer time, so as to cover the sampler with a protective layer [152]. When using a sampler with a cone angle of 120°, the stability of the plasma and the ion extraction were found to be optimum. In the intermediate vacuum, the beam of the sampled ions expands. From this beam the central part is sampled with a second aperture (skimmer) and then led into the high vacuum of the mass spectrometer. The cone angle of the skimmer is ca. 55". The vacuum in the mass spectrometer is maintained with the aid of a diffusion or a cryopump. A high ion transmission is obtained at a distance of 5-10 mm between sampler and skimmer. In the intermediate vacuum high-energy ions, radicals, molecules and electrons give rise to a number of reactions, by which a series of compounds are formed, namely:

(a) Metal compounds MO-MO++e M + C1, N, . . .

__t

MCl', MN', . .

(b) Argon compounds

-

Ar + H,O --+ ArO', ArH', ArOH; Ar + C1, N, . . . ArCl', ArNH', . . . 2Ar Ar; + e

(6.1 1)

The presence of these cluster ions gives rise to spectral interferences with the analyte ion signals. This is especially the case with quadrupole mass spectrometers, which have only unit mass resolution. In order to keep these inteiferences to a

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

229

tolerable level, acids such as HCl, H,PO,, and H,SO, should not be present in the measurement solution and thus must be replaced by HNO,. However, also the working conditions such as the sampling depth, the aerosol gas flow and the parameters of the ion optics influence the degree of interferences. In commercial equipment, mostly quadrupole mass spectrometers are used. They have the advantage that no electrical field is required at the sampler for realizing a sufficiently high ion transmission. In the ion beam, one often provides a photon stop so as to prevent UV radiation from entering the detector, and uses several electrostatic lenses to focus the ion beam. The mass spectrometer itself consists of 4 equidistant and parallel rods (diameters 10-12 mm), between which a highfrequency field is applied and superimposed on a DC field. The transmission for anions with a given mle thus becomes maximum at a certain value of the field and the system then acts as a filter for this species. This can, however, be done in a rapidscanning mode. The spectral information can further be stored in a multichannel analyzer. Apart from quadrupole instruments, also high-resolution double-focussing mass spectrometers can be used. With such very expensive spectrometers, spectral interferences in a number of cases can be successfully eliminated, however, at the expense of ion transmission. The latter can only be compensated for by applying a high voltage at the sampler. Recently, time of flight mass spectrometers have also bee proposed for ICP-MS [1531 and potential for the use of ion traps exists. For ion detection in mass spectrometry,electron multipliers and pulse counters normally are applied. However, other detectors can also be used, such as microchannel plates. The operation of the plasma and of the mass spectrometeras well as of the data acquisition are computer-controlled. An external computer is mostly used for calibration, for interference as well as for drift corrections, graphics and spectrum treatment. Equipment including an ICP combined to a quadrupole mass spectrometer has been commercially available since 1982 and an apparatus using a double-focussing mass spectrometer and an ICP since 1987.

6.4.1.2 Analytical figures of merit ICP-MS has the advantages of all ways of sample introduction known from ICPAES, the possibilities for an easy calibration with synthetic solutions or by standard addition as well as the possibilities of rapid and flexible multielement determinations. Its power of detection is at the same level for most of the elements and is much higher than in ICP-AES. Also isotope dilution can be applied for calibration. (a) ThelCP niuss spectru (Fig. 6- 15) in the case of quadrupole mass spectrometers have a resolution of 1 dalton. Accordingly, cluster ions may cause considerable spectral interferences with analyte ions, especially at low masses. Cluster ions may be formed as a result of different processes and may stem from:

230

J.A.C. BROEKAERT

5 ." 3

----L 0

E

I.

N 0.. 0 0 -

v,

7

1.

8

1. o

00-

0

NO

0

r.

4

N

0

c

dd

0

,

+2{

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

23 1

0

Solvents and acids: H', OH', H,O+, . . . , NO+, NO2', .. ., C1' (in the case of HCl), SO', SO;, S0,H' (when residual H,SO, is present).

0

Gases from the stmounding atmosphere: 0;. CO', CO;, N;, NH', NO', . ..

0

Reactions of the above mentioned species with argon: ArO', &OH+, . . ., ArCl', . . ., Ar;.

These cluster ions cause interferences especially in the mass range below 80 dalton and may seriously hamper the determination of the light elements. Moreover, also a number of compounds of the analyte ions with different species are found. Compounds such as MO', MCl', MOH', MOH;,. ..are formed as a result of the dissociation of nitrates, sulfates or phosphates in the ICP or they are formed by reactions of analyte ions with solvent or oxygen in the plasma and eventually also in the intermediate vacuum. The formation of these species very much depends on the working conditions, as it has been extensively studied by Horlick et al. [154] and others [ M I . For the recognition of interferences in ICP-MS, the use of the isotope ratios of the elements can be very useful. The ICP-MS spectra accordingly are much less sophisticated than the ICP emission spectra, however, the resolving power of the mass spectrometers used in most cases also is much lower. Due to the different kinds of cluster ions which may be formed, spectral interferences in many cases will occur and correction methods will have to be worked out. In the ICP mass spectra the spectral background is low and is mostly determined by the dark current of the detector used or by scatteringof ions in the mass spectrometer. Other than ICP-AES, the ICP itself only slightly contributes to the spectral background, by which in ICP-MS it will be much easier to apply standard addition; (b) For the optimization of ICP-MS with respect to power of detection, freedom of spectral interferencesand of signal enhancements or suppression as well as of analytical precision, the operation parameters of the ICP, namely, the gas flow rates, the power, the position of the sampler and the parameters of the ion optics have to be optimized. The nebulizer gas flow is much more important than the outer and the intermediate gas flow. Its influence on the plasma temperature, as well as on the droplet size and accordingly, on the power of detection and the nebulization effects is similar as in ICP-AES. However, the effect is much larger, which can be understood from the fact that the sampling location is much more defined than in ICP-AES. Moreover, also the formation and decay of cluster ions and the ion energies are severely influenced. The

232

J.A.C. BROEKAERT

Figure 6-16. Energy maxima for 68Cu+and A 0 ions as a function of the aerosol gas flows [1551; power: 1.5 kW

latter is also the case for cluster ions, as shown for ArO+ and Cu+ in Fig. 616. The aerosol gas flow also influences the geometry of the aerosol channel and accordingly, also the ionic number densities at the sampling aperture and the analyte signals. The power influences the plasma volume more than the plasma temperature and has to be optimized together with the aerosol gas flow and the sampling position (Fig. 6-17). The voltages at the different ion lenses must be optimized so as to get maximum transmission, which is eventually a compromise for elements with widely different masses; (c) The detection h i t s of ICP-MS are at the sub-ngJml level (Table 6-3). The detection limits for most elements are of the same order of magnitude. The fact that for some elements the detection limits are higher is due to isobaric interferences. This applies for As (interference of 7 5 A ~with + 40Ar35C1'),for Se (interference of "Se' with 40Ar40Ar+)and for Fe (interference of 56Fe+ with &Arl60'). For these elements the acid present in the analyte solution also is important. Also for the elements of which the sampler is made (e.g. Ni, Cu, ...), the limits of detection may be higher. In the case of elements with high ionization energies such as the halogens, low detection limits may also be obtained with negative ions (e.g. for C1+: 5 and for C1-: 1 nglml) [156]. The analytical precision mainly depends on the stability of the aerosol production, of the ionization in the plasma, of the ion sampling and of the detection. Relative short-term fluctuations are of the order of 1%. Further improvement just as in ICP-AES can also be achieved with the aid of a reference line [157,158]. This may occur especially when the salt content in the analyte solutions is high, as it is in the case with sample decomposition by fusioii with fluxes. Then salt deposits at the nebulizer, the torch or the sampler may lead to drifts. Whereas deposits at the nozzle of the nebulizer can

1,O

1,l

1,2

1,3l/min 1,L

0,9

1,0

1,l

1,2

lJI/minl,L

0,9

1,O

1,t

1,3I/minl,4

Aerosol gas flow

1)

Figure 6-17. The influence of the aerosol gas flow and the power on the signals of Ga+, Ge+, As+, In+, Sn+, Sb+, TI+, Pb+ and Bi+ (sampler position of 15 mm above the coil) (reprinted with pemiission from Ref. [2101)

0,9

w

w w

J.A.C. BROEKAERT

234

Table 6-3. Detection limits in ICP-AES and ICP-MS Element

Ag

AI As AU

B Cd

Ce co Cr Ge Fe Hg In La Li Mg Mn

Detection limit ICP-AES [SS] 7 20

50 20 5

3 50 6 6 50 5 20 60

10 80

Pb

0.1 1 10 40

Se

70

Te Th Ti

40

Ni

U V W

zn

60

0.03 0.2 0.04

0.06 0.04 0.06 0.05 0.01

0.3 0.02

-

0.02 0.07

0.05 0.1 0.7 0.1

0.05 0.8 0.09 0.02

4 250 5

0.03 -

30

0.05 0.2

2

efficiently by avoided by wetting the aerosol gas, deposits at the sampler are more problematic. Therefore, the total salt contents of the solutions should not be above 0.1-0.5 dl00 ml depending on the salts used. These effects lead not only to signal depressions and signal fluctuations but also to memory effects. In the case of solutions with low salt contents they are only 1-2 min (before the signals go back to < 1% of the sample signal).

Znteifereizces as a result of changes in the matrix composition may be due to changes in nebulization, in the ionization in the plasma, in the geometry of the aerosol channel or in the ion energy. The first are known as nebulization

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

235

effects and have been discussed earlier (see Section 6.3.2.1). By increasing the aerosol gas flow, nebulization effects decrease and at the same time the influences of the analyte on the geometry of the aerosol channel go back.

As in ICP-MS mass signals are measured, a differentiation between different isotopes of an element becomes possible. At the low mass resolution of quadrupole mass spectrometers (about 1 dalton), this leads to a series of isobaric interferences. Therefore, corrections with the aid of suitable software have to be made in the evaluation of the spectra. These kinds of interferences do not much change with the working conditions. This is not the case with the spectral interferences arising from doubly charged ions, background species or cluster ions, which depend very much on the working conditions. The background species at low masses lead to considerable interferences [ 1591: for example, 28Si+interferes with I4NL4N+, 31P+with 14NOH+,and 80Se+with 40Ar40Ar+.Species like 40Arl60+ not ony interfere with 56Fe+but as a result of a series of isotopic combinations and hydrides they lead to spectral interferences for a whole series of transition metals (53Cr,54Cr,54Fe,55Mn,56Fe, 58Ni,58Fe,and 59C0). When HCl is present, Cl', ClO+, ClN+,Cl', and ArCl+ species will give rise to inteiferences for further transition metals. At fixed working conditions, these interferences again are rather constant. The occurrence of doubly charged ions and cluster ions with the analyte ions greatly depends on the power and the aerosol gas flows. They are especially important for elements such as Ba, Sr and Mg which have rather low ionization energies and have thermally rather stable oxides [ 1591. Many inteiferences are mentioned in the literature. For instance, the isotopes of T i with the masses 46,47, 48,49 and 50 have oxide ions with I6O+,which interfere with 62Ni+, 63Cu+,65Cu+and 66Zn+and these interferences widely differ with the plasma locations sampled [MI; (d) Isotopic dilution can be used well in ICP-MS. This enables it to eliminate a series of systematic errors as well as to perform tracer studies. This technique [ 1601 can be applied for each element which has at least two stable or longliving isotopes. A known amount of the element with a known but different isotopic composition as compared to the one where the sample is added and intensively mixed. The isotope ratio (R: isotope l/isotope 2)) is then given bv: (6.12)

Np is the number of atoms of the element to be determined in the sample and NAthe number added. up and aA are the abundancees of the isotope (1) and (2) in the sample and in the added amount, respectively. Accordingly, from R we can calculate Np

236

.

J .A .C BROEKAERT

Isotopic dilution ICP-MS was very useful for studies on Pb (see Ref. [ 1611). Also tracer studies with Fe have been performed in biological materials [ 1621. The precision of the determination of isotopic compositions is of the order of 1% when the respective isotopic concentrations do not differ by more than one order of magnitude; (e) Apart from pneumatic nebulization of aqueous solutions some alrernative methodsfor sanzple introduction are particularly useful for ICP-MS. Similar to ICP-AES the analysis of organic solutionsis somewhat more difficult [ 1631. The use of ultasonic nebulization is also known from ICP-AES, but due to risks for high water loading of the aerosol and the related influences on the pressure in the intermediate stage, it is still more difficult to optimize than in the case of ICP-AES [164]. A new approach lies in the use of high-pressure nebulization [165]. It promises to be ideally suited for a combination of ICP-MS and HPLC, which is one of the strongest tools in speciation now available. With the formation of volatile hydrides, as it is known from AAS [166] and from ICP-AES (see Ref. [56]), one can considerably improve the detection limits for elements such as As, Se, and Sb. As shown in 11671, improvements were obtained also for Pb. They are due to the increased sampling efficiency but also the production of a water-free analyte flow resulting in a decrease of cluster formation. However, also in ICP-MS the many chemical interferences in the hydride formation reaction may hamper the accuracy in the case of real samples. From ICP-AES electrothermal evaporation (ETV) is known to be very useful for the analysis of micro-samples (see Sections 6.2 and and 6.3.2.2). In the case of ICP-MS it brings the additional advantage of introducing a dry analyte vapor into the plasma. It has been found to be extremely useful for the elements of which the detection limits are high, as a result of spectral interferences with cluster ions. In the case of 56Fe,which is interfered by 4 0 A ~ O Park + , et al. [ 1681showed that the detection limit could be considerably improved by ETV. Also the direct insertion of samples into the plasma has been described for the case of ICP-MS [169,170].

As techniques for direct solids sampling, spark ablation [171] and especially laser ablation have been investigated. The latter is commercially available and is very suitable for the analysis of electrically non-conducting samples also as it enables rapid survey analysis of sample inclusions. It has been shown for Sic, for example, that with a Nd-YAG laser operated at 110 Hz and an energy of some 0.1 J, a sample ablation of 1-10 ng and detection limits in the 0.1 pg/g range can be obtained [79,172].

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

237

6.4.1.3 Applications ICP-MS has found use in all areas where ICP-AES is applied but where further progress in power of detection was required. This is the case in trace analysis for geological samples and then especially for hydrogeological samples as well as for trace determinations in metals, in biology and medicine as well as in environmental analysis. The latter in particular is true when it comes to speciation work. (a) In the case of geologicalsamples, the determination of traces of rare earths in minerals has been described [ 173,1741,when ICP-AES is not suitable because of interferences or of limited power of detection. Calibration in most cases is done with standard addition and sample concentrations of < 0.1% were used. A further example is the determination of Pt subsequent to pre-enrichment by a "fire assay" with NiS [175]. Also the analysis of hydrogeological samples has been widely applied, as described by Garbarino and Taylor [ 1761. They applied isotopic dilution in the determination of Ni, Cu, Sr, Cd,Ba, TI and Pb in pure water samples. Also alternative sample introduction techniques such as ETV for the determination of Tl 11771 or suspension techniques for the analysis of powders of coal [ 1781 have been investigated; (b) In nietuls und ceiantic niutr-ices trace determinations down to the sub-pg/g level are possible as the analyte concentrations may be up to 5 g/l. In contrast to ICP-AES, considerable signal changes may occur already at this sample concentration and have to be considered in the calibration. ICP-MS is especially useful for determinations in metals with linerich emission spectra. They include high-temperature alloys which are of use for reactor technology or in aeronautics [179]. McLeod et al. [181], however, showed with the example of nickel alloys that also in ICP-MS, finally the spectral interferences stemming from matrix lines limit the power of detection. Therefore, procedures for matrix removal, as possible with extraction, in the case of uranium [182] will bring progress in power of detection and in accuracy. Also direct solids sampling has been applied. In uranium as nuclear fuel, direct determinations can be performed by laser ablation coupled to ICP-MS [180]. It was also of use for the case of ceramics [ 1721. With arc ablation coupled to ICP-MS detection limits at the 0.1-1 pg/g were obtained in steels [183]; (c) For biological atid ntedical saniples ICP-MS allows it to considerably enlarge the number of elements which can be directly determined in body fluids. Indeed, in serum, e.g. only Na, Ca, Mg, Fe, Cu and Zn can be directly determined with ICP-AES whereas with ICP-MS any other elements also can be determined. Thus ICP-MS will be a strong tool also for the study of bioavailability of trace elements. ICP-MS was used already fairly early for the determination of normal concentrations of trace elements in clinical

238

J.A.C. BROEKAERT

samples [185]. In proteins Mg, Al, Cr, Mn, Fe, Ni, Cu, Zn, and Se have been determined. In urine, a good agreement with results of other techniques was found for elements with masses higher than 81 (Pb, Cd, Hg, and T1) but for As, Fe and Se, deviations were found. The latter could be eliminated by removing C1 from the analyte solutions by precipitation. It also has been shown that by internal standardisation the accuracy of ICP-MS as well as the precision could be considerably improved [158]. ICP-MS was also used for the determination of Pb in blood [186] and for bioavailability studies of Pb [81]. Flow injection allows it to work with small sample volumes or to avoid difficulties stemming from samples with a high viscosity or with a high salt content [ 1881. By isdtopic dilution, metabolic studies with stable isotopes can be performed [ 1891, which is a very promising technique for medical studies; (d) For environmental arralysis ICP-MS is a most promising technique, because of its high power of detection. Especially for drinking water a determination of almost any relevant element can be directly performed. For the case of waste waters, the possible interferences were studied in detail by Herzog and Dietz [ 1901, who described optimization procedures for minimizing the interferences, in addition to factorial analysis studies for the treatment of large data collectives. For direct determinations in seawater, its salt content is too high. Here liquid-liquid extraction procedures or sorption techniques should be applied to isolate the elements to be determined. Beauchemin et al. [I911 described the use of a SiO, column loaded with 8-hydroxyquinoline, which enabled it to perform a 50-fold pre-enrichment of Ni, Cu, Zn, Mo, Cd, Pb, and U. In river water Na, Mg, K, Ca, Al, V, Cr,Mn, Cu, Zn, Sr, Mo, Sb, Ba, and U could be determined directly and Co, Ni, Cd, and Pb could be measured subsequent to the above mentioned pre-enrichment [ 1921. Isotopic dilution also delivered the most accurate results [ 1931. Also for the analysis of marine sediments [1941 and the characterization of the according reference materials ICP-MS was very useful. With ICP-MS also Sn could be determined in environmentally relevant samples [ 1951 and even its speciation is possible. For this aim it is to be mentioned that ICP-MS coupled directly to HPLC offers unique possibilities.

6.4.1.4 Outlook Further trends in ICP-MS lie in the development of new instrumentation, both from the side of the plasma as well as from the spectrometer and in the improvement of sample introduction. With respect to the first point, the use of MIPS as sources for mass spectrometry has to be mentioned, as it has been described, for example, by Caruso et al. [196].

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

239

With this low-power plasma the possibilities for sample introduction are limited, which is less often the case when high-power microwave plasmas are used, as recently mentioned by Furuta et al. [197]. Also high-power MIPS operated with nitrogen, such as the MINDAP described by Wilson et al. [198], could be very useful. The use of alternative gases in general is interesting, as it opens up the possibility of reducing interferences in particular cases. In this respect the use of mixed-gas ICPs [199] and also the use of helium ICPs, as described by Montaser et al. [200] are promising. Apart from quadrupole mass spectrometers, the high-resolution double-focussing mass spectrometer certainly enables it to cope with a number of interferences; however it is very expensive. Here, the use of ion traps, time-of-flight systems, double quadrupoles, etc. is to be further investigated. Finally, in the field of sample introduction, much of the achievements in ICPAES may deliver the inspiration to cope with particular analytical problems. Due to its high power of detection, the coupling of ICP-MS to chromatography certainly is a challenge for speciation work. Here, innovative work such as the optimization of new sampling techniques is underway. In summary, it can be stated that with ICP-MS a powerful method is available which has the high power of detection of graphite furnace atomic absorption on one hand, with the multielement capability of plasma spectrometry, on the other hand.

6.42 Glow discharge muss spectrometry Glow discharges became especially attractive for trace analysis since they are used as ion sources for mass spectrometry. In the sources developed for emission, spectrometry ions can be sampled very easily with the aid of a skimmer, as it is done in ICP-MS. Both quadrupole-based as well as double-focussing mass spectrometers can be used and for both approaches instruments are commercially available. The scope and the analytical figures of merit of glow discharge mass spectrometry have been described in papers of Harrison et al. (see Ref. [ 151) and of Jakubowski and Stiiwer (Fig. 6-18) [ 161. In the case of DC discharges, both with a pin cathode as well as with a flat cathode, a sputtering equilibrium and constant ion signals are obtained after a certain time, by which both sequential as well as simultaneous determinations in compact electrically-conductive solids are possible. Detection limits in the case of quadrupole mass spectrometers are in the ng/g region and for the light elements they may be limited by spectral interferences from clusters [201]. However, it could be shown that in the case of steels, reliable analyses at sub-pg/g concentrations can be performed. In the case of a double focussing mass spectrometer, the detection limits are considerably lower, as shown by work on analysis of materials used in microelectronics [202]. In both cases relative sensitivity factors, which in mass spectrometry reflect the influence of the matrix mainly as a result of phenomena in

J.A.C. BROEKAERT

240

~7 ~~

'

SOURCf

OPTICS

FILTER ,L

d

m n l f f f BUS

0AKING

DIFRRENIIAL PUMPING

FLOW

Figure 6-18. Glow discharge mass spectromelry including a Grimm-type glow discharge and a quadruple mass spectrometer; (1) exit aperture, (2) skimmer, (3) ion optics, (4) quadrupole rod system, (5) ion detector (reprinted with permission from Ref. [16])

the ion source as well as in ion transmission, are much closer to 1 [203] as compared to the spark source values. Glow discharge mass spectrometry with flat cathodes has the unique feature for depth profiling, which is important for the analysis of protective layers on working materials and for the characterization of multilayer materials now used in microelectronics. For the latter, it has been recently shown in an impressive way how multilayers can be sputtered and analyzed by mass spectrometry using a Grimm-type glow discharge [204]. Accordingly, it can be understood that glow discharge mass spectrometry is also a very powerful tool for the analysis of dry solution residues which are formed by the evaporation of microvolumes of analyte solutions of low total salt contents on a suitable carrier. In the case of the noble metals, one can make use of cementation in the case of copper targets to fix the analyte, so that it can be enriched and sputtered reproducibly. There has been found a way to fix pg amounts of Ir on a copper plate prior to their determination with GD-MS (Fig. 6-19) 12051. As r.f. glow discharges become available, they can be expected to become also of use as sources for mass spectrometry. They will certainly enable both direct bulk and depth-profile analysis of solid ceramics, as has already been demonstrated in the case of emission spectrometry. Further impulses for the use of glow discharges as sources for plasma mass spectrometry may also come from the use of other types of mass spectrometers [206]. This approach will be particularly valuable at first in model studies on ion formation. The latter as well as the modelling of the sputtering itself, as described in a paper of Van Straeten et al. [207], are actual topics of methodological studies by now.

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

0.1

1.

0.01 0.1

I 1

10 1 0 0 loo0

Ir content. ng

-

24 1

time, s

Ir

-b&

ground

-_--- detection limit

Figure 6-19. Determination of pg amounts of Ir by dry solution residue glow discharge mass spectrometry; (a) calibrationcurve obtainedwith disk-shapedcathode, quadratic regression, (b) single ion monitoring profile of 1 pg Ir (reprinted with permission from Ref. [205]

6.5 Conclusion In comparison with other methods of elemental analysis, plasma atomic spectrometry has a number of interesting analytical features. Its features are to be compared with those of atomic absorption spectrometry, spectrophotometry and fluorimetry as well as with X-ray spectrometry and electrochemical methods, in terms of power of detection, analytical accuracy in terms of systematic errors, but also with respect to its instrumentation and operational costs when it comes to its application in trace analysis. A survey of the analytical figures of merit of the atomic spectrometric methods is given in Table 6-4. 6.5.1 Power of detection

The detection limits for plasma emission spectrometry are at the 1-20 ng/inl level and for ICP-mass spectrometry at the sub-ngml level. Therefore the latter is one of the most sensitive techniques in elemental analysis. Only for the light elements, such as Mg, B, et al. or for interfered elements such as Fe, ICP-AES and in the latter case, other methods such as total reflection X-ray fluorescence (for a feature article, see Ref. [208]) may be more suitable. For solids analysis, both ICP-AES and MS make use of solutions, apart from some techniques allowing direct solids sampling. Therefore, both the loss in power of detection as well as the time involved with sample dissolution are disadvantages. This point also applies for AAS and for electrochemical methods, not however, for glow discharges which especially in the case of direct solids mass spectrometry, now is the most sensitive method in many cases.

J.A.C. BROEKAERT

242

Table 6-4. Figures of merit of methods for atomic spectrometric elemental determinations [I91 ~

Method

Power of detection

Matrix effects

costs

Atomic absorption * flame * furnace

++ +++

+ +++

++

Atomic emission * flame * arc/spark * inductively coupled plasma * microwave discharges * DC plasmas * glow discharge * laser

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

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

Laser atomic fluorescence

+++

++

+++

Laser enhanced ionization

+++

+

+++

+++

+++ ++

+++ +++

+++ ++

+++ ++ +++

Mass spectrometry * themiionic * inductively coupled plasma * glow discharge

X-rayspectroinetry

* fluorescence * total reflection * electron micmpmbe

+++ +++

++ +++ +++

+

++

+

+

+++

++

+ ++ ++ ++

++

+ low, ++ medium, +++ very high 6.5.2 Ititegerences As compared to graphite furnace AAS, interferences in ICP-AES and ICP-MS are low. In ICP-AES, they are mostly the result of spectral interferences, by which a matrix separation especially in the case of matrices with linerich spectra (e.g. Zr)is the only solution and this despite the availability of highly sophisticated instrumental background correction. In ICP-MS the influence of matrix constituents on the signals is much larger. However, when spectral interferences do not occur, which can be intelligently tested at the hand of the isotope patterns, calibration by standard addition often can solve the interference problem. In the case of the

PLASMA OPTICAL EMISSION AND MASS SPECTROMETRY

243

light elements especially, cluster ions may cause spectral interferences. This is also the case for a number of heavy elements, but here a control is more easily possible and as more isotopes are present, the problem can often be solved. In the case of solids, low detection limits often cannot be obtained with a determination of the trace constituents in the presence of the matrix, as a result of the fact that ICP-MS only can tolerate low salt contents. Here, combined analytical procedures including matrix removal must be applied. Then, however, apart from ICP-MS, also total reflection X-ray fluorescence becomes very useful as its absolute detection limits are in the pg-range and multielement determinations are very rapidly done (for further literature, see Chapter 1). However, the light elements here cannot be determined. Graphite furnace AAS is a cheaper but monoelement solution for the problem as well as are electroanalytical methods. In the case of direct solids analysis, glow discharges especially in mass spectrometry have a high power of detection and are highly accurate, especially when compared to earlier spark source mass spectrometry.

65.3 Economic aspects Whereas ICP-AES in the latest years became considerably cheaper, as a result of the rationalization of the instruments design and the development of lowconsumption ICPs with lower gas and power consumption, ICP-MS remains an expensive method. However, when considering the multielement capacity and the power of detection, it becomes affordable for routine analytical laboratories. For a number of analytically challenging problems such as speciation' in life sciences and biological problems as well as for the characterization of refractory metals for microelectronics, it often becomes the only analytical approach available. Also glow discharge mass spectrometry is an expensive method, however, at least in combination with quadrupole mass spectrometers, its instrumental costs become similar to other direct solids analysis methods. It is much more reliable than the earlier well-known spark source mass spectrometry.

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CHAPTER 7

Instrumental neutron activation analysis (INAA)

ZEEV B.ALFASSI Department of Nuclear Engineering. Beti Gurion University of the Negev. Beer-Sheva 84I20. Israel

Contents 7.1 7.2

7.3

7.4

7.5

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 254 Basic nuclear physics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 7.2.1 Nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 255 Radioactivedecay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 7.2.2 7.2.3 Kinetics of decay of radioactive nuclides . . . . . . . . . . . . . . . . . . .260 Kinetics of formation of radioactive nuclides by irradiation . . . . . . . . . . 261 7.2.4 The chart of the nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . 265 7.2.5 Gamma detection systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 NaI(TI) - scintillation detector . . . . . . . . . . . . . . . . . . . . . . . . 268 7.3.1 Solid-state ionization detector . . . . . . . . . . . . . . . . . . . . . . . . . 268 7.3.2 7.3.3 The shape of y spectrum . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 IiTadiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 7.4.1 Neutronsources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 7.4.2 Samples introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 Instrumental neutron activation analysis (INAA) . . . . . . . . . . . . . . . . . . . . 280 7.5.1 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 7.5.2 Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 280 Nuclear interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 7.5.3 7.5.4 A test case - INAA of trace elements in silicon . . . . . . . . . . . . . . . .285 7.5.5 EpithermalINNA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 Fast neutrons INAA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296 7.5.6 7.5.7 Cyclic INAA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 299 7.5.8 Prompt Gamma Neutron Activation Analysis (PGNAA) . . . . . . . . . . . 301 7.5.9 Depth profiling by INAA . . . . . . . . . . . . . . . . . . . . . . . . . . . 302

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7.1 Introduction Chemical analysis by nuclear activation is an elemental analysis, i.e. it determines the contents of the various elements, but cannot tell in what chemical forin (compounds, valence states) they are. The analysis is based on a reaction of the analysed element with nuclear projectiles (neutrons, accelerates small charged particles, for example protons, deuterons, 3He or a particles, or gamma photons). The reaction can be written in the same form as a chemical reaction: Target

+ projectile

-+ light product

+ heavy product

or in the more concise form of writings of nuclear physics: Target (projectile, light product) heavy product. Thus, for example, the first production of artificial radionuclide by Joliot and Curie was done by the reaction of alpha particles with aluminum metal.

27Al+ a +30P+ n

( a is the4He nucleus)

or in the physics notation 27Al (a,n)30P. The light product is similar to the projectile, a neutron, a small charged particle or a photon. The basis of the nuclear activation method for chemical analysis, is the measurement of the amount of either the light or the heavy product produced in a known flux of projectiles for a known length of time. The amount of the products is proportional to the number of the target atoms, and hence, the measurement of the amount of the product yields the amount of the target atoms. The amount formed of the products is too small to be measured chemically (except in very rare cases [ 11). and the only way to measure them is by the nuclear physics methods of pulse countings. The light product can be measured due to its energy, if it is a photon or high kinetic energy charged particle, or due to its reaction if it is a neutron. The common denominator of all these processes, is that they must be done at a very short time after the formation of the product, otherwise the light product will lose either its energy or its identity, by reaction with the surrounding media. Consequently, the measurement of the light particle must be done during the bombardment of the target with the projectiles. This kind of measurement is called Prompt Activation

Analysis. In the case where the heavy product is radioactive, its amount can be measured by its radioactivity. This is the more common case, Since the radioactivity can be measured after the end of the interaction between the target and the beam of the projectiles, this method of analysis is called Delayed Activation Analysis. Since the heavy product must be radioactive, not all elements can be measured with each projectile. However, by the choice of the bombarding projectile, every element can

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255

be determined. Since many elements are activated simultaneously, the radioactivity measurement must yield both the identity of the radioactive nuclides and their amounts. The sometimes used term radioisotopes is wrong, since when saying isotopes we should say of what element. Nuclide is the term in nuclear physics which is parallel to the atom in chemistry. It consists of a nucleus (composed of neutrons and protons) and electrons around it. As the identity of radionuclides (short term for radioactive nuclides) in a mixture cannot be determined from pemission (unless very few are in the mixture), almost only y ray emitters are used in activation analysis. The radionuclides used for identification are called Indicator Radionuclide and abbreviated IRN. Only a few - p--emitting IRNs as 32Pare used in activation analysis. In this case the phosphorus should be separated from the other radionuclides in the activated sample. The analysis by nuclear activation with activity measurements only after chemical separation, is named Radiochemical Activation Analysis (RAA), whereas analysis by nuclear activation with direct measurement of the nuclear activity is named Instrumental Activation Analysis (IAA). RAA is used not only for p- and p’ emitters, but also for y-emiting IRN’s when they are masked by the more active nuclides in the sample. Since the most common projectile is neutron, this chapter will be devoted to Instrumental Neutron Activation Analysis (INAA).

7.2 Basic nuclear physics This chapter gives only briefly the basic knowledge. Readers who are interested in further reading can use common textbooks [ 2 4 ] . 7.2.1 Nuclides

The nucleus of an atom is composed of protons and neutrons. A general term to describe a particle in the nucleus (either proton or neutron) is nucleon. The total number of nucleons in the nucleus is called the mass number - A (A = N + 2, when N is the number of neutrons in the nucleus, and Z is the number of protons in the nucleus). Z is called the atomic number. A nucleus is defined unequivocally by its A and 2 values. The usual way to write a nucleus is i X . Nuclides of constant Z but different values of A are called Isotopes. The writing of Z is parallel to the use of the chemical symbol of the element. For example, oxygen nucleus has 8 protons and consequently the use of the symbol for oxygen is the same as writing Z = 8. If we write for the nucleus the symbol 0 we do not have to write the subscript 8, although in some cases we will do it for clarity purposes. Oxygen has three stable isotopes with 8,9 and 10 neutrons. They are written as l60, 170and l80.A common misuse is the designation of any nuclear species, as e.g. 14C, 13N,1 7 0 , by the word isotope. The word isotope should be used only in connection with a special element, as e.g. saying that l60,1 7 0 and l80are the three stable isotopes of oxygen or saying that

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34Cl,35Cl,36Clare isotopes of the same element. However, "F and ''0 are not isotopes. The general term to designate any nuclear species is nu.clide. A general term for radioactive nuclide should be radionuclide and not radioisotope, although it is correct to say for example that "'1 is a radioisotope of iodine. 7.2.2 Radioactive decuy

The neutrons and protons are held together in the nucleus due to the nuclear forces, These forces together with the Coulombic force lead to the result that only special combinations of neutrons and protons are stable. This is clear from the fact that each element has only a limited number of stable isotopes. Saying that some nuclides are unstable does not mean that they cannot be formed, it means that they are transformed to the other nuclides which have lower energy (i.e. lower mass according to Einstein's equation of mass and energy). This transformation of nuclides is done by the transformation of a proton to a neutron or vice versa, thus changing the ratio neutrons/protons. The transformation of nuclides can be done also by emission of an alpha particle (4Henuclide), however, this emission occurs only for the very heavy nuclides and it is almost unimportant in activation analysis. Very few radionuclides are transformed by emission of a neutron or a proton or by a spontaneous fission. All the transformationsof neutron +proton and proton +neutron are called /?-decays. They are characterized by the mass number - A - remains unchanged, whereas the atomic number is changed by f l . There are three transformations of that type. A neutron is transformed to a proton by a /?- process.

The proton remains in the nucleus, while the electron (p-) and the antineutrino (Ti) are emitted outside from the nucleus, with high kinetic energies. The electron

is written as /?- and not e-, to show that it is not an atomic electron. A proton can be transformed into a neutron by two processes. One process is p' decay, in which a proton is transformed into a neutron and a positron p' = e+ emitted from the nucleus, an entity similar in mass to electron but which has an opposite charge. p+ +n + / ? + + u

In this process there is also an emission of a neutrino (v). Another process is the reaction of a proton with one of the surrounding electrons in the atom to yield a neutron. This process is called electron capture (EC). Neutrino and antineutrino are very small particles which have no charge, and consequently are very difficult to be detected. They are not important at all to activation analysis, except for their effect on the /? energies, which will be explained later.

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Most of the radionuclides are not pwe ,8 emitters, but they emit simultaneously also a y photon (more accurately the y emission is a very short time of less than lO-’Os after the ,8- emission). Gamma-rays are electromagnetic waves, as light and radiowaves, but they have much higher energies (much shorter wave lengths). They have usually lower energies than X-rays, although the main difference between X-rays and y-rays are their sources. X-rays are due to atomic transitions (transitions between different energy levels of electrons), where y rays are due to nuclear transitions (transitions between different energy levels of the nucleons). The ,8 decay does not yield in most cases the ground state of the product nuclide, but forms it in an excited state. The excited state nuclide decays very rapidly (in most cases) to the ground state by the emission of y rays. A very important difference between p processes and y decay (besides the difference in A Z, f1 in the /3 process and 0 in the y decay) is the fact that in /3 processes there is emission of two particles, p- and V or p’ and v, while in y decay there is emission of only one photon. There are cases of emission of a few photons froin the same nuclide, but these are successive emissions and not a simultaneous emission as in the /3 processes. The importance of this difference lies in the fact that each nuclear decay is a transformation between two discrete energy states, resulting in a definite energy released in the process. If only one particle is emitted from the nuclei, this particle has a defined energy. When two particles are emitted, the released energy is distributed between them, and each particle can have a spectrum of energies ranging from zero up to a maximal energy, equal to the released energy. Since the emitted y photons have definite energies, they can be used in order to identify their emitters, by the measurement of the energy of the photons. As /3 particles do not have definite energies, they cannot usually be used to identify their emitters. In nuclear activation analysis, we need to identify the different radionuclides (in order to know from which nuclide they were formed), in addition to the measurement of their activities. This is the reason why nuclear activation analysis is employing almost exclusively the measurement of the spectrum of emitted y photons. In a few cases, where the IRN (Indicator Radionuclide) is a pure-p- emitter it can be measured either by chemical separation of this element (Radiochemical Activation Analysis - RAA) or for long-lived IRNs, by long waiting periods between the end of the irradiations and the starting of the counting of the activity. This waiting time is usually called “cooling time.” The end of irradiation is called also “pile out” in case of nuclear reactor irradiations, or EOB (end of bombardment) in case of accelerators. Some of p’ emitters do not have deexciting y photons. However, they are not measured by their p’ emission due to its short range, but rather by the 5 11 keV y produced in the annihilation process. When a positron collides with an electron, the masses of both particles annihilate and is transformed to two photons of 5 11 keV each. Since each /?+-emitter,emits the 51 1 keV y line, 51 1 keV y line can be used only for quantitative measurement, but not for identification. Thus, in many cases, NAA

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with IRNs which are only p' or /?-emitters are done only after chemical separation of the element. If there are only a few (2-3) non-y-emitter IRNs in the sample with quite different half-lives, their separate activities can be measured even without chemical separation by measuring the activities at different times (measuring the decay curve), and extracting the various activities from the time dependence of the measured activity. The y photons emitted are due to the deexcitation of the nuclide produced in the p process. However, this deexcitation has not to be always by only one photon. Moreover, not all /3 decays lead to the same level of excitation. These two facts lead to the possibilities that one radionuclide can have more than one kind (energy) of photons, and that the number of photons should not be equal to the number of nuclides which as been decayed (disintegrated). The number of photons of specific energy emitted per 100 disintegrated nuclides is called the intensity of that y line, in percent. Let us examine different cases of y lines intensity in four examples of radionuclides.

1. 28A1decays completely to a 1.78 MeV excited state of %i. This level decays to the ground state by only one photon. Thus, it means that 28A1has only one y line of 1.78 MeV with intensity of 100%. This fact can be drawn in the following scheme (called decay scheme) of decreasing energies;

energies 4.64

..-I--

p -= 2.86 MeV 1-78..--.-I..I...... .....-

28

Si* (1.78 MeV ) y (1.78MeV)

28

Si

2. 24Nadecays almost completely (> 99.8%) to one level of 24Mg. However, this level decays to the ground state only by two successive photons. The first photon of 2.75 MeV is followed by a second photon of 1.39 MeV. This decay scheme explains why 24Nahas two y lines of 2.75 MeV and 1.39 MeV, each with 100% intensity;

3. 27Mg decays to two different excited levels, resulting in two p- with different energies 1.59 MeV (31%) and 1.75 MeV (69%). The higher excited level (1.01 MeV) decays only 98% directly to the ground state while 2%

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(0.31 . 0.02 = 0.006 = 0.6% from the total nuclides disintegrated) decays to the lower excited state. The lower excited level decays completely to the ground state by one photon. This decay scheme explains why 27Mghas three y lines of 1.01 MeV (31%), 0.84 MeV (69%), 0.18 MeV (0.6%);

4. 38Cldecays directly to three different states of 38Ar, both to two excited states of 38Ar and to the ground state of 38Ar. This is the reason for 38Cl having thee different energies p- particles (remember, it is only maximal energy, since each p- leads to spectrum of 0-energies, up to Emaximum), 4.81 MeV (53%), 2.77 MeV (9%) and 1.77 MeV (38%). The higher excited state (3.77 MeV) decays almost completely to the lower excited state and

0.03 % )

E (MeV)

260

ZEEV €3. ALFASSI

only very little (0.06% of the 38%) decays directly to the ground state. Consequently, 38Clhas three y lines 3.77 MeV (0.03%), 2.10 MeV (38%) and 1.60 MeV (38% + 53% = 91%). This data together with more physical data on the nuclides can be found in books collecting all the decay schemes of the nuclides, however, for application of radionuclides it is sufficient to use collection of all the y lines of the radionuclides without the detailed decay scheme [8,9]. These collections are for all known radionuclides, however, for the most cases of activation analysis it is better to use a smaller collection of radionuclides, only of those potentially formed by the specific kind of activation [7,10].

7.2.3 Kinetics of decuy of radioactive riuclides The decay of radioactive nuclides is a statistical process. Thus, the number of atoms decaying per unit time (rate of decay) is proportional to the number of atoms of that specific radionuclide present in our sample. If the number of nuclei (atoms) of a specific radionuclide at time t is N*, their rate of disappearance (decay or disintegration) is given by the equation

--dN' -AN* dt

The constant A, which is different for each radionuclide, is called the decay constant of the radionuclide. Integration of Eqn. (7.1) lead to the decay equation of radionuclides - the equation describing the time dependence of the number of atoms of the specific radionuclide: N * ( t )= N p ' (7.2) where N,' is the number of atoms at time chosen as t = 0. Eqn. (7.2), which is the same as the integrated equation of any first-order chemical process, indicates the existence of constant life for the half of the atoms, which is called the half-life. It means that independent of the value of N;, it takes the same time (for a specific radionuclide) for disintegration of half of the atoms, leaving only AZ/2 atoms of that radionuclide. Substituting N*(t) by N;/2 in Eqn. (7.2) leads to the correlation between the half-life (tllz) and the decay constant (A)

N*(t)= N,"/2 ==$

= ln2/A

or

t , / 2= 0.693/A

Eqn. (7.2), the decay equation, can be written also with tIl2 instead of A:

(7.3)

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

26 1

The advantage of Eqn. (7.4) on (7.2), is that it gives easier “feeling” for the extent of the decay. Thus for example, if the half-life is t1,2 = 2.5 days, we know that after 2.5 days, only half of the original atoms remain, and after 5 days, the number of original atoms reduces to one-fourth. Eqn. (7.2) was easier to use when people were using a table of exponents. Nowadays, with computers, there is no difference which equation is used. In tables of data, only tIl2 - the half-life is given. In order to use Eqn. (7.2), A should be calculated from Eqn. (7.3). Data on half lives and y line intensities of all nuclides can be found in several sources. The most common source to non-experts is the CRC Haildbook of Chentistiy and Physics.

