QUANTITATIVE ENERGY-DISPERSIVE ELECTRON PROBE X-RAY MICROANALYSIS OF INDIVIDUAL PARTICLES 287 Chul-Un Ro Department of Chemistry, Inha University 253, Yonghyun-dong, Nam-gu, Incheon 402-751, Korea ABSTRACT An electron probe X-ray microanalysis (EPMA) technique using an energy-dispersive X-ray detector with an ultra-thin window, designated low-z particle EPMA, has been developed. The low-z particle EPMA allows the quantitative determination of concentrations of low-z elements such as C, N, and O, as well as higher-z elements that can be analyzed by conventional energydispersive EPMA. The quantitative determination of low-z elements (using full Monte Carlo simulations, from the electron impact to the X-ray detection) in individual environmental particles has improved the applicability of single-particle analysis, especially in atmospheric environmental aerosol research; many environmentally important atmospheric particles, e.g. sulfates, nitrates, ammonium, and carbonaceous particles, contain low-z elements. The low-z particle EPMA was applied to characterize loess soil particle samples of which the chemical compositions are well defined by the use of various bulk analytical methods. Chemical compositions of the loess samples obtained from the low-z particle EPMA turn out to be close to those from bulk analyses. In addition, it is demonstrated that the technique can also be used to assess the heterogeneity of individual particles. INTRODUCTION In the last decade, both the technological background and the data evaluation methods, i.e. X-ray spectra analysis and quantitative determination of sample composition, have been significantly improved for electron probe microanalysis. One of the most essential breakthroughs in this field was the appearance of commercial silicon-based spectrometers equipped with thin polymer windows which improve the transmission of low-energy X-rays, as well as newly designed highresolution energy dispersive detectors such as microcalorimeters. Another important improvement can be found in the recently developed evaluation models for quantitative analysis, which can handle various types of target samples. The ultimate goal of this scientific effort has been to increase the X-ray detection efficiency, to broaden the energy range of the X-rays to be analyzed, to decrease the irradiated and excited volume in the specimen and to obtain maximum information about sample composition and structure with application of adequate quantitative model for fast calculation. Electron probe X-ray microanalysis (EPMA) equipped with an energy-dispersive X-ray (EDX) detector can be used to simultaneously detect the morphology and the constitution elements (within a microscopic size volume) of a sample. This advantageous analytical capability of the EPMA has been used successfully in atmospheric aerosol research. One of the principal aspects that limit the efficient application of EDX detectors lies in the fact that the detection of low-z elements, such as C, N, and O, is hindered by the absorption of characteristic X-rays by the
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beryllium window of the detectors. It is well known that this technical difficulty can be neglected by using thin polymer windows instead; their thickness is approximately 200 nm, they are commercially available, and they have been introduced in routine analysis for several years. The morphology and the composition of environmental microparticles are heterogeneous, therefore detailed quantitative information about both major and trace constituent elements is required. The characteristic X-ray lines of the main components, which are mostly low-z elements (Z<9), undergo extremely strong attenuation while propagating through the particle volume, therefore the estimation of the matrix effect is important. Quantitative determination of low-z elements is a necessary development for further research of individual particles, firstly because these elements (C, N, O) are abundantly present e.g. in atmospheric particles, and secondly because quantitative information is necessary for speciation of individual microscopic particles; indeed, many environmental particles contain low-z elements in the form of nitrates, sulfates, oxides or mixtures including a carbon matrix. 288 Furthermore, beam variation EPMA has also been developed, which permits one to get information on the depth heterogeneity with respect to chemical composition, of single particles. Some internally mixed individual aerosol particles are created by heterogeneous atmospheric reactions in air, for example, CaSO 4 particles are produced by the reaction between CaCO 3 particles and gaseous SOx. If the reaction is not complete, then CaSO 4 species would be expected to exist more on the surface of the particles and CaCO 3 more in the core. By varying the excitation voltage, we could obtain useful information on the heterogeneity of e.g., artificially generated heterogeneous CaSO 4 -CaCO 3 individual particles. In this work, we wish to introduce a newly developed EPMA technique, named low-z particle EPMA, which allows the quantitative determination of low-z elements and demonstrate that the technique can also be used to assess the heterogeneity of individual particles. LOW-Z PARTICLE EPMA Cautious sampling of airborne particles is the first step in the application of low-z particle EPMA, which aims for the quantitative determination of elemental concentrations, both for low- Z and higher-z elements, of individual aerosol particles. Low-Z particle EPMA measurements are carried