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

Nitrate nitrogen and oxygen isotopes have been widely used to trace the nitrogen biogeochemical cycle by identifying NO3− sources. An improved method of anion exchange was developed to measure δ15N-NO3− 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 Ag2O used as a neutralizer was reduced, time was saved, and operation became simpler than before. Meanwhile, analytical precision remained identical to previous studies. KNO3 solutions were prepared at 0.2, 5 and 25 mg-N L−1 from KNO3 standard salt (δ15N = +6.27‰), and the average δ15N 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 δ15N-NO3− 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 δ15N in NO3− than original techniques.