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Graduate Theses and Dissertations Iowa State University Capstones, Theses and Dissertations
2019
Development of ionic liquid-based stationary phases for gas Development of ionic liquid-based stationary phases for gas
chromatographic separations chromatographic separations
He Nan Iowa State University
Follow this and additional works at: https://lib.dr.iastate.edu/etd
Part of the Analytical Chemistry Commons
Recommended Citation Recommended Citation Nan, He, "Development of ionic liquid-based stationary phases for gas chromatographic separations" (2019). Graduate Theses and Dissertations. 17755. https://lib.dr.iastate.edu/etd/17755
This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Graduate Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected].
Development of ionic liquid-based stationary phases for gas chromatographic separations
by
He Nan
A dissertation submitted to the graduate faculty
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
Major: Analytical Chemistry
Program of Study Committee:
Jared L. Anderson, Major Professor
Robbyn K. Anand
Alexander Gundlach-Graham
Emily A. Smith
Arthur H. Winter
The student author, whose presentation of the scholarship herein was approved by the program
of study committee, is solely responsible for the content of this dissertation. The Graduate
College will ensure this dissertation is globally accessible and will not permit alterations after a
degree is conferred.
Iowa State University
Ames, Iowa
2019
Copyright © He Nan, 2019. All rights reserved.
ii
TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ............................................................................................................. iv
ABSTRACT .....................................................................................................................................v
CHAPTER 1. INTRODUCTION ..............................................................................................1 1.1 A Brief Overview of Ionic Liquid-Based Stationary Phases for Gas
Chromatographic Separations ......................................................................................... 1 1.2 A Brief Overview of Multidimensional Gas Chromatography ....................................... 5 1.3 Developments and Applications of IL-Based Stationary Phases in Gas
Chromatography .............................................................................................................. 6 1.4 Applications of IL-Based Stationary Phases in Comprehensive Two-Dimensional
Gas Chromatography .................................................................................................... 10 1.5 Organization of the Dissertation ................................................................................... 12
1.6 References ..................................................................................................................... 13
CHAPTER 2. EXAMINING THE UNIQUE RETENTION BEHAVIOR OF VOLATILE
CARBOXYLIC ACIDS IN GAS CHROMATOGRAPHY USING ZWITTERIONIC LIQUID
STATIONARY PHASES ..............................................................................................................19 2.1 Abstract ......................................................................................................................... 19
2.2 Introduction ................................................................................................................... 19 2.3 Experimental Section .................................................................................................... 22
2.4 Results and Discussion .................................................................................................. 25
2.5 Conclusions ................................................................................................................... 39
2.6 Acknowledgments ......................................................................................................... 40 2.7 References ..................................................................................................................... 40
CHAPTER 3. EVALUATING THE SOLVATION PROPERTIES OF METAL-
CONTAINING IONIC LIQUIDS USING THE SOLVATION PARAMETER MODEL ...........44 3.1 Abstract ......................................................................................................................... 44
3.2 Introduction ................................................................................................................... 45 3.3 Experimental Section .................................................................................................... 47 3.4 Results and Discussion .................................................................................................. 51
3.5 Conclusions ................................................................................................................... 60 3.6 Acknowledgments ......................................................................................................... 61 3.7 References ..................................................................................................................... 61
CHAPTER 4. ARGENTATION GAS CHROMATOGRAPHY REVISITED:
SEPARATION OF LIGHT OLEFIN/PARAFFIN MIXTURES USING SILVER-BASED
IONIC LIQUID STATIONARY PHASES ...................................................................................65 4.1 Abstract ......................................................................................................................... 65
4.2 Introduction ................................................................................................................... 65 4.3 Experimental Section .................................................................................................... 67 4.4 Results and Discussion .................................................................................................. 68
iii
4.5 Conclusions ................................................................................................................... 77 4.6 Acknowledgments ......................................................................................................... 77
4.7 References ..................................................................................................................... 77
CHAPTER 5. TUNABLE SILVER-CONTAINING STATIONARY PHASES FOR
MULTIDIMENSIONAL GAS CHROMATOGRAPHY ..............................................................80 5.1 Abstract ......................................................................................................................... 80 5.2 Introduction ................................................................................................................... 81
5.3 Experimental Section .................................................................................................... 83 5.4 Results and Discussion .................................................................................................. 85 5.5 Conclusions ................................................................................................................... 93 5.6 Acknowledgments ......................................................................................................... 94 5.7 References ..................................................................................................................... 94
CHAPTER 6. LIPIDIC IONIC LIQUID STATIONARY PHASES FOR THE
SEPARATION OF ALIPHATIC HYDROCARBONS BY COMPREHENSIVE TWO-
DIMENSIONAL GAS CHROMATOGRAPHY ..........................................................................98 6.1 Abstract ......................................................................................................................... 98
6.2 Introduction ................................................................................................................... 99 6.3 Experimental Section .................................................................................................. 102 6.4 Results and Discussion ................................................................................................ 108
6.5 Conclusions ................................................................................................................. 120 6.6 Acknowledgments ....................................................................................................... 121
6.7 References ................................................................................................................... 121
CHAPTER 7. GENERAL CONCLUSIONS.........................................................................126
APPENDIX A. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 2 ........128
APPENDIX B. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 3 ........138
APPENDIX C. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 4 ........142
APPENDIX D. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 5 ........156
APPENDIX E. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 6 ........172
iv
ACKNOWLEDGMENTS
I would like to express my special appreciation to my advisor, Dr. Jared Anderson for his
guidance during graduate study. I consider myself very fortunate to have the opportunity to
pursue my Ph.D. under his supervision. I would like to sincerely thank him for offering me the
opportunity to work in his lab during a difficult time of my life and providing guidance
throughout my graduate school training. Without his guidance and support, I would not have
been able to achieve my aspirations as an analytical chemist. His dedication in scientific research
and pragmatic approach always inspires me to pursue my own scientific career.
I would like to acknowledge my committee members, Drs. Robbyn K. Anand, Alexander
Gundlach-Graham, R. Samuel Houk, Emily A. Smith, and Arthur H. Winter for their guidance
and support in my research. I would like to thank my previous and current group members:
Honglian Yu, Cheng Zhang, Omprakash Nacham, Kevin Clark, Jiwoo An, Stephen Pierson,
Xiong Ding, Deepak Chand, Marcelino Verona, Miranda Emaus, Ashley Bowers, Chenghui Zhu,
Qamar Farooq, Gabriel Odugbesi, Han Chen, Philip Eor, Nabeel Abbasi, for their suggestions
and being good friends. I would like to thank our many visiting scholars for working with me in
various projects. I also would like to extend my appreciation to Drs. James H. Davis Jr., Aaron
Rossini, Cecilia Cagliero, Kosuke Kuroda, and their research groups for the collaboration and
contributions to my research.
I would like to thank my parents for their love and support. This dissertation would not
have been possible without their support and encouragement. Last but not least, I would like to
thank all my friends, colleagues, the department faculty, and staff for making my time at Iowa
State University a wonderful journey.
v
ABSTRACT
Ionic liquids (ILs) are a class of molten salts that fulfill many of the requirements of GC
stationary phases including negligible vapor pressure, high thermal stability, wide liquid range,
and tunable viscosity. The chemical structure of ILs can be tailored to exhibit a wide range of
solvation properties for the selective separation of various types of analytes. The work presented
in this dissertation is focused on the development of various types of IL-based stationary phases
with unique selectivities and high thermal stabilities to further expand the capability of one-
dimensional gas chromatography (1D-GC) and comprehensive two-dimensional (2D) gas
chromatography (GC × GC).
A series of ILs and zwitterionic liquids (ZILs) containing sulfonate functional groups
were employed as GC stationary phases to separate volatile carboxylic acids (VCAs). The highly
polar and acidic nature of VCAs significantly limits the number of currently available GC
stationary phases, which are all largely based on acid-modified polyethylene glycol. In this
study, it is shown that this class of ZILs exhibit strong retention of VCAs with excellent peak
symmetry. Unique chromatographic selectivity toward VCAs is also demonstrated by tuning the
structural features of the ZILs. The solvation properties of the three ZILs and their structural
homologues were characterized using the Abraham solvation parameter model.
The solvation properties of eight room temperature ILs containing various transition and
rare earth metal centers (e.g., Mn(II), Co(II), Ni(II), Nd(III), Gd(III), and Dy(III)) are
characterized using the Abraham solvation parameter model. These metal-containing ILs
(MCILs) consist of the trihexyl(tetradecyl)phosphonium cation and functionalized
acetylacetonate ligands chelated to various metals. Depending on the metal center and chelating
ligand, significant differences in solvation properties were observed. MCILs containing Ni(II)
vi
and Mn(II) metal centers exhibited higher retention factors and higher peak asymmetry factors
for amines (e.g., aniline and pyridine). Alcohols (e.g., phenol, 1-octanol, and 1-decanol) were
strongly retained on the MCIL stationary phase containing Mn(II) and Dy(III) metal centers.
Silver ion or argentation chromatography utilizes stationary phases containing silver ions
for the separation of unsaturated compounds. In this study, a mixed-ligand silver-based ionic
liquid (IL) was evaluated for the first time as a gas chromatographic (GC) stationary phase for
the separation of light olefin/paraffin mixtures. The selectivity of the stationary phase toward
olefins can be tuned by adjusting the ratio of silver ion and the mixed ligands. In addition, a
stationary phase containing silver(I) ions was successfully designed and employed as a second
dimension column using comprehensive two dimensional gas chromatography (GC × GC) for
the separation of mixtures containing alkynes, dienes, terpenes, esters, aldehydes, and ketones.
Compared to a widely used non-polar and polar column set, the silver-based column exhibited
superior performance by providing better chromatographic resolution of co-eluted compounds.
Lipidic ILs possessing long alkyl chains as well as low melting points have the potential
to provide unique selectivity as well as wide operating ranges. A total of eleven lipidic ILs
containing various structural features (i.e., double bonds, linear thioether chains, and
cyclopropanyl groups) were examined as stationary phases in comprehensive two-dimensional
gas chromatography (GC × GC) for the separation of nonpolar analytes in kerosene. Compared
to a homologous series of ILs containing saturated side chains, lipidic ILs exhibit improved
selectivity toward the aliphatic hydrocarbons in kerosene. The palmitoleyl IL provided the
highest selectivity compared to all other lipidic ILs as well as the commercial
SUPELCOWAX10 column. This study provides the first comprehensive examination into the
relation between lipidic IL structure and the resulting solvation characteristics.
1
CHAPTER 1. INTRODUCTION
1.1 A Brief Overview of Ionic Liquid-Based Stationary Phases for Gas Chromatographic
Separations
Gas chromatography (GC) is one of the most efficient, reliable, and robust techniques for
the analysis of volatile and semi-volatile compounds. Efficient and prompt gas chromatographic
analysis of target analytes is mainly dependent on the performance of the GC column. Although
there have been major instrumental improvements, there still is a strong demand of highly polar,
inert, selective, and thermally stable GC columns for analytically challenging compounds such as
volatile amines, free fatty acids, and polychlorinated biphenyls (PCBs) [1]. In addition, the
physicochemical properties such as melting point, viscosity, and surface tension are also critical
to produce highly efficient GC columns. Polydimethylsiloxane (PDMS) and polyethylene glycol
(PEG) are the most widely used stationary phases, and used in commercially available Rtx-5,
SPB-50, and DB-Wax column series.
Ionic liquids (ILs) are salts with melting points at or below 100 °C [2]. ILs are typically
prepared using a nitrogen- or phosphorous-containing organic cation and an organic or inorganic
anion (see Figure 1.1). Since the first introduction of IL-based GC columns in 1999, ILs have
been successfully employed as stationary phases due to their negligible vapor pressure, high
thermal stability, high polarity, and tunable selectivity [3]. The ILs can be modified with
different functional groups to undergo various solvation interactions and exhibit unique
chromatographic selectivity as GC stationary phases. A wide range of IL-based columns have
been commercialized by Supelco (now MilliporeSigma) including SLB-IL60, SLB-IL111, and
SLB-ILPAH columns. The developments and applications of IL-based stationary phases were
discussed in several excellent review articles [4, 5].
2
Figure 1.1 Representative cation and anion structures of ionic liquids. The R groups contains
different functional groups.
The selectivity of the GC columns is mainly influenced by the polarity of the stationary
phase. The modern-day wall coated open tubular (WCOT) GC columns are generally classified
as nonpolar (e.g., DB-1 consisting of 100% PDMS) and polar (e.g., DB-Wax containing PEG-
based stationary phases or DB-225 containing cyanopropyl modified PDMS-based stationary
phases) columns. McReynolds constants are widely accepted in academia as well as the industry
to describe the polarity of the GC columns (see Figure 1.2). Recently, a new polarity scale
system named polarity number (PN) was invented by Mondello and coworkers [6]. The PN value
is determined using overall polarities derived from McReynolds constants and is used in the
naming system of the commercial IL-based columns from Supelco (e.g., SLB-IL59 and SLB-
IL100). Unlike the conventional PDMS- and PEG-based stationary phases, ILs can be modified
with different functional groups and can undergo multiple types of intermolecular interactions.
These polarity scales are excellent indicators for the selection of GC columns for specific
applications. However, these polarity scales are often times overlooked the specific types of the
intermolecular interaction. A recent study showed that GC stationary phases possessing
3
Figure 1.2 Chemical structures and polarities of selected GC stationary phases. The polarity
values (130, 948, 2324, and 5150) of stationary phases are obtained from Ref. [6]. The number
50 for SPB-50 indicates the percentage of phenyl group within the PDMS structure, while the
number 111 for SLB-IL111 represents the PN value of the stationary phase.
similar PN values exhibited different selectivity to a wide range of analytes [7]. In comparison,
Abraham solvation parameter model characterizes the solvation properties of the IL-based
stationary phases through five different types of intermolecular interactions, namely π-π or n-π
interaction, polarizability/dipolarity, hydrogen bonding basicity and acidity of the stationary
phase, and dispersive interactions. The Abraham solvation parameter model provides insights
into the solvation properties of the IL-based GC stationary phases. The commercial IL-based
columns such as SLB-IL61, SLB-IL61, and SLB-IL100 were thoroughly investigated using the
solvation parameter model by Lenca and Poole [8-10].
In addition to the selectivity, a successful GC analysis is also strongly influenced by other
important parameters, such as inertness, the operating temperature range, peak asymmetry, and
column efficiency. For the IL-based stationary phases, inertness has been a major focus of recent
development of IL-based columns such as the introduction of new SLB-IL111i column by
Supelco. Grob test is widely adopted for the evaluation of the inertness and the separation
performance for a wide range of analytes.
4
The thermal stability of the stationary phases determines the range of the analytes can be
resolved by GC. For example, highly polar analytes with high boiling points (e.g., long chain
free fatty acids, glycerols) are difficult to be analyzed by GC due to the lack of GC column with
sufficiently high polarity and thermal stability. Thermal gravimetric analysis (TGA) is widely
used for determining the thermal stability of wide range of chemicals. However, it was found in
the recent study that the maximum allowable operating temperature (MAOT) can be 100 °C
lower than their experimental TGA decomposition temperatures [11]. The thermal stability of the
stationary phases can be more accurately evaluated by recording the detector response (e.g.,
flame ionization detector) when a temperature gradient was applied (e.g., 40 to 400 ºC at 2
ºC/min) [12].
The melting point of stationary phase is an important physical property since it largely
dictates the minimum operating temperature of the resulted GC column. ILs with low melting
points are highly desirable and are commonly obtained by incorporating symmetry-breaking
regions and alkyl side chains with different lengths [13-15]. Analytes typically interact with IL-
based stationary phases through either an adsorption- or partition-type mechanism [16-18].
Greater separation efficiencies were typically provided by the partition-type retention
mechanism. When the oven temperature is lower than the melting point of the IL-based
stationary phase, the molecular interaction between the analytes and stationary phase is likely to
be dominated by adsorption. Differential scanning calorimetry is typically utilized to determine
the melting point of IL-based stationary phases [14].
Physicochemical properties of ILs such as viscosity and surface tension/wetting ability
are also very important to obtain GC columns with highly efficiency. Typically, ILs with higher
viscosity are preferred to prepare IL-based GC columns [19, 20]. The viscosity of ILs are usually
5
determined using a falling ball or rotational (cone and plate) viscometer [12, 21]. To produce a
highly efficient GC column, a homogenous layer of IL on the inner surface of capillary wall is
highly desirable. To generate the homogeneous film instead of scattered droplets, the similarity
of surface tension between the ILs and the capillary inner wall is critical. Since the
physicochemical properties including surface tension are determined by the chemical structures
of ILs, different chemical and physical modifications on the capillary inner wall were used to
produce highly efficient columns [22-24]. The surface tension of the uncoated and coated
capillary inner wall can be examined by measuring capillary dynamics of water/ethanol mixtures
[25]. Recent developments of the IL-based stationary phases showed promising advancements
for pushing the limit of the GC analysis.
1.2 A Brief Overview of Multidimensional Gas Chromatography
Multidimensional gas chromatography (MDGC) is a powerful technique to achieve
advanced separation of volatile and semi-volatile compounds in complex matrices [26-28].
MDGC typically involves connecting two columns possessing different separation mechanisms
to improve the peak capacity as well as the resolution of unresolved regions of the 1D-GC
separation [29, 30]. Heart cutting multidimensional gas chromatography (H/C MDGC)
commonly uses a flow-switching valve to transfer a selected segment of the primary column
effluent into a second dimension column to better resolve the heart-cut region. In comparison,
comprehensive two-dimensional (2D) gas chromatography (GC × GC) connects two columns
possessing different selectivities to maximize peak capacity and all compounds eluted out from
the first dimension column are transferred as pulses into a second dimension column using a
thermal or flow modulation system. In the most applications, sample analytes are first separated
on a long non-polar column (15-30 m) and then injected into a shorter and narrower second
6
dimension column containing a polar or analyte selective stationary phase (see Figure 1.3).
Several excellent review articles regarding the fundamentals of H/C MDGC and GC × GC
separations have been published [26, 31-34]. Although the development of advanced
instrumentation (e.g., modulators and detectors) and analytical methods can improve separation
results, the performance of the GC stationary phase including selectivity, thermal stability, and
inertness still strongly affects quality of MDGC analysis regarding the resolution, detection limit,
retention order, and analyte distribution.
Figure 1.3 Schematic representation of comprehensive two-dimensional gas chromatography
(GC × GC) set-up
1.3 Developments and Applications of IL-Based Stationary Phases in Gas Chromatography
Due to the ever-growing demand for the high resolution, high sensitivity, and information
rich analysis of complex samples such as petrochemicals, flavors, fragrances, and pharmaceutical
raw materials, constant developments of GC columns with unique selectivity, high inertness, low
bleed, and wide temperature working range are needed.
1.3.1 Developments and Applications of IL-Based Stationary Phases with Unique Selectivities
Due to the high polarity and excellent thermal stability, IL-based stationary phases have
been utilized to resolve a wide range of analytically challenging compounds mostly highly polar
7
compounds with high boiling points and structural similarities including long chain fatty acids,
PCBs, polycyclic aromatic sulfur heterocycles (PASHs), and essential oils [35, 36]. Most
recently, Armstrong and coworkers introduced a new series of IL-based stationary phases
(commercially available as Watercol 1460, 1900, and 1910 columns) for the analysis of trace
water content a range of 12-3258 ppm [37-39]. These IL-based stationary phases enabled the GC
analysis for efficient and accurate quantification of water in the industrial products such as
petrochemicals and pharmaceutical compounds. Cagliero et al. demonstrated that these water
compatible IL-based columns can be routinely used for the direct analysis of samples with water
as the main solvent making them very promising in particular for the analysis of fragrances and
essential oils [40].
