Use of kinetic plots for the optimization of the separation time in ultra-high-pressure LC

The kinetic-plot approach, in which experimental to and N-values are extrapolated to the performance at maximum system pressure by increasing the column length, was validated by coupling 250 x 3 mm columns packed with 3 pm particles. The extra-column volume introduced by coupling columns could be neglected with respect to the peak volumes. Plate numbers of up to 132 000 were experimentally achieved by coupling four columns. The maximum deviation between the experimental and predicted plate numbers was 7% for two coupled columns, and decreasing to 0.1% for four coupled columns. Kinetic plots were used to find the conditions to separate a critical pair, with a preset value for the effective plate number, in the shortest possible time. For high-efficiency separations yielding 100 000 effective plates, the optimum critical-pair retention factor was around 4.5. Kinetic plots are presented to find the optimal column length to obtain the fastest possible 100 000 effective-plate separation, taking into account the effect of mobile-phase viscosity on column pressure, and consequently the optimum column length.