7.2.4 Kinetics offorniatiori of radioactive nuclides by irradiation When a thin target is bombarded with a beam of projectiles, the rate of nuclear transformations (number of produced nuclides per unit time) is proportional to the beam intensity (I- number of incident particles per unit time), proportional to the target nuclei density (n - number of target nuclei per unit volume) and the thickness of the target (dz). The proportionality to the thickness of the target is limited to sufficiently small dx, such that both the intensity of the beam and the energy of the projecticles remain practically unchanged. The proportionality constant is assigned u and is called the reaction cross-section. This name comes from a simple model, when assuming that each geometrical collision leads to a nuclear reaction. In this case, u is the geometrical cross-section of the collision pair - ~ ( r+,r2)2. dN* -= u .I . n - dx dt

(7.5)

Since the nucleus radius is few times fm cm), the cross sections are in the order of 10-24cm2. This is the reason why it has been accustomed to express crosssections in units of barns: 1 barn = 10-24cm2. Eqn. (7.5) assumes that the beam cross-section is smaller than the target size facing the beam, and consequently each projectile particle is transversing through the target. In the case where the target size is smaller than the beam cross-section, the beam intensity, I, should be replaced by the product 4 . A, where 4 is the beam flux (number of projectile particles per unit area and per unit time), and A is the area of the target facing the beam. dN* -=u-q5-A.n dt

edx

A dz is the volume of the target and consequently 11 . A dz is the total number of target atoms - N:

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262

Since we are speaking of areaction of a specific nuclide, N is the total number of the atoms of that nuclide. Eqn. (7.6) is used mainly for irradiation in a nuclear reactor, where the target is located inside a uniform flux of neutrons. Eqn. (7.5) is used in cases of a narrow beam from charged particle accelerators. In the case of prompt activation analysis, the only important number is the number of nuclei which were transformed by the nuclear reaction. Hence, the number of nuclei transformed is given by the integration of Eqns. (7.5) or (7.6) over the irradiation time. However, for delayed activation analysis, the important factor is the number of nuclei decaying during the measurement period, which is delayed and done sometimes a long time after the end of the irradiation. In calculating the number of nuclei of the newly formed radionuclide at the end of the irradiation, it will be wrong to integrate those equations. The radionuclides are not only formed during the irradiation, but they are also decaying. Combining Eqns. (7.6) and (7.1) leads to the real equation for the rate of the change of the number of radioactive nuclei during the irradiation:

dN' -dt

- U - 4 . N - A . N*

(7.7)

Integration of Eqn. (7.7) with the initial condition N,* 10, leads to the equation for the number of radioactive nuclei at the end of irradiation for time t,:

(7-8) ti is the irradiation time. EOI stands for end of irradiation. For cases where Ati << l(ti << flI2), the term in the brackets is approximately equal to Ati(e-A'i 1 - Ati for Ati << 1) and Nso, = a $ N t i , i.e. the rate of formation multiplied by the irradiation time, since the decay is negligible. For irradiation-time, long relative to the half-life of decay (ti >> t , / 2 ==+Ati >> I), the term in the brackets is equal to 1, which is its maximal value. This is the maximum number of nuclei which can be formed. Longer irradiation does not increase the number of radioactive nuclei, since the rate of formation by the nuclear reaction is equal to the rate of disappearance by radioactive decay. This maximal number of formed radionuclei is named the saturation value or the number at saturation - NZal:

-

The saturation value can be increased by increasing the flux of the bombarding particles or the mass of the target, but not by longer irradiation. Usually we speak of the mass of specific elements in the target and not of the number of atoms of this element, so it is desirable to transform N - the number of atoms, to nz - the mass of specific elements in the activated samples. The number of

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

263

atoms in n i grams of element with atomic weight M is N = . N,, where N , is the Avogadro number. In contrast to usual chemical measurements, where all natural isotopes of the same element react almost in the same way, it is not true for nuclear activation. For example, let us look at chlorine. Chlorine has two stable natural isotopes: 3sCl with 75.77% abundance and 37Cl with 24.23% abundance. Irradiation with thermal neutrons leads to a ( n ,y) reaction forming the two radionuclides 36Cl and 38Cl. 36Cl cannot be used as IRN for activation analysis due to the absence of y radiation (36Clis a pure ,8 emitter) and its long half-life. (IRN-Indicator Radionuclide is a radionuclide used for identification and quantification of an element in activation analysis). Thus, not the total number of chlorine atoms should be used in Eqns. (7.6-9), but rather the number of 37Cl atoms alone. This is done by multiplying N with the abundance of the parent of the IRN (0.2423 for 37Cl). Using .f for the abundance, Eqn. (7.9) gets the form: (7.10) The number of the nuclei of the IRN at the end of irradiation is given from Eqn. (7.8) as N ; =~ N;, ~ (1 - e-”“) (7.1 1) The measurement of the 7 activity (the rate that the y rays are emitted by the activated sample) is done not immediately after the end of irradiation, but rather after some time, which is called the decay time - t d . The decay time (called also “cooling” time) can be quite short, only the time required to transfer the sample from the irradiation port to the measurement station, or a long time. Long decay times are used when we are interested in IRNs with medium to long half-lives (they are called also medium to long-lived IRNs). The high activity of the short-lived radionuclides at short times after irradiation obscures the activity of the longer-lived radionuclides. Consequently, in order to measure medium and long-lived IRNs, their measurement is delayed. This is the reason why the decay time is called also “cooling time.” For measurement of the concentrations of many elements, the y activity is measured several times (at least three) after different decay times. Since the number of atoms of the IRN at the beginning of the decay time is N, the number at the end of the decay period, t d , is given by the equation (according to Eqn. (7.2)): (7.12) N,* = N& = N;oI . eXtd The subscript EOD stands for end of decay, and the subscript SOC means start of counting. The measurement of the y activity is called counting, since it is done by the counting of the pulses formed in the y-ray detection systems, due to the interaction of the y-photons with the detector.

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264

The counting is done during time - t, called the counting time. The number of atoms of the radionuclides still undecayed at the end of the counting time N& is given by (according to Eqn. (7.2)

N ; ~ ,= N , . ~~ X ~L= N*sat (1

- e-'ti)

. e-Xtde-'tc

(7.13)

The number of atoms decaying in the counting period (AN,) is given by the difference in the number of atoms at the beginning of the counting period N& and the number of atoms at the end of this period (N&)

A N , = N <-~~i~ = N sat * (1 - e-"i) . e-Xld (1 - e-"9

(7.14)

The actual number of events of decay recorded by the detection system is smaller than A N , due to three different factors:

1. The intensity of the specific y line (number of photons emitted per 100 atoms decayed) is in many cases smaller than 100%; 2. Not every y photon emitted reaches the detector. The photons are emitted isotropically and only those in the direction of the detector will be recorded by the detection system;

3. some of the photons reaching the detector will not interact with it. Others will lose only part of their energy in tlie detector, and consequently will not be recognized as originated from the specific IRN. The last two factors are united together in a factor called the geometric efficiency of the detector - 6. This factor depends on the detector, the energy of the y line and the distance of the sample from the detection. For large samples, E depends also on the sample size. The factor E is determined experimentally by counting the activity of calibrated standards (which can be bought commercially), for which the rate of disintegration is accurately known. Thus, the number of counts recorded by the detection system, C, is given by the equation

c=c

I, . A N , = c I, - N ; ~(1 - e-'k)

. e-"d (I

- e-Ak)

(7.15)

where I, is the intensity of the specific y line. Thus, for activation in a uniform flux the number of measured counts is given by the equation

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

265

For a beam smaller than the target, 6 is replaced by I / A where A is the area of the target. A is expressed usually by A = n~,~,,,~,~/pdx. It should be noted that here the mass is the total mass of the sample and not the mass of the determined element. pdx is the common term of nuclea physicists in expressing thickness; its units are g.cm-2. Eqn. (7.15) shows that the number of counts recorded is proportional to the mass of the element responsible for the specific y lines, and this is the basis of the activation method.

7.2.5 The chort of the nuclides Similar to the periodic table of elements which gives all known elements with their main properties in a systematic way, the known nuclides are described in the chart of the nuclides (suggested by E. Segre). Figure 7-1 gives a small part of the chart of the Nuclides [ 13. The chart is constructed such that the nuclides in the same row have the same Z (i.e. they are the isotopes of the same element). The nuclides in the same column have the same number of neutrons. The number at the left of each row is the Z of all nuclides in that row. The number at the bottom of each column is the number of neutrons in the nuclides in that column. The darkened squares represent the natural nuclides (mostly stable ones). It can be seen from the chart that the stable isotopes are forming approximately an ascending diagonal. This line of the natural nuclides is called the stability line. Nuclides which are above this line (left to this line) have an excess of neutrons, and decay by p- process. Nuclides below the stability line (right to the stability line) have an excess of protons, and decay by either p' or E.C. (written as E ) processes or by both. Some of the nuclides inside the stability line are unstable (having an odd number of either neutrons or protons and mainly odd numbers of both). These nuclides decay sometimes by both p- and p+ processes. All p processes occur along the descending diagonal. Some of the squares are divided

into two (or in some cases even into three). These are cases where the excited states of the nuclides are relatively long-lived. In most cases excited nuclides decay extremely rapidly (in less than s), however, some excited states live longer. Usually, if the half-life of the excited states is longer than 1 ms (this is an

266

ZEEV B. ALFASSI

Figure 7-1. A part of the chart of nuclides (reproduced wilh permission from the Karlsruher Nuklidkarte - 1981) .

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

267

arbitrary criterion, some use a limit of 1 s while others use 1 p s ) the excited state is called a metastable state or an isomer of the nuclide. This situation exists both for stable and radioactive nuclides. For example, the stable nuclide 77Se(natural abundance - 7.7%) has an isomer which is written as 77n'Sewith half-life of 17.5 s. The radionuclide 82Br(4,2 = 35.34 h) has an isomer 82n1Br with half-life of 6.1 m. In the case of radionuclides, sometimes the isomer has a longer half life than the ground state itself, as for example "Br (il,2 = 17.6 m) and 80"1Br(t1,2 = 4.42 h). The isomer state in this case is longer-lived than the ground state. It is called the isomeric state since its energy is higher. Most isomeric states decay by emission of -y photons, however, there are metastable states of radioactive nuclides which decay also by ,B processes. The number given immediately under the name of the nuclide is its natural abundance (its percent in nature, taking the sum of all the isotopes of this element in nature as 100%) in case of a natural (existing in nature, but not always stable although in most cases it is stable) nuclide. Thus the table shows that there are four natural isotopes of sulfur with the concentrations of 95.02% (32S), 0.75% (33S), 4.24% (34S) and 0.02% (36S). In the case of a radionuclide this number is its half-life where s stands for seconds, m = minutes, d = days and a = years. The numbers given in the bottom of the squares are the types of decay and the energies of the emitted particles or photons. Not all the y photons are listed, only the main ones. In the case of a natural nuclide the numbers in the bottom of the squares are activation cross sections. For most of the squares there is only one number, which is the cross section, 0 , for ( T Z , ~ ) reaction with reactor neutrons. For some nuclides there are two numbers in the same line, for example in the case of 23Na. These are the (n, 7) cross-sections for the formation of the two isomers of 24Na. The reaction 23Na (~z,y)~""Nahas a cross-section of 0.40b, whereas the 23Na(n,-y)24Nareaction has a cross-section of 0.13b. Since 24"'Na is very short-lived and since it decays completely to 24Na (it is not true for all metastable states, e.g. ""'Mn decays mainly directly to "Cr and only 9% goes through the route 52mMn3 52Mn Cr. I.T. designates isomeric transition) it means that practically 24Nais formed with a cross-section of 0.40 + 0.13 = 0.53b. Some natural nuclides have two lines (see e.g. 32S and 33S) or even three lines (see e.g. 35Cl)in the bottom of the square. These are nuclides which have relatively high cross-sections also for other reactions with reactor neutrons. In this case o means u for ( n , y )reaction, whereas in the other line states the referred reaction. For the case of %, for example, cross-sections are given also for (12, a)and (12, p ) reactions.

7.3 Gamma detection systems In activation analysis we need a detection system for the y photons which will measure the total number of photons together with their energy distribution, i.e.

2G8

ZEEV B. ALFASSI

we need to have a y-ray spectrometer. These measurements are done mainly by the use of two solid-phase detectors working on different phenomena. 7.3.1 NaI(T1) - sciritillatiori detector

A scintillation radioactivity detector consists of a scintillator or phosphor, optically coupled to a photomultiplier tube. The most common scintillator for y-ray measurements is a large crystal of NaI activated with 0 . 1 4 2 % T1. The y photon reacts with the detector, ejecting electrons. These electrons produce excitation or ionization in the scintillator crystal. De-excitation of the scintillator occurs via fluorescence in about 0.2 ps by the T1’ activator (visible light). The small percentage of T1, the “activator,” is added to shift the wave-length of the emitted light by the detector to longer wave-lengths for two reasons: (1) in order to reduce the self-absorption of the emitted de-excitation light by the NaI crystal, and (2) the shift from UV light to visible light increases the sensitivity of the photomultiplier to the emitted light. The most popular size for routine y-ray spectrum measurements is a 7.5 cm diameter, 7.5 cm high cylinder. It requires approximately 30 eV of energy deposited in NaI crystal to produce one light photon. The light photon ejects an electron from the photocathode, and this electron is accelerated towards the dynodes in the photomultiplier by electric voltage. In each dynode the electron ejects more electrons (typically 3 - 4 , multiplying the electron current. Usually there are 10 dynodes and each electron ejected from the photocathode produces about lo6 electrons. It takes on the average about 10 light photons to release one photoelectron at the photocathode of the multiplier. Thus, it takes approximately 300 eV of y energy deposited in NaI to release one photoelectron. Since the number of photoelectrons is proportional to the y-energy, the number of final electrons in the pulse, the voltage of the electric pulse, is proportional to the energy of the 7 photon. The pulse voltage obtained from the photomultiplier is quite high (50-1000 mV) and only modest further amplification by a pulse amplifier is required. The electric pulses are sorted according to their energies by a multi-channel pulse height analyzer which usually sorts pulses of 0-10 volts. Each energy range is fitted to one channel. The spectrum of counts per channel is actually an energy spectrum of the 7 rays.

7.3.2 Solid-stute ionizutiori detector The simplest idea for measurement of radioactive decay is by the use of the property of the main emitted particles or photons as ionization radiation. When an ionizing radiation strikes a non-conducting or semi-conducting material it forms in it electrons and holes or cations. The quantity of electrons formed is proportional to the energy of the striking photon or charged particle. The amount of energy required to raise an electron from the valence band to the conduction band in

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

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a semiconductor is considerably smaller than the same energy in insulators, or the energy required to form an electron-ion pair in the liquid or gaseous phases. Thus, for the same amount of energy absorbed in the detector by the photon, more electrons are formed in a semiconductor material than in insulator. The larger number of electrons reduces the statistical fluctuation in the number of electrons and hence reduces the width of the radiation peak in the detector (measured as FWHM = full width at half maximum of the peak). Later, we will show comparison of 7-ray spectra taken with both kinds of detectors which will clarify these terms. The smaller FWHM leads to much higher resolution. The currents formed by the ions produced in the detector by the ionizing radiation is smaller than that formed in the scintillation detector due to the high multiplication factor in the photomultiplier. Hence, before measurement of the currents in the solid state ionization detectors they have to be amplified. In order to reduce losses in the pulse current and to reduce the rise time of the pulse, a preamplifier located as close as possible to the detector is used for initial amplification of a detector output signal. Since the capacitance of the solid state detector depends upon a high voltage bias, a charge-sensitive preamplifier should be used. The voltage pulse produced at the output of the preamplifier is proportional to collected charge and independent of detector capacitance. However, the output pulse from the preamplifier is too low for sorting by the multichannel-analyzer (MCA). Further amplification is done by the main amplifier, which also serves to shape the pulse. In order to reduce the noise due to the current leakage of the electrons in the conduction band agitated by thermal excitation, the semiconductor crystal is cooled to liquid nitrogen temperature. Of all semiconductor materials, germanium and silicon are almost exclusively used for modern y-ray spectrometry, since only for them adequate pure material can be prepared. The impurities which are not fourvalent will produce either an almost free electron in the conduction band or a “hole,” leading to increased leakage current - a noise, a current not due to ionizing radiation. Even silicon and germanium could not be prepared pure enough to be used as a y-ray detectors in the first stages. However, their impurities could be small enough to be compensated by the lithium ion drifting method developed in 1960. The method is used to effectively compensate p-type (acceptor type containing an excess of trivalent impurities) grown crystals of silicon and germanium. The small lithium ions are pulled into the crystals by electric field and high temperature. Due to its very low ionization potential, lithium acts as donor impurities compensating the excess of holes. The lithium ions, having a high mobility, are drifted under the influence of the local fields in such a way that the number of lithium donor atoms compensate everywhere in the crystal exactly the acceptors of the original materials. These lithium drifted detectors are called “extrinsic detectors” and written as Si(Li) (pronounced “silly”) and Ge(Li) (pronounced “jelly”). Lithium ions have high mobility in germanium at room

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temperature, thus Ge(Li) detectors must be kept always cooled at liquid nitrogen temperature (77 K), immediately after the desired compensation is obtained. A schematic diagram of a solid state ionization detector is given in Fig. 7-2, and the schematic diagram of the measurement system is given in Fig. 7-3. In silicon the mobility of lithium ions at room temperature is low enough so that Si(Li) detectors O

5

4 Figure 7-2. A schematic diagram of a Ge(Li) detector: (1) original Li doped region (n-type); (2) Li-drifted active region; (3) original p-type germanium; (4) charges formed by y-ray; ( 5 ) the out signal 1

Figure 7-3. A schematic diagram of HPGe, Ge(Li) or Si(Li) detection system: ( I ) HPGe, Geki) or Si(Li) crystal; (2) end cap; (3) Field Effect Transistor preamplifier: (4) metal housing: ( 5 ) VacuumIpressure release: (6)liquid nitrogen filling tube; (7) cold finger to cool the crystal: (8) gas absorbant. in ordcr to keep vacuum: (9) liquid nitrogen; (10) vacuum heat-insulation

27 1

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

can be stored temporarily at room temperature without cooling. It should be remembered that for measuring y-ray spectra, both are to be at 77°K during the measurement. In the mid 1970s, advances in germanium purification technology made available high purity germanium, with an impurity level as low as lO"atoms/ cm3, which can be used for y-ray spectrometry detection without lithium drifting. These intrinsic germanium detectors are usually called HPGe (abbreviation for high purity germanium). The outstanding feature of these detectors is that they do not have to be kept at liquid nitrogen temperature constantly. HPGe detectors must be cooled to 77°K only during the measurement, to reduce the leakage current due to thermal excitation of electrons to the conducting band. HPGe and Ge(Li) detectors are virtually identical from the practical point of view of measurement. However, due to the more convenient use of HPGe they completely replace Ge(Li) detectors in contemporary y-ray spectroscopy. Due to germanium's higher atomic number compared to silicon, the cross section (probability of interaction) for Compton scattering is about 50-60 times in Ge. As the main interaction of y rays with material above 100-200 keV is via Compton scattering, the main detector for y-ray spectrometry is the Ge detector (either Ge(Li) or HPGe). Si(Li) detectors are used for the measurement of X-rays (up to 40-60 keV). Another detector for X-rays or low energy y-rays is a planar (thin) Ge detector. Si(Li) has better discrimination for y-rays, whereas the planar Ge detector has a higher efficiency of measurement in the range 40-150 keV. The main peiformance characteristic of a nuclear detector is its resolution, expressed as the full width of the peak at half of its maximum (abbreviated FWHM). The narrower the peak is (lower FWHM), the better is the ability of the detector to separate two close peaks, i.e. better resolution. The Ge detectors feature very high resolution as compared to NaI(T1). At 1332 keV (this is one of the y-rays of 6oCo, and is used usually to characterize the FWHM of a detector) the resolution of a good HPGe detector is about 1.8 keV while the resolution of 3"x3" NaI(T1) detector is about 60 keV. Figures 7-4 and 7-5 give examples of activated sample (4) and radioisotope standards (60Co+ 137Cs)measured both with NaI(T1) - line a, and with HPGe-line b, to see the different resolutions. However, the efficiency (the fraction of y photons hitting the detectors which appear in the photopeak of the spectrum) of NaI(T1) is larger, and the efficiency of a Ge detector is given usually relative to NaI(T1). The first Ge(Li) detectors were small and their efficiency relative to NaI(Tl) was less than 10% (for 1332 keV). However, nowadays, HPGe detectors with about 100 mm diameter were cdnstructed and their relative efficiency to NaI(Tl) 3" x 3" are almost 60% at 1332 keV. The relative efficiency decreases with increasing energy due to the higher Z of Iodine. For most applications of activation analysis where there are multitude of y lines, the resolution is more important than efficiency and Ge detectors are the most used ones.

-

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channel Figure 7-4. Gamma-ray spectrum of neutron activated biological sample; (a) taken with a 3"x3" NaI(Tl) detector; (b) taken with a Ge(Li) detector

4

c,

c a 0

0

energy Figure 7-5. Gamma-ray speclruln of a mixed 137Cs + 6oCosource; (a) taken with NaI(T1) detector; (b) taken with a HPGe detector

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One way for improving the counting efficiencies for both NaI(T1) and Ge detectors is the use of well detectors. These are specially built detectors, in which a cylindrical well has been drilled in the center of the cylindrical crystal, or that the crystal has been grown to get this shape. The sample is put for counting into the well. The detector surrounds the sample from almost all directions and the geometric factor for radiation-detector interaction is almost 4n. However, for each direction the thickness of the detector is smaller than for the usual detector, decreasing the efficiency for higher energy. However, for not too high energies, the use of well counters increases the efficiency of counting.

7.3.3 The shape oj y spectrum In optical (absorption and emission) spectrometry we know that if a species has only one excited state, its spectrum will appear as a single line without any background. This is due to the yes/no quantum characteristic of the energy transfer. The energy difference between the two states are either transferred or not transferred, but it can not be transferred in parts. A photon of light in the IR, visible or UV range can be absorbed totally or not at all, but it cannot lose only part of its energy. The situation is different for high energy y-rays. Low energy y-rays interact with matter by the photoelectric process. In this process the 7 photon interacts with one electron of the material, losing all its energy to this electron. The transferred energy is higher than the binding energy of the electron, and thus the electron is ejected from the atom with kinetic energy equal to this difference (hvpho,,,,,-bindingenergy of the electron). However, since the range of high kinetic energy electrons is considerably shorter than that of 7-rays, the electrons will lose their kinetic energy inside the detector material. From the point of view of the detector, all the photon energy has been absorbed in it, resulting in the same energy absorbed for all photons (assuming that all photons are of the same energy). The situation is different for higher energy y-rays. The cross-section (probability of reaction = rate constant) for the photoelectric process decreases with increasing energy of the photon. For higher energy photons, the main interaction with matter is via Compton scattering. In germanium the photoelectric effect is the dominant process for y interaction, up to about 200 keV, whereas from 200 keV and up, the Compton scattering becomes more important. In the Compton scattering the photon interacts with what might be called a free electron (mainly electrons from outer shells, whereas a photoelectric process occurs with inner shell electrons, mainly from the most inner one, the K shell). The photon transmits only part of its energy. The enegy the electron receives is more than sufficient to eject it from the atom and it moves with the excess energy as kinetic energy, losing it by collisions at very short distances. The photon retains part of its energy by changing its frequency, since E = hv, and scatters in a different direction. In the Compton process the energy of the original photon is shared between two particles (the

274

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ejected electron and the scattered photon), and consequently there is a continuous distribution of energies of the scattered photon. In order to conserve both energy and momentum, the y photon cannot lose all its energy (in the photoelectric effect the momentum conservation is compensated by the recoiling of the atom from which the electron was ejected, while in Compton scattering it is an interaction with a “free” electron). The maximum energy that the photon can lose is given by the equation: E,/<1+ moc2/2E,) (7.16) where E, is the energy of the initial photon and in, is the electron rest mass. mo2 is the rest mass of an electron in energy units and it is equal to 0.5 11 MeV. The main difference between the photoelectric and Compton processes with respect to their responses in the detector material is that the electrons (in both processes) lose all their energy in the detector, due to their short range, while the scattered photon might escape from the crystal without further interaction. In y spectrometry we are measuring the full-energy peak, called the photopeak, as this energy will be the only one to appear if the only interaction process is the photoelectric absorption. The photopeak results from the gamma photons losing all their energy by a photoelectric absorption, or by a Compton scattering followed by photoelectric absorption of the scattered lower energy y photon in the detector (the photoelectric absorption can be after one scattering or several scattering, all of them within the detector crystal). However, the scattered y photon from the Compton process might escape from the detector crystal, leaving in the detector less energy than the full energy peak. This escape of the scattered photon, not only reduces the photopeak, but its main disadvantage is the formation of a background for lower energy peaks, since part of the energy (those given to the electron) is absorbed in the detector. For one y energy source, the Compton scattering produces a continuum background ranging from zero up to a maximum, called the Compton edge, given by Eqn. (7.16). This results from the fact that the minimum energy that the scattered photon can have is not zero but rather given by Eqn. (7.17).

For E, >> m02,Eminimum approaches a value of m 0 2 / 2 i.e. 256 keV. Thus for high energy y the Compton edge will be separated from the photopeak by about 256 keV. For one-energy source the range between the photopeak and the Compton edge is almost free of counts. However, most real samples have many different y energies and the Compton continuum stretches from zero up to the Compton edge of the highest y energy. If the energy of the measured photon is above 1.022 MeV, a third process can be also operative in the interaction of the photon with the detector - a process called pair production. In this process the photon energy is transformed, under

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the influence of the field of a nucleus, into a matter in the form of a pair electronpositron (transformation of energy into matter-antimatter pair). Since the rest mass of each electron or positron is 0.511 MeV, the threshold of this reaction is 1.022 MeV to conserve energy. The cross-section for pair production is very low below 1.5-1.6 MeV. The excess energy is shared between the kinetic energies of the electron and the positron. Since the ranges of positrons and electrons are very short, this excess energy (E, - 1.022 MeV) will be deposited in the detector. When the positron lost all its kinetic energy, it reacts with an electron in an annihilation reaction to form two y photons of 0.5 11 MeV each (transformation of matter-antimatterinto electromagnetic energy). Each of these 5 11 keV photons can escape from the detector without any interaction, or lose part of its energy by Compton scattering, or lose all its energy in the detector (either by photoelectric absorption or by successive Compton scattering and photoelectric absorption). If one of the 51 1 keV photons deposits all its energy in the detector, while the other one totally escapes from the detector, the energy absorbed in the detector will be 511 keV less than the full energy peak. With y photons of above 1.6 MeV we usually see also this peak of E, - 5 11 keV, where E, is the photopeak energy. This peak is called single-escape peak, due to one 5 11 keV photon escaping from the detector. Another peak in the spectrum is the double-escape peak, rising from the two 5 11 keV photons escaping from the detector. The energy of this peak is E, - 1.022 MeV. When analyzing y-rays spectrum to find the radionuclides in the analyzed spectrum, every time when we find a peak with energy ETy> 1.6 MeV which we know belongs to some radionuclide, we have to remember that in the list of the peaks found there are some which do not belong to other radionuclides, but are either single- or double-escape peaks. We saw that 28A1has a y energy of 1.778 MeV. Thus, we expect to see in 28A1y-ray spectrum peaks of 1.778 MeV, 1.269 MeV (single-escape= 1.778 - 0.511) and 0.758 MeV (double-escape peak). 24Nahas two y lines of 2.75 and 1.39 MeV. We expect to have four lines in the y-ray spectrum 2.75 MeV, 2.24 MeV (single-escape), 1.73 MeV (double-escape) and 1.39 MeV. Although the 1.39 MeV is above the threshold of 1.022 MeV for pair-production, its cross-section for this process is quite low and pair-production contributes very little to the interaction of the 1.39 MeV photon with the detector. Hence in most cases we will not see the single and double escape peaks of this photon. Figure 7-6 gives the measured y-ray spectra of 24Na. The pair production and pair annihilation can occur also in the air and in the material surrounding the detector. This process will form 7 rays of 5 11 keV which will be detected by the detector, although our radionuclide does not emit 511 keV photons.. The ratio of escape peaks to the photopeak depends on the detector size (influencing the probability of the y escape) and on the energy of the 7 photon (affecting the chance that the photon will interact by pair production). In order to know if a peak is a photopeak or SE/DE (single-, double-escape), it is preferable to start the

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276

1

L

L 3

I

I

anorgv

Figure 7-6. Gamma-ray spectrum of 24Naradioisotope taken with a HPCe detector; (1) photopeak of 2754 keV; (2) single escape peak of 2243 keV (2754-51 1 keV); (3) double escape peak at 1732 tieV (2754-1022 keV); (4) photopeak at 1369 keV; (5) 51 1 keV positron annihilation

analysis of the spectrum from the highest energy peak. For each peak we can remove from the list of peaks its SE and DE peaks. An important artefact in many y-ray spectra is sum-peak, a peak which is due to the summation of two y photons. If two photons are interacting with the detector, and the time between them reaching the crystal is smaller than a time characteristic to the detector, the detector treats both of them as a single photon and the energy deposited is the sum of the energies of the two photons. The two photons can be due to different radionuclides, or to the same radionuclide which has more than one y line. The sum-peak is referred to as "pile-up" and can be reduced (relative to the photopeaks) by reducing the count rate either by removing the measured sample further from the detector or by analyzing a smaller sample. Lower rate of counting increases the average time between two photons reaching the detector, and thus decreasing the chances of pile-up. In the spectrum of only one y line there is also a peak with energy given by Eqn. (7.17). This peak is due to 180"backscattering from external sources, mainly the shielding of the detector, of the original photon. The backscattered photon reacts with the detector, giving a peak of its full energy. For a source with various y-ray energies, this backscatter peak can range from 70-256 keV, thus adding to the Compton continuum rath'er than forming a distinct peak. The Compton background is an intrinsic background due to the physical characteristics of the detector and the interaction of radiation with matter. An additional source of background is an external one, due to other radioactive sources than our sample and cosmic radiation. This external background can be minimized by

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

.

277

the use of shielding, done mainly from lead due to its high 2 and mainly its high density. Usually the detector is located inside a lead “castle.” Lead shielding configurations suitable for a germanium detector are available commercially, usually in the form of a hollow cylinder with a sliding lid or revolving door for access. In many cases where the Compton continuum interferes with the measurements of mainly small peaks, it can be reduced by the use of Compton suppression method. In this method the germanium detector is located inside an annular scintillation detector or inside a ring of scintillation detectors. Many of the scattered Compton y-rays which escape from the germanium detector (in the case of Compton continuum), are depositing some energy in the surrounding detectors, whereas those photons which lose all their energy in the germanium detector do not cause any signal in the scintillation detector/s. The system is operated in the anticoincidence mode, i.e. the multichannel analyzer (MCA) is recording only the events which occur only in the germanium detectors. If within a fixed time the pulse in the germanium detector is followed with a pulse in the scintillation detector (indicating that the germanium pulse is in the Compton continuum and not part of a photopeak) this event is discarded. The scintillation detector/s are usually NaI(Tl), although in some systems it is a crystal of bismuth germanate (Bi,Ge,O,, - abbreviated BGO) due to its higher efficiency for absorption of ?-rays (since it has a higher 2-element). However, BGO crystals are more expensive and are not used in most systems. A schematic diagram of Compton suppression system is given in Fig. 7-7.

Figure 7-7. Schematic diagram of Anti-coincidence Compton Suppression 7 spectrometer; (1) a HPGe delector with liquid nitrogen cooling; (2) NaI(TI) detectors with photomultipliers.

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7.4 Irradiation 7.4.1 Neutron sources

Three main sources of neutrons are available: 1. Research nuclear reactors (nuclear reactors used as neutron sources);

2. Ion and electron accelerators including neutron generators; 3. Radioactive sources. Research nuclear reactors have the highest neutron fluxes, but are limited concerning on-site determination and their price and availability. Consequently, nuclear reactors will be used for neutron activation analysis of very minute amounts. When site irradiation is important, neutron generators or radioactive sources are used. For trace elements determination, nearly only research nuclear reactors are used. Research nuclear reactors are usually large devices in which fissionable material, almost exclusively 23sU, is fissioned into two nuclides with simultaneous emission of neutrons which further induce fissions in a chain reaction. The fissionproduced neutrons are very energetic. The cross-section for neutron-induced fission of fissionable nuclides increases with decreasing energy of the neutrons, and in order to increase the neutron activity, moderators which slowed down the neutrons are added to the reactor. To reflect back some of the neutrons which leaked from the reactor core, reflectors are used. The fission process releases large amounts of energy, mainly due to the stopping of the recoiling two fissioned particles, and the system is cooled by a coolant (either liquid or gas). The nuclear reactors are categorized according to their fuel, moderator, coolant, reflector and configuration. Almost all research nuclear reactors (neutron sources) are heterogeneous reactors in which the fuel is in the form of rods. The fuel is enriched 23sU (natural uranium has only 0.7% of 235U- the fission material). Most research reactors had 93-99% 235U. Many of the reactors have rods which are U-A1 alloys, however, some of the newer designs (mainly those converted to 20% 23sU) are of the uranium-silicide type. Triga reactors operate with uranium-zirconium hydride fuel, which, due to their large negative temperature coefficient of reactivity, allow the operation of the reactor in pulses. In Light Water Reactor (LWR), ordinary water (H20) is used both as a moderator and as a coolant. The reflector is mainly graphite but there are also Be or H 2 0 reflected reactors. The construction is either pool type or tank-in-pool type. Due to the relative high cross-section for capturing of thermal neutrons by H atoms, the flux of neutrons in light water reactors always contain a large fraction of fast and epithermal neutrons. The available power is in the range of 10-5000 kW with neutron fluxes of 5~1014-1.5-10'8n~m-2 s-'. Many reactors are unique in their design, however, there are some commercial types which are

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more common, the American TRIGA and the Canadian Slowpoke. TRIGA reactor is a popular multi-purpose research reactor. About 50 of them are operating with power levels of 18 kW to 3 MW with fluxes of 7~10*s-3~10'7n~m-2s-'. The most common types are those of 250 kW and 1 MW. They are of the pool type, graphite reflected with uranium-zirconium hydride fuel with 235Uenrichment of 10-70%. The Slowpoke reactor is a low-power (20 kW)reactor designed specifically as a teaching aid with additional purposes of activation analysis and production of small amounts of radionuclides. The system is designed to operate remotely. It can be provided with up to five irradiation sites in the core with flux of 10'6n.m-2 s-' and five further tubes outside the reflector with half of that flux. Heavy water research reactors are tank types, have usually enriched uranium fuel, heavy water moderator and cooled, and heavy water and graphite reflected. Due to the low cross-section for thermal neutrons absorption by D and 0, they are characterized by well thermalised neutron flux (very little epithermal and fast neutron fluxes except inside the core). Due to the lower moderation power of D compared to H, the physical size of heavy water reactors is large and hence they have large available irradiation volumes. Their power is usually between 10 and 26 MW (fluxes of up to 2 x 10'8n.m-2 s-'). 7.4.2 Saniples introduction The way of introducing a sample into the neutron flux depends on the physical structure of the reactor. It is essential that the introduction of the sample will not affect the operation of the reactor. The irradiation site maybe within the reactor core or outside in the moderator/reflector region. If the reactor is an open pool type with access from above, vertical tubes or ropes can be installed to lower the samples down, inside the core or closed to the side. The samples are closed in sealed ampules in order not to be in contact with the water surrounding the core. The ampules are usually made of aluminium due to its corrosion-resistance and short-lived activation products. For short irradiations, polyethylene capsules can also be used. The manual loading of samples is neither quick nor reproducible in time, when short irradiations are performed. In order to have quick and reproducible sample introduction, mechanical systems are used. Two types of mechanical systems are used: chain driven racks and pneumatic devices. The latter one is the most common one, due to the shorter loading and unloading time, and less maintenance and failures of operation. In the pneumatic device, the sample is pushed along a tube with pressurised gas (air or nitrogen). The transfer time depends on the distance transferred. In many systems the transit time is a second or less. The pneumatic device can be automated very easily. Figure 7-8 depicts schematically the pneumatic system. In cases of closed tank reactors, the irradiation is done either by pneumatic device or with neutron beams.

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3

1

3

4

Figure 7-8. Schematic layout of a fast transfer system for INAA: (1) reactor; (2) y spectrometer; (3) valves; (4) sample changer; the valves can allow also direct transfer of rabbit from reactor to detector or indirect transfer through the sample changer for decay

7.5 Instrumental neutron activation analysis (INAA) 75.1 Techniques INNA is one of the simplest techniques for trace element analysis. Samples are inserted into polyethylene or polypropylene cans for short irradiation or into a quartz vial inserted in an aluminum can for long irradiation. PVC cans or glass vials cannot be used, since they will become very radioactive due to the formation of 38Cl or 24Na,respectively. For short irradiation times, each sample is “sent” alone to the reactor by means of a pneumatic operated tube - the rabbit system. For long irradiation times many samples can be lowered into the reactor and left there for the required time. After the samples are removed from the reactors, they are counted (means taking ?-rays spectrum) immediately or the 7-ray spectrum is taken after some “cooling” (decay) time, to reduce the high radioactivity level due to short-lived radionuclides. Before counting, the vials are washed to remove surface contaminants. In some cases the samples are counted inside the vial, while in others, the samples are taken out of the vial before the measurement, due to inteiferences from trace elements in the vial.

752 Calculation In general, the first calculation step consists ofyeak identification, i.e. to identify which peaks are photopeaks (discarding S.E.,D.E.and sum-peaks), and to identify the radioactive species which are producing them. There are tables of 7-ray energies listed in order of increasing energies for ease of identification [7-lo]. Some tables give the energies of all known radionuclides, while others give only for radionuclides which can be formed in one form of nuclear activation as, e.g. in a nuclear reactor. For better identification, the tables give the half-life of this radionuclide, its probability of formation and other y lines of the same radionuclide. Thus, if we find one y peak of some radionuclide, but not its other lineflines, it

INSTRUMENTALNEUTRON ACTIVATION ANALYSIS (INAA)

28 1

means a wrong identification. The different lines have also to be checked to be in the reported ratios, taking into account the relative intensities of the y lines and the efficiency of the detector at those energies. Nowadays, most MCAs are computer (PC)-based and have in their data a nuclide library. However, most libraries check only for the energy of the photopeak with a preset tolerance of 1-2 keV. Some commercial computer programs check also for the presence (in the right ratios) of the other lines of the same radionuclide. Most programs do not treat the identification through half-lives. It should be noted that that S.E. and D.E. energies are not listed in most libraries and this can lead to misidentifications. Figure 7-9 gives an example of y-ray spectra measured from an activated sample.

14

101.

nl

O

1

-

d

3

E lo3 l o 3 -. n

a 0 0

I

I

800

1600

channel

2400.

Figure 7-9. Gamma-ray spectrum of neutron-activated geological sample measured with a HPGe system 15 min after end of irradiation; (1) 3084 keV-49Ca; (2) 2754 keV-24Na; (3) 2573 keV-SE of 49Ca; (4) 2243 keV-Se of "Na; (5) 2113 keV-56Mn; (6) 2062 keV-DE of 49Ca; (7) 1811 keV-56Mn; (8) 1779 keV-28A1; (9) 1732 keV-DE of 24Na; (10) 1434 keV-s2V; (11) 1369 keV-24Na; (12) 1268 keV-SE of 28A1; (13) 1014 keV-27Mg; (14) 846 keV-56Mn + "Mg; (15) 757 keV-DE of 28A1; (16) 511 keV-pC annihilation;(17) 320 keV-s'Ti

The second step in the calculation is calculation of the net counts under the peak (plotted as number of counts in each channel per the channel number). Various methods exist for the evaluation of the net peak "area" (really it is the net peak counts). The simpler methods define the channels which belong to this peak and Ci, where Ci is the counts in sum all the counts in those peaks. Total = ErZL channel number i, L and u are the lower and upper channels belonging to the peak. However, these counts also contain background (Compton and external)

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which should be subtracted. The background is estimated by taking the average of counts in 3-5 channels in both sides of the peak, multiplying by the number of channels in the peak.

