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

Abstract

AbstractSeveral 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 information on the chemical composition of each individual particle, accurate determination of x‐ray intensities is critical. The energy‐dispersive x‐ray spectra under 1 keV are complex. The L and M lines of heavier elements can strongly overlap with light‐element K lines. Also, the transmission of the detector window and the shape of the bremsstrahlung background should be taken into account. Because of the strong overlap of the characteristic lines, the ‘top‐hat’ filter used for on‐line evaluation of conventional computer‐controlled EPMA spectra cannot be applied for thin‐window EPMA spectra under 1 keV. Therefore, the spectra were processed using a non‐linear least‐squares fitting algorithm (AXIL). The applicability and the limitations of the method were tested by a series of measurements of particulate standards in the size range of 0.3–10 µm, 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 composition of each individual particle was approximated using a new data evaluation method (EP‐PROC) based on an iterative Monte Carlo simulation with combination of successive approximation. The dependence of the K/L intensity ratios of the elements Ca–Zn on the size and composition of the particles was studied extensively by Monte Carlo simulations and measurements of standard 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 concentrations was improved significantly, especially when K, Ca, Ti or Cr were present at high concentrations. Copyright © 2001 John Wiley & Sons, Ltd.