Capillary isoelectric focusing (CIEF) for the characterization of humic substances

Preparative solution isoelectric focusing was used to fractionate 50 mg of a soil fulvic acid (FA); the harvested fractions were characterized with UV-Vis spectroscopy, gel permeation chromatography and capillary zone electrophoresis (CZE) and showed a distribution in the created pH gradient of the focused fulvic fractions as a function of their molecular weight. Borate complexation was used in CZE to compare the relative acidities of the harvested fractions. Only 32% of the total FA did not interact with the used ampholytes (precipitation), and this FA fraction was mainly composed of low-molecular-weight compounds as shown with CZE. A capillary-free solution isoelectric focusing (CIEF) method was then adapted to the analysis of the same FA, and parallelisms were found in the repartition of the focused fulvic fractions. The potential of capillary electrophoresis in the isoeletric focusing mode (CIEF) is the rapidity of the analysis (40 min) and the use of samples in the microliters range; CIEF was used to characterize humic substances from different natural environments.