The Advent and Potential Impact of Ionic Liquid Stationary Phases in GC and GCxGC

Jul 31, 2009

Ronald E. Majors
Choices of available stationary phases for GC have been fairly constant for many years. The same basic types of columns that do analogous separations can be obtained from any of a large number of sources, worldwide. Recent research has indicated that unique new substances have been developed that will play an important role in GC column technology. These substances are ionic liquids (ILs). ILs are solvents in which the constituents consist entirely of ions. By definition, they are pure salts that have melting points below 100 °C. However, when used as GC stationary phases, melting points in the range of ~ –40 °C to 50 °C are preferable. ILs have a number of properties that make them exceptional stationary phases. For example, their viscosity can be varied over a broad range, they can have high thermal stabilities, they can be coated on fused-silica capillaries with high efficiencies, they have unique solvent properties, and they can be immobilized and crosslinked (1–7). Indeed, it was noted early on that IL stationary phases had a dual nature in that they separated nonpolar analytes as if they were nonpolar stationary phases and simultaneously separated polar analytes as if they were polar stationary phases (8).

Figure 1
Another very important aspect of IL stationary phases is that their physico-chemical properties are almost infinitely tunable. Tunability is a characteristic that is unavailable to all other classes of GC stationary phases. With relatively simple synthetic modifications or changes to an IL's cation, anion, the substituents thereon, and their linkage chains, one can alter and control whatever solvent and selectivity characteristics that are desired (9–11).

Types of ILs for GC Use

Figure 2
Figure 1 shows the structures of typical "tunable," high-stability cations that have been shown to be particularly useful as GC stationary phase components. Figure 2 shows the structure of two of the more common anions used in IL–GC formulations. The bis(trifluoromethane)sulfonamide anion (NTf2-) tends to produce ILs with lower melting points and somewhat lower polarities than the triflate anion (TfO-). Also, it provides excellent peak shapes for nonhydrogen bonding or weakly hydrogen bonding analytes, but tends to produce tailing peaks for alcohols, carboxylic acids, and amines. The tailing peaks of these strong hydrogen bonding analytes can be minimized or eliminated by masking the effect of the NTf2- anion by using a cation containing an amide moiety (see trigonal cation in Figure 1e) or by utilizing the triflate anion. Examples of these behaviors will be shown throughout this monograph. The cations (Figure 1) can be further selected and varied to emphasize or deemphasize any known solvation interactions including: n/π, dipolar, H-bond acidity, H-bond basicity, and dispersion interactions. Clearly, the hydrocarbon linkage chains (connecting the charged moieties) produce less polar stationary phases than polyethylene glycol types. Shorter linkage chains result in more polar stationary phases than analogous longer chains. Imidazolium cations have a delocalized positive charge in contrast to phosphonium and pyrrolidinium cations (Figures 1b and 1c). The NTf2- anion has a more delocalized charge and is more hydrophobic than the triflate anion.

Table I: Areas where ionic liquid stationary phases will impact GC
While it is difficult to predict the future, it is clear that IL stationary phases will have a direct impact on specific areas of GC. Four representative areas are listed in Table I. Examples of IL-based separations involving each of these "impact areas" will be presented and discussed.