Gas Chromatographic Separation of Isotopic Molecules Using a Cavitand-Impregnated Ionic Liquid Stationary Phase

Abstract

Cavitands, which are a class of compounds with deep open-ended cavities, are known to exhibit remarkable molecular recognition ability through host-guest interactions because of their unique structures. It is known that isotopic molecules can be differentiated in the small spaces provided by completely closed capsules in solution. To determine if this subtle molecular recognition ability extends to cavitands, we have developed a new method to facilitate the use of cavitands as stationary phases (SPs) in gas chromatography (GC). These involve a "pseudo heterogeneous two-phase system"; specifically, ionic liquids (ILs) were used as solvents to coat three cavitands with slightly different structures onto GC columns. When cavitand-coated columns were compared with columns coated with only ILs, it was clear that cavitands not only extended the utilization but also substantially improved separation efficiency of the columns. It was found that cavitand-coated columns can effectively separate many different types of isotopic compounds including aromatic hydrocarbons (mixture of chlorobenzene-h(5) and chlorobenzene-d(5), mixture of 1,2-dichlorobenzene-h(4) and 1,2-dichlorobenzene-d(4)), alcohols (methanol from its corresponding methanol-d, d(3), d(4)), ether, pyridine, and acetonitrile. The results also show that by modifying functional groups of the cavitand, from Et to C(11)H(23) or from amide to carboxylate, we can fully retain the molecular recognition ability of the cavitand. The drastic changes in the polarity of the SP from medium polar to nonpolar, or to polar, greatly extends the applicability of these cavitand-coated SPs. Compared to other GC SPs that are known to separate isotopic molecules, these cavitand-coated SPs can separate a relatively wider range of isotopic compounds at relatively lower temperature, with shorter column length and higher efficiency.