A mechanical separator for separating a fluid sample into first and second phases is disclosed. The mechanical separator includes a float having a passageway extending between first and second ends thereof with a pierceable head enclosing the first end of the float, a ballast longitudinally moveable with respect to the float, and a bellows extending between a portion of the float and a portion of the ballast. The bellows is adapted for deformation upon longitudinal movement of the float and the ballast, with the bellows isolated from the pierceable head. The float has a first density and the ballast has a second density greater than the first density. The bellows is structured for sealing engagement with a cylindrical wall of a tube, and the pierceable head is structured for application of a puncture tip therethrough. The separation device is suitable for use with a standard medical collection tube.