Hydrodynamic relaxation and sample concentration in field-flow fractionation using permeable wall elements.

The advantages of hydrodynamic relaxation in field-flow fractionation, in which an injected sample is driven rapidly toward its equilibrium distribution by flow, are described relative to conventional field-driven relaxation. A new concept for achieving hydrodynamic relaxation, based on the use of permeable wall elements (or frit elements) embedded in the channel walls, is introduced. Here an auxiliary substream of carrier fluid, permeating uniformly into the FFF channel near the inlet, drives the sample, entrained in its own substream, close to its equilibrium configuration. Such frit elements can also be used to enrich the sample at the outlet. Equations are derived and plots are provided for the position of the splitting plane dividing the two substreams; this position defines the strength of the hydrodynamic relaxation. Variations in shear through these frit-modified end regions are also formulated and plotted. The effects of frit elements on band broadening are discussed. It is concluded that permeable wall elements in many configurations may be broadly applicable to FFF and related methods for improved sample introduction, increased separation speed, reduced risk of sample adhesion to the wall, improved flow stability, and sample enrichment.