Comparison of two methods for the determination of nitrogen and oxygen isotope composition of dissolved nitrates

Two methods of sample preparation for isotopic analyses of nitrates dissolved in water are presented: (i) chemical conversion of NO3– to N2 and CO2, and (ii) bacterial reduction of NO3– to N2O, followed by the measurement of nitrogen and oxygen isotope composition of these gases. Both methods have been successfully used for routine isotope analyses of dissolved nitrates in different types of water. The chemical conversion method, requiring ca. 300 mi mol of NO3– yields relatively good precision, in the order of 0.3‰ for delta15N, and 0.6‰ for delta18 O. The bacterial reduction method was modified in this study to accommodate samples of intermediate size (ca. 30 mmol of NO3–) to provide sufficient amount of N2O gas for isotope analysis using conventional IRMS. The method demonstrated satisfactory results, although sample preparation was more complicated and required access to microbiological laboratory.