18-crown-6-tetracarboxylic acid as a chiral additive for the simultaneous separation of o-, m- and p-enantiomers of phenylalanine family by capillary electrophoresis

Simultaneous separation of 12 o-, m- and p-positional enantiomers of tyrosine and fluorophenylalanine has been achieved by the use of optically active 18-crown-6-tetracarboxylic acid (18C6H(4)) as an additive in the background electrolyte. It has been investigated that the separation conditions such as pH, electroosmotic flow, the concentration of 18C6H(4) and applied voltages have critical influence on the simultaneous separation. Based on the information of electropherograms obtained, the interaction between the chiral selector and positional enantiomers has been discussed. It was shown that a-methyl group in the amino acid enantiomers make the resolution be reduced, and that o-, m- and p-substituents have an influence on the resolution.