D.J. Pinckney R.L. Wershaw 1973 <p>Humic acids, the most abundant organic components of&nbsp;natural water systems, are complex mixtures of molecular aggregates of&nbsp;different chemical and physical properties. The first step in the study of&nbsp;such a mixture is the fractionation of the mixture. The most common&nbsp;approach with humic acids is to attempt to obtain a molecular weight&nbsp;fractionation by gel-permeation chromatography. However, since the&nbsp;preponderance of evidence indicates that the components of humic acid&nbsp;do not fulfill the basic criteria of uniform shape and chemical structure&nbsp;necessary for obtaining a molecular fractionation on gel-permeation&nbsp;media, molecular weight distributions in humic acids cannot be<br />evaluated by this method. A fractionation dependent upon chemical&nbsp;structure can be obtained by the manipulation of elution conditions on<br />a gel-permeation column. This procedure provides a beginning in the&nbsp;isolation and identification of discrete components of humic acids.</p> application/pdf en U.S. Geological Survey The fractionation of humic acids from natural water systems article