Selective capillary electrophoresis separation of mono and divalent cations within a high-surface area-to-volume...

In this study, the separation of inorganic mono and divalent cations using multi-lumen silica capillaries (MLCs) of 126 channels, each with either 4 or 8 μm inner diameter, was investigated using capillary electrophoresis and on-capillary capacitively coupled contactless conductivity detection (CE-C⁴D). MLCs provided sufficiently high surface area-to-volume ratios to generate significant wall ion-exchange interactions for the divalent cations, which significantly affected resultant selectivity, whereas monovalent cations were predominantly separated by simple electrophoresis. The resultant hybrid selectivity was seen for both 4 and 8 μm channel multi-lumen capillaries, without any preconditioning or capillary wall modification. Remarkably, the electrophoretic mobilities for the divalent cations Mg²⁺ and Ca²⁺ were reduced 7.5 times compared to those determined using a single channel open tubular capillary of 50 μm i.d., providing much improved selectivity. Apparent electrophoretic mobilities of divalent cations increased as the concentration of BGE increased, while those of monovalent cations decreased parallel to electroosmotic mobility. These results show the electrostatic interaction between the divalent cations and the silica wall. At least, this specific separation of mono- and divalent cations were clearly observed with a mixture standards solution of less than 200 μmol L^-1. Using a MLC with 126 × 8 μm i.d. channels and 49.1 cm in length, together with a 20 mmol L^-1 MES/His BGE, containing 2 mmol L^-1 18-crown-6, monovalent cations (NH₄⁺, K⁺ Na⁺ and Li⁺) and divalent cations (Ca²⁺ and Mg²⁺) could be completely separated within 4 min. For monovalent cations, on-capillary detection using C⁴D provided calibration curve (0-200 μmol L^-1) correlation coefficients in the range R² = 0.995-0.999, and limits of detection of 2.2-6.6 μmol L^-1. Relative standard deviations for migration times were less than 0.6%, and recoveries ranged from the 93.8%-105.4%. The new method was applied to the separation and quantitative determination of monovalent and divalent cations in drinking waters and soil extracts.