J Sep Sci 2011 34 888-900 líquidos ionicos

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Review Article
Ionic liquid stationary phases for
gas chromatography
This article provides a summary of the development of ionic liquids as stationary phases
for gas chromatography beginning with early work on packed columns that established
details of the retention mechanism and established working methods to characterize
selectivity differences compared with molecular stationary phases through the
modern development of multi-centered cation and cross-linked ionic liquids for high-
temperature applications in capillary gas chromatography. Since there are many reviews
on ionic liquids dealing with all aspects of their chemical and physical properties, the
emphasis in this article is placed on the role of gas chromatography played in the design
of ionic liquids of low melting point, high thermal stability, high viscosity, and variable
selectivity for separations. Ionic liquids provide unprecedented opportunities for
extending the selectivity range and temperature-operating range of columns for gas
chromatography, an area of separation science that has otherwise been almost stagnant
for over a decade.
Keywords: Gas chromatography / Ionic liquids / Solvation parameter model /
Stationary phases
DOI 10.1002/jssc.201000724
1 Introduction
The first separation by gas chromatography using a molten
salt (ionic liquid) stationary phase was described by Barber
et al. in 1959 [1]. These authors used stearate salts of several
bivalent metals at temperatures slightly above their melting
point to obtain thermodynamic information for several
compounds. Organic ionic liquids of the type currently used
as stationary phases for gas chromatography were first
described by Gordon et al. in 1966 [2]. These authors studied
the retention characteristics of various compounds on
quaternary ammonium nitrate, picrate, and bromide salts
just above their melting points. The term ionic liquid was
not in use at that time and the salts studied by Gordon et al.
were more commonly known as organic molten salts or
fused salts. Organic molten salts were considered novel
materials at that time and little was known of their general
properties, as can be gleaned from the comprehensive
review of fused salts in organic chemistry published by
Gordon in 1969 [3]. Inorganic molten salt chemistry was
well established at this time, and inorganic salts and their
eutectic mixtures had been evaluated as both stationary
phases and additives to organic stationary phases up to a
decade earlier [4]. Inorganic molten salts afforded stable
liquid systems with wide temperature ranges but showed
little potential for the separation of organic compounds,
except, for example, in the work of Schurig [5], which led to
the use of metal chelates for the separation of structural,
configurational, and optical isomers. Inorganic molten salts
were mostly too reactive and generally poor solvents for
organic compounds to have much impact on the develop-
ment of gas chromatography. The early development of
inorganic and organic molten salts in gas chromatography
is described in detail by Poole et al. [6].
The first systematic study of the separation properties of
an ionic liquid stationary phase employed the room
temperature ionic liquid ethylammonium nitrate, which
was shown to separate a wide range of volatile organic
compounds at temperatures below 1201C [7]. An interesting
feature of this stationary phase was the very low retention of
n-alkanes compared with polar compounds of similar vola-
tility. Poor column efficiencies, however, meant that the
development of practical applications was unlikely. The first
ionic liquid to demonstrate acceptable chromatographic
properties and useful separation selectivity was ethylpyr-
idinium bromide [8]. This salt had a useful liquid
temperature range from 110 to 1601C and exhibited chro-
matographic efficiencies similar to those of conventional
stationary phases. In the liquid state, it retained the
compounds of intermediate polarity by a partitioning
mechanism but compounds at either extreme of the polarity
scale (e.g. n-alkanes and alcohols) were retained largely by
interfacial adsorption [9]. These preliminary studies provi-
ded a green light for the studies that followed but already
Colin F. Poole1
Salwa K. Poole2
1Department of Chemistry,
Wayne State University,
Detroit, MI, USA
2Detroit District Laboratory, US
Food and Drug Administration,
Detroit, MI, USA
Received October 12, 2010
Revised November 20, 2010
Accepted December 14, 2010
Correspondence: Professor Colin F. Poole, Department of
Chemistry, Wayne State University, Rm. 183, 5101 Cass Avenue,
Detroit, MI 48202, USA
E-mail: cfp@chem.wayne.edu
Fax: 11-313-577-1377
& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com
J. Sep. Sci. 2011, 34, 888–900888
raised several interesting questions. The organic molten
salts then known generally exhibited small liquid tempera-
ture ranges of little interest for gas chromatography.
Methods were needed to assist in the identification of
organic salts with wide temperature liquid ranges desirable
for gas chromatography from the potentially enormous
number of candidate organic salts. The general retention
mechanism required clarification since it was unclear
whether retention of organic compounds occurred by
partition, by interfacial adsorption, or by a combination of
the two extreme models. The influence of temperature and
solute structure on retention needed to be established and
quantitative comparisons made with molecular solvents.
The column characterization methods widely used at the
time failed to produce meaningful results for organic salts
and the reasons for this would have to be traced and more
reliable approaches identified. In particular, the system of
McReynolds’ phase constants did not seem to produce
results with credibility for the comparison of stationary
phase properties or as a basis for column selection in
method development. There was also a concern that the
‘‘chemical inertness’’ of organic salts could not be guaran-
teed and the search for salts suitable for use at high
temperatures in gas chromatography might prove fruitless
if this was accompanied by chemical transformation of
samples. These were the questions that dominated the next
phase in the development of ionic liquid stationary phases
for gas chromatography.
2 Packed columns
Poole’s group was largely responsible for the introduction
and characterization of the first generation of ionic liquids
(which they called liquid organic salts) suitable for gas
chromatography and for the studies of their fundamental
properties aimed at answering the above questions. These
ionic liquids were mainly alkylammonium and alkylphos-
phonium salts with weak nucleophilic anions (Table 1), a
number of which had liquid temperature ranges that
exceeded 1001C (Table 2) [9–17]. The liquid temperature
range for an organic salt depends on its melting point or
glass transition temperature at the low end and the
decomposition temperature or volatility of the ionic liquid
at the high end of the temperature scale. A low melting
point is a desirable property for a stationary phase to
facilitate the separation of samples of a wide volatility range.
This focussed attention on the development of room
temperature ionic liquids [18]. General empirical rules have
been proposed to predict melting points [19–23] but none of
these methods is fully satisfactory or broadly applicable. The
discovery of low-melting-point salts is still, in part,
serendipidous. Qualitatively, it seems that low symmetry,
bulkiness, and charge delocalization or shielding for one or
both ions together with weak hydrogen bonding between
ions favors the formation of salts with low melting points.
