ELECTRICAL FIELD-FLOW FRACTIONATION IN PARTICLE SEPARATION .1. MONODISPERSE STANDARDS

In its original implementation, electrical field-flow fractionation (EFFF) was carried out in membrane-walled channels with the electrodes placed externally to the flow channel. The poor separation efficiency of this system left the technique largely unattended for about two decades. In the present study, we describe a new and simple approach to EFFF, which demonstrates the technique's ability to carry out rapid, high-resolution separations of colloidal samples in aqueous suspensions. The present channels are bounded by the smooth and rigid graphite electrodes which allow for the application of small voltages, typically less than 2 V, across the thin (178 mum) separation space defined by a Mylar spacer. Although this arrangement generates nominal fields of the order of 100 V/cm, polarization of the electrodes considerably reduces the effective field across the bulk of the channel to less than 1% of the nominal value. Nevertheless, under conditions of low ionic strength the system is shown to retain and separate polystyrene (PS) latex standards with sizes ranging from 60 to 10 000 nm. For small particles of comparable zeta-potential, separating in the ''normal'' mode of EFFF, the size selectivity S(d), is approximately 0.7. As with other FFF techniques, EFFF displays a transition from ''normal'' to ''steric'' behavior; the critical diameter for this transition is highly dependent on ionic strength, with values ranging from approximately 500 nm in deionized water to approximately 1200 nm in 133 muM aqueous NaCl under an applied voltage of 1.37 V and a flow of 1 mL/min. Although the stop-flow period customarily observed immediately after sample injection is shown to perturb the field, it still has a positive effect on resolution under most conditions. Thus, stop-flow times of approximately 5 min will permit the baseline resolution of a 272-nm latex from soluble contaminants in a 2-min separation step.