This research work mainly deals with the application of confocal Raman spectroscopic technique to study in vivo human skin penetration of sunscreen products, as there are a lot of controversies associated with their skin penetration. Healthy human volunteers were tested for penetration of two commercial sunscreen products into their volar forearm skin for a period of 2 h. Measurements were taken before and after application of these sunscreen products. All the confocal Raman spectra were pre-processed and then subjected to multivariate two-dimensional principal component analysis and classical least squares analysis to determine the skin penetration of these sunscreens in comparison to the "sunscreen product spectrum" which was considered as the control. Score plots of principal component analysis of confocal Raman spectra indicated clear separation between the spectra before and after application of sunscreen products. Loading plots showed the maximum differences in the spectral region from 1590 to 1626 cm-1 where the characteristic peak of the pure sunscreen products was observed. Classical least squares analysis has shown a significant penetration to a depth of 10 μm in the volar forearm skin of healthy human volunteers for both these sunscreen products. The results confirm that the penetration of these tested sunscreen products was restricted to stratum corneum and also prove that confocal Raman spectroscopy is a simple, fast, nondestructive, and noninvasive semi-quantitative analytical technique for these studies.
Keywords: confocal Raman spectroscopy; in vivo human skin studies; multivariate analysis; sunscreen skin penetration.