Electron ionization mass spectrometry fragmentation and multiple reaction monitoring quantification of autoxidation products of α- and β-amyrins in natural samples

Abstract : Rationale: Riverine particulate organic matter is generally considered to be refractory with respect to further decomposition in the ocean. In order to check the validity of this paradigm, there is a real need for tracers sufficiently stable and specific to monitor the degradation of terrestrial higher plant material in the environment. 3β‐Hydroxy‐urs‐12‐en‐11‐one and 3β‐hydroxy‐olean‐12‐en‐11‐one (autoxidation products of α‐ and β‐amyrin) were previously proposed for such use. Methods: Electron ionization mass spectrometry (EIMS) fragmentation pathways of 3β‐hydroxy‐urs‐12‐en‐11‐one and 3β‐hydroxyolean‐12‐en‐11‐one trimethylsilyl (TMS) derivatives were investigated. These pathways were deduced by: (i) low‐energy collision‐induced dissociation (CID)‐gas chromatography/tandem mass spectrometry (GC/MS/MS), (ii) accurate mass measurement and (iii) deuterium labelling. Quantification of these compounds in total lipid extracts of natural samples was then carried out in multiple reaction monitoring (MRM) mode. Results: CID‐MS/MS analyses, accurate mass measurement and deuterium‐labelling experiments allowed us to elucidate EIMS fragmentations of 3β‐hydroxy‐urs‐12‐en‐11‐one and 3β‐hydroxyolean‐12‐en‐11‐one TMS derivatives. Some specific fragmentation pathways, useful in addition to chromatographic retention times for further characterization, could be selected. As an application of some of the described fragmentations, TMS derivatives of these oxidation products were characterized and quantified in MRM mode in different natural samples. Conclusions: EIMS fragmentations of 3β‐hydroxy‐urs‐12‐en‐11‐one and 3β‐hydroxyolean‐12‐en‐11‐one TMS derivatives exhibit specific fragment ions, which appear to be very useful for the quantification of these oxidation products in natural samples (riverine particulate matter, wet and dry deposited atmospheric particles).