A chromatographic device comprises a tube, a chromatographic medium, a piston assembly, and a locking mechanism. The tube comprises first and second opposite ends and an inner surface. The inner surface of the tube at least in part defines an intermediate region. The intermediate region is generally between the first and second ends. The chromatographic medium is within the intermediate region. The piston assembly is within the tube and is positioned generally between the first end of the tube and the chromatographic medium. The piston assembly defines a passage for flow of process fluid. The piston assembly comprises a piston body portion. The locking mechanism is adjacent the piston assembly and is in engagement with the inner surface of the tube in a manner to prevent movement of the piston assembly relative to the tube axially toward the first end of the tube without preventing movement of the piston assembly relative to the tube axially toward the second end of the tube.