Kinetic performance of a 50 mm long 1.8 μm chiral column in supercritical fluid chromatography

Reduced plate heights (h(r)) of <2 were observed for the first time during the chiral separation of enantiomers, on sub-2 mu m particles with supercritical fluid chromatography (SFC). The enantiomers of trans-stilbene oxide, were separated on a 4.6 x 50 mm, 1.8 mu m R,R-Whelk-O1 column, with hr as low as 1.93. The plumbing of a commercial SFC instrument was modified to create a low dispersion version. Without the modification performance was considerably worse. vanDeemter like plots of reduced plate height vs. flow rate, for trans-stilbene oxide, indicate that the optimum flow varied with% modifier. On a 4.6 x 250 mm, 5 mu m R,R- Whelk-O1 column, the optimum flow was >4mL/min for 5% methanol in CO2, decreasing to <2 mL/min for 40% methanol (more than a factor of 2). For a 4.6 x 50 mm column packed with 1.8 mu m particles the optimum appeared to be near, or >5 mL/min with 2.5%, 5%, and 10% methanol, decreasing to between 3 and 3.5 mL/min at 40% methanol. This is the first time such shifts have been characterized. Since the solutes were the same in all cases, the differences are likely due to changes in solute diffusion coefficients caused by changes in modifier concentration, and pressure. Pump pressure requirements sometimes exceeded 500 bar. It is shown that a 5 mL/min flow rate is inadequate for use with 1.8 mu m particles in a 4.6 mm ID column format. Instead, it is suggested to decrease the ID of the column to 3 mm, where the optimum flow rates are on the order of 2 mi./min with decreased tubing variance. Nevertheless, a number of sub-1 min chromatograms are presented. (C) 2016 Elsevier B.V. All rights reserved.