Reduction of end effect-induced zone broadening in field-flow fractionation channels

A channel configuration for the elimination of end effects in field-flow fractionation (FFF) channels is simulated and demonstrated for a microfabricated FFF system. In field-flow fractionation, the carrier liquid and sample particles are transferred from a point injection to the full breadth of the rectangular channel using a triangular end piece at the inlet. The nonuniformity in streamline length generated by this end piece results in an increased instrument-related plate height. An additional contribution from the end piece at the outlet of the channel further increases the total band broadening. This paper presents a novel approach to minimize end-effect contributions to plate height by fabricating microstructures in the channel end sections to redistribute the flow streams and force streamline lengths to be more uniform. Numerical analysis of the flow profile and sample dispersion (including spreading of particles due to diffusion and advection) is carried out to investigate the optimized microstructure column size, shape, and placement in the end pieces. The configuration obtained from the numerical simulation results is used to design a prototype device. Experimental measurement of the plate heights for this prototype with an on-chip impedance-based detector shows marked improvement in performance due to the presence of the microstructures in comparison to conventional FFF channel geometry with an average 50% reduction in plate height. The redesigned inlet triangle results in a uniform transition of the point-injected sample into a thin and straight band across the width of the channel at the start of the rectangular section of the fractionation channel.