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less than 100 °C are known as ionic liquids (ILs) [1]. Salts which are liquid at
room temperature (≤25 °C) are known as “room-temperature ionic liquids (RTILs)”
[2]. ILs have “unique” properties, such as low volatility, non-flammability, wide
liquid range, broad solubility properties, and high thermal stability, which make
them versatile compounds that are useful in both academia and industry [3].
One of the crucial applications of ILs is their use as stationary phases for gas
chromatography (GC) [4–9]. Molten salts which were stearate salts of bivalent metals were used as GC stationary phases for the first time by Barber et al. in 1959
[10]. Later, quaternary ammonium and phosphonium salts, such as ethylammonium
nitrate, ethylpyridinium bromide, and tetraalkyl phosphonium salts, were used as GC
stationary phases for packed and open tubular formats [11–15]. However, these ILs
showed narrow liquid ranges, low column efficiencies, and poor thermal stabilities,
which limited their practical applications as GC stationary phases. Toward the end of
the last century, monocationic ILs with an imidazolium-based cation were developed
and evaluated as GC stationary phases by Armstrong and coworkers [4, 5]. These stationary phases showed improved performance compared to the previously reported
molten-salt stationary phases. These monocationic IL stationary phases were studied
in detail, and some fundamental understanding was developed during these studies
[16]. Further, a new class of dicationic ILs was synthesized and evaluated as GC stationary phases [6–8, 17]. This generation of ILs showed improved thermal stabilities,
higher viscosities, and excellent performance as GC stationary phases compared to
the monocationic ILs. The continued research led to the invention of tricationic IL
stationary phases having performance akin to dicationic ILs [9]. This new generation
of dicationic, tricationic, and even tetracationic ILs was shown to be excellent GC
stationary phases, and many were commercialized by Supelco (now MilliporeSigma)
beginning in 2008. These stationary phases showed very high thermal stabilities and
more opportunities for structural modification [7–9]. The molecular structures of the
commercial IL stationary phases in the columns along with their allowable operating
temperatures are given in Table 6.1.
The dicationic and tricationic ILs have properties that make them unique and
highly useful as GC stationary phases. These ILs show higher thermal stability than
comparable traditional polarity columns containing polymeric stationary phases [18].
Low volatility helps in low column bleeding at high temperatures and makes ILs
particularly useful for GC-MS applications [19]. ILs show good wettability, which
makes it easier to coat them on the inner walls of fused silica capillary columns [4].
High viscosity is beneficial to keep the IL coated on the inner walls of the fused silica
capillary column, especially at high temperatures [20, 21]. ILs have a wide liquid
range, which makes them useful for performing gas–liquid chromatography over a
wide temperature range [4, 17].
In addition, IL stationary phases show many advantages over traditional GC stationary phases. ILs show multiple solvation interactions, while traditional stationary
phases often show one dominant type of interaction [16]. ILs show an unusual dualnature behavior, separating both polar and nonpolar compounds through a wide range
of interaction mechanisms [4, 22]. They perform like nonpolar stationary phases
while separating nonpolar compounds and at the same time retain polar analytes
M. Talebi et al.
less than 100 °C are known as ionic liquids (ILs) [1]. Salts which are liquid at
room temperature (≤25 °C) are known as “room-temperature ionic liquids (RTILs)”
[2]. ILs have “unique” properties, such as low volatility, non-flammability, wide
liquid range, broad solubility properties, and high thermal stability, which make
them versatile compounds that are useful in both academia and industry [3].
One of the crucial applications of ILs is their use as stationary phases for gas
chromatography (GC) [4–9]. Molten salts which were stearate salts of bivalent metals were used as GC stationary phases for the first time by Barber et al. in 1959
[10]. Later, quaternary ammonium and phosphonium salts, such as ethylammonium
nitrate, ethylpyridinium bromide, and tetraalkyl phosphonium salts, were used as GC
stationary phases for packed and open tubular formats [11–15]. However, these ILs
showed narrow liquid ranges, low column efficiencies, and poor thermal stabilities,
which limited their practical applications as GC stationary phases. Toward the end of
the last century, monocationic ILs with an imidazolium-based cation were developed
and evaluated as GC stationary phases by Armstrong and coworkers [4, 5]. These stationary phases showed improved performance compared to the previously reported
molten-salt stationary phases. These monocationic IL stationary phases were studied
in detail, and some fundamental understanding was developed during these studies
[16]. Further, a new class of dicationic ILs was synthesized and evaluated as GC stationary phases [6–8, 17]. This generation of ILs showed improved thermal stabilities,
higher viscosities, and excellent performance as GC stationary phases compared to
the monocationic ILs. The continued research led to the invention of tricationic IL
stationary phases having performance akin to dicationic ILs [9]. This new generation
of dicationic, tricationic, and even tetracationic ILs was shown to be excellent GC
stationary phases, and many were commercialized by Supelco (now MilliporeSigma)
beginning in 2008. These stationary phases showed very high thermal stabilities and
more opportunities for structural modification [7–9]. The molecular structures of the
commercial IL stationary phases in the columns along with their allowable operating
temperatures are given in Table 6.1.
The dicationic and tricationic ILs have properties that make them unique and
highly useful as GC stationary phases. These ILs show higher thermal stability than
comparable traditional polarity columns containing polymeric stationary phases [18].
Low volatility helps in low column bleeding at high temperatures and makes ILs
particularly useful for GC-MS applications [19]. ILs show good wettability, which
makes it easier to coat them on the inner walls of fused silica capillary columns [4].
High viscosity is beneficial to keep the IL coated on the inner walls of the fused silica
capillary column, especially at high temperatures [20, 21]. ILs have a wide liquid
range, which makes them useful for performing gas–liquid chromatography over a
wide temperature range [4, 17].
In addition, IL stationary phases show many advantages over traditional GC stationary phases. ILs show multiple solvation interactions, while traditional stationary
phases often show one dominant type of interaction [16]. ILs show an unusual dualnature behavior, separating both polar and nonpolar compounds through a wide range
of interaction mechanisms [4, 22]. They perform like nonpolar stationary phases
while separating nonpolar compounds and at the same time retain polar analytes