Handbook of X-Ray Spectrometry Second Edition, Revised and Expanded edited by René E Van Grieken University of Antwerp Antwerp, Belgium Andrzej A Markowicz Vienna, Austria Marcel Dekker, Inc New York ã Basel TM Copyright â 2001 by Marcel Dekker, Inc All Rights Reserved Copyright © 2002 Marcel Dekker, Inc ISBN: 0-8247-0600-5 First edition was published as Handbook of X-Ray Spectrometry: Methods and Techniques This book is printed on acid-free paper Headquarters Marcel Dekker, Inc 270 Madison Avenue, New York, NY 10016 tel: 212-696-9000; fax: 212-685-4540 Eastern Hemisphere Distribution Marcel Dekker AG Hutgasse 4, Postfach 812, CH-4001 Basel, Switzerland tel: 41-61-261-8482; fax: 41-61-261-8896 World Wide Web http:==www.dekker.com The publisher offers discounts on this book when ordered in bulk quantities For more information, write to Special Sales=Professional Marketing at the headquarters address above Copyright # 2002 by Marcel Dekker, Inc All Rights Reserved Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage and retrieval system, without permission in writing from the publisher Current printing (last digit): 10 PRINTED IN THE UNITED STATES OF AMERICA Copyright © 2002 Marcel Dekker, Inc Preface to the Second Edition The positive response to the first edition of Handbook of X-Ray Spectrometry: Methods and Techniques and its commercial success have shown that in the early 1990s there was a clear demand for an exhaustive book covering most of the specialized approaches in this field Therefore, some five years after the first edition appeared, the idea of publishing a second edition emerged In the meantime, remarkable and steady progress has been made in both instrumental and methodological aspects of x-ray spectrometry This progress includes considerable improvements in the design and production technology of detectors and in capillary optics applied for focusing the primary photon beam The advances in instrumentation, spectrum evaluation, and quantification have resulted in improved analytical performance and in further extensions of the applicability range of xray spectrometry Consequently, most of the authors who contributed to the first edition of this book enthusiastically accepted the invitation to update their chapters The progress made during the last decade is reflected well in the chapters of the second edition, which were all considerably revised, updated, and expanded A completely new chapter on microbeam x-ray fluorescence analysis has also been included Chapter reviews the basic physics behind x-ray emission techniques, and refers to extensive appendices for all the basic and generally applicable x-ray physics constants New analytical expressions have been introduced for the calculation of fundamental parameters such as the fluorescence yield, incoherent scattering function, atomic form factor, and total mass attenuation coefficient Chapter outlines established and new instrumentation and discusses the performances of wavelength-dispersive x-ray fluorescence (XRF) analysis, which, with probably 15,000 units in operation worldwide today, is still the workhorse of x-ray analysis Its applications include process control, materials analysis, metallurgy, mining, and almost every other major branch of science The additional material in this edition covers new sources of excitation and comprehensive comparisons of the technical parameters of newly produced wavelength-dispersive spectrometers Chapter has been completely reconsidered, modified, and rewritten by a new author The basic principles, background, and recent advances are described for the tubeexcited energy-dispersive mode, which is invoked so frequently in research on environmental and biological samples This chapter is based on a fresh look and follows a completely different approach Copyright © 2002 Marcel Dekker, Inc Chapter reviews in depth the available alternatives for spectrum evaluation and qualitative analysis Techniques for deconvolution of spectra have enormously increased the utility of energy-dispersive x-ray analysis, but deconvolution is still its most critical step The second edition includes discussions of partial least-squares regression and modified Gaussian shape profiles Chapter reviews quantification in XRF analysis of the classical and typical ‘‘infinitely thick’’ samples In addition to being updated, the sections on calibration, quality control, and mathematical correction methods have been expanded Chapter 6, on quantification for ‘‘intermediate-thickness’’ samples, now also includes the presentation of a modified version of the emission-transmission method and a discussion of both the accuracy and limitations of such methods Chapter is a completely original treatment by a new author of radioisotope-induced and portable XRF It discusses semiconductor detectors, including the latest types, analyzes in detail the uncertainty sources, and reviews the recent and increasingly important applications Since the appearance of the first edition, synchrotron-induced x-ray emission analysis has increased in importance Chapter was updated and modified by including a comprehensive review of the major synchrotron facilities Although its principles have been known for some time, it is only since the advent of powerful commercial units and the combination with synchrotron sources that total reflection XRF has rapidly grown, mostly now for characterization of surfaces and of liquid samples This is the subject of the substantially modified and expanded Chapter The new authors have taken a radically different approach to the subject Polarized-beam XRF and its new commercial instruments are treated in detail in a substantially revised and expanded Chapter 10 Capillary optics combined with conventional fine-focus x-ray tubes have enabled the development of tabletop micro-XRF instruments The principles of the strongly growing microbeam XRF