The value of 3 in the last equation can be increased or decreased, but it is about an optimal value. Lower numbers will increase the statistical error in the evaluation of the background, while larger numbers will increase the possibility that the following channels belongs already to other peaks. The radioactive decay is not an absolute process but rather of a statistical nature, and consequently the larger the number of counts the more accurate is the result. The standard deviation of each decay count (as long as the counting time is small compared to the half-life of the measured readionuclide) is equal to the square root of the number of counts. The number of net counts is given by

N = net counts = Total

-

Background = T - B

The standard deviation of the net counts aN and the relative standard deviation % un are given by the equations: UN =

m=

%

ON =

100 * m

/

N = 100 - m

/

N

7.5.2.1 Calibration and calculation The amount of a nuclide in a sample can be calculated from the counts of the radionuclide produced from its neutron activation by the use of Eqn. (7.15) if the physical constants: u, f,I,, q5 and A are known and the geometry factor c is measured by using calibrated radioactive standards, which can be bought commercially. These standards are counted in 47r detector, i.e. a detector which surrounds the source from all directions such that any ,f3 particle or photon interacts with detector. e depends on the distance of the source from the detector and the energy of 7 line. For a fixed distance, c is a maximum type function of the energy, the maximum being at 100-150 keV. For higher energies than the maximum, log E is approximately a linear function of the log (energy), although for more accurate interpolation, a quadratic dependence of log c on log (energy) is used. Standard sources often used for calibration are "'Am (60 keV), 131Ba (81, 302, 356 and 383 keV), 13'Cs (661 keV), 6oCo (1773 and 1332 keV), 22Na (1275 keV) and 88Y(898 and 1836 keV). These standard radioactive sources are used also for the calibration of the number of channel vs. the appropriate y energy. The standard radioactive sources are supplied with a certificate, certifying their activity at a fixed date. The activity is given either in pCi (3.7 . lo4 dps = disintegrations per

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second) or in kBq (1 kBq = lo3 Bq, 1 Bq = 1 dps 3 1 pCi = 37 kBq). The activity of the source at the measurement is calculated by the use of Eqn. (7.2). A(2) = A, . exp(-At)

where A ( t ) is the activity at the measurement time, A, is the certified activity and t is the time elapsed from the date of certification to the measurement. f,I, and X are tabulated and known very accurately. However, cr is a function of the energy of the neutrons and the real term in Eqn. (7.15) should not be g . 4 but rather the integral so” a(E)n(E)dE, where n ( E ) is the energy-flux density or the flux distribution function q3 = n(E)dE.These functions n(E) and o ( E ) are

so”

not always known and sometimes they are changed with time and with place in the reactor. In order to overcome this problem it has been used to calculate the results of neutron activation analysis by the use of Comparators. A comparator is a standard which contains known amounts of the element to be determined in the sample. If the sample (denoted by subscript s) and the comparator (denoted by subscript c) are irradiated simultaneously and counted under the same conditions (the number of counts is given by C), then Eqn. (7.15) gives: Ill,

= 171,

’

Cs D, C, D, M,

- * -’ -

where D is the decay factor D = exp(-Xtd), td being the time between the “pile out” and the beginning of the counting and M is the measurement time factor M = 1 - exp(-At,) where t, is the counting time, without connection to the c subscript for the comparator. This method of comparators is the most accurate one, and it is the best one to use if the concentration of a single (or even a few) element(s) in the sample is/are required. It should noted that the sample and the comparators should be of similar size and composition to compensate for unequal self-absorption of the measured -y photons in the sample or comparator. This problem of self absorption is almost negligible if both sample and comparators are small. The main problem in the use of comparators for each element is when INAA is used for multi-element analysis, which is the usual case. In some samples, more than 25 elements are determined simultaneously (more accurately, by three irradiations for different periods of times followed by 5-6 countings at different times after the irradiations for various periods of times) [ll-161. For these multi-element determinations, it is difficult to prepare comparators of all the elements besides that it becomes impractical due to the large space required for all the comparators. Thus for multi-element analysis, either multi-elemental comparators have to be used, or one to two element(s) comparators are also used for calculation of other elements. Some multi-elements standards (comparators) of different types (geological, biological, etc.) were prepared either by a group

284

ZEEV B. ALFASSI

of scientists or by government or international organizations. However, these standards (SRM = Standard Reference Materials) are not recommended as primary standards due to some large uncertainties in some elements and problems with homogeneity of small samples (of the SRMs) used in NAA. The use of one or two elements to be comparators for all elements has become more widely used in recent years. The advantage of using two elements or an element with two isotopes which are activated is that the use of two activated nuclides, if chosen well, can give more information on the energy distribution of the neutron flux in the reactor [17-201. The most popular method is the so-called I;, method, in which k, values were determined for most of the elements according to the equation

The subscript c and s stand for comparator and sample/element respectively. 0 t h is the cross-section for reaction with thermal neutrons. The most popular comparator is Zr,which has two active isotopes, used to minotor the thermal and epithermal flux simultaneously. The calculation of the amount of the different elements involves both the thermal and epithermal neutrons.

75.3 Nuclear inteiferances There are two main kinds of interferences in the calculation of trace element concentrations by INAA. The first one is the formation of the same radionuclide from two different elements. This is impossible if all neutrons are thermal ones, since then the only possible reaction is (n,y). However, all reactors have some higher energy neutrons, which can also induce other reactions. Examples of this are the determination of Mg in the presence of Al, Cr in the presence of Fe or A1 in the presence of Si or P. Mg can be determined only through its less abundant isotope 26Mg( 7 2 , ~ ) 27Mg. The cross-section for this reaction is rather low, making this measurement not too sensitive. It has interference since 27Mgcan be formed from non-thermal neutrons reacting with A1 in a ( 1 1 , p ) reaction: 27Al( n ,11) 27Mg. Similarly, the determination of chromium by the 'Cr (n,7) "Cr is interfered by the non-thermal neutron reaction 54Fe(n,a)'lCr. A1 is determined by 28A1,27Al (n,.y)28A1, which can be produced also by the reactions 31P (n,cr) 28A1, and 28Si ( n , p ) 28A1. In animal flesh it was found that the contribution of P to 28A1is more than ten times than the 28A1from Al, in most reactors [21-241. Later, we will show how it is possible to overcome this interference. Another kind of interference is from two radionuclides having very close y lines. Examples of this kind of interference are the 846.8 keV line of 56Mn and the 843.8 keV line of 27Mg. Nowadays, good spectrometers are able to separate these two peaks, but this was impossible 10 or 15 years ago when germanium

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

285

detectors have lower resolution and still today some detectors cannot separate them sufficiently well. If they cannot be separated by the spectrometer, there are two ways of overcoming this interference.

1. Since the half-life of 56Mn (2.56 h) is longer than that of 27Mg (9.45 m), decay of two hours will leave practically only "Mn (the activity of 27Mg will decrease by a factor of 6647 whereas 56Mn activity will be decreased by less than a factor of two). Thus, measurements of the sample after the irradiation and after decay of 2-3 h will give the activities of both 56Mn (from the delayed measurement) and 27Mg (by subtracting the corrected activity of s6Mn from the first measurement);

2. The activities of both radionuclides can be calculated using the fact that 27Mghas also another y line at 1014 keV. The ratio of the activities of the two lines can be measured in a pure sample of Mg, and thus the 843.8 keV activity of 27Mgcan be calculated from the 1014 keV activity. Subtracting this activity from the combined (843.8 + 846.8) keV peak yields the activity of 56Mn 846.8 keV peak. 75.4 A test case - INAA of trace elemerits in silicon

Silicon has three naturally stable isotopes 28Si(92.21%), 29Si (4.70%).and 30Si (3.09%). Thus only 30Si produces a radionuclide by the usual (n,y) reaction of thermal neutrons. 31Siproduced by 30Si (12, y) 31Sireaction has a 2.62 h half-life and very little y-rays, an intensity of only 0.07%(1.26 MeV). Considering the low abundance, the relatively low cross-section (0.28 b) and mainly the low intensity of the y-ray, 31Sidoes not present a large problem in instrumental analysis by y-ray spectrometry. However, due to the presence of fast and epithermal neutrons in the reactor, the main activity induced in silicon is due to 28A1,29Al, and 27Mgformed by the reactions 28Si ( n , p ) 28A1, 29Si (n, p ) 29Al and 30Si (n, a) 27Mg. As the half-lives of these radionuclides are short (2.3 m, 6.6 m, and 9.5 m, respectively) instrumental activation is possible for radionuclides with half-lives longer than 1-2 h. In any case, the activity induced in nuclides with shorter half-lives is usually not sufficient to measure the required purity for electronic grade semiconductors, due to the small amount of radioactive atoms which can be formed until reaching saturation. In most NAA studies, waiting for the decay of 31Siwas also done. As there is no background from the Si matrix after about a day or two and since the concentrations of the trace elements are low, the ways to increase the sensitivity (decrease the lower limit of detection) are to irradiate larger samples, or to increase the total number of bombarding neutrons (higher flux, longer irradiation times, or both). In the HFR reactor at Petten, Netherlands, a special irradiation facility was constructed for irradiation of silicon samples of Philips, in which silicon wafers

286

ZEEV B. ALFASSI

Table 7-1. Limit of detection of trace element in Si by Instrumental Neutron Activation Analysis (ppbW) ~

Element Na Mg A1

Ref. [26] 0.5

c1

K Ca

sc Ti V Cr Mn Fe

co Ni cu

zn Ga Ge

0.15 10 O.ooOo3 3

0.1 3000

500 5 100

~

~~

~

~

Ref. [28] and Ref. 1291

Ref. [30]

0.3

0.1

0.6

2000 0.003 1000

< 0.002 30

10 0.002 15 0.3 0.0005 0.15

< 0.56 0.015 0.0015

1 0.1 50 0.1 2000 0.5 1

0.02

0.1

4 0.1

1.6 0.00 1

3 0.1

0.1

0.2

0.3

0.05

1

As

< 0.033

Se Br Rb Sr Y Zr Nb Mo Ru Pd Ag Cd in

0.002 < 0.0024 0.01 0.15

0.02

0.001

0.02

0.1 0.0 1

0.008

4

200 0.15

500

1.5 0.006 0.00 15 0.06 0.003 0.015

Sn

0.2

< 0.18 0.004

2

0.01 0.1 0.01

0.08 0.3 0.3

0.004

Sb

Te

Ref. [27]

0.0 1

0.02 0.2

0.006

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

287

Table 7-1. (Continued) Element

Ref. I261

I

8.0 0.0005 0.06 0.00015 O.OOO9 0.006 0.01 O.ooOo3 0.00007 0.006 0.000 1 300 0.003 0.04 0.003 0.0002

cs Ba La Ce Pr Nd

Sm Eu Gd Tb

DY Ho Er Tni Yb Lu Hf Ta

Ref. [27]

Ref. [28] and Ref. [29]

0.05 10 0.01

1.5 0.0007 0.1

0.01

0.0oO4 0.002

0.05

0.006

0.001

0.00004 0.0003

0.0005

20 0.05 0.02

0.005 0.0 1 o.Ooo1

W Re 0s Ir

< 0.025

Pt

0.004

Au

< 0.0016

0.0005

Hg

O.OOO6 3 300 0.0002 0.001

o.ooo1 0.7

0.1 0.3

0.0015 0.02

TI Pb Th U

Ref. [30]

0.0003 0.0007 0.000004

0.0002 0.005 0.0008

Note: Reference [26] irradiated wafers of 15 cm diameter and 0.5 m m thickness (20.6 g ) with flux of 4 ~ 1 0 n.cm-2s-* '~ for 72-96 h and counted for 6-8 h; Ref. [27] irradiated samples of 100 mg for up to 6 hr at 5 ~ 1 0 cni-2.s-1; '~ Ref. [28,29] used 1 g Si and irradiated for 72 h at 2 . 3 ~ 1 0 ncm-2s-1; '~ Ref. [30] irradiated 5-10 g samples for 11 d at 2x lOI3 n.cm-2.s-1.

288

ZEEV B. ALFASSI

of up to 15 cm diameter can be irradiated with 4 ~ 1 0 thermal ’~ n.cin-*.s-’ and the irradiation is done for 72 to 96 h 1251. They also used large Ge(Li) detectors (100 to 150 cc) and counted for 8 to 16 h. Thus, they got the lowest detection limits. Table 7-1 summarizes some of the limits of detection given in the literature for Si matrix. For other elements, e.g. C1 and Mn,shorter irradiation was used, between 0.5 to 2.0 h. Special correction has to be done in the case of determination of Na due to the reaction %i (n,pa) 24Na,as was shown by Niese [31] by irradiating very pure Si in a reactor core with and without cadmium cover to absorb the thermal neutrons. The contribution of the 28Si(??,pa) 24Nareaction to 24Nain a silicon matrix depends on the spectrum of the neutrons in the reactor core, as very high energy is required for this reaction and the Q value exceeds that of 14 MeV neutron generator 131). Niese found [31] that the contribution of 28Si( ~ 7 , p c r )24Na is equal to 0.7 ppb. For the HRF reactor at Petten, Netherlands, the contribution is 0.55 [26] or 0.6 ppb [25]. Revel et al. [28] found this contribution of 28Si(n,pa) 24Nato be less than 0.4 ppb. Haas et al. [32] warned against the common use of wrapping the sample for irradiation in A1 foil, as they showed that there is a recoil of 24Nafrom the A1 wrapping to the sample, due to the high recoil energy from the 27Al(n, a)24Nareaction, In many other uses it is not important, but due to the very low concentrations of Na in pure Si, this contribution might be a considerable one. Several important elements cannot be determined by this method, including mainly the light elements H, Li, Be, B, C, N, 0 and F as they do not form radioisotopes or they are too short-lived. Phosphorous cannot be determined in that way due to a lack of 7 emission from the only produced 32P, however, 32P p rays can be determined destructively by the extraction of the phosphomolybdic complex in the presence of hold-back carriers of Ta and Au [33]. The chemical yield of the extraction was found to be 83.8 5 4 % and the decontamination factors were 79 to 6500 for the different elements studied. The limit of detection was found to be 3 x lo-” g. They [33] did their study for phosphorus-doped silicon and Alfassi and Yang [34] showed that for this case, there is no need for chemical separation and 10-20 d of cooling are sufficient to ensure that the only ,8 emitter is 32P(with an accuracy of 1-2%, which is much better than the reproducibility of the extraction yield). The limit of detection should be about the same, as both methods use scintillation counting for the measurement of the p activity, although Alfassi and Yang used a discriminator which reduces the detection limit by a factor of two. 75.5 Epithermal INNA In the usual instrumental neutron activation analysis (INAA), the whole reactor neutron energy spectrum is used. However, in some cases, the use of part of the neutron spectrum is preferable; these systems are characterized by large differences

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

.

.

289

in the activation cross-sections for the desired and the interfering nuclides in the various parts of the energy spectrum. The required trace elements are activated with part of the neutron spectrum while the inteifering major elements are activated more strongly with the other parts of the spectrum, and thus, it is preferable to avoid this second part. The neutron energy spectrum in a nuclear reactor is usually divided, for convenience, into three portions, and their relative abundance are dependent on the reactor structure. The usual most abundant fraction is the one of thermal neutrons, i.e. those neutrons which are in thermal equilibrium with the moderator atoms. Their most expected energy is equal to kT (where k is Boltzmann’s constant, T is the neutron temperature), which at room temperature is equal to about 0.025 eV. The neutrons with energy above those of the thermals are divided into fast neutrons, those which come directly form fission and have not been moderated at all with energy mainly above 1 MeV, and epithermal neutrons, i.e. partly moderated and having energy between tenths of eV and 1 MeV. When the whole reactor neutron energy spectrum is used for activation, the main contribution is from the thermal neutrons due to their usually higher cross-section [(n, y) reactions]. A special case where advantage can be taken of the epithermal and fast neutrons is the case where the required trace elements are activated more strongly relative to the major elements by the epithermal or fast neutrons. Most major elements in geology and biology follow the l/v cross-section rule (their activation cross-section is inversely proportional to the square root of the neutron energy) throughout the whole energy spectrum. On the other hand, many of the less abundant elements have, in addition to their thermal activation, large activation cross-section resonances in the epithermal energy region, and consequently can be activated preferentially in this region. Similarly, several of the less common elements can be activated by other neutron reactions besides the common (n, y) reaction. These (12, p ) and (n,a) reactions require higher energy than thermal (and in most cases, they did not occur also in the epithermal region and are induced only by fast neutrons). A simple example for the advantage of using neutron filters (thermal neutron absorbers) in activation analysis is seen in Fig. 7-10, which shows the -pray spectra of a sample of blood serum, activated with reactor neutrons once with a cadmium wrapping and once without any absorber (bare irradiation) [35]. In the case of activation without Cd absorber, the Comptons of the major elements Na and C1cover the peaks of bromine and iodine (bromine can be determined only after a delay of several days while the shorter-lived iodine cannot be determined instrumentallyand can be determined only after chemical separation). The activation with epithermal neutrons (Cd cover) shows clearly the peaks of Br and I. A more complicated sample (an air dust) irradiated with and without a Cd cover is shown in Fig. 7-11.

ZEEV B. ALFASSI

290

a

z4Na 1111

SO’.

, . ... . . ..

.. .. .. -.%I ?’ ,. .. (SE) .. .. .. . .. .. ...... .. .* . .............. -.:-............................. .................. .*:.. 127

.:8%‘. 5?L

aa5 *

11L5

62Br

7:a

a*

* I~SL

IOU

;*

’

500

I000

1500

ENERGY (hew

Figure 7-10. Gamma-ray spectrum of neutron-activatedblood sample taken with a Ge(Li) detector; (a) irradiated bare with reactor neutrons; (b) imdiated in a Cd shield

The counts are lower in the epitherrnal activation, but new peaks can be seen. Due to the presence of both thermal and epitherrnal neutrons, the product q5 in Eqn. (7.15) should be replace by the term dt,,- 0 t h + 4epi- I,, where q5epi and I, are defined by the equations [I, is called the resonance integral of the (n,y) reaction].

-

I, = J1MeV cr(E)d E 0.55 e~

E

q5epi = total epitherrnal flux/ J 1MeV d E / E 0.55 e V

The boundaries of 0.55 eV and 1 MeV were chosen since neutrons with energies above 1 MeV are categorized as fast neutrons rather than epitherrnal neutrons. The 0.55 eV was chosen as the low boundary of epitherrnal neutrons since it is the cut-off in neutron absorption by Cd, the main filter used to capture thermal neutrons. The epitherrnal activation properties of a nuclide can be expressed by the ratio I,/ath or by means of the absorber ratio. An absorber ratio is the ratio of the

29 1

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

.lo5

.104 01 c,

C

a 0 u .

- 102

I +ne rgy

Figure 7-1 1. Gamma-ray spectrum of neutron-activated air-dust after 10 days cooling; (a) inside a Cd sheet; (b) irradiated bare; (1) 1732 keV-DE of 24Na; (2) 1595 keV-140La; (3) 1474 keV-82Br; (4) 1369 l ~ e V - ~ ~ N(5) a ; 1691 keV-'"Sb; (6) 1099 keV-"Fe; (7) 1317 keV-82Br; (8) 1044 keV-82Br; (9) 889 keV-"Sc; (10) 828 keV-82Br; (11) 777 keV-82Br; ; (12) 690 keV-82Br, (13) 619 keV-82Br; (14) 554 keV-82Br

activities of this nuclide irradiated once bare (without a cover) and once covered by an absorber of thermal neutrons. Thus the cadmium ratio is given by

.

. l o = l + -4th uih I o depi I o

4th ah -t4epi

=

~

*

The most important fact is not only how the insertion of an absorber of the thermal neutrons influences the activity of the specific measured nuclide, but also how it influences the interfering nuclides. Thus, it would be advantageous to analyze an element by epithermal neutron activation rather than by using the whole spectrum of reactor neutrons for activation (ENAA vs. RNAA as it is most usually written, epithermal and reactor neutron activation analysis) only if its ratio of resonance integral to thermal neutron cross-section IJg,,, is larger than this ratio for the interfering elements. Several criteria were suggested to measure the advantage of ENAA over RNAA.

7.5.5.1 Advantage factor Britne arid Jirlow's Advantage Factor - Brune and Jirlow [36] suggested to use

292

ZEEV B. ALFASST

as an advantage factor for ENAA activation the ratio between the cadmium ratio of the measured nuclide and the interfering nuclide.

where & is the cadmium ratio as defined previously; the superscript 0 and i stand for the measured and the interfering nuclides, respectively. This is the most used advantage factor and several tables of this factor for many nuclides appeared in the literature for cadmium absorbers as well as for boron absorber [7,36-391. Parry’s Improvemetit Fuctor - Parry [40] pointed out that while the advantage factor describes well the increase in the signal-to-noise ratio, it does not consider the decrease of the activity of the analyzed element due to the elimination of the activation by the thermal neutrons and hence does not treat the larger error resulting from the lower counting statistics. Parry suggested that the true criteria should be the improvement in the detection sensitivity. The lower detection limit LD for a radioactivity measurement, i.e. the minimal signal which can be detected above the background at 95% confidence level is given by the equation [41]

L D = 2.33dB where B is the background activity. The minimal detected mass in activation analysis (sensitivity) is given by

M D= A/LD = A / 2 . 3 3 J B where A is the specific activity of the analyzed element under the experimental condition for the activation and detection. Since A is proportional to the activity of the analyzed element and since B is due mainly to the interfering nuclide. Parry suggested that the improvement factor ,fp is given by fp=

JEZL

Ben2 arid Ryuri’s Advurituge Factor. - Bem and Ryan [42] followed the same trend of thinking as Parry; however, they suggested that the advantage factor should describe the improvement of the relative standard deviation of the counts. If the net counts (base line corrected counts) is IV and the background is B, the advantage factor is given by

where the subscripts 1: and 0 stand as the superscripts before.

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

293

When the background is mainly due to the interfering radionuclide, Bemm and Ryan’s advantage factor is equal to that of Parry. The coirelation of Parry’s advantage factor to that of Brune and Jirlow can be seen from the equation

f ,= .fB,/& Therefore, Parry’s advantage factor is always smaller than that of Brune and Jirlow. T i m arid Ehrrnunn’s Generalized Advantage Factor - Tian and Ehmann [43] criticized Bemm and Ryan’s criterion (and consequently also Parry’s) on the grounds that in practice, in INAA, the limit on the number of counts is not due to the activity of the analyte, but rather, to problems associated with high counting rate. High count rate leads to inferior resolution and also to problems of pile up and inaccurate measurement of the counting live time. In order to overcome these problems, the samples are measured quite far from the detector or are irradiated for shorter times. When the sample is activated with only epithermal neutrons, the total activity of the sample is reduced considerably, and hence, the counting efficiency can be increased by using smaller sample-detector distances, or the total counts can be increased by using larger samples or larger iiradiation times. If the increase in counts (due to either count efficiency or size of the sample or length or irradiation) is given by G2,then the generalized Tian and Ehmann’s advantage factor is given by .fm = G

&,

JEBG

if G = 1, fTE = fp, and if G = then fTE = .fB,. The last case is the practical one, since in both the thermal activation and in the epithermal activation the counting efficiency is usually chosen to obtain the maximum total counting rate allowable by the dead time correction device. This generalized approach by Tian and Elimann [43] gives a more firm basis for the widely used definition of Brune and Jirlow.

7.5.5.2 Thermal neutron absorbers The main absorbers for the thermal neutrons are cadmium and boron due to their high cross-section for reaction with thermal neutrons. The variation of the cross-section with the energy is different for Cd and B, hence an intelligent choice of an absorber (also sometimes called a filter) will lead to optimized detection of some nuclides [a]. In some cases, one absorber is used and in others a combination of two absorbers, e.g. Cd + B [40,44,45]. In some experiments, filters which absorb the epithermal neutrons in some regions are used, allowing more selectivity for some elements [46].

294

ZEEV B. ALFASSI

Since the main absorbers are boron and cadmium, it is very important to compare them from the technical point of view. The absorber can be used as a covering sheet wrapping the sample in it, mainly in the case of cadmium from which metallic sheets are commercially available, or as a capsule built from these materials, or as a mixture with the sample, used with B,O, [47]or as a permanently lined installation inside one of the irradiation ports of the reactor. The use of absorber-lined ports has the disadvantages that (1) scattered thermal neutrons can come from angles which are not covered by the lining of the absorber, leading to lower absorber ratios, while when the absorber is used as a capsule or wrapping, it is covered from all angles, and (2) the capsule in which the sample is held during the pneumatic transfer and which usually is from polyethylene, leads to partial thermalization of the epithermal neutrons. On the other hand, the use of an absorber-built capsule or wrapping suffers from the disadvantages of the absorption reactions. Cadmium is activated to short- and long-lived nuclides and the unloading and unpacking of the sample for medium- and long-lived radionuclides measurement faces radiation safety problems due to the high radiation dose. Short-lived (tl,2 < 20-30 s) radionuclides cannot be measured at all in cadmium capsules since the short halflife prohibits the safe unpacking of the vessel and the activity of the absorber is too high to allow measurement together with the filter. While the absorption of neutrons by loB does not lead to radioactive products, the reaction loB ( n , ~ ) 7Li is very exoergic (Q = 2.792 MeV) and the samples are heated considerably. Stroube et al. [48] found that in the 20 MW reactor at the National Bureau of Standards which is now called National Institute of Standards and Technology (USA), thermal heating of the boron nitride vessel limited the length of irradiation for freeze-dried foods to 4 s and prevented completely safe irradiation of wet food. When biological samples are irradiated, this heating accelerates the thermal decomposition of organic compounds, producing high pressure in the sample container, when sealed airtight, which may explode and contaminate or even ruin the irradiation port. In other cases, elements may be volatilized and lost. In order to avoid these effects, the time or irradiation should be limited. Williamson et al. 1451 measured the temperature inside a polyethylene rabbit inserted into a Cd-lined irradiation port and found that the temperature reached an equilibrium value of 90°C in about 7 min. When a BN capsule was irradiated in the same position, the measured temperature was 120°C in about 3 min and continued rising. Ehmann et al. [49] irradiated rock samples in a boron carbide filter for 20 h, keeping the sample in heat-sealed quartz ampules. Quartz, being a poor thermal conductor, keeps the sample from being highly heated, however, this solution is good for geological samples, but will probably not suffice for biological samples. Chisela et al. [50] studied the temperature in a sintered BC capsule in an air-cooled irradiation facility and found the capsule to reach steady-state temperatures of 163' C and 194" C for 4.0 MW and 5.0 MW reactors, respectively.

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

295

When a permanent installation from powdered B4C was done with water coolant, the temperature reached not more than 50°C. The use of permanent lining of absorber has also the disadvantage of reducing the total flux of the neutrons in the reactor and of excessive use of the nuclear fuel. A possible advantage of boron over cadmium is the reuse of the same filter in subsequent irradiation. Cadmium filters cannot usually be reused, at least immediately, due to the long-lived radioactivity produced in cadmium during irradiation. The activity produced in boron filters is small and is only due to contamination in the boron. However, the use of boron filters is limited in many cases to not high doses of neutrons due to structural failure of the capsule, probably due to excessive formation of helium gas from the loB (72, a ) 7Li reaction. Cadmium filters are easily done from metallic cadmium sheets of about 1 mm thickness. Boron is a difficult material to machine and Stuart and Ryan [51] prepared boron shields by forming a mixture of boron carbide powder and paraffin wax. The mixture was heated to 70°C (paraffin melting point = 56°C) and cast into cylindrical forms. A central hole was made in the form as it solidified by using a heated rod of appropriate diameter. The hole was not done through the whole length of the cylinder, in order to obtain a cylindrical capsule with a central cavity closed at one end. The other end was closed with a top made from the same material. Parry [38] used the same method, however, instead of using boron carbide, she used B powder. It should be mentioned that the paraffin causes a small thermalization of the epithermal neutrons. The best machinable refractory boron compound is BN [52], and consequently, many of the studies with boron filters were done using BN capsules. Ehmann et al. [49] suggested not using BN due to the relatively high cross-section of 1.81 b for the I4N (n,p) I4C reaction which leads to formation of an appreciable amount of the long-lived radioactive 14C, but rather, to use boron carbide, another refractory compound of boron. Unfortunately, boron carbide is extremely hard and is not machinable, and a capsule can be made only by the hot pressing process. One of the disadvantages of boron filters is the impurities found in boron powder as discussed in detail by Bem and Ryan [42]. However, if a boron capsule is used together with a permanently installed Cd lining, the interferences due to the activities of '*Al, 56Mn, and 38Cl from the boron contaminants are significantly reduced [38]. Both cadmium and boron have high absorption cross-sections for low-energy neutrons; however, the energy dependence of the cross-sections differ considerably. Cadmium approaches a perfect sharp filter for the thermal region and has some resonance in the epithermal range, whereas boron behaves as almost aperfect l/v absorber with no s h a 7 energy cut-off. Although the cross-section for neutron capture by boron is lower than the cross-section for absorption by cadmium in the lower energy range of 0.01 to 1 eV, it can be compensated for by using thicker boron absorbers. A 0.25 cm thick boron shield is sufficient to stop practically all the thermal neutrons. The

296

ZEEV B. ALFASSI

effective cut-off energy is almost independent of the thickness of the Cd absorber while it increases considerably with the thickness of the boron absorber.

7.5.6 Fust neutr'ons INAA The reactions occurring with fast neutrons with energy usually in the MeV range should be looked upon in two ways: (1) the use of these reactions for the determination of some elements, and (2) the possible interference of these reactions in the determination of some elements by (n,7 ) reaction, due to the formation of the same nuclide as was shown earlier dealing with nuclear inteiferences. These interferences can be solved only by the use of double irradiation, one with a bare core, and one inside a Cd or B filter, and calculating the contribution of each element. The same treatment is usually done for the use of ( 1 2 , ~ ) and (n,a) reactions in the determination of some elements. The main advantage of these reactions is that they produce nuclides different from those produced by (n,y) reactions. Consequently, it may lead to a faster determination in the case of producing a short-lived nuclide rather than the longlived one produced in ( 1 2 , ~ reaction. ) In other cases, it may enable the determination of elements which cannot be measured via (n,y) reactions, since the radionuclide produced is only a ,f?emitter.

7.5.6.1 Rapid determination of iron Figure 7-12 shows the y-ray spectrum of an iron sample irradiated for a short time [53]. As can be seen, the peak of 847 keV y-rays of 56Mndue to 56Fe(n,p) 56Mn is considerably higher than the '*Fe (n,7) 59Fe 1099 keV. Hence the use of (12, p ) reaction for determination of iron has a higher sensitivity in the case of short irradiation and counting. However, 56Mn is formed also from manganese by the 55Mn( 1 2 , ~ )56Mnreaction. The concentrations of both iron and manganese can be found by double irradiation, one sample with reactor neutrons (without any filter) and one sample with epithermal neutrons (with cadmium absorber). If the specific activity (measured counts under the experimental set-up for 1 g of the element) for irradiation with reactor neutrons will be FR and MR for iron and manganese, respectively, and similarly for epithermal neutrons F , and ME, then the activity of 1 g sample containing P,% of iron and PM% of manganese will be

where CR and CEare the activities induced by reactor neutrons and epithermal neutrons, respectively. The solution of these two equations gives

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

-

297

15

Q

E

E m

c

u

b

10

9)

a a

c. C

J 0

u

5

"Fe

z X

500

1000

Energy (keV) Figure 7-12. Gamma-ray spectrum of iron irradiated with reactor neutron (irradiation 10 min, decay - 5 min and count - 6 min)

7.5.6.2 Determination of phosphorus and silicon Thermal neutron activation cannot be used for the determination of phosphorus and silicon. Radiative capture ( q y ) reaction with the only stable isotope of P leads to formation of 32Pwhich is a pure ,B emitter. In the case of silicon [stable isotopes 28Si (92.2%). 29Si (4.7%). and 30Si (3.1%)], the only radionuclide produced by the (n,y) is 31Si which is almost only a /3 emitter. Its very low intensity of 7-rays (1266 keV - 0.07%) together with the low abundance of 30Si and the low cross-section for radiative capture (0.11 b) enable only the determination of relatively large amounts of silicon. However, activation with epitherrnal neutrons leads also to the formation of 28A1via both 31P( 7 2 , ~ ) 28A1and 28Si( 7 2 , ~ ) 28A1, and of 29Al by the 29Si (n.,p) 29Al reaction. 28A1 is produced also by the 27Al ( n , ~28A1 ) reaction. This leads to a procedure for determination of Si from the activity of 29Al, which is produced only from silicon, using the activities of 28A1 from activation with a Cd filter and without a filter to determine the concentration of both aluminum and phosphorus. However, the activity of 29Alproduced from Si is almost two orders of magnitude less than the activity of 28A1produced from it. Thus, the use of 29Alwill both limit the minimal amount of silicon that can be determined and will reduce the accuracy of the measurement. Another problem associated with the measurement of 29Alis that its main y line is the 1273 keV which suffers from the interference of the single escape peak of the more abundant 28A1 at 1268 keV. When the concentration of Si is high, silicon can be determined by the 29Si

298

ZEEV B. ALFASSI

( n , p ) 29Alreaction as was done by Hancock [54] for the measurement of silicon in pottery using a Cd shield to decrease the formation of 28Al. The samples were allowed to decay for 17-20 min before counting, for further decreasing of the 28A1 activity. Ordiigh et al. [55] measured via "A1 the concentration of silicon in very small inhomogeneous lymph node samples. The concentration of P was determined spectmphotometrically by the molybdenum blue method and hence the concentration of Si and A1 can be found from the 28A1activity induced by both epithermal activation (Cd cover) and reactor neutron irradiation. Another way was suggested by Alfassi and Lavi 1561 who used the simultaneous determination of 27Mgand 28A1,each of them for all neutrons reactor activation and irradiation with only epithermal neutrons, to simultaneously measure Mg, Al, Si, and P. Each of these radionuclides can be formed by three reactions. The concentration of

the four elements Mg, Al, Si and P can be obtained from a solution of the four equations for the four measured activities (the 844 keV due to 27Mgand 1778 keV due to 28A1each without a filter and with a cadmium absorber). The specific activity (measured counts per 1 g of the element under the experimental set-up) will be assigned by three letters, the first one giving the target element, the second one the element formed, and the third one ( R or E ) will show if the activation has been done by reactor neutrons (without any absorber) or by epithermal neutrons (with a cadmium absorber). Thus, for example, SAR means the specific activity of 28A1produced from silicon by reactor neutrons. The activities measured per 1 g of sample are also represented by three letters, the first one is always C, and the second and third have the same meanings for specific activities. If the concentration of Mg, Al, Si, and P are given by fmg, fAl, fsi, and fp in weight fractions, then the four appropriate equations are CMR CME CAR CAE

+ AMR - f A [ + SMR . fsi MME . f,, + AME FA, + SME fsi AAR fA[ + SAR fsi + PAR f p AAE fA[ + SAE fsi + PAE f p

= MMR . f,,

= =

=

*

*

*

*

*

*

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

299

7.5.7 Cyclic lNAA Several elements can be measured by INAA only through the measurements of very short-lived isotopes (e.g. F which is activated only to "F with 11.0 s half-life). For several others, while the measurement through medium-lived radioisotopes is possible, the analysis can be considerably simpler and faster by the measurements of the short-lived radionuclides. For example, measurement of 77n1Se(17.4 s) instead of 75Se (120.4 d). At least 38 elements produce short-lived nuclides (half life 5 60 s) by thermal neutron bombardment. The use of short irradiation increased also the number of samples which can be measured per day and made activation analysis a more sound method economically. Due to short half-life, the number of radioactive nuclei formed in saturation is small and consequently, the counting has a large statistical ei-ror. In order to increase the number of counts, cyclic activation analysis should be used. In cyclic INAA (CINAA), a sample is irradiated for a short time, rapidly transferred to a detector for counting, and the entire process is repeated for a number of cycles [57-591; the y-ray spectrum of each cycle is recorded to yield a cumulative spectrum. Four periods of time are important in cyclic activation: the irradiation time, t,; the time between the end of irradiation and the start of counting; i.e. the decay time, due usually to the time required for transfer of the sample from the irradiation position to the counting station, t,; the counting time, t3; and the transfer time back to the irradiation position, t,. The cycle period T is given by

T = t , + t, + t , + t, The number of counts for the first cycle CI is given by the equation where (Y is the saturation number of atoms and Fi are the time factors, respectively.

(7.18)

Fl = 1 - e-"'l

F2 -- e - A b

F3 = 1 - e-xi3

is the efficiency of the detector, I is the intensity of the 7 line of interest, and X is the decay constant of the nuclide of interest (A = ln2/t1,,). In the second counting period, there is the same number of counts due to the second irradiation, plus what was left from the first irradiation E

C2 = C1+ F4 - G, = C,(l + F,) where F4 = Similarly for the nth cycle, we obtain an expression which is the sum of a geometric series.

300

ZEEV B. ALFASSI

and the total number of counts accumulated in all the n cycles is given by

(7.20)

CTcan be optimized by choosing proper times t , , t,, t,, and t4 which lead to the optimal time-factors Fl,F2,F,, and F4. The optimization can be done by plotting Eqn. (7.20) and finding the maximum. When dealing with optimization, the first question that arises is what quantity is to be optimized and what are the limitations. The answer to the limitation factors is simple, the transfer times and the total time of the analysis. We want the total time of the analysis, t , = nT, to be short enough in order to enable the measurement of a large number of samples. The important quantity to optimize is a combination between the signal and the signal-to-noise ratio. It is not only the signal-to-noise ratio which is important and should be optimized 1601, since having a small signal with a very large signal-to-noise ratio still means a large statistical error in the activation analysis determination. Spyrou and Kerr [60] suggested that the quantity to be optimized is S/@ where S is the total counts, CT,due to the measured nuclide, and R is the total counts due to the interfering nuclides - the background. Spyrou et al. [61] suggested that a more realistic quantity to optimize in order to obtain the timing parameter required is the increase in the relative error or the precision, i.e. the quantity to be optimized is s/&FB. 7.5.7.1 Replicates vs. cyclic activation Guinn [62] suggested another method similar to the cyclic activation which will use the same analysis time but will lead to a better precision of the determination than the cyclic activation analysis for short-lived isotopes; this method can be called the method of replicates. Guinn pointed to the fact that in each cycle the main counts of the measured nuclide is due to the last irradiation, since this is the way timing parameters are determined. However, this is not true for the background radiation which has a considerably longer half-life. The simple conclusion is to irradiate and measure the sample only one time using n sepamte samples of the analyte instead of making 11 cyclic measurements on one sample of this material. The method of replicates has also fewer problems of dead time and pile up which in cyclic activation increases with each cycle. Also the same correction is applied for each replicate, while a different one is applied for each cycle. The main disadvantage of the method of replicates is the large time required for the preparation of n samples for each material to be analyzed and the large number of rabbits and containers required. Guinn suggested that this method can

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

30 1

be used for n < 5 to 10 due to the tedius preparation of 72 samples for larger n. Guinn pointed out that this method will be especially suited for sample materials that are not very homogeneous where the method of replicates is averaging the inhomogeneities. P~UTY [63] used this method for the measurements of short-lived nuclides in activation analysis of geological material due to their inhomogeneous character, By this method Parry measured rhodium and silver in some geological samples and reference materials and found that the detection limit was improved using 20 replicates by a factor of 4.4 to 4.7 in accordance with the theoretical factor of n'/' or 4.5). She found that the total analysis time for 1 sample (20 replicates) was about 1 hr (about 30 min weighing and 25 min analysis time). The other solution of using larger samples (i.e. combining n. samples together) cannot be used because of problems of dead time when dealing with large samples.

(a

7.5.8 Prompt Gamma Neiuron Activutioti Aiialysis (PGNAA) Prompt 7-ray measurement in neutron activation is particularly useful for non-destructive analysis of elements which absorb neutrons but do not produce radioactive products, or produce radionuclides with either too long or too short half-live, or produce only ,&emitters. Nuclear properties and the abundance of the elements in common matrices lead to PGNAA finding its main application in the determination of light non-metals (H, C, N, Si, P, S), or trace elements with high thermal-neutrons capture cross-section (B, Cd, Gd). Most of these light non-metals do not form radionuclides on capturing thermal neutrons by their most abundant isotopes (H, C, N, S) or they form only or mainly /3 emitters (P, Si). The application of PGNAA as a routine method of elemental analysis is usually limited to the above mentioned elements, and only in a few laboratories. The PGNAA irradiation cannot be done in the reactor core or near it since the detector should be close to the sample, in order to measure relative high fractions of the emitted 7-rays. The PGNAA is done with neutron beams from the reactor with flux of about 10' ncm-2as-1 as compared to 5x10*3-2x10'4flux in the core, used for delayed activation analysis. Also the efficiency in counting the 7-rays is usually lower than INAA due to two reasons: (1) the distance sample-detector is higher in PGNAA, in order to prevent high background and damages to the detector by the radiation induced by the beam, and (2) many of the prompt 7 lines are of higher energies than the y photons of the radionuclides in INAA. The higher energies lead to lower sensitivities. For most elements, INAA has better sensitivity than PGNAA, and PGNAA is limited mostly to elements which cannot be measured by INAA. Another disadvantage of PGNAA compared to conventional neutron activation analysis is that in PGNAA only one sample can be irradiated and measured at one time. For most samples for multi-element analysis, a irradiation time of several hours is required for each sample, thus only limited throughput is available.

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ZEEV B. ALFASSI

New advances of PGNAA with reactors is the use of cold neutron beams. Cold neutrons are applied by passage of the neutrons from the reactor through a liquid hydrogen cooled target (as e.g. Bi filter, or D20).The lower energy neutrons have two advantages: (1) almost only ( n , 7)reactions with higher cross-sections, as most cross-sections are proportion to E l / * , and (2) cold neutrons are better reflected and hence can be transferred with high efficiency to large distances using a neutrons guide tube (parallel to optical fibers). The last advantage leads to considerably lower background since the sample and detector can be located quite far from the reactor. PGNAA finds most of its applications in ~ F Ivivo and in situ measurements using a radiosotope neutron source, e.g. Pu-Be.

