TLC Analysis of Analgesic Drugs

The standard compounds will all be available as solutions of 1 g of each dissolved in 20 mL of a 50 50 mixture of methylene chloride and ethanol. The purpose of the first reference plate is to determine the order of elution (R values) of the known substances and to index the standard reference mixture. Several of the substances have similar Revalues, but you will note a different behavior for each spot with the visualization methods. On the sample plate, the standard reference mixture will be spotted, along with Naproxen sodium and several solutions that you will prepare from commercial analgesic tablets. These tablets will each be crushed and dissolved in a 50 50 methylene chloride-ethanol mixture for spotting. 

Two methods of visualization will be used to observe the positions of the spots on the developed TLC plates. First, the plates will be observed while under illumination from a short-wavelength ultraviolet (UV) lamp. This is best done in a darkened room or in a fume hood that has been darkened by taping butcher paper or aluminum foil over the lowered glass cover. Under these conditions, some of the spots will appear as dark areas on the plate, while others will fluoresce brightly. This difference in appearance under UV illumination will help to distinguish the 

It is possible to use several developing solvents for this experiment, but ethyl acetate with 0.5% glacial acetic acid added is preferred. The small amount of glacial acetic acid supplies protons and suppresses ionization of aspirin, ibuprofen, and naproxen sodium, allowing them to travel upward on the plates in their protonated form. Without the acid, these compounds do not move. 

In some analgesics, you may find ingredients besides the five mentioned previously. Some include an antihistamine and some contain a mild sedative. For instance, Midol contains N-cinnamylephedrine (cinnamedrine), an antihistamine, and Excediin PM contains the sedative methapyrilene hydrochloride. Cope contains the related sedative methap nilene fumarate. Some tablets may be colored with a chemical dye. 

Column Chromatography, Sections 19.1-19.3 Thin-Layer Chromatography Identification of Drugs 

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