ANALYSIS OF SUNBLOCK LOTIONS
Introduction:
Light is composed of tiny units called photons, just as matter is composed of tiny units
called atoms. Each photon has a wavelength and energy associated with it. A photon with a long
wavelength has low energy, while a photon with a short wavelength has high energy. (Does an
ocean with long smooth waves or short choppy waves have more energy?) Visible light, which
our eyes can detect comprises a very small portion of the light spectrum. The different types of
light are divided into gamma, x-ray, ultraviolet (UV), visible (vis), infrared (IR), microwave, and
radiowaves, depending on the wavelength of the photon.
Wavelength (nm)
gamma
ray
1
X-ray
10
Ultraviolet
2
Visible
10
-2
10
4
Infrared
10
6
Microwave
10
8
12
10
10
10
Radio
V I B G Y O R
High Energy
Low Energy
Ultraviolet-visible Spectrophotometry (UV-vis) is the study of the transmission or
absorbance of light in the ultraviolet and visible regions of the light spectrum. Transmittance
tells how much light pass through a substance while absorbance tells how little light passes
through a substance. For example, a piece of thick black paper has zero transmittance or 100%
absorbance of visible light. A clear colorless piece of glass has close to100% transmittance and
0% absorbance of visible light.
In this experiment, spectrophotometry is used to find the absorbance of sunscreens at
different wavelengths. UV light of about 300 nm causes the most severe sunburn. An effective
sunscreen is one that absorbs UV light in the 300 nm region, thus preventing the UV-light from
reaching and damaging your skin. The sunscreen SPF (sun protection factor) indicates the
effectiveness of the sunscreens at absorbing the UV radiation. In order to be effective, active
ingredients, such as benzophones, phenyl salicylate, and p-amino benzoic acid (PABA), must
exhibit an intense absorption maximum centered around 300 nm while transmitting the longer
wavelength tanning radiation.
We will be using an instrument called a ultraviolet-visible spectrometer (UV-vis), which
allows you to measure the transmittance or absorbance of a substance at different wavelengths in
the ultraviolet and visible region of the light spectrum.
Goal: To determine the most effective suntan lotion through spectrophotometric analysis.
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Safety:

Always wear goggles in the lab.

Wash your hands with soap and water if you get any solution on you.

The quartz cuvets are very expensive. Be careful not to break them.
Procedure:
Collecting reference and dark spectra
1. Fill the cuvet 1/2 full with your blank solution (isopropanol).
2. Wipe off the outside of the blank cuvet with the special paper provided. This will remove
fingerprints, dripped isopropanol, and other substances that may interfere with the analysis by
absorbing light.
3. Insert the cuvet into the sample port of the instrument and note the position of the uniquely
marked face. In the future, always insert your cuvet the same way. Make sure the cuvet is
completely inserted. In some instruments, there will be some resistance before the cuvet is
all the way in.
4. Press Reference and then Store.
5. Remove the cuvet.
6. Insert the black cuvet into the sample port. Press Dark and then Store.
7. Remove the black cuvet. You are now ready to collect your data!!!
Collecting Data
8. Fill the cuvet 1/2 full with one of the sunscreen solutions and wipe the cuvet with the special
paper. Record the identity of the solution in your data table.
Note: The prepared solutions contain a precisely weighed amount of
sunscreen that has been diluted with isopropanol so the absorbance can be
accurately measured.
9. Place the cuvet in the sample port, making sure the marking is oriented the same as it was for
your background. If you do not remember exactly how you inserted it, contact the MCL staff
or your teacher for help.
10. Press Scan
11. Save your file by selecting File and then Save Spectral Values. Type in a name for the
spectrum and click OK. Record the name of your file in your data table.
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12. Determine the absorbance at 300 nm by clicking the black + button in the window in the
upper right. Then click the red crosshair in the graph window and trace along the spectrum to
approximately 300 nm. Press left or right diamond of the bigger diamond in the upper right
window to fine tune the wavelength to 300 nm. The exact wavelength and absorbance are
printed in this window. If the crosshair is already on 300 nm, move it off 300 nm and then
back to 300 nm. This is necessary to get the absorbance reading for your current spectrum
instead of the previous spectrum.
13. Record the value for the absorbance at 300 nm in your data table. (Do not fill in the "Color
in Printout" column yet).
14. Using a pipet, transfer the solution from the cuvet to the waste container. Rinse the cuvet
with isopropanol and allow it to dry for about 30 seconds.
15. Repeat steps 8-14 for each solution to be analyzed. When recording the absorbance at 3000
nm, remember to move the crosshair off 300 nm and the back to 300 nm or you will not get
the correct number.
Printing your data
16. To view all your spectra on the screen at once, select Open Spectrum Overlay in the file
menu. Select one of your saved files from the list and press Open. Keep track of which file
is which color in the table below. Repeat until you have opened all your files.
17. Print your spectra.
Clean-up
18. Wash your hands with soap and water.
Data:
Sunscreen solution
Name of Saved File
Absorbance @ 300 nm Color in printout
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Analysis and Questions:
1. What is the purpose of running the "blank" cuvet containing only the solvent and the “dark”
cuvet (ocean optics only) at the beginning of the experiment?
2. Why is it necessary to hold the cuvet by the "top" and wipe it with the kimwipe?
3. What suntan lotion blocked the wavelengths around 300 nm most effectively? How can you
tell?
4. The UV-vis spectrometer measures absorbance of light between approximately 200-800 nm.
Why are we most interested in the lower wavelengths in this experiment?
5. Overall, which suntan lotion will give you the most protection? Explain your choice.
6. Which suntan lotion would you choose and why?
7. How much greater should the absorbance be for an SPF 30 sunscreen than an SPF 15
sunscreen? Is your data consistent with this?
1. 8. The Generic SPF 8 sunscreen has a cost of $4.95 for a 454g bottle and the name brand
SPF 8 has a cost of $9.95 for a 454 g bottle. Calculate the cost per gram for each, and
using the absorbance values, analyze the relative worth of each suncreen.
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