out on scanning electron microscope equipped with an ultra-thin window EDX detector. Since EPMA measurements on individual particles are carried out automatically in point analysis mode and the localization of the particles is based on inverse backscattered electron contrast, a Ag foil is one of the best choices for collection of particles. To achieve optimal experimental conditions such as a low background level in the spectra and high sensitivity for low-z element analysis, a 10 kv accelerating voltage is chosen. The beam current is 1.0 na for all the measurements. In order to obtain statistically enough counts in the X-ray spectra and to minimise the beam damage effect on sensitive particles, a typical measuring time of 10 s is used. The cold stage of the electron microprobe allows the analysis of particulate samples at liquid nitrogen temperature (around 193 C), which minimises contamination and reduce beam damage to the samples as well. A more detailed discussion on the measurement conditions can be found elsewhere [1,2]. Morphological parameters such as diameter and shape factor are calculated by an image processing routine. These estimated geometrical data are set as input parameters for the
quantification procedure. The net X-ray intensities for the elements are obtained by nonlinear least squares fitting using the AXIL program [3] and they are used as input data to determine the elemental concentrations by utilizing Monte Carlo calculation. A number of well-developed and rigorously tested quantification procedures are available in EPMA [e.g. ZAF and φ(ρz) methods], especially for the analysis of bulk samples. However, these procedures are limited for low-z element analysis of individual atmospheric microparticles (in view of the small size and variable shape of the particles and the important matrix effect for low-z elements); therefore a quantification method, which employs a Monte Carlo simulation in combination with successive approximations, has been developed [4]. It is based on a modified version of the single scattering CASINO Monte Carlo program [5], which is designed to assess, for low-energy beam interaction, the generated X-ray and electron signals. The modified version of the CASINO program allows the simulation of electron trajectories in spherical, hemispherical and hexahedral particles located on a flat substrate [1,4]. The simulation procedure determines also the characteristic and continuum X-ray flux emitted from the substrate material and the influence of the substrate material on the energy distribution of the exciting electrons. The algorithm to iteratively determine the elemental concentrations from measured X-ray intensities has been implemented using a MS Visual C++ compiler. The quantification procedure provides results accurate within 12 % relative deviations between the calculated and nominal elemental concentrations when the method is applied to various types of standard particles such as NaCl, Al 2 O 3, CaSO 4. 2H 2 O, Fe 2 O 3, CaCO 3, and KNO 3, except for C and K where the characteristic X- rays overlap with those from the Ag substrate [6]. A more detailed description of the algorithm for the successive approximations can be found elsewhere [7]. 289 The determination of chemical species in atmospheric environmental particles was done in a way that fully utilized the information contained in their X-ray data. The chemical composition of any single particle is never exactly the same as that of others. It is also rather rare to see particles composed of only one pure chemical species. Also, particles constituting of two or more chemical species have different compositions. The low-z particle EPMA can provide quantitative information on the chemical composition, and particles can be classified based on their chemical species. Using the low-z particle EPMA method, molar concentrations of major chemical species in individual environmental particles can be determined. For example, the molar concentrations of ammonium sulfate and ammonium nitrate in a single particle can be measured in particles internally mixed with ammonium sulfate and ammonium nitrate species. When particles are composed of several chemical species so that the number of equations is smaller than the number of chemical species to be determined, the quantitative analysis of each chemical species can be ambiguous; however, many particles are composed of one or two major chemical species and thus this technique can provide direct observation of atmospheric chemistry for airborne particles in more detail. The analytical procedure for determining chemical species, and the way to perform classification, are described in more detail elsewhere [8,9]. This classification procedure takes a substantial amount of time if done manually, since we analyze several thousands of particle data for each environmental sample. Thus, an expert system that can determine chemical species from the elemental concentration data has been developed. The expert system is implemented by macro programming that is done by using MS Visual Basic interpreter available in MS Excel software. The expert system runs on IBM-PC compatible computers and uses input and output files in the format of MS Excel files. The inputs to this
