元素分析仪-气体同位素质谱法分析硫酸钙样品的硫同位素组成

硫酸盐硫同位素的常规分析方法是将硫酸盐转化为硫酸钡后搭配双路进样SO2法,该法易于操作、数据稳定,但样品用量大、费时费力,需要繁杂的前处理,无法满足微量分析发展方向的需求。本文以石膏为例,以元素分析仪-气体同位素质谱法(EA-IRMS)直接测定硫酸钙样品硫同位素比值,对同一样品分别采用:①硫酸钙与V2O5混合后包裹于锡杯中密封,直接进行元素分析仪-气体同位素质谱分析;②硫酸钙充分溶于去离子水中,向溶有硫酸钙样品的液体中加入沉淀试剂BaCl2,将生成的硫酸钡沉淀滤出后,用去离子水清洗2~3遍,烘干后与V2O5混合包裹于锡杯中密封再进行质谱测定。实验选取了13件δ34S值变化范围介于-20‰^+30‰之间的天然石膏样品,将获得的硫同位素比值进行对比,二者δ34SV-CDT绝对差值在0.00‰~0.24‰,表明同一样品的硫同位素比值结果在误差范围内基本一致。与常规分析方法相比,本文建立的直接在线分析时无需任何化学前处理,只需直接加入适量的V2O5,V2O5和氧气中的外部氧在瞬间燃烧的过程中替代了硫酸钙本身的氧,生成的SO2气体的氧是均一的,其硫同位素比值能代表样品的硫同位素组成,无需进行氧同位素的校正。经过验证表明,硫酸钙样品的直接在线分析是完全可行的。

Analysis of Sulfur Isotope Composition of Gypsum Samples by Elemental Analyzer-Isotope Mass Spectrometry

BACKGROUND: The 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.OBJECTIVES: To develop a method for determination of sulfur isotope compositions in gypsum directly through continuous flow elemental analyzer-isotope mass spectrometry(EA-IRMS).METHODS: Two 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.RESULTS: The 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.CONCLUSIONS: Compared 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.