Discontinuous electrokinetic chromatography of parabens using different substituted resonances as pseudostationary...

Resorcarene derivatives, negatively charged even at moderate pH, were synthesized and employed as pseudostationary phases to achieve mobilities exceeding that of the electroosmotic flow. Under these conditions, a discontinuous electrolyte system was developed which allows the separation of four uncharged homologous 4-hydroxybenzoic esters (parabens) within a zone of resorcarene electrolyte, and the detection of these UV active compounds in a resorcarene-free zone, free from the high UV background absorbance of the resorcarenes. Resorcarenes, with differently charged functionalities (carboxylate and phosphate groups) to provide the electrophoretic mobility and with alkyl residues of different chain lengths responsible for the chromatographic interactions with the analytes, were tested and compared in terms of mobility and selectivity. Only the resorcarene phosphates exhibited sufficient mobilities at low pH exceeding the mobility of the electroosmotic flow (EOF). Retention factors of the parabens were found to increase with increasing chain length of the alkyl residues attached to the resorcarene. However, maximum selectivity was observed for an intermediate chain length (C8). An equation for the calculation of retention factors in discontinuous electrokinetic chromatography (EKC) is presented.