Metal salt additives (e.g., cobalt chloride or silver nitrate) were reported to possess
significant effect on the selectivity of the stationary phase and were widely used in packed GC
columns [41, 42]. As shown in Figure 1.4, silver ions possess vacant orbitals and can undergo
selective and reversible π-complexation with unsaturated hydrocarbons such as alkenes and
alkynes [43, 44]. Since majority of GC columns involved from packed column to WCOT
column, metal salt additives were out staged due to the limited solubility and the difficulty of
homogenous mixing of salts in the traditional stationary phases materials [45, 46]. In
comparison, ILs are organic molten salts possessing high solubility of the metal salts, while
maintaining low melting point which is suitable for the application as GC stationary phases.
Figure 1.4 Schematic representation of silver-olefin complex
8
1.3.2 Developments and Applications of Highly Inert IL-Based Stationary Phases
The analysis of volatile acids and amines using GC is still challenging due to their high
polarity and acidic/basic nature [47, 48]. Due to their importance in the modern chemical
industry and pharmaceutical analysis, highly inert GC column with unique selectivities are
needed for a better separation from various solvents, improved peak symmetry, accurate
quantification, and reduced analyte adsorption. IL-based stationary phases attracted significant
interest due to their high polarity, high thermal stability, and unique selectivities. To better utilize
these advanced features and broaden the application of IL-based stationary phases, column
inertness is a factor that should be always improved. It was reported that due to the superior
inertness, the DB-Wax column was considered as a better choice for the analysis of volatile
compounds from coffee samples, despite the IL-based column also provided excellent resolution
and column efficiency [49]. A new series of highly inert IL-based columns (e.g., SLB-IL60i and
SLB-IL111i) were introduced in 2016 by Supelco. These columns were used for the separation
and quantification analytes in fragrance and essential oils and exhibited improved peak
symmetry, narrower peak widths, and lower column bleed, compared to previous generation
columns [50].
1.3.3 Developments and Applications of Highly Thermally Stable IL-Based Stationary Phases
Highly thermally stable polar GC stationary phases are in strong demand for the analysis
of polar compounds with high boiling points such as long chain fatty acids, polycyclic aromatic
hydrocarbons (PAHs), and PCBs. By far, majority of widely used polar columns are prepared
using PEG-based stationary phase with a MAOT of 240-250 °C (e.g., DB-Wax and
SUPELCOWAX10). Recently, DB-HeavyWAX was introduced by Agilent to further extend the
9
MAOT up to 280 °C [51]. IL-based stationary phases can provide higher polarity as well as
comparable thermal stability. To further broaden the applications of IL-based GC columns, the
column bleed at elevated operating temperatures should continue to be improved. By far, most of
the commercially available IL-based stationary phases are prepared from ILs consisting of
dicationic imidazolium or phosphonium cations combined with triflate or
bis[(trifluoromethyl)sulfonyl]imide anions. It was reported that DB-5HT column was preferred
instead of SLB-IL59 column, despite better resolution of analytes can be provided by SLB-IL59
column, because column bleed occurred at lower temperature for SLB-IL59 and prevented an
accurate quantification of the analytes [52]. In addition, to improve the detection and
identification of analytes in complex matrices, more sensitive and selective detectors such as
triple quadrupole time of flight mass spectrometer (TOFMS), electron capture detector, and
nitrogen phosphorus detector have been more and more widely applied in 1D-GC and MDGC
employing IL-based columns. Therefore, IL-based stationary phase with high thermal stability
and low column bleed are needed for a wider range of applications and further improvement of
the separation performance.
Monocationic IL-based stationary phases (e.g., 1-benzyl-3-methylimidazolium triflate)
exhibited MAOT up to 260 °C [20]. Dicationic imidazolium-based ILs were subsequently
developed and their MAOT were found to be significantly higher up to 400 °C compared to the
monocationic ILs [12]. Polymeric Ionic Liquid (PIL)-based stationary phases were used to
further increase the thermal stability and reduce the pooling or agglomerate of the ILs at elevated
oven temperatures. Zhang et al. studied the crosslinked PIL-based stationary phase as a second
dimension column for the GC × GC separation of the kerosene and diesel samples to achieve
better separation performance as well as higher MAOT of 325 ºC) compared to a commercial
10
PEG-based column [53]. Pojjanapornpun et al. evaluated the separation performance of new
highly inert commercial IL-based columns for the separation of fatty acid methyl esters
(FAMEs) using GC × GC [54]. Compared with the previous generation SLB-IL111 column, the
new SLB-IL111i exhibited significantly less column bleed at elevated oven temperatures and
thus better suited for the analysis of FAME with high profiling speed and good repeatability.
The thermal decomposition mechanisms of dicationic ILs including imidazolium- and
phosphonium-based ILs was investigated [11, 55]. The thermal stability of ILs was found to be
strongly affected by the carbon-heteroatom bonds such as C-O, C-N, and C-P as well as position
and number of linkage chain substituents. Recent studies have shown that the perarylated
compounds such as triarylsulfonium- and tetraarylphosphonium-based ILs exhibited excellent
thermal stability where the ILs were able to be heated at 300 ºC for 90 days with no observable
mass loss[56, 57]. The aryl substituents were found to suppress the common decomposition
pathway such as Hoffman elimination. These perarylated ILs are highly promising to exceed the
current limit of the thermal stability of IL-based stationary phases for the separation of high
boiling point aromatic compounds such as PAHs and PCBs [58].
1.4 Applications of IL-Based Stationary Phases in Comprehensive Two-Dimensional Gas
Chromatography
Since Liu and Philips first introduced GC × GC using a polyethylene glycol (PEG) ×
methyl silicone (polar × non-polar) column set, various types of GC stationary phases have been
utilized to separate extremely challenging samples [59]. IL-based stationary phases were first
introduced for GC × GC in 2006 [60]. These highly polar and thermally stable IL-based
stationary phases with unique selectivities have been employed as first dimension or second
11
dimension columns for GC × GC separations of a wide range of complex samples including
FAMEs, PCBs, PASHs, and essential oils [61].
GC × GC separations employing IL-based stationary phases have been mainly used for
the analysis of FAMEs. For example, Delmonte et al. utilized a SLB-IL111 (200 m × 0.25 mm ×
0.20 µm) × SLB-IL111 (2.5 m × 0.10 mm × 0.08 µm) column set for the GC × GC analysis of
FAMEs [62]. Compared to a previous study utilizing a 200 m SLB-IL111 column, improved
separation and easy identification of the FAMEs were achieved through GC × GC separation.
Zeng et al. developed a GC × GC/MDGC combined system to separate FAMEs employing a
wide range of IL-based columns (e.g., SLB-IL76, SLB-IL100, and SLB-IL111) [63]. Compared
to PDMS or PEG-based phases, IL-based columns provided expanded separation space as well
as better separation for complex fatty acid samples. By far, the best separation result of PCBs,
which are probable human carcinogens, was achieved by Zapadlo et al. using a GC ×
GC/TOFMS system with IL-based columns to separate and identify 196 out of 209 PCB
congeners [64, 65]. Antle et al. examined the retention behavior of 119 PASHs and their
alkylated homologues and observed an improved separation of alkylphenanthrene/anthracenes
and chrysenes using a GC × GC system employing a SLB-IL60 column [66]. Tranchida et al.
used a GC × GC system employing a SLB-5ms × SLB-IL60 column set and a high-speed triple
quadrupole MS detector to analyze contaminated mandarin and spearmint essential oils [67].
Resmethrin I/II were successfully separated from interfering substances with the aid of an IL-
based second dimension column. Manzano et al. used a GC × GC/TOFMS system employing a
SLB-IL60 × Rxi-17 column set for the analysis of thia-arenes and aza-arenes in samples
containing 45 PAHs [68]. Compared to 1D-GC/MS analysis, GC × GC/TOFMS system with IL-
12
based column was successfully used to fully resolve the less abundant but relatively more polar
aza-arenes and reduce the risk of false positives and overestimations.
1.5 Organization of the Dissertation
Chapter 2 describes the development and application of zwitterionic ionic liquid-based
stationary phases for the separation of volatile carboxylic acids. The ZILs with sulfonate
functional group exhibit strong retention of volatile carboxylic acids with excellent peak
symmetry. Unique chromatographic selectivity toward VCAs is shown by modifying the
structural features of the ZILs. The Abraham solvation parameter model was used to characterize
the solvation properties of these ZILs and their structural homologue ILs.
Chapter 3 describes the evaluation of the solvation properties of eight MCILs containing
various transition and rare earth metal centers (e.g., Mn(II), Co(II), Ni(II), Nd(III), Gd(III), and
Dy(III)) using the Abraham solvation parameter model. These metal-containing ILs (MCILs)
consisting of the trihexyl(tetradecyl)phosphonium cation and functionalized acetylacetonate
ligands were prepared to a series of highly efficiency GC columns with unique selectivities
toward a wide range of analytes. This study provides unique insight into how the solvation
properties of ILs incorporating with different transition and rare earth metal centers into their
structural make-up.
Chapter 4 describes the development and application of silver-containing IL-based stationary
phases for the separation of paraffin/olefin mixtures. The selectivity toward olefins can be tuned
by changing the ratio of silver ion and the mixed ligands. Nuclear magnetic resonance (NMR)
spectroscopy was used to characterize the coordination behavior of the silver-based IL as well as
provide an insight into the retention mechanism of light olefins on silver-containing IL-based
stationary phases.
13
Chapter 5 expands the application of silver-based IL-based stationary phases for GC × GC. A
silver-containing IL-based stationary phase was successfully designed and employed as a second
dimension column in GC × GC to separate sample mixtures containing alkynes, dienes, terpenes,
esters, aldehydes, and ketones. Compared to a widely used non-polar and polar column set, the
silver-based column exhibited superior performance by providing better chromatographic
resolution of co-eluted compounds.
Chapter 6 describes the application of eleven lipidic IL-based stationary phases with long alkyl
side chains as well as low melting points for the separation of aliphatic hydrocarbons. Compared
to a homologous series of ILs containing saturated side chains, lipidic ILs exhibit improved
selectivity toward the aliphatic hydrocarbons in kerosene. The Abraham solvation parameter
model was used to evaluate the solvation properties of the lipidic ILs.
Chapter 7 provides a summary of all research projects.
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19
CHAPTER 2. EXAMINING THE UNIQUE RETENTION BEHAVIOR OF VOLATILE
CARBOXYLIC ACIDS IN GAS CHROMATOGRAPHY USING
ZWITTERIONIC LIQUID STATIONARY PHASES
Modified and reprinted from J. Chromatogr. A 2019, 1481, 127-136
Copyright © 2019, Elsevier
He Nan, Kosuke Kuroda, Kenji Takahashi, Jared L. Anderson
2.1 Abstract
For the first time, gas chromatographic (GC) stationary phases consisting of zwitterionic
liquids (ZILs) possessing sulfonate functional groups were utilized for the analysis of volatile
carboxylic acids (VCAs). The highly polar and acidic nature of VCAs significantly limits the
number of currently available GC stationary phases, which are all largely based on acid-modified
polyethylene glycol. In this study, it is shown that this class of ZILs exhibit strong retention of
VCAs with excellent peak symmetry. Unique chromatographic selectivity toward VCAs is also
demonstrated by tuning the structural features of the ZILs. The solvation properties of the three
ZILs as well as a structurally similar conventional monocationic IL were characterized using the
Abraham solvation parameter model.
2.2 Introduction
Volatile carboxylic acids (VCAs) including volatile fatty acids such as butyric acid and
lactic acid are important for the production of food, cosmetics, fuels, and pharmaceuticals [1-3].
Gas chromatography (GC) is most commonly used for the separation and quantification of
individual acids in acylated lipids. Derivatization of VCAs by various methods such as acylation
and alkylation is typically performed to increase the volatility of these compounds and make
20
their analysis feasible by GC. In addition, derivatization can also reduce analyte adsorption to
improve peak separation and peak symmetry since the free carboxyl group often undergoes
strong interaction with the stationary phase, resulting in tailing peaks that often complicate
quantification [4-6]. Polar stationary phases such as polyethylene glycol (PEG) or cyanopropyl-
modified polydimethylsiloxane (PDMS) have been widely used for the analysis of derivatized
compounds such as fatty acid methyl esters (FAMEs) [7-9]. However, the derivatization process
can often be undesirable and may lead to incomplete conversion, multiple by-products, and the
introduction of side reactions [10]. Despite the broad application of derivatization techniques in
the analysis of VCAs, there remains a strong need for separating VCAs in their free acid form.
However, the highly polar and acidic nature of VCAs severely restricts the number of
commercially-available GC stationary phases. In the analysis of VCAs, the most widely used
columns (e.g., FFAP, Stabilwax-DA, and Nukol) are all based on acid-modified PEG-based
stationary phases [11-13]. More inert and selective GC stationary phases that provide excellent
peak symmetry and high thermal stability are needed for the direct analysis of VCAs.
Ionic liquids (ILs) are a class of molten salts with melting points lower than 100 °C [14].
IL-based GC stationary phases have been commercialized and successfully applied for the
analysis of FAMEs [15, 16]. These stationary phases can provide selectivity based on chain
length, number of unsaturated units, and location or geometries (e.g., cis or trans) of the double
bonds. In addition, studies have shown that structural modification of IL-based stationary phases
can be used to tune the polarity and improve the selectivity toward polar analytes such as
FAMEs [17]. However, IL-based stationary phases are not yet routinely used for the direct
analysis of VCAs due to the challenge of creating highly selective, inert, and thermally stable
materials.
21
ILs possessing high hydrogen bond basicity have been successfully used for the
extraction of alcohols, alkaloids, and acids (e.g., butyric acid and methacrylic acid) as well as in
the dissolution of cellulose [18-22]. The strong intermolecular interactions are attributed to
hydrogen bonding between the acids and IL anions [23]. ILs with high hydrogen bond basicity
values (0.8-1.1 in the Kamlet-Taft scale) usually contain chloride, carboxylate,
methanesulfonate, or dialkylphosphate anions [22-25]. Among them, ILs with methanesulfonate
anions possess relatively high hydrogen bond basicity values (0.7-0.8 in the Kamlet-Taft scale),
while also tending to be less hygroscopic. In our quest to develop highly selective stationary
phases suitable for the separation of VCAs, we have focused on the development of ILs
possessing sulfonate functional groups. ILs with methanesulfonate anions have rarely been
investigated as GC stationary phases due primarily to their high melting points (approximately
75 - 80 °C for the 1-butyl-3-methylimidazolium methanesulfonate IL) [15]. New structural
designs and synthetic approaches are needed to incorporate the methanesulfonate anion into ILs,
while also matching important requirements of GC stationary phases such as wide working
ranges, excellent inertness, and high thermal stability.
Recently, the synthesis of room temperature zwitterionic liquids (ZILs) in which both
imidazolium cation and sulfonate anion units attach to the parent molecule was reported [26, 27].
Inspired by the fact that these compounds can be prepared as liquids at room temperature, three
ZILs incorporating oligoether or alkyl side chain substituents were designed and examined as
GC stationary phases. It was found that VCAs strongly retained on these stationary phases and
exhibited excellent peak symmetry compared to separations performed on a commercial HP-
FFAP column. It was observed that the selectivity was largely dependent on the structural
features of the ZILs. Using the Abraham solvation parameter model, the effects of ZIL structural
22
modification on the overall solvation properties were studied. The structurally-tuned ZIL-based
stationary phases provide unique selectivity, strong retention, excellent peak symmetry, and a
relatively wide working range suitable for the analysis of volatile VCAs.
2.3 Experimental Section
2.3.1 Materials
Ethyl acetate, 2-nitrophenol, and butyraldehyde were purchased from Acros Organics
(Morris Plains, NJ, USA). Ethyl benzene was bought from Eastman Kodak Company (Rochester,
NJ, USA). Bromoethane and 1-butyl-3-methylimidazolium methanesulfonate ([BMIM+][MeSO3-
]) was purchased from Alpha Aesar (Ward Hill, MA, USA). Benzene was purchased from EMD
chemicals (Gibbstown, NJ, USA). Acetic acid, toluene, and N,N-dimethylformamide were
purchased from Fisher Scientific (Pittsburgh, PA, USA). Naphthalene, 1-bromohexane, 2-
chloroaniline, p-cresol, and p-xylene were obtained from Fluka (Steinheim, Germany).
Octylimidazole was purchased from IoLiTec (Heilbronn, Germany). Acetophenone, acrylic acid,
aniline, acetonitrile, benzaldehyde, benzonitrile, benzyl alcohol, bromobutane, 1-bromooctane,
bis[(trifluoromethyl)sulfonyl]imide, 1-butanol, 1,4-butanesultone, 1,4-dioxane, 1-chlorooctane,
1-chlorohexane, 1-chlorobutane, 1,2-dichlorobenzene, cyclohexanol, cyclohexanone, 1-decanol,
formic acid, lactic acid, levulinic acid, methylimidazole, 1-iodobutane, isobutyric acid,
isohexanoic acid, isovaleric acid, ethyl phenyl ether, methyl caproate, methacrylic acid, 1-
nitropropane, n-butyric acid, n-hexanoic acid, n-heptanoic acid, n-octanoic acid, n-valeric acid 1-
octanol, octylaldehyde, 1-pentanol, 2-pentanone, propionitrile, 1,3-propanesultone,
dichloromethane, phenol, pyridine, pyrrole, m-xylene, o-xylene, 2-propanol, propionic acid and a
standard mix containing ten VCAs were bought from MilliporeSigma (St. Louis, MO, USA). A
HP-FFAP column (30 m × 250 µm × 0.25 µm) was purchased from Agilent Technologies (Santa
23
Clara, CA, USA). A SLB IL-111 column (30 m × 250 µm × 0.20 µm) and untreated fused silica
capillary (I.D. 250 µm) were purchased from MilliporeSigma (Bellefonte, PA, USA).
2.3.2 Synthesis of Zwitterionic Liquids and Conventional Ionic Liquids
Synthetic procedures for the ZILs were previously reported [26, 27]. The structures of the
conventional ILs and ZILs are shown in Figure 2.1. Briefly, 0.2 mol sodium hydride was
suspended in tetrahydrofuran (THF) under argon gas. Imidazole (0.1 mol), dissolved in 30 mL
THF, was added dropwise to the sodium hydride solution. After stirring for 24 h at room
temperature, 1-bromo-2-(2-methoxyethoxy)ethane (0.1 mol) was added to the solution. The
resulting suspension was filtered after stirring for 6 h at 70 °C to remove the white precipitate.
