Compound-specific stable isotope analysis (CSIA) is an analytical technique for determining small isotope ratio variations of individual analytes from complex mixtures that offers unique insights into sources and fate of organic compounds. However, due to the online conversion of analytes into a measuring gas such as carbon dioxide any structural information of analytes is lost during the analysis. At the same time, correct interpretation of isotope data requires baseline separation of analytes without coelution of the unknown compounds. This is of particular importance in liquid chromatography (LC) coupled to isotope ratio mass spectrometry (IRMS) due to its system-inherent low peak capacity. Furthermore, LC-IRMS is hampered by the restriction to purely aqueous eluents. This limits the use of already established chromatographic separations based on the use of organic eluents, which make up over 90 % of applied LC-methods. Finally, LC-IRMS can so far only be used for carbon isotope analysis but not for nitrogen, which is an element potentially providing very useful complementary information for the abundant anthropogenic nitrogen-containing compounds. In the presentation, new instrumental developments to overcome these core limitations will be discussed together with still existing obstacles and first successful application examples.
LC was combined for the first time with simultaneous analysis by IRMS and high-resolution mass spectrometry (HRMS), enabling the direct identification of unknown or coeluting species. We thoroughly investigated and optimized the coupling and showed that it provided reproducible results in terms of resulting peak areas, isotope values, and retention time differences for the two mass spectrometer systems [1].
A possibility to overcome the eluent restriction is offered by coupling two dimensional LC (2D-LC) with LC-IRMS. Adaptation of a conventional 2D-LC set-up allows LC-methods taken from the literature to be used with organic additives in the first dimension, without need for any method development. The analyte of interest is then transferred via heart-cut modulation onto the second dimension, where the organic solvents are separated from the analyte prior to oxidation. First development and optimisations of an 2D-LC-IRMS system in this ongoing work will be presented.
Finally, as an intermediate step towards an LC-IRMS interface capable of nitrogen isotope measurements, the current state of the technical development and first applications for nitrate N isotope measurements will be shown.
Literature:
[1] R. G. H. Marks et al., Anal. Chem. 2022, 94, 2981.
