UV Analysis of Sunscreens

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1 UV Analysis of Sunscreens Texas A&M University Organic Lab 238 The Electromagnetic Spectrum The UV region is the range of frequencies just higher than those for visible light UV-Vis energies correspond to electronic transitions - the energy needed to excite an electron from one molecular orbital to another. The key relationship between energy and wavelength is: E = h = hc/ 1

2 Regions of Ultraviolet Radiation Most sunscreens protect primarily against UVB radiation and much less against UVA radiation. Skin Cancer and Skin Aging What s the harm in sunburn? A person's risk for melanoma--the most serious form of skin cancer--doubles if he or she has had five or more sunburns. The two most common nonmelanoma skin cancers, basal cell carcinoma (BCC) and squamous cell carcinoma (SCC), are directly correlated with sun accumulation over many years. Sunburn results when the amount of exposure to the sun, or other ultraviolet light source (e.g. tanning lamps and welding arcs etc.), exceeds the ability of the body's protective pigment, melanin, to protect the skin. Sunburn destroys cells in the outer layer of the skin, damaging tiny blood vessels underneath. Burns deeper into the skin's layers also damage elastic fibers in the skin, which over time and with repeated sun overexposure, can result in the appearance of yellowish, wrinkled skin. 2

3 Sunscreens Sunscreen, also commonly known as sunblock or sun cream, is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet (UV) radiation on the skin exposed to sunlight Sunscreens can act in two ways: 1. Absorption of the ultraviolet light by organic compounds which undergo electronic excitation by the absorption. 2. Reflection and scattering of the light by small particulates of inorganic (or possibly organic) solids. In this experiment, you are going to investigate the UV absorption properties of several sunscreen preparations. Various Sunscreens Obtain a UV-VIS spectrum for one sunscreen preparation Compare to data from other lotions (other students) Compare absorbance vs SPF (sun protection factor) determined by the manufacturer Determine which lotions have greater absorption in the UVA region 3

4 SPF Values SPF values are supposed to indicate the amount of UV radiation required to cause sunburn on skin with the sunscreen on, relative to the amount required without the sunscreen. Using human subjects is subjective (amount of sunburn cannot be determined until hours after exposure), expensive, and difficult on the subjects (sunburn cancer) Spectroscopic methods are complicated by needing to take into account degradation of sunscreen compound by the UV and the effectiveness of specific wavelengths to cause sunburn. In today s experiment, we will consider the relative effectiveness of the different sunscreens at absorbing radiation in both UVA and UVB regions. Active Ingredients in Commercial Sunscreens 4

5 Stock Solutions 1.00 g of the lotion was mixed with warm isopropyl alcohol. The mixture was diluted to 100 ml and allowed to stand until any insoluble materials settled out. Your sample will come from the clear solution containing the dissolved organic compounds. Summary for Sample Preparation 1. Use small cork ring to protect your 5-mL volumetric flask. 2. Use 50 l of your assigned Sunscreen Stock Solution. 3. Dilute with reagent grade isopropyl alcohol to 5 ml volume. 4. Stopper the flask and mix well. 5. Transfer some of your diluted sample to a disposable cuvette. 6. Move to spectrometer to obtain the UV spectrum. 5

6 Sample Dilution Obtain 50 L of the stock solution using a micropipette attached to a Drummond micropipette filler (one per lab room) Dilute to 5.0 ml in a volumetric flask Stopper and mix well before using Preparation of Sample for UV 1. Use a disposable pipette to transfer about 1 ml of your diluted solution into a disposable cuvette. 2. Wipe the cuvette with a Kimwipe to remove any fingerprints or solution from the cuvette faces. 3. Move to the Spectrometer with your cuvette. 6

7 Measuring the Absorption Place the cuvette into the holder on the spectrometer such that the light will pass through solution that is the full width of the cuvette As soon as the sample is in place, an absorbance spectrum will be displayed on the computer screen. Use the cursor line to find and record absorbance values at 310 and at 360 nm. Go to File/Print. When spectrum has printed, label it with your name and solution #. 7

8 Data Collection You will need to complete your Data table by getting information on the absorbances of the other sunscreen lotions from other students in the class. Do not leave lab until you have this data. Clean Up Pour the solution in the cuvette into the appropriate waste bottle Place cuvette in the container for used cuvettes Pour solution in volumetric flask into waste container Rinse volumetric flask with 2 small portions of isopropyl alcohol Drain as much alcohol as possible from the flask and return flask to your bin 8

9 Data Analysis Before you leave the lab today you should: Make sure you have recorded absorbance data for your sample and also for each of the other preparations analyzed by other students. Work with your partner to determine which of the preparations had larger absorbance in the UVA region and then use Table IV from your in-lab observations to deduce which chemical components are likely responsible for that increased absorbance. Since all samples were prepared in the same way, you can simply compare absorbance values. However, you will need to calculate molar extinction coefficients in the post-lab questions for this experiment. 9

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