After filtration, the solvent was removed by rotary evaporation to yield the crude product, which
was further purified by distillation. A fraction was collected at 105 °C under reduced pressure to
obtain 1-(2-(2-methoxyethoxy)ethyl)-1H-imidazole (OE2IM). OE2IM (0.1 mol) was
subsequently dissolved in 40 mL acetonitrile. 1,4-butanesultone (0.1 mol) was added dropwise to
the solution under nitrogen gas. The mixture was subsequently refluxed for 40 h and the solvent
removed by rotary evaporation. The residue was washed several times with diethyl ether by
decantation followed by drying of the product under vacuum at 50 °C for 24 h to obtain ZIL 1 3-
(1-(2-Methoxyethyl)-1H-imidazol-3-ium-3-yl)butane-1-sulfonate (OE2IMC4S) as a colorless
viscous liquid. ZIL 2 3-(1-octyl-1H-imidazol-3-ium-3-yl)propane-1-sulfonate (C8IMC3S) and
ZIL 3 3-(1-octyl-1H-imidazol-3-ium-3-yl)butane-1-sulfonate (C8IMC4S) was similarly prepared
using octylimidazole with 1,3-propanesultone or 1,4-butanesultone. 1H NMR data and the
synthetic procedures of ILs R1 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide
24
([BMIM+][NTf2-]) and R3 1-(2-methoxyethyl)-3-propylimidazolium methanesulfonate
([OE2IMC3+][MeSO3
-]) are provided in the appendix A.
2.3.3 Preparation of Zwitterionic Liquid-based GC Columns
Five-meter untreated fused silica capillary columns (I.D. 250 µm) were prepared by the
static coating method. The coated ZIL-based columns were conditioned from 40 to 110 °C at 3
°C min-1 and held for two hours. A 0.1 mg mL-1 naphthalene standard solution in
dichloromethane was used to determine the column efficiencies at 100 °C. The abbreviation of
ILs and corresponding characteristics of the prepared columns is shown in Table 2.1. No surface
modification or deactivation process was used in the preparation of the columns.
Table 2.1 Abbreviations of conventional ionic liquids and zwitterionic liquids and corresponding
characteristics of the prepared columns examined in this study
Column Abbreviationa RTILa Solubility in
Dichloromethane
Film
Thickness
(µm)
Efficiency
(Plates/
Meter)b
IL R1 [BMIM+][NTf2-] Yes Soluble 0.28 2200
IL R2 [BMIM+][MeSO3-] No Soluble 0.28 -c
IL R3 [OE2IMC3+][MeSO3
-] Yes Soluble 0.28 -c
ZIL 1 OE2IMC4S Yes Suspended small
liquid droplets 0.20 1000
ZIL 2 C8IMC3S Yes Soluble 0.20 2000
ZIL 3 C8IMC4S No Soluble 0.28 -c
HP-FFAP -d -d -d 0.25 3100
SLB-IL111 -d -d -d 0.20 3400
a RTIL is the abbreviation for room temperature ionic liquid. b Naphthalene was used to determine the column efficiency at 100 °C. c The efficiency values were lower than 1000 plates/meter. d Information not relevant to the commercial columns.
25
2.3.4 Preparation of Analyte Standards
The acid standards were prepared in acetonitrile at a concentration of 1 mg mL-1. A
standard mix of VCAs was purchased from MilliporeSigma and were diluted to 0.3 mg mL-1
using acetonitrile. For experiments involving the solvation parameter model, a list of the 46
probe molecules and their corresponding solute descriptors is provided in Table A1 (see
appendix A). All analytes were prepared in dichloromethane and injected individually at three
different oven temperatures (50, 80, and 110 °C). Analytes possessing low boiling points
exhibited low retention on the stationary phase at higher temperatures whereas others exhibited
very strong retention (i.e., beyond 3 hours). As a result, not all probe molecules could be
subjected to multiple linear regression analysis at each oven temperature studied. Analyze-it
software (Leeds, UK) was used perform multiple linear regression analysis and statistical
calculations.
2.3.5 Instrumentation
All GC measurements utilized to characterize the IL-based stationary phases were
performed on an Agilent 7890B GC with a flame ionization detector (FID) or an Agilent
7890B/5977A GC/MS system. Helium was employed as the carrier gas at a constant flow of 1
mL min-1. The inlet and FID detector temperatures were held at 250 °C using a split ratio of
20:1. The injection volume was 1 µL. The hydrogen and air flow of the FID detector was held at
30 and 400 mL min-1, respectively.
2.4 Results and Discussion
To evaluate IL stationary phases possessing the methanesulfonate anion for the analysis
of VCAs, two reference conventional ILs, namely, IL R1 ([BMIM+][NTf2-]) and IL R2
([BMIM+][MeSO3-]), were selected. The [NTf2
-] anion within IL R1 has been widely used in
26
Figure 2.1 Structures and abbreviations of the ILs examined in this study. R1, R2, and R3 are
conventional ILs and 1, 2, and 3 are zwitterionic ILs.
commercial IL-based GC stationary phases, such as SLB-IL61, SLB-IL100, and SLB-IL111
[28]. IL R2 shares the same cation ([BMIM+]) as IL R1, but possesses the methanesulfonate
anion (see Figure 2.1). A mixture containing ten VCAs was separated on two columns prepared
using ILs R1 and R2 as stationary phases. As shown in Figure A1, the VCAs exhibited low
retention and eluted from the IL R1 column within 6 min. In addition, analytes such as n-
hexanoic acid and n-heptanoic acid exhibited asymmetric peaks. In comparison, acids were
observed to strongly retain on the IL R2 column with good peak symmetry. The peak asymmetry
factor values of n-hexanoic acid and n-heptanoic acid on IL R2 column are 1.05 and 1.08,
respectively, which is much lower than that on IL R1 column (4.32 and 7.48, respectively).
These results indicate the unique selectivity that the methanesulfonate anion imparts in the
analysis of VCAs.
27
A major challenge was encountered when examining the solute retention time
reproducibility of IL R2. As shown in Table 2.2, the retention factors varied significantly for
VCAs when multiple injections were performed on IL R2 (%RSD ranging from 6.8 to 11.3%).
In comparison, the reproducibility improved dramatically for IL R1 and ranged from 0.04% to
0.26%. It is worth highlighting that the melting point of IL R2 is approximately 75 to 80 °C. The
poor retention time reproducibility may be related to the slow mass transfer resulting from the
prevailing gas solid chromatography mechanism. Due to the limited minimum and maximum
allowable operating temperature as well as poor retention time reproducibility of this stationary
phase, IL R2 is not suitable for routine VCA analysis.
Our attention shifted toward investigating structurally-similar ZILs that were liquids at
room temperature and capable of retaining VCAs with good peak symmetry while also
exhibiting high retention time reproducibility. Three ZILs were examined and prepared as
columns for the separation of VCAs. As shown in Figure 2.1, ZILs 1, 2, and 3 possess different
side chain substituents. ZIL 1 contains a relatively polar oligoether side chain, while ILs 2 and 3
possess an n-octyl side chain substituent. ZILs 1 and 2 were both liquids at room temperature.
Interestingly, ZIL 3 was a solid after synthesis, even though the alkyl spacer of ZIL 3 is only one
carbon longer than that of ZIL 2. These changes of the melting points can be related to the length
of the spacer chain and the resulting peak symmetry of the ZIL [29, 30]. All of these ILs were
prepared as wall coated open tubular columns and their efficiencies provided in Table 2.1.
Among them, only columns of IL R1 and ZIL 2 possessed column efficiencies higher than 2000
plates/meter. The column of ZIL 1 was prepared with a column efficiency of 1000 plates/meter,
likely due to the limited solubility of 1 in dichloromethane and its tendency to form suspended
liquid droplets in the coating solution.
28
Tab
le 2
.2 C
om
par
ison o
f re
tenti
on f
acto
rs f
or
sele
cted
vola
tile
car
box
yli
c ac
ids
on
four
dif
fere
nt
IL-b
ased
sta
tionar
y p
has
es a
nd t
he
HP
-FF
AP
colu
mn a
t 100 º
C
Pro
be
mole
cule
IL
R1
IL
R2
Z
IL 1
Z
IL 2
H
P-F
FA
P
Form
ic a
cid
0.9
8 ±
0.0
1
-a 65.2
8 ±
0.3
6
68.0
8 ±
0.1
5
4.1
9 ±
0.0
1
Ace
tic
acid
1.0
5 ±
0.0
1
51.9
6 ±
4.0
1
21.2
5 ±
0.0
6
29.2
7 ±
0.1
6
3.0
0 ±
0.0
1
Lac
tic
acid
-a
-a -a
-a 144.8
6 ±
0.0
2
Acr
yli
c ac
id
2.0
1 ±
0.0
1
132.4
3 ±
14.9
8
50.0
5 ±
0.1
4
77.6
6 ±
0.3
0
8.2
9 ±
0.0
1
Pro
pio
nic
aci
d
1.6
4 ±
0.0
1
62.9
4 ±
4.7
7
21.9
6 ±
0.1
4
42.8
7 ±
0.0
4
4.9
2 ±
0.0
1
Isobuty
ric
acid
1.8
4 ±
0.0
1
57.7
5 ±
4.6
8
17.0
4 ±
0.0
8
44.4
2 ±
0.0
6
5.7
0 ±
0.0
1
Met
hac
ryli
c ac
id
2.5
5 ±
0.0
1
124.2
0 ±
11.0
6
38.8
9 ±
0.2
2
87.1
2 ±
0.1
3
11.0
3 ±
0.0
1
n-B
uty
ric
acid
2.6
5 ±
0.0
1
86.2
0 ±
8.6
3
26.3
1 ±
0.0
6
67.9
1 ±
0.1
6
8.0
9 ±
0.0
1
Isov
aler
ic a
cid
3.3
5 ±
0.0
1
94.5
1 ±
9.0
3
24.7
7 ±
0.0
2
83.1
3 ±
0.1
2
10.2
7 ±
0.0
1
n-V
aler
ic a
cid
4.5
7 ±
0.0
1
134.8
9 ±
13.4
7
36.0
6 ±
0.1
0
123.0
0 ±
0.2
9
15.0
9 ±
0.0
1
Isoh
exan
oic
aci
d
6.4
9 ±
0.0
1
171.2
1 ±
17.7
7
39.9
9 ±
0.1
3
176.9
0 ±
0.3
6
21.9
0 ±
0.0
1
n-H
exan
oic
aci
d
7.6
6 ±
0.0
1
202.9
9 ±
21.1
7
47.4
9 ±
0.0
8
215.2
7 ±
0.0
1
27.5
8 ±
0.0
3
n-H
epta
noic
aci
d
12.7
7 ±
0.0
1
305.8
6 ±
34.5
3
62.2
3 ±
0.4
0
384.2
2 ±
0.4
3
49.5
6±
0.0
1
n-O
ctan
oic
aci
d
20.9
8 ±
0.0
1
450.7
9 ±
30.6
7
82.2
3 ±
0.2
8
693.0
1 ±
0.2
9
88.6
5±
0.1
1
Lev
uli
nic
aci
d
170.2
3 ±
0.0
1
-a -a
-a 310.6
1 ±
0.1
3
a Com
pound d
id n
ot
elute
or
was
not
obse
rved
in t
he
chro
mat
ogra
m.
IL R
1, [B
MIM
+][
NT
f 2- ];
IL
R2, [B
MIM
+][
MeS
O3- ];
ZIL
1, O
E2IM
C4S
; Z
IL 2
, C
8IM
C3S
. T
he
colu
mn d
imen
sion f
or
ILs
R1
and R
2 i
s
5 m
× 0
.25 m
m ×
0.2
8 µ
m, w
hil
e th
e co
lum
n d
imen
sion
for
ZIL
s 1 a
nd 2
is
5 m
× 0
.25 m
m ×
0.2
µm
. A
com
mer
cial
HP
-FF
AP
colu
mn (
5 m
× 0
.25 m
m ×
0.2
5 µ
m)
was
use
d f
or
com
par
ison p
urp
ose
s.
29
2.4.1 Retention and peak symmetry of volatile carboxylic acids
To examine the retention characteristics of the stationary phases, fifteen VCAs were prepared as
standards and subjected to separation on four IL-based columns (R1, R2, 1, and 2) as well as a
commercial HP-FFAP column (see Table 2.2). ZIL 3 was not included due to its limited column
efficiency. A comparison can first be made between the ZILs and reference ILs. It can be
observed in Table 2.2 that the retention factor of VCAs on the three ILs sharing the same
sulfonate functional group in the anion (ZILs 1, 2, and IL R2) are considerably higher than that
of IL R1 containing the [NTf2-] anion. This result indicates that the sulfonate functional group
plays a very important role in the retention of VCAs. It is important to note that the retention
time stability of the two ZILs is significantly better than IL R2. When these IL-based stationary
phases are compared to the HP-FFAP column, the retention factors of the acids are much higher
on ZILs 1 and 2 despite their smaller film thickness (see Table 2.2). Figure 2.2 shows that the
elution orders of analytes including acetic acid, propionic acid, and isobutyric acid on ZILs 1 and
2 and the HP-FFAP column were vastly different, further showcasing the unique selectivity
offered by the ZIL stationary phases. As shown in Table 2, lactic acid was observed to elute only
from the HP-FFAP column while levulinic acid was observed to elute from the IL R1 and HP-
FFAP columns.
To evaluate the inertness and loading capacity of the IL-based columns, peak asymmetry
factors of VCAs were examined after injecting the analytes at different concentration levels, as
shown in Table 2.3. Two commercial columns, namely, SLB-IL111 and HP-FFAP, were tested
where the SLB-IL111 column possessed the same column dimension as the two ZIL-based
columns and the HP-FFAP column possessed a 25% higher film thickness. The SLB-IL111
stationary phase consists of a dicationic imidazolium-based IL paired with the [NTf2-] anion and
30
Figure 2.2 Chromatographic separations of a VCA mixture using the (A) HP-FFAP, (B) ZIL 1,
and (C) ZIL 2 columns. Analytes: 1, formic acid; 2, acetic acid; 3, propionic acid; 4, isobutyric
acid; 5, n-butyric acid; 6, isovaleric acid; 7, n-valeric acid; 8, isohexanoic acid; 9, n-hexanoic
acid; 10, n-heptanoic acid. Helium was employed as the carrier gas at a constant flow of 1
mL/min. The inlet temperature was held at 250 °C using a split ratio of 20:1. The following
temperature program was used: initial oven temperature, 60 °C; 5 °C/min ramp to 150 °C and
held 20 min for HP-FFAP and ZIL 1 columns and 60 min for ZIL 2 column. The mass
spectrometer was operated in electron ionization mode (EI) at 70 eV for all analyses. Data were
acquired in SCAN mode (mass range: 40-300 m/z). Note: formic acid was not observed on the
ZIL 1 and 2 columns. Using the ZIL 2 column, n-heptanoic acid eluted after 80 min.
31
shares some structural similarities with IL R1 ([BMIM+][NTf2-]). Three different concentration
levels (0.1, 1, and 10 mg mL-1) were examined for acetic acid, propionic acid, n-butyric acid, n-
valeric acid, and n-hexanoic acid on the four columns. As shown in Table 2.3, SLB-IL111
produced the highest peak asymmetry factors with the difference being more significant at lower
concentration levels (i.e., 0.1 and 1 mg mL-1). This result is consistent with previous
observations of peak tailing for later eluting compounds on the IL R1 stationary phase (see
Figure A1), which has some structural similarities to SLB-IL111. This further supports the
argument that imidazolium-based ILs containing [NTf2-] anions are not ideal stationary phases
for the analysis of VCAs in their free acid form.
ZILs 1 and 2 produced excellent peak symmetry of VCAs at sample concentrations of 0.1
and 1 mg mL-1 and were comparable to the HP-FFAP column. When analytes at a concentration
of 0.1 mg mL-1 were separated, the best peak symmetry was obtained using the HP-FFAP
column. However, for the early eluting compounds such as acetic acid, propionic acid, and n-
butyric acid at 1 and 10 mg mL-1, the peak asymmetry factors were better with the ZIL 1
stationary phase compared to the HP-FFAP column. ZIL 2 also showed better peak asymmetry
factors for early eluting compounds at 10 mg mL-1 compared to the HP-FFAP column.
2.4.2 Separation of volatile carboxylic acids on zwitterionic liquid-based GC columns
A mixture of ten VCAs (formic acid, acetic acid, propionic acid, isobutyric acid, n-butyric acid,
isovaleric acid, n-valeric acid, isohexanoic acid, n-hexanoic acid, n-heptanoic acid) was used to
compare the separation performance of the ZIL-based stationary phases to a commercial HP-FFAP
stationary phase. When ZILs 1 and 2 (30 m × 0.25 mm × 0.2 µm) were compared with the HP-
FFAP column (30 m × 0.25 mm × 0.25 µm), the retention factors of all analytes were higher on
the ZIL-based stationary phases despite their smaller film thickness, as shown in Figure 2.2.
32
Table 2.3 Peak asymmetry factors of five selected VCAs on four different stationary phases
Columna Probe Molecules Peak Asymmetry Factorb
0.1 mg mL-1 1 mg mL-1 10 mg mL-1
ZIL 1 Acetic acid 1.44 1.38 4.48
5 m × 0.25 mm × 0.2 µm Propionic acid 1.22 1.31 2.74
n-Butyric acid 1.42 1.27 1.19
n-Valeric acid 1.53 1.35 1.25c
n-Hexanoic acid 1.60 1.80 1.32c
ZIL 2 Acetic acid 1.88 1.63 4.21
5 m × 0.25 mm × 0.2 µm Propionic acid 1.52 1.47 3.70
n-Butyric acid 1.25 1.45 3.19
n-Valeric acid 1.34 1.38 2.94
n-Hexanoic acid 1.23 1.36 2.26
SLB-IL111 Acetic acid 2.10 6.15 5.44
5 m × 0.25 mm × 0.2 µm Propionic acid 2.81 5.06 7.29
n-Butyric acid 2.59 4.87 4.95
n-Valeric acid 2.58 4.57 2.45
n-Hexanoic acid 2.63 3.69 2.03
HP-FFAP Acetic acid 1.11 1.57 7.64
5 m × 0.25 mm × 0.25 µm Propionic acid 1.14 1.46 5.77
n-Butyric acid 1.09 1.33 4.01
n-Valeric acid 1.06 1.18 1.18
n-Hexanoic acid 1.06 1.00 1.49c a ZIL 1, OE2IMC4S; ZIL 2, C8IMC3S; SLB-IL111, 1,5-Di(2,3-dimethylimidazolium)pentane
bis(trifluoromethanesulfonyl)imide. b The peak asymmetry factor is defined as the ratio of the peak half-width in the rear and the
front of the chromatographic peak measured at 10% of the peak height. Unless otherwise noted,
all peaks exhibited tailing peak shapes. c Indicates a fronting peak.
Furthermore, unique separation selectivity on the ZIL-based columns was observed. In the case
of the HP-FFAP column, the elution order was dependent on the boiling point of the VCAs,
except for formic acid. Formic acid possesses the lowest pKa value among the analytes resulting
in strong interaction with the stationary phase and long retention times [31]. The elution order of
acetic acid, propionic acid, isobutyric acid, n-butyric acid, and isovaleric acid on the ZIL-based
columns was different compared to the HP-FFAP column. Isobutyric acid eluted first from the
ZIL 1 column despite the boiling point of isobutyric acid being higher than acetic acid and
33
propionic acid. Isovaleric acid co-eluted with propionic acid on the ZIL 1 column, while acetic
acid eluted after isobutyric acid, propionic acid, and isovaleric acid.The elution order of VCAs
on ZIL 2 was similar to the HP-FFAP column, except for isobutyric acid eluting earlier than
propionic acid.
To evaluate the effect of the oligoether and alkyl side chain substituent on the selectivity
toward VCAs, ZILs 1 and 2 were compared. As shown in Figure 2.2, VCAs retained much
longer on ZIL 2 compared to ZIL 1, despite the column dimensions and separation conditions
being the same. For example, n-heptanoic acid eluted from ZIL 1 within 30 min but was strongly
retained on ZIL 2 and eluted after 80 min. Isobutyric acid eluted first from the ZIL 1 column,
while acetic acid eluted first on the ZIL 2 column. In addition, the elution order of n-butyric acid
and isovaleric acid was reversed on these two columns. These results demonstrate that minor
structural modifications to the ZILs can influence the selectivity toward highly volatile VCAs.
