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A simple and reliable method reducing sulfate to sulfide for multiple sulfur isotope analysis

Abstract Rationale

Precise analysis of four sulfur isotopes of sulfate in geological and environmental samples provides the means to extract unique information in wide geological contexts. Reduction of sulfate to sulfide is the first step to access such information. The conventional reduction method suffers from a cumbersome distillation system, long reaction time and large volume of the reducing solution. We present a new and simple method enabling the process of multiple samples at one time with a much reduced volume of reducing solution.

Methods

1 mL reducing solution made of HI and NaH2PO2 was added to a septum glass tube with dry sulfate. The tube was heated at 124 °C and the produced H2S was purged with inert gas (He or N2) through gas‐washing tubes and then collected by NaOH solution. The collected H2S was converted to Ag2S by adding AgNO3 solution and the co‐precipitated Ag2O was removed by adding a few drops of concentrated HNO3.

Results

Within 2‐3 hours, a 100 % yield was observed for samples with 0.2 to 2.5 μmol Na2SO4. The reduction rate was much slower for BaSO4 and a complete reduction was not observed. International sulfur reference materials, NBS‐127, SO‐5 and SO‐6, were processed with this method, and the measured against accepted δ34S values yielded a linear regression line which had a slope of (0.99 ± 0.01) and a R2 value of 0.998.

Conclusions

The new methodology is easy to handle and allows us to process multiple samples at a time, and it demonstrated good reproducibility in terms of H2S yield and for further isotope analysis. It is thus a good alternative to the conventional manual method, especially when processing samples with limited amount of sulfate available.

Authors:   Lei Geng, Joel Savarino, Clara Savarino, Nicolas Caillon, Pierre Cartigny, Shohei Hattori, Sakiko Ishino, Naohiro Yoshida
Journal:   Rapid Communications in Mass Spectrometry
Year:   2017
Pages:   n/a
DOI:   10.1002/rcm.8048
Publication date:   22-Dec-2017
Facts, background information, dossiers
  • samples
  • solution
  • reductions
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