a method is described for separating and processing liquid - borne particles within an aliquot thereof following injection into a field flow fractionator . said fractionation method may be employed also to capture , for subsequent segregation , specific predefined classes of such particles . the unique fractionation method disclosed contains means to control the applied transverse flow at each designated location along the length of said channel . in one embodiment of the method a separate compartment lies below each distinct location and corresponding membrane supporting permeable frit segment of the fractionator , providing the individual means to control the localized flow through the membrane section thereabove . employment of a corresponding concentric compartment implementation achieves the same type of compartmentalized cross flow when applied to a hollow fiber fractionation means .