Analyzing Sunscreens
Learning Goals:
1. To practice important solvation and dilution techniques.
2. To make connections between absorbance of EM radiation and chemical structure.
3. To explore Beer's law and its importance in spectroscopy.
Abstract:
When one thinks of the energy delivered to the Earth from the sun, it is common to consider only the light that is visible to the human eye. However, the sun's rays consist of a wide variety of wavelengths in the electromagnetic spectrum. One of the most troubling types of electromagnetic radiation is ultraviolet (or UV). UV rays are not visible to the human eye, and are characterized by wavelengths shorter than those of visible light (between 1 and 400 nm). UV rays are responsible for giving skin a tanned color when one sits in the sun. However, these same rays also induce premature aging of the skin, and-In some cases-the genetic mutations that eventually give rise to skin cancer.
In this lab you will apply the techniques and mathematics of spectroscopy to an investigation of UV-blocking ability of sunscreens. By analyzing the absorbance in the UV range of a sunscreen solution, you will be able to make predictions about its chemical composition and strength. You will also apply your data to an application of Beer's Law: an important relationship between concentration and absorbance.
| Materials | Chemicals |
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Procedure:
(1) Making the Sunscreen Solution
Begin by placing a beaker on the electronic balance. Tear the scale so that it reads 0.000 g (this will subtract the beaker's mass from subsequent measurements).
Now, very carefully, add a drop of sunscreen to the beaker. Your goal should be to deposit between 0.025 g and 0.075 g of sunscreen. Make sure that you record the exact mass of sunscreen added in your notes. If your sunscreen is too thick to drip, try moving a small amount into the beaker with a stirring wand.
Add a few drops of water to the beaker and swirl/stir until the mixture takes on a milky consistency. Try to break up the sunscreen blob as much as possible.
Now, place the magnetic stirrer in the beaker and place the beaker on the stirring plate. Slowly turn up the power on the stirrer. Now, slowly add about 50 ml of isopropyl alcohol to the beaker. Allow this solution to mix until most of the sunscreen flecks have disappeared.
Using the funnel, pour the beaker's contents into the volumetric flask. Using the same funnel, add additional isopropyl alcohol to the flask until the fluid's meniscus is just above the etched line in the glass (this will mean that there is exactly 100 ml of solution present).
(2) Making Dilutions
Set up your two burettes on the burette stand. Note: the burettes need to be closed (this means that the turning piece at the bottom needs to be perpendicular to the glass tube).
Using the rinsed funnel, fill the burette on the right with your remaining isopropyl alcohol.
Fill the burette on the left with your sunscreen solution.
Obtain 6 cuvettes. You may wish to label these with small pieces of tape as A, B, C, D, E and "blank". Record the contents of each in your notes.
Fill the "blank" cuvette with isopropyl alcohol. Note: "filling" a cuvette really only requires that it be two thirds full (or one third empty, for the pessimists out there).
Fill the cuvette labeled "E" with your sunscreen solution. Set these two cuvettes aside.
You will now make a series of dilutions of your original sunscreen solution. Make a table in your notes with three headings: "name", "Parts Sunscreen Solution" and "Parts Isopropyl Alcohol," like the one below:
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NAME
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Parts Sunscreen Sol'n
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Parts Isopropyl Alcohol
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A
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2
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4
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B
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2
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2
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C
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4
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2
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D
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8
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2
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E
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Pure
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N/A
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Use the burettes to drop 2 milliliters of sunscreen solution and 2 milliliters of isopropyl alcohol into a beaker (note: "parts" = "milliliters" in this lab). Swirl this slightly to stir and then fill cuvette "A" with the solution. Empty the beaker and repeat the process, using the parts listed for cuvettes B, C, and D.
If you would like to make additional dilutions in additional cuvettes, you will improve your results and may be eligible for extra credit. Be sure to record the name and the parts of each of the solutions you used.
(3) Scanning your cuvettes.
Follow the procedure that your teacher showed you for using your spectrophotometer. Some basic guidelines will be discussed here.
Blank the spectrophotometer using the cuvette filled with just isopropyl alcohol. This is the background for all of your sunscreen scans.
