Analysis of Analgesic Mixtures An Organic Chemistry Experiment Ned H. Martin University of North Carolina at Wilmington, Wilmington. NC 28406 Students seem to appreciate experiments performed on samples that are within their realm of experience more than those involving "some chemical from a bottle in the stockroom." Furthermore, they enjoy the challenge of unknowns, including the reward of working out experimental conditions themselves. Designed to take cognizance of these observations, the experiment described herein involves the analysis of one of several commercial analgesic mixtures (for examples: APC, Bromo-SeltzeP, Empirin*, Excedrinm, or Vanquish*) by silica gel TLC, followed by preparative (thick) layer chromatographic separation and spectroscopic analysis (UV, IR, NMR, and MS). The experiment can he performed without loss of coherence by omitting up to three of the spectral analyses. This experiment also serves as an introduction to semi-micro techniques (extraction, filtration, chromatographic and spectral analyses) that may he overlooked in the traditional organic laboratory course. One key difference between the experiment described here and others involving TLC analysis of drugs ( 1 5 )is that in this experiment, the student is responsible for devising a suitable solvent system for the TLC separation. T o facilitate this process, a list ofelutropic strengths (6)of the solvents available for eluantsl is provided to each student. The students are told that in binary mixtures of solvents, the elutropic strength of the mixture varies continuously between the en values of each of the components (7). Students are asked to seek a solvent system (single or binary) that provides separation of three components and has Rf values within the range of 0.1-0.9.Most quickly "discover" that the Rr correlates with the elution strength of the solvent and use simple (single) solvents tc determine the optimum @range. Then, they devise
818
Journal of Chemlcai Education
mixtures which should have similar @values (based on their useful oversimolification of linearity between solvent composition and eiution strength), and-test them for resolving abilitv. Manv successful solvent systems have been developed, and most apply equally well to the thick layer separation, although Rfvalues may vary. Students gain a much better understanding of the nature of the adsorption process and an appreciation for the range of solvent polarities by arriving a t an appropriate solvent system (often a mixture) by this method than if a predetermined, arbitrary solvent system were suggested. Students are encouraged to work in pairs, which accomplishes several objectives: first, it speeds the analysis; secondly, teamwork demands cooperation and develops in the students a sense of responsibility. Another major difference between this experiment and others utilizing TLC for drug analysis is that this experiment includes preparative thick layer separation, which permits isolation of sufficient quantities of the components to allow subsequent spectroscopic investigation. Thick layer plates (1000 p ) can either be purchased or prepared by the students. Zones can be located on both TLC and PTLC using either shortwave ultraviolet light (254 nm) or iodine vapor. Some convenient techniques for these procedures have been described ~reviouslv(8). The experiment requires one 3-hr laboratory for the TLC analysis (determining a solvent system and matching &values with standards) and preparation of thick layer plates, and another lab period for the PTLC separation, including eluting the comoonents from the isolated zones. T h e "free time" during d'rvelupment of the thick layer plate is well spent watchine a c d d u l demonstration of'kwett's rlawical column chromatography experiment ( 9 ) ,with separation monitored
bv TLC. If time oermits. a demonstration of other chromat&raphy equipment, such as automatic fraction collertnrs and H1'I.C instruments, is appropriate. Spectral analysis of t h e .. . analgesic components can be performed a s time permits in subsequent laboratory sessions, o r a s t h e students learn each technioue. T h e soectra of t h e standards are r u n bvstudents. then kept o n file for t h e common use by all, s o t h a t each stud e n t m u s t determine onlv two soectra. C are m a d e available a s S t a n d a r d s for t h e T ~ anaiysis methvlene chloride solutions. T h e followine substances a r e provided: acrtylsalirylic arid.'cai(einr.:' ph$acet~n, salicylic acid.l acetmninophen.'