To further investigate the effect of the zwitterionic structure on the selectivity of IL-based
stationary phases, IL R3 was synthesized as a structural homologue of ZIL 1 (see Figure 2.1). As
shown in Figures 2.2 and A1, the elution order of VCAs on IL R3 was more similar to ZIL 2
despite IL R3 being more structurally similar to ZIL 1.
2.4.3 Separation of Grob mixture
To investigate the separation performance of ZILs and compare them to commercial
stationary phases, the Grob mixture consisting of alcohols, esters, and amines was separated. As
shown in Figure 2.3, three 30 m columns of ZIL 1 (30 m × 0.25 mm × 0.20 µm) and ZIL 2 (30 m
× 0.25 mm × 0.20 µm) as well as the HP-FFAP (30 m × 0.25 mm × 0.25 µm) were examined. In
comparing ZIL 1 to the HP-FFAP column, all eleven analytes eluted with excellent peak
34
Figure 2.3 Chromatograms of the Grob mixture using the (A) HP-FFAP, (B) ZIL 1, and (C) ZIL
2 column. Analytes: 1, decane; 2, dodecane; 3, 2,6-dimethylaniline; 4, dicyclohexylamine; 5,
2,3-butanediol; 6, methyl decanoate; 7, methyl undecanoate; 8, methyl laurate; 9, 1-octanol; 10,
2,6-dimethylphenol; 11, 2-ethylhexanoic acid. All separations were performed using an Agilent
7890B GC-FID system. Helium was used as the carrier gas at a constant flow of 1 mL/min. The
temperature program consisted of the following: initial oven temperature, 60 °C; 5 °C/min ramp
to 150 °C and held for 20 min. The inlet and FID detector temperatures were held at 250 °C and
a split ratio of 20:1 was employed. Note: Dicyclohexylamine and 2,3-butanediol were not
observed to elute from the IL 2 column.
35
symmetry on both columns, with the exception of tailing peaks being observed for
dicyclohexylamine (4). Decane (1) and dodecane (2) eluted earlier on ZIL 1 compared to HP-
FFAP (see Figure 2.3). Interestingly, compared to the alkanes and fatty acid methyl esters, the
retention factors of 2,6-dimethylaniline (3), 2,3-butanediol (5), 2,6-dimethylphenol (10), and 2-
ethylhexanoic acid (11) were significantly higher on ZIL 1 despite its smaller film thickness. As
revealed in Figures 3.3A and 3.3B, the elution order of several analyte pairs such as 2,6-
dimethylaniline (3) and dicyclohexylamine (4), 2,3-butanediol (5) and1-octanol (9), as well as
2,6-dimethylphenol (10) and 2-ethylhexanoic acid (11) were reversed on the ZIL 1 and HP-
FFAP columns.
In a side-by-side comparison of ZILs 1 and 2 in Figures 2.3B and 2.3C, the elution order
of 2,6-dimethylphenol (10) and 2-ethylhexanoic acid (11) were reversed. Dicyclohexylamine (4)
and 2,3-butanediol (5) did not elute from the ZIL 2 column. In comparison, these analytes were
well separated on the ZIL 1 stationary phase. These results showcase the highly retentive
character of ZIL 2 toward very polar analytes such as amines and alcohols.
2.4.4 Solvation properties of zwitterionic liquids
The Abraham solvation parameter model has been successfully utilized to characterize a
broad range of IL-based GC stationary phases [32-35]. This approach utilizes a linear free-
energy relationship to describe the contribution of individual solvation interactions of the
stationary phase toward retention and provides insight into solute/solvent interactions.
𝐿𝑜𝑔 𝑘 = 𝑐 + 𝑒𝐸 + 𝑠𝑆 + 𝑎𝐴 + 𝑏𝐵 + 𝑙𝐿 (1)
As indicated in Equation 1, k represents the retention factor of each analyte on the
stationary phase at a specific temperature. The solute descriptors including E, S, A, B, and L are
36
defined as: E, the excess molar refraction determined from the solute’s refractive index; S, the
solute dipolarity/polarizability; A and B, the solute hydrogen bond acidity and basicity,
respectively; and L, the solute gas hexadecane partition coefficient measured at 298 K. The
solute descriptors E, S, A, B, and L have been previously determined and are described in Tables
A1 and A2 [36]. The c term represents the intercept of the regression line. The coefficients e, s,
a, b, and l represent the system constants and are used to indicate the strength of each solvation
interaction. The system constants are defined as: e, electron lone pair interactions provided by
the IL stationary phase; s, the dipolarity/polarizability of the stationary phase; a and b, the
hydrogen bond basicity and acidity of the IL stationary phase, respectively; and l describes the
dispersion forces/cavity formation of the stationary phase. As shown in Table 2.4, system
constants were determined for the conventional ILs and ZILs examined in this study at three
different oven temperatures (50 ºC, 80 ºC, and 110 ºC).
The first comparison was made between the IL-based columns and the commercial DB-
FFAP column (equivalent to HP-FFAP). The hydrogen bond basicity of the DB-FFAP stationary
phase (a = 2.53 at 80 °C) is higher than that of IL R1 ([BMIM+][NTf2-], a = 1.68 at 80 °C).
However, when DB-FFAP and IL R1 are compared to ILs with sulfonate functional groups (i.e.,
ZILs 1-3 and IL R2), it can be observed that these compounds possess much higher hydrogen
bond basicity (e.g., a = 3.95 at 80 °C for ZIL 1). This indicates that ILs containing sulfonate
functional group are more hydrogen bond basic compared to the ILs containing the [NTf2-] anion
and the commercial DB-FFAP stationary phase. This result is in agreement with previous reports
that have described the high hydrogen bond basicity behavior of the tetra-n-butylammonium
methanesulfonate [TBA+][MeSO3-] IL (a = 3.76) compared to IL R1 ([BMIM+][NTf2
-]) and
[BMIM+][TfO-] (see Table A3). These results are further corroborated by the chromatographic
37
Table 2.4 System constants of the studied ionic liquids and commercial DB-FFAP and SLB-
IL111 stationary phases obtained by the solvation parameter model
Stationary
Phase
Temperature
(°C)
System Constants
c e s a b l n a R2 a F a
DB-FFAPc 80 -3.24
(0.04)b
0.22
(0.02)
1.65
(0.03)
2.53
(0.04) 0
0.56
(0.01) 35 0.99 3750
SLB-IL111c -d 0.35 0.17 0.66 0 0 0.15 -d -d -d
IL R1
[BMIM+]
[NTf2-]c
80 -d 0 1.65 1.68 0.33 0.60 -d -d -d
IL R3
[OE2IMC3+]
[MeSO3-]
50 -3.14
(0.11)
0.20
(0.08)
2.49
(0.10)
5.21
(0.14)
0
(0.12)
0.53
(0.02) 24 0.99 506
80 -3.10
(0.09)
0.17
(0.06)
2.25
(0.07)
4.50
(0.11)
-0.07
(0.09)
0.44
(0.02) 23 0.99 636
110 -2.94
(0.10)
0.14
(0.06)
2.02
(0.09)
3.80
(0.10)
-0.26
(0.11)
0.36
(0.01) 17 0.99 429
ZIL 1
OE2IMC4S 50
-2.77
(0.11)
0.40
(0.06)
2.02
(0.10)
4.72
(0.12)
-0.27
(0.11)
0.37
(0.02) 16 0.99 472
80 -2.93
(0.11)
0.67
(0.07)
1.86
(0.09)
3.95
(0.10)
-0.42
(0.11)
0.31
(0.02) 19 0.99 494
110 -3.43
(0.09)
0.82
(0.08)
1.81
(0.10)
3.69
(0.09)
0.34
(0.10)
0.22
(0.01) 16 0.99 566
ZIL 2
C8IMC3S 50
-3.14
(0.0)
0.05
(0.06)
2.01
(0.08)
5.26
(0.13)
-0.09
(0.10)
0.64
(0.02) 27 0.99 582
80 -3.14
(0.11)
0.05
(0.08)
1.88
(0.10)
4.64
(0.11)
-0.15
(0.13)
0.52
(0.02) 30 0.99 496
110 -2.90
(0.11)
0.07
(0.07)
1.67
(0.09)
4.03
(0.10)
-0.36
(0.12)
0.42
(0.02) 25 0.99 406
ZIL 3
C8IMC4S 50 -e -e -e -e -e -e -e -e -e
80 -e -e -e -e -e -e -e -e -e
110 -3.25
(0.32)
0.35
(0.17)
1.84
(0.18)
4.60
(0.24)
-0.79
(0.32)
0.45
(0.03) 10 0.99 92
a n, number of probe analytes subjected to multiple linear regression analysis; R2, correlation
coefficient; F, Fisher coefficient. b The values in brackets are the reported standard deviations. c Data were obtained from references [38-40]. d These values were not reported in the references. e Data were not able to be generated from the model due to a limited number of probe molecules
that eluted at 50 °C and 80 °C.
38
behavior in which acetic acid (2) and propionic acid (3) retained strongly on the ZILs 1 and 2
stationary phases, while exhibiting good peak symmetry (see Figure 2.2).
To further evaluate the effect of different structural features on the solvation properties, the
system constants of ZILs 1, 2, 3, and IL R2 can be compared. Among them, ZIL 2 is more
hydrogen bond basic (a = 4.64 at 80 °C) and is even higher than that of ZIL 1. In addition, ZIL 2
exhibits stronger dispersive-type interactions (l = 0.52 at 80 °C) compared to ZIL 1 (l = 0.31 at
80 °C). These results are consistent with the previous observation that VCAs retained much
longer on ZIL 2 compared to ZIL 1, especially for the compounds with longer alkyl chains such
as n-hexanoic acid and n-heptanoic acid. To investigate the effect of the zwitterionic structure,
the solvation properties of IL R3 were evaluated. As a structural homologue of ZIL 1, IL R3 is
more hydrogen bond basic (a = 4.50 at 80 °C) but is less cohesive than ZIL 1. This result
indicates that the alkyl chain between the cation and anion in ZIL 1 plays a more diminished role
in dispersive-type interactions compared to free alkyl side chain of IL R3. Furthermore, ZIL 1
exhibited the highest lone pair and π-electron interaction capabilities (e = 0.67 at 80 °C) among
all the ILs as well as the FFAP column (e = 0.22 at 80 °C). In comparison, IL R3 which is the
homologue of ZIL 1 possesses a much lower lone pair and π-electron interactions (e = 0.17 at 80
°C). This result indicates that conventional IL and ZIL with the same functional groups possess
vastly different solvation properties. IL R3 were observed to exhibit the highest
polarity/polarizability (s = 2.25 at 80 °C). The hydrogen bond acidity (b term) values of IL R3
and ZILs 1-3 were negative, since there are no acidic hydrogens that can act as hydrogen bond
donors. These results indicate that zwitterionic structure and different structural features strongly
influences the solvation properties.
39
2.4.5 Thermal stability of zwitterionic liquid stationary phases
The maximum allowable operating temperature (MAOT) of the stationary phases was
examined by heating the columns for 1 hour at different temperatures (e.g., 100 °C, 150 °C, 200
°C, and 250 °C) and monitoring the column bleed at these temperatures. As shown in Figure
A2A, significant column bleed was produced during heating up to 225 °C. To further investigate
the thermal stability of the column after each heating step, the GC oven was reset to 100 °C after
each heating increment and the column efficiency determined using naphthalene standard
solution in dichloromethane. Since the column efficiency is dependent on both the retention time
and peak width of the analyte, the MAOT can be estimated by observing changes in analyte
retention time as well as decreased separation efficiency. As shown in Figure A2, there was no
observable change in the column efficiency upon heating to 225 °C. However, beyond 225 °C a
decrease in the retention time of naphthalene was observed on ZIL 1. After ZIL 1 was heated to
250 °C, a significant drop in the retention time as well as significant peak broadening was
observed. The MAOT of ZIL 1 was estimated to be between 200 °C and 225 °C. Using the same
process, the MAOT of ZIL 2 was also determined to be between 200 °C and 225 °C, as shown in
Figure A3. These results are lower than the decomposition temperatures of 282 °C and 315 °C
for ZILs 1 and 2, respectively, measured by thermogravimetric analysis. It is widely known that
TGA results generally provide an overestimation of the thermal stability for GC stationary
phases [37]. The manufacturer recommended MAOT for the HP-FFAP column is 240°C, which
is considerably higher than the ZIL-based stationary phases.
2.5 Conclusions
A new class of room temperature ZILs with sulfonate functional groups was employed
for the first time as GC stationary phases in the separation of VCAs. Compared to a widely used
HP-FFAP column, the VCAs were observed to exhibit higher retention on the ZIL-based
40
columns while producing excellent peak symmetry. In addition, unique chromatographic
selectivity toward highly volatile VCAs (e.g., acetic acid and isobutyric acid) was demonstrated
by incorporating oligoether or alkyl side chain substituents within the chemical structure of the
ZILs. Using the solvation parameter model, the effect of structural modification on the hydrogen
bond basicity and cohesivity of the ZIL-based stationary phases was evaluated. The structurally-
tuned ZIL-based stationary phases with sulfonate functional groups provided strong retention,
excellent peak symmetry, wide working range, and unique selectivity in the analysis of VCAs.
These stationary phases can serve as attractive alternatives to the acid-modified PEG stationary
phases when stronger retention of VCAs and different selectivity is needed.
2.6 Acknowledgments
JLA acknowledges funding from Chemical Measurement and Imaging Program at the
National Science Foundation (Grant number CHE-1709372). KK thanks KAKENHI (18K14281
from the Japan Society for the Promotion of Science) and Leading Initiative for Excellent Young
Researchers (from Ministry of Education, Culture, Sports, Science and Technology-Japan). The
authors thank Len Sidisky from MilliporeSigma for preparing 30 meter columns of ZILs 1 and 2.
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44
CHAPTER 3. EVALUATING THE SOLVATION PROPERTIES OF METAL-
CONTAINING IONIC LIQUIDS USING THE SOLVATION PARAMETER MODEL
Modified and reprinted from Anal. Bioanal. Chem. 2018, 410, 4597-4606
Copyright © 2018, Springer
He Nan, Liese Peterson, Jared L. Anderson
3.1 Abstract
Ionic liquids (IL) have been utilized as gas chromatography stationary phases due to their
high thermal stability, negligible vapor pressure, wide liquid range, and the ability to solvate a
range of analytes. In this study, the solvation properties of eight room temperature ILs containing
various transition and rare earth metal centers (e.g., Mn(II), Co(II), Ni(II), Nd(III), Gd(III), and
Dy(III)) are characterized using the Abraham solvation parameter model. These metal-containing
ILs (MCILs) consist of the trihexyl(tetradecyl)phosphonium cation and functionalized
acetylacetonate ligands chelated to various metals. They are used in this study as gas
chromatographic stationary phases to investigate the effect of the metal centers on the separation
selectivities for various analytes. In addition, two MCILs comprised of tetrachloromanganate and
tris(trifluoromethylphenylacetylaceto)manganate anions were used to study the effect of
chelating ligands on the selectivity of the stationary phases. Depending on the metal center and
chelating ligand, significant differences in solvation properties were observed. MCILs containing
Ni(II) and Mn(II) metal centers exhibited higher retention factors and higher peak asymmetry
factors for amines (e.g., aniline and pyridine). Alcohols (e.g., phenol, p-cresol, 1-octanol, and 1-
decanol) were strongly retained on the MCIL stationary phase containing Mn(II) and Dy(III)
metal centers. This study presents a comprehensive evaluation into how the solvation properties
45
of ILs can be varied by incorporating transition and rare earth metal centers into their structural
make-up. In addition, it provides insight into how these new classes of ILs can be used for
solute-specific gas chromatographic separations.
3.2 Introduction
Ionic liquids (IL) are salts with melting points under 100 ºC [1]. ILs have attracted
attention from various areas of interdisciplinary research due to their high thermal stability, low
vapor pressure, and unique separation and reaction selectivities [2, 3]. ILs have been successfully
employed as stationary phases for gas chromatography, solvents for organic synthesis, solvents
for liquid-liquid extraction, sorbent coatings for solid-phase microextraction, and membrane
materials for selective filtration [3-7]. Gas chromatography (GC) columns utilizing IL-based
stationary phases have been commercially available for several years and often exhibit unique
selectivities compared to polydimethyl(siloxane) (PDMS) and poly(ethylene glycol) (PEG)-
based stationary phases [8-10]. Unique separation selectivities towards target analytes (e.g., fatty
acid methyl esters (FAMEs)), polyaromatic hydrocarbons, and petrochemicals) have been
demonstrated by introducing various functional groups to the IL structures or by creating
dicationic/tricationic ILs [11-13].
Recently, our group has sought to incorporate transition or rare earth metals (e.g., Ni(II),
Co(II), Mn (II), and Dy(III)) into hydrophobic ILs in an effort to exploit their paramagnetic
properties for various analytical and bioanalytical applications [14-16]. Interestingly, depending
on the choice of the metal center, the metal-containing ionic liquids (MCILs) exhibit vastly
different extraction efficiencies toward target analytes. For example, the Ni(II)-based MCIL was
shown to have superior extraction of E. Coli cells, while the Mn(II)-based MCIL possessed
46
excellent extraction efficiency toward phenolics, insecticides, and polycyclic aromatic
hydrocarbons [14, 15]. A limitation of these studies is that they do not utilize a platform that
permits a systematic investigation into the types and magnitude of intermolecular interactions
between the analytes and MCILs. Inverse GC analysis is an ideal tool that can be exploited to
examine the unique solvation properties offered by MCILs.
The first use of GC stationary phases containing metal centers dates back to the mid-
1950s [17, 18]. Most of these solid stationary phases were incorporated into packed columns [19-
23]. Cobalt(II) chloride and nickel(II) chloride were added to a packed column, which favored
the separation of oxygen containing compounds from other polar organic substances [19-21].
Rhodium (II), palladium(II), and silver(I) compounds were used as stationary phase additives for
the separation of paraffins and olefins [24-26]. The use of metal containing stationary phases was
further extended after the invention of open-tubular capillary columns [27, 28]. To obtain liquid
stationary phases, the metals were dissolved as inorganic salts in a classic liquid stationary phase,
in an effort to eliminate gas-solid adsorption [29-32]. Wasiak and co-workers reported various
structurally modified PDMS-based stationary phases containing metal ions (e.g., Ni(II), Co(II),
and Cu(II)) [33, 34]. However, the utilization of metal containing stationary phases for selective
separation can be limited by the low solubility of the inorganic salt in the stationary phases. For
example, when the concentration of Ni(II) or Co(II) in the mercaptopropylmethyl polysiloxane-
based stationary phase reached the level of 0.007 or 0.04 mol mol-1 (molar ratio of metal and
thiol group), the stationary phase changed from a viscous liquid to a gum-like material, resulting
in decreased separation performance with increasing adsorptive characteristics of the stationary
phase [33].
