Introduction to X-Ray Spectrometric Analysis

Eugene P. Bertin RCA Laboratories David Sarnoff Research Center Princeton, New Jersey

Springer Science+Business Media, LLC

Library of Congress Cataloging in Publication Data

Bertin, Eugene P 1921-

Introduction to X-ray spectrometry analysis.

Includes bibliographies and index. 1. X-ray spectroscopy. I. Title.

QD96.X2B46 543\085 77-27244 ISBN 978-0-306-31091-1 ISBN 978-1-4899-2204-5 (eBook) DOI 10.1007/978-1-4899-2204-5

This limited facsimile edition has been issued for the purpose of keeping this title available to the scientific community.

Photograph of W. C. Röntgen from Otto Glasser, Dr. W. C.

Röntgen (2d ed.; Springfield, Illinois: Charles C Thomas,

1958); courtesy of Mrs. Otto Glasser and the publisher.

Photograph of H. G. J. Moseley from Mary Elvira Weeks, Discovery of the Elements (6th ed.; East on, Pennsylvania: Journal of Chemical Education, 1960); courtesy of the publisher.

10 9 8 7 6 5

© Springer Science+Business Media New York 1978 Originally published by Plenum Press, New York in 1978

All rights reserved

No part of this book may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher

Introduction to X-Ray Spectrometric Analysis

Wilhelm Conrad Rontgen

(1845-1923)

Discoverer of x-rays

Henry Gwyn Jeffreys Moseley

(1887-1915)

Founder of x-ray spectrometry

Preface

X-ray fluorescence spectrometry has been an established, widely practiced method of instrumental chemical analysis for about 30 years. However, although many colleges and universities offer full-semester courses in optical spectrometric methods of instrumental analysis and in x-ray dif­fraction, very few offer full courses in x-ray spectrometric analysis. Those courses that are given are at the graduate level. Consequently, proficiency in this method must still be acquired by: self-instruction; on-the-job training and experience; "workshops" held by the x-ray instrument manu­facturers; the one- or two-week summer courses offered by a few uni­versities; and certain university courses in analytical and clinical chemistry, metallurgy, mineralogy. geology, ceramics. etc. that devote a small portion of their time to applications of x-ray spectrometry to those respective disciplines.

Moreover, with all due respect to the books on x-ray spectrometric analysis now in print, in my opinion none is really suitable as a text or manual for beginners in the discipline. In 1968, when I undertook the writing of the first edition of my previous book, Principles and Practice of X-Ray Spectrometric Analysis,* my objective was to provide a student text. However, when all the material was compiled, I decided to provide a more comprehensive book, which was also lacking at that time. Although that book explains principles, instrumentation, and methods at the begin­ner's level, this material is distributed throughout a mass of detail and more advanced material. That 679-page book and its I079-page second edition are simply too voluminous and comprehensive for novices. X-Rays, Electrons, and Analytical Chemistry, by Liebhafsky, Pfeiffer, Winslow,

* The publication details for this and the other books referred to here are given in the Suggested Reading list at the end of Chapter I.

v

vi Preface

and Zemany (566 pages), is, by the authors' own expressed intent, a graduate-level book and contains much advanced material. Probably the best beginners' book, Adler's X-Ray Emission Spectrography in Geology, devotes only 165 of its 258 pages to x-ray fluorescence spectrometry, the remainder to electron-probe microanalysis. Other books on general x-ray spectrometric analysis are unsuitable for beginners for one or more of the following reasons: (I) they may contain material that is either extraneous or too advanced; (2) they may not give elementary explanations of prin­ciples, instrumentation, and methods; (3) they may lack practical laboratory material, such as specimen preparation and operation of the pulse-height selector; (4) they may devote undue space to specialized topics of particular interest to their author(s); and/or (5) they may lack adequate treatment of energy dispersion. Of course, books on limited aspects of the field, such as quantitative analysis or energy dispersion, are unsuitable for beginners.

