Measurement of Particle Density, Porosity, and Size Distributions by Sedimentation/Steric Field-Flow Fractionation: Application to Chromatographic Supports

A procedure is developed here for measuring the density and density distribution of polydisperse populations of near-spherical particles having diameters >1 µm using a combination of sedlmentatlon/sterlc (Sd/St) FFF and microscopy. This procedure is an extension of a recent calibration method designed to measure the size and size distribution of populations of particles of known density by Sd/StFFF using latex calibration standards. Both procedures are based on a density compensation principle in which particles of different densities are assumed to be retained Identically if the field strength is adjusted to be inversely proportional to the density difference Δρ between particle and carrier. Following the development of the new calibration procedure, the above methods are applied to a group of five porous silicas used as chromatographic supports. The fractionation of the supports into narrow size classes by Sd/StFFF is demonstrated by the electron microscopy of collected fractions. Seven sizes (4–29 µm) of polystyrene latex particles are chosen for calibration. Values of diameter and ∆ρ selectlvitles generally fall in the ranges 0.72–0.77 and 0.15-0.25, respectively. By positioning retention time (from Sd/StFFF) and particle diameter (from microscopy) data for different silica fractions on the calibration plots, the density and thus the specific pore volume V_P of each fraction can be determined. The data suggest a small increase in V_P with size, which may be due in part to system band broadening. The average V_P values, unaffected by band broadening, are in reasonable agreement with those from outside sources.