Method for characterization of selectivity in reversed-phase liquid chromatography : III. Retention behaviour in gradient-elution chromatography:Application to the chromatography of pesticide compounds

Abstract

The method introduced recently for the characterization and prediction of absolute and relative retention in reversed-phase liquid chromatography under isocratic conditions has been applied to gradient-elution chromatography. The method is based on two indices (one lipophilic, n ce, and the other polar, q i to characterize the behaviour of each solute in methanol-water mobile phases of various compositions. The indices n ce and q i of phenylurea and triazine herbicides were found to be composed of additive contributions of structural elements (functional groups) in the molecules of the solutes. Using the present approach, it is possible to predict retention volumes in gradient-elution chromatography with a precision comparable to those under isocratic conditions ( i.e., less than 10% relative for most compounds tested). Simultaneous adjustment of the initial mobile phase composition and the slope of the linear gradient makes it possible to control to a certain extent the selectivity in gradient-elution chromatography using binary solvent gradients and offers an alternative approach to the use of multi-solvent gradients for selectivity control. This method may be especially useful in optimizing gradient-elution separations of pairs of compounds with positive differences in both indices n ce and q i, Δ n ce > 0 and Δ q > 0. Selection of the optimal gradient profile is illustrated on a practical example of gradient-elution chromatography of phenylurea herbicides.