Characterization of natural aquatic colloids (<5 nm) by flow-field flow fractionation and atomic force microscopy

Flow-field flow fractionation (FIFFF) coupled to a UV detector and to atomic force microscopy (AFM) has been used, for the first time, to characterize ultrafine natural colloids (<5 nm) from selected freshwaters. FIFFF-UV measures a "weight diffusion coefficient distribution" and the corresponding "weight hydrodynamic diameter distribution" was calculated by applying FIFFF theory and the Stokes-Einstein equation. In addition, FIFFF has been used to prepare fractions of very narrow size range for AFM analysis. AFM measures number distribution of particle height (related to radius), and these were calculated. Both raw and transformed data show good agreement between the techniques, with conversion of the UV data to a number-weighted distribution giving better agreement and reduced errors. The small differences between the corrected data from UV analysis and the raw AFM data are either due to the fundamental differences in the analytical techniques, that is, measurement of hydrodynamic properties (FIFFF) or properties after sorption to a solid phase (AFM), or are due to the assumptions of the Stokes-Einstein equation not being met, that is, the fine natural colloids are not spherical or permeable. The methodology offers a means of quantifying fine colloid nonsphericity and permeability.