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

Herein, a method was developed to measure the ammonia oxidation rate () and the nitrite oxidation rate () of water and sediment samples using a coupled stable isotope tracing and sulfamic acid reduction (SIT-SAR) method. NH was used as a tracer to determine the ammonia oxidation rates () by calculating the concentrations of produced NO and NO during incubation, while NO was used as a tracer to determine the nitrite oxidation rates () by calculating the increase of NO during incubation. NO was chemically reduced to N with 15 mmol·L sulfamic acid (SA). NO was first reduced to NO with a zinc-cadmium reducing agent, and then NO was subsequently reduced to N with SA. The produced N was measured by a membrane inlet mass spectrometer (MIMS). Under optimized experimental conditions, this method provides a sensitive (detection limit: 0.5 μmol·L) and precise (relative standard deviation: 4.80% for NO, 3.82% for NO) approach to quantify the concentrations of NO (0.5-150 μmol·L) and NO (0.5-120 μmol·L) in water and sediment samples over a wide range of salinities (0-30‰) with excellent calibration curves ( ≥ 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 and simultaneously.