Distinction among isomeric unsaturated fatty acids as lithiated adducts by electrospray ionization mass spectrometry using low energy collisionally activated dissociation on a triple stage quadrupole instrument

Abstract

Features of tandem mass spectra of dilithiated adduct ions of unsaturated fatty acids obtained by electrospray ionization mass spectrometry with low-energy collisionally activated dissociation (CAD) on a triple stage quadrupole instrument are described. These spectra distinguish among isomeric unsaturated fatty acids and permit assignment of double-bond location. Informative fragment ions reflect cleavage of bonds remote from the charge site on the dilithiated carboxylate moiety. The spectra contain radical cations reflecting cleavage of bonds between the first and second and between the second and third carbon atoms in the fatty acid chain. These ions are followed by a closed-shell ion series with members separated by 14 m/z units that reflect cleavage of bonds between the third and fourth and then between subsequent adjacent pairs of carbon atoms. This ion series terminates at the member reflecting cleavage of the carbon–carbon single bond vinylic to the first carbon–carbon double bond. Ions reflecting cleavages of bonds distal to the double bond are rarely observed for monounsaturated fatty acids and are not abundant when they occur. For polyunsaturated fatty acids that contain double bonds separated by a single methylene group, ions reflecting cleavage of carbon–carbon single bonds between double bonds are abundant, but ions reflecting cleavages distal to the final double bond are not. Cleavages between double bonds observed in these spectra can be rationalized by a scheme involving a six-membered transition state and subsequent rearrangement of a bis-allylic hydrogen atom to yield a terminally unsaturated charge-carrying fragment and elimination of a neutral alkene. The location of the β-hydroxy-alkene moiety in ricinoleic acid can be demonstrated by similar methods. These observations offer the opportunity for laboratories that have tandem quadrupole instruments but do not have instruments with high energy CAD capabilities to assign double bond location in unsaturated free fatty acids by mass spectrometric methods without derivatization.