EXPERIMENT 8 THIN LAYER CHROMATOGRAPHY AND MELTING POINT DETERMINATION: DETECTION OF CAFFEINE IN VARIOUS SAMPLES Additional Resources http://orgchem.colorado.edu/hndbksupport/TLC/TLC.html http://coffeefaq.com/caffaq.html Materials Needed TLC equipment: 1 5-cm x 8-cm TLC plate, 2 capillary micropipettes, TLC tank and lid, ruler, UV lamp Extraction Equipment: spatula, 2 small test tubes, Pasteur pipet and bulb Chemicals: caffeine, ethanol, dichloromethane, TLC solvent (5% acetic acid in ethyl acetate) Meltemp apparatus and capillary tube Three of the following samples for analysis: diet cola, diet lemon-lime soda, analgesic pill, instant coffee, instant iced tea Purpose In this experiment, thin-layer chromatography (TLC), a standard technique for the qualitative analysis of mixtures, will be introduced. First, a sample of caffeine will be authenticated by measuring its melting point. The authentic caffeine will then be used as a standard for the TLC analysis one of the following items: a diet soda, tealeaves, instant coffee, cocoa powder, or an analgesic pill. The caffeine present in these substances must first be separated from dissimilar materials in the substance using a process called extraction. Extraction simply involves treating the substance with an organic solvent that will dissolve out the caffeine and other soluble organic materials but leave insoluble matter (such as cellulose in leaves and inorganic binders in a pill) behind. Background Caffeine. Caffeine is a heterocyclic (i.e., has atoms other than carbon in a ring) amine found in many plant materials including tealeaves, coffee beans, kola nuts, cocoa beans, and guarana seeds. Compounds found in nature are referred to as “natural products” and amine natural products are called alkaloids. Hence, caffeine is an alkaloid. Many alkaloids, including caffeine, have pharmacological effects. Some other examples are morphine, codeine, cocaine, and nicotine. Caffeine-containing beverages, mainly tea and coffee, have been used for their stimulant effect for over a thousand years. It was only recently that caffeine became available to consumers in essentially pure form (No-Doz). Pure caffeine was first isolated from coffee beans in 1821 by Pierre Robiquet. Caffeine is also a diuretic and a vasoconstrictor (constricts blood vessels). Recognition of the vasoconstriction effect suggested caffeine’s use as a treatment for headaches especially migraines. (Although it has recently been argued by some that it is caffeine withdrawal that is the root cause of the great majority of migraines.) In fact, it is often added to headache remedies, including Excedrin and other OTC analgesics and is also a key ingredient in the new prescription drug, Fioret. Caffeine’s side effects and addictive nature lead many people to use decaffeinated coffee, cola, etc. The caffeine can be removed from coffee beans by extracting the beans with an organic solvent (the old method) or with supercritical carbon dioxide. The caffeine so removed can then be isolated and purified and used in other products. In fact, decaffeination of coffee beans is the primary source of pure caffeine used in stimulant pills (No-Doz), headache medications, and non-cola caffeinated soft drinks such as Mountain Dew.
Thin-Layer Chromatography. In a TLC experiment, a small spot of the sample to be analyzed is made at one end of a glass or plastic plate that has been coated with a thin layer of silica gel (SiO2(s)). In a process known as “development” the plate is then immersed spot-end-down in a pool of solvent (the exact solvent used depends on the sample and is determined by experimentation). The solvent is allowed to move up the plate by capillary action (the silica gel “soaks it up”). Compounds present in the sample are carried up the plate by the solvent. However, different compounds generally move at different rates. Therefore, if the sample is a mixture of compounds it will separate into a series of spots at...
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