Journal of Isotopes (Jun 2024)
The Optimization of Hypobromite Oxidation Combined with Sodium Azide Chemical Methods and Determination of the Nitrogen Isotopic Composition of Dissolved Ammonium in Seawater
Abstract
Ammonium is a key dissolved inorganic nitrogen (DIN) in the biogeochemical cycle of nitrogen in the ocean. Depending on the environment，the 15N natural abundance (δ15Nair ) of NH4+ can provide insights into N sources and mechanisms of N transformations in terrestrial ecosystems. A variety of methods have been developed for δ15Nair determination for NH4+ during the past several decades, such as the steam distillation method，ammonia diffusion method，ion exchange method and chemical conversion method. The former three methods are not unsuitable for low NH4+ concentration in seawater samples. In this study，a pretreatment method was developed for determination of nitrogen isotopic composition in ammonium based on the chemical method by hypobromite oxidation combined with sodium azide. To determine the reagent blank source and the effection on the yield rate and the value of δ15Nair, the appropriate reagent additions, NH4+ concentration were analyzed. There is no interference from any of the nitrogen-containing compounds tested. Higher concentration samples are readily diluted to lower concentration or taken a small volume. When the NH4+ concentration ranged from 5 to 50 μmol/L，the yield rate of ammonium converting to N2O was greater than 92%. And the calibration curves of ammonium isotopic standards with different abundances showed a good linearity The slope was 0.465 which was close to the theoretical values of 0.5. The standard deviation of δ15Nair measurements was 0.35‰ (n=5)，and the difference between the measured value and the reference value was 0.26‰. This method was used to determination of the nitrogen isotopic composition of dissolved ammonium in seawater, and δ15Nair (NH4+) value were between 11.46‰ to 22.31‰ . The standard deviation were between 0.20‰ to 0.62‰ (n=5). This method meet the requirements of ammonium nitrogen isotope analysis in seawater.
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