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Whole humic acid extracts are usually too polydisperse for reliable molecular-weight measurement to be made in the ultracentrifuge by the sedimentation velocity technique. Consequently, the humic acid used in this study was fractionated with respect to molecular weight into fractions of low polydispersity by extensive use of gel-permeation chromatography and other fractionation techniques. The sedimentation and diffusion coefficients of the fractions were determined and molecular-weight values calculated. These values ranged approximately from 2 × 103 to 1.5 × 106, the higher figure not necessarily representing the upper limit for these substances. On the basis of the frictional parameters calculated from the experimental data, it is proposed that the molecule adopts the solution conformation of a randomly coiled polymer in which branching may be significant, particularly at higher molecular weights. [ABSTRACT FROM AUTHOR]

Three common gel-chromatography materials have been calibrated for humic acid, using fractionated samples the molecular weights of which were calculated by combining sedimentation coefficients determined on the ultracentrifuge with diffusion coefficients. The gel materials were found to have the following fractionation ranges for humic acid when tris buffer was used: Sephadex G-100, 2500 to 100000; Bio Gel P-150, below 1000 to 40000; Sepharose 6B, 1500 to 2500000. Borax buffer appeared to extend the range of Sephadex G-100 to lower molecular weights. A brief comparison is made of the retention behaviours of humic acid, proteins, and dextrans on the gel materials. [ABSTRACT FROM AUTHOR]