Enhanced capillary zone electrophoretic separation of dinitrophenyl-amino acid derivatives through control of electroosmotic flow by the buffer cation

Abstract

The effect of Mg2+, Cd2+ and Zn2+ are evaluated as buffer additives to reduce electroosmotic flow (EOF) to facilitate the capillary zone electrophoretic (CZE) separation of 2,4-dinitrophenyl (DNP) derivatives of amino acids. As the divalent cation concentration increases EOF decreases and reduction of EOF follows the order Zn2+>Cd2+>Mg2+. The electrophoretic mobility of the DNP-amino acid derivatives remains constant for a given buffer and pH over a change in divalent cation concentration. The effect of the divalent cation buffer additive is consistent with cation-exchange at the silanol sites on the fused-silica wall. Higher buffer pH favors greater cation-exchange and reduced EOF. Separation of multicomponent mixtures of DNP-l-amino acid derivatives in the presence of Mg2+, Cd2+ and Zn2+ are compared at pH 7.00. Derivative migration time, resolution and peak shape are enhanced and are best for Mg2+ as the buffer additive although Mg2+ must be used at a higher concentration than either Cd2+ or Zn2+. Raising the buffer pH to 9.25 in the presence of Mg2+ increases migration time and resolution but analysis time is also increased.