ABSTRACT
In liquid chromatography, the retention volumes and the resolution of sample compounds with very different retention can be widely controlled by the appropriate selection of mobile phase so as to achieve adequate chromatographic separation within a reasonable time. Solvent-programmed elution has become a well-established routine in liquid chromatography. Nevertheless, the fundamental aspects of the theory and their potential for a better understanding of the influence of gradient profile on chromatographic behavior of the sample compounds have not yet been recognized widely. The influence of the composition of the mobile phase on capacity ratios is controlled by the separation mechanism, which differs in the three principal modes of liquid chromatography—adsorption, partition, and ion-exchange. The separation mechanism in a practical system can be quite complex and often two or more mechanisms are operating simultaneously. The exponential gradients became less important with the introduction of modern versatile gradient-elution liquid chromatographic instrumentation, where linear gradients or gradients of any desired type can be produced.
