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

Diffusion coefficients of the Suwannee River fulvic acid (SRFA) obtained us ing fluorescence correlation spectroscopy (FCS), pulsed-field gradient nucl ear magnetic resonance spectroscopy (PFG-NMR), and flow field-flow fraction ation (FIFFF) were compared as a function of pH (4.0-8.5) and ionic strengt h (5-500 mM). Diffusion coefficients of the SRFA ranged between 1.9 and 3.5 x 10(-10) m(2) s(-1). These values were fairly constant as a function of b oth pH and ionic strength and comparable to the limited literature values a vailable. Polydispersity data are shown indicating that there is some degre e of size and chemical heterogeneity for this humic sample including a smal l fraction of SRFA components with a diffusion coefficient smaller than I x 10(-1)0 m(2) s(-1). The results imply that the majority of SRFA components have hydrodynamic diameters between 1.5 and 2.5 nm.