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 (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.