DIRECT DETERMINATION OF CARBON, OXYGEN AND NITROGEN-CONTENT IN COAL USING THE ELECTRON-MICROPROBE

Recent advances in electron microprobe technology together with the development of synthetic crystals have enabled development of techniques for direct light element (C, O, N) analyses of coal macerals. The analytical results are both accurate compared to ASTM methods and highly precise, and provide an opportunity to assess the variation in coal chemistry at the micrometre scale. Our experiments show that analyses using a 10 kV accelerating voltage and 10 nA beam current yield the most reliable data and result in minimum sample damage. High sample counts were obtained for C, O and N using PC2, a nickel-carbon pseudocrystal (d = 9.5 nm), as an analysing crystal. Vitrinite isolated from anthracite rank coal proves the best carbon standard and was more desirable than graphite which has higher porosity, whereas lower rank vitrinite is too heterogeneous to use routinely as a standard. No significant carbon or oxygen X-ray peak to be constant over a range of beam sizes and currents for counting times to 160s, and no correction factor was necessary in determination of oxygen content. Probe-determined carbon contents agree closely with those reported from ASTM analyses and some minor deviations are attributed to heterogeneity of the vitrinite on the micrometre scale. Our results show that the electron microprobe technique can provide accurate compositional data for both minor and major elements in coal without the necessity and inherent problems associated with mechanical separation of macerals.