Applications of other FFF methods

There is a healthy overlap in the capabilities of the different FFF subtechniques, and it is consequently not surprising to find that other FFF approaches are also applicable to polymers. A case in point is flow FFF, which is the most universal of all FFF subtechniques [43,44]. 

In order to generate a cross-flow of carrier liquid in a flow FFF channel, the walls of the channel must be permeable to the liquid. For this reason the walls of flow FFF systems are made up of frit-membrane combinations. Generally, the upper wall is a frit, and the lower wall is a frit covered by a membrane having the desired molecular-mass cutoff limits. 

The flow FFF system is universal because all conceivable components are driven to the accumulation wall by the cross-flow irrespective of electrical charge, molecular mass, or other special properties. The method should be applicable to any sample as long as an appropriate membrane can be found with the needed molecular-mass cutoff and with stability against swelling or decomposition in the carrier solvent utilized. 

While in theory flow FFF should provide a resolving power and performance comparable to that of thermal FFF, the implementation of this subtechnique has been slow because of the difficulty of fabricating thin uniform channels using frit and membrane materials. There is presently no commercial equipment available except by special order (FFFractionation, Inc.). 

For completeness we note that two other FFF subtechniques can be applied to certain polymeric materials, although applications are so far limited. Sedimentation FFF is the most notable example. For this system the driving force (centrifugally induced sedimentation) is directly proportional to molecular mass in a form that is calculable from first principles (see eqn 8.7). Accordingly, molecular mass distributions can in theory be obtained by calculation without empirical calibration. This principle has been successfully applied to the determination of the molecular mass and particle size distribution of numerous colloidal particles including viruses, latices, emulsions, liposomes, protein aggregates, and water-borne colloids [5,7,9]. However, as noted earlier, sedimentation FFF is not applicable to many polymers of interest because sedimentation forces (even in a powerful centrifuge) are not adequate to drive the components to the accumulation wall of the FFF channel. Thus molecular masses of less than 10 cannot be well 

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