Sorbents, layer preparation and precoated plates

Physicochemical properties, that is the hardness and polarity of silica gel, are related to particle size (pm), surface area (m /g), pore size (A), and presence or absence of additives. The surface area of silica gel adsorbent for TLC is typically 300-400 mVg. and pore sizes range from 20 to 150 A. Adsorbent with larger surface area (smaller particle size) will generally give better resolution, but a slower 

During manufacture, inorganic and organic contaminants may be introduced into TLC silica gel. During slurry preparation or handling of a finished plate, additional contaminants may be added. Slurries or plates can be washed in chloroform-methanol (1 1 v/v) or the mobile phase to help reduce contaminants. Silica gel is usually prepared as an aqueous slurry of approximately 2 parts water to 1 part powder, prior to application to a glass plate. When properly purified, silica gel tends not to catalyze the reactivity of labile substances. 

Cellulose Cellulose is a polymerized polysaccharide characterized by the cellobiose unit. The presence of free OH groups in cellulose permits hydrogen bonding with low-molecular-weight liquids such as alcohols or water. Cellulose is useful for the separation of hydrophilic substances primarily by the mechanism of normal-phase partition chromatography. For a discussion of separation mechanisms and the cellulose-water complex , see Ref. 176. 

TLC cellulose powders consist of two basic types, native fibrous and microcrystalline. An example of the fibrous type is Macheiy-Nagel MN-300 and of the microcrystalline, Avicel . The fibrous type is best for the transfer of procedures originally designed for paper chromatography. Because of the particulate nature of TLC microcrystalline cellulose powder, spots formed on plates of this cellulose are more compact than in paper chromatography, and separations are more rapid. Because of the fine particulate nature of microcrystalline cellulose, it may act partly by the mechanism of adsorption chromatography. Separation capabilities of both types have been reported [15]. Cellulose sorbent can be prepared as a 15-35% 

Sephadex Sephadex layers are prepared from modified dextran gels for the separation of hydrophilic solutes such as nucleic acids and peptides. The mechanism of separation is partition chromatography governed by size exclusion in the swollen gel containing pores of controlled dimensions. The gels, a layer spreader and special equipment for developing layers are available from Pharmacia Fine Chemicals. 

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