Abstract: A chromatographic device comprising a column body having a first end and a closed second end. The device further includes a piston inside the column body, a chromatographic medium, a rod, and a locking mechanism. The chromatographic medium is within the column body and between the piston and the second end. The rod has a longitudinal rod axis and is operatively engageable with the piston for pressing the piston in the column body. The locking mechanism is adapted for locking the rod relative to the column body in a manner to prevent movement of the rod relative to the column body in an outward longitudinal direction away from the piston without preventing movement of the rod relative to the column body in an inward longitudinal direction toward the piston. Preferably, the chromatographic device includes a spring for exerting a constant axial compression against the piston to prevent the formation of voids in the packing medium.