[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

Soils or sediments that are the most investigated by

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soils, or sediments – that are the most investigated by XRF – accurate chem- ical information is necessary including major, minor, and trace elements. The major composition of individual microparticles including low atomic number (low- Z ) elements can be quantitatively determined using electron-probe X- ray microanalysis (EPMA) [359, 360]. However, on the basis of the major element composition natural and anthropogenic particles cannot be identified unambiguously (consider e.g., soil-derived and fly-ash particles in atmospheric aerosol). Therefore, the accurate knowledge of the minor and trace composi- tion of individual particles is of high importance for source profiling studies, as they offer more distinctive features for the identification of different parti- cle sources. As X-rays originating from conventional diffraction X-ray tubes or synchrotron sources can be focused to form micrometer-sized beams [361], the examination of microparticles became possible using microbeam XRF [362]. Minor elements can also be detected using micro-PIXE in individual parti- cles, but for achieving fully quantified results, it has to be combined with other methods [363].
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628 S. Kurunczi et al Using photons or charged particles for excitation, the measured X-ray in- tensities of elements depend non-linearly on the concentrations, and they are not independent from each other. For this reason, no explicit mathematical ex- pression can be given for calculating concentrations from measured intensities. The problem has to be solved in an iterative way. The conventional quantifi- cation methods [364] are not suitable for single-particle analysis, because they are not flexible enough for various experimental and sample conditions, such as the analysis of microparticles having irregular shapes and heterogeneous compositions. The most widely used analysis technique for single particle analysis is EPMA and the quantitation methods for such application were developed earlier and will be briefly summarized here. The quantification methods in EPMA, such as classical ZAF and φ ( ρz )-based procedures, aim to correct for matrix and geometric effects, observed in particulate matter, that are even the most pronounced for light-element X-rays. The most reliable and widely used quantification method for microparticles is the so-called particle-ZAF algorithm developed by Armstrong and Buseck [365]. However, the particle- ZAF method based on the use of bulk standards introduces large errors for light element analysis (that are major components of environmental samples). This is mostly because of the large absorption correction needed for low- Z element quantification and the difference between the bulk standard and single particle in their behavior under electron bombardment. In addition, when the average atomic number of the substrate significantly differs from that of the particle, the side-scattering correction of the φ ( ρz ) function is reasonable only if the electron excitation volume is smaller than the particle itself. It has been
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