Separation Mechanisms for Nanoscale Spheres and Rods in Field-Flow Fractionation

Separation mechanisms for spheres and rodlike particles in classical field-flow fractionation (FFF) are studied using a Brownian dynamics simulation. For spheres, simulation results for mean elution time are found to be in good agreement with experimental data and the steric inversion theory of Giddings [1]. Modeling of particle separation for rods is compared with spheres of equal diffusivity. The simulation shows that nanotube scale particles elute by a normal mode mechanism up to aspect ratios of about 500, based on a particle diameter of 1 nm. At larger sizes, the rods also show a steric deviation from normal mode elution, but in the opposite sense as for spheres. The different behavior is attributable to the effect of particle shape. Extension of the steric mode theory for spheres to rods illustrates a potential steric mode separation that can be used to separate rods based on chirality.