The spectral stability of several sunscreening agents on stratum corneum sheets

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Abstract
Film layers of seventeen commercially available sunscreen products and sixteen active ingredients on stratum corneum sheets were spectrophotometrically monitored before and after simulated solar irradiation. Fixed irradiation doses were given within the daily terrestrial limits. From the changes in the absorption spectra after irradiation the spectral stability was determined. The spectral stability reflects the photochemical stability and the assay quickly provides an insight in the sunscreen''s stability towards (solar) radiation in situ. Good to excellent spectral stability was observed for the benzophenones, para-aminobenzoic acid and its esters, homomenthylsalicylate, guanine, 2-phenylbenzimidazol-5-sulphonic acid (potassium salt) and guiazulene. Moderate spectral stability was observed in case of the cinnamates and 3-[4-methylbenzylidene] camphor. Poor spectral stability was exhibited by phenylsalicylate and 4-isopropyldibenzoylmethane. Among the commercial products good to poor spectral stability was established. We recommend the performance of this assay on every sunscreen agent of cosmetic formulation which may be exposed to the sun or any other ultra-violet/visible radiation source.

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