75.9 Depth piufiling by INAA Depth profiling means the measurement of concentration of a trace element as a function of its distance from the surface of the sample. This can be done by a destructive method in which the whole sample is activated and then divided into a number of sections of varying distances from the surface, either by gradual slow dissolution or by gradual milling and grinding. (For each layer, its thickness and -y or ,L3 activity of the various elements are measured). Many samples are too important to be destroyed in the measurement. Using (n, a) or (12, p) reaction, the depth profiling can be done non-destructively. The emitted charged particle (aor p ) has a definite kinetic energy. This particle, while transversing in the material, from its place of formation inside the sample to the detector positioned in front of the sample, is losing part of its energy, depending on the distance it travelled in the sample. From the measured energy, the distance from the surface can be calculated. Thus, the energy spectra measured for this particle is related to the concentration depth profile of the element, which is responsible for this nuclear reaction. In order to enable the measurement of low concentration, the nuclear reaction should have a high cross-section of the order of at least 10 b. These high cross-sections rule out most elements and only very few elements can be determined. The method was developed in 1972 by Ziegler et al. [64] for determination of boron due to the high cross-section of the loB ( 1 2 , a) 7Li (0= 3837 b.) This cross-section is five orders of magnitude larger than that of almost any other stable nuclides for ( n , a ) reaction (except 6Li). The measured sample is placed in a vacuum chamber (pressure less than 5 ~ 1 0 torr). - ~ They needed only about 10" neutrons to detect concentrations of 100 ppm boron in a 2 cm Si wafer. The 7Li is produced in both ground state (6%) and in the 479 keV excited state (94%). The energy is distributed between the a particle and the 7Li according to their masses (1471 and 839 keV for 4Heand 7Li in the reaction leading to the excited state). The bombarding thermal neutron passes through the Si wafer (170 pm thick) with negligible attenuation, so that the reaction efficiency is independent on

INSTRUMENTAL NEUTRON ACTIVATION ANALYSIS (INAA)

303

the depth. The neutron beam was collimated to produce a spot 1 cm radius at the target chamber (neutron flux 2.3 x los nan-2.s-1). The targets were mounted at 45" angle to the neutron beam. The a particles were detected by a surface barrier detector subtending an angle of 19 msr and was especially fabricated to have low y and electrons background and to be able to measure the spectrum of the cr particles in the presence of 7Li, although this leads to worse energy resolution for the a particles (FWHM -20 keV instead of 12 to 14). The depth of the boron is determined by the energy loss of the 1471 keV a particles, using the known specific energy loss, dE/dx. An energy, E, is parallel to depth given by the equation

Ziegler et al. [64]measured the energy spectra of 10'' to 10l6 loB atoms per square centimeter and calculated a concentration resolution of 3 ppm, and depth resolution of 20 nm. Muller et al. [65] used the high flux reactor of ILL, Grenoble with collimated flux of 1 . 0 4 ~lo9 n.cm-'-s-' and 1.5 cm diameter, together with a detector with resolution of better than 16 keV for the 1471 keV peak. For a counting time of 1000 min, 10" loB atoms per square centimeter will lead to about integral counts of 110 counts. This is enough for measuring total boron concentration but not for profile determination and they estimated that for profile determination the minimum required concentration is 10l2 loB atoms per square centimeter. They measured the concentration depth profiles of boron in silicon wafers implanted with lo'*, lOI3, and l O I 4 loB atoms per square centimer; the accuracy of the depth determination is about 25 nm. It should be noted that this is for loB atoms while they are only 20% of the natural boron atoms. Kvitek et al. [66] studied the boron concentration profile in Si using both the spectra of the a particles and the 7Li particles. As can be expected from the higher dE/ds, the spatial resolution obtained with the 839 keV 7Li particle is better than the one obtained by using the 1431 4He line (by almost 30%). However, in a latter work, they explained [67] that in spite of the better resolution of the 7Li profiling, the fact that the 7Li line is located in the high background region of the measured spectra often makes the profile analysis by this method quite difficult. In order to obtain better resolution, they offered to measure the a spectra with different tilting angles. Biersack et al. [68] discussed the achievable depth resolution and the main sources of uncertainties. He pointed out four main inherent uncertainties: (1) the uncertainty in the angle 8 between the point of reaction and point of detection as the particle travel distance L, L = x/ cos 0 where 5 is the depth of the target atom; (2) the uncertainty in the energy resolution; (3) the uncertainty in energy loss due to energy loss straggling, assuming a fixed path length L; and (4) uncertainty due to multiple angular scattering. They concluded that for (72, a) reaction, depth

304

ZEEV B. ALFASSI

resolution of 5 nm is feasible, while 10 nm depth resolution is feasible for (72,p) reactions. Besides B, the only element which can be determined in this way using the natural abundance is lithium with 7.4%. 6Li, having a cross-section of 940b for the reaction 6Li ( 1 2 , a)T,with (Y energy of 2055 and T energy of 2728 keV. Helium can be determined not for natural He, but for 3He-doped substrates (stable isotope, natural abundance 0.0001%). Similarly, Na and Be can be determined with the radioactive isotopes 22Naand 7Be, respectively, due to their high cross-section. Downing et al. [69] gave a detailed description of the neutron depth profiling system and sumamrized the available data in various systems in the world. Special attention was given to minimize the contamination of the thermal neutron beams with fast neutrons and y-rays. The amount of the y-rays and fast neutrons was reduced by filtering the beam through 200 mm of sapphire single crystal and 80 mm long silicon single crystal which lead to a cadmium ratio for gold of greater than lo4 and a 7 intensity of 200 mrad/h. The neutron depth profiling (NDP) technique is quantitative and has few interferences in silicon-based materials. As a result, it is often used for calibration for other techniques. Other instrumental methods, such as SIMS, Fourier transform infrared spectrometry (FTIR), and Auger emission spectrometry (AES) have greater sensitivity than NDP in most applications and better resolution in some matrices, but each of the other techniques is subject to highly interactive chemically and electronically induced artifacts. NDP can be used to calibrate the other methods and after calibration, the better resolution of the other method can be used. Rutherford backward scattering (RBS)and NDP complement each other as they both nondestructively analyze different compositions. RBS measures concentration profiles of heavy atoms in matrices of light atoms while NDP is limited to some light elements.

References 1 Clarke, W.B., Koekebakker. M., Ban; R.D., Downing, R.D., and Fleming, R.F., Appl. Radiat. Isot., 38 (1987) 735.

2 Harvey, B.G.. Introductionlo Nuclear Physics and Chemistry.2d ed., Prentice-Hall,Englewood Cliffs, NJ, 1969. 3 Friedlander, G., Kennedy, J.W., Macias, E.S.,and Miller, J.M., Nuclear and Radiochemistry, 3rd ed, Wiley, New York, 1981. 4 Choppin, G.R. and Rydberg, J.. Nuclear Chemistry, Pegamon Press, Oxford, 1980. 5 Ehmann, W.D. and Vance, D.E.,Radiochemistry and Nuclear Methods of Analysis, Wiley, New York, 1991.

6 Verles, A. and Kiss, I., Nuclear Chemislry, Elsevier, Amsterdam, 1987.

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7 Alfassi. Z.B., in: Activation Analysis, Vol. 2, Z.B. Alfassi (Ed.). CRC Press, Boca Raton, FL, 1990, p. 35.

8 Erdtmann, G. and Soyka, W., The Gamma Rays of the Radionuclides, Verlag Chemie, Weinheim. 1979. 9 Reus. U. and Westmcier, W., Atomic Data and Nuclear Data Tables, 29 (1983) 1-192, 193406. 10 Yule, H.P., in: Nondestructive Activation Analysis, S. Amid (Ed.), Elsevier, Amsterdam, 1981, p. 113. 11 Zhuk, L.I., Kist, A.A., Mikholskaya, I.N.. Osinskaya, N.S., Tilayev. T.. Tursunbayev, S.I., and Agzamova, S.V.. J. Radioanal. Nucl. Chem., 120 (1988) 369. 12 Hancock, R.G.V., Grynpas, M.D., and Alpert, B., J. Radioanal. Nucl. Chem., 110 (1987) 283. 13 Cunningham, W.C. and Stroube, WJ., Jr., The Science of the Total Environment, 63 (1987) 29. 14 Dams, R., Robbins. J.A., Rahn, K.A., and Winchester, J.W., Anal. Chem., 42 (1970) 861. 15 Wyttenbach, A., Bajo, S., and Tobler, L., J. Radioanal. Nucl. Chem.. 114 (1987) 137.

16 Simons, A. and Landsberger, S., J. Radioanal. Nucl. Chem., 110 (1985) 555. 17 De Corte, F., Speecke, A., and Hoste, J.. J. Radioanal. Chem., 3 (1969) 205.

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21 Glascock, M.D., Tian, W.Z., and Ehmann, W.D., J. Radioanal. Nucl. Chem., 92 (1985) 379. 22 Alfassi, Z.B. and Lavi, N., Analyst, 109 (1984) 959.

23 Landsberger, S., Arendt, A., Keck, B., and Glascock, M., Trans. Am. Nucl. Soc., 56 (1988) 230. 24 Kratochvil, B., Motkosky, N., Duke, M.S.M., and Ng, D., Can. J. Chem., 65 (1987) 1047. 25 Theunissen, M.J.J., Jaspers, HJJ., Hansen, J.M.G., and Verheijke. M.L., J. Radioanal. Nucl. Chem., 113 (1987) 391.

26 Verheijke, M.L., Jaspers, H.J.J., Hansen, J.M.G., and Theunissen, MJJ., J. Radioanal. Nucl. Chem., 113 (1987) 397. 27 Lindstrom, R.M., in: Microelectronic Processing: Inorganic Materials Characterization, L.A. C a p e r (Ed.), A E S Symposium Series No. 295, American Chemical Society, Washington, DC, 1986, p. 294. 28 Revel, G., Deschamps, N., Dardenne, C., Pastol, J.L., Hania, B., and Dinh Ngugen, H., J. Radioanal. Nucl. Chem., 85 (1984) 137. 29 Revel, G., Euroanalysis VII Conference Proceedings 1987, E. Roth (Ed.), Editions Physiques, Paris, 1988, p. 167. 30 Fujingawa, K. and Kudo, K., J. Radioanal. Chem., 52 (1979) 411.

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31 Niese, S., J. Radioanal. Chem., 38 (1977) 37. 32 Haas. E.W., Hofmann. R., and Richter, F., J. Radioanal. Chem.. 69 (1982) 219. 33 Jaskolska. H. and Rowlinska, L.. J. Radioanal. Chem., 26 (1975) 31. 34 Alfassi, Z.B. and Yang, M.H.,J. Radioanal. Nucl. Chem., 132 (1989) 99. 35 Lavi, N. and Alfassi, Z.B., Analyst, 109 (1984) 361; Etzion, Z., Alfassi, Z., Lavi, N., and Yagil, R., Biol. Trace Ele. Res., 12 (1987) 411. 36 Brune, D. and Jirlow, K.. Nukleonik, 6 (1964) 242. 37 Gladney, E.S.,Perrin, D.R.. Balagna, J.P., and Warner, C.L., Anal. Chem., 52 (1980) 2128. 38 Parry, S.J., J. Radioanal. Chem., 72 (1982) 195; J. Radioanal. Nucl. Chem., 81 (1984) 143. 39 Alfassi. Z.B., J. Radioanal. Nucl. Chem., 90 (1985) 151. 40 Parry, S.J.. J. Radioanal. Chem., 59 (1980) 423. 41 Currie, L.A., Anal. Chem., 40 (1986) 587. 42 Bem, H. and Ryan, D.E., Anal. Chim. Acta, 124 (1971) 373. 43 Tian, W.Z. and Ehmann, W.D.. J. Radioanal. Nucl. Chem., 84 (1984) 89. 44 Rossitto, F., Terrani, M., and Terrani, S., Nucl. Instr. Meth., 103 (1972) 77. 45 Williamson, T.G., Penneche, PI., Hostica, B., Brenizer, J.S.. and Ngugen, T.L., J. Radioanal. Nucl. Chem., 114 (1987) 387. 46 Tokay, R.K.. Skalberg, M., and Skamemak, G.,J. Radioanal. Nucl. Chem., 96 (1985) 265. 47 Rosenberg. R.J., J. Radioanal. Chem., 62 (1981) 145. 48 Stroube, W.B., Jr., Cunningham, W.C.. and Lutz, GJ., J. Radioanal. Nucl. Chem., 112 (1987) 341. 49 Ehmann. W.D., BrUckner, J., and McKown, D.M., J. Radioanal. Chem., 57 (1980) 491.

50 Chisela, F., Gawlik, D., and Bratter, P., J. Radioanal. Nucl. Chem., 98 (1987) 133. 51 Stuart, D.C. and Ryan, D.E.,Can. J. Chem., 59 (1981) 1470. 52 Soreq, H. and Griffin, H.C., J. Radioanal. Chem., 79 (1983) 135.

53 Alfassi, Z.B. and Lavi, N., J. Radioanal. Nucl. Chem., 84 (1984) 363. 54 Hancock, R.G.V. J. Radioanal. Chem., 69 (1982) 313. 55 OrdOgh, M., Orban, E., Miskovitz, G., Appel, J., and Szabo, E., Int. J. Appl. Radiat. Isot., 25 (1974) 61. 56 Alfassi, Z.B. and Lavi, N., Analyst, 109 (1984) 959. 57 Anders, O.U., Anal. Chem., 32 (1960) 1368; 33 (1961) 1706. 58 Caldwell, R.I., Mills, W.R., Jr., Allen, L.S., Bell, P.R., and Heath, R.L., Science, 152 (1966) 457.

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61 Spyrou, N.M., Adesarnni, Ch.. Kidd, M., Stephens-Newsham, L.G., Ortaoval, A.Z., and Ozek. F., J. Radioanal. Nucl. Chem., 72 (1982) 155. 62 Guinn, V.P., Radiochem. Radioanal. Lett, 44 (1980) 133. 63 Parry. S.J., J. Radioanal. Nucl. Chem., 75 (1982) 253.

64 Ziegler, J.F.. Cole. G.W.,and Baglin, J.E.E.. J. Appl. Phys.. 43 (1972) 3809. 65 MUller, K., Henkelrnann, R., and Boroka, H., Nucl. Instr. Meth., 129 (1979) 557. 66 Kvitek, J., Hnatowicz, V., and Kotas, P., Radiochern. Radioanal. Lett., 24 (1976) 205. 67 Cerevna, J., Hnatowicz, V., Hoffmann, J., Kosina, Z., Kvitek, J., and Onheiser, P., Nucl. Instr. Meth., 188 (1981) 185. 68 Biersack, J.P., Fink. D., Henkelmann. R.. and Miiller, K.. Nucl. Inslr. Meth., 149 (1978) 93. 69 Downing, R.G., Fleming, R.F., Langland, J.K., and Vincent, D.H., Nucl. Instr. Meth., 218 (1983) 47.

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CHAPTER 8

Radiochemical neutron activation analysis

ZEEV B. ALFASSI

Deparlnieiil of Nuclear Eiigineeriiig,Ben Guriom Umiversiry of the Negev, Beer-Sheva 84102, Israel

Contents 8.1 8.2

8.3

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 312 Samples dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.2.1 Geological samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312 312 8.2.2 Metal samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 8.2.3 Biological samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Radiochemical separations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 Radiochemical separations in material sciences . . . . . . . . . . . . . . . . 317 8.3.1 RNAA of geological and environmentalsamples . . . . . . . . . . . . . . .326 8.3.2 RNAA of biological samples . . . . . . . . . . . . . . . . . . . . . . . . . 340 8.3.3

8.1 Introduction While instrumental neutron activation (INAA) is more popular than RNAA, due to its simplicity, RNAA (Radiochemical Neutron Activation Analysis) can determine much lower concentrations than INAA, since the interference from other radionuclides are reduced considerably. Whereas INAA includes only activation and activity measurement, RNAA includes also dissolution of the sample and chemical separation of the analyte solution. In some cases, the interferences can be removed by decay (“cooling”) and INAA can be used. However, in many cases the IRNs (Indicator RadioNuclides) are shorter-lived than those of the interferences, and in other cases the decay time is too long to be practical.

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The use of radiochemical separations in neutron activation analysis complements the analysis by INAA, enabling the measurement of additional elements of low concentration. Thus for example, Chatt et al. [ 11, in their measurement of trace elements in food samples, measured by INAA the elements As, Br, Ca, C1, Co, Cr, Fe, I, K, Mg, Mn, Na, Rb, Sc, Se, Sn and Zn, while by RNAA they measured the concentration of As, Au, Co, Cu, Fe, Hg, Mo, Sb, Se and Zn. Some of the elements were determined also by RNAA, although they were already determined by INAA, since higher precision is obtained for these elements by the RNAA method. Similarly, Zeisler et al. [2], in their measurement of trace elements in human liver, measured many elements by INAA, but for some elements they had to use RNAA. They measured several elements by RNAA, although only one of them, Sn, could not be measured at all by INAA in this system. For the other elements, the RNAA method increases the precision of the measurement. Yinsong et al. [3] determined trace elements in human hair and some internal organs. Br, Ca, C1, Co, Cr, Fe, Mg, Mn, Na, Rb, S, and Se were determined by INAA, whereas RNAA was used for the determination of As, Cd, Cu, Hg, and Sn. Xan and Ehmann [4] measured by RNAA the concentration of As, Cd, Cu, and Mo after at least 17 other elements were determined by INAA of the same sample. To summarize, in order to achieve the ultimate sensitivity of the NAA technique, it is frequently necessary to separate the element(s) of interest from the other radionuclides in the sample. An important question is what is the difference between RNAA and separation for the purpose of enrichment in any other method of determination of trace elements. There are three differences justifying a separate chapter for RNAA:

1. The interferences from the major elements are different for various measuring techniques. Thus, while silicon is interfering with ICP-AES measurements [6], it does not interfere with NAA measurements. Another example is the GaAs matrix. The only long-lived radionuclide formed from the matrix in GaAs activation is 74As [7]. Thus, for measuring the activities of the t r m elements’ IRNs (indicator radionuclide - the radionuclide used for the measurement of the concentration of the trace element), it is sufficient to remove only the As. This was done [6] by evaporation of AsC1,. The gallium remaining in the solution did not inhibit the measurement of the activities of the INRs, while for many other methods of trace analysis, Ga also has to be removed;

1

2. The multi-element character of NAA means that the trace elements do not have to be separated one by one; it is sufficient to separate it into several groups. The groups are designed so that in each, there is no interference between the various radionuclides [8,9]. In many cases even separation into groups is not needed, and it is sufficient to remove the major interferences. Egger and Krivan [lo] in their determination of trace elements in aluminum,

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

311

removed only the interference of 24Na(by sorption on hydrated antimony pentoxide- HAP - column) and were then able to measure 38 trace elements, all in concentrations of less than 1 ppb;

3. This difference is indicated in the name radiochemical separation. We are separating radionuclides, which has the disadvantage of radiation hazards and requires sometimes partial work in hot-cells. However, from the analytical point of view, it has two enormous advantages, not shared by any other analytical method. In usual (non-nuclear) analytical chemistry of trace elements, there are two main sources for errors: (1) contamination by reagents, vessels and the atmosphere, which leads to too high results; (2) adsorption of the trace elements on the vessels’ walls, a phenomenon characterized by very low concentration solutes, which results in too low values of measured concentration. The first source of error, i.e. contamination, can be diminished and in many cases almost eliminated by working in “clean” rooms with purified reagents, however, it requires long work, and much time and money. These errors do not exist in RNAA, or are relatively easily eliminated. In RNAA the probed sample is irradiated without the addition of any reagent. The dissoluting and separating reagents are added only after the end of the irradiation. Any contamination in the vessels, reagents or the surrounding atmosphere is not radioactive, and consequently carries no influence on the measurement results. However, it should be noted that the statement that RNAA is free from problems of “blanks” refers primarily to blanks due to reagents. An almost blank-free system occurs only when the sample can be irradiated as a single integral place, with subsequent etching of the surface to eliminate surface contamination, which is not characteristic of the total material. It should be noted that the surface etching is done after the irradiation. This procedure is usually done for RNAA of ultrapure metals and some composite materials. Sometimes it can also be done for geological materials [ 111, but in this case there are problems of homogeneity. When the sample is dissolved (after irradiation), carriers of all the elements measured are also added to the dissolution vessel. This considerably increases the concentration of these elements, and although it does not reduce the absolute amount absorbed to the suiface of the vessels and tubes, it renders their fraction negligible. The absolute amount absorbed is about constant, due to saturation of the absorption sites. Increasing the total amount makes the fraction absorbed negligible. The addition of carriers not only considerably reduces the fraction lost by adsoiption (or volatilization during dissolution), but also causes the trace elements to behave as expected from larger concentration. Many elements behave differently in the microscale (concentration of pdml or less) than in the macrolevel (higher concentration). Elements

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ZEEV B. ALFASSI

that at macroscale are concentrated in one phase in liquid-liquid extraction, may at microlevel be split between the two fractions. Addition of carrier materials eliminates problems which exist in other methods of trace analysis. The carriers added should not be only of the probed elements, but also those of the interfering elements (referred to as the holdback carriers), to maximize the separation of the measured elements from the interfering elements. 8.2 Samples dissolution

Samples that were irradiated for a long time might have suffered considerable radiolysis, leading to formation of gaseous products. It is advisable, for this reason, to cool the irradiation container before opening it. Most long-irradiations are done in quartz ampules (quartz is chosen, because it contains very little activable elements, whereas glass contains sodium, and hence, the glass and metal ampules will be too radioactive and will be radiation hazards; polyethylene becomes brittle after long irradiation). It is usual to cool the ampule in liquid nitrogen before opening. The irradiated sample together with carriers are transferred to the decomposition vessel. It is important to remember that we are dissolving, and later separating, radioactive materials. In many cases we will prefer faster solubilization or separation methods in order to decrease the decay of the radionuclides. Due to the high radiation hazards in some samples, work should be done behind a lead shield, with as little bodily exposure to radiation as possible. The sample decompositions depend on the type of sample analysed. The various methods for sample dissolutions were recently reviewed by Bajo [ 121, and only a relatively short description will be given here.

8.2.1 Geological samples Geological samples are decomposed either by fusion with Na202+ NaOH in

Zr or Ni crucibles, or by dissolution with strong acids. It was usually preferable to decompose silicate samples by fusion, due to its relative short time. However, dissolution with mineral acid can be considerably speeded up by the use of a microwave oven [13]. Acid mixtures (HNO,, HClO,, HF) sometimes with additional small amounts of H202are also used to mineralize geological samples. The dissolution is usually done either in a platinum crucible or in pressurised decomposition vessels. The use of microwave oven [13] shortens the dissolution to 5-10 min.

8.2.2 Metal samples Metals are usually dissolved in a mixture of acids. Aquaregia can dissolve almost all metals. However, in many studies they can be dissolved also in HF + HNO, mixtures [9, lo]. HF + HNO, mixture with a higher proportion of HF is used also for surface etching.

RADIOCHEMICALNEUTRON ACTIVATION ANALYSIS

313

8.2.3 Biological samples Biological (organic) samples are digested by several methods which can be divided into two groups: dry and wet ashings. Dry ashing in furnaces between 400 and 700°C is used mainly due to the simplicity of the method, and its suitability for large samples and large number of samples. The samples are added to porcelain or quartz crucibles, the crucibles are inserted into a furnace and the temperature is gradually raised until reaching 40&700"C. The ash is then dissolved with a few ml of dilute acid, mainly HC1. However, it should be remembered that considerable losses of several elements may occur during this ashing procedure. The losses can be due either to volatilization or retention on the crucible surface, even after dissolving the ash with more concentrated acid [ 141. In many cases it was found that the addition of some salts considerably reduced those losses [ 151. The losses due to volatilization can be eliminated by dry ashing in closed system, and collecting the volatile compounds in a cold finger [161. Anotherdry way to digest the biological materials in a closed system is by burning them in oxygen by the Schoniger technique (oxygen flask method) [17-191. This method is especially suitable to small samples. However, this method is not suitable for As, Pb and Bi, which react with the platinum holder [20] or even with the quartz holder [21]. Desai et al. [21] found that Mn, Cu, Zn, Sb, Na, and Hg were also found on the holder and are not completely recovered. The burning in a static system is restricted to small amounts of sample, due to the small amounts of oxygen in the vessel, while dynamic burning (oxygen or air flow) allows combusion of large quantities of sample, and consequently, a larger ratio of trace elements, to be determined, to the surface of the apparatus. The combusion can also be done by a commerciallyavailable apparatus called Trace-0-Mat (Anton Paar, Graz, Austria and H. Kurner, Neuberg, Germany). The decomposition process takes about 50-60 min. This system was found to give recoveries 2 95% for all elements studied in biological standard reference materials [22], human serum [23] and also in oil and fats [24]. This method suffers from the disadvantage that only one sample can be digested each time, whereas in dry ashing in oven, several samples can be processed simultaneously. In the second generation of this apparatus, Trace0-Mat 11, a higher sample throughput is available and it is possible to process 10 samples per hour [25]. More popular than the dry ashing in the case of RNAA, is the wet ashing. It was common to rapidly dissolve dry biological material by boiling nitric acid with HClO, or H,SO, or both of them. H2S0, is sometimes added in order to reduce the danger of explosion due to HClO,. Care should be taken not to allow the boiled mixture to evaporate to dryness, otherwise it can explode, although several papers note that the solution was heated to dryness [26]. However, most laboratories avoid the use of HClO,, and used HNO, or HNO,/H,SO, mixtures. If the obtained solution is not clear, H,02 is added dropwise to the hot solution until obtaining a clear

3 14

ZEEV B. ALFASSI

solution [27,28]. The use of microwave oven for heating the acid mixtures with the sample leads to rapid dissolution in 5-10 min [29]. Bajo et al. [30] described a simple apparatus for wet ashing of all organic materials by a H,SO,/HNO, mixture. The digested material is kept with H,SO, at a high temperature of 300" C (with HNO, it cannot be kept at this high temperature, due to its lower boiling temperature), and small amounts of HN03 is constantly self-feeded to the reaction mixture. The dissolution is done overnight automatically and no supervision is required. Hot concentrated sulfuric acid destroys most organic compounds primarily by charring (dehydration), however in the presence of H,O, or HNO, a clear colorless solution is obtained. Nitrates are more soluble than sulfates, and the use of H2S0, has the disadvantage of possible formation of insoluble sulphates. This is especially true for Ca-rich samples. For elements which form insoluble sulfates it is preferable to use a HNO, + H,O, mixture. Heating in open systems can lead to vaporization of more volatile elements, such as Hg and 0 s . The use of refluxing in a Bethge apparatus is recommended [31]. Other methods of wet ashings in closed systems involve the use of teflon-covered stainless steel pressure bombs [31-341 or closed quartz tube [34]. In some cases, the vaporization of some elements are even used to obtain partial separation already in the digestion step. In many laboratories the usual technique is that 100-500 mg of sample are heated with acid mixtures overnight at about 140-170°C either in a high pressure teflon decomposition vessel (bomb), or in a refluxing system. Most of the wet dissolutions of biological materials were done in acidic media. Shimizu et al. [35] suggested the use of tetramethylammonium hydroxide for solubilization of biological material. This compound is a strong base equal to NaOH or KOH and is available with very high purity (ppb levels of contaminants). In order to digest many samples simultaneously, it is preferable to have a large metal block in which all the samples are inserted. The metal block can be heated and controlled continuously. Thus ashing time and temperature are controlled. 8.2.3.1 Determination of chemical yield A common pitfall in radiochemical separation is a yield of less than 100%. The best case of radiocheniial separation is a method which was shown to have reproducible yield close to 100%. Usually this recovery yields measurements are done by radiotracers. However, even if radiotracer experiments show that the recovery is near loo%,it is recommended to measure it also in the unknown sample. If the recovery yield is shown to be close to loo%, it does not have to be measured in each experiment. However, in cases in which the yield is lower, it will never be very precisely reproducible, and the recovery yield should be measured in each analysis. One of three methods are usually used to measure the recovery yields. The first two are the same in principle. Before starting the digestion procedure, carriers are added to the irradiated material. These are already macroamounts and

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

315

can be determined more easily and more accurately. As the carriers behave in the same way as the probed element in the sample, the recovery yields should be the same. Thus, the recovery yield of the carrier element is taken as the recovery yield of the determined element in the sample. In the first method, the carrier, which is not in a trace amount, can be separated and determined by classical analytical methods, such as gravimetry, calorimetry or some titration methods. In the second method, an advantage is taken of the laboratory activation analysis techniques. After measuring the radioactivity induced by the original activation of the sample, the separated fraction carriers, together with the traces from the sample, are reactivated by irradiation with neutrons, and the amount of the carriers is determined by INAA. The amount originated from the sample is negligible compared to the added carrier, and the INAA of the separated fraction yields the amount of the separated carrier. As the amount of the added carrier is known, the ratio gives the recovery yield. A more direct method is the use of radiotracers, added together with the tracers to the sample before the digestion. This requires the use of radioisotopes which are not the same as those produced by the neutron activation, and which can be measured simultaneously by y-ray spectrometry, due to different y-ray energies. Thus, for example, Co is determined in NAA via6’Co, which has y-rays of 1 173 and 1332 keV. Its recovery can be measured with the use of 57C0(EY= 122 or 136 keV) or of 5 6 C ~ (E, = 847 or 1238 or 2598 keV). Zr is determined either by 95Zror 97Zr,while 88Zr can be used as a radiotracer for determination of the recovery yield. Since the radiotracers are not those produced by neutron activation, they have to be cyclotron produced, or sometimesproduced by fission as, e.g. I3’I for radiotracing of I, or by other methods as, e.g. I3lI produced from neutron-irradiationof Te. It is preferable to use cyclotron produced 1231and not I3*I,since it can be formed in the neutron-activated sample either from uranium by fission or from Te. Thus, 56C0is formed by cyclotrons via the 56Fe ( p , 9 t ) 56C0reaction or through the 55Mn(3He, 2n) 56C0reaction. **Zr is produced via the ”Y (p,2n) 88Zrreaction. However, in some cases it is possible to use even neutron-producedradionuclides as radiotracers in RNAA. This is the case when neutron activation of the naturally occurring isotopes of one element is leading to the formation of at least two radioisotopes, with considerably differenthalf-lives. For example, the irradiation of Zr leads to the formation of 95Zr( tlIZ= 64 d, E, = 757 keV) and 97Zr(16.8 h, 508 or 1148 keV). Irradiating our studied sample for a long time of several days and letting the sample to decay (“cool”) for two weeks leads to complete decay of 97Zrand measurements of Zr content with ”Zr as IRN (only of the 97Zractivity remains). After these two weeks of “cooling,” 97Zrcan be added to the sample together with the carriers, and the digestion-separation will be started. y7Zrwill be the radiotracer used to measure the recovery yield of Zr in the digestion-separation process. There are several elements where this scheme of using only neutron-produced radionuclides can be done. Other examples are Mo, Ru,Cd, In, Sn,Sb, etc.

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316

8.3 Radiochemical separations Radiochemical separations can be grouped according to several criteria, for example, methods of separation, areas of application, or complete separations, versus separation of a few elements, or according to the separated elements. From a practical point of view, it seems that the best choice of classification is according to the areas of application. It is true that many separation schemes can be used in various fields, and it will be very instructive to look on all the schemes, irrespective of the reader’s own interest. However, it seems less practical. The main methods for separation are:

1. Separation on columns; 2. Solvent extraction; 3. Precipitation; 4. Distillation.

The most popular method is separation by columns. The use of columns, which is liquid-solid phase extraction, is preferable to liquid-liquid extraction due to its rapidity, higher efficiency, ease of work and lesser danger of radiation hazards and radioactive contamination. Most of the columns used are ion exchange columns, however, other columns are using ion retention media such as active carbon [36,37 J or C,,-bonded silica gel [38]. For hydrated antimony pentoxide-HAP, the most popular packing material used for removal of 24Na, which is a very important process in many samples, also called polyantimonic acid (V), the mechanism of operation not being clear in spite of several studies r39-421. The packing materials are mainly oxides, as for example, acidic aluminum oxide (AAO), hydrated manganese dioxide (HMD), hydrated antimony pentoxide (HAP), tin dioxide (TDO), or synthetic ion exchange resins, which are made of polymeric organic materials with a large number of acidic or basic functional groups. Cation exchange resins contain usually sulfonic acid group (R-SO,-H) or carboxylic group (R-COOH), where R is the polymeric hydrocarbon. Anion exchange resins contain secondary, tertiary or quartenary amines. They are usually supplied in the C1form, but the anion can be changed by washing with appropriate acid. Anion exchangers are often preferred over cation exchangers, due to higher selectivity for many cations, as a result of complex formation. However, cations which do not form strong negative complexes are separated on cation exchange columns. Girardi et al. [43] summarized the data on retention of many metal ions on 11 different column materials with various eluting solutions. The results are presented schematically in periodic tables.

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

3 17

Solvent extraction is very selective and is used frequently for single element separations, although procedures for multi-elements separation based on solvent extraction were also devised [44]. Some of the procedures used both column chromatographyand solvent extraction [45]. Girardi and Pietra [46] reviewed many of the schemes for multi element separations based on solid-liquid (columns) and liquid-liquid extractions. Solvent extraction is based on transforming some of the metal ions into compounds that are more soluble in immiscible organic solvents than in water. In most cases this is done by addition of organic compounds which form metal chelates. By using chloroform solution of various chelates, gradual separation of the radionuclide mixture can be achieved [47]. The precipitation agents are mainly the same ones used in the basic qualitative inorganic analysis course in first-year chemistry. C1- is used for Ag and Hg, COT2 and SO;2 are used for Ba, Sr and Ca. OH- is used for Fe, Sc and rare earth elements (REE). The REE is precipitated also by F-. S-2 is used to precipitate Cu, Ni, Zn, Cd, Au, Ag and platinum group elements. However, some organic chelates are also used to precipitate some elements, e.g. the precipitation of Cd, Co, Cr, Fe, 0, Ni, Se, Ti,V and Zn from solutions of digested biological material [48]. Several radionuclides which form volatile compounds can be separated from the mixture of the IRNs by distillation of these compounds. Hg, C1, Br and I can be separated by volatilization of the element itself. C1, Br and I can be distilled also as HCl, HBr and HI. As, Se and Sb can be distilled as halides from HC1-H2S04or HBr-H2S04solutions [7]. Several reviews on the RNAA of geological, biological and atmospheric samples were published in recent years. This is the reason that this chapter will start with metallurgical samples.

8.3.1 Radiochemical sepasatiom it! material scietices 8.3.1.1 Removal of the main interference Egger and Krivan [101studied the concentrations of trace elements in high-purity aluminum by NAA. The main activity induced in aluminum is due to the short-lived 28A1, via the radiative neutron capture reaction 27Al ( n , ~28A1, ) and it is not a serious interference since decay of a few hours will reduce this activity to zero (the half-life tIl2 of 28A1is 2.246 m). However, most of the reactors used for activation have also part of the flux of neutrons, which were not yet completely thermalized. The fission-induced neutrons have high kinetic energy, and they are thermalized in the moderator before reacting with the sample. The fast neutrons, due to their high kinetic energy, can react also in other reactions besides the radiative capture, mainly reactions with emission of small particles. The fast neutrons reacting with aluminum yields 24Na( tl,2 = 15.03 h), via the 27Al( 12, a)24Nareaction. The high background due to Compton scattering of the high energy y photons of 24Nais a

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serious limiting factor for the INAA determination of very low concentration trace elements via IRNs with t,,253 days. Egger and Krivan [lo] showed that removal of *"a (> 99%) was sufficient for determination of 18 elements with concentrationlevels of 0.1 ppb. For concentration levels of 1 ppb, an extra 20 elements can be measured. The 24Nais removed by passing HF/HCI solution of the digested irradiated aluminum on a 1.5 cm height HAP (hydrated antimony perntoxide) column, at a rate of about 1ml/min. More than 99% of the Na was retained on the column, whereas 48 elements were eluted with yields of above 99% and an additional five elements with yields of 93-98.5%. HAP columns or HAP powder in batch are utilized in many uses of RNAA to remove 24Na interference 139-421. However, most studies use 6-12 M HC1 for which retention of Ta, As and Se, besides Na, were also found. Egger and Krivan showed that the use of HC1-HF (10 ml concentrated HCl + 1 ml40% HF) followed by elution with concentrated HCI leads to quantitative recovery also of these elements. 8.3.1.2 Separation of a single element Although silicon is one of the most important impurities in high-purity metals, very few studies were able to determine its concentration at the level of ppm or below. The main limiting factor for all these studies is the high blank involved in decomposition of the sample and silicon separation. Due to the high concentration of Si in the earth crust (28%), it is almost impossible to obtain a detection limit below 1 ppm, even using clean rooms and specially purified reagents. Only activation analysis does not suffer from the blank problem. However, silicon is not an easy element for delayed neutron activation analysis. Silicon has three natural stable isotopes 28Si,29Siand 30Si. Only the last one, which is the least abundant (3.1%), gives a radionuclide on radiative capture of neutrons. The cross-section for this reaction, while not too small, is still smaller than for most of the elements. However, the main problem in determining silicon by NAA stems from the fact that its indicator radionuclide emits very few 7-rays. Its y intensity is 0.07% (meaning that for every 10000 decaying 31Si nuclides, only seven photons of y-rays are emitted). Thus, the activity of 31Si must be determined by its /3 activity. While different y-ray emitters can be distinguished by their different energy ?-rays using HPGe or Ge(Li) detectors, the activities of a mixture of p emitters can be separated into its components only for small numbers of components (- 3 4 ) with sufficiently different energies. Another way to distinguish between different p emitters is by the analysis of the decay curve (activity vs. time) and convolution to different half-lives. However, the analysis of decay curves is accurate only for at most, 3-4 components with sufficiently different half-lives. 32P,which has a relatively long-life, can be measured after long decay [49,501 for various matrices. 31Si has a relatively short halfliife (2.62 h) and in order to determine Si in metals, 3'Si should be completely

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

3 19

separated from any other radionuclides, and then its P activity should be measured. Rouchaud et al. [51] determined the concentration of silicon in molybdenum by RNAA, separating the 3 1Si from all other radionuclides. After irradiation, the molybdenum was digested by alkaline fusion with NaN0,-NaOH (2: l), which was found to attack both molybdenum and silicon. The fused cake is dissolved in hot water. The solution is neutralized and H202is added to transform the molybdenum quantitatively to peroxy ,complexes needed for the following separation. To the solution are added HC1 and HF, to form HCl(O.03 M), HF (0.1 M) solution, which is passed on an anion exchange column (Dowex 1 X8,C1- form), and the column is washed with an addition 25 ml of HC1 (0.03 M/HF 0.1 M). Most of the trace elements are eluted and on the column remain Si, Mo and its daughter Tc, and W. Silicon was later eluted with 30 ml of HCl(8 M)/HF (0.1 M). In order to concentrate the 31Sito smaller volume, to allow higher detection efficiency, the solution was brought to pH 9 and passed through a small column of hydrated alumina, on which ,lSi remained fixed and counted. The removal of all radioactive interferences was checked by looking for y activity. It was found that no 7 or X-ray emitter was present. In a later paper, Rouchaud et al. [52] developed a similar method for Fe and Al, using this time cation exchange Dowex 50 x8 for the retainment of the matrix. Si is eluted with HF + HCl + acetone, and Si is finally retained on A1203for counting. Schmid and Krivan [53] developed another process for separating solely Si. This process is based on distillation of 31Sias SiF, from HF + HBr solution, followed by measurement of the /3 activity, with the very efficient liquid scintillation counting system. The HBr was added to the sample before its decomposition, in order to prevent distillation of As together with the Si. They used this method for determination of Si in vanadium and niobium. 8.3.1.3 Group separation of several elements Since several IRNs can be determined simultaneously, as long as they do not interfere with each other, there is no need for complete separation. Park et al. [54] studied the concentration of 13 trace elements in high purity molybdenum by separating them into three groups, which are separated from the major interferences, which are Mo and W. A cation exchange column (Dowex 50) in dilute HC1 medium is used to remove Mo and W, which are only weakly adsorbed on this column, while many of the impurity elements are strongly absorbed. Na and K are eluted by using additional dilute HCI. Nine trace elements were eluted with 6 M HCI. Together with Mo and W, 239Npand 233Pawere also eluted, which are the IRNs for U and Th, respectively, due to p- decay following the (n,y) reaction.

Np and Pa were separated from Mo and W using anion.exchange (Dowex-1) in

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320

Dipslcd Mo. hcaced lo dryness

1. Heat to dryness

2. + 16 mL 13% HzO,

3.

+ 15 mL conc. HNO,

0.5M HCI i0.3%

H202

Rb,Zr, 1%

Group

Zn.CO.La

Na

u

l-l

Dowcx

column

7M

HNO,

(0.3% H203

1M HCI

Figure 8-1. Park et al. [541 method for separation of 13 elements into lhree groups for determination of Lrxe elements in molybdenum

strong nitric acid media. Pa and Np are strongly absorbed on the anion exchange resin, while molybdenum and tungsten are not. Figure 8-1 gives a schematic representation of the process of separation of Park et al. [54]. Before counting, each eluate was concentrated by partial evaporation. Theimer and Krivan [55] developed a group separation for the determination of 20 elements in high-purity molybdenum (Fig. 8-2). They developed two modifications, the first one involves removal of Mo and its daughter Tc and the main contamination of W, by absorption on anion exchange resin (Dowex 1x8). Five other elements are also absorbed on the column, but 20 elements are eluted and can be determined simultaneously in the eluate. The second modification allows determination of only 11 elements, although separation into two groups is done. The advantage of this special modification is the separation of 233Pa(the IRN of Th) only with Sc, enabling determination of small amounts of Th. Both modifications are based on elution with various concentrations of HF,

321

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

(1 mLconc.HF+ 0.5 mL conc. HNO3)

evaporation Procedure B

Procedure A .r

dissolution (2 mL 20 M HF/ 3%H2W

(0.5 mL 20 M HFI 0.5 mL 30% H 2 W

elution with 27 mL 20 M HF/3% H202 v v

r

elution with 20 mL 2 MHF/3% H202

'-r

elution with 25 mL

OE

Nb,Re,Sb, Ta, Tc. W,Zr, (Sn)

1 M HF/lM NH4F

adsorbed: Hf, Mo,

W c (

I

'I

eluted: Ag, Cd, Co, Cr, Cs, Cu.Fe, Ga. XnJr, K, Mn. Na, Np, Pa, Rb, Ru, Sc. Se, Zn,0,

(Sn). (W

Eluate 1: Ag. Co, Cr.Cs, Fe. Rb. Ru,Se,Zn,(Sn)

Eluate 2: Pa;Sc

Figure8-2.Flow chart of the two procedures for radiochemical separation of trace element in molybdenum (reproduced Ref. [551, with permission)

322

ZEEV B.ALFASSI

similarly to all procedures developed in recent years by Krivan and coworkers for determination of trace elements in metals. The flow chart for the two radiochemical separation procedures are given in Fig. 8-2. The two methods are based on Mo, Tc, and W absorbing on anion exchange column in HF media, while the trace elements are eluted with HF. For selective separation of Pa (together with only Sc), lower concentrations of HF are used. The elements given in parentheses are eluted but not completely, so they are found both on the column and in the eluate. The elements determined via medium-lived IRNs and which had to be in eluate 1 according to their distribution ratios, as e.g. Cu, Ga, K, Mn and Na, do not appear in procedure B, since about 1%of the technetium is eluted with this fraction and masking the activity of these IRNs. However, if (NH,),S20, is added to the sample solution prior to its evaporation, in order to oxidize all technetium species to the heptavalence state, less than 0.01% of the technetium is found in fraction 1, and also the other elements can be determined in this fraction. A similar technique was developed by Krivan's group to determine the content of 19 elements in high purity tungsten [56]. However, a change was done in order to measure the concentration of Ta and Sb, which in the previous processes were adsorbed on the anion exchange resins together with Mo, Tc and W, and could not be determined. Ta and Sb are extracted with organic solvent (dichloreothane-DCE) after forming complexes with diantipyrlmethane (DAM), prior to the introduction of the solution of the dissolved irradiation tungsten on the anion exchange column. In order to measure the P content via 32P, only /3--emitter, 32P is substoichiometrically extracted as molybdophosphate with tetraphenylarsonium chloride in dichoromethane [57]. The flow chart of this separation is given in Fig. 8-3. Caletka et al. [58] developed a radiochemical NAA procedure for determination of 26 elements in niobium by irradiation and processing of two samples, one for measurement of medium-lived IRNs (Indicator Radionuclide - the radionuclide used for the determination of an element in NAA), by irradiation for 12 h. The second sample was irradiated for 5 days for the sake of determination of long-lived IRNs. In the determination of medium-lived IRNs, they wanted to measure also 56Mn ( t I l 2= 2.58 h) and 65Ni ( i l l , = 2.52 h). This is possible since the main activity produced from the matrix e4"'Nb) is short-lived ( 11,, = 6.24 m) and one hour of cooling leaves a sample that is not too radiactive to handle. For matrices where the main activity is longer-lived, IRNs with half-lives of a couple of hours can be determined only by pre-irradiation separation of the matrix, or treatment in special hot laboratories, where very high radioactivity can be handled. Different schemes of separations were devised for the medium and long-lived IRNs, however, they have many common features. Both are using solvent extractions followed by separation on columns. The same two solvent extractions were used in the two procedures, and the only difference is in the columns and acids used for separation of the last aqueous phase into three groups. The first extraction step is for cations which form

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

1

Surface etching

323

I

Carriers ~~~~

Evaporation near to dryness, dissolution in 12M HF I

in DCE,

I

1

Aqueousphase

r.

