A set of standard samples consisting of low alloy steels, tool steels and a graphite standard was prepared carefully for measurements by polishing (down to 0.3μm alumina powder) and cleaning of the surfaces. Photographs from optical and scanning electron microscopical images were taken in order to provide information about the microstructure. The measured carbon fluorescent intensities are interpreted in view of this information as well as compared to calculated data based on fundamental parameter models. Thereby excitation by iron and carbon photoelectrons as well as secondary enhancement effects by iron K and L lines and other matrix elements were taken into account.

Results suggest that the count rates are not only a function of the total carbon content, but also of graphite segregations and to some extent of the specimen preparation procedure. Graphite particles are easily washed out and the little remaining holes may remain partially covered with carbon or filled with polishing residuals, which leads to reduced count rates.