Zoya Spolnik

Optimization of Measurement Conditions of an Energy Dispersive X-Ray Fluorescence Spectrometer with High-Energy Polarized Beam Excitation for Analysis of Aerosol Filters:

A new commercial energy dispersive X-ray fluorescence spectrometer (EDXRF), applying a three-dimensional geometry and high-energy excitation, was optimized for the quantitative analysis of aerosols deposited on filters. The preliminary results are presented here. First-order calibration curves were obtained for 20 elements deposited on the filters. The accuracy of the applied method and of the obtained calibration curves was checked by the measurement of a standard reference material from NIST. The precision of the analysis for the majority of the analytes was better than 10%. Due to the obtained low detection limits, it is possible to determine the analytes usually present at very low concentrations in ambient air, such as, e.g., Cd, Sb, Cr, and V. It is also possible to decrease significantly the time of analysis or the time of the sampling.

Enhancement of electron-induced X-ray intensity for single particles under grazing-exit conditions

Abstract Grazing-exit electron probe microanalysis (GE-EPMA) was performed for single Al2O3 and atmospheric particles, deposited on a flat Si substrate coated by gold, by using an aperture (1 mm in diameter) in front of an energy-dispersive X-ray detector. Silicon Kα X-rays from the Si substrate were strongly observed at an exit angle of ∼45°. However, they disappeared at grazing-exit angles about 0° and only the X-rays from particles were detected. Furthermore, Al Kα and O Kα intensities from single Al2O3 particle were enhanced approximately three- and sixfold at the grazing-exit angles (∼1°), respectively, in comparison with those at large angle (∼7°). The background intensities at the energy of Al Kα and O Kα almost monotonously decreased with decreasing exit angle. As a result, the intensity ratios of Al Kα and O Kα X-rays to the background intensities were enhanced five- and sixfold, respectively. This enhancement is considered to be caused by the interference effect of both directly detected X-rays and reflected X-rays on the flat substrate. The similar results are also obtained for Al Kα, Si Kα, K Kα and Ca Kα emitted from single atmospheric particle. The significance of the matrix effect in the particle is also pointed out.

Appraisal of measurement methods, chemical composition and sources of fine atmospheric particles over six different areas of Northern Belgium

Daily and seasonal variation in the total elemental, organic carbon (OC) and elemental carbon (EC) content and mass of PM(2.5) were studied at industrial, urban, suburban and agricultural/rural areas. Continuous (optical Dustscan, standard tapered element oscillating micro-balance (TEOM), TEOM with filter dynamics measurement system), semi-continuous (Partisol filter-sampling) and non-continuous (Dekati-impactor sampling and gravimetry) methods of PM(2.5) mass monitoring were critically evaluated. The average elemental fraction accounted for 2-6% of the PM(2.5) mass measured by gravimetry. Metals, like K, Mn, Fe, Cu, Zn and Pb were strongly inter-correlated, also frequently with non-metallic elements (P, S, Cl and/or Br) and EC/OC. A high OC/EC ratio (2-9) was generally observed. The total carbon content of PM(2.5) ranged between 3 and 77% (averages: 12-32%), peaking near industrial/heavy trafficked sites. Principal component analysis identified heavy oil burning, ferrous/non-ferrous industry and vehicular emissions as the main sources of metal pollution.

Quantification in grazing-emission X-ray fluorescence spectrometry☆

Abstract In grazing-emission X-ray fluorescence (GEXRF) spectrometry wavelength-dispersive detection can be applied. Much softer radiation and hence lighter elements than in total-reflection X-ray (TXRF) spectrometry can thus be detected. We used simulations to investigate methods of quantification of GEXRF results involving soft characteristic radiation. From these studies, it is concluded that for ultra-thin layers, e.g. the sub-monolayer amounts encountered in semiconductor contamination analysis, calibration plots are linear. For thicker layers, quantification should be performed very carefully because of deviations from linearity due to absorption of radiation and to oscillations in the calibration curve. These oscillations are caused by interference of fluorescence radiation emitted directly towards the detector and radiation reflected at the sample–substrate interface. Suggestions for a judicious choice of measurement conditions are made and the benefits of internal standardisation are discussed.

New experimental equipment for grazing-exit electron-probe microanalysis

New grazing-exit electron-probe microanalysis (GE-EPMA) equipment is developed. In GE-EPMA, characteristic x rays are measured at the grazing-exit angle. X rays emitted from deep positions in the substrate are reduced under grazing-exit conditions; therefore, surface-sensitive analysis is possible with low background. In previous equipments, the sample holder was tilted to change the exit angle. In this new equipment, the energy-dispersive x-ray detector is moved to change the exit angle, and the analyzed position is stable even if the exit angle is changed. Therefore, this equipment is useful especially for particle analysis. The new GE-EPMA equipment is applied to Pd–Se–Te single-particle analysis. Although it was difficult to measure the Se Kα line at an exit angle of 45° due to the large Au Lβ radiation emitted from the Au substrate, Se Kα was measured without any Au signals at the grazing-exit angle near zero.