expert system are the concentration data for individual particles that are given as MS Excel files. The concentration data are the outputs of our iterative Monte Carlo calculation program that obtains atomic concentrations of particles from X-ray spectral data. The outputs of the expert system are the chemical species and formula concentrations of each particle, particle groups with similar chemical compositions, and distributions of particle groups in the different size range. Its feasibility is confirmed by applying the expert system to data for various types of standard particles and a real atmospheric aerosol sample. By applying the expert system, the time necessary for chemical speciation becomes significantly shorter, and detailed information on particle data can be saved and extracted when more information is needed for further analysis. A detailed description of the expert system that tries to mimic the logic used by experts can be found elsewhere [9]. 290 An exemplar secondary electron image obtained form an Asian Dust sample collected in Korea is shown in Figure 1, where chemical species of individual particles are also denoted. The final analytical results obtained are the morphological and chemical information on individual particles. NaNO 3 Mg(NO 3,SO 4,Cl) Ca(CO 3,NO 3 )/Aluminosilicates Ca(CO 3,NO 3,SO 4 )/FeOx Figure 1. An exemplar secondary electron image on which chemical species of individual Asian Dust particles are denoted. (Ca,Mg)(CO 3,NO 3 )/Aluminosilicates Aluminosilicates NaNO 3 Cu Ca(NO 3 ) 2 Eventually, the low-z particle EPMA technique can provide size-segregated relative abundances of chemical species observed in aerosol samples, which is valuable for the characterization of atmospheric environmental aerosol samples. However, information on size-segregated mass fractions of chemical species using a single particle analysis could be useful considering that most aerosol analyses are done by the use of bulk analyses that provide data for mass fractions of chemical elements, ions, and/or organic species. By obtaining mass fractions of chemical species from the low-z particle EPMA analysis, bulk and single particle analyses will provide complementary information on chemical compositions of aerosol samples. We analyzed two certified reference materials (CRM) for Chinese loess soil (called CJ-1 and CJ-2) using the low-z
particle EPMA technique. Elemental concentrations of samples CJ-1 and CJ-2 are reported to have been defined using various bulk analytical techniques such as ICP/AES, AAS, XRF, NAA, and PIXE [10]. In Table 1, are shown relative abundances and weight concentrations of chemical species obtained from a single particle analysis of 1,000 particles for each CRM sample. Since we know the chemical compositions of individual particles, the densities of the individual particles can be estimated. With the combination of information on chemical compositions, densities, and sizes of individual particles, mass fractions of chemical species observed in two CRM samples can be obtained. In Table 2, elemental concentrations deduced from our analysis are listed with those from bulk analyses. Those elemental concentrations obtained from single particle and bulk analyses are not much different, in spite of totally different approaches employed in the two methods. It is necessary to identify the uncertainty involved in the low-z particle EPMA analysis. 291 Table 1. Relative abundances and weight concentrations of chemical species obtained from low-z particle EPMA analysis of 1,000 particles for each CRM sample. % in number wt % chemical chemical CJ-1 (%) CJ-2 (%) species species CJ-1 (%) CJ-2 (%) carbon-rich 13.75 1.97 carbonaceous 12.34 4.14- organic 6.07 1.45 SiO2 55.65 53.18 SiO2 17.35 10.14 Al2O3 12.12 19.03 SiO2/C 1.24 - CaCO3 11.39 7.90 AlSi 36.80 56.42 Fe2O3 1.55 5.36 AlSi/C 1.49 4.87 K2O 0.76 1.64 AlSi/CaCO3 2.35 1.24 MgO 2.31 3.68 AlSi/misc. 1.12 5.80 MgCO3 1.13 1.14 CaCO3 11.65 9.83 others 2.74 3.92 CaCO3/misc. 5.20 4.66 others 2.97 3.62 sum 100.0 100.0 sum 100.0 100.0 Table 2. Elemental concentrations deduced from low-z particle EPMA and bulk analyses. CJ-1 (China Loess) (wt %) CJ-2 (Simulated Asian Mineral Dust) (wt %) Element low-z particle low-z particle bulk bulk analysis Element EPMA EPMA analysis Na 0.51 1.33±0.06 Na 0.51 1.38±0.08 Mg 2.04 1.57±0.06 Mg 2.79 1.57±0.06 Al 7.39 6.01±0.17 Al 10.61 5.88±0.16 Si 29.97 27.7±0.6 Si 26.20 28.0±0.4 K 0.78 1.94±0.10 K 1.44 1.70±0.08 Ca 5.93 5.83±0.23 Ca 3.86 5.33±0.22 Ti 0.01 (0.36) Ti 0.22 (0.46) Fe 1.25 2.94±0.09 Fe 3.95 3.02±0.12
CHEMICAL SPECIATION OF SINGLE PARTICLES, HETEROGENEITY 292 Since the technique can provide the quantitative elemental concentrations of individual particles, the determination of chemical species in single particles is possible. This includes pure particles containing only one major chemical species, and internally mixed particles containing two or more chemical species. Furthermore, in optimal cases even molar concentrations of the different chemical species in internally mixed particles can be determined. Because many atmospheric particles contain only one or two chemical species, low-z particle EPMA could provide more details about airborne particles of environmental interest. This capability of the low-z particle EPMA has been proven useful for the characterization of various types of environmental aerosol samples [11-15]. In addition, it is of primary importance to have an analytical tool to distinguish chemical species in the surface region from that of the core region in individual microparticles, because the analysis would allow the direct and more conclusive investigation of the nature of atmospheric reactions, which some airborne particles may experience. For example, sea salt can react with NOx to produce sodium nitrate particles in the air. Also, the atmospheric reaction between soil particles and SOx