Ion‐exchange resin and denitrification pretreatment for determining δ15N‐NH4+, δ15N‐NO3−, and δ18O‐NO3− values

Rationale There has never been a highly sensitive method for simultaneously measuring the δ 15 N and δ 18 O values of nitrate ions (NO 3 − ) and the δ 15 N values of ammonium ions (NH 4 + ) in particulate matter using denitrifying bacteria. In this study, we explored a method that combines use of an anion‐exchange resin and denitrifying bacteria to make such measurements. Methods The δ 15 N‐NH 4 + values of samples obtained using the hypobromite and denitrifying bacteria method were measured by isotope ratio mass spectrometry. Tests (effect of flow rate, breakthrough, and acid concentration) were conducted to verify the removal of NO 3 − using an AG1‐X8 anion‐exchange resin for NH 4 + measurements and the enrichment of NO 3 − . For aerosol samples, the optimized method was used to measure the δ 15 N‐NO 3 − , δ 18 O‐NO 3 − , and δ 15 N‐NH 4 + values of atmospheric particulate matter (PM 2.5 , aerodynamic diameter < 2.5 μm). Results The δ 15 N‐NO 3 − and δ 18 O‐NO 3 − values measured following extraction with 1–6 mol/L HCl, at sample flow rates of 1–2 mL/min, with total anion amounts of less than 2.2 mmol, and in concentration tests were found to be in very close agreement with reagent values. The precisions and the accuracies of the δ 15 N‐NH 4 + and δ 15 N‐NO 3 − values were in all cases less than 1‰. In addition, the accuracies for the δ 18 O‐NO 3 − values were less than 1.4‰ and generally acceptable. The δ 15 N‐NH 4 + , δ 15 N‐NO 3 − , and δ 18 O‐NO 3 − values in six PM 2.5 samples were similar to those reported in previous studies. Conclusions Our proposed method for removing anions using AG1‐X8 resin, for isotopic analysis using denitrifying bacteria, and for concentrating samples containing low concentrations of NO 3 − will make it possible to perform high‐precision and accurate analyses easily and inexpensively. These methods are applicable not only to aerosols, but also to samples from diverse locations such as rivers, oceans, and Antarctica.