Accurate Quantification of ⁴⁵ N ₂ O with a Membrane Inlet Mass Spectrometer by Optimizing the Temperature of a Cold Trap

Membrane Inlet Mass Spectrometry (MIMS) is extensively applied in the study of nitrogen cycling processes. However, robust methods for the quantitative detection of N₂O by this technique are still scarce. In this study, acidified azide was employed to reduce ¹⁵NO₂ ^- to generate ⁴⁵N₂O, which was used to make a standard curve for qualifying ⁴⁵N₂O with MIMS. Key parameters affecting the azide reduction, including reagent concentration, reaction time, reaction mode, and salinity, were systematically evaluated and optimized. Then, the generated ⁴⁵N₂O dissolved in water was introduced into the MIMS via a cold trap maintained at -88 ± 2 °C. This specific temperature capitalizes on the difference in the freezing points of N₂O (-91 °C) and CO₂ (-78 °C), thereby effectively minimizing CO₂ interference during N₂O measurement. Under optimized conditions, the linear fitting between ⁴⁵N₂O ion current intensity and expected ⁴⁵N₂O concentration showed an excellent linearity in the range of 0.1-100 μmol/L (<i>R</i> ² = 0.998), with a detection limit of 0.1 μmol/L. This method achieves accurate (relative error: 4.688%) and precise (relative standard deviation: 1.560%) detection of ⁴⁵N₂O and holds potential for future application in tracing microbial N₂O source-sink processes across diverse habitats.