A multi-modal mass spectrometric approach for lipid analysis enabled by online fractionation of complex mixtures in a solid-phase extraction cartridge.

This study aimed to develop a direct infusion analytical platform for analyzing complex lipid mixtures using a solid-phase extraction (SPE) cartridge. The SPE cartridge was developed in-house and coupled to a contained-electrospray ionization (cESI) source, which enabled online fractionation of lipids followed by real-time mass spectrometry (MS) analysis of eluates. The SPE cartridge was packed with a silica sorbent. We used multiple solvents with varying polarities to sequentially elute and characterize different lipids. To stabilize the electrospray signal derived from nonpolar organic solvents, our SPE-cES-MS platform utilized a co-axial spray system that allowed an ammonium acetate containing lipase enzyme to be delivered from a second electrospray capillary. In its fullest implementation, the SPE-cESI-MS platform was applied to fractionate a sample mixture containing 18 different lipids using hexane, ethanol, and water for sequential elution from the silica sorbent. Together, the three elution steps enabled 93.0% of the lipids in the mixture to be detected, with overall improvement in signal intensities. In contrast, without the miniaturized SPE front-end, direct analysis of the 18-component lipid mixture resulted in the detection of 85.7% of the lipids. The solvents employed were optimized by evaluating their selectivity for polar and nonpolar lipids. The limits of detection associated with the elution of nonpolar lipids (e.g., trilinolein) from the polar silica surface were determined to be 0.62 µM and 0.84 µM for the elution of polar oleic acid from the same sorbent. This work represents a rapid and affordable analytical approach for analyzing complex lipid mixtures in situations where conventional chromatography cannot be used, including analysis on a portable mass spectrometer.