Elucidation of the Double-Bond Position of Long-Chain Unsaturated Fatty Acids by Multiple-Stage Linear Ion-Trap Mass Spectrometry with Electrospray Ionization

Linear ion-trap (LIT) MS² mass spectrometric approach toward locating the position of double bond(s) of unsaturated long-chain fatty acids and toward discerning among isomeric unsaturated fatty acids as dilithiated adduct ([M - H + 2Li]⁺) ions are described in this report. Upon resonance excitation in a LIT instrument, charge-remote fragmentation that involves β-cleavage with γ-H shift (McLafferty rearrangement) is the predominant fragmentation pathway seen for the [M - H + 2Li]⁺ ions of monoenoic long-chain fatty acids. The fragmentation process results in a dilithiated product ion of terminally unsaturated fatty acid, which undergoes consecutive McLafferty rearrangement to eliminate a propylene residue, and gives rise to another dilithiated adduct ion of terminally unsaturated fatty acid. In addition to the above-cited fragmentation process, the [M - H + 2Li]⁺ ions of homoconjugated dienoic long-chain fatty acids also undergo α-cleavage(s) with shift of the allylic hydrogen situated between the homoconjugated double bonds to the unsaturated site. These fragmentation pathways lead to two types of C{single bond}C bond cleavages that are allylic (α-cleavage) or vinylic, respectively, to the proximal C{single bond}C double bond, resulting in two distinct sets of ion series, in which each ion series is separated by a {single bond}CH₂CH{double bond, short}CH{single bond} (40 Da) residue. These latter fragmentations are the predominant processes seen for the polyunsaturated long-chain fatty acids. The spectrum feature dependent on the position of unsaturated double bond(s) affords unambiguous assignment of the position of double bond(s) of long-chain unsaturated fatty acids.