Scan each of your sunscreen cuvettes making sure that you save each scan under a name you'll remember. For example, "Yourname_A.icn" for cuvette "A" is much clearer than "171202_6.icn". When you are finished using the machine, allow another group to scan their samples. If you used a disk to store your scans, take it with you. Ask you teacher about disposal instructions for your chemicals.
Analysis of Results
(1) Looking at the scans.
Open all of your sunscreen scans (A, B, C, D and E for most) so that you can look at them together. If your program shows the graphs as "wavelength vs. absorbance" you will be able to drag a pointer along the screen. Wherever it intersects the graphs, it will display each of their absorptions at the current wavelength.
For Example:
The text at the bottom displays the file names that correspond to the different sunscreen solutions and their respective absorptions at 310 nm. Note how the vertical line in the cross is also positioned at 310 nm, thus selecting this wavelength.
Record the absorptions of the five solutions at their highest point in your notes or data table. This need not be a sharp peak like the one in the graph above, just the location of the curves' maximum values. Make sure that you know which absorbance value corresponds to which concentration. Note: although the curves have different concentrations, they will all peak at about the same point.
(2) Doing the Math
It is now necessary to calculate the concentrations of each of your sunscreen solutions in terms of milligrams of sunscreen per milliliter of solution. Let's first figure out the concentration of your original solution (sometimes called a "stock" solution). Take the number of grams of sunscreen you used and multiply it by 1000 (this converts the grams into milligrams). Now, since you made 100 ml of solution, divide the mass of the sunscreen in milligrams by this number. For the purpose of explaining the following procedure, this lab handout will use a stock concentration of 0.500 mg/ml (your value should be close, but probably won't be identical; anything from 0.250 mg/ml to 0.750 mg/ml should work fine). You will need to refer to the data table you made earlier. Use the following formula to calculate the concentration of the different dilutions.
[(stock concentration) x (parts sunscreen)] ¸
[(parts sunscreen) + (parts isopropyl alcohol)]Calculating the concentration of sample "A"
(0.500 mg/ml x 2) / (2 + 4) = (1.00 mg/ml)/(6)
= 0.167 mg/mlRepeat this calculation for the other solutions you made.
Make another data table that compares the concentration of samples A, B, C, D and E to their peak absorptions that you found earlier. You will be asked to make a graph using these values later.
(3) Making a Beer's Law Plot
For more information on Beer's Law and making a Beer's Law plot, CLICK HERE.
Beer's law demonstrates the direct relationship between concentration and absorbance of solutions under ideal conditions. You will test this relationship by graphing your concentration and absorbance data as points on a graph. The concentration value will serve as the "x" and the absorbance will serve as the "y" for your (x,y) data points. Do not worry about putting units of absorbance on your graph, but do include all other necessary graph features (title, labeled axes, etc.).
Draw a best-fit line through the data and calculate its slope. Your teacher may allow you to use graphical analysis software to plot your data. If so, use the "add trendline" feature to draw the line. Select the "linear" option and display the equation of the line on your graph.
(1) Do the raised portions of your scans correspond to the UV region? What does this tell us about the sunscreens?
(2) Most sunscreens use separate chemicals to block different regions of the UV wavelengths. Each chemical will provide its own absorbance curve, and together these curves "meld" into the single curve produced by your scan. Based on the bumps/peaks in your scans, how many active chemicals would you say are contributing to the sunscreens absorbance?
(A) Extra Credit:
Color the regions that you think correspond to the different chemicals. Using the active ingredients list on the back of your sunscreen, see if you can identify which bump belongs to which chemical. You may need to do some research to determine what parts of the UV Spectrum the chemicals block. It may also be useful for you to know the breakdown of the UV range into its A, B and C components. Your teacher may have some useful information about these investigations.(3) What is the slope of your graph? What are the units of the slope? What does this tell us? Compare your slope to the slope of another lab group's graph, preferably one who used a different SPF sunscreen. Is there any relationship between the slope and SPF?
(B) Extra Credit:
Make a graph of all the groups' slopes and SPF values. Use the various sunscreens' SPF values as the "x" and the slope as the "y". Is there a relationship between the two?
