Quantitative Profiling of Major Neutral Lipid Classes in Human Meibum by Direct Infusion Electrospray Ionization Mass Spectrometry

PURPOSE. The purpose of this investigation was to better understand lipid composition in human meibum. METHODS. Intact lipids in meibum samples were detected by direct infusion electrospray ionization mass spectrometry (ESI-MS) analysis in positive detection mode using sodium iodide (NaI) as an additive. The peak intensities of all major types of lipid species, that is, wax esters (WEs), cholesteryl esters (CEs), and diesters (DEs) were corrected for peak overlapping and isotopic distribution; an additional ionization efficiency correction was performed for WEs and CEs, which was simplified by the observation that the corresponding ionization efficiency was primarily dependent on the specific lipid class and saturation degree of the lipids while independent of the carbon chain length. A set of WE and CE standards was spiked in meibum samples for ionization efficiency determination and absolute quantitation. RESULTS. The absolute amount (mu mol/mg) for each of 51 WEs and 31 CEs in meibum samples was determined. The summed masses for 51 WEs and 31 CEs accounted for 48 +/- 4% and 40 +/- 2%, respectively, of the total meibum lipids. The mass percentages of saturated and unsaturated species were determined to be 75 +/- 2% and 25 +/- 1% for CEs and 14 +/- 1% and 86 +/- 1% for WEs. The profiles for two types of DEs were also obtained, which include 42 alpha, omega Type II DEs, and 21 omega Type I-St DEs. CONCLUSIONS. Major neutral lipid classes in meibum samples were quantitatively profiled by ESI-MS analysis with NaI additive.