Sulfur isotopic analysis of 3–10 micromole samples of SO2 from sulfides, sulfates, and whole rocks using conventional combustion and mass spectrometric techniques

Both the CuO [Grinenko, V.A., 1962. The preparation of sulphur dioxide for isotope analysis. Zn. Neorgan. Khim. 7, 2478–2483; Fritz, P., Drimmie, R.J., Nowicki, V.K., 1974. Preparation of sulfur dioxide for mass spectrometer analyses by combustion of sulfides with copper oxide. Analytical Chemistry, 46, 164–166.] and V2O5–SiO2 [Yanagisawa, F., Sakai, H., 1983. Thermal decomposition of barium sulfate–vanadium pentaoxide–silica glass mixtures for preparation of sulfur dioxide in sulfur isotope ratio measurements. Analytical Chemistry, 55, 985–987; Ueda, A., Krouse, H.R., 1986. Direct conversion of sulphide and sulphate minerals to SO2 for isotope analyses. Geochemical Journal, 20, 209–212.] methods of `off-lineʹ SO2 production from sulfide and sulfate minerals for sulfur isotope measurements are shown to be suitable for samples containing as little as 3 μmoles of S. δ34S values relative to a laboratory standard fall within ±0.1‰ for samples larger than 10 μmoles, and ±0.4‰ (maximum observed value) for smaller samples. Preheating of reagents, particularly native Cu that is used to reduce SO3 to SO2, is necessary for sulfide samples that produce less than ∼10 μmoles SO2. The pretreatment reduces blank effects and potential contaminant interferences. The CuO–Cu method is also suitable for combustion of whole rock samples containing as little as 150 ppm S. δ34S reproducibilities are not as good as for mineral separates, but are within ±0.5‰ for samples that contain between 170 and 350 ppm S.