Detection Limits of Grazing-Exit EPMA for Particle Analysis

Abstract. In conventional electron probe microanalysis (EPMA), the electron-induced X-rays are measured at large take-off angles of about 45°. In the grazing exit EPMA (GE-EPMA) method, they are measured at small angles (< 1°). X-rays emitted from deep positions can not be detected at grazing exit angles due to refraction effects at sample-vacuum interface; therefore, it is possible to measure X-rays emitted only from near the surface with a low background. GE-EPMA is especially suitable for the analysis of particles deposited on a flat sample carrier. The detection limits of GE-EPMA were investigated for artificial particles (Al2O3, Fe2O3 and PbO2 , particle sizes: 1 ∼ 18 μm) deposited on flat sample carriers of Au thin films–Si wafers. The detection limits improved with decreasing exit angle. The detection limits for characteristic X-rays at an exit angle of approximately 1.1° were 2–4 times lower than at 45°. A minimum detection limit of ca. 0.1% was obtained for Al in small particles.

Grazing-exit electron probe X-ray microanalysis of ultra-thin films and single particles with high-angle resolution

Abstract Ultra-thin metallic films, artificial particles and aerosols were analyzed by grazing-exit electron probe X-ray microanalysis (GE-EPMA). It is beneficial to measure the characteristic intensity emitted from these specimens at very small exit angles. Such an approach reduces the high background originated from the substrate. The reduction of background X-rays is very important for thin film analysis. Therefore, it is shown that GE-EPMA is a powerful tool for localized analysis of thin-films and the surface of the sample. It is also found that GE-EPMA solves the problem of the overlapping of X-ray lines emitted from the substrate and the particle or the aerosol. Performing the measurement of the characteristic radiation from the particle under grazing exit conditions allows particles smaller than the electron beam diameter to be analyzed. Thus any limitations of the electron beam size can be removed.

Comparison of grazing-exit particle-induced X-ray emission with other related methods☆

Abstract Particle-induced X-ray emission (PIXE) is recognized as a trace analysis method; the Bremsstrahlung background, however, still limits the detection power in the low-energy region of spectrum. The grazing-incidence or grazing-exit arrangements are considered to be useful in order to reduce the background intensity. We measure grazing-incidence PIXE (GI-PIXE) and grazing-exit PIXE (GE-PIXE) using an identical sample vacuum chamber to evaluate the performance of both methods. The sample used is a standard sample (Ca, Fe, Ni and Zn) prepared on Si wafer by the spin-coating method. In GI-PIXE mode, a collimated 2.5 MeV proton beam irradiates the surface of the sample at grazing incident angles. However, it was difficult to reduce sufficiently the background intensity in the GI-PIXE spectra because of the Bremsstrahlung background induced by secondary electrons. In the GE-PIXE mode, proton-induced X-rays are measured at grazing-exit angles. The background intensity is reduced, because only the X-rays emitted from the near-surface layer can be detected in the grazing-exit arrangement. Furthermore, the result obtained for an Au–Cu thin film sample by GE-PIXE method was compared with the result obtained by other related methods: grazing-exit X-ray fluorescence and grazing-exit electron probe microanalysis. Similar results were obtained by different methods, indicating that the same phenomena occur in all grazing-exit X-rays measurements. One of the merits of the grazing-exit microprobe analysis method is in microanalysis using a small diameter electron probe; however, the damage by electron irradiation is more severe than that in GE-PIXE.

Localized thin-film analysis by grazing-exit electron probe microanalysis

Abstract We have studied the application of grazing-exit electron probe microanalysis (GE-EPMA) for surface and thin-film analysis. In this method, characteristic X-rays are measured at small take-off angles of less than 1°. Under grazing-exit conditions, the X-rays emitted from deep inside the sample are not detected because they are stopped by a slit mounted in front of the energy-dispersive X-ray detector. Hereby, it becomes possible to perform localized surface analysis with GE-EPMA. We applied this method to thin-film analysis of a small surface area. The exit-angle dependence of the characteristic X-ray intensities was measured for thin films of Cr and Ti. Thickness and density of thin films were determined by fitting the experimental plots with theoretically calculated curves. Differences were found in the densities of two Cr thin films deposited by magnetron sputtering and vacuum evaporation. The advantage of GE-EPMA is that non-destructive thin-film analysis of small surface areas can be incorporated in simple scanning electron microscope (SEM) analysis.

Behavior of the continuous X-ray background in grazing-exit electron probe X-ray microanalysis

Abstract The energy distribution of the background radiation originating from two kinds of substrate materials has been studied using Electron Probe X-Ray Microanalysis (EPMA) at grazing-exit angles. The different behavior between tendencies of background reduction for the silicon and the gold substrates at the grazing exit angle is explained using a critical energy concept. It is also shown experimentally that a gold substrate results in a lower background intensity than a silicon substrate in the energy region 5–7 keV, while on the other hand, for elements with X-ray lines of 2–3 keV, it is advantageous to perform the analysis when such elements are deposited on the silicon substrate.