Measurement of rare isotopologues of nitrous oxide by high‐resolution multi‐collector mass spectrometry

Rationale Bulk and position‐specific stable isotope characterization of nitrous oxide represents one of the most powerful tools for identifying its environmental sources and sinks. Constraining 14 N 15 N 18 O and 15 N 14 N 18 O will add two new dimensions to our ability to uniquely fingerprint N 2 O sources. Methods We describe a technique to measure six singly and doubly substituted isotopic variants of N 2 O, constraining the values of δ 15 N, δ 18 O, ∆ 17 O, 15 N site preference, and the clumped isotopomers 14 N 15 N 18 O and 15 N 14 N 18 O. The technique uses a Thermo MAT 253 Ultra, a high‐resolution multi‐collector gas source isotope ratio mass spectrometer. It requires 8–10 hours per sample and ~10 micromoles or more of pure N 2 O. Results We demonstrate the precision and accuracy of these measurements by analyzing N 2 O brought to equilibrium in its position‐specific and clumped isotopic composition by heating in the presence of a catalyst. Finally, an illustrative analysis of biogenic N 2 O from a denitrifying bacterium suggests that its clumped isotopic composition is controlled by kinetic isotope effects in N 2 O production. Conclusions We developed a method for measuring six isotopic variants of N 2 O and tested it with analyses of biogenic N 2 O. The added isotopic constraints provided by these measurements will enhance our ability to apportion N 2 O sources.