Factors Influencing the Separation of Ionic and Non-Ionic Chemical Natural Compounds in Plant Extracts by Capillary Electrophoresis

Abstract

Capillary electrophoresis represents an alternative to the common liquid chromatographic technique in natural product research because of the high-resolution separations obtained by several migration modes. Natural molecules such as flavonoids, flavonoid-O-glycosides, coumarins, phenolic acids, glucosinolates or desulphoglucosinolates, have been resolved by several electrophoretic techniques (free solution capillary electrophoresis with a no-complexing or complexing electrophoretic buffer and by micellar electrokinetic capillary chromatography with a cationic or anionic surfactant). In free solution capillary electrophoresis (CE), the selectivity of the flavonoid-O-glycosides, using a borate buffer, may be explained by in-situ borate complexation of both the sugar moiety and the cis-1, 2-hydroxyl groups on the flavonoid skeleton, and also by the ionization of hydroxyl groups on the flavonoid skeleton in such alkaline pH conditions. In particular, the magnitude of the borate complexation depends on the number of boration sites on the sugar moiety and, consequently, on the sugar configuration. Micellar electrokinetic capillary chromatography (MECC) based on sodium dodecylsulfate (SDS) and cetyltrimethyiamrnonium chloride (CTACI) micelles was developped for the separation of uncharged natural compounds (coumarin derivatives or desulphoglucosinolates) or charged solutes (phenolic carboxylic acids or glucosinolates). The high efficient separations allowed the resolution of closely related structures.