Flow field-flow fractionation (flow FFF) is a mild separation technique capable of fractionating macromolecules and colloids in the range from 1 to 0.001 µm. Detailed FFF theory and the principles of fractionation, FFF sub-techniques [11-16] and instrumentation are discussed elsewhere [14-18]. Macromolecular size fractionation is based on the molecular diffusion coefficients in the carrier medium in a long, ribbon-like hollow channel, the floor of which is a semi-permeable membrane. Two forces are applied perpendicular to each other: a cross flow that enters through the channel ceiling and exits through the membrane floor and the second flow, called channel flow, runs parallel to the membrane axis. Typically, a small aliquot of a sample such as the fulvic acids under investigation is injected into the fractionation channel. Channel flow is momentarily stopped, and the cross flow is introduced. This cross flow facilitates the accumulation of macromolecules against the membrane, which is impermeable to macromolecules larger than the membrane molar mass cutoff but lets the cross flow fluid pass through. During this relaxation period, steady state equilibrium is achieved between the cross flow driving force and macromolecular back diffusion corresponding to the field induced velocity and the molecular diffusion coefficients. This leads to a diffusion gradient in which the small macromolecules are located away from the membrane wall while large macromolecules are positioned close to the wall.