Determination Trace Elements...Determination of trace elements by chromatographic methods em- ploying atomic plasma emission spectroscopic detection Speciation of trace elements with

Determination of Trace Elements Edited by Zeev B. Alfassi

3 Balaban Publishers VCH +b Weinheim . New York Base1 - Cambridge Tokyo

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Determination of Trace Elements

Edited by Z. B. Alfassi

3 Balaban Publishers

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

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

NE: Alfassi, Zeev B. [Hrsg.] ISBN 3-527-28424-9

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 ma- chine 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.

call change the electrical properties of the devices.

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 par- ticular 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.

In the semiconductor industry, very minute amounts of impurities can drasti-

In order to understand the effect of trace elements in biology or to obtain purer

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

2

3

4

5

6

7

8

9

10

11

12

13

Systematic errors in trace analysis by G. Tolg and P. Tschopel Limits of detection and accuracy in trace elements analysis by C. J. Kirchmer . . . . . . . . . . . . . . . . . . . . . . Sampling and sample preparation by J. R. W. Woittiez and J. E. Sloof . . . . . . . . . . . . . . Separation and preconcentration of trace elements by K. Terada . . . . . . . . . . . . . . . . . . . . . . . . Determination of trace elements by atomic absorption spectrometry by 1. Z. Pelly . . . . . . . . . . . . . . . . . . . . . . . . Plasma optical emission and mass spectrometry by J. A. C. Broekaert Instrumental neutron activation analysis (INAA) by Z. B. Alfassi . . . . . . . . . . . . . . . . . . . . . . . Radiochemical neutron activation analysis by Z. B. Alfassi . . . . . . . . . . . . . . . . . . . . . . . Determination of trace elements by electron spectroscopic methods byM. Polak . . . . . . . . . . . . . . . . . . . . . . . . l h c e element determination by electrochemical methods by R. von Wandruszka . . . . . . . . . . . . . . . . . . . . Determination of trace elements by chromatographic methods em- ploying atomic plasma emission spectroscopic detection

Speciation of trace elements with special reference to the use of radio- analytical methods by H. A. Das . . . . . . . . . . . . . . . . . . . . . . . . Trace elements in environmental and health sciences by G. V. Iyengar and V. Iyengar . . . . . . . . . . . . . . . .

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

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

by P. C. Uden . . . . . . . . . . . . . . . . . . . . . . .

1

39

59

109

145

191

253

309

359

393

425

46 I

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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1.1 General aspects of extreme trace analysis . . . . . . . . . . 2 1.1.2 Direct instrumental determination methods . . . . . . . . . 3 1.1.3 Multi-stage procedures . . . . . . . . . . . . . . . . . . . 3 1.1.4 Further general important statements . . . . . . . . . . . . 4 Systematic errors and their avoidance . . . . . . . . . . . . . . . . . 4 1.2.1 Volatilization . . . . . . . . . . . . . . . . . . . . . . . . 6 1.2.2 Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.2.3 Blanks from vessels, vessel materials and working tools . . 9 1.2.4 Blanks from the reagents . . . . . . . . . . . . . . . . . . 13 1.2.5 Blanks from airborne dust . . . . . . . . . . . . . . . . . 15 1.2.6 Contamination by sample handling . . . . . . . . . . . . . 18 1.2.7 Problems due to changes of the valency state . . . . . . . . 19 Systeinatic errors during the analytical procedure . . . . . . . . . . . 20 1.3.1 Sampling, sample storage and pretreatment . . . . . . . . . 20 1.3.2 Decomposition . . . . . . . . . . . . . . . . . . . . . . . 22 1.3.3 Separation . . . . . . . . . . . . . . . . . . . . . . . . . 27

