CHARACTERIZATION OF HUMAN SKIN CHEMISTRY 295 ble with the following typical formulation for skin cleansing products: mineral oil, 50% beeswax, 10% lanolin, 4% and fatty acid esters used as emulsifiers, 1%. The approximately 35% fraction of water in the original formulation is not revealed in this infrared spectrum since the sample was allowed to dry "naturally" on the prism face, as it would upon the skin, prior to the analysis. Spectral traces recorded immediately after applying cosmetics to the prisms do show intense absorption bands for water, but these quickly disappear. The second trace from the top of Figure 6 characterizes the skin "toner" preparation. It contains, in addition to the mineral oil and fatty-ester components common to many cosmetic formulas, a polymeric ingredient of highly polar character as indicated by the strong absorption band in the region between 1100 and 1200 cm -•. Similarly, the third and fourth traces from the top of Figure 6 present internal reflection infrared spectra of the skin "mask" and "conditioner" products, respectively, actually used by our volunteer subject. Noting the strong absorption bands in the regions between 3000 cm -• and 3600 cm -•, and between 1000 cm -• and 1200 cm -•, both indicative of the O-H bond containing groups of polyols, it can be readily seen that these products contain highly hydroxylated components. The next-to- last spectrum of Figure 6 characterizes the "night cream" preparation which was used, showing a band sequence and relative intensity ratio which indicates this typical com- position for such products: stearates and palmitates, 3% mineral oil, 3% fatty alcohols and other hydroxylated materials (such as polypropylene glycol), about 5% and both animal and vegetable fatty esters, about 6%. The remaining quantity, water, had dried from the prepar,ation before its analysis. The lowermost trace of Figure 6 presents once again a "blank" baseline spectrum for a KRS-5 internal reflection prism. This prism was used throughout the skin analysis series and also a series of direct characterizations of different cosmetics, including those represented in both Figures 6 and 7. It is easily noted, comparing this final trace of Figure 6 with the "blank" baseline for the same prism at the top of Figure 5, that there has been some degradation in the transmission of infrared energy through the prism. This is especially apparent at the higher fre- quencies which are more subject to scattering losses caused by degradation of the optical quality of the prism's surface. Soft KRS-5 salt prisms are more easily damaged during multiple, between-sample cleaning operations (using cotton swabs and acetone to remove deposits on the prism face) and more susceptible to "etching" by water than are the germanium prisms preferred for this work. Figure 7 collects additional infrared spectra of typical cosmetic products having the following general purposes: facial washing cream, hand cream, deep cleanser, suntan cream, moisture cream and a perfumed ointment. The facial washing cream is characterized by an internal reflection spectrum compatible with a formulation contain- ing approximately 50% mineral oil, 15% beeswax, 5% emulsifiers and 30% water (which evaporated prior to recording this trace). The hand cream spectrum presented in Figure 7 would be compatible with a formulation having this typical composition: stearic acid, 15% sorbitol and fatty alcohols, 10 to 12% and triethanolamine, 2% the remainder (nearly 72% water) dried from the preparation prior to this analysis. The "deep" cleansing product provides an infrared spectrum consistent with the following proportion of ingredients: isopropylpalmitate, 2 % saturated fatty acids (such as stearic acid), 2% unsaturated fatty acids (such as oleic acid) polyols (such as sorbitol), 2% triethanolamine, 1% and the remaining 80+ % of the product water evaporated from the specimen prior to its analysis. The most complex of the infrared spectra collected in Figure 7 is for the suntan cream, where the numerous sharp absorption bands reflect

i I Figure 7. Additional infrared spectra of typical cosmetic products, applied directly to the face of a KRS-5 internal reflection plate. (•) Facial washing cream (•) Hand cream ¸ "Deep" cleanser(• Suntan cream (•) Moisture cream (• Per- fumed ointment. 296