This book is intended to be a general-purpose text for all types of instruction: (I) self-instruction for technicians new to x-ray spectrometry, and for technicians having practical experience in x-ray spectrometry but little or no formal instruction and therefore lacking a basic understanding of the field; (2) workshops and short courses; (3) university courses partially devoted to x-ray spectrometry; and (4) full-semester undergraduate courses. The book also constitutes a concise summary of all aspects of the subject for scientists and technicians in other disciplines who want to evaluate the applicability of the method to their own fields. The book is intended par­ticularly for those who are not content simply to operate the instrument and perform the analyses, but who want explanations of the principles, instrument, and method. .

This book has sprung from two sources: my previous book, both editions of which were very well received, and more important, from my lectures, since its inception in 1966, at Professor Henry Chessin's annual summer short course on x-ray spectrometry at the State University of New York at Albany. Attendees at this course come from private, institutional, university, industrial, and government laboratories throughout the United States and Canada, and frequently from foreign countries. They represent the full range of experience from complete novices to highly competent analysts. I have benefited greatly from my association with these people.

The book presupposes no knowledge of x-ray spectrometry, but does require a familiarity with the language of physical science and with simple algebra. The emphasis is on x-ray fluorescence spectrometry for chemical analysis (as distinguished from x-ray spectrometry for atomic and molecular structure), particularly as applied on standard commercial wavelength-

Preface vii

dispersive and energy-dispersive instruments. Full consideration is given to energy dispersion, and a chapter is devoted to electron-probe microanalysis.

In addition to providing answers-or how to find them-an important objective of a book of this type is to raise questions that otherwise might not occur. For example, the novice is not likely to realize that his analyses may be affected by the orientation of grind marks on solid specimens, or by particle size and distribution in powders, or by the kind of acid used in preparation of liquids!

The principles underlying instrumentation, methods, and techniques are given in elementary descriptive, rather than mathematical terms. Elementary explanations or definitions are given of such subjects as atomic structure, radioisotopes, crystal structure and Miller indexes, ion exchange, and cathode-ray tube displays. Essential working mathematical equations are included, but are not derived. Essential laboratory procedures are given in detail sufficient to serve as instructions. Operation of instrumentation is described in some detail. Specific instruments or applications are not described except when they illustrate general instrumentation, methods, or techniques. An understanding of general principles and instrumentation should lead to proficiency in specific instruments and applications.

Much of the text and many of the figures and tables are taken from my Principles and Practice of X-Ray Spectrometric Analysis. However, much new material is added to provide an overall book much more concise and better suited to pedagogy than the previous book.

The book is divided into I I chapters: Chapters I and 2 describe the nature and properties of x-rays. Chapter 3 gives a general introduction to x-ray spectrometric analysis, and Chapters 4-6 go into the details (excitation, wavelength dispersion, detection and readout, x-ray intensity measurement, energy-dispersive x-ray spectrometry). Chapter 7 discusses qualitative and semiquantitative analysis, Chapter 8 deals with performance criteria, and Chapter 9 takes up quantitative analysis. Specimen preparation and pre­sentation are discussed in Chapter 10, and Chapter ] I describes electron­probe microanalysis. The chapters vary widely in length. By far the most detailed treatment is given to explanation of basic concepts for which comprehensive explanations are not given in other books on x-ray spectro­chemistry.

At the end of each chapter are some selected references for further study, mostly to other books. ] also recommend that the reader have access to my Principles and Practice . .. for further study. Moreover, many labora­tory procedures are described in sufficient detail to serve as instructions for self-training experiments.

viii Preface

It is my objective-and hope-that the reader who carefully studies this book will benefit in three ways. He/she should acquire a working proficiency permitting intelligent operation of the instrument and per­formance of analyses previously developed by a competent x-ray spectro­chemi~t. He/she should be able to "graduate" profitably to more com­prehensive and advanced books. Finally, he/she should acquire a basic foundation of knowledge that will serve as a mental "framework" or "file" to which knowledge can be added systematically as it is acquired by laboratory experience, reading books and journals, and attending technical meetings.

Eugene P. Bertin Harrison, New Jersey

Contents

Chapter 1. Excitation and Nature of X-Rays and X-Ray Spectra 1.1. History . 1.2. Definition . . . . 1.3. Properties . . . . 1.4. Units of Measurement 1.5. X-Ray Tubes .....

