Safe, accurate, and precise sulfur isotope analyses of arsenides, sulfarsenides, and arsenic and mercury sulfides by conversion to barium sulfate before EA/IRMS

Abstract

The stable isotope ratios of sulfur (δ³⁴S relative to Vienna Cañon Diablo Troilite) in sulfates and sulfdes determined by elemental analysis and isotope ratio mass spectrometry (EA/IRMS) have been proven to be a remarkable tool for studies of the (bio)geochemical sulfur cycles in modern and ancient environments. However, the use of EA/IRMS to measure δ³⁴S in arsenides and sulfarsenides may not be straightforward. This difculty can lead to potential health and environmental hazards in the workplace and analytical problems such as instrument contamination, memory efects, and a non-matrix-matched standardization of δ³⁴S measurements with suitable reference materials. To overcome these practical and analytical challenges, we developed a procedure for sulfur isotope analysis of arsenides, which can also be safely used for EA/IRMS analysis of arsenic sulfdes (i.e., realgar, orpiment, arsenopyrite, and arsenian pyrite), and mercury sulfdes (cinnabar). The sulfur dioxide produced from of-line EA combustion was trapped in an aqueous barium chloride solution in a leak-free system and precipitated as barium sulfate after quantitative oxidation of hydrogen sulfte by hydrogen peroxide. The derived barium sulfate was analyzed by conventional EA/IRMS, which bracketed the δ³⁴S values of the samples with three international sulfate reference materials. The protocol (BaSO₄-EA/IRMS) was validated by analyses of reference materials and laboratory standards of sulfate and sulfdes and achieved accuracy and precision comparable with those of direct EA/IRMS. The δ³⁴S values determined by BaSO₄-EA/IRMS in sulfdes (arsenopyrite, arsenic, and mercury sulfdes) samples from diferent origins were comparable to those obtained by EA/IRMS, and no sulfur isotope fractionations were introduced during sample preparation. We report the frst sulfur isotope data of arsenides obtained by BaSO₄-EA/IRMS.