FEASIBILITY OF COMBINED WAVELENGTH ENERGY-DISPERSIVE COMPUTER-CONTROLLED SCANNING ELECTRON-MICROSCOPY FOR DETERMINING TRACE-METAL DISTRIBUTION

The utility of wavelength-dispersive and energy-dispersive electron probe microanalysis (WD-ED EPMA) for quantifying the distribution of seven trace metals (Cr, Ni, As, Se, Cd, Hg, and Pb) among coal minerals and ash particles was investigated. The technique is limited to analyzing particles greater than about 7 mum in diameter because of the relatively intense beam conditions (25 kV, 100 nA) required to attain reasonable detection sensitivity; statistical limits of detection are generally <300 ppm for a WD spectrometer (xenon-filled detector) and 100s counting time. The application of WD EPMA to a glass standard, compositionally analogous to coal ash, indicates that Cr, Ni, As, and CD can be determined routinely at concentrations ranging from about 200-500 ppm with reasonable accuracy and precision. Lead, with a consensus value of 418 ppm, was detected in the glass, but could not be quantified accurately with the general analysis conditions employed. The metals Ni and Se were detected and quantified in pyrite during a cursory application of automated WD-ED EPMA to minerals in the Illinois No. 6 and Pittsburgh No. 8 Argonne premium coals. WD-ED EPMA and computer-controlled scanning electron microscopy can be combined to determine trace metal distribution among the inorganic constituents of coal and ash as a function of particle size.