an apparatus for sequentially fractionating a centrifuge tube includes a capillary tube and a means for applying positive pressure . the capillary tube has an o - ring at the lower end thereof . as the capillary tube is placed within the centrifuge tube , the o - ring forms a seal within the tube . movement of the capillary tube within the centrifuge tube places the liquid in the centrifuge tube under pressure , thus forcing the liquid to flow up through the capillary tube and into a chamber . a chase fluid is then pumped horizontally through the chamber to force the liquid therein through an exit port and into a fraction collector . the apparatus and method of the present invention may be entirely automated and controlled by a single microprocessor .