Capillary electrophoresis for aluminum ion speciation: Optimized separation conditions for complex polycation mixtures

Abstract

Aluminum chlorohydrates (ACH) are used in numerous applications and commercial products on a global scale including water treatment, catalysis or antiperspirants. They are complex mixtures of water soluble aluminum polycations of different degrees of polymerization, that are difficult to separate and quantify due to their susceptibility to depolymerize in solution when placed out of equilibrium, which is inherent to any separation process. We recently achieved the first capillary electrophoresis separation and characterization of ACH oligomers using 4-morpholineethanesulfonic acid (MES) as background electrolyte counter-ion. MES stabilizes the separated ACH oligomers during the electrophoretic process leading to highly repeatable and fast separations. In this work, the separation of ACH oligomers was further studied and perfected by varying the ionic strength, MES concentration and pH of the background electrolyte. Complex electrophoretic behavior is reported for the separation of Al13, Al30 and Na+ ions according to these experimental parameters. The transformation of the electropherograms in effective mobility scale and the use of the slope-plot approach are used to better understand the observed changes in selectivity/resolution. Optimal conditions (700 mM MES at 25 mM ionic strength containing 0.1 mM didodecyldimethylammonium bromide for dynamic capillary coating, pH 4.8) obtained for the separation of ACH oligomers are used for the baseline separation of samples difficult to analyze with other methods, including different molecular, aggregated and colloidal forms of aluminum from the Al13, Al30 and Na+ mixture, validating the rationale of the approach.