Chromatography

Chromatography is one of the most important laboratory techniques used for separation and/or chemical analysis of complex mixtures. It is mostly implemented in science subjects such as chemistry and life sciences, especially in biochemistry. Chromatography comes from the Greek words, chroma meaning 'color' and graphein meaning 'to write'. The credit for inventing chromatography goes to the Russian botanist, Mikhail Semyonovich Tsvet, who used it for the first time in 1901, while he was researching on plant pigments. Later on, many other scientists have developed several methods of chromatography following the basic principles.

In every chromatographical method, there are two phases: the stationary phase or the column packing material (solid or liquid) and the mobile phase or the solvent (usually liquid or gas). Stationary phase refers to the bed or support substance that are fixed, whereas mobile phase refers to the medium, through which the mixture to be separated moves in a particular direction. The mixture gets separated by distributing its component molecules between the two phases. Distribution may take place on the basis of their structure, size, shape, charge or molecular weight. Chromatography can be 'analytical' or 'preparative' based on the main objective for which it has been followed. Analytical chromatography makes use of only a small amount of a mixture and determines the components of that particular mixture. Preparative chromatography, on the other hand, is followed as a purification process, where the components of a mixture are separated and used for further studies.

Chromatography methods are further classified depending upon three basic factors: type of the stationary phase, medium of the mobile phase and mechanism of separation. Let's look into each of the classification and their types.

Classification Based on Stationary Phase: The chromatographic methods are classified on the basis of shape of the stationary phase. There are basically two methods; first is column chromatography, where the stationary phase is in the form of a tube and second is planar chromatography, where the stationary phase is in the form of plane or present on a plane support.

Classification Based on Mobile Phase: Chromatographic methods are also classified depending upon the medium of the mobile phase. There are two methods under this category and they are gas chromatography and liquid chromatography. The stationary phase medium used for both gas and liquid chromatography methods is either liquid or solid.

Classification Based on Separation Mechanism: In this category, the chromatographic methods are classified based on how the component molecules of the mixture are separated. The various chromatographic methods under this category include: adsorption, partition, ion-exchange, affinity and exclusion.

Chromatography technique has many advantages over other techniques in analytical studies. It is very sensitive and reliable (provided that the method is carried out carefully without any contamination). The main advantage is that complex mixture can be separated accurately by using only a few micrograms of the sample. Also, separation takes less time as compared to other techniques. The other advantage is that equipment setups are simple and easy. Because of these advantages, chromatography has become an important technique for forensic science and other clinical studies. The disadvantage of this technique is that since the method is very sensitive, improper setup or contamination, even in nanograms, will give different results
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