Thin Layer Chromatography:
Analysis of Isolated Caffeine and Aspirin
Reading assignment: Fessenden, R.J., Fessenden, J.S., Feist, P.
ed.; Brooks/Cole: Pacific Grove, 2001, pp 133-138.
In last week’s experiment you attempted to isolate aspirin and caffeine (figure 1) from the
over-the-counter drug, Anacin®. This week your goal is to gather evidence to determine
the purity of the compounds isolated and to prove whether or not you successfully
isolated caffeine and aspirin.
The structures of caffeine and aspirin.
Chromatography is the general method used to separate mixtures of two or more
compounds. Some examples include column and gas chromatography, which you will
learn later this semester, and thin layer chromatography (TLC), the focus of today’s
experiment. TLC is an extremely simple, fast, and inexpensive technique involving
separation of compounds based on differences in
polarity, adsportivities and solubilities
TLC has many uses including determination of the
number of components
in a mixture,
of the components in a mixture, to monitor the progress of a reaction, and
to measure the effectiveness of a purification. In today’s lab you will use TLC to
determine the identity of the compounds isolated in last week’s experiment and also to
make a determination of the purity of these compounds.
As you have learned from reading Fessenden and Fessenden, chromatography typically
involves the use of two phases: a
and a highly polar
The mixture of compounds to be separated/analyzed is dissolved in
an easily evaporated solvent and placed on the adsorbent. The eluent travels via capillary
action through the stationary phase and as it travels past the mixture of compounds an
equilibrium is established for each component of the mixture between the molecules of
that component which are adsorbed on the solid and the molecules which are in solution.
In principle, the components will differ in solubility and in the strength of their
adsorption to the stationary phase and some components will be carried farther up the
plate than others. Generally, polar molecules adsorb more tightly to the polar stationary
phase than non-polar molecules and consequently travel a shorter distance up the plate.