Elucidating the Shape Selectivity for Polycyclic Aromatic Hydrocarbons In Reversed-Phase Liquid Chromatography

Reversed-phase liquid chromatography (RPLC) is the foremost technique for the separation of analytes that have very similar chemical functionalities, but differ only in their molecular shape. This ability is crucial in the analysis of various mixtures with environmental and biological importance including polycyclic aromatic hydrocarbons (PAHs) and steroids. A large amount of effort has been devoted to studying this phenomenon experimentally, but a detailed molecular-level description remains lacking. To better understand the mechanism of shape selectivity in RPLC, particle-based simulations using efficient Monte Carlo sampling algorithms and accurate force fields were carried out. The retention of aromatic hydrocarbons ranging in size from benzene to 6-ring PAHs was examined for octadcylsilane RPLC systems with various surface coverages. The simulations yield precise and accurate separation factors for these complex analytes.  Structural analysis of the simulation trajectories shows that the stationary phase acts as a very heterogeneous environment where analytes with different shapes prefer different spatial regions with specific local bonding environments of the ODS chains. However, these favorable retentive regions cannot be described as pre-existing cavities. Rather, the chain conformation in these local stationary-phase regions adapts to accommodate the analytes.