EDS quantification of light elements using osmium surface coating

This study demonstrates the validity of a thin osmium coating for quantitative energy-dispersive spectroscopic (EDS) analysis, particularly for light elements such as O (and potentially C and N) in natural/synthetic minerals. An osmium coating prepared by chemical vapor deposition provides an extremely thin and uniform layer whose thickness can be controlled simply by coating time. Because of the high reproducibility and reliability of the osmium coating process, users have no difficulty in evaluating the actual coating thickness, which enables strict and precise absorption corrections (for the coating layer), even for low-energy characteristic X-rays, which are susceptible to attenuation by the coating layer itself. Our results show that oxygen concentrations in silicate and oxide minerals can be quantified correctly when using the osmium coating, whereas quantification using a carbon coating afforded values that were a few wt% lower than stoichiometry, probably due to the uncertainty of the actual coating thickness (i.e., the absorption correction was incorrect). The ability to accurately quantify oxygen may stimulate new analytical applications, such as the estimation of Fe²⁺/Fe³⁺ concentrations and water content in minerals. Furthermore, the Os-coated samples prepared for EDS analysis are also suitable for electron back-scattered diffraction (EBSD) analysis without re-polishing and re-coating, which are usually routine but time-consuming tasks in the case of carbon-coated samples.