Determination of sulfur isotopic ratio by HR-ICP-MS method

Sulfur is an important geochemical element that is part of many natural compounds. Sulfur takes part in most natural processes. In this case, a change in the valency of this element can occur. The change in sulfur valency is accompanied by isotope fractionation, which gives rise to noticeable shifts in the isotopic composition of this element. Thus, sulfur is one of the important geochemical indicators that can be used to reconstruct the redox conditions of various geological processes, including sedimentation processes in marine basins. For many decades, the traditional method of analyzing the isotopic composition of sulfur in sulfides remains the method involving the oxidation of sulfur to SO2 and the subsequent measurement of the 34S/32S ratio in gas using a mass spectrometer (Giesemann et al., 1994; Grassineau et al., 2001; Studley et al., 2002, etc.). However, with the advent of the inductively coupled plasma method of multi-collector mass spectrometry, new in isotope geochemistry, the corresponding methodological direction began to develop in which this method is used to analyze the sulfur isotopic composition (Krupp et al., 2004; You and Li, 2005; Clough et al ., 2006; Mason et al., 2006; Craddock et al., 2008 and others). The results presented in these works suggest that the MC-ICP-MS method can be used to solve certain problems of studying the natural variations in the sulfur isotopic composition. Therefore, we created optimized methodic for measuring isotope ratio of sulfur. The experimental work was carried out on the instrument base of the laboratory of SEC "Geothermohronology" of the Institute of Geology and Oil and Gas Technologies at Kazan Federal University (IGiNGT KFU, Kazan) and was aimed at creating a set of analytical procedures for chemical sample preparation and mass spectrometric analysis methods. The chemical preparation of pyrite samples included the stage of their acid decomposition using concentrated inorganic acids, which ensured the quantitative transfer of the sulfur of the sample into solution, as well as the stage of obtaining pure sulfur preparations based on the use of ion exchange chromatography. The optimal methodological scheme of mass spectrometric measurement of the 34S / 32S ratio was determined on a Neptune Plus multi-collector mass spectrometer. It takes into account the influence of factors unfavorable for analysis of the isotopic composition of sulfur, such as interference overlays of ions of oxygen, carbon, and nitrogen compounds, as well as the effect of instrumental mass discrimination. The work was supported by the Ministry of Science and High Education of the Russian Federation contract No. 14.Y26.31.0029 in the framework of the Resolution No.220 of the Government of the Russian Federation.