Atomic Spectrometry Update. X-ray fluorescence spectrometry

Contents

• 1 Reviews

• 2 Instrumentation
  • 2.1 General instrumentation and excitation sources
  • 2.2 Detectors

• 3. Spectrum evaluation, matrix correction and calibration procedures
  • 3.1 Spectrum evaluation
  • 3.2 Matrix correction and calibration procedures

• 4 X-ray optics

• 5 Synchrotron radiation

• 6 Total reflection X-ray fluorescence spectrometry (TXRF)

• 7 Portable and mobile XRF

• 8 On-line XRF

• 9 Applications
  • 9.1 Sample preparation
  • 9.2 Preconcentration techniques
  • 9.3 Geological
  • 9.4 Environmental
  • 9.5 Archaeological and forensic
  • 9.6 Industrial
  • 9.7 Clinical and biological
  • 9.8 Thin films
  • 9.9 Chemical state analysis and speciation

• References

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Abstract

This annual review of X-ray fluorescence covers developments over the period 2000–2001 in instrumentation and detectors, matrix correction and spectrum analysis software, X-ray optics and microfluorescence, synchrotron XRF, TXRF, portable XRF and on-line applications as assessed from the published literature. The review also covers a survey of applications, including sample preparation, geological, environmental, archaeological, forensic, biological, clinical, thin films, chemical state and speciation studies. During the current review period, publications have demonstrated the development of sub-100 nm X-ray beams for SR microprobe analysis together with the wider use of WD spectrometers in this application. There is evidence of an extension of the application of XRF as a reference technique, with XRF increasingly being used in modern laboratories in place of older wet-chemical methods, and computer-modelling studies continue to be popular in extending the understanding of various XRF phenomena. Some interesting work has been undertaken in the measurement of radiative Auger effects using high-resolution WDXRF instruments. However, the potential for future developments in XRF is illustrated by research into ultra-high resolution microcalorimeter detector devices, which are still at the experimental stage and have not yet progressed to the status of useful practical devices.

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1 Reviews

X-ray fluorescence spectrometry has had yet another vigorous year and has seen the publication of a number of comprehensive reviews. With an unimpeachable author line of Jenkins, Gould and Gedcke, Applied Spectroscopy Reviews published a comprehensive review of the technique¹ covering the analysis of metals, alloys, minerals, ores, ceramics, archaeological samples, catalysts, forensic and environmental science samples, trace elements in natural water and techniques for the analysis of small samples. During the current review period, the companion Analytical Chemistry review of X-ray spectrometry was published,² with a scope that extends beyond the present review to cover PIXE, EPMA and XAS. A number of additional reviews also appeared in technical, trade and national publications including Jordan,³ Janssen and Flock⁴ (in German), Polesello⁵ (in Italian), and Matherny and Pliesovska⁶ (in Hungarian).

Other, more general, contributions to the field of XRF spectrometry during the current review period include the work of Sieber,⁷ who published a review of the use of X-ray techniques within the National Institute of Standards and Technology covering a wide range of applications and techniques, including elemental analysis, microanalysis, metrology, lithography and basic research in physics. A more specific development is that all X-ray spectrometry techniques require reference materials, not the least to demonstrate the traceability of their analytical results. Carpenter et al.⁸ published a review of the composition of specific Corning glass reference materials to which a variety of techniques contributed, including XRF.

2 Instrumentation

2.1 General instrumentation and excitation sources

In addition to providing information about the bulk composition of a material, analysts are increasingly asked for speciation of elements present. To satisfy such demands Geyer et al.⁹ optimised a cold cathode tube as an excitation source for soft X-ray spectrometry. Different cathode materials (aluminium, titanium and steel) were investigated with the titanium option offering a stable discharge over a long period of time, making it suitable for practical work. Volkov et al.¹⁰ published work on detection limits of heavy elements obtained with K series radiation induced by electron accelerator bremsstrahlung. The electron energy varied from 1 to 5 MeV making it possible to determine element concentrations down to 10⁻⁸–10⁻¹⁰% m/m. Rangsten et al.¹¹ offered a micro-machined field-emitting structure for a miniature X-ray source. Pillay and Peisach¹² published an overview of applications associated with the development of charge-induced XRF using ion beams.

Other instrumental developments included a wide angle geometry EDXRF spectrometer with secondary target and direct excitation modes offered by Yokhin.¹³ The system allowed switching between secondary target and direct excitation modes with a single stationary tube and was claimed to provide 3–7 times better efficiency in both modes when compared with existing similar configurations. Mitra et al.¹⁴ described the experimental determination of the optimum geometry for minimum background from scattered bremsstrahlung measured by a Si(Li) detector in an EDXRF system with an X-ray tube and secondary target. Bakulin and Durbin¹⁵ described a high efficiency logarithmic spiral monochromator made from pyrolytic graphite with a 200-fold improvement in collection efficiency, while maintaining the intrinsic graphite energy resolution. Parker and Willis¹⁶ provided some interesting observations for those users with the latest generation of WDXRF spectrometers with closely coupled X-ray tube–sample optics. Such configurations offer significantly higher intensities, lower detection limits and shorter counting times for the same level of precision: however, with decreasing distance in the optics, errors in sample positioning may lead, in theory, to intensity errors exceeding ±1%. Readers with inclined tube (side or end-window) sequential spectrometers were advised that the measured intensities follow a second order parabolic relationship rather than the inverse square law. However, those with end-window simultaneous spectrometers were informed that the inverse square law has important implications for analytical error.

2.2 Detectors

A review of gamma- and X-ray detectors by Knoll¹⁷ presented the full chronological history from 1950 of all the types of detector used for these spectroscopic regions. The author included the latest trends in portable, thermoelectrically cooled and silicon drift X-ray detectors and the more recent high-resolution cryogenic X-ray detectors.

The excitement of three years or so ago, when ultra-high resolution energy dispersive (ED) detectors based on microcalorimeters or superconducting tunnel junction (STJ) devices were being claimed to be close to wider availability and commercial viability, was tempered by reality in the past year. Newbury and co-workers at NIST¹⁸ described their microcalorimeter ED detector, for which they claimed an energy resolution of 3 eV at 1.5 keV and a count rate of 500 cps. These authors combined the detector with a polycapillary X-ray optic to yield an effective collection area of 5 mm². At this stage, their system remains limited and in early development, although a number of microanalysis applications were presented. Silver et al.¹⁹ described a germanium-based microcalorimeter, which also used an X-ray focusing optic to improve the solid angle of collection and which was applied to laboratory astrophysics and microanalysis problems. Meier et al.²⁰ described improvements to the all-important cryogenic system that provided active thermal feedback, resulting in improved thermal stability at the ultra-low (<300 mK) operating temperatures needed for their ED system, based upon a phase transition thermometer. Despite these developments, which improved energy resolution at 1.5 keV from 230 eV to 133 eV, a conventional Si(Li) detector would deliver a better energy resolution with a much greater active area and considerably less complexity. The most recent publication from the cryogenic spectrometer company CSP²¹ described an improved adiabatic dilution refrigerator (ADR), which delivered a temperature of 100 mK and lasted 30 h between regeneration cycles. An energy resolution of 43 eV at 5.9 keV was achieved using a microcalorimeter in conjunction with a mechanical cooler together with this new ADR, although there are still considerable challenges in putting together a successful and complete working system for practical micro- and XRF analysis. Mitsuda et al.²² developed a microcalorimeter using a Ti–Au transition edge sensor (TES), which, they claimed, should lead to improved energy resolution at higher temperatures (300–500 mK) compared with the temperature of 80 mK required for microcalorimeters. Although the detector was shown to be sensitive over the 3–25 keV energy range, a dismal energy resolution of only 550 eV at 5.9 keV was reported—a figure that can be approached by some types of gas proportional counter! On a more positive note, Monfardini et al.²³ claimed an impressive energy resolution of 5.4 eV at 6 keV for their microcalorimeter. The authors claimed their energy resolution figure to be the best published to date.

The second type of ultra-high energy resolution cryogenic ED detector is based upon superconducting tunnel junction (STJ) devices. Kurakado, who has been at the forefront of this field throughout its brief history, produced an excellent and comprehensive tutorial²⁴ in which the principles, operational detail and performance of these important detectors were reviewed. Nakamura et al.²⁵ proposed an improvement to the processing of STJ signals in which the rising part of the amplifier output signal was used. This resulted in a significant improvement in energy resolution in comparison with the use of the decaying signal. Improvements in STJ fabrication using oxygen plasma etching of the device layers^26,27 led to reduced leakage current and improved energy resolution. These workers reported energy resolutions at 5.9 keV of 41, 58, 65 and 129 eV for devices with dimensions of 20 × 20, 100 × 100, 200 × 200 and 500 × 500 µm², respectively. Morohashi et al.²⁸ reported the fabrication of a tantalum-based STJ, which was designed for its greater stopping power for higher energy X-rays. An improved internal method for suppressing background current was reported by Nakagawa et al.,²⁹ who used a micro-machined microstrip coil rather than a conventional external electromagnet. Finally, Otani et al.³⁰ used Monte Carlo (MC) simulation and a device model to reproduce successfully observed spectra from niobium-based STJs. The authors anticipated improved STJ detectors using this approach and through improvements to their model.

The review year has seen little activity in silicon-based semiconductor ED detectors, with the only real interest having been in drift detectors and pixellated devices such as CCDs. A review by Lechner et al.³¹ on silicon drift detectors (SDDs) acted as a valuable update of these relatively new detectors. The authors presented early results on large area SDDs in which the use of off-chip electronics was described. This work indicates where future improvements will be found to overcome the solid angle limitations of present day detectors. A new, novel architecture for a controlled drift detector was proposed and initially characterised³² and Castoldi et al.³³ carried out a detailed study of the mechanism used to confine the liberated charge in 3-D wells. Struder³⁴ provided a useful review of imaging X-ray spectrometers incorporating pixellated SDD or CCD energy dispersive X-ray detectors. The greatest interest in these devices is in the field of astrophysics and high-energy physics or laser research.³⁵ In order to understand fully the ED and spatial data resulting from the use of CCDs, the measurement and modelling of the local energy response of CCDs remains the subject of study.^36,37 An investigation of transient currents in Si PIN detectors³⁸ identified their relationship to capture cross-sections of traps, which yielded a useful insight into some of the problems that occur when using such X-ray detectors. A further study³⁹ of electron-beam-induced currents in Si PIN detectors provided useful data to study boundary effects in two commercially available silicon PIN diodes.

As with silicon-based ED detectors, the review year has seen little activity in the field of compound semiconductor ED detectors. A review of the current use and future potential in space applications of GaAs and TlBr semiconductor ED detectors was presented by Bavdaz et al. at the European Space Agency.⁴⁰ Other work on CdZnTe strip detectors⁴¹ and a pixellated CdZnTe detector⁴² concentrated on imaging, but reflected the importance of using ED detectors to provide maximum information. A study by Soo et al.⁴³ concentrated on the diffusion by heat treatment of copper into CdTe, which has a bearing on detector performance when certain rear contacts are used. An improved, lower cost method for growing detector grade GaAs films up to 500 µm thickness was described^44,45 with details of how electron irradiation was found to reduce the amount of doping required. The films were characterised physically and electrically with and without Schottky barriers on them, although no data from actual X-ray detectors were reported. Finally, Kocsis⁴⁶ presented the important characteristics of thallium-lead iodide, which led the author to propose this semiconductor as a new room temperature X-ray detector material.

X-ray detectors made from diamond offer some novel and interesting possibilities. A review⁴⁷ of the development of CVD diamond detectors indicated their potential benefits in hostile environments such as those found in the nuclear industry and in high-energy physics experiments. Such detectors remain in the early stages of development or the subject of basic investigations of the detector material.⁴⁸

An impressive energy resolution at 5.9 keV of 8.9% was reported^49,50 for a gas proportional scintillation counter, which compared favourably with the energy resolution of 20% or more that is typically achieved with conventional Xe-filled gas proportional counters. The authors also presented data on the effect of detector voltage on the low-energy tailing of measured peaks and predicted this effect would be worse for higher energy X-rays. Krajcar Bronic and Grosswendt⁵¹ presented a detailed study of the effect of gas composition, pressure and detector voltage on gas gain and energy resolution in conventional argon-filled gas proportional counters. The best energy resolution was obtained with low gas gain (100–200) and results were presented of the effect on energy resolution of various gas mixtures and partial pressures.

