Influence of <i>δ</i> <sup>18</sup> O of water on measurements of <i>δ</i> <sup>18</sup> O of nitrite and nitrate
Kanae Kobayashi, Keitaro Fukushima, Yuji Onishi, Kazuya Nishina, Akiko Makabe, Midori Yano, Scott D. Wankel, Keisuke Koba, Satoshi Okabe
Abstract
Rationale Oxygen isotope ratio measurements of NO 2 − and NO 3 − by the azide method and denitrifier method are sensitive to the δ 18 O value of the sample water. However, the influence of δ 18 O H2O on those measurements has not been quantitatively evaluated and documented so far. Therefore, we investigated the influence of δ 18 O H2O of a sample on the δ 18 O analysis of NO 2 − and NO 3 − . Methods We prepared NO 2 − and NO 3 − standards (with known δ 18 O NO2‐ and δ 18 O NO3‐ values) dissolved in waters having different δ 18 O H2O values ( δ 18 O H2O = −12.6, 25.9, 56.7, and 110.1‰). Nitrite and nitrate were converted into N 2 O using the azide method and the denitrifier method, respectively. The isotope ratios of the generated N 2 O were measured with a Sercon purge‐and‐trap gas chromatography/isotope ratio mass spectrometry (PT‐GC/IRMS) system. The measured δ 18 O values of the produced N 2 O were plotted against known δ 18 O NO2‐ and δ 18 O NO3‐ values to evaluate the influence of exchange of an oxygen atom with H 2 O during the conversion of NO 2 − into N 2 O and NO 3 − into N 2 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 18 O H2O value of water than the denitrifier method. Thus, the intercept of the standard calibration curve must be corrected to account for differences in δ 18 O H2O . Abiotic NO 2 –H 2 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 NO 2 − by the azide method are highly sensitive to δ 18 O H2O as a result of significant oxygen isotope exchange between NO 2 − and H 2 O. Therefore, to obtain the most accurate measurements water with the same δ 18 O H2O value as that of the sample must be used to make the NO 2 − and NO 3 − standards.