Chromatography

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Chromatography is a method of separating substances that make up a liquid or gaseous mixture. One use of it is to measure or identify low concentrations of substances, such as pollutants in air or water. Another use is to separate and identify products of chemical reactions. Chemists use this method to separate pure substances from impurities.
Chromatographic methods are based mainly on a process called adsorption. A mixture passes through a solid or liquid material that adsorbs to substances. This material is called an adsorbent. A liquid or gas added to the mixture helps move it through an adsorbent. Various substances are adsorbed at different rates of speed, so the substances in a mixture separate from one another as the mixture moves through the material. Common chromatographic methods are liquid column, thin layer, and gas chromatography.
Liquid Chromatography
For compounds that cannot be volatilized readily, the liquid chromatograph can be used instead of the gas chromatograph. The stationary phase consists of a finely powdered solid adsorbent packed into a thin metal column and the mobile phase is made of an eluting solvent forced through the column by a high-pressure pump. The mixture that needs be analyzed is injected into the column and monitored by a detector. Many different liquid chromatograph packing and eluting solvents are available to achieve the desired resolution.
Thin-Layer and Paper Chromatography
A layer of adsorbent also can be spread on a glass plate, instead of packed into a column, for analytical purposes. By means of a thin capillary tube, the plate is spotted with a solution of the mixture that is to be resolved, and the solvent is allowed to evaporate. An eluting solvent is then allowed to move up the plate by capillary action, drawing the components of the mixture along by varying degrees. Spraying it with an oxidizing agent develops the plate, so that each component becomes charred and ...