Sulfur isotope analyses using 3× elemental analysis/isotope ratio mass spectrometry: Saving helium and energy while reducing analytical time and costs

Rationale

Helium (He) and energy shortages have caused price increases and reduced their availability. Using three combustion reactions per acquisition of carbon and nitrogen isotope ratios saves 50% He and energy during the elemental analysis/isotope ratio mass spectrometry (EA/IRMS). This approach needs to be tested for sulfur isotope (δ³⁴S) analyses.

Methods

A new method to measure δ³⁴S in three sequential combustion reactions within one EA/IRMS acquisition was developed. The same material or blank samples could be used in the three reactions. After SO₂ was used, a N₂ purging method was employed to prolong the lifetime of the valves in the EA/IRMS interface. The 3×EA/IRMS was applied to measure δ³⁴S in precious samples, such as Ag₂S from acid-volatile and chromium-reducible sulfur extracted with a multiple-port setup.

Results

The 3×EA/IRMS-δ³⁴S method was validated with replicate analyses of international reference materials and laboratory standards with a wide range of mineralogical compositions and δ³⁴S values. The method provided a strategic advantage for the δ³⁴S measurements of small precious samples (measured between blanks). The accuracy and precision of the 3×EA/IRMS values effectively matched those obtained using conventional EA/IRMS, with good agreement between the mean ± SD values and the recommended values with their uncertainties.

Conclusions

Compared with the conventional EA/IRMS, the proposed method provides accurate and precise δ³⁴S measurements of the sulfate and sulfide samples while saving approximately 50% of He, energy, SO₂ reference gas, O₂, analysis time, and cost. Notably, 3×EA/IRMS can provide up to three δ³⁴S values unaffected by memory effects.