3. Spectrum evaluation, matrix correction and calibration procedures

3.1 Spectrum evaluation

A novel application of a genetic model to the fitting of peaks in EDXRF spectra was described by Brunetti and Golosio.⁵² The characteristics and performance of this model were compared to the conventional non-linear Marquardt–Levenberg (ML) fitting algorithm using one simple synthesised spectrum and two real spectra. The real spectra were from a reference soil and a glass reference material. The new genetic procedure used a simple Gaussian peak shape as a gene and generated a typical population of 100 solutions to which the evolution operators were applied. A simple fitness function was used, based on the absolute channel-by-channel difference between the predicted and the measured spectrum, to permit selection of the best chromosomes (solutions) to be retained in the next generation. The number of generations used was in the range 1000–5000. The genetic algorithm was claimed to be the superior of the two, although the performance was similar and both methods failed to fit at least one significant peak or had substantial errors in the evaluation of the real spectra. The authors claimed that the genetic algorithm was more adaptable than the ML algorithm because constraints and different peak models could be introduced more easily. The authors recognised that their algorithm had not been optimised for speed, which was reflected by the time of about 1 min taken on a 700 MHz Pentium 3 PC to reach a solution for a fit to just 20 peaks. This is an interesting and potentially widely applicable approach to the problem, but the speed and error rates of the technique will have to be improved substantially before it supplants existing well-accepted procedures. A further report⁵³ on the theory and application of a genetic algorithm to real and synthetic PIXE spectra was shown to give good agreement with other peak fitting procedures.

The processing of X-ray and gamma ray spectra poses its own particular problems and these were investigated⁵⁴ for the case of spectra from CdTe and CdZnTe detectors. The importance was stressed of using real peak shapes in the fitting function as there are typically large amounts of low-energy tailing on peaks, which must be fitted accurately in order to get reliable peak area data. Similar findings on the importance of including peak distortions were reported by Kondrashov et al.,⁵⁵ who also recommended the use of a least moduli rather than a least squares method when fitting peaks with high peak-to-background ratio.

It is a truism that, no matter how good the peak fitting algorithm, the overall performance can only be as good as the detector response function. A new MC code was developed⁵⁶ in order to obtain a better understanding of the peak shape function from a Si(Li) detector. The model simulated individual elastic and inelastic processes within the crystal and the effect on incomplete charge collection in various areas of the detector crystal. The authors concentrated on the energy range <5 keV and found a good agreement between the simulated and measured spectra.

A numerical model and correction procedure was developed⁵⁷ for the correction of iodine K escape peaks from NaI scintillation detectors used in dual energy X-ray absorption studies of human bone density. Full details of the escape probability function and the correction algorithms were given, including the numerical values of all the parameters used. The intensity ratio of the unresolved I Kα + Kβ escape peak from the 59.4 keV peak from the ²⁴¹Am source was determined to be 0.069, which compared favourably with the literature value of 0.067. As with all good escape peak corrections, the algorithm developed here not only removed from the spectrum the I K escape peaks, but also reinstated the escape peak intensity back into the full energy parent peak.

A semi-empirical method was described by Szaloki et al.⁵⁸ for the practical determination of the detector efficiency curve for a Si(Li) and for two Si PIN detectors over the energy range 1–35 keV. The authors used tight collimation to define closely the geometry of an in-house system in which the detector could be easily mounted to measure the spectrum from samples of pure elements or their compounds excited by an ¹²⁵I or ⁵⁵Fe radioisotope source. The fundamental parameter (FP) correction technique was used to calculate the true pure element intensity from the measured data. The efficiency model incorporated the bulk detector efficiency and the contributions from metal contact layers, dead layer and the beryllium entrance window. The commercially supplied Si PIN detectors were found to have an insignificant metal contact and dead layer of <0.1 µm but were found to differ greatly in bulk detector efficiency with measured values of 145 and 205 µm, both of which were significantly less than the value given by the manufacturer.

3.2 Matrix correction and calibration procedures

The choice of which calibration and matrix correction expression to use has always been the subject of discussion. Thus, the assertion that the fundamental algorithm really was the best and the one from which all other theoretically valid expressions could be derived was the subject of a polemic.⁵⁹ The authors questioned the original conclusions and strongly defended use of the expression whose correctness had been called into question. Needless to say, such discussions will continue whilst practitioners continue to use successfully the expression and software that they have validated for their own application. A matrix correction method for treating ultra-light elements that cannot be measured directly by XRF has been described,⁶⁰ in which the unanalysed element was treated as an elimination component in the calculation of alpha coefficient for use in the de Jongh–Norrish expression. The unanalysed element was treated as the balance component when analysing unknowns and the approach was shown to work well for the determination of B₂O₃ in two types of glass and of Li₂O in lithium potassium niobate crystals.

A so-called substitution–dilution method, based on the dilution–addition method, was described,⁶¹ in which the unknown sample was diluted/substituted by a multi-element reference material. Interestingly, it was claimed that the unknown sample and the reference material did not need to be similar in composition or mineralogy. Using diluent/sample dilution factors greater than 10, the method produced a linear calibration model and was applied successfully to the analysis of a binary alloy and a geological sample. The method appears to offer no major benefit for geological materials over the well-established fusion methods using external, synthetic calibration samples.

The use of scattered background measurements for matrix correction in XRF is long established and widely used, particularly when the matrix comprises light elements. Thus, the Compton-scattered tube lines from the secondary target excitation were used to deduce the mass absorption coefficient (MAC) for the measured samples, which were then used to correct the analyte line intensities.⁶² The theoretical basis of this approach was presented in useful detail and the authors were able to establish the basic relationship of MAC at the analyte peak to MAC at the Compton line energy from the Compton peak intensity of a single silicon sample. The calibration procedure did not require an iterative solution and the authors reported its successful application to the analysis of a glass reference material and rare medieval glass samples. An alternative to using Compton scatter was described⁶³ in which MAC values were derived from a quadratic function relating them to the continuum backscatter intensity. The new method was claimed to be more suitable than the traditional use of Compton scatter as an internal standard line, although the measurement of the Compton peak is simpler and very reliable, which is likely to result in its continuing widespread use.

The level of interest in chemometric methods for XRF data seen in recent years was not maintained during the review year. Notwithstanding, a very interesting use of chemometrics and EDXRF for the non-invasive identification of compounds in closed polyethylene containers was reported.⁶⁴ The authors claimed that the use of characteristic XRF lines and the normal correction procedure using backscatter radiation failed to make full and optimum use of all the data in the EDXRF spectrum from the sample. The proposed method used characteristic XRF, Compton and Rayleigh scatter data in a principal component analysis in conjunction with regularised discriminant analysis to establish a reliable classification scheme for unknown samples of importance to the pharmaceutical industry. The new method was also the subject of a patent application.⁶⁵

Finally, the important topic of matrix correction using the fundamental parameter (FP) approach continues to generate interest. The detailed mathematics and calibration features of the FP method and some of the practical approaches that can be adopted for the solution of the FP correction equations was reviewed in detail by Kitov.⁶⁶ It is normally the case for FP correction algorithms that the excitation spectrum is either measured or calculated. However, an interesting method was proposed⁶⁷ in which the excitation spectrum was described by means of measurements taken from a set of either thick or thin pure element samples. The method was found to be better than when a semi-empirical or adjusted measured spectrum was used in the FP calculation, although only analyses of AgI and AgBr were performed. Rather unsurprisingly, the measurement of Ag and I in AgI using an X-ray tube high voltage of 35 kV was found to yield highly inaccurate results for Ag as the iodine K intensities from which the I concentration and correction were derived would be expected to be pitiful with such a small overpotential for the I K absorption edge.

A new model was developed⁶⁸ to predict successfully the bremsstrahlung spectrum produced from a wide variety of thick targets and electron beam accelerating voltages between 10 and 25 kV. The model calculated corrections for electron backscattering, target self absorption and the efficiency of the Si(Li) detector. Although not specifically aimed at XRF, this model may find use in predicting excitation spectra used in FP correction procedures.

4 X-ray optics

One of the important developments in recent years in X-ray microfluorescence (XRMF) has been the development of glass capillary ‘focusing’ optics. Studies originally centred on the development of single (monocapillary) optical elements, but an important advance was made with the introduction of polycapillary focusing devices, commonly referred to as ‘Kumakhov’ lenses and comprising bundles of glass capillaries. It is noteworthy, therefore, that the originator of these devices, Kumakhov, presented a review⁶⁹ of capillary optics and their use in X-ray analysis for both fluorescence and diffraction applications. By ‘focusing’ a beam of 0.5–1 mm down to 5–10 µm, the X-ray flux on the sample was claimed to increase by 2–3 orders of magnitude. In combination with a 50–100 W microfocus X-ray tube, these devices are capable of achieving flux densities equivalent to those found on a synchrotron beamline. In combination with a novel type of X-ray tube, new possibilities were reported in the development of high efficiency portable X-ray spectrometers. The concept for such a portable instrument was described by Langhoff et al.,⁷⁰ whose interest was in a combination of a high brilliance low powered X-ray tube, a capillary optical system and a non-cryogenic X-ray detector for applications in the analysis of works of art. A further development using polycapillary optics was described by Ding et al.,⁷¹ who coupled such a device to a position-sensitive detector to provide high energy and spatial resolution and high detection efficiency. Yan et al.⁷² also discussed the application of polycapillaries to X-ray microanalysis and Worley et al.⁷³ reported that the retrofitting of a monolithic polycapillary assembly to a microfluorescence instrument resulted in an intensity gain of 125 at 4 keV in comparison with a pinhole collimator, although this advantage was reduced at higher energies because, it was suggested, the capillary was shorter than the optimum length. Nevertheless, a minimum capillary focal spot of 36 µm (FWHM) was achieved, compared with 50 µm for the pinhole, showing that the former device could achieve greater spatial resolution.

These developments in polycapillary devices should not mask the continuing interest in monocapillary devices. Xie and colleagues⁷⁴ presented a theoretical model of the focusing properties of monolithic lenses, making comparisons with experimental measurements. Measurements with a capillary showed that for the Mo Kα line, a focal spot of 30 µm gave an increase in power density of about 1500 compared with an aperture of equivalent size placed at the focal distance. With such an arrangement, detection limits for transition elements down to 10⁻¹³ g were reported. Haschke and Theis⁷⁵ also considered the advantages of mono- and poly-capillary lenses, and Carpenter⁷⁶ described capillary optics coupled to a low power micro-focus source.

Tapered capillaries offer certain advantages as focusing devices, and a wave-optics treatment of X-rays passing through such devices was published by Kukhlevsky et al.,⁷⁷ using the Fresnel–Kirchhoff theory and the method of images. The predicted appearance of diffraction fringes in both the near- and far-field zones of the waveguide output was confirmed experimentally.

In terms of the development of X-ray microfluorescence instrumentation, Bichlmeier et al.⁷⁸ evaluated the performance of components for a compact system, including mono- and poly-capillary lenses coupled to a molybdenum anode minifocus X-ray tube, and Si-drift chamber and Si PIN-diode detectors. The aim was to optimise the performance of an instrument capable of use on-site for the determination of trace elements in glass and silver matrices. This investigation was extended to the use of mini (250 µm) focus and conventional (1 mm focus) X-ray tubes. Kogan⁷⁹ claimed a patent for a parabolic multilayer mirror and Wang et al.⁸⁰ described, in a Chinese language paper, an optical system incorporating a large area transmission grating and a pre-optics assembly comprising a toroidal mirror, slit and cylindrical mirror, designed for soft X-ray emission spectra. Position sensitive detectors were featured in a paper by Hu et al.⁸¹ in conjunction with PIXE and XRMF instrumentation, and these authors reported the energy resolution of some transition element fluorescence lines. The Cr Kβ/Mn Kα interferences and Si Kα satellite lines were resolved and a detection limit for Ti of 10⁻⁹ g was reported.

In order to enhance the quality of images or maps obtained by XRMF, Lankosz and Sieber⁸² measured the flux distribution of the primary X-ray beam using a simple procedure based on scanning a wire. Recognising that the result of such a scan is a convolution of the beam profile with the shape of the wire, these authors developed a non-linear least squares procedure to model the profile using a parametric function (a sigmoid function was found to be the best) to describe the flux distribution. Modelling was undertaken for a pinhole collimator and the method had the advantage that the flux profile could be measured in the sample position and that results could also be used to evaluate the quality of a pinhole collimator.

A different approach to improving XRF distribution maps was developed by Togami et al.,⁸³ who developed a numerical method based on maximum likelihood principles to relate pixel intensities to mineral composition. The method was evaluated using XRF images obtained using a scanning X-ray microscope on a granite comprising predominantly quartz, biotite, plagioclase and K-feldspar to obtain maps of the major elements, including Al, Ca, Fe, K and Si. Scruggs et al.⁸⁴ also reported developments in techniques for XRF mapping based on spectral data recorded using EDXRF to measure the full spectrum, permitting data to be reanalysed by setting up regions-of-interest covering element lines that were not included in the initial assessment of data.

