Paternò–Büchi Reaction Mass Spectrometry Enables Positional Assignment of Polymethylene-Interrupted Double Bonds in Food-Derived Lipids

Fatty acids (FAs) containing polymethylene-interrupted (PMI) double bonds are a component of human foods; however, they present a significant analytical challenge for de novo identification. Covalent adduct chemical ionization and ozone-induced dissociation mass spectrometry (MS) methods enable unambiguous assignment of PMI-FA double bond positions, however, no method has been reported with electrospray ionization (ESI) platform using off-the-shelf systems. In the current work, we studied the Paternò–Büchi (PB) fragmentation patterns of PMI-FA and triacylglycerol (TG) by analyzing several known food sources. PB-MS/MS and MS³ enabled complete double bond location assignments, including the isolated double bond in PMI-FA and triacylglycerols. Sea urchin (“uni”), oyster, pine nut, and ginkgo nut were characterized for their signature PMI-FA, 20:2­(5Z,11Z), 22:2­(7Z, 15Z), 18:3­(5Z,9Z,12Z), and 20:3­(5Z,11Z,14Z), respectively. Quantitative analyses of the relative abundance of these PMI-FA led to results similar to reference methods. 18:3­(5Z,9Z,12Z) was enriched at the sn-1/sn-3 position in pine nut major TG.