A Method for Accurate Quantification of

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.