The sample is injected into the moving phase, a gas or liquid stream which flows over the stationary phase, a porous solid or a solid support coated with a liquid. The various components of the sample are adsorbed (see adsorption) by the stationary phase at different rates, and separation occurs. Each component has a characteristic velocity relative to that of the solvent, and so can be identified. There are two main types of chromatography: liquid-solid and gas-liquid.
Liquid-solid chromatographyIn this, the solid is packed into a tube, the sample is added at the top, and a liquid eluant is allowed to flow through; the different fractions of effluent are collected. A variation of this method is ion-exchange chromatography, in which the solid is an ion-exchange resin from which the ions in the sample are displaced at various rates by the acid eluant. Other related techniques are paper chromatography (with an adsorbent paper stationary phase) and thin-layer chromatography (using a layer of solid adsorbent on a glass plate).
Gas-liquid chromatographyThis is the most sensitive type of chromatography. In it, a small vaporized sample is injected into a stream of inert eluant gas (usually nitrogen) flowing through a column containing nonvolatile liquid adsorbed on a powdered solid. The components are detected by such means as measuring the change in thermal conductivity of the effluent gas.
Thin-layer chromatography represents a method of quickly and accurately separating the components of a mixture. It is carried out using slurries of finely divided adsorbants, such as silica gel or kieselguhr, applied to glass plates or plastic sheets in a thin, uniform layer by a spreading procedure, usually with the aid of a commercial applicator as shown in (A). The applicator consists of a hollow metal block with exit gates on each of two opposite long faces. A section through the applicator (B) shows how the internal, rotating reservoir chamber, with a wide longitudinal slot, fits into this block. Flow of the suspension does not begin until the chamber is rotated to its open position (C). Simultaneously, the applicator is pulled across a series of plates laid out on a mounting board (D). The applicator is self-adjusting to the surface of the plates because it rides on the plate during the costing procedure.
Related category• ANALYTICAL CHEMISTRY
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