I. Szalóki

Thermal stability of beam sensitive atmospheric aerosol particles in electron probe microanalysis at liquid nitrogen temperature

Abstract Thin-window electron probe X-ray microanalysis offers new analytical possibilities for low-Z detection (i.e. elements with low atomic number; such as C, N and O). However, the quantitative analysis of individual particles raises some practical questions concerning the technique. From the analytical point of view, beam damage is one of the most important problems due to its big impact on the analysis of individual atmospheric particles. The dependence of the beam-damage effect on the type of collection substrate was studied using standard aerosol particles. Different metallic substrates were rigorously tested in relation to the beam damage effects to different kinds of beam sensitive particles. Ammonium sulfate, ammonium nitrate, sodium nitrate as well as sulfuric acid droplets were analyzed using a liquid nitrogen cooled sample stage on different metallic substrates such as Be, Al, Si and Ag. The obtained results confirm that the use of Be as a collection surface offers some advantages in order to minimize the damage to beam sensitive particles, as suggested in earlier research.

Polycapillary-optics-based micro-XANES and micro-EXAFS at a third-generation bending-magnet beamline.

A focusing system based on a polycapillary half-lens optic has been successfully tested for transmission and fluorescence micro-X-ray absorption spectroscopy at a third-generation bending-magnet beamline equipped with a non-fixed-exit Si(111) monochromator. The vertical positional variations of the X-ray beam owing to the use of a non-fixed-exit monochromator were shown to pose only a limited problem by using the polycapillary optic. The expected height variation for an EXAFS scan around the Fe K-edge is approximately 200 microm on the lens input side and this was reduced to approximately 1 microm for the focused beam. Beam sizes (FWHM) of 12-16 microm, transmission efficiencies of 25-45% and intensity gain factors, compared with the non-focused beam, of about 2000 were obtained in the 7-14 keV energy range for an incoming beam of 0.5 x 2 mm (vertical x horizontal). As a practical application, an As K-edge micro-XANES study of cucumber root and hypocotyl was performed to determine the As oxidation state in the different plant parts and to identify a possible metabolic conversion by the plant.

X-ray fluorescence microtomography and polycapillary based confocal imaging using synchrotron radiation

This work illustrates the development of X-ray fluorescence tomography and polycapillary based confocal imaging towards a three-dimensional (3D), quantitative analytical method with lateral resolution levels down to the 2-20 μm scale. Detailed analytical characterization is given for polycapillary based confocal XRF imaging, which is a new variant of the 3D micro-XRF technique. Applications for 2D/3D micro-XRF are illustrated for the analysis of biological (zooplankton) and geological samples (microscopic inclusions in natural diamonds and fluid inclusions in quartz). Based on confocal imaging, fully three-dimensional distributions of trace elements could be obtained, representing a significant generalization of the regular 2D scanning technique for micro-XRF spectroscopy. The experimental work described in this paper has been carried out at the ESRF ID18F microfluorescence end-station and at HASYLAB Beam Line L.

Novel application of an in situ radiotracer method for the study of the formation of surface adlayers in the course of Cr(VI) reduction on a gold electrode

The electroreduction of Cr(VI) species to Cr(III) at noble metal electrodes in acidic solutions takes place via a ce (chemical-electron-transfer) mechanism where the heterogeneous chemical step which precedes the charge transfer involves anions (e.g. HSO 4 /SO 2 4 ) leading to the formation of intermediate surface complexes. In this paper, we report some new findings obtained by in situ radiotracer and voltammetric studies of the simultaneous adsorption of anions (HSO - 4 /SO 2 4 labeled with 35 S. Cl - labeled with 36 Cl) and Cr-containing species (labeled with 51 Cr) on a gold electrode in the course of dichromate reduction in 1 mol dm 3 HClO 4 supporting electrolyte. Moreover, special attention is paid to present a detection and calculation method elaborated for the quantitative evaluation of the surface excess of radiolabeled Cr species via measurement of the intensity of low energy X-rays (E= 4.90 keV) emitted by 51 Cr. From our experimental results it can be stated that: (i) at E < 1.20 V the electroreduction of Cr(VI) particles presumably proceeds via a ce mechanism to yield a gold surface covered with an intermediate surface adlayer containing Cr(VI) species and added anions (HSO - 4 /SO 2 4 , ClO 4 ); (ii) no data indicating the embedding of Cl ions into the surface film could be detected; (iii) the maximum surface excess of Cr-containing species (Γ max = 1.2 x 10 9 mol cm 2 ) as well as the molar ratio between the Cr species and HSO - 4 /SO 2 4 ions (m 6 at E = 0.8 V) attests that the coverage of the gold surface with intermediate complexes does not exceed one monolayer.