Before considering any applications, thought must be given to both sample preparation and calibration. In the case of sample preparation, Colletti and Havrilla⁸⁵ considered factors that affected trace element microanalysis and investigated micro-pellet, slurry and dried spot techniques as well as aperture size, capillary optics and tube excitation conditions. Of importance in all microanalytical techniques is the provision of suitable reference materials, and a novel micro-structured reference material was developed by the Institute of Reference Material Measurements, Geel, Belgium, as described by Watjen et al.⁸⁶ This reference material comprised strip patterns of different widths from 2 to 100 µm of permalloy (81% m/m Ni, 19% m/m Fe). Because these patterns were very well defined, with edge profiles 0.5 µm high and shape irregularities and undulations of <0.1 µm, this reference material could be used directly to convert an XRF microfocus line scan into a spot size and beam profile.

Turning now to applications of XRMF, it is interesting that this technique is beginning to be used in a similar range of applications as the main stream XRF technique, as summarised in Section 9. Haberkorn and Beck⁸⁷ used this technique to study a wide range of sample types, including meteorites, jasper, coated glass and reference materials. The interest in glass coatings was the determination of the thickness of ytterbium layers on glass from the XRF intensity, using for calibration a sample for which the layer thickness had been determined by XRD reflectometry. These authors also proposed the analysis of samples mounted within glass capillaries (presumably for support), where the achievable element range extended from S to U, compared with Na to U for samples analysed under vacuum. A forensic application was reported by Charpentier and Desrochers⁸⁸ in which XRMF was used to analyse gunshot residues in the area surrounding a bullet hole. The microfluorescence instrument incorporated a rhodium anode microfocus tube and used a 500 µm collimator. Results were obtained following the examination and analysis of residues remaining on chromatographic paper into which shots from selected hand-guns had been fired. Metal ions in spruce wood were analysed by Sunden and colleagues⁸⁹ with the aim of measuring the distribution of ions in spruce wood sections and elucidating the species to which the ions were attached. Polycarbonate foils of different thickness were the subject of the study by Wegrzynek,⁹⁰ who used a capillary optics EDXRF system offering a spatial resolution of 30 µm. Line scans were used to determine the elements Ba, Cr, Fe, Sr and Ti and to assess differences in concentration associated with foil thickness.

The XRMF technique continued to make an important contribution in the field of art and archaeometry, with important contributions continuing to be published by the Antwerp group, including the use of both EPMA and XRMF for the quantitative analysis of 16th–17th century archaeological glass vessels.⁹¹ A partial least squares (PLS) regression technique, based on the careful selection of training sets of data was used to quantify the major element composition (SiO₂, K₂O and CaO) to an error smaller than 3% for EPMA and 5% for XRMF data. The group also reported the interpretation and use of inter-element correlation graphs, obtained by scanning XRMF to characterise individual particles. In the first contribution to this study, Somogyi et al.⁹² developed a theoretical model, verified by measurements on the reference material NIST SRM K961, in which characteristic line intensities for several elements were calculated as a function of beam position for spheres, spherical shells and rectangular blocks having a silica or carbon matrix. The model showed that, when self absorption of the XRF signal was important, the shape of a correlation plot of one element as a function of another was characteristic of the topology of the particle: its slope depended on the concentration ratio of the investigated pair of elements and the width varied with the composition and density of the matrix. In the complementary paper, Somogyi et al.⁹³ used this theoretical model to analyse individual oil and coal fly ash particles. The absorption characteristics of the residual matrices of the particles could be estimated from the width of the correlation diagrams, and inter-element concentration ratios from the slopes. These data were compared with values determined by measuring a thin glass calibration reference material NIST SRM 1832.

Other developments based on X-ray microfluorescence using a synchrotron as excitation source are described in Section 5.

5 Synchrotron radiation

Research published in the current review period continued to take advantage of the higher beam intensities available at third generation synchrotron facilities. There was also a continuing trend in upgrading older synchrotron instrumentation. An example of recent developments in beam lines was the work of Beckhoff et al.⁹⁴ describing an extension in the energy range available at the BESSY II synchrotron from 1.5 to 10.5 keV with plans to extend this further to 50 keV by using radiation from a 7 T wavelength shifter. This will allow a substantial extension in the number of elements that can be excited from their K lines. This beam line was used as a primary standard for the absolute calibration of detectors (efficiency and response function characteristics), and has been applied to photodiodes, semiconductor detectors and superconducting tunnel junction devices, achieving relative uncertainties in detector parameters of better than 1%. Chantler et al.^95,96 monitored the fluctuations in a monochromatic bending magnet beam line at the Photon Factory, Tsukuba, using matched ion chambers. Results allowed them to measure the fluctuations in beam stability, an important, but often neglected aspect of measurements using synchrotrons. A status report on technical and instrumental aspects of the X-ray microscopy facility at the ESRF (0.2–7 keV range) was given by Susini et al.⁹⁷ This microscopy facility incorporates three undulators, uses a zone plate focusing lens and provides transmission and scanning X-ray microscopes. The design, fabrication and testing of Fresnel zone plates used at this facility for high resolution X-ray fluorescence microscopy was described by David et al.,⁹⁸ reporting that an exceptional spatial resolution of less than 100 nm could be achieved. The capabilities of this instrument were demonstrated by recording sulfur XRF images in zinc sulfide nanostructures. Pollmann et al.⁹⁹ reported on the development at the Advanced Photon Source (Chicago) of a circularly polarised X-ray microprobe using a Bragg-diffracting diamond phase retarder to achieve a spot size of 4 × 2 µm² in the energy range 5–10 keV. This facility was used to map, in two dimensions, magnetic domains in HoFe₂. Preliminary results were presented by Hayakawa et al.¹⁰⁰ of an X-ray microprobe system at the undulator Beamline 39XU, at SPring-8 (the expected beam size <1 µm) using a pair of elliptical aspherical total reflection mirrors in Kirkpatrick–Baez orientation. This microprobe was capable of achieving a spot size of 2 × 5 µm² with a photon flux of 10¹⁰ photons s⁻¹. Yamamoto et al.¹⁰¹ used an X-ray fluorescence imaging microscope with a Wolter-type objective mirror giving a magnification of 13 on beam line 39XU at SPring-8. Two dimensional element maps were recorded by comparing two images obtained by irradiating samples with monochromatic photons (ΔE/E about 10⁻⁴, in the energy range 6–10 keV) selected to excite the sample above and below the absorption edge of a particular element. Eba et al.¹⁰² used a special Ge(220) analysing crystal in Rowland geometry (radius of 150 mm) using monochromatic X-rays from an undulator beam line at SPring-8 (Harima, Japan). Because of the excellent energy resolution of this spectrometer, this instrument could be used for chemical state analysis.

Turning now to further applications of SR beamlines, Kempenaers et al.¹⁰³ used SR XRMF to measure the degree of heterogeneity of inorganic trace constituents in selected reference materials. The aim of this work was to estimate the minimum mass of sample for homogenous measurements. Barrea and Bonzi¹⁰⁴ presented measurements of L-fluorescence cross-sections to contribute to an improvement in the accuracy of fundamental parameter correction procedures. Mono-energetic synchrotron radiation at 13 and 14 keV was used as the excitation source to attain a reduction in experimental uncertainties in the measurement of these L line cross-sections. Data showed excellent agreement with established parameters. When SR XRF is combined with capillary electrophoresis, the technique becomes element specific and Mann et al.¹⁰⁵ demonstrated the application of this technique in the determination of complexes of Fe(III), Co(II), Cu(II) and Zn(II). The authors claimed a direct on-line capability and, compared with ICP-MS, no volatilisation of the eluent was required. Reported SR XRF detection limits were 500 pg, with the potential of a 2–3 orders of magnitude improvement after optimisation of collection geometry and incident intensity.

De Jesus et al.¹⁰⁶ proposed Nerium oleander as a bio-indicator for environmental pollution monitoring in Rio de Janeiro, Brazil. They collected, washed and analysed the leaves of the plants and found that elements, such as Fe, Ti, V, Zn, are present at higher concentrations in areas with high traffic flow, whereas Co, Cu, Mn, Ni did not show a significant increase compared with samples from a rural site. Surprisingly, Pb was not found at enhanced levels, but this was expected because lead-free gasoline has been in use in Brazil for many years. Several publications dealt with atmospheric aerosols in Siberia using both SR XRF and INAA with the aim of better understanding the sources of pollution.^107–110 Some of this work involved directly sampling the convection column of taiga forest fires using a selection of instruments, including an impactor mounted on a helicopter. Another investigation of particulate material, specifically agglomerates of 10 nm (nominal) Fe₃O₄ particles embedded in thick epoxy resin at an overall concentration of 0.03% m/m, was undertaken by Thorpe et al.¹¹¹ Using a 2 µm probe step size, results showed that spherical agglomerate particles in the size range from 100 to several thousand nm were present, formed by magnetostatic attraction in agreement with independent TEM measurements. The authors claimed that, by using smaller step scans, the resolution could be improved significantly with the potential of detecting particle masses down to sub-fg levels. Molders et al.¹¹² reported results from a recently commissioned beam line at Louisiana State University offering an 18.8 × 7 µm² X-ray probe mapping facility, which was used to investigate the composition and chemical state of low-Z elements present in particles emitted by automobile engines.

Fluid inclusions in geological samples were measured by SR XRMF analysis,^113–115 with applications including reconstruction of the relative chronology of hydrothermal events, related to mineralisation processes, fluid transport properties, the composition of porphyry ore-forming and crustal brines. Zhan et al.,¹¹⁶ in a Chinese language paper, investigated the effect of sampling depth on characteristic X-ray intensity by SR XRMF, relevant to the analysis of fluid inclusions. Interference problems arising from the overlap from the uranium L lines on Rb, Nb and Mo K-lines were investigated by Phedorin et al.¹¹⁷ This work was part of a geochemical investigation of the oil-producing rocks from the Bazhenov formation, and advantage was taken of the capability of tuning the energy of the SR beam to excite, selectively, individual elements to avoid line overlap problems. A further geological application was the determination of Ba, Ce, La, Nb, Rb, Sr, Y and Zr in lunar basalts from the Apollo 12, 14 and 15 missions by Tarasov et al.¹¹⁸ Phedorin et al.¹¹⁹ undertook a comparison of SR XRF, INAA, ICP-MS, AAS and conventional XRF in the analysis of sediment samples and concluded that synchrotron radiation-excited XRF was the best technique when evaluating a number of analytical parameters. This is not a conclusion with which experts in the other named techniques are likely to agree.

SR XRF techniques were also used in a range of medical applications: Huang et al.¹²⁰ undertook a microprobe study of human bone tumor slices; Chen et al.¹²¹ analysed trace elements in human liver samples after separation by protein electrophoresis; and Pinheiro et al.¹²² used a SR microprobe technique to analyse human teeth prepared as 1 mm thick longitudinal slices. Ektessabi et al.¹²³ mapped the distribution of Fe in a single neuron taken from patients with Parkinson's disease. In this interesting application, the authors found that the Fe content of the melanin pigment granules was an order of magnitude higher than that of control samples. In a second contribution, Ektessabi et al.¹²⁴ mapped the distribution of metal elements (Cr, Fe and Ti) leached into tissue adjacent to a total hip replacement prosthesis. Nicolis et al.¹²⁵ measured the Zn content of single human hair strands from patients under parenteral nutrition to provide information on the progress of the treatment.

Biological research involving SR XRF included the work of Liu et al.,¹²⁶ who measured trace elements in mouse plasma protein taken from animals that had been treated with increasing amounts of cisplatin. Increased concentrations of Cu and S and decreased Zn were detected compared to control samples, implying that cisplatin exposure leads to a marked loss of kidney Cu and a moderate rise in kidney Zn. Selenium in rat brain at a level of 124 ng g⁻¹ was determined by Schulmann-Choron et al.¹²⁷ to provide information on the role of this element in retarding ageing pathologies in humans.

A number of other SR XRF applications were reported in the review period. Techniques for the characterisation and provenancing of artefacts of cultural heritage (including statues in bronze and silver, coins, historic glass and ink) were discussed in a paper by Janssens et al.¹²⁸ These authors used both SR XRMF and laboratory XRMF instrumentation that could be easily transported to the museum or archaeological site. Petrified wood from Poland was investigated by Kuczumow et al.¹²⁹ by SR XRMF and micro XRD. The elements Ca, K and Ti were found to be uniformly distributed in the silica matrix, whereas Fe was concentrated in clear inclusions and As, Cr, Cu and Zn were found in other inclusions present in samples. McHugo et al.¹³⁰ identified the preferred sites for the precipitation of metal impurities in silicon. They found that the poor performance of solar cells correlated with the presence of metal impurities. Finally, Hayakawa et al.¹³¹ analysed trace impurities in synthetic diamonds grown in the presence of metal solvents. The results suggested that Co was preferentially dissolved in the (111) growth sectors and that Co concentrations decreased by one order of magnitude in the same growth sector if Ni was present in the solvent.

