New type of filaments for improved accuracy of multiple sulfur isotope analyses by electron-impact gas-source mass spectrometry

Rationale

The analysis of the three sulfur stable isotope ratios (³³S/³²S, ³⁴S/³²S, ³⁶S/³²S) is routinely performed by gas-source isotope ratio mass spectrometry (IRMS) on the SF₆ gaseous molecule, collecting SF₅⁺ ions at m/z ~ 127, 128, 129 and 131. High precision and accuracy are commonly achieved owing to a lack of correction because fluorine has only one isotope and the inert nature of the SF₆ molecule. The analysis of the ³⁶S/³²S ratio is, however, complicated by the low abundance of ³⁶S (~0.015%) and the possible occurrence of trace amounts of fluorocarbon compounds leading to ¹²C₃F₅⁺ ions at m/z ~ 131, i.e. where ³⁶SF₅⁺ ions are collected.

Methods

We used gas-source high-resolution IRMS to better characterize the nature of possible interferences, and we tested novel types of filaments in order to investigate their influence on possible interferences.

Results

We confirm that the ¹²C₃F₅⁺ ion represents the main isobaric interference at m/z ~ 131. We also demonstrate that tungsten fluoride adducts are formed from the reaction of fluorine ions derived during fragmentation of the SF₆ molecule with the hot tungsten filament. These reactions lead to the formation of e.g. WF₅⁺, WF₄⁺, WF₃⁺ ions, including doubly charged ions. WF₄⁺⁺, in particular, leads to isobaric interference on m/z ~ 128, 129 and 131 from ¹⁸⁰WF₄⁺⁺, ¹⁸²WF₄⁺⁺ and ¹⁸⁶ WF₄⁺⁺ ions, respectively. Because ¹⁸⁰W (0.12%) is at low abundance, its influence on δ³³S measurements would remain negligible, but ¹⁸²W (26.5%) and ¹⁸⁶W (28.4%) lead to scale contraction for both δ³⁴S and δ³⁶S.

Conclusions

Rather than correcting for these interferences, or working at high mass resolution, we suggest avoiding W isobaric interferences by using other types of filaments, with initial reports on both pure Re filaments and Y₂O₃-coated W filaments.