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

Abstract

RationaleThe 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.MethodsWe 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.ResultsWe 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.ConclusionsRather 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.