An apparatus for treating an aqueous sample stream includes analyte ions. The apparatus comprises an ion exchange barrier; a sample stream flow channel; an ion receiving stream flow channel adjacent to the sample stream flow channel and separated therefrom by said first ion exchange bather. Stationary flow-through ion exchange packing is disposed in the sample flow channel of the same charge as the ion exchange bather. The ion exchange packing comprises a mixture of a first ion exchange portion with strong ionizable groups and a second ion exchange portion with weak ionizable groups of the same charge. First and second electrodes are in electrical communication with the sample stream flow channel and ion receiving flow channel.