This disclosure is directed to a cap for obtaining a target analyte from a suspension. The cap introduces a magnetic field or a magnetic gradient to the tube to draw the target analyte bound to a particle to the cap. In one aspect, a cap includes a magnetic insert and a receiving piece. The magnetic insert includes a stopper and a magnet extending from the stopper; and, the receiving piece, which is configured to hold the magnetic insert, includes a receiving stopper and a sheath. The sheath may include imaging slides on opposite sides of the sheath. In another aspect, the cap may include a stopper and an embedded magnet. The cap may include an analysis piece on a bottom end of the stopper. In yet another aspect, the cap may include a fluid compartment and a filter at a bottom end of the stopper.