X-ray Spectrometry - Analytical Chemistry (ACS Publications)

Anal. Chem. 2004, 76, 3445-3470

X-ray Spectrometry Imre Szalo´ki

Institute of Experimental Physics, University of Debrecen, Bem te´ r 18/a, H-4026 Debrecen, Hungary Ja´nos Osa´n

KFKI Atomic Energy Research Institute, P.O. Box 49, H-1525 Budapest, Hungary Rene´ E. Van Grieken*

Department of Chemistry, University of Antwerp, B-2610 Antwerp, Belgium Review Contents Overview Detection Instrumentation and X-ray Optics Quantification and Fundamental Data Tomography and Holography TXRF Electron Probe Microanalysis Particle-Induced X-ray Emission X-ray Absorption Spectrometry Applications Sample Preparation ED XRF Micro-XRF TXRF EPMA PIXE XAS Standards Literature Cited

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OVERVIEW The present review outlines the newest developments and improvements in the main fields of X-ray spectrometry (XRS), published in the period 2002-2003: algorithms for spectral evaluation and data processing methods, detectors, excitation devices, optical instruments, quantitative models for X-ray fluorescence (XRF) analysis and electron probe microanalysis (EPMA), holography and XRF tomography methods, absorption spectrometry, and applications in each analytical subfield. We aspired to present a selected citation of the characteristic papers, which represent the current trends in XRS. During our review period, similar publications highlighting the field were presented by Potts et al.; they published two review papers in Journal of Analytical and Atomic Spectrometry, in 2002 (A1) and in 2003 (A2), and cited over 250 articles, representing the most relevant papers, which have had great impact in the main fields of the XRS and its applications. EPMA is one of the most common worldwide spread quantitative X-ray analytical methods; it has become a standard analytical tool for microanalysis and particle analysis in basic and applied research, such as environmental analysis, and its importance increased during the past decade. In a review article, Love (A3) discussed the recent status of EPMA, the expected future 10.1021/ac0400820 CCC: $27.50 Published on Web 05/08/2004

© 2004 American Chemical Society

progress, and some typical latest developments in the related software and hardware elements. The author predicted how this would effectively affect the analytical capability of the technique. The new key instrument in EPMA is the detection device; its efficiency was drastically improved in the past decade due to the new X-ray optical elements such as polycapillary lenses for focusing the secondary X-rays to the detector surface. The paper also highlighted the most advantageous features of the other detection technologies (e.g., CCD cameras) and the data evaluation models. In the low-energy region of X-rays, the most progressive X-ray sources are the plasma-based X-ray lasers. Daido (A4) published a detailed review of all the technical and theoretical aspects of this source device, for wavelengths between 50 and 6 nm, pointed out the importance of applications, gave examples for X-ray holography, soft X-ray interferometers, soft X-ray microscopy, and other applications, and summarized possible future perspectives. This technique can provide highly resolved (submicrometer) size distributions with a picosecond-range time resolution; therefore, it will become a powerful analytical tool for material science. The practical importance of XRS is based on its worldwide applications in different experimental (environmental, geology, biology, etc.) sciences. This practical aspect of X-ray analysis was focused upon by Pukhovski (A5), who outlined the applicability and effectiveness of XRF analysis in the agrochemical services in Russia. The aim of this project was to determine the trace and major elements in plants and forages. The newest developments and trends in CdTe and CdZnTe detectors for Xand γ-ray applications were summarized by Limousin (A6). The author discussed the physical and technical advantages of this type of detectors, their integration and good spectral performance, and their high modularity, overviewed all the detection properties of the CdTe and CdZnTe materials, and made a comparison between them. Because of its excellent detection properties, the CdTe is a suitable material for both X- and γ-ray detection in material sciences, as a monolithic detector, and in pixelized form in medical applications. The article contains many figures and photos demonstrating visually the flexible applicability of this detector material. Finally, the author concluded that CdTe and CdZnTe are strong candidates for future high-resolution imaging and high-efficiency detection of hard X-rays. In the past decade, the greatest hit in detector technology was the appearance of lowAnalytical Chemistry, Vol. 76, No. 12, June 15, 2004 3445

temperature solid-state detectors, such as the superconducting tunnel junction (STJ), calorimeters, and superconductive phase transition thermometers. Angloher (A7) overviewed the most important characteristic properties of these detector types and emphasized that, originally, the low-temperature detectors were developed for achieving a low-energy threshold and high-energy resolution. In practice, the cryogenic detectors have not reached yet the possible theoretical limits of their parameters. The first free-electron laser facility was built in the 1970s and now this type of electromagnetic source is becoming more and more important due to the high level of coherence of the beam and the possibility of generating ultrashort beam pulses. Pellegrini and Sto¨hr (A8) issued an overview of X-ray free electron lasers, their operating principles, measuring properties, and possible applications. The electromagnetic field is emitted by a branch of electrons traveling between undulator magnets and superimposed in their phase, which results in an extremely increased amplitude of the coherent field. Generating extremely short X-ray pulses yields a new horizon of measuring technology based on XRS, namely, the study of individual fundamental interactions between X-ray photons and atoms, molecules, and clusters. The studies of these topics have a great importance in chemistry, material science, biology, and plasma physics. A general overview about synchrotron sources was published in a book (A9) edited by Mills. The book consists of 10 chapters dealing with the operating principle and the fundamentals of synchrotrons, producing modes of X-ray beams for microanalysis, microscopy, imaging, scattering, macromolecular crystallography, and various applications and recent examples, obtained at third-generation synchrotron facilities. The capillary focusing technique becomes an essential element in all type of XRS setups, to form the excitation and secondary X-ray beam, for improving the detection efficiency and decreasing the analyzed sample volume. An extensive review on capillary optics, their applicability, and the recent progress in their fabrication has been published by Bilderback (A10). The paper give an overview of the working principle of different capillary types, their most common forms, special application cases, and the possible future trends and manufacturing technologies. The article helps the average reader to understand the main elements of the theory and the practical aspects of mono- and polycapillaries. The aim of the author was to summarize the state of the art and experiences of the 2nd International Capillary Optics Meeting, which took place at the University of Antwerp in 2001. Currently, the smallest beam size produced by waveguides for coherent X-rays is around 37 nm × 300 nm, which could be the instrumental basis of nanoscale analytical investigations. Finally, in this general highlighting section, we cite a special application of XRS: small-angle scattering (SAS) of X-rays on macromolecules, as detailed in ref A11. The authors outlined the main advantageous property of the method, which lies in its ability to provide structural information on disordered systems, i.e., biological macromolecules. The review considers the fundamentals of SAS: instrumentation, mathematical methods for data analysis, and modeling techniques, shows some application examples for different types of macromolecules, and gives a brief account of the new measuring technologies, such as time-resolved investigation and coherent and single-molecule scattering with synchrotron excitation. Injuk and Van Grieken (A12) published a general, statistical overview of the literature 3446

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on XRS for the period from 1990 to 2000, using three different databases: Chemical Abstracts, Web of Science, and Analytical Abstracts. The observed trends in the number of publications in the different subfields led to the conclusion’s hot topics, which have increasing impact on the XRS literature: micro X-ray analysis, application of synchrotron radiation, and some special research topics such as models for matrix correction, X-ray absorption nearedge structure (XANES), X-ray tube spectra, and X-ray holography. From the early 1970s up to the end of the 1990s, the total annual number of related papers increased from 120 to 5400, demonstrating the worldwide utilization of XRS methods in basic research and different interscience applications. A comprehensive review was published by Suortti and Thomlinson (A13) about the status of medical application of synchrotron radiation and all the modern measuring technological aspects that are available for XRS, such as the following: extended X-ray absorption fine structure (EXAFS) analysis, absorption contrast imaging, phase, refraction and scattering contrast imaging, X-ray microscopy, X-ray microtomography, X-ray microfluorescence analysis, small-angle scattering, and radiation therapy with synchrotron radiation. DETECTION In this review period, the development and improvement of the basic type of cryogenically cooled detectors such as STJ and transition edge sensors (TES) (the latter are also called as microcalorimeters) was continued, in view of their favorable highresolution spectral properties. The thermoelectrically cooled detectors have ∼30 times better energy resolution than can be achieved theoretically by conventional semiconductor-based energydispersive detectors. The fundamental reason is that the excitation states in the cryodetector material are between 10-5 and 10-3 eV, while in the Si-based semiconductor detectors, the typical activation energy is ∼3.8 eV. That means that a photon with 1 keV energy generates ∼260 charge carriers in a Si detector, while, in an STJ detector, this process leads to ∼3 × 105 charge carriers. This results in the significant differences in energy resolutions: the low-temperature superconductive detectors allow a 2-4 eV energy resolution in the X-ray energy range of 0-4 keV, while the best value for Si(Li) detectors is ∼120 eV at 5.9 keV energy. This value is close to the theoretical lower limit. Ho¨hne and colleagues (B1) published an overview of the operating elements and of the main spectroscopic features of the cryodetectors. The operating principle of the STJ detectors is based on the split of the Cooper pairs of electrons generated by the absorbed X-ray radiation in the detector junction. The microcalorimeter consists of a small gold absorber mounted on a superconductive goldiridium sensor that senses the warming up of the gold absorber. The latter type of superconducting detectors provides a better energy resolution than can be achieved by STJ detectors; those are around 10-15 and 2-5 eV at X-ray energy of 6 keV, respectively. The most crucial point in the use of cryodetectors is the cooling, since the normal working temperature of a few millikelvins must be achieved. The authors of ref B1 presented a vibration-free cooling device based on a two-stage pulse system, which achieves a 4 K temperature. To obtain less than 100 mK, an adiabatic demagnetization refrigerator was added that can be mounted in a scanning electron microscope. To demonstrate the spectroscopic capability of their microcalorimeter system, they

