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

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

Oct 1, 2009
By: Daniel W. Armstrong, Tharanga Payagala, Leonard M. Sidisky
LCGC Europe
Volume 22, Issue 9

KEY POINTS

Choices of available stationary phases for gas chromatography (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

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)sulphonamide 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 1(e)] or by using the triflate anion. Examples of these behaviours 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 1(b) and 1(c)]. The NTf2₂^– anion has a more delocalized charge and is more hydrophobic than the triflate anion.

Table 1: 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.

Polar IL Stationary Phases with Low Bleed and High Thermal Stability

Figure 3

Polyethylene glycol (PEG) wax-type coatings are popular and ubiquitous polar stationary phases in GC. The "bleed temperature" can vary somewhat with the molecular weight of the polymers used, and possibly with the pretreatment procedures used on the fused-silica capillary. Typically, the upper temperature limits for these columns are in the 240–280 °C range, depending upon the manufacturer. Figure 3 shows a comparison of the thermal stability–bleed profiles between a typical PEG GC column and a phosphonium IL-based column of approximately the same polarity [see structure in Figure 1(b)]. The "wax" stationary phase starts to show significant bleed at 280 °C and by 350 °C this stationary has been stripped completely from the fused-silica capillary. At the same temperature (350 °C), the phosphonium IL column is just beginning to have detectable bleed. Indeed the lower bleed of many IL columns and smaller, simpler fragmentation at high temperatures results in less interference and lower limits of detection for many GC–mass spectrometry (MS) applications. Figure 4 shows that there are minimal changes in the retention times of a rapeseed oil fatty acid methyl ester (FAME) mixture on the phosphonium IL column after 80 h of conditioning at 300 °C.

12 3