Oxygen interference with membrane inlet mass spectrometry may overestimate denitrification rates calculated with the isotope pairing technique

Membrane inlet mass spectrometry (MIMS) is used to measure gas concentrations in aqueous samples, and is often employed to measure N2 in denitrification studies. While most denitrification studies using MIMS have employed the N2:Ar method, MIMS is increasingly used with the isotope pairing technique (IPT), an alternative method that calculates denitrification based on 29N2 and 30N2 concentrations, measured as mass to charge ratios (m/z) 29 and 30. Dissolved O2 is known to interfere with m/z 28 and 40 (28N2 and Ar, respectively) in the MIMS, and in this study, we show that the effect on m/z 29 and 30 is also significant. This O2 effect may result in artificially high denitrification rates with IPT if O2 varies between samples. In lab‐based experiments, m/z 29 and 30 were impacted immediately as O2 was depleted. The resulting effect on simulated denitrification rates was minor if final O2 concentrations remained above 50% initial O2, but increased by more than 100 μmol m−2 h−1 as O2 fell below 50%. Similarly, denitrification rates from sediment incubations were an order of magnitude higher when analyzed in the presence of O2, compared with analysis without O2 using an inline furnace with reduction column. We further show that this effect is variable over the life of the MIMS, and/or with different ion sources. Given these uncertainties, we recommend using an inline furnace to remove O2 for all IPT experiments. Alternatively, the magnitude of the O2 effect should be assessed often to determine the range of minimal O2 interference.