and its applications are now covered thoroughly in an additional chapter, Chapter 11 Particle-induced x-ray emission analysis has grown recently in its application types and particularly in its microversion Chapter 12 discusses the physical backgrounds, instrumentation, performance, and applications of this technique The sections dealing with the applications were substantially expanded Although the practical approaches to electron-induced x-ray emission analysis— a standard technique with wide applications in all branches of science and technology— are often quite different from those in other x-ray analysis techniques, a treatment of its potential for quantitative and spatially resolved analysis is given in Chapter 13 The new and expanded sections deal with recent absorption correction procedures and with the quantitative analysis of samples with nonstandard geometries Finally, the completely updated and revised Chapter 14 reviews the sample preparation techniques that are invoked most frequently in XRF analysis The second edition of this book is again a multiauthored effort We believe that having scientists who are actively engaged in a particular technique covering those areas in which they are particularly qualified outweighs any advantages of uniformity and homogeneity that characterize a single-authored book The editors (and one coworker) again wrote three of the chapters in the new edition For all the other chapters, we were fortunate to have the cooperation of truly eminent specialists, some of whom are new contributors (see Chapters 3, 7, 9, 10 and 11) We wish to thank all the contributors for their considerable and (in most cases) timely efforts Copyright © 2002 Marcel Dekker, Inc We hope that novices in x-ray emission analysis will find this revised and expanded handbook useful and instructive, and that our more experienced colleagues will benefit from the large amount of readily accessible information available in this compact form, some of it for the first time An effort has been made to emphasize the fields and developments that have come into prominence lately and have not been covered in other general books on x-ray spectrometry We also hope this book will help analytical chemists and other users of x-ray spectrometry to fully exploit the capabilities of this powerful analytical tool and to further expand its applications in material and environmental sciences, medicine, toxicology, forensics, archeometry, and many other fields Rene´ E Van Grieken Andrzej A Markowicz Copyright © 2002 Marcel Dekker, Inc Preface to the First Edition Scientists in recent years have been somewhat ambivalent regarding the role of x-ray emission spectrometry in analytical chemistry Whereas no radically new and stunning developments have been seen, there has been remarkably steady progress, both instrumental and methodological, in the more conventional realms of x-ray fluorescence For the more specialized approaches—for example, x-ray emission induced by synchrotron radiation, radioisotopes and polarized x-ray beams, and total-reflection x-ray fluorescence— and for advanced spectrum analysis methods, exponential growth and=or increasing acceptance has occurred Contrary to previous books on x-ray emission analysis, these latter approaches make up a large portion of the present Handbook of X-Ray Spectrometry The major milestone developments that shaped the field of x-ray spectrometry and now have widespread applications all took place more than twenty years ago After wavelength-dispersive x-ray spectrometry had been demonstrated and a high-vacuum x-ray tube had been introduced by Coolidge in 1913, the prototype of the first modern commercial x-ray spectrometer with a sealed x-ray tube was built by Friedmann and Birks in 1948 The first electron microprobe was successfully developed in 1951 by Castaing, who also outlined the fundamental concepts of quantitative analysis with it The semiconductor or Si(Li) detector, which heralded the advent of energy-dispersive x-ray fluorescence, was developed around 1965 at Lawrence Berkeley Laboratory Acceleratorbased particle-induced x-ray emission analysis was developed just before 1970, mostly at the University of Lund The various popular matrix correction methods by Lucas-Tooth, Traill and Lachance, Claisse and Quintin, Tertian, and several others, were all proposed in the 1960s One may thus wonder whether the more conventional types of x-ray fluorescence analysis have reached a state of saturation and consolidation, typical for a mature and routinely applied analysis technique Reviewing the state of the art and describing recent progress for wavelength- and energy-dispersive x-ray fluorescence, electron and heavy charged-particle-induced x-ray emission, quantification, and sample preparation methods is the purpose of the remaining part of this book Chapter reviews the basic physics behind the x-ray emission techniques, and refers to the appendixes for all the basic and generally applicable x-ray physics constants Chapter outlines established and new instrumentation and discusses the performances of wavelength-dispersive x-ray fluorescence analysis, which, with probably 14,000 units in operation worldwide today, is still the workhorse of x-ray analysis with applications in a wide range of disciplines including process control, materials analysis, metallurgy, Copyright © 2002 Marcel Dekker, Inc mining, and almost every other major branch of science Chapter discusses the basic principles, background, and recent advances in the tube-excited energy-dispersive mode, which, after hectic growth in the 1970s, has now apparently leveled off to make up approximately 20% of the x-ray fluorescence market; it is invoked frequently in research on environmental and biological samples Chapter reviews in depth the available alternatives for spectrum evaluation and qualitative analysis; techniques