Light element analysis of individual microparticles using thin-window EPMA

The determination of the concentration of light elements, such as carbon, n itrogen and oxygen, in e.g, atmospheric aerosol particles is important to s tudy the chemical behaviour of atmospheric pollution. The knowledge of low- Z element concentrations gives us information on the speciation of nutrient s (species having nutritional value for plants) and toxic heavy metals in t he particles. The capability of the conventional energy-dispersive EPMA is strongly limited for the analysis of low-Z elements, mainly because the Be window in the EDX detector hinders the detection of characteristic X-rays o f light elements such as C, N, O and Na. WDS is suitable for analysis of li ght elements, but the measurement of beam sensitive microparticles requires the minimisation of the beam current and the measurement time. A semi-quan titative analytical method based on EPMA using an ultra-thin window EDX det ector was developed. It was found that the matrix and geometric effects tha t ale important for low-energy X-rays can be reliably evaluated by Monte Ca rlo calculations. Therefore, the quantification part of the method contains reverse Monte Carlo calculation done by iterative simulations, The method was standardised and tested by measurements on single particles with known chemical compositions. Beam-sensitive particles such as ammonium-sulphate a nd ammonium-nitrate were analysed using a liquid nitrogen cooled sample sta ge. The shape and size of the particles, which are important for the simula tions, were determined using a high-magnification secondary electron image. Individual marine aerosol particles collected over the North Sea by a nine -stage Berner cascade impactor were analysed using this new method. Prelimi nary results on five samples and 4500 particles show that the method can be used to study the modification of sea-salt particles in the troposphere.