Separations of metal ions by capillary electrophoresis using complexation with metallochromic ligands

This work deals with separations of metal ions by capillary electrophoresis (CE) using metallochromic ligands and with some related problems in separation and detection. In the first part, some important instrumental factors relevant to CE of metal ions using metallochromic ligands were investigated. The positioning of the separation capillary relative to the electrode in the electrolyte vial and buffering of the background electrolyte (BGE) in CE have been found to have a crucial influence on the magnitude of pH changes of the BGE in the capillary. On-capillary measurement of the pH changes accomplished by the addition of a suitable pH indicator dye to the BGE and employing absorbance detection in the visible region showed that pH changes of more than 2.5 pH units occurred when the capillary end was positioned close to the electrode surface. This effect was explained by direct observations of pH changes around a platinum electrode placed in a photometric cell serving as a BGE reservoir which showed a zone of altered pH being generated close to the electrode and spreading over time through the surrounding BGE by thermal convection. Light emitting diodes (LEDs) in the range from 563 to 654 nm were examined as light sources for on-capillary detection in CE. The detector linearity and noise for the LEDs were compared to mercury and tungsten lamps used with interference filters. Both a theoretical model, taking into account both stray light and polychromatic light, and experimental results showed stray light as the major factor which determines linearity under typical CE operating conditions. Noise was found to be same or better for the LEDs as for the other sources. In the second part, several factors concerning chemistry of separations of metal ions by CE were studied. Methods for analysis of metallochromic ligands by CE and micellar electrokinetic capillary chromatography were developed using an uncoated fused silica capillary and an alkaline BGE containing a zwitterionic additive and a strong complexing agent (EDTA) which was crucial for obtaining good peak shapes. Further, it was shown that ligand purity can play a crucial role in the separations of metal ions by CE in the form of kinetically labile complexes using addition of the ligand to the BGE. Using separation of uranium(VI) and lanthanides with arsenazo III as a model system, solute-wall interactions were studied and found to vary for a number of capillary wall chemistries. Least analyte adsorption was observed in an untreated bare fused silica pre-run coated with a hydrophilic polymer, whilst the frequently used hydroxide treated capillary exhibited greatest adsorption. The applicability of polymer capillaries (polypropylene and polytetrafluoroethylene) for CE of metal ions/complexes was also investigated. Complexation equilibria in the BGE were studied and the secondary ligand type, pH and concentrations of secondary ligand and arsenazo Ill were found to have a strong influence on the peak shapes and on the selectivity and sensitivity. Mechanisms for these effects are discussed. A validated method is presented for determination of Ba\ and Sr\" in the presence of up to 40000 fold excess levels of calcium(II) and magnesium(II) using an electrolyte containing sulfonazo Ill with a limit of detection of approx. 50 ppb for both metals. Analytical potential of the general approach of using metallochromic ligands in CE is discussed."