THEORY OF FIELD PROGRAMMED FIELD-FLOW FRACTIONATION WITH CORRECTIONS FOR STERIC EFFECTS

This paper deals with the principal perturbation to ideal normal-mode elution of particles in field-flow fractionation (FFF). This perturbat ion is due to the finite size of particles undergoing migration in the FFF channel. The effects of a first-order correction for particle siz e are examined. Equations are derived for retention time, fractionatin g power, and steric inversion diameter for operation at constant field strength, as well as under conditions of both exponential and power p rogrammed field decay. Useful limiting equations for fractionating pow er are derived and their validity is confirmed for typical experimenta l conditions. The derived equations are necessary for the future devel opment of a systematic optimization strategy for the selection of oper ating conditions for particle size analysis by FFF. Calculations confi rm our previous conclusion that the fractionating power for exponentia l held programming varies strongly with particle size; this variation is only slightly reduced by steric perturbations. The uniform fraction ating power of power programming is slightly disturbed by steric effec ts although fractionating power remains much more uniform than for exp onential programming. It is shown that a higher uniformity in fraction ating power can be gained by manipulating the parameters of power prog ramming but that no improvement is possible with exponential programmi ng. Phenomena giving rise to higher order perturbations and to seconda ry relaxation are discussed and the conditions identified under which these effects are minimized.