Retention and peak shape of acidic substances in capillary supercritical fluid chromatography with binary and ternary mobile phases

Abstract

AbstractAcidic model compounds were chromatographed on a cyanopropyl capillary column using binary and ternary mobile phases with supercritical carbon dioxide as the primary mobile phase component. When a binary mobile phase was used, large amounts (> 15%) of methanol in carbon dioxide were needed to elute the most polar analytes. Formic and citric acid were tested as modifier additives. The addition of formic acid to the eluent resulted in virtually no positive effects on the chromatographic behavior of the acidic substances. Addition of small amounts of citric acid to the modifier did, on the other hand, significantly improve peak shapes of all acidic compounds tested. Retention could then be adjusted by changing the concentration ratio between carbon dioxide and the methanol‐citric acid solution. The effect of the ternary citric acid additive is probably caused by deactivation of stationary phase active sites and possibly also facilitation of solute mass transfer across the boundary between the chromatographic phases. Finally, methanol could be replaced by less toxic ethanol as a modifier in binary and ternary mobile phases, without noticeable effects on the chromatographic behavior.