Pretreatment method and apparatus to remove matrix ions from a liquid sample, prior to separation of the sample analytes (e.g. by chromatography), by flowing the liquid sample into a sample compartment and stopping the flow. The sample compartment has a wall comprising an ion exchange membrane having exchangeable ions of the same charge as the matrix ions to be removed. A regenerant liquid stream flows through a regenerant flow compartment on the other side of the membrane from said parked liquid sample. Matrix ions in the parked liquid sample are transported across the membrane into the regenerant flow compartment. Suppression may be performed electrolytically and/or chemically. A concentrator column may also be used.