| Abstract
| - Capillary electrophoresis (CE) and fluorescence correlationspectroscopy (FCS) were employed to determine electrophoretic mobilities and hydrodynamic sizes of three humicsubstances (IHSS aquatic fulvic acid (FA), IHSS aquatichumic acid (HA), and IHSS peat humic acid (PHA)) as afunction of pH and ionic strength. A slight aggregationcorresponding to the formation of dimers and trimers wasobserved at low pH using fluorescence correlationspectroscopy (FCS). For example, for the peat humic acid,diffusion coefficients decreased from 2.1 × 10-10 m2 s-1at pH 4 to 2.4 × 10-10 m2 s-1 at pH 11. For all three humicsubstances, electrophoretic mobilities were also shownto decrease significantly below pH 6. Calculated zeta potentialsobserved at high pH of −69 mV (FA), −62 mV (HA), and−63 mV (PHA) decreased to −39, −50, and −47 mV,respectively, under slightly acidic pH (4.5−4.8) conditions.No evidence of ionic strength induced aggregation wasfound using fluorescence correlation spectroscopy (FCS): diffusion coefficients increased slightly (<25%) withincreasing ionic strength (up to 1 M). Negative electrophoreticmobilities decreased to a maximum measured ionicstrength of 0.18 M. Above this ionic strength, no peakswere observed due to an increased HS adsorption to thecapillary wall and an important decrease in electroosmoticflow. Interpretation of electrophoretic mobilities determinedby CE is complicated by the fact that under certainconditions, HS appeared to be complexed by CE buffersystems, including MES, BES, and AMPSO.
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