Steric-mode field-flow fractionation

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...

The techniques of field-flow fractionation appear to be well suited to colloid analysis. The special subtechnique of sedimentation FFF (SdFFF) is particularly effective in dealing with colloidal particles in the diameter range from 0.02 to 1 using the normal or Brownian mode of operation (up to 100 jU-m using the steric-hyperlayer mode). As a model sample for the observation of aggregate particles by SdFFF, of... 

Data Analysis. The computer program used for data analysis was developed at the Field-Flow Fractionation Research Center. The underlying theory is similar to that discussed by Giddings et al. (4). For normal mode characterizations, the fractograms are converted to particle size distributions by using developed theory. However, for steric mode analyses, calibration curves are required (15, 20). 

Field-flow fractionation is a family of high-resolution techniques capable of separating and characterizing colloids and macromolecules. In normal FFF, the particles form a Brownian-motion cloud that extends a short distance into the channel. Separation is possible because the solvent flows at different velocities at various points within the channel. The smaller particles, whose cloud protrudes out into the faster laminae, are transported more rapidly than the larger particles, so that the two populations are soon separated. In the steric mode of operation, which happens when the protrusion of particles into the flow stream is determined by their physical... 

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