47
By combining metal-containing anions with a bulky phosphonium cation, the resulting
MCILs possess a high concentration (ranging from 1.4 to 1.8 mol L-1) of transition and rare earth
metal centers (e.g., Ni(II), Mn(II), Dy (III), and Nd(III)) and are liquids at room temperature
making them ideal gas-liquid chromatography (GLC) stationary phases. In this study, the
solvation properties of eight room temperature MCILs were investigated using the Abraham
solvation parameter model. Seven of the MCILs possess different transition and rare earth metal
centers as well as different chelating ligands. In addition, one MCIL lacking any ligand within
the anion component was used for comparison. Significant differences in solvation properties of
MCILs were observed depending on the choice of metal center as well as the presence and type
of the chelating ligands. Fifteen meter columns possessing high efficiency were prepared using
Mn(II) and Dy(III)-based MCILs. The separation selectivity of the MCIL-based GC columns
toward different analyte groups were compared with commercial PDMS (Rtx-5) and IL-based
(SLB IL-111) columns. Vastly different separation selectivities for a wide range of analytes,
particularly amines and alcohols, was observed. The results from this study are the first to
provide an understanding into how the structural basis of MCILs affect their solvation
characteristics and how their unique structural properties can be exploited in selective solute-
specific gas chromatographic separations.
3.3 Experimental Section
3.3.1 Materials
Eight MCILs were examined in this study. Their chemical structures are shown in Figure
3.1. They consist of six hexafluoroacetylacetonate-based MCILs, namely IL 1,
trihexyl(tetradecyl)phosphonium tris(hexafluoroacetylaceto)nickelate(II) ([P66614+]
48
[Ni(II)(hfacac)3-]); IL 2, [P66614
+] tris(hexafluoroacetylaceto)cobaltate(II) ([Co(II)(hfacac)3-]); IL
3, [P66614+] tris(hexafluoroacetylaceto)manganate(II) ([Mn(II)(hfacac)3
-]); IL 4, [P66614+]
tetrakis(hexafluoroacetylaceto)dysprosate(III) ([Dy(III)(hfacac)4-]); IL 5, [P66614
+]
tetrakis(hexafluoroacetylaceto)gadolinate(III) ([Gd(III)(hfacac)4-]); and IL 6, [P66614
+]
tetrakis(hexafluoroacetylaceto)nyodymate(III) ([Nd(III)(hfacac)4-]). Additionally, IL 7 [P66614
+]
tris(trifluoromethylphenylacetylaceto)manganate(II) (Mn(II)(tfmphacac)3-]) and IL 8 [P66614
+]2
tetrachloromanganate(II) ([MnCl42-]) were used to examine the effect of chelating ligands. All
MCILs were synthesized and characterized according to previously reported methods [14, 35].
Butyraldehyde, 1-chlorobutane, ethyl acetate, methyl caproate, and 2-nitrophenol were
purchased from Acros Organics (Morris Plains, NJ, USA). Bromoethane was purchased from
Alpha Aesar (Ward Hill, MA, USA). Ethyl benzene was purchased from Eastman Kodak
Company (Rochester, NJ, USA). Acetic acid, N,N-dimethylformamide and toluene were
purchased from Fisher Scientific (Pittsburgh, PA, USA). 2-chloroaniline, p-cresol, naphthalene,
o-xylene, p-xylene, and 1-bromohexane were purchased from Fluka (Steinheim, Germany); and
benzaldehyde, 1-chlorohexane, 1-chlorooctane, cyclohexanol, cyclohexanone, 1-iodobutane, 1-
nitropropane, octylaldehyde, 1-pentanol, 2-pentanone, propionitrile, 1-decanol, acetophenone,
aniline, benzonitrile, benzyl alcohol, 1-bromooctane, 1-butanol, 1,2-dichlorobenzene,
dichloromethane, 1,4-dioxane, 1-octanol, phenol, pyridine, pyrrole, m-xylene, 2-propanol, and
propionic acid were purchased from Sigma Aldrich (St. Louis, MO, USA). All analytes were
used as received. Untreated fused silica capillary (I.D. 250 µm) and a SLB IL-111 column (30 m
× 250 µm × 0.20 µm) were obtained from Supelco (Bellefonte, PA, USA). The Rtx-5 column (30
m × 250 µm × 0.25 µm) was purchased from Restek (Bellefonte, PA, USA). The two purchased
commercial columns were cut to 15 m for the comparison with MCIL-based GC columns.
49
Figure 3.1 Chemical structures of the eight metal-containing ILs examined in this study.
3.3 2 Preparation of GC Columns
Five or fifteen-meter untreated fused silica capillary columns were coated with MCILs
using the static coating method. The MCIL coating solution was prepared at a concentration of
0.45% (w/v) in dichloromethane in order to produce an approximate film thickness of 0.28 µm.
The coated capillary columns were conditioned from 40-110 °C at 3 °C/min and held for two
hours. The column efficiency was determined using naphthalene at 100 °C. The list of prepared
columns is shown in Table 3.1. All columns had efficiencies ranging from 1800 to 3700
plates/meter. Compared to most traditional ILs containing the [P66614+] cation, it was observed
50
Table 3.1 Characteristics of metal-containing ionic liquid based stationary phases examined in
this study.
IL
No. Abbreviation
Metal
Center
Film Thickness
(µm)
Length
(m)
Efficiency
(Plates/ Meter)
1 [P66614+][Ni(hfacac)3
-] Ni(II) 0.28 5 2800
2 [P66614+][Co(hfacac)3
-] Co(II) 0.28 5 2700
3 [P66614+][Mn(hfacac)3
-] Mn(II) 0.28 5 2700
0.28 15 3700
4 [P66614+][Dy(hfacac)4
-] Dy(III) 0.28 5 2400
0.28 15 3700
5 [P66614+][Gd(hfacac)4
-] Gd(III) 0.28 5 2200
6 [P66614+][Nd(hfacac)4
-] Nd(III) 0.28 5 2700
7 [P66614+][Mn(tfmphacac)3
-] Mn(II) 0.28 5 2400
8 [P66614+]2[MnCl4
2-] Mn(II) 0.28 5 1800
that the MCILs examined in this study possessing the hexafluoroacetylacetonate (hfacac) or
trifluoromethylphenylacetylacetonate (tfmphacac) ligand exhibited more superior wetting ability
on the surface of the untreated capillary.
3.3.3 Preparation of Probe Solute Standards and Chromatographic Conditions
The analyte standards were prepared in dichloromethane at a concentration of 1 mg/mL.
A mixture of analytes was prepared using fifteen different compounds with a concentration of 1
mg/mL. All separations were performed on an Agilent 7890B gas chromatograph with a flame
ionization detector. Helium was used as the carrier gas with a flow rate of 1 mL/min. The
injector and detector temperatures were held at 250 °C. The detector used hydrogen as a makeup
gas at a flow rate of 30 mL/min and air flow was held at 400 mL/min.
A list of the 46 analytes and their corresponding solute descriptors is provided in Table
A1 (see appendix A). All probe molecules were dissolved in methylene chloride and injected
individually at 3 different oven temperatures (50, 80, and 110 °C). Analytes possessing low
boiling points exhibited low retention at higher temperatures whereas others exhibited very
51
strong retention on the stationary phase (in some cases, beyond 3 hours). As a result, not all
probe molecules could be subjected to regression analysis at the temperatures studied. Multiple
linear regression analysis and statistical calculations were performed using the program Analyze-
it (Leeds, UK)
3.4 Results and Discussion
The MCILs examined in this study (see Figure 3.1) were carefully selected to compare
the effect of metal centers and chelating ligands on the solvation properties. All eight MCILs
contain the same phosphonium cation. Six of the MCILs (ILs 1-6) contain the hfacac chelating
ligand but with different metal centers (e.g., Ni(II), Co(II), Mn(II), Dy(III), Gd(III), and Nd(III)).
IL 7 contains the tfmphacac chelating ligand, while IL 8 consists of the [MnCl4]2- anion and
lacks any chelating ligand. In this study, the solvation properties of all eight MCILs were
examined using the solvation parameter model developed by Abraham in the 1990s [36-38]. This
model, represented by Equation 1, has been used extensively to examine the solvation properties
of a wide range of ILs.
𝐿𝑜𝑔 𝑘 = 𝑐 + 𝑒𝐸 + 𝑠𝑆 + 𝑎𝐴 + 𝑏𝐵 + 𝑙𝐿 (1)
As shown in Equation 1, k is the retention factor of each probe molecule on the MCIL
stationary phase at a specific temperature (50 ºC, 80 ºC, or 110 ºC). The solute descriptors (E, S,
A, B, and L) of the 46 probe molecules have been previously reported and are listed in Table A1
(see appendix A). The solute descriptors are defined as: E, the excess molar refraction calculated
from the solute’s refractive index; S, the solute dipolarity/polarizability; A, the solute hydrogen
bond acidity; B, the solute hydrogen bond basicity; and L, the solute gas-hexadecane partition
coefficient determined at 298 K. Multiple linear regression analysis was performed using the
solute descriptor of probe molecules and their retention factors to determine the intermolecular
interaction between the probe molecules and IL-based stationary phases. The c term is the
52
intercept of the regression line. The system constants (e, s, a, b, and l) are used to characterize
the strength of each solvation interaction. The system constants are defined as: e, the ability of
the stationary phase to interact with analytes by electron lone pair interactions; s, a measure of
the dipolarity/polarizability of the stationary phase; a, the hydrogen bond basicity of the IL
stationary phase; b, the hydrogen bond acidity of the IL stationary phase; and l describes the
dispersion forces/cavity formation of the IL. The system constants of all eight MCILs at 50 ºC,
80 ºC, and 110 ºC are listed in Table 3.2. For the majority of the MCILs studied, the system
constants exhibit a smooth decrease as the column temperature is increased. The multiple linear
regression fits are statistically sound, as represented by the Fisher coefficients which range from
400-696.
3.4.1 Effect of Metal Center on System Constants
Since ILs 1-8 possess the same [P66614+] cation, the variation in system constants can be
attributed to the different counteranions. Among ILs 1-6, the Dy(III)-based MCIL (IL 4)
possessed the highest dipolarity/polarizability (s = 1.74 at 80 °C), while the Nd(III)-based MCIL
(IL 6) exhibited the lowest dipolarity/polarizability value (s = 1.17 at 80 °C). The Mn(II)-based
MCIL (IL 3) exhibited by far the highest hydrogen bond basicity (a = 2.34 at 80 °C) among the
six MCILs containing the hfacac chelating ligands. Conversely, the Ni(II)-based MCIL (IL 1)
possessed the lowest hydrogen bond basicity (a = 0.57) at 80 °C (see Table 3.2). In the case of
the hydrogen bond acidity (b term) for ILs 1-6, all values were positive with the Nd(III)-based
MCIL (IL 6) producing a hydrogen bond acidity (b = 0.90 at 80 °C) nearly 5 times higher than
that of the Co(II)-based MCIL (b = 0.20 at 80 °C). ILs 1-6 were observed to exhibit similar
dispersive type interactions and can be regarded as moderately cohesive stationary phases. The
transition
53
Table 3.2 System constants of the studied metal-containing ionic liquids obtained using the
solvation parameter model.
Stationary
Phase/Tempera
ture (°C)
System constants
c e s a b l n a R2 a F a
IL 1 Trihexyl(tetradecyl)phosphonium tris(hexafluoroacetylaceto)nickelate(II)
50 -3.09
(0.09)b
-0.59
(0.08)
1.79
(0.10)
0.91
(0.10)
0.49
(0.13)
0.79
(0.02) 36 0.99 440
80 -2.96
(0.08)
-0.56
(0.07)
1.63
(0.09)
0.57
(0.09)
0.28
(0.11)
0.60
(0.02) 37 0.99 451
110 -3.09
(0.07)
-0.43
(0.05)
1.48
(0.07)
0.29
(0.06)
0.26
(0.09)
0.57
(0.02) 33 0.99 486
IL 2 Trihexyl(tetradecyl)phosphonium tris(hexafluoroacetylaceto)cobaltate(II)
50 -2.81
(0.09)
-0.56
(0.08)
1.61
(0.11)
1.62
(0.14)
0.28
(0.14)
0.77
(0.02) 30 0.99 427
80 -2.86
(0.07)
-0.47
(0.06)
1.49
(0.08)
1.17
(0.10)
0.20
(0.11)
0.66
(0.01) 34 0.99 617
110 -2.68
(0.07)
-0.34
(0.05)
1.26
(0.07)
0.81
(0.09)
0.14
(0.10)
0.54
(0.01) 31 0.98 411
IL 3 Trihexyl(tetradecyl)phosphonium tris(hexafluoroacetylaceto)manganate(II)
50 -3.18
(0.10)
-0.46
(0.09)
1.51
(0.11)
2.66
(0.15)
0.89
(0.15)
0.82
(0.03) 31 0.99 467
80 -2.97
(0.07)
-0.40
(0.06)
1.33
(0.08)
2.34
(0.10)
0.51
(0.11)
0.69
(0.02) 31 0.99 633
110 -3.03
(0.09)
-0.38
(0.07)
1.33
(0.10)
1.75
(0.09)
0.25
(0.14)
0.60
(0.02) 26 0.99 463
IL 4 Trihexyl(tetradecyl)phosphonium tetrakis(hexafluoroacetylaceto)dysprosate(III)
50 -2.95
(0.09)
-0.68
(0.09)
1.86
(0.13)
1.82
(0.20)
0.68
(0.18)
0.77
(0.02) 30 0.99 406
80 -2.85
(0.07)
-0.57
(0.07)
1.74
(0.11)
1.41
(0.16)
0.50
(0.14)
0.65
(0.02) 30 0.99 558
110 -2.88
(0.08)
-0.43
(0.07)
1.67
(0.08)
1.38
(0.09)
0.38
(0.12)
0.52
(0.02) 25 0.99 452
IL 5 Trihexyl(tetradecyl)phosphonium tetrakis(hexafluoroacetylaceto)gadolinate(III)
50 -2.98
(0.09)
-0.64
(0.08)
1.86
(0.11)
1.78
(0.14)
0.70
(0.14)
0.76
(0.02) 34 0.99 493
80 -3.01
(0.07)
-0.53
(0.06)
1.67
(0.09)
1.16
(0.11)
0.61
(0.12)
0.65
(0.02) 36 0.99 583
110 -3.04
(0.07)
-0.45
(0.06)
1.51
(0.08)
0.86
(0.09)
0.48
(0.10)
0.56
(0.01) 34 0.99 493
54
Table 3.2 (continued)
Stationary
Phase/Tempera
ture (°C)
System constants
c e s a b l n a R2 a F a
IL 6 Trihexyl(tetradecyl)phosphonium tetrakis(hexafluoroacetylaceto)nyodymate(III)
50 -2.73
(0.07)
-0.40
(0.06)
1.27
(0.08)
1.50
(0.10)
1.15
(0.11)
0.75
(0.02) 31 0.99 444
80 -2.69
(0.08)
-0.40
(0.07)
1.17
(0.08)
1.12
(0.08)
0.90
(0.11)
0.62
(0.02) 29 0.99 474
110 -2.73
(0.07)
-0.35
(0.06)
1.04
(0.08)
0.81
(0.10)
0.80
(0.11)
0.55
(0.02) 26 0.99 403
IL 7 Trihexyl(tetradecyl)phosphonium tris(trifluoromethylphenylacetylaceto)manganate(II)
50 -3.09
(0.09)
-0.43
(0.08)
1.66
(0.10)
2.91
(0.14)
-0.25
(0.13)
0.82
(0.02) 36 0.99 463
80 -3.04
(0.09)
-0.35
(0.07)
1.51
(0.08)
2.19
(0.11)
-0.28
(0.11)
0.70
(0.02) 34 0.99 400
110 -3.05
(0.07)
-0.23
(0.05)
1.27
(0.07)
1.38
(0.10)
-0.08
(0.10)
0.60
(0.02) 31 0.99 507
IL 8 Trihexyl(tetradecyl)phosphonium tetrachloromanganate(II)
50 -3.00
(0.09)
-0.29
(0.08)
1.94
(0.11)
3.88
(0.14)
-0.73
(0.14)
0.76
(0.02) 31 0.99 565
80 -2.96
(0.07)
-0.18
(0.06)
1.75
(0.09)
3.30
(0.11)
-0.71
(0.12)
0.63
(0.02) 33 0.99 614
110 -3.11
(0.07)
-0.14
(0.06)
1.63
(0.08)
2.76
(0.09)
-0.66
(0.10)
0.56
(0.01) 32 0.99 696
a Note: n, number of probe analytes subjected to multiple linear regression analysis; R2,
correlation coefficient; F, Fisher coefficients. b The values in brackets are the reported standard deviations.
metal based MCILs (ILs 1-3) possess different molar ratios of metal center and chelating ligand
than the rare-earth metal based MCILs (ILs 4-6). However, no distinct trends in system constants
can be observed based on the molar ratio of metal center and chelating ligand.
3.4.2 Effect of Chelating Ligand on System Constants
To examine the effect of the chelating ligand on the system constants, two MCILs (IL 7
and IL 8) were selected to compare with ILs 1-6. IL 7 contains the tfmphacac chelating ligand
while IL 8 possesses the [MnCl42-] anion and does not contain any chelating ligand (see Figure
55
3.1). ILs 3, 7, and 8 are all Mn(II)-based MCILs (see Figure 3.1 and Table 3.2). MCILs with
hfacac ligands (ILs 1-6) possess positive values for the hydrogen bond acidity (see Table 3.2).
However, IL 8 exhibits a large negative value (b = -0.71 at 80 ºC). This result seems to indicate
that the hydrogen bond acidity of MCILs can be strongly influenced by the chelating ligand or
the absence of a ligand. The system constants for [P66614+]-based ILs with various anions (e.g.,
chloride, bis[(trifluoromethyl)sulfonyl]imide, triflate, and tetrafluoroborate) were previously
reported [39]. The hydrogen bond acidity (b term) of all reported phosphonium ILs were
negative, since there are no acidic hydrogens that can act as hydrogen bond donors. In addition,
the value of hydrogen bond acidity was also negative or close to zero for the [P66614+]
tetrachloroferrate IL, which possesses structural similarity to IL 8 [40]. Interestingly, IL 7
containing the tfmphacac ligands also possesses a negative hydrogen bond acidity value (see
Table 3.2), which is opposite to IL 3 with the hfacac ligand, despite the structural similarities.
This result indicates that the phenyl groups may influence the proton donating capability of the
tfmphacac ligand.
3.4.3 Separation of Analyte Mixtures using MCIL-Based GC Columns
A mixture of fifteen analytes was subjected to isothermal separation (80 ºC) on four
different columns containing the same column length (15 m), including the commercial Rtx-5
and SLB IL-111 columns and two MCIL-based columns. The Mn(II) and Dy(III)-based MCILs
were selected as representative stationary phases due to the highest hydrogen bond basicity and
dipolarity/polarizability, respectively, among the six MCILs with hfacac ligands. As shown in
the Figure 3.2A, the separation was performed on the Rtx-5 column, which is among the most
widely used PDMS-based GC columns. All analytes were separated except for acetophenone and
56
Figure 3.2 Chromatographic separation of a mixture containing 15 analytes on different
columns: (A) Restek Rtx-5 column, (B) Supelco SLB IL-111 column, (C) IL 3 Mn(II)-based
MCIL column, and (D) IL 4 Dy(III)-based MCIL column. Analytes: 1, nitropropane, 2, pyridine,
3, N,N-dimethylformamide, 4, 1-chlorohexane, 5, ethylbenzene, 6, p-xylene, 7, cyclohexanol, 8,
cyclohexanone, 9, 1,2-dichlorobenzene, 10, 1-chlorooctane, 11, acetophenone, 12, 1-octanol, 13,
nitrobenzene, 14, 2-chloroaniline, 15, 1-bromooctane. Separation conditions: flow rate, 1
mL/min, isothermal separation at 80 ºC. The cyclohexanol and 1-octanol eluted out from Mn(II)
and Dy(III)-based MCIL columns with a broad and tailing peak, which is difficult to distinguish
from the baseline. The N,N-dimethylformamide eluted out from Dy(III)-based MCIL column
after 40 min, which was not shown in the chromatogram.
