INSTRUMENTATION IN THE COSMETIC I,ABORATORY 325 Ultraviolet spectrophotometry can be employed as a quantitative tool and a means of identification for any compounds that exhibit aromatic character (e.g., compounds containing a benzene ring or conjugated double 1,onds). This is a very valuable tool as all that is required in many in- MENTHOL AND PERFUME SHAVING PREPARATION 150 •C. 5.5 lbs. He METER 15% CARBOWAX 600 ON 60-80 MESH CELITE MENTHOL Figure 2. 90' GREEN STANDARD E 80 •o 70 • 40- STANDARD • COLOR STANDARDS RECORDING SPECTROPHOTOMETER I0- WAVELENGTH IN MILLIMICRONS 460 ,560 660 760 Figure 3. stances to obtain a quantitative determination of a specific compound is to dissolve the sample in an appropriate solvent and read its absorbance at the analytical wavelength. Obviously, this affords a dramatic saving

326 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS in time, as no separation or isolation procedures are necessary. Ultra- violet spectrophotometry requires small quantities of sample. Con- centrations of only a few parts per million can usually be detected and measured. In general, the precision of this technique is in the range of plus or minus 1 per cent absolute. When the product being analyzed contains compounds which exhibit interfering absorptions at the analytical wavelength being measured, appropriate extraction and/or ion-exchange techniques will remove the interfering constituents. Compounds exist whose ultraviolet absorption peak maxima shift with changes in pH. This phenomenon can be utilized for the analysis of bacteriostats such as hexachlorophene, bithionol and halogenated anobials by employing differential ultraviolet spectrophotometry. The ultraviolet absorption maximum of hexachlorophene shifts from 300 millimicrons in alcohol containing 4 per cent glacial acetic acid to 314 millimicrons when alcohol containing 4 per cent ammonium hydroxide is used (1, 2). PRODUCT CONTAINING H/XACHLOROPHENE DIFFERENTIAL ULTRAVIOLET SPECTRUM I cm. CELL • A•COHOL /• .... " • SAMPLE • , • REFERENCE 4% 360 z6o WAVELENGTH I" MILLIMICRONS Figure 4. 'l'herefore, if identical aliquots of an alcoholic solution of a product con- raining hexachlorophene are treated as previously described and the acid in solution is used as the reference sample, the only absorption at 314 millimicrons should be that ofhexachlorophene. All absorption effects of interfering constituents in the sample are automatically compensated for and the result is a valid quantitative method for the determination