show some electron-excited X-ray spectra in the energy range of 0-2.5 keV in which the Si-KR (1.740 keV) and W-MR (1.776 keV) are resolved. The research group of Newbury (B2) published their current results obtained in NIST with a new microcalorimeter, which was developed to detect the characteristic radiation of the light elements in a scanning electron microscope. To increase the detection efficiency, polycapillary optics were installed between the electron-bombarded sample and the microcalorimeter window, which yielded a gain factor in solid angle of ∼300. The best resolution was found to be 2 eV for Al KR1,2 peaks and 4.5 eV for Mn-KR1 peak. The weakest point of the cryogenic detectors is the limited count rate due to the low heat capacity of the detector body. To solve this essential problem, they constructed an electrothermal feedback system to reduce the pulse decay time; this allowed a 800 counts/s rate, that is comparable to the 3000-5000 counts/s value for Si-based energy-dispersive detectors in maximum resolution mode (highest shaping time). The authors emphasized that the natural widening of the X-ray peaks is less than 1 eV and the best achievement is now 2 eV; it is possible to improve this current value technically soon. A German research group published (B3) their results about the experimental characterization and evaluation of an STJ detector (the detector area was 141 × 141 µm2); it was tested with synchrotron radiation in the energy range 180-1600 eV. The authors investigated the response of the STJ and found some artifacts in the detected spectra due to the geometrical construction of the detector substrate and the different connections in which the excited phonons caused peaks. Experimental comparison of the STJ and a Si(Li) detector showed that the great advantage of the STJ is the better efficiency; on the other hand, the artifacts appearing in STJ spectra make difficulties during the evaluations. Veldkamp and colleagues (B4) investigated an STJ detector designed for the soft X-ray energy region from 100 to 900 eV with 12 and 25 eV fwhm energy resolution, respectively. Because the count rate that can be achieved by STJ is more than 1 order of magnitude higher in comparison with TES detectors, the STJ is more suitable for synchrotron applications, due to the high photon flux. The STJ detector consisted of four junctions deposited on the same bulk Si holder, and those were constructed as serial layers of Nb/Al/ Al2O3/Al/Nb in the size range from 70 × 70 to 200 × 200 µm2. The authors determined the degradation of the energy resolution by increasing the detector count rate: it started from 14 eV at 230 counts/s, increased up to 20 eV at 40 kcounts/s and went up to 33 eV at 80 kcounts/s, due to the high pileup level. Day and co workers (B5) reported on a revolutionary new type of broadband superconducting detector, which suits well the requirements of fabrication of pixilated array detectors planned for future X-ray astronomy missions. The basic concept of the operating principle is that the impedance of a superconductive meandered microwave resonator depends strongly on the number of excited Cooper pairs. A microfabricated resonator forms the surface of the detector, and it is coupled to an oscillator through a waveguide. If an X-ray photon impinges into this superconductive detector surface, the resonance frequency and the shape of the resonant curve of the resonator change. Practically, this curve is a function between the oscillator frequency and the loaded power of the resonator from the oscillator. The authors deducted from their experiments that the resolving power is less than 5 eV for 6-keV

X-ray photons. Since the structure and fabrication of this detector is very simple, it seems to be suitable for producing 1000 pixel array detectors in the future. The research group of Hilton published (B6) on the development and construction of arrays of TES for application in submillimeter X-ray astronomy and XRS for material science. The TES was fabricated on the surface of a Si3N4 film, which was mounted on top of a Si wafer. and the Si3N4 film held the Mo/Ci bilayer that formed the material of the TES. To achieve best thermal isolation of the TES material, the Si3N4 film holder was formed on 12 legs that kept the film away from the Si holder. Significantly improving superconductive quantum interference devices (SQUID) is one of the possible ways to increase the detection efficiency of the TES and STJ detectors. The efficiency of the single unit of cryogenic X-ray sensors is limited due to physical reasons; therefore, the multiplication of these chips offers a possible improvement; however, for this the readout procedure must be improved. To solve this basic problem, Lanting and co-workers published (B7) on a frequency-domain SQUID multiplexer for reading out TES array sensors. The readout system of the TES array sensors is operated with an ac bias with individual frequency for each sensor. This solution is necessary because the individual wiring to each sensor could lead to such high heat conductivity that the TES should be out of the superconductive state. The sensors absorb the signal power that results in the change of its resistance, which modulates the signal current. Because each sensor has a characteristic, individual frequency, the frequency distribution of the signal corresponds to the sensors; i.e., electronically the sensors can be identified, and this causes a change in the bias signal. The authors’ readout system that led the TES sensor signal to the room-temperature electronic devices had 32 channels, and the best applied frequency range was found to be from 380 kHz up to 1 MHz. A special biological application of STJ detectors in the energy range of 0.1-3 eV was reviewed by Fraser et al. (B8); it demonstrated the excellent analytical capability of this type of superconductive detector. The STJ had an area of 30 × 30 µm2 with 100-nm-thick Ta layers and 30-nm Al layers, and the Ta/Al multilayer was deposited on a polished SiO2 substrate. The latest development was a 10 × 32 Mo array TES with a cycle cooling system for the temperatures below 100 mK. Friedrich and co-workers also presented (B9) a pixilated multichannel STJ detector for soft X-ray spectroscopy; it has a 3 × 3 matrix sensor with 200 × 200 µm2 Nb-Al-Al2O3-Al-Nb layers and produces a detector with 15-eV resolution below 1 keV. The maximum count rate was 100 000 counts/s at the 100 mK operating temperature that was obtained by a two-stage adiabatic demagnetization refrigerator. They used their STJ for L-edge absorption spectroscopy of V, Mn, and Ni, involved in metalloenzymes in solutions with concentrations of a few hundred ppm. The signal-to-noise ratio was ∼25; this did not allow observation of the fine structure of vanadium L2 and L3 edges. However, it was suitable to identify the energy position of the edge in the order of 0.5 eV. The silicon drift detectors (SDD) with an integrated field effect transistor extend the detected count rate up to several hundred thousand cps, while keeping the energy resolution below 300 eV in the energy range of 1-30 keV. An international research group (B10) utilized these advantageous spectroscopic properties of SDD in a time-resolved X-ray absorption fine structure (XAFS) experiment in Hasylab. They used a seven-element SDD for in Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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situ and ex situ analysis of catalysts containing low concentrations of metals. The only essential requirement for this experiment was the high detection capacity of the SDD, which allowed performing the XAFS energy scan within 10 min. During this observation period, they were able to investigate the reduction and oxidation process of metals by quantitative evaluation of the white line of the absorption spectra. To design energy-dispersive Si-based detectors with high operating speed and position-sensitive resolution, modeling of charge transport processes taking place in the detector material is needed theoretically and numerically. To solve this principal problem, Fowler et al. (B11) studied the pixilated Si detectors and used a Monte Carlo simulation code to describe the charge transport: (i) how much the charges leak to the neighboring pixels, (ii) how long it takes for the charges to reach the surface, (iii) which portion of the charge is lost, and (iv) what kind of design provides the optimal operating performance. A similar method was chosen by a Swedish research group (B12) for studying the charge transport and X-ray photon absorption in a three-dimensional (3D) X-ray pixel detector. The laser-based microfabrication technique allows constructing 3D X-ray detectors that are faster and more sensitive compared to the conventional one- or two-dimensional drift detectors, due to the short distance between the collecting electrodes. Based on their simulation results and modeling of the charge spread in the detector material, they suggested that a bias voltage of ∼1 V fully depletes the device. The simulated detector layout was arranged in hexagonal structure where n- and p-type semiconductor rods were implanted parallel to each other. That construction was designed for dental application in order to reduce the dose rate during illumination, and the authors found a lower charge sharing than for conventional planar (2D) drift detectors. Finally in this subtopic, we mention the work of Mathieson et al. (B13) about the charge sharing in a pixilated Si detector; they used a simulation code to model the basic interaction in the Si crystal between X-ray generated electron-hole pairs and the Si matrix and the charge transport in the X-ray energy range of 13-36 keV. In the experiments, they used a pixilated 16 × 16 matrix Si detector with 300-µm pixel size, mounted on Si wafer. Comparison of the calculation and measurements allowed the deduction that the dominant physical effect in the charge sharing is the diffusion of the charge carriers; therefore, this unwanted effect can be reduced by increasing the operation voltage. The authors pointed out that their simulation model and its representative computer code can be extended to investigate other type of detectors. The accurate evaluation of particle-induced X-ray emission (PIXE) spectra, especially to determine the fluorescence yields of the elements, necessitates exact knowledge of the response function of the energy-dispersive Si detector. To solve this problem, Puc and colleagues (B14) developed a new X-ray spectrum fitting procedure, using Gaussian and Voigt functions to describe the characteristic lines itself and additional exponential tail and step functions for the low-energy side of the characteristic lines to simulate the incomplete charge collection in the detector crystal. For precise fundamental parameter calculations, exact knowledge of the inner structure and sizes of the used Si(Li) detector is required; otherwise, large errors are introduced in the final analytical results. For example, to determine indirectly the detector inner geometry and the distance between the Si(Li) crystal and the detector 3448

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window, Hopman and Campbell (B15) proposed a new procedure, using a 55Fe radioactive point source. They scanned the X-ray source in front of a Si(Li) detector and evaluated all the energydispersive X-ray spectra measured in the individual geometrical positions of a 2D raster. On the basis of their calculations and measurements, the error generated by the incorrect knowledge of the solid angle for detection should be ∼20% relative. They performed also both area and profile scans by moving a tantalum screen with a circular aperture in the x-y direction, driven by a stepping motor system. The authors presented a complete mathematical proof for the geometrical setup and its contribution to the detected X-ray intensities. Quantitative applications require the accurate energy-dependent description of the full-peak efficiency of the applied energy-dispersive detector. The research group of Uzonyi (B16) performed an energy calibration of an ultrathinwindow Si(Li) detector for the energy range from 0.28 to 22.1 keV, similarly to what was published in ref B15 for EPMA. The result was necessary for PIXE analysis at a scanning nuclear microprobe facility. The method for the determination of the optimal values of the detector parameters, such as the thickness of ice on the Si crystal and dead layer, the grid ratio on the detector window, etc., was based on the comparison of measured and calculated characteristic intensities. The parameters were achieved by minimization of these intensity differences for C, N, O, Al, Si, Cl, Na, P, K, Ti, Fe, Cu, Mo, and Ag; these elements were in a pure form or as chemical compounds. The error of the efficiency function was estimated as 10-20 and 5-10%, for the energy ranges 0.28-1 and 1-22.1 keV, respectively. Combination of capillary plates for guiding the X-rays and microstrip gas chambers seems to be an excellent solution for X-ray imaging and X-ray diffraction experiments; that has been published by Ochi and co-workers in ref B17. The effective area of the detector was 95 × 95 mm2 and the spatial resolution was better than 100 µm, while the maximum count rate was found to be above 105 counts/s/mm2 and the time resolution less than 1 µs. Using this highly performing 2D gaseous detector combined with fast data readout electronics, the authors developed the so-called continuous rotation photograph (CRP) method that yields within a few tens of minutes a diffraction data set that allows determination of the 3D structural model of the analyzed crystal. Bateman and colleagues (B18) developed a new gaseous imaging pixilated detector, which uses connector pins as anode. Readout rates for 2D detectors above 1 MHz required very complex and extensive electronics; therefore, the main advantage of the pixilated form is that the data reading can be carried out in parallel. In this case, the total data reading rate should reach the few hundred gigahertz. The current status of the most important top gaseous detectors based on microstrip gas chamber has been reviewed by Sauli (B19). The gaseous detectors are advantageous since they have a high-rate capability and can be obtained with a large area; their position sensitivity is good, and they can be fabricated easily with photolithographic technology. However, the main disadvantageous spectral property is in their low-medium density that strongly limits the field of applications. A special construction of gaseous detectors was reviewed by Charpak (B20), namely, the so-called Micromegas; their operating principle is based on the amplification of an electron avalanche in a short (100 µm) electrode gap at atmospheric pressure. The working param-