6 Total reflection X-ray fluorescence spectrometry (TXRF)

During the current review period, the 8th Conference on TXRF and Related Techniques took place in Vienna, Austria, the proceedings from which are expected to be published as a special issue during the coming year. Unlike the previous conference, which focused on semiconductor applications, this one had an equal balance with trace chemical analysis. An account was published by Streli,¹³² who reviewed the development of TXRF at the Atominstitut, Vienna. The author described the historical progress made from first experiments in the laboratory, showing the detection power of the technique, to the use of modern X-ray sources including SR and special X-ray tubes to excite efficiently low Z elements.

Laser ablation was applied by Spanke et al.¹³³ as a sampling technique for the analysis of solids by TXRF. The sample material, ablated by a pulsed Nd∶YAG laser, was collected directly on a quartz or Plexiglas reflector. The analytical capabilities of the combined method were investigated for metallic samples, minerals and gallstones. It was shown that the combined method could be used for the rapid characterization of solids on the 10 µm scale, without the need for laborious sample preparation steps.

The Brazilian group of Nascimento¹³⁴ published results obtained with an in-house TXRF spectrometer using the conventional arrangement of a double reflector collimator, comprising a combination of collimator and high-energy cut-off. The authors reported a detection limit of 4 ng ml⁻¹ for Zn using a molybdenum target X-ray tube.

The first time use of a wavelength dispersive spectrometer in a TXRF arrangement was reported by Awaji et al.¹³⁵ This was carried out at the SPring-8 Beamline BL16XU, which is the most brilliant source currently available and is, therefore, highly suitable for the exploitation of the WD technique. The reported detection limits for metallic impurities on silicon wafer surfaces were 10⁹ atoms cm⁻² or 100 fg in absolute mass. The results were compared with ED-TXRF, which, as might have been expected, provided a factor of 10 lower detection limits. Nevertheless, the better energy resolution and higher count rates obtained using WD-TXRF are likely to open new fields of application for TXRF spectrometry.

Progress on wafer surface analysis was reported by several groups. A study of Al impurities was published by Baur et al.¹³⁶ from SSRL, Stanford, who investigated resonantly-enhanced X-ray Raman scattering. This scattering was observed when the excitation energy was set just below the Si-K edge and was found to dominate the background around the Al K line. The energy and angular dependence of this Raman background were studied in order to establish optimised measurement conditions. Detection limits of 6 × 10⁹ atoms cm⁻² after optimisation were reported. The group at SSRL further published¹³⁷ detection limits for Ni of 8 × 10⁷ atoms cm⁻² obtained at the beamline dedicated to routine semiconductor industry use, meeting the key industry requirements of cleanliness and stability. Streli et al.¹³⁸ reported the first results on the detection of low Z elements at the Physikalisch-Technische Bundesanstalt (PTB) plane grating monochromator undulator beamline at BESSY2, Berlin. Detection limits for elements Z = 6–12 were found to be in the low picogram range. This was the first recording of such low detection limits for C and N, which were reported to be 0.5 and 0.8 pg, respectively.

During the review period, interest in the application of SR TXRF at less well known SR facilities was reported. Simabuco et al.¹³⁹ used the Brazilian NSLS synchrotron source to analyse rainwater samples for which they obtained detection limits of, for example, 1.3 ng ml⁻¹ for Zn. At the Beijing SR source, the diffusion process of MoO₃ on stable Al₂O₃ and SiO₂ thin films was investigated.¹⁴⁰ In addition, further investigations of trace elements in cells of lung and cervical cancer were performed with SR TXRF.¹⁴¹

During the current review period, several publications dealt with surface reference materials and the correlation between TXRF and other methods. This is perhaps a recognition of the analytical maturity and acceptance of TXRF in this challenging area. A new method for providing reference materials for surface quantification was described by Stevie et al.,¹⁴² who used ion implantation through a removable layer to place the peak of the implant at the interface between layer and substrate. The proposed method was claimed to provide a known concentration at the interface after removing the layer and its effectiveness was established using both SIMS and TXRF spectrometry. Mori et al.¹⁴³ studied a specific sample preparation method for reference materials and its influence on high-accuracy analysis. The authors found that the usual method of immersion in alkaline hydrogen peroxide (IAP) was suitable for producing surface reference materials that could then be used for cross-checking TXRF measurements with other analytical techniques such as AAS. Lazzeri et al.¹⁴⁴ produced reference samples by spin-coating and characterized them by TXRF spectrometry, TOF-SIMS, XPS and SEM. The authors reported that the lateral distribution and the physical status of the contaminants could seriously affect the results. The correlation between TXRF spectrometry and surface SIMS was investigated by Smith et al.¹⁴⁵ Results for Fe were found to be in good agreement for concentrations in the range 8 × 10⁹ to 2 × 10¹⁵ atoms cm⁻² but differences were found for W and Cu when the contamination was not uniform across the surface or extended to a significant depth below the surface.

Istratov et al.¹⁴⁶ reviewed Fe contamination in silicon technology. In addition to presenting the fundamental physical properties of Fe in silicon, the experimental techniques used to measure trace contamination levels were compared. TXRF spectrometry was employed in a study¹⁴⁷ of the removal, using a remote hydrogen plasma, of Fe impurities from the silicon substrate.

Barium diffusion properties in silicon at 800 °C were studied with TXRF, VPD-TXRF and TOF-SIMS by Boubekeur et al.¹⁴⁸ Vereecke et al.¹⁴⁹ investigated silicon nitride films with TXRF after treatment with a vapor phase decomposition–solution collection method.

Urbach and de Bokx¹⁵⁰ presented explicit formulae for the intensities of emitted X-ray fluorescence radiation from multilayer samples under grazing emission conditions. Effects such as multiple reflections and secondary fluorescence were taken into account and the derived expressions were compared with experimental results. Using the same grazing emission technique, silicon oxynitride films were characterised¹⁵¹ and the molecular weight of the conducting polymer was studied.¹⁵² Glancing incidence and glancing take-off XRF spectrometry was used by Tsuji et al.¹⁵³ to study the thermal diffusion of nickel into a gallium arsenide substrate. The diffusion was clearly found by measuring the takeoff angle dependence of characteristic X-ray intensities at a grazing incidence angle of 2.5 mrad. The diffusion profile was found to be 15 nm by fitting calculated curves to experimental plots. In a related study Tsuji et al.¹⁵⁴ compared grazing incidence and grazing exit PIXE. The bremsstrahlung background, induced by secondary electrons, was significantly reduced only in the grazing exit geometry. Another benefit of grazing exit PIXE was that it generated substantially less sample damage than the equivalent grazing exit electron microprobe technique.

The many different fields, including some new areas, to which TXRF was applied during the review period is very impressive. Applications in organic and inorganic chemistry were reported, the first being an investigation of colloidal humic substances by Exner et al.,¹⁵⁵ who combined asymmetric flow field-flow fractionation with TXRF spectrometry. This combined technique was used to determine heavy metals and to study metal ion exchange equilibria, particularly for seepage water and sewage sludge samples. Mixed crystal growth and structure determination of tetrakis(3,4-dimethylphenyl)-imidodiphosphate salts with alkali cation ratios of K⁺/Rb⁺(1∶5) and Rb⁺/Cs⁺ (1∶1) were studied by Bock and Heigel,¹⁵⁶ who analysed the cation ratios by TXRF spectrometry. Greaves et al.¹⁵⁷ determined the metal–ligand stoichiometries for inorganic complexes using TXRF spectrometry. The authors claimed that their new method was faster, more convenient and more precise than existing methods.

In the application of TXRF spectrometry to aerosol analysis, Schmeling et al.¹⁵⁸ characterized filter samples taken by a remote piloted aircraft near Tenerife. Chemical information about the distribution of aerosols in the lower troposphere was obtained by TXRF spectrometry of filter samples. These data enabled the origin of three different air masses to be identified: clean marine air, anthropogenically influenced air from Europe and dust-laden air from the Sahara. Streit et al.¹⁵⁹ characterized the size fractionated aerosol from Jungfraujoch, Switzerland, which was sampled directly on silicone oil-coated quartz reflectors and measured with TXRF spectrometry. Sixteen elements from S to Zr and Pb were determined in order to establish the best cut-off diameter (which was found to be 1 µm) to distinguish between geogenic and anthropogenic particles. Chemical speciation of metals and sulfur in urban dust was performed by Varga et al.^160,161 by sequential leaching followed by analysis using TXRF spectrometry.

Biological and environmental samples were analysed by Messerschmidt et al.¹⁶² after separation and enrichment of Pd and Au. Reductive co-precipitation with Hg of trace and ultra-trace elements followed by complete evaporation of the Hg made it possible to determine Pd and Au with detection limits of 2.5 ng l⁻¹ in urine. Mansoor et al.¹⁶³ found a correlation between plasma total homocystein and Cu in patients with peripheral vascular disease and concluded that the atherogenicity of homocysteine may be related to copper dependent interactions. Greaves et al.¹⁶⁴ used TXRF spectrometry to monitor drug-related Pt levels in serum and urine samples from cancer patients after undergoing chemotherapy with Pt-containing drugs. Samples were prepared by simply spiking with elements absent from these matrices, diluting 1∶1 with water and then irradiating with cut-off filtered or monochromatic X-rays. Sensitivity curves were derived based on the use of Co as an internal standard and by ratioing to the Compton scatter peak. The method was shown to be appropriate to the routine monitoring of Pt in patients. TXRF spectrometry has also been used in the determination of free ¹¹⁵In labels during the preparation of labelled oligopeptides.¹⁶⁵ Sobrado et al.¹⁶⁶ studied the leaf secretion composition of the mangrove species Avicennia germinans in relation to salinity and found TXRF spectrometry to be the preferred analytical technique. Wamwangi et al.¹⁶⁷ determined trace elements in pollen, bee tissue and honey in Kenya using TXRF spectrometry. The geographical distribution of As and other toxic trace elements in groundwater of the Argentine Pampas plain was studied by Vazquez et al.¹⁶⁸ The authors found the levels to be higher than the maximum allowable upper limits for drinking water. Ebert et al.¹⁶⁹ analysed single, inclusion-free crystals of garnet after chemical decomposition and found TXRF spectrometry to be a suitable method for mineralogical and geological micro-samples.

Various industrial applications of TXRF spectrometry were reported during the review period. Trace impurities in graphite and silicon carbide were determined after homogenous liquid–liquid extraction, which was used as a preconcentration technique by Yamaguchi et al.¹⁷⁰ Metal additives and Br in recycled thermoplastics from electronic waste were determined by Fink et al.¹⁷¹ The samples were dissolved in an organic solvent and the corresponding solutions or suspensions subsequently analysed. The authors found this method to be less time-consuming than the conventional digestion of the polymer matrix. Schmidt et al.¹⁷² reported the removal of embedded contamination in chlorinated rubber coatings on concrete surfaces using a high power diode laser. The remaining surfaces were then analysed by a variety of measurement techniques, including optical microscopy and TXRF spectrometry, in order to establish the effectiveness of the removal of the coating.

In the field of cultural heritage, TXRF spectrometry was applied by Vandenabeele et al.¹⁷³ to the analysis of Egyptian masks. The samples were obtained by the gentle microsampling method, originally developed by Klockenkämper, without leaving any visible damage. Micro-Raman spectroscopy and TXRF were performed on the resulting samples, giving complementary organic and elemental information. Casting material sampled from historical violins was analysed by Staat et al.¹⁷⁴ using TXRF spectrometry and IR spectroscopy to obtain elemental composition as well as to determine the inorganic and organic molecules of the binding media, pigments and additives. This interesting combination of spectroscopic techniques showed that Si was present in the form of silicone rubber.

7 Portable and mobile XRF

Portable XRF (PXRF) is one of a select group of analytical instruments that can be used for in situ field measurements and one of a larger group suitable for use in a field laboratory. In the latter context, Hou and Jones¹⁷⁵ presented a review of field instrumentation in atomic spectroscopy with particular interest in environmental monitoring and industrial process control. The main emphasis of this review was on laser-induced breakdown spectroscopy and AAS (using a tungsten coil), but PXRF, immunoassay, capillary electrophoresis and electrochemical sensors were recognised as rival techniques.

Anderson et al.¹⁷⁶ undertook a comparison of techniques used for the analysis of industrial soils, the elements of interest being Cd, Cr, Cu, Mn, Ni, Pb, V and Zn. In addition to ICP-AES and AAS laboratory methods, a comparison was made between direct standardless PXRF and ICP-AES with agreement for many determinations being within 10%. However, a large systematic bias was observed in the V determinations by PXRF, thought to be caused by spectrum interference, and Cd could not be determined satisfactorily by either technique.

