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; (b) an electromagnet (121, 122) having a winding (122) connected to the current source and a gap between at least one pair of poles (107, 108) each of which has a corrugated outer surface and (c) a microchannel (102) used as flow-through cell. The microchannel (102) is configured and dimensioned to receive an amount of magnetic particles to be retained within it and to allow flow of a liquid therethrough. The liquid carries molecules or particles to be captured by means of the magnetic particles. A portion of the microchannel (102) is inserted in the gap.