eters are excellent since the energy resolution can be characterized by 5.4% of fwhm at energy of 22.1 keV; the position resolution was found to be 12 µm. Since the construction of this detector type is very simple, large-area (40 cm × 40 cm) detectors for 2D X-ray or γ-ray imaging can be manufactured easily and this is suitable for radiology purposes. Orthen et al. (B21) published also on a gaseous 2D detector (MicroCAT) for X-ray imaging; it has a microspacer to keep a constant distance (∼200 µm) between the gas gain and the readout structure. The detector consists of a mosaic structure of single cells with four pieces of chargecollecting electrodes, and the size of the largest working model is about 56 × 56 mm2. A systematic investigation was performed on microstrip gas counters (MSGC) by Bateman and colleagues (B22) to disclose the dependence of the energy resolution on the bias potential, gas filling, and construction of the strip geometry. The work was motivated by the excellent spectroscopic features of the MSGC that implies high-energy resolution combined with a very high data capture rate and a millimeter-range spatial resolution. In their test equipment, the electrode pattern was manufactured by lithography with a size of 50 × 50 mm2, in which the anode strips had 10-µm width and the size for the cathode strips was 90 µm on a repeat pitch of 300 µm. The authors studied experimentally the fwhm for different detector parameters and X-ray energies, e.g., gas mixture, drift potential, and gas gain that allowed determining the optimized operating parameters. The charge-coupled device-based area X-ray detector (CCD) becomes more and more important in two-dimensional XRS when high spatial resolution is necessary. An international research group gave (B23) a detailed description about all the aspects of CCD detectors in XRS, citing over 200 relevant papers. The authors guide the readers through the operating principle of different CCD cameras and through the properties of the main signal components: phosphorus plates, fiber optics and lenses, image intensifiers, and the position-sensitive matrixes that convert the X-ray energy to light or electron-hole pairs and accumulate and record the image. The authors overview all the possible solutions for these light or X-ray optical elements and compare their general and special features, advantages and disadvantages, and applicability and pay special attention to the data readout algorithm from the pixilated semiconductor matrix detector. They take into account all types of realized detectors, their calibration modes, and characteristics, like spatial resolution, intensity corrections, energy and angular corrections, geometrical distortions, background subtraction, and quantum efficiency. The most promising development is directed toward arrays that consist of direct radiation-detecting pixels, each with its own processing electronics, especially the two-layer architecture in which the X-ray absorber semiconductor layer is connected by metal layers, pixel by pixel, for converting the generated charges to the second layer. A new CCD detector system was designed by Phillips et al. (B24) for X-ray imaging measurements with high spatial resolution and sensitivity. A GdO2S2 phosphor screen converts the X-ray signals into an optical image that is coupled to the CCD sensor having an area of 4.9 cm × 8.6 cm with 4000 × 7000 × 12 µm2 pixels. In this device, a single 12-keV X-ray photon generates ∼100 electron charges, while the total noise per pixel is ∼30 e- and the quantum efficiency is higher than 0.6 at one X-ray photon per pixel. The readout procedure of the full image takes less than 4 s, and the

best spatial resolution was 50 µm. The authors tested the detector system and studied the dynamic range (the ratio of the largest signal that can be measured to the single X-ray photon-generated signal); it was found to be 6400 for a 12 keV X-ray energy during a 10-s exposition time. The best utilization of a CCD camera is to use it for imaging purpose in an X-ray microscope as was emphasized by Ohigashi et al. (B25). In their application, the CCD camera was used for an X-ray full-field fluorescence microscope with which a two-dimensional elemental analysis could be performed without changing the energy of the excitation X-rays and using the K-edge subtraction method. To increase the detector illumination, a Wolter-type mirror was set between the irradiated sample and the CCD detector. The authors tested the CCD-based microscope by the analysis of diamond inclusions. The scintillatorbased detectors are important in X-ray and γ-ray imaging since the scintillator materials convert the X-rays to light signals. Moses (B26) outlined the recent status and development of inorganic scintillator materials and reviewed the desired spectroscopic properties, such as high density and atomic number for appropriate attenuation of X-rays, high-level light output, short decay time, mechanical ruggedness, etc. The author reviewed the newest most promising materials, such as LaBr3:Ce, LaCl3:Ce, and RbGd2Br7:Ce, that possess a high luminous efficiency with an appropriate energy resolution and short decay time (∼50 ns). Due to their wide application fields, energy-dispersive detectors operating at room temperatures are in the focus of current research. Owens et al. (B27) published their newest results about the response function of a HgI2 detector having a 7-mm2 surface area and a 0.5-mm thickness. The measurements were carried out both in laboratory conditions and at synchrotrons using a 50 × 50 µm2 beam size and 15-keV monoenergetic X-rays. The fwhm energy resolution was found to be 600 eV at 5.9 keV; that value increases up to 6 keV at 100 keV. The operating temperature is significant since the fwhm energy resolution depends strongly on the detector temperature. The basic problem of this type of detector is the improper hole collection causing significant tailing in at the low-energy part of the characteristic X-ray peaks; this effect increases with the thickness of the detector material. Due to the high atomic number of Hg, the thickness (i.e., high absorption of HgI2) can be kept at low values of ∼0.5 mm. Portable spectrometers are the typical application area of the PIN detectors cooled by Peltier effect. Ferrero and co-workers (B28) published a typical in-field application with a PIN detector for paint analysis. The portable system was built with a miniaturized X-ray generator and equipped with a PIN diode having a 7-mm2 effective area, 300-µm detector thickness, 25-µm beryllium window, and an energy resolution of ∼200 eV at 5.9 keV X-ray energy. The author demonstrated the feasibility and effective capability of the SiPIN detectors in archaeology, in the restoration and conservation processes. CdZnTe detectors can be effectively utilized for hard X-ray and γ-ray spectroscopy due to the high atomic number and density of this detector material; therefore, one of the promising areas is medical radiography, as published in ref B29. The authors used this detector for mammography purposes and investigated the spectroscopic response using a 242Am radioisotope. They pointed out that main spectral distortion effects, such as transmission of the primary X-rays through the detector crystal, escape of the secondary X-rays, and carrier trapping, are not significant in Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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the energy range of 10-40 keV, when using a 2-mm-thick CdZnTe detector. Van Eijk (B30) published a general overview of one of the important areas of using X-ray imaging devices, namely, medical investigations. The author lists all the applications of the inorganic scintillation materials in this field and the main imaging techniques that utilize X- and γ-rays. In medical investigations, the ultimate goal is to project the inner structure of the human body to a two-dimensional position-sensitive imaging detector. The typical flat-panel detector consists of amorphous Si diodes, mounted on a glass plate, and on top of it a CsI:Tl scintillator plate is deposited. Still, the paper listed more than 20 different scintillator materials used as imager. The sizes of such plates are between 30 and 60 cm. The article describes the image intensifiers and one-dimensional array detectors used for tomography investigations. Finally, the author gave an overview of the physical principle of different scintillation materials and the influence of the compositions on the operating parameters. INSTRUMENTATION AND X-RAY OPTICS Research group of Pfeifer (C1) published an outstanding paper about a new coherent X-ray point source in the nanometer range, based on the use of resonant coupling of synchrotron beams into a nanostructure; this setup is capable of generating coherent X-rays. A two-dimensional nanostructure was fabricated by electron beam lithography, and it was tested by a parallel undulator beam achieving a resonant mode for X-ray propagation in a 2D X-ray waveguide. The nanodevice acts as a coherence filter, since the exciting beam is fully coherent. The authors emphasized that the coupling and prefocusing devices must have improved performances. and they predicted that this type of nanosize X-ray source will become more important in nanoscale analysis because guided waves can be used to create hard coherent X-rays with extremely small spot size (several tens of nanometers in diameter). For understanding the chemical properties and structure of various materials that contain low-Z elements such as C, N, and O, one of the most effective analytical tools is the fluorescence yield X-ray absorption spectroscopy. The method uses monochromatic and tunable soft X-rays to excite the sample. Fisher et al. (C2) presented a special optical layout that contained spherical multilayer reflection mirrors for enhancement of the carbon K edge signal detected by a proportional counter. The mirror had a spherical optic of 100-mm diameter and a radius of 70.6 mm. The X-rays were focused to a 1-mm spot on the sample after passing through a 19-mm hole in the mirror, which collected ∼16% of the solid angle from the sample. The mirror material was constructed of vanadium and carbon layers; each pair of layers had a thickness of 2.25 nm. A new focusing X-ray element was designed by Price and colleagues (C3), based on Wolter-type microchannel plate (MCP) grazing incidence X-ray optics. The great advantageous of this type of X-ray optical element is the large effective area and the low mass, and these properties offer an ideal application area for planetary X-ray imaging. The MCP is formed by a square cross sectional glass core surrounded by a lead glass cladding. The authors modeled their MCP prototype by a Monte Carlo simulation code, and they experimentally found the X-ray imaging with optimal intensity. The applicability of tabletop laboratory X-ray spectrometers equipped with an X-ray tube source was always limited due to the low excitation and detection efficiency origi3450

