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

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.

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.

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.

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.