Determination of Sulfur in Fossil Fuels by Isotope Dilution Thermal Ionization Mass Spectrometry

Abstract

Total sulfur has been measured in 13 petroleum and 14 coal Standard Reference Materials (SRMs) by isotope dilution thermal ionization mass spectrometry. These materials are suitable as primary and quality control standards for determining the sulfur content of oils and coals by X-ray fluorescence and high-temperature combustion instrumentation. Samples were spiked with enriched stable S-34 and combusted in sealed Carius tubes using nitric and hydrochloric acids, The oxidized sulfur was reduced to H2S, precipitated as As2S3, and then dissolved in an ammoniacal solution of As2O3. A portion of this solution, equivalent to 1.5 mu g of sulfur, was added to a single Re filament coated with silica gel. The amount of sulfur in the samples was determined from the S-32/S-34 ratio by measuring the (AsS+)-As-75-S-32/(AsS+)-As-75-S-34 molecular ions in a Faraday detector. A total of 158 sulfur procedural blanks covering a 10-year period show an approximate log-normal distribution with a grand mean of 0.26 mu g of sulfur. This is a negligible correction for most of the data reported here. The total uncertainty (95% confidence interval) for homogeneous materials such as oils is about 0.5%, and for less homogeneous materials such as coals is 1-4%.