Introduction
Ionic liquids are organic salts utilized for various industrial applications. Their unique and tunable physicochemical properties are unlike any other solvent. They typically remain liquid over wide temperature ranges, and tend to exhibit low melting points, good thermal stability, negligible vapor pressure, and high viscosity. These properties also make them ideal candidates as stationary phases for gas chromatography (GC). In fact, a quick literature search reveals a great deal of development aimed at preparing columns using ionic liquid stationary phases.
While monocationic ionic liquids can be employed for industrial applications, it was discovered that dicationic and polycationic ionic liquids make suitable GC stationary phases.1 Currently, there are seven different commercialized capillary GC columns which use ionic liquid stationary phases. Their main advantage is that they offer different separation properties compared to columns prepared with polysiloxane polymer and polyethylene glycol stationary phases. They also exhibit lower column bleed, higher thermal stability, greater resistance to damage from moisture and oxygen, and longer life time when compared to columns of similar polarity.
McReynolds and Abraham Methods
The polarity of a GC stationary phase can be estimated using the McReynolds method, in which retention indices (I) are determined for five test probes, representing different compound classes.2 The relationships of probes to compound classes are summarized in Table 1. Each probe relates to a set of solute-stationary phase interactions. Combining the five retention indices can then be used to determine the polarity of the stationary phase. However, this approach cannot fully differentiate individual interactions since the retention of probes is not driven by a single interaction, but is most often due to several simultaneous interactions. Thus, the imprecise property of ‘polarity’ alone is probably not sufficient to characterize GC stationary phases. In contrast, the solvation parameter model (SPM) can quantitatively evaluate the individual intermolecular interactions between a substance and the stationary phase.3 Used for many years to characterize HPLC phases, the SPM is also applicable for characterizing GC stationary phases.4-6 This model is described for GC by the Abraham equation: log k = c + eE + sS + aA + bB + lL where k is the retention factor of a solute on the stationary phase at a specific temperature; c is the model intercept; the capital letters (E, S, A, B, and L) represent the solute descriptors that are probe-specific parameters determined for many substances; and the lowercase letters (e, s, a, b, and l) are referred to as the system constants, in which all information concerning the solvation properties of the stationary phase is represented.7 Table 2 lists the correlation between system constants and the capacity of the stationary phase for various interactions.
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