In the Laboratory
A Simple Preparative Method to Evaluate Total UV Protection by Commercial Sunscreens Shouei Fujishige,* Sumiko Takizawa, and Kaoru Tsuzuki Laboratory for Polymers and Textiles, Tokyo Kasei University, Kaga 1-18, Itabashi, Tokyo 173-8602, Japan; *
[email protected] During the course of experimental work on the UV spectroscopy of polymer thin films, it was found that the optical transparency of a cellulose membrane in the UV region is equivalent to that of a quartz plate as shown in Figure 1. Based on this fact, we would like to show that commercial cellulose membrane such as a Visking tubular membrane can be used as a cell to hold thin creamy samples to obtain UV absorption spectra without any pretreatment. A previous paper in this Journal presented the active ingredients in various sunscreens for absorption analysis after extraction of the contents with 2-propanol (1). This procedure was successfully applied to analyze the chief ingredients of the sunscreens. However, we found that some of the UV spectra of 2-propanol extracts from popular commercial products show little absorbance. We wondered if the active ingredients were extracted with 2-propanol insufficiently owing to their limited solubility in the solvent used. This suggested the development of another preparative method for sunscreen just as it comes from the tube of the commercial product, without any pretreatment, in order to evaluate the total UV protection power for the skin over two wavelength regions, UVB (290–315 nm) and UVA (315–400 nm).
Absorbances were measured on a Shimadzu UV-240 at 25 °C using a pair of special quartz cells with 1-cm path length. A Visking Tubular Membrane 36/32 from Union Carbide Corp. with a film thickness of 0.0203 mm was used as purchased. The tubular membrane was cut into pieces about 4 cm long, which could contain about 0.2 g of the creamy samples. The membrane containing the sample was squeezed on a flat plate with a cylindrical polytetrafluoroethylene rod to attain a thin film for UV spectroscopy. The Visking tubular membrane used in this experiment contains a small amount of glycerin to maintain a semipermeable membrane structure for dialysis use. This amount of glycerin does not disturb the measurement, especially in the UV region; in fact, it increases the compatibility of the membrane surface with various sunscreens. This compatibility is important to attain uniform thickness of the sunscreens. If the sample is still not thin enough, we can repeat the squeezing with the cylindrical rod to obtain a sufficiently thin layer. In Figure 2, a typical spectrum thus measured is shown in a comparison to one obtained on a 2-propanol extract of the same commercial product, Kose’s Suncare (SPF60/PA++). This is another type of approach to evaluate the net protection
Figure 1. A comparison of the UV spectrum of a tubular cellulose membrane with that of a quartz cell (2-window, thickness 1 mm) used for 2-propanol solutions and of a quartz plate of 2-mm thickness.
Figure 2. A typical comparison of the UV spectrum of a commercial product, Kose’s Suncare (SPF60/PA++), measured on 2-propanol solution ( ) to that obtained by the proposed method ( ).
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Journal of Chemical Education • Vol. 78 No. 12 December 2001 • JChemEd.chem.wisc.edu
In the Laboratory
power of the total ingredients of commercial sunscreens for the skin. This preparative method is recommended to all people who are concerned with UV protection of commercial sunscreens, not only because of its simplicity, but also its capability to
allow the measurements over two wavelength regions without the need to purchase cells made of high-quality quartz. Literature Cited 1. Abney, J. R.; Scalettar, B. A. J. Chem. Educ. 1998, 75, 757.
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