Electrodialysis as a sample preparation tool for capillary electrophoresis of small anions - A proof of principle

The use of capillary electrophoresis (CE) for biomedical applications or control of waste water effluents is often restricted, due to the complex matrix embedding trace concentrations of the analyte. Selective removal of interfering ionic species is challenging when analyte and matrix are similar in size and charge. An electrodialysis method applying a home-made module for sample desalting is described. Chloride, sulfate and nitrate ions were employed at 50 or 100 mM (referring to concentrations in real samples from clinical or environmental origin) to simulate matrix compounds. Sulfanilic acid was selected as a model analyte ion at millimolar or sub-millimolar concentrations. Process analysis was performed with ion chromatography coupled on-line. For a simple monitoring of the process in application, conductivity measurement in the sample compartment could be successfully established. Within 45 minutes, the matrix concentration was decreased by a factor of about 10, whilst the analyte was only diluted by a factor of about 3. The rate of the matrix depletion was constant over the whole process and controlled by the constant current generated from the voltage source. Speeding up the process by applying higher current densities failed, due to resulting thermal stress at the membranes. Electrodialysis results were independent of both the matrix anion and matrix cation identity, indicating a good robustness against variations of the sample matrix.