Ultra-high thermal stability perarylated ionic liquids as gas chromatographic stationary phases for the selective separation of polyaromatic hydrocarbons and polychlorinated biphenyls

Abstract

Ionic liquids (ILs) are well-known in the field of separation science for their unique selectivity when used as stationary phases in gas chromatography (GC). While a significant amount of knowledge has been attained in correlating structural features of an IL to separation selectivity, developments in producing IL-based stationary phases suitable for high temperature GC studies have lagged behind. Column bleed is a result of the stationary phase undergoing volatilization/decomposition at high temperatures and is undesirable in separations coupled to GC/MS. It has been well-known that traditional classes of ILs with long alkyl side chain substituents are susceptible to Hofmann elimination at elevated temperatures. In this study, a new class of IL stationary phases containing perarylated cations exhibiting improved thermal stability are introduced. These ILs were used to prepare wall-coated open tubular columns with high column efficiency and produced very low bleed at temperatures up to 350 °C. Their unique chemical structures provide stronger π-π interactions compared to many commercially-available stationary phases. To exploit the unique interactions provided by these stationary phases, the separation of two classes of environmentally hazardous aromatic compounds, namely, polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), was examined. Both classes of compounds contain structural isomers with high boiling points that are often challenging to separate. The perarylated sulfonium and phosphonium IL-based stationary phases exhibited excellent thermal stability as well as unique selectivity toward isomers of PAHs as well as toxic PCB analyte pairs.