57
1-octanol, which possess similar boiling points (202 and 195 ºC). Since the commercial SLB IL-
111 column is much more polar than the Rtx-5 column, the observed retention behavior of these
analytes on this column is different (see Figure 3.2B). Analytes containing nitro, amine, or
hydroxy groups were strongly retained by the SLB IL-111 column compared to the Rtx-5
column. Acetophenone and nitrobenzene were observed to co-elute on the SLB IL-111 column,
while N,N-dimethylformamide (DMF) exhibited a strong tailing peak starting at 11.1 min (see
Figure 3.2B).
As shown in Figures 3.2C and 3.2D, the MCIL-based columns exhibited very different
retention characteristics compared to the Rtx-5 or SLB IL-111 columns. A consistent retention
order among analytes including 1-chlorohexane, ethylbenzene, p-xylene and 2-chloroaniline can
be observed on all four columns. However, most of the remaining analytes such as
cyclohexanone, 1,2-dichlorobenzene, 1-chlorooctane, and nitrobenzene were more strongly
retained on the MCIL-based columns. Furthermore, the retention order of certain analyte pairs
was reversed depending on the incorporated metal center. It has been previously reported that the
retention of analytes can be modified by incorporating metal containing salts [19, 29]. When
MCILs with different metal centers were employed as the GC stationary phase, interesting
chromatographic retention characteristics can be observed. As shown in Figure 3.2C, pyridine
eluted after nitropropane, 1-chlorooctane, and 1-bromooctane on the Mn(II)-based MCIL column
(IL 3). However, pyridine eluted before these three analytes on the Dy(III)-based MCIL column
(IL 4). The retention order of cyclohexanone and 1,2-dichlorobenzene was reversed on the two
MCIL-based columns. Therefore, the separation selectivities offered by the MCIL-based
columns are strongly affected by the metal center incorporated in the MCIL.
58
Table 3.3 Comparison of retention factors of selected analytes on eight different MCIL-based
stationary phases with varying metal centers and chelating ligands at 80 ºC (ILs 1–7). IL 8 does
not contain a metal chelating ligand. See structures of MCILs in Figure 3.1.
IL 1 IL 2 IL 3 IL 4 IL 5 IL 6 IL 7 IL 8
Probe molecule Ni Co Mn Dy Gd Nd Mn Mn
1-Nitropropane 2.2 2.1 2.0 3.2 2.3 2.2 1.4 1.5
Pyridine 119.8 16.9 25.9 3.7 2.2 4.7 1.3 9.7
N,N-
Dimethylformamide 17.3 12.0 31.0 64.6 43.2 94.2 5.3 5.4
1-Chlorohexane 1.2 1.4 1.4 1.7 1.2 1.4 1.2 1.0
Ethyl benzene 1.2 1.4 1.3 1.7 1.2 1.4 1.3 1.3
p-Xylene 1.3 1.5 1.5 1.9 1.3 1.5 1.4 1.3
Cyclohexanol 2.5 4.0 15.2 7.9 5.0 8.3 5.3 7.0
Cyclohexanone 8.5 7.5 7.7 13.5 9.7 9.8 4.2 3.0
1,2-Dichlorobenzene 6.6 7.2 7.1 8.5 6.4 6.7 8.4 10.2
1-Chlorooctane 5.7 6.3 6.0 7.2 5.2 5.7 5.7 4.2
Acetophenone 28.7 25.8 26.2 41.7 30.3 31.1 19.9 15.7
1-Octanol 11.1 18.7 123.9 44.7 22.6 47.6 24.6 30.3
Nitrobenzene 31.1 29.4 28.6 43.8 32.5 29.9 27.6 27.7
2-Chloroaniline 24.3 29.8 30.5 32.2 27.7 27.3 46.9 144.7
1-Bromooctane 10.0 11.4 10.8 12.7 9.0 10.0 10.9 8.2
3.4.4 Effect of Metal Centers and Chelating Ligands on the Separation Selectivity for Selected
Analytes
The retention factors of 15 selected analytes on the eight 5 m columns with different
MCIL-based stationary phases are listed in Table 3.3. The retention factors of 1-chlorohexane,
ethylbenzene, p-xylene, 1,2-dichlorobenzene, 1-chlorooctane, and 1-bromooctane were similar
among all eight MCIL-based columns. The retention factor of pyridine on the Ni(II)-based MCIL
column (IL 1) was 119.8, which is an order of magnitude higher than all other MCIL-based
columns. Cyclohexanol and 1-octanol exhibited the highest retention on IL 3 (Mn(II)-based
MCIL) and lowest retention on the Ni(II)-based MCIL column (IL 1). The Dy(III)-based MCIL
59
Table 3.4 Effect of metal center and chelating ligand of MCILs on the selectivity of chosen
solute pairs at 80 °C.
IL 1 IL 2 IL 3 IL 4 IL 5 IL 6 IL 7 IL 8
Solute pairb Ni Co Mn Dy Gd Nd Mn Mn
Naphthalene/p-xylene 18.9 16.8 17.9 18.3 18.9 16.7 18.8 22.4
1-Octanol/1-butanol 23.9 23.2 40.0 26.8 21.7 26.0 24.1 18.3
1-Butanol/p-xylene 0.4a 0.5a 2.1 0.9a 0.8a 1.2 0.7a 1.3
Naphthalene/benzonitrile 1.5 1.7 1.7 1.4 1.4 1.6 2.0 2.4
1-Bromooctane/benzene 39.1 34.9 38.8 31.6 31.7 25.1 40.2 50.2
1-Nitropropane/1-pentanol 2.0 1.1 0.2a 0.9a 1.0 0.5a 0.6a 0.5a
Butyraldehyde/benzene 1.9 1.4 1.5 1.9 1.9 1.6 1.7 0.5a
Cyclohexanone/cyclohexanol 3.4 1.9 0.5a 1.7 1.9 1.2 0.8a 0.4a
Methyl caproate/1-butanol 9.4 5.0 1.3 3.8 4.4 2.8 2.4 1.0
Octylaldehyde/1-pentanol 9.7 5.3 1.2 4.1 4.6 2.6 2.6 1.3
Pentanone/1-pentanol 1.4 0.7a 0.2a 0.7a 0.7a 0.4a 0.3a 0.2a
Benzyl alcohol/naphthalene 0.5a 0.4a 0.4a 0.4a 1.3 1.8 2.1 2.4
2-Chloroaniline/naphthalene 1.0 1.2 1.2 0.9a 1.1 1.1 1.8 5.0
Pyridine/naphthalene 4.8 0.7a 1.0 0.1a 0.1a 0.2a 0.1a 0.3a a By the definition of selectivity, the value should not be smaller than unity. However, in some
cases, the solute pairs exhibited reversed elution order, which makes it is impossible to report
selectivities greater than one for all MCIL-based columns. b Additional solutes from those listed in Table 3.3 were used to make the selectivity comparisons.
exhibited unique selectivity toward analytes with carbonyl and nitro groups (e.g., 1-nitropropane,
nitrobenzene, cyclohexanone, and acetophenone), as the retention factor of these analytes are
among the highest on this column. N,N-dimethylformamide was strongly retained on the IL 6
(Nd(III)) and IL 4 (Dy(III)) MCIL-based columns. These results further demonstrate the unique
separation selectivities offered by the different metal centers. A comparison of IL 3 and IL 8
shows that the [MnCl42-] anion provides lower retention factors of alcohols, while exhibiting
significantly higher retention of 2-chloroaniline, but lower retention of pyridine and N,N-
dimethylformamide.
Interesting separation behavior can be further illustrated by examining the selectivity of
solute pairs. As shown in Table 3.4, the selectivities of selected analyte pairs were notably
60
affected by the choice of MCILs with different metal centers and chelating ligands. The retention
order of butanol/p-xylene, 1-nitropropane/1-pentanol, and pyridine/naphthalene were reversed on
different MCIL-based columns. The Ni(II)-based MCIL (IL 1) showed strong selectivity toward
pentanone, 1-nitropropane, and pyridine compared to all the other MCILs. Alcohols (e.g., 1-
butanol, 1-pentanol, and cyclohexanol) were more strongly retained on the Mn(II)-based MCIL
column (IL 3). Interestingly, the retention order of cyclohexanone and cyclohexanol was the
same on all three Mn(II)-based MCILs (ILs 3, 7, and 8), but were reversed on all other MCIL
stationary phases. The selectivity between 2-chloroaniline and naphthalene was the highest on
the IL 8 column (possessing the [MnCl42-] anion). The IL 8 stationary phase also showed
reversed retention order for butyraldehyde and benzene compared to all the other MCIL-based
columns.
3.5 Conclusions
In this study, the solvation properties for a total of eight MCILs were investigated for the
first time using the Abraham solvation parameter model. Different solvation properties of MCILs
was observed depending on the metal centers and chelating ligands. The Mn(II)-based MCIL (IL
3) possessed the highest hydrogen bond basicity, while the Dy(III)-based MCIL (IL 4) exhibited
the highest dipolarity/polarizability. A separation of a test mixture of analytes with various
functional groups were compared using two MCIL-based columns, a commercial PDMS column,
and a commercial IL-based column. The retention factors of analytes such as 1-chlorohexane,
ethylbenzene, and p-xylene remained consistent on four columns. However, the retention factors
of other analytes were vastly different on the MCIL-based columns compared to the commercial
PDMS and IL-based columns. The system constants obtained for MCILs in this study provides
insight for the future design of solute-specific GC stationary phases using room temperature ILs
61
with various metal centers. Furthermore, this study describes the type and magnitude of
intermolecular interactions between different analyte groups and MCILs possessing transition
and rare earth metals. This enhanced understanding of solvation characteristics can be exploited
in future analytical and bioanalytical applications involving these very unique and interesting
materials.
3.6 Acknowledgments
The authors acknowledge funding from Chemical Measurement and Imaging Program at
the National Science Foundation (Grant number CHE-1709372). Stephen Pierson and Gabriel
Odugbesi are acknowledged for their assistance in the preparation of MCILs and coated capillary
column
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65
CHAPTER 4. ARGENTATION GAS CHROMATOGRAPHY REVISITED:
SEPARATION OF LIGHT OLEFIN/PARAFFIN MIXTURES USING SILVER-
BASED IONIC LIQUID STATIONARY PHASES
Modified and reprinted from J. Chromatogr. A 2017, 1523, 316-320
Copyright © 2017, Elsevier
He Nan, Cheng, Zhang, Amrit, Venkatesh, Aaron J. Rossini, Jared L. Anderson
4.1 Abstract
Silver ion or argentation chromatography utilizes stationary phases containing silver ions
for the separation of unsaturated compounds. In this study, a mixed-ligand silver-based ionic
liquid (IL) was evaluated for the first time as a gas chromatographic (GC) stationary phase for
the separation of light olefin/paraffin mixtures. The selectivity of the stationary phase toward
olefins can be tuned by adjusting the ratio of silver ion and the mixed ligands. The maximum
allowable operating temperature of these stationary phases was determined to be between 125 ºC
and 150 ºC. Nuclear magnetic resonance (NMR) spectroscopy was used to characterize the
coordination behavior of the silver-based IL as well as provide an understanding into the
retention mechanism of light olefins.
4.2 Introduction
Light olefins are among the most important feedstocks employed in the production of
various industrial chemicals including polymers (e.g., polyethylene and polypropylene),
oxygenates (e.g., ethylene glycol and propylene oxide), and important chemical intermediates [1,
2]. Processes commonly result in final products containing positional and/or geometric isomers
of alkenes and alkynes. Gas chromatographic (GC) separations are among the most widely used
methods for the analytical scale separation of olefin/paraffin mixtures. Due to the similar polarity
66
and volatility of paraffins and olefins containing the same carbon number, these separations can
be very challenging using conventional GC stationary phases [3, 4]. Gas solid chromatography
(GSC) employing an alumina porous layer open tubular (PLOT) column has been widely
employed for the separation of light olefins [5]. However, compared to liquid stationary phases,
the adsorbent materials used in GSC possess a number of drawbacks including limited sample
capacity, chemical and/or geometrical inhomogeneity, and generally requires higher temperature
for faster mass transfer [6]. The development of stationary phases with unique selectivity toward
olefins can provide alternatives to the chromatographic columns available today.
Silver ions possess vacant orbitals that undergo selective and reversible π-complexation
with unsaturated hydrocarbons [7-9]. Silver ion (or argentation) chromatography has been widely
used for the separation of lipids with units of unsaturation [10-12]. In an approach first reported
by Gil-Av and coworkers, silver nitrate was dissolved in benzyl cyanide or triethylene glycol and
used as a stationary phase for the separation of olefins by GC [13]. However, these stationary
phases exhibited low thermal stabilities [14, 15]. Therefore, GC stationary phases possessing low
vapor pressure, high thermal stability, and high selectivity toward light olefins are needed. Ionic
liquids (ILs) are molten salts composed of an organic cation and an inorganic or organic counter
anion with the melting points less than 100 °C. IL-based stationary phases possess various
advantages including negligible vapor pressure, wide liquid ranges, tunable viscosity, and high
thermal stability [16, 17]. ILs have attracted increased attention as solvents for silver ions to
provide both high selectivity and fast diffusion for membrane-based separation of olefins [18,
19]. Binnemans and coworkers reported a new class of mixed-ligand silver-based ILs for the
electrodeoposition of silver coatings [20]. By coordinating silver ion with different ligands, the
melting points of these silver-based ILs are substantially lower than the corresponding silver salt
67
without any ligand, facilitating faster diffusion of olefins. These compounds are interesting due
to the fact that the IL may help stabilize the silver ion and provide customizable melting points
based on the ligands used.
For the first time, a silver-based IL was applied as a GC stationary phase for the
separation of light olefin/paraffin mixtures using wall coated open tubular (WCOT) columns.
The selectivity of the stationary phase toward olefins was tuned by adjusting the ratio of silver
ion and ligands. NMR spectroscopy was used to characterize the silver-based IL structure and
aid in understanding the retention mechanism of olefins.
4.3 Experimental Section
4.3.1 Materials
Silver oxide, bis[(trifluoromethyl)sulfonyl]amine, acetonitrile, 1-butylimidazole, 1-
chlorobutane, hexane , and 1-hexene were purchased from Sigma-Aldrich (St. Louis, MO, USA).
Lithium bis[(trifluoromethyl)sulfonyl]imide [Li+][NTf2-] was obtained from SynQuest
Laboratories (Alachua, FL, USA). The reagent 1-methylimidazole was purchased from Fluka
(Steinheim, Germany). Trans-2-hexene , cis-2-hexene , 1-hexyne , 2-hexyne , 3-hexyne , 1,5-
hexadiene, and 2,4-hexadiene were obtained from Alfa Aesar (Ward Hill, MA, USA).
Naphthalene and untreated fused silica capillary tubing (I.D. 250 µm) were purchased from
Supelco (Bellefonte, PA, USA). All chemicals were used as received. A 30 m × 250 µm HP-5ms
column (df = 0.25 µm) and a 30 m × 530 µm alumina PLOT column were obtained from Agilent
Technologies (Santa Clara, CA, USA).
4.3.2 Synthesis of silver-based ionic liquids and preparation of GC columns
The chemical structures and melting points of the ILs examined in this study are shown
in Table C1. The synthetic procedure used to produce the mixed-ligand silver-based IL
68
([Ag+(MIM)(BIM)][NTf2-]) was obtained from previously published work [20]. The detailed
synthetic procedure along with 1H NMR data for the silver-based ILs and 1-butyl-3-
methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [BMIM+][NTf2-] are shown in the
appendix C. GC columns were prepared using the static coating method; all procedures and
instrumentation details are provided in the appendix C.
4.4 Results and Discussion
4.4.1 Selectivity of [Ag+][NTf2-] and [Ag+(MIM)(BIM)][NTf2
-] IL stationary phases
To compare the selectivity of the silver-based stationary phases toward olefins, a gas
mixture of hexane and 1-hexene was subjected to separation on four stationary phases consisting
of neat [Ag+][NTf2-], neat [Ag+(MIM)(BIM)][NTf2
-] IL, and mixtures of the two (see Table 4.1).
The [Ag+][NTf2-] salt was selected due to a previous report demonstrating higher solubility of
aliphatic hydrocarbons in salts with [NTf2-] anions [21]. For comparison purposes, an IL
stationary phase containing no silver ion ([BMIM+][NTf2-]) (column 2) and an IL stationary
phase containing a mixture of [Ag+][NTf2-]/[BMIM+][NTf2
-] (column 3) were evaluated. Hexane
and 1-hexene were separated with a resolution of 2.04 on column 1 (see Table 4.1). However, a
strong interaction between 1-hexene and the [Ag+][NTf2-] stationary phase produced a tailing
peak, as shown in Figure 4.1A. The retention mechanism of 1-hexene on column 1 is likely to be
dominated by adsorption, due to the crystalline nature of [Ag+][NTf2-]. When hexane and 1-
hexene were subjected to column 2 containing the [BMIM+][NTf2-] IL stationary phase, no
separation was observed (see Figure 4.2B). It has been reported that the dissolution of silver
compounds into ILs provides faster diffusion of olefins [19]. Thus, column 3 was prepared using
a mixture of [Ag+][NTf2-] and [BMIM+][NTf2
-] at a molar ratio of 1:4 (see Table 4.1).
69
Table 4.1 List of IL stationary phases examined in this study.
Column
No. Stationary phase
Film
thickness
(µm)
Efficiency
(plates/
meter)
Retention
factor of
1-hexene
Resolution
of hexane
and 1-
hexene
1 [Ag+][NTf2-] 0.23 - 10.99 2.04
2 [BMIM+][NTf2-] 0.28 2200 0.05 0a
3
[Ag+][NTf2-] +
[BMIM+][NTf2-]
(Mixing ratio: 1:4)b
0.28 1000 0.37 2.36
4 [Ag+(MIM)(BIM)][NTf2-] 0.28 2600 0.10 1.23
5
[Ag+(MIM)(BIM)][NTf2-] +
[Ag+][NTf2-]
(Mixing ratio: 1:1)b
0.28 1500 1.37 11.27
6
[Ag+(MIM)(BIM)][NTf2-] +
[Ag+][NTf2-]
(Mixing ratio: 1:3)b
0.28 1500 3.68 8.26
aHexane and 1-hexene peaks co-eluted. bIL mixtures are based on molar ratios of the two salts.
Column 3 exhibited limited selectivity toward 1-hexene and low column efficiency (see Figure
4.1C and Table 4.1). As the molar ratio of [Ag+][NTf2-] and [BMIM+][NTf2
-] IL was increased to
4:1, the retention factor for 1-hexene increased to 0.79, compared to 0.37 for column 3 (see
Table C2). However, the resolution of the pair decreased due to the significant peak tailing (see
Figure C3, Appendix C). The mixture was separated on column 4 containing the mixed-ligand
([Ag+(MIM)(BIM)][NTf2-]) IL with a resolution of 1.23 (see Figure 4.1D). It has been previously
reported that a higher concentration of silver ion provides higher selectivity towards compounds
containing units of unsaturation [10]. To study this effect, columns 5 and 6 were prepared using a
mixture of [Ag+(MIM)(BIM)][NTf2-] and [Ag+][NTf2
-] at molar ratio of 1:1 and 1:3,
respectively. As shown in Table 4.1, an increase in the concentration of silver ion produced
higher retention factors for 1-hexene. This result indicates that the selectivity of the silver-based
ILs toward olefins can be tuned by the amount of silver ion in the stationary phase.