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Errors in analytical results . . . . . . . . . . . . . . . . . . . . . . 40 Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Measuring trace concentrations . . . . . . . . . . . . . . . . . . . . 42 The problem of detection . . . . . . . . . . . . . . . . . . . . . . . 2.5.1 Random error of blank responses . . . . . . . . . . . . . . 42

detection) . . . . . . . . . . . . . . . . . . . . . . . . . . 46 48 50

Practical applications . . . . . . . . . . . . . . . . . . . . . . . . . 52

Reporting results at small concentrations . . . . . . . . . . . . . . . 53 Conclusions and recommendations . . . . . . . . . . . . . . . . . . 56 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Changes in trace element composition . . . . . . . . . . . . . . . . 62 3.2.1 Element specific changes . . . . . . . . . . . . . . . . . . 63 3.2.2 Sample specific changes . . . . . . . . . . . . . . . . . . 76 Pre-sampling considerations . . . . . . . . . . . . . . . . . . . . . 77

Basic rules for the recognition and elimination of systematic ei-rors . 29

42

Errors of the first kind- the critical level (aposteriol-i detection) 43 Errors of the second kind - the limit of detection (a priori

Limits to the use of the definitions of L, and L, . . . . . . Regression theory approaches to the problem of detection .

2.5.2 2.5.3

2.5.4 2.5.5

VII

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 . . . . . . . . . . . . . . . . . . . . . . . . . . Establishment of analytical control . . . . . . . . . . . . . Sampling error in a test portion . . . . . . . . . . . . . . . Uniformity of laboratory samples . . . . . . . . . . . . . .

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 4.1.4 Separation and preconcentration of trace elements by flotation . . . . 4.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2 Principle . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.3 General procedures . . . . . . . . . . . . . . . . . . . . .

3.4.1 3.4.2 3.4.3

Coprecipitation with inorganic precipitants . . . . . . . . . Coprecipitation with organic collectors . . . . . . . . . . .

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

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

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

VIII

5.7 5.8 5.9 5.10 5.1 1 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 . . . . . . . . . . . . . . . . . Radiation sources . . . . . . . . . . . . . . . . . . . . . . . . . . . Wavelength selection system . . . . . . . . . . . . . . . . . . . . . Atomization by flame . . . . . . . . . . . . . . . . . . . . . . . . . Instruments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrothermal atomization . . . . . . . . . . . . . . . . . . . . . . Hydride generation . . . . . . . . . . . . . . . . . . . . . . . . . . Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Instrumental background corrections . . . . . . . . . . . . . . . . . Modifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Automation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atomic spectrometry with plasma sources . . . . . . . . . . . . . . 6.1.1 Historical development . . . . . . . . . . . . . . . . . . . 6.1.2 Optical emission spectrometry . . . . . . . . . . . . . . . 6.1.3 Plasma mass spectrometry . . . . . . . . . . . . . . . . .

6.2.1 Arc and spark sources . . . . . . . . . . . . . . . . . . . 6.2.2 Flames . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2.3 DC plasma jets . . . . . . . . . . . . . . . . . . . . . . . 6.2.4 Inductively coupled plasmas . . . . . . . . . . . . . . . . 6.2.5 Microwave discharges . . . . . . . . . . . . . . . . . . . 6.2.6 Sample introduction for plasma spectrometry . . . . . . . 6.2.7 Discharges under reduced pressure . . . . . . . . . . . . . Plasma optical emission spectrometry . . . . . . . . . . . . . . . . . 6.3.1 Atomic emission spectrometry . . . . . . . . . . . . . . . 6.3.2 ICP-Atomic emission spectrometry . . . . . . . . . . . . . 6.3.3 MIP-Atomic emission spectrometry . . . . . . . . . . . . 6.3.4 Glow discharges . . . . . . . . . . . . . . . . . . . . . . 6.3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . Plasma mass spectrometry . . . . . . . . . . . . . . . . . . . . . . 6.4.1 ICP mass spectrometry . . . . . . . . . . . . . . . . . . . 6.4.2 Glow discharge mass spectrometry . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5.1 Power of detection . . . . . . . . . . . . . . . . . . . . . 6.5.2 Interferences . . . . . . . . . . . . . . . . . . . . . . . . 6.5.3 Economic aspects . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Basic nuclear physics . . . . . . . . . . . . . . . . . . . . . . . . .

