Electron ionization mass spectrometry fragmentation pathways of trimethylsilyl derivatives of isomeric allylic alcohols derived from HBI alkene oxidation.

Abstract

C25 tri-unsaturated highly branched isoprenoid (HBI) alkenes are produced by a number of marine diatoms around the world yet are very easily oxidized during senescence to yield several isomeric allylic 9-hydroperoxides. Elucidation of the electron ionization mass spectrometry (EIMS) fragmentation pathways of the trimethylsilyl (TMS) derivatives of the alcohols (obtained by reduction of the corresponding 9-hydroperoxides) is essential for their characterization and quantification in natural samples. EIMS fragmentation pathways of TMS derivatives of isomeric allylic alcohols resulting from NaBH4 reduction of photo- and autoxidation products of HBI alkenes were investigated. These pathways were deduced by: (i) low-energy collision-induced dissociation gas chromatography/tandem mass spectrometry (CID-GC/MS/MS) and (ii) deuterium labelling. CID-MS/MS analyses and deuterium labelling allowed us to elucidate EIMS fragmentations of TMS derivatives of several isomeric allylic alcohols resulting from NaBH4 reduction of HBI alkene oxidation products and to propose some specific fragment ions for differentiating individual isomers. As an application of some of the described fragmentations, these oxidized compounds in phytoplanktonic cells collected from the Antarctic were characterized and quantified in multiple reaction monitoring (MRM) mode. EIMS fragmentations of TMS derivatives of several isomeric allylic alcohols resulting from NaBH4 reduction of HBI alkene oxidation products are shown to be strongly dependent on the position and configuration of double bonds, allowing simple, yet robust differentiation of individual isomers in natural samples.