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

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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.