Field-flow fractionation an alternative to size exclusion

Field-flow fractionation was developed about the same time as SEC the first FFF paper was published in 1966 [1]. However, major instrumentation problems were not solved as quickly for FFF as they were for SEC as instrumentation became available for SEC, there was then less incentive for the development and application of FFF instrumentation. For almost twenty years, the only active group involved in the development of FFF for polymer analysis was the University of Utah group from which the FFF concept originated. However, as the demands placed on polymer characterization tools become more stringent and the applications more exotic, the FFF approach is gaining increased attention from a widening circle of users. 

It is remarkable that FFF has remained technically competitive with SEC despite a limited development effort that has certainly amounted to less than one percent of that devoted to SEC. Major advances in FFF technology can now be expected as new research groups are attracted to the methodology. 

For context, we point out here that FFF is a broad family of techniques applicable to macromolecules, colloids, and particles of diverse types extending over a mass range from a few hundred to 10 dalton [3-8]. FFF has perhaps become best known for high-resolution colloid separation and characterization [5,7,9]. For this task, a specific subtechnique termed sedimentation FFF is most often utilized [10]. For polymer analysis, another subtechnique, thermal FFF, is most commonly employed [11,12]. 

The object of FFF is to find means for differentially partitioning polymers or other species between different parts of the flow profile so they will be swept along at different velocities. Since there is only one phase in the channel, ruling out the use of phase distribution forces, and since there is no packing or porous structure to generate entropic differences as in SEC, an external influence must 

Separation occurs with each polymer component established in a condition very close to the steady-state distribution relative to the accumulation wall [14]. The greater the force exerted on a component by the field, the closer it is driven toward the wall under steady-state conditions. If the components of a mixture, such as the different molecular mass components of a polydisperse polymer, are driven toward the accumulation wall at different force levels, they will form steady-state layers next to the wall having different thicknesses. This is very similar in effect (but not in the mechanism for achieving it) to differential 

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Field-flow fractionation an alternative to size exclusion chromatography... 

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