Alkaline persulfate oxidation as an intermediate step for the development of a wet chemical oxidation interface for compound-specific δ15N analysis by LC-IRMS

For the measurement of compound-specific isotope ratios by liquid chromatography isotope ratio mass spectrometry (LC-IRMS), complete mineralization of organic compounds to a single species of measurement gas is required so that isotopic fractionation can be minimized and corrected by identical treatment with standards. The established use of peroxydisulfate in an acidic environment has its limitations, especially when it comes to the complete oxidation of nitrogen-containing compounds with aromatic ring systems. Under acidic oxidation conditions, ammonium and nitrate were identified as the main nitrogen containing mineralization products of the oxidation of different model compounds. In contrast to the oxidation in an acidic environment, alkaline peroxydisulfate oxidation leads to nitrate as a final mineralization product. The concept of alkaline oxidation was transferred from large-scale batch experiments to a commercially available oxidation reactor used in LC-IRMS systems. The obtained nitrate recoveries indicate that alkaline oxidation could be a promising step towards the measurement of compound-specific nitrogen isotope ratios by LC-IMRS. In our work, we show that alkaline peroxydisulfate oxidation allows faster and more complete mineralization of nitrogen-containing compounds. For several model compounds, 63 to 100% of the initially present nitrogen was converted to nitrate within a reaction time of 43 s.