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nating from the geometrical setup. This problem can be partially overcome by using a focusing system for both primary and secondary X-rays before and after the sample excitation. The optimized shape of highly oriented pyrolytic graphite (HOPG) and a new method for its production were described by Grigorieva and Antonov (C4). Because of the high reflectivity value, the pyrolytic graphite offers a very efficient focusing and monochromatizing device to collect primary and secondary X-rays in XRF analysis. The authors reviewed a new fabrication method that overcame the limitation in focusing parameter of bent HOPG crystals. They tested experimentally the new HOPG for both primary and secondary X-ray beams and found an intensity gain between 2 and 20, and the focal spot diameter was 0.8-4 mm depending on the shape and dimension of the HOPG. The researchers expect intensive application of this new X-ray optical element in medical applications, since the bent focusing element gives a chance of decreasing the X-ray intensity, which results in lower absorbed dose in living tissues. Kolmogorov and Trounova (C5) also presented a new double torodial concave focusing system based on a HOPG X-ray lens and monochromator. The monocrystal pyrolytic graphite has a larger reflection parameter value, by 1 order of magnitude, than other types of crystals, providing a higher X-ray flux for detection. The authors measured the X-ray flux density distribution in the focal point, and the fwhm was found to be ∼1 mm. For testing they used a commercial tabletop X-ray generator, Si(Li) detector, and Ag anode tube at 50 kV high voltages and 30 mA current for measurements of different mineral and rock samples. The LLD for the spectrometer setup having the above parameters was found to be ∼0.1 ppm for Rb, Sr, Y, Zr, and Nb. The parabolic refractive lenses are the most spectacular tools for hard X-ray imaging in X-ray microscopy; they are applied in many fields of science and allow producing magnified X-ray imaging with 100-nm resolution. The international research group of Schroer (C6) reviewed their experiences on Be and Al refractive lenses, the achievable microscopical properties, and application possibilities. Two attractive fields for utilization of the spectroscopic advantages of these X-ray optical elements were demonstrated: (i) X-ray microscopy for magnified tomography, and (ii) hard X-ray lithography. The tomography experiment was carried out with 10.6 × magnification, with E ) 20.65 keV X-ray energy, while the lithography capability of the X-ray microscope was tested by comparison with a SEM image of a gold lithographic test mask deposited on silicon wafer. The newest X-ray optical element is the refractive lens for focusing hard X-rays, to be applied mainly at synchrotron beamlines. Lengeler and co-workers (C7) published their newest results on self-constructed parabolic refractive lenses made from Al and Be for submicrometer focusing, for X-ray absorption, and for phase-contrast imaging. In the article, they reviewed the general features of the refractive lenses and practical properties and showed some experimental examples of X-ray micrographs of a Ni mesh using Al and Be lenses (N ) 120) at 25 keV X-ray energy. The authors expected that the newest Be lenses will be able to achieve a lateral resolution below 80 nm. A novel solution for combined X-ray optics (C8), is the achromatic Fresnel zone plate, which consists of a refractive lens and a Fresnel zone plate. This newly designed X-ray optics provides a resolution comparable to the simple Fresnel zone plate (that achieved an optimal resolution of ∼25 nm), but a significant improvement of

2 orders of magnitude in spectral bandwidth was achieved. The importance of this result is that the resolution improvement of X-ray optical elements is strongly limited by different optical aberrations (chromatic and achromatic) of lenses and zone plates, which allow using this combined optics for X-ray lithography purposes. A new X-ray microscopy method, developed by Larson and colleagues (C9), is based on differential aperture X-ray microscopy (DAXM) to disclose the structure of materials with submicrometer spatial resolution in three dimensions, using a polychromatic X-ray beam. The method requires a CCD camera to accumulate the depth-resolved diffraction pattern intensity and position as a 2D image. The measuring algorithm is as follows: (i) the CCD camera collects a diffraction image while a Pt wire with 50-µm diameter is in a certain position between the camera and the sample, (ii) the wire moves to an other position and a new data collection is started, etc. The differences between the collected 2D images allow determining the 3D structure of the investigated microobject associated with the Laue diffraction pattern with submicrometer lateral resolution. The paper showed an example for the 3D structural X-ray microscopy measurements: depth dependence of elastic strain tensors in a 25-mmthick [001] oriented Si plate that was bent to a radius R ) 3.3 mm. The authors predicted that their novel technology (DAXM) enhances the possible field of applications, and it can be extended for the purposes of geology, environmental, biological, and structural mechanics, and general research of the physics of materials. The only limitation of the method is that the penetration depth will be restricted in case of high-Z elements in the investigated material and the spatial resolution will be reduced for low scattering angles. The keywords for the future of the XRS research are short, coherent, brilliant, and tuned pulses of X-ray beams in a wide energy range, with as small as possible beam sizes. These parameters are essentially necessary to discover the structure of materials (with X-ray holography, tomography, and diffraction methods), through atomic-scale maps, for example: macromolecules such as proteins, superconductive materials, etc. Service (C10) gave an overview on these open questions of modern XRS and the possible future solutions for achieving the above technical aims. One of the possible fourth-generation X-ray sources should be the short-pulse photon source, where a laser can produce shorter X-ray pulses (100-200 fs) in an alternating magnetic field than can be arranged in conventional synchrotrons. The other top idea for this aim is the recirculation Linac source in which the electron branches are led back to a successive spiral and are used repeatedly. In comparison to the conventional synchrotron storage rings, the possible advantages of this hybrid solution is that the length of electron branches can be manipulated without limitation, allowing one to achieve shorter X-ray pulses down to 100 fs and even shorter. The research group of Bjeoumikhov summarized in their article (C11) the physical and spectroscopic properties of a new generation of polycapillary lenses and gave an overview of the manufacturing technologies and the most essential application areas. The ultimate motivation for the development of X-ray capillary lenses is to achieve a higher spectral efficiency and smaller spot size for the output X-ray beam. Now, the best performance for polycapillary lenses for higher photon energies is ∼19 keV, while the best peak intensity gain is ∼3000 and the focal spot size is ∼20-30 µm. The authors showed

that, for effective focusing for a broad energy band, a new combined geometrical structure is necessary, using different channel sizes for the individual capillaries. This combined structure provides significantly better transmission efficiency than what can be achieved by a conventional polycapillary lens. In a special issue of X-Ray Spectrometry, selected papers have been published dealing with the current status of capillary optics. Vincze and Riekel (C12) gave a general overview about the status and the top results of the capillary techniques used for X-ray beam focusing in the European Synchrotron Reasearch Facility (ESRF) in Grenoble, France. They showed that a minimum lowest diameter of ∼100 nm for capillary optic is achievable; however, the fabrication technique for reproducibility is not stable yet. The extremely low value of output spot size of capillaries reduces the other sizes of the capillary optics; i.e., the optimal length should be 2 mm while the original beam spot should be from 0.7 to 0.1 µm. The glass capillaries suffer from some disadvantages when used in synchrotron experiments: (i) the extremely high photon flux after the insertion devices induces 10 kW‚cm-2 and this causes disastrous thermal damage in the glass material, resulting changes in refractive properties; (ii) the high flux changes as well as the geometrical size of the capillary tube that results in a change of optical parameters. To avoid these structural problems, Hirsch (C13) developed a metal capillary optics using two different new fabrication methods based on a special high-pressure technology. One of the methods was performed by applying two pressing plates between which a wire was placed; after the compression procedure, the plates were disassembled, the wire was simply lifted out, and finally the two plates were aligned together to form the capillary. The X-ray optical properties were tested with synchrotron radiation and a CCD camera for recording the output X-ray beam distribution: the gains of the capillary were found to be between 90 and 100 in a 10-µm beam size at photon energies of 12 keV. The author predicted that this type of capillary should find widespread use at both synchrotron facilities and simple tabletop X-ray machines. In the last 5 years, polycapillary optics for XRS has been developed significantly, due to the wide variety of application areas: large-area collimated beams, small focused beam for protein crystallography, medical applications, etc. The reproducibility and quality of the optics has been markedly improved, as was reviewed by MacDonald and Gibson (C14). They outlined the main optical features, operating principles, and lens quality analysis techniques and finally showed some typical application examples. The state of the art of capillary technology has been outlined and an overview given in a paper of Gao and Ponomarev (C15). The greatest advantage of this optical element is that it offers a large solid angle for collecting X-rays and thus allows the same analytical efficiency as can be reached by using more powerful X-ray sources; therefore, the application of capillaries allows designing and constructing compact portable X-ray spectrometers for field and in situ analysis. The article discussed the recent technology of capillary production, performance, and general optical properties and gave some spectroscopic examples about the typical setup and possible future developments. The authors predicted some possible improvement of the capillary parameters, e.g., a higher total number of channels in polycapillaries, up to 107, and a lower spot size than 10 µm, in a wide energy range of 1-25 keV. Freund reviewed (C16) the state of the art of Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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the top X-ray devices applied in ESRF for a great variety of experimental requirements, providing a small spot size, polarization, an extremely intense X-ray flux, etc. For highly focused X-ray beams, different X-ray optical elements are used such as Fresnel zone plates, refractive lenses, capillaries, and bent surface systems such as Kirkpatric-Baez systems. The review article gives an overview of the recent instrumental developments in the field. A Ukrainian research group (C17) proposed a new solution for a compact X-ray source based on Compton scattering; this idea is based on the interaction between an electron beam produced by a storage ring and an intensive laser pulse accumulated in an optical resonator. They estimated that this newly proposed X-ray facility generates an X-ray beam of intensity ∼2.6 × 1014 s-1 and spectral brightness of ∼1012 phot/0.1% bw s-1 mm-2mrad-2 in the energy range from 10 keV to 0.5 MeV. The article is an example of how to utilize an old electron storage ring with a new technological idea for improved spectroscopic parameters. The extended use of intense third-generation synchrotron sources needs the focusing of a hard X-ray beam for which one of the best solutions is the refractive lens. The first refractive lens was fabricated in such a way that a series of simple holes were drilled in aluminum, but today planar microfabrication technology is used for producing microlenses with low absorption of hard X-rays. An international research group (C18) developed a new diamondbased refractive lens technology using a 200-µm-thick CVD diamond substrate formed by electron lithography and reactiveion etching steps. The sizes of the series of the lenses were 300 × 80 µm2, with vertical focal lengths of 250, 500, and 1000 mm, which provides 67-78% transmission for 17.5 keV photon energy. A novel combined flux-enhancement X-ray optical setup was introduced by the research group of Bongaerts (C19); their method was based on the application of a one-dimensional lineartransmission Fresnel zone plate (FZP) for coupling monochromatic X-rays to the entrance of a waveguide. The optical solution resulted in a 54 times higher factor for X-ray intensity as without a zone plate. The FZP was fabricated by electron lithography and wet chemical ion etching on a Si membrane; the height of the ridges of the FZP was 5.5 µm and the sizes of the lens aperture were 200 µm perpendicular to the ridges and 2.5 mm along the ridges. A Japanese research group (C20) constructed a high-resolution X-ray imaging microscope, which was used as well as X-ray phasecontrast microscope at Spring-8. The essential part was a zone plate made of Ta, having a radius and width of the outermost zone of 7.07 µm and 50 nm, respectively; the number of zones was 5000, and the outer diameter is 1 mm; the focal length and the numerical aperture were 40 cm and 1.24 × 10-3, respectively, at 10 keV X-ray energy. When the X-ray microscope operated as phase-contrast microscope, a phase zone plate was placed in the back focal plane of the zone plate objective. For decreasing the high coherence of the X-ray beam, a rotatable diamond diffuser was applied that was made from diamond paste (the average particle size was 6 µm) having thickness equivalent to a ∼30 µm diamond. The principle of phase-contrast microscopy is that phase changes of the X-ray beam introduced by the investigated object are transformed into changes in intensity by shifting the phase of the central order of the diffraction spectra. The phase plate was made of Au having thickness of 1 µm deposited on Kapton film (20 µm), which is capable of shifting the X-ray phase by one-quarter of a period at 3452

