Summer Research Program for Science Teachers

Caren Birchwood-Taylor

Bayside High School

2000

How may natural substances be analyzed as standards using thin-layer chromatography (TLC)?

This laboratory activity is to be used with the students who have already mastered most science process skills, would be familiar with simple laboratory equipment, and could accurately measure basic quantities. The goal of this activity is for students to produce chromatograms of various vegetable extracts and standards , then create a database of the results.

Thin-layer chromatography (TLC) would be used  in this activity. In this technique, a glass plate is coated with a thin layer of a solid. Samples are then placed on the solid, and the plate is immersed in a solvent. Some components of the sample have a high affinity for the solid and move more slowly, others have a high affinity for the solvent and move more rapidly. [9-12 Content Standard B- Properties of matter] The result is a separation of the mixture into its components (Bodner and Pardue,1995) . The students would first boil the vegetables in a small amount of water to extract the pigments. Then they would place a small amount of the samples on the silica. The TLC plates would then be immersed in a solvent until the solvent; line is just below the point where the samples were applied. The plates would then be removed from the developing chamber and dried by passive evaporation. Then they would be stained and immediately placed face down on a flatbed scanner. Several standards and vegetable extracts would be used as samples. The students would create a database of the results and compare the chromatograms. The lesson plan attached represents the TLC procedure for the standards cholesterol, cholesterol acetate, and cholesterol pelargonate (Johnson, 2000).

Objectives

Students will be able to:

i)       load a thin-layer chromatography plate

ii)      develop a thin-layer chromatograph

iii)    create a database of chromatographs by scanning them to interface with a computer software program

Materials

Cholesterol, cholesterol acetate, cholesterol pelargonate, silica gel TLC plates, toluene, 10ml glass pipets, 2 propanol, heptane, iodine crystals, scanner, computer, printer

[Teaching Standard D- Make accessible science materials]

CAUTION: TOLUENE IS AN IRRITANT

Students will work in groups of 3. Each group will use a different sample.

Procedure:

1)     Dissolve 100mg of each sample in 1.00ml of toluene.

2)     Dilute with toluene in a 1:2 ratio.

3)     Place a small drop of diluted sample on silica and immerse in 10:90   2 propanol/heptane until the solvent line is just below the point where samples were applied.

4)     Allow plates to dry by passive evaporation.

5)     Stain plates with iodine.

6)     Remove plates and place face down on a flatbed scanner.

7)     Measure the distance the spot moves. Measure the distance the solvent front moves. Calculate the R_f values using the formula R_f = distance spot moves/distance solvent front moves. Save this information on the software program. [Content Standard Unifying Concepts- Change, constancy, and measurement]

Summary:

i)       Explain the process of TLC in terms of  adhesive forces. [9-12 Content Standard B- Motions and forces]

ii)      Describe the visual differences between the chromatograms of cholesterol and its two derivatives.

iii)     What are the differences in their R_f  values?

 Homework/Extension:

Write a short paper describing how coffee is decaffeinated  by extraction. [9-12 Content Standard E- Understandings about science and technology]

 References

Bodner,G.M. and Pardue,H.L. 1995. Chemistry - An Experimental Science.  2^nd ed.  New York: John Wiley and Sons, Inc.