Effect of urea on the ion-association capillary electrophoresis separation of the anionic metal complexes using hydrophobic tetraalkylammonium as ion-association agent.

The effect of urea as an electrophoretic buffer solution modifier on the ion-association (IA) capillary electrophoresis (CE) separation of four anionic metal complexes of Al(III), Co(III), Cr(III), and Fe(III) with 2,2'-dihydroxyazobenezene-5,5'-disulfonate (DHABS) using a hydrophobic ion-association agent, tetrapentylammonium bromide, was studied. The mutual separation of the four anionic metal-DHABS complexes was not achieved without the addition of urea in the electrophoretic buffer solution. However, the addition of 1.5 M urea in the electrophoretic buffer solution brought about a complete separation of the four metal complexes. The ion-association constants between all metal-DHABS complexes and tetrapentylammonium in an aqueous urea solution were smaller than those in a neat aqueous solution. This indicates the hydrophobic interaction contributing to the ion-association between analytes and ion-association agent during IA-CE separation processes can be controlled with the addition of urea to the electrophoretic buffer solution. Another advantage of adding urea was a substantial enhancement of separation efficiency with a reduction of the half-bandwidth of the peaks. Also, a reduction of the electrophoretic mobility of the electroosmotic flow when urea was added was much less than when organic solvents were used.