a new field flow fractionation process for the separation of particles using a modified channel structure to achieve hydrodynamic relaxation without involving a stop - flow procedure and thus provide improved speed of operation as well as improved sample concentration , the new improved channel structure comprising a thin channel having one or more permeable wall sections contained in the enclosing wall or walls of the channel , the permeable wall sections making up not more than 25 % of the total wall area of the channel , and adjustable flow control valves for controlling the flowrate of any fluid at the inlet end of the channel and independent flow control valves for controlling the flow rate of the fluid being introduced into or withdrawn from the permeable wall sections .