Rationale: The Δ³⁶S standard deviation (SD) measured in a conventional isotope-ratio mass spectrometer (IRMS) such as MAT 253 is at c.a. 0.1‰ to 0.3‰. At this precision, it is difficult to resolve the origin of non-mass-dependent sulfur (NMD-S) isotope fractionation in the tropospheric sulfate aerosol and in Martian meteorites or small deviations from the canonical mass-dependent fractionation laws. Interfering ions (originated from fluorination and/or mass spectrometer ion source itself) with m/z at 131 of ³⁶SF₅⁺ by the community as the cause of the poor precision, but the exact ion species has not been identified or confirmed.
Methods: Here we examined the potential interfering ions by using the Thermo Scientific MAT 253 Ultra, a high-resolution (mass resolving powers up to 40,000) stable isotope-ratio mass spectrometer, to measure the SF₆ working gas and SF₆ gases converted from IAEA-S1 Ag₂S reference materials via a fluorination system.
Results: We found that there is a resolvable peak to the right of the ³⁶SF₅⁺ peak for both the SF₆ working gas and SF₆ generated by the fluorination system. The peak is identified as the ¹²C₃F₅⁺ ion, generated inside the instrument during the ionization process. By minimizing the presence of carbon-bearing compounds (e.g., organic matter in sample or helium gas, glue or o-ring in sample tubes, or carbon-bearing gases inside the mass spectrometer), we were able to achieve a Δ³⁶S SD of 0.046‰ (n=8) for SF₆ zero-enrichment and 0.069‰ (n=8) for overall measurement start from sliver sulfide IAEA-S1.
Conclusions: Minimizing the presence of carbon-bearing compounds and avoiding the interfering signals from ³⁶SF₅⁺ via MAT 253 Ultra high‐resolution isotope-ratio mass spectrometer, we can improve Δ³⁶S measurement precision by 2 to 5 folds, which helps to open new territories for research using quadruple sulfur isotope composition.
 

non-mass-dependent sulfur (NMD-S) isotope fractionation,interfering ions,Δ36S measurement precision