Pretreatment Method for Nitrogen Isotopic Analysis of Nitrate in Seawater

A suitable method for the pre-treatment of dissolved nitrate samples in seawater for nitrogen isotopic analysis was established. In this method, the seawater sample was first processed by removing nitrite and ammonium. Then, Devarda's alloy was used in sample for conversion of dissolved nitrate to ammonium. The sample was distilled and the ammonium condensate was collected with zeolite. After distillation, the collected condensate was filtered and then prepared for determining nitrogen isotopic values. Some experiments designed to optimize various parameters of the method were conducted. The distillation condition, effects of salinity on nitrogen isotopic analysis, absorption of ammonium onto zeolite, and an improved method on a large volume of seawater were discussed. The results showed that the distillation step had an average recovery of (104.9 ± 4.2)% (n = 6) when distillating every 300 ml aliquot of the sample under strong alkaline condition with 0.5 g Devarda's alloy and a distillation time of 30 min. The nitrogen isotopic fractionation decreased markedly when salinity was increased from 0% to 0.5%; furthermore increase (1.0%–3.5%) showed little effect. The absorption of ammonium onto zeolite had a high yield of (95.96 ± 1.08)% (n = 6) in average. An improved collection method was used to process a large volume of seawater with several distillations and had good effect on analysis. The method had been applied to analyze water samples collected from the Yangtze River estuary. The analytical result could indicate the dissolved nitrate in seawater for δ15N analysis. This method could provide valuable information about the source and cycle mechanism of dissolved nitrogen in estuary waters.