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EOS-d₉ (d18:1-d₉/32:0/18:2) is intended for use as an internal standard for the quantification of EOS by GC- or LC-MS. EOS is a ceramide found in the outer layer of the epidermis in mammals.¹ It is comprised of an ω-hydroxy very long-chain fatty acid (C28-36) esterified to the essential fatty acid linoleic acid and glucosylated to sphingosine. The consecutive regio- and stereospecific oxygenation of the linoleate portion of EOS by 12(R)-lipoxygenase (12(R)-LO) and eLOX3 is essential for the maintenance of the epidermal barrier to prevent water loss. Following oxygenation, the oxidized linoleate is hydrolyzed, leaving the ω-hydroxy end of the very long-chain fatty acid to covalently bind the protein layer, forming the corneocyte lipid envelope and sealing the gap between the extracellular lipid lamellae and the cornified cell envelope of the corneocyte.

1. Zheng, Y., Yun, H., Boeglin, W.E., et al. Lipoxygenases mediate the effect of essential fatty acid in skin barrier formation: A proposed role in releasing omega-hydroxyceramide for construction of the corneocyte lipid envelope J. Biol. Chem. 286(27),24046-24056(2011).