High-accuracy measurement of 36SF5+ signal using an ultrahigh-resolution isotope ratio mass spectrometer

Rationale

The Δ³⁶S standard deviation measured in a conventional isotope ratio mass spectrometer such as MAT 253 is ca 0.1‰ to 0.3‰. At this precision, it is difficult to resolve the origin of non-mass-dependent sulfur isotope fractionation in tropospheric sulfate aerosol and in Martian meteorites or small deviations from the canonical mass-dependent fractionation laws. Interfering ions with m/z at 131 of ³⁶SF₅⁺ are suggested 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 a Thermo Scientific ultrahigh-resolution isotope ratio mass spectrometer to measure SF₆ working gas and SF₆ gases converted from IAEA-S1/2/3 Ag₂S reference materials.

Results

We found that there are two resolvable peaks to the right of the ³⁶SF₅⁺ peak when a new filament was installed, which are ¹⁸⁶WF₄²⁺ followed by ¹²C₃F₅⁺. However, only the ¹²C₃F₅⁺ interference peak was observed after more than three days of filament use. ¹²C₃F₅⁺ is generated inside the instrument during the ionization process. Avoiding the interfering signals, we were able to achieve a Δ³⁶S standard deviation of 0.046‰ (n = 8) for SF₆ zero-enrichment and 0.069‰ (n = 8) for overall measurement start from silver sulfide IAEA-S1.

Conclusions

Aging the filament with SF₆ gas can avoid the interference of ¹⁸⁶WF₄²⁺. Minimizing the presence of carbon-bearing compounds and avoiding the interfering signals of ¹²C₃F₅⁺ from ³⁶SF₅⁺, we can improve Δ³⁶S measurement accuracy and precision, which helps to open new territories for research using quadruple sulfur isotope composition.