Dissertation: Mass Spectrometry of Drug Derivatives: A Contribution to the Characterization of Fragmentation Reactions by Labelling with Stable Isotopes

The proper identification of anabolic androgenic steroids with GC/MS in anti-doping research remains an important topic and stable isotope labelling is a convenient and straightforward way to increase confidence in detection through mass spectral structure characterization. The different labelling methods for the introduction of [2H9]TMS and 18O presented in this work were shown to be suitable for the interpretation of benzophenone and hydroxy steroid GC/MS data. Together with HRMS and MS/MS experiments, fragmentation pathways were elucidated and unexpected differences to previously described assumptions uncovered for both substance classes. The practical applicability was confirmed with the self-explanatory mass shifts observed in the respective mass spectra and the comparatively fast preparational steps of labelled derivatives even with low amounts of analyte. Especially the newly developed 18O-labelling method proved to be valuable for confirmatory analysis. It can be employed independently from silylation procedures and thus also for LC-MS approaches, and no migratory tendencies as opposed to TMS groups were observed in the mass spectra. All labelling methods described in this work can be performed with the usual laboratory equipment within a few hours. When used for the characterization of unknown metabolites, the number of reference standards required for unequivocal identification, is expected to be narrowed down and unnecessary laborious and time-consuming synthesis avoided. With the help of isotopically labelled derivatives, differences in the mass spectra of structurally closely related analytes can be determined, which plays a role in the characterization of similar metabolic patterns of endogenous and exogenously administered steroids. In these cases, costly gas chromatography-combustion-isotope ratio mass spectrometry (GC/C-IRMS) for compound identification can be reduced to a minimum or even replaced. Future work may focus on metabolite identification with the developed stable isotope labelling methods or fragment ion elucidation of other relevant steroid subclasses, such as 17-alkyl or 1,4-diene steroids. The use of protecting groups may be helpful to establish derivatization procedures to isotopically label every single functional group in steroids separately. Furthermore, the 18O-labelling method can be further optimized to be applicable for other compound classes as well and should also be tested in LC-MS(/MS) approaches.