Effect of dissolved organic nitrogen contamination on δ15N–NH4 determination in water samples by modification of the diffusion method with gas-phase trapping

Abstract

Rationale: The isotopic tracer technique is widely used to identify the sources and fate of nitrogen (N) in order to understand the N cycle and contamination in water environments. The stable isotope ratio of ammonium is expected to greatly enhance the tracing analysis by combining it with the traditional nitrate isotope ratio. Diffusion followed by gas-phase trapping is the most commonly applied method for ammonium isotope ratio measurement. Although dissolved organic nitrogen (DON) is abundant in natural water and its breakdown in the diffusion procedure has been reported, the interference of DON with the measurement of ammonium isotope ratios has not been fully examined. Methods: This study aims to test the effect of DON contamination by using organic N compounds, viz. humic acid and alanine. A series of diffusion experiments was conducted at a temperature of 80°C for a maximum of 7 days. Ammonia was transferred from alkaline solution and trapped with an acidic filter. This method was applied for samples with ammonium concentrations between 0.5 and 2.0 mg-N/L. Results: There was no difference between the ammonium N stable isotope ratios for samples with and without added DON compounds; the fractionation between the observed value and the actual value was negligible, in the range of 0.2 to 1.0‰. The modifications of previous studies, i.e. shorter diffusion period, no vigorous shaking and using gas-phase trapping, successfully avoided any breakdown of DON in fresh water samples. Conclusions: The modified method provides high precision and accuracy and it is recommended for the analysis of anthropogenically influenced water samples, such as paddy fields, ground water, rivers and lakes.