Chromatography is a process used for separating the substances that make up a mixture, when the concentration of such substances is very low. Thus chromatography is used in forensic science to detect and measure presence of specific substances in very small quantities in objects collected as evidence.
Chromatography achieves separation of different substances in a mixture by passing the mixture through an adsorbent material. The adsorbent material has the property of attracting various substances in the mixture to different extents, causing them to separate from one another.
There are several chromatographic methods. Three of the most commonly used are liquid-column, thin-layer, and gas chromatography. In liquid-column chromatography, a column of adsorbent material in a tube is used. A sample of a mixture under examination is placed at one end of the column. Then a fluid called an eluant is poured through the column. The individual substances in the mixture move through the column at varying rates because of the difference in their tendency to be absorbed. This makes it possible to collect each substance separately as it emerges from the column or is removed from the adsorbent material.
Thin-layer chromatography uses a thin film of adsorbent material on a flat plate. A sample of the mixture under study is applied near one edge of the plate. This edge is brought into contact with the eluant, which moves upward through the adsorbent material. The substances in the mixture separate on the plate.
Gas chromatography is used in analysing gases and substances that can be easily converted into gases when heated. It is similar to gas chromatography except that a gas such as helium is used as the eluant to move the gaseous mixture through a column of adsorbent material.