1 1

Ta,Sb,(Re)

1

16M or 31M HF

fraction B

1 determination Figure 8-3. Flow chart for (he radiochemical separalion of trace elements in tungsten (reproduced from Ref. [56],with permission)

chelates with dithizone and are extracted with chloroform. This fraction includes Cu, Au, Pd and Pt in the medium-lived IRNs and Ag, Se in the long-lived IRN samples. The second extraction of metals is for those which form complexes with diantipyrlmethane (DAM). The organic solvent is dichloreothane. This extraction is used to remove tantalum and niobium, and the organic phase including them is discarded. The elements remaining in the aqueous phase are separated into three groups using two columns in series. A group is absorbed on each column, and the third group contains the elements which were eluted from the two columns. For the long-lived, the first column is the reversed phase di (2-ethylhexyl)orthophosphoric acid on a solid support and then a column of anion exchange resin-Dowex 1. For the medium-lived IRNs the anion exchange-Dowex 1 is the first column, followed by an HAP (Hydrated Antimony Pentoxide) column. In high concentration HCl solution, HAP retains only the Na ions, however, under the conditions used here,

ZEEV B. ALFASSI

324

Oissolulion in the HF-HNO] mixture

r--l Carriers

drgness. assolution in K)M HF

I 1 E.lmcllon

1

Extrachon wlth O.COSM dlthizone. Washlng with CHC13

I Wllh

Cu ,AU ,Pd, Pl

1

1

Dorri-1-column

w. ~

0 ini.zri .

HAP-column Na. K

‘r’ I

I

Figure 8-4. The two procedures for RNAA of trace elements in niobium (a) for medium-lived IRNs; (b) for long-lived IRNs (based on figures in Ref. [%I, with permission)

325

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

K also remained on the HAP column. The flow chart of the separation is given in Fig. 8-4. A most thorough determination of the trace elements in ultra pure metal was done for aluminum [59]. Previously, we saw that 38 elements were determined in levels of ppbs, by selective removal of 24Na with a HAP column. Later, the same group extended this study, and in order to improve the limits of detection, an additional separation into 10 groups was done [59]. Forty-three elements have limits of detection below 10 ppb, and for U and Th the limits of detection are 50 ppt (5 x lo-"). The separation scheme is given in Fig. 8-5. First, Na' is retained on

I

Activalion ( - l W mp SamOlOl Surfaca doamtamination I

Sample dnompovlbn

' Eluoon rnlh l2molllHCLnOmolllHF S:l O0W.I 1 x 0

C.a d s o m : (AS). Au. BI. Cd. CO. (Cul. fe. Ga. np. I. (In). MO. W. A& sb. (S1I. sn. Tr. w. zn

'

"'I

(+

1+

"2 (Iraciion Co. (Cu). (In). C) (AS).

"2 Fe.

Nb. W

Ga. Mo (banion 0 )

-

EluaCs L h k i i 01 Ilwnde ions ri l h bong acid

Elution win tZmoll1 HCI

ads0rb.d: Np. Pa (Iractlon A)

Eluate

0.8moi/l

nci

CI. (Nal. NI (Iractbn HI Ca. Cs. (Cu). K. hln. Fib. (bacllon I)

-

I

Ce. W.Eri Eu. Gd. Ho. La. Lu. Nd. Pr. Sc. Sm. Tb. Trn. Y. Yb

(fraclion J)

Figure 8-5. Procedure of separation of 52 elements in irradiated ultra pure aluminum to 10 fractions

HAP (hydrated antimony pentoxide). With 12 M HC1 elution, this is the only cation to be retained. Twenty elements from the eluate are adsorbed on anion exchanger resin (Dowex lXS), and later divided into five groups - by elution of four different groups with different elutions. The fifth group are those that are retained on the column after the four eluants. To the original eluate from the Dowex 1x8column, boric acid is added and the solution is introduced into another Dowex 1 x 8 column.

326

ZEEV B. ALFASSI

The masking of the F- anions by the boric acid, leads to retaining of Pa and Np (IRNs of Th and U) on the column. In order to change the medium of the eluted solution for further separation, the solution is evaporated to dryness and dissolved in 0.1 M HC1. The solution is added to cation exchanger Dowex 50 X8 and washed with 0.1 M HCI. The eluate is one group of five elements, while the 27 elements remained on the column are separated into three groups by two elutions. The flow chart of the separation in Fig. 8-5 shows the separation of 52 elements as studied by radiotracers. Not all of them were found in the ultra-pure Al. Only those not in brackets were actually found in the irradiated Al.

8.3.2 RNAA of geologicti1 arid ciiviroiiinciitul suniples 8.3.2.1 Removal of tlie main interferences This method was not used for geological samples, but it was used for trace element determination in sea or river waters [60,61]. Ndiokwere and Guinn [60] irradiated preconcentrated river water (by evaporation either at room temperature or at low temperature under reduced pressure) and removed the main interferences after the irradiation. 24Nawas removed by a HAP column, 82Brwas removed by oxidation to Br, followed by the extraction of Br, into CHCI,. This allows the determination of elements with IRNs which have half-lives larger than a couple of hours, after a decay of 12-14 h (mainly for the decay of Grancini et al. 1611 removes besides 24Na and 82Br, also 32P. 24Na was removed an a HAP column, which was found to retain also Ta (in 12 M HCl), 32P was removed by an AAO (Acidic Aluminum Oxide) column. This column removes also As, W and part of Mo, Ta, Sn,Br and Sb. The remaining Br is removed by oxidation with H20, and distillation of the Br2 formed. C1, I and part of 0 s are removed also in that way.

8.3.2.2 Separation of one special element The separation of a single element is done usually in order to obtain lower limits of detection for elements which are important for some reason. Thus for example, Munita et al. [62] determined the low concentration of thorium in some geological samples. In the section on material science, we saw that Th was determined via the IRN 233Pa. It is done for many geological samples instrumentally, without any chemical separation (INAA). For microamounts of Th,long irradiation times and often long “cooling” times are required to allow the decay of the matrix activity. However, a fast determination can be done using 233Thas IRN ( t1,2= 22.3 min). Its low-energy -pray (main peak = 86 keV) cannot be detected in the presence of the matrix and other contaminants activity, and it can be measured only after complete isolation of Th. 233Th was adsorbed on a cation exchange column (Bio-Rad AG 50W) which was previously saturated with thorium cations. The interfering radionuclides were eluted with a dilute solution of Th in 0.5 M HCl. Singh and

RADIOCHEMICALNEUTRON ACTIVATION ANALYSIS

327

Sawant 1631 used RNAA for determination of Se in geological environmental and food samples. Sediments, phosphate rocks and dust deposits were dissolved in HNO, + H,SO, + HF mixture. The solution is evaporated to small volume and HC1 is added. Se is reduced to elemental red selenium with ethyl-a-isonitroso acetoacetate in 6.5 M HCl. This reagent was found to reduce Se compounds selectively and quantitatively within 10 min on heating the mixture at about 60' C. Cooling in ice lead to settling of the red colloidal precipitate of Se. 8.3.2.3 Separation of a few elements Many studies were done on the determination of precious metals in geological samples. Nadkarni and Morrison 1641 digested the irradiated sample (200-500 mg) by peroxide fusion and the noble metals are separated on Srafion NMRR selective chelating ion exchange resin. However, yields are not well reproducible and reirradiation of each sample is necessary to determine them [65]. Only Pd, Pt, Ir and Au can be determined by this process. The use of the ion exchanger Srafion NMRR was questioned by Stockman 1661 who cannot repeat the results of Nadkarni and Morrison, and attributed it to changes in manufacturing of the resin. There are also some previous results which contrast the specificity of this resin for d8 ions [67,68]. Cocherie et al. [69] developed a procedure for the determinationof all platinum group elements (=PGE - platinum, ruthenium, rhodium, palladium, osmium, iridium and palladium) except Rh. Ag is also determined. Ir is determined instrumentally by biparametric y - y coincidence up to levels of 0.5 ppb. The recovery yields are not reproducible by this method, but instead of determining the recovery yield by reirradiation together with the carriers, they used direct yield determination by the use of radiotracers. This method must use different radioisotopes than those produced by the neutron activation. Thus for example, Pd is determined by lWPd (loSPd(n,y) '@Pd), whereas its recovery is measured with 'O'Pd. These radioisotopes are added to the irradiated sample before the digestion and they are digested together. Different separation procedures were used for different rock materials, according to their matrix. We will use as an example the chromites, which are geological samples with large concentrations of QO,. An important step is the removal of the radioactivity of the matrix. The irradiated samples together with carriers and the radiotracers are digested by alkali fusion. The fused cake is dissolved by 12 M HC1 and is brought to pH 7. Some of the elements remain as a precipitate of hydroxides. The Cr is in the solution. The solution is filtered and the Cr is precipitated as barium bichromate by bringing it to pH 5 and the addition of BaCl,. The previously precipitated hydroxides are dissolved in HCI and added to the filtrate from the barium bichromate precipitate. Pd, Pt, Au and Ag are precipitated from this solution together with Se and Te by reduction to the elemental state with SnCl,. Se and Te solutions are added together with SnCl, solution. The solution is filtered and Pt, Pd, Au and Ag IRNs are determined on the filter paper together

328

ZEEV B. ALFASSI

with measurement of their radiotracers. Thus both their contents and their recovery yields are measured in the same measurement. In this method 0 s and Ru were not determined, since 0 s is very volatile in hot acid solutions, and since there is no good radiotracer for Ru. 0 s and Ru are determined by second, longer, irradiation of another fraction of the sample. In the shorter irradiation the radiotracer 9 7 R is~ the same as the IRN. In the longer irradiation the IRN used is Io3Ru ( t l , , = 39.4 d). The 9 7 R formed ~ in the activation is decayed at the cooling time ( f1,2 = 2.88 d) of two to three weeks. At the end of the cooling time, new 9 7 R is ~ added to the irradiated sample and can be used for determination of the recovery yields. The Ru concentration is determined from the activity of lo3Ru. The digestion process is similar with one exception. In the first fraction after dissolution of the fused cake it is heated to remove H,O2, while for these two elements this step is omitted. Most of the Ru is found in the hydroxide precipitate and it is used for its determination, whereas most of the 0 s (90%) is found in the filtrate. The hydroxide precipitate is dissolved in acid and Ru is precipitated with Se and Te by SnCI, reduction. From the filtrate, Cr is precipitated as BaCr,O, and the 0 s is retained on an ion-exchange column (Srafion NMRR). Chai et al. 1701 developed two different method for radiochemical separation of the noble metals. They found that Nadkarni and Morison were right and that Srafion NMRR resin can be used to retain 0 s and Ir. Ten ml of 0.05 M HCl eluted 100% of Sc, Cs, and Fe, whereas only 10% of 0 s were eluted. Nadkarni and Morrison reported that thiourea can be used to elute the noble metals, whereas Stockman argued that thiourea elutes only gold, while Ir remains on the resin. Chai et al. [70] found that Ir and Re can be eluted with NH,, whereas only one-fourth of 0 s was eluted and none of Au or Pt. Chai et al. [70]found that all PGE including Au is retained by various productions of this resin. Chai et al. suggested a scheme for radiochemical separation for noble metals which is given in Fig. 8-6. They found that their digestion process forms mainly Ir(1II) which is not absorbed onto the chelate resin. However, oxidation of the chelated Ir with hot H202transforms all Ir to the +4 oxidation state, which is absorbed on another column with the same resin. An alternative process for the separation of the noble metals suggested by Chai et al. [70]is based on extraction of noble metals with long chain primary amines (19-23 carbon atoms) in dichloroethane. All the noble metals are extracted in high yields into the organicphase. The base metals were only negligibly extracted. Oddoneet al. [713 measured very low levels of platinum group elements in standard rock material, as well as coal, ash and botanical samples. They determined the concentration of all platinum group elements (PGE - ruthenium, rhodium, palladium, osmium, iridium and palladium besides platinum itself) and also gold. The samples were fused with N%O, + NaOH. The 0 s and Ru were successfully oxidized to tetraoxides (OsO, by KMnO, and RuO, by Ago) and extracted with CCl,. The remaining cations were retained as the chlorocomplexes in HCl solution on anion exchange resin. Pd, Pt

329

RADIOCHEMICALNEUTRON ACTIVATION ANALYSIS

I

ca. 200 ng srnple irradiated in f,facto_r2 s - l for 60 hrs at 8x10 n.un

.

1-

add c a r r i e r of os I r Ru,Re,Pt, and anticarrier of Sc,Cr,etc.

]decayed for 1 day melted in ruffle furnace at at 65OoC for 20 min

7 :z

1 rinse the melt with hot m t e r ,

Be;,: ‘$3

- - .- .

acidify with H=l and m k e solution transparant

1

adjust pH to 1.5 with 7”Hj

1

transfer solutibn to chelate ion exchange colum 1,100 mesh,&x90 mn

I resin phase and base metals

1

count i ng

heat and adjust .1

transfer solution to chelate ion exchange colum I I ,10(mesh,g8x90 mn

m resin phase

+.

count 1%

Figure 8-6. Radiochemical separationprocedure of noble metals based on chelate ion exchange resin (reproducedfrom Ref. [701, wilh permission)

and Au were eluted with 0.1 M HCl aqueous solution of thiourea. Ir and Rh remains on the resin. Each fraction was counted with y-ray spectrometry to determine the concentration of each element. As the separation yields are not reproducible, the yield of the recoveries have to be determined for each element by removal of the extracting or eluting solvents or the resin, reirradiating and determining the amount of each element vs. its amount added as carrier. Two radiochemical separation methods are based on fire-assay procedures which are more frequently used to preconcentrate the low concentrations of noble metals in large samples of ores [72,73]. Hillard [74] irradiated with epithermal neutrons 500 mg samples and collected Ir, Au, and Ag in 1 g lead buttons using the mini fire-assay technique. The main contaminants found in the buttons were As and Sb.

330

ZEEV B. ALFASSI

They were removed by heating the button with a mixture of Na202 and NaOH. The resulting 0.2 g lead-bead is counted in a Compton-suppression spectrometer. The chemical (recovery) yields are very irreproducible (15-6095 for Ir in two-thirds of the samples, or 47-77% for Au) and the carrier yields are determined for each sample by reirradiation of the lead beads with neutrons. Parry et al. [75] developed a method for fire-assay of 50 g samples with NiS in order to preconcentrate PGE before analysis. Later, in order to reduce the blank value due to chemicals in the fusing mixture, they irradiated the sample before separation, followed by radiochemical separation via the NiS-fireassay technique. However, special hot labs should be available to handle irradiated 50 g geological samples. Pietra et al. [76] developed many radiochemical separation procedures, and one of them was developed for determination of noble metals and Hg in geological samples. The method involves the fusioii in a closed system of the sample in a Ni boat with recovery of the distilled Hg. The fused sample is dissolved in H,SO,. 0 s and Ir are distilled as tetraoxides from this solution. The other precious metals are separated on the ion exchanger IONAC SR3. Another group besides the noble metals, which are determined many times by RNAA, is the rare earth elements (REE) group. Zilliacus et al. [77] described two procedures for the separation of the REE group from irradiated geological samples. A simple and faster method was developed for samples with concentrations above 0.5 ppm, while a method with more separation steps was developed for samples with lower concentrations. The more time-consuming long process is required for obtaining a clear REE fraction. The two processes are described in Fig. 8-7. Both methods are based on fusion with Na202and cycles of precipitation as hydroxides and as fluorides. In the simple method there is one more step of removal of Sc and other metals by extraction with tributylphosphate. For the lower concentration samples, Si02 is removed by precipitation with gelatin in acidic solution, the main interferences are adsorbed on an anion exchange column in HCI-medium, while the REE are eluted. Sc is removed by extraction with ether from SCN- solution. Purification of the REE fraction is done by a cycle of precipitation of hydroxides and fluorides. A similar process was developed also by Laul et al. [78]. They don’t have the step of Sc removal by extraction; however, they are doing in the final step, three cycles of hydroxides-fluorides precipitation. Meloni et al. [79] dissolved the samples in HCI/HF mixture, converted the digested material to chlorides by drying and dissolving in HCl, and used cation exchange column for separation. Two washings (0.5 M HCI + 0.1 M oxalic acid, and 2 M HNO,) were done to remove all interfering elements. The REE are collected on the third elution with 6 M HC1. However, their method has problems when analyzing geological samples with high silicate concentrations. A similar method was developed by Wandless and Morgan [80]. They did not use the fluoride-hydroxide precipitation cycles in order to save time, and used more

33 1

RADIOCHEMICALNEUTRON ACTIVATION ANALYSIS Irradiated sample Fuse with Na202 + carriers Dissolvc the melt in H20

I

I

Method 1

Add HCI until clear green solution Liquid t o wastePrecipitate to waste

-

I I

I

100 x 10 rnm Dowex-2 column

I . I

3

Y1StC

Dissolve with HCI Add thiocyanate solution Extract uitb ether

Liquid to wnstc-

Precipitate with NaOH

Org. phase tci

-

Centrifuge -Liquid

to waste

3

Dissolve with HC1 Precipitate with Relatine Pass the solution through

Liquid t o waste-

I I

Precipitate with NH

Effluent solution Precipitate with NH

Method 2

Dissolve with HCI Precipitatc with NH4F solution -Liquid I t o waste Dissolve with I l j B 0 3 + HCI Org. phase Extract with TBP I t o waste Precipitatc lanthanoids with HH4F

-

I

I

Dissolve with HCI Precipitate lenthanoids with NHhF at pH 5

Figure 8-7. Separation scheme for the analysis of REE in geological samples (taken from Ref. [77]. with permission)

the anion exchange column. The method is based on the precipitation of the REE hydroxides both at high pH and at pH 9 in ammoniacal solution. SiO, is removed by precipitation with gelatin in acidic solution. Fe is removed by an anion exchanger in the C1- form from 8 M HC1 solution. Zr, Sc and Hf are removed by an anion exchanger in the SCN- form from 0.8 M SCN- + 0.5 M Cl- solution. The only remaining interference is Crt3 which is removed by precipitation of the REE with excess 8 M NaOH. The complete scheme is given in Fig. 8-8. Bishop and Hughes [Sl] used Na202+ NaOH for digestion. To increase REE recovery and recover REE not precipitated in the first step, they added FeC1, and the REE is coprecipitated with ferric hydroxide. The REE is separated using cation exchange column of Biorad AG 50W-X8. The other elements are removed by three elutions (1 M HCI, 2 M HNO,, 1 M H2S04)and REE is eluted in the fourth elution by 5 M HCI. In order to reduce the volume of the eluate (300 ml) but not to zero, 5 g conc. H,SO, is added and the solution is heated until only concentrated H2S04remains. Loron and Ottenlo [82] removed most metals with anion exchange column. However, they add a DEP-celite resin column to remove scandium. REE is precipitated as fluorides together with CaF,. Stosch et al. [83] used the same process and the only change is removal of scandium by precipitation with phytic acid. Koeberl et al. [84] developed a completely different separation process. The irradiated material is digested with 2 M HCl and the mixture is separated to solution and undigested residue. The residue is decomposed with HF/H,S04. It is separated

ZEEV B. ALFASSI

332

.

Sample REE corr~ers

2

Fusion wilh NoOH, No201

-Leach with H$

Supernale Si. AI,

K.

ppl

2' REE. Ni, Fe. Mg. Zr, Sc, HI

Rb. Cs

-NHrCI*NHLOH

Supernole

10

pH9

PPl

Mg. Co. Ba. Sr. Ni

~

PPl

502

REE. Fe, Cr. Zr. Sc. HI

IzFLin Supernote

REE Fe, Cr. Zr, Sc. HI

-6M 200-400

HCI

IWSh

CI-lam -Evopomte

10dryness

100-xx)mesh

SUpQrnOW

PPt RE€ I-HCI RE€

Figure 8-8. Separation procedure flowchart for RNAA of REE in geological samples (reproduced from Ref. (80).with permission)

again, and the residue is dissolved in 7 M HCl, evaporated to dryness and dissolved in 2 M HCl. The two fractions dissolved in 2 M HC1 are added together, brought to 9 M HC1 and then loaded on anion exchange column. The elute is extracted with tributylphosphate,and the REE remaining in the aqueous phase is precipitated with oxalic acid solution. Parthasarathy et al. [SS] separated the REE into two groups rather than to one group, as all other processes. They separated it into a group of light REE and a group of heavy REE. The process is different from all those previously described and the flowchart is given in Fig. 8-9. Other elements besides REE and PGE were also separated in irradiated geological samples, usually 2-3 elements. Garg et al. [86] determined the concentration of Rb, Cs and Ta in mica samples by RNAA. Some other elements were determined without any chemical separation

333

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS HREE

LREE I r r a d i a t e d s o l u t i o n + 20 mg Mn Add 10 m l cc. %NO3 and KBrO.,

+

20

mg La.

&

Iln02 p r e c i p i t a t e 2 3 3 ~ a/ r e j e c t /

S o ht i o n

r

MnOz precipitate 2 3 3 ~ a/ r e jectl Hydroxide p r e c i p i t a t e D i s s o l v e i n cc. H C 1 and c o n v e r t t o c 1' form. Take i n 10 ml of BN ltCl: Add KCIOJ t o o x i d i z e NplIVl to Np/Y/. P a s s t h r o u g h pre-equilibrated Dowex-1 ,column

Add 2 0 mg Mn+KBr03 1 Solution Carry o u t 3 c y c 1 c s Of roll-

Solution Reject

I

Resin 239Np reject

I r r a d i a t e d s o l u t i o n + 20 mg Mn + 20 mg La t 2 0 m Y. Carr o u t t h e s e p a r a t i o n of 213Pa and 1 3 9 N p as shown i n LREE. The e f f l u e n t from t h e Dowex-1 column a f t e r t h e separ a t i o n of 239Np is e v a p o r a t e d t o d r y n e s s and c o n v e r t e d t o NO: form. The f i n a l r e s i d u e i s t a k e n i n 10 m l 2 . 5 8 7H lINO m e t h a n o l m i x t u r e . Pass t h e s o l u t t o n through a p r e e q u i l i b r a t e d Dowex-1 / 2 0 c c / column.

Ef I h e n t

Ef Eluent reject

Eluate Evaporate t o d r y n e s s and precipitate Y-oxalate

Column/Resin/ E l u t e w i t h 50 m l 10%i~ H N O ~ methanol

-

Resin LREE u p t o cd

Evaporate to d r y n e s s . Take t h e r e s i d u e i n w a t e r . Add o x a l l c a c i d and p r e c i p i t a t e La-oxalate

Figure 8-9. Flowchart for RNAA of REE by separation of two fraclions, light and heavy REE (LREE and HREE) (reproduced from Ref. [8S].wilh permission)

(INAA). The irradiated sample was dissolved by fusing with sodium peroxide. Rb, Cs, Cd and Hf were precipitated by sodium tetraphenylborate. Ta was separated by solvent extraction using N-benzoyl-N-phenyl hydroxylamine and cupferron in chloroform. Van der Sloot et al. [87] developed a method for separation for arsenic, selenium and antimony in geological and environmental samples. The method is based on dissolution of the irradiated samples in an acid digestion bomb, evaporation to near dryness, uptake in 6 M HCl and separation of As, Se and Sb from the interfering elements by the formation of the hydrides. The hydrides of these three elements are volatile and can be separated by distillation. The hydrides are formed by sodium borohydride (NaBH,). The hydrides formed are swept out by an inert carrier gas and collected on an active carbon column. To ensure complete volatilization, the elements have to be present as As"', Sb"' and Se'". To achieve this, a reducing agent has to be added before hydride formation. The three elements cannot be determined together due to very different half-lives of the IRNs. For determination of Sb and As the samples were irradiated for one hour and cooled for 24 hr. Reducing to the I11 state was done by 10%ascorbic acid + 0.2 M KI solution. Se (together with Sb also) are determined after 24 hr of irradiation and cooling for 3 weeks. The reduction is done by refluxing with 6 M HCl. Drabaek et al. [88] measured the concentration

334

ZEEV B. ALFASSI

of Hg, As and Se in coal, sediments and biological samples. After digestion, Hg is separated from the solution by distillation as HgCl,. HgCI, is distilled only above 120' C, while HCl would distill off already at lower temperatures. In order to solve this problem, HCI is formed it! sitic at the higher temperature by the reaction of HClO, with glycine. Hg recovery is measured by electrolysis, using gold and platinum electrodes. Hg is electrodeposited on a gold foil, and its amount is determined by weighing. As (111) and As (IV) remaining in the solution is distilled as the bromides after addition of HC1 and HBr. In order to measure recoveries, either Se or As carriers were added, but not both of them. Their recoveries were determined by precipitation with ammonium hypophosphite and weighing of the precipitate. They suggested that if both yields have to be determined simultaneously,it can be done using XRF. Rammensee and Palme [89] used extraction with molten metallic iron from silicates to separate the contents of geological matrices into two fractions,those quantitatively dissolved in iron and those dissolved in silicates. Those which dissolved only partly in iron cannot be determined in this method. Only one separation step was needed in which the geological sample is mixed with iron and with silicates (2:l) and heated to 1580"C. After cooling, the metal spherule is manually separated. The first y-measurement can be made about 3 hr after the end of irradiation (the term pile-out is used many times instead of end of irradiation, in the case of irradiation in a nuclear reactor). All the 24Naactivity, one of the main interferences in geological samples, is concentrated in the silicate phase and siderophile elements (iron-loving) with half-lives from several hours to 100 hr (Cu, As, Sb, Re, Pt, Au, W, Mo) can be determined simultaneously in the iron phase. The main long-lived activities in geological specimen are due to Fe and Co. As these elements are concentrated in the iron phase, litophile (silicate-loving)elements can be more accurately determined. In principle, this is a similar method to the fire-assay technique. However, there are two advantages in that method, according to the authors. More elements than only the noble metals can be extracted, and since the recoveries are quantitative, no yield measurement should be done, simplifying the analysis.

8.3.2.4 Group separation methods Several group separation methods were developed for geological samples. For space reasons, not all of these procedures can be described here, and only a few of them will be given as examples. The main difference between the various separation procedures depends on which geological samples are used. Some methods are good only for some special geological samples, while in other samples some elements with relatively high concentrations interfere with the determination of other elements in the same group. Morrison et al. [90] described a procedure for chemical group separations, which, with the use of a Compton suppression Ge(Li) detector, can determine 45 elements in geological samples. The flow chart of this process is given in Fig. 8-10. The

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

335

Sample Carriers (Se,Zn,Lo,As,Br and HF+I$S04

1

I

Residue

I

Dlssolvad In 8N HCL

76

82

A s , Br

Solulion HAP botch rxtracllon ond column arlractlon

Passed through o 10cm

-

column of Dowar' 1 ~ 8 . 2 0 0 400 mesh

1

Evaporated

Rekdue ;;s$rd

65Zn, 6 9 mZn,1Z%b,124Sb

In

197

"Fe 239

197n

Hgi

,6oCo :4Cu

Np I

Hg

I87

,%a

W ,"Mo

Extracte with 10 8.20 rnl TBP I

I Organic phase (group 5 ) ] 181

Hf,

97

48

Zr,

40C,47SC,17kf,

Sc "Po

1 Aqueous phase (group 6 11 I

42K, asRb,"4Cs, '7Sc , l"Ea 85Sr, "Ice ,'47Nd!'3Sml15ZE~

'54Eu,1''Gd ,16aTb:46La ,'%o 170Trnr'Yb

,IT7Lu,%r

, *,'Ca

Figure 8-10. Flowchart for chemical separation of 45 trace elements in geological samples into six groups appropriate for simultaneous counting (reproduced from Ref. 1901, with permission)

first group is the one including volatile elements and volatile fluorides. Elements adsorbed to HAP in 8 M HC1 formed the second group. Those adsorbed to anion exchange resin in 8 M HCl formed the third and fourth groups. The separation between these two groups is done by elution with 0.5 M HC1. The elements not adsorbed on the anion exchange in the 8 M HCl are separated into two groups by extraction with tributylphosphate (TBP). Laul et al. [91] developed a RNAA method for trace elements in terrestrial rocks and stony meteorites. They stated that their main emphasis is minimizing chemical procedure and maximizing the -pray spectrometry aspect. However, their separation is longer than that of Morrison et al. [go] and they measure only 16 elements. The flowchart of their separation is given in Fig. 8-11. The scheme involves four different separation techniques: distillation, solvent extraction, ion exchange columns, and precipitation. Allen et al. [92] developed a RNAA method for 39 elements in small or precious geological samples. However, it involves many separation steps, as they

ZEEV B. ALFASSI

336

Exirocfion

6 N HCI

K2H,h,O

&

Oroonic

Pr-i;cipilaie

-Ion -exchonoe 2N H

, r I

-1

Aaueous

Voldtile bromides Preci ilolion

Supernaie

I

@-

b

SuDernote '

Ion - exc hog.

I

E

, Pr+:i:itatiye

Supernole Precioitoiion

c0,sc

Supernole discord

Figure 8-1 1. Chemical separation of trace elements in geological samples, leading to determination of 16 elements (reproduced from Ref. [go], with permission)

separated chemically the sample into 12 groups. For some elements with very low concentrations, they used NaI(Tl) detector. Three chemists can finish the chemical separations and sample preparations within 7 hr after pile out. One NaI(T1) detector and two Ge(Li) detectors were used for counting. The process involves only solvent extractions and precipitation and is given in Fig. 8-12. Keays et al. [93] developed a very detailed method for RNAA determination of 20 elements in terrestrial, lunar and meteoritic samples. In order to determine very small concentrations, the method separates the determinand to many small groups and the power of 7-ray spectrometry is not really utilized. $met et al. [94]developed a group separation for RNAA determination of 24 elements in a wide variety of silicate rocks and minerals. The samples were decomposed in a HF-HNO, mixture in a PTFE-lined bomb, and it was separated into soluble and insoluble fluorides. The soluble fluorides were separated into three groups by sequential elution (0.1 M HF, 3 M HC1+ acetone, 12 M HCl) from a cation exchange column. The insoluble fluorides (Ca, Sr, Ba, REE and part of Rb and Cs) were dissolved and purified from iron and scandium activities by extraction with TBP. The scheme of separation is given in Fig. 8-13. The elements written in bold were determined, whereas the usual type letter elements are those that are in more than one fraction and cannot be determined. In some samples their concentration is too

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

337

Solid G

A i r m orolnlc

Figure 8- 12. Allen el al. I921 procedure for chemical separation of 39 trace elements in small geological samples into 12 groups (reproduced from Ref. 1921. with permission)

..

IRRAOlAlED SAMPLE 1250 rnpl CARRIEIIS DISSOLVE 111 LO% IIF 111i03

I EVAfQRAIE

- 01 1.1 IIF -

-

-

- 12 1.1 HCI

111 01 1.1 IIF

Zr,Sb,lif,Ta ,W,Pa. sc,cr

T

I

ORG PH

-

3 I 4 llCl ACEIOI*E I10 I 0 v/v

, OISSOLVE

I

-

Cu,Zn,Ga,Fe

-

pH:L,. E D l A ,

1 AQ pH.1,

Sc,Fc

rii s

EOlA.

rmw w t m i l C O

SUPERNAIE

I'IIECIII WllH HJPII'D

[

Na.Cr.Co

-

I - - 1 PR~CIPITATI?

PREClPllAlE

Rb,Cs,i(

R b, C s, I( I C0I.IOIFIE __

I

SUPERNAIE

REE,Ca, Sr, Ba. Na.Cr.Co

Figure 8-13. Chemical separatioii procedure for groups of trace elements in geological samples (reproduced from Ref. 1941. with permission)

high and they might interfere with the determination of the other elements in the group. For these cases, the authors developed a more thorough scheme, in which for each fraction there is a method to remove the interferences. However, these further steps should be done, only if y-spectrometry will not be sufficient. Sun et al. [95] developed a procedure which will enable the determination of all fourteen REE together with other elements (22) in a small geological sample (62.1 mg of lunar sample). The samples were digested with HF + HNO, + HC10, mixture. After heating to almost dryness, the residue was dissolved in 1m19 M HC1. The irradiated mixture was separated into 12 groups, using a HAP column in series

338

ZEEV B. ALFASSI

with an anion exchange column. Two consecutive elutions of the anion exchange column were done (9 M HCl and 0.5 M HCl). The 9 M eluate was extracted with trifluorothenolyacetone in isoamyl alcohol to remove Ti, which is extracted into the organic phase. The aqueous phase was further extracted with cupferron in chloroform. The remaining elements in the aqueous phase are separated into 7 groups by reversed phase chromatography columns of dL(2-ethylhexyl) phosphoric acid (HDEP) on a kel-F support. Two columns of HDEHP were used, first column of 50% HDEHP was eluted successively with 0.05 M HC1 and 9 M HC1. The 9 M HCl eluate was loaded on 10% HDEHP column and eluted successively with five eluants (0.15 M HCl, 0.21 M HNO,, 0.4 HNO,, 1.5 M HNO, and 5 M HNO,). Pietra et al. [76] developed 22 different RNAA procedures for environmental and biological samples. Many of these procedures are for several elements or for single elements; however, some of these are for group separations. The most extensive one (50 elements) was used only for biological samples. The next most extensive (39 elements) procedure was used also for geological samples, and the scheme of 01-05 g sample

DlSSdVQ

in tellan

Wash 50 cm’ CUM H N 4

y’ ccartb- -Sc. Fe. In Hf. NO

!s

-B-N_a &.c_s, !&

a&.%Q

Figure 8- 14. Radiochemical separation scheme for the delermination of 39 elements from geological samples (reproduced from Ref. [76], with permis-

sion)

339

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

the separation is depicted in Fig. 8-14. The samples were dissolved in acid mixtures in a teflon bomb, and after drying, dissolved in 0.1 M HNO,. 0.1 M HNO, was used also as the only eluant. The trace elements were separated into five groups by the use of five different columns-acidic aluminum oxide (AAO), tin dioxide (TDO), copper sulfide (CUS), cadmium oxide (CDO) and hydrated antimony pentoxide (HAP). The separation time for this procedure is about 2 hr. In another scheme where the geological material is dissolved in 6 M H E HAP column was first used, followed by cation exchange column, and later, anion exchange column to separate the mixture into 4 groups in which 35 elements can be determined. As in the previous scheme, in order to simplify the procedure and to make it easier for automation, they use only one eluant (6 M HF). Vasconcellos and Lima [96] developed procedures which is somewhat between several elements and group separation. It is based on the use of three columns-hydrated atminony pentoxide (HAP), anionic resin column and a reverse phase chromatography column Sample

carriers

( Zn, Co, S c , Sm)

HF H C l O r R t s i d u r dissolved n EN HCI "Na , TaI1"Rb, I n Sb, 1z4Sb,2 3 3 p g , 4 6 s ~

1

Rock solution

#= h=

b= h =

+=

h=

"K,

E I 1I ue nI

"Sc,

4'Sc,131h,

"5,I4'Ce, 147Nd153Sm I 5 2 Eu, Tb, I 40 La 16 6 H ,'

1sgYb,177Lu,' I C r , 4 7 C a , '"HI

Figure 8-15. A simple group separation scheme for geological samples (reproduced from Ref. [96], with permission)

Figure 8-16. A system for rapid separation through a series of column (reproduced from Ref. [96], with permission)

ZEEV B. ALFASSI

340

with tributylphosphate on a kieselguhr support. Several elements appeared in more than one fraction, but all the REE appear in one group and can be measured. The same is true for Na, Ta, Zn, Fe, Co, Cu, Ga, K, Ba, and Cr. The flow chart of the separation is given in Fig. 8-15. Figure 8-16 shows the construction of the three columns in series. The elution is speeded up by suction with a vacuum pump. This series of columns can be used when the eluants is the same for all columns, as in the scheme of Pietra et al. 176). The determination of geological samples was reviewed previously by Laul [97] and Fardy [98]. Chai [99] reviewed the RNAA of platinum group elements.

8.3.3 RNAA of biologicul samples

8.3.3.1 Removal of the main interferences The main activities of irradiated biological material are 38Cl ( t , / 2 = 37.2 m, a = 0.06 6, (T u is the product of the cross section in the natural isotopic abundance which gives the actual probability for formation of the IRN), 24Na ( t I l 2= 14.96 h, . a = 0,53b), 42K( t l I 2= 12.36 h, D . ci = 0.98b) and 32P.32Pis only /3- emitter and its activity can be stopped by a thin absorber which will absorb only a small fraction of the y activity. This absorber should be made of low Z elements in order to diminish the formation of X-rays by the stopping of the p particles. The best absorber will be 5-10 mm thick polyethylene. 38Clis many times removed by decay. In cases where short-lived activity has to be measured, 38Clis removed by one of three methods: (1) dissolution in a mixture of acids, followed by evaporation to dryness, where 38Clis removed as HC1 vapours; (2) precipitation as AgCl with AgNO, solution; and (3) adsorption on an anion exchange column. The main interferences are 24Naand 42K. 24Nais usually the major one as its concentration is larger and its CT a is 5.4 times larger. 24Naand also 42K are usually removed by hydrated antimony pentoxide in high concentration HC1 media [26,39]. Although some other columns [101,102] or composite-HAP columns [103,104] were also used, they are not advantageous to HAP columns. The only disadvantage of HAP columns is that since Na adsorption is irreversible, new resin has to be used after a few separations. The removal of Na by HAP is quantitative and quite fast. Nadkarni and Ehmann [ 1051removed from digested irradiated cigarette tobacco 24Naand 42K on HAP columns. They measured after the removal, the activity of La, As, Br, Se, Cs, Sb, Cr, Ag, Se, Zn, Fe, and Co. Murthy and Ryan [lo21 determined several elements in biological reference material after removal of 24Naby adsorption to a polymer containing cryptand 221B.Piedade-Guei-reiroet al. [ 1061 removed 24Naby HAP column in 12 M HCI media from mineralized biological standard reference material in order to measure Ca, Fe, Zn, Co, and Ba. Maihara and Vasconcei10s [lo71 measured Cu, Zn, Sb, and La in milk powder and bread after removal of 24Nafrom the digested sample in 8 M HCI by a HAP column. D

+

+

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

34 1

8.3.3.2 Separation of a single element Single-element separation from all other radionuclides in the mineralized irradiated samples allows the determination of very small concentrations of this element. The activity can be measured with a low-resolution high-efficiency NaI(Tl) welldetector. Different single elements were determined in that way: V [ 108,1091, Cr [110],Mn [111],Cu [112-116],Zn [117],As [115,118,119J,Se[63,120-124], Mo [125], Ag [126], Cd [122], I [128,129], Cs [130], La [131], Pt [132-1381, Hg [ 139,1401. Pietra et al. [76]developed procedures for single-elementseparation €or the following elements in biological materials: Ca, Ga, Mg,Hg, I, P,Sn and TI. In the following section, examples of the large number of procedures will be given.

Vanadiuni - Both Cornelis et al. [lo81 and Simonoff et al. [log] used RNAA to determine V in blood serum by solvent extraction with cupferron. Simonoff et al. [ 1091 dissolved the sample in HCl and NH,VO, was added as carrier and 48V02Clwas added for chemical recovery measurement. V was extracted from the aqueous phase by cupferron in chloroform. Chroniiuni - The samples are dissolved in a mixture of HNO, + H2S04 + HF + HClO, (not all simultaneously). The Cr in the solution is in the +6 oxidation state. KMn0, is added to oxidize any remaining Cr to the +6 oxidation state. HCl is added to the cooled solution and Cr is extracted into chloroform containing 5% tribenzylamine (TBA). Cr (VI) is back extracted into the aqueous phase by 2 M NaOH solution.

+ HNO, mixture. The solution was diluted with water, and the solution was neutralized to pH 69 with NH,OH. Addition of ammonium persulfate and boiling the solution lead to precipitation of MnO,.

Murzguwese [111J - Samples were mineralized with HClO,

Copper - Dybczynski et al. [112] dissolved the samples in HN03-HC10, mixture with vanadium (IV) as oxidation catalyst. After pH adjustments the solution was loaded on a LIX 70 column, and the column was eluted with 1 M NaNO, + 0.24 M NH,PO, adjusted to pH N 3.4. Copper retained on tbe column and was eluted with 4 M HC1. Rajadhyaksha and Turel [ 1131separated copper from solution of digested material by substoichiometric extraction. Copper was substoichiometrically extracted with chloroform containing 2-mercaptobenzothiazole. Whitley et al. [1141 extracted Cu from the aqueous phase with neocuproine in chloroform. Cortes et al. [1151mineralized the biological material with HNO, + H202, evaporated to dryness, and the residue dissolved in 6 M HCl. 24Nawas removed by a HAP column. Na2S0, is added to ensure that all Cu is in the +I oxidation state, and KCNS solution is added to precipitate CuCNS.

342

ZEEV B. ALFASSI

Arsenic - Cortes et al. [ 1151 add also H,S04 to the digesting mixture for determination of As (in addition to HNO, + H,O, used for Cu determination). To eliminate excess oxidants, SnO, is added. Then KI is added in order to precipitate arsenic. The arsenic is soluble in toluene, and it was extracted into it. Comparetto et al. [118] precipitated arsenic from the solution of the digested material by reduction with sodium hypophosphite. May and Picot [ 1191used first a cation exchange to remove most of the cations, and then As is precipitated as As$,. Pietra et al. [76] separated As by the formation of the volatile ASH,, by reduction with Zn + SnC1, + K1 in 10 M HCl media, and retaining the ASH, on an AgNO, trap.

