(Comparison of sulphur isotope ratio measurements by various techniques, and the delta ³⁴S values of some sulphur standards.

Three preparation techniques of SO₂ for the isotopic ratio measurements of geological samples are described: 1) Cu₂O method: Ag₂S is combusted by Cu₂O at 800oC under vacuum conditions. Ag₂S reacts with Cu₂O: Ag₂S + 2Cu₂O rt arrow SO₂ + 2(Ag,Cu), 2) Thermal decomposition of BaSO₄: a mixture of BaSO₄, V₂O₅ and SiO₂ (1:10:10 in weight) is heated 900oC under vacuum conditions with metal copper wire as a reductant to convert SO₃-SO₂. V₂O₅ acts as a flux and lowers the decomposition T of BaSO₄: BaSO₄ + 6V₂O₅ (melt) rt arrow BaV₁₂O₃₀ + SO₂ + O₂, 3) Kiba reagent method: rock powders and other geological samples are decomposed at 280oC under vacuum by Kiba reagent (phosphoric acid containing Sn²⁺). Sulphate, sulphide and carbonate in the samples are extracted as SO₂, H₂S and CO₂, respectively. The H₂S is then converted to SO₂ by reaction with heated Cu₂O. The delta ³⁴S values of two working standards (MSS-2 and MSS-3) obtained by the three techniques for the last 13 years are compared. The differences between MSS-2 and MSS-3 of delta ³⁴S values were reproducible to + or - 0.3per mille on method 2, and + or - 0.5per mille on the other methods. The delta ³⁴S values of the three working standards (MSS-2, MSS-3 and MSS-4) by the three techniques were compared under the same conditions. The most favourable results were obtained by method 2. With the delta ³⁴S values of the three working standards; +21.5per mille (MSS-2) , +3.5per mille (MSS-3) and +4.5per mille (MSS-4), respectively, relative to the Canon Diablo troilite. These values are in good agreement with the previous values.-A.K.