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10 keV X-ray energy. For testing, they recorded phase images of polystyrene particles demonstrating the feasibility of the method for studying the inner structure of natural biological samples without sample preparation. A special construction of an X-ray spherically shaped focusing analyzer was reported by Tirao et al. (C21). This equipment was designed for a back-diffraction measuring geometry at synchrotron applications. The analyzer was constructed by positioning 2000 small Si crystals on a concave glass substrate in such a way that a Si plate was divided and cut into 2000 equilateral triangles glued into the surface of a concave glass. The advantages of this technique are that a higher intensity can be achieved in comparison with nonfocusing systems and that simultaneously a good energy resolution is available. Those properties offer possibilities to investigate low-energy satellite lines in X-ray emission spectra, natural line width of fluorescence spectra, and high-resolution fluorescence XANES experiments. A centimeter-sized miniature X-ray source was discussed by Ribbing and colleagues (C22); it has an extremely small size and needs very low power. These special operating and geometrical properties could be achieved by using a Peltier cooled Si detector and a special X-ray source based on the field emission effect. The cathode of the miniature X-ray source was a diamond membrane with a pyramidal tip. and the anode was a metal film deposited on another diamond membrane. The diamond has many advantageous physical and chemical properties, which make it suitable for construction of the electrode in a miniature X-ray tube because of the (i) controllable conductivity by doping, (ii) chemically robust material, (iii) very high thermal conductivity, and (iv) high transmission for X-rays. The maximum field emission current was 10 µA, and the accelerating voltage 15 kV. The possible application field of this type of X-ray source is the in situ field analysis where the use of portable X-ray spectrometers is advantageous. This type of X-ray tube can be vacuum encapsulated to create a compact small X-ray source. For the routine use of capillaries in XRS, all of their optical parameters have to be known, which can be achieved by testing under laboratory conditions, as proposed by Vincze and co-workers (C23), who investigated monolithic polycapillary half-lenses for monochromatic synchrotron radiation. The gain of the tested optical element was found to be 600 (ratio of the flux densities with and without capillary optics), and the size of the outgoing X-ray beam was between 37 and 53 µm in the energy range of 6-19 keV. For the determination of the intensity distribution of the output X-ray beam, a Cu and Fe-Cr-Ni wire with 10-µm diameter was scanned in both vertical and horizontal directions and the emitted characteristic X-rays were detected. An approximately linear relationship was found between the fwhm values in vertical and in horizontal directions and the beam energy, and the authors determined experimentally both the energy dependence of the gain factor and the transmission efficiency of the polycapillary half-lens. Finally, they checked the results theoretically by Monte Carlo simulation and investigated the analytical capability for microfluorescence and micro-XANES analysis. The average achievable absolute detection limit was found to be 60 fg to 1 pg, and the achieved relative detection limit was about 0.3-200 ppm depending on the atomic number of the elements.

QUANTIFICATION AND FUNDAMENTAL DATA The neural network offers an effective tool for the solution of nonlinear problems such as the concentration calculation based on fundamental equations. The model and application of Luo (D1) demonstrate the effectiveness of this combined method (neural network with fundamental parameter (FP) method), which is based on a learning algorithm, in order to search the minimum difference between measured and calculated intensity values when varying the quantitative composition of the sample. The author compared the analytical capability of the NNFT with classical fundamental approaches, and the predicted errors were found to be systematically lower. Luo supposed that neural networks can better calibrate the nonlinear matrix effects in complex multivariate samples and demonstrated that the neural network system is a powerful tool in correction with nonlinear matrix effects. The accurate and correct fitting procedure of energy-dispersive (ED) X-ray spectra requires a complex model with a combination of Gauss, shelf, and tail functions (D2). The research group of Van Gysel found a significant dependence of tail parameters on the attenuation coefficients of the detector material, and they proposed a mathematical description that takes into consideration the relationship of tail parameters and X-ray energy. The method resulted in more robust spectra fitting procedure and a decrease of the number of fitting parameters. The general applicability of the method was demonstrated and successfully tested on ED spectra recorded by HPGe and Si(Li) detectors. One of the basic problems of FP approaches is the estimation of the X-ray properties (attenuation, average atomic number, etc.) of the “dark matrix”, which is nondetectable for the conventional ED detectors. Wegrzynek and co-workers (D3) improved the backscattered fundamental parameter (BFP) procedure for portable ED X-ray spectrometers using monochromatic excitation based on radioactive sources. This BFP method is established on the use of coherent and incoherent scattering intensities for the estimation of the essential FPs of the dark matrix, in which the undetectable (with conventional ED detectors) elements (i.e., C, N, O, etc.) are involved. The algorithm was tested by in situ analysis with a portable spectrometer equipped with a Si-PIN detector and a 109Cd radioactive source for sample excitation, which allowed using one standard calibration without a priori knowledge of the composition of the dark matrix. The X-ray spectrum fitting is sensitive to the value of the Fano factor; therefore, the determination of the accurate value of the variable is a critical point. To solve this problem, Owens et al. (D4) showed experimentally that the secondary electrons generated by the low-energy X-rays resulted in systematically lower value of the Fano factor. To avoid this fundamental problem, the authors determined the Fano factor from the probability analysis of the pulse heights in the detector. The experimental checking of the theoretical estimation showed good agreement with the earlier theoretical values. D'Angelo and co-workers (D5) developed a new version of standardless ED XRF analysis providing elementary concentrations. The procedure was based on the synchrotron radiation (SR) XRF and conventional XRF analysis for the case of small sample mass and size; i.e., the characteristic radiation of the substrate could not be neglected and had to be considered in the algorithm. The analysis requires the exact knowledge of the substrate composition and the description of the excitation beam. A novel algorithm was

published by Garcia et al. (D6), on the basis of the FP method, using selective excitation with variable X-ray wavelengths and integral counting of secondary fluorescence radiation (SEICXRF). The model respects the secondary excitation process, resulting in increased fluorescence in the output, and the fluorescence background fall effect (FBF), which occurs in the SEICXRF excitation and detection mode. The consideration of this FBF effect in the model calculation for the net fluorescence intensity is possible for the determination of both high and low concentrations of sample elements. The authors presented all the details of their theoretical calculations and deductions, allowing the average reader to follow the mathematical demonstration. The recently used theoretical formalisms of the FP method applied for quantitative elementary analysis neglect the contribution of double interactions to the characteristic X-ray intensities. Tirao and Stutz published (D7) a theoretical calculation and experimental results for the estimation of second-order processes, including one elastic or inelastic scattering event and one photoelectric absorption. In the calculations, various experimental conditions were considered such as different sample thicknesses, atomic numbers, incident and outgoing angles, and incident photon energies. The obtained results for scattering contributions to the KR1 fluorescence line vary from about 1% to 50% for light elements in the energy range from 20 to 120 keV. They concluded that the contribution of photoelectric elastic scattering is predominant in the low X-ray energy range while, at high energy, the Compton photoelectric process is more important. The basic concept, the experimental performance, and the calculation algorithm of SEICXRF have been described in much detail by Figueroa (D8). The excitation energy is tuned by a double-crystal monochromator, and when one of the edges of one of the elements is reached, the detected integrating signal jumps up, due to the higher excitation probability. On the basis of the energy of the jumps in the integrated X-ray spectra, the sample elements can easily be identified and evaluated. The author calculated the FPbased model for the jump height in both cases of primary and secondary excitations and gave a complete physical and mathematical description of what takes place in the sample. The greatest analytical limitation of the method is that, around the absorption edge, the intensity starts to oscillate due to the structure of the absorption edge; however, the simplicity of the required detector system is advantageous. The uncertainty of XRF analysis is one of the central problems of quantitative elementary determination in complex samples since the concentrations can occur in wide range of magnitude. Wegrzynek and co-workers (D9) studied the uncertainty of the quantification procedure for elementary determinations in a soil matrix, using ED XRF and the emission-transmission measuring method. They used a Mo anode tube with a Mo target for excitation in a Cartesian geometry setup and analyzed elements from Z ) 11 to Z ) 39, for K lines and from Z ) 47 to Z ) 92 for L lines, using a conventional Si(Li) detector. The authors took into consideration all the possible sources of error and gave a precise mathematical estimation for the contribution of each possible uncertainty source. The main benefit of the calculation of the combined uncertainty is in the possibility to reduce the contribution of the critical parameter to the errors, i.e., optimize the analytical procedure. Borkhodoev (D10) investigated also the uncertainty of the FP approaches: on Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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one hand, he considered the impact of photoelectrons on the enhancement of the characteristic fluorescence lines, and on the other hand, he estimated the influence of the uncertainties of the fluorescence yield, absorption jump ratio, and probabilities of line series emissions. The author presented an optimized algorithm for numerical integration by wavelengths and for the contribution of X-ray tube Bremsstrahlung radiation. A Monte Carlo (MC) simulation code was used by Li et al. (D11) to investigate the geometric and matrix effects on intensity/intensity ratios measured by EPMA. For the experiments, both synthetically produced artificial particles (KCl, K2SO4, KHSO4) and biomass burning particles were used in order to compare the calculated intensity ratios obtained by the CASINO simulation code developed in the University of Sherbrooke (Canada) and the University of Antwerp (Belgium). The modified CASINO frame program allows determining the single-particle composition quantitatively by iterative approximations without applying standard samples, using a thinwindow ED detection setup for recording the fluorescence signal from low-Z elements. The simulation of the whole X-ray spectra and the characteristic intensities of sample elements with comparison to the measured ED X-ray spectra, called the reverse Monte Carlo (RMC) method, is an alternative way to quantify the ED analysis. The principal requirement of this algorithm is a relevant simulation code, which describes correctly all the physical processes between X-ray photons, electrons, and the atoms of the sample and the detector material. Llovet and co-workers (D12) published a new simulation algorithm for solving the general mathematical problem for electron excitation of thick targets using ED detection. The transport of electrons and photons was simulated by the MC code PENELOPE that described the Bremsstrahlung emission as well. The energy of emitted photons was estimated by numerical cross section tables, and their angular distribution was represented by an analytical expression with parameters determined by fitting benchmark shape functions obtained from partial-wave calculations. The simulated relaxation cascade process of the excited atoms after the ionization of an inner shell proceeds through emission of characteristic X-rays and Auger electrons completed when all vacancies have migrated to M, N, etc., shells. The nonirradiative Coster-Kroning and the Auger effects as well as all the possible steps of photon transport are considered such as coherent and incoherent scattering, photoelectric effect, and generation of Bremsstrahlung emission. To test the simulation algorithm, the authors performed an electron-excited experiment using electron microprobe equipment with 20 keV electron beam energy and a conventional Si(Li) detector having a 7-µm Be window. The detector response was constructed by a Gaussian distribution for peak shape with an energy-dependent fwhm, which was approximated by independent measurements of pure elements samples. The agreement between simulated and measured X-ray spectra is acceptable in the energy range from 3 to 15 keV, where the efficiency function of the detector is constant; however, for a high-Z element, ∼10% systematic difference was found, which is caused by some unconsidered physical interactions in the electron-electron and electron-nucleus processes. The basic assumption of the FP approaches for quantitative X-ray analysis is based on the use of parallel X-ray beams for both excitation and secondary beams; however, nowadays the different X-ray optical elements allow using 3454

