Characterization and comparison of HPLC columns for gradient elution

The type of the stationary phase for reversed-phase liquid chromatography significantly affects the sample polarity range that can be covered using gradients of acetonitrile or of methanol in water The polarity range available for gradient separations of samples containing compounds differing in the lipophilic parts of the molecules can be characterized by "lipophilic capacity, Pl, based on the retention of retention standards with regular differences in lipophility, such as homologous alkylbenzenes. 'Standard' Pl is defined as a quantity independent of the column dimensions and efficiency. It is controlled mainly by the chemistry of the bonded phase and by the properties of the support materials, but depends significantly also on the nature of the organic solvent in the mobile phase. The gradient lipophilic capacity and the sample polarity range that can be eluted during a "standardized" gradient run were used to compare the gradient-elution performance of more than 20 columns packed with various C8, C18 and other bonded phase materials on polymeric, monolithic, non-porous and superficially porous supports with various shielding of the residual silanol groups, including "aqueous phase" materials designed for the applications in highly aqueous mobile phases. The "lipophilic capacity" approach can be extended to characterize the suitability of various columns for gradient separations of oligomers and lower polymers, as demonstrated on the example of lower poly(ethylene glycols).