UV Analysis of Suntan Lotions

Introduction:

Erythema (sunburn) is caused most severely at about 300 nm. An effective sunscreen would be one that completely blocked the radiation in the vicinity of this wavelength. Sunscreens and suntan lotions are rated with protection factors. In order to be effective, active ingredients, such as benzophones, phenyl salicylate, and p-amino benzoic acid, must exhibit an intense absorption maximum centered around 300 nm while transmitting the longer wavelength tanning radiation.

Spectrophotometry may be used to find the absorbance of sunscreens at various wavelengths. Standard solutions of known SPF value will be run on a scanning UV-VIS spectrophotometer to create a standard curve. Solutions of unknown SPF can then then be run and compared to the standards to calculate its SPF value.

Purpose:

The purpose of this lab is to determine the SPF value of unknown suntan lotions through spectrophotometric analysis.

Materials:

UV-VIS spectrophotometer

Droppers

Quartz cuvet

Kim wipes

Solvent (1-propanol)

Standard solutions

Unknown solutions

Safety:

  • Always wear goggles and an apron in the lab.
  • Alcohols are flammable and irritating. Dispose of waste in the sink with large amounts of water. Wash hands thoroughly and avoid contact with eyes.

Procedure

1. With the sample compartment empty, turn on the power switch located at the rear of

the instrument. Let the instrument undergo the “Self Diagnostics.”

2. Press the “Make a measurement” pad.

3. Press the “Scan” pad.

4. Place a quartz cuvet with solvent (blank) into the cell holder such that the frosted sides

are placed in front and behind the solution relative to the front of the instrument. This

orientation must always be followed for this instrument.

5. Press the “Ref” pad.

6. Rinse the cuvet twice with the standard solution to be run.

Then fill the cuvet with the standard solution and insert it into the cell holder.

7. Press the “Test” pad.

8. Move the cursor with the left or right arrow pad until the largest Absorbance value is

displayed.

9. Turn on the printer by pressing the “1” pad.

10. Press the “Print” pad.

11. Press the “Accept” pad.

12. Repeat steps 6-11 with the other standard and unknown solutions. Remember to

label the print-outs with the sample name.

Data:

Sample Name

/ Absorbance at 300 nm / SPF

Calculations:

On graph paper or a computer, plot a graph of absorbance (y) vs. SPF (x) for the standard solutions. Insert a best fit line. Since the instrument was zeroed with the solvent, the line should go through the origin.

Using the absorbance value for the unknown samples and your graph, determine the SPF values of the unknowns. Record these values in the data table. Be sure to attach the print-outs and the graph.

Questions:

1.What suntan lotion blocked the light at 300 nm most effectively? How can you tell?

2.Overall, which suntan lotion would you chose? Explain your choice.

UV Analysis of Suntan Lotions

Teacher Notes

Lab Time:about 30 minutes (more time for graphing)

Preparations:

Time:30 minutes.

To prepare the solutions, first dissolve 1 g of the suntan lotion in 500 mL of n-propanol. Mix 30 mL of this solution with 400 mL of n-propanol to dilute the solution. If the solutions are too concentrated, they will absorb all of the light at 300nm.

Considerations:

This lab allows students to compare SPF values, brands of lotion, and cost of the lotion with respect to the absorbance of ultraviolet radiation. The suntan lotion solutions can be saved indefinitely. Suntan lotions must be "PABA free" to dissolve in the n-propanol. Lotions made for children are usually "PABA free."

Acknowledgements:

DeMenna, J. “Determination of SPF factor in Sunscreen Lotions by UV-Spectrophotometry.” Buck Scientific.

Last updated 8-01