A characteristic feature of ionic liquids is the virtual
absence of significant vapor pressure over a wide tempera-
ture range. The long range Coulombic forces present in
ionicliquids resist the escape of ions into the gas phase.
Ionic liquids have been distilled in high vacuum but this is a
tediously slow processes and of limited technical interest
[24]. The absence of significant vapor pressure at high
temperatures makes ionic liquids good candidates for gas
chromatography where the upper operating temperature
limit is established by the onset of significant column bleed
(determined by the vapor pressure of the ionic liquid) or
thermal decomposition of the ionic liquid. The thermal
stability of ionic liquids is often indicated by thermal
gravimetric analysis or by gas chromatography (column
bleed profile) during a rapid temperature program. Both
methods result in an optimistic assessment of the
temperature range that an ionic liquid can be used in gas
chromatography. These methods are suitable as a scouting
tool to estimate a range of temperatures for more detailed
study. For chromatographic applications, a series of long
isothermal segments (usually several hours) followed by the
measurement of retention for a suitable well-retained
compound at a lower temperature is preferred. Changes in
retention or chromatographic performance after an
isothermal segment indicate a temperature that is too high
for practical applications. The highest isothermal tempera-
ture at which the column can be maintained at for a
reasonable time (perhaps, 8–12 h) without change in the
Table 1. Alkylammonium and alkylphosphonium organic salts used as stationary phases for packed column gas chromatography
Cation Anion Number of salts Reference
1-Methyl-3-ethylimidazolium Chloride [9]
Tri-n-butylbenzylphosphonium Chloride [9]
Tetra-n-alkylammonium Tetrafluoroborate 4 [9, 10]
Alkylammonium Thiocyanate 23 [11]
Alkylammonium 4-Toluenesulfonate 18 [12]
Tetra-n-butylphosphonium Varied 9 [13]
Tetra-n-butylammonium Sulfonates with proton donor/acceptor substituents 11 [14]
Tetra-n-alkylammonium Sulfonates with proton donor/acceptor substituents 15 [15]
Tetra-n-alkylammonium Alkane- and perfluoroalkanesulfonates 14 [16, 17]
Benzene- and perfluorobenzenesulfonate 2 [16, 17]
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peak shape or retention of a standard used for column
evaluation at a lower temperature is a better indication of a
safe upper temperature limit for gas chromatography. This
is the method used to indicate the upper temperature limit
for the ionic liquids summarized in Table 2. In many cases,
the upper temperature limit is established by the thermal
stability of the ionic liquid rather than its vapor pressure.
Two modes of thermal decomposition are known for tetra-
alkylammonium salts: the reverse Menshutkin reaction
(elimination of RX where X is the anion) and the less
common Hofmann elimination (olefin elimination). These
reactions were discussed by Gordon [3] and studied exten-
sively by Criddle and Thomas using pyrolysis gas chroma-
tography [25]. Tetraalkylphosphonium salts are intrinsically
more thermally stable than the tetraalkylammonium salts
and decompose by a more complex mechanism [26, 27]. In
general, anions of low nucleophilicity with effective charge
delocalization are usually more thermally stable and
simultaneously of lower melting point [28–30], properties
that are favorable for gas chromatography.
Studies on the retention mechanism were initiated to
establish how ionic liquids interact with organic compounds
and to understand the influence of temperature on the
model for these interactions [31]. Retention in gas chro-
matography can occur by gas–liquid partition, interfacial
adsorption, or a combination of both mechanisms [17,
31–34]. For moderately polar and polar compounds, reten-
tion by most ionic liquids is dominated by gas–liquid
partitioning, sometimes exclusively, with the relative
contribution of partitioning for mixed retention mechan-
isms increasing at higher temperatures. For n-alkanes and
compounds of low polarity, interfacial adsorption (adsorp-
tion at the gas–liquid interface) is often important, in some
cases the dominant retention mechanism. The relative
contribution of interfacial adsorption in mixed retention
mechanisms generally declines in importance at higher
temperatures. Interfacial adsorption was an important
retention mechanism for ethylammonium and propyl-
ammonium nitrates [31, 35], alkylammonium thiocyanates
[31, 36], alkylammonium sulfonates when the anion
contains hydrogen-bonding substituents, 1-ethanol-3-
methylimidazolium tetrafluoroborate and hexafluorophos-
phate and 1-ethyl-3-methylimidazolium diethylphosphate
[37], n-acryloyloxypropyl-N-methylimidazolium and metha-
cryloxyhexyl-N-imidazolium bromides [38], 1-ethyl-3-
methylimidazolium 4-toluenesulfonate [39], 1-methyl-3-
Table 2. Liquid temperature range for representative alkylammonium and alkylphosphonium ionic liquid stationary phases
Ionic liquid Temperature-operating limits (1C) Liquid
range (1C)
Lowera) Upper
Tetra-n-butylammonium
Perfluorooctanesulfonate r.t. 220 4200
Tris(hydroxymethyl)methylamino-2-hydroxy-1-propanesulfonate r.t. 180 4160
4-Morpholinepropanesulfonate r.t. 180 4160
Octanesulfonate r.t. 180 4160
Perfluorobenzenesulfonate 51 210 159
2-[bis(2-Hydroxyethyl)amino]ethane-sulfonate r.t. 170 4150
4-Toluenesulfonate 55 200 145
3-(Cyclohexylamino)propanesulfonate r.t. 160 140
Benzenesulfonate 78 210 132
Tetrafluoroborate 162 290 128
Trifluoromethanesulfonate 112 240 128
Picrate 90 200 110
Butanesulfonate 50 160 110
Methanesulfonate 79 180 101
Tris(hydroxymethyl)methylaminopropanesulfonate 110 210 100
Tetra-n-butylphosphonium
4-Toluenesulfonate 44 230 186
Chloride 83 230 147
4-Toluenesulfonate
Tetra-n-pentylammonium 55 190 135
Tri-n-propylammonium 75 180 105
Tetra-n-ethylammonium 85 190 105
Di-n-ethylammonium 105 210 105
n-Ethylammonium 121 225 104
Tri-n-ethylammonium 78 180 102
Tri-n-butylammonium 82 180 98
Trimethylammonium 93 190 97
a) r.t., salt is a liquid at room temperature.
