Direct Major (C, O, N) and Minor Element Analysis of Macerals by Electron Microprobe

Abstract

Recent advances in electron microprobe technology together with the development of synthetic crystals has enabled development of techniques for direct light element (C, O, N) analysis of coal macerals. The analytical results are both accurate compared to ASTM methods and highly precise, providing an opportunity to assess variation in maceral chemistry without the neccessity of physical concentration. Utilizing a nickel-carbon pseudocrystal (d = 9.5 nm) as an analyzing crystal, a 10 kV accelerating voltage and a 10 nA beam current yields the most reliable data and results in minimum sample damage. We used vitrinite isolated from an anthracite as the carbon standard, BN as the nitrogen standard, MgC0₃ as the oxygen standard, and BaS0₄ as the sulphur standard. Probe-determined carbon and oxygen contents agree closely with those reported from ASTM analyses; some minor deviations are attributed to heterogeneity of the vitrinite on the micrometer scale. The microprobe-determined nitrogen content of our samples suggests that nitrogen is distributed heterogeneously even within pure vitrinite. We cite examples of microprobe-determined chemical analysis of macerals contained within high-, medium-, and low-volatile bituminous coals to illustrate the utility of the method. These results demonstrate that the electron microprobe provides a rapid, accurate, and relatively inexpensive method for obtaining compositional data for both minor and major elements in coal without the necessity of and inherent problems associated with mechanical separation of macerals.