Chromatography, << `kroh` muh TOG ruh fee, >> 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 (attracts to its surface) 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. See Absorption and adsorption .
Common chromatographic methods are liquid column, thin layer, and gas chromatography.
Liquid column chromatography
uses a column (tube), usually made of glass. Chemists fill the column with an adsorbent material. They then add the mixture to be separated and a liquid carrier at the top of the col-umn. The substances move down through the column at different speeds.
Thin layer chromatography
uses a plate–that is, a thin, flat sheet of glass or other material–coated with an adsorbent film. Chemists place a drop of the mixture on one end of the plate, then stand that end of the plate in a shallow pool of liquid. The liquid travels up the film, moving the mixture along with it. The substances separate from one another as they are adsorbed.
Gas chromatography
uses a column of adsorbent material to separate gases and substances that are easily converted into gases by heating. Chemists use special equipment to inject the mixture and gaseous carrier into the column. The gas most often used to move a mixture through an adsorbent is argon.
Computers play an important role in the analysis of chromatographic data, especially in gas chromatography. Computers can be used to determine how much of a substance is in a mixture. They can also help identify an unknown mixture by looking for various properties of the possible ingredients.
