Preparation of sulfur dioxide from sulfates for isotopic analysis

A convenient method for extraction of sulfur dioxide for sulfur isotopic analysis, from natural sulfates (SO42-, BaSO4, SrSO4 and CaSO4) is described. A sulfate reacts with NaPO3 in vacuum at 8000C; SO3 thus obtained is then reduced to sulfur dioxide on copper heated to 7500C. It has been experimentally shown that the reaction takes place with complete yield within twenty minutes, SO2 contains only small quantities of CO2 and H2O as impurities, σ18O of sulfur dioxide reflects the oxygen isotopic composition of the sulfate. The last property has been utilised to evaluate roughly σ18O of sulfates by measuring σ66 and σ50. It has been observed that samples of sulfates with different σ18O, if roasted at 8000 for half an hour, acquire the same σ18O value due to isotopic exchange with atmospheric oxygen; this can be utilised in the exclusive sulfur isotopic analysis where only one value σ66 or σ50 is necessary and additive correction for σ18O in the SO2 samples. (author)