Effects and mechanism characterization of ionic liquids as mobile phase additives for the separation of matrine-type alkaloids by liquid chromatography.

The mechanism for the separation of matrine-type alkaloids (sophoridine, sophoramine, sophocarpine, matrine, and oxymatrine) on a C(18) column with a mobile phase containing room temperature ionic liquids (RTILs) as additive was studied in the current work by investigating several factors that influenced chromatographic behavior, including mobile phase pH, buffer solution concentration, ionic liquid concentration, and length of alkyl groups in RTILs. Peak efficiency was selected as the criterion for chromatographic efficiency whereas peak efficiency was also selected for triethylamine (TEA). The addition of RTILs in a mobile phase demonstrated superior effects on the separation of these basic compounds because of the increase in peak efficiency. The difference between RTILs and TEA and the changes in retention factors caused by the additives concentration, depending on the acidity of the mobile phase, showed the dual nature of the ionic liquids. This allowed both the constituting anion and cation to participate in the retention mechanism. Furthermore, the chromatographic behaviors of analytes with a mobile phase containing RTILs having different lengths of alkyl groups at different concentrations, complied with the stoichiometric displacement model for retention (SDM-R). This clarified the significant role of competitive adsorption between solutes and additives on the stationary phase.