Abstract
Oxygen isotope ratio measurements of NO2− and NO3− by the azide method and denitrifier method are sensitive to the δ18O value of the sample water. However, the influence of δ18OH2O on those measurements has not been quantitatively evaluated and documented so far. Therefore, we investigated the influence of δ18OH2O of a sample on the δ18O analysis of NO2− and NO3−. We prepared NO2− and NO3− standards (with known δ18ONO2‐ and δ18ONO3‐ values) dissolved in waters having different δ18OH2O values (δ18OH2O = −12.6, 25.9, 56.7, and 110.1‰). Nitrite and nitrate were converted into N2O using the azide method and the denitrifier method, respectively. The isotope ratios of the generated N2O were measured with a Sercon purge‐and‐trap gas chromatography/isotope ratio mass spectrometry (PT‐GC/IRMS) system. The measured δ18O values of the produced N2O were plotted against known δ18ONO2‐ and δ18ONO3‐ values to evaluate the influence of exchange of an oxygen atom with H2O during the conversion of NO2− into N2O and NO3− into N2O, respectively. The degree of oxygen isotope exchange was 10.8 ± 0.3% in the azide method and 5.5 ± 1.0% in the denitrifier method, indicating that the azide method is more susceptible to artifacts arising from differences in the δ18OH2O value of water than the denitrifier method. Thus, the intercept of the standard calibration curve must be corrected to account for differences in δ18OH2O. Abiotic NO2–H2O equilibrium isotope effect experiments yielded a rate constant of (1.13 ± 007) × 10−2 (h−1) and an equilibrium isotope effect of 11.9 ± 0.1‰ under the condition of pH = 7.5, 30°C, and 2.5% salinity. Oxygen isotope ratio measurements of NO2− by the azide method are highly sensitive to δ18OH2O as a result of significant oxygen isotope exchange between NO2− and H2O. Therefore, to obtain the most accurate measurements water with the same δ18OH2O value as that of the sample must be used to make the NO2− and NO3− standards.