J. Sep. Sci. 2011, 34, 888–900890 C. F. Poole and S. K. Poole
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ethylimidazolium chloride [31], triethanolammonium thio-
cyanate [11], and tetra-n-ethylammonium 4-toluenesulfonate
[40]. The contribution of partition and interfacial adsorption
to the retention mechanism can be evaluated experimentally
by varying the volume of ionic liquid in the column and
observing the change in retention [33, 34, 37, 41–43]. For a
purely partitioning mechanism, retention increases in
proportion to the change in volume, whereas for adsorption
at the liquid interface the change in retention is propor-
tional to the surface area of the liquid, which is not corre-
lated with the volume of liquid. For mixed retention
mechanisms, it is possible to obtain accurate values for the
gas–liquid partition coefficients by linear extrapolation of
the above plots to an infinite liquid volume but the gas–li-
quid adsorption constant is more difficult to determine
because the surface area of the stationary phase is generally
unknown and difficult to measure for typical gas chroma-
tographic conditions. In fact, gas–liquid chromatography is
the most widely used technique to determine gas–liquid
partition coefficients for ionic liquids and the thermo-
dynamic properties derived from them, such as activity
coefficients [38–40, 42, 44]. The importance of the above
studies was that they demonstrated that the well-established
retention models developed for gas chromatography could
be applied to ionic liquids, facilitating their classification
and comparison with conventional non-ionic stationary
phases. The understanding gained for the retention
mechanism led to the first purposefully designed practical
application of an ionic liquid stationary phase, the use of
tetra-n-ethylammonium 4-toluenesulfonate for the single-
column separation of n-alkylaromaticsin gasoline [40]. The
primary requirement for this separation is the elution of
benzene after dodecane and the elution of n-pentylbenzene
in a reasonable time. For this ionic liquid, the n-alkanes are
retained weakly (largely by interfacial adsorption) and
benzene and the alkylaromatics by partitioning. Optimiza-
tion of the stationary phase loading, column length, and
temperature allowed the separation to be achieved in under
10 min.
Early attempts to classify ionic liquids employed the
Rohrschneider–McReynolds approach using the retention
index differences for a series of prototypical compounds
measured on the ionic liquid and a hydrocarbon reference
stationary phase, squalane, to establish the contribution of
individual intermolecular interactions to the retention
mechanism [33, 44–46]. In addition, the sum of the reten-
tion index differences for five of the McReynolds’ proto-
typical compounds, or the free energy of solution for a
methylene group, was suggested as a general polarity scale
[33, 42, 47]. These approaches are still used in contemporary
studies although it was shown some time ago that they are
unsuitable for column characterization [44, 48–51]. In the
case of ionic liquids, they often suggested low selectivity for
polar interactions when compared with molecular stationary
phases and misclassified them as having similar separation
properties to stationary phases with quite different chemical
compositions. Failure to account for the contribution of
interfacial adsorption for the n-alkane retention index stan-
dards was an important contributing factor in the misclas-
sification, but more important from a fundamental point of
view was the demonstration that the phase constants for the
prototypical compounds were composite values of the
expected contribution from intermolecular interactions of
the compound with the stationary phase as well as the
interactions of the n-alkane with the stationary phase. In
addition, singular interactions cannot be associated with a
particular prototypical compound, and the interpretation of
the phase constants was ambiguous and often misleading.
At the present time, the principal method used for column
characterization is the solvation parameter model set out
below in its modern form [43, 52–55].
log SP ¼ c þ eE þ sSþ aAþ bBþ lL ð1Þ
All information concerning the solvation properties of the
stationary phase is represented by the five system constants
indicated by the lower case letters in italics in Eq. (1). They
are defined with respect to the complementary properties of
the solute descriptors, the capital letters in Eq. (1), as the
contribution of lone pair electron interactions, e, dipole-type
interactions (orientation and induction), s, hydrogen bond
basicity, a, hydrogen-bond acidity, b, and cavity formation
and dispersion interactions, l, to some free-energy-related
retention property of a series of probe compounds, SP. The
values for the system constants are obtained by multiple
linear regression analysis for a series of compounds with
known descriptor values. Correct application of Eq. (1)
requires careful selection of the number and identity of the
probe compounds, as discussed elsewhere [54–57]. Our
interest here is not how the system constants are obtained,
but their description of the solvent properties of ionic liquid
and molecular liquid stationary phases.
Poole and Poole [58] calculated the system constants for
38 ionic liquids (mainly tetra-n-alkylammonium and tetra-n-
alkylphosphonium salts) at about 1201C. The dependent
variable (SP in Eq. 1) was the gas–liquid partition coefficient
fully corrected for interfacial adsorption. For the same
conditions, Poole’s group have also provided system
constants for ten tetra-n-alkylammonium alkanesulfonate
and perfluoroalkanesulfonate ionic liquids [59]. This data-
base can be compared directly with a similar collection of
system constants for 23 common non-ionic stationary
phases determined at the same temperature [60]. The
general ranges for the system constants for the ionic liquid
and non-ionic stationary phases are summarized in Table 3.
The ionic liquids are all dipolar and strong hydrogen-bond
bases but not hydrogen-bond acids (except for 1-ethyl-3-
hydroxypyridinium salts). Alkylammonium salts with
anions containing hydrogen-bond acid functional groups
(e.g. 3-[trisfhydroxymethylgmethyl]amino-1-propylsulfonate)
seem to prefer self-association over solute-ionic liquid
hydrogen-bonding interactions with the ionic liquid acting
as a hydrogen-bond acid. Most of the ionic liquids in the
database have s system constants between 1.5 and 2.1 and a
system constants between 3.0 and 4.0. The exceptions are
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the tetra-n-alkylammonium salts with pentacyanopropio-
nide, picrate, perfluoroalkanesulfonate, and penta-
fluorobenzenesulfonate anions (low a and s system
constants). These ionic liquids have properties similar to
polar non-ionic stationary phases. Tetra-n-alkylammonium
and tetra-n-alkylphosphonium chloride, bromide, and nitrite
salts have larger than average a system constants and
average or above average s system constants. The hydrogen-
bond basicity of these ionic liquids is primarily a property of
the anion and is influenced by the size and charge locali-
zation on the anion. Ions that can delocalize charge (e.g.
picrate, pentacyanopropionide, perfluoroalkanesulfonate,
etc.) are weaker hydrogen-bond bases and less dipolar than
other ionic liquids. The halide anions have relatively small
atomic radii and no mechanism for charge delocalization.
They are the most basic of the ionic liquids in the database.
Figure 1 shows the effect of the anion on gas–liquid parti-
tioning for a series of tetra-n-butylammonium salts [61, 62].
The 20 anions are ordered by increasing a system constant.
