Chemistry: Thin Layer Chromatography

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Introduction
Thin layer chromatography is a classical case of adsorption or solid/liquid chromatography or planar chromatography. In planar chromatography, the stationary phase is applied on a flat surface and movement of mobile phase is due to the capillary action. The stationary phase is normally a polar absorbent and the mobile phase can be a single solvent or combination of solvents. Adsorption is a concentration dependent process and adsorption coefficient is not constant, in contrast to partition coefficient (liquid/liquid chromatography). Hence, if the concentration of sample is more than the absorptive capacity of stationary phase, the separation of components of the mixture will be poor. TLC is a useful tool for separating and identifying …show more content…

The covalent network of these adsorbents creates very polar material. Silica gel is acidic in nature and thus offers poor separation of basic samples. However, Alumina is available in all forms (neutral, basic, or acidic). However, it cannot separate large sample sizes unlike silica gel under a given layer thickness. Alumina being more reactive than silica gel also limits its use. Fig.1: Structure of silica (www.ecompounds.com) …show more content…

In addition, the coated plate must be physically stable to ensure consistent chromatographic properties. Stability of the layer of stationary phase both in the dry and wet form is achieved by the use of a binder. Binders facilitate the adsorbent sticking to the support. Common binders are Gypsum (G) or Plaster of Paris (CaSO4.2H2O). It is used in about 10-15% by weight of gel. However, due to high solubility of the gypsum binder in aqueous solutions, they are not recommended for use with mobile phase containing water more than 20%. Silica gel plate with gypsum binder is given the designation "silica gel G". Some organic binders like carboxy methyl cellulose, starch (1-2%), and polyvinyl alcohol (1-5%) are also used. These give a stronger binding but get charred on treatment with strong sulphuric acid solutions and

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