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 84

Another, very interesting property of the system is disclosed in figure 26. At about 25˚C, if the optimum velocity is employed together with the column of minimum length, then the analysis time is independent of the composition of the solvent mixture. This, at first sight, appears to be non sequitur until it is realized that at about 25˚C, the change in optimum velocity due to solvent composition adjustments is exactly compensated by proportional changes in minimum column length. It is also interesting to note that at temperatures above 25˚C, the longest analysis times are realized by the more polar solvent mixtures. However, at temperatures below 25˚C, the converse applies, the least polar solvent mixtures produce the longest analysis times.

It must be emphasized that although the shape of the graphs that are presented will be similar for different enantiomeric pairs, the actual values for column length and analysis times will differ widely between different chiral isomers. In addition, the results will be very different if solvents that associate (e.g. methanol/water mixtures or acetonitrile/water) are employed as the mobile phase. In order to develop the same algebraic procedure with such solvents, the association constant of the solvent pair must be known so that the ternary mixture of water–unassociated–with–methanol, methanol–unassociated–with–water and the water–methanol–associate can be calculated. The contribution of each of the three mobile phase components to solute retention must then be taken into account.

It follows, that by employing the standard thermodynamic relationship between distribution coefficient and temperature, in conjunction with other relationships arising from established chromatography theories, the optimum conditions for many practical analyses can often be determined

Such calculations may require a considerable amount of basic data, pertinent to the separation, to be determined experimentally, which may be costly in time and money. However, if the separation has sufficient economic importance, whether it be for product control or pollution monitoring the expense may may well be worthwhile and often essential.