LAB #10: THIN LAYER CHROMATOGRAPHY OF ANALGESICS
Chromatography is one of the most useful methods of separating and purifying organic compounds. There are many types of chromatography but most depend on the principle of adsorption.
Various types of chromatography includes:
- liquid/ column
- thin layer (TLC)
- Gas chromatography Paper
The two important components of chromatography are the adsorbent and the eluent.
- Adsorbents are usually solid materials that will attract and adsorb the materials to be separated. Adsorbents are also called the stationary phase.
- The eluent is the solvent, which carries the materials to be separated through the adsorbent. The eluent is also called the mobile phase.
Chromatography works on the concept that the compounds to be separated are slightly soluble in the eluent and will spend some of the time in the eluent (or the solvent) and some of the time on the adsorbent. When the components of a mixture have varying affinities for the eluent, they can then be separated from one another. The polarity of the molecules to be separated and the polarity of the eluent are very important. Changing the polarity of the eluent will only slightly affect the solubility of the molecules but may change the relative attraction for the adsorbent. Affinity of a substance for the eluent versus the adsorbent allows molecules to be separated by chromatography.
Paper chromatography is often used as a simple separation technique. In paper chromatography, the adsorbent is the paper itself, while the eluent can be any number of solvents. When the paper is placed in a chromatography chamber, the eluent moves up the paper by capillary action. Organic molecules that are "spotted" onto the paper strip separate as they are carried with the eluent at different rates. Those molecules that have a polarity closest to the polarity of the eluent will move up the strip the fastest.
The choice of eluent is the most difficult task in chromatography. Choosing the right polarity is critical because this determines the level of separation that will be achieved. Different samples will spend varying amounts of time interacting with the paper and the solvent. Through these different interactions, the samples will move different distances along the chromatography paper. The distance a sample moves aling the chromatography paper is compared to the overall distance the solvent travels - this ratio is called the Rf value or rate of flow.
Introduction to Analgesics
Analgesics are substances that relieve pain. The most common of these is aspirin, a component of more than 100 nonprescription drugs. In the present experiment analgesic tablets will be analyzed by thin-layer chromatography to determine which analgesics they contain and whether they contain caffeine, which is often added to counteract the sedative effects of the analgesic.
In addition to aspirin and caffeine the most common components of analgesics are, at present, acetaminophen and ibuprofen (Motrin). Phenacetin, the P of the APC tablets and a former component of Empirin, has been removed from the market because of deleterious side effects. In addition to one or more of these substances each tablet contains a binder, often starch or silica gel. To counteract the acidic properties of aspirin, an inorganic buffering agent is added to some analgesics. Inspection of labels will reveal that most cold remedies and decongestants contain both aspirin and caffeine in addition to the primary ingredient.
To identify an unknown by TLC, the usual strategy is to run chromatograms of known
substances (the standards) that are likely to be present in the unknown and the unknown at the same
time. If the unknown has one or more spots that correspond to spots with the same R ’s as the
standards, then those substances are probably present.
Proprietary drugs that contain one or more of the common analgesics and sometimes caffeine
are sold under the names of Bayer Aspirin, Anacin, Datril, Advil, Excedrin, Extra Strength Excedrin, Tylenol, and Vanquish.
Chromatographic Separation Explained
Calculating Rf factor
This is a video that explains the calculations.
This is a GREAT video. It explains and shows what we will do in this lab very well. Watch carefully, second by second. It is so good, you should watch it at least twice!
View the procedure handout attached at the bottom of this page. Please note that we will not look at the kinetics of crystal violet, but of another, very similar molecule. The principles operate similarly to the discussion above. Apply what you learned about crystal violet to the molecules and reactions under investigation.