Analysis of Sulfur Isotope Composition of Gypsum Samples by Elemental Analyzer-Isotope Mass Spectrometry
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ABSTRACT
BACKGROUNDThe conventional method for the measurement of sulfur isotopes in sulfate includes mainly barite conversion coupled with dual-inlet SO2 methods. These methods are facile and reliable. However, the large amounts of samples, the length of time needed, the laborious experimental work, and complicated preprocessing are impractical for the development direction of micro-analysis.OBJECTIVESTo develop a method for determination of sulfur isotope compositions in gypsum directly through continuous flow elemental analyzer-isotope mass spectrometry (EA-IRMS).METHODSTwo different preparation methods were used. (1) Direct measurement of sulfur isotopes was carried out by mixing calcium sulfate powder with V2O5 in a tin cup using EA-IRMS. (2) Calcium sulfate was fully dissolved in deionized water. The precipitation reagent BaCl2 was added to the liquid in which the calcium sulfate sample was dissolved. After the precipitated barium sulfate was filtered out, it was washed 2 to 3 times with deionized water. Samples were dried and mixed with V2O5 in a sealed tin cup and then determined by mass spectrometry.RESULTSThe δ34S values of 13 gypsum samples ranged from -20‰ to -30‰, and the results from replicate measurements were compared for these two methods, yielding the absolute difference in δ values were 0.0-0.2 permil, indicating that the sulfur isotope ratios of the same sample were basically identical.CONCLUSIONSCompared with the conventional analysis method, the application of 'no pretreatment' to analyze the sulfur isotopes of gypsum was conducted successfully. Adding V2O5 to samples directly, during the process of instantaneous combustion, the oxygen in calcium sulfate are substituted by the external oxygen in V2O5 and oxygen gas, and the obtained oxygen of SO2 gas is uniform. Therefore, sulfur isotope ratios of SO2 can completely represent those of the calcium sulfate, making it unnecessary for oxygen isotope correction. The direct online analysis of sulfur isotopes in calcium sulfate samples using EA-IRMS is verified to be feasible.
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