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wide solid angles. To resolve these discrepancies, Malzer and Kanngieβer (D13) proposed a new method for calculating the primary and secondary X-ray fluorescence beam divergences using MC integrating calculations. The authors introduced the “equivalent angles” for representation of the angularly distributed X-rays; this new approach contributed to the smaller differences between the results of the MC simulation and fundamental parameter calculation. Kanngieβer (D14) emphasized that quantification procedures and methods became more important in X-ray analysis, since up to now a great variety of different type of X-ray spectrometers have been developed which are equipped with special excitation modes, various X-ray optical elements, and different types of detectors. From this point of view, the most effective operating and evaluation mode can be achieved by applying the general or special form of the FP method (FPM). The author surveys the general features of the quantification procedures and special properties for FPM and MC approximations and discusses some special problems such correction for nonparallel X-ray beams and spectrum evaluation in the soft X-ray region. A complete analytical solution has been offered by Ro and co-workers (D15) for EPMA for single particles using a reverse MC simulation algorithm that is capable of estimating the concentration values of particle elements by successive approximations. The main idea of this algorithm was that the concentration set of the analyzed material is carefully varied with an appropriate strategy, as far as the simulated X-ray intensities of the sample elements became approximately equal to the measured intensities. The kernel of the representing software was the modified CASINO program that describes mathematically the physical process taking place in the excited sample. The CASINO code was adapted to the experimental conditions of thin-window EPMA, taking into consideration all the possible electron-electron and atom-electron interactions and the generation processes of characteristic emission and Bremsstrahlung. The output of the algorithm is a complete set of concentration of particle elements from C up to U; however, the method was developed above all for quantitative standardless determination of major elements that are low-Z elements in environmental aerosol and sediment particles. The authors demonstrated the analytical reliability of the MC approach by analysis of standard particles (Al2O3, CaCO3, KNO3, SiO2, NaCl, CaSO4‚2H2O, BaSO4, Fe2O3, (NH4)2SO4, NH4NO3) produced artificially. Excellent agreement was found between calculated and nominal concentrations, for different particle sizes and for a wide range of elements (6 e Z e 50). Schalm and Janssens (D16) proposed a simple method to eliminate the determination of detector efficiency function, which causes a variable factor in time and result uncertainties and in systematic errors in quantitative EPMA. The low-Z detection suffers from ice contamination on the semiconductor crystal, which drastically influences the soft X-ray detection probability due to the high attenuation of the ice layer. The authors developed a new method for calculation of the main efficiency parameters of the Si(Li) spectrometer, such as thickness of the ice layer, and all the structural elements of Si(Li) detector (dead layer, conductive layer, window thickness, etc.), based on the evaluation of one multielement glass standard sample containing elements in the analyzed atomic number range. The research group of Vekemans reviewed (D17) the application of RMC procedures for XRF analysis at the

ESRF ID18F beamline; this method is an algorithm similar to the one described for thin-window EPMA in ref D15; however, in this work, internal reference elements were necessary for the calculation since, in this case, the low-Z elements were not detectable. Both refs D17 and D15, dealing with the analytical questions of RMC methods, presented some examples demonstrating the flexible and unconstrained analytical capability of this quantitative approach. The RMC should be seen as one of the possible candidates for general unlimited quantification solutions for X-ray analysis in the future. Vincze et al. (D18) published as well on the application of RMC for micro XRF analysis of individual fly ash particles (with 20-60-µm diameter) using SR for excitation. Their iterative strategy was based on the assessment that the particles were homogeneous. They applied an iterative RMC approximation for quantitative evaluation similar to the method applied in refs D15 and D17. The average uncertainty level of the quantitative results was in the range of 5-30%. Finally, in this subsection, we review a special quantitative analytical application of high-intensity SR radiation, published by Kempenaers et al. (D19). The ultimate goal of their study was to disclose the nature of the heterogeneity of inorganic trace elements in a heterogeneous matrix, i.e., in a 1-mm-thick glass wafer and NIST SRM 1577a standard sample. The excited sample mass was between 10 and 100 ng, and the size of the X-ray beam varied from 5 to 150 µm. The heterogeneity analysis was based on repeated measurements of a small sample mass, while scanning the sample surface in order to evaluate the suitability of the samples as reference material for trace element analysis. For the quantitative characterization, they calculated the variance of the obtained intensity, supposing different distributions of the trace elements in the sample, and they developed an MC code to simulate the X-ray response of the sample for different types and degrees of the microheterogeneity. In the field of spectra evaluation, a novel algorithm has been developed by Bennun and co-workers (D20) for identification and quantification of the X-ray signal based on the maximum likelihood estimation concept, for cases when a small characteristic signal is present on a high background level. The basic limitation of the method is in the assumption that the signal is proportional to the acquisition time; that means the procedure can be applied to total reflection XRF (TXRF), PIXE analysis, and XRF, with low counting levels. Instead of using net peak areas to calculate the concentrations, the authors introduced a new estimation mode for elementary concentrations, which was based on probability properties of the X-ray spectra, and the method requires the knowledge of the peak position and the peak and background distribution. On the basis of this model, they constructed a new definition of the limit of detection, which is less than the conventional value. To test the reliability of the new method, they performed some experiments for the determination of the Hg content in mercurygold amalgams under TXRF and 109Cd-excited XRF conditions. Generally, 3D analysis of microparticles can be done by XRF microtomography. However, Rindby and co-workers (D21) proposed a new calculation model to get information on the threedimensional distribution of constituent elements in fly ash particles using simple micro-XRF spectroscopy. Their method utilizes the special feature of this type of particle (i.e., the high degree of symmetry) for their special reconstruction algorithm, which requires only a 2D elemental map performed by high lateral

