Separation of some metallochromic ligands by capillary zone electrophoresis and micellar electrokinetic capillary chromatography

Abstract

Methods for the separation of several metallochromic ligands by capillary zone electrophoresis and micellar electrokinetic capillary chromatography were developed using an uncoated fused-silica capillary and an alkaline background electrolyte containing ethylenediaminetetraacetic acid (EDTA) and a zwitterionic additive. These additives were used to suppress sorption of the analytes on the capillary wall by interaction with sorbed metal ions present as impurities in the reagents used and also through polar interactions of the analytes. In model experiments it was shown that the addition of calcium or zinc ions to the background electrolyte reduces the electroosmotic flow, probably due to their sorption on silanol groups. They also have detrimental effects on the peak shape of most of the analytes. However, a separation of some metallochromic ligands could be achieved which involved complex equilibria with calcium or zinc added to the background electrolyte in an excess over the EDTA and with citrate added in an excess over the metals. Both methods yielded separation efficiencies up to approximately 500 000 theoretical plates but differed substantially in selectivity and ultraviolet-visible spectra of the separated ligands. Further, it was shown that analytes present as free ligands or as metal complexes that exhibit low effective mobilities and/or solubilities in the background electrolyte can be analysed after addition of sodium dodecylsulfate to the background electrolyte in a micellar separation mode.