for deconvolution of spectra have enormously increased the utility of energy-dispersive x-ray analysis, but deconvolution is still its most critical step Chapters and review the quantification problems in the analysis of samples that are infinitely thick and of intermediate thickness, respectively Chapter is a very practical treatment of radioisotope-induced x-ray analysis, which is now rapidly acquiring wide acceptance for dedicated instruments and field applications Chapter reviews synchrotron-induced x-ray emission analysis, the youngest branch, with limited accessibility but an exponentially growing literature due to its extreme sensitivity and microanalysis potential Although its principles have been known for some time, it is only since the advent of powerful commercial units that total reflection x-ray fluorescence has been rapidly introduced, mostly for liquid samples and surface layer characterization; this is the subject of Chapter Polarized beam x-ray fluorescence is outlined in Chapter 10 Particle-induced x-ray emission analysis is available at many accelerator centers worldwide; the number of annual articles on it is growing and it undergoes a revival in its microversion; Chapter 11 treats the physical backgrounds, instrumentation, performance, and applications of this technique Although the practical approaches to electron-induced x-ray emission analysis, now a standard technique with wide applications in all branches of science and technology, are often quite different from those in other x-ray analysis techniques, a separate treatment of its potential for quantitative and spatially resolved analysis is given in Chapter 12 Finally, Chapter 13 briefly reviews the sample preparation techniques that are invoked most frequently in combination with x-ray fluorescence analysis This book is a multi-authored effort We believe that having scientists who are actively engaged in a particular technique covering those areas for which they are particularly qualified and presenting their own points of view and general approaches outweighs any advantages of uniformity and homogeneity that characterize a single-author book Three chapters were written by the editors and a coworker For all the other chapters, we were fortunate enough to have the cooperation of eminent specialists The editors wish to thank all the contributors for their efforts We hope that novices in x-ray emission analysis will find this book useful and instructive, and that our more experienced colleagues will benefit from the large amount of readily accessible information available in this compact form, some of it for the first time This book is not intended to replace earlier works, some of which were truly excellent, but to supplement them Some overlap is inevitable, but an effort has been made to emphasize the fields and developments that have come into prominence lately and have not been treated in a handbook before Rene´ E Van Grieken Andrzej A Markowicz Copyright © 2002 Marcel Dekker, Inc Contents Preface to the Second Edition Preface to the First Edition Contributors X-ray Physics Andrzej A Markowicz I Introduction II History III General Features IV Emission of Continuous Radiation V Emission of Characteristic X-rays VI Interaction of Photons with Matter VII Intensity of Characteristic X-rays VIII IUPAC Notation for X-ray Spectroscopy Appendixes I Critical Absorption Wavelengths and Critical Absorption Energies ˚ II Characteristic X-ray Wavelengths (A) and Energies (keV) III Radiative Transition Probabilities IV Natural Widths of K and L Levels and Ka X-ray Lines (FWHM), in eV ˚ V Wavelengths of K Satellite Lines (A) VI Fluorescence Yields and Coster–Kronig Transition Probabilities VII Coefficients for Calculating the Photoelectric Absorption Cross Sections t (Barns=Atom) Via ln–ln Representation VIII Coefficients for Calculating the Incoherent Collision Cross Sections sc (Barns=Atom) Via the ln–ln Representation IX Coefficients for Calculating the Coherent Scattering Cross Sections sR (Barns=Atom) Via the ln–ln Representation X Parameters for Calculating the Total Mass Attenuation Coefficients in the Energy Range 0.1–1000 keV [Via Eq (78)] XI Total Mass Attenuation Coefficients for Low-Energy Ka Lines XII Correspondence Between Old Siegbahn and New IUPAC Notation X-ray Diagram Lines References Copyright © 2002 Marcel Dekker, Inc 56 Wavelength-Dispersive X-ray Fluorescence Jozef A Helsen and Andrzej Kuczumow I II III IV V VI VII Introduction Fundamentals of Wavelength Dispersion Layout of a Spectrometer Qualitative and Quantitative Analysis Chemical Shift and Speciation Instrumentation Future Prospects References Energy-Dispersive X-ray Fluorescence Analysis Using X-ray Tube Excitation Andrew T Ellis I II III IV V Introduction X-ray Tube Excitation Systems Semiconductor Detectors Semiconductor Detector Electronics Summary References Spectrum Evaluation Piet Van Espen I II III IV V VI VII VIII IX X Introduction Fundamental Aspects Spectrum Processing Methods Continuum Estimation Methods Simple Net Peak Area Determination Least-Squares Fitting Using Reference Spectra Least-Squares Fitting Using Analytical Functions Methods Based on the Monte Carlo Technique The Least-Squares-Fitting Method Computer Implementation of Various Algorithms References Quantification of Infinitely Thick Specimens by XRF Analysis Johan L de Vries and Bruno A R Vrebos I II III IV V VI Introduction Correlation Between Count Rate and Specimen Composition Factors Influencing the Accuracy of the Intensity Measurement Calibration and Standard Specimens Converting Intensities to Concentration Conclusion References Copyright © 2002 Marcel Dekker, Inc For the determination of trace elements in lichen material, 200 mg of the collected samples are digested with mL HNO3, 0.2 mL, H2O2, and 0.05 mL HF in a PTFE vessel From the digested sample, targets can be prepared by pipetting 