J a m e s W. Pavlik University of WisconsinRiver Falls River Falls, ~ i s c o n s i n ~ 4 0 ~ ~
TLC Detection of Caffeine in commercial Products
Application of thin-layer chromatography to the analysis of commercially available products provides a practical and interesting introduction to this chromatographic technique. This report describes an introductory that in. experiment . volves the tlc detection of caffeine in a variety of familiar pharmaceutical products. The experiment takes on added significance to our students since it is preceded by the larpescale isolation of caffeine from tea or coffee by solvent extraction and suhlimation.~~2 Based on this experience, students can devise their own small-scale extraction procedure and use their own sample of sublimed caffeine to determine a suitable solvent system. During a pre-laboratory discussion, thin-layer chromatography is introduced and demonstrated and an extraction procedure is discussed. Our procedure given below is designed to remove aspirin since this is a known constituent in most of the products screened. Students are asked to report the presence or absence of caffeine in each product and also to note the Rr of any additional spot. Since half of the products listed below show a second component under the conditions used, this not only provides an additional criterion for judging laboratory technique, hut leads to active diiruisions about t h e identity of the spot. In\ari3hlv. some hludenrs suggest aspirin a i t h e irlentit? ot 111esecond spot. Chromatographing their unknowns alongside a pure sample 'Mohrig, J . R., and Neckers, D.C., "Laboratory Experiments in Organic Chemistry," Reinhdd Corp., New Yark, 1968, p. 4. 2Adams, R., Johnson, J. R., and Wilcox, C. F., "Laboratory Experiments in Organic Chemistry" (6th ed.), The Macmillan Co., New York, 1970, p. 108. 3Jars suitable for solvent chambers can bc obtained from Fisher Scientific Ca., Catalog No. 2-892C with cap No. 2-883-25,
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Journal ol Chemical Education
of aspirin, however, eliminates this possibility and confirms the efficacy of the extraction procedure. Experimental Determination of o Soluent System for Caffeine. A suitable solvent system for caffeine is determined by spotting a solution of caffeine in chloroform on Silica Gel thin-layer plates (microscope slides dipped in a Silica Gel G slurry-35 g Silica Gel in 100 ml of chloroform/methanol in a ratio of 2 to I ) , developing in solvents of increasing polarity, and visualizing in a chamber of iodine.3 Typical R, values for caffeine in a variety of solvents are: benzene, 0; ether, 0.1; ethyl acetate, 0.15; chloroform, 0.4; acetone, 0.6; chloroform/acetane (1to I ) , 0.5. Extraction and Analysis. One finely powdered tablet and 3 ml cone. ammonia are thoroughly mixed in a small centrifuge tube and extracted with 1 ml of chloroform. The two phases are thoroughly mixed by placing a spatula in the tube and rapidly rotating the shank of the spatula between the thumb and index finper. The phases are allowed t o separate (centrifuge if necessary) and a portion of the lower layer is transferred with a dropping pipet to a clean vial and sootted on a thin-laver olate aloneside a soot of authentic caffeine. The plate is developedin ehloroform/acetone ( I to I ) and visualized in an iodine chamber.
Results Product Anacin APC Aspirin (any brand) Bufferin Cope Emperine Exeedrin No-Doz
Caffeine
Remarks
Positive Positive Negative Negative Positive Positive Positive Positive
No other spots observed Second mot a t 0.85 No spatsbbserved Nospots observed Second intense spat a t 0.15 Second spot a t 0.6 Second spot a t 0.75 No other spots observed
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