Ignoring the anions with extensive charge delocalization,
the variation of retention for the solutes which are weak
hydrogen-bond bases (benzene, pyridine, and 1-nitropro-
pane) is weakly affected only by variation of the anion type
in contrast to the hydrogen-bond acidic n-butanol, which
varies significantly with the choice of the anion. Thus, the
selection of the anion has a significant effect on the
separation of solutes that are hydrogen-bond acids as illu-
strated for the chromatographic separation of a varied group
of solutes on similar columns coated with tetra-n-butyl-
ammonium trifluoromethanesulfonate, 4-toluenesulfonate,
and methanesulfonate (Fig. 2) [6]. The dipolar/polarizable
solute pyridine (although pyridine is a reasonably strong
hydrogen-bond base, none of the ionic liquids is hydrogen-
bond acid, and therefore, its hydrogen-bond basicity has no
effect on the selectivity of the separation) elutes at almost
the same time on the three columns. On the other hand, the
solutes which are significant hydrogen-bond acids (n-buta-
nol and 2-methyl-propanol) are shifted substantially in good
agreement with the relative hydrogen-bond basicity of the
three anions. Varying the type of alkylammonium cation for
4-toluenesulfonate salts shows a general increase in reten-
tion with an increase in molecular weight of the cation, but
hardly any noticeable change in selectivity as cations with
N–H groups are replaced by N-R groups [45]. Interestingly,
the system constants for the tetra-n-butylammonium and
tetra-n-butylphosphonium ionic liquids with a common ion
are identical within statistical uncertainty, indicating that for
these analogous salts the identity of the cation is unim-
portant to their solvation properties. Alkylphosphonium
salts, however, tend to be more thermally stable, and may be
more suitable for gas chromatography when higher
temperatures are required.
A notable characteristic of the ionic liquids summarized
in Table 3 is the similar magnitude of the l systemconstant
for ionic liquids with weakly associated anions and the low-
polarity non-ionic stationary phases. The l system constant
provides a measure of the capability of a solvent to dissolve
higher members of a homologous series and in gas chro-
matography is an indication of the peak spacing between
homologs. For ionic liquids, the equilibrium distance
between ions is controlled primarily by Coulombic forces.
These distances are comparatively large for ionic liquids
composed of bulky ions, resulting in relatively low cohesion
forces and separations of low-polarity compounds resem-
bling those obtained on non-ionic stationary phases of low
polarity. It is noteworthy that the l system constant for tetra-
Table 3. Typical ranges for the system constants at 1211C for
alkylammonium and alkylphosphonium ionic liquids
[57, 58] and non-ionic stationary phases [59]
System constant Range
Ionic liquids Non-ionic liquids
e (Electron lone pair interactions) 0.07–0.50 0–0.0.37
s (Dipole-type interactions) 1.4–2.1 0–2.1
a (Solvent hydrogen-bond basicity) 1.4–5.4 0–2.1
b (Solvent hydrogen-bond acidity) 0 0
l (Cohesion and dispersion
interactions)
0.44–0.55 0.37–0.58
(Associated anions) 0.26–0.37
Figure 1. Variation of the partial molar Gibbs free energy of
solution (kcal/mol) for (A) benzene, (B) pyridine, (C) 1-nitropro-
pane, and (D) 1-butanol on tetra-n-butylammonium ionic liquids
with different anions at 1211C. Identification of anions: PIC,
picrate; PCP, pentacyanopropionide; FBuS, perfluorobutanesul-
fonate; FMS, trifluoromethanesulfonate; BFS, pentafluorobenze-
nesulfonate; MOPSO, 2-hydroxy-4-morpholinopropanesulfonate;
BZS, benzenesulfonate; SCN, thiocyanate; PTS, 4-toluenesulfo-
nate; SuL, sulfamate; NAT, nitrate; MOPS, 4-morpholinopropa-
nesulfonate; CHES, 2-(cyclohexylamino)ethanesulfonate; CAPS,
3-(cyclohexylamino)-1-propanesulfonate; MES, methanesulfo-
nate; ETS, ethanesulfonate; HS, hexanesulfonate; Br, bromide;
NO2, nitrite; and Cl, chloride.
J. Sep. Sci. 2011, 34, 888–900892 C. F. Poole and S. K. Poole
& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com
n-pentylammonium 4-toluenesulfonate is about the same as
for a poly(dimethylsiloxane) stationary phase. When either
or both ions in an ionic liquid are capable of strong ion–ion
intermolecular interactions, such as association by hydrogen
bonding, there is an increase in the cohesion of the ionic
liquids, resulting in retention properties for low-polarity
solutes similar in magnitude to those observed for polar
non-ionic solvents. In addition, for polar solutes, additional
intermolecular interactions with individual ions are possible
and result in increased retention and selectivity.
Principal component analysis was used to demonstrate
that the ionic liquids summarized in Table 3 possessed
separation characteristics that could not be duplicated by
non-ionic stationary phases (Fig. 3) [52, 58, 60]. The first two
principal components accounted for 95.6% of the variance
and Fig. 3 should be a reliable representation of the data
contained in the system constants database. The 55
stationary phases are grouped into three clusters with three
ionic liquids behaving independently (1, tetra-n-butyl-
ammonium perfluorobenzenesulfonate; 2, tetra-n-butyl-
phosphonium chloride; and 3, tetra-n-butylammonium
chloride). Group 1 contains non-ionic stationary phases with
weak polar interactions (group membership is indicated in
the figure legend). Group 2 contains the polar non-ionic
stationary phases and the ionic liquids with charge deloca-
lized anions. Group 3 contains the remaining ionic liquids
and is well separated from groups 1 and 2, indicating their
unique separation characteristics. This confirmed an
important goal of the early development of ionic liquid
stationary phases, namely that they could provide a range of
reference stationary phases with different separation char-
acteristics to the non-ionic liquid stationary phases in
common use. In addition, it confirmed that ionic liquids
could also be used to replace certain polar polymeric
stationary phases with materials having similar separation
characteristics, a defined structure, and wider liquid
temperature ranges.
Ionic liquids are widely used as inert solvents for
organic synthesis. In a few cases, however, secondary reac-
tions have been demonstrated in which the ionic liquid is a
reactant as well as a solvent [63, 64]. For 1,3-dialkylimida-
Figure 2. Variation of retention for a varied group of compounds
with different functional groups on three similar columns of
tetra-n-butylammonium trifluoromethanesulfonate (A), 4-tolue-
nesulfonate (B) and methanesulfonate (C). Each column was
3.5 m� 2 mm id packed with 10% w/w of ionic liquid on
Chromasorb W-AW. Temperature is 1201C and nitrogen flow
rate 15 mL/min. The ionic liquids are arranged in order of
increasing basicity of the anion determined by the solvation
parameter model. Identification: 1, benzene; 2, n-butanol; 3, n-
pentanone; 4, 1-nitropropane; 5, pyridine; 6, 2-methyl-2-penta-
nol; 7, 1-iodobutane; 8, 1-octyne; 9, dioxane; and 10, cis-
hydrindane.
