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Factors influencing the accuracy of the denitrifier method for determining the oxygen isotopic composition of nitrate

Zhang, Man, Shi, Jia-chun, Wu, Lao-sheng
Journal of Zhejiang University 2019 v.20 no.1 pp. 49-58
denitrifying microorganisms, dissolved oxygen, models, nitrates, nitrogen, oxygen, stable isotopes
The denitrifier method is widely used as a novel pretreatment method for the determination of nitrogen and oxygen isotope ratios as it can provide quantitative and high-sensitivity measurements. Nevertheless, the method is limited by relatively low measurement accuracy for δ¹⁸O. In this study, we analyzed the factors influencing the accuracy of δ¹⁸O determination, and then systematically investigated the effects of dissolved oxygen concentrations and nitrate sample sizes on estimates of the δ¹⁵N and δ¹⁸O of nitrate reference materials. The δ¹⁸O contraction ratio was used to represent the relationship between the measured difference and true difference between two reference materials. We obtained the following main results: (1) a gas-liquid ratio of 3:10 (v/v) in ordinary triangular flasks and a shaking speed of 120 r/min produced an optimal range (1.9 to 2.6 mg/L) in the concentration of dissolved oxygen for accurately determining δ¹⁸O, and (2) the δ¹⁸O contraction ratio decreased as nitrate sample size decreased within a certain range (1.0 to 0.1 μmol). Our results suggested that δ¹⁸O contraction is influenced mainly by dissolved oxygen concentrations in pure culture, and provided a model for improving the accuracy of oxygen isotope analysis.