Ionic liquids (ILs) are a class of organic salts that meet many of the requirements of GC stationary phases including high thermal stability, high viscosity, and tunable selectivity through the modification of the chemical structure. IL-based columns, when incorporated either in the first or second dimension, can offer unique selectivity compared to polydimethyl(siloxane) and poly(ethyleneglycol) derived GC stationary phases for the separation of complex samples by multidimensional gas chromatography. In addition, IL-based columns are emerging as superior choices for applications requiring high polarity as well as high thermal stability. The present contribution provides an overview on IL-based stationary phases for multidimensional gas chromatography with an emphasis on developments in the period from 2012 to early 2018. The analysis of various analytes (e.g., fatty acids, polycyclic aromatic sulfur heterocycles, and biodiesels) in complex matrices as well as the developments of new IL-based stationary phases for multidimensional gas chromatography are described.

Introduction

Multidimensional gas chromatography (MDGC) is one of the most useful tools available for the analysis of volatile and semi-volatile analytes in complex mixtures. MDGC typically combines two or more independent separation mechanisms to increase the peak capacity and improve the resolution of unresolved regions within the 1D chromatographic separation. Heart cutting multidimensional gas chromatography (H/C MDGC) typically connects two columns using a flow-switching device which directs a specific segment of the primary column effluent into the secondary column to improve the separation of the heart-cut region. In comparison, comprehensive two-dimensional gas chromatography (GC × GC) connects two columns possessing different selectivities (i.e., nonpolar × polar or polar × nonpolar column configuration) to maximize peak capacity. All eluted compounds from the first column are transferred as a pulseinto a second column. For more information regarding the fundamentals of instrumental principles for GC × GC and H/C MDGC, readers are encouraged to consult a number of 40 excellent review articles [1-4]. The analysis of complex samples such as petrochemicals, food additives, forensic samples, fragrances, and pharmaceutical compounds requires more and more powerful analytical tools due to the increasing demand for high resolution, high sensitivity, and information rich analysis. The improvement of analyte detection limits, peak resolution, and peak capacity requires the constant development of detectors, modulators, data analysis software, and GC stationary phases that exhibit unique selectivity in MDGC. Recent developments of instrumentation and data analysis methods in MDGC have been reviewed by Synovec and co48 workers [5]. Although advanced instrumentation and methods can improve separation results, the selectivity, thermal stability, and inertness of the GC stationary phase always plays an important role in the resolution, retention order, and analyte distribution in MDGC. of IL-based stationary phases have been commercialized and have received increasing attention due to their high thermal stability, low vapor pressure, and unique separation selectivities [7]. The chemical structure of ILs can be tailored to undergo various solvation interactions and ultimately provide unique chromatographic selectivities. In addition, the chemical structures of ILs can be modified to produce highly viscous products possessing high thermal stability. It is of continued interest to design new IL-based stationary phases that can be coated to possess high separation efficiencies as well as provide enhanced maximum allowable operating temperatures (MAOT) that rival other commercially available stationary phases. Compared to more 60 conventional stationary phases based on polydimethyl(siloxane) (PDMS) and poly(ethyleneglycol) (PEG), IL-based stationary phases are commonly considered as highly polar phases based on the overall polarities derived from their McReynolds constants [8]