Observations on Anomalous Retention in Steric Field-Flow Fractionation

Abstract

We discuss two anomalies that have characterized our recent work in steric field-flow fractionation (steric FFF): (a) retention has been found to vary with flow rate, contrary to simple theory; and (b) low density cells elute earlier than theory predicts. After reviewing the theory, we describe experiments in both gravitational and centrifugal fields applied to two spherical beads (microporous silica and polystyrene latex) of essentially equal sizes but different densities. The velocity dependence of retention is verified for both kinds of particles, but we find that retention depends dramatically on density as well. We speculate that these anomalous dependencies originate in a velocity-dependent lift force acting to pull the particle away from the wall. This hypothesis is supported by the observation that an increase in field strength increases retention. We conclude that a controllable field strength (as in a centrifuge) is an important asset in steric FFF, permitting increased separation speed and allowing the option of separating particles on the basis of density as well as size.