One of the well-known difficulties with in situ PXRF is that results are affected by surface roughness effects. When analysing real samples, the effect of surface roughness is usually to introduce an additional air gap between sample and PXRF analyser. In addressing this problem, Gauvin and Lifshin¹⁷⁷ developed a Monte Carlo program that simulated the X-ray spectrum from samples having a rough surface. The shape and intensity of such X-ray spectra were shown to be strongly influenced by changes in the generation and absorption of X-rays as the beam was moved across the sample. Bos et al.¹⁷⁸ developed a new calibration procedure for small samples of irregular shape based on conventional calibration samples and small modifications to existing procedures and software. Typical errors were of the order of 1.4 to 1.5%. A different approach to surface shape correction was applied to ancient ceramics by Leung et al.¹⁷⁹ These workers used Y as an ‘outer marker’ by coating an appropriate solution on a Mylar membrane. Characteristic peaks of Y Kα and Y Kβ were then recorded with the piece of pottery covered with the membrane with correction coefficients calculated for elements of interest from test samples analysed at discrete distances up to about 10 mm from the analyser.

One application where PXRF continues to flourish is in the analysis of museum and archaeological samples, and, in this field, Uda et al.¹⁸⁰ analysed some pigments on the Funerary Stele of Amenemhat (ca. 2000 BC) exhibited in the Egyptian Museum, Cairo, and on the walls of a rock-cut tomb in Thebes, Egypt. Measurements were made with a home-made XRD instrument and a commercial PXRF under ‘touch-free’ conditions. Hunite (a white Ca–Mg carbonate pigment) and an As-bearing yellow pigment were detected. Further work, which in addition made use of laboratory measurements by PIXE, was reported by the same group.¹⁸¹ In another archaeological application, a portable EDXRF instrument incorporating a calcium or lead-anode X-ray tube was used by Cesareo et al.¹⁸² to determine Cl and S in frescoes and stone monuments. Detection limits were reported to be 0.04% m/m Cl and 0.03% m/m S.

There are some exciting possibilities in the use of PXRF to analyse dust and Sterling et al.¹⁸³ described a rapid on-site XRF method for the analysis of dust wipes for clearance and risk assessment purposes. Comparative measurements were made by laboratory AAS with no statistically significant difference found between the two sets of results; the highest correlation being found when determinations by the two techniques were paired between samples that did not contain paint chips.

The investigation of heavy metals in soils and hazardous waste is probably one of the more popular applications of PXRF and the cone penetrometer has featured in previous reviews in this series. Further work in this field was reported by Elam et al.,¹⁸⁴ in which the compact nature of modern detectors and X-ray tubes made it possible to incorporate an analytical device into a cone penetrometer to measure heavy metal contaminants directly in soils at depths of up to 50 m. This device was used to map the contamination from Pb at munitions plants without the requirement for drilling, or sample removal. The results were in agreement with conventional sampling methods. This device was the subject of a patent application.¹⁸⁵ Mobile analysis of hazardous metals in transuranic waste solidified in Portland cement was described by Goldstein¹⁸⁶ in an application promoting EDXRF as a rugged technique for use in a mobile laboratory. Using a conventional analytical procedure, detection limits were reported in the range 2–20 µg g⁻¹ and were adequate to meet the action levels of all the elements of interest, except Be (which could not be detected), Hg and V. Strategies for the investigation of contaminated sites were discussed by Ridings et al.¹⁸⁷ using a commercially available instrument to evaluate sampling, collection and geographic modelling of contaminant elements, including As, at a disused plunge dip. Results were compared with concentrations in samples collected using the US EPA protocol and analysed by ICP-AES and ICP-MS to make recommendations for enhancing the strategy for future investigations.

Other applications do not feature as strongly in this year's review: however, the determination of Pb in paint is a particular forté of PXRF, as further demonstrated by Koyak et al.¹⁸⁸ in a US government sponsored field trial of PXRF Pb measurement technologies. The particular analytical parameters of interest were bias and precision, which were estimated by a linear, heteroscedastic model relating PXRF measurements to laboratory analyses. In contrast, the performance of PXRF in the non-destructive identification of chemical elements in materials was addressed by Fiorini and Longoni¹⁸⁹ using a new version of the silicon drift ED detector. Initial quantitative measurements were made in the analysis of metal alloys. The attractions of portable XRF analysis were demonstrated by Potts et al.¹⁹⁰ in a study of in situ analysis of As contamination at an abandoned arsenic works that is now an industrial heritage site. In addition to the known presence of As in the soil, PXRF was used to highlight a hazard from efflorescent deposits containing As on parts of industrial buildings sheltered from the weather.

Finally, space research continues to be one of the most exotic applications of PXRF and, in this sphere, a special ²⁴⁴Cm source was prepared by Radchenko et al.¹⁹¹ to perform chemical analyses of the Mars atmosphere and surface rocks by alpha backscattering, alpha-proton excitation and X-ray fluorescence spectrometry. Details of the design and fabrication of this source were presented. Muses-C is a mission to a near-Earth asteroid and Okada et al.¹⁹² described the X-ray spectrometer that will be used to measure the composition of the asteroid surface, using solar X-rays as the excitation source. It was also planned that the solar spectrum would be used to measure surface abundance of Al, Ca, Fe, Mg, Si and Ti of Eros as part of the Near Earth Asteroid Rendezvous mission, as described by Starr et al.¹⁹³ To address the controversy about the survival of water or hydrous minerals on the Moon, an XRD/XRF instrument is under development by Sarrazin et al.¹⁹⁴ and could be used in conjunction with a drilling system capable of penetrating a few tens of centimetres into the lunar regolith to search for ice or hydrous minerals.

8 On-line XRF

Despite claiming each year that the number of research publications in this area seriously underestimates the extent of activity in this field, we never seem to be able to stimulate more publications. This year is no exception. However, Haupt et al.¹⁹⁵ reported on the automated monitoring of stack gas emissions by EDXRF, using a continuous sampling system to monitor the emission of hazardous metal particulate material from a smelter site. This followed developments in the on-line monitoring of aerosols reported by the same group from the University of Hamburg last year.¹⁹⁶ In a demanding application, Ujihara et al.¹⁹⁷ used an in situ technique to measure the equilibrium composition of Zn in gallium solution using a sample holder composed of carbon. Results were found to be in agreement with the liquidus curve of the Ga–Zn phase diagram. The development of an on-line EDXRF automatic control system for rare earth extraction processes was also described by Li et al.¹⁹⁸ in conjunction with simulation software to optimise the extraction process.

9 Applications

9.1 Sample preparation

Interest continues in the analysis of liquids by drying microdroplets. Igarashi et al.¹⁹⁹ described work with various combinations of chelating agents, homogeneous liquid–liquid extraction followed by the XRF analysis of microdroplets deposited on filter paper. The methods described were offered for the preparation of samples containing trace metals such as body fluids, environmental contaminants in waters and functional materials. Turner et al.²⁰⁰ preferred to deposit their microdroplets onto thin polymer support film and addressed the problem of waters with differing levels of total dissolved solids by acidifying to 5% v/v with nitric acid.

Other preparation techniques included a fusion method for refractory nickel-based alloy powders described by Vuchkova and Jordanov²⁰¹ using a Li₂B₄O₇ flux with the addition of Li₂CO₃, KNaCO₃ and NaNO₃ in a muffle furnace at 1200 °C. The method was tested for the analysis of Cr, Fe, Ni and Si. Kirk and Bondarowicz²⁰² revisited the preparation of aluminium alloys to overcome Si smearing during abrasion. A new process was reported by Zhou et al.²⁰³ for the separation and determination of sulfur with different valence in a polysulfide mixture. The effect of temperature and pH values on the separation were discussed.

9.2 Preconcentration techniques

The advantages of using solid retention media as a means of achieving preconcentration were studied by Foulkes et al.²⁰⁴ for the determination of trace metals by EDXRF. They acknowledged the often overcomplicated techniques reported by some users and developed new methodologies to reap the benefits of a calibration for light matrices. Nagata et al.²⁰⁵ reported the use of modified silica gel as a separation method for non-essential heavy metals such as Cd, Hg and Pb. Tyutynnik et al.²⁰⁶ developed a sorption XRF method for the determination of Os by means of a wide-range calibration with a detection limit of 10⁻⁶% m/m.

Other techniques included a chemofiltration enrichment method by De Vito et al.²⁰⁷ for the analysis of Eu(III), Gd(III) and Sm(III) based on using o-(3,6-disulfo-2-hydroxy-1-naphthylazo)-benzenearsonic acid (Thorin) retained on a polyamide membrane. Takahashi et al.²⁰⁸ continued their interest in liquid–liquid extraction with diethyldithiocarbamate by the publication of a method for the analysis of 36 metal ions.

9.3 Geological

The bulk of the abstracts received during the year under review concerned the routine application of XRF to support geological investigations. For example, XRF was used by Mizumura et al.²⁰⁹ to study seabed elevation changes via ratio matching, and the excellent precision of the technique was put to use in the provision of geochemical evidence to test alternative plate tectonic models.²¹⁰ The application of XRF to the analysis of geological materials is well established and the technique is often now seen, and indeed used, as a reference technique for determining chemical composition for the purposes of validating a new analytical method.²¹¹ The use of XRF in the trace determination of REEs in geological and environmental samples was also reported in the context of a wider review of this subject.²¹² In an introductory review of the use of isotope-source EDXRF for the elemental analysis of sediment samples, Boyle²¹³ highlighted the ability of that technique to provide rapid screening with little or no sample preparation. The author noted that, in such applications, performance in terms of accuracy and precision could often be related to the appropriateness of the sampling strategy adopted.

A study of the determination of Nb at low concentrations in geological pressed powder samples by WDXRF, following extraction, was published.²¹⁴ A rhodium X-ray tube and a LiF (100) analysing crystal were used in combination with a scintillation counter for the detection of Nb Kα fluorescence. In the first method, standard additions were made to the sample (4 g), which was dried, ground and pelletised with wax (0.4 g). The net intensities observed by XRF using this method were corrected by ratioing to the rhodium Kα Compton scatter net count rate. In the second method, Nb was extracted from the rock and preconcentrated on a strong quaternary ammonium ion-exchange membrane for 48 h, washed with water and analysed directly. Detection limits for the methods were reported as 0.2 µg g⁻¹ and 10 ng, respectively. The second method was considered to be faster than the first with RSDs (n = 6) of 7% or better. Results reported for the analysis of 15 international reference materials were in good agreement with certified values. The pelletisation approach was used in two applications of EDXRF to the analysis of silicates. An ²⁴¹Am excitation source was utilised with a Si(Li) detector for the rapid detection of Ba, Fe, Sr and Zr in powdered rocks, soils and sediments.²¹⁵ Samples (0.25 g) were powdered and mixed with boric acid and pelletised (25 mm × 2 mm). A measurement time of 1000 s was adopted and an analytical precision of better than 10% relative was achieved. A pressed-powder pellet approach was also described by Saini et al.²¹⁶ to achieve the rapid analysis of silicate rocks. Using this method, it was claimed that 15 trace elements could be determined along with 10 major oxide components with acceptable accuracy and precision. Energy dispersive XRF has also been applied to the direct analysis of powdered soils and via pelletisation with cellulose.²¹⁷ Excitation was achieved using molybdenum and rhodium anodes allowing the determination of 10 and 15 metals respectively. The XRF methods were compared with an alternative procedure based on instrumental NAA.

While it is difficult to discern advances in the theory or practice of XRF in many studies, the trend towards the use of XRF in combination with ICP-MS to achieve complementary information on concentration range is perhaps most notable.^218–220 X-ray fluorescence spectrometry is also now frequently used in combination with other techniques that provide structural information. Examples include the utilisation of XRF in conjunction with XRD and EPMA in the study of elemental distribution in the oxide phases of cobalt-rich oceanic crusts,²²¹ with Mossbauer spectroscopy, XRD and XAFS in the characterisation of natural chromite samples²²² and with Mossbauer spectroscopy and TEM in the study of the natural magnetic properties of haematite.²²³

9.4 Environmental

Chemical analysis for environmental applications features extensively in the literature during this review period. Our companion ASU publication by Cave et al.²²⁴ covers all atomic spectroscopy techniques for environmental applications. Landsberger et al.²²⁵ presented an overview of multi-elemental analysis of solid wastes and leachates by neutron activation analysis, XRF and ICP-AES. This same group from the University of Texas²²⁶ also studied Cu and Pb contamination from industrial activities and firing ranges. A German language review by Hahn²²⁷ considered methods used for monitoring Ni in the workplace. Kotas and Stasicka²²⁸ surveyed Cr occurrence in the environment and the challenges for the analyst working at trace- or ultra-trace level. Current methods for Cr speciation were discussed with particular emphasis on sampling, storage, handling and separation.

