Quantitative investigation on the stacking of metal ions induced by another metal ion based on moving substitution boundary electrophoresis

Abstract

In this paper, a novel model for the stacking of metal ions via another metal ion was developed based on a dynamic equilibrium system (DES) composed of moving substitution boundary (MSB) and moving chelation boundary (MCB). With Cu²⁺, Co²⁺ and EDTA as an example, a substitution reaction took place between Cu²⁺ and [Co-EDTA]²⁻, and resulted in the formation of MSB, and Co²⁺ released from [Co-EDTA]²⁻ reacted with EDTA, which resulted in the formation of MCB and [Co-EDTA]²⁻, the produced [Co-EDTA]²⁻ reacted again with Cu²⁺. The reaction between Cu²⁺, [Co-EDTA]²⁻, and Co²⁺, EDTA were repeated. The concentration of Co²⁺ and [Co-EDTA]²⁻ was stacked accompanying the formation of MSB and MCB until the above two reactions reached equilibrium. Relevant experiments were performed and the results indicated that: (1) Cu²⁺ can be used for the stacking of Co²⁺ and higher than 70-fold preconcentration of Co²⁺ was achieved in capillary electrophoresis (CE); (2) the concentration of Cu²⁺ had a key role in the stacking of Co²⁺, a higher Cu²⁺ concentration resulted in a better focusing efficiency; (3) the initial concentration and zone length (L_initial) of [Co-EDTA]²⁻ were the major factors on enrichment. Furthermore, experiments based on MSB were performed with multi-metal ions (such as Cu²⁺, Co²⁺, Ni²⁺ and Pb²⁺), which demonstrated the potential application of the MSB-based technique for the separation and stacking of multi-metal ions.