In situ radiotracer and voltammetric study of Zn electrosorption on noble metal electrodes. Part I. A novel method for the evaluation of the surface excess of X-ray emitting species in sub-monolayer coverages

In this work, we report a calculation model elaborated for the evaluation of the surface excess of radiolabeled species via measurement of the intensity of low-energy X-rays, and present some new findings obtained by the radiotracer and voltammetric studies of the Zn2+ electrosorption on polycrystalline gold. The preliminary investigations of the underpotential deposition (upd) of Zn2+ ions demonstrates the extremely high sensitivity of the radiotracer technique based on the detection of the Cu–Kα1,2 X-radiation and assures the reliability of the model calculation procedure. The experimental results reveal that: (i) the upd of Zn2+ ions on polycrystalline gold is measurable even from solution containing 5×10−8 mol dm−3 Zn2+ over the entire potential range where the discharge of protons or water molecules, i.e. hydrogen evolution takes place; (ii) the fractional coverage of the surface obtained under non-equilibrium (transient) conditions in the upd region (from 0.05 to −0.35 V) is no more than 1.5%.

Application of chemometric methods for classification of atmospheric particles based on thin-window electron probe microanalysis data

Abstract Conventional single-particle electron probe microanalysis (EPMA) is widely used for evaluating the sources of atmospheric aerosol. The method is capable of simultaneously detecting the chemical composition and the morphology of each particle. Computer-controlled automatic EPMA allows the analysis of huge numbers of individual particles. Cluster as well as factor analysis are used for the classification of particles based on the obtained data set. However, the method is not able to detect low- Z elements (C, N, O), therefore, e.g. organic particles can only be identified by their typical inorganic content and high background. Using a thin-window X-ray detector, the capabilities of EPMA can be extended to determine low- Z elements. The recently developed quantification method based on Monte Carlo simulations is capable to evaluate elemental concentrations in single microscopic particles, including C, N and O. It was shown that also chemical species can be determined from the obtained concentrations. Hierarchical and non-hierarchical cluster analysis, as well as principal component analysis were applied for the classification of particles based on low- Z EPMA data. A mixture of standard particles as well as atmospheric aerosol samples were used to test the classification methods. Different input data (X-ray intensities or elemental concentrations) and scaling functions were used for the chemometric methods. Cluster and factor analysis appear to be efficient tools for classification of particles based on low- Z EPMA data. As an example, atmospheric ammonium sulphate and organic sulphur were classified in separate groups, which was not possible by conventional EPMA.

Empirical equations for atomic form factor and incoherent scattering functions

An accurate fitting of linear combinations of exponential functions was carried out to Hubbells tabulated data of atomic form factor and incoherent scattering functions of all (Z = 1-100) elements. The validity range of momentum transfer for form factor is varied from q 1 = A -1 to q 2 = 15 A -1 and no limitation for the incoherent scattering function is required. The number of applied parameters is seven except for elements Z = 1-7 for the form factor. In order to achieve low deviations between Hubbells data and fitted values, the range of q was divided into two and three parts. The average deviations are within 1-2%.

Determination of Cr and Mn in aluminium wires and sheets by XRF, NAA and AAS

The fundamental parameters method (FPM) for X-ray fluorescence (XRF) has been applied to determine the MN, Cr and Sr components of aluminium wires and sheets. The results are compared with neutron activation analysis (NAA) and atomic absorption spectrometry (AAS). The FPM does not require standards, however, it requires the spectra of the pure element for the concentration determination.