Plasma sources and sampling . . . . . . . . . . . . . . . . . . . . .

155 156 159 164 172 173 178 180 183 187 188 192 192 194 199 201 202 203 204 205 207 209 215 217 217 220 223 224 226 226 227 239 241 241 242 243 254 255

IX

7.3

7.4

7.5

8.1 8.2

8.3

9.1 9.2

9.3

7.2.1 Nuclides . . . . . . . . . . . . . . . . . . . . . . . . . . 255 7.2.2 Radioactive decay . . . . . . . . . . . . . . . . . . . . . 256 7.2.3 Kinetics of decay of radioactive nuclides . . . . . . . . . . 260

7.2.5 The chart of the nuclides . . . . . . . . . . . . . . . . . . 265 Gamma detection systems . . . . . . . . . . . . . . . . . . . . . . . 267 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

7.4.1 Neutron sources . . . . . . . . . . . . . . . . . . . . . . 278 7.4.2 Samples introduction . . . . . . . . . . . . . . . . . . . . 279

Instrumental neutron activation analysis (INAA) . . . . . . . . . . . 7.5.1 Techniques . . . . . . . . . . . . . . . . . . . . . . . . . 280 7.5.2 Calculation . . . . . . . . . . . . . . . . . . . . . . . . . 280 7.5.3 Nuclear interferences . . . . . . . . . . . . . . . . . . . . 284

7.5.5 EpithermalINNA . . . . . . . . . . . . . . . . . . . . . . 288 7.5.6 Fast neutrons INAA . . . . . . . . . . . . . . . . . . . . 296 7.5.7 CyclicINAA . . . . . . . . . . . . . . . . . . . . . . . . 299

7.5.9 Depth profiling by INAA . . . . . . . . . . . . . . . . . . 302 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 309 Samples dissolution . . . . . . . . . . . . . . . . . . . . . . . . . . 312 8.2.1 Geological samples . . . . . . . . . . . . . . . . . . . . . 312 8.2.2 Metalsamples . . . . . . . . . . . . . . . . . . . . . . . 312 8.2.3 Biological samples . . . . . . . . . . . . . . . . . . . . . 313 Radiochemical separations . . . . . . . . . . . . . . . . . . . . . . 316 8.3.1 Radiochemicalseparationsinmaterialsciences . . . . . . 317 8.3.2 RNAA of geological and environmental samples . . . . . . 326 8.3.3 RNAA of biological samples . . . . . . . . . . . . . . . . 340 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 359

9.2.1 Basic principles . . . . . . . . . . . . . . . . . . . . . . . 363 9.2.2 Surface sensitivity and depth profiling . . . . . . . . . . . 369 9.2.3 Chemical-state information . . . . . . . . . . . . . . . . . 374 9.2.4 Quantitative analysis . . . . . . . . . . . . . . . . . . . . 375 XPSjAES applicationsintraceelementdetermination . . . . . . . . 384

7.2.4 Kinetics of formation of radioactive nuclides by irradiation 261

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Irradiation 278

280

7.5.4 A test case - INAA of trace elements in silicon . . . . . . . 285

7.5.8 Prompt Gamma Neutron Activation Analysis (PGNAA) . . 301

General review of X-ray photoelectron and auger electron spectroscopies 362

X

9.4 10.1 10.2

10.3 10.4 10.5 10.6 11.1 11.2

11.3

11.4

11.5

11.6

Summary and conclusions . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . Non-stripping methods . . . . . . . . . . . . . . . . . . . . . . . . Potentiometric stripping . . . . . . . . . . . . . . . . . . . . . . . . Adsorptive stripping voltammetry . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analytical information from interfaced chromatography with specific element detection . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.1 Interelement selectivity . . . . . . . . . . . . . . . . . . . 11.2.2 Elemental sensitivity and limits of detection . . . . . . . . 1 1.2.3 Dynamic measurement range . . . . . . . . . . . . . . . .

11.3.1 Non-spectroscopic detectors . . . . . . . . . . . . . . . . 1 1.3.2 The Flame Photometric Detector (FPD) . . . . . . . . . .