70
Figure 4.1 GC separation of hexane and 1-hexene using columns 1-6. See Table 4.1 for a
description of each column stationary phase composition. aIL mixtures are based on molar ratios
of the two salts. Analytes: 1, hexane; 2, 1-hexene. Separation conditions: Length of the column,
5 m; flow rate, 1 mL min-1; oven temperature, 100 ºC.
4.4.2 Separation of saturated and unsaturated hydrocarbons using mixed-ligand silver-based IL
([Ag+(MIM)(BIM)][NTf2-] and [Ag+][NTf2
-]) stationary phase
To further test the separation capability of the mixed-ligand silver-based IL stationary
phase, nine probe molecules consisting of alkane/alkene/alkynes and their isomers were
separated on a 10 m coated column prepared using a mixture of [Ag+(MIM)(BIM)][NTf2-] and
[Ag+][NTf2-] at a molar ratio of 1:1 (see Figure 4.2). Separations were also performed on HP-
5ms and alumina PLOT columns for comparison. Table C3 lists the retention factors of these
nine probe molecules on the three different columns. As shown in Figure 4.2A, the 30 m HP-5ms
71
column exhibited limited selectivity toward all alkenes and alkynes. A more polar PEG-based
column (SUPELCOWAX10 30 m × 250 µm × 0.25 µm) were tested for the separation of the
nine analytes. Limited selectivity toward alkenes and alkynes was shown (see Figure C4,
Appendix C). The analytes were nearly all baseline resolved on the alumina PLOT column, as
shown in Figure 4.2B. A higher separation temperature and flow rate was used. As shown in
Figure 4.2C, all analytes were baseline resolved on the silver IL stationary phase, except for 1-
hexene and cis-2-hexene (which were also poorly resolved on the alumina stationary phase).
Importantly, 1-hexyne did not elute from the IL column. A likely reason for this is the ability for
silver salts and terminal alkynes to form stable alkynyl silver compounds [22]. Alkenes and
internal alkynes were more strongly retained on the silver-based IL stationary phase compared to
hexane. As shown in Figure 4.2C and Table C3, cis-2-hexene exhibited a higher retention factor
compared to trans-2-hexene, which is in good agreement with previous reports for the retention
order of lipids in silver ion liquid chromatography [12]. In addition, the silver-based IL column
provided comparable separation performance toward the geometric isomers of 2,4-hexadiene
compared to the alumina PLOT column (see Figure C5, Appendix C). Moreover, internal
alkynes (e.g., 2-hexyne and 3-hexyne) were more retained than the alkenes (e.g., 1-hexene,
trans-2-hexene, and cis-2-hexene). The retention factor for 1,5-hexadiene on the silver-based IL
column was 22.26, which is much higher than that on the alumina PLOT column (1.35). These
results demonstrate that the mixed-ligand silver-based IL stationary phase possesses unique
selectivity toward various classes of olefins.
72
Figure 4.2 Gas chromatographic separation of nine probe molecules on (A) 30 m HP-5ms
column or 30 m SUPELCOWAX10 column, (B) 30 m alumina PLOT column, and (C) 10 m
column consisting of [Ag+(MIM)(BIM)][NTf2-] and [Ag+][NTf2
-] at a molar ratio of 1:1.
Analytes; 1, hexane; 2, trans-2-hexene; 3, 1-hexene ; 4, cis-2-hexene; 5, 1,5-hexadiene; 6, 2,4-
hexadiene (mixture of three possible isomers); 7, 3-hexyne; 8, 2-hexyne ; 9, 1-hexyne .
Separation conditions: (A) and (C) flow rate, 1 mL min-1; oven temperature, 100 ºC; (B) flow
rate, 10 mL min-1; oven temperature, 160 ºC.
73
4.4.3 Thermal stability of silver-based IL stationary phase
The thermal stability of the best performing IL column prepared using a mixture of
[Ag+(MIM)(BIM)][NTf2-] and [Ag+][NTf2
-] at a molar ratio of 1:1 was examined using thermal
gravimetric analysis (TGA). As shown in Figure 4.3A, the IL lost 1% of its total mass at 175 ºC
and 5% of total mass at 290 ºC. The maximum allowable operating temperature (MAOT) of the
IL stationary phase was estimated using a previously reported method [23]. Briefly, a three meter
coated capillary was conditioned to 100 ºC, 125 ºC, 150 ºC, 175 ºC, and 200 ºC. After each
conditioning step, a mixture of hexane and 1-hexene was separated at 100 ºC under a constant
flow rate of 1 mL min-1. As shown in Figure 4.3B, when the silver-based IL stationary phase was
conditioned from 125 ºC to 150 ºC, the selectivity slightly decreased. When the column was
conditioned to 175 ºC and 200 ºC, a significant loss in selectivity was observed suggesting that
volatilization/decomposition may have occurred in this temperature range. Therefore, the MAOT
of the IL stationary phase was between 125 ºC and 150 ºC. This is considerably higher than the
previously reported stationary phases consisting of silver nitrate in in benzyl cyanide or
triethylene glycol (MAOT of 65 ºC) [24].
4.4.4 Characterization of silver-based IL and silver olefin complexes using NMR
To gain additional understanding into the coordination environment of the mixed-ligand
silver-based IL used in this study, we have applied 109Ag solid-state NMR spectroscopy. 109Ag is
a spin-1/2 nucleus with high isotopic abundance (48.2%), but it possesses a low gyromagnetic
ratio () which gives rise to a low Larmor frequency (18.6 MHz at 9.4 T). The acquisition of
NMR spectra of low- nuclei such as 109Ag is challenging due to their low inherent sensitivities,
long relaxation times and experimental difficulties associated with working at low Larmor
74
Figure 4.3 (A) Thermal gradient analysis (TGA) of silver-based IL and (B) separation of hexane
and 1-hexene using a three meter silver-based IL WCOT column consisting of a mixture of
[Ag+(MIM)(BIM)][NTf2-] and [Ag+][NTf2
-] at a molar ratio of 1:1 after each step of column
conditioning. Analytes: 1, hexane; 2, 1-hexene. Separation conditions: column length, 3 m; flow
rate, 1 mL min-1; oven temperature, 100 ºC.
75
frequencies. One simple way to circumvent these problems is to indirectly detect the 109Ag NMR
signal using a high- nucleus such as 1H or 19F. However, indirect detection in solution NMR
with HMBC or HSQC pulse sequences requires 1H-109Ag scalar (J-) couplings which are likely
vanishingly small in the ILs studied here. To address this problem, we used fast magic angle
spinning (MAS) solid-state NMR spectroscopy to characterize the silver-based ILs. Solid-state
NMR uses dipolar (spatial) couplings to mediate magnetization transfers between 1H and 109Ag
and this allows proton detected 1H-109Ag 2D hetero-nuclear correlation (HETCOR) spectra to be
obtained in short experiment times.
Figure 4.4 shows the 109Ag solid-state NMR spectra of [Ag+][CH3SO3-],
[Ag+(ACN)][NTf2-] and [Ag+(MIM)2][NTf2
-] IL. Each 109Ag NMR spectrum was obtained from
the projection of a proton detected 1H-109Ag 2D HETCOR spectrum. The [Ag+(MIM)2][NTf2-]
IL exists as a solid at room temperature and has a higher melting point than
[Ag+(MIM)(BIM)][NTf2-], so the former has been used a representative compound to
characterize this class of silver-based ILs using solid-state NMR spectroscopy. [Ag+][CH3SO3-]
was used as a setup compound for 109Ag solid-state NMR and was included here for
comparison.[25] Figure 4.4 illustrates that there are clear differences in the 109Ag chemical shifts
and these correlate with the silver coordination environment. The crystal structure of
[Ag+][CH3SO3-] shows the silver ion is coordinated by five oxygen atoms and this complex
shows a 109Ag peak at 53.3 ppm. [Ag+(ACN)][NTf2-] and [Ag+(MIM)2][NTf2
-] have coordination
numbers of 4 (2 oxygen and 2 nitrogen) and 2, and have 109Ag chemical shifts of 148.1 ppm and
423.1 ppm, respectively. The trends in the silver chemical shifts observed here are consistent
with those previously reported where lower coordination numbers and replacement of oxygen
76
Figure 4.4 1D 109Ag projections from 1H-109Ag 2D solid state NMR spectra of (top to bottom)
[Ag+][CH3SO3-], [Ag+(ACN)][NTf2
-] and [Ag+(MIM)2][NTf2-].
with nitrogen in the silver coordination sphere both lead to more positive 109Ag chemical
shifts.[26, 27]
With an understanding of the coordination environment for the mixed ligand silver-based
ILs, it is also important to further examine their selectivity toward the olefinic isomers. In order
to understand the retention mechanism of olefinic isomers, various amounts of [Ag+][NTf2-]
were added to 1-hexene and cis-2-hexene and the 1H chemical shift of the vinyl group as a
function of silver concentration was followed by NMR. This allowed the binding constants (KB)
for the olefin-silver binding to be estimated (see SI). The binding constant for 1-hexene was
found to be higher than that of cis-2-hexene (see Figure C8). This result indicates that 1-hexene
possesses stronger interaction with the silver-based IL stationary phase compared to cis-2-
hexene. This is in good agreement with chromatographic separation data obtained in this study.
This approach will be used in the future to screen additional mixed ligand silver-based ILs to
generate compounds that can achieve better resolution of these two analytes.
77
4.5 Conclusions
In conclusion, mixed-ligand silver IL stationary phases exhibiting tunable selectivity
toward olefins were developed and possess an improved thermal stability compared to previously
reported stationary phases containing silver ion. This study establishes a basis for the use of
highly selective silver ILs as GC stationary phases for the separation and identification of various
classes of olefins and their isomers.
4.6 Acknowledgments
The authors acknowledge funding from Chemical Measurement and Imaging Program at
the National Science Foundation (Grant number CHE-1413199). Prof. James H. Davis Jr. is
thanked for his kind gift of [Ag+][NTf2-].The authors acknowledge funding from the Chemical
Measurement and Imaging Program at the National Science Foundation (Grant number CHE-
1413199).
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Heteroleptic silver-containing ionic liquids, Dalton Trans., 41 (2012) 6902-6905.
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[21] D. Camper, C. Becker, C. Koval, R. Noble, Low pressure hydrocarbon solubility in room
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80
CHAPTER 5. TUNABLE SILVER-CONTAINING STATIONARY PHASES FOR
MULTIDIMENSIONAL GAS CHROMATOGRAPHY
Modified and reprinted from Anal. Chem. 2019, 91, 4969-4974
Copyright © 2019, American Chemical Society
Israel D. Souza, He Nan, Maria Eugênia C. Queiroz, Jared L. Anderson
5.1 Abstract
To achieve high separation power of complex samples using multidimensional gas
chromatography (MDGC), the selectivity of the employed stationary phases is crucial. The non-
polar × polar column combination remains the most popular column set used in MDGC.
However, resolution of mixtures containing light analytes possessing very similar properties
remains a formidable challenge. The development of stationary phases that offer unique
separation mechanisms have the potential to significantly improve MDGC separations,
particularly in resolving co-eluted peaks in complex samples. For the first time, a stationary
phase containing silver(I) ions was successfully designed and employed as a second dimension
column using comprehensive two dimensional gas chromatography (GC × GC) for the separation
of mixtures containing alkynes, dienes, terpenes, esters, aldehydes, and ketones. Compared to a
widely used non-polar and polar column set, the silver-based column exhibited superior
performance by providing better chromatographic resolution of co-eluted compounds. A mixture
of unsaturated fatty acids (UFA) was successfully separated using a GC × GC method in which
the elution order in the second dimension was highly dependent on the number of double bonds
within the sample.
81
5.2 Introduction
Multidimensional gas chromatography, including comprehensive two dimensional gas
chromatography (GC × GC), offers high peak capacity in the analysis of complex samples that
are often poorly separated in conventional one-dimensional (1D) GC [1-5]. The selection of
column sets that maximize peak capacity is a major task in optimizing GC × GC methods. The
non-polar × polar column set constitutes the majority of published GC × GC methods [6, 7].
However, this popular column combination does not always provide the best selectivity in the
separation of structurally similar compounds that exhibit nearly identical chromatographic
behavior. For example, the separation of complex samples containing paraffins, olefins, and
aromatics remains a challenge using the commercially-available column sets [8].
Since the first introduction of GC × GC by Liu and Philips using the polyethylene glycol
(PEG) × methyl silicone (polar × non-polar) column combination, significant progress has been
made in the development of GC stationary phases that provide specific molecular interactions
and unique selectivity to separate extremely challenging samples [9, 10]. For example, the HP-1
(dimethylpolysiloxane) × HT-8 (8% phenyl (equiv.) polycarborane-siloxane) column set was
used to separate polychlorinated biphenyls and toxaphene components into groups according to
the number of chlorine substituents [11, 12]. Stationary phases containing liquid crystals have
been shown to separate compounds according to the planarity of target analytes [13, 14]. The
development of cyclodextrin-based stationary phases and their adaptation to 2D separations [15,
16] have facilitated improved enantiomeric separation to determine the chiral composition of
monoterpenes in Australian tea tree (Melaleuca alternifolia). However, choosing the appropriate
second dimension column is a challenge since it must enable fast separation and high selectivity
to produce sharp peaks, satisfactory separation power, and high peak capacity [17].
82
Instrumental or stationary phase modifications are sometimes required in order to make
primary columns compatible with secondary columns. For example, a column set composed of
cyclodextrin × BP-20 (PEG) was reported for the enantiomeric separation of monoterpene
hydrocarbons and oxygenated monoterpenes [16]. It was shown that the cyclodextrin chiral
stationary phase was not suitable in the second dimension since high plate numbers and long run
times are needed to obtain sufficient enantioresolution [17]. Shellie and Marriott circumvented
this limitation by applying subambient pressure (vacuum outlet) conditions at the end of the
secondary column to speed up the separation [18]. Using this approach, a GC × enantio-GC
method was developed using a DB-5 (5% diphenyl-dimethylpolysiloxane) × cyclodextrin
column set. Short analyte retention times with adequate enantioresolution (Rs ~ 1.0) was
achieved on a 1 m cyclodextrin-based second dimension column. This example highlights that
adaptation of new stationary phases with unique selectivities can further improve the separation
performance of GC × GC.
To resolve complex samples containing light paraffins and olefins with similar polarity
and volatility using 1D GC, alumina porous layer open tubular (PLOT) columns are often used
[19]. Although alumina PLOT column was used in the valve-based heart-cutting
multidimensional GC experiments by Shellie and coworkers, this column has not been
successfully employed in GC × GC due to its requirements of higher temperatures and flow rates
[1, 6, 20]. Stationary phases that facilitate separation by argentation chromatography have
potential since silver(I) ion possesses unique selectivity towards unsaturated hydrocarbons [21,
22]. However, the adaptation of silver-based stationary phases to GC × GC remains a challenge
due to strong π-complexation between analytes and the silver-based stationary phase, which can
result in slow mass transfer and wrap-around. To the best of our knowledge, no GC × GC
83
method has been reported to date that uses an analyte-selective silver-containing stationary
phase.
In this technical report, we develop the first class of silver-based stationary phases that
are compatible with GC × GC separations. Using a stationary phase comprising a mixture of
customized silver-based ionic liquids (ILs) and conventional imidazolium-based ILs, the Ag+
concentration was tuned to facilitate chromatographic resolution of a wide variety of analytes.
Additional parameters including structural composition of silver-based IL, film thickness, and
column length were studied and optimized.
5.3 Experimental Section
5.3.1 Chemicals and Materials
Bis[(trifluoromethyl)sulfonyl]amine (99%), silver oxide (99%), dichloromethane
(99.8%), acetonitrile (99.9%), 1-butylimidazole (C4IM) (98%), 1-decyl-2-methylimidazole
(C10MIM) (97%), 1-chlorobutane (99%), all analytes used to evaluate the columns (see Table
D1, Appendix D), and a mixture of unsaturated fatty acids (UFA) commercially available as
polyunsaturated fatty acids (PUFA No.2, animal source) were purchased from Sigma Aldrich
(St. Louis, MO, USA). 1-Bromooctane (98%) was obtained from Acros Organics (Morris Plains,
NJ, USA). 1-Methylimidazole (MIM) (99.0%) was purchased from Fluka (Stainheim, Germany).
A SUPELCOWAX10 (30 m, 0.20 mm ID, 0.20 µm - PEG) column and untreated fused silica
capillary tubing (ID 0.25 mm) were obtained from Supelco (Bellefonte, PA, USA). A Rtx-5MS
column (30 m, 0.25 mm ID, 0.25 μm - 5% diphenyl-dimethylpolysiloxane) was purchased from
Restek (Bellefonte, PA, USA).
84
5.3.2 Synthesis of Silver-based IL
The silver-based ILs were synthesized based on a previously reported procedure from the
literature [23-25]. Briefly, [(C10MIM)(MIM)Ag+][NTf2-] was prepared through a chelation
reaction performed between [(ACN)Ag+][NTf2-] and the MIM and C4IM ligands. Silver-based
ILs with other combinations of ligands were also prepared using the same procedure. The
chemical structures of the silver-based ILs used in this study are shown in Figures 5.1a and D1.
The 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C4MIM+][NTf2-], 1-
octyl-3-methylimidazolium [C8MIM+][NTf2-], and 1-decyl-3-methylimidazolium
[C10MIM+][NTf2-] ILs were prepared using previously reported methods[26] and characterized
by 1H NMR (see Figures D2-D4). A detailed description of synthetic procedures and
characterization is included in the appendix D.
5.3.3 Preparation of GC Columns and Probe Mixtures
To obtain the coating solution, the silver-based IL was mixed with the
conventional IL and this mixture was dissolved in dichloromethane. Two meter segments of
untreated fused silica capillary (ID, 0.25 mm) were coated by the static coating method at 40 °C.
Additional details of this procedure are described in the appendix D. Probe mixtures were
prepared by sealing 3 microliters of each compound in a 20 mL-headspace vial. Then, 1 µL of
the headspace was injected into the GC × GC system with a split ratio of 5:1.
The UFA mixture was diluted in dichloromethane at a concentration level of 10 µg mL-1.
Then, 1 µL of the sample solution was injected into the GC × GC system with a split ratio of
100:1. The chemical structures of UFAs are listed in Figure D5.
85
5.3.4 GC × GC-flame ionization detector (FID) Analysis
Two-dimensional chromatographic separations were performed on a homebuilt GC × GC
instrument assembled on an Agilent 6890 GC equipped with a split/splitless, an FID detector,
and a two-stage cryogenic loop modulator. The first dimension employed a Rtx-5MS column (30
m, 0.25 mm ID, 0.25 μm) and the second dimension a silver-based IL column (1.2 m, 0.25 mm
ID, 0.15 μm) or SUPELCOWAX10 (1.2 m, 0.2 mm ID, 0.2 µm) column. Hydrogen was used as
the carrier gas with the inlet pressure set at 9.32 psi and the column flow rate of 1.2 mL min-1. A
full description of the chromatographic instrumentation is included in the appendix D.
