Quantification of skin penetration of caffeine and propylene glycol applied topically in a mixture by tMCR-ALS of depth-resolved Raman spectra.

The quantitative determination of topically applied substances in the skin is severely limited and represents a challenging task. The porcine skin ex vivo was topically treated with a gel containing caffeine (CF) and propylene glycol (PG), and depth-resolved Raman spectra were recorded with two confocal Raman microscopes. We applied a novel tailored multivariate curve resolution-alternating least squares (tMCR-ALS) method to the selected spectral regions (512-604 and 778-1148 cm -1 ) of gel-treated skin and quantitatively determined the concentrations of CF and PG in the stratum corneum (SC). The highest concentration of CF (181 mg/cm 3 ) was found at the surface, while PG (384 mg/cm 3 ) was found at 10% SC depth, indicating the formation of a reservoir at the superficial SC. The concentrations of CF and PG decreased monotonically and reached the detection limit at ≈60% and ≈80% SC depth, respectively, indicating that neither permeate the SC. This article is protected by copyright. All rights reserved.