CHROMATOGRAPHY Chromatography is a technique for separating and identifying the components of a mixture. Many different forms of chromatography are used but they all work on the same principle. The components of the mixture have different affinities for two phases: a stationary phase and a mobile phase and so are separated as the mobile phase moves through the stationary phase. A component which has a strong attraction for the mobile phase will move quickly, whereas a component with a strong attraction for the stationary phase will be held back. If the stationary and mobile phases are carefully chosen, the different components will move at different speeds and so be separated effectively. Polar compounds, for example, are more likely to move quickly when the mobile phase is a polar solvent. Two main types of chromatography There are two main types of chromatography: partition and adsorption chromatography. Partition chromatography Chromatography using a non-volatile liquid stationary phase held on an inert solid surface is known as partition chromatography. The components distribute themselves between the two phases according to their relative solubility. Paper chromatography and gas liquid chromatography are examples. The more soluble or volatile the component, the faster it will move. Adsorption chromatography Chromatography which uses a solid stationary phase and a mobile liquid or gas phase is known as adsorption chromatography. Some components of the mixture are attracted to the solid surface and the other components which are less strongly bonded travel faster with the mobile phase. Thin-layer chromatography is an example. As the stationary phase is generally a polar solid, the more polar solutes are more readily adsorbed than the less polar solutes. Thin-layer chromatography (TLC) Thin-layer chromatography is an example of adsorption chromatography. It follows the same procedure as paper chromatography, with small spots of the test solutions placed on the base line using a capillary tube. The stationary phase is a thin layer of absorbent particles of alumina or silica supported on a glass or thin plastic plate and the mobile phase is a liquid solvent. The different components separate and can be identified. The technique is used in qualitative analysis to determine whether a substance is pure. TLC is a type of planar chromatography.