Competitive binding of a Tris run buffer with chiral crown ether in chiral capillary electrophoresis

In capillary electrophoresis of primary amine racemates using (+)-(18-crown-6)-tetracarboxylic acid (18C6H(4)) as a chiral selector, chiral recognition emanates from the differences in the complex formation between 18C6H(4) and the two protonated amine enantiomers. The presence of buffer constituents such as tris(hydroxymethyl)amino-methane (Tris) or Na+, capable of forming complexes with 18C6H(4), is thus detrimental to the chiral separation of primary amines. Such a competitive binding of buffer constituents was studied by comparing the electrophoretic mobilities of racemic analytes obtained in Tris/citric acid and triethylamine/citric acid buffers. We developed a simple fitting method to determine the competitive binding constant and applied it to the Tris buffer system. The competitive binding constant of Tris with 18C6H4 obtained at pH 3.0 was 27 +/- 4.