Figure 3. Score plot for 55 stationary phases. Group member-
ship: 1, squalane, SE-30, OV-3, OV-105, OV-11, OV17, OV-22, OV-
25, PPE-5, QF-1, and OV-7; 2, DEHPA, U50HB, OV-275, OV-330,
CW-20M, EGAD, QPIC, QPCP, DEGS; and 3, remaining ionic
liquids not included in group 2 or behaving independently (see
text).
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zolium salts, these reactions often proceed through the base-
catalyzed formation of a carbine ion that may not be relevant
for typical conditions employed in gas chromatography. For
these conditions, more probable reactions include nucleo-
philic displacement, acid/base catalyzed transformations,
and oxidation by easily reduced anions (e.g. nitrate). These
reactions are expected to be facilitated by the high
temperatures, significant residence times, and high
concentration of ionic liquid to sample typical of separations
by gas chromatography. A further possibility is the
destruction of the stationary phase by reaction with the
injection solvent, solvent impurities, or matrix components.
In early studies with tetra-n-alkylammonium tetra-
fluoroborate salts, it was noted that alcohols, phenols, and
alkylamines were retained excessively and in some cases
irreversibly [9, 10]. Similar observations (including severe
peak tailing) were made for compounds with hydrogen-
bonding functional groups on imidazolium-based ionic
liquids containing trifluoromethanesulfonate, hexa-
fluorophosphate, and bis(trifluoromethylsulfonyl)imide
anions [46, 65, 66]. Columns prepared with tetra-n-butyl-
ammonium methanesulfonate are rapidly degraded by
injection of 1-bromoalkanes [61]. On columns prepared with
alkylammonium thiocyanates, peaks were not always
observed for pyridine and 1-iodobutane [11]. The recovery of
injected mass for n-hexanol and benzaldehyde was shown to
depend rather critically on the sample size and temperature
for columns prepared from ethylammonium 4-toluene-
sulfonate [6]. For tetra-n-butylammonium chloride, picrate,
methanesulfonate, trifluoromethanesulfonate, and 4-tolue-
nesulfonate ionic liquids and the n-butylammonium, di-n-
butylammonium, and tri-n-butylammonium 4-toluene-
sulfonate ionic liquids the only significant chemical reac-
tions observed were nucleophilic displacement of halogens
from saturated carbon and the degradation of alkanethiols
on some ionic liquids [67]. Strong nitrogen bases exhibit
poor peak shapes on the same ionic liquids. In general,
except as noted above, the mass recovery of samples (e.g.
hydrocarbons, halobenzenes, nitro compounds, weak
nitrogen bases, ketones, aldehydes, ethers, esters, alcohols,
phenols, carboxylic acids, etc.) injected onto ionic liquids is
complete. Reactivity and undesirable interactions (poorpeak
shapes) for polar compounds on ionic liquid stationary
phases is a topic requiring continuous assessment in gas
chromatography.
3 Wall-coated, open-tubular columns
Most of the early development studies using ionic liquids
employed packed column technologies because virtually all
ionic liquids of interest provide stable films above a
minimum coating amount on packed column supports
and ionic liquids are good deactivating agents minimizing
undesirable interactions between the support and the polar
analytes. The modern practice of gas chromatography is
dominated by wall-coated, open-tubular columns which
offer much higher efficiency (plate counts) and can be made
chemically inert using column technology developed over
the past half century [43]. Successful column preparation
requires the creation of a thin and homogeneous film of
stationary phase on the column wall that resists forming
droplets as the column temperature is varied. Adequate
wetting of the glass surface (film formation) requires that
the surface tension of the stationary phase is less than the
critical surface tension of the column wall, usually fused
silica. The stability of the stationary phase film on the
curved wall surface and over a wide temperature range
depends on the viscosity of the stationary phase and its
variation with temperature. To maintain control of the film
thickness, the static coating method is generally used. This
requires that the stationary phase is soluble in a volatile
organic solvent and free of moisture (many ionic liquids are
hygroscopic and readily absorb moisture which leads to
blockage or film breakup when subject to the low vacuum
used in the static coating method). For poly(siloxanes) and
poly(ethylene glycols), which dominated the modern prac-
tice of gas chromatography, there is an accumulation of
experience in both stationary phase synthesis and column-
coating technology that facilitates column preparation in a
manufacturing or laboratory environment. This is not the
case for ionic liquids although Armstrong’s group, with
others, have made considerable progress in the design of
polymeric ionic liquids and multication ionic liquids with
properties better matched to those required for column
preparation [68, 69]. Early attempts to exploit the ionic
liquids used for packed columns were only partially
successful for open-tubular columns because of the
mismatch in properties with those required for column
preparation. A characteristic of these columns was a limited
temperature-operating range due to unstable film formation
at higher temperatures [10, 68]. Film formation/stability
could be improved by surface roughening using chemical
etching of glass surfaces with ammonium bifluoride [70, 71]
or deposition of a thin layer of sodium chloride on glass or
fused silica surfaces [46, 65, 72] but without changes in
stationary phase chemistry these columns lacked suitable
temperature-operating ranges and general robustness to
compete with conventional non-ionic stationary phases for
practical applications.
