Application of retention modeling in chiral method development. I. Selection of isocratic composition for preparative...

Preparative chiral separations are carried out using chiral stationary-phases (CSP) employing isocratic composition mode to take advantage of stacking multiple injections within a single continuous operation. Development of the separation method, however, is not conducted directly in the preparative systems. Chromatographic systems at analytical scale are set up to screen multiple CSPs with various mobile-phases (MP) to detect a suitable CSP-MP combination. For faster method screening, solvent-gradients are implemented - operating from low to higher modifier composition, e.g. 5 to 70%. Once the right CSP-MP pair is detected, the isocratic method for preparative separation is developed through further experimental trials in the analytical system. The scope of the trial steps is generally limited to detecting a "good-enough" separation condition through one or two isocratic experiments. Ideally, the analyst should scout all possible isocratic conditions to detect the most suitable method; which, however, is not possible in high-throughput separation laboratories. In this report we demonstrate the utility of a simple set of algebraic equations, supported by an experimental protocol, in generating complete isocratic method options based on minimum number of experimental trials. The approach presented here was developed for chiral separation with supercritical-fluid chromatography. We also suggest an approach to identify an isocratic composition for the purification step. The process proposed in this report should be useful in developing better preparative separation methods in high-throughput laboratories.