RETENTION BEHAVIOR OF POLAR COMPOUNDS USING POROUS GRAPHITIC CARBON WITH WATER-RICH MOBILE PHASES

The retention factors of polar compounds (mono-, di- and trisubstituted aromatic derivatives) were measured on porous graphitic carbon (PGC), alkyl-modified silicas and an apolar copolymer (PRP-1) with water-methanol mobile phases. It was first shown that the mobile phase effects were similar with the three sorbents and that the comparison of retention factors extrapolated to aqueous mobile phases (k(w)') could give information on stationary phase-solute interactions. The functional group contribution was examined. For aromatic derivatives containing hydrophobic substituents (alkyl and chloro groups), correlations with octanol-water partition coefficients were obtained for the three sorbents. For aromatic derivatives containing polar substituents, these correlations were obtained only for alkylsilicas and PRP-1. On PGC, the retention factor increased with increase in the number of polar substituents and was shown to depend on both the held and the mutual resonance effects of the different substituents on the aromatic ring. The results indicate that electronic interactions are more important than hydrophobic interactions in the retention mechanism of polar compounds. The parametrization of the polarity of the solutes, taking into account field and resonance effects, was carried out using local dipolar moments and the overall electron-excess charge density. The analyte retention factors could be predicted through correlation between log k(w)' and the electron-excess charge density.