Online analysis of isotopic signatures of nitrate, nitrite and ammonium in aqueous samples by isotope ratio mass spectrometry

硝酸盐、亚硝酸盐和铵是土壤和水生态系统中氮循环的关键化学物种。它们在植物营养、微生物代谢和生态系统功能中的重要作用早已为人所知，并得到了广泛的研究。亚硝酸盐是氮素主要转化过程的界面，越来越多的证据表明，尽管亚硝酸盐在土壤和水中很少观察到，有时在氮循环研究中也被忽视，但它与许多氮素途径的控制密切相关。土壤溶液和水体中硝酸盐、亚硝酸盐和氨氮中15N的自然丰度水平对阐明这些物种的来源和转化过程具有重要意义。到目前为止，只有离线方法可以在自然丰度水平下分析水溶液中的硝酸盐和/或铵。建立了一种在线四极质谱(QMS)法选择性分析标记的铵、亚硝酸盐或硝酸盐的15N丰度。然而，由于质谱仪的精度有限，这种方法不足以在自然丰度水平下进行同位素比率分析。因此，到目前为止，由于缺乏足够有效地分析N底物自然丰度的方法，使得能够分析足够大的样本量以实现足够的时间和空间分辨率的数据，以提高对N循环的理解，阻碍了阐明N周转过程与N池的相互作用的手段。该项目的目的是开发一种在线分析水溶液中自然丰度下的硝酸盐、亚硝酸盐和铵氮同位素比值的方法。该方法将基于用于质量管理体系方法的化学转化。将使用气体渗透膜将分析物从液相连接到同位素比质谱仪。与离线方法相比，该设置将允许自动样品制备和分析，从而显著提高样品吞吐量。这最终将为N循环研究的实质性进展铺平道路。

Nitrate, nitrite and ammonium are key chemical species of the N cycle in soil and aquatic ecosystems. Their prominent role in plant nutrition, microbial metabolism and ecosystem functioning is long known and extensively studied. Nitrite is an interface for the main N transformation processes and there is growing evidence that nitrite, although rarely observed in soils and waters and sometimes neglected in N cycle studies, is highly relevant for the control of many N pathways. Natural abundance levels of 15N in nitrate, nitrite and ammonium in soil solution and water bodies are important to elucidate the origin and transformation processes of these N species. To date, only off-line methods are available to analyze nitrate and/or ammonium from aqueous solutions at natural abundance levels. An on-line method using quadrupole mass spectrometry (QMS) has been developed for the selective analysis of 15N abundances of labelled ammonium, nitrite or nitrate. However, this method is not precise enough for isotope ratio analysis at natural abundance levels due to the limited accuracy of the mass spectrometer. As a result, until now the means to elucidate the interaction of N turnover processes and N pools is hampered by the lack of methods to analyze natural abundance of N substrates efficiently enough to allow the analysis of large enough sample numbers to achieve an adequate temporal and spatial resolution of data to improve the understanding of the N cycle. The aim of the project is to develop an on-line method for analyzing nitrate, nitrite and ammonium N isotope ratios at natural abundance by isotope ratio mass spectrometry from aqueous solutions. The method will be based on the chemical conversions used for the QMS approach. A gas permeable membrane will be used to interface the analytes from liquid phase into the isotope ratio mass spectrometer. The setup will allow automated sample preparation and analysis and thus increase sample throughput considerably as compared to off-line methods. This ultimately will pave the way for substantial progress in N cycle research.