300 mL of the solution onto a Mylar film and drying (Calliari et al., 1995) A very effective method for the digestion of organic materials is the high-pressure digestion in specially designed bombs Usually, a small amount of the pulverized sample is placed in a quartz or Teflon vessel and some milliliters of acid are added The tube is then placed inside a high-pressure chamber and exposed to pressures of about 200 bar under nitrogen atmosphere, at temperatures of about 260 C Under such high pressures, the organic material should be completely decomposed Several drops of the resulting solution can be pipetted onto a foil or target, dried, and analyzed The main drawbacks of this procedure are the substantial effort and the requirement of a suitable instrument Usually, one digestion needs about h and includes slow heating and cooling down of the sample chamber Furthermore, such instruments, working under high pressure, should be kept in special rooms in order to avoid accidents by possible explosions (Tschopel, 1983; ă Schmeling et al., 1997) The destruction of biological materials with reactive gases is rarely carried out, although it seems to be quit efficient for some materials In one study for example, 90% of plant material was oxidized with HNO3 vapor within a short time and only a simple allglass apparatus was used for the vapor-phase oxidation (Thomas and Smythe, 1973) The addition of HClO4 can accelerate the reaction and ensure complete oxidation Almost the same technique was applied by another group for the determination of Zn in brain tissue by digestion in PTFE vessels under pressure (Klitenick et al., 1983) The sample digestion by microwave plays a more and more important role (AbuSamra et al., 1975; Kingston and Jassie, 1986; Skelly and di Stefano, 1988), and the available digestion systems become better equipped Automatic ventilation to remove acid fumes is already available in all new equipments The interior of the oven is, in comparison to the conventional household devices, usually acid resistant to prevent rusting and to increase the lifetime of the device Systems of 1000 W and greater power are in use for a fast and complete destruction of materials that are difficult to decompose Several metalfree materials ensure that contaminations from the vessels are kept low With digestion vessels of tetrafluorometoxil (TFM), the memory effects due to adsorption and diffusion are almost absent after two predigestion cycles of 30 with concentrated HNO3 (Noltner et al., 1990) For vessels made of perfluoroalcoholoxil (PFA), a day HNO3 vapor exposure is enough to reduce the blank values under the detection limits of XRS (Knapp, 1985) In using microwave digestion systems, care should be taken as well, because the pressure inside the vessels increases rapidly during the digestion procedure Opening the vessels too soon after finishing the procedure, without careful releasing of the inside pressure, can create hazards Especially when working with highly concentrated acids, special care is required The vessels should be smoothly cooled down and stand for a while before removal of the sample Usually, HNO3, in concentrated or diluted form, is recommended for the microwave digestion of organic materials Neither mixtures of HNO3 with H2SO4 nor HNO3 with HClO4 are advised for use for decomposition by microwave H2SO4 has a quite high evaporation point in comparison to HNO3 and a substantial increase of the pressure inside the vessel can occur during the digestion event, which might lead to explosions HClO4 reacts explosively with incompletely reduced organic material, which can cause dangerous injuries In several studies, explosions were reported resulting from overpressurizing the digestion bombs (Matthes et al., 1983; Fernando et al., 1986) Copyright © 2002 Marcel Dekker, Inc Also, the relation between acid amount and sample as well as the complete reagent amount should be selected carefully For safety reasons, it is better to start with small amounts of material and acid and increase this by careful observation of their behavior A detailed discussion about working with microwave digestion systems can be found in the book by Kingston and Haswell (1997) The major advantage of the microwave technique is the short time required for a complete reaction By digestion with microwaves, the sample is directly attacked and the used vessels or container not need to be heated up A destruction biological material can be performed within a minimum of time compared to other technique, where containers and sample chambers are included in the heating process The wet digestion methods (by microwave or high-pressure ashing) are considerably more rapid, with reaction times of about 2–3 h, compared to at least h necessary for a complete decomposition by dry ashing A comparison of different sample preparation methods for the analysis of organic material showed that slurry preparation needs min, microwave digestion 45 min, dry ashing 24 h, and open wet digestion 36 h (Miller-Ihli, 1988) In combination with high pressure, the microwave digestion technique shows quantitative recovery for elements, which would be lost by volatilization during an open digestion Also, traces of rare earth elements show good recoveries at the ppb level, even in the presence of organic material that is extremely difficult to digest Several studies dealt with the destruction and analysis of certified biological reference materials Hay (V-10, IAEA) samples, for example, were digested in a microwave oven with HNO3 and H2O2 Five hundred milligrams of the powdered material were mixed with mL concentrated HNO3 and mL H2O2 and heated three times for each at 300 W After each heating cycle, the vessel was allowed to cool down and the pressure released Finally, the sample was exposed for at 300 W and at 600 W The complete procedure needed no more than 30 and showed good