ASYMMETRICAL FLOW FIELD-FLOW FRACTIONATION AS A METHOD TO STUDY THE BEHAVIOR OF HUMIC ACIDS IN SOLUTION

Asymmetrical flow field-flow fractionation (AsFlFFF) is used to measur e the size distributions of humic acids in solution. Shifts in the siz e distribution are used to study the behavior of these amphiphilic mac romolecules in solution with changes in pH, ionic strength, and humic acid concentration. Humic acid concentrations are increased by on-chan nel focusing of the injected samples. As the concentration is increase d, the humic acids aggregate, but the effect varies with solution cond itions. As the concentration of calcium chloride is increased, size di stributions broaden and become multimodal. The decrease in size is con sistent with reports that divalent cations induce conformational chang es by the formation of intramolecular bridges between carboxylate moie ties; multimodality indicates intermolecular interactions as well. Low ering the pH also promotes intermolecular interactions, with severe ag gregation occurring below pH 4. The work demonstrates the utility of A sFlFFF in studies of the hydrodynamic behavior of amphiphilic macromol ecules. (C) 1997 John Wiley & Sons, Inc.