a free floating plastic channel for sedimentation field flow fractionation is suspended in a centrifuge rotor filled with a compensating liquid . the channel is constructed of a plastic central hub assembly fitted with a plastic outer ring preferably of a lower density than the hub . the hub contains a shallow channel on its outer surface and is interference - fitted to the outer ring to insure a liquid tight seal at zero force field . with the liquid totally surrounding the hub - outer ring assembly , stresses on the plastic parts are essentially equalized even under high force fields and leakage from the channel at the hub - ring interface is greatly reduced .