Multistage tandem mass spectrometry of anionic phosphatidylcholine lipid adducts reveals novel dissociation pathways

The gas-phase fragmentation reactions of various anionic adducts of phosphatidylcholine lipids have been examined by electrospray ionization and multistage tandem mass spectrometry (MSn) in a linear quadrupole ion trap mass spectrometer. The MS/MS and MSn fragmentation reactions of the anionic adducts were found to be highly dependent on the identity of the anions (A). Consistent with previous reports, MS/MS of phosphatidylcholine lipid ions formed by anionic adduction with formate, acetate and chloride resulted in a dominant product ion corresponding to the loss of (A + CH3) from the [M + A]− precursor ions. In contrast, MS/MS of other anion adducted precursor ions gave rise to multiple products, including [M + A(A + CH3)]−, [M + A(AH)]−, [M + A(AH + CH3)]−, [M + A(AH + (CH3)3N)]−, [M + A(A + (CH3)3NCHCH2)]− and [M + A(CH3)3N)]−. In order to gain evidence for the structures of these product ions, and the mechanisms by which they were formed, each of the products formed by MS/MS were subjected to further analysis by MS3 and MS4. The MS/MS and MSn fragmentation reactions of several deuterium isotope labeled phosphatidylcholine lipid standards, as well as authentic or structurally analogous lipid standards corresponding to the proposed product ion structures were also examined to gain evidence to support the proposed mechanisms. The results from this study indicate that great care must be taken to control the identity of the anions employed during lipid isolation, purification and separation, in order to allow the development and application of sensitive and robust neutral loss scan methods for mass spectrometry based lipid identification and quantitative analysis.