1.5.1. Conventional High-Power X-Ray Spectrometer Tubes . 1.5.2. Other X-Ray Tubes . .

1.6. The Primary X-Ray Generator. . . . . 1.6.1. Alternating Current . . . . . . 1.6.2. Transformers, Rectifiers, Filters . 1.6.3. X-Ray Tube Power Supply. . .

1.6.3.1. AC-Line Input . . . . 1.6.3.2. High-Potential Power Supply . 1.6.3.3. X-Ray Tube Filament Supply 1.6.3.4. Operation ........ . 1.6.3.5. Stabilization ....... . 1.6.3.6. Safety and Protective Devices

1.7. The Continuous Spectrum ] .8. The Characteristic Line Spectrum

1.8.1. Atomic Structure . 1.8.2. Origin and Nature. . . . 1.8.3. Notation . . . . . . . . 1.8.4. Wavelength and Photon Energy. 1.8.5. Intensity . . . . . . . . . . . 1.8.6. Electron (Primary) Excitation . . 1.8.7. X-Ray (Secondary, "Fluorescence") Excitation

1.8.7.1. Principles. . . . . . . . . . . . . 1.8.7.2. Relationship of Absorption Edges and Spectral-Line

Series ................... . 1.8.7.3. Excitation with Polychromatic X-Rays .... . 1.8.7.4. Comparison of Primary and Secondary Excitation

1.8.8. Other Excitation Modes Suggested Reading . . . . .

ix

]

4 5 6 8 8

11 ]2 ]2 13 16 ]6 16 18 18 21 22 23 26 26 34 38 39 43 46 48 48

50 5] 53 55 58

x Contents

Chapter 2. Properties of X-Rays

2.1. X-Ray Absorption ..... . 2.1.1. Mass-Absorption Coefficients . 2.1.2. X-Ray Absorption Phenomena 2.1.3. Absorption Edges . . . . . . 2.1.4. X-Ray Cross Section 2.1.5. Comparison of X-Ray and Optical Absorption . 2.1.6. Significance of X-Ray Absorption . 2.1.7. The Inverse-Square Law

2.2. X-Ray Scatter . . . . . . . . 2.3. X-Ray Diffraction by Crystals.

2.3.1. The Bragg Law . . . . 2.3.2. Miller Indexes (hkl) . .

2.4. Auger Effect; Fluorescent Yield Suggested Reading . . . . . .

Chapter 3. X-Ray Emission Spectrometric Analysis: General Introduction

59 59 61 62 65 65 67 67 68 71 71 74 78 81

3.1. Nomenclature . . . . . . . . . . . . . . 83 3.2. Introduction. . . . . . . . . . . . . . . 84 3.3. Wavelength-Dispersive X-Ray Spectrometry . 87

3.3.1. Principle . . . . . . . . . . . . . 87 3.3.2. Basic Instrument and Method 87 3.3.3. 'Manual, Semiautomatic, and Automatic Spectrometers 91

3.3.3.1. Introduction ............ 91 3.3.3.2. Sequential Automatic Spectrometers. . 93 3.3.3.3. Simultaneous Automatic Spectrometers. 97 3.3.3.4. Special Spectrometers . 99

3.4. Energy-Dispersive X-Ray Spectrometry . 101 3.4.1. Principle . . . . . . . . . . . 101 3.4.2. Basic Instrument and Method 101

3.5. Appraisal of X-Ray Spectrometric Analysis . 103 3.5.1. Scope . . . . . . . . . . . 103 3.5.2. Advantages . . . . . . . . . . . . 104 3.5.3. Disadvantages and Limitations . . . 110 3.5.4. Wavelength and Energy Dispersion Compared 113

3.6. Personnel Safety . 115 Suggested Reading . . . . . . . . . . . . . . . . 119

Chapter 4. Excitation; Wavelength Dispersion

4.1. Excitation . . . . . . . . 4.1.1. Electron Excitation 4.1.2. Ion Excitation. . . 4.1.3. Radioisotope Excitation

121 121 122 122

Contents xi

4.1.4. X-Ray Tube Excitation (X-Ray "Fluorescence") 126 126 128 132 137 138 138 140 140 140 149 152 155 157

4.1.4.1. General ......... . 4. J .4.2. Theory-Excitation Equations 4.1.4.3. Practice-Operating Conditions

4.2. Wavelength Dispersion 4.2.1. Radiation Path . 4.2.2. Collimators. . . 4.2.3. Analyzer Crystals

4.2.3.1. Introduction 4.2.3.2. Features . . 4.2.3.3. Other Wavelength-Dispersion Devices 4.2.3.4. Curved-Crystal Spectrometers .

