A novel gas chromatography–combustion–isotope ratio mass spectrometry derivatization method for nitrogen isotope analysis of nitrate in water

Rationale The stable nitrogen isotopic composition of nitrate (NO 3 − ) can be an effective tool to identify NO 3 − sources and understand nitrogen cycling. However, the usefulness of this isotopic tool in environment research has been limited by complex analytical procedures for the nitrogen isotopic analysis of NO 3 − . We aimed to develop a novel method for the simple and rapid measurement of the δ 15 N value of NO 3 − in water. Methods Water NO 3 − was transformed into nitrobenzene for nitrogen isotopic analysis by gas chromatography–combustion–isotope ratio mass spectrometry (GC/C-IRMS). The derivatization procedure was optimized to maximize the nitrobenzene yield. 1.5-fold 98% sulfuric acid and 0.5-fold benzene with respect to the water sample were used to derive nitrobenzene with only 1 min of vortex at room temperature. The nitrobenzene derivatization product (dissolved in benzene) was then separated by GC, followed by combustion to N 2 for δ 15 N analysis by IRMS. Results The minimum amount of nitrogen required for accurate determination of the δ 15 N value was 8.6 ng at an m/z 28 intensity of 150 mV. The standard deviation of the GC/C-IRMS method was ≤0.5‰ (three replicates) even at a low NO 3 − concentration of 9.58 mg L −1 . The various standards and samples with different δ 15 N values exhibited good agreement with the assigned δ 15 N values obtained by using the proposed method. Conclusions Compared with previous methods, the newly developed method provided shorter analysis time and easy operation, and eliminated the need for the removal of chloride and nitrite. The GC/C-IRMS derivatization method offers a fully operational tool for δ 15 N measurement of NO 3 − in water.