Chemical conversion of nitrate and nitrite to nitrous oxide for nitrogen and oxygen isotopic analysis in freshwater and seawater
Chemical conversion of nitrate and nitrite to nitrous oxide for nitrogen and oxygen isotopic analysis in freshwater and seawater
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DOI：
10.1021/ac050528s
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发表时间：
2005-09-01
影响因子：
7.4
通讯作者：
Altabet, MA
中科院分区：
文献类型：
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作者：
McIlvin, MR;Altabet, MA
我们提出了一种对环境浓度下海水和淡水的硝酸盐和亚硝酸盐进行氮和氧天然同位素丰度分析的新方法。该方法包括使用海绵镉将硝酸盐还原为亚硝酸盐，并使用乙酸缓冲液中的叠氮化钠进一步还原为一氧化二氮。对于单独的亚硝酸盐分析，可以简单地绕过镉还原步骤。使用自动化系统从水样中清除一氧化二氮并低温捕获，随后释放到气相色谱柱中。然后通过开放式分流器在连续流同位素比质谱仪上分析分离出的一氧化二氮。本文描述了从硝酸盐、亚硝酸盐或两者中获得可重复的自然丰度水平氮和氧同位素比所需的基本方案和反应条件，以及支持这些结论的结果。对于浓度范围为 40 至 0.5 μM（亚硝酸盐更好）的硝酸盐样品，发现氮气的标准偏差小于千分之 0.2，氧气的标准偏差小于千分之 0.5，50 mL 样品的空白为 2 nmol。氮和氧同位素分馏和氧原子交换在每批分析中都是一致的。没有任何海水基质的干扰。迄今为止，只有另一种方法可以测量海水中硝酸盐氧同位素丰度，并且没有一种方法可以在硝酸盐存在的情况下单独测量亚硝酸盐。与其他方法相比，该方法可能被证明更简单、更快速，并且可以获得较低浓度的硝酸盐和亚硝酸盐的同位素信息。
We present a novel method for nitrogen and oxygen natural isotopic abundance analysis of nitrate and nitrite of seawater and freshwater at environmental concentrations. The method involves the reduction of nitrate to nitrite using spongy cadmium with further reduction to nitrous oxide using sodium azide in an acetic acid buffer. For separate nitrite analysis, the cadmium reduction step is simply bypassed. Nitrous oxide is purged from the water sample and trapped cryogenically using an automated system with subsequent release into a gas chromatography column. The isolated nitrous oxide is then analyzed on a continuous flow isotope ratio mass spectrometer via an open split. This paper describes the basic protocol and reaction conditions required to obtain reproducible natural abundance level nitrogen and oxygen isotopic ratios from nitrate, nitrite, or both, and the results obtained to support these conclusions. A standard deviation less than 0.2 parts per thousand for nitrogen and 0.5 parts per thousand for oxygen was found for nitrate samples ranging in concentration from 40 to 0.5 mu M (better for nitrite), with a blank of 2 nmol for 50-mL samples. Nitrogen and oxygen isotopic fractionation and oxygen atom exchange were consistent within each batch of analysis. There was no interference from any seawater matrixes. Only one other method published to date can measure the nitrate oxygen isotopic abundance in seawater and none that do so for nitrite alone in the presence of nitrate. This method may prove to be simpler, faster, and obtain isotopic information for lower concentrations of nitrate and nitrite than other methods.