Zinc - Shah and Haldah [ 1171 separated Zn by substoichiometric extraction of the Zn-isonitrosoacetophenone complex into chloroform at pH 7.5-8.5. The time required for radiochemical separation and counting is less than 35 min.

Seleniicnz - Damsgaard et al. [120] separated Se by precipitation after reduction with ascorbic acid, followed by extraction with methyl isobutyl ketone from HC1 solution. Itawi and Turel [121] precipitated Se (IV) from HCl solution with 1amidino-Zthiourea. The Se precipitate is dissolved in HNO, and further purified by extraction with chloroform containing 2-mercaptobenzothiazole. Kalouskova et al. [122] also used extraction of Se. To ensure that all Se (VI) is converted to Se (IV) they heated it to dryness in HC1 media. They extracted into toluene the benzoselenodiazole formed in the reaction with o-phenylenediamine. Navarete et al. [124] separated Se by adsorption of its complex with ammonium pyrrolidine dithiocarbamate (APDC) on active carbon at pH 1. A similar method was used previously by Lavi et al. [ 1411 when coprecipitating Se(PDC), with Pb(PDC), pre-irradiation in order to determine Se through its short-lived isotope 77mSe.

Silver - Bowen and Sugari [ 1261separated Ag by precipitation of AgCl. The silver is further purified by dissolution in 18 M ammonia and scavenged with one drop of 5% Fe(NO,), solution. The silver, remaining in the solution, is precipitated with Na,S solution. Ag2S precipitate is dissolved in hot 8 M HNO, and reprecipitated.

Cadnziunz - Itawi and Turel [1271extracted substoichiometrically Cd (11) complex with 2-mercaptobenzothiamle (HMBT) into methyl isobutyl ketone. The HMBT is added in an alcoholic solution.

Zodine- Dermelj et al. [1291determined trace quantities of I in tobacco, by combustion of the sample in oxygen atmosphere in Schoniger technique. This is followed by selective oxidation to I, with NaNO,. The I, is extracted to CCl,, and selectively reduced with Na,SO, and I- is back-extracted into an acidic aqueous phase. The I- is then re-oxidized and re-extracted. The isolation procedure takes 15-20 min.

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

343

Pietra et al. [76] used combustion distillation to separate I. I, is distilled from the combustion vessel onto a column of hydrated manganese dioxide kept at room temperature. Heating the column to 300° C leads to release of I,, which is trapped on a silver-coated quartz wool. The overall separation time is 20 min. Cessiimi - Biran and Guinn [130] separated Cs by removal of 24Na with a HAP column, and adsorption of Cs on ammonium molybdophosphate (AMP) solid by stirring 50 mg AMP in the solution. Ninety-nine percent of the Cs is adsorbed on the AMP precipitate, removed by filtering.

Plutirzim - Platinum is determined usually as gold due to the chain of reactions: I9'Pt (12, y) 199Pt% IWAu. Although Ig9Aucan be formed also from gold Ig7Au ( n . , ~lg8Au ) (n,y) Ig9Au, its contribution can be subtracted by the measurement of both I9*Au and IwAu. Esposito et al. [132] separated gold by using anion exchange column in HCl media. The chlorocomplex of Au is strongly bound to the column even in 0.5 M HC1 media. The chlorocomplex of Au is strongly bound to the column even in 0.5 M HCl. Au is eluted with 0.1 M HCI containing thiourea. Zeisler and Greenberg [133,134] separated gold by heating the solution of the digested material to above 250"C. At these temperatures, gold compounds decompose and an auto-reduction to elemental gold takes place. The process was done sufficiently slow, to allow the gold to precipitate in the form of fine particles agglomerating to the form of a nugget, or in the form of a leaflet accumulating to a leak-like precipitate, without the inclusion of the matrix activity. Kucera and Drobnik [ 1351 separated gold as metal by coprecipitation with selenium after reduction with ascorbic acid in highly acidic media. Xilei et al. [136] separated gold by electrolysis on niobium cathode (with a graphite anode) at 800 mV. The recovery yield is determined by reirradiation. Czauderna [ 1371 separated gold, for determining Pt, either by precipitation with dimethylglyoxime or by extraction with ethyl acetate, Taskaev et al. [ 1381 determined Pt by Au separation by extraction with Cu(DDC), into chloroform. (DDC = diethyldithiocarbamate).

Mercury -Grimanis and Kanias [1391 separated Hg, after wet-digestion with H2S04/ HNO, mixture, by quantitative extraction of HgI, from 10 M H2SO4-O.025 KI media into toluene. HgI, is back-extracted into aqueous phase containing 0.034 M EDTA + 5% ammonia. Biso et al. [140] separated mercury by combination of two precipitation steps. In the first step elemental mercury is precipitated by reduction with hydroxylamine in the presence of ammonia. The precipitate is filtered and dissolved in HNO,. Hg(SCN), is precipitated in the second step by the addition of NH4SCN solution. Pietra et al. [76] used combustion-distillation technique to separate mercury. The distillate is further purified by precipitation of HgS with the addition of concentrated solution of thioacetamide.

344

ZEEV B. ALFASSI

8.3.3.3 Separation of several elements Many articles have been published on simultaneous determination of a few elements by RNAA. We will summarize a very small part of these articles. They can be divided according to the kind of the matrix, the kind of the elements or the type of the separation. We will not go according to any of these categories, but just report on some of these studies, as examples. Fardy [98] summarized in table form the studies from 1980-1988.

R a w Earth Elements - Siripone et al. [37] used open wet-mineralization with the addition of Mg(NO,), to prevent losses. The solution of the mineralized biological material is adjusted to pH 2.5, APDC (ammonium pyrrolidonedithiocarbamate) is added and the precipitate is refiltered through a 50 mg carbon on filter. The pH is adjusted to 0.2 and refiltered, later adjusting the pH to 2.5, reducing with ascorbic acid followed by APDC + oxine and filtration. Afterwards the pH is adjusted to 5.0, reducing with ascorbic acid, precipitating with APDC + oxine + cupfei-ron, filtering through a carbon filter. Lastly, pH is adjusted to 7.5, precipitating with APDC and filtered through a carbon filter. Before last and the last precipitates contain REE + some other elements. Tjioe et al. [142] determined 8 W E in biological samples together with 15 other elements. Most fractions contain one to two elements and only the REE fraction contains 8 elements. The samples are digested with H$04 + H202in the presence of REE-carriers (50 pg of each of the eight REEs). Br and Cr are distilled as Br, and chromyl chloride. Na and Rb are absorbed on a HAP column (called in this paper - PAA-polyantimonic acid) in 8 M HC1. Loading the eluant on anion exchange column and eluting with 8 M HCl results in the REE with 32P,42K, 47Ca and 47Sc. REE is precipitated with NaOH in the presence of EDTA. Pietra et al. [76] separated REE from digested material in HNO, by heating to dryness and dissolving in 6 M HE The solution is loaded on a column of anhydrous manganese dioxide (AMD) and eluting with 6 M HE REE, Ag and Se are retained on the column. Lepel and Laul [143] separated REE from biological samples by a method similar to that used for geological materials. The sample is fused with N+02 + NaOH, dissolved in H20, acidified and then brought to approximately pH 10 with NH40H in order to precipitate REE hydroxides. NH40H is added in excess to dissolve Ni hydroxide. The precipitate is dissolved in 10 M HCl and loaded onto anion exchange columns. Fe and Np remain on the column while REE is eluted with the 10 M HCl. The REE is purified by three cycles of hydroxide precipitation +dissolution in HCl +precipitation as fluorides ---tdissolution in H,B03 + HNO,. Collecchi et al. [ 1441 separated REE using cation exchange column. The aciddigested material is heated to dryness, dissolved in 0.5 M HCI and loaded on cation exchange column. The column is washed with 0.5 M HCl and eluted by 0.1 M

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

345

H , q 0 4 followed by elution with 2 M HNO,. Third elution with 6 M HC1 includes the REE.

Orher elenieiirs: Pietra et al. [76] have several procedures for separation of several elements. For example, for As, Sb, Cu, they used digested material in 6 M HC104 and two columns. The first column of copper chloride retains Cu and Sb, whereas the second column of tin dioxide retains As. Rajadhyaksha and Turel [1451 determine Cu, As, Au, Se, Hg, Co, Zn, Ca, Fe, P, Cr, Na and K by a long series of precipitations and solvent extractions. Woittiez et al. [ 1461 used two columns to separate Cd, Mo, Cr. and Co. Cd and Mo are retained together on an anion column while Cr and Co are adsorbed on the chelating resin chelex-100. Vasconcellos et al. [147] developed several rapid separations for determination of a few different elements. As, Hg, Sb and Se were determined by the use of an anion exchange column in 6 M HCI to retain Hg and Sb on the column. The eluate was loaded on a tin dioxide (TDO) column and eluted with 3 M HC1. As and Se were retained on the column. Se and Hg alone were determined by retention of Se on a TDO column and extraction of Hg with Ni(DDC), in chloroform. Yinsong et al. [3,148] developed a rapid separation of As, Cd, Cu, Hg and Zn by extracting Cu and Hg with Zn(DDC),/CHCl, and successively extracted As, Cd and Mn from H2S0,/KI media by methyl isobutyl ketone. Kobayashi [ 1491determined Cd, In and Sn by substoichiometricextraction of indium with NadDC/CHCl,. Indium isotopes are the IRNs for the three elements. 'lSrnInfor Cd, 114mIn for In and 113rnIn for Sn. Fajgelj et al. [150] determined Se, Mo,As and Sb by RNAA by four successive extractions, so that each element was separated alone. Taskaev et al. [ 151developed a extraction method for determination of Hg, As, Cd, Cu, and Zn. Hg and Cu were extracted with dithizone solution in chloroform. Further extractions with trioctylamine/tolueneand NaDDWCHCl, was used to isolate Hg and Cu, respectively. As was reduced to As (111) and extracted with NaDDC/CHCl, from 2 M HC1 media. The same NaDDC/CHCl, was used to extract Cd and Zn from pH 11 aqueous solution. Greenberg [2,152] used hydrated manganese dioxide column to absorb Cr, As, Se, Mo, Ag, Sn and Sb. In order to minimise 32Padsorption, the column was preconditioned with They developed also successive extractions to isolated Cr and Sn in separate fractions. Chatt and coworkers [l] separated As, An, Co, Cn, Fe, Hg, Mo, Sb, Se, and Zn by coprecipitation with CuS + ZnS. Fardy and Mingguang [38] determined Se and Hg by adsorption of their complexes with pyrrolidiiie dithiocarbamate on either C18bonded silica gel or activated carbon. Grimanis and Pertessis-Keis [1531 isolated Hg and Se by solvent extraction with toluene from H,SO,-HBr solution. Firstly, Hg is extracted from 7.5 M H,SO,-O.Ol M HBr, and then Se from 7 M H,SO,-l M HBr. Taskaev [1541 extracted V and Mo by N-benzoyl-N-phenylhydroxylaminein toluene, from acidic solution. Adjusting the pH to 5, Cu was extracted with

346

ZEEV B. ALFASSI

Pb(DDC),/CHCI,. Mn was next extracted with NaDDCKHCl,. Finally Rb and K are precipitated as tetraphenylborates by addition of solution of sodium tetraphenylborate and cooling to 0" C. Dermelj et al. [ 1551extracted Se (after reduction to Se (IV) by heating in 6 M HCl solution) with CCl, containing 4-nitro-o-phenylenediamine. From the aqueous phase at pH- 0.2, Cu, Cd and Co were extracted with NaDDC/ CCl,, while Zn remains in the aqueous phase. Zn was further purified by two extractions. The mixture of Cu, Cd, and Co was separated to each element alone by further extractions. Gharib et al. [156] separated three small groups of trace elements from milk. In the distillate they determined Hg and Br, (separately), Ag and Se were deposited on a copper foil. In the remaining solution they determined Co, Cr, Fe, Mo, Sb, and Zn. Tian and Ehmann [4]determined As, Cu, Cd, and Mo by RNAA. Cu, Cd, and Mo were extracted with NaDDCKHCl, and As was separated from the aqueous phase by tin dioxide column in 10 M HCl-0.2 M HF solution. Dang et al. [ 1571isolated Fe, Co, and Se, each separately, from milk. Se was distilled as SeBr, from HBr solution, collected in chilled water and precipitated as elemental Se with H,SO,. Fe was precipitated as hydroxide in the presence of ammonia. It was further purified by extraction of its chlorocomplex with isopropylether, back extracted and finally precipitated as oxinate. Co hydroxide was precipitated by KOH, and Co was further purified by precipitation as potassium cobaltinitrite. The same group published earlier a single-element separation for Mn, As, Mo, Cu and Zn in milk [ 1581. Nakahara et al. [ 1591 separated As, Cu, Mn, and Zn from human blood serum into two groups: As + Cu and Mn + Zn by extraction with APDCKHC, at different pHs. Meloni et al. [ 1601 adsorbed Na and K by HAP from 1 M HNO, solution, Cu was deposited on a column of grains of metallic Cd. Mn and Br were determined in the eluate from these two columns. After counting, 32Pwas removed by SnO, column and La and Zn were determined in the eluate. Cornelis [161] precipitated together Au, Cu, Zn, As, Sb, Mn, and Hg, as sulfides or oxinates. Samsahl [ 1621 developed a method to separate Br, Hg, Sb, As, and Se by distillation and three anion-exchanges columns. The three columns have the same packings, but were conditioned and later eluted with different acids. This process instead of using one column, and changing the eluants, makes it easier for automation. Br is separated by distillation and absorbed in NaOH. Hg is retained on an anion exchanger in the HSO, form. Sb is retained on the same column in the C1form, whereas As and Se are retained on the column in mixed C1- and Br- forms. A similar system was used by Woittiez and Iyengar [ 1631. They either separated together Mo, As, Cr,Sb, Sn, and Se on hydrated manganese dioxide column (from 1 M HNO, solution) or use three anion exchanger columns with different eluants (1.8 M HSO; for Mo, 0.4 M HBr for Cd and 11.3 M LiCl for Zn and Fe). The final eluant is divided into two fractions containing Co + Cu + Ni and Cr + Mn by the chelating resin - Chelex 100.

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

347

8.3.3.4 Group separation Many group separations were developed for biological materials. Some of them divide the sample to a few groups measured on a Ge(Li) or HPGe detector, while others made very elaborate separations to many groups to enable activity measurements with a well-type NaI(T1) detector in order to measure very small amounts. Figure 8-17 gives the flow chart of Pietra et al. [76] procedure for the determination of 50 elements. The procedure starts with distillation of the elements (CI, Br, I, O L l g,ramplc

Distillate

5 c d 4 M HCI

Wash 30 c d 4M HCI

30 crr? 6M H C l q

Rb. K

Ts

8'" w Bi

Figure 8-17. Pietra et al. 1761 scheme for radiochemical separation for the determination of 50 elements in biological samples (reproduced from Rcf. [76],with permission)

348

ZEEV B. ALFASSI

biological Ash dissolved in HBr

2 mL 10M HCI + 1% HzOz

18 mt 3M HC1+0.2% HzO2

'-4

As.Cu.Se

I

GmupII

Remaining

on column Y

-

Au. Cd.

Hg, Mo.Zn

MD

Peristaltic puap Sr

A Sample soluiIon/Elumt (LSI IIClO,)

Co, Cs, Fe,

Ilg, Zn

Cmting

vessel

Figure 8-19. Determinationof 16 trace elements in biological samples by separation into four groups using three columns (reproduced from Ref. [164], with permission)

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

349

-

Wrl ashinp of sample In Iml conc. HzSOt. * l . S m l HZOtSml 6N HCI ( l o t a l volume 7.5 d ; 9 N H' I 5 N SO:-( L N CI-)

Volaliliralim CI Br I 1 (Tell Sc" ,Hg"

Cs' Rb'

RbMP

Ocm)

HAP,

(1.5cm)

Asv Ng: Wf

X)ml 6N NaOH

7.5ml HzO

volume 35,Oml ( includlng 25ml 01

@---Total

,

deod volume

!mm Ihe !hrce columns (2.2N H'11.7N No', 1.7N CI2.2N.

so? 1.

8 i o l l o e A G l x B d t.400:B i o R r r 63

,,Na+

I'

(3Ocm)

Poi-

(6 em)

Sc"'

(Ca) ] R . E .

NaOH/HIO~ (aulomallc lllrallon pH 2 )

---6N

Washlnp solution lSml pH 2-1.9

( 6 d

x

Fe"'

APDC/Char coal

(311 cm) Co"

HCEHP

(6 cm)

R.E.

Chrler lOOd

(18 cm)

Cr"'

(Nil

3

Mntl

1

Effluent (85-90m1, p~ 6 )

K'

Figure 8-20. A half automated procedure for almost complele separation of 25 trace elements in biological samples (reproduced from Ref. [1651, with permission)

0s) and chlorides-bromides (Sb, Sn, Hg, Au, As, Se, Ge, Re, Ru). The chloridesbromides are separated into six groups (not more than two elements in the same group) by the use of two columns (first anion exchanger and then tin dioxide-TDO) with successive elutions. An arrow out of the column means elution. Elements with an-owsto the column are retained on the column. Elements in brackets are present in more than one group. The non-volatileelements/chlorides/bromideswere separated using successive elutions with six columns: anion exchanger, CuS, TDO, acidicaluminum oxide-AAO, and cation exchanger. The procedure of Pietra et al. [76]

ZEEV B.ALFASSI

350

FI, C a , C u , Z n , G o , Mo. Cd. S b . A u , Hg

Grouol

Evoooorole lo new dryness Add I ml I8 M H S O Diilil lwice rim I ml 01 48 Of. ond 1 ml of 30% HpOZ Distil lwicr with Sml of H P

Her

h a , Se

Oissolrc *I ( I +I I HNOj POIS through AAO column Woih rilh 50ml of HNO) W I )

Elllucnl

Sc, CI 80.

,

LO,

CS , , Sm,

Rb.

Ce

Group

Eu

3

Figure 8-21. Yeh et al. [166] scheme for RNAA determination of 21 elements (reproduced from Ref. [166], with permission)

for 39 elements determination by five columns with single eluant described for geological material was used also for biological material (Fig. 8-14). Although 50 elements can be separated and determined, most studies do not need so much information, and most studies measured 10-20 elements in more simple methods. Pietra et al. [76] have several schemes for this purpose. Van Renterghem and Cornelis [23] determined 10 elements in human serum. Their procedure divides 13 elements (Na, K, and Br were not determined in the serum) into three groups by using an anion exchange column in the Br- form with two elutions. The scheme is given in Fig. 8-18. Sixteen trace elements were determined by the same groups using three columns and one eluant [1641as can be seen in Fig. 8-19. Schuhmacher et al. [165] developed an almost complete scheme, in order to enable measuring the activities with well-type NaI(Tl) detector, to measure the contents of 25 trace elements in small biological samples (1-15 mg). The scheme is based on consecutive 14columns divided into three groups (3,7,4).For all columns in one group, the same eluant was used. Between the second and third group the eluate is automatically titrated to pH 2. The scheme is given in Fig. 8-20. Figure 8-21 gives the scheme of Yeh et al. [1661for group separation,measuring 21 elements, not including Na and Br

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS

35 1

which can also be measured. This scheme includes distillation of Br as an element, a HAP column, As and Se distillation as bromides and an acidic aluminum oxide AAO column. Saiiisahl 11671 developed an automatic system for rapid separation. This system was the basis for development [ 1681 of an automatic group separation system for the determination of about 32 elements (other eight can be measured but were not detected in the biological materials). Several later studies were devoted to development of automatic systems (see for example Ref. [46,169-173]), however, it is good only for very specialized laboratories and this trend was neglected in the last decade. The scheme of the separation by ion exchange columns developed by Samsahl and coworkers [ 167,1681 is the basis for most present separations.

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132 Esposito, M., Col1ecchi.P.. Oddone, M., and Me1oni.S.. J. Radioanal. Nucl. Chem.. 113 (1987) 437. 133 Zeisler. R. and Greaibcrg. R.R.. J. Radioanal. Chem.. 75 (1982) 27. 134 Greenberg. R.R.. Fleming. R.F., and Zeisler, R.. Environment Itit.. 10 (1984) 129. 135 Kucera. J. and Drobnik, J.. J. Radioanal. Chem., 75 (1982) 71. 136 Xilei. L., Heydorn, K.. and Rietz, B.. J. Radioanal. Nucl. Chem., 160 (1992) 85. 137 Czauderna, M.. J. Radioanal. Nucl. Chem., 89 (1985) 13. 138 Taskaev. E., Karaivanova, M., and Trigorov, T., J. Radioanal. Nucl. Chem., 120 (1988) 75. 139 Grimanis. A.P. and Kanias, G.D., J. Radioanal. Chem., 72 (1982) 587. 140 Bso, J.N., Cohen, I.M., and Resnizki. S.M., Radiochem. Radioanal. Lett., 58 (1983) 175. 141 Lavi. N., Mantel, M., and Alfassi, Z.B.. Analyst, 13 (1988) 1855. 142 Tjioe. P.S.. Volkers. KJ.. Kroon. J.J., and De Goeij. J.M.M., J. Radioanal. Chem.. 80 (1983) 129. 143 Lepel. E.A. and Laul. J.C, J. Radioanal. Nucl. Chem., 113 (1987) 275. 144 Collecchi, P., Esposito. M., Meloni, S.. and Oddone. M., J. Radioanal. Nucl. Chem., 112 (1987) 473. 145 Rajadhyaksha. M.M. and Turel. Z.R., J. Radioanal. Nucl. Chem., 156 (1992) 341. 146 Woittiez, J.R. and de la c r w Tangona, M., J. Radioanal. Nucl. Chem., 158 (1992) 313. 147 Vasconcellos, M.B.A.. Maihara. V.A., Favaro. D.I.T.. Armelin, MJ.A., Toro, E. Corks. and Ogris. R., J. Radioanal. Nucl. Chem.. 153 (1991) 185. 148 Zhuang, G.S., Yinsong. W.. Min. Z., Wei, Z., and Jianhua, Y.. J. Radioanal. Nucl. Chcm.. 129 (1989) 459. 149 Kobayashi. K., Analyst, 113 (1988) 1121. 150 Fajgelj, A., Dermelj, M., Byme, A.R., and Stegnar, P.,J. Radioanal. Nucl. Chem., 128 (1988) 93. 151 Taskaev, E., Penev. I., and Kinova, L., J. Radioanal. Nucl. Chem., 120 (1988) 83. 152 Greenberg, R.R.. Anal. Chem., 58 (1986) 2511. 153 Grimanis, A.P. and Pertasis-Keis, M., J. Radioanal. Nucl. Chem.. 113 (1987) 445. 154 Taskaev, E., J. Radioanal. Nucl. Chern., 118 (1987) 111. 155 Dermelj. M., Byme, A.R.. F m k o , M., Smodis, B.. and Stegnar, P., J. Radioanal. Nucl. Chem., 106(1986)91. 156 Gharib. A., Rahimi, H., Pyrovan, H., Raoffi, Chem., 89 (1985) 3 1.

NJ., and Taherpoor, H., J. Radioanal. Nucl.

157 Dang. H.S.,Desai, H.B.. Kayaslh. S.R., Jaiswal, D.D., Wadhwani, C.N., and Somasundamn, S., J. Radioanal. Nucl. Chem., 84 (1984) 177. 158 Dang, H.S.. Desai. H.B.. Jaiswal, D.D., Kayasth, S.R., and Somasundararn, S., 1. Radioanal. Chem., 77 (1983) 65.

RADIOCHEMICAL NEUTRON ACTIVATION ANALYSIS 159 Naliahara, H., Nagame. Y., Yoshizawa, Y., Oda. H.. Gotoh. S.. and Murakami, Chcm., 54 (1979) 183.

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160 Mcloni. S.. Caramella-Crcspi. V., and Fassi. G., J. Radioanal. Chem., 34 (1976) 113. 161 Cornelis, R., J. Radioanal. Chem.. 15 (1973) 305. 162 Samsahl. K.. Anal. Chem.. 39 (1967) 1480. 163 Woittiez, J.R. and lyengar. G.V..Fresenius Z. Anal. Chem., 332 (1988) 657. 164 Xilei. L., van Renlerghem, R.Cornellis, and Mees, L., Anal. Chim. Acta, 211(1988) 231. 165 Schuhmacher, J., Maier-Borst, W.. and Hauser. H.. J. Radioanal. Chem., 37 (1977) 503. 166 Yeh.

S.J.,Chen, P.Y., Ke, C.N., Hsu, S.T.. and Tanaka, S., Anal. Chim. Acta, 87 (1976)

167 Samsahl, K.. Nukleonik, 8 (1966) 252. 168 Samsahl. K.. Wester. P.O., and Landstrtlm. O., Anal. Chem.. 40 (1968) 181. 169 Goode, G.C.. Baker, C.W., a i d Bro0keN.M.. Analyst, 94 (1969) 728. 170 Samsahl, K., Sci. Total Environ., 1 (1972) 65. 171 Elek, A., J. Radioanal. Chem., 16 (1973) 165. 172 Liverns, P.. Cornelis, R., and Hoste, J., Anal. Chim. Acta, 80 (1975) 97. 173 Gaudry, A., Maziere, B., and Comar. D., J. Radioanal. Chem., 29 (1976) 77. 174 Wester, P.O. Brune, D.. and Samsahl, K.. Int. J. Appl. Radiat. Iso.. 15 (1964) 59.

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CHAPTER 9

Determination of trace elements by electron spectroscopic methods

MICHA POLAK Departmetit ofhiaterials Eiigiiieeritig,Beti-Gurioti Utiiversity ofthc Negev, Beer-Sheva84105,Israel

Contents 9.1 9.2

9.3 9.4

Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .359 General review of X-ray photoelectronand Auger electron speclroscopies . . . . . . . 362 9.2.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 363 9.2.2 Surface sensitivity and depth profiling . . . . . . . . . . . . . . . . . . . . 369 Chemical-slate information . . . . . . . . . . . . . . . . . . . . . . . . . . 374 9.2.3 9.2.4 Quantitativeanalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375 XPS/AES applications in trace element determination . . . . . . . . . . . . . . . . . 384 Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 389

9.1 Introduction A great variety of modern spectroscopic methods of surface analysis are now available, which in contrast to classical techniques, can furnish quantitative, atomiclevel chemical information about elements composing the surface region. Either electrons, ions or photons are used to excite the surface atoms, typically under ultra-high-vacuum conditions to avoid surface contamination. Each technique is usually named after the emitted particle which is analyzed and detected by it. Thus, in electron spectroscopy electrons carrying information on the quantum states of surface atoms are measured. Using electrons has the following advantages

1. Most important in surface analysis, the short mean-free-path (A-0.4 -3 nm) of slow electrons in solids (energies up to -2 kiloelectron volts), make them suitable for probing surfaces, i.e. obtaining surface-selective information. Slow electrons emitted from atoms composing deep layers loose energy by collisions, and do not contributeto the characteristicspectrum lines;

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2. Relatively easy determination of electron energy and angular distributions;

3. Electrons disappear from the test chamber after analysis;

4. Efficient detection and counting. Since their advent before a quarter of a century the two most widely used techniques for surface chemical analysis, X-ray Photoelectron Spectroscopy (XPS 01‘ ESCA) and Auger Electron Spectroscopy (AES), have been applied successfully in numerous material issues in metallurgy, thin-film semiconductors, ceramics and polymers. In addition to basic research on well-defined samples, such as mechanisms of molecular adsorption and reactions on atomically-clean single-crystal surfaces, both techniques have been very useful in ‘real world’ surface analysis, addressing practical problems in coiTosion, catalysis, adhesion, microelectronics, etc. Probably this category of industrial applications, which includes also trouble-shooting and quality control, has contributed the most to overwhelming developments, experimental-instrumental as well as theoretical, in these methods during the years. Two other powerful method, which together with XPS and AES constitute the so-called “Big Four” surface analysis techniques, use ions for probing the surface chemistry: SIMS (Secondary Ion Mass Spectrometry) and ISS (Ion Scattering Spectroscopy). Part of the many available methods can provide information on physical properties, such as atomic-level structure of surfaces and thin films (adsorbate geometry, for example), surface vibrations and electronic states. While structural information cannot be obtained in conventional XPS/AES experiments, the high “chemical surface sensitivity” inherent in these techniques enables the detection and analysis of a small fraction of a monolayer on a solid surface or a few atomic layers below it. Although new developments in spectrometer technologies have also included improved sensitivities, under normal operation, both can’t be considered as trace analysis techniques. In particular, depending on spectrometer performances and the specific element analyzed, it is not possible to detect less than 10-2-10-4 impurity concentration in the bulk. However, if a trace analyte originally present in a solid, gas or a solution is deposited as a very thin film on a solid surface (see Fig. 9- l), these surface sensitive methods with their intrinsic high absolute detection power, become suitable for trace analysis with good accuracy and precision. (The absolute detection limit can reach lo3-lo4atoms using AES with highly-focused and relatively intense electron-beam, see Section 9.2.4.3). Obviously, since the information comes from the outermost few atomic layers, for best results, diffusion of the deposited or adsorbed material into the bulk of the carrier should be absolutely prevented. Then, detection limits can be comparable to those achieved with more conventional trace analysis techniques, such as those described in other chapters. Indeed, reported detection sensitivities in the parts-per-billion (and in some cases N

N

DETERMINATION OF TRACE ELEMENTS

36 1

Figure 9-1. Illustration showing the principle of applying surface-sensitive techniques, such as electron spectroscopy, in trace element analysis: the analyte, which originally is homogeneously distributed in the bulk (a), should be deposited as a very thin layer (preferably in the monolayer range) on a suitable solid surface (b). While the sensitivity of XPS/AES may be insufficient to detect Ihe aiialyle atoms in case (a), a bulk-sensitive technique (having several orders of magnilude larger sampling-depth) is expected to give the same signal intensity for (a) and (b)

even higher) range, make XPS and AES attractive tools in environmental, biomedical or geochemical applications. While in most cases the analyte is “artificially” trapped from solution on a suitable surface, in atmospheric environmental studies, for example, elemental or compound surface accumulation occurs “naturally.” In any case, these techniques can’t detect less than about 0.1-1% of a monolayer. The number of publications concerning the use of these powerful techniques in trace analysis is quite limited. (This does not mean that there are not unpublished works done routinely in analytical laboratories!). Therefore, a principal objective of this chapter is to draw the attention of a wide audience to the merits of XPS/AES in trace analysis applications. It should be noted that conventional XPS seems to be advantageous in trace analysis since electron beam damage in AES (e.g. by electron-stimulated desorption) can usually be much more severe than damage caused by X-ray irradiation, especially in cases of organics and non-metallic thin layers. Obviously,the issues of XPS/AES quantificationand characteristicdetection limits deserve special emphasis in the context of trace element analysis. Other useful aspects of these methods, such as analysis of elemental chemical-state and in-depth distributions, are discussed as well. Electron spectroscopic instrumentation, which is beyond the scope of this review, and additional spectroscopic issues can be found

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in numerous books specific to these techniques [ 11. Briefly mentioned are some nonconventional electron spectroscopic experiments with improved sensitivity, which were proposed in the literature. Before the concluding section of the chapter, the performance of conventional XPS/AES in trace analysis is demonstrated, and procedures for analyte separation and deposition/trapping on solid surfaces of appropriate carriers are reviewed. 9.2 General review of X-ray photoelectron and Auger electron spectroscopies

These techniques are based on energy analysis of slow electrons which are emitted from atoms excited by X-rays and an electron beam in XPS and AES, respectively. All elements, except H and He, can be detected, and besides their qualitative and quantitative analysis, AES and XPS can furnish specific chemical-state information. Since in both spectroscopies, peak overlap is quite rate, simultaneous multielement analysis is straightforwardly feasible. Microprobe analysis, namely, analysis of small areas and determination of the lateral distribution of elements across an inhomogeneous surface, is another advantage. Using a focused electron beam, AES can provide much higher spatial resolution compared to XPS, although recently, remarkable progress has been made also in microprobe XPS. Furthermore, combining XPS/AES with controlled inert ion bombardment provides quite detailed information on in-depth concentration distributions (“sputter depth-profiling”). Together with the former capability, they furnish what can be called a “three-dimensional chemical analysis.” Table 9-1. Characteristics of X-ray photoelectron spectroscopy and Auger electron spectroscopy Elements detected

Quant. ChemicalSampling Spatial Detection limits’ Quant. depth resolution3 [conc.] anal. anal. state [monolayersl [mono~ayers] accuracy’ precision information

XPS 2 > 2

2-10

5-50 ,urn

10-2-10-4 10-’-10-~

20-30%

f5%

Directly from “chemicalshifts” in core levels

AES 2 > 2

2-10

5-100 nm

10-2-10-4 10-1-10-3

20-30%

55%

Mainly from CVV lineshape and fine structure

’ The range represents variations in elemental detection sensitivity. It can change depending on the spatial resolution and the spectrometerperformance. Detection limits can be improved by a factor

of 10-100 with non-conventionalelectron spectroscopicexperiments mentioned in the text.

* Achievable by using standard procedures based on published elemental sensitivity factors. Higher

-

accuracy is possible by using locally produced standard specimen, for example. In conventional XPS 2 mm.

DETERMINATION OF TRACE ELEMENTS

363

For all these reasons, both methods have become very popular for practical surface analysis. Table 9- 1 summarizes principal characteristics of the two methods. Other advantages include the ease of use and operation, and the ability to analyze “real surfaces,” without the need for any special sample preparation (in comparison, trace analysis involves the “artificial” preparation of surface layers). Thus, vast amounts of support data have been accumulated during the years and are available in various handbooks [2,3], textbooks [ 13 and specialized journals [4]. Before introducing the two electron spectroscopies in some detail, it should be noted that for achieving a correct and comprehensive solution of a surface problem, often combination of several complementary techniques is desirable. 9.2.1 Basic pr-iiiciples

There are three basic stages in an XPS or AES experiment (see Fig. 9-2): 1. Excitation of sample atoms resulting in the emission of characteristic (relatively slow) electrons from the surface region;

2. Energy analysis of the emitted electrons by an electrostatic analyzer; 3. Detection and amplification of the electron signals to produce an “electron spectrum.”

Figure 9-2. The three principal stages in XPS/AES experiments: (a) excitation and electron emission. A is the electron mean-free-path; the effective probing (“escape”) deplh, &a, can be decreased by choosing lower electron take-off angles (Y), thus enhancing surface sensitivity, see Section 9.2.2.2;(b) electron energy analysis; (c) electron signal detection

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Figure 9-3. The basic atomic mechanism involved in photoelectron (photoemission) spectroscopy. Electron binding energies are usually referenced to the Ferrni-level (Ef) of the solid (metal in this case); Ev - vacuum level.

The fundamental well-known physical phenomena behind the first stage are the photoelectric effect in case of XPS (Fig. 9-3) and the Auger process in AES (Fig. 97). In particular, in XPS, the energy of a photon (hv) is transferred to an electron in an atomic orbital having binding-energy E,. If hv exceeds E, (plus a few eV work-function, q5), the electron is emitted outside the solid surface with characteristic kinetic energy, E , = hv - E, - 4 (9.1) Since core-level energies are unique to each element, and hv is fixed (typically Mg or Al-Ka), measurements of photoelectron energies constitute means to identify elements composing the surface. This includes all elements with 2 > 2 (Fig. 94), and since line overlappings are quite rare, qualitative analysis with XPS is frequently an easy task. Figure 9-5 shows an example of XP spectra measured for air-exposed Na P-alumina [6]. Besides the photoelectron lines of Al, 0, Na and C (contamination layer), characteristic Auger lines appear in the spectra. These X-ray induced Auger electron spectral lines, denoted XAES, often contain useful and complementary information on chemical states [7],and can be used in highsensitivity quantification [8]. The photoelectron spectrum shows in Fig. 9-6 is taken from one of the early studies demonstrating the applicability of XPS in trace analysis [9]. In spite of the quite low bulk concentration (1 ppm) of the trace ions, after ion-exchange absorption decent signal-to-noise ratios have been observed for all the elements tested except cadmium and barium. (The Ba line obscured by the nearby oxygen Auger lines, can be resolved by using AI-Ka instead of Mg-Ka). Electron emission in AES (or in XAES) involves a somewhat more complex process compared to photoemission. The Auger transition is shown schematically

365

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I

1

1

10

20

30

I

I

1

40

50

60

I

I

I

100

110

I20

3 >-

ZW

104

z W

(3

z 0

lo3

z, m Io20

70

80

90

ATOMIC NUMBER, 2

Figure 9-4. Binding energies of core-level electrons vs. elemental atomic number

BINDING ENERGY, eV

Figure 9-5. Binding energy, eV

in Fig. 9-7. After an initial ionization, induced by a few kV electron beams (or Xrays) with energy Ep > E l , the excited atom can relax by the transfer of an electron from higher levels (e.g. E,) to fill the El core-hole. In the final stage, the extra energy (= E2 - El) can be released as a photon (X-ray emission) or radiationlessly by ejection of an electron from E, or from higher orbitals. The kinetic energy of this so-called Auger electron is given approximately by,

(In case of a solid, the kinetic energy is slightly reduced by the work-function, 4). All elements with 2 > 2 can emit Auger electrons and are quite easily identified since line overlapping is not common and several principal Auger transitions occur for most of them (Fig. 9-8).

MICHA F'OLAK

366 c u2 p

Pb4f

Blndlnq Energy, r V

Figure 9-6. X-ray photoelectronspectra of a mixlure of ions (1 ppm in solution) adsorbed on acrylic acid grafted polypropylene[91

Figure 9-7. Illustration of the mechanism leading to Auger electron emission: I -electron bombardment (X-rayin XAES), I1 - atomic ionization, I11 - intra-atomic relaxation, and 1V ejection of an Auger electron: (schematic energy-level diagram of a free atom)

Since Auger signals are relatively weak (but sharp) and superimposed on a large, but more gradually varying background (see Fig. 9-9 and Section 9.2.4.3), a standard approach for data acquisition is to take the derivative of the Auger energy spectrum (using modulation and phase-sensitive detection). AES data acquired in a study of single-crystal Na @-alumina[ll] are shown in Fig. 9-10. In this special case of a highly anisotropic superionic conductor (but an electrical insulator) the electrical field due to the impinging electron beam, triggers fast Na ion migation from the bulk towards a certain surface (Fig. 9-1Oc). The surface-sensitivityof AES enabled

DETERMINATION OF TRACE ELEMENTS

367

90

85 80 15 70

65 60 55

50 45 40 35 30

25 20 15 10

5

0

200

400

600

800 1000 1200 1400 1600 ELECTRON ENERGY (eV)

1800 2000 2200 2400

Figure 9-8. Auger elcclron energies of thc cieinenis [3]: The speclroscoDic nolalions KLL. LMM and MNN refer to the three shells involved in the Auger innsition ( E l ,Ez and E3 respectively, see Fig. 9-7)

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Kinetic

Figure 9-9. Schematicrepresentation of the energy spectrum of back-scatteredsecondary and primary electrons emitted from a solid surface which is subjected to bombardment by an electron beam with energy Ep(taken from Ref. [lo])

quantitative characterization of this unusual diffusion process. However, it should be noted that such perturbation caused by the electron beam is usually unacceptable in AES quantitative analysis of surfaces. The next example presented here is relevant to trace analysis since it deals with the common phenomenon of the spontaneous accumulation of bulk impurities at the surface (or interface) layers of metals and alloys ('surface segregation'). It has wide technological implications [12], such as in the case of sulfur, which is a common impurity in nickel and exhibits extensive surface and grain-boundary segregation at elevated temperatures, often leading, in the latter case, to embrittlement of the metal. AES has been used to study the kinetics of segregation of sulfur at the surfaces of a Ni-9% A1 alloy containing about 200 ppm S in the bulk ( [13] and Fig. 9-11). While without annealing, no sulfur signal could be detected, as can be seen, after about 20 min of 700" C anneal, the S intensity starts to exceed the Ni signal. Argon sputtering revealed that the sulfur is confined to the outermost surface layer, and intensity analysis (next section) can be used to estimate its coverage. Thus, a spontaneous (or artificial) accumulation of a low-concentration element as a very thin layer enables its characterization by AES (or XPS).As already mentioned, this forms the basis for utilization of these techniques in trace element analysis.

DETERMINATION OF TRACE ELEMENTS

1

1

1

200 400 600

369

1

800

1

I

I

loo0 lzoo 1400

ELECTRON ENERGYeV. Figure 9-10. AES measurements of single-crystalNa P-alumina demonstratingelectron-inducedNa ion migration to the surface: (a) spectrum of the as-inserted sample acquired with relatively low beam current density: (b) spectrum acquired after short Ar bombardment ('sputtering') removing mainly carbon surface contamination, and (c) the spectrum resulting from several minutes of relatively high current electron bombardment done after (b) [I 11

9.2.2 Surface sensitiviry and depth profiling 9.2.2.1 The probing depth A key factor in the electron spectroscopy sensitivity to surface atoms, namely,

its shallow probing depth, is the use of slow electrons with relatively short inelastic mean-free-path (IMFP, A) as the information carriers. Thus, the residual number of electrons ( N ) , left with their initial energy after traveling a distance d in a solid is given by N = Noexp(-d/A) (9.3)

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--.---.-.-LA--

__._------

AN--

0

60

120

180

240

300

Annealing time (rnin.1

Figure 9-11. Studying sulfur surface segregation in Ni-9% A1 alloy [13]: ratios of sulfur to nickel AES line intensilies as a function of time at three leinperalures

where N o is the initial number, such as that produced inside the solid by the photoelectric and the Auger processes. This attenuation is due to energy losses by inelastic collisions of the moving electrons with the matrix (A is the average distance between successive events). Consequently, these electrons do not contribute to the characteristic signal. As electrons are emitted deeper within the sample, the probability of escaping outside the surface is reduced exponentially. (95% of the detected signal intensity originates from atoms within the so-called informationdepth, 3 4 . Depending on the nature of the solid matrix (metal, insulator, etc.), X values vary significantly with the electron kinetic energy (Fig. 9-12). In particular, it exhibits a minimum value of several angstroms at several tens eV kinetic energy, so that for highest surface sensitivity, lines around this energy range should be measured. (In photoemission this can be readily achieved with a tunable excitation source, i.e. synchrotron radiation). Recently, advanced theoretical approaches [ 151, some of which include also effects of elastic scattering [16], have been introduced, giving more accurate estimations for the attenuation of Auger or photo electrons in solids. In particular, the “attenuation length” (AL) or “escape-depth” (ED) to be used in quantification of thin overlayers (Section 9.2.2.2) is equivalent to the average net distance traveled by an electron between inelastic as well as elastic scattering events. However, it has been claimed that in quantitative AES analysis (Section. 9.2.4) IMFP should be used rather than AL [ 171. The rather complex issue of accurate definitions and comparison of these three parameters has been addressed by Jablonski and Ebel [ 181.

