Removal of contamination in helium for precise SF₆‐based Δ³⁶S measurements

Abstract

Rationale Quantifications of quadruple sulfur isotopic compositions (δ34S, Δ33S, and Δ36S) 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 Δ36S measurements at high precision and accuracy have always been technically difficult due to the low abundance of 36S (~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. Methods An SF6 fluorination and purification system was built. Laboratory SF6 reference gas and international Ag2S 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 Δ36S using the SF6‐based isotope ratio mass spectrometry method. Results Without proper purification of helium carrier gas, large errors in Δ36S measurements were found. Measured Δ36S values of SF6 with trace contamination from helium were >10‰ higher than expected values. Using a newly developed purification strategy, the difference in Δ36S values of SF6 before and after passing through the gas chromatography is less than instrumental errors (<0.2‰). Our improved method yielded an overall Δ36S precision for IAEA‐S1 of 0.12‰ (n = 6). This precision is comparable to that found by other laboratories around the world. Conclusion Our simple two‐step cryogenic method significantly improved the accuracy and precision of Δ36S measurements and is therefore recommended for future determination of quadruple sulfur isotopic compositions in natural samples.

Toward the Origins of Quadruple Sulfur Isotope Anomalies in Modern Sulfate: A Multitracer Approach and Implications for Paleo- and Planetary Atmospheres

“…Multiple measurements of international reference Ag 2 S materials (IAEA-S1) that were fluorinated to SF 6 show that external errors of δ 34 S, Δ 33 S, and Δ 36 S are better than 0.1, 0.007, and 0.13‰, respectively. 23 In this study, we found that the Thode solution reduction step led to small and correctable isotopic fractionation for δ 34 S (∼0.5‰) and Δ 33 S (∼0.15‰), while changes in Δ 36 S are negligible and within our analytical uncertainty (Figure S1). Such fractionation problems were eliminated when we used a new reduction apparatus.…”

“…The observed near-zero Δ 36 S values in this study are consistent with the relatively low concentration of levoglucosan in our sampling period, as expected by previous works that define a negative correlation between Δ 36 S and levoglucosan (Figure A,B). Such linkage is not an analytical artifact because contaminations of carbon-rich materials (via the formation of 36 SF 5 + -isobaric 12 C 3 F 5 + fragments in the IRMS) are expected to lead to positive rather than negative Δ 36 S anomalies . In addition, if carbon contamination did occur during analytical processes, carbon-rich coal samples would not show near-zero Δ 36 S values .…”

“…No unexpected or unusually high safety hazards were encountered. The produced SF 6 was purified by a gas chromatography (GC) system equipped with a 5 A mol-sieve column and a Hayesep-Q column . In the purification step, contamination in the GC carrier gas (helium; 99.999%) was effectively removed by our newly developed method to ensure precise quadruple sulfur isotope measurements, especially for Δ 36 S. SF 6 was finally measured by an isotope ratio mass spectrometer (IRMS; MAT 253 Plus, Thermo Fisher Scientific) installed at GIGCAS.…”

“…The produced SF 6 was purified by a gas chromatography (GC) system equipped with a 5 A mol-sieve column and a Hayesep-Q column . In the purification step, contamination in the GC carrier gas (helium; 99.999%) was effectively removed by our newly developed method to ensure precise quadruple sulfur isotope measurements, especially for Δ 36 S. SF 6 was finally measured by an isotope ratio mass spectrometer (IRMS; MAT 253 Plus, Thermo Fisher Scientific) installed at GIGCAS. Multiple measurements of international reference Ag 2 S materials (IAEA-S1) that were fluorinated to SF 6 show that external errors of δ 34 S, Δ 33 S, and Δ 36 S are better than 0.1, 0.007, and 0.13‰, respectively …”

“…23 BrF 5 , a hazardous material, was stored in a Kel-F (poly(chlorotrifluoroethylene)) tube connected to a high-vacuum system, and excessive BrF 5 was removed by a waste trap (filled with KOH pellets) connected in the same system. 23 No unexpected or unusually high safety hazards were encountered. The produced SF 6 was purified by a gas chromatography (GC) system equipped with a 5 A mol-sieve column and a Hayesep-Q column.…”