Effects of π-π interactions on the separation of PAHs on phenyl-type stationary phases

Phenyl‐type stationary phase surfaces are useful for the separation of highly aromatic compounds because of the extensive intermolecular forces between the π‐electron systems. For this reason, we studied the retention behaviour and selectivity of polycyclic aromatic hydrocarbons (PAHs) on Synergi polar‐RP and Cosmosil 5PBB chromatography columns using methanol/water, acetonitrile/water, benzene spiked (0.5%) methanol/water, and benzene spiked (0.5%) acetonitrile/water mobile phases. These four solvent systems were employed because π‐π. interactions between the aromatic solute (i.e., PAH) and the aromatic stationary phase should be inhibited in mobile phases that are also π electron rich, and hence a competitor for the analyte. Our results showed that the acetonitrile mobile phases were substantially stronger eluents than the methanol mobile phases, which was consistent with the premise that retention of aromatic compounds is sensitive to π‐π. interactions. Aside from changes in absolute retention, selectivity of the PAHs was also generally greater in methanol rather than acetonitrile mobile phases because the methanol did not attenuate the π‐π. bonding interactions between the PAH and the stationary phase; but, despite this, the retention behaviour of the Synergi polar‐RP column was similar to that observed on C₁₈ columns. The excessive retention times of the Cosmosil 5PBB column were decreased dramatically when acetonitrile was used as the mobile phase; however, selectivity between structural isomers was lost.<br />