GAS-PHASE ION/ION REACTIONS FOR ENHANCED LIPID ANALYSIS

Heightened awareness regarding the implication of disturbances in lipid metabolism with respect to prevalent human-related pathologies demands analytical techniques that provide unambiguous structural characterization and accurate quantitation of lipids in complex biological samples. The diversity in molecular structures of lipids along with their wide range of concentrations in biological matrices present formidable analytical challenges. Modern mass spectrometry (MS) offers an unprecedented level of analytical power in lipid analysis, as many advancements in the field of lipidomics have been facilitated through novel applications of and developments in electrospray ionization tandem mass spectrometry (ESI-MS/MS). ESI allows for the formation of intact lipid ions with little to no fragmentation and has become widely used in contemporary lipidomics experiments due to its sensitivity, reproducibility, and compatibility with condensed-phase modes of separation, such as liquid chromatography (LC). Owing to variations in lipid functional groups, ESI enables partial chemical separation of the lipidome, yet the preferred ion-type is not always formed, impacting lipid detection, characterization, and quantitation. Moreover, conventional ESI-MS/MS approaches often fail to expose diverse subtle structural features like the sites of unsaturation in fatty acyl constituents or acyl chain regiochemistry along the glycerol backbone, representing a significant challenge for ESI-MS/MS. To overcome these shortcomings, various charge manipulation strategies, including charge-switching, have been developed to transform ion-type and charge state, with aims of increasing sensitivity and selectivity of ESI-MS/MS approaches. Importantly, charge manipulation approaches afford enhanced ionization efficiency, improved mixture analysis performance, and access to informative fragmentation channels.

Here, gas-phase ion/ion chemistry was developed to transform conventional lipid ion types formed upon direct ESI into structurally informative ion types entirely within the mass spectrometer. Explicitly, gas-phase anionic to cationic charge switching chemistries were first developed for fatty acid profiling, as unambiguous structural elucidation and relative quantitation were achieved. Extensions of this gas-phase charge switch derivatization strategy to glycerophospholipids (GPLs), including ether GPLs, and fatty acid esters of hydroxy fatty acids demonstrates the versatility and flexibility of the ion/ion platforms. In an alternate approach, gas-phase proton transfer ion/ion reactions were employed for the gas-phase separation, concentration, and identification of cardiolipins (CLs) from total lipid extract. In total, benefits for lipid structure elucidation and enhanced detection efficiencies have been demonstrated utilizing the reported gas-phase ion/ion platforms.