HUMIC SUBSTANCES Capillary Zone Electrophoresis

Humic substances (HS) which are widespread in the soil, present structural complexity and polyelectrolyte properties. Many techniques and methods have been used to obtain more information about them, as reviewed by Davies and Ghabbour in 1999. The characterization of HS has been the focus of intense research for many years because the organic matter in soil contributes to the quality of the soil more than the other constituents. Nowadays, one of the main goals is the separation of HS into fractions which can then be studied independently. The aim is to determine HS structure, which remains unknown, despite the enormous efforts and remarkable progress in this Reld. HS exhibit properties similar to weak acid polyelectrolytes, presenting a wide range in molecular weights, solubilities and acid strength. They are sparingly soluble in acid and slightly acidic solutions, but solubility increases with pH and they are only fully soluble alkaline solutions. Many general hypothetical models of HS structure have been proposed, and most of these have been discussed by Shevchenko and Bailey. As HS play an important role in environmental chemistry, it is important to know their properties. HS are very important as regards the quality and productivity of a soil and in retention of metalions and pollutants by the environment. Dissolved humic material has a tendency to interact with these compounds, complexing metal ions or pollutants, which may alter their fate and transfer in the soil. The mechanisms involved in the interaction of these compounds with HS are not clear and may vary depending on the physicochemical properties of the compounds, soil pH, redox status and the heterogeneous structure of HS. The effect of pH, for example, is related to the degree of humic R-COOH and R-OH dissociation; deprotonation increases the polarity of the humic material and altering its structure. Electrophoretic methods have been applied for the purpose of HS studies since the 1960s. The Rrst work on the separation of HS by a capillary technique was performed in 1991 using capillary isotachophoresis. However, few really suitable methods for the separation of HS exist. Nevertheless, to understand HS behaviour, it is useful to perform separation into fractions and determine the structure of each fraction. Capillary zone electrophoresis (CZE) has been applied by Rigol et al. in 1994 for this purpose.