11.3.4 Flame emission detection . . . . . . . . . . . . . . . . . . 11.3.5 Atomic absorption detection . . . . . . . . . . . . . . . . 11.3.6 Atomic plasma emission spectroscopy (APES) . . . . . . . Classes of atomic plasma emission chromatographic detectors . . . . 1 1.4.1 The microwave-introduced electrical discharge plasma

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 . . . . 1 1.4.6 The plasma electrodeless discharge afterglow . . . . . . . The plasma-chromatograph interface . . . . . . . . . . . . . . . . . 11.5.1 Gas chromatographs . . . . . . . . . . . . . . . . . . . .

11.6.1 GC-AED detection of non-metallic elements . . . . . . . . 11.6.2 GC-MIP detection . . . . . . . . . . . . . . . . . . . . .

Element-selective gas chromatographic detection . . . . . . . . . . .

11.3.3 Atomic spectroscopic detectors . . . . . . . . . . . . . . .

(MIP) detector . . . . . . . . . . . . . . . . . . . . . . .

Analytical GC applications . . . . . . . . . . . . . . . . . . . . . .

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

11.7.1 HPLC-ICP detection . . . . . . . . . . . . . . . . . . . . 11.7.2 HPLC-DCP detection . . . . . . . . . . . . . . . . . . . . 1 1.7.3 HPLC-MIP detection . . . . . . . . . . . . . . . . . . . .

11.8 11.9 Chromatographic detection by plasma-mass spectrometry . . . . . .

11.9.1 HPLC-plasma mass spectrometry (MS) . . . . . . . . . . 1 1.9.2 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 . . . . . . . . . . . . . . . .

12.2.1 Basic equations of radiotracer experiments in a closed system and their applications . . . . . . . . . . . . . . . . . . . .

12.2.2 12.2.3 Isotopic exchange between a solid and an aqueous solution

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

11.7 Liquid chromatographic applications . . . . . . . . . . . . . . . . .

Supercritical fluid chromatographic (SFC) applications . . . . . . . .

12.2 Principles of radiotracer methods . . . . . . . . . . . . . . . . . . .

Isotopic exchange in one phase . . . . . . . . . . . . . . .

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

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

XI1

12.4.4

12.4.5

Leaching experiments on granular solids by means of radio- tracers and a previously radioactivated aliquot . . . . . . . Measurement of the in situ diffusion coefficient and distribution constant in (partly) wetted soils and granular wastes . .

499

501 12.5 Chemical speciation of trace elements . . . . . . . . . . . . . . . . 504

12.5.1 Survey . . . . . . . . . . . . . . . . . . . . . . . . . . . 504 12.5.2 Radiometric determination of conditional extraction constants 5 10 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 evap- oration AAS . . . . . . . . . . . . . . . . . . . . . . . . 518

13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 544 13.2 Need for trace element analysis of biomaterials . . . . . . . . . . . . 545

13.2.1 BTER. a multi-disciplinary science . . . . . . . . . . . . . 546 13.2.2 Trace element speciation and bioavailability . . . . . . . . 548

13.3.1 The “Bio” sources of analytical errors . . . . . . . . . . . 549 13.3 Biological standardization . . . . . . . . . . . . . . . . . . . . . . 549

13.3.2 Presampling factors . . . . . . . . . . . . . . . . . . . . . 550 13.4 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

13.5 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

13.6 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 . . . . . . . . . . . . . . . 575 13.7.1 Nutritional and metabolic factors . . . . . . . . . . . . . . 575 13.7.2 Nutritional surveillance of trace elements . . . . . . . . . . 576 13.7.3 Recommended dietary allowances (RDA) . . . . . . . . . 576

13.8 Trace elements and high altitude populations . . . . . . . . . . . . . 577 13.8.1 Iodine and selenium . . . . . . . . . . . . . . . . . . . . 579

