Quadrupolar Effects on the Retention of Aromatic Hydrocarbons in Reversed-Phase Liquid Chromatography

Abstract

Because of their large molecular quadrupole moments, aromatic hydrocarbons can give rise to strong electrostatic interactions with polar molecules such as acetonitrile and water. Such interactions may have some significant effects on the retention and separation of the aromatic compounds in reversed-phase liquid chromatography. We report here an experimental investigation of these effects in which we studied the retention behavior of a series of aromatic hydrocarbons in comparison with that of aliphatic hydrocarbons which have no appreciable quadrupole moments. The elution order of benzene, cyclohexane, and n-hexane was determined and shown to follow inversely the increasing magnitude of the molecular quadrupole moments. A linear dependence of retention on molecular polarizability was observed for both the aliphatics and aromatics, the latter being consistently less retained than the former under the same chromatographic conditions. The molecular polarizability and quadrupole moment were regressed as independent variables against the logarithmic retention factors of both the aliphatic and aromatic solutes, and an excellent correlation was found with all five different chromatographic systems studied. These results clearly show that, while the retention of aromatic hydrocarbons is primarily governed by the dispersion interactions with the nonpolar stationary phase, the electrostatic interactions of the aromatic solutes with the polar mobile phase act to reduce the retention and thus contribute favorably to the control of the relative retention and selectivity in reversed-phase liquid chromatography.