Simultaneous optimisation of gradient time, gradient shape and initial composition of the mobile phase in the high-performance liquid chromatography of homologous and oligomeric series

Abstract

A general approach for optimisation of non-linear gradient elution in reversed-phase and in normal-phase chromatography was suggested. This should be suitable especially for separations of more complex samples containing compounds with repeat units such as members of homologous or oligomeric series where more or less regular retention increase between the adjacent peaks is observed. The approach is based on predictive calculations of the retention and of the resolution of the individual pairs of compounds in the sample mixture. Isocratic or gradient-elution retention data acquired in a few initial runs under different conditions are employed to determine the parameters of retention equations describing the dependence of the retention factor, k, on the composition of the mobile phase and on the number of repeat structural units in homologous or oligomeric series. The approach allows us to optimise simultaneously the initial composition of the mobile phase, the time (volume) and the shape of the gradient to achieve required resolution for all components of the sample mixture in minimum time. Using non-linear gradients, band spacing and peak capacity in the chromatogram may be improved with respect to the elution with linear gradients. For better practical convenience, optimised curved shape of the gradient can be substituted by corresponding multisegmented linear profile. The approach is illustrated by examples of reversed-phase gradient-elution separation of n-alkyl-3,5-dinitrobenzoates and of normal-phase gradient-elution separation of lower oligostyrenes on a silica gel column.