The influence of sample matrix on the accuracy of nitrite N and O isotope ratio analyses with the azide method. (8th January 2020)

• Record Type: Journal Article
• Authors: Granger, Julie; Boshers, Danielle S.; Böhlke, John K.; Yu, Dan; Chen, Nengwang; Tobias, Craig R.
• Is Part Of: Rapid communications in mass spectrometry. Volume 34:Number 1(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-01-08
• Subjects: Mass spectrometry -- Periodicals; 543.65
• DOI: 10.1002/rcm.8569
• Languages: English
• ISSNs: 0951-4198

Abstract

Abstract : Rationale: The isotope ratios of nitrogen ( 15 N/ 14 N) and oxygen ( 18 O/ 16 O) in nitrite (NO2 − ) can be measured by conversion of the nitrite into nitrous oxide (N2 O) with azide, followed by mass spectrometric analysis of N2 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 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 N2 O produced from the reaction of NO2 − with azide. The N2 O isotope ratios were measured by GC/IRMS. Results: Under the experimental reaction conditions, the conversion of NO2 − into N2 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 NO2 − and water during the reaction. Similarly, aqueous NO2 − samples preserved at elevated pH values resulted in the incomplete conversion of NO2 − into N2 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 N2 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 NO2 − then N2 O revealed no influence of sample salinity on the N isotope ratios of the N2 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 NO2 − measured with the azide method.