Herein, a method was developed to measure the concentrations of ⁴⁵N₂O and ⁴⁶N₂O in water and sediment sample using a coupled stable isotope tracing and ferric iron oxidation (SIT-FIO) method. Mixture of ¹⁴NH₂OH and ¹⁵NH₂OH at ratio 1:1, used as a tracer, was chemically oxidized to ⁴⁵N₂O and ⁴⁶N₂O by ferric iron (Fe³⁺). The produced ⁴⁵N₂O and ⁴⁶N₂O was measured by a membrane inlet mass spectrometer (MIMS). Under optimized experimental conditions, this method provides a sensitive (detection limit:12.5 nmol·L^-1) and precise (relative standard deviation: ≤5％) approach to quantify the concentrations of ⁴⁵N₂O (25-25000 nmol·L^-1) and ⁴⁶N₂O (12.5-12500 nmol·L^-1) in water and sediment samples over a wide range of salinities (0-30‰) with excellent calibration curves (R²≥0.999). This method can be successfully applied to measure the N₂O production rates contributing by nitrification and denitrification in water and sediment along the salinity gradient. Overall, this SIT-FIO method is the first to determine ⁴⁵N₂O and ⁴⁶N₂O simultaneously and accurately using Membrane Inlet Mass Spectrometer with low cost.
 

isotope tracing, ferric iron oxidation, 45N2O, 46N2O, membrane inlet mass spectrometer