Young Guns: Ionic Liquids for GCxGC

Comprehensive two-dimensional gas chromatography (GC×GC) is a step up from standard GC and involves connecting two distinct columns each possessing stationary phases with different selectivities to achieve high resolving power.

My laboratory is interested in designing new generations of stationary phases that can be used for targeted analyte separations. In the pursuit of new stationary phase materials, there are a number of properties that must be met. The stationary phase must exhibit high thermal stability in order to provide low column bleed at high temperatures. This feature is particularly important when coupling GC to mass spectrometry detectors. The phase material should be chemically inert, ideally possess high viscosity, and be capable of effectively wetting the surface of fused silica capillary columns.

While the basic features above are important, the resulting separation selectivity is the most important property exploited by the separation scientist. Polysiloxanes are the most popular GC stationary phases due their tunable polarities and excellent long-term stability. My laboratory has been involved with the synthesis and development of ionic liquid (IL) stationary phases in GC×GC. In particular, we have focused on employing these materials towards complex samples, particularly petrochemicals.

ILs are interesting because their overall physico-chemical properties are dependent on the cation and anion combination. I like to think of ILs as non-molecular solvents that contain two personalities (cation and anion). When analytes solvate into ILs, they can interact through hydrogen bonding interactions, dispersive-type interactions, π−π interactions, and dipolar interactions. These interactions originate from the cation and/or anion and can be highly customizable. Our understanding of how ILs solvate analytes has been greatly improved using linear free energy relationships, such as the Abraham solvation parameter model.

My view of ILs has morphed considerably in the 15 years that I have worked in the field; at first, most of the research was based on a collection of nearly a half dozen ILs. As our understanding of IL structure/separation selectivity has improved, my group has ventured to exploit our synthetic capabilities to rationally design new IL structures that possess the aforementioned essential properties but that also include structural motifs that impart the desired selectivity to the separation. Let’s consider the separation of kerosene by GC×GC. The most common approach would be to employ a nonpolar primary column (5% phenyl, 95% methyl polysiloxane [HP-5]) and a polar second dimension column (polyethylene glycol [PEG]). Such a column set can often provide good selectivity of the nonpolar, aliphatic compounds while also providing good resolving power of aromatics. However, when higher temperatures are required, most standard PEG columns begin to show unwanted activity above 270°C. You might expect that the high thermal stability of ILs could address this issue presented by PEG-based stationary phases. However, while ILs can generally be regarded to as “polar” stationary phases, most general IL stationary phases provide good separation selectivity of aromatic compounds, but provide little to no separation of the aliphatic compounds. This result can be advantageous if you are seeking to utilize the separation space for the resolution of aromatic compounds. My group was interested in exploring the structural characteristics of ILs that might impart the needed selectivity to also separate the nonpolar fraction of kerosene.

In recent work from our laboratory, we have shown that the cationic component of the IL plays a vital role in providing the separation of nonpolar aliphatic molecules. Specifically, we synthesized highly alkylated phosphonium and imidazolium cations (monocationic and dicationic platforms), paired these with a number of different anions, and observed that ILs possessing low cohesive forces generally provided the necessary selectivity to separate the nonpolar aliphatic molecules in GC×GC. The role of the anion in the GC×GC analysis of petrochemicals is not as pronounced as the cation, but choosing weakly-coordinating anions that produce low melting salts is important. We have identified monocationic and dicationic ILs that have increased the thermal stabilities well above 300°C, but we are not yet satisfied. We will strive to increase thermal stability while preserving selectivity. We are also interested in exploiting this “structural tunability” feature of ILs and expanding it to the separation of other complex samples.

If you are interested in IL stationary phases for GC×GC analysis, please remember that, just like polysiloxanes, no two ILs are the same. As the popularity of these unique stationary phases expand, I am confident that new generations will be designed to meet the needs of GC×GC users. Applications involving IL stationary phases in GC×GC are still in their infancy but I believe that their future is very bright and that this class of stationary phases will find an important niche in the separation scientist’s toolbox.

The new sheriff in town  by Daniel Armstrong

Wrangling orthogonality in multidimensional chromatography  by Michelle Camenzuli

Reigning in multidimensional data  by James Harynuk

Vacuum-uv lone star versus ms  by Kevin Schug

Rodeo champs  by Mark Schure

Young guns: ionic liquids for gcxgc  by Jared Anderson