Coupled Stable Isotope Tracing and Sulfamic Acid Reduction (SIT-SAR) Method to Determine the Ammonia and Nitrite...

Herein, a method was developed to measure the ammonia oxidation rate (R_a) and the nitrite oxidation rate (R_n) of water and sediment samples using a coupled stable isotope tracing and sulfamic acid reduction (SIT-SAR) method. ¹⁵NH₄⁺ was used as a tracer to determine the ammonia oxidation rates (R_a) by calculating the concentrations of produced ¹⁵NO₂^- and ¹⁵NO₃^- during incubation, while ¹⁵NO₂^- was used as a tracer to determine the nitrite oxidation rates (R_n) by calculating the increase of ¹⁵NO₃^- during incubation. ¹⁵NO₂^- was chemically reduced to ²⁹N₂ with 15 mmol·L^-1 sulfamic acid (SA). ¹⁵NO₃^- was first reduced to ¹⁵NO₂^- with a zinc-cadmium reducing agent, and then ¹⁵NO₂^- was subsequently reduced to ²⁹N₂ with SA. The produced ²⁹N₂ was measured by a membrane inlet mass spectrometer (MIMS). Under optimized experimental conditions, this method provides a sensitive (detection limit: 0.5 μmol·L^-1) and precise (relative standard deviation: 4.80% for ¹⁵NO₂^-, 3.82% for ¹⁵NO₃^-) approach to quantify the concentrations of ¹⁵NO₂^- (0.5-150 μmol·L^-1) and ¹⁵NO₃^- (0.5-120 μmol·L^-1) in water and sediment samples over a wide range of salinities (0-30‰) with excellent calibration curves (R² ≥ 0.999). This method was a successful application to estuarine water and sediments along the salinity gradient. Overall, the SIT-SAR method provided a rapid, accurate, and cost-effective means to determine R_a and R_n simultaneously.