•j. Soc. Cosmet. Chem., 31,361-366 (December 1980) Sunscreens at skin application levels: direct spectrophotometric evaluation BORIS M. CUMPELIK, c/o Van Dyk & Co., Inc., IVilliam & Main Streets, Belleville, NJ 07109. Received January 29, 1980. Presented at the Annual Scientific Meeting of the Society of Cosmetic Chemists, December 6- 7, 1979. Synopsis The method utilizes the standard spectrophotometric procedure to establish absorbance of the active ingredient(s) of the tested sample, and is accompanied by an assay of the sample "as is" applied in the form of a thin film deposited between two clean, empty, matched cells after the volatiles were driven off. The weight and thickness of the film are momentarily ignored in order to record the true spectral position and shape of the whole absorbing system. By transposing this graph over the previously recorded one, a meaningful calculation of actually transmitted UV energy is possible. INTRODUCTION Many promising sunscreen preparations carefully prepared and assayed instrumentally, show variable efficacy in human tests. Also, identical levels of the same UV absorber perform differently when formulated as creams, lotions, oil or as hydro-alcoholic product. Studies of solvent polarity and its influence on spectral position of the UV absorbing compounds in a single solvent system (Figure l) pointed the way to investigation of multiple solvent systems (1,2). The UV absorbing agents are incorporated into preparations with a great number of cosmetic materials: oils, gums, polymers, emulsifiers, soaps or alcohols, often accom- panied by varying amounts of water. Each of these acts as a co-solvent for the particular system. Naturally, sunproducts are designed to give the best possible skin protection, but each formulator aims to give the consumer some special unique quality he could see, smell and feel well enough to make it his or her choice the second time. As a result, the number and the ratio of ingredients of each product vary from the next brand, affecting the performance accordingly, even though the amount of the UV absorber was equal. When a sunscreen is applied to the skin, evaporation takes place and the non-volatiles increase and drastically change their ratio to the UV absorber, forming thus an entirely new absorbing system. 361

362 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lOO I I I I I I I I 9o ] UV-B & UV-A REGION 80 (ERYTHEMAL R.) "B" 7O 60 5O 40 30 20 10 200nm in H20 in IPA I I I 240 280 320 (TANNING R.) "A" 360 400 Figure 1. Absorption curve showing spectral shift of PABA in isopropanol vs. in water (5mg/cm/L in each case). Aromatic acids are greatly affected by spectral shift due to water and pH changes, with considerable loss of absorptivity. UV-B = erythemal region UV-A = tanning region. (PABA-- p-aminobenzoic acid by Nobelkrut, Abbofors, Sweden.) In the course of a routine spectrophotometric assay, the UV absorber of the product is greatly diluted in volumetric steps--usually with isopropanol, ethanol or methanol--in order to bring it within the low operative range of the instrument. Obviously, such a massive alcoholic dilution removes not only all traces of contributory absorptivity of the formulation itself, but--and this is important--it obliterates its original polar influence on the UV absorber. Having no way of "seeing" that influence instrumentally, but fully confident of the amount of UV absorber in the formulation, sunproduct is sent for human evaluation, which often shows much higher, or lower SPF than aimed for. To minimize the chance of failure before embarking on costly stability and human tests, the following method is suggested. PROCEDURE First, the usual spectrophotometric assay is carried out using a spectroquality alcohol such as isopropanol (IPA) for dilution of sample and as reference, calculating absorbance of the sample in terms of lg/lcm/L and expressing its in vitro efficacy, i.e.