Tandem Mass Spectrometry for Collisional Activation of Alkali-Metal-Cationized Fatty Acids: A Method for Determining Double Bond Location

A new method is described for structural determination of fatty acids by using fast atom bombardment (FAB) and tandem mass spectrometry. Fatty acids are desorbed by FAB as molecules cationized with alkali metal Ions (Cat), specifically as (M + Cat)₊ and (M + 2Cat - H)₊ ions. The (M + Li)₊ and (M + Na)₊ Ions and the (M + 2Cat - H)₊ Ions, where Cat = Ll₊, Na₊, K₊, Rb₊, and Cs₊, undergo remote charge site fragmentations when collisionally activated at high translational energy. These remote site fragmentations, as those Induced by collisional activation (CA) of (M - H)_- ions of fatty acids, are useful for determining locations of double bonds. But unlike activation of (M - H)_- Ions, activation of the alkall-metal-catlonized acids leads to remote charge site fragmentations even for polyunsaturated acids with tour or more double bonds and for acids In which the unsaturation Is located near the carboxyl moiety. Decompositions of (M + Cat)₊ and (M + 2Cat - H)₊ ions contrast with decompositions of (M + H)₊ ions; these latter Ions do not undergo analogous remote site fragmentations. The detection limit for obtaining a CAD spectrum of the (M + 2LI - H)₊ Ions of oleic acid Is 10 ng.