Readers interested in the analysis of aerosols and air particle pollution will be reassured by a feasibility study reported by Foster,²²⁹ that it is possible to generate batches of nearly identical multi-element aerosol-on-filter reference materials using an ultrasonic nebulizer. Sets of such calibration filters were used to transport a primary calibration for harmful elements in workplace air from a master XRF spectrometer to other X-ray systems with different configurations. A vacuum cleaner was used by Lisiewicz et al.²³⁰ to collect dust samples from 27 domestic and workplace environments in Warsaw, Poland, to investigate toxic and potentially toxic elements by granulometry and EDXRF. Chimidza and Moloi²³¹ identified sources of aerosol particles in three locations in eastern Botswana. Chaaban et al.²³² conducted a preliminary evaluation of selected transport-related pollutants in the urban atmosphere of Beirut, Lebanon. The association between particulate and gas-phase components of urban air pollution and daily mortality in eight Canadian cities was reported by Burnett et al.²³³ Neutron activation analysis and XRF were used by Var et al.²³⁴ to study the concentration, trend and seasonal variation of metals in the atmosphere of 16 Japanese cities. Sources were characterised as crustal elements (Al, Ca, Fe, Mn, Sc and Ti), anthropogenic (As, Cr, Cu, Ni, Pb, V and Zn) and a marine element (Na). Laboratories involved in proficiency testing schemes for air pollution monitoring may be interested in the work of Kucera et al.,²³⁵ who investigated the homogeneity of new IAEA reference air filters and bulk air particulate matter (APM) for up to 20 elements of importance in such studies.

The mass and element concentrations of PM 10 and PM 2.5 airborne particulates in five Chilean cities were reported by Kavouras et al.,²³⁶ with factor analysis applied to identify and quantify their sources. Tolocka et al.²³⁷ investigated the chemical composition of PM 2.5 at 4 sites in the USA. Teflon filters were reported to lose nitrate to a greater extent than the heat-treated, quartz-fibre filters used.

The application of XRF to the study of the pollution of groundwater was reported by Sbarato and Sanchez,²³⁸ who used preconcentration techniques and an internal standard method for the determination of As in first and second-aquifer wells. Kot et al.²³⁹ published work on the determination of various elements in saline mine water. The development of a fibre optic-based, heavy metal ion detection system was seen by Prestel and colleagues²⁴⁰ as having potential in monitoring networks for heavy metals. Gharaibeh et al.²⁴¹ offered a method for the removal of Cd(II), Cu(II), Ni(III) and Pb(II) from aqueous solutions using a solid by-product from Jordanian oil shale refining.

Several authors investigated fly ash during this review period. Oki and Inumaru²⁴² burnt three brands of pulverised coal to form ash samples to test the effect of temperature and pressure on ash release characteristics in a coal gasifier. A combination of XRF to determine total Cu and solid electron paramagnetic (spin) resonance spectroscopy for Cu(I) enabled Weber et al.²⁴³ to study the role of copper(II) chloride in the formation of organic chlorine in fly ash. Agyei et al.²⁴⁴ used XRF and XRD to investigate phosphate ion adsorption from aqueous solution by fly ash and slag. Campbell et al.²⁴⁵ tested the acid generating potential of waste rock and coal ash in New Zealand coal mines to provide mine management with a classification of problematic and stable waste rock types. X-ray and other techniques were also used by Fermo et al.²⁴⁶ and Seong-Ki et al.²⁴⁷ to characterise municipal solid waste incinerator fly ash.

The ability of the XRF technique to satisfy demands for the analysis of more elements at lower concentrations in soils is amply demonstrated in this review period. EDXRF was used by dos Anjos et al.²⁴⁸ for the determination of µg g⁻¹ levels of Br, Ca, Cu, Fe, Ga, K, Mn, Pb, Rb, Sr, Ti, Y, Zr and Zn in soil treated with organic compounds of urban waste. Gunicheva et al.²⁴⁹ showed that the concentration of organic matter in a soil affected the intensity of analytical lines of rock forming elements. Two publications by Hungarian analysts^250,251 determined trace element distributions in eight particle size fractions of soil. The combination of XRF and XRD was used to demonstrate that toxic elements tended to accumulate in the smallest particle size fraction, which could easily be resuspended by wind as dust particles, thereby posing a danger to the health of people. Muntau et al.²⁵² concentrated on the analytical aspects of sampling and sample treatment in the determination of Cd, Cu, Pb and Zn in soil. Alov²⁵³ used a “standardless” semi-quantitative technique to monitor heavy metals in Moscow soils. The spatial variability of element composition was found to be very high and was thought to be one of the reasons for loss of lime, poplar and pine trees in Moscow in recent years. Katahira et al.²⁵⁴ looked at metal element distribution in surface soil and plants in Kyoto City. The XRF results indicated that grasses might be used for remediation of polluted soil. Workers in Cuba^255,256 used EDXRF and AAS in studies on soils and vegetables. Savichev and Sorokin²⁵⁷ showed that absorption correction and standard background methods were more accurate than external standard techniques for the analysis of micronutrients and heavy metals in soils. Agricultural soils from ten countries around the Baltic Sea were surveyed by Reimann et al.²⁵⁸ for 41 elements by WDXRF to relate soil compositions to regional geochemical maps. Solecki and Chibowski²⁵⁹ determined Sr and Y in a study of soil mineralisation conditions. XRF was used by Diegor et al.²⁶⁰ to address concerns about incomplete sample digestion experienced in the analysis of sediment and soil samples by ICP-MS. Paz-Gonzalez et al.²⁶¹ found evidence of Cu and Zn contamination, thought to be due to the traditional use of animal manures as fertilisers in north-west Spain. Blaser et al.²⁶² reported data for As, Cr, Cu, Ni, Pb and Zn in Swiss forest soils. The data suggested that a critical evaluation of trace element enrichment factors was a better tool to assess soil polluted with trace elements than the use of maximum allowable concentrations for topsoil samples.

Pollution of Izmir Bay, Turkey was investigated by measuring trace elements of the surface sediments. Atgin et al.²⁶³ used a combination of XRF, AAS and instrumental neutron activation to show differences between inner, middle and outer bays with higher concentrations found in the inner bay. Bakac²⁶⁴ collected suspended sediment particles from a site located close to industrial and geological areas in the Gediz river, also in Turkey, to analyse 15 elements by EDXRF, gamma spectroscopy and the collector chamber method. Varimax-rotated factor analysis was used to identify industry and radioactivity/mine/agriculture as factors. Vertical gradients in particle distribution and element composition were reported by Kleeberg and Schubert²⁶⁵ under oxic and anoxic conditions in a eutrophic lake in Germany. Solecki and Chibowski²⁶⁶ analysed trace amounts of some heavy metals in bottom sediments of three lakes in south eastern Poland.

Other environmental applications included the use of an APDC precipitation and EDXRF by Alvarez et al.²⁶⁷ for the determination of several metals in rainwater. Hori et al.²⁶⁸ developed an X-ray transmission method to obtain a density profile of ice cores of the Dome Fiji in Antarctica. Racz et al.²⁶⁹ determined Cs and Se in cultivated mushrooms using radionuclide XRF techniques to demonstrate uptake of these elements from the doped culture medium. The influence of Pb on plant growth was demonstrated by Eun et al.,²⁷⁰ who used AAS and XRF to analyse root meristem in maize seedlings. Chemical analysis of dated ombrotrophic peat cores from the Sonnenberger Moor, Harz, Germany, was used by Kemter and Frenzel²⁷¹ to determine the effect of early mining and smelting on tropospheric aerosols. Concentrations of Cu and Pb were shown to follow closely changes in mining activity back to pre-medieval times. S and Fe geochemistries of Holocene coastal peats in NE Germany were studied by Dellwig et al.²⁷² for palaeoenvironmental reconstruction. Residential wood emissions were analysed by McDonald et al.²⁷³ to determine emission rates and develop chemical emission profiles that represented both appliances and woods typically used in wood burning communities.

9.5 Archaeological and forensic

The non-destructive capabilities of XRF are particularly suited to examinations in art and archaeology where the sample is unique or its integrity has significant technical or aesthetic value. This is perhaps best exemplified in the examination of works of art, where the forensic aspects of the measurement can provide historical insight.²⁷⁴ Radionuclide excitation sources are key components that allow the practical operation of portable XRF instruments, and these have increasingly been used in examination of paintings in situ. A portable XRF analyser weighing 1800 g was described, which was based on an ²⁴¹Am source (1.8 MBq), a Si-PIN photodiode Peltier-cooled X-ray detector, an amplifier and a multi-channel pulse height analyser. The system was applied to the characterisation of the blue colorant in an original 1682 Japanese votive picture. The blue colorant was identified as “Smalt” which contains As, Ca, Co, Fe and Ni as major elements. However, it is rare that a single technique is able on its own to provide all the information that might be sought in investigations of this type. Thus, XRF was used in combination with Raman spectroscopy in the identification of mineral pigments in modern art²⁷⁵ and binding media and varnishes used in medieval paintings and manuscripts.²⁷⁶ Similarly, EDXRF was used in conjunction with X-ray microanalysis to characterise archaeological ceramics from the Andes. The combination of the two techniques was used to confirm the extent of ceramic exchange between coastal sites in the first millennium BC in Peru. A comparison of PIXE and XRF for the elemental analysis of Japanese coins was also published.²⁷⁷ It was found that selective filtering was necessary to achieve successful PIXE analysis, and that XRF was the preferred approach. The relative concentration of low-level elements was found to be associated with their place of manufacture. A review of the relative merits of PIXE, XRF and ICP-MS in the analysis of archaeological artefacts²⁷⁸ and of EDXRF and X-ray microanalysis²⁷⁹ may also be of interest in this context.

The relationship between elemental composition of coins and their historical or geographical origin was the subject of a number of studies. These included the examination of Japanese medieval coins,²⁸⁰ ancient Dacian coins from Romania,²⁸¹ Hungarian coins from the 15th century,²⁸² Ayyubid and Mamluk dirhams²⁸³ and silver coins from the time of Alexander the Great.²⁸⁴ In the last case, the effect of the silver corrosion layer on the surface of the coins on the analytical results for 12 elements was studied before and after removal of the corrosion product.

There were similar investigations of the relationships between elemental composition and classification of glass artefacts. These include the application of radioisotope-excited XRF of 8th century Polish, Brandenburg and Saxon glassware²⁸⁵ and EDXRF analysis of Celtic glasses.²⁸⁶

In a study of porcelain by Wu et al.,²⁸⁷ different compositional patterns were found for samples from various Chinese dynasties and for different usage. It was claimed that the method employed produced a highly efficient method of classification. In a related study by Leung et al.,²⁸⁸ using principal components analysis in the examination of EDXRF data derived from 41 pieces of Dehua porcelain, it was shown that most of the samples were distributed in three areas corresponding to the source of production.

9.6 Industrial

There is no doubt that X-ray fluorescence is one of the key components of the industrial chemist's armoury for elemental determinations, due to the inherent flexibility of the technique in the provision of qualitative and/or quantitative data in laboratory, automated, on-line and in situ environments. The focus, however, in this section is the advancement in XRF technology or practice, and for a comprehensive discussion of industrial applications of XRF and other atomic spectrometric techniques, the reader is referred to the companion ASU review on industrial analysis.²⁸⁹

X-ray fluorescence spectrometry has for decades played an important role in metallurgical applications, and developments in this field continue to appear in the primary literature. The non-destructive analysis capability and good precision that can be achieved by XRF make it attractive as a technique for examining precious metals and their alloys. The technique was employed for the analysis of silver and palladium brazier filler metals using a fundamental parameter method.²⁹⁰ Results obtained by this approach for a synthetic sheet sample were in good agreement with those achieved by GD-MS using a relative sensitivity factor approach.

Process and quality control are major issues in the production of steel and ferrous alloys. Examples published in the year under review include the determination of Cr, Mn, Ni, P, S and Si in rolled and cast steel,²⁹¹ process control in the steel casting industry,²⁹² and the quality control of superalloys for the determination of 16 elements by XRF using a variable alphas calibration mode.²⁹³ The determination of major and minor constituents in ferroalloys such as FeSi and FeMn using alpha matrix corrections was also reported.²⁹⁴ Electrolytic separation was combined with XRF in a method for the quantitation of non-metallic aluminium inclusions in steel samples.²⁹⁵ The inclusions were measured using the Al Kβ line and, following data deconvolution to correct for low spectral resolution, LODs of 1.8% m/m for alumina and 0.4% m/m for aluminium nitride in the binary mixture was achieved. Sixteen samples of two low-alloy steels with a total amount of insoluble Al in the range 0.003 to 0.013% m/m were analysed using this procedure. An X-ray method for the direct observation of changes in the expansion and contraction states inside of the coking process were described in a Japanese patent application from Kawasaki Steel Corporation.²⁹⁶ The transmission of soft X-rays through a coal layer in parallel with and at the same temperature as the coke oven was detected continuously using an X-ray tube and detector located outside the oven. The cross-sectional pictures obtained at different operating temperatures gave an indication of the behaviour of the expansion and contraction of the coal-filling layer.

