Diffusion Coefficients and Polydispersities of the Suwannee River Fulvic Acid:  Comparison of Fluorescence Correlation Spectroscopy, Pulsed-Field Gradient Nuclear Magnetic Resonance, and Flow Field-Flow Fractionation

Abstract

Diffusion coefficients of the Suwannee River fulvic acid (SRFA) obtained using fluorescence correlation spectroscopy (FCS), pulsed-field gradient nuclear magnetic resonance spectroscopy (PFG-NMR), and flow field-flow fractionation (FlFFF) were compared as a function of pH (4.0−8.5) and ionic strength (5−500 mM). Diffusion coefficients of the SRFA ranged between 1.9 and 3.5 × 10 - 10 m 2 s - 1 . These values were fairly constant as a function of both pH and ionic strength and comparable to the limited literature values available. Polydispersity data are shown indicating that there is some degree of size and chemical heterogeneity for this humic sample including a small fraction of SRFA components with a diffusion coefficient smaller than 1 × 10 - 10 m 2 s - 1 . The results imply that the majority of SRFA components have hydrodynamic diameters between 1.5 and 2.5 nm.