Chromatography is a technique used to separate compounds based on differences of partitioning between two phases, the stationary phase and the mobile phase.

The phase in which the compounds linger or on which the compounds adsorb is called the stationary phase, whereas the mobile phase is the solvent that carries the solutes to be analyzed. In traditional column chromatography, the mixture flows through the stationary phase, and the compounds partition between the stationary and mobile phases based on their structures and polarities. Solutes that interact weakly with the stationary phase move more quickly through the column and exit first. Solutes interacting more strongly with the stationary phase move slowly through the column and elute later.

There are several types of chromatography, each harnessing a different chemical property to achieve separation. Partition and adsorption chromatography are the two main types of chromatography. In one form of partition chromatography, both the stationary and mobile phases are liquids, and separation occurs based on the differential migratory velocities of solutes. In another form of partition chromatography, an inert gas carries the mixture through a tube stuffed with oil-coated inert solids. The migration rates of the components depend on their solubility in oil and their vapor pressure. In adsorption chromatography, the stationary phase is solid, and compounds are separated based on the differential adsorption of the solute molecules to the stationary phase. Thin-layer chromatography and column chromatography are the two main types of adsorption chromatographic techniques covered in the subsequent lessons.