11.2: Chromatographic Methods: Classification

Chromatographic techniques are classified in three ways: the classification is based on the physical state of the stationary and mobile phases, how the mobile phase and the stationary phase contact each other, or through the chemical or physical processes that isolate the components of the sample. Typically, the mobile phase is either a liquid or gas, while the stationary phase is either a solid or a liquid layer applied to a solid surface.

Chromatographic techniques are typically named by starting with the type of chromatography, then the mobile phase, and then the stationary phase. For example, in gas-solid chromatography, the type of chromatography is gas, the mobile phase is gas, and the stationary phase is solid. Similarly, in liquid-liquid chromatography, the liquid is the mobile phase, and another liquid or a liquid-coated solid is the stationary phase. Other examples include gas chromatography and supercritical fluid chromatography, which focus on the mobile phase alone.

The second type of classification also depends on the geometry of the material supporting the stationary phase. The most common classes are column chromatography and planar chromatography. In column chromatography, the stationary phase is packed into or adhered to the inside of a narrow tube (column), and the mobile phase passes through the stationary phase under gravity or pressure. In contrast, planar chromatography features the stationary phase supported on a flat surface, with the mobile phase traveling over the stationary phase by capillary action.

The third classification type, based on the chemical or physical mechanism of solute separation, includes techniques such as adsorption, partition, ion exchange, and size-exclusion chromatography. In adsorption chromatography, the separation depends on the relative adsorption affinities of solutes for the solid stationary phase. In partition chromatography, separation depends on the solubilities of solutes in liquid-state phases. Ion-exchange chromatography separates solutes based on their attraction to the stationary phase. In size-exclusion chromatography, solutes travel different path lengths through the column depending on whether they fit into pores in the stationary phase.