Ionic cyclodextrins in ionic liquid matrices as chiral stationary phases for gas chromatography

High-resolution performance of pillar[6]arene functionalized with imidazolium ionic liquids for gas chromatography

Pillar[n]arenes (P[n]A, n = 5-10) have attracted much attention because of their highly symmetric pillar-shaped architecture with π-electron rich cavity. Nevertheless, the use of ionic liquid functionalized P[n]A in chromatography has not been reported up to data. This work reports the investigation of the imidazolium ionic liquids functionalized pillar[6]arene (P6A-C10-IM-C8[NTf2]) as the stationary phase for gas chromatography (GC). The statically coated P6A-C10-IM-C8[NTf2] column (0.25 mm i.d.) showed moderate polarity and high column efficiency of 4733 plates/m determined by n-dodecane at 120 °C (k = 2.29). Owing to its unique amphiphilic conformation, the P6A-C10-IM-C8[NTf2] showed good column inertness and resolving capability for a wide range of analytes and isomers. Particularly, the P6A-C10-IM-C8[NTf2] column exhibited distinctly advantageous performance for the challenging isomers of halogenated benzenes, benzaldehydes, phenols and anilines over the common commercial columns, namely 5% phenyl methyl polysiloxane (HP-5) and 35% phenyl methyl polysiloxane (HP-35). In addition, it exhibited good column repeatability and reproducibility with RSD values on the retention times less than 0.05% for run-to-run, 0.38% for day-to-day and 2.94% for column-to-column, respectively. This work demonstrates the promising future of ionic liquid P[n]A stationary phases for chromatographic separations.

Evaluation of Cryogen-Free Thermal Modulation-Based Enantioselective Comprehensive Two-Dimensional Gas Chromatography for Stereo-Differentiation of Monoterpenes in Citrus spp. Leaf Oils

This study evaluates the applicability of enantioselective gas chromatography (eGC) and enantioselective comprehensive two-dimensional gas chromatography (eGC×GC) coupled with flame ionization detection for the stereospecific analysis of designated chiral monoterpenes within essential oils distilled from the leaves of Citrus hystrix (CH), C. limon (CL), C. pyriformis (CP), and C. microcarpa (CM). A cryogen-free solid-state modulator with a combination of enantioselective first-dimension and polar second-dimension column arrangements was used to resolve potential interferences in Citrus spp. leaf oils that can complicate the accurate determination of enantiomeric compositions. Interestingly, considerable variations were observed for the enantiomeric fractions (EFs) of the chiral terpenes. (+)-limonene was identified as the predominant enantiomer (60.3-98.9%) in all Citrus oils, (+)-linalool was the major enantiomer in CM (95.9%), (-)-terpenin-4-ol was the major isomer in CM (66.4%) and CP (61.1%), (-)-α-pinene was the dominant antipode in CL (55.5%) and CM (92.1%). CH contained (-)-citronellal (100%) as the pure enantiomer, while CL and CP have lower proportions (9.0-34.6%), and citronellal is absent in CM. The obtained enantiomeric compositions were compared and discussed with results from eGC using the same enantioselective column. To our knowledge, this work encapsulates the first report that details the EFs of these chiral monoterpenes in Citrus spp. leaf oil.

Performance and selectivity of amphiphilic pillar[5]arene as stationary phase for capillary gas chromatography

Pillar[n]arenes possess highly symmetrical and rigid pillar-shaped architecture with π-electron rich cavity that afford their reliable host-guest recognition interactions towards matched guests. In this work, a novel amphiphilic pillar[5]arene (P5A-C10-2NH₂) was designed, synthesized and employed as the stationary phase for capillary gas chromatography. To date, they have not been reported in the field of chromatography. The P5A-C10-2NH₂ capillary column (10 m × 0.25 mm i.d.) was prepared by static coating method. Its capillary column exhibited moderate polarity and column efficiency of 2265 plates/m determined by naphthalene at 120 °C. As evidenced, the P5A-C10-2NH₂ column achieved advantageous separation performance for a mixture of 24 analytes of diverse types and exhibited different chromatographic selectivity from two pillar[5]arene derivatives columns and commercial HP-35 column with 35%-phenyl-methylpolysiloxane. Moreover, the P5A-C10-2NH₂ column baseline resolved more than a dozen positional and cis-trans isomers. Furthermore, the separation mechanism of P5A-C10-2NH₂ column was discussed by quantum chemical calculations. In addition, the P5A-C10-2NH₂ column had high thermal stability and excellent separation repeatability 0.01-0.04% for run-to-run, 0.03-0.17% for day-to-day and 3.2-3.9% for column-to-column. The special amphiphilic structure and high resolution for various analytes reveal the good potential of pillararenes as a new class of stationary phases for chromatographic analyses. Moreover, the TPG column achieved improved thermal stability over the GIL column and excellent repeatability.

