Correction factors for the effect of shape and thickness of SEM-EDS microanalysis of asbestos bundles and fibres by Monte Carlo simulation

SEM-EDS quantitative microanalysis of asbestos mineral fibres still represents a complex analytical issue because of the variable fibre shape and small thickness (<5 µm) compared with the penetration of the incident electron beam. Size and shape of micro- and sub-micrometric particles may cause large errors in chemical quantification due to particle effects on the generation and measurement of X-rays intensity from the sample. These effects are related to the elastic scattering of electrons in the finite size (mass) of the fibre, with the scattering being strongly influenced by the average atomic number. For a given mean atomic number, the thickness of the particle is the main factor affecting X-rays intensity, with a component related to the particle shape that biases the contributions of absorption and fluorescence to the correction routine. To overcome these issues empirical methods were developed, however they are cumbersome and need characterized standards for thickness, geometry and composition. Here we present the results of Monte Carlo investigation for the thickness and shape effects on SEM-EDS and microprobe analysis of asbestos bundles. Crocidolite, amosite, tremolite-asbestos, chrysotile, anthophyllite-asbestos and actinolite-asbestos were simulated and correction factors for X-ray microanalysis were proposed. Monte Carlo simulation was used to investigate electron transport, X-ray generation and detection in asbestos bundles of variable thickness lying on a pure carbon holder. We report the results obtained on 100 µm long bundles of fibres of square section and thicknesses from to 0.1 µm to 10 µm. Realistic experimental conditions, such as sample geometry, SEM set-up and detector physics were taken into account. An electron probe of 40 nm in diameter was simulated, focussed in parallel illumination onto the surface of the bundle of fibres, in a mid position with respect to the edges. The modelled EDS detector has a resolution of 130 eV measured at Mn Kα, an elevation angle of 40°, and an azimuthal angle of 0°. The influence of thickness and shape on the simulated spectrum was investigated for electron beam energies of 5, 15 and 25 keV. A strong influence of the asbestos bundles thickness was observed. In general, the X-ray intensities as a function of bundle thickness showed a considerable reduction below about 0.5 µm at 5 keV, 2 µm at 15 keV, and 5 µm at 25 keV, with a non-linear dependence. Specific correction parameters, k-ratio, for the asbestos bundle thickness effect are here presented.