The discovery of the dialkylimidazolium ionic liquids
[73] was an important catalyst in furthering interest in ionic
liquids since these ionic liquids and their analogous alkyl-
pyrolinium, N-alkylpyrrolidinium, N-alkylpyridinium, and
N-alkylisoquinolinium salts with bulky and or charge-delo-
calized anions have provided the greatest number of room
temperature and low-melting-point ionic liquids with high
thermal stability [18, 74]. Quaternary alkylphosphonium [75,
76] and N-alkylguanidinium [77] salts with bulky and/or
charge-delocalized anions were shown to posses similar
properties. Ionic liquids with 1-vinyl-3-alkylimidazolium
cations can be crosslinked in the presence of a free radical
catalysts of either the precoated ionic liquid [76, 78] or by
prepolymerizing the ionic liquid which is then coated on the
J. Sep. Sci. 2011, 34, 888–900894 C. F. Poole and S. K. Poole
& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com
column [79, 80] to obtain stationary phases of enhanced
thermal stability. The extent of crosslinking is important in
maintaining kinetic performance. It is worth noting that
these stationary phases are not immobilized by reaction
with the column wall, but crosslinking increases the
apparent viscosity of the ionic liquids, resulting in more
robust and stable films at higher temperatures. So far, it
seems that crosslinking does not allow films of various
thicknesses to be easily prepared and attempts to prepare
thick film columns resulted in stationary phases with poor
kinetic performance. Advances in thermal stability and film
stability have been achieved by utilizing dicationic [75,
78–84] and tricationic [85] ionic liquids, which have also
been crosslinked in some cases. On account of their high
thermal stability and high viscosity, these ionic liquids,
some of which are liquid at room temperature, represent the
most stable ionic liquid columns described so far with
temperature-operating limits around 3501C. Some ionic
liquid columns based on polymerized ionic liquids and
multication ionic liquids are commercially available but
general composition information is withheld from the
customer [86]. Columns coated with binary mixtures of ionic
liquids were shown to conform to the general model for
ideal mixing, facilitating optimization of solvation proper-
ties to obtain separations corresponding to intermediate
compositions between the extreme properties of the ionic
liquids used to prepare the mixtures [87, 88]. In these
studies, the cation component of the ionic liquids was the
same and only the anion was varied.
The solvation parameter model has been used to char-
acterize many of the new ionic liquid stationary phases for
open-tubular columns discussed above [65, 75, 76, 78, 81,
84, 85, 88]. System constants were usually determined at
two or three temperatures in the range of 40–1101C. The
system constants at 1001C are summarized in Table 4.
Some values were obtained by interpolation of data sets
lacking experimental values at 1001C. Some models
reporting unrealistic values for the system constants were
not included in Table 4. For example, negative coefficients
for a and b system constants represent an impossible
chemical model because it implies that the gas phase is a
stronger hydrogen-bond acid or base than the stationary
phase [65, 79, 80]. The statistical method of determining
system constants for modest data sets is susceptible to
problems from mixed retention mechanisms (which affect
individual compounds differently), inclusion of compounds
which show phase overloading and/or irregular peak
shapes, contain a small number of extreme numerical
values for the dependent variable, or contain significant
crosscorrelation among the descriptors. The dependent
variable in all the above studies is the retention factor which
is not corrected for contributions from interfacial adsorp-
tion. To facilitate a comparison with a large database of
system constants for commercially available open-tubular
columns coated with poly(siloxane) and poly(ethylene
glycol) stationary phases [89], the ranges for the system
constants are summarized in Table 5. There is a significant
overlap of the system constants for the ionic liquids and the
polar stationary phases in the conventional stationary
phase database. None of the ionic liquids is selectivity
equivalent to the conventional stationary phases but
several have similar selectivity. The ionic liquid 1-butyl-3-
methylimidazolium trifluoromethanesulfonate and the
tricationic ionic liquid with a tris(2-hexanamido)ethylamine
core, tri(n-propyl)phosphonium substituents, and bis(tri-
fluoromethylsulfonyl)imide anions are similar in their
separation properties to the poly(ethylene glycol) stationary
phases. The ionic liquids 1-(4-methoxyphenyl)-3-methyl-
imidazolium trifluoromethanesulfonate, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide,
1,11-di(3-methylimidazolium)-2,6,9-trioxaundecane bis(tri-
fluoromethylsulfonyl)imide, and the tricationic ionic liquid
with a benzene core, 3-butylimidazolium substituents, and
bis(trifluoromethylsulfonyl)imide anions are similar in their
separation properties to the poly(cyanopropylphenyldimethyl-
siloxane) and poly(biscyanopropylsiloxane) stationary
phases. An advantage of the ionic liquid stationary phases is
their higher thermal stability compared with the conven-
tional phases. A notable feature of several ionic liquids
summarized in Table 4 is their significant hydrogen-bond
acidity with b system constants between 0.50 and 1.62. Since
none of the commonly used poly(siloxane) and poly
(ethylene glycol) stationary phases are hydrogen-bond acids
[89, 90], this feature of the ionic liquids indicates an addi-
tional possibility for selectivity optimization. Since virtually
all aromatic compounds and aliphatic compounds with
functional groups are hydrogen-bond bases, their separa-
tions can be adjusted by this interaction on ionic liquids,
which has no parallel for conventional stationary phases.
For the imidazolium-based ionic liquids, the C-2 hydrogen
is one source of hydrogen-bond acidity. The ionic
liquids with the highest b system constant contain cations
with additional amide or hydroxyl substituents and anions
with weak hydrogen-bond basicity or structural features
that minimize ion–ion associations. The lack of anion
diversity of the ionic liquids summarized in Table 4 (most
contain either bis(trifluoromethylsulfonyl)imide, trifluor-
omethanesulfonate, or tris(pentafluoroethyl)trifluorophos-
phate anions) suppresses the selectivity space that should be
possible with a wider range of anion types. This can be seen
by comparison of the system constant ranges summarized
in Tables 3 and 5. The ionic liquids summarized in Table 4
provide only a modest extension of the selectivity space
beyond the possible range using conventional stationary
phases with the exception of their hydrogen-bond acidity.
