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.

Ionic liquid stationary phases for gas chromatography

“…A rapidly developing application of ionic liquids is their ability to dissolve or form fluid dispersions with materials difficult to handle using molecular solvents. In gas chromatography, this has resulted in the use of ionic liquids to dissolve or disperse methylated cyclodextrins 93, ionic cyclodextrins 94, single‐walled carbon nanotubes 95, fullerenes 96, cavitands 97, and metallomesogens 98. Ionic cyclodextrins dissolved in di‐ or tricationic ionic liquids demonstrated a broader enantioselectivity, higher thermal stability, and higher efficiency for the separation of enantiomers than a commercially available column containing the analogous non‐ionic cyclodextrin selector dissolved in a poly(siloxane) stationary phase 94.…”

“…The dispersion of carbon particles (nanotubes or fullerenes) in ionic liquids facilitated variation of the relative retention of compounds able to interact with the carbon particles 95, 96. Ionic liquids containing cavitands (compounds with deep open‐ended cavities) demonstrated a remarkable separation of isotopic compounds based on the molecular recognition ability of the additive through formation of host–guest interactions and the high efficiency of the columns 97.…”

The use of novel ionic liquid‐in‐water microemulsion without the addition of organic solvents in a capillary electrophoretic system

“…In the recent years, IL has received more attention thanks to negligible vapor pressure, good thermal stability and high electrical conductivity [17]. Because of intriguing physicochemical properties, IL has shown tremendous applications in various analytical fields, such as sample preparation [18], electrochemistry [19], chromatography [20], spectroscopy [21] and an unorthodox matrix for analysis [22]. Among them, increasing number of investigations by employing IL in CE have provided significant advantages in the sample treatment [23], chiral analysis [24,25], the use of PSP [26,27], the addition of chemical modifier [28] and the support coating [29].…”

Binding of the ionic liquid cation 1-alkyl-3-methylimidazolium to p-tetranitrocalix[4]arene probed by fluorescent indicator displacement

“…Finally, Tran et al. [16] demonstrated the gas chromatographic separation of isotopic species such as halobenzenes and alcohols using a cavitand-impregnated IL stationary phase.…”