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


LCGC North America
pp. 596-605


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.


ADVERTISEMENT

blog comments powered by Disqus
LCGC E-mail Newsletters
Global E-newsletters subscribe here:




 

LCGC COLUMNISTS 2014

Sample Prep Perspectives | Ronald E. Majors:

LCGC Columnist Ron Majors, established authority on new column technologies, keeps readers up-to-date with new sample preparation trends in all branches of chromatography and reviews developments in existing technology lines.

LATEST: The Role of Selectivity in Extractions: A Case Study

History of Chromatography | Industry Veterans:

With each installment of this column, a different industry veteran covers an aspect of the evolution and continued development of the science of chromatography, from its birth to its eventual growth into the high-powered industry we see today.

LATEST: Georges Guiochon: Separation Science Innovator

MS — The Practical Art| Kate Yu:
Kate Yu is the editor of 'MS-The Practical Art' bringing her expertise in the field of mass spectrometry and hyphenated techniques to the pages of LCGC. In this column she examines the mass spectrometric side of coupled liquid and gas-phase systems. Troubleshooting-style articles provide readers with invaluable advice for getting the most from their mass spectrometers.

LATEST: Mass Spectrometry for Natural Products Research: Challenges, Pitfalls, and Opportunities


LC Troubleshooting | John Dolan:

LC Troubleshooting sets about making HPLC methods easier to master. By covering the basics of liquid chromatography separations and instrumentation, John Dolan, Vice President of LC Resources and world renowned expert on HPLC, is able to highlight common problems and provide remedies for them.

LATEST: LC Method Scaling, Part I: Isocratic Separations

More LCGC Chromatography-Related Columnists>>

LCGC North America Editorial Advisory Board>>

LCGC Europe Editorial Advisory Board>>

LCGC Editorial Team Contacts>>


Source: LCGC North America,
Click here