SULFUR ISOTOPIC ANALYSIS OF 3-10 MICROMOLE SAMPLES OF SO2 FROM SULFIDES, SULFATES, AND WHOLE ROCKS USING CONVENTIONAL COMBUSTION AND MASS SPECTROMETRIC TECHNIQUES

Abstract

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-line' 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.