COMBINED EFFECTS OF MOLECULAR-SIZE AND ELECTROENDOSMOTIC FLOW ON THE CAPILLARY ELECTROPHORETIC BEHAVIOR OF HUMIC SUBSTANCES

The electroendosmotic flow (EOF), generated by the migration of solvat ed ions near the charged capillary surface, is an important factor in determining the capillary electrophoretic behaviour of humic substance s (HS). We investigated the electrophoretic mobilities of HS fractions of reduced molecular-weight polydispersity extracted from peat and fr om a spodosol either in the presence or after suppression of the EOF W hen the EOF was not suppressed, HS migrated to the cathode in spite of their negative charge. Fractionation of HS according to molecular siz e was achieved in polyacrylamide-coated capillaries filled with 0.05 M tris-phosphate buffer. In uncoated capillaries filled with the same b uffer, all fractions had very close mobilities. Addition of polyethyle ne glycol MW 4000 at concentrations above its entanglement threshold c aused the migration times of larger molecules to increase more than th ose of smaller molecules. The separation was a linear function of mole cular size up to 75 000 g mol(-1) for peat HS and to 50 000 g mol(-1) for HS extracted from the spodosol.