Effects of substituted cyclodextrins on the separation of aromatic sulphonic acids by capillary zone electrophoresis

Effects of the addition of various additives in the working electrolyte on the selectivity of capillary electrophoretic separation of naphthalenesulphonic acids used as intermediates in the production of synthetic dyes were investigated. Cyclodextrins form inclusion complexes with various compounds and are not only excellent chiral selectors, but have been also successfully applied for separations of positional isomers. In this work, methyl-beta-cyclodextrin, heptakis(2,6-di-O-methyl)-beta-cyclodextrin, heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin, (2-hydroxypropyl)-beta-cyclodextrin and (2-hydroxypropyl)-gamma-cyclodextrin were studied as isomeric selector additives and compared with unsubstituted beta-cyclodextrin. In addition to the size of the cyclodextrin cavity, the number and type of the substituents in the cyclodextrin molecules strongly affect the separation of isomeric naphthalenesulphonic acids, but the effect of the substituted cyclodextrins on the separation selectivity is different for various types of sulphonic acids. Best separations of non-substituted naphthalenesulphonic acids were achieved in a borate buffer with methyl-beta-cyclodextrin, whereas the running buffer with non-substituted beta-cyclodextrin provides superior separation of amino and hydroxynaphthalenesulphonic acids.