30 May / June 2014
Evaluation of a New Ionic Liquid GC Stationary Phase with PEG- Like Selectivity
by Leonard M. Sidisky, Greg A. Baney, James L. Desorcie, Gustavo Serrano, Daniel L. Shollenberger, and Katherine K. Stenerson Supelco- Division of Sigma Aldrich, 595 North Harrison Road, Bellefonte, PA 16823
Ionic liquids are a class of solvents with low melting points that consist of organic cations associated with inorganic or organic anions. (Figure 1) These compounds exhibit ideal properties for a stationary phase in gas chromatography such as very low vapour pressure and high thermal stability [1]
The application of new dicationic liquids as stationary phases improved the efficiency as well as the thermal stability of the phases. [2] These unique combinations of cations and anions can provide a variety of different and unique selectivity’s when used as capillary GC stationary phases.
This work focuses on the SLB-IL60 ionic liquid phase which has selectivity similar to a polyethylene glycol (PEG) or Wax phase as was previously discussed by Zeng,
et.al. [3], Cagliero, et. al [4] Ragonese, et. al [5] and Poole [6]. By injecting a series of non-polar and polar analytes, a number of the ionic liquid columns were evaluated. The new phase based on the phosphonium ionic liquids was applied to several challenging applications usually analysed on a WAX phase. The unique selectivity and high thermal stability of the new phase allowed the development of unique and improved GC separations for these applications.
Results and Discussion
Polarity Classification of Ionic Liquid GC Phases
In order to obtain a single parameter for comparing the relative polarity of the ionic liquid phases to classical and conventional GC phases a polarity scale was suggested by Professor Luigi Mondello of the University of Messina, Italy. This scale is visually represented in Figure 2. Each column is characterised with a series of five probes plus several n-alkane markers to determine the retention index for each probe. The first five McReynolds Probes were used as the marker compounds as these are the traditional probes that have been used to benchmark polarity of GC phases for many years. The compounds are: benzene,
Figure 1: Structure of the SLB-IL60 - 1,12-di(tripropylphosphonium)dodecane bis(trifluoromethylsulfonyl)imide
Figure 2: Visual Polarity Comparison of Ionic Liquid and Conventional GC Phases
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