RESUMEN
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.
RESUMEN
Measurements of the stable isotope ratios of nitrogen (15N/14N) and oxygen (18O/16O) in nitrate (NO3-) enable identification of sources, dispersal, and fate of natural and contaminant NO3- in aquatic environments. The 18O/16O of NO3- produced by nitrification is often assumed to reflect the proportional contribution of oxygen atom sources, water, and molecular oxygen, in a 2:1 ratio. Culture and seawater incubations, however, indicate oxygen isotopic equilibration between nitrite (NO2-) and water, and kinetic isotope effects for oxygen atom incorporation, which modulate the NO3- 18O/16O produced during nitrification. To investigate the influence of kinetic and equilibrium effects on the isotopic composition of NO3- produced from the nitrification of ammonia (NH3), we incubated streamwater supplemented with ammonium (NH4+) and increments of 18O-enriched water. Resulting NO3- 18O/16O ratios showed (1) a disproportionate sensitivity to the 18O/16O ratio of water, mediated by isotopic equilibration between water and NO2-, as well as (2) kinetic isotope discrimination during O atom incorporation from molecular oxygen and water. Empirically, the NO3- 18O/16O ratios thus produced fortuitously converge near the 18O/16O ratio of water. More elevated NO3- 18O/16O values commonly reported in soils and oxic groundwater may thus derive from processes additional to nitrification, including NO3- reduction.