Chiral stationary phases and applications in gas chromatography

Chiral compounds are ubiquitous in nature and play a pivotal role in biochemical processes, in chiroptical materials and applications, and as chiral drugs. The analysis and determination of the enantiomeric ratio (er) of chiral compounds is of enormous scientific, industrial, and economic importance. Chiral separation techniques and methods have become indispensable tools to separate chiral compounds into their enantiomers on an analytical as well on a preparative level to obtain enantiopure compounds. Chiral gas chromatography and high-performance liquid chromatography have paved the way and fostered several research areas, that is, asymmetric synthesis and catalysis in organic, medicinal, pharmaceutical, and supramolecular chemistry. The development of highly enantioselective chiral stationary phases was essential. In particular, the elucidation and understanding of the underlying enantioselective supramolecular separation mechanisms led to the design of new chiral stationary phases. This review article focuses on the development of chiral stationary phases for gas chromatography. The fundamental mechanisms of the recognition and separation of enantiomers and the selectors and chiral stationary phases used in chiral gas chromatography are presented. An overview over syntheses and applications of these chiral stationary phases is presented as a practical guidance for enantioselective separation of chiral compound classes and substances by gas chromatography.

Chemical Vapor Deposition of Ionic Liquids for the Fabrication of Ionogel Films and Patterns

Film deposition and high-resolution patterning of ionic liquids (ILs) remain a challenge, despite a broad range of applications that would benefit from this type of processing. Here, we demonstrate for the first time the chemical vapor deposition (CVD) of ILs. The IL-CVD method is based on the formation of a non-volatile IL through the reaction of two vaporized precursors. Ionogel micropatterns can be easily obtained via the combination of IL-CVD and standard photolithography, and the resulting microdrop arrays can be used as microreactors. The IL-CVD approach will facilitate leveraging the properties of ILs in a range of applications and microfabricated devices.

Recent advances of the ionic chiral selectors for chiral resolution by chromatography, spectroscopy and electrochemistry

Ionic chiral selectors have been received much attention in the field of asymmetric catalysis, chiral recognition, and preparative separation. It has been shown that the addition of ionic chiral selectors can enhance the recognition efficiency dramatically due to the presence of multiple intermolecular interactions, including hydrogen bond, π-π interaction, van der Waals force, electrostatic ion-pairing interaction, and ionic-hydrogen bond. In the initial research stage of the ionic chiral selectors, most of work center on the application in chromatographic separation (capillary electrophoresis, high-performance liquid chromatography, and gas chromatography). Differently, more and more attention has been paid on the spectroscopy (nuclear magnetic resonance, fluorescence, ultraviolet and visible absorption spectrum, and circular dichroism spectrum) and electrochemistry in recent years. In this tutorial review as regards the ionic chiral selectors, we discuss in detail the structural features, properties, and their application in chromatography, spectroscopy, and electrochemistry.

Recent advances in the enantioseparation promoted by ionic liquids and their resolution mechanisms

More and more various chemical media are being applied in enantioseparation; among them, ionic liquids (ILs) have attracted the long-term attention in this decade as green designable solvents. This paper provides comprehensive overview for the applications of ILs in chiral extraction, gas chromatography, liquid chromatography, capillary electrophoresis and other techniques for enantioseparation. Additionally, the important resolution mechanisms based on ILs have also been summarized and discussed. This review focuses on the latest development of enantioseparation methods by using ILs in various modes, leading to meaningful and valuable information to related fields and thus promotes further research and application of reported methods.

