Illustration ST 16 shows one of the simplest forms of chromatography, namely thin layer chromatography.
Chromatography is the separation of a homogeneous solid - liquid mixture, based on differences in the equilibrium distribution of the various dissolved substances between the two phases. In chromatography a distinction is made between the stationary phase (the solid phase) and the mobile phase (the elutant). Each component of the mixture undergoes an equilibrium distribution between the two phases. A component that is to be found chiefly in the mobile phase moves more quickly than a component that shows a greater affinity for the stationary phase. A difference arises in the rate of movement of the components and so these can be separated.

To start a thin-layer chromatography experiment a small quantity of the mixture to be analysed is put onto the thinlayer plate using a capillary tube. The plate comprises a carrier, e.g. glass or aluminium, onto which a thin layer of a few tenths of a millimetre is coated. This layer forms the stationary phase and is made of either modified or non-modified silica gel (SiO₂), aluminium oxide (Al₂O₃) or cellulose. Once the solvent has evaporated the sample spot is formed.

Next, the thin-layer plate is put into the developing tank.

This tank contains a shallow layer of the mobile phase, a few millimetres deep, so that the sample spot remains above the liquid surface. The elutant (the mobile phase) rises as a result of the capillary action of the stationary phase and carries the various components with it. The capillary action is however counteracted by gravity. As a result the mobile phase moves more and more slowly as it rises up the plate, and finally its speed falls to zero.

The experiment is terminated long before tis situation is arrived at. After detection, the place where a compound is found on the resulting chromatogram is given by a socalled R_f value, defined as :

The different components in the mixture have different R_f values as a result of their differing affinities for the two phases.

The Illustration shows the separation of a black ink mixture from a felt-tip pen. The ink is first extracted from the pen using a solvent (in this case methanol). The extract is then put onto the thin-layer plate and eluted with an 80/20 methanol / water mixture that also contains 0.5 mol/l sodium chloride. The various components of the black ink separate out over a large part of the plate according to their affinities for the mobile and the stationary phases. The results show that the black ink from a felt-tip pen contains various dyes. The experimental R_f value for the first dark blue spot is 0.11 ( = a/f) whilst the red spot that is furthest away from the starting point has an R_f value of 0.70 (b/f).
The higher the spot is transported up the plate by the mobile phase, the greater the R_f value.

If this experiment is carried out under standard conditions it is possible to identify the dyes from their R_f values. In the example detection of the components is simple because we are dealing with coloured dyes. For colourless components other identification procedures are needed to see the spots, e.g. viewing under ultra-violet light.