Migration behavior of metal complexes in capillary zone electrophoresis: Interpretation in terms of quantitative structure–mobility relationships

Significant advances in metal ion analysis by capillary zone electrophoresis (CZE) have occurred as a consequence of using metal complexes with various organic and inorganic ligands. Metal–ligand complexes that contribute to metal speciation in solution have also gained the attention of researchers. An understanding of the molecular properties that control the separation and further insight into the migration mechanism call for a systematic analysis of relationships between migration parameters and charge and size characteristics of metal complexes. To perform such an investigation, a number multiparametric migration models derived from a generally valid equation for electrophoretic mobility as a function of charge density were developed. The models operating with tabular or readily calculated structural descriptors (in particular, metal atom electronegativity or effective charge) as well as with the formal charge and ligand number were evaluated using numerous sets of experimental migration data for inorganic and organic ligand complexes. Consistent – in a great many instances – approximation results confirm the separation mechanism for metal complexes in CZE as governed basically by differences in charge-to-size parameters, present a valuable and convincing selection of such parameters, possessing a definite physical meaning, and owing to the general validity of the multivariate regression approach, open new possibilities in its application to more complex (i.e., electrokinetic chromatographic) systems.