DETERMINATION OF TRACE ELEMENTS

371

Energy (eV1

Figure 9- 12. Dependence of the inelastic mean-free-path (in monolayers)on the electronenergy; dots -experimental values for elements; curve -a least square fit to an analytical expression often used in the literature [ 141

9.2.2.2 Quantification of thin overlayers and composition gradients In many cases of practical importance, modifications in chemical composition and bonding are not confined to the outermost surface layer (monolayer), but extend to depths of several nm or even much more. Air oxidation products, corroded metals or multilayered electronic materials are just a few examples. (Predeposition for trace analysis can result in a several atomic layer-thick analyte, although better accuracy is expected for the monolayer range). The inherent shallow probing depth of XPS/AES obviously limits the information-depth obtainable. But even for very thin films (< 3 4 one often wants to know the thickness and the detaileddistribution of elements and chemical states as a function of depth. Equation (9.3) forms the basis for estimation of overlayer thickness from measured substrate line-intensities before and after deposition. Likewise, this simple attenuation law can be readily applied to derive expressions for XPS/AES intensities from elements composing a thin overlayer or a submonolayer, by which thickness and coverage, respectively, can be evaluated from experimental data. For the case of signal intensity (I) from a submonolayer, one obtains,

I = OIo[1 - exp(-u/X sin Y ) ] ,

(9.4)

where I"is the intensity from the same element as a bulk sample, 6 is the coverage (monolayer fraction), u denotes its thickness, and 4' ' is the angle between the surface

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and the electron take-off direction (XsinY is the “effective escape depth,” see Fig. 9-2). The linear relationship expected between the element signal intensity and its coverage (which, obviously, is linear with the deposited weight) can be used in trace analysis in coiljunction with calibration curves. When the ratio between line intensities from deposited element or adsorbate ( I Aand ) substrate element (In) is measured (a procedure that usually yields higher precision), its dependence on coverage is generally not linear (except at low coverages) [19]:

--8, 1 - OA

- IAIg { 1 - exp[-u/X,(E,)

sinY]}

IBIi{ 1 - exp[-u/XA(EA) sinY]}

(9-5)

Equations pertinent to quantitation of coverage on rough (non-polar) surfaces with complex structures are given in [21]. Non-destructive depth profiling can be accomplished through angle-resolved measurements of spectral intensities obtained by titling the sample with respect to the analyzer entrance, as illustrated in Fig. 9-2. Enhanced sensitivity for surface elements can be achieved by using low electron take-off angles (“grazing-angle”). Another non-destructive procedure for extracting in-depth information (such as layer thickness or relative location of an element) compares intensities of low and high energy lines of the same element since they reflect different information depths. As a reference, ratios of intensities measured for the pure element (or for a homogeneous surface region) are used. In this case too, one is limited by the electron information-depth, and for deeper concentration gradients sputter etching by inert ions is commonly used. The sample surface is eroded by a 0.5-5 kV ion beam and the newly exposed atomic layers x e analyzed by XPS or AES (see Fig. 9-10 above). Approximate sputtering rate calibration can be accomplished by measuring the time it takes to sputter, under the same conditions a chemically similar layer of known thickness. Alternatively, the sputter depth can be calculated from the measured ion-beam current (J,Am-’), the sputtering yield of the sample (5’ in atoms/ion, from literature) and the sputtering time, t [22], z = SJMt(lOOOepNAn),

(9.6)

where M ,n,N, and p are respectively, the molecular weight, the number of atoms per molecule, Avogadro’s number and the layer density (kg m-3). Several factors limit the practical depth resolution during depth profiling and result in a certain broadening of interfaces compared to the true profile. This limited depth resolution is associated with bombarded-induced changes of topography (roughening of polycrystalline metal layers) and of chemical composition of the eroded surface due to preferential sputtering of an element [23]. Another smearing effect is associated with the finite probing (escape) depth of the photo or Auger electrons (see above). An example for very high interface sharpness is the case of Ta,O, on Ta developed as

DETERMINATION OF TRACE ELEMENTS

373

I

1

Depth. z .nm

Figure 9-13. Oxygen AES depth profile of a 28.4 nm TaZOs layer on Ta, used as a reference material for testing depth profiling resolution qualily. The depth width (oblained with 2 kV Ar+) over which the signal intensities change from 84 to 16%of their plateau values, 1.40 nm, marks the resolution [22].

v)

w

k cn z w !-

z w

-z -I

v)

0.

x

No ( KLL) 0

I00 200 SPUTTERING TIME. ( S C )

Figure 9-14. XPS peak area vs. spultering time (500 V Art) measured for Na /3-alumina [6]

a reference material for depth profiling (Fig. 9-13). More typical depth resolutions are in the range of several tens nm. Finally, Fig. 9-14 presents an XPS/Ar depth profile measured for air-exposed Na-b alumina (see the initial spectrum in Fig. 9-5), from which the existence of carbon overlayers and extensive sodium segregation from the bulk towards the surface are inferred (the segregation of the highly mobile Na' is attributed to its preferential interactions with atmospheric H,O [6].)

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374

9.2.3 Chemical-state irformation One of the principal advantages of XPS, realized already during its earliest days, is the relatively simple characterization of chemical bonding of surface atoms. Thus, oxidation states of ions, for example, are manifested as “chemical-shifts” in corelevel binding-energies, typically 1-5 eV relative to the energy corresponding to the pure element. In AES (or XAES) such information is somewhat less straightforward, although when valence levels are involved in the Auger process, the remarkable lineshape features observed can be used as fingerprintsfor chemical-state identification. Figure 9-15 shows 3d photoelectron spectra measured for arsenic in three known chemical states together with a spectrum of a black precipitate which appeared under certain conditions during the preparation of arsine for trace-analysis [24]. Clearly, the relatively wide range of XPS chemical-shifts enables in this case to easily identify the cationic valences, and the black precipitate is definitely elemental arsenic. Likewise, anions exhibit characteristic line chemical-shifts, as demonstrated by Briggs et al. [25] in trace analysis of IO;/I- (Fig. 9-16). Considering the observed

-

1

48

1

1

1

46

1

44

1

1

42

1

1

1

40

1

1

38

Binding Energy ( e V )

Figure 9-15. XPS Chemical-shifts - application to cations: arsenic 3d spectra for As2S04, Asz03, elemental arsenic and black precipitate formed during preparation of arsine for trace analysis [24]

375

DETERMINATION OF TRACE ELEMENTS I-

Figure 9-16. XPS “Chemical-Shifts”- application to anions: iodine 3d5/2 spectra measured for 1 p g 10; and I- in 10 pI of KIO3 and KI solution [25]

signal-to-noise ratios, much lower quantities of these ions can be detected by XPS. In many cases the shift is proportional to the ionic valence, with higher binding energy corresponding to larger positive charge. In other cases contributions from electronic relaxation (hole screening energy) lead to deviations from this regularity. In the Auger process, with a two-hole final-state, extra-atomic relaxation can be dominant, leading in cases of conductors, to shifts significantly larger than those observed in photoemission (if the initial hole is in the inner shell). Sodium, for example, is characterized by a very narrow range of photoelectron chemical-shifts. Na Auger lines. on the other hand, exhibit a relatively wide range of kinetic energies reflecting different chemical environments which induce diverse relaxation. Spectra of metallic sodium vs. Na+ on Na &alumina are shown in Fig. 9-17 [ 111. Other examples are summarized in Table 9-2. In such cases, examination of shifts in the energy difference between photo- and Auger electrons in the same XP spectrum (“Auger parameter”), which depend only on extra-atomic relaxation, is sometimes more useful in chemical-bonding identification than either spectroscopy alone [7]. 9.2.4 Quarrtitative analysis Two basic approaches to quantification of XPS and AES, namely relating line intensities to composition, have been established. One is based on first-principle calculations, while the more common and practical second approach uses empirical elemental sensitivity factors. A major problem in quantitative analysis is the accurate measurement of the number of photoelectrons or Auger electrons contributing to characteristic spectral lines [26,27]. In particular, it is necessary to separate these electrons from background electrons appearing in the raw data, which are produced by various processes discussed in Section 9.2.4.3.

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MICHA POLAK

Figure 9-17. Chemical effecls in AES Na (KLL) spectra measured for a Na @-aluminasurface [ 1 I]: (a) Na': (b) metallic Na produced by intensiveelectronbombardment(nole the different line shape and energy, and the plasmon loss structures)

DETERMINATION OF TRACE ELEMENTS

377

Table 9-2. Photoelectron and Auger energy shifts' in selected elements [7] ~

Element Mn Zn Ga Ge As

cd In Sn

~~

Photoelectron2

Auger electron

BE shift (eV)

KE shift (eV)

-1.2

6.2

-0.5 -2.0 -3.2 -3.0 -0.4 -0.8

4.2 6.2 6.7 6.4

-1.4

3.9

5.5

2.6

Shift corresponds to energy in the elemental fonii less that in the oxide fomi. 2The average is taken of two values: 21)2/3and 3d-jl2, or 3d-j/2 and 4d.

9.2.4.1 First principles (theoretical sensitivity factors) The photoelectron and Auger currents which vary linearly with atomic concentration can be expressed by means of a product of several terms related to the excitation process, to electron transport in the solid and its emission outside, and to instrumental parameters. For the case of a homogeneous surface region (with 12 as the number of atoms of the element of interest per unit volume of sample), the detected photoelectron signal current, corresponding to a particular line, reads

I = n@u4!~Xsin Y T ,

(9.7)

where CP - the X-ray flux on the sample (replaced by I,, primary electron beam current,

in AES); (T

- total photoionization cross-section (the corresponding term in AES is the product of three terms: (i) the cross-section for direct ionization by the primary electrons; (ii) primary electron backscattering factor which adds to I , and increases with the matrix atomic number [28], and (iii) the probability for Auger process following ionization which competes with X-ray emission);

4 - angular factor dependent upon the angle between photon and detected electron directions (irrelevant in AES);

T - instrumental collection efficiency related to characteristics of the electron energy analyzer and the detector.

378

MICHA POLAK

The problem of several unknown parameters in this equation is partially overcome by taking relative intensities of lines in the spectrum correspondingto two elements ( A , B): ll,A/lzg = ( 1 A / 1 5 ) ( s 5 / s A ) (9.8) where S, defined according to Eqn. (9.7), is the “Atomic Sensitivity Factor” usually expressed relative to a certain line from a particular element (S = 1). In estimating S, various approximations have been made, assuming invariance of certain terms. For example, one XPS handbook [2] uses S values calculated from theoretical photoelectric cross-sections (Fig. 9- 18) multiplied by energy correction terms only. The latter factor has been modified in a subsequent study [29], and compared to empirically derived sensitivity factors (see below). Likewise, first principle calculations have been made in quantitative AES [30].

9.2.4.2 Empirical sensitivity factors In deriving a set of sensitivity factors empirically, intensity ratios of spectral lines either from pure element standards or from various compounds have to be carefully measured. The first approach is more common in AES with the strongest

10

100 BINDING ENERGY, *V

1000

Figure 9- 18. Photoelectric cross-sections and theoretical sensitivity factors vs. electron bindingenergies [291

DETERMINATION OF TRACE ELEMENTS

379

1“““‘“”’””1

Ep :SkeV

Alomlc number

Figure 9-19. Empirical “Alomic Sensitivity Factors” based on measured peak-to-peali intensities of the differential Auger spectrum for pure elements relative to Ag (MsNsN45) intensity ( [28], compiled from Ref. [3])

I

10

I

I

I

I

I I l l

I

100

I

I

,

,

,

,

1000

BINDING ENERGY. eV

Figure 9-20. X P S empirical and theoretical “Atomic Seiisilivil y Factors” for 3d5/2 transitions [29]

380

MICHA POLAK

Ag line chosen as primary standard (SAg(MsN45N45)=1, Fig. 9-19), while in the second approach XPS intensity data from 135 compounds of 62 elements have been used with SFls= 1 1291. The significant discrepancies noted between the XPS empirical and theoretical values (Fig. 9-20), indicate the existence of fundamental errors in implementation of the latter approach [29]. When pure element standards are used as a basis for first-order approximation in quantitative analysis, it is desirable to include “matrix-correction-factors” as second-order approximation calculated from first principles. For the general case of a multi-element sample, the molar fraction of each element is given by [28],

where in XPS, the matrix dependent F factor consists of two contributions, namely, atom densities and attenuation lengths, while in AES it includes electron backscattering contributions as well. Calculated matrix factors for pairs of elements published in the literature [28,31] show that usually this correction cannot be ignored in both XPS and AES. It should be noted that measuring the pure element intensities (I”) in the same spectrometer (and under the same experimental conditions!) is preferable to using published values. In this third route to quantification, a relatively high degree of accuracy can be obtained by the use of locally produced “real” internal standards, namely, measuring relative intensities from adequately prepared sample (scribed under UHV, for example) with a chemical matrix similar to that of the test specimen. However, since such samples are often not available, application of one of the above-mentioned quantification methods is quite common. Of particular importance in quantitative analysis is the accurate measurement of photoelectron or Auger current, namely the correct line intensities in a spectrum. Therefore, severe efforts have been made to develop appropriate procedures for background subtraction, deconvolution of overlapping lines (“curve fitting”) and other data processing methods [l, 26,271. Recently, the validity of traditional methods for background subtraction from XPS lines has been studied by comparison to first-principle intensity calculations [32]. In particular, a much higher accuracy has been achieved using a novel method based on a detailed description of the physical processes involved. Finally, it should be recalled that all the above equations for XPS/AES quantification apply only to homogeneous compositions (at least within the information depth, 3X). Otherwise, modifications should be made in accordance with expressions for overlayer (or “underlayer”) intensities such as reviewed in Section 9.2.2.2. For example, Wagner [33] introduced a set of “monolayer sensitivity factors” derived in a simple way from the corresponding (empirical) bulk factors [29].

-

DETERMINATION OF TRACE ELEMENTS

381

9.2.4.3 Detection sensitivity Despite its importance, this somewhat complex issue is rarely addressed in XPS/ AES textbooks or review articles. However, in the context of trace analysis, it deserves a detailed discussion, based here primarily on theoretical studies by Cazaux [8,34,35]. Thus, this section describes the principal factors affecting detection sensitivity in XPS and AES, highlights means for its enhancement and presents some estimations of detection limits. A pulse-counting signal (common in XPS) is confidently detectable in the presence of background if its amplitude is three times larger than the background noise. Thus, the relative detection limit is given by 2,

= 3(J?BG/t)”210-’

(9.10)

where IBG and I” are the numbers of counts of the background and the pure-element signal, respectively, and t is the measurement time per channel. Clearly, to optimize (minimize) z ,, the signal intensity and the acquisition time should be maximized as much as possible, and the background intensity minimized. However, due to intrinsic limitations in both XPS and AES (see below), x,, turns out to be about at best. Suggested modifications to conventional XPS/AES experiments, which can enhance overall sensitivity, will be mentioned too. Before discussing the complex issues of background and signal intensities, possible improvements in data acquisition are briefly addressed. Increasing the duration of the experiment enhances sensitivity (by t’/*),but there are limitations due to X-ray (or electron) induced surface modifications (desorption, dissociation, etc.). Furthermore, surface contamination by residual-gas adsorption can also limit data acquisition time, depending on the gashrface reactivity. Simultaneous multi-channel detection instead of the more conventional sequential acquisition is another means to improve sensitivity. Finally, it should be mentioned that various mathematical manipulations of raw data (e.g. “digital smoothing,” see Ref. [36]) aimed to reduce noise levels and enhance sensitivity, have been developed. For example, in a recent AES study of phosphorus in silicon, a detection limit of 0.04 at. 9% P was reported, compared to 0.09% achieved without mathematical treatments [37].

9.2.4.4 XPS and AES signal intensities First-principle expressions for the signal intensities were given before (see Eqn. (9.7). Increasing the X-ray flux (higher voltage on the anode is more effective than higher current), or the primary electron-beam current in AES, improves the sensitivity. As already mentioned, radiation damage sets physical limitation to the highest possible dose of photons (or electrons) incident on the sample suiface (see below). Furthermore, 5, is inversely proportional to the photoionization crosssection which varies quite considerably from element to element (see Fig. 9-18), depending on the photon energy to binding-energy ratio. Similar effects on x , in AES are caused by the ionization cross-section and Auger transition probability.

382

MICHA POLAK

9.2.4.5 Background intensity Besides instrumental electronic effects there are three distinct, matrix-related contributions to the background of an XPS line: 1. Photoelectrons (excited by characteristic hv) which are inelastically scattered before leaving the surface. This includes also contributions from neighboring photoelectron lines at higher kinetic energies, and therefore can depend also on composition;

2. Elastically scattered photoelectrons excited by non-characteristic X-rays. (The use of X-ray monochromator to improve spectral resolution, reduces the overall sensitivity since it attenuates the principal radiation but eliminates only this contribution to the background); 3. Photoelectrons excited as in case 2, but which have undergone inelastic scattering (as in case 1). In conventional XPS, this contribution is negligible compared to the other two. All three contributions have been evaluated quantitatively by Cazaux [8], and the derived expressions together with that for the signal intensity allow estimation of detection limits (via Eqn. (9.10) in XPS and XAES. Considerable improvement in sensitivity is predicted for an XAES experiment with continuous synchrotron radiation (CXAES). In particular, the signal can be maximized and the background reduced to its physically lowest level (only from inelastically scattered valenceband photoelectrons), by using intense, tunable X-rays with adjustable bandwidth. Then, calculations give detection limits of for boron and indium, and respectively, in silicon [8], i.e. more than two orders of magnitude higher sensitivity than estimated for conventional XPS. Clearly, such performance makes CXAES a potentially more attractive tool for trace element analysis. In AES, excitation is done by electron bombardment, and the background accompanying a particular line is much more intense than in XPS. It consists of three contributions (see Fig. 9-9 in Section 9.2.1):

1. Backscattered primary electrons which were inelastically scattered; 2. Auger electrons coiresponding to higher kinetic energy transitions, which were inelastically scattered;

3. The “true” secondary electrons.

DETERMINATION OF TRACE ELEMENTS

383

Figure 9-21. Variation of Auger signal-to-noise with electron beam energy (E,) for (a) thin unsupported carbon film, and (b) the same film on copper (bulk) [34]. U is defined as E&b, where E b denotes the binding energy of the level initially ionized in the Auger transition.

All three contributions increase with decreasing primary electron energy. Their quantitativeestimation together with the signal behavior give the variation of signalto-noise with primary energy, which is compared to experimental results for cases of supported and unsupported thin film in Fig. 9-21. Clearly, sensitivity is improved in the latter configuration, and can reach a factor of 10 at higher Auger electron energies [34]. Nevertheless, the estimated detection limit in conventional AES (w is obviously not sufficientfor bulk analysis of traceelements. Considerable suppression of noise can be achieved by correlating AES with Electron Energy Loss Spectroscopy [38], as suggested recently [34,39] and verified experimentally for Fe on Si [40]. This coincidence spectroscopic technique, which under certain experimental conditions may have sensitivity improved by about two orders of magnitude, seems to be another attractive new tool for trace analysis. Finally, the optimization of the incident beam spot size in microanalytical techniques, such as AES and spatially resolved XPS,is briefly reviewed. Generally, for a given electron (or photon) dose received by the sample, the minimum concentration detectable (znl)is obtained by using the largest achievable spot size, while for detecting the minimum (absolute) number of atoms, ym = N a , (N is the number of atoms in the analyzed volume), the smallest achievable spot must be used [34,35]. Thus, considering only beam parameters, yn1is proportional to 6*/IiI2, where 6 is the diameter of the incident beam spot and I, is its intensity. Obviously, one has to take into consideration also the critical dose received by the sample. All these factors affecting AES sensitivity, including comparison to XPS performance,

MICHA POLAK

384

A

\

\

Y

\ I

\ I

..

-

4C-2

40-~

(i

....-

YPP . and ....... ...... ." A F P

Eioiirp T m and ahcnliitp *I I ' a " . " , 0-33 , . a . Rdativp "m.1.Y ""Yv.".ym detertinn limitc " in . . I . . . " . .

Y

z

nc fiinrtinn nf Puritatinn v-

I " Y"

* " . . . I . . . . -I.--."..-..

doses received by the sample for different incident spot sizes (6). The calculation assumes simultaneous data acquisition and signal-lo-baclrgroundintensity ratio of 10 for XPS and 0.1 for AES, while the same values have been chosen for olher parameters: atomic density = 5 x 10'2at./cm', X = 3 nm, cross-section = 5 x lo-*' cm' and (see Eqn. (9.7) in collection efficiency of the spectrometer/detector system T = Section 9.2.4.1). For the same spot size in X P S and AES there are three different situations depending on the relative critical dose received by the sample (D" in XPS,P in AES) which is the limiting factor of sensitivity: (i) Da = ff: the advantage of XPS over AES is its better signal-to-backgroundratio:(ii) D' > Dc: the detection limits of XPS correspond to an abscissa to the right of thal corresponding to the Auger critical The advantageof X P S can reach 2-3 orders of magnitude; (iii) Dx
are summarized in Fig. 9-22. Such estimations can guide users of individual or combined XPS/AES apparatus in choosing conditions for optimal operation.

9.3 XPS/AES applicationsin trace element determination After reviewing various aspects of XPS and AES in general, this section focuses on their performance and limitations in trace element analysis, including reported procedures for analyte trapping to form thin layers on appropriate carriers. Various experimental factors have to be considered and studied when optimizing conditions for such an application: 1. Deposition efficiency on different carriers;

DETERMINATION OF TRACE .ELEMENTS

385

2. Kinetics of deposition and possible (unwanted) diffusion of analyte inside the carrier material:

3. Lateral homogeneity of the deposition and of the carrier depending on its preparation and subsequent surface treatment; dependence of analytical reproducibility on the homogeneity. (Better accuracy is expected for deposited layers in the monolayer range, compared to quantification of sputter profiles, Section 9.2.2.2); 4. Stability of the deposited film:

(a) in air (during transfer to the test chamber), and (b) under electron (or X-ray) irradiation (see Section 9.2.4.3);

5 . Appropriate calibration of the analytical procedures (see Section 9.2.2.2);

6. Experimental (instrumental, spectral) conditions for maximal (overall and surface) sensitivity. As an instructive example, trace arsenic determination by XPS [24], representing one of the early developments in this field, will be described now in some detail. Environmental and food chemistry, water quality control and steel analysis are areas in which arsenic high sensitivity trace analysis is required. In this study, volatile arsine was produced by the reaction of sodium borohydride with arsenic solution, and tapped as mercury arsenide on mercury chloride impregnated filter paper of constant area in a specially designed apparatus. Arsine pickup was measured as function of HgCI, concentration (maximum at 15% - see Fig. 9-23a) and impregnation time (Fig. 9-23b). The chosen analytical procedure was repeated for solutions with several arsenic concentrations, and the measured As to carbon XPS line-intensity ratio was used to obtain a calibration curve (Fig. 9-24), most of which exhibits linearity (see Section 9.2.2.2). It should be noted, however, that choosing carbon signal for quantitative analysis may introduce errors, since this element appears as a common contaminant on most surfaces. Nevertheless, good precision (- 10% for 5 replicate samples) and good accuracy (9.8 f 0.9 microgram As per grain vs. 10 f 2 of the standard reference material) have been reported. A significant advantage of the volatilization method is the capability to lower the detection limit by using larger solution volumes. This was demonstrated by using a 165 ml sample giving a detection limit of 300 ppt (Fig. 9-25a) compared to 3 ppb achieved with the standard 1 ml samples. It was concluded that the XPS/volatilization method is quite competitive with other trace analytical techniques [24]. Two additional advantages of the method have been demonstrated in this study:

386

MICHA POLAK 1

I

1 ‘. Hg CI,

I

I 40

I

I

I

80

I

I20

Impregnation Time ( m i n 1 Figure 9-23. Optimizing conditions for trace arsenic determination by volatilization and XPS [24]: (a) arsine pickup by impregnated paper as a function of percent HgC12 in the impregnating solution. Arsenic concentration - 1 ppm, impregnation time - 30 rnin; (b) arsine pickup as function of impregnation time for 15% HgCl2 solution (5% As

= (IAS / I c > / (IAS/~c)max).

1. The capability of simultaneous multielementanalysis (see Fig. 9-25b). Chemically similar species, which can be volatilized simultaneously, often exhibit distinct spectral features;

2. The relatively simple identification of the analytechemical-statevia core-level “chemical-shifts’’(see Fig. 9-15). On the other hand, attempts to perform As trace analysis by Auger Electron Spectroscopy faced major difficulties (see Table 9-3). As already mentioned, the number of publications dealing with XPS/AES applications in trace element analysis is quite small in spite of its long established promising potential. Table 9-3 summarizes procedures, capabilities and some problems reported in those studies.

387

DETERMINATION OF TRACE ELEMENTS

60-

I

I

-

0

40X

a

L,

20-

I

I

200

I

I

1

600

4 00

I

I

800

I

I000

P p b As

Figure 9-24. Calibration c w e for quantitative trace arsenic analysis. Since one ml of sample has been used, the concentrationcan also be read as nanograms of arsenic [241.

A53d

b

I

45

I

I

43

1

41

EB(eV) Figure9-25. (a) Enhancement of detection limit in arsenic trace analysis: As 3d spectrum recorded for arsenide trapped from a solution 300 parts per trillion in arsenic using large (165 ma) sample volume: (b) Multielement trace analysis; simultaneous deiection of As, Se, Sb and Sn (100 ppb each in 1 ml sample) using the XPS/Volatilization method [24].

388

MICHA POLAK

Table 9-3. Summary of developments in trace element determination by XPS and AES‘ Element Carrier and analyte Detection Comments (ref.) depositioii technique limit2 As [24] Mercury-chloride Elemental arsenic produced at high arsenic impregnated paper concentration; problems with AES: due to (deposit mercury charging, metals had to be used as carriers arsenide); instead of paper, but As was not trapped Volatilization at trace levels Se, Sn, Mercury -chloride Simultaneous analysis Sb, [24l impregnated paper (deposit mercury arsenide); Volatilization Controlledelemental selectivity by chelate Pb, Cu, Glass fiber surfaces chemistry; cation specific; use of TLC plates Hg. ”4 coated with chelating failed due to inward diffusion. ditiocarbamate; Reaction 1411 with ions in solution (“two dimensional ion-exchange”) Inward diffusion of Ag suggests the use of Pb. Ag, Acrylic-acid grafted Cu, Fe, polypropylene; 2D ionthinner grafts: Ba not detected due to Ca, Cd, exchange from solution competitive reactions among the ions and overlapping with Ox Auger lines; cation Hg PI specific. Complication of quantitative analysis, partially Pb, Ba Cleaved calcite (scratched surface); adsorption from because of carbon contamination. Wetting [201 solution (evaporation problems solved by light scratching the calcite in air). surface or soaking it in water for long time. Pb, Bi, Hg-coated Pt electrode; Cation specific; multicomponent trace analysis of solutions in a single experiment Cd [42] electrochemical even when intermetalliccompounds are deposition formed. Problems with quantification because of preferential layering. A1 strip etched by HCI; Large chemical shifts for I- vs. 10; 1 (I-/ identification; Some decomposition of 10; to Adsorption from solution 10;) (evaporation in air) I-; small volume sample (lo-* ml); Uneven 1253 deposition on untreated Al, but longer etching (rougher surface) led to higher detection limits; no competition effects in multielement analysis of both cations and anions. F Soft analysis conditions (low electron and ionA1203, MgO, ZrOz or sputteringcurrents and energies to prevent F 143,441 Er2O3 vapor deposited thin films on glassy carbon desorption;best performance with 50 nm Volatilization as evaporated MgO; precision: 10% (CH3)3 S F S Soft analysis conditions to prevent S Ag vapor deposited on segregation; precision: 10% W3.441 glassy carbon; volatilization I

The studies in the last two rows were done with AES including sputter depth profiling; all thc rest were done using XPS (no sputtering). Lowest concentrations measured. Actual detection limits can be lower, and in some cases they were estimated.

DETERMINATION OF TRACE ELEMENTS

389

Finally, it should be noted that in the characterization of environmental materials, such as atmospheric particulate air pollution material, surface analysis techniques can be applied “naturally,” i.e. without the need for any preconcentration or deposition procedures. Representative examples are a Scanning Auger Microprobe (SAM) study combined with argon sputtering of the surface layers of single particles of a coal fly ash [45],and XPS studies of the chemical state of sulfur in atmospheric aerosols, revealing the presence of sulfate, sulfite and chemisorbed SO, and SO, [46]. Actually, application of XPS or AES in studying surface predominance of elements and organic compounds on the particle surface [47], circumvents drawbacks inherent in the more traditional procedures of solvent leaching followed by trace analysis, using, for example, atomic absorption spectrometry for metals. In particular, the latter approach lacks important features characteristic to SAM, namely, the high spatial and depth resolutions (see Table 9-1), which are crucial for detailed and meaningful analysis of such heterogeneous systems having microscopical dimensions. When this is combined with chemical-state information, easily obtainable in XPS/AES, understanding of fundamental mechanisms for complex environmental materials becomes possible.

9.4 Summary and conclusions The main objective of this chapter is to elucidate and demonstrate capabilities and merits of modern electron spectroscopic methods applied to trace analysis. In particular, if a trace element is trapped as a very thin deposit (preferably in the monolayer range), the two most widely used surface analytic methods, Auger Electron Spectroscopy and X-ray Photoelectron Spectroscopy, can provide quantitative determination with high sensitivity (detection limits < ppb), as well as good accuracy and precision. Continuous efforts in manufacturing more advanced instruments, have resulted in better overall performance, and improvements in data acquisition and manipulation have led to enhanced sensitivity. Various parameters, affecting the signal-to-background relative intensities (and hence the detection limit) have been reviewed above, with emphasis on its optimization including minimum detectable mass considerations. In this context, it has been claimed that what can be considered as the ultimate goal, namely the chemical detection of a single atom (!), will become possible with Auger analysis using a very highly focused (and intense) electron beam. Surface selectivity can be enhanced by analyzing lines corresponding to lowenergy (several tens eV) electrons collected in small take-off (“grazing”) angles. All this can serve the technique user as guidelines for getting the best performance out of a given spectrometer (purely technical/instrumental considerations, which can help in designing a spectrometer, are beyond the scope of this review). Two non-conventional electron spectroscopic experiments, which were proposed in the literature as potentially yielding improved sensitivity, were mentioned. This can make electron spectroscopies even more attractive tools in trace element analysis.

390

MICHA POLAK

However, one should be aware of potential problems. Surfaces should be treated very carefully, especially when submonolayer analysis is concerned. Thus, contamination from surrounding gases usually affects XPS/AES line intensities used for elemental quantification and may modify composition and chemical-state (this sets limits on acquisition times even under ultra-high vacuum). Several procedures used for trace analyte deposition on surfaces were described above. While suitable c m i ers were usually chosen, i.e. substrate materials that do not allow inward diffusion, none of these experiments avoided exposure of the surface to atmosphere before analysis. Definitely, for improved accuracy and precision, direct interfacing of the spectrometer with the deposition (preparation) chamber is desirable. Another factor to be considered when dealing with surfaces is the stability of the layer under photon or electron irradiation, which also limits the possible duration of data acquisition time. Material specific, stimulated processes such as decomposition and desoi-ption, are more likely to occur in AES than in conventional XPS, which was the technique chosen in most of the reported electron spectroscopic studies of trace elements. Prominent advantages of these methods include also multielement simultaneous analysis via commonly well-spaced spectral lines, and chemical-state information accessible via small, but characteristic and measurable shifts or shape changes in the lines. In this context, the existing vast amount of support data, as well as availability and relatively easy use of equipment, are further justifications for choosing these techniques for quantitative analysis of trace elements. It seems that the full promising potential of XPS and AES in such applications has not yet been fully recognized. Hopefully, this review will contribute to this goal.

References 1 Seah, M.P. and Briggs, D. (Eds.), Practical Surface Analysis, Wiley, New York, 1990.

2 Wanger, C.D., Riggs, W.M., Davis, L.E., Moulder, J.F., and Muilenberg, G.E. (Eds.), Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer (Physical Electronics), Eden Prairie, MN, 1979; Moulder, J.F., Stickle, W.F., Sobol, P.E., Bomben, K.D., Handbook of X-Ray Photoelectron Spectroscopy,Perkin-Elmer (Physical Electronics), Eden Praire, MN, 1992.

3 Davis,L.E., MacDonald, N.C., Palmberg,P.W., Riach,G.E., and Weber, R.E. (Eds.), Handbook of Auger Electron Spectroscopy, Perkin-Elmer (Physical Electronics),Eden Prairie. MN, 1978. 4 I n particular. Journal of Electron Spectroscopy and Related Phenomena; and Surface Science Spectra. 5 Feldman, L.C. and Mayer, J.W., Fundamentals of Surface and Thin Film Analysis, NorthHolland, Amsterdam, 1986. p. 223.

6 Grinbaum, Y,Livshits, A., and Pol&, M., Appl. Surface Sci., 25 (1986) 203-212.

7 Wanger, C.D. and Joshi, A., J. Electron Spectrosc. Relat. Phenom., 47 (1988) 283-313. 8 Cazaux, J. Appl. Surface Sci., 10 (1982) 124-140. 9 Czuha. M. and Riggs. W.M., Anal. Chem.. 47 (1975) 1836-1838.

DETERMINATION OF TRACE ELEMENTS

39 1

10 Langeron, J.P., Surf. Interface Anal., 14 (1989) 381-387. 11 Polak, M. and Livshits, A.. Appl. Surface Sci., 10 (1982)446-454. 12 Polak, M.. in: Surface Segregation Phenomena. PA. Dowben and A. Miller (Eds.). Chapter 11, CRC Press, Baco Raton, FL, 1990. pp. 291-325. 13 Arkush, R.. Talianker, M.. and Polak, M., Scr. Met. Mater.. 24 (1990) 297-300. 14 Seah, M.P. and Dench, W.A., Surf. Interface Anal.. 1 (1079) 2-1 1. 15 Tanuma, S., Powell, C.J., and Penn. D.R., Surf. Inlerface Anal., 17 (1991) 91 1-926. 16 Werner, W.S.M., Gries, W.H.,and Stori. H.,Surf. Interface Anal., 11 (1988) 627632. 17 Jablonski, A., Surf. Interface Anal., 15 (1990) 559-566. 18 Jablonski, A. and Ebel, H., Surf. Interface Anal., 17 (1988) 627-632. 19 Deviation from linearity is sometimes ignored in calibration curve analysis (e.g. Ref. [20]). 20 Bancroft, G.M., Brown. J.R.. and Fyfe. W.S.,Anal. Chem.. 49 (1977) 1044-1047. 21 Fulghum, J.E. and Linton, R.W., Surf. Interface Anal., 13 (1988) 186-192. 22 Seah, M.P.. J. Vac. Sci. Technol. A, 3 (1985) 1330-1337. 23 Hoffman, S., in: [ll, Chapler4, pp. 143-200. 24 Carvalho, M.B. and Hercules, D.M., Anal. Chem., 50 (1978) 2030-2034. 25 Briggs, D., Gibson, V.A., and Becconsall. J.K., J. Electron Spectrosc. Relat. Phenom., 11 (1977) 343-347.

26 Grant. J.T..Surf. Inlerface Anal.. 14 (1989) 271-283. 27 Powell, CJ. and Seah. M.P., J. Vac. Sci. Technol. A, 8 (1990)735-763. 28 Seah. M.P., in: [l], Chapter 5. pp. 201-256. 29 Wagner, C.D., Davis. LE., Zeller, M.V., Taylor, J.A., Raymond, R.H., and Gale, L.H., Surf. Inlerface Anal., 3 (1981) 21 1-225.

30 Mroczkowski. S. and Lichtman, D., Surface Sci., 127 (1983) 119-134. 31 Hall. P.M. and Morabito, J.M., Surface Sci., 83 (1979) 391-405. 32 Tougaard, S. and Jansson, C., Surf. Interface Anal., 19 (1992) 171-174. 33 Wagner, C.D., J. Electron Spectrosc. Relat. Phenom., 32 (1983) 99-102. 34 Cazaux, J., Surface Sci., 140 (1984) 85-100. 35 Cazaux, J., Scann. Electron. Micros., 111 (1984) 1193-1202. 36 Seah, M.P. and Cumpson, PJ., Appl. Surface Sci., 62 (1992) 195-198. 37 Procop. M. and Weber. E.-H., Surf. Interface Anal., 15 (1990) 583-584. 38 Electron Energy Loss Spectroscopy (EELS) is based on Ihe excitation of core-level electrons to empty electronic States by means of a beam of electrons which loose the corresponding energy. (See Fig. 9-9 in Section 9.2.1). Expressions for detection limits in EELS and AES are similar, but the numerical values differ [34]. 39 Cazaux. J., Jbara, O., and Kim, K.H., Surface Sci., 247 (1991)360-374.

392

MICHA POLAK

40 Gerely. G., Surf. Interface Anal., 19 (1992) 1 4 . 41 Hercules, D.M.. Cox, L.E.. Onisick. S.. and Carver, J.C., Anal. Chem.. 45 (1973) 1973-197s. 42 Brinen, J.S. and McClure, J.E., Anal. Chem., 5 (1972) 737-743; Brinen, J.S. and McClure, J.E., J. Electron Speclrosc. Relat. Phenom., 4 (1974) 243-248. 43 Macho. K., Garten, R.P.H.. Klockow. D., and Tolg. G.. Surf. Interface Anal., 12 (1988) 574-575. 44 Macho, K.. Garten, R.P.H., Bubert. H.. Auffarth, J., Klockow, D., and Tolg, G.. Surf. Interface Anal., 9 (1986) 5 16.

45 Hock, J.L. and Lichtman. D., Environ. Sci. Technol., 16 (1982) 423. 46 Clark, W.E.. Landis. D.A.. and Harker, A.B., Atmos. Environ.. 10 (1976) 637. 47 Adams, F. and De Waele, J., Surf. Interface Anal., 12 (1988) 551-564.

CHAPTER 10

Trace element determination by electrochemical methods

RAY VON WANDRUSZKA Department of Chemislry, Uiiiversity ojldaho, Moscow, Idaho, USA

Contents 10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2 Anodic and cathodic striQping voltammetry . . . . . . . . . . . . . . . . . . . . . . . 10.2.1 Electrodes.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.2 Stripping waveforms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 Film stripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.4 Cathodic slripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.5 Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 Non-stripping methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.4 Poteiitiomelric stripping . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.5 Adsorptive stripping voltammetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

393 394 395 397 399 400 400 405 405

408 416

10.1 Introduction Electrochemical methods of analysis encompass a wide variety of techniques, divided among the realms of potentiometry and voltammetry. Potentiometric methods (with one important exception) usually do not have limits of detection much below the micromolar level, so in the area of trace analysis, voltammetry generally holds the edge. The sensitivity of voltammetric methods can be greatly enhanced by the inclusion of a preconcentration step in which the analyte is accumulated at the electrode by either faradaic or non-faradaic processes. The analytical signal is generated by a subsequent stripping step.

394

RAY VON WANDRUSZKA

This type of stripping analysis, in the form of anodic, cathodic, or adsorptive stripping voltammetry, is undoubtedly the most successful method for electrochemical trace analysis of the elements. However, potentiometric stripping analysis, which combines analyte accumulation with a potentiometric stripping procedure, gives excellent results for some analytes and is gaining popularity. In addition to the stripping methods, “traditional” voltammetry (not involving preconcentration), including some polarographic techniques, can also give the low limits of detection that classify it as a trace method in a number of cases.

10.2 Anodic and cathodic stripping voltammetry Anodic stripping voltammetry (ASV) [ 11 involves the cathodic deposition of an element on the working electrode, constituting a preconcentration step, followed by anodic redissolution (Fig. 10-1). The current arising in this oxidative stripping I I

I

l a .

Time

Figure 10-1. Applied potentials in anodic stripping of a metal at a mercury electrode

process is the analytical signal, which is linearly related to the original concentration of the analyte in the sample solution. The reductive preconcentration

0 + tie +R

(10.1)

when carried out at a cathodic potential that is sufficiently large to lead to immediate reduction of 0 to R upon arrival at the electrode surface, and hence a resident surface concentration of 0, that is effectively z r o , produces a limiting electrolysis current, i, given by

i, = nFAD,,-c,.

