Analysis of Analgesics

This is a favorite exercise for manual writers because students are supposed to be interested in practical exercises that link with their daily experiences. Could be. In the few years I've done this it has worked quite well. Its major limitation if you're on a tight budget is the cost of thin-layer chromatography plates. Students examine the purity of the caffeine they extracted from tea in the previous exercise and identify aspirin, caffeine, and acetaminophen in commercial pain relievers. I no longer include phenacetin, since it has been banned since 1983, although comparing the structures of phenacetin and acetaminophen with their migration on the plates was instructive. To keep the exercise simple, I also ignore ibuprofen, naproxen, and ketoprofen, all of which are now available over-the-counter.

I had students use 2.5 x 10 cm TLC plates from EM Science with a 0.25 mm layer of Silica Gel 60 F-254 (fluorescent indicator). This proprietary layer has an organic binder that makes the layers extremely tough. You can write on them fairly firmly with pencil without damaging the surface. The solvent mixture I use is isopropyl ether/acetic acid (97:3 v/v). Aspirin moves well up the plate, acetaminophen moves slightly off the origin and caffeine doesn't move appreciably.

Students prepare a developing chamber by putting 10 mL of the solvent mixture in a 400-mL beaker and covering tightly with aluminum foil. They are given a standard solution containing 2% each of aspirin and acetaminophen in extraction solvent (ethanol/methylene chloride, 50:50 v/v). They dissolve their caffeine preparations in 2 mL of extraction solvent and prepare unknowns by crushing about an eighth of a tablet in 2 mL of extraction solvent. Using a 5-microliter capillary pipet, they apply two microliters of the standard mixture, their caffeine, and their unknown to the plate about 1 cm from the bottom. For unknowns, I use aspirin, Tylenol, Anacin (aspirin and caffeine), Excedrin (aspirin, acetaminophen, and caffeine), and Aspirin-free Excedrin (acetaminophen and caffeine).

Plates are developed in the beaker with the foil cover until the solvent front reaches about 1 cm from the top (about fifteen to twenty minutes). After drying in the hood, spots are located under a short wavelength UV lamp.

The write-up asks students (1) to draw a sketch of their TLC plate, (2) to calculate the Rf of each component, (3) to identify known components in their unknowns, (4) to indicate (and, by comparison with their caffeine, to identify) components in their unknowns other than the three standards, and (5) to comment on the purity of their extracted caffeine.

Revised 9/1/06