Since biogenic and abiotic marine particles with calcite or aragonite as their main matrix, which are important in the context of Global Climate Change, can be chemically and morphologically heterogeneous, and the average composition and the average aerodynamic diameter do not describe well the population of the particles, the application of micro-analytical methods should be useful. Electron probe X-ray microanalysis (EPMA) is capable of simultaneously detecting the chemical composition and morphology of a microscopic volume, such as a single environmental particle. In automated EMPA, the electron beam automatically scans over a loaded filter while the backscattered electron signal is monitored. At the pixels where this signal exceeds a certain threshold, a particle can be assumed to be present. In an automated way, this particle will be located, sized and analysed in a few seconds. In this way, data for thousands of particles are accumulated in a short time. Application of subsequent statistical processing techniques, like cluster analysis, will classify the particles in specific particle types. In this way the relative abundance ofcalcite and other particles can be measured representatively. For each particle type, the average composition, shape and size are registered. The methodology for this has been developed earlier for high-Z elements.
In recent studies at the University of Antwerp, it was found that excitation interactions between electrons and the matrix atoms and the geometric and matrix effects on electron-induced X-ray signals for light elements in individual microparticles could be described by Monte Carlo simulation. By the application of a quantification method, which employs this Monte Carlo simulation combined with successive approximations, at least semi-quantitative specification of the chemical compositions can be done. This EPMA technique especially allows determining the concentration of low-Z elements such as carbon, nitrogen and oxygen, as well as the heavier elements which are observed using conventional energy-dispersive EPMA, including e.g. Sr. Conventional energy-dispersive X-ray detectors are not suitable for low-Z element analysis mainly because their Be window, used for protecting the semiconductor detector surface from contamination, absorbs low-energy X-rays and thus hinders the detection of the low-Zelement X- rays. The determination of low-Z elements in individual environmental particles allows to improve the applicability of the single particle analysis; many important marine particle types, including calcite and carbonaceous particles, contain low-Z elements, which have not been characterised using the conventional EPMA. It is obvious that e.g. the carbon content might give clues about the biogenic origin of a single micrometer-size calcite particle.
However, it is also of prime importance to have an analytical tool to distinguish chemical species in the surface region, from those of core region, in individual particles of micrometer size. In the proposed grant period, we will also study and apply a new analytical methodology, which can characterize the surface layers (including organic layers) on individual CaCO3 particles, using "variable-energy" EMPA. When one uses X-ray excitation in individual particles with electron beams of variable energies (e.g. from soft beams of 5 keV to hard beams of 25 keV), the probing depth is varied, and one obtains information about the variability of the composition with depth, i.e. also about surface layers. For quantification, this approach requires again complicated simulations of the electron interactions using Monte Carlo calculations for every particle, but this methodology has recently been developed. Hitherto, surface layers on marine CaCO3 particles have not been studied directly, because of the technical problems involved. Yet this information will be very valuable. One may expect that the residual organic layer on plankton skelet