Field-Flow Fractionation

Field-flow fractionation (FFF) represents a whole family of methods, separating particles due to differences in size (or the diffusion constant) and no other property. In contrast to Analytical Ultracentrifugation, also a method fractionating by particle size, particle density plays no part in FFF: Diffusion constants remain the only target property in FFF.

Field-flow fractionation is a chromatographic method. Separation is achieved by the influence of diffusion on elution time. Elution occurs by means of a laminar flow through the channel, exhibiting a parabolic flow profile. As illustrated in Fig 1, particles particles elute at a rate dependent on their proximity to the wall. A transverse separation field attempts to displace particles towards an accumulation wall, opposed by the particles' back diffusion. In this way, separation occurs dependent on the particles' diffusion properties.

Principle of Field-flow fractionation
Figure 1: Principle of Field-flow fractionation

For particle dimensions greater than a nominal 0.5 µm threshold, the centre of mass of a particle will extend farther into the parabolic profile than the diffusion of smaller particles. This is called inversion to the sterical mode of field-flow fractionation. This effect must be taken into account when choosing channel dimensions for the system in question.

Other problems arise when the particles are not spherical. Rod-shaped particles experience some restrictions in their degrees of freedom in the vicinity of the accumulation wall. This effect leads to considerable errors (orders of magnitude) in calculated results.

As the FFF theory is still subject to controverse scientific discussions, a characterisation by means of detection is often preferred to a characterisation by elution time. Consequently, static light scattering detectors are readily applied and also dynamic light scattering is currently introduced into the market as a means of characterisation.

Influence of the separation field in Flow Field-flow fractionation
Figure 2: Influence of the separation field in F-FFF

The separation field is arbitrary, making FFF a large family of methods. Most common are thermal, sedimentational and magnetic fields, or a hydrodynamic field (transverse flow). We use an asymmetric flow field-flow fractionation with a combined absorbance/viscometry/refractive index detection