Validation of the combined ATR-FTIR/tape stripping technique for monitoring the distribution of surfactants in the stratum corneum.

The physical presence of surfactants in the skin is linked to their skin irritation potential. Combined ATR-FTIR spectroscopy and tape stripping experiments in vitro on porcine ear skin were used to investigate the spatial distribution of sodium lauryl ether sulfate (SLES) in the stratum corneum and to assess its effects on conformational order of stratum corneum intercellular lipids, secondary structure of keratin and skin hydration. It was possible to monitor the spatial distribution of SLES in the stratum corneum for the first time by subtracting spectra of untreated from treated skin samples and without the need of a perdeuterated form. This method of analysis was evaluated by addressing potential error sources such as differences in removed amounts of corneocytes and intra-individual changes in stratum corneum composition as a function of depth. The obtained results indicate a penetration of SLES into deep layers of the stratum corneum. Furthermore, SLES treatment led to significantly decreased skin hydration levels, whereas the secondary structure of keratin remained nearly unaffected. The reliability of this semi-quantitative method of analysis was confirmed by receiving a coefficient of determination of 0.9963 after making a correlation of deep depended absorbances of two different characteristic bands with different absorption coefficients.