The anions used provide suitable physical properties for gas
chromatography (low melting points and high thermal
stability) but the identification of new anions with the ability
to extend the selectivity space would be useful. So far, the
solvation properties of ionic liquids have been studied at
comparatively low temperatures (o1101C), whereas a large
number of likely applications for ionic liquids are expected
to be at much higher temperatures. Intermolecular inter-
actions have a significant temperature dependence and it
J. Sep. Sci. 2011, 34, 888–900 Gas Chromatography 895
& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com
Table 4. System constants for ionic liquids at 1001C for open-tubular columns
Ionic liquid System constants Reference
e s a b l
(i) Monocations
1-Benzyl-3-methylimidazolium
Trifluoromethylsulfonate 0 1.70 2.41 0 0.47 [67]
1-Butyl-3-methylimidazolium
Bis(trifluoromethylsulfonyl)imide 0 1.60 1.55 0.24 0.49 [81]
Trifluoromethanesulfonate 0 1.39 2.35 0 0.49 [65]
Hexafluorophosphate 0 1.54 1.37 0 0.44 [65]
1-Butyl-2,3-dimethylimidazolium
Bis(trifluoromethylsulfonyl)imide 0.09 1.58 1.57 0.11 0.48 [65]
1-Hexyl-3-methylimidazolium
Tris(pentafluoroethyl)trifluorophosphate 0 1.46 0.49 0.61 0.42 [76]
1-Nonyl-3-methylimidazolium
Bis(trifluoromethylsulfonyl)imide 0 1.52 1.43 0 0.60 [78]
1-Octyl-2,3,4,5-tetramethylimidazolium
Bis(trifluoromethylsulfonyl)imide 0.24 1.40 1.56 0 0.50 [65]
1-(4-Methoxyphenyl)-3-methylimidazolium
Trifluoromethanesulfonate 0.28 2.05 2.03 0.16 0.38 [67]
1-Butyl-1-methylpyrrolidinium
Bis(trifluoromethylsulfonyl)imide 0 1.44 1.55 0 0.48 [81]
Tris(pentafluoroethyl)trifluorophosphate 0.19 1.46 0.60 0.60 0.44 [76]
1-(6-Aminohexyl)-1-methylpyrrolidinium
Tris(pentafluoroethyl)trifluorophosphate 0.21 1.73 1.66 0.29 0.38 [76]
1-Ethoxycarbonylmethyl-1-methylpyrrolidinium
Tris(pentafluoroethyl)trifluorophosphate 0 1.65 0.49 0.84 0.39 [76]
1-(2-Hydroxyethyl)-1-methylpyrrolidinium
Tris(pentafluoroethyl)trifluorophosphate 0.22 1.59 0.65 1.62 0.37 [76]
Bis(trifluoromethylsulfonyl)imide 0.22 1.81 1.65 0.97 0.37 [76]
1-(2-Hydroxyethyl)-3-methylimidazolium
Tris(pentafluoroethyl)trifluorophosphate 0 1.79 0.71 1.51 0.33 [76]
Bis(trifluoromethylsulfonyl)imide 0.13 1.99 1.81 1.03 0.37 [76]
Trihexyl(tetradecyl)phosphonium
Tris(pentafluoroethyl)trifluorophosphate �0.31 1.30 0.45 0.27 0.62 [76]
(ii) Dications
1,4-Di(3-methylimidazolium)butane
Bis(triflioromethylsulfonyl)imide 0.20 1.69 1.57 0.33 0.37 [81]
1,9-Di(3-methylimidazolium)nonane
Bis(trifluoromethylsulfonyl)imide 0.11 1.64 1.50 0.15 0.43 [81]
1,9-Di(1-methylpyrrolidinium)nonane
Bis(trifluoromethylsulfonyl)imide 0.23 1.49 1.48 0 0.42 [81]
1,12-Di(3-benzylimidazolium)dodecane
Bis(trifluoromethylsulfonyl)imide 0 1.47 1.44 0.52 0.46 [81]
1,11-Di(3-methylimidazolium)-2,6,9-trioxaundecane
Bis(trifluoromethylsulfonyl)imide 0.12 1.68 1.65 0 0.44 [84]
Trifluoromethanesulfonate 0.45 1.95 2.72 0.22 0.31 [84]
1,14-Di(3-benzylimidazolium)-3,6,9,12-tetraoxapentadecane
Bis(trifluoromethylsulfonyl)imide 0.09 1.55 1.54 0 0.46 [84]
1,11-Di(3-hydroxyethyl)-2,6,9-trioxaundecane
Bis(trifluoromethylsulfonyl)imide 0.37 1.27 1.38 0.83 0.35 [84]
1,9-Di(3-hydroxyethylimidazolium)nonane
Bis(trifluoromethylsulfonyl)imide 0.29 1.44 1.34 0.76 0.40 [84]
1,14-Di(3-hydroxyethylimidazolium)-3,6,9,12-tetraoxapentadecane
Trifluoromethanesulfonate 0.27 1.76 2.42 0 0.36 [84]
(iii) Tricationic (all bis[trifluoromethylsulfonyl]imides)
Core 5 Mesitylene R 5 3-benzyl-1-imidazolium 0.10 1.87 1.61 0.58 0.39 [85]
Core 5 benzene R 5 3-methylimidazolium 0.18 1.51 1.42 0 0.42 [85]
R 5 3-butylimidazolium 0.09 1.56 1.57 0.15 0.48 [85]
J. Sep. Sci. 2011, 34, 888–900896 C. F. Poole and S. K. Poole
& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com
would be helpful to know how the selectivity of ionic liquids
changes at higher temperatures and what, if any, differences
in trends exist compared with conventional stationary
phases. Suitable probe solutes for use with the solvation
parameter model have been proposed and used to char-
acterize conventional open-tubular columns at temperatures
up to 2601C [55, 91, 92].
A rapidly developing application of ionic liquids is their
ability to dissolve or form fluid dispersions with materials
difficult to handle using molecular solvents. In gas chro-
matography, this has resulted in the use of ionic liquids to
dissolve or disperse methylated cyclodextrins [93], ionic
cyclodextrins [94], single-walled carbon nanotubes [95],
fullerenes [96], cavitands [97], and metallomesogens [98].
Ionic cyclodextrins dissolved in di- or tricationic ionic
liquids demonstrated a broader enantioselectivity, higher
thermal stability, and higher efficiency for the separation of
enantiomers than a commercially available column
containing the analogous non-ionic cyclodextrin selector
dissolved in a poly(siloxane) stationary phase [94]. Initial
attempts to use neutral cyclodextrins dissolved in 1-butyl-3-
methylimidazolium chloride were less successful due to the
possible formation of inclusion complexes with the ionic
liquid that diminished the enantioselectivity compared with
an analogous column prepared from the selector dissolved
in a poly(dimethylsiloxane) stationary phase [93]. An alter-
native approach for the separation of enantiomers employed
ionic liquids in which the chiral center was contained in the
cation (N,N-dimethylephedrinium-based ionic liquids) [99]
or (alanine tert-butyl ester ionic liquids) [100] containing the
bis(trifluoromethylsulfonyl)imide anion. The ease of synth-
esis of the ionic liquids with opposite stereogenic centers
facilitates the evaluation of the separation mechanism and
their high column efficiency and good thermal stability is an
indication that the design of task-specific ionic liquids with
specific enantioselectivitycould be a useful approach for
extending the range of enantiomer separations by gas
chromatography. The dispersion of carbon particles (nano-
tubes or fullerenes) in ionic liquids facilitated variation of
the relative retention of compounds able to interact with the
carbon particles [95, 96]. Ionic liquids containing cavitands
(compounds with deep open-ended cavities) demonstrated a
remarkable separation of isotopic compounds based on the
molecular recognition ability of the additive through
formation of host–guest interactions and the high efficiency
of the columns [97].