4.2.4. Spectrogoniometer Alignment . Suggested Reading . . . . . . . . . . . . .

Chapter 5. Detection and Readout; X-Ray Intensity Measurement

5.1. X-Ray Detection. . . . . . . . 159 5.1.1. Principles. . . . . . . . 159 5.1.2. Pulse-Height Distributions 162 5.1.3. Gas-Filled Detectors . . . 164

5.1.3.1. Structure. . . . 164 5.1.3.2. Types of Gas-Filled Detectors; Gas Amplification 166 5.1.3.3. Phenomena in the Detector Gas Volume 169 5.1.3.4. Detector Output; Escape Peaks . 172

5.1.4. Scintillation Counters. . . . . . 173 5.1.5. Lithium-Drifted Silicon Detectors 176 5.1.6. Detector Characteristics. . . 178

5.1.6.1. Quantum Efficiency 179 5.1.6.2. Plateau. . 179 5.1.6.3. Dead Time . . . . 181 5.1.6.4. Resolution . . . . 184 5.1.6.5. Comparison of Detectors 187

5.2. Readout Components. . . . . . 189 5.2.1. Introduction ...... 189 5.2.2. Preamplifier and Amplifier 192 5.2.3. Pulse-Height Selector 192

5.2.3.1. Instrument and Principle . 193 5.2.3.2. Pulse-Height Distributions 194 5.2.3.3. Pulse-Height Distribution Curves ]97 5.2.3.4. Pulse-Height Selector Settings 201 5.2.3.5. Applications and Limitations . . 207 5.2.3.6. Automatic Pulse-Height Selection 209 5.2.3.7. Pulse-Height Selection Problems 212

5.2.4. Ratemeter-Recorder 217 5.2.5. Scaler-Timer 218 5.2.6. Computer. . . . . 220

xii Contents

5.3. X-Ray Intensity Measurement . . 5.3.]. Measurement Techniques . 5.3.2. Background. . . . .

5.3.2.1. Nature. . . 5.3.2.2. Measurement 5.3.2.3. Reduction

221 221 223 223 224 226 228 Suggested Reading . . . . .

Chapter 6. Energy-Dispersive X-Ray Spectrometry

6.1. Energy-Dispersive X-Ray Spectrometry. . . 231 6.1.1. Introduction ........... 231 6.1.2. Multichannel Pulse-Height Analyzer. 232 6.1.3. Instrument Functions . . . . . . . 235

6.1.3.1. General ......... 235 6.1.3.2. Spectrum Display and Labeling. 237 6.1.3.3. Spectrum Processing. 242 6.1.3.4. Quantitative Analysis. . . . . . 245

6.2. Nondispersive X-Ray Spectrometry ...... 246 6.3. Energy-Dispersive X-Ray Diffractometry-Spectrometry . 248

Suggested Reading . . . . . . . . . . . . . . . 253

Chapter 7. Qualitative and Semiquantitative Analysis

7.1. Qualitative Analysis .. . . . 255 7.1.1. Recording the Spectrum . . . . . . 256 7.1.2. Identification of Peaks . . . . . . . 261

7.1.2.1. X-Ray Spectrometer Tables 261 7.1.2.2. Identification of Peaks. 269

7.2. Semiquantitative Analysis . . 273 7.2.1. General. . . . . . . 273 7.2.2. Comparison Analysis 275 Suggested Reading . . . . . 277

Chapter 8. Problems and Performance Criteria

8.1. Sources of Error. . 279 8.2. Matrix Effects . . . . . . . . . . . . 281

8.2.1. Introduction ......... 281 8.2.2. Absorption-Enhancement Effects 282

8.2.2.1. General ....... 282 8.2.2.2. Nonspecific Absorption Effects 284 8.2.2.3. Specific Absorption-Enhancement Effects 287 8.2.2.4. Secondary Absorption-Enhancement Effects 291 8.2.2.5. Unusual Absorption-Enhancement Effects . 292