(10.2) 6 where n is the number of electrons exchanged, F is the Faraday constant, A the electrode surface area, Do the diffusion coeffficient of the oxidized species, C: its bulk concentration, and S the thickness of the diffusion layer around the electrode. The electrolysis current reaches a maximum value because of diffusion limits of 0

TRACE ELEMENT DETERMINATION BY ELECTROCHEMICAL METHODS

395

and R in solution. To increase i, the diffusion layer thickness, 6,may be decreased, and this is easily achieved by vigorously stii-ring the solution. If analyte deposition proceeds for a sufficiently long time to measurably effect C;, the electrolysis current displays a time dependence of the form [2] i(i) = kIe-k”I

(10.3)

where k’ and k” are constants that depend on the experimental parameters. 10.2.1 Electrodes

The nature of the anodic stripping process is highly dependent on the type of electrode used. The most widely used electrode material for ASV analyses is mercury, and it is commonly employed in one or two configurations: the hanging mercury drop electrode (HMDE), or the thin mercury film electrode (TMFE). These two electrodes are primarily distinguished by their shape and their volume. The HMDE is spherical and has a relatively large volume, while the TMFE is flat and has a small volume. It is customary and permissable in analytical practice to ignore the curved nature of the HMDE surface, effectively presuming an infinitely large mercury drop, and treat it as if it were flat. With this assumption in place, a fast anodic scan proceeding from the deposition potential produces a stripping current peak, i, given by i, = knin213ADK 112I / 112 C:t (10.4) where k is a constant, 172 is the mass transfer coefficient, v is the potential sweep rate, and t is the electrolysis time. Eqn. (10.4) shows the linear dependence of the analytical signal on the analyte bulk concentration and on the preconcentration time, affirming the need of careful control oft. The typical thickness of a TMFE is 10 pm, produced by electrolytically plating mercury from a solution of its salt onto a substrate (usually glassy carbon) [3]. Electrodes of greater thickness may be used, but if the mercury layer is much thicker than the diffusion layer, the TMFE response becomes virtually identical to that of the HMDE. At a thin TMFE, the diffusion of the reoxidized species into the solution is entirely linear, but the finite volume of the mercury layer now exerts an effect. If a mercury layer of 1 pm or less is used, two conditions that simplify the dependence of the stripping current are met. The first of these is that the preconcentration ratio be large (i.e. C,/C,’ is large, where C, is concentration of analyte in the electrode), and the second is that diffusion of amalgamated analyte in the electrode be negligible, If, in addition, a fast sweep rate is used, then the following stripping current dependence applies [4,5].

i, = kn2ACRlv

(10.5)

RAY VON WANDRUSZKA

396

where k is a constant and 1 is the electrode thickness. Equation (10.5) can be written [6] in terms of the bulk analyte concentration, the electrolysis time, t, and the mass transfer coefficient, 772:

i, = k'nin2ACztu

(10.6)

Solid electrodes are not widely employed in stripping voltammetry, but in some instances, especially with metals that do not amalgamate, glassy carbon electrodes can be used to good advantage. Wax impregnated carbon electrodes are a somewhat cheaper alternative, while carbon paste electrodes are being increasingly applied in specific analyses. These latter electrodes are cheap and offer possibilities for modification for special purposes [7].Their construction is further discussed in the section on adsorptive stripping voltammetry later in this chapter. Various new developments of electrodes for voltammetric analysis include claymodified electrodes for ion-exchange voltammetry [8]. The large surface area of these electrodes allows them to provide limits of detection in the picomolar range. The similar zeolite-modified electrodes are noted for their selectivity based on size exclusion [9]. Solid carbon electrodes can be modified by the attachment of polymer films. Examples include a glassy carbon electrode onto which a polysalicylic acid film was electropolymerized, giving a 40-fold improvement in stripping response for copper [lo]. Wang et al. [1 11 have constructed an interesting device consisting of a mercury-coated carbon-foam composite. This electrode gives low background currents and offers the large surface area usually associated with zeolites. Short accumulation times in quiescent solutions can therefore be used. A novel development for ASV in highly resistive media without supporting electrolytes [12], consists of a thin mercury film electrodeposited on a glassy carbon electrode inteifaced to an ion-exchange membrane. The other surface of the membrane, which may be regarded as a solid polymer electrolyte, is in contact with an electrolyte solution containing the reference and counter electrodes. Screen printed electrodes (SPEs) must count among the most interesting and promising developments in modern electrode technology. Tliese devices do not constitute a new and different class of electrodes based on their mode of operation, but rather on the way in which they are produced. They are small, cheap, and disposable, and they may be adapted to most existing electroanalytical techniques. Their importance lies in their suitability for field studies, decentralized environmental monitoring, clinical testing, and food analysis. SPEs consist of thin carbon or metal films printed on an inert support such as PVC or alumina. They may have any desired shape and can be chemically modified, not unlike traditional electrodes. Wring et al. [ 131and Craston et al. [ 141have published accounts of SPE fabrication in the laboratory, which allows the experimenter full

TRACE ELEMENT DETERMINATION BY ELECTROCHEMICAL METHODS

397

control of variables such as size and shape. In circumstances where this is not important, the basic device may be purchased in a drug store. Figure 10-2 shows a diagrammatic representation of a type of SPE. Insulating layer

/

Sobpate

Working electrode

/

Conlacls

Re&ce

electrode

Figure 10-2. Diagram of screen printed electrode

The first application of SPEs to voltammetric trace metal analysis was recently reported by Wang and Tian [15]. They used inexpensive commercial carbon and reference electrode strips printed on PVC. The carbon strips were coated with mercury, in analogy with the production of TMFEs. Scanning electron micrographs showed that this coating in SPEs, as in TMFEs, consists of individual spherical microdroplets of 1-2 pm diameter. The electrodes were used for both voltammetric and potentiometric stripping analyses of trace amounts of lead and cadmium. Limits of detection of 30 ppt Pb and 50 ppt Cd were obtained with ASV, and 1 ppb Pb and 0.5 ppb Cd with potentiometric stripping (see discussion of potentiometric stripping analysis below). 10.2.2 Stripping waveforms The use of a differential pulse waveform is recommended for anodic stripping, because it minimizes the contribution of the double layer charging current to the measured stripping current, thereby increasing the sensitivity of the technique. As shown in Fig. 10-3, a regular series of square potential pulses (30-50 mV amplitude) is superimposed on a voltage ramp. The current is measured both before and during the latter part of each pulse, and the difference of the two measurements is recorded. The double layer charging current decays during the early portion of the pulse, leaving a relatively pure faradaic current. If the potential pulse, AE, has a magnitude less than about 120/n mV, then a reversible electrode reaction at a HMDE gives a maximum current difference of [ 161

n2F2

DkI2 (10.7) 4RT (7F t)'/* where t is the time since the application of the pulse. For pulsed voltammetric stripping from a TMFE, the electric charge in the deposition step Qnl has been invoked [ 171: (2, = I z F CAI~ ~ (10.8)

Ai,,,,= -AACRAE-

RAY VON WANDRUSZKA

398

SO ms f------)

-E

I

time Figure 10-3. Potential waveform and current response in differential pulse voltammetry

giving a maximum stripping current

Ai,,, = 0.138Q,/tP

(1 0.9)

where t, is the pulse duration. A technique that is gaining popularity in stripping analysis is square wave (sw) voltammetry, which was developed from square wave polarography [18,19]. The waveform consists of a square wave superimposed on a staircase potential (Fig. 10-4), and it is carried out at substantially higher frequencies (10&300 Hz) than differential pulse stripping. The technique is especially useful for adsorptive stripping voltammetry (vide irfiu), since this involves species present in a surface monolayer and does not require diffusion from the electrode interior. This aspect makes very high sweep rates possible, resulting in greater stripping currents. The diffusion of the reaction product away from the electrode remains a salient feature, of course. Another attractive attribute of sw stripping is the short analysis time - the shipping step is usually complete in little more than a second. A final advantage of the high frequencies used in sw stripping is that interference by dissolved oxygen becomes less of a problem when reductive analyte stripping is employed. The reduction of oxygen, which depends upon diffusion of the dissolved gas to the electrode, contributes little to the overall stripping current [20-231 The sw differential peak current, i, is linearly related to the surface concentration, r, of the analyte. If this adsorbed species provides the major portion of the sw current, the expression is

i,= IiAr

(10.10)

TRACE ELEMENT DETERMINATION BY ELECTROCHEMICAL METHODS

399

I I

:-E I

cumnt MeaSurcd

#-.

Differential cumnt

Figure 10-4.Potential waveform and current response insquare wave voltammetry

I' is related to the bulk concentration, C*,of the analyte [24]

r = 0.739DLf2C8t'l2

(10.11)

where t is the preconcentration time. To be determined by ASV, metals must be soluble in mercury, have a lower reduction potential than mercury, and have a higher reduction potential than water or other reducible species present in the sample. The first of these requirements eliminates nonamalgamating metals such as Fe, As, Si, Ni, Mo, Ti, Te, Sb, and Au. The alkali and alkaline earth metals, rare earths, aluminum, and transition metals such as Ti(II), Mn(II), V(II), Th(IV), and Zr(1V) have standard reduction potentials that are too low. Conversely, the reduction potentials of Au, Pt, and Pd are too high, making their reoxidation from mercury impossible. The possibility of interference because of (near) equipotential stripping is very real in ASV, and analysts should consult standard potential tables such as those published by Bard et al. [25].

10.2.3 Film stripping Elements that do not form amalgams at the HMDE or TMFE clearly cannot be determined by the ASV methods based on these electrodes. A related technique, termedfilm stripping, may be used for these species [26]. Nonamalgamating metals are deposited cathodically [27] and stripped anodically in film stripping voltammetry for metals (FSVM). Alternatively, the preconcentration step involves

RAY VON WANDRUSZKA

400

an electrochemical change of oxidation state of the metal, followed by precipitation with an added agent, A [28].

The insoluble compound is stripped in the opposite electrochemical direction. 10.2.4 Cathodic stripping

In cathodic stripping voltammetry (CSV), anionic species are deposited anodically and stripped cathodically. This usually involves oxidation of the electrode material (mercury) and its subsequent reaction with the anionic analyte: M -+ M + + e M’+A- 2 MA Cathodic stripping of MA completes the analysis. The distinction between CSV and adsorptive stripping (discussed later in this chapter) is often only semantic, although the term “adsorptive” is not usually applied when compound formation as described above, is involved. 10.25 Inteiferences

The most widely encountered interference in stripping voltammetry is the overlap of stripping peaks of different elements present in the sample. Many of the examples quoted in Tables 10-1-3 address this problem for individual analytes, mostly through judicious choice of accumulation potential, stripping sweep range, and solution conditions. Two techniques used in ASV/CSV and adsorptive stripping deserve special mention. The first is irradiation with UV light [29-311, which is useful for the destruction of organic surfactants that block the electrode surface by adsorbing onto it. The second is referred to as medium exchange and involves removal of the electrode from the sample solution after the accumulation step [32-361. The stripping process is canied out in a blank solution, free of potential interferences. The removal of oxygen from the analyte solution is always an important practical consideration in stripping voltammetry, especially in cases where sfiipping is not carried out with a high frequency (sw) waveform. Oxygen is usually driven from the sample solution by purging with pure nitrogen, but chemical methods can also be used. A recent report on flow systems notes [37] that citric acid under UV radiation is an effective oxygen scavenger, giving better results than conventional removal with nitrogen.

TRACE ELEMENT DETERMINATION BY ELECTROCHEMICAL METHODS

401

Table 10-1.Species determined by anodic aid cathodic stripping voltammetry aid other voltammetric inethods A. Metallic and Seniimetallic Species Analyte Supporting electrolyte NHJOH, NH3NOj, N b O A c (pH 5 or 7). KNO3 0.2 M KNO3

0.1 M HzSOj

As

0.5 M NaOAC + HN03, pH 4 0.2 M H2 0.1 M NHOj + 0.9% NaCl HCI or HClO4 with NazSO3 HCI 7 N HCI 0.36 N H2W4 + 40mg/ml Se 1 NHCl

Au

0.1 M HCI 1 M HCI

Bi

Acid medium wilh excess mecumus ion Seawwatet filtered through 0.45 pm sieve acidified with 0.05 M HCI pH 5.6 acetate

Bi, Cd, Co. Cu,Fe, Ni, Pb, Sn, TI

Various media, i d . CI-, OAc-, ClO,-. NH;

Cd

Acetate buffer pH 4.64.7 HNO3: acetate buffer HCI-sodium dodecylsulfate

0.05 M N&OAc 0.1 M tartaric acid 0.1 M acetate buffer

Cd. Cu. Pb

0.02 M HCI Acetate pH 4.8; ammonia pH 8 5% HCI

1:1 benmne/niethanol, 0.1 M NaC104 Acetate buffer, pH 5.7 HCI, pH 2 Unmodified low salinity

solutions

HNo3 PH < 2 HCI or HOAc Cd, Cr, Ni. Pb,

Ref.

ASV at W E E

39

FSVM with LS on graphite. C P E lod 2.5 x 10-9M GC and F’I ring electrode: lod M, no intermetallic compounds FSVM with Ison graphite PI electrode DPASV at CPE ng/ml range ASV and DPASV ASV, codeposition with copper ASV at Au-film electrode CSV at HMDE lod 2 ppb DPCSV at HMDE in presence of CuCl2; Ir 0.2-20 ppb DPASV at CPE nghol range Trioctylphosphineoxide-modified GCE 50 lod 0.6 ppb I n situ Hg deposition on Pt RDE to 5 x M Bi Ir 2.5 x lit situ Hg depos. on G C E ASV; ppb range

40 41 42

43 43 44 45 46 47 48 43,49 51 52

HMDE: adsorptive wave of benzoylphenylhydroxyamine complex: lod 2 x lO-’M ASV a! CPE

53

Interactions with humid acid Preconcentrationonto controlled-pore glass chronidtogrdphyc o h i n . eluted with HNOj; DPASV at GCE Adsorptive wave of iodide brilliant green complex; lod 0.16 ppb ASV at Ni-MFE, range 5 x ID-‘” lo 10-7MF%.2x10-10to 10-7MCd ASV at cellulose acetate coated electrode lod 7 x 10-toM pb, 1.3 x loA9Cd carbon SPE lod 30 ppt Pb,50ppt Cd DPASV on TMFE; no deaeration needed

55 56

d 0.5 ppb ASV at HMDE; I ASV: lod 1 x lo-’ M DPASV at W E E interference by Cu and Zn in ppb range DPASV at HMDE ASV at HMDE; ppb range; medium exchange with high salinity samples DPASV at HMDE lod 0.1 ppb ASV at TMFE 50-min deposition for ppb levels; samples UV irradiated

Ammonia buffer, pH 9.6

DPASV; lod Cd, Ni. Zn 5 x lo-* M, pb 3 x IO-’M; Ir Cr 6.6x lo-* to 1 x lo-’ M

0.1 M HOAc + 0.1 M NaOAc

Ce”

Zn Ce

Comments

+ 4H10 = Ce(OH)J + 4H+ + e

54

57 58

15 60

61 62 63 64 36 65 31

66

67

RAY VON WANDRUSZKA

402 Table 10-1 (Continued) A. Metallic and Semimetallic Species Analyte Supporting electrolyte

co

Cr

Cu

1.25 M NaSCN + 2.5 M KOH 0.4 M N k O H t 0.05 M NQCI pH 4.5 acetate buffer

2.5 M NaOH + 0.2 M Na2H2EDTA + HNO3,pH 8 1.3 M niangsnese sulfate 0.1 M NaF, 0.01 M NaCIO7. 10-3M NH7. -. 10-4 MN H ~ F H2SO4. H20,0.9% NaCl 6.8 phosphate buffer: 0.03 M acetylacetone Solid polymer electrolyte

0.5 M NaCl Seawater 0.1 M H N q + 10-sM Hg(N03)2 Seawater, pH 4.9 1 0 - 3 ~HCI 0.07 M acetate buffer pH 4.0 0.05 M NaCl pH 5 acetate buffer Filtered seawater, 0.45 pni niillipo~tfilter 0.1 M acetate buffer pH 5.8-6.0 Seawater at natural pH; pH 5.8 acetate M e r Seawater at natural pH orpH 1 Seawater Cu, Cd. Pb, Bi,

pH 5.5-5.8 acetate

Zn Cu, Co,a, Hg, Ni, Pb, 2n

0.1 N HCIOJ

Fe

H3BO3 + NaOH, pH 8

Ga

1.5 M KOH + 0.006M H2 tafl tO.001 M NazS 0.5 M NaSCN + 4.2 M NaCIO4

pH 5.3 acetate buffer 0.1 M KSCN + 0.025 N HCI +20 ppb Cu (PH 3) 1 M HzS04

Comments

Ref.

LS on graphite; lod 2 x lo-' M Forms CoR3;R = 2-nitroso-I-naphthol

68 69 70

2-(3.5-dibromoZ-pyridy)azo-5-(diethyl. amino)phenol complex: single sweep phenol complex (1 2);lod 5 x lo-" M M LS on graphite; lod 1 x

71

TMFE lod 1 ppb ASV at TMFE and HMDE: electrocatalyzed reoxidation of Cu by C102-

72 73

DPASV at CPE and HMDE Single sweep adsolptive wave of acetylacetone complex Ion exchange membrane replaces electrolyte; lod Cu 0.02 ppb, Pb 0.05 ppb /n$fu Hg deposition on GCE: electrode awg DPASV at rotating Hg-coated GCE FSVM by Isv at graphite electrode

43 74

76 77

Automated ASV

78

ASV at Hg-coated Nafion electrode

80

ASV at Hg-coated GC tubularelectrode; potable, battery-powered equipment ASV at RDE lod Cd and Pb 0.002 pgA, Cu 0.004 pgfl ASV at HMDE

81

82

ASV and DPASV at WIGE

84

Some UV radiation; compare in sibr Hg-film GCE to HMDE

85

Subtractive mode DPASV at twin Hg-coated GCE disks; lod Cd 0.025 ppb. Cu 0.067 ppb DPASV; lod < I ppb in working soln., < 3 ppb in effluent ASV at GCE; deaeration not necessary

86

Fez* + 30H = Fe(OH)3 + e; iod 1 x 1 0 - 7 ~ Is on graphite; lod 2 x lo-' M

12 75

83

87 88 89 68

SW stripping of Zn-Ga intermetallic mipound at HMDE lod 1 x lo-* M Adsorptive wave of eriochrome blackT complex; lod 0.42 ppb DPASV at W E

91

ASV at PI riiig and GC disk with Au film:

93

90

92

lod lOppb pH 2 buffer Various elearoly~es; NaOH, HCI, KNO3 + HNO3. NaC104 t HC104. KSCN + HClO4

Subtractive-mode DPASV; twin rotating Au electrodes; 0 . 0 4 4 0 p p b range Rotating GCE: KSCN + HCIO, pH 2 preferred

94 95

TRACE ELEMENT DETERMINATION BY ELECTROCHEMICAL METHODS

403

Table 10- 1 (Continued) A. Metallic and Semimetallic Species A~ialyte Supporting eleclrolyte

Seawater + HNO3. pH 2.5; 0.005 M HClO4 stripping solution 0.1 M KSCN HNO3

Neutral elec~rolyte

In

Acetatebromide pH 4.65 Chlorofumi/EtOH/H~O (1:4:1),0.005 M NaOAc, 0.06 M KBr, 0.06 M HCI

Mo(V1)

0.1 M HClOj 3 M NaCl in pH 5 acetate pH 5.8-6.7 phosphateborate buffer Acetate buffer

Ni

ammonium taltrate buffer

Mn Mo

SCN media

0.03 M KOB + 10-6M H2D

Pb

HOAc-KNOj Seawater 0.01 M H ~ S O J 0.1 M NaN03 + HCIO, 0.01 M HNO3 0.1 M KNO3

Pu Rh(II1)

HN03 pH 3.0 buffer

Ru Sb

Se(IV)

S e w ) , Se(W

1.5 M KCI + 0.5 M H2SO4 +4 x lo-' MR pH 2.2-2.3 HzSO4 NaBtlJ, 0.05 M NaOH HCI, HCIO4, Hzso4, HNOJ 0.2 M HCI

pH 8.25 ammonia buffei Various electrolytes: N € ~ C I Obuffer J pH 8

Comments

Ref.

ASV at WlGE with medium exchange lod 0.005 ppb

96

~ M ASV at WIGE lod 4 . 0 lo" DPASV at WIGE and G C E lod at WlGE 10 ppb, at GCE lppb Constant current stripping from Au electrode coated with Hg; lod 40 ppb Oxidn. peak of tetraphenyl borate monitored; lod < 8 x 10-9M DPASV at HMDE and ASV at TMFE, bod 0.5 pptr Liquid extraction followed by ASV at HMDE

97 98

at pt; iod I ~ 1 0 - 7 ~ ASV at HMDE ASV a1 HMDE

102 103 104

Catalytic current of Mo-ferron complex with oxidant DPP sensitized by ethanediamidoxime lod 2 ppb ASV at Pt and Au electrodes: lod 5 x 10-* M FSVM of Ni(DH)2; H2D = dimelhylglyoxinie; lod 2 x 10-9 M Soil digest sample ASV at TMFE CSV of PbO2 film on conducting.~ glass electrode; low ppb level Pb/fulvic, huniic complexes pH 6; filtered CSV at conductive tin oxide electrode; lod 2 x lo-* M ASV at ultramicroelectrode; no deaeration Hg on silica-carbon deposit; Iod 10 nM Hg film on poly(3-methylthiophene) electrode; lod 0.05 ppni DF'SV at AS Rh(NH3)g: ASV at ultramicroelectrode; to M level Oscillopolarographic;catalysis of Ce(IV)/AS(III) reaction; lod 7.9 x lo-" M

105

99 100

101

62

106

107

108 109 110 111 112 113

114 115 116

117 118 119

(sbcl6y- + R* = R(SbCb) + 2e: R = diodamine C ASV at HMDE DPCSV a1 HMDE; lod 1 ppb

120

CSV at HMDE either as adsorbed hexahaloselenous acid or HgSe film; Iod adsorption: 7~10"Mlodfilm5~1O-~M After Dentvl alcohol extraction: lod 1 nM M SW C'SV it HMDE; I d 3x DCP. NPP and DPP ASV and CSV at graphite and GCE

123

121 122

124 125 126 127

404

RAY VON WANDRUSZKA

Table 10-1 (Continued) A. Metallic and Semimetallic Species Analyte Supporling electrolyte

Comments

Ref.

~~

Sn

Sn, Pb

n

1.2 N HCI. ascorbic acid, 0.1 M KCI HBr. HzSO4, EtOH, hydraziniuni hydroxide 1 M HCI + 0.0015 M [(Cz Hs 0hPSz INi Heniatein, dioxane, N h O H . pH 2-3 Acetate buffer + EDTA

ASV at HMDE no R intelference

128

sirrr Hg deposition on G C E lod 0.05 ppb Lod 5xlO-*M

129

CPE; Sn done anodically, Pb calliodically

131

CSV at TMW; in presence of Cu,bi,

132

fti

130

Pb,cd TI

U

Acetate buffcr + EDTA pH 4.6 acetate + EDTA 2.5 M (Nk&S04 + NH40H pH 8.5 0.001 M PIPM buffer. sodiuni sulfite, 0.002 M catechol (pH 8.6)

V W'

0.06 M HCI

Zn

pH 1-2 HNOj 5% HCI with N h O H , pH 4 KBr

Zn,Pb. Cd, Cu

B. ANIONIC SPECIES 0.1 M HCIO,, 1:l EtOHIH20 Br-, CI-. IBr-. CI-

HCIO, 1.8 M HzSO4

CI-

0.1 M K N a + 0.002 M H N q 0.1 M KNO3 + 0.1 M H3Cit 0.08 M HNO3,80% EtOH

CN-

0.1 M K N q , CuSO4

Cr04-

pH 4 HNO3

1-

4 x lo-, M ascorbic acid; 0.01 M HNO3. 0.01 N HNO3.0.05% ascorbic acid pH 4.7 acetate buffer MdNOoh 0.1 M NaOAc, 0.1 M HOAc

DPASV at HMDE and Hg-plated GCE ASV at HMDE anion exchange preconc. M LS on graphite; lod 2 x

133 134 135

CSV at twin CFEs

136

DW; catalytic current with bmiate; lod 5 ppb Catalytic Hz wave of p-acetylcarboxylazo complex; lod 8 x lo-'' M DPASV at HMDE: ppb level ASV at HMDE; lod 0.5 ppb GCE semidifferential; lod ppb range

137 138 139 61 140

I-. l40 pg/l Br-, CSV at HMDE; 8 @ 177 @gA C1- detected CSV at HMDE CSV at TMFE

141

HMDE (PI); lod 5 x M Mercury-pool electrode; lod 5 x M CSV at HMDE (pl); temp = 2 'C ASV in presence of Cu at HMDE lO-'OM level M CSV at HMDE (Pt); lod 2 x

144 145 68

DPCSV at HMDB; many interferences; Sub-ppb level DPCSV at SMDE

148

ASV at Agniicroelecrode; lod 4 x IO-*M lod 5 X lo-* M CSV at HMDE (R); LS a1 HMDE; lod 4 x lO-'M

150 151 68

142 143

146

147

149

mi-

pH 4-10.6 (HNO3: KOH) Potasium antimony taretrate; ammonium heptamolybdate; HCI

CSV of Cu-phosphate film Adsorptive wave of ternary heteropoly acid

152 38

S2-

1 M KNO3.1 M NaOH

CSV at HMDE after HgS formation; LodIppb Indirect method. excess Hg; lod 8 x lo-* M CSV; low ppb level

153

0.4 M KlO3 0.2 M NaOH

68 154

Abbreviations: ASV - anodic stripping voltammetry; CFE - carbon fiber electrode; CPE - carbon paste electrode; CSV - cathodic stripping voltamnietry; DASV - differential anodic stripping voltammetry; DCP - d.c. polarography; DPP differential pulse polarography; DPASV - differential pulse anodic stripping voltammetry; DPCSV - dilferential pulse cathodic stripping voltammetry; FSVM - film strippng voltammetry for metals; GC - glassy carbon; GCE - glassy carbon S -linear scan (sweep); MFE -mercury film electrode; RDE - rotating electrode; lod 41limit of detection; Ir -linear range; L disk electrode; SMDE - static mercury drop clectrode; SPE - screen printed electrode; TMFE -thin mercury film electrode; WIGE - wax-impregnated graphite electrode.

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405

10.3 Non-stripping methods Although accumulative preconcentration followed by stripping is by far the most effective electrochemical method for trace element analysis, single-sweep voltammetric techniques that do not involve analyte accumulation have also been able to reach low limits of detection. Adsorptive wave voltammetry (AWV) is a non-stripping method that has been reported in some recent trace analyses. The technique should not be confused with adsorptive stripping voltammetry, which is discussed later in this chapter. AWV involves a normal voltammetric sweep without preconcentration, often in the form of a polarographic procedure. The analyte is treated with a complexing agent that renders it surface-active, and its adsorption to the electrode occurs during the sweep. In the course of this sweep, and immediately following the adsorption, analyte oxidation or reduction takes place. The resulting adsorptive faradaic process can be recognized by having a maximum in the currenthemperature curve and a minimum in the electrocapillary curve [38]. Examples of adsorptive wave analyses are included in Table 10-1, which also lists particulars of metal and nonmetal determinations by ASV and CSV. 10.4 Potentiometric stripping Potendometric stripping analysis (PSA) was developed as a method for the determination of trace metals by Jagner and GranCli [ 1551 and has been reviewed by Jagner [156] and by Hansen [157]. The technique shares with ASV a cathodic faradaic deposition process, which preconcentrates the analyte in the working electrode. The TMFE is the customary electrode in PSA. The stripping procedure (Fig. 10-5), however, is carried out without the impression of an external potential, and the electrode potential, rather than the reducing current, is the measured quantity. This spontaneous potential, which arises as the amalgamated analyte is reoxidized by a solution borne oxidizing agent such as oxygen, provides a qualitative measure for its identification: n n M(Hg) + -A M"' + -Amm m where A is the oxidizing agent. Besides oxygen, Hg(I1) is also frequently used as the oxidizing agent, in which case it is added as a mercury salt to the analyte solution. This has the added advantage that it can also serve to form the TMFE mercury film by codeposition during preelectrolysis. Some analytes require strong oxidizing agents for the chemical reoxidation step. For instance, Hg(I1) can be analyzed by PSA, when KMnO, is added as the oxidant [158]. The characteristic potential in PSA is given by the Nernst equation __t

(10.12)

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406

1 I

Em. Potcntiostatic accumulation

potentiornetric stripping M(Hg)--t M * + C'

Time

Figure 10-5. General potential-timecurve in potential stripping analysis

where {M"'}, is the steady-state surface activity of the metal ion at the electrode. In some instances, PSA can be used for compounds that are anodically deposited and reductively stripped. Examples of this are selenide and sulfide, for which an ingenious re-reduction process was devised by Christensen et al. [1591. They used a mercury drop electrode which contained amalgamated sodium as the reducing agent. This was produced electrolytically by subjecting a sodium salt solution to -1.94 V vs. SCE prior to the experiment. The analyte (Sz- or Se2-) was then deposited from the sample solution as the mercury salt at the electrode surface. This obviously required a more anodic potential (e.g. -0.3 V vs. SCE) and resulted in some reoxidation of sodium. However, a sufficient quantity remained in the mercury drop to effect reduction of the surface bound chalcogen after electrolysis ceased:

-

H++ 2Na(Hg) + HgSsUfiace

2Na'

+ Hg + HS-

The electrode assumed the characteristic potential of the analyte couple and maintained it until the surface was stripped clean. The quantitative measure in the potentiometric stripping step is the time required for the reoxidation of the analyte. This time is usually short, ranging from milliseconds to seconds, by virtue of the small volume of the TMFE (and hence the s m d quantity of amalgamated analyte), and because the the electrode is rotated to minimized the thickness of the diffusion layer. The latter expedient promotes transport of A to the electrode, and removal of M"' from it. If a constant rate of rotation (or stirring) is used, which is identical for deposition and stripping, if the sample temperature remains constant, and if the concentration of A does not change significantly during the experiment, the stripping time, t, depends only on the original analyte concentration and on the deposition time, t,:

t, = k[M""]td

(10.13)

Potentiometric strippingcurves thus obtained therefore consist of a series of plateaus (Fig. 10-6), positioned at potentials characteristic of the analyte, and of a length

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407

Figure 10-6.Potentiometricstrippingcurves obtainedafter2 min (A) and 4 min (B) of potentiostatic deposition at -1.10 V vs. S.C.E.for a sample containing 1 ppm Cd2', Pb2+,and Cu2+, and 80 ppm Hg2' in 1 M HCl. [From Jagner. D., Analyst, 107 (1982) 593. With

permission.]

linearly related to its concentration. The measured quantity in PSA is time, while in ASV it is current. This imparts an analytical advantage of PSA, since time is inherently more accurately and precisely measurable than current. Some serious interferences in ASV are less so in PSA, making the latter technique more robust in certain environments. The presence of other electroactive species is a case in point, notably oxygen. While deaeration is routinely practiced in voltammetry to eliminated O2 interference, oxygen is the common reoxidizing agent in PSA and there is no call for its removal. Interference by low concentrationsof surface active agents is also less serious in PSA than in ASV. These compounds have a tendency to adsorb onto the electrode surface, effecting a partial blockage that impairs the voltammetric stripping current. It does, however, affect electrode access by analyte (deposition) and by oxidizing agent (stripping) in a similar fashion, resulting in virtually unchanged stripping times. Lastly, PSA stripping times are independent of electrode area (see Eqn. (10.13), dispensing with the need to control this parameter. On the negative side, Eqn. (10.13) is only valid if diffusion layer thicknesses during deposition and strippingare constant and equal. This imposes rather stringent hydrodynamicdemands on PSA and necessitatescareful control of electroderotation or stirring rates. It should also be recognized that redox couples of which both members are soluble can produce a signal in PSA if the concentrations are high enough. A case in point is the H,BO;/BH; couple, which gives rise to an untidy strippingplateau between ca. - 1.4 V and - 1.28 V vs. SCE23'.

408

RAY VON WANDRUSZKA

The instrumentation used for PSA must be able to accommodate analysis times of the order of milliseconds and should provide the appropriate sample rates and data storage facilities. Sampling around 40 kHz is sufficient for most applications, but this clearly lies beyond the realm of stripchart recorders. However, these instrumental demands can easily be met by modern electronic data capture devices. PSA has wide application in elemental analysis, as shown in Table 10-2. Very low limits of detection, (down to 0.01 ppb) can be achieved, and linear dynamic ranges that extend from the 1.o.d. to the ppm level are common. The relative insensitivity to interference by electroactive species and surfactants makes the technique eminently suitable for complex matrices such as seawater. The above discussion of potentiometric stripping should bring to mind the older technique of chronopotentiometry, in which electrode potential and time are also the measured quantities, but which proceeds under the influence of an externally imposed constant current. This method has suffered some bad press [179,180], and probably justifiably so, because its accuracy is severely limited by double-layer charging effects inherent in all electrochemical techniques that involve the flow of current in the analytical step. Unlike in PSA, dissolved oxygen constitutes a real interference in chronopotentiometry. Despite these reservations, stripping chronopotentiometryhas seen some analytical use, especially the cathodic variant (for a further discussion of cathodic stripping voltammetry, see Section 10.5). For further details see Table 10-2.

10.5 Adsorptive stripping voltammetry Adsorptive strippingvoltammetry (AdSV) is a relatively new analytical technique that takes advantage of the tendency of many compounds to adsorb to electrode surfaces. This may happen under open circuit conditions, or may occur more extensively at certain applied potentials. In neither case, however, is the process a faradaic one and no charge passes across the electrode-solution interface. The adsorption constitutes an effective preconcentration process that lays down the analyte in (usually) a monolayer on the electrodesurface, where it is readily available for the subsequent analytical stripping step. As in related methods, stripping may in principle be anodic or cathodic in nature, but in AdSV of the elements it is usually a cathodic process. As a consequence of its usual electrochemical direction in elemental analysis, AdSV is sometimes referred to as adsorptive cathodic stripping voltammetry (ACSV). Recent reviews of the technique provide considerable detail [181,182]. Adsorptive preconcentration of analytes leads to vastly improved limits of detection, making AdSV a true trace technique. Reported limits of detection usually lie in the 10-7-10-8 M range, although values down to the picomolar level are not uncommon [183,184]. Not surprisingly, it has found most extensive use in the

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409

Table 10-2. Species determined by potentiometric strippinganalysis Element

Cominents

Ref.

Ag, Au, Hg As(II1)

ASC, graphile eleclrode; NH4CI/NH40H; 30 ppb Au lod PSA: dep. direct on GCE; Au codeposited for conductivity;2 mn preelectrolysis @ -0.05 V; 4.3 nM lod PSA; Mg(OH)2 coprecipitation;8 min preelectrolysis @ -0.9 V 0.003 nM lod PSA; 32 rnin preelectrolysis;0.04 nM lod PSA; Hg(I1) oxidant; 30 rnin preelectrolysis @ - 1.3 V; ng/l lod CSC; Hg-coated C-fiber electrode CSC: DMG complex; pH 8.5 borate buffer; 60 s adsorption 0.1 nM lod PSA; 4 min preelectrolysis; 20 nM lod CSC; quinolin-8-01 complex; 1.8 nM lod PSA; in beer and wine; acidified with HCl; 8-16 rnin preelectrolysis @ - 1.2 V PSA; pH 9 tartrate; sub-ppb lod range PSA; KMn04 oxidant; 5 nM lod PSA; in urine; KMnO4 trap; preelectrolysis @ -0.7 V; 2 ng/l lod; rsd 4-896 PSA; reductive stripping; 5 min preelectrolysis @ 4 . 6 V,GCE and Pt electrode; MnO2 deposited; hydroquinone reductant; 0.1 pM lod CSC; 3 pA constant current; DMG complex; Hg-coated C-fiber electrode; 40 ng/l lod CSC; DMG complex; pH 8.5 borate buffer; 60 s adsorption 0.1 nM lod PSA; 32 rnin preelectrolysis; 0.06 nM lod PSA; in urine; Hg(I1) oxidant; 0.2 M HCI; 16 rnin preelectrolysis @ -0.95 V; 1 &Ilod PSA; complexometric titration PSA; in urine; 2 rnin preelectrolysis @ - 1.25 V; 1 nM lod PSA; in whole blood, serum; 2 rnin preelectrolysis; 25 nM lod PSA; reductive stripping; 1-2 rnin preelectrolysis; Na(Hg) reductant CSC; quinolin-8-01 complex; 1.6 nM lod PSA; 16 rnin preelectrolysis; 0.25 nM lod PSA; sample sparged with Nz; Hg(I1) oxidant; pH 4.7 lod (ppb); Zn 0.03, Pb 0.03. Cd 0.01 PSA; 16 rnin preelectrolysis@ -1.4 V vs Ag/AgCI; 0.25 nM lod PSA; biological tissue; HNO3 digestion; Hg(I1) oxidant

160

Bi(II1) Cd Cd, Pb co

cu Cu, Cd, Pb Cu, Pb, Zn, Cd

Hg Mn(I1) Ni

Pb

Pb, Bi(II1) Pb, Cd

s2-,Se2U Zn Zn, Pb, Cd

Zn, Pb, Cd, Cu

161 162 163 164 163 165 166 165 167 168 158 169,170 171 172 165 163 173 174 175 176 177 165 163 166 159 178

ASC anodic stripping chronopolentiomelry;CSC: cathodic stripping chronopotentiornetry; DMG: dimethylglyoxime;lod: limit of detection;potentials quoted vs. SCE, unless otherwise stated; GCE: glassy carbon electrode

RAY VON WANDRUSZKA

410

voltammetric analysis for compounds of pharmaceutical and medical interests, because these generally have more surfactant character than small ions. Their greater tendency to adsorb onto the electrode makes them more suitable for AdSV. Anson and Brown have described the adsorption and strippingprocesses in AdSV and have identified five classes of adsorbates that interact with electrodes in different ways [ 185,1861. For the purpose of this chapter, which deals with elemental analysis, the discussion will be confined to the classes that include adsorbed elements and some other small inorganic ions. In all cases, adsorption from aqueous solution involves an initial dehydration of the adsorbate. This entails an energy expenditure which must be compensated to render the overall process energeticallyfavorable. As a consequence, small ions such as alkalis and alkaline earths, which have high hydration energies, have little or no tendency to adsorb on electrodes. Larger ions, including Cs', R4N+,R3NH+,H,PO;, NO, PF;, and ClO;, are only weakly hydrated and easily adsorb onto oppositely charged electrodes. Some sulfur and nitrogen containingions such as S2-, HS-, SO:, S,Oi-, NCO- ,and NCS-, adsorb onto mercury electrodes through strong covalent interactionsinvolving donation of electrons to empty mercury orbitals. This type of bonding can even occur to slightly negatively charged electrode surfaces. Large metal ions such as Cd(II), Hg(II), In(II), Pb(II), Tl(II), and Zn(I1) do not adsorb by themselves, but can do so through the intervention of anionic bridging ligands. In this mechanism, ions such as I- and NCS- are the primary adsorbed species on a mercury surface, but can act as d'' anionic bridges anchoring the cation in a secondary fashion. Transition metal ions that form strong isocyanato complexes experience especially strong adsorption to mercury surfaces. With an overall negative charge, species such as &s-Cr(NH,),(NCS); have a strong tendency to adsorb even onto positive electrodes. The interaction is assumed to occur chiefly through the sulfur atom [ 1871. Complexes in which the metal has the possibility to undergo intermetallic bonding with a mercury surface constitute a final class of adsorbed compounds. Reactions are comparableto those between dissolved species, leading to similar surface complexes such as [188]; Co(CN):-

2 Co(CN)!-

+ CN-

Theoretical considerations in AdSV assume monolayer coverage or less in the adsorption process. While adsorbate-adsorbentinteractions are obviously dominant, lateral interactions between adsorbed species cannot be ignored when the monolayer is approached. If is the surface concentrationof analyte and r,,,is the concentration

TRACE ELEMENT DETERMINATION BY ELECTROCHEMICAL METHODS

411

conesponding to monolayer coverage, then a fraction surface coverage 0 = r/T,, can be defined. The simple Langmuir adsorption isotherm then becomes

0 BC' = 1-0

(10.14)

where B is the adsorption coefficient and C' is the bulk analyte concentration. No account is taken for lateral interactions in this case. The introduction of an interaction parameter, 9,in the Frumkin isotherm addresses this problem [1891:

(10.15) AdSV mostly utilizes hanging mercury drop electrodes (HMDE), which have the advantage of being easily and reproducibly renewable. This renewable nature is important because the stripping step is usually reductive, and in the case of metal ions, the products may amalgamate and contaminate the electrode. An entirely different type of electrode that has been given increasing attention for elemental analysis by AdSV is the chemically modified electrode (CME). It is especially important for the analysis of metals that are incompatible with mercury. In CMEs, the analyte is entrapped at the electrode surface by chemical means involving a modifying agent such as a ligand or an ion exchanger. This agent is made part of the electrode surface by incorporating it in a polymer coating attached to the electrode surface [190] by dipping, spin coating, or electrodeposition. The modifying agent may be a part of this polymer backbone, or be introduced to it through subsequent functionalization. An interestingapproach to the production of CMEs has been pioneered by Guadaloupe and AbruAa [1911, who used bifunctional and multifunctional polymer films containing electroactivecenters and coordinating groups. The former induces precipitation of the polymer on the electrode surface, and the latter binds strongly and selectively with the metal ion of interest (Fig. 10-7). The stripping current is generated by reduction of the immobilized metalhigand and is related to the bulk concentration of the analyte. Functionalized polymer films permit synthetic variations to taylor the electrode for particular applications. There are two general approaches to the preparation of such films [192]. In the first, the ligand is part of the polymer backbone itself [1931, which is formed by the copolymerization of, for instance, vinylferrocene (the redox center) and vinylpyridine (the ligand). In the second approach, the ligand is incorporated by ion exchange. A pyridinium group, for instance, may be used to attract an anionically charged ligand such as sulfonated bathophenanthroline. This ligand has the added advantage of producing a multi-use electrode suface [ 1941, since it has a high affinity for Cu+,but readily releases Cu2+. Cleaning of the electrode therefore requires merely a number of anodic scans.

RAY VON WANDRUSZKA

412

+xe-

+

- xe-

+

.X)+

Figure 10-7. Electrochemical reduction reduction involving complexed metal and ligand bound to polymer backbone at electrode surface. [From: Guadaloupe, A.P. and Abrufla, H.D., Anal. Chem., 57 (1984) 142. With permission.]

Carbon paste electrodes have been popular in voltammetry because of their wide applicability, ease of preparation, and low cost. Their working surface consists of a paste of graphite powder mulled with a hydrophobic agent such as light oil or a-bromonaphthalene. They have seen most use in the determination of organic species, but chemically modified versions are also suitable for elemental analysis. Wang et al. [ 1951 have incorporated a cation-exchange resin directly in the carbon paste, and used the electrode for the determination of Cu(I1). Baldwin et a]. [ 1961 have reported a nickel specific CPE which contains dimethylglyoximein the carbon paste. They found a limit of detection of 50 nM Ni, linearity over 3 orders of magnitude of concentration, and little or no interference from other metal ions. Exposure to acid regenerated the surface, making this a multi-use electrode, Modification of electrode surfaces with microorganisms, specifically algae, has recently been introduced by Wang and coworkers [197,198]. The organisms bioaccumulate metal complexes through sorption of functional groups in the cellular matrix and are generally of greater interest for their selectivity than their sensitivity. Gold(II1)and copper(I1) were determined with limits of detection of 2.4x lO-'M and 2 x 10-6M, respectively. Fairly sensitive determination of H202 with a horseradishroot-modified carbon paste bioelectrode, based on hydrogen peroxidase activity in tissue, was recently reported [ 1991. A limit of detection of 3 x 10-7M was achieved. The usual sequence of reactions in AdSV involves the formation of a metal-ligand complex which has increased surfactantproperties relative to the aiialyte alone. The complex adsorbs onto the electrode, and this accumulation step may be allowed to

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413

Table 10-3. Species determined by adsorptive stripping analysis Analyte Electrode

Ligand/Reagent

Comments