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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 m2 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 m2 s-1. The results imply that the majority of SRFA components have hydrodynamic diameters between 1.5 and 2.5 nm.

High performance size exclusion chromatography (HPSEC) was used to obtain the molecular weight distributions of natural organic matter (NOM) from two South Australian drinking water sources. The NOM was separated into five nominal molecular weight fractions (<500, 500-3K, 3K-10K, 10K-30K, and >30K) using ultrafiltration membranes prior to HPSEC analysis. The use of HPSEC as a tool for NOM characterization was compared with an independent method, flow field-flow fractionation (FlFFF), which separates molecules via a different mechanism. Unlike HPSEC, which uses a porous gel with a controlled pore size distribution to separate molecules, FlFFF uses hydrodynamic and molecular diffusion principles to separate molecules on the basis of molecular size, in the absence of a porous gel. The comparison was made using the following parameters: weight-average molecular weight (M w ), numberaverage molecular weight (M n ), peak molecular weight (M p ), polydispersivity (M w /M n ), and molecular weight range (80% confidence limits). Within the technical limitations of each method, good agreement was obtained between HPSEC and FlFFF for the different fractions. Although solutegel interactions were identified with the HPSEC system, the validation of the technique with FlFFF indicates that HPSEC can provide useful and reliable molecular weight distributions of NOM in drinking water supplies.

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Sedimentation field-flow fractionation (SdFFF) with UV detection is used to systematically investigate the effect of traditional membrane filtration and centrifugation procedures on the isolation of specific size fractions from soil suspensions. Both procedures were used to isolate the nominal <0.45 and <0.2 microm fractions from a clay soil suspension. Results showed that the membrane filtration approach seriously underestimated the total mass of particulate matter present as compared to the centrifugation approach. This has serious implications forthe interpretation of results for "colloidal" and "soluble" fractions from soil suspensions and other environmental matrices obtained using the standard membrane approach. The results also show that sedimentation FFF has great potential as a robust and relatively mild technology for studying size distributions in the "colloidal" range for soil suspensions and other aquatic matrices.