In the Laboratory
A New Glow on the Chromatography of M&M Candies
W
Kurt R. Birdwhistell and Thomas G. Spence* Department of Chemistry, Loyola University, New Orleans, LA 70118; *
[email protected] Paper chromatography of food dyes is a popular experiment in many beginning laboratories at the university and high school level. Several articles in this Journal involve the separation and identification of food dyes (1–4). In particular, paper chromatography was used to confirm the presence of yellow dye #5 in M&Ms (5). In this experiment, dyes in M&Ms and other foodstuffs are separated by paper chromatography and identified based on a comparison of color and Rf with those of standard FD&C dyes (6 ). We have expanded this lab by adding an additional identification technique. We have students determine whether the FD&C dyes fluoresce and, if they do, determine the color of the fluorescence under a long-wavelength hand-held UV lamp. Students can then examine M&M candies directly under the UV lamp and observe that indeed most of them appear to fluoresce. Students then examine their dry M&M dye chromatograms under UV light to determine which components contribute to the fluorescence. This additional step obviously provides more supporting data for identification of the standard dyes and introduces students to an important new phenomenon (fluorescence). Fluorescence quenching may also be discussed, as no fluorescence can be observed while the chromatograms are damp. This experiment removes some of the bad connotations fluorescing substances may have in the minds of students. After all, students have been eating these tasty fluorescing substances for years. Experiment Preparation and development of chromatograms follows the procedure described by Markow (4). Students prepare two chromatograms using 10.2 × 13.3-cm Whatman cellulose chromatography paper. They take their dried chromatograms to a darkened room and examine the spots on the chromatograms using a hand-held UV lamp. They use one chromatogram to determine the Rf value and color and fluorescence characteristics of seven standard food dyes; the second is used to separate and identify the food dyes in the coating of M&M candies. Please refer to the online lab documentation for details of the experimental procedure.W Hazards Use caution when handling UV lamps, as UV light is damaging to your eyes. Students should use safety glasses and should not look directly at the UV lamp to see if it is on during chromatogram observation. Results The fluorescence characteristics and Rf values of the standard food dyes are summarized in Table 1 and the colors of the fluorescence observed for the M&M candies are listed in Table 2. Both the Rf values and fluorescence colors of the dyes in the M&M candy coating mixture agree with those observed for the standard dyes alone. Students are able to use the fluorescence colors to confirm their assignments on the basis of Rf values and color alone. Some students note that
Table 1. Characteristics of FD&C Food Dyes Under 365-nm UV Light
Dye Name
Color in Room Light
Yellow 6
Yellow
No
—
Yellow 5
Faint yellow
Yes
Yellow (faint)
.53
Blue 1
Greenish blue
Yes
Pink (faint)
.91
Fluoresce?
Color
Rf .94
Blue 2
Dark blue
Yes
Blue (bright)
.32
Red 3
Pinkish red
Yes
Orange (bright)
.16
Red 40
Faint red
Yes
Pink
.02
Green 3 fcf
Greenish blue
Yes
Pink (bright)
.86
Table 2. Fluorescence of Dr y M&M Candies Color of M&M
Fluoresce?
Color under 365-nm UV Light
Blue
No
Yellow
Yes
— Orange
Red
Yes
Dark red
Brown
Yes
Red
Green
Yes
Brownish yellow
Orange
Yes
Brownish
the fluorescence aids in locating spots that were difficult to see otherwise, especially the blue dye used in green M&Ms. A summary of typical student results is given online.W The students are able to complete this procedure and identify the dyes in a little over two hours. This is one of the most enjoyable labs for the students during the first-semester general chemistry lab. Acknowledgments We would like to thank our colleagues teaching general chemistry lab at Loyola University for helpful comments and the students of general chemistry I lab in the fall of 2000 for doing the experiment as we were modifying it. Supplemental Material Notes for the instructor, student materials, and experimental procedures are available in this issue of JCE Online. W
Literature Cited 1. 2. 3. 4. 5. 6.
Ondrus, M.G.; Brice, L. K. J. Chem. Educ. 1985, 62, 798–799. McKone, H. T.; Ivie, K. J. Chem. Educ. 1980, 57, 321–322. Bird, E. W.; Sturtevant, F. J. Chem. Educ. 1992, 69, 996–998. Markow, P. G. J. Chem. Educ. 1988, 65, 899–900. Kandel, M. J. Chem. Educ. 1992, 69, 988–989. The structures of the dyes are found in Pavia, D. L.; Lampman, G. M.; Kriz, G. S.; Engel, R. G. Introduction to Organic Laboratory Techniques A Microscale Approach, 2nd ed.; Saunders: Philadelphia, 1995; pp 351–355.
JChemEd.chem.wisc.edu • Vol. 79 No. 7 July 2002 • Journal of Chemical Education
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