agreement with the certified results (Noltner et al., 1990) Very short digestion times of were found to be sufficient for the decomposition of NIST standard material Oyster Tissue (SRM 1566a) and Bovine Liver (SRM 1577a) in a closed PFA bomb with HNO3 (Stripp and Bogen, 1989) Lichen samples were studied after collection from different places and were proposed as biomonitors The collected material was first dried and then acid-digested For this procedure, 200 mg of lichen material were treated with mL of HNO3, containing the internal standard, 0.2 mL H2O2, and 0.05 mL HF in closed Teflon vessels by microwave digestion The final targets were prepared by pipetting 300 mL of the decomposed sample onto Mylar foil The detection limits were reported between 0.1 mg=L for Cu and 10 mg=L for S (Calliari et al., 1995) For the determination of Cr in seed material (e.g., barley seedlings), the samples are acid-digested using a microwave oven with 1200 W power Then, 300–400 mg of the roots and leaves are placed into a PTFE vessel and decomposed with 3–4 mL HNO3 The sample is then presented as a thin film mounted on Mylar foil to the EDXRF instrument (Calliari et al., 1993) A special energy-dispersive miniprobe multielement analyzer (EMMA) was developed for the determination of Pb and other traces in peats The samples can be analyzed directly or after acid digestion in Teflon bombs in a microwave oven with mL H2O2, mL HNO3, and mL HF The comparison of both methods showed satisfying results (Cheburkin and Shotyk, 1996) Also, for the determination of animal and plant tissue by PIXE, a microwave digestion procedure might be applied In this case, the digestion can be even performed in a Copyright © 2002 Marcel Dekker, Inc conventional household microwave oven in closed Teflon vessels with high-purity HNO3 as the reagent A high acid concentration of 14 M should be applied in order to reduce the dilution effect of the sample and ensure sufficient sample material for the analysis of traces The reaction vessels are filled with vegetable oil (7.5 mL to 100 mg of the sample) to prevent damage of the antenna After digestion, 10 mL of the resulting solution are pipetted onto a 1.5-mm-thick Kimfol polycarbonate film The film was previously treated with 14 M HNO3 and 0.05% polyvinylpyrrolydone solution to make it hydrophilic and to achieve a small spot The results obtained by Pinheiro et al (1989) showed an accuracy of better than 5% and a matrix reduction factor of In body fluids with very low trace element content, it is often recommended to enrich the traces with a preconcentration step and to separate the matrix Cell fluids and blood serum, for example, show a relatively thick layer of a low-Z-element matrix after evaporation, which increases the detection limits In principle, all preconcentration methods, which are applicable for water analysis, are also practicable for organic fluids However, it should be considered that the amounts of such samples are usually much smaller, and working in microscale is often required Very low contents of Cr (0.3 ng=mL) in plasma can be determined after complexation with APDC and extraction with methyl isobutyl ketone After evaporation of the organic solvent, the residue is dissolved in acid and deposited on a thin polycorbonate foil (Simonoff et al., 1985) APDC can be also applied for the analysis of hair samples For that, the hair samples are first digested with a combination of HClO4 and HNO3 (1:5 v=v), later distilled, and then precipitated with ADPC, whereas Y acts as the internal standard and coprecipitation agent The resulting precipitate is filtered through a Nuclepore membrane, dried, and measured The detection limits are around 0.4 mg=g for Pb and Ni and around 2.2 mg=g for Fe (Eltayeb and Van Grieken, 1989, Eltayab and Van Grieken, 1990) For the determination of Se in serum of patients with liver cirrhosis by PIXE, Te was chosen as a coprecipitant and internal standard, and good results were reported (Cesaril et al., 1989) As for the preconcentration of traces from water samples, various kinds of ion-exchange resins have been proposed for biological samples as well With the application of cellulose-hyphan, for example, the detection limits might be reduced by about a factor of 10, by separating the trace elements (Agarwal et al., 1975) Protein material can be preferably isolated by gel electrophoresis (Szokefalvi-Nagy et al., 1987) ă C Sample Preparation for Analysis with Spatial Resolution For the analysis of microsamples with spatial resolution special sample preparation methods were developed They are well known in the field of electron microprobe analysis and several books dealing with this subject are available (Hayat, 1970; Hall et al., 1974; Echlin and Galle, 1975; Reed, 1975; Erasmus, 1978; Hayat, 1980; Revel, 1984) By application of microprobe analysis, some regulations should be strongly considered regarding collection and handling of the sample in order to avoid systematic errors For biological material, the time of sample collection is often of high importance; for example, it should be considered that by working with tissue samples, enrichment of Na, Cl, and Ca is taking place after the death of the cell and decreasing concentrations of Mg and K will influence the results also (von Zglincki et al., 1985) To avoid problems in this direction, the time delay between sampling and analysis should be kept as short as possible Copyright © 2002 Marcel Dekker, Inc Furthermore, during analysis of the sample, the specimen will be transformed into another state Usually, a high vacuum is required for the analysis, and to avoid evaporation of tissue water, these must be withdrawn or immobilized by suitable procedures For ultrastructural studies with transmission electron microscopy, ‘‘wet chemical techniques’’ might be applied For such procedures, the sample is chemically fixed, the cellular