Gas-Phase Ion/Ion Reactions Involving Tris-Phenanthroline Alkaline Earth Metal Complexes as Charge Inversion Reagents for the Identification of Fatty Acids

Fatty acids (FA) play vital biological roles as energy sources, signaling molecules and key building blocks of complex lipids in cell membranes. Modifications to FA structure and composition are associated with the onset and progression of a number of chronic diseases. Consequently, the sensitive detection and unambiguous structure elucidation of FA is integral to the advancement of biomedical sciences. Recent advances in FA analysis have taken advantage of wet chemical derivatization to enhance detection and drive unique fragmentation in tandem mass spectrometry protocols. Here, we significantly further this approach through demonstrating gas-phase charge inversion of singly deprotonated FA ions, [M - H](-) , using doubly charged tris-phenanthroline alkaline earth metal complexes, [Cat(Phen)(3)](2+) (Cat = Mg2+, Ca2+, Sr2+, or Ba2+). Metal cationized FA, [M - H + Cat](+) are obtained after the gas-phase ion/ion reaction. Low-energy collision-induced dissociation (CID) of the [M - H + Ca](+) cations facilitates double bond localization for a variety of monounsaturated and polyunsaturated FAs. Ultimately, detailed characterization presented unambiguous distinction among FA double bond positional isomers, such as n-3 and n-6 isomers. The method was successfully used to identify the FA profile of corn oil, including double bond localization for unsaturated FAs present.