Separation of aromatic sulphonic acids by CZE in coated and non-coated capillaries

Capillary zone electrophoresis (CZE), besides ion-pairing mode HPLC and salting-out HPLC, is well-suited for the analysis of aromatic sulphonic acids widely used as intermediates in the production of synthetic dyes and optical brighteners. The separation selectivity in CZE of many aromatic acids, including positional isomers, can be controlled by the type and concentration of a cyclodextrin additive. The influence of the concentration of beta-cyclodextrin in the working electrolyte on the separation of the positional isomers of naphthalene (poly-)sulphonic acids, and their amino- and hydroxy derivatives, by CZE was studied, both in non-coated fused silica capillaries and in capillaries coated with polyacrylamide. The migration time scale was calibrated using 4-alkylbenzenesulphonic acids as the calibration standards. Limiting mobilities of the free acid anions and of their complexes with beta-cyclodextrin were calculated and the effect of the inclusion guest-host complex formation on the CZE separation was quantitatively characterized. The migration order. in coated capillaries is reversed with respect to CZE in non-coated fused silica capillaries, the separation selectivity is different and the separation of polysulphonic acids such as naphthalene tri- and tetrasulphonic acids is significantly accelerated.