processes are arrested, and the cell contents are immobilized The tissue water can then be removed by immersion in an organic solvent The dehydrated specimen is then infiltrated with a suitable resin for sectioning purposes A more detailed discussion about such procedures can be found in Chapter 13 and in the works of Glauert (1974), Steinbrech and Zierold (1987), and Hayat (1989) Synchrotron-radiation XRF might be a suitable technique for microanalysis as well (see Chapter 8) Gilfrich et al (1991) used synchrotron-radiation XRF for the determination of elements in tree rings, which are good indicators for the geological and atmospheric conditions during the growth of the tree SRXRF offers the advantage that the primary beam can be focused on small parts of the sample and sample preparation requires only polishing of the tree slices The concentrations of manganese in living leaves were determined by Fukumoto et al (1992) In this study, the leaf was directly placed into the instrument and analyzed The influence of acid rain and the exposure to severe x-ray radiation of the leave was also monitored in the same study VI ATMOSPHERIC PARTICLES The amount of publications dealing with the analysis of atmospheric particles by x-ray fluorescence increased in the last years and showed the suitability of this technique for such samples XRS is still the most popular technique for this kind of specimen, especially for routine analysis, in the field of x-ray analysis Several studies were also carried out with TXRF or PIXE, but easy handling of the sample and less effort make the conventional XRF more attractive, despite its higher detection limits Heavy metals are nearly always present in the particulate phase of air and can be easily separated by filtration or impaction High enrichment factors are reached by filtration of large air volumes, and homogeneously loaded filters are an ideal target for XRS Nevertheless, there are several requirements, which should be considered carefully before reliable results can be obtained The main problems which occur are bound to particle size and x-ray absorption effects due to the filter media (‘‘filter penetration effect’’) If the particle size distribution of a particular element is known, the particle size effect can rather easily be corrected However, in most cases, the particle size distribution is unknown or difficult to measure If this is the case, the particle size effect should be kept low by selection of suitable filter materials, which collect particles on the surface like Nuclepore or Millipore filters The absorption of x-rays by the filter medium depends on the depth distribution of the particles, which can be calculated approximately by measuring the fluorescence intensities from the back and front sides of the loaded filter The mass absorption coefficient of the particles can be evaluated by transmission measurements and, later, the particle attenuation can be calculated (Adams and Van Grieken, 1975) If thin membranes, like Nuclepore or Millipore, are selected for the collection, the absorption effects in the filters are small and often negligible above keV, because, Copyright © 2002 Marcel Dekker, Inc normally, all particles should stay at the filter surface However, for light elements such as Si, P, S, and as well as for elements having very small particle sizes, like Pb and S, the absorption effect needs to be corrected by using a suitable correction procedure When choosing thick filters, for the collection of large particle amounts, an easy and proper way to minimize serious absorption effects is the so-called sandwich geometry (Van Grieken and Adams, 1975) This technique is based on simple inward folding of the loaded filter to obtain a kind of sandwich The heterogeneous distribution of the particles is minimized, despite the higher absorption effect for the filter in this geometry, as it is assumed that all particles are on the surface and therefore in the center of the sandwich, which also averages the unknown deposition depth of the particles Both methods (the back–front measurement and sandwich geometry) show accurate results when the absorption cross section of the filter is not excessively high The particle size distribution on the filter surface influences the accuracy of the correction more than the depth distribution inside the filter (Davis et al., 1977) For the establishment of filter reference material for aerosol particles, different types of filter were tested with a high-volume aerosol sampler As conditions for the reference material were claimed, low particle size effects, the filter thickness should not exceed 0.5 mg=cm2 and the lateral uniformity of the filter must be within certain limits Fiber filters were generally excluded from this study, as they collect particles in their bulk Only two types of filter (polycarbonate and cellulose nitrate membranes) fulfilled these conditions and showed an almost uniform deposition onto the surface, which was determined by analysis of different spots from different places at the loaded surface (Watjen et al., ă 1993) With ltration of the atmospheric particles, usually all particle sizes present in the aerosol down to the filter cutoff recorded and particle size effects can easily occur With stacked-filter units, operating with different filters of different pore sizes, and dichotomous samplers, some kind of size fractionation can be established With such collection methods, the particle size effects can almost be overcome, as the particles are divided into two or more fractions For particles smaller than mm, the particle size effects are negligible in general, and, therefore, the determination of elements with a low Z number is also possible If impactors with several stages are applied for the aerosol collection, the size distributions of the particles become more uniform and fulfill the requirements Another