Influence of δ18 O of water on measurements of δ18 O of nitrite and nitrate.

RATIONALE Oxygen isotope ratio measurements of NO2- and NO3- by the azide method and denitrifier method are sensitive to the δ18 O of sample water. However, the influence of δ18 OH2O on those measurements has not been quantitatively evaluated and documented so far. Therefore, we investigated the influence of δ18 OH2O of sample on the δ18 O analysis of NO2- and NO3- . METHODS We prepared NO2- and NO3- standards (with known δ18 ONO2- and δ18 ONO3- values) dissolved in waters having different δ18 OH2O values (δ18 OH2O = -12.6, 25.9, 56.7, and 110.1‰). Nitrite and nitrate were converted to N2 O using the azide method and the denitrifier method, respectively. The isotope ratios of the generated N2 O were measured with a Sercon PT-GC/IRMS system. The measured δ18 O values of the produced N2 O were plotted against known δ18 ONO2- and δ18 ONO3- values to evaluate the influence of exchange of an oxygen atom with H2 O during the conversion of NO2- to N2 O and NO3- to N2 O, respectively. RESULTS The degree of O 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 δ18 OH2O value of water than the denitrifier method. Thus, the intercept of the standard calibration curve must be corrected to account for differences in δ18 OH2O . Abiotic NO2 - H2 O 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. CONCLUSIONS Oxygen isotope ratio measurements of NO2- by the azide method are highly sensitive to δ18 OH2O as a result of significant oxygen isotope exchange between NO2- and H2 O. Therefore, to obtain the most accurate measurements the same δ18 OH2O value as that of the sample must be used to make the NO2- and NO3- standards.