Standardless quantitative EPMA of microparticles with irregular shapes
Résumé
Quantitative electron probe microanalysis (EPMA) is usually performed using analysis based on standards in which matrix effects are corrected by conventional ZAF or φ(ρz) models. However, standard based models are developed for bulk specimens with plane surfaces and involve many assumptions about the thickness and morphology of the sample. These models are no longer accurate when quantifying the composition of microparticles. In fact, microparticles can have irregular shapes and high surface roughness. Thus, the generation and the emission of X-rays inside the particle can be significantly different than in a bulk polished sample. Moreover, analysis based on bulk standards does not take into account the loss of X-ray signal caused by the transmission of electrons through the microparticle.
Quantitative standardless procedures where X-ray intensities are calculated and used as virtual standards enable to overcome these limitations. Standardless models based on Monte Carlo (MC) simulation were shown to be the most viable approaches to deal with nonconventional samples such as porous media, thin films and microparticles. Accurate X-ray intensity calculation can be obtained after including the geometry of the particle in the simulation. However, in some cases, particles can have very complicated shapes that would take a tremendous time and effort to describe accurately. Moreover, particles such as those used in catalysts or in the nuclear fuels, often exhibit surface roughness which adds more complexities in the construction of their geometry. If not taken into account, rough surfaces can be responsible for a loss in the measured X-ray intensities and might lead to multiple uncertainties in the quantitative results. In order to cope with this difficulties, peak-to-background (P/B) correction methods can be used to correct for the intensity loss due to surface roughness. Standardless approaches based on P/B method were reported in a number of studies and showed that they are suitable for the quantification of micro-sized particles.