~nndcnlicvlamide. M m t of theanalceric mixtures contain three comp&ents; Excedrin" ha8 four. S t u d e n t s a r e told t h a t t h e correct identification of t h e components is less important t h a n proving their presence; this eliminates t h e temptation t o check t h e contents o n t h e label. Experimental Students obtain two pills of their"favoriteWanalgesic directly from their respective bottles on which the list of ingredients has been marked. After thoroughly grinding the pills in a mortar with a pestle, the powdered analgenic is partitioned between 3-5 ml each of water and methylene chloride. If small separatory funnels are notavailahle. the extractions can he performed in 4 dram screw cap vials.Vhe oreanie (hottuml laver is transferred toaclean vialand dried brieflv with
20cm glass plates with a ICMMcoating of silica gel with a fluorescent indicator. These plates can be either student-prepared ( 8 ) or commercially-obtained. An applicator can be made from s disposable pipet hydrawing one endti, a capillary. Students are encouraged to practice "streaking" with the applicator using methyleneehloride on a paper towel until they feel confident in their ability to apply a straight streak of uniform width (5-8 mml. Once the preparative plates have heen developed (using thesame solvent system devised for TLC separation), the zones are visualized by UV and 12 vapor (81 and Rt values are calculated. Each zone is scraped from the plate onto separate, labelled sheets of glassme weighing paper. Isolation of the adsorbed components can be done conveniently using a scintexd glass filter funnel, a rubber filter adapter, a hell jar. 25-ml Erlenmeyer flasks, and a water aspirator vacuum source. One at a time, the components are eluted from the s e r a ~ e dzones usine a solvent more oolar than thesolvent emoloved . . in the senamtion. Tvnicailv. of 10%of methanol to the sol. , addition .~ vent used previously works well, and 10-15 ml is sufficient for each component. After two washes with 5-8 ml of solvent. the flask is labelled and allowed to stand open in a hood. Within a week, solvent has evaporated, and samples can he taken for spectral analysis. Students are permitted to do whatever spectral analyses they choose with the samples. as long as they gather sufficient data to cunfirm the identity of the components. As the class learns a new spectroscopic technique (UV. IR, NMR, and MS), students generally anal- their comnonents hv that teehnioue and comoare their results
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algesic, including a spectral comparison to standards, is required of each student. by the following laboratory period Thin layer chromatography is performed using commercial plastic-backed fluorescentsilicagel thin layer sheets,cut toapproximately 4 em x 10 em. Wide mouth sample bottles (lfi a.1containing a truncated 12.6em diameter diseof iilter paperareused as developkg chambers. Capillary tuhesdrawn from disposable pipets are used to aoolv the samoles to be analvzed. Visualization of zones is done hv ti\; iight and/& iodine vapo;. Preparative (thick) layer chromatography is done using 20 em X
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Students select their eluants from the following: hexane, carbon tetrachloride, toluene, ethyl ether, methylene chloride, acetone, ethyl acetate. acetonitrile, isopropanol, and ethanol. Student preparation, recrystallized. Studentextracted, recrystallized. Often found a s a hydrolysis product of acetylsalicylic acid. Powdered Tylenol" tablet. eBromo-Seltzefl and other euervescenl snalgeslcs should be extracted only in a separatory funnel to allow for the releaseof pressure. A convenient filter can be made by stuffing a very small wad of cnlton into a disposable pipet. To hasten the fillration, pressure can be applied using a pipet bulb.
Results S t u d e n t s rarely misidentify t h e components of their analgesic mixture, with t h e exception of acetylsalicylic acid, which sometimes hydrolyzes t o salicylic acid. T h e activity (water content) of t h e silica eel aooears . . t o be resoonsible. Generallv... students gather more d a t a t h a n is required for positive identification and learn more soectroscoov in t h e orocess. T h e onlv instructions given for the lab write-;; are t h a t students should assume t h a t they a r e writing a technical report from a n analytical laboratory t o a customer. T h e reports are usually quite well written a n d complete.
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Literature Ctted
To.. Boston. 1985. p. 61
Volume 58
Number 10
October 1981
819
