Electrospray ionization and low energy tandem mass spectrometry of polyhydroxy unsaturated fatty acids.

Negative ion electrospray ionization, fast-atom bombardment, and low energy tandem mass spectrometry were used for the analysis of dihydroxy-eicosatrienoic acids, which contain a vicinol diol and three nonconjugated double bonds, dihydroxy-eicosatetraenoic acids, which contain a conjugated triene structure, and trihydroxy-eicosatetraenoic acids, which contain a vicinol diol and a conjugated tetraene structure. In general, the product ion spectra were qualitatively similar for both modes of ionization, but electrospray ionization was strikingly more efficient in generation of abundant carboxylate anions that could be collisionally activated to yield product ion spectra. Collision-induced dissociation fragmentation mechanisms were described generally by α-hydroxy fragmentations directed by relative positions of double bonds and were consistent with stable isotope labeling studies. Rearrangement of the conjugated triene system in dihydroxy-eicosatetraenoic acids may be described by formation of a cyclohexadiene structure. Fragmentations that involve a two-proton transfer were described best by intramolecular oxidation of a hydroxy substituent to an enolate that resulted in an extended conjugated system. Collision-induced dissociation spectra obtained for the polyhydroxy unsaturated fatty acids, which are isobaric within each class, were uniquely descriptive of individual structures.

Analysis of hydroxy fatty acids as pentafluorobenzyl ester, trimethylsilyl ether derivatives by electron ionization gas chromatography/mass spectrometry.

Electron ionization (EI) gas chromatography/mass spectrometry (GC/MS) analysis of pentafluorobenzyl ester-trimethyl sllyl ether (PFB-TMS) derivatives of hydroxy-subshtuted fatty acids provides structural information comparable to that obtained in analysis of methyl ester-trimethyl silyl ether (Me-TMS) derivatives. Use of this derivative eliminates the need to prepare two separate derivatives, the PFB-TMS derivative for molecular weight determination by electron capture ionization (negative ions) analysis and the Me-TMS derivative for structural determination by EI GC/MS analysis. The relative abundance of fragment ions observed during EI GC/MS analysis of these derivatized unsaturated fatty acids indicates the location of the -OTMS substituents relative to double bond positions in those cases studied. The most abundant fragment ions are observed when the compound contains an unsaturation two carbon atoms removed from the -OTMS ether carbon (the ß-OTMS position). The "saddle effect" observed in the GC/MS analyses of some derivatized monohy- droxy unsaturated fatty acids is suggested to be due to a thermally allowed pericyclic double bond rearrangement and indicates the presence of a conjugated diene one carbon atom removed from the -OTMS ether carbon (the α-OTMS position). The saddle effect is most prominent for fatty acids that contain additional unsaturation separated by a single methylene unit from the conjugated diene moiety.