Spatial resolution of electron probe X-ray microanalysis on sections of organic (biological) material

A locally enhanced element concentration influences the result of an X-ray microanalysis at a neighbouring position. This influence was investigated for the first time systematically in organic (biological) material using sections of epoxy resin (thickness 0.5–2.5 μm) containing a layer of pure gold. Wavelength and energy dispersive spectrometers were applied to analyse the X-rays generated by 15–35 keV electrons. Characteristic X-rays could be detected up to distances of several μm from the gold layer. For example, for a 2.4 μm thick section and 35 keV electrons the measured apparent gold concentration was above 0.1% (weight% per dry mass) at a distance of 10 μm. Thus, the lateral resolution may be not better than a multiple of the section thickness. The apparent gold concentration at a given distance is proportional to the specimen thickness and increases with increasing electron energy. Monte Carlo simulations confirm the experimental results. The influence of a local enrichment depends on the particular specimen properties (e.g. thickness, density, mean atomic number), the electron energy, and the geometry of the detector with respect to the specimen.