Centrifugal field flow fractionation:

Centrifugal field flow fractionation:

Centrifugal FFF is basically a new name given to Sedimentation FFF after further developments in it. The specialty of this Centrifugal FFF is that it involves the use of centrifugal force as the externally applied force which is the basic element required in the separation of the analyte particles. Channel used in this technique is in the form of a ring which spins at 4900 rpm. 

This technique is effective in the separation, characterization and purification of micron-sized particles of any type of analyte. It allows for the separation of particles with only a 5% difference in size.

Mode of elution

In Centrifugal or sedimentation FFF the elution is carried out by hyperlayer mode. The other two modes of elution fail to explain that for the particles having the size less than 1 µm the retention ratio increases with the increase in velocity of flow. In other words the transport velocity is variable for the particles whose size is less than 1 µm. There is another mode of elution by which separation occurs in Centrifugal FFF and is called as lift-hyperlayer mechanism. And in this mechanism we assume that there are no interactions between the particles and the walls of the channel. The separating forces are the external applied field and the other force hydrodynamic forces, these forces are collectively called as hydrodynamic-lift forces. These forces are originally involved in focusing the particles into an equilibrium position according to their characteristics like density and size. Equilibrium position of the particle and the channel thickness both are related to the retention ratio.

Construction 

The centrifugal field flow fractionation technique is carried out in open channel; this channel is allowed to rotate under the action of the centrifugal force those particles which are more dense than the mobile phase migrate towards the outer walls. In the channel the continuously flowing mobile phase intercepts in those regions of the walls where the flow is slow. Smaller particles have the ability to resist the externally applied gravitational force so the smaller particles are further way form the wall as compared to the larger particles. So the smaller particles are eluted first because these are intercepted by the faster flow streams.

At start the SFFF were made up of metals so that they can endure the speed of the rotors. Channels were also made from the stainless-steel and welded metal spacer were used this type of construction gave opportunity to expose the channel for cleaning and changing the configuration of channel. 

Another approach made in laboratories was employed that uses a stainless-steel ring which is split and is fitted in a centrifuge bowl which is also made up of metal and has an outer wall which is polished. The importance of using this split ring system is that the size of the ring can be increased by the increasing motor speed. But the major problem here is that these rings are difficult to fabricate and are costly. 

The recent studies involve the use of plastic channel in the metal ring.

So two basic types of channels used are as follows

• “Continuous ring” channel

• “Floating” channel

“Continuous ring” channel

Another important approach involves a ring with a channel whose design is like a continuous ring which is made up of a biochemically engineered plastic like Delrin acetyl resin or Noryl polyphenylene oxide plastic. As the force field the elasticity of the plastic ring causes it to enlarge with a centrifuge bowl made of titanium. In this arrangement the channel is inserted inside the polished surface of a metal rotor bowl by lapped bands backed up with an O-ring seal.

Floating channel

Floating plastic approach is a more flexible arrangement in which the whole channel assembly is immersed in a fluid present in a centrifuge bowl.