Characterisation and prediction of retention in isocratic and gradient-elution normal-phase high-performance liquid chromatography on polar bonded stationary phases with binary and ternary solvent systems

Abstract

The retention of phenylurea herbicides on bonded nitrile- and amino-silica gel columns was studied in mixed mobile phases containing 2-propanol or dioxane in n-hexane. Under isocratic conditions, a three-parameter retention equation describes adequately the dependence of the retention factors on the concentration of a polar solvent in binary mobile phases. Parameters of this equation can be used for the prediction of elution volumes in gradient elution with binary solvent gradients. If dry solvents and controlled temperature are used, if the migration of sample compounds corresponding to the gradient dwell volume is accounted for and if the initial concentration of the polar solvent in gradient elution is 3% or more, the differences between the predicted gradient elution volumes and the experimental data were, in most cases, in between 0.1 and 0.25 ml, i.e., less than 5%. With gradients starting from pure non-polar solvents, the differences are more significant because of preferential adsorption of the polar solvents during the gradient. The adsorption isotherms of 2-propanol and of dioxane in n-hexane on the nitrile-bonded phase are less steep and column saturation occurs at a higher concentration of the polar solvent than with the unmodified silica gel column. Based on three experimental retention factors, retention can be predicted in isocratic and gradient elution with ternary mobile phases.