receives considerable attention in the atmospheric environment society. And thus, if gaseous or aqueous NOx or SOx species react with sea-salt or dust particles in air and if the atmospheric reactions are not completely finished, then it is expected that the product of the reactions would exist in the surface layer and the original chemical species in the core region. Therefore, the existence of different atmospheric reactions would be directly proven if we could characterize both regions in individual particles. However, since the analysis volume of individual microparticles is quite small (pg-range in mass), quantitative analysis of surface and core regions in individual particles has been a real challenge. Recently, a methodology based on EDX-EPMA was developed that can analyze chemical species both in surface and core regions of individual particles [16]. The idea was to investigate heterogeneous individual particles with different primary electron beam energies, i.e. X-ray photons obtained with different primary electron beam energies carry information on the chemical compositions for different regions in the particles, mainly because of the different excitation volumes according to the energies of primary electron beam. The excitation volume of the elements is decreased with the decrease of the primary electron beam energy. Artificial heterogeneous CaCO 3 -CaSO 4 particles were synthesized, i.e. particles with CaSO 4 in the surface region and CaCO 3 in the core. X-ray spectra were obtained at 5, 10, 15, and 20 kv electron-accelerating voltages for a heterogeneous CaCO 3 -CaSO 4 spherical particle of 1.5 µm diameter. The measured characteristic X-ray intensities for the elements in the particle were observed to vary differently with the variation of primary electron beam energies. From the observation of different (characteristic X-ray intensity variation according to the variation of electron beam energies) trends for the elements, these X-ray spectra were observed to contain information on chemical species and heterogeneity of the particle. When simulated spectra calculated by our modified Monte Carlo program were produced, the similarity between the simulated and experimental spectra is remarkably obvious. The Monte Carlo calculation almost perfectly simulates the X-ray intensity variations for the elements according to the variation of the primary electron beam energies.
293 By the application of the Monte Carlo calculation, even the thickness of CaSO 4 surface region of the artificially generated CaCO 3 -CaSO 4 particles can be determined. In Figure 2, ratios of simulated-to-measured intensities with the variation of the CaSO 4 surface thickness for a spherical CaCO 3 -CaSO 4 particle of 1.5 µm diameter are shown for a 15-kV primary electron beam energy. For oxygen, the ratios of simulated-to-measured intensities are relatively constant with the variation of the CaSO 4 thickness, mainly because of their small compositional differences between the two chemical species. However, the ratios for carbon and sulfur between the simulated and measured intensities are strongly dependent on the thickness of the surface CaSO 4 region. Furthermore, the ratios for sulfur decrease as the thickness of CaSO 4 region decreases, whereas the ratio for carbon increases as the thickness of CaSO 4 region decreases. For the heterogeneous CaCO 3 -CaSO 4 particles, the sulfur species is in the surface region and carbon is in the core region. Therefore, if the assumed CaSO 4 surface thickness for the Monte Carlo calculation is thicker than the real one, then the calculated intensities are larger than the measured ones for sulfur, whereas they are smaller for carbon. From the result in Figure 2, the good match between the simulated and measured data is in the range of 160-200 nm thickness of the surface region. SUMMARY In this work, we introduced a newly developed single particle analytical technique, named low-z particle EPMA. The low-z particle EPMA allows the quantitative determination of concentrations of low-z elements such as C, N, and O. The quantitative determination of low-z elements (using full Monte Carlo simulations, from the electron impact to the X-ray detection) in individual environmental particles has improved the applicability of single-particle analysis, especially in atmospheric environmental aerosol research; many environmentally important atmospheric particles, e.g. sulfates, nitrates, ammonium, and carbonaceous particles, contain low-z elements. In addition, an expert system that can perform chemical speciation from the elemental composition data obtained by the low-z particle EPMA has been developed. Finally, chemical compositions of loess samples obtained from the low-z particle EPMA were compared with those from bulk analyses. In addition, we introduced a methodology based on EDX-EPMA that can analyze chemical species both in surface and core regions of individual particles. The idea was to investigate heterogeneous individual particles with different primary electron beam energies, i.e. X-ray photons obtained with different primary electron beam energies carry information on the chemical compositions for different regions in the particles, mainly because of the different excitation volumes according to the energies of primary electron beam. When this technique was applied to the analysis of artificially generated heterogeneous CaSO 4 -CaCO 3 individual particles, it was demonstrated that even the thickness of surface layer can be estimated.
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