An apparatus for retaining magnetic particles within a segment of a flow-through cell during flow of a fluid through the cell comprises (a) optionally, an electrical current source; (b) an electromagnet having a winding connected to the current source and an air gap between at least one pair of poles each of which has a corrugated outer surface and (c) a flow-through cell 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 is inserted in air gap.