Investigation of solute-wall interactions in separation of uranium(vi) and lanthanides by capillary electrophoresis using on-capillary complexation with arsenazo iii

The separation of uranium(VI) and lanthanides by capillary electrophoresis (CE) in the form of anionic complexes with arsenazo III in aqueous background electrolytes was found to depend strongly on the chemistry of the surface of the internal wall capillary. A recently developed separation scheme for these complexes was used as a model for the evaluation of the solute-wall interactions in which a number of capillaries with different charge, polarity, hydrophobicity, and structure of bonded layer of the internal wall surface were compared. Pretreatment of bare fused silica (FS) capillaries with acid or alkali was found to be very important: After treatment with sodium hydroxide, the observed adsorption of the analytes was higher than for untreated FS capillaries. Capillaries with a chemically bonded neutral hydrophilic layer also showed considerable adsorption of the analytes. Strong adsorption of the negatively charged amphiphilic molecules of ligand or metal complexes was observed for coated capillaries with a positively charged wall, as well as for capillaries with a hydrophobic wall. Dynamic coating of the bare FS capillary with Carbowax 20M provided better peak shapes and shorter migration times owing to suppression of electroosmotic flow and blockage of adsorption sites on the internal capillary wall. The best separation of complexes of uranium and lanthanides with arsenazo III was achieved using an untreated bare FS capillary dynamically coated with Carbowax 20M (350,000 theoretical plates for uranyl and 63,000 theoretical plates for lanthanum).