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 ( 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. 15 NH 4 + was used as a tracer to determine the ammonia oxidation rates ( R a ) by calculating the concentrations of produced 15 NO 2 - and 15 NO 3 - during incubation, while 15 NO 2 - was used as a tracer to determine the nitrite oxidation rates ( R n ) by calculating the increase of 15 NO 3 - during incubation. 15 NO 2 - was chemically reduced to 29 N 2 with 15 mmol·L -1 sulfamic acid (SA). 15 NO 3 - was first reduced to 15 NO 2 - with a zinc-cadmium reducing agent, and then 15 NO 2 - was subsequently reduced to 29 N 2 with SA. The produced 29 N 2 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 15 NO 2 - , 3.82% for 15 NO 3 - ) approach to quantify the concentrations of 15 NO 2 - (0.5-150 μmol·L -1 ) and 15 NO 3 - (0.5-120 μmol·L -1 ) in water and sediment samples over a wide range of salinities (0-30‰) with excellent calibration curves ( R 2 ≥ 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.