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 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.

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)  

Product recovery when using normal phase solvents is best carried out by bulking the fractions and removing the solvent in a rotary evaporator under reduced pressure. For reverse phase solvents that have a high water content, recovery can be best achieved by passing the fraction through a reverse phase, C18, column of high capacity. The solute and solvent is adsorbed, and the solute and solvent content of the fraction can be recovered by displacement with another solvent, and the solute, now concentrated is recovered by evaporation. Solvent Hazard Unless solvent recycling can be employed, which is not always possible, the operation of large diameter columns inevitably involves the use of large quantities of solvent. It follows, that there may be a possibility of both fire and toxicity hazards. As a consequence, the solvent should be selected with care and if possible the entire chromatograph, including the solvent supply, electrically grounded, and

Aqueous Solvent Mixtures When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and the solvent composition, was tested with aqueous solvent mixtures it was found that the relationship identified by Purnell and Laub and Katz et al failed. It was suspected that the failure was due to the solvent strongly associating with the water and, in fact, an aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. The solvent mixture was thus, a ternary system and thus the linear

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