Although there are a variety of secondary reasons to consider the gradient mode of elution, the primary reason is that it provides a solution to the so-called ’general elution problem’ in liquid chromatography. This general elution problem stems from the fact the free energy of transfer between the mobile and stationary phases varies roughly linearly with properties of the solutes (e.g., number of methylene units and a homologous series of alkylbenzenes), where as the retention factor increases exponentially with increasing free energy of transfer. Table 1 shows data to this effect for a homologous series of alkylbenzenes. The increasing difference in retention factors between adjacent homologs as the homolog number increases results in a chromatogram like that shown in Figure 1 when isocratic conditions are used in RPLC.