Ion-exchange resin and denitrification pretreatment for determining δ15 N-NH4 + , δ15 N-NO3 - , and δ18 O-NO3 - values.

Abstract

There has never been a highly sensitive method for simultaneously measuring the δ15 N and δ18 O values of nitrate ions (NO3 - ) and the δ15 N values of ammonium ions (NH4 + ) 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. The δ15 N-NH4 + 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 NO3 - using an AG1-X8 anion-exchange resin for NH4 + measurements and the enrichment of NO3 - . For aerosol samples, the optimized method was used to measure the δ15 N-NO3 - , δ18 O-NO3 - , and δ15 N-NH4 + values of atmospheric particulate matter (PM2.5 , aerodynamic diameter < 2.5 μm). The δ15 N-NO3 - and δ18 O-NO3 - 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 δ15 N-NH4 + and δ15 N-NO3 - values were in all cases less than 1‰. In addition, the accuracies for the δ18 O-NO3 - values were less than 1.4‰ and generally acceptable. The δ15 N-NH4 + , δ15 N-NO3 - , and δ18 O-NO3 - values in six PM2.5 samples were similar to those reported in previous studies. Our proposed method for removing anions using AG1-X8 resin, for isotopic analysis using denitrifying bacteria, and for concentrating samples containing low concentrations of NO3 - 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.