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for evaporation. The use of a paper wick is depicted on the right-hand side of figure 29. The saturated solvent vapor in the chamber not only prevents solvent evaporating from the plate surface but partly controls the retention mechanism by surface deactivation. The solvents are selectively adsorbed on the surface of the stationary phase causing the solutes to interact, not with the native silica surface, but with the silica surface covered with the most strongly interacting solvent. It should be emphasized, however, that the equilibrium between the solvent vapor and the plate will not be the same as the equilibrium between the solvent and the plate. For example, for a binary mixture of solvents having concentrations of solvent in the gas phase of c1 and c2 respectively,   where (x1) is the molar fraction of solvent (1) and (x2) is the molar fraction of solvent (2)  

the surface corresponding to the area (Y). Finally, the remaining solvent (C) with the weakest interactions with the stationary phase continues to migrate and cover the surface with a layer of solvent (C) in the area (Z). It is seen that the distribution system, which results from the frontal analysis of the three mobile phase components is now quite complex. The solutes will interact during the separation process. In the first section (X) solutes will be distributed between the ternary solvent mixture (A), (B) and (C) and the surface covered with solvent (A). In the next section (Y) the solutes will be distributed between a binary solvent mixture of (B) and (C) and a surface covered with solvent (B). Finally, distribution will take place in section (Z) between pure solvent (C) and a surface covered with solvent (C). Even this is an over-simplification, as the composition of the mobile phase in each section will not be constant but will decrease along the plate.

solvent to pass to the pump, air again dissolves in the solvent. The solvent is filtered through a stainless steel or sintered glass filter to remove any solid contaminants. Depending on the type of solvent programmer that is employed, the supply from each reservoir may pass either to a pump or to a valved blending device. Solvent reservoirs are not usually thermostatted but, when necessary, the solvent can be brought to the column temperature by the use of an appropriate heat exchanger. The solvent containers are often situated in an enclosure that protects the user from toxic solvent vapors such as chloroform or aromatic hydrocarbons. Such enclosures also isolate the solvents from atmospheric moisture. The Gradient Programmer The High Pressure Programmer There are two basic types of solvent programmer. In the first, the solvent mixing occurs at high pressure and in the second the solvents are premixed at low pressure and then passed to the pump. The high pressure

nbsp; The Effect of Temperature and Solvent Composition on the Separation Ratio of the Two Enantiomers Curves demonstrating the change in separation ratio of the two enantiomers with temperature and solvent composition, calculated from equation (49)  are shown in figure (21). Despite the dominant effect of solvent composition on capacity ratio, the effect of solvent composition on the separation ratio is much smaller, and the dominant effect is now the operating temperature. This stresses the importance of temperature for selectivity control in chiral separations. It is very interesting to note that there is a temperature at which the solvent composition has no effect on the separation ratio whatsoever (ca 43˚C). Figure 21. Curves Relating the Separation Ratio of the Two Enantiomers with Temperature and Solvent Composition It is clear

the piston progresses to the right, solvent is displaced to the column system and, simultaneously, fresh solvent is withdrawn from the solvent reservoir into the right hand chamber. When the piston arrives at the extent of its travel, a step in the driving cam is reached and the piston is very rapidly reversed. As a result  the contents of the chamber on the right-hand side are displaced into the left-hand chamber. This situation is shown in the lower part of figure 8. The transfer rate of the solvent to the left-hand chamber is 100 times faster than the delivery rate to the column and consequently reduces the refill-pulse very significantly. In addition, if a solvent gradient is being used and the right-hand chamber is being  filled with a solvent mixture, excellent mixing is achieved during the refill of the left-hand chamber. It is clear, however, that there will not be a smooth transition from one solvent concentration to the next but will be a step-wise change. An

the solute and the stationary phase surface. There are three different surfaces on which a molecule can interact by sorption and three different surfaces from which molecules of solvent can be displaced and allow the solute molecule to penetrate. In any separation all the alternatives are possible but it is more likely that for one particular solute, one type of interaction will dominate. the various types of interaction are depicted in figure 43. Where there are multi-layers of solvent, the solvent that interacts directly with the silica surface is the most polar, and consequently constitutes the first layer. Depending on the concentration of the polar solvent the next layer may be a second layer of the same polar solvent as in the case of ethyl acetate. If, however, the quantity of polar solvent is limited, then the second layer might consist of a less polar component of the solvent mixture. If a ternary mixture of solvents is used, the nature of the surface, and the solute