GC-MS for Analyte Identification in Isotope Ratio Mass Spectrometry (Combined GC-MS/IRMS)

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Over recent years we have been advancing compound-specific isotope analysis (CSIA) to trace sources of organic groundwater contaminants, and to decipher their natural transformation from compound-specific isotope values. Gas chromatographic (GC) separation of target compounds is followed by online conversion to CO2 for direct isotope analysis of 13C/12C by isotope ratio mass spectrometry (IRMS). While IRMS is uniquely suited to determine accurate isotope ratios, structural information is lost during combustion to CO2. Hence, the identity of analytes cannot be confirmed independently. Early applications of CSIA were little affected, because they targeted contaminated sites, where analytes were present in high concentrations (mg/L) compared to background organic matter. Hence, pollutants stood out as characteristic peaks in GC-IRMS chromatograms and could be reliably identified by their retention times. We have been striving to expand the CSIA approach also to so-called micropollutants: pesticides, pharmaceuticals and consumer care products which occur in much smaller concentrations (low micrograms/L to nag/L range). In GC-IRMS chromatograms their peaks are, therefore, typically embedded in a contiuum of co-eluting peaks from organic matter leading to prominent research challenges. (i) Analysis of field samples warrants unique compound identification, yet frequently shows subtle changes in chromatographic retention times making them unreliable for analyte identification. (ii) At the same time, our current research efforts would greatly profit from structural elucidation of these interferences to optimize approaches for their targeted removal. This application, therefore, requests funding for crucial instrumentation to be combined with existing IRMS instrumentation: a GC with a conventional quadrupole mass spectrometer (qMS) and an interface for compound conversion prior to IRMS analysis. This equipment will allow splitting the GC flow so that one part goes through the interface into the IRMS for isotope analysis, while the other part is directed to the qMS to enable simultaneous structural identification. Not only will this setup support our measurements at natural abundance that depend on identification of target analytes and background interferences. It will also benefit a second research avenue that we pursue with funding by the International Atomic Energy Agency (IAEA): the identification of micropollutant metabolites by application of a stable isotope-labelled pulse where radioactive labelling is impossible. We will lead a Coordinated Research Network to trace the fate of 13C labelled antibiotics, metabolites and antibiotic resistance in an agricultural field experiment. Here, GC-IRMS instrumentation will be able to detect chromatographic peaks of 13C-labelled metabolites as “needles in the haystack”, while GC-MS analysis would enable simultaneous structural elucidation of these unknown metabolites.