优化次溴酸盐-叠氮化钠化学法测定海水中铵态氮同位素

The Optimization of Hypobromite Oxidation Combined with Sodium Azide Chemical Methods and Determination of the Nitrogen Isotopic Composition of Dissolved Ammonium in Seawater

利用次溴酸盐将海水样品中的NH_(4)^(+)氧化为NO_(2)^(−)并与叠氮化钠反应转化为N_(2)O,测试其氮同位素组成,探讨转化过程中试剂添加量、NH_(4)^(+)浓度、溶解有机氮(DON)含量及高浓度样品稀释对测试结果的影响,确定最佳实验条件,同时绘制不同丰度铵态氮同位素标样校准曲线。结果表明,在最佳反应条件下,NH_(4)^(+)浓度范围在5~50μmol/L之间,其NH_(4)^(+)-N_(2)O转化率均在92%以上,且水样中DON含量对NH_(4)^(+)-N_(2)O转化率和δ^(15)Nair值准确度无显著影响。4种丰度铵态氮同位素标样校准曲线斜率为0.465,相关系数R^(2)为0.999,相关性良好。铵态氮同位素标样δ^(15)Nair值标准偏差为0.35‰(n=5),测量值与参考值的差值为0.26‰,精密度与准确度符合环境样品测试要求。5个海水样品铵态氮δ^(15)Nair值范围为11.46‰~22.31‰,标准偏差在0.20‰~0.62‰(n=5)之间。

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(δ^(15)Nair)of NH_(4)^(+) can provide insights into N sources and mechanisms of N transformations in terrestrial ecosystems.A variety of methods have been developed for δ^(15)Nair determination for NH_(4)^(+) 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 NH_(4)^(+) 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 ofδ^(15)Nair,the appropriate reagent additions,NH_(4)^(+) 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 NH_(4)^(+)concentration ranged from 5 to 50μmol/L,the yield rate of ammonium converting to N_(2)O 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 δ^(15)Nair 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 δ^(15)Nair(NH_(4)^(+))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.