Investigation of Adsorption and Retention of Charged Compounds In RPLC

Abstract: The adsorption isotherm of two weak bases, Promethazine hydrochloride and Propranolol hydrochloride, were determined with isocratic reversed-phase liquid chromatography, with a 60 w% methanol in 20 mM sodium acetate buffer pH 4 as the mobile phase, and calculated by the elution by characteristic points method. The data obtained from the method were then fitted into the Langmuir isotherm and the electrostatically modified Langmuir. Propranolol fitted reasonably good into the models while Promethazine was not as good. When Promethazine and Propranolol were together in the same sample, there was indication of competition of the adsorption sites. For comparing retention and peak shape between a C18 column and a mixed mode column, Waters XBridge C18 and Thermo Scientific Acclaim WCX-1, were tested in gradient elution with 11.32 mM sodium acetate buffer and 10–70 % methanol. The mixed-mode column gave significantly better peak shapes, while the retention time were longer compared to the C18 column.