Identification of Δ6-monounsaturated fatty acids in human hair and nail samples by gas-chromatography–mass-spectrometry using ionic-liquid coated capillary column

Lipids found in human sebum contain specific fatty acids such as sapienic (cis-6 16:1), cis-8 18:1 and sebaleic (cis-5, cis-8 18:2) acids. These fatty acids belong to the n-10 series and the initial step involved in their synthesis is the desaturation of palmitic acid by the Δ6-desaturase to form sapienic acid. The occurrence in human hair and nail of sapienic (cis-6 16:1), cis-8 18:1 and sebaleic (cis-5, cis-8 18:2) acids has not been reported to our knowledge nor has the formation of Δ6-monounsaturated fatty acids from other saturated fatty acids such as stearic acid. The pre-requisite for such identification is the ability to separate cis-6 from cis-8 monounsaturated fatty acid derivative (i.e. cis-6 18:1 from cis-8 18:1 methyl esters) by gas-chromatography (GC) and such separation is not achievable using cyanoalkyl based highly polar capillary columns. In the present study, we used the 100 m SLB-IL 111 ionic liquid based capillary column recently commercialized by Supelco (Bellefonte, PA). The identification was performed by gas-chromatography–mass-spectrometry (GC–MS) with electronic impact (EI) ionization using 4,4-dimethyloxazoline (DMOX) derivatives. Baseline separation between critical cis-6 18:1 and cis-8 18:1 isomers was obtained allowing unambiguous identification based on MS fragmentation and pure standards. In sebum, hair and nail samples, sapienic, cis-8 18:1 and sebaleic acids were found and more importantly, petroselinic acid was identified in these human tissues for the first time. In addition, we identified in hair and nail lipids cis-6 14:1, cis-6 15:1, iso-cis-6 16:1, aiso-cis-6 17:1 and cis-6 17:1 as their DMOX derivatives based on molecular ion as well as diagnostic ion fragments at m/z 167, 180 and 194. Possible biosynthesis scenario is postulated to explain the occurrence of these Δ6-monounsaturated fatty acids in human sebum, hair and nail lipids.

► UnusualΔ6-monounsaturated fatty acids were identified for the first time by GC–MS in animal samples.
► High-resolution separation was achieved using ionic liquid based capillary column.
► Identification of ethylenic double bond location was achieved using DMOX derivatives.
► Putative biosynthesis pathway explaining the occurrence of these fatty acids in human nails, hair and sebum.