Compositional Analysis of Wax Esters in Human Meibomian Gland Secretions by Direct Infusion Electrospray Ionization Mass Spectrometry

Wax esters (WE) are one of the predominant lipid types in human meibomian gland secretions (meibum) and account for 40-50 % of total meibum lipids. Recently, we managed to quantify 51 isomeric groups of intact WE in normal human meibum samples by direct infusion electrospray ionization mass spectrometry (ESI-MS), with each WE peak in the MS spectrum corresponding to one isomeric group (Chen et al, Invest Ophthalmol Vis Sci 54(8):5730-53, 2013). However, the information of the isomeric composition in each group was not obtained. In this study, tandem mass spectrometry (MS/MS) was applied to quantify relative amounts of these isomers using the intensities of the corresponding diagnostic ions after appropriate correction and normalization. This data was combined with the previous obtained mole fraction of each isomeric group to total WE in human meibum to determine the corresponding percentage of each isomer. A total of 23 of the most abundant WE peaks of different molecular weights (corresponding to 85.3 % of the total amount of WE) in human meibum were studied and resulted in quantification of 92 WE species. The quantitative information of composition of WE in human meibum will help better understand their role in the tear film.