Table 4. Continued.
Ionic liquid System constants Reference
e s a b l
R 5 3-benzylimidazolium 0 1.97 1.78 0.39 0.43 [85]
Core 5 triethylamine R 5 3-methylimidazolium 0 1.58 1.51 0.31 0.45 [85]
R 5 3-butylimidazolium 0 1.43 1.29 0.16 0.46 [85]
R 5 3-benzylimidazolium 0 1.10 1.37 0.30 0.46 [85]
R 5 3-(2-hydroxyethyl)imidazolium 0.22 0.45 0.70 0 0.57 [85]
Core 5 tris(2-hexanamido)ethylamine
R 5 3-methylimidazolium 0.16 2.10 2.50 0.17 0.37 [85]
R 5 3-butylimidazolium 0.10 1.45 1.84 0 0.45 [85]
R 5 3-benzylimidazolium 0 1.69 1.93 0 0.42 [85]
R 5 tri(n-propyl)phosphonium 0.14 1.72 2.17 0 0.44 [85]
(iv) Polymeric
Poly(1-nonyl-3-vinylimidazolium-co-Di-1,9-[3-vinylimidazolium]nonane)
Bis(trifluoromethylsulfonyl)imide 0 1.54 1.41 0.31 0.54 [78]
Poly(1-hexyl-3-vinyl)imidazolium
Bis(trifluoromethylsulfonyl)imide �0.35 1.76 1.38 0.95 0.49 [88]
Table 5. Typical ranges for the system constants at 1001C for
imidazolium, pyrrolidinium, and phosphonium (mono-,
di-, and tricationic and polymeric) ionic liquids and
conventional stationary phasesa)
System constant Range
Ionic
liquids
Non-ionic
liquids
e (Electron lone pair interactions) �0.35 to 0.45 �0.46 to 0.39
(0–0.29)
s (Dipole-type interactions) 1.10–2.72 0.07–1.90
(1.30–2.00)
a (Solvent hydrogen-bond basicity) 0.45–2.72 0–2.21
(1.35–2.50)
b (Solvent hydrogen-bond acidity) 0–1.62 0
(0–1.00)
l (Cohesion and dispersion interactions) 0.31–0.62 0.45–0.65
(0.37–0.50)
a) Values in parenthesis indicate the common range for ionic
liquids.
J. Sep. Sci. 2011, 34, 888–900 Gas Chromatography 897
& 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com
Academic research can create interest in new materials
for gas chromatography, their commercialization can open
the door to a broader understanding of their capabilities, but
their importance to the practice of gas chromatography is
driven by growth in their applications. This would seem to
be the next phase in the evolution of ionic liquid stationary
phases together with further studies aimed at improving
column-operating characteristics and extending the selec-
tivity range of the ionic liquids known to date. Qi and
Armstrong [101] demonstrated that columns prepared with
a partially crosslinked mixture of 1-vinyl-3-nonylimidazo-
lium bis(trifluoromethylsulfonyl)imide, 1,9-di(3-vinylimida-
zolium)nonane bis(trifluoromethylsulfonyl)imide, and a
poly(dimethyldiphenylsiloxane) in equal proportions were
more effective at separating essential oils containing
compounds of a wide polarity range than columns coated
with 1,9-di(vinylimidazolium)nonane or poly(ethylene
glycol). Ragonese et al. [102] demonstrated higher resolution
in a shorter time between fatty acid methyl esters and
alkanes in biodiesel on a commercially available open-
tubular column coated with 1,9-di(3-vinylimidazolium)no-
nane bis(trifluoromethylsulfonyl)imide compared with a
similar column coated with poly(ethylene glycol). An
increasing use of ionic liquids is found in series coupled
columns and comprehensive multidimensional gas chro-
matography for the separation of model mixtures [103–105],
diesel fuel [104], alkyl phosphonates [105, 106], and poly
(chlorinated biphenyl) congeners [107]. Comprehensive
multidimensional techniques employ two or three columns
separated by modulation interfaces. To achieve a useful
increase in peak capacity with respect to a single column,
the coupled columns should be of different selectivities.
Ionic liquids were selected for the above methods because
they provide an opposite selectivity to a non-polar or
moderately polar conventional stationary phase and because
of their high-temperature stability compared with conven-
tional polar stationary phases. This selectivity difference is
usually described in terms of ‘‘orthogonality’’ but in reality
stationary phases differ in the intensity of specific inter-
molecular interactions, and even the most different
stationary phases cannot be stated to be ‘‘orthogonal’’ [108].
Although orthogonality maximizes the peak capacity, it does
not necessarily result in better separations unless the
polarity of the sample covers a sufficiently wide range to
spread across the selectivity space, if not, large areas of the
retention space remain empty. Many practical separations,
therefore, will require columns with appropriate selectivity
differences for the separation to be achieved. A good
example is the separation of diesel fuel into narrow bands
containing the saturates, monoaromatics, and diaromatics
by comprehensive two-dimensional gas chromatography
using an ionic liquid stationary phase trihexyl(te-
tradecyl)phosphonium bis(trifluoromethylsulfonyl)imide as
the first (or primary column) and a poly(di-
methyldiphenylsiloxane) column, HP-5, as the second (or
secondary) column [104]. By improving the separation
between groups while maintaining adequate within group
separations, a detailed analysis of the fuel composition was
obtained in a relatively short time for such a complex
mixture. In addition, noteworthy is the separation of 208
of the 209 congeners of poly(chlorinated biphenyl)
using comprehensive two-dimensional gas chromatography
with a poly(dimethyldiphenylsiloxane) primary column
and 1,12-di(tripropylphosphonium)dodecane bis(tri-
fluoromethylsulfonyl)imide ionic liquid secondary column
[107]. The ionic liquid stationary phase provided higher
selectivity than cyanopropylsiloxane stationary phases for
the separation and the higher thermal stability of the ionic
liquid facilitated faster separations. Applications of ionic
liquids in the literature are still not common but the
commercial availability of open-tubular columns of different
selectivities is expected to facilitate rapid growth in the next
phase of the evolution of ionic liquid stationary phases for
gas chromatography.
The views expressed in this article are entirely those of the
authors and do not necessarily represent the views of the US Food
and Drug Administration.
The authors have declared no conflict of interest.
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