Chromatography is a laboratory technique that is often employed to separate mixtures. This is usually done by dissolving the mixture in a specialized fluid known as the mobile phase. The mobile phase is now made to carry the mixture through a structure made up of another specialized material known as the stationary phase. Since the mobile phase carries different constituents at different speeds, the mixture is effectively separated over the stationary phase. The differential retention on the stationary phase is primarily affected by the partition coefficient of the chemical compounds in the mixture. It can be noted that the principle of chromatography is that when the molecules of a mixture are applied on the surface of the stationary phase, they are separated from each other by the moving mobile phase.
It is important to note that chromatography can be both analytical and preparative. The key motive of preparative chromatography is to separate the constituents of a mixture for later use (and can, therefore, be viewed as a way of purifying the mixture). In analytical chromatography, relatively small quantities of the mixture are used. The primary purpose of this process is to either determine the identity of one of the analytes in the mixture or to measure the relative proportions of the analytes that constitute the mixture.
Types of Chromatography
Column chromatography is an important type of chromatography in which the stationary phase is placed within a tube. The solid particles of the stationary phase are coated with a liquid stationary phase and can even fill up the entire inside volume of the tube, resulting in a ‘packed’ column. The stationary phase can also be concentrated along the walls of the tube. This results in a hole at the centre of the tube, creating an unrestricted path for the mobile phase to flow.
In this type of chromatography, a small quantity the analyte mixture (which is usually a liquid solution) is placed on a strip of chromatography paper in the form of a small dot or a thin line. The strip of chromatography paper is then placed in another container (which contains a shallow layer of a suitable solvent). Now, the solvent rises through the strip of chromatography paper, meets the dot/line containing the sample, and carries it upward by acting as the mobile phase. In this type of chromatography, the strip of chromatography paper acts as the stationary phase. It can also be noted that the strip of chromatography paper is usually made up of cellulose.
Thin-layer chromatography (often abbreviated to TLC) is a type of chromatography in which the stationary phase is a thin layer of a suitable adsorbent. Common examples of materials used in the stationary phases of TLC setups include cellulose, silica gel, and alumina. It can be noted that multiple samples can be separated on the same layer simultaneously via thin-layer chromatography.
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