Nonaqueous packed capillary electrokinetic chromatographic separations of large polycyclic aromatic hydrocarbons and fullerenes.

Abstract

Polycyclic aromatic hydrocarbons (PAH) and a fullerene mixture (C60/C70) were separated with capillary columns (50 microns ID) packed with a reversed-phase packing (octadecylsilica, 3 microns diameter) using electrokinetic pumping. Nonaqueous (acetonitrile modified with methylene chloride or tetrahydrofuran, THF) mobile phases were used for these experiments. The effects of mobile phase composition on such factors as electroosmotic flow, plate height, and capacity factor (k') are reported. The less polar solvents methylene chloride and THF produced predictable reductions in k' when used to modify an acetonitrile mobile phase. Large amounts of the less polar modifiers (50% v/v) also resulted in a fourfold decrease in flow rate. This meant that even with a decrease in k', the retention time increased. Nonaqueous capillary electrokinetic chromatographic (CEC) separations gave efficiencies as high as 160,000 plates/m. Use of nonaqueous mobile phases provided small currents which in turn diminished the role of heating effects on efficiency. The nonaqueous system also provided greater solubility for the hydrophobic solutes. A Van Deemter plot for an acetonitrile mobile phase was obtained that exhibited expected trends in plate height with flow rate and k'. Solvent rinses with water and THF are shown to have only small effects on retention and flow rate.