8.2.3. Surface-Texture, Particle-Size, and Heterogeneity Effects . 294 8.3. Spectral Interference . . . . . . . . . . . . . . . . . . .. 298

Contents

8.4. Precision . . . . . . . . . 8.4. J. Definitions . . . . . 8.4.2. Elementary Statistics . 8.4.3. Counting Error . . . 8.4.4. Counting Strategy and Figure of Merit 8.4.5. Analytical Precision

8.5. Sensitivity. . . . 8.6. Resolution. . . .

Suggested Reading

Chapter 9. Quantitative Analysis

9.1. Introduction . . . . . . . . 9.2. Calibration-Standard Methods .

9.2.1. General Method .... 9.2.2. Special Calibration Methods

9.3. Internal-Standardization Methods .. 9.3.1. Internal-Standard Method .. 9.3.2. Other Standardization Methods

9.4. Standardization with Scattered X-Rays 9.5. Matrix-Dilution Methods . . . . . . 9.6. Thin-Film Methods. . . . . . . . . 9.7. Standard Addition and Dilution Methods 9.8. Miscellaneous Experimental Correction Methods 9.9. Mathematical Correction Methods ..

9.9.1. Geometric Methods ..... . 9.9.2. Empirical-Correction Methods 9.9.3. Absorption-Correction Methods . 9.9.4. Influence-Coefficient Methods . . 9.9.5. Variable-Takeoff-Angle Method . 9.9.6. Fundamental-Parameters Method 9.9.7. Multiple-Regression Method

9.10. Films and Platings . . . . 9.11. Trace and Micro Analysis

Suggested Reading . . . .

Chapter 10. Specimen Preparation and Presentation

10.1. Introduction . . 10.2. Specimen Forms . . 10.3. Standards . . . . .

10.3.1. Permanence. 10.3.2. Sources ...

10.4. Solids. . . . . . . 10.4.1. Scope, Advantages, Limitations 10.4.2. Presentation . 10.4.3. Preparation . . . . . . . . .

xiii

299 299 301 305 310 313 316 322 325

327 331 331 334 338 338 343 344 348 351 353 355 357 358 360 366 368 373 376 380 381 386 389

393 396 397 397 398 401 40J

. 402 403

xiv

10.5. Powders and Briquets .. ..... 10.5.1. Scope, Advantages, Limitations 10.5.2. Preparation 10.5.3. Precautions

10.6. Fusion Products . 10.7. Liquids .....

10.7.1. Scope, Advantages, Limitations 10.7.2. Liquid-Specimen Cells

10.8. Supported Specimens . . . . . . 10.8.1. General Techniques . . . 10.8.2. Ion-Exchange Techniques.

10.9. Special Specimen Forms. . . 10.9.1. Selected-Area Analysis 10.9.2. Radioactive Specimens 10.9.3. Dynamic Systems . 10.9.4. Gases and Vapors. Suggested Reading . . . .

Chapter 11. Electron-Probe Microanalysis

11.1. Introduction . . . . . . . . . . . . . 11.2. Instrumentation. . . . . . . . . . . . 11.3. Interaction of the Electron Beam and Specimen. 11.4. Modes of Readout and Display . . . .

11.4.1. Cathode-Ray Tube Displays ... 11.4.1.1. The Cathode-Ray Tube . 11.4.1.2. Line and Raster Scanning 11.4.1.3. Deflection Modulation Displays . 11.4.1.4. Intensity Modulation Displays

11.4.2. Measurement at a Point . . 11.4.3. Measurement along a Line . 11.4.4. Measurement over a Raster 11.4.5. Color Displays .

11.5. Specimen Considerations 11.6. Quantitative Analysis 11.7. Performance . . . . . . 11.8. Applications . . . . . . 11.9. Comparison with X-Ray Fluorescence Spectrometry

Suggested Reading . . . . . . . . . . . . . . .

Contents

405 405 407 409 411 413 413 415 419 419 423 426 426 434 435 435 436

439 440 444 446 446 448 450 451 453 453 454 456 461 462 464 468 469 470 473

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 475