天然海水硝酸盐氮同位素组成的蒸馏法测定

Ammonia distillation method for nitrogen isotope analysis of nitrate in seawater

建立了测定天然海水中硝酸盐氮同位素组成的蒸馏法,该方法主要是在碱性条件下利用戴氏合金将海水中的硝酸盐还原为氨,后利用稀盐酸吸收生产的氨,将得到的氨吸收液浓缩后干燥结晶,利用同位素比值质谱仪测定所得晶体的氮同位素组成。研究中开展了戴氏合金添加量及氨吸收溶液在不同条件下干燥结晶对氮同位素测值的影响研究。结果表明,戴氏合金添加量为3.0g及60℃下直接干燥结晶为最佳的实验条件。所建立的氨蒸馏法氮空白值仅为(0.90±0.19)μmol,低于此前文献报道的氮空白值;氮同位素组成(δ15 N)空白值为(-14.7±4.1)×10-3。运用所建立的氨蒸馏法实测得到的硝酸盐δ15 N值与氨扩散法、硝酸盐直接测定法得到的数值非常吻合,进一步证明所建立氨蒸馏法的可靠性。改进后的氨蒸馏法适用于硝酸盐浓度在2～50μmol/dm3内的天然海水硝酸盐氮同位素组成的测定,方法的标准偏差为±0.3×10-3。

A distillation method for determining 15N abundance of nitrate in seawater was set up in this study.The method mainly involved the conversion of nitrate to ammonia using Devarda＇s alloy and the trapping of the ammonia with diluted HCl.After drying,nitrogen isotopic composition of the ammonium salts was measured by an isotopic ratio mass spectrometer.In this study,the amount of Devarda＇s alloy for nitrate reduction and the conditions for ammonium salts drying were assessed.A total of 3.0 g of Devarda＇s alloy was needed to completely reduce nitrate to ammonia in 2 dm3 seawater,and the drying of ammonium salts at 60 ℃ was appropriate for sample preparation for isotopic measurements.The overall nitrogen blank of our method was（0.90±0.19）μmol,and lower than those reported previously.The δ15N value of nitrogen blank was（-14.7±4.1）×10-3.Nitrogen isotopic composition determined by this method was well consistent with that measured by the ammonia diffusion method,demonstrating the reliability of this method.The ammonia distillation method was applicable for measurement of nitrogen isotopes in seawater with the nitrate concentration of 2~50 μmol/dm3.The precision for nitrogen isotope determination was within ±0.3×10-3.