Structural characterization of triacylglycerols as lithiated adducts by electrospray ionization mass spectrometry using low-energy collisionally activated dissociation on a triple stage quadrupole instrument

We describe features of: tandem mass spectra of lithiated adducts of triacy lglycerol (TAG) species obtained by electrospray ionization mass spectromet ry (ms) with low-energy collisionally activated dissociation (CAD) on a tri ple stage quadrupole instrument. The spectra distinguish isomeric triacylgl ycerol species and permit assignment of the mass of each fatty acid substit uent and positions on the glycerol backbone to which substituents are ester ified. Source CAD-MS2 experiments permit assignment of double bond location s in polyunsaturated fatty acid substituents. The ESI/MS/MS spectra contain [M + Li - (RnCO2H)](+), [M + Li - (RnCO2Li)](+), and RnCO+ ions, among oth ers, that permit assignment of the masses of fatty acid substituents. Relat ive abundances of these ions reflect positions on the glycerol backbone to which substituents are esterified. The tandem spectra also contain ions ref lecting combined elimination of two adjacent fatty acid residues, one of wh ich is eliminated as a free fatty acid and the other as an alpha,beta-unsat urated fatty acid. Such combined losses always involve the sn-2 substituent , and this feature provides a robust means to identify that substituent. Fr agment ions reflecting combined losses of both sn-1 and sn-3 substituents w ithout loss of the sn-2 substituent are not observed. Schemes are proposed to rationalize formation of major fragment ions in tandem mass spectra of l ithiated TAG that are supported by studies with deuterium-labeled TAG and b y source CAD-MS experiments. These schemes involve initial elimination of a free fatty acid in concert with a hydrogen atom abstracted from the ol-met hylene group of an adjacent fatty acid, followed by formation of a cyclic i ntermediate that decomposes to yield other characteristic fragment ions. De termination of double bond location in polyunsaturated fatty acid substitue nts of TAG is achieved by source CAD experiments in which dilithiated adduc ts of fatty acid substituents are produced in the ion source and subjected to CAD in the collision cell. Product ions are analyzed in the final quadru pole to yield information on double bond location. (C) 1999 American Societ y for Mass Spectrometry.