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K.B. Green–Church, C. Zeigler, J.J. Nichols, K.K. Nichols; Identification and Analysis of Polar and Non–Polar Lipids From Meibomian Gland Secretions Using Electrospray Ionization Mass Spectrometry . Invest. Ophthalmol. Vis. Sci. 2005;46(13):4421.
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To use electrospray ionization time–of–flight mass spectrometry (ESI–TOF) to (1) develop protocol to collect and analyze human meibomian gland lipid, and to (2) identify and compare the chemical structures of polar and non–polar lipids from meibomian gland secretions to current understanding of lipid composition.
Meibomian gland secretions were collected using a sterile 1.5mm curette probe. The secretions were obtained from one male and female subject (mean age ± 48.5) and stored in a 2:1 chloroform/methanol solution. Meibomian gland secretions were directly infused at 20 µL/min to a Micromass LCT equipped with an orthogonal electrospray source (Z–spray) which was operated in positive ion mode.
Inter–subject agreement of lipid profiles in healthy patients was good and little change was observed in the mass spectrum after storage of 1 month. Identification of lipids observed in the mass spectrum was further characterized by a combination of accurate mass analysis, lipid database searching, Collision–induced dissociation and GC–MS. Once a lipid was identified, the molecular formula and structure was verified by the commercial purchase of the lipid and compared with the unknown. Table 1 lists the representative peaks observed in the study and the subsequent identification of lipids to date using accurate mass analysis and CID analysis. There are approximately 10 – 20 more peaks in the mass spectra to identify using the same methods described above. Of significance is the identification of oleamide.
Other investigators have proposed that a major component of meibomian gland lipid is oleic acid (similar molecular weight to oleamide). Our studies have not indicated even trace amounts of oleic acid present in human lipid samples. The identification of oleamide, an amide rather than a carboxylic acid, might change theories of proposed mechanisms of dry eye and warrants further investigation.
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