Removal of contamination in helium for precise SF₆ -based Δ³⁶ S measurements

Quantifications of quadruple sulfur isotopic compositions (δ³⁴ S, Δ³³ S, and Δ³⁶ S) of sulfur-bearing compounds in nature are valuable for providing new insights into the Earth's evolution such as the crust-mantle cycle, oxygenation of atmosphere and oceans, and the origin and evolution of early life. SF₆ -based isotope ratio mass spectrometry is the most widely used method of quantification, but Δ³⁶ S measurements at high precision and accuracy have always been technically difficult due to the low abundance of ³⁶ S (~0.01%). In this paper, we identify a major source of isobaric interferences (i.e., contamination in helium carrier gas in the gas chromatography purification step) and propose a simple strategy to solve this problem.

An SF₆ fluorination and purification system was built. Laboratory SF₆ reference gas and international Ag₂ S standard (IAEA-S1) were used as reference materials to test our method. Contamination from helium carrier gas (99.999%) was purified by a simple two-step cryogenic method to allow for accurate and precise measurements of Δ³⁶ S using the SF₆ -based isotope ratio mass spectrometry method.

Without proper purification of helium carrier gas, large errors in Δ³⁶ S measurements were found. Measured Δ³⁶ S values of SF₆ with trace contamination from helium were >10‰ higher than expected values. Using a newly developed purification strategy, the difference in Δ³⁶ S values of SF₆ before and after passing through the gas chromatography is less than instrumental errors (<0.2‰). Our improved method yielded an overall Δ³⁶ S precision for IAEA-S1 of 0.12‰ (n = 6). This precision is comparable to that found by other laboratories around the world.

Our simple two-step cryogenic method significantly improved the accuracy and precision of Δ³⁶ S measurements and is therefore recommended for future determination of quadruple sulfur isotopic compositions in natural samples.