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M. Talebi et al.
the anion part of the IL [16]. Anions with high nucleophilicity (e.g., halides) show
high (a) values compared to the less nucleophilic anions (e.g., [PF 6 ]
– , [NTf 2 ]
– , [16].
All the commercial IL stationary phases (SLB-IL series) contain the [NTf 2 ]
– anion,
and hence, they show similar (a) values [36]. However, their (a) values are higher
compared to the polysiloxane-based nonpolar GC stationary phases [36]. The dipolarity (s) is weakly dependent on the type of cation, linkage chain, and anion [16,
36]. All the SLB-IL series stationary phases showed similar (s) values, only IL-100
being slightly higher than others [36]. The hydrogen-bond acidity (b) of SLB-IL
phases was observed to increase with an increase in polarity (except SLB-IL59)
[36]. The polysiloxane-based and wax stationary phases do not possess significant
hydrogen-bond acidity (b) values compared to IL phases of all polarities [36].
6.2.3 Characterization Using Test Mixtures
Retention behavior, separation efficiencies, and selectivities of IL stationary phases
can be observed by analyzing different test mixtures composed of a variety of compounds having different functional groups and polarities. This procedure is mainly
followed in the early development of new IL GC stationary phases, and it is a quick
method to determine the effect of structural modifications on the selectivities and
polarities of IL stationary phases. Different test mixtures, such as the Grob mixture, fatty acid methyl esters (FAME) isomer mixture, and polar compound/alkane
mixtures, are used for stationary phase characterization studies [7–9, 18].
The Grob test mixture consists of 12 different components: dicyclohexylamine,
methyl decanoate, methyl undecanoate, methyl laurate, decane, undecane, 1nonanal, 1-octanol, 2-ethylhexanoic acid, 2,3-butanediol, 2,6-dimethylaniline, and
2,6-dimethylphenol. Different components in this mixture are useful to evaluate the
separation efficiency, adsorptive activity, hydrogen bonding interactions, and inertness (acid/base characteristic) of the column [37, 38]. The mixture of polar compounds (alcohols, amines, ketone, etc.) and straight-chain alkanes can be used to
evaluate the relative polarity of IL stationary phases [7, 9, 18]. As the polarity of the
stationary phase increases, the relative retention of polar compounds with respect
to the nonpolar alkanes increases. This quick method is useful in designing new
stationary phases and understanding the influence of different structural variables on
the polarities of IL phases. An example of this approach to examine the effects of
cations on the polarities of ILs is shown in Fig. 6.3.
The polar test probe 2-octanone (compound 1 in Fig. 6.3) elutes before C15 on the
phosphonium IL column, while it elutes after C16 on the imidazolium column and
after C17 on the pyrrolidinium column. The elution order of 2-octanone shows that
ILs with the tripropylphosphonium cation are less polar compared to analogous ILs
with a dimethylimidazolium or methylpyrrolidinium cation [18]. The same procedure
is used to investigate the effects of linkage chain lengths, substituents, and anions on
the polarities of ILs.
M. Talebi et al.
the anion part of the IL [16]. Anions with high nucleophilicity (e.g., halides) show
high (a) values compared to the less nucleophilic anions (e.g., [PF 6 ]
– , [NTf 2 ]
– , [16].
All the commercial IL stationary phases (SLB-IL series) contain the [NTf 2 ]
– anion,
and hence, they show similar (a) values [36]. However, their (a) values are higher
compared to the polysiloxane-based nonpolar GC stationary phases [36]. The dipolarity (s) is weakly dependent on the type of cation, linkage chain, and anion [16,
36]. All the SLB-IL series stationary phases showed similar (s) values, only IL-100
being slightly higher than others [36]. The hydrogen-bond acidity (b) of SLB-IL
phases was observed to increase with an increase in polarity (except SLB-IL59)
[36]. The polysiloxane-based and wax stationary phases do not possess significant
hydrogen-bond acidity (b) values compared to IL phases of all polarities [36].
6.2.3 Characterization Using Test Mixtures
Retention behavior, separation efficiencies, and selectivities of IL stationary phases
can be observed by analyzing different test mixtures composed of a variety of compounds having different functional groups and polarities. This procedure is mainly
followed in the early development of new IL GC stationary phases, and it is a quick
method to determine the effect of structural modifications on the selectivities and
polarities of IL stationary phases. Different test mixtures, such as the Grob mixture, fatty acid methyl esters (FAME) isomer mixture, and polar compound/alkane
mixtures, are used for stationary phase characterization studies [7–9, 18].
The Grob test mixture consists of 12 different components: dicyclohexylamine,
methyl decanoate, methyl undecanoate, methyl laurate, decane, undecane, 1nonanal, 1-octanol, 2-ethylhexanoic acid, 2,3-butanediol, 2,6-dimethylaniline, and
2,6-dimethylphenol. Different components in this mixture are useful to evaluate the
separation efficiency, adsorptive activity, hydrogen bonding interactions, and inertness (acid/base characteristic) of the column [37, 38]. The mixture of polar compounds (alcohols, amines, ketone, etc.) and straight-chain alkanes can be used to
evaluate the relative polarity of IL stationary phases [7, 9, 18]. As the polarity of the
stationary phase increases, the relative retention of polar compounds with respect
to the nonpolar alkanes increases. This quick method is useful in designing new
stationary phases and understanding the influence of different structural variables on
the polarities of IL phases. An example of this approach to examine the effects of
cations on the polarities of ILs is shown in Fig. 6.3.
The polar test probe 2-octanone (compound 1 in Fig. 6.3) elutes before C15 on the
phosphonium IL column, while it elutes after C16 on the imidazolium column and
after C17 on the pyrrolidinium column. The elution order of 2-octanone shows that
ILs with the tripropylphosphonium cation are less polar compared to analogous ILs
with a dimethylimidazolium or methylpyrrolidinium cation [18]. The same procedure
is used to investigate the effects of linkage chain lengths, substituents, and anions on
the polarities of ILs.