Further optimisation of the denitrifier method for the rapid 15N and 18O analysis of nitrate in natural water samples

Abstract

Rationale: This study aims to develop a simplified denitrifier method for the δ15N and δ18O analysis of nitrate (NO3−) in natural water samples combining the method of Zhu et al (Sci Total Environ. 2018; 633: 1370–1378) and the original denitrifier method of Sigman et al (Anal Chem. 2001; 73: 4145–4153). Unlike in the aforementioned methods, the aerobic cultivation was performed without the addition or removal of nitrate in the liquid medium. We remove the nitrate contained in the nutrient medium as N2O in the gas phase by an additional purging step after incubation overnight before the water sample is injected. This eliminates the need for another preparation step, thus saving working time. Methods: The δ15N and δ18O values of dissolved NO3− were determined using a Delta V Plus isotope ratio mass spectrometer coupled to a GasBench II sample preparation device that included a denitrification kit. Results: After optimising the influencing factors (i.e., purging gas, purging time, and type of crimp seals), the method yielded high accuracy and precision (standard deviations were generally ≤0.7‰ for δ18O values and ≤0.3‰ for δ15N values), confirming the suitability of this procedure. Finally, the potential applicability of the method was demonstrated by measuring the isotopic composition of NO3− in natural water samples. Conclusions: The denitrifier method for converting NO3− into N2O for isotope analysis was optimised. This allowed the sample preparation time to be further reduced. The complete working time for sample preparation, including all steps, takes 10 min per vial if 60 vials are prepared in one run. The water samples are ready for isotope analysis on the fourth day after preparation has started. Isotope measurements can be performed up to 14 days after preparation.