possibility for overcoming effects bound to different particle sizes and to obtain reliable results is the analysis of filters as pressed pellets For this, the loaded filters are pelletized and sometimes also previously mixed with binder and analyzed (Vigayan et al., 1997) Coal fly ash, for example, was ground, mixed with poly(vinyl alcohol) binder, and pelletized (Bettelini and Taina, 1990) In one study, it was shown that pressing the sample into a pellet after grinding to a powder yields better reprodu´ cibility than applying the sample as a thin film (Matherny and Balgava, 1993) The preparation of fusions is another way to eliminate particle size effects (Pella et al., 1978; Balcerzak, 1993) Good results were obtained by fusion of $70 mg of small dust samples with lithium fluoride and lithium tetraborate (20:80) To yield a homogenous microbead, the sample was treated at 1050–1100 C for in a muffle oven Subsequently, the fusion was poured into a specially designed mold and finally analyzed in a normal sample holder, modified with a spacer in order to fix the sample (Moore, 1993) However, it should always be considered that each additional sample preparation step might introduce blank values to the material or dilute the sample Especially by working in the Copyright © 2002 Marcel Dekker, Inc trace element region, which is usually the case for atmospheric particle samples, this can cause serious problems One point, which should not be underestimated, is the partial or complete loss of volatile components of the sample Especially in the wavelength-dispersive mode, voltaile compounds such as Cl, Br, and, in particular, S may be lost during evacuation of the sample chamber This effect is even more severe when working with PIXE Hereby, the sample is irradiated with a highly focused charged particle beam under high vacuum and losses of about 50% of sulfur may occur In addition to the losses of sulfur due to volatilization, chemical reactions can take place and lead to significant losses of S when 2-MeV protons or 18-MeV a-particles are used (Hansen et al., 1980) For the collection of aerosols onto filters, a variety of filter materials is available Each filter material provides special properties with respect to collection efficiency, mechanical stability, hygroscopy, and so forth and every selection is usually a compromise among the filter properties, the collection purpose, the available costs, and the compatibility with the analytical technique to be applied Before choosing a filter material for the collection of atmospheric particles, the blank values of the material itself must be determined The filter should consist of materials with low blank values for the required elements Furthermore, the spectral background should be reasonably low in order to allow a reliable trace element determination Cellulose membrane filters, for example, especially those made of cellulose nitrate and acetate, are unsuitable for the determination of P, S, K, and Ca, as they show high blank values of these elements (Krivan et al., 1990) Very practicable are Nuclepore membranes because their blank values are very low and the particles are collected only on the surface Unfortunately, the price for these filters is quite high Whatman 41 filters have a high collection efficiency and are mechanically stable, clean, and reasonable in price However, the high hygroscopicity requires a controlled humidity for weighing and handling of the filter They also show a much higher background signal in comparison to the Nuclepore membranes The interpretation of the filter efficiency is often complicated, as the test data are usually based on unloaded or ‘‘clean’’ filters However, during a sampling event, the filters change their properties depending on the particle load With increasing amounts of collected particles, the efficiency increases as well, despite the higher flow resistance of the loaded filter In general, the real efficiency of the filter is higher than in the published data Furthermore, the collection efficiency increases with higher surface velocity and larger particle size For the Whatman 41 filter, for example, the collection efficiency is quite low for particle sizes of 0.26 mm and surface velocities of 1.5 cm=s With surface velocities of 100 cm=s, the collection efficiency increases to 95% for both small and large particles A 10-mm pore-size Teflon filter shows collection efficiencies of 60–90% for low surface velocities and particle sizes between 0.003 and 0.1 mm For 0.5- and 1-mm pore-size Teflon filters, the efficiency is always > 99.99%, independent of surface velocity and particle size (Lippmann, 1978) Membranes with different pore sizes can be used in stacked-filter units for sizefractionated collection or with dichotomous samplers Quartz-fiber filters present another alternative They show high collection efficiencies and are very clean and less hygroscopic than cellulose filters Particles within the range of 100–0.1 mm are retained on this filter Furthermore, they are heat resistant Haupt et al (1995) tested different techniques for the preparation of standards for the direct analysis of quartz-fiber filters One preparation technique was based on the generation of aerosols and their deposition onto the filter surface The other one required spotting of a multielement Copyright © 2002 Marcel Dekker, Inc standard solution onto the filter and subsequent air-drying The aerosol generation procedure showed much better accuracy and is, therefore, preferable for the preparation of standards Glass-fiber filters are not recommended for trace analysis of inorganic components in aerosols They are much less expensive than the quartz filters, but very high and variable blank values of diverse elements are their major drawback However, they are quite useful for the collection and determination of organic compounds Sometimes, these filters are also