Quantification of free fatty acids in human stratum corneum using tandem mass spectrometry and surrogate analyte approach

The free fatty acids (FFAs) are one of the major components of the lipids in the stratum corneum (SC), the uppermost layer of the skin. Relative composition of FFAs has been proposed as a biomarker of the skin barrier status in patients with atopic dermatitis (AD). Here, we developed LC-ESI-MS/MS method for simultaneous quantification of a range of FFAs with long and very-long chain length in the SC collected by an adhesive tape (D-Squame). Method based on derivatization with 2-bromo-1-methylpyridinium iodide (BMP) and 3-carbinol-1-methylpyridinium iodide (CMP) allowed highly sensitive detection and quantification of FFAs using multiple reaction monitoring (MRM). For the quantification, we applied a surrogate analyte approach and internal standardization using isotope labeled derivatives of FFAs. Adhesive tapes showed presence of several FFAs, which are also present in the SC, a problem encountered in previous studies. Therefore, the levels of FFAs in the SC were corrected by using C12:0 which was present on the adhesive tape, but not detected in the SC. Method was applied on the SC samples of patients with AD and healthy subjects. Quantification using MRM allowed sufficient sensitivity to analyze FFAs of chain lengths C16-C28 in the SC collected in only one tape strip.