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

RATIONALE

Oxygen isotope ratio measurements of NO and NO by the azide method and denitrifier method are sensitive to the δ O value of the sample water. However, the influence of δ O on those measurements has not been quantitatively evaluated and documented so far. Therefore, we investigated the influence of δ O of a sample on the δ O analysis of NO and NO .

METHODS

We prepared NO and NO standards (with known δ O and δ O values) dissolved in waters having different δ O values (δ O  = -12.6, 25.9, 56.7, and 110.1‰). Nitrite and nitrate were converted into N O using the azide method and the denitrifier method, respectively. The isotope ratios of the generated N O were measured with a Sercon purge-and-trap gas chromatography/isotope ratio mass spectrometry (PT-GC/IRMS) system. The measured δ O values of the produced N O were plotted against known δ O and δ O values to evaluate the influence of exchange of an oxygen atom with H O during the conversion of NO into N O and NO into N O, respectively.

RESULTS

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 δ O value of water than the denitrifier method. Thus, the intercept of the standard calibration curve must be corrected to account for differences in δ O . Abiotic NO -H O equilibrium isotope effect experiments yielded a rate constant of (1.13 ± 007) × 10 (h ) 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 NO by the azide method are highly sensitive to δ O as a result of significant oxygen isotope exchange between NO and H O. Therefore, to obtain the most accurate measurements water with the same δ O value as that of the sample must be used to make the NO and NO standards.