The influence of sample matrix on the accuracy of nitrite N and O isotope ratio analyses with the azide method

Abstract

Rationale: The isotope ratios of nitrogen (15N/14N) and oxygen (18O/16O) in nitrite (NO2−) can be measured by conversion of the nitrite into nitrous oxide (N2O) with azide, followed by mass spectrometric analysis of N2O by gas chromatography isotope ratio mass spectrometry (GC/IRMS). While applying this method to brackish samples, we noticed that the N and O isotope ratio measurements of NO2− are highly sensitive to sample salinity and to the pH at which samples are preserved. Methods: We investigated the influence of sample salinity and sample preservation pH on the N and O isotope ratios of the N2O produced from the reaction of NO2− with azide. The N2O isotope ratios were measured by GC/IRMS. Results: Under the experimental reaction conditions, the conversion of NO2− into N2O was less complete in lower salinity solutions, resulting in respective N and O isotopic offsets of +2.5‰ and −14.0‰ compared with seawater solutions. Differences in salinity were also associated with differences in the fraction of O atoms exchanged between NO2− and water during the reaction. Similarly, aqueous NO2− samples preserved at elevated pH values resulted in the incomplete conversion of NO2− into N2O by azide, and consequent pH-dependent isotopic offsets, as well as differences in the fraction of O atoms exchanged with water. The addition of sodium chloride to the reaction matrix of samples and standards largely mitigated salinity-dependent isotopic offsets in the N2O product, and nearly homogenized the fraction of O atom exchange among samples of different salinity. A test of the hypobromite–azide method to measure N isotope ratios of ammonium by conversion into NO2− then N2O revealed no influence of sample salinity on the N isotope ratios of the N2O product. Conclusions: We outline recommendations to mitigate potential matrix effects among samples and standards, to improve the accuracy of N and O isotope ratios in NO2− measured with the azide method.