Comprehensive Structural Characterization of Lipids by Coupling Paternò-Büchi Reaction and Tandem Mass Spectrometry.

Since the invention of soft ionization methods, in particular electrospray ionization (ESI), mass spectrometry (MS) has become the method of choice for both qualitative and quantitative analysis of lipids from complex samples. A large number of lipids can be readily detected from a single mass spectrum free from molecular fragmentation that may complicate spectral interpretation. This has been the driving force for MS to play a predominant role in lipidomics. However, elucidation of the detailed lipid structures, especially the location of carbon-carbon double bond (C=C), remains challenging for MS-based lipid analysis workflows. Here we describe the coupling of photochemical derivatization of C=C via Paternò-Büchi (PB) reaction with tandem mass spectrometry (MS/MS) to identify C=C locations in unsaturated lipids and quantify lipid C=C location isomers. The PB reaction can be conducted online in ~30 s, which transforms a C=C into the oxetane ring structure. Subjecting PB products of lipids to MS/MS leads to the formation of abundant C=C-specific fragment ions upon low energy collision-induced dissociation.