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1 mm I.D. was employed, packed with silica gel having a particle diameter of 10 m. The mobile phase was n-hexane at a flow-rate 50 ml per min. The solutes of interest are naphthalene and pyrene, the first two peaks. The two solutes are well separated and, as they have no permanent dipole, and as dispersive interactions with the silica gel are weak, they are selectively retained almost exclusively by induced dipole interactions. These interactions occur between the strong dipoles of the silanol groups on the silica gel surface and the induced dipoles on the aromatic nucleus resulting from their proximity to the silanol groups. To ensure that polar interactions dominate in the stationary phase the mobile phase consists of the dispersive solvent n-hexane

. Firstly, the solute molecule can interact with the adsorbed solvent layer and rest on the top of it. This type of interaction is called sorption interaction and occurs when the molecular forces between the solute and the silica are relatively weak compared with the forces between the solvent molecules and the silica. The second type of interaction is where the solute molecules displace the solvent molecules from the surface and interact directly with the silica gel itself, for example, the silanol groups. This type of interaction is called displacement interaction and occurs when the interactive forces between the solute molecules and the silica surface are much stronger than those between the solvent molecules and the silica surface. This type of interaction would occur if the solute was strongly polar such as an alcohol, which would interact more strongly with the polar silanol group than the dispersive chloroform layer. An example of Sorption Interaction is depicted in figure

nbsp; .   In this way the strongly polar silanol groups are methylated and assume dispersive characteristics that do not produce peak tailing. Although the major contributors to adsorption by the support are the silanol groups, a residual adsorption results from the presence of trace quantities of heavy metals such as iron. which can be largely removed by acid washing prior to silanization. All three types of support are commercially available. None of these supports, however, are completely devoid of adsorptive properties and in

adhere to one another by means of glass formed from the silica and the sodium carbonate. As the original Celite structure is disrupted, the material exhibits a wide range of pore sizes which differs significantly from the material that was calcined in the absence of sodium carbonate. This materials is sold under the name of Chromosorb W together with two similar materials called Chromosorb G and Chromosorb S. The residual deleterious adsorptive properties of the support are due to silanol groups on the surface and these can be removed by silanization. The support is treated with hexamethyldisilazane which replaces the hydrogen of the silanol group with a trimethylsilyl radical. The reaction proceeds as follows

nbsp; The carbon content of a bonded phase is often used to determine the efficacy of bonding, but its value must be used in conjunction with a knowledge of the surface area of the native silica in order to arrive at a meaningful conclusion. The amount of material bonded to the silica will depend, not only on the efficiency of the reaction, but also on the number of silanol groups that were available with which it could react; ipso facto it will also depend on the surface area of the parent silica. The carbon content of the bonded phase is usually determined by micro-analysis and the result expressed as %w/w of the combined bonded organic material and the silica gel. Consider a bonded phase where the carbon content is (y)%w/w coated with a hydrocarbon moiety having (n) carbon atoms per aliphatic chain (e.g., for the dimethyl octyl brush phase, n=10).

excess of the chlorosilane is then added to the silica dispersion together with 5 ml of pyridine. The pyridine acts as scavenger for the hydrochloric acid released during the reaction. The mixture is refluxed for about 5 hours and the product is then filtered on a sintered glass filter, washed sequentially with toluene, tetrahydrofuran (THF), methanol, methanol water (50:50 v/v) and finally with methanol and dried under suction. The bonded phase now needs end-capping; that is, any unreacted silanol groups are treated with a small molecular weight silanizing reagent to react with those hydroxyl groups that were stearically unavailable to the larger reagent due to exclusion. To end-cap the product, the bonded phase is refluxed for two hours in a mixture of 100 ml of toluene and 25 ml of hexamethyldisilazane. The product is again filtered free of the reaction liquid mixture and washed sequentially with, toluene tetrahydrofuran, methanol, methanol water (50:50 v/v) and finally with