4.7. Lipidomics Analysis Using Sebum Measurements and Liquid Chromatography-Mass Spectrometry after Tape-Stripping
Sebum levels were measured in triplicate on adjacent skin using a Sebumeter SM815 (Courage and Khazaka, Cologne, Germany). The average of three measurements was calculated. For lipidomics, the stratum corneum was sampled with 5 subsequent polyphenylene sulfide tape strips (Nichiban, Tokyo, Japan), of which the first one was discarded. Pressure was applied to each tape with a D500 D-squame Pressure Instrument (CuDerm Corporation, Dallas, TX, USA). A 16 mm-diameter hole was punched out from the section that was pressed onto the skin, and the tape was stored in chloroform:methanol (2:1). For extraction, tapes were shaken at 40 °C for one hour each in chloroform:methanol (2:1), chloroform:methanol:water (1:2:0.5), chloroform:methanol (1:1), and heptane:isopropylalcohol (1:1). The solvent was collected, and a liquid–liquid extraction was performed with the addition of 0.25 M potassium chloride. The organic layer was washed with chloroform, filtered with 0.45 µm PVDF syringe filters (Grace, Deerfield, IL, USA), and concentrated. Analysis was performed as described by Boiten et al. [