126 JOURNAL OF COSMETIC SCIENCE coating was obtained by treating the surface of the ultrafine zinc oxide with methyl hydrogen polysiloxane at a 6% level. The titanium dioxide was in the form of ultrafine particles (size 0.03-0.05 l•m). (To simplify the terminology in this manuscript, we termed this as uncoated, although, as usual, it had a small percentage of aluminum oxide as a coating.) The silicone coating was obtained by treating the surface of the ultrafine titanium dioxide with methyl hydrogen polysiloxane (2%), and the polyethylene coating by treating TiO 2 with oxy- genated polyethylene (3%). All metal oxides were provided by U.S. Cosmetics Corp. (Dayville, CT). CHEMICALS DMPO (5,5-dimethyl-l-pyrroline-N-oxide) and H202 were purchased from Aldrich Chemical Co. (Milwaukee, WI). SPIN TRAPPING The spin-trapping technique was used in this work to study the free radicals generated by metal oxides upon exposure to light. This technique typically involves the addition of a reactive short-lived free radical across the double bond of a diamagnetic "spin trap" to form a much more stable free radical, a "radical adduct." The radical adduct then can be examined with electron paramagnetic resonance (EPR) spectroscopy, which very specifically detects molecules with unpaired electrons, such as free radicals. The EPR spectral parameters of the radical adduct reflect, to a varying degree, the nature of the trapped radical, and under favorable conditions, these parameters can be used to identify the species of radical that was trapped (5,6). DMPO was chosen to be the spin trap in this study since radical adducts of DMPO usually provide distinctive spectral parameters, which facilitates the identification of the radical. In addition, in aqueous solutions the lifetimes of DMPO adducts of oxygen- centered radicals usually are longer than those of most other spin traps. For example, the hydroxyl radical adduct, DMPO/eOH, has a lifetime of over 40 minutes in an aqueous solution, which was sufficiently long for this study. DMPO (100 mM) was vortexed with an aqueous suspension of an oxide (zinc or tita- nium, 20 mg/ml), drawn into gas-permeable Teflon tubing, and then inserted into a quartz EPR tube for exposure to a light source (described below) for 30 seconds. The EPR spectrum of the sample was recorded immediately. To assist in the identification of the trapped radical, a well-characterized system for generating eOH radical also was studied: an aqueous solution containing DMPO (100 mM) and H202 (2 mM) that was exposed to UV light for 30 seconds to produce the DMPO/*OH adducts. All concen- trations listed are final. The signal intensity of the EPR spectrum is related to the amount of the radicals trapped, and hence, can be used as an indication of the amount of radicals generated by the oxides. Because several different reactions are involved with spin trapping and the stability of the spin adducts, absolute quantitation is very difficult to achieve. By using similar conditions for all of the samples, as was done in this study, however, one can usefully compare relative signal intensity among different experiments. Each experiment

REACTIVITY OF SUNSCREEN COATINGS 127 described in this study was repeated at least three times. The variation in signal intensity between experiments usually was less than 15%. EPR MEASUREMENTS The EPR spectra of the radical adducts were recorded at room temperature on a Bruker ER-220 EPR spectrometer immediately after the samples were exposed to the light source. Typical instrumental settings were: microwave frequency, 9.6 GHz incident microwave power, 20 roW scan time, 2 m time constant, 0.1 s modulation amplitude, 1 gauss scan range, 100 gauss modulation frequency, 100 kHz. The EPR spectra were collected, stored, and manipulated using software developed in our laboratory, installed on IBM-compatible computers. ILLUMINATION The light source was a xenon high-pressure lamp (Oriel Corp., model 66023) equipped with quartz condenser lenses. The spectral distribution of the xenon lamp is very similar to that of sunlight except with two additional strong infrared peaks at 850 and 900 nm. The infrared radiation was partly eliminated by a heat-absorbing water filter with quartz windows (thickness of water layer 75 mm). The photon fiuence rate in the visible range from 400 to 700 nm was measured with a LI-COR quantometer radiometer LI 189. This fiuence rate was 12,000 pmol m -2 s -1. The total UV content in the radiation reaching the sample was evaluated to be about 3000 pmol m -2 s -1. In consecutive experiments, various ranges of short-wavelength radiation were cut off using long-pass glass color filters (Edmund Scientific). The following filters were used (Schott reference numbers given in brackets): 345 nm (WG345), 385 nm (GG385), 435 nm (GG435), 455 nm (GG455), 550 nm (OG550), and 695 nm (RG695). In the following text, "white light" means the light from the xenon lamp without any filter. For the exposure of the metal oxides to room light, the samples were prepared in quartz EPR tubes in the same way as for exposure to white light, and the tubes were laid on the laboratory bench for the specified time before taking EPR measurements. The fiuence rate of the "room light" was about 12 pmol m - s RESULTS In the presence of unfiltered white light from the xenon lamp, the untreated zinc oxide (Figure 1C) and titanium dioxide (Figure 2B) generated intermediates that produced spin adducts with spectral characteristics of hydroxyl radicals (Figure 1G). Following addition of sodium formate into the reaction system, the EPR signal of the formate adduct was recorded (Figure 3C), suggesting that the intermediate species generated by metal oxides is very likely to be the hydroxyl radical (7). The presence of a silicone surface treatment had a very strong effect on the relative amount of species that were trapped (Figure 1C vs Figure 1D, and Figure 2B vs Figure 2C), while treatment with polyethylene essentially had no protective effect (Figure 2B vs Figure 2D).