A METHOD TO EXTRACT AMMONIUM AND NITRATE FROM FRESHWATER FOR NITROGEN ISOTOPE ANALYSIS
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摘要: 这是一种基于连续流同位素质谱分析法测定水体氨氮和硝态氮氮同位素的预处理方法。该方法先利用阴阳离子树脂交换法对水体氨氮和硝态氮进行浓缩,然后凯氏定氮法将氨氮和硝态氮转化为氨气,用过量稀硫酸接收。接收液加入氢氧化钡去除过量的硫酸根,滤液冷冻干燥后获得纯净的硫酸铵固体,可满足连续流同位素质谱仪分析的进样要求,从而减少了氮同位素分析所需的氮量。实验结果表明,离子交换法对氨氮和硝态氮的吸附和洗脱效率均高于95.1%,洗脱液的转化效率高于95.0%,不会产生明显的同位素分馏。不同类型(雨水、河水、湖水和井水)、不同浓度(0.035~11.047 mg/L)的氨氮和硝态氮水样的氮同位素分析结果表明,该方法具有较好的可靠性。Abstract: This paper presents a simple and rapid method to concentrate ammonium and nitrate from freshwater samples for an on-line isotope analysis using an elemental analyzer coupled to a mass spectrometer. Ammonium and nitrate are collected by passing the water-sample through pre-filled, disposable, ion exchanging resin columns. The ammonium and nitrate in sample solutions are separated as ammonium into diluted H2SO4 by sequential distillation techniques. Then, the excrescent SO42- in the distillate is precipitated directly as an insoluble salt of BaSO4. (NH4)2SO4 is collected by freezing-drying filtrate, which is subsequently combusted in an elemental analyzer for δ15N determination. The sorption efficiency of 001×7 cationic resin and 732 cationic resin for 2 L sample solutions with ammonium and nitrate concentration of 0.5, 1.0, 5.0 mg/L is >95.1%, elution recovery is >95%, showing insignificant isotope fractionation during precipitation. Analyses of the solutions prepared from (NH4)2SO4 and KNO3 distributed, and also the rain, river, lake and well samples showed that the method can produce good quality of isotope measurements.
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Key words:
- nitrogen isotope /
- ammonium /
- nitrate /
- ion exchange resin
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