Polycyclic aromatic hydrocarbons (PAHs) are a unique group of compounds for investigations of chromatographic behavior since numerous isomers exist that differ only in the arrangement of the aromatic rings, which provides different molecular shapes and often different degrees of nonplanarity. The influence of molecular shape of PAHs in chromatographic separations was first recognized over 40 years ago for liquid crystalline stationary phases in gas chromatography (GC) and for selected C₁₈ phases in reversed-phase liquid chromatography (RPLC). RPLC C₁₈ phases prepared using trifunctional (polymeric) rather than commonly used monofunctional silanes (monomeric) provide pronounced shape selective separations of PAH isomers. Investigations in the 1980s and early 1990s focused on elucidating the origin of the unique shape selective separation capabilities of the polymeric C₁₈phases. LC methods for determination of PAHs were based on these polymeric C₁₈phases, and LC column manufacturers provided “PAH columns” based on this unique shape selectivity. A molecular shape descriptor, denoted as the length-to-breadth ratio (L/B), and descriptors for nonplanarity were correlated with chromatographic retention. We recently expanded our investigations of shape selective separations with studies of retention behavior of over 350 PAHs, polycyclic aromatic sulfur heterocycles (PASHs), methyl-PAHs, and methyl-PASHs in RPLC, normal-phase (NP) LC, and GC. In RPLC, we expanded our investigations of shape selectivity using polymeric C₁₈phases and correlated L/B and nonplanarity with retention for various isomeric PAHs and PASHs. In NPLC using a polar aminosilane phase, the separation of PAHs is generally related to the number of aromatic carbons; however, nonplanarity of a PAH isomer decreases retention relative to planar isomers. Similar behavior is also observed for PASHs and methyl-PASH in NPLC. In GC, the early liquid crystalline stationary phases exhibited profound shape selectivity, but they were limited by the usable temperature range. A mixed liquid crystalline/methylpolysiloxane phase extended the upper temperature limit. Phases with 50% phenyl methylpolysiloxane exhibit improved PAH isomer separations compared to the commonly used 5% phenyl methylpolysiloxane phases, and some GC stationary phases are now customized for PAH separations. In this presentation, we will describe our current understanding of shape selectivity in LC and GC separations of PAHs and PASHs.

Walter Wilson

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