INFLUENCE OF CROSS-FLOW HYDRODYNAMICS ON RETENTION RATIO IN FLOW FIELD-FLOW FRACTIONATION

The axial velocity profile in flow field-flow fractionation (flow FFF) differs from that in most forms of FFF, because analytes are polarized near the accumulation wall by a crossflow of carrier instead of a gradient in chemical potential. In accordance with the Navier-Stokes equations, this crossflow alters the axial velocity profile in an open parallel plane channel from a parabola to one which is skewed toward the accumulation wall. Because of this skewing, the commonly used expression for the retention ratio in open parallel plate channels, which is based on a parabolic profile, is incorrect for flow FFF. An alternative expression for retention ratio R, which accounts for this skewing, is derived. The expression depends on the reduced zone thickness, lambda, and on the Reynolds number, Re = \U0\w/2-nu, where \U0\ is the magnitude of the crossflow velocity, w is the channel width and nu is the kinematic viscosity. In the limiting but important case in which R varies linearly with lambda, the error between this expression for R and that based on the parabolic profile is less than 1%, as long as Re < 0.030. Experimental values of Re in open parallel plate channels are shown to be at least one order of magnitude smaller than this threshold value. Consequently, the expression for the retention ratio based on the parabolic profile is adequate in most instances. In the most restrictive case considered, it is shown that crossflow hydrodynamics should not perturb transport rates provided that the analytes elute within 10 void volumes.