Development of an LC-IRMS interface for the component specific determination of the nitrogen isotopic ratios of organic compounds via wet chemical oxidation/reduction

近年来，化合物特定稳定同位素分析（CSIA）已成功地应用于食品真实性控制、体育兴奋剂分析以及环境污染物降解途径的阐明等领域。除了广泛使用气相色谱耦合同位素比质谱法（GC- irms）外，液相色谱法还可以用于分离极性，而不是直接GC相容的化合物。随着LC-IRMS界面的可用性，可以直接从水基质中分离极性化合物。目前市售的LC- irms接口利用LC系统洗脱的分析物的湿化学氧化。过硫酸氢盐常用于界面体系中，通过硫酸盐自由基将分析物分子中的碳氧化为二氧化碳。在第二步中，二氧化碳在气透膜的帮助下从液体流动相中分离出来，并通过氦载气流直接转移到IRMS系统中。与GC-IRMS相比，LC-IRMS系统到目前为止仅限于碳同位素比的测定，因为在氧化过程中没有形成用于IRMS测量的均匀的氮反应产物。含氮有机物经硫酸盐自由基处理后的氧化产物尚未有详细的研究。估计反应产物一方面是硝酸盐和亚硝酸盐等水溶性化合物，另一方面会形成气态氮氧化物。本文将对不同种类含氮化合物的氧化产物进行研究。为了能够确定有机化合物中氮的同位素比率，所含的氮必须转化为IRMS测量的单一物种，例如元素氮。因此，水溶性氮将被还原为气态氮氧化物，通过膜系统将其与液相分离。在第二个还原步骤中，气态氮氧化物将在铜的帮助下进一步还原为元素氮。在该项目中，将比较分析物氧化后亚硝酸盐和硝酸盐湿化学还原的不同途径。此外，还将研究分析物在湿化学氧化过程中的转化率以及系统中不同氮种的还原。最后，利用元素分析同位素比质谱（EA-IRMS）系统和国际同位素标准物质对LC-IRMS系统可获得的不同模型化合物的氮同位素比进行验证。

During the last years, compound specific stable isotope analysis (CSIA) has successfully been applied in the field of food authenticity control, doping analysis in sports and in the elucidation of degradation pathways of environmental pollutants. In addition to the widespread use of gas chromatography coupled to isotope ratio mass spectrometry (GC-IRMS), liquid chromatography can be applied to separate polar, not directly GC compatible compounds. With the availability of LC-IRMS interfaces, it became possible to separate polar compounds for example directly from an aqueous matrix. Currently commercially available LC-IRMS interfaces utilize wet chemical oxidation of the analytes eluting from the LC system. Peroxydisulfate is commonly used in the interface system to oxidize the carbon in the analyte molecules to carbon dioxide via sulfate radicals. In a second step the carbon dioxide is separated from the liquid mobile phase with the help of gas permeable membranes and directly transferred into the IRMS system by a helium carrier gas stream. In contrast to GC-IRMS, LC-IRMS systems are restricted to the determination of carbon isotopic ratios until now, as no uniform nitrogen reaction product for the IRMS measurement is formed during the oxidation. The oxidation products of nitrogen containing organic compounds after sulfate radical treatment have not been studied in detail yet. Estimated reaction products are on the one hand water soluble compounds like nitrate and nitrite, on the other hand gaseous nitrogen oxides will be formed. The oxidation products of different classes of nitrogen containing compounds will be studied in this work. To be able to determine isotope ratios for nitrogen from the organic compounds, the contained nitrogen has to be converted into a single species for the IRMS measurement e.g. elemental nitrogen. Therefore the water soluble nitrogen species will be reduced to gaseous nitrogen oxides, which are separated from the liquid phase by a membrane system. In a second reduction step, the gaseous nitrogen oxides will be further reduced to elemental nitrogen with the help of copper. In the project, different pathways for the wet chemical reduction of nitrite and nitrate after the oxidation of the analytes will be compared. In addition, the conversion rates during the wet chemical oxidation of the analytes and the reduction of the different nitrogen species in the system will be studied. Finally, the nitrogen isotopic ratios which can be obtained for different model compounds with the LC-IRMS system will be validated with the help of an elemental analysis isotope ratio mass spectrometry (EA-IRMS) system and international isotope reference material.