resolution micro-XRF analysis. The algorithm was capable of distinguishing the symmetric outer shell from the asymmetric inner shell of the fly ash particles. The application of FP programs for quantitative X-ray analysis in XRF, EPMA, and X-ray photoelectron spectroscopy needs the calculation of basic atomic data, such as photo absorption coefficients, coherent and incoherent scattering, and mass attenuation coefficients, in wide energy range. This necessity motivated the group of Ebel (D22) to analyze systematically the available databases and to construct a new fitting procedure describing Scofield’s numerical values with fifthorder polynomial functions with maximum deviations of 0.1%. The results have a great impact on the FP algorithm and programs. The database can be found at the following website: http:// www.ifp.tuwien.ac.at/forschung/horst.ebel. In the past decade, the improvement of the ED low-energy X-ray detection technique has led to the necessity of more accurate knowledge of the soft X-ray lines such as K lines of low-Z elements, L lines of medium elements, and M lines of high-Z elements, i.e., relative intensities and exact energies. The knowledge of intensity ratio of soft X-ray peaks is indispensable for the correct quantitative separation of X-ray peaks and the application of the fundamental calculation of elementary concentrations. For exploring this “white” area of XRS, Wendt (D23) studied experimentally the low-energy X-ray spectra (excited by electrons) using a Si(Li) detector equipped with an ultrathin polymer window and determined the I(Mγ)/I(MR) intensity ratio for elements in the atomic range from Z ) 39 to Z ) 56. Our opinion is that, in the future, more intensive activity has to be deployed by the XRS community in this field, in view of the rapid development of soft X-ray detectors, such as superconducting cryogenic-type sensors and conventional semiconductorbased thin-window detectors. The lack of fluorescence cross sections for L shells of high-Z elements hinders the effective application of the new spectroscopic techniques and the calculation of the sample compositions, etc. The absence of suitable databases motivated the work of Mittal and Singh (D24) to develop two empirical relations, one between the L shell cross sections and the photon energy (EL1 < E < EK) and the other between the L shell cross section and the atomic number (57 e Z e 92). For both energy and atomic number dependences, they used logarithmic-type polynomial functions. The calculated database of polynomial parameters allowed constructing software that calculates the general cross sections for interelement and interenergies with interpolation on single existing cross section data. The empirical relationship between measured intensities of the ED spectra and the unknown attenuation parameters of the sample is the basis of some quantification procedures especially when the major elements of the matrix are not detectable. For this relationship, Bao published (D25) a new simple expression where the second-order polynomial function describes the relationship between the mass attenuation coefficients and the inverse of the incoherent scattering intensity. The proposed function is very simple, and it is valid over a wide range of atomic numbers, from Li to Rb. The expression was tested with a Rigaku wavelengthdispersive (WD) X-ray spectrometer equipped with a LiF analyzing crystal using different standard materials. Each quantification algorithm suffers from the lack of a priori knowledge of the density distribution of the investigated samples, especially in the case of tomography methods. Heismann and co-workers (D26) developed Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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a new algorithm for estimation of mass density and atomic number, which is based on two individual measurements on the sample using different spectral weighting of excitation X-ray beam or the detector efficiency. The authors tested the method in a dual-energy computed tomography experiment of an organic sample, and they found that the density distribution reflects the morphology of the objects. For their recent experimental setup, they obtained an absolute precision of 0.1 for Z and 20 mg/cm3 for density. The accurate and reliable evaluation of ED X-ray spectra demands knowledge of the response function of the detectors. Papp (D27) reviewed the state of the art for the Si(Li) and HPGe detectors and their spectroscopic features and described some special effects that take place at the surface of the Si or Ge crystal and front contact material. The deduction of the author is based on the quantitative investigation results with a static electron spectrometer and the model of the possible electron transport effects in semiconductor material. The author proposed a model for the response function instead of the frequently applied Hypermet function, since it can explain the origin of the exponential tail and other discrete structures on the low-energy side of the primary peaks. Intense X-ray sources such as synchrotron facilities require development of X-ray detectors that are capable of handling a high flux and of improved data evaluation methods to treat pile-up effect. Laundy and Collins (D28) published a new method for this problem, the so-called pulse-overlap model for describing the loss of counts that cause high count rates in the detector. The model treats mathematically and more generally the counting losses than the conventional dead-time model does, and they give also the error of the measured intensities. In an appendix, the authors present a complete and detailed statistical model calculation. For quantitative PIXE analysis, fundamental data are essentially necessary, especially the K X-ray production cross sections. The K shell ionization is sometimes accompanied by other shell ionizations, which yields changes in the I(KR)/ I(Kβ) ratios and shifts of the characteristic energies. Ozafra´n and co-workers (D29) published their investigations for quantitative measurements of these effects on Al, Si, S, Cl, K, Ca, Ti, Cr, Fe, and Cu, with bombardment of 12C ions of 14-50 MeV energy. The energy shift was found to be from 10 to 50 eV, depending on the target element and the bombarding energy; however, the intensity ratios do not change significantly. Overall, they determined the cross sections for fluorescent production for these elements, and comparison with theoretical calculations shows an acceptable agreement when the multiple vacancy production was taken into consideration. The lack of fundamental data for the M shell of high-Z elements causes difficulties in quantitative analysis of 60 < Z elements. For the solution of this basic problem of quantitative PIXE, the research group of Ferna´ndez (D30) published M-shell production cross sections for W, Au, Pb, Bi, Th, and U, generated by 0.3-0.7-MeV energy protons in steps of 50 keV. The experimental data were compared with results of plane wave Born approximations (PWBA) and ECPSSR, and they found the PWBA resulted in higher values than the experimental data. As an alternative scattering process, resonant Raman scattering (RRS) should contribute to the X-ray attenuation process in special cases; that effect has not been taken into consideration up to now in different quantitative model calculations. Karydas et al. (D31) quantitatively investigated the resonant Raman scattering 3456

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induced by 1.5-MeV energy protons in Ti, V, Cr, Cs, La, Ce, and Ba. To minimize the level of Rayleigh and Compton scattering, a 90° geometry was used, and two different experiments were carried out, in reflection and transmission modes. On the basis of the results, the authors concluded that RRS effect should be one of the reasons responsible for the discrepancies between experimental and theoretical values in the fine structure of the absorption edge. Complex quantitative PIXE analysis combined with imaging was developed by an Australian research group (D32), who performed high-sensitivity PIXE analysis of the composition of minerals and of the inclusions in diamonds. To obtain immediately the concentration values during the probe scanning, they developed the so-called dynamic analysis, which transforms directly the spectral information accumulated in a Ge detector to a set of concentrations under the frame of an IDL operation platform. TOMOGRAPHY AND HOLOGRAPHY The emission tomography methods suffer from a lower intensity, by several orders of magnitude, in comparison with absorption tomography. In the former case, the primary X-ray beam excites the sample voxels and the secondary photons produced by the fluorescence process and scattering are detected; in the latter case, the beam transmits through the sample volume and the measurement of the attenuation of the beam is the basis of the tomography reconstruction. Brunetti et al. (E1) developed a new algorithm for x-y scanning reconstruction, using Compton radiation of the sample. This method considers and corrects the self-absorption effects without knowledge about the elementary composition of the sample. A new algebraic reconstruction technique for XRF tomography was reviewed by Chukalina and co-workers (E2). Their model was based on the measurement of the intensity of the monochromatized primary X-rays before penetrating into the sample and of the transmitted X-rays after the sample, while the excited characteristic X-ray signal was detected by a Si-type semiconductor detector. The collected information was sufficient to calculate the attenuation coefficients in every voxel as is done in conventional tomography techniques. The authors showed that, using this information in the basic equation of the fluorescent intensities for every sample element and sample voxel, the system of equations can be solved by an appropriate numerical iteration algorithm. The detection efficiency of X-ray tomography methods can be increased if, instead of ED detectors, a CCD camera is used with a scintillation plate, allowing one to record in a 2048 × 2048 pixelated detector matrix; this offers edge-enhanced and phasesensitive measurements. This technique can characterize properties of the inner structure (composition, volumetric structure) of the studied object. The research group of Stampanoni (E3) developed this type of detector system for SR excited microtomography with high spatial resolution, from 715 × 715 µm2 to 7.15 × 7.15 mm2, with a magnification from 4× to 40× . The authors performed some experiments with this setup and achieved 0.7µm optimal spatial resolution, illustrating the recording with edgeenhanced radiographic projections. Hard X-rays are an ideal tool for exploring the volumetric structure at submicrometer scale; this requires excellent performance of the CCD detector system, which converts the X-ray image into the visible range onto a CCD camera with the help of suitable microscopic optics. Stampanoni

et al. (E4) published a new setup for edge-enhanced microtomography, called a Bragg magnifier, which utilizes double asymmetrical Bragg diffraction for producing a hard X-ray image with magnification factors up to 100 × 100 while the pixel size was less than 200 × 200 nm2. The operating energy range was between 21 and 23 keV, which was suitable for 2D analysis of nanoscale structure of human bones. An example of Compton scattering tomography was published by Baloguna and Cruvinel (E5); they used a 137Cs isotope with 200 MBq activity. Although the γ energy of 137Cs is ∼662 keV, which is far from X-ray energy range, the method should be applicable for hard X-ray energies for smaller sample sizes. The scanner equipment consists of a simple sample moving setup, radioactive source, and NaI scintillator detector, which is suitable to distinguish the incoherent and coherent parts of the detected spectra. Because the structure of the specimen and the variation of the sample density influence significantly the level Compton and Rayleigh scatterings, the detected spectra collected in different source-sample-detector positions carry information on the compaction of the sample material. The authors studied different soil structure with this method. X-ray holography is one of the most promising and simplest instrumental methods for investigating the material structure at atomic scale, resolving atomic distances. Faigel and co-workers (E6) summarized the most characteristic features of their XRS method, discussed the recent developments and experimental techniques, and showed some examples for demonstrating the capabilities of X-ray holography. The basic requirements of X-ray holography determine the experimental conditions: (i) in the analyzed area, each atom has to have the same atomic environment and they must have the same orientation; (ii) monochromatic emission X-ray is necessary; (iii) the radiation emitted by the atoms must be incoherent; (iv) the sample size must be much smaller than the detector-sample distance. The X-ray holography method can be used mostly for the study of single crystals and macromolecules, but with the appearance of different variants of holographic techniques, using γ-rays and Bremsstrahlung can be expected. Quantitative phase tomography and reconstruction of a small tip was performed by McMahon’s group (E7), with a spatial resolution below 900 nm. For the experiment, a coherent X-ray beam with 1.83 keV energy was used and the imaging device was a Ni Fresnel zone plate creating the X-ray image on the surface of a CCD camera having 1024 × 1024 resolution. The magnification was ∼160 while the efficiency of the zone plate was ∼2.5%. Because the different X-ray beams produced by the Fresnel zone plate interfere on the surface of the CCD, this detector records the phase relations between different X-ray paths, which originate from different points of the zone plate. After carrying out the tomography experiment with this setup, the authors reconstructed the phase tomogram with submicrometer-scale resolution resulting in a 3D picture of the investigated tip. Antonie and his research group (E8) also reviewed phase-contrast X-ray microtomgraphy for disclosing the detailed 3D structure of papers by means of detection of the refractive index of the material with a 1-µm spatial resolution. The method is based on near-field coherent imaging, which gives phase contrast in the localized changes in the refractive index, i.e., borders between sample matrix and inclusions. The basic requirement for phase-contrast tomography is the spatially coherent X-ray beam, which is obtained at third-

