Theoretical comparison of the performance of gradient elution chromatography at constant pressure and constant flow rate

The theory of gradient elution usually assumes a constant flow rate. This works extends it to gradients performed under constant pressure drop and variable flow rates. The peak capacity is derived under both constant flow rate and constant pressure gradient chromatography by integrating the rate of increase of the peak capacity from the hold-up time to the end of the gradient time. Assuming that the eluent mixture is incompressible, the chromatographic system isothermal, the pressure has no effect on the retention pattern, and neglecting the contributions of the instrument to the total pressure drop and the total peak width, it is found that both modes of gradient elution chromatography are strictly equivalent, provided that the elution time of the last eluted compound and the volume gradient are kept the same in both cases.