Separation of Glycolipids/Sphingolipids from Glycerophospholipids on TiO2 Coating in Aprotic Solvent for Rapid Comprehensive Lipidomic Analysis with Liquid Microjunction Surface Sampling-Mass Spectrometry

In lipidomic analysis by direct mass spectrometry (MS), high abundance lipids with high ionizability (such as glycerophospholipids) would cause ion suppression to lipids with poor ionizability and low abundance (such as glycolipids, sphingolipids, or glycerides), which largely limits the detection coverage for lipidomics. In this work, TiO2-based liquid micro-junction surface sampling (LMJSS) coupled with MS was used for separation of glycerides, phospholipids and glycolipids/sphingolipids in biological samples and rapid analysis of lipids in different classes with high lipidome coverage. We found that, in nonaqueous aprotic solvents, lipids with a glycosyl or sphingosine group could be selectively separated from lipids with a phosphate group (selectivity >10) after being coenriched on TiO2 by tuning the solvent composition. Accordingly, a selective multistep extraction method was developed by loading the biosamples on TiO2 slides in neutral aprotic solvent, and sequentially eluting glycerides in pure acetonitrile, glycerophospholipids in 6% ammonia-94% acetonitrile (v/v) and glycolipids/sphingolipids in 5% formic acid-95% methanol (v/v) by LMJSS probe from TiO2 slide. Each eluate from TiO2 slide was directly delivered by LMJSS to MS for analysis. The total detection time with three desorption steps would be controlled in 3 min. The method performance for each lipid class was evaluated using lipid standards, including matrix effects (107-128%), RSDs (0.4-16%), linearity (0.98-0.99), detection limits (5-3000 ng/mL), the adsorption equilibrium constants (10(2)-10(4)) and adsorption capacity (1-38 mu g/mm(2)) of TiO2 coated slides to lipids. Finally, the TiO2-based-LMJSS-MS method was applied to lipidomic analysis for blood plasma and brain tissue, and compared with direct infusion MS. Results showed that (2-5)-fold more sphingolipids/glycolipids and 40-50 more glycerophospholipids/glycerides were identified in both plasma and brain extract by the new method comparing with direct infusion MS method. Detected lipids were quantified with standard addition calibration method, and the absolute quantitation results measured by TiO2-based-LMJSS-MS were verified with that by the traditional LC-MS method (correlation coefficient >0.98, slope of correlation line = 0.87-1.05).