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

Abstract

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_2O sources. Methods: We describe a technique to measure six singly and doubly substituted isotopic variants of N2O, 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_2O. Results: We demonstrate the precision and accuracy of these measurements by analyzing N_2O 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_2O from a denitrifying bacterium suggests that its clumped isotopic composition is controlled by kinetic isotope effects in N_2O production. Conclusions: We developed a method for measuring six isotopic variants of N_2O and tested it with analyses of biogenic N_2O. The added isotopic constraints provided by these measurements will enhance our ability to apportion N_2O sources.

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