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.