generation synchrotron sources with undulator devices. For the reconstruction algorithm, the inverse Radon transformation was used while the volume size obtained was 2048 × 512 × 2048 voxels, stored as 512 images of 2048 × 2048 pixels. The group of Nishino (E9) proposed a new method for X-ray holography measurements based on the use of a two-energy twin image removal algorithm, which allows performance of atomicrange resolution X-ray holography. For detection, they used a ZnSe single crystal and pointed out that this technique can be applied for both a laboratory generator setup and for X-ray pulses produced by free-electron lasers. A special application for a tomography experiment was published by Appoloni and coworkers (E10) for the determination of porosity parameters of different amorphous materials. The samples were illuminated by an X-ray tube generated beam of 80-µm beam size. On the basis of the set of reconstructed absorption coefficients, the sample porosity and pore size were calculated for all analyzed voxels. The authors analyzed two samples, sandstone and ceramic foam, which were prepared in 5 × 8 mm2 sizes. Because of the great variety of tomography techniques that are widespread in material science, medical diagnosis, and environmental and biological analysis, this 2D analytical method appeared for educational use as well. For this reason, the international group of Claesson (E11) developed such tomographic equipment based on detection with a CZT solidstate detector and using a standard-level personal computer. The ultimate goal of this new tomographic setup was to offer training and teaching possibilities for different medical courses. The detector was a commercial cryogenically cooled CZT with 3 × 3 mm2 area and 2-mm effective thickness, and the source of X-rays was 241Am with 10 mC activity. Control of the movements and the data acquisition processes were performed by the LabVIEW system, and for purposes of data reconstruction, the MATLAB standard routine was applied. In their experiments performed on artificial samples, the best volumetric resolution achieved was 1 mm in the case of 100 × 100 mm2. An appropriate application for XRF microtomography (XFMT) was published (E12); the impurities of a SiC shell over the nuclear fuel particle, which had a kernel made of UCO, UC2, ThO2, or UO2, were measured. The kernel is covered by an SiC layer, which should be incorporated by different metals such as Fe or Ni during the preparation of the particles and it could contain fission products as well after hard irradiation of the cells by neutrons (∼1025 neutrons/m2). The authors developed an XFMT measuring procedure for 3D analysis of the shell contamination level, using a Fresnel zone plate for focusing the X-ray beam of 10.5 keV energy and a Si PIN diode for detection. The best lateral resolution was found to be ∼150 µm for Ni, Zn, Cu, Fe, and Cr in the shells, which had 50-100-µm thickness on the particle (the diameter was 600-700 µm). One of the greatest advantages of the micro X-ray tomography procedure is that it can easily be applied in the in situ measuring mode; an example has been published in ref E13. They investigated a metal powder sample (Cu and Fe) during the sintering process taking place in a furnace in front of the end of a beamline of 35-45-keV X-rays, the tomograms were recorded with a CCD camera having 1024 × 1024 pixels, and the achievable spatial resolution was found 2.5 µm. A simple X-ray holographic experimental setup was reviewed by Chen and co-workers (E14) for soft X-rays. The small sample was mounted in the path of the primary X-ray beam while Analytical Chemistry, Vol. 76, No. 12, June 15, 2004

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the other part of the primary beam hit the photoresist plate, which acted as a 2D detector for recording the holographic interference patterns. The illuminating X-ray beam was formed by a Fresnel zone plate and a 700-µm pinhole, the recorded holograms were magnified by an atomic force microscope (AFM), and finally the image of the AFM was analyzed by a common scanning microdensitometer. Chukalina et al. published a general description (E15) of the X-ray microfluorescence tomography and gave an overview of the status and the main elements of the algebraic reconstruction technique, which are able to present the quantitative distribution of different constituting elements in the studied object. The published quantitative model consists of a nonlinear system of equations describing the mathematical relationship between the atomic density distribution and the measured fluorescence signals and the sample parameters. The system of equations can be solved by numerical iterative approximations, to provide the reconstructed elementary distribution. TXRF Total reflection X-ray fluorescence analysis is a special spectroscopic technique, which utilizes the total reflection of X-rays when the incident beam impinges under a very small angle, avoiding the main portion of continuous scattered background in the detected spectra. The method can be applied when the sample has a flat and smooth surface or the sample material can be transferred onto such a substrate surface. Wobrauschek and his research group (F1) reviewed a new TXRF spectrometer with rotating anode tube (4 kW), a small-angle X-ray scattering camera, and a new elliptically bent multilayer designed to collimate and monochromatize the divergent X-ray beam. The focal length of the multilayer optics was between 150 and 500 mm. The fluorescence signals of the sample elements were detected by a Si drift chamber detector with 5-mm2 active area, 7-µm Be window, and 148-eV resolution at an X-ray energy of 5.9 keV. The detection limits for Co was found to be 300 fg, which is remarkable compared to 1 pg obtained with a 2-kW fine-focus standing anode tube and 10 fg using monochromatic SR. One of the great advantages of TXRF is that the minimum sample mass required for performing the measurement is less for almost any other kind of analytical technique. This can be utilized when an extremely small amount of sample mass is available: for example, small biological objects, painting chips, and archaeological specimens. For rapid analysis of excavated Etruscan pottery samples, the research group of Cariati (F2) developed a new sample preparation technique for TXRF analysis. Small amounts of ceramic sample were ground by an agate mortar and ball mill; then high-purity water was added and the suspension was dropped onto the substrate. The simplified sample preparation technique was tested in comparison with the result of ICP analysis of standard NIST samples, and reasonable agreement was obtained between the two series of concentration sets. Kro¨pfl and colleagues (F3) used TXRF to analyze natural biofilms grown on a polycarbonate substrate in Lake Velence (Hungary). The ultimate goal was to explore the accumulation and the enrichment factors for different metals (Ca, Fe, K, Mn, Sr, Ti, Zn) from the lake water, with the aim of using the biofilms as a tool for biomonitoring, to control continuously the water quality. The direct TXRF measurement of biofilms on the substrate introduces some errors in the final 3458

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quantitative results due to the variation of biofilm thickness and lack of internal standard. To increase the photon flux onto the sample surface, Tsuji and Wagatsuma (F4) developed a new optical setup using an additional Si reflector, positioned above the sample holder, which reflects the part of the excitation X-ray beam whose trajectory passes just above the sample. They changed the tilting angle relative to the surface of the substrate holder, to find the maximum fluorescence signal. The principle of the effect is similar to the waveguides, such as capillary optics, where the wall of the capillary tubes reflects totally the X-rays. The advantage of this solution is that it does not require an expensive polycapillary lens, and changing the orientation of the reflector provides the optimum analytical performance. Finally, the authors suggested using more than one reflector acting as a waveguide optical element. TXRF analysis with SR was used for analysis of atmospheric particles collected with stacked filters by Matsumoto et al. (F5), for the fine and coarse aerosol fractions. After the TXRF measurements, they performed principal component analysis to identify the emission sources of the particles. An Argentinean group (F6) utilized TXRF for analysis and identification of polymer samples. The matrix of polymers consists of carbon oxygen and hydrogen; hence, the conventional ED XRF detection mode does yield characteristic peaks for the matrix. However, Compton and Rayleigh scattering yields a significant signal originating from the sample material, but the sample holder gives only a very weak intensity due to the total reflection geometry arrangement. The authors analyzed the data with multivariate statistical methods and were able to distinguish the different sample materials with a high probability. The most brilliant X-ray sources for TXRF are synchrotrons due to the highly monochromatic and well-formed beam. The group of Simon (F7) reviewed a new TXRF beamline at ANKA, applying bending magnet and retractable doubled multilayer monochromator to provide a high photon flux with very low background. The best minimum detection limit they achieved was found between 20 and 100 ppb, while the energy range for the exciting X-ray beam was from 1 to 25 keV. To obtain a small spot size for the excitation beam, a monocapillary was applied to achieve 20-µm beam size. A significant improvement in analytical ability of TXRF was published by Sakurai and co-workers (F8); they applied the WD detection technique with a Ge220 Johansson crystal and YAP:Ce detector. Both the energy resolution (6-7 eV for Ni, Co K lines) and minimum detection limit (3 µg for nickel) were improved using a SR X-ray source. Because the WD detection offered only a sequential acquisition mode in contrast to ED detection, they deducted that the best detection performance will be a reasonable combination of these methods in the future, uniting the advantageous analytical features of both detection modes. A novel X-ray microimaging technique was presented by Sakurai and Eba (F9) based on the combination of grazing-incidence excitation geometry and parallel optics for detecting X-rays; they were capable of recording XRF images with 20-µm resolution. Surprisingly, the acquisition time was extremely short, i.e., 1-2 min. A Peltier cooled CCD (1000 × 1018 pixels with 12-µm2 size) camera was the detector, equipped with a microchannel plate as well as a capillary-assembled plates in front of the detector, to act as collimators. The great advantage of this new method is that the variation of the incident angle of the X-ray beams indicates different segmentation of the elements versus

depth in the excited substrate in the 2D image. The TXRF measuring setup is suitable for performing depth analysis in the substrate by varying the angle of incident beam to the analyzed surface. Huh and co-workers (F10) performed SR TXRF experiments to study the depth profile of impurity concentrations in a Si wafer, at a few tens of nanometers depth under the surface. They calculated the angularly dependent fluorescence intensity and compared them to the experimentally determined values for Ni and Fe. They introduced an iterative solution to find the levels of impurities; in this procedure, they compared the theoretically calculated characteristic X-ray intensities and the measured values; this procedure was capable of determining the optimum value. TXRF is one of the suitable methods for thin-layer analysis for implanted layers in a semiconductor wafer. Klockenka¨mper and co-workers (F11) published a comparison for layer analysis for different methods such as grazing-incidence XRF, Rutherford backscattering (RBS), and TXRF analysis for Co ions (∼1017 atoms cm-2) in the nanoscale surface layer of Si wafer. The aim of the work was to record the depth profile of the implanted layer by all the analytical methods and to compare these results in order to find the accuracy of the methods for this task. They found significant differences in the profiles; only the RBS and TXRF results yielded a similar quantitative dependence of the implanted atomic density versus depth. The work calls attention to the lack of suitable standard samples. Environmental applications motivate the development of more and more complicated and specialized analytical methods because sometimes one procedure is not able to give the complete answer to an analytical question. In this case, a reasonable combination of different instrumental methods has to be used; such type of application was published by Kurunczi and co-workers (F12) for Hg determinations on Ag-coated filters. They analyzed the filters by micro-XRF, TXRF, and conventional XRF methods to explore the macrodistribution of Hg on the filters, to determine exact average concentration of Hg in the filter, and to estimate the filtering efficiency for Hg accumulation in the filter. The ultimate goal of these investigations was to approximate the Hg emissions from different combustion plants to the atmosphere and indirectly to the soil and water environment. ELECTRON PROBE MICROANALYSIS Based on the recent advances in electron beam instruments and X-ray detectors, an increasing interest was encountered in the field of low-voltage EPMA (LV-EPMA). Field-emission gun electron beam instruments such as scanning electron microscopes (SEMs) have high-brightness electron guns with excellent performance at low beam energies, E0 e 10 keV. The recently developed high-resolution ED X-ray spectrometers, like the microcalorimeter detector, provide high-resolution (