The Thermodynamics of Chromatography - Other Thermodynamic Methods that are Used for Studying Chromatographic Systems > Optimum Operating Conditions for Chiral Separations in Liquid Chromatography > The Effect of Temperature and Solvent Composition on the Minimum Column Length > Page 79

The Effect of Temperature and Solvent Composition on the Minimum Column Length

The minimum column length will be the product of the required efficiency (figure 22) and (Hmin) (figure 23) which is shown for different temperatures and solvent compositions in figure 24. The dependence of the minimum theoretical plates on temperature and solvent composition dominates the magnitude of the product and, thus, the curves in figure 24 take a form similar to those in figure 22. The higher the temperature and the stronger the ethanol concentration, the smaller the magnitude of (k'). Thus, more theoretical plates will be required to resolve the enantiomers and thus a longer column will be necessary. At a temperature of 5˚C and at an ethanol concentration of 5%v/v, the column need only be about 5 mm long(a length of column that is impractical to pack and operate). Contemporary columns, shorter that 2 cm are extremely difficult to operate efficiently.



Figure 24 The Minimum Column Length that will Produce the Required Efficiency

The minimum column length that will provide the minimum analysis time for this particular separation is not in the practical range of column lengths normally available. Consequently, the optimum column length must be a compromise between, that which is theoretically desirable, and that which is practically possible, and thus the shortest column available would be chosen.