Free Books and Brochures

progresses through the chromatographic system, albeit through a column or along a plate, only while it is in the mobile phase. This process, whereby the substances are moved through the chromatographic system, is called chromatographic development. There are three types of chromatographic development, elution development, displacement development and frontal analysis. Elution development is now virtually the only development technique employed in both GC and LC although some displacement development is occasionally used in preparative LC.   In TLC, the development process is confused by the frontal analysis of the multicomponent solvent that occurs as the mobile phase moves through the system. In contrast, the solutes are transported across the plate by elution development. This apparent paradox will be explained in detail in due course. Displacement Development Displacement development is only effective with a solid stationary

or a combination of both. The same rules apply; if the solvent interacts more strongly with the surface than the solute then the solute interacts with the adsorbed layer of solvent by sorption.   Figure 45. The Adsorption Isotherms of a Homologous Series of Aliphatic Alcohols If, on the other hand, the solute interacts more strongly with the reverse phase than the layer of solvent molecules then the solute will displace the solvent and interact directly with the surface by displacement. In, general, those solutes that elute early in the chromatogram will interact by sorption, those that elute late in the chromatogram will interact by displacement and at some intermediate point in the elution scale, solute stationary phase interactions will probably involve both sorption and displacement. Bi-layer adsorption is also possible with reverse phases but, at this time, experimental evidence of this does not appear to be available in the literature

These two types of interaction are shown in figure 42. Displacement 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. Sorption is depicted as a solute molecule on interacting with each solvent layer and can not interact strongly enough with the silica gel surface to displace the solvent.. Mobile phases consisting of mixtures of polar and dispersive solvents frequently produce surface bi-layers when used with silica gel as a

nbsp; It is seen that a wide range of sorption and displacement processes can occur between the solute and the stationary phase surface. There are three different surfaces available for interaction by sorption and three corresponding different surfaces available for interaction by displacement. All the alternatives are possible but it is more likely that for any particular solute, one type of interaction will dominate. The various types of interaction are depicted in figure 25. In multi-layer adsorption the most polar solvent is the one that

nbsp; Experimental Support for the Sorption and Displacement Process Scott and Kucera (15) carried out some experiments that demonstrated, sorption and displacement interaction. They dispersed about 10 gram of silica gel in a solvent mixture made up of 0.35%w/v of ethyl acetate in n-heptane. From the adsorption isotherm in figure 21, it is seen that at 0.35w/v of ethyl acetate more than 95% of the first layer of ethyl acetate has been established and very little of the second layer was formed. This concentration was chosen to ensure that,

n-hexane and 24% v/v 2-propyl alcohol at a flow-rate of 1.0 ml/min.. The steroid hormones are mostly weakly polar, consequently, the separation on silica gel, will be based primarily on polarity. The silica, however, was heavily deactivated by a relatively high concentration of the moderator 2-propyl alcohol and thus the silica gel surface would be covered with isopropanol and thus the interacting surface would be virtually pure 2-propyl alcohol. Whether the interaction is by sorption or displacement is difficult to determine. It is likely that the early peaks interacted by sorption and the late peaks possibly by displacement. Ionic Interaction Chromatography Ionic interaction chromatography, or ion chromatography as it is usually called, is typically carried out employing ion exchange resins as the stationary phase. There are some silica based ion exchange materials available, but the bonded silicas tend to be unstable in the presence of high salt