Evaluation of energy-dispersive x-ray spectra of low-Z elements from electron-probe microanalysis of individual particles

Several applications require the determination of low-Z elements such as C, N and O in individual microparticles. Electron-probe microanalysis (EPMA) using a thin-window energy-dispersive x-ray detector offers the possibility to detect light elements. To achieve at least semi-quantitative informatio n on the chemical composition of each individual particle, accurate determi nation of x-ray intensities is critical. The energy-dispersive x-ray spectr a under 1 keV are complex. The L and M lines of heavier elements can strong ly overlap with light-element K lines. Also, the transmission of the detect or window and the shape of the bremsstrahlung background should be taken in to account. Because of the strong overlap of the characteristic lines, the 'top-hat' filter used for on-line evaluation of conventional computer-contr olled EPMA spectra cannot be applied for thin-window EPMA spectra under 1 k eV. Therefore, the spectra were processed using a non-linear least-squares fitting algorithm (AXIL). The applicability and the limitations of the meth od were tested by a series of measurements of particulate standards in the size range of 0.3-10 mum, made from analytical-reagent grade chemicals. The composition and thickness of the thin window of the detector were modelled using the transmission data given by the manufacturer. The elemental compo sition of each individual particle was approximated using a new data evalua tion method (EP-PROC) based on an iterative Monte Carlo simulation with com bination of successive approximation. The dependence of the K/L intensity r atios of the elements Ca-Zn on the size and composition of the particles wa s studied extensively by Monte Carlo simulations and measurements of standa rd particles. By taking into account the L-line contributions of K, Ca, Ti or Cr in the spectral evaluation, the estimation of the light-element conce ntrations was improved significantly, especially when K, Ca, Ti or Cr were present at high concentrations. Copyright (C) 2001 John Wiley & Sons, Ltd.