Ionic liquids as stationary phases in gas chromatography: determination of chlorobenzenes in soils

The present research focuses on the evaluation of different ionic liquid (IL) stationary phases in gas chromatography. The different IL columns were evaluated in terms of peak resolution (Rs) and peak symmetry for the separation of the chlorobenzenes. The determination of chlorobenzenes in soil samples by means of the optimal IL stationary phase (SLB-IL82) is proposed as an application. Soil pretreatment was based on a simplified quick, easy, cheap, effective, rugged, and safe extraction procedure and a large injection volume via a programed temperature vaporizer working in solvent vent mode. The retention time of the chlorobenzenes increased as the polarity of the IL column decreased. SLB-IL82 is the stationary phase that provides the best values as regards Rs and asymmetry factor. Soil sample blanks were spiked with the analytes before subjecting the sample to the extraction process. The existence of a matrix effect was checked and the analytical characteristics of the method were determined in a fortified garden soil sample. The method provided good linearity, good repeatability and reproducibility values, and the LODs were in the 0.1-4.7 μg/kg range. Two fortified soil samples were applied to validate the proposed methodology.

Imidazolium-based ionic liquids grafted on solid surfaces

Supported ionic liquids (SILs), which refer to ionic liquids (ILs) immobilized on supports, are among the most important derivatives of ILs.

Evaluation of the enantioselectivity of glycogen-based synergistic system with amino acid chiral ionic liquids as additives in capillary electrophoresis

In this paper, two novel amino acid chiral ILs, tetramethylammonium-l-arginine (TMA-l-Arg) and tetramethylammonium-l-aspartic acid (TMA-l-Asp), were applied for the first time in CE enantioseparation to evaluate their potential synergistic effect with glycogen as chiral selector. As observed, significantly improved separation of tested enantiomers were obtained in the chiral ILs/glycogen synergistic systems compared to the single glycogen separation system. Several primary parameters affecting the enantioseparation, such as amino acid ILs (AAILs) concentration, glycogen concentration and buffer pH, were systematically investigated. An achiral tetramethylammonium hydroxide ionic liquid (TMA-OH) modified separation system was also evaluated to validate the superiority of the novel chiral ILs/glycogen synergistic systems. To further optimize the overall synergistic systems, the effect of three other parameters, including buffer concentration, applied voltage and capillary temperature were simultaneously analyzed by a central composite design (CCD), and excellent enantioseparations were achieved with the optimized parameters. The results indicate that the application of chiral ILs/glycogen synergistic systems is a promising way in chiral separation science.

Ionic liquids coated multi-walled carbon nanotubes as a novel pseudostationary phase in electrokinetic chromatography

A new CE system using ionic liquids coated multi-walled carbon nanotubes (ILs-MWNTs) as pseudostationary phase was developed for the simultaneous determination of four flavonoids, four phenolic acids and two saponins. Several parameters affecting the separation were studied, including the choice of ILs, ILs-MWNTs concentration, the respective use of ILs and MWNTs, buffer pH, SDS concentration and borate content. Results revealed that the addition of ILs-MWNTs in running electrolytes enhanced the separation of target compounds compared to conventional micelle because the surface of carbon nanotubes interacted favorably with the analytes. Under the optimum conditions, a baseline separation was achieved for these analytes within 11 min in a 41.5 cm of effective length fused-silica capillary. At a voltage of 28.0 kV, the separation was carried out in a 10mM borate buffer (pH 9.0) containing 100mM SDS, 6% propanol and 4 μg mL(-1) ILs-MWNTs. All calibration curves showed good linearity (r(2)>0.9990) within the test ranges. The intra- and inter-day precisions as determined from standard solutions were below 3.30% and 6.23%, respectively. The recoveries for ten compounds were found to range from 85.5 to 101.8%. The method was successfully applied for the determination of three types of compounds in Qishenyiqi dropping pills. Our experimental results indicated that the proposed method offered new opportunities for the analysis of complex samples.

Application of Quaternary Aminated β-Cyclodextrin Ionic Liquid Stationary Phase in Capillary Gas Chromatography

Two kinds of quaternary aminated β-cyclodextrin ionic liquids were synthesized and used as stationary phases in capillary gas chromatography. The influences of different counter ions (tetra- borofluoride, bis-trifluoromethanesulfonylimide) were studied. The results indicated that quaternary aminated β-cyclodextrin ionic liquids possessed good column coating properties and a chromatographic separation abilities. The Grobs mixture, alkanes, alcohols and aromatic position isomers, such as xylene, nitrotoluene, dichlorobenzene and cresol were separated well. These quaternary aminated β-cyclodextrin ionic liquids have potential to separate some complex compounds including alcohol, amine, ester etc.