An improved method of ion exchange for nitrogen isotope analysis of water nitrate

Abstract

Nitrate nitrogen and oxygen isotopes have been widely used to trace the nitrogen biogeochemical cycle by identifying NO 3 − sources. An improved method of anion exchange was developed to measure δ 15N-NO 3 − in fresh water by continuous-flow elemental analyzer/isotope ratio mass spectrometry (EA-IRMS). We used a custom-built exchange resin column, a peristaltic pump and the oven-drying method in our experiments. Consequently, the amount of Ag 2O used as a neutralizer was reduced, time was saved, and operation became simpler than before. Meanwhile, analytical precision remained identical to previous studies. KNO 3 solutions were prepared at 0.2, 5 and 25 mg-N L −1 from KNO 3 standard salt (δ 15N = +6.27‰), and the average δ 15N values of the solutions after having been absorbed on and subsequently stripped from anion columns were +6.62 ± 0.22‰ ( n = 6), +6.38 ± 0.09‰ ( n = 6), and +6.26 ± 0.07‰ ( n = 6), respectively. In addition, the “natural” water sample δ 15N-NO 3 − showed consistency in comparison to standards, and the mean standard deviation by the different approaches was 0.08‰. Accordingly, by these improvements the anion exchange resin technique is demonstrated to be more suitable for measuring δ 15N in NO 3 − than original techniques.