6 Gas Chromatography Columns Using Ionic Liquids …
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lengths [17, 27]. Different linkages, such as alkane, polyethylene glycol, and partially fluorinated carbon chains, can be used to connect cationic head groups [7, 28].
Instead of a straight chain linkage, branched linkage chains can be used as a “tether,”
and they can have different substituents [21]. Such ILs are known as branched-chain
dicationic ILs [21, 29]. Hence, different cations can be synthesized by using different
combinations of these structural moieties, and then, these cations can be paired with
different anions: bis(trifluoromethanesulfonyl)imide ([NTf 2 ]
– ), perfluorooctanesulfonate ([PFOS]
– ), triflate ([OTf]
– ), hexafluorophosphate ([PF 6 ]
– ), among others [17,
23]. The structure–property relationship of ILs is studied by systematic variation of
structural moieties and examining changes in their physicochemical properties [20,
21, 23]. Understanding the effects of different structural modifications on physicochemical properties is essential if one wants to introduce or vary any desired property
of an IL.
6.2 Selectivity of Ionic Liquid Stationary Phases
Physicochemical characterization of ionic liquids is essential to understand the effects
of different structural variations and their behavior when used as stationary phases.
One type of characterization is known as “inverse GC” in which different analyte
probes are used to understand the solvent properties of GC stationary phases at different temperatures [8, 16]. Different models and methods used for characterization
of IL GC stationary phases are explained in the following three sections.
6.2.1 Rohrschneider–McReynolds Parameters
Early on, Rohrschneider and McReynolds developed a system to characterize and
compare different stationary phases used in GC [30, 31]. For characterization, the
retention behavior of five “informative” probe analytes is compared on different
IL stationary phases. The probes are benzene (X) for π–π interactions, 1-butanol
(Y) for hydrogen-bond donor and acceptor interactions, 2-pentanone (Z) for weak
hydrogen-bond acceptor and dipole interactions, nitropropane (U) for polar interactions, and pyridine (S) for basic or strong hydrogen-bond acceptor interactions.
Kovats retention indices of all five probe molecules are determined on the IL GC
stationary phase to be characterized and a squalane stationary phase column of the
same dimensions at the same isothermal temperature. The McReynolds constants for
the probes represent the difference between the Kovats index of the probe on the IL
stationary phase and the Kovats index of the same probe on the squalane stationary
phase. The overall or average polarity (P) of the IL stationary phase is measured by
taking the sum of five McReynolds constants. The polarity number (PN) of an IL stationary phase is calculated by Eq. 6.1, in which P SLB-IL100 is the average polarity of
135
lengths [17, 27]. Different linkages, such as alkane, polyethylene glycol, and partially fluorinated carbon chains, can be used to connect cationic head groups [7, 28].
Instead of a straight chain linkage, branched linkage chains can be used as a “tether,”
and they can have different substituents [21]. Such ILs are known as branched-chain
dicationic ILs [21, 29]. Hence, different cations can be synthesized by using different
combinations of these structural moieties, and then, these cations can be paired with
different anions: bis(trifluoromethanesulfonyl)imide ([NTf 2 ]
– ), perfluorooctanesulfonate ([PFOS]
– ), triflate ([OTf]
– ), hexafluorophosphate ([PF 6 ]
– ), among others [17,
23]. The structure–property relationship of ILs is studied by systematic variation of
structural moieties and examining changes in their physicochemical properties [20,
21, 23]. Understanding the effects of different structural modifications on physicochemical properties is essential if one wants to introduce or vary any desired property
of an IL.
6.2 Selectivity of Ionic Liquid Stationary Phases
Physicochemical characterization of ionic liquids is essential to understand the effects
of different structural variations and their behavior when used as stationary phases.
One type of characterization is known as “inverse GC” in which different analyte
probes are used to understand the solvent properties of GC stationary phases at different temperatures [8, 16]. Different models and methods used for characterization
of IL GC stationary phases are explained in the following three sections.
6.2.1 Rohrschneider–McReynolds Parameters
Early on, Rohrschneider and McReynolds developed a system to characterize and
compare different stationary phases used in GC [30, 31]. For characterization, the
retention behavior of five “informative” probe analytes is compared on different
IL stationary phases. The probes are benzene (X) for π–π interactions, 1-butanol
(Y) for hydrogen-bond donor and acceptor interactions, 2-pentanone (Z) for weak
hydrogen-bond acceptor and dipole interactions, nitropropane (U) for polar interactions, and pyridine (S) for basic or strong hydrogen-bond acceptor interactions.
Kovats retention indices of all five probe molecules are determined on the IL GC
stationary phase to be characterized and a squalane stationary phase column of the
same dimensions at the same isothermal temperature. The McReynolds constants for
the probes represent the difference between the Kovats index of the probe on the IL
stationary phase and the Kovats index of the same probe on the squalane stationary
phase. The overall or average polarity (P) of the IL stationary phase is measured by
taking the sum of five McReynolds constants. The polarity number (PN) of an IL stationary phase is calculated by Eq. 6.1, in which P SLB-IL100 is the average polarity of