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 delta N-15 and delta O-18 values of nitrate ions (NO3-) and the delta N-15 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. Methods The delta N-15-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 delta N-15-NO3-, delta O-18-NO3-, and delta N-15-NH4+ values of atmospheric particulate matter (PM2.5, aerodynamic diameter < 2.5 mu m). Results The delta N-15-NO3- and delta O-18-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 delta N-15-NH4+ and delta N-15-NO3- values were in all cases less than 1 parts per thousand. In addition, the accuracies for the delta O-18-NO3- values were less than 1.4 parts per thousand and generally acceptable. The delta N-15-NH4+, delta N-15-NO3-, and delta O-18-NO3- values in six PM2.5 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 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.

Automated summary

The study introduced a new method combining anion-exchange resin, denitrifying bacteria, and samples with low concentrations of NO3- to achieve high-precision and accurate analysis. This method can be easily and inexpensively applied to aerosols as well as samples from various locations such as rivers, oceans, and Antarctica.