Reporting requirements for sulfur isotope measurements on environmental samples

Full text: Throughout the last few years, an increasing number of studies have appeared in the literature, in which sulfur isotope values are reported with accuracies and precisions of better than ±0.2 per mille, but with minimal information on the employed extraction procedures, the measuring gas used, and the reference materials analyzed. Such information is, however, essential for comparison of sulfur isotope values generated in different laboratories. For water samples where SO42- is the predominant sulfur compound, precipitation of BaSO4 is straightforward and can be easily achieved without significant isotope fractionation. If reduced inorganic sulfur species such as H2S, HS-, or S2- are present, significant errors can be introduced during sampling, storage, or pretreatment of the samples. If sulfur compounds need to be extracted from solid samples by wet chemical extraction techniques, uncertainties of more than ±0.5 per mille may be introduced. Interestingly, reported δ34S values seem to differ dependent upon whether SO2 or SF6 was used as a measuring gas. This discrepancy can be more than 1 per mille for very high or low δ34S values and is thought to result mainly from cross-mixing effects during SO2 based analyses with dual inlet isotope ratio mass spectrometers. This suggests that some commonly used SO2 based methods for determining δ34S values have an inherent error resulting in a scale contraction. Therefore, it is important that the utilized measuring gas and its generation technique are reported. It is also imperative that authors report the obtained values for all international reference materials utilized in their study, especially since the accepted values for some of these reference materials have changed through time. Reporting of the assigned values for international reference materials is particularly important for sulfur isotope determinations via continuous flow isotope ratio mass spectrometry (CF-IRMS). In this increasingly popular analytical technique, international reference materials are often used for normalization of obtained raw δ34S values because of the inability to perform an independent correction for mass 66 contributions via 32S16O18O. Considering the problems listed above, it is imperative that authors reporting sulfur isotope data for environmental samples provide detailed information regarding (1) the reproducibility of their extraction procedure for the sulfur compound of interest, (2) the measuring gas used, and (3) the obtained or assigned values for reference materials utilized in their study. This would greatly facilitate comparisons of sulfur isotope data generated in different laboratories world-wide. (author)