Interpretive optimisation of organic solvent content and flow-rate in the separation of beta-blockers with a Chromolith RP-18e column.

The chromatographic performance of a Chromolith RP-18e column was comprehensively examined for a group of basic drugs (beta-blockers), eluted with isocratic ACN-water mixtures at increasing flow-rate up to 6 mL/min. As the flow-rate increases at fixed mobile phase composition, peak distribution (selectivity) is maintained, but the relative peak widths increase. This reduces the resolution below satisfactory values for closely eluting compounds. With the monolithic column, flow-rate becomes thus an important factor to be optimised, in addition to the mobile phase composition. Since, theoretically, retention factors (k) are independent of the flow-rate, the classical quadratic model relating log k with the solvent content allows the prediction of the retention at any combination of organic solvent content and flow-rate. The small deviations found for the most retained compounds were corrected by including, in the quadratic model, an additional term correlating linearly log k with the flow-rate. Peak shape and resolution changes were predicted by taking advantage of the approximated linear relationships between peak half-widths and retention times, which offered similar coefficients for peaks eluting at different organic solvent contents and flow-rates. The accuracy of the predictions in critical conditions was experimentally verified to be satisfactory.