Field-Flow Fractionation (FFF) is an analytical separation technique in which the components of a small sample are eluted sequentially from the channel outlet, much like in chromatography[1,2]. This technique is capable of separating and characterizing species in a wide dimensional range (0.01-100μm). As in classic chromatography, sample components elute at given retention times, which are related to various physicochemical properties of the retained species. This relationship can be so rigorously predictable that measurements of retention times can yield these properties for each fractionated component. This is a major distinguishing feature of FFF with respect to classical chromatography that can neither directly predict retention for a given component nor provide chemical information from measured retention parameters. Whereas chromatography exploits differences in partition between the mobile and stationary phases to separate sample components as they are carried along a column, FFF separation is achieved within the mobile phase alone. The separation device takes the form of a thin, parallel-walled channel, across the thickness of which is applied a field of some type. Due to viscous drag, the mobile phase velocity profile across the channel thickness is parabolic, or near-parabolic, with highest fluid velocity near the channel center and zero velocity at the walls. The field acts to drive susceptible sample components toward one of the walls, and therefore into relatively slowly moving fluid. For particles smaller than about a micron in diameter, an exponential concentration profile results from the opposite influence of the field-driven and diffusive transport mechanisms. Particles that interact strongly with the field form thin zones adjacent to the wall, and are confined to very slow moving fluid close to the wall. Particles that interact less strongly with the field form more diffuse zones, and sample faster fluid streamlines in addition to those close to the wall. The net result is that particles that interact less strongly with the field are carried to the channel outlet more quickly than those that interact more strongly. Furthermore, the quantitative theoretical foundation of FFF allows the determination of the strength of interaction of particles with the field as a function of their elution time.
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