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).
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 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.
Perspectives in Modern HPLC: Michael W. Dong is a senior scientist in Small Molecule Drug Discovery at Genentech in South San Francisco, California. He is responsible for new technologies, automation, and supporting late-stage research projects in small molecule analytical chemistry and QC of small molecule pharmaceutical sciences. LATEST: Seven Common Faux Pas in Modern HPLC