Apparatus for retaining magnetic particles within a segment of a flow-through cell during flow of a fluid through the cell. The apparatus comprises (a) an electrical current source (12) (b) an electromagnet (13) having a winding (14) connected to the current source (12) and an air gap (23) between at least one pair of poles (21, 22) each of which has a corrugated outer surface and (c) a flow-through cell (18) which is configured and dimensioned to receive an amount of magnetic particles to be retained within the flow-through cell and to allow flow of a liquid through the flow-through cell. The liquid carries molecules or particles to be captured by means of the magnetic particles. A portion of the flow-through cell (18) is inserted in air gap (23).