Field flow fractionation

Field-flow fractionation, abbreviated FFF, is a separation technique where a field is applied to a fluid suspension or solution pumped through a long and narrow channel, perpendicular to the direction of flow, in order to cause separation of the particles present in the fluid, dependent on their differing "mobilities" under the force exerted by the field. It was invented and first reported by J. Calvin Giddings. The method of FFF is unique to other separation techniques due to the fact that it can separate materials over a wide colloidal size range while maintaining high resolution. Although FFF is an extremely versatile technique, there is no "one size fits all" method for all applications.

In field-flow fractionation the field can be asymetrical flow through a semi-permeable membrane, gravitational, centrifugal, thermal-gradient, electrical, magnetic etc. In all cases, the separation mechanism is born from differences in particle mobility (electrophoretic, when the field is a DC electric field causing a transverse electrical current flow) under the forces of the field, in equilibrium with the forces of diffusion: an often-parabolic laminar-flow-velocity profile in the channel determines the velocity of a particular particle, based on its equilibrium position from the wall of the channel. The ratio of the velocity of a species of particle to the average velocity of the fluid is called the retention ratio.

Fundamental Principles

Field Flow Fractionation is based on laminar flow of particles in a solution. These sample components will change levels and speed based on their size/mass. Since these components will be travelling at different speeds, separation occurs. A simplified explanation of the setup is as follows. The sample separation occurs in a thin, ribbon-like, channel in which there is an inlet flow and a perpendicular field flow. The inlet flow is where the carrier liquid is pumped into the channel and it creates a parabolic flow profile and it propels the sample towards the outlet of the channel.

Fractogram

A graph of a detection signal vs. time, derived from an FFF process, in which various substances present in a fluid get separated based on their flow velocities under some applied external influence, such as an electric field.

Often these substances are various particles initially suspended in a small volume of a liquid buffer and pushed along a fractionation channel by more of the pure buffer. The varying velocities of a particular species of particles may be due to its size, its mass, and/or its distance from the walls of a channel with non-uniform flow-velocity. The presence of different species in a sample can thus be identified through detection of a common property at some distance down the long channel, and by the resulting fractogram indicating the presence of the various species by peaks, due to the different times of arrival characteristic of each species and its physical and chemical properties.

In an electrical FFF, an electric field controls the velocity by controlling the lateral position of either a charged (having electrophoretic mobility) or polarized (being levitated in a non-uniform field) species in a capillary channel with a hydrodynamically parabolic flow-velocity profile, meaning that the velocity of the pumped fluid is highest midway between the walls of the channel and it monotonically decays to a minimum of zero at the wall surface.

External links

• Diagram
• Example
• Postnova Analytics GmbH
• Wyatt Technology GmbH
• Wyatt Technology Europe GmbH
• FFF-Platform