The application of XRF to the determination of the carbon content in coal and ash was described.²⁹⁷ A rhodium anode side window tube was employed to provide C Kα excitation in a WD spectrometer fitted with a multilayer monochromator (2d = 12.2 nm) that was used for dispersion. Samples either untreated or diluted with boric acid were pressed as layers of 0.16 g cm⁻² thickness onto a boric acid support. Relative precisions for the determination of 1.8, 1.5 and 15% were reported for undiluted, diluted coal and ash, respectively. Schnurer et al. at the Atominstitut, Vienna²⁹⁸ reported the generation of femtosecond X-ray fluorescence from light element matrices such as carbon and boron. K-shell vacancies were created in these materials using a few-cycle-driven coherent laser harmonic generation source. It was claimed that this methodology allowed a means of probing the evolution of the microscopic environment of selected atoms in chemical and biological reactions by following the “chemical shift” of peaks associated with inner-shell electronic transitions in time-resolved XRF spectra. The structure and properties of carbonaceous nanotube materials synthesised in an arc discharge were characterised using XRF and electron microscopy.²⁹⁹ X-ray fluorescence spectra were recorded at different angles and these provided evidence of angular dependence in the short wave spectral region that was attributed to anisotropy of the chemical bonding in the carbon nanotubes.

Methods for the analysis of polymeric materials by XRF continue to be published. Examples included the use of radioisotope-excited EDXRF for the estimation of metal content in natural latex,³⁰⁰ a comparison of EDXRF and NAA for the determination of Ba, Cr, Fe and Ti in polycarbonate foils³⁰¹ and a comparison of EDXRF, AAS and ICP-AES for the quantification of heavy metals in technical waste plastics.³⁰² Several papers cited the utility of XRF in application to the study of metal-ion uptake or exchange on or through polymeric membranes.^303–306 The application of XRMF to the non-destructive analysis of polymers and catalysts was also described.³⁰⁷

X-ray fluorescence is widely used in the study of catalysts, and a significant volume of work was published in this field during the year under review. However, much of the work involved relatively routine applications of existing analytical methodology to the study of new catalyst systems. Thus, methods for the analysis of fused iron catalysts for ammonia synthesis,^308,309 alumina-supported potassium fluoride catalysts,³¹⁰ palladium–cerium alumina-supported catalysts³¹¹ and zeolites^312–316 were reported. The determination of zeolite and silicate in detergent products and in raw materials by WDXRF was also discussed.³¹⁷ A study of the effect of cationic polysaccharides on polydimethylsiloxane deposition onto hair via a surfactant-emulsified (shampoo) formulation was carried out using XRF directly as the diagnostic technique.³¹⁸ It was reported that the cationic charge level appeared to control the build up of silicone oil deposition after multiple shampooing treatments.

Other applications of XRF in the analysis of chemicals reported during the review period included a standard method for the determination of sulfur in petroleum products,³¹⁹ the characterisation of timber treatment plant sludge³²⁰ and the detection of catalytic metals in drug substances.³²¹ Energy dispersive XRF was utilised in the non-destructive surface analysis of nuclear fuel element plates.³²² Samples collected by dry rubbing of the entire surface of both sides of the fuel plates were analysed using a ¹⁰⁹Cd radioisotope excitation source to excite the L lines of the U analyte. A Si(Li) detector offering a resolution of 165 eV (at 5.9 keV) was used for measurement and a detection limit of <1 µg of U was reported.

The study of ceramic and refractory materials by XRF remains a topic of research activity. The focus of much activity has centred on developments in calibration strategy. For example, glass-like standard specimens based on bismuth and boron oxides were proposed for the analysis of dopants in dysprosium titanate materials.³²³ Bismuth was used as the internal standard, and calibration equations for the determination of Dy, Nb, Ti and Zr were derived. A fundamental parameter method for the characterisation of refractories containing silicon carbide was described.³²⁴ As a consequence of the conversion of SiC to SiO₂ during the glass beading sample preparation method, the use of a silica–alumina binary calibration was inappropriate. Instead, the gain-on-ignition (GOI) was included in the theoretical matrix correction coefficients calculation as a base component, and satisfactory results were achieved. A new XRF spectrometer with special optical components was described and applied to the analysis of ceramic materials.³²⁵ As part of this work the advantages and limitations of the commercially available UniQuant software package for semi-quantitative applications in this field were explored. A method based on partial least squares regression for spectral evaluation and quantitation was proposed for the determination of the major constituents of cement.³²⁶ Thus, Al₂O₃, CaO, Fe₂O₃, SiO₂ and SO₃ were estimated using a low-resolution benchtop EDXRF system equipped with a low power X-ray tube for excitation and a gas-filled proportional counter for detection. A mean relative error of better than 5% was achieved for all constituents determined.

Practical methods for the analysis of refractory oxide materials by XRF continue to be published. These have included: the use of JRRM reference materials (Technical Association of Refractories, Japan) for the analysis of spinels³²⁷ and refractories containing zirconia;³²⁸ a chemical validation of a method for the XRF determination of B in ceramics and boracic raw materials;³²⁹ and sample preparation régimes for the determination of the composition of alumina–chrome refractories.³³⁰ The advantages of XRF in comparison with ICP-AES for the detection of P in ceramics were illustrated by Marina and Lopez.³³¹ It was reported that XRF offered superior performance in terms of LOD and simpler sample preparation requirements. However, in the determination of sulfate content in cement reported by Brachtel and Rickmann,³³² ICP-AES was preferred to an XRF method because of possible interference from other S oxidation states in the latter approach. A rapid XRF method for the determination of chlorides in building materials was published.³³³ Potassium chloride was added to match the upper limit permitted by Spanish regulations for cements and it was claimed that this allowed a quick test to be applied to other materials such as concrete, mortars and plasters. The performance of an automated laboratory XRF system used in quality assurance and control at the Bosenberg Portland cement works was assessed by Voge.³³⁴ The raw meal, clinker and cement compositions were assessed by XRF and XRD as part of a range of laboratory tests. Full quality assurance was attained for the system including a system availability of 98.6%.

The use of XRF in the study of the structure of refractory materials also received attention during the review period. Synchrotron excited XRF was employed for the characterisation of Cu- and Fe-implanted alumina.³³⁵ The chemical state of the Cu and Fe species were studied as a function of annealing, and it was reported that changes in the observed XRF peak ratios for Fe were due to the formation of Fe₂O₃ on heating. The use of XRF to predict cavity formation in sintered zinc oxide was reported by Watanabe and Narukawa.³³⁶ It was found that the X-ray fluorescence intensity for Al and Si correlated in a linear manner with cavity occurrence, thus allowing a prediction of cavity formation rate before sintering. Samples of graphite fluoride intercalated with benzene were examined by Okutrub et al.³³⁷ using XRF before and after heating to 150 °C in a spectrometer vacuum chamber. The C Kα differential spectra of the samples were used to characterise the electronic state of the C atoms in the benzene molecule inside the graphite fluoride matrix. The differential spectrum was found to be distinct from the spectrum of solid benzene by the presence of additional maxima that indicated the interaction between the benzene and the graphite fluoride matrix. It was noted that the layers of the matrix contained extensive regions of both completely fluorinated and graphite-like domains. Major and minor elements of nanometer powder zirconium oxide–cerium oxide–lanthanum oxide were determined using a powder-pelleting sample preparation approach.³³⁸ The XRF intensities observed were found, as might have been expected, to increase with increasing grinding time or pressure, but no correlation with temperature treatment was noted. It was concluded that agglomeration of the nanometer powder was responsible for this effect.

A tutorial review was published which surveyed a wide range of techniques used for the characterisation of semiconductor materials.³³⁹ Several X-ray analytical techniques were assessed including conventional and TXRF, XRD and X-ray topography, and the strengths and weaknesses of each approach were considered. The dissolution of Cu precipitates in Czochralski-grown silicon was studied by SRXRF.³⁴⁰ Copper was introduced to the silicon matrix, allowed to precipitate and the dissolution monitored after low temperature annealing. It was found that Cu could be released from these sites at temperatures as low as 360 °C and this was considered a potential source of contamination and degradation of integrated circuit devices. A report of the use of soft X-ray emission spectroscopy for the study of the formation of Si nanocrystals in silica and silica/silicon multilayers may also be of interest.³⁴¹ The non-destructive capability of XRF was also used to good effect by Joseph et al.³⁴² in the trace element characterisation of rubies and sapphires. They reported that EDXRF could be used to distinguish between natural and synthetic rubies on the basis of the presence or absence of Fe.

9.7 Clinical and biological

X-ray fluorescence is widely used for elemental determinations in clinical and biological investigations, and a large number of papers were published during the year under review. Key applications included the measurement of toxic and essential elements in, for example, blood, serum, urine, bone and vital organs, usually as part of larger programmes involving metabolic, population, ageing or occupational exposure studies. Only the most important advances in terms of detail in XRF technique are highlighted in this section.

The application of radioisotope-excited EDXRF to the analysis of plants was the subject of a review citing 41 references.³⁴³ The ²⁴¹Am source was considered particularly useful for the determination of Ag, Cd, Dy, Mo and Sn, while ¹⁰⁹Cd and ²³⁸Pu were found more appropriate to particular determinations. Radioisotope-excited EDXRF was also employed in the determination of Cu in pupae and adults of the Colorado potato beetle.^344,345 The contact fungicide Kuprikol 50 is used to control powder mildew and is a source of Cu. The influence of Cu was studied using a ²³⁸Pu excitation source and was considered in the context of control measures against the beetle, and its resistance to pyrethroid insecticides. A biochemical and trace element study of the role of Zn in influencing the toxic effects of organophosphorus insecticides was published.³⁴⁶ X-ray fluorescence spectrometry was used to determine serum and hepatic levels of Cu, Fe, Se and Zn, resulting from a controlled dietary feed, in rats treated with chlorpyrifos. The co-administration of zinc restored serum and liver marker enzymes to within normal limits was reported, and this could provide the basis of a method for mediation of the toxic effects of organophosphates. The effect of lithium augmentation on the trace element profile of diabetic rats was studied using EDXRF.³⁴⁷ The interactions of Br, Cu, Fe, K, Rb and Zn in four groups of lithium-treated rats and a control population were examined in respect of blood serum levels. A significant depression in K and Rb levels and a significant elevation in Br levels was observed in a comparison of lithium diet-treated and untreated diabetic groups.

Lead is widely recognised as a toxic element that is implicated, in chronic cases, in causing brain damage, and there is considerable interest in establishing non-invasive in vivo methods for the measurement of body Pb burden. There was a noticeable increase in activity in research concerned with the accuracy and repeatability of bone-lead measurements in the year under review. Todd and co-workers^348–350 made significant contributions to the understanding of these issues in a series of publications addressing calibration methods for ¹⁰⁹Cd excited K-shell EDXRF measurements. For example, an investigation was made of the factors contributing to the differences in Pb response between the calibration sample matrix and the human bone.³⁵¹ This “coherent conversion factor” (CCF) was considered, a priori, to be a function of scattering angle, excitation energy and the elemental composition of the respective matrices. The CCF was calculated for published compositions of bone, assumed and assessed plaster of Paris reference samples, and synthetic apatite. It was concluded that the synthetic apatite matrix was most representative of bone mineral, and that errors arising from impurities, coherent scatter from non-bone tissues, and the individual subject geometry had only minor or negligible effects on the CCF. The contamination of the calibration sample matrix of plaster of Paris by both Pb and non-Pb contaminants was also assessed in a further study.³⁵² A new anthropomorphic phantom was developed for the purpose of calibrating the response of in-vivo bone-Pb EDXRF measurements. A ¹⁰⁹Cd source was used to excite K-shell radiation over a 30 min measurement period and a detection limit of 20 µg g⁻¹ was reported based on a background measurement. Advantages cited for this approach for in-vivo XRF determinations included the reproduction of the anatomy of the human leg with similar radiological characteristics to those of human tissue, and the yielding of a realistic reference sample for intercomparison studies. According to one paper, the validation of K-shell XRF measurement in the determination of Pb via lead-doped phantoms and bare bone specimens was repeatedly demonstrated, but relatively few studies have compared XRF with chemical measurements of actual intact cadaver limbs containing skin and soft tissue.³⁵³ Eight amputated human legs were examined directly by XRF and, after dissection, the Pb levels in the bare bones were estimated using XRF and ICP-MS. Close agreement between the two techniques was reported for both the tibia and patella. The effect of measurement location on tibia Pb measurement uncertainty was considered.³⁵⁰ Replicate XRF measurements were performed on 10 cadaver legs at multiple locations and on 9 bare tibia dissected from them. It was reported that there was limited evidence for localised regions of low tibia Pb levels. The investigation of variability in the measurement of ¹⁰⁹Cd radioisotope excited K-shell XRF tibia Pb both from cadavers and in-vivo with volunteers was also reported,³⁴⁹ and the repeatability and reproducibility of tibia Pb measurements in a study of occupational exposure in a battery-making workforce was examined.³⁵⁴ In the same context, a paper concerning the accumulation and retroactive exposure assessment of bone lead concentrations in smelter workers was published.³⁵⁵

