On-capillary ion-exchange preconcentration of inorganic anions using open-tubular capillaries followed by elution with a transient isotachophoretic gradient

A new method is described for the elution of inorganic anions in open-tubular capillary electrochromatography (in which an anion-exchange stationary phase is adsorbed to the capillary wall) after preconcentration via ion-exchange interactions. The method utilises a transient isotachophoretic gradient, established using two electrolytes. The first contains a low mobility, weak ion-exchange competing anion, with the second containing a higher mobility strong ion-exchange competing anion. Analytes are adsorbed onto the stationary phase in the weakly eluting electrolyte, but are efficiently desorbed in the stronger eluting electrolyte. By selecting the appropriate mobilities and concentrations of the competing anions, it is possible to obtain highly efficient elution of the analytes from the concentrator column, resulting in a peak compression/focusing effect. The principle of this method for peak compression and elution from the column was demonstrated for iodide, which could be eluted as a peak of width less than 0.1 s, correlating to an elution volume of approximately 40 nL. The potential of this method was demonstrated using bromide and nitrate, which were preconcentrated on a coated section of the capillary, eluted with an isotachophoretic gradient, and separated by capillary zone electrophoresis. Detection limits obtained for iodide, bromide and nitrate using this method were 2.24 nM, 0.86 μM, and 0.32 μM, respectively.