A Method for Accurate Quantification of ¹⁵N-Enriched Nitric Oxide with Membrane Inlet Mass Spectrometer

Nitric oxide (NO), a key nitrogen cycle intermediate, is an important atmospheric pollutant. However, a reliable method for the accurate determination of NO remains scarce. Here, we report a novel isotopic approach employing acidified potassium iodide (KI) to reduce ¹⁵NO₂^- to ¹⁵NO, which is then quantified via a membrane inlet mass spectrometer (MIMS). The effects of the reducing agent concentration, settling time, light intensity, salinity, and ¹⁵NO₃^- concentration on KI reduction were systematically assessed. Under optimized conditions, standard calibration curves demonstrated excellent linearity (R² = 0.996) for ¹⁵NO quantification over a range of 0.1 to 200 μmol L^-1, with a detection limit of 0.1 μmol L^-1. This method provides accurate (relative error: 4.39%), precise (relative standard deviation: 3.12%), and cost-effective ¹⁵NO analysis for understanding the biogeochemical process of NO dynamics in the environment.