The influence of sample matrix on the accuracy of nitrite N and O isotope ratio analyses with the azide method
Rationale The isotope ratios of nitrogen ( 15 N/ 14 N) and oxygen ( 18 O/ 16 O) in nitrite (NO 2 − ) can be measured by conversion of the nitrite into nitrous oxide (N 2 O) with azide, followed by mass spectrometric analysis of N 2 O 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 NO 2 − 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 N 2 O produced from the reaction of NO 2 − with azide. The N 2 O isotope ratios were measured by GC/IRMS. Results Under the experimental reaction conditions, the conversion of NO 2 − into N 2 O 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 NO 2 − and water during the reaction. Similarly, aqueous NO 2 − samples preserved at elevated pH values resulted in the incomplete conversion of NO 2 − into N 2 O 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 N 2 O 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 NO 2 − then N 2 O revealed no influence of sample salinity on the N isotope ratios of the N 2 O 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 NO 2 − measured with the azide method.