Separation behavior of amino acid enantiomers in ligand exchange micellar electrokinetic chromatography

The separation of amino acid enantiomers using a Cu(II) complex with L-hydroxyproline as a chiral selector has been investigated by ligand exchange micellar electrokinetic chromatography. It was found that anionic surfactants like sodium dodecyl sulfate (SDS), sodium n-decyl sulfate, and sodium n-tetradecyl sulfate can not only improve the resolution of D,L-enantiomers, but also can change their migration orders. The migration order of o-, m-, and p-positional isomers of fluorophenylalanine and tyrosine was reversed when the concentration of SDS exceeded a certain concentration. Tween 20 improved the separation efficiency, but the migration time became longer. Cetyltrimethylammonium bromide can change the direction of electroosmotic flow, but it has no significant improvement on resolution. Organic modifiers cause the decline of resolution, but not the reversal of enantiomer migration order (EMO). A possible mechanism of the reversal of EMO is presented. (C) 1999 John Wiley & Sons, Inc.