Anomalous behavior of selected methyl-substituted polycyclic aromatic hydrocarbons in reversed-phase liquid chromatography

Abstract

In the reversed-phase liquid chromatographic separation of polycyclic aromatic hydrocarbons (PAHs) on C 18 phases, methyl-substituted PAHs are expected to elute after the unsubstituted parent PAH based on the increase in hydrocarbonaceous contact area of the methyl-substituted PAHs. However, we have observed that several methyl-substituted PAHs elute prior to the parent compound, e.g., 1-methylperylene, 1-methylpicene and 13-methylpicene. To investigate this anomalous retention behavior, the retention characteristics of all methyl-substituted isomers of chrysene, picene and perylene were compared on a series of sixteen commercially prepared C 18 phases. The anomalous retention behavior was observed only on polymeric C 18 phases ( i.e., those prepared using trifunctional silanes) whereas the methyl-substituted PAHs elute after the parent PAH, as would be expected, on monomeric C 18 phases ( i.e., those prepared using monofunctional silanes). The early elution of some of these methyl-substituted isomers is related to the non-planarity of these PAHs due to the presence of the methyl group in the so-called “bay-region” of the PAH structure. The non-planarity of these methyl-PAHs can be characterized by the dihedral angle of distortion between the aromatic rings.