13.9 Referencevaluesfor traceelement s i n humanspecimens . . . . . . . 580 13.9.1 Reference values vs normal values . . . . . . . . . . . . . 580 13.9.2 Referenceconcentrationsinclinicalspecimens . . . . . . . 580 13.9.3 Trace element content in Reference Man . . . . . . . . . . 581

13.10 Reference parameters for data interpretation . . . . . . . . . . . . . 586 13.1 1 The future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 586

CHAPTER 1

Systematic errors in trace analysis

G . ToLG'i2 and P . TSCHOPEL~

'I~istitur fur Spektrochcniie utid atigewatidte Spektroskopie 2~~-PIa t i ck - l t i s t i tu t fur Metallforschung. Luboratorium fur Reinststoflatialytik Butisen-Kirchhoff-Strasse 13. 0.44139 Dortmutid I . Germany

Contents

1.1 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.4 Basic rules for the recognition and elimination of systematic errors . . . . . . . . . . . 1.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.2

1.3

1.3.3 Separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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 opti- mization 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 particu- larly 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

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.

samples.

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


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 ob- tained 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 quali- fications 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


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;

vessels, reagents and airborne dust during the analytical procedure; (b) Contamination of the sample and/or the sample solution by tools, apparatus,

(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);

incomplete atomization, overlap of peaks); ( f ) Influences of the matrix on the generation of the analytical signals (e.g.

(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 volatilization and adsorption as well as the contamination due to the three most important blank sources: vessels, reagents and dust [ 17-24].

6 G . TOLG AND P. TSCHOPEL

Table 1-1. Elements and compounds whichcanbe separated by volatilization (20-1,000' C) (after Bachmann and Rudolph [25])

Elements Gaseous elements, Te. Sn, Pb, T1, P, As, Sb, S, Se, Br, J, Zn, Cd,

Oxides of As, S , Se, Te, Re, Ru, Os, Zn, Cd, Hg

Fluorides of 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

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

Hg

Hydrides of

I .2 .I Volatilization Losses of elements by volatilization mainly occur at high temperatures. How-

ever, 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 evapo- rating solutions or when performing decomposition 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


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. Adsorption of %o(II)-ions (adapted from [6]);c = 2 x 10-5mo~; AA glass, (7. 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 analytical problems is not possible. However, the losses can be monitored very easily with the aid of radioactive isotopes, provided they are available [32].

ooquartz, AOOpH 1.5, A.OpH9.

8 G. MLG AND P. TSCHOPEL

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

(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.

often lower than those occurring with neutral or alkaline solutions;


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 investiga- tion [6]

Element Glassy carbon FTE Quartz Borosilicate (sigradur G) suprasi@ glass

B Na

A1 Si Ca Cr Mn Fe c o cu zn As cd Sb Pb

Mg

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 1 0 - ~

-

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

main main 600

main main lo00

3 6

200 0.1

1 2-4

0.5-22 1

7-9 3-50


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 con- sists 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.


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 purification with 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 re- flection X-ray fluorescence spectrometry) [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 inevitable risk 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

12 G . TOLG AND P. TSCHOPEL

0 10 20 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 Ih 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. Ultrapurification of some other reagent such as H202, hydrazine, and AsCl, can be performed with the aid of sublimation at low temperature [65,66].

14 G. T ~ L G AND P. TSCHOPEL

A- n

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 Fe A1 Pb Mg Zn

HzO subb. 0.01 0.04 0.3 5 0.05 0.02 5 0.02 50.04 HCI 10M subb. 0.01 0.07 0.6 0.07 - < 0.05 0.2 0.2 HCI 12 Mp.a. 0.1 1 100 10 0.5 14 8

HNO3 15Msubb. 0.001 0.25 0.2 5: 0.005 5 0.002 0.15 0.04 HNO3 15 Mp.a. 0.1 2 25 10 0.5 22 3

HF54% subb. 0.01 0.5 1.2 2 0.5 1.5 1 HF 48% p.a. 0.06 2 100 5 4 3 5

Determination Trace Elements...Determination of trace elements by chromatographic methods em- ploying atomic plasma emission spectroscopic detection Speciation of trace elements with

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