Possibility of Calculating System Maps Using Gradient Elution Reversed-Phase Liquid Chromatography

Summary

Gradient elution is explored as a rapid method for calculating system constants of the solvation parameter model for reversed-phase liquid chromatography. Linear solvent-strength theory is used to relate gradient retention times to system properties and to predict isocratic retention factors for the calculation of system maps. The combination of linear solvent strength theory and the solvation parameter model provides only a poor model for gradient retention times. A significant source of uncertainty in these models is the variation of the solute-dependent ratio (column hold-up time / gradient steepness parameter), and to a lesser extent, non-linearity in the plots of retention factors (log k) against mobile phase composition. Reasonable agreement between gradient predicted and experimental isocaratic retention factors was obtained when plots of the isocratic retention factors (log k) against mobile phase composition are linear. But even in this case, small systematic differences in the calculated system constants with increasing volume fraction of the strong solvent were observed. When plots of the isocratic retention factors against mobile phase composition are non-linear, the retention factors estimated by linear solvent strength theory yield system constants that contain significant error, producing fictitious system maps. Consequently, none of the methods studied is recommended for the fast and reliable calculation of system maps or solute descriptors by gradient elution reversed-phase liquid chromatography.