A new method was proposed, based on Monte Carlo simulation model results for a prototype instrument that combined K- and L-shell XRF measurement for bone Pb determinations.³⁵⁶ The system would comprise a ¹⁰⁹Cd excitation source and germanium and Si(Li) detectors for K and L lines, respectively. The results of the simulation indicated that this instrument would yield a better LOD and provide for the possibility of obtaining information on near-surface bone content in addition to bulk concentration. The use of two commercial digital spectroscopy systems, which were claimed to offer improved analytical performance in bone-Pb determination in comparison with the conventional analogue amplifier ADC system for EDXRF, were described.³⁵⁷ Improvements in measurement precisions of 27 and 11% were reported for the digital systems and LODs were achieved which were between 4 and 10 times better than an optimised conventional instrument. The feasibility of establishing a normalised XRF calibration that would account for inter-patient variability for finger bone Pb content was investigated by experiment and Monte Carlo simulation.³⁵⁸ The Pb X-ray intensities were normalised to the coherent scatter signal. It was found that this method produced a fixed Pb concentration within 5–10% of the mean over a physiologically relevant range of bone sizes and tissue thickness. It was noted that this introduced variation was acceptable, as it was less than the uncertainty typically achieved for in vivo determinations. Monte Carlo simulation was used to develop a theoretical model of a 180° geometry for in vivo XRF for the measurement of Pt concentrations in head and neck tumours.³⁵⁹ The model took into account excitation source, collimator phantoms and detector specifications. The results were compared with experimentally derived XRF data from an existing instrument developed in Swansea. Although the models were in agreement, there were differences between the simulated and experimental spectral shapes obtained. These were attributed to the assumption of a free electron model for Compton interactions that was likely to underestimate results. Details of a related Monte Carlo simulation study concerning the detection of Cd in deep body organs such as the kidney in vivo using ¹³³Xe as the excitation source were also published.³⁶⁰ A Monte Carlo simulation of the critical operational parameters, such as tube voltage, filtration and polarisation, that were of importance in improving detection limits for the in vivo polarised XRF measurement of renal mercury, was conducted.³⁶¹ The purpose of developing the simulation approach was to allow the development of a tool for mild to moderate occupational exposure.

A non-invasive method for the measurement of Pt in human kidneys by in vivopolarised XRF spectrometry was reported by Kadhim et al.³⁶² The X-ray beam from a radiotherapy treatment unit was polarised using a copper–silicon bilayer and this beam was, with the addition of a 0.25 mm tin filter, used to excite Pt X-ray fluorescence from cisplatin chemotherapy drugs used to treat cancer in the kidney. An LOD of 16 µg g⁻¹ Pt was achieved using this instrument for a kidney depth of 3 × 3 cm² and a measurement time of 2000 s. The development of hydrophilic cross-linked co-polymers as tissue equivalent or phantom materials for breast cancer detection by in vivo XRF was also the subject of investigation.³⁶³

High resolution XRF was used in the characterisation of metalloproteins and model compounds.³⁶⁴ A few studies reported the application of XRF to the analysis of hair samples. These included the determination of trace elements in the hair of children from urban environments,³⁶⁵ a screening method for Pb toxicity,³⁶⁶ a study of the relationship of hair Ca content to the incidence of coronary heart disease³⁶⁷ and the use of trace elements to monitor breast cancer patient status.³⁶⁸

An XRF method for the in vivo determination of trace elements in skin was described.³⁶⁹ This system comprised a tungsten anode X-ray tube operating at 15 kV, 23 mA with collimation and a molybdenum beam-hardening filter of 0.2 mm thickness. This yielded a quasi-monoenergetic output of approximately 13 keV and 1.4 keV FWHM. It was reported that the system could be used to detect Cu, Fe and Zn at levels of the order of 5, 10 and 20 µg, respectively, per g of skin tissue using a 2000 s measurement period with skin entrance doses of <16 mSv.

9.8 Thin films

Compositional changes in two-stage processed chalcopyrite thin films have been widely reported during the current review period. Alberts and co-workers^370–372 studied selenisation in metallic precursors. The use of more sensitive Lα lines revealed a segregation of In away from, and Cu towards, the upper part of the layer with increasing selenisation temperature. The same group³⁷³ also worked on the rapid thermal treatment of metallic precursors to demonstrate their influence on structural properties and they³⁷⁴ reported details of the distribution of Ga in Cu(In,Ga)Se₂/CdS/ZnO solar cell devices.

XRF and other techniques have been used in studies on the structure of thin films. Baumann et al.³⁷⁵ combined XRF and Rutherford backscattering spectrometry in a report on high frequency tunable devices. Jo³⁷⁶ grew Bi₄Ti₃O₁₂ on IrO₂/SiO₂/Si substrates and showed that the cation content of the films was dependent on the oxygen mixing ratio during deposition. The structure of a passivating sulfide layer on Ge(001) was studied by Lyman et al.³⁷⁷ using XRF and X-ray standing waves. Kurmaev et al.³⁷⁸ reported the conversion of organic–inorganic polymers into ceramics induced by ion irradiation and Saidov et al.³⁷⁹ worked on epitaxial layers of (Ge²⁻)_(1 − x)(ZnSe)_(x) solid solutions. Carbon nitride films deposited on Si(100) were studied by Izumi et al.³⁸⁰ using XPS, electron probe analysis, FTIR and X-ray absorption and emission spectrometry. SEM and EDXRF techniques enabled Tarntair et al.³⁸¹ to report the preparation and field emission properties of quasi-aligned SiCN nanorods. Kukli et al.³⁸² used XRF, XPS and XRD to study the composition of yttrium oxysulfide thin films in controlled growth experiments by atomic layer deposition.

The principles of XRF for coating thickness testing were reviewed by Wittkopp et al.³⁸³ in a German language paper on the analysis of multi-layer systems. Those able to read French may benefit from studies by Broll et al.³⁸⁴ on thickness measurements of Au films on Si substrates. Parbhoo et al.³⁸⁵ offered a robust method to calibrate an X-ray spectrometer to determine silicone coating weight and thickness. Remmel and Werho³⁸⁶ developed a method to measure both composition and thickness of barium strontium titanate films of 60 nm or less to assist in the manufacture of advanced memory devices.

A new application for EDXRF using a theoretical mathematical model was presented by Szaloki et al.³⁸⁷ for the quantitative evaluation of sorption phenomena at solid–liquid interfaces on a polycrystalline Au substrate of radiolabelled Zn²⁺ ions. Theoretical simulations were performed by Winarski et al.³⁸⁸ during studies of the electronic structure of polyimide films (PMDA-ODA) prepared on Si substrates. The observed C and O Kα X-ray emission spectra were compared with X-ray photoelectron valence band spectra to identify certain spectral features.

Other applications included analysis of powder microsamples of multi-element mono- and poly-crystals by Jurczyk et al.³⁸⁹ Hukka and Zhang³⁹⁰ used EDXRF to monitor Cl levels in amorphous carbon films and Cuevas et al.³⁹¹ measured the GeO₂ radial concentration in silica glass preforms.

9.9 Chemical state analysis and speciation

Soft X-ray fluorescence measurements featured in the literature included a study by Kurmaev et al.³⁹² of a single crystal of organic antiferromagnetic 2,4,6-triphenylverdazyl. C and N Kα X-ray emission lines were measured and compared with X-ray photoelectron valence band spectra and de Mon density-function theory calculations. Butorin et al.³⁹³ presented work on the character of doped holes in La_(1.9)Sr_(0.1)CuO₄ at the oxygen K edge. The same team³⁹⁴ also published their studies on tunable excitation soft X-ray spectroscopy of high-T_c superconductors showing the technique to be a powerful tool for probing the local electronic structure at inequivalent O sites. Duda et al.³⁹⁵ used polarised resonant soft X-ray emission to observe band-like and excitonic states of O in CuGeO₃. A study of the effects of inter-atomic multi-atom resonant photoemission of C and F Kα transitions in LaF₃,Ti_xNb_1 − xC was published by Moewes et al.³⁹⁶ Venturing into ultra-soft spectra, Okotrub et al.³⁹⁷ obtained spectra from ortho- and meta-carborane, C₂B₁₀H₁₂. Ab initio self-consistent field quantum-chemical calculations of these molecules were performed to interpret B Kα and C Kα spectra. A simple theoretical approach was given by Fujikawa and Kawai³⁹⁸ to the study of extended X-ray emission fine structure observed in XRF spectra based on a one-step quantum mechanical formula derived by earlier workers (Almdladh and Hedin).

Kβ/Kα X-ray intensity ratio studies on the valence electronic states of 3d-transition metals in some of their compounds were reported by Pawlowski et al.³⁹⁹ Mukoyama et al.⁴⁰⁰ expanded on earlier work on Kβ/Kα X-ray intensity ratios with calculations based on the discrete-variational Xα molecular orbital method. They found that the calculated results for Cr and V compounds were strongly correlated with the effective number of 3d electrons. Mukoyama⁴⁰¹ also published his work on Kα_1,2 X-ray emission lines of Cr and its compounds with a new method to estimate the line width for a single vacancy in 3d elements by the use of the asymmetry index. Gamblin and Urch⁴⁰² quantified the splitting of the Kβ peak of first row transition metal compounds into Kβ₁ and Kβ_1,3 using a consistent curve fitting procedure. Trends were identified and related to the oxidation state of the metal and its chemical environment. The team from Materials Science and Engineering at Kyoto University⁴⁰³ also published work on high resolution soft X-ray absorption spectroscopy for chemical state analysis of Mn. Yoshio et al.⁴⁰⁴ used high resolution XRF spectral data to support Ni valence results in LiNi_xMn_2−xO₄ cathode material. Yablonskikh et al.⁴⁰⁵ reported on magnetic circular dichroism in XRF of Heusler alloys at threshold excitation after studies on Mn 3d states.

Hallak⁴⁰⁶ measured L shell PIXE spectra to calculate relative intensities of Lγ_2,3/Lα, Lγ_2,3/Lβ₁ and Lβ₁/Lα lines of some rare earth elements by impact of 2 MeV ⁴He²⁺ ions. In a separate publication⁴⁰⁷ he also reported the effect of fluorescence and Coster–Kronig yields on L subshell X-rays of Dy, Er, Gd, Ho, Lu, Sm, Tb, Tm and Yb.

Observations with a conventional WDXRF spectrometer of the radiative Auger X-ray emission spectra for P, S and Si in AlPO₄, Li₂SO₄ and SiO₂ in the energy regions where the KL_2,3L_2,3 RA spectra would be expected were published by Abrahams et al.⁴⁰⁸ This transition due to the ³P state is parity-forbidden in the Auger spectra of light elements, however, the X-ray spectra revealed peaks with kinetic energies of the Auger electrons. Further work by this group⁴⁰⁹ compared ‘real’ and radiative Auger spectra with X-ray absorption spectra to assist in the understanding of this interesting work. Arenholz et al.⁴¹⁰ also studied Auger electron emissions from Fe₂O₃.

Other applications included a systematic EDXRF study by Sawhney et al.⁴¹¹ of the relative X-ray intensity ratios of the K and L series lines from compounds of Fe, Pt and U to examine the chemical state. Batrakov et al.⁴¹² studied the oxidation state of U from the shifts of the Lα₁ and Lβ₁ emission lines. Soni and Poonia⁴¹³ reported their work on satellite spectra from L₃M_x−M_xN_4,5 (x = 1–5) transitions in X-ray emission spectra of Cd, Mo, Pd, Ru, Sn and Te. Cl Kα shifts were used by Dolenko et al.⁴¹⁴ as features to study the electronic structure of TiCl₄. Ozen et al.⁴¹⁵ published work on lanthanum lutetium gallium garnets doped with Cr³⁺ and Nd³⁺ ions. A combination of energy shifts and line broadening was used by Banas et al.⁴¹⁶ to study solid state effects in L–O X-ray transitions induced by O, S and Si. Kβ, Lα and Lβ emissions from Mo were studied by Acharya et al.⁴¹⁷ to see the effect of V₂O₅ incorporation in a MoO₃ matrix.

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