336 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 2. Erythemal flux of midsummer sun ture programmed from 200/300øC at 10øC/min.) Since sunscreens are mostly aromatic esters, they need not be silanized beforehand. One mi- croliter injected at 10 a X 16 attenuation usually gives a good resolution. Quantitative Evaluation of a Sunscreen Figure 4 shows the spectrophotometric curve of one commercial prep- aration. This curve was made on a Beckman DB double-beam spectro- photometer, having 2 matched quartz cells. Samples should be well mixed before weighing. In the case of aerosol spray sunscreens, precool- ing of both the can and the receiving container is advisable to minimize evaporation loss during transfer. All glassware used must be cleaned in advance with hot (80-100øC) chromic acid mixture and rinsed several times with hot water, then finally with alcohol. Quartz cells must be cleaned also in hot chromic acid mix- ture followed by rinsing with distilled water, then alcohol. The utmost importance of clean glassware. and especially of the quartz cells, must be

ANALYSIS OF SUNSCREENS 337 Figure 3. Absorption curves of four most widely used screens projected on erythemal flux of midsummer sun emphasized. The plastic squeeze bottles so commonly used in labora- tories today should never be used in this work. (The plasticizers used in the manufacture of plastic squeeze bottles are readily extracted by the alcohol they hold.) Since most plasticizers also absorb in the ultraviolet range, they would grossly interfere with results. A 0.5000-g sample of sunscreen is weighed and dissolved in isopro- panol (Spectrograde), then transferred to a 100-ml volumetric flask. If heating is necessary, cool to room temperature before adjusting to exact volume. This 5.000 g/liter concentration is then further diluted (10 ml/ 100 ml flask) to 0.5000 g/liter. (The instrument should be scanned man- ually to see whether a further dilution of the sample is necessary.) The absorbance curve of the sunscreen is carefully recorded in 5-rev. steps from 292.5-372.5 mt•. When the whole curve has been read, the ob- tained values are multiplied, so as to represent 1 g/liter/1 cm, and the results are recorded in the second column of the calculation sheet.