Field-flow fractionation accumulation wall

Figure 13.6. Separation principle of field-flow fractionation (FFF) is based on physical interactions of particles within an applied field and subsequent field-induced migration to the FFF channel wall ( accumulation wall ). Molecules, depending on their size and diffusion coefficient, are distributed over different velocity lines of axial flow, and they separate accordingly. Larger particles possess less diffusional motion and higher interaction with the applied field hence, they will be caught up in slower-moving streams near the channel wall and elute later than smaller particles.

In FFF systems, the separation along the flow axis is caused by the perpendicular field, whose crucial role is recognized by the word "field" 1n field-flow fractionation. The applied field Interacts with entrained particles, forcing them to accumulate at one wall (the accumulation wall) of the channel. Since the flow velocity near any wall is reduced by frictional drag, the downstream displacement of the particles 1s retarded. Retardation (or retention) is greatest for those particles forced most closely to the wall. Consequently, particles are separated according to the different forces exerted on them by the applied field. These forces normally depend on particle size, leading to a size-based separation. 

Fig. 26. Schematic design of field flow fractionation (FFF) analysis. A sample is transported along the flow channels by a carrier stream after injection and focusing into the injector zone. Depending on the type and strength of the perpendicular field, a separation of molecules or particles takes place the field drives the sample components towards the so-called accumulation wall. Diffusive forces counteract this field resulting in discrete layers of analyte components while the parabolic flow profile in the flow channels elutes the various analyte components according to their mean distance from the accumulation wall. This is called normal mode . Particles larger than approximately 1 pm elute in inverse order hydrodynamic lift forces induce steric effects the larger particles cannot get sufficiently close to the accumulation wall and, therefore, elute quicker than smaller ones this is called steric mode . In asymmetrical-flow FFF, the accumulation wall is a mechanically supported frit or filter which lets the solvent pass the carrier stream separates asymmetrically into the eluting flow and the permeate flow which creates the (asymmetrical) flow field...

In field-flow fractionation, components that interact strongly with the applied field are driven to the accumulation wall. Carrier flow elutes components protruding into the channel prior to those compressed near the accumulation wall. 

This method (FlFFF) is another member of the field-flow fractionation family, with measurement capabilities somewhat like those of SdFFF (18). The basic field-flow fractionation separation process is retained in this method. Particles separate because they are intercepted by different flow stream velocities near the accumulation wall. However, in this FFF method, particles equilibrate at distances from the wall strictly as a function of their size (Stokes radius). The nearness to the wall is a balance of the cross flow in the channel pushing particles toward the wall and normal diffusion tending to move them away. 

Fig. 1 Principle of field-flow fractionation. 1—Solvent reservoir, 2-carrier liquid pump, 3—injection of the sample, 4— separation channel, 5—detector, 6—computer for data acquisition, 7—transversal effective field forces, 8—longitudinal flow of the carrier liquid. A—Section of the channel demonstrating the principle of polarization FFF with two distinct zones compressed differently at the accumulation wall and the parabolic flow velocity profile. B—Section of the channel demonstrating the principle of focusing FFF with two distinct zones focused at different positions and the parabolic flow velocity profile. C—Section of the channel demonstrating the principle of steric ITF with two zones eluting at different velocities according to the distance of their centers from the accumulation wall.

Fig. 4 Optimized SdFFF fractogram of ES Cells. Representative fractogram of ES cell suspensions after SdFFF elution. Elution conditions Flow injection of 100 pL of ES suspension flow rate, 0.6 mL/min (sterile PBS, pH 7.4) and external multigravitational field, 40 (O.lg spectrophotometric detection at 254 nm). Fractions were collected as follows PFl, 3 min 40 sec/4 min 15 sec PF2, 4 min 20 sec/4 min 50 sec PF3, 5 min 0 sec/5 min 50 sec. ER corresponds to the end of channel rotation. In this case, the mean externally applied field strength was equal to zero gravity thus RP, a residual signal, corresponds to the release peak of reversible cell accumulation wall sticking. (View this art in color at www.dekker.com.)...

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