Prediction of HPLC chromatograms under linear and curved gradient elution by linear gradient trials with initial hold time

It is demonstrated that initial hold time (t_I) before gradient elution may be taken as an extension of dwell time (t_D) of the chromatograph. On the basis of this property, we deduce the expressions for retention time (t_R) and band compression factor (G_Z) under linear gradient with t_I which is also called scouting gradient in this work. In the case of linear solvent strength model (LSSM) and when the analyte is eluted from the column after the end of linear gradient, an algebraic expression for G_Z is deduced. A nonlinear fit based on the Levenberg-Marquardt algorithm was applied to the experimental data of t_R acquired from six scouting gradient runs to obtain the retention parameters in LSSM as well as quadratic solvent strength model (QSSM). The plate number of the column was also calculated from the gradient elution experimental results. With the retention parameters and plate number obtained, the chromatograms under linear and curved gradient elution were predicted in the case of chiral and reversed-phase HPLC separation. The experimental and predicted chromatograms are consistent, and QSSM is generally better than LSSM for the prediction, especially in the case of chiral separation.