Hydrodynamic vs. focusing relaxation in asymmetrical flow field-flow fractionation

Two different sample relaxation methods (focusing relaxation and hydrodynamic relaxation) in asymmetrical flow field-flow fractionation (AFIFFF) are examined by using a conventional asymmetrical channel and a frit inlet asymmetrical channel. In this report, sample recovery using a protein standard is studied, comparing focusing relaxation and hydrodynamic relaxation process at various field strengths and outflow rate conditions. It is shown that a conventional AFIFFF channel provides a better resolution than a frit inlet asymmetrical flow field-flow fractionation (FI-AFIFFF) channel, with sample recoveries up to 80% under moderate field strength conditions. Focusing relaxation appears to provide flexibility in selecting high speed run conditions without losing resolution, since sample relaxation is expected to be achieved more completely than hydrodynamic relaxation. The FI-AFIFFF channel utilizing the hydrodynamic relaxation provides a relatively poor resolution and separation speed; however, it performs well under higher field strength conditions with higher sample recoveries than the conventional channel and it is remarkably convenient in system operation by using stopless flow sample injection method. The low separation speed can be overcome by a simple application of a field programming technique to FI-AFIFFF.