Improved photometric detection of metal ions by capillary zone electrophoresis after precapillary complexation

Precapillary reactions using complexing reagents are increasingly being used to carry out separations of metal ions by capillary zone electrophoresis. To compensate for the sensitivity limitations of on-column photometric detection, several approaches were evaluated. Parameters affecting the sensitivity response of a direct detection system, such as the detection wavelength, sample size, applied voltage, and electrolyte concentration, were examined to produce the best possible sensitivity for metal ions complexed with cyclohexane-1,2-diaminetetraacetic acid. Using injection in the electrokinetic mode, a 2-fold increase in detectability over hydrodynamic (gravity) injection was obtained for certain metal analytes that corresponds to the detection limits in the sub-mu g/l range. A characterization and optimization of photometric detection in an indirect format have been conducted. An organic dye, combined with the carrier electrolyte to impart an absorbance background in the visible range and selected with due account for its spectral and electrophoretic characteristics, was found to provide micromolar-level sensitivities. Feasibility of on-line (stacking) sample concentration for metal complexes has also been demonstrated. (C) 1998 Elsevier Science B.V. All rights reserved.