An unknown mixture is pushed passed what is called the stationary phase in a continuous stream, it can be a solid in the case of TLC, or a liquid as is the case in many forms of GC. The exact nature of the stationary phase is not so important, what is important is that the individual components of the unknown mixture interact with the stationary phase in some way. Ones that interact more strongly and prefer to stay associated with the stationary phase versus in solution (or in the gas phase) will move slowly and be held back in the stream. Species that interact weakly move more swiftly through the stream. In this manner the different components of the mixture can be separated out by the amount of time it takes for them to elute : move through a column of stationary phase.
A measure of how strongly or weakly a compound is retained is the Retention Factor (RF) or Retention Time. In TLC the RF of a compound can be used to identify it from tabulated values. The retention factor is the ratio of the distance traveled by the compound up the plate versus the distance traveled by the solvent front. The RF for a given compound is (relatively) unique as it depends upon the structure and chemistry of that compound.
In this instructable I will describe how one can prepare their own silica gel TLC (Thin-Layer Chromatograph) plates. In this form of chromatography the stationary phase is a thin layer of silica, a type of finely divided silicon dioxide, deposited on a glass slide. The analyte is "spotted" on the plate using an eye-dropper or micropipette and the whole plate is placed in a beaker with a small amount of solvent in the bottom such that the solvent level is just above the bottom of the plate. The solvent moves up the plate by capillary action, pulling the pigment along with it. The different chemical species in the pigment interact with the silica in differing ways and this affects the degree to which they are pulled up the plate, which is how the separation is effected.
A more basic version of this substitutes a strip of sturdy paper in place of the TLC plate, in which case the cellulose of the paper fills the role of the stationary phase instead of silica. Paper chromatography has its limitations, however, usually making themselves visible as smearing or poor separation. This is why thin layer chromatography is usually employed. It operates in much the same way as paper chromatography in so far as development, however peak separation is generally better (amongst other things).