Combined use of isopropylamine and trifluoroacetic acid in methanol-containing mobile phases for chiral supercritical fluid chromatography.

In chiral supercritical fluid chromatography (SFC), mobile-phase additives are often used to improve enantioseparations and peak shapes. An acidic or basic additive is chosen, depending on the nature of the compound. This work highlights the simultaneous use of the acidic additive trifluoroacetic acid (TFA) and the basic additive isopropylamine (IPA) in supercritical fluid chromatography for enantioseparations. To evaluate the combination of TFA and IPA, 59 chiral pharmaceutical compounds were analyzed on four polysaccharide-based chiral stationary phases (CSPs): Lux® Cellulose-1, Lux® Cellulose-2, Lux® Cellulose-4 and Lux® Amylose-2. The results show that an important increase in enantioselectivity of the chromatographic system can occur when combining trifluoroacetic acid and isopropylamine in the mobile phase (MP), compared to the individual use of these additives. However, the combination of isopropylamine and trifluoroacetic acid in a supercritical methanol-containing mobile phase can also lead to problems as a result of the formation of salt complexes between the two additives. Combining the additives trifluoroacetic acid and isopropylamine and taking the appropriate measures to avoid salt formation, i.e. reducing the additives' concentrations, can lead to simpler chiral SFC screening conditions that display even broader enantioselectivity.