The Thermodynamics of Chromatography - Other Thermodynamic Methods that are Used for Studying Chromatographic Systems > Optimum Operating Conditions for Chiral Separations in Liquid Chromatography > Effect of Temperature and Solvent Composition on the Optimum Velocity > Page 82

 

Figure 25 Curves Relating Optimum Mobile Phase Velocity to Temperature for a Range of Different Solvent Mixtures.

The curves illustrate the interesting interactive effect of temperature and solvent composition on the diffusivity and capacity ratio of the first eluted enantiomer and consequently on the optimum velocity. Solvent composition and temperature have comparable impact on the mobile phase velocity, the highest velocity occurring at the highest temperature and with the most polar solvent mixture.

It might appear that the shortest analysis time would be achieved at the highest optimum velocities, (i.e. at the highest temperature and the most polar solvent mixture). However, as shown in figure 20, the higher temperatures and more polar the solvent mixture the lower the capacity ratios, and lower separation ratios as shown in figure 21. These, in turn, demand high column efficiencies to achieve the separation as shown in figure 22. It also follows, that the demand for higher efficiencies evoke the use of longer columns as shown in figure 24. As a result, even with the increased mobile phase velocity at the higher temperatures and more polar solvent mixtures, the column length has a greater impact on analysis time than the higher mobile phase velocities. As a consequence, the analysis times are greater at higher temperatures and with more polar solvent mixtures. Curves relating analysis time to temperature and solvent composition are shown in figure 26.