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Field Flow Fractionation
Field flow fraction consists of a wide variety of separation methods for which are used for the purification and characterization of superamolecular compounds like macromolecules, colloids, particles, gels, emulsions and cells. So it has applications especially in the field of biological sciences. It consists of various sub-techniques and separation devices which are used for the separation and characterization of the particles ranging in size from 10-3 to 102 µm. 










These sub-techniques of the FFF are capable of separating the particles on the basis of their physical properties like charge, mass, size and density. The basic concept of Filed flow fractionation for the first time was given in 1960s by J. Calvin Giddings. Field flow fractionation is considered to be the hybrid of chromatography, electrophoresis and ultracentrifugation all of which are field-driven methods. 






Like ultracentrifugation is requires a field force and like chromatography it is an elution technique based on the differences in the flow rates and the separation is carried out in a single liquid phase. The separation in the FFF is carried out by non-uniform flow of carrier liquid and the field force applied perpendicular to the carrier. The analyte is dissolved in a carrier fluid and then it is pumped through an empty channel which is 50- 500 µm thin. And the flow of the carrier fluid forms a parabolic Newtonian flow velocity profile.


Basic concept
The basic concept of the FFF is that the formation of the parabolic flow profile which causes the reduced speed of the flow near the walls of the channel. The flow speed is maximum at the centre of the channel. The use of the applied physical field is that when it is applied perpendicularly to the flowing fluid it derives the analyte to the walls of the channel. This causes the development of the diffusive forces because of the greater analyte formation at the walls of the channel and as a result of the operation of these diffusive forces the analyte is driven back to the centre of the channel. This causes the forces to balance each other and the steady state equilibrium is attained. Retention of the analyte occurs when the analyte resides in the velocity zones of the carrier liquid which are slower than the average velocity of the carrier liquid. This causes different analytes to reside in different velocity zones which causes the separation. In this whole process the main role is played by the diffusive forces.






The basic theory about the working principle of the field flow fractionation technique basically relates the retention time with the external field force exerted on the particle of the analyte. So this theory actually relates the properties of the applied field like direction and magnitude with that of the properties of the analyte or the carrier fluid like flow rate. And there are three basic elution methods based on this theory which are as follows:
• Brownian elution mode
• Steric mode
• Hyperlayer or focusing mode






Each type of elution is used in a different type of field floe fractionation technique as field flow fractionation technique has many sub-types given below:
1. Flow FFF
2. Asymmetric FFF
3. Thermal FFF
4. Split Flow Thin Cell Fractionation 
5. Centrifugal FFF






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