applied for the analysis of one single component in the aerosol sample Lead, for example, can be determined in total suspended air particulates, after collection onto glass-fiber filters without problems LaFisca et al (1991) cut disks of 32-mm diameter from the filters and irradiated them successfully Koutrakis et al (1992) collected household dust onto Teflon filter with a Harvard impactor The fine-particle mass was gravimetrically determined and, subsequently, the concentrations of elements associated with the fine mass determined by XRS In some cases, filters may also be used for the collection of reactive gases For this purpose, the filter is impregnated with special chemicals which retain selected components of the gases Hydrogen sulfide, for example, can be trapped by coating the filter with ferric ion solution The excess Fe is removed from the filter surface by washing with appropriate solvents, whereas the Fe sulfide precipitation stays on the filter Sulfur can be determined directly or indirectly by measuring the Fe x-radiation Suitable standards are available for such measurements (Leyden et al., 1984) Because XRF is a well-established method, which shows good reliability and easy handling, the technique is widespread for the determination of aerosols The use of radioisotope sources for the excitation makes it independent of the laboratory placement and suitable for field application The samples can be analyzed in situ and contaminations and losses, which might occur during transport and handling, are avoided or kept at a minimum (Gilfrich and Birks, 1978) In addition to conventional XRF, TXRF is more frequently applied for the analysis of aerosols To utilize the low detection limits and to reduce the matrix effects originating from the collection material, filter samples need to be digested With this procedure, element concentrations between 0.2 ng=m3 for Cu and ng=m3 for Mn were determined (Einax et al., 1994; Schmeling and Klockow, 1997) By collection with suitable impactor systems (e.g., Battelle impactor), the aerosols are separated directly onto the polished sample carriers required for the instrument, and detection limits of ng for Cr and 15 ng for Ca were obtained (Injuk and Van Grieken, 1995; Klockenkamper et al., 1995) ă VII SAMPLE SUPPORT MATERIALS In addition to the specimen preparation procedure, the position of the sample inside the spectrometer plays an important role for a reliable and precise analysis It should be ensured that the sample and the calibration standards are in the same form and position for analysis, in order to avoid deviations and ensure reliable quantification Usually, the sample size is determined by the size of the sample holder, in which the specimen is presented to the spectrometer In some cases (e.g., for customized instruments), the sample chamber is adapted for the nondestructive analysis of special objects like archeological and art specimens Usually, the sample holders of commercial instruments are cylindrical with a diameter of 5.1 cm and allow placing samples with a maximum thickness of cm Smaller samples can be fixed by a special mask, which is inserted to the conventional holder The bottom of the sample holder is normally Copyright © 2002 Marcel Dekker, Inc Table Degradation Resistance Properties of Selected Thin-Film Materials Material Contaminants Poor degradation resistance for Mylar P, Ca, Zn, Sb Strong, mineral acids (HCl, HNO3) Polypropylene Al, Si, Ti, Cu, Fe Oxidizing, concentrated acids; aqua regia Polyethylene Oxidizing, concentrated acids; alcohols; esters; ketones Polycarbonate Oxidizing, concentrated acids; alcohols; esters; ketones; aliphatic and aromatic hydrocarbons; mineral, vegetable and animal oils Polystyrene Esters; ketones; aliphatic and aromatic hydrocarbons; mineral and vegetable oils Kapton Strong, mineral acids; alkalines Formvar Acids covered with a thin film to prevent losses of the sample and contamination of the spectrometer As ideal targets for the XRS analysis, samples prepared as thin films are used For infinitely thin samples all interelement and mass absorption effects are negligible However, in practice, infinitely thin means that for most x-rays, the sample thickness should be between 10 and 200 mm, which is difficult to obtain Figure Analyte line transmittance for various thin-film substances and thicknesses (From Ref 182 Reprinted by permission of American Laboratory.) Copyright © 2002 Marcel Dekker, Inc With the use of thin-film supports, special conditions for the materials are required The films should be stable under measuring conditions and have low impurities, especially for the desired elements The film should be thin enough to provide the highest degree of transmittance, particularly for low concentration levels and low-energy photons Furthermore, the material should show a high resistance for degradation, which means that the specimen is retained safely in an XRF cup during the measurement and shows chemical resistance against acids, organic materials, thermal softening, tearing, and stretching For some materials these properties, are listed in Table Figure illustrates the effect of thin-film thickness on analyte-line transmittance for some substances (Solazzi, 1985) Except Teflon, almost all of the presented materials show transmission better than 90% for photons of keV or more, whereas in the low-energy region, absorption effects are 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(1996): X > < s1 x; Zị ẳ di ẵexpxị 1i ; x xi Sx; Zị ẳ iẳ1 > : s2 x; Zị ẳ ẵZ s1 x; Zị t2 g1 x? ?? þ t2 g2 ? ?x? ? þ s1 ? ?x1 ; ZÞ x1 < x 66ị where g1 x? ?? ẳ expẵt1 x1 x? ?? and g2 x? ?? ẳ expẵt3 ? ?x1 À x? ?Š The... combination of analytical functions to calculate F (x, Z): x x1 > f11 x; Zị ẳ a expb1 x? ?? ỵ ðZ À aÞ expðÀcxÞ; > > < f12 ? ?x; ZÞ ¼ f11 ? ?x1 ; ZÞ expðÀb2 ? ?x1 À x? ?Š; x x x2 74ị F1 x; Zị ẳ f13 x; Zị ẳ f12 x2