5.4 Results and Discussion
5.4.1 Optimization of silver-based IL stationary phase composition and column parameters
Ionic liquids are molten salts with melting points lower than 100 ºC [27]. IL-based
stationary phases possess numerous properties such as high thermal stability, low viscosity, and
unique chromatographic selectivity that make them useful in 1D GC and MDGC separations [28-
30]. One of the most advantageous properties of ILs is the ability to customize unique stationary
phases by judicious choice of cations/anions.
Although a silver-based IL stationary phase [(C4IM)(MIM)Ag+][NTf2-]) was reported for
the conventional 1D GC separation of unsaturated hydrocarbons, its direct adaptation to GC ×
GC provided numerous challenges [25]. When separations were performed using this column in
the second dimension, asymmetric peaks and excessive retention of analytes were observed (see
Figure D6a). Since high silver ion concentration results in strong π-complexation towards
unsaturated compounds, a stationary phase containing the neat silver-based IL was deemed to be
not compatible with GC × GC separations.
To overcome the limitation presented by the neat silver-based IL column, the selectivity
86
and retention power of the stationary phase was tuned by dissolving the silver-based IL
([(C4IM)(MIM)Ag+][NTf2-]) in a conventional IL ([C10MIM+][NTf2
-]) in an effort to reduce its
retentive nature. As shown in Figures D6b-D6f, five silver-based IL columns were prepared
using mixtures of the silver-based IL and the conventional IL at ratios ranging from 1:10 to 1:50
(w/w). A column containing the neat [C10MIM+][NTf2-] IL stationary phase was also used for
comparison purposes (see Figure D6g). As the concentration of silver ion in the stationary phase
was lowered, peak broadening and wrap-around in the second dimension decreased significantly
(see Figure D6). However, when the stationary phase containing the lowest silver ion
concentration [(C4IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:50 (w/w) was examined, the
selectivity in the second dimension was completely lost (see Figure D6f). By comparing the
second dimension chromatographic resolution (R) values of selected analytes, it can be observed
that the higher chromatographic resolution of n-hexane (1) and 1-hexene (2) (R1,2 = 0.5) was
obtained on the [(C4IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:20 (w/w) column, and the lower
peak broadening and higher chromatographic resolution of methyl 4-pentenoate (3), methyl
pentanoate (4), methyl 3-pentenoate (5), and methyl 2,4-pentadienoate (6) (R3,4 = 1.2, R5,6 = 3.3)
was obtained on the [(C4IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 (w/w) column.
Therefore, capillary columns suitable for the optimal separation of hydrocarbons and esters were
prepared using stationary phases containing the silver-based IL:conventional IL at ratios of 1:20
and 1:30 (w/w), respectively.
Conventional ILs with different lengths of alkyl side chain substituents (e.g.,
[C4MIM+][NTf2-], [C8MIM+][NTf2
-], and [C10MIM+][NTf2-]) as well as silver-based ILs
comprised of different ligands (e.g., MIM, C4IM, and C10MIM) were tested to identify the
optimal stationary phase composition. A probe mix was used to determine the resolution of
87
selected analytes (R1,2, n-hexane and 1-hexene; R5,6, methyl 3-pentenoate and methyl 2,4-
pentadienoate) to elucidate the optimal structural features for the silver-based ILs and
conventional ILs. Three second dimension columns were prepared by dissolving the
([(C4IM)(MIM)Ag+][NTf2-]) IL in different conventional ILs. As shown in Figure D7a, the
[C10IM+][NTf2
-] IL used to dissolve the silver-based IL provided the best chromatographic
resolution of the analyte pairs. Second dimension columns were then prepared using different
silver-based ILs comprised of various ligands (e.g., [(C4IM)(MIM)Ag+][NTf2-],
[(C10MIM)(MIM)Ag+][NTf2-], and [(C10MIM)(C4IM)Ag+][NTf2
-]) dissolved in the
[C10IM+][NTf2
-] IL. As shown in Figure D7b, higher chromatographic resolution values were
obtained with the stationary phase consisting of [(C10MIM)(MIM)Ag+][NTf2-] in the
[C10MIM+][NTf2-] IL. It was also observed that the solubility of the [(C10MIM)(MIM)Ag+][NTf2
-
] IL was higher in the [C10MIM+][NTf2-] IL compared to the [C8MIM+][NTf2
-] and
[C4MIM+][NTf2-] ILs.
Due to the strong π-complexation between analytes and the silver-based stationary phase,
the film thickness and the column length need to be optimized to be compatible with the first
dimension column. The effects of film thickness and column length on the GC × GC separation
were also investigated. As shown in Figure D8a, the silver-based IL column with a film
thickness of 0.15 µm exhibited narrower peak widths and increased chromatographic resolution
compared to a column containing a 0.28 µm film thickness. Regarding the length of the second
dimension column, the 120 cm column provided the highest chromatographic resolution values
of methyl 3-pentenoate and methyl 2,4-pentadienoate (see Figure D8b), while minimizing wrap-
around.
88
In the final step, the maximum allowable operating temperature (MAOT) of a 120 cm
segment of the [(C10MIM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 (w/w) IL stationary phase
was determined. As shown in Figure D9a, the column was heated slowly in a GC oven and an
ultra-sensitive flame ionization detector was used to detect any volatilization/decomposition of
the stationary phase. To further evaluate the thermal stability, the column was conditioned to
different temperatures for 1-hour. After each conditioning step, a mixture of methyl 2,4-
pentadienoate and methyl 3-pentenoate was separated using GC × GC and the second dimension
resolution values were compared. As shown in Figure D9, significant column bleed and loss of
chromatographic resolution was observed at temperatures above 180 °C. It was also observed
that the silver-based IL column could be reused for approximately 700 injections without
significant loss of chromatographic resolution or efficiency when operating below this
temperature. The enhanced thermal stability of the silver-based IL stationary phase can be
attributed to the chelating ligands (C10MIM and MIM) and the [C10MIM+][NTf2-] IL which
provides stability when subjected to abrupt heating cycles [31, 32].
5.4.2 Separation of analyte mixtures using GC × GC with silver-based IL 2D column
An optimized silver-based column was prepared using a mixture of silver-based IL
([(C10MIM)(MIM)Ag+][NTf2-]) and conventional IL ([C10MIM+][NTf2
-]). As shown in Figure
5.1, a mixture of thirty-three analytes containing esters, aldehydes, and ketones: (1)
Propionaldehyde, (2) butyraldehyde, (3) pentanal, (4) Hexanal, (5) heptanal, (6) octanal, (7)
benzaldehyde, (8) acetone, (2) butanone, (10) pentanone, (11) 3-pentanone, (12) 2-hexanone,
(13) cyclohexanone, (14) ethyl acetate, (15) methyl acetate, (16) methyl butyrate, (17) ethyl
butyrate, (20) isopropyl butyrate, (21) methyl 4-pentenoate, (22) methyl pentanoate, (23) methyl
2,4-pentadienoate, (24) methyl 3-pentenoate, (25) methyl tiglate, (26) ethyl pentanoate, (27)
89
isoamyl acetate, (28) Propyl tiglate, (29) ethyl hexanoate, (30) propyl tiglate, (31) isopropyl
tiglate, (32) ethyl heptanoate, and (33) heptyl acetate (see Table D1 for structures) was separated
using GC × GC with the Rtx-5MS and silver-based IL [(C10MIM)(MIM)Ag+][NTf2-
]/[C10MIM+][NTf2-] 1:30 (w/w) column set. To compare and benchmark the separation
performance of this column set, the same mixture was analyzed using a Rtx-5MS ×
SUPELCOWAX10 column set, as shown in Figure 5.1c. The Rtx-5MS ×
[(C10MIM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 (w/w) column set provided better
chromatographic resolution of analytes, especially for early eluted compounds, compared to the
Rtx-5MS × SUPELCOWAX10 column set. For example, the cluster of analytes possessing low
boiling points and similar polarities (e.g., butyraldehyde (2), 2-butanone (9), 2-pentanone (10), 3-
pentanone (11), ethyl acetate (14), and methyl acetate (15)) was better resolved by the Rtx-5MS
× silver-based IL column set. The second dimension chromatographic resolution between
butyraldehyde (2) and 2-butanone (9); pentanal (3) and methyl butyrate (16); and hexanal (4) and
propyl 2-hexanone (12) was found to be 2.2, 3.8, and 6.3, respectively, using the silver-based IL
and 0.9, 1.6, and 4.9 using the SUPELCOWAX10 as second dimension column (Table D2).
When the Rtx-5MS × SUPELCOWAX10 column set was employed, the separation in the first
dimension is based on the boiling point of each analyte and the separation in the second
dimension is largely based on polarity, mainly dipolar and electron lone pair interactions [33,
34]. In comparison, when the Rtx-5MS × silver-based IL column set was used, the separation
mechanism offered in the second dimension is strongly influenced by π-complexation between
the silver ions and the double or triple bonds of the analytes. To further validate the effect of the
silver IL in the second dimension column, a column set using a neat conventional IL column
(containing no Ag+) was used for GC × GC separation of the analyte mixture. As shown in
90
Figure 5.1 a) Chemical structures of (I) silver-based IL ([(C10MIM)(MIM)Ag+][NTf2-]) and (II)
imidazolium-based IL ([C10MIM+][NTf2-]) and GC × GC-FID chromatograms of esters,
aldehydes, and ketones obtained using the following column sets: b) Rtx-5MS (30 m, 0.25 mm
ID, 0.25 μm) × [(C10MIM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 (w/w) (1.2 m, 0.25 mm
ID, 0.15 μm) and c) Rtx-5MS (30 m, 0.25 mm ID, 0.25 μm) × SUPELCOWAX10 (1.2 m, 0.2
mm ID, 0.2 μm). Inlet pressure: 9.32 psi; Split ratio: 5:1; Temperature program: 40 °C to 44 °C
at 2 °C/min; then increased to 100 °C at 5 °C/min and held for 3 min. Modulation time: 10 s.
Peak identification: See Table D1 for additional information.
Figure D10, the selectivity towards the analytes in the second dimension was completely lost
compared to the Rtx-5MS × silver-based IL column set.
An additional analyte mixture composed of alkanes, alkenes, alkynes, cycloalkanes, and
terpenes: (34) n-pentane, (35) 2,4-hexadiene, (36) 3-Methyl-1,4-pentadiene, (37) 1,5-hexadiene,
(38) 1,3-hexadiene, (39) 1-hexene, (40) cis 2-hexene, (41) 3-hexene, (42) n-hexane, (43) 2,3-
dimethyl-1,3-butadiene, (44) benzene, (45) 2-hexyne, (46) 1-hexyne, (47) 3-hexyne, (48)
toluene, (49) n-octane, (50) m-xylene, (51) o-xylene, (52) p-xylene, (53) 1,8-nonadiene, (54) 1-
91
nonene, (55) n-nonane, (56) myrcene, (57) α-terpinene, (58) γ-terpinene, and (59) terpinolene
was subjected to GC × GC separation using both column sets, as shown in Figure 5.2. It can be
observed that the analytes were better resolved and distributed using the Rtx-5MS × silver-based
IL column set. In addition, it was found that the retention time of analytes eluted in the second
dimension column was correlated to the number of units of unsaturation within the analyte.
Nonane (55), 1-nonene (1 double bond) (54), and 1,8-nonadiene (2 double bonds) (53) eluted in
1.56 s, 2.25 s, and 3.03 s, respectively. It can also be observed that compounds with low boiling
points and similar polarities (compounds 34 to 48) were better resolved using the Rtx-5MS ×
silver-based IL column set. For example, the analyte group containing 3-methyl-1,4-pentadiene
(36), 1-hexene (39), 3-hexene (41), and n-hexane (42) were better separated using the Rtx-5MS
× silver-based IL column set. The 2,4-hexadiene (35) and 1,3-hexadiene (38) pair was not
separated by either column set. In addition, the probes 2,3-dimethyl-1,3-butadiene (43) and
benzene (44), which co-eluted on the Rtx-5MS × SUPELCOWAX10 column set, were well
separated using the Rtx-5MS × silver-based IL column set (R43,44 = 6.5). The analytes 2-hexyne
(45), 1-hexyne (46), and 3-hexyne (47) exhibited a highly distinctive and interesting separation
pattern. It is important to note that the 1D analysis of 1-hexyne was not possible using the neat
silver-based IL column reported by Nan et al [25]. due to its propensity of undergoing an
irreversible complexation reaction with the silver-based IL stationary phase, resulting in its
tenacious retention. However, for the [(C10MIM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:20
(w/w) IL column, the strength of π-complexation was effectively tuned allowing all alkynes to
elute, but sufficient enough to provide high selectivity. As observed previously, when a neat
conventional IL column was employed in the second dimension, the selectivity (compounds 34
to 59) was completely lost (see Figure D11).
92
Figure 5.2 GC × GC-FID chromatograms of alkanes, alkenes, alkynes, dienes, cycloalkanes, and
terpenes using the column sets: a) Rtx-5MS (30 m, 0.25 mm ID, 0.25 μm) ×
[(C10MIM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:20 (w/w) (0.9 m, 0.25 mm ID, 0.15 μm) and
b) Rtx-5MS (30 m, 0.25 mm ID, 0.25 μm) × SUPELCOWAX10 (0.9 m, 0.2 mm ID, 0.2 μm). Inlet
pressure: 9.32 psi; Split ratio: 5:1; Temperature program: 25 °C held for 3 min, increase to 44 °C
at 2 °C/min; then increased to 90 °C at 5 °C/min, and held for 2.3 min. Modulation time: 10 s. See
Table D1 for peak identification.
5.4.3 Separation of unsaturated fatty acids using GC × GC with silver-based IL column
To further validate the application of the GC × GC method using a silver-based IL
column, a UFA sample was analyzed using the SUPELCOWAX10 ×
[(C10MIM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 (w/w) column set. The UFA sample is
composed of long chain fatty acids (from 14 to 22 carbons) with 0 to 6 units of unsaturation. The
sample was initially separated using a conventional 1D system in order to determine the elution
order of the analytes (see Figure D12a). The UFA sample (fraction from 1tR = 15 to 1tR = 47 min,
UFAs from C14:0 to C18:3) was subjected to GC × GC separation, as shown in Figure 5.3. The
silver-based IL stationary phase provided good selectivity for the analytes within the sample as
evidenced by UFAs containing the same carbon chain length being well separated. As an
example, the second dimension retention times of C18:0 (0 double bond), C18:1n7 (1 double
93
Figure 5.3 GC × GC-FID chromatogram of an unsaturated fatty acid sample obtained using
SUPELCOWAX10 (30 m, 0.25 mm ID, 0.25 μm) × [(C10MIM)(MIM)Ag+][NTf2-
]/[C10MIM+][NTf2-] 1:30 (w/w) (0.4 m, 0.25 mm ID, 0.15 μm) column set. Inlet pressure: 9.32;
Split ratio: 100:1; Temperature program: isothermal mode 180 °C for 47 min; Modulation time:
10 s. For peak identification, refer to Figure D5. The peaks labeled with (*) refer to interferent
compounds present within the purchased sample.
bond), C18:2n6 (2 double bonds), and C18:3n3 (3 double bonds) were found to be 2.27 s, 3.55 s,
6.48 s, and 8.57 s, respectively. UFAs possessing more than 20 carbon atoms were not studied
since they exceeded the MAOT of the column. Overall, UFAs (from C14:0 to C18:3) were well
separated and exhibited unique retention behavior using this column set. This study is the first
time in which UFAs were separated by a GC × GC using an analyte-selective silver column
where the retention order in the second dimension is highly correlated to the number of double
bonds.
5.5 Conclusions
In summary, a GC × GC compatible silver-based stationary phase was successfully
developed. The unique chromatographic selectivity and GC × GC compatibility of the stationary
phases were achieved through careful structural design and mixing of the silver-based IL and
conventional ILs as well as the optimization of film thickness and column length. This study will
94
guide the design and development of new generations of silver-based stationary phases with high
thermal stability capable of providing unique selectivity for a broader range of analytes
possessing similar polarity within complex samples, such as long chain UFAs in food products
and isomers of long chain unsaturated hydrocarbons within petrochemicals. Furthermore, the
approach of employing analyte-selective components within a tunable stationary phase further
demonstrates the unique features offered by ILs in separation science.
5.6 Acknowledgments
The authors acknowledge funding from Fundação de Amparo à Pesquisa do Estado de
São Paulo (FAPESP) – Grant number 2017/24721-0 and 2016/01082-9. JLA acknowledges
funding from the Chemical Measurement and Imaging Program at the National Science
Foundation (CHE-1709372).
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98
CHAPTER 6. LIPIDIC IONIC LIQUID STATIONARY PHASES FOR THE
SEPARATION OF ALIPHATIC HYDROCARBONS BY COMPREHENSIVE TWO-
DIMENSIONAL GAS CHROMATOGRAPHY
Modified and reprinted from J. Chromatogr. A 2016, 1481, 127-136
Copyright © 2016, Elsevier
He Nan, Cheng Zhang, Richard A. O’Brien Adela Benchea, James H. Davis Jr., and
Jared L. Anderson
6.1 Abstract
Lipidic ionic liquids (ILs) possessing long alkyl chains as well as low melting points have
the potential to provide unique selectivity as well as wide operating ranges when used as
stationary phases in gas chromatography. In this study, a total of eleven lipidic ILs containing
various structural features (i.e., double bonds, linear thioether chains, and cyclopropanyl groups)
were examined as stationary phases in comprehensive two dimensional gas chromatography (GC
× GC) for the separation of nonpolar analytes in kerosene. N-alkyl-N′-methyl-imidazolium-based
ILs containing different alkyl side chains were used as model structures to investigate the effects
of alkyl moieties with different structural features on the selectivities and operating temperature
ranges of the IL-based stationary phases. Compared to a homologous series of ILs containing
saturated side chains, lipidic ILs exhibit improved selectivity toward the aliphatic hydrocarbons
in kerosene. The palmitoleyl IL provided the highest selectivity compared to all other lipidic ILs
as well as the commercial SUPELCOWAX 10 column. The linoleyl IL containing two double
bonds within the alkyl side chain showed the lowest chromatographic selectivity. The lipidic IL
possessing a cyclopropanyl group within the alkyl moiety exhibited the highest thermal stability.
The Abraham solvation parameter model was used to evaluate the solvation properties of the
99
lipidic ILs. This study provides the first comprehensive examination into the relation between
lipidic IL structure and the resulting solvation characteristics. Furthermore, these results establish
a basis for applying lipidic ILs as stationary phases for solute specific separations in GC × GC.
6.2 Introduction
Comprehensive two-dimensional gas chromatography (GC × GC) is one of the most
powerful tools available for the separation and identification of volatile and semi-volatile organic
compounds in complex mixtures [1-3]. This technique involves connecting two columns
possessing different selectivities (i.e., nonpolar × polar or polar × nonpolar column
configuration) in order to maximize orthogonality [4, 5]. Compared to conventional gas
chromatography (1D-GC), GC × GC provides increased peak capacity to resolve sample
components from complex matrices. For example, the separation of complex petrochemicals can
be improved by applying combinations of various polydimethyl(siloxane) (PDMS) and
poly(ethyleneglycol) (PEG) derived stationary phases (i.e., DB-1 × BPX-50, DB-1 × DB-1701,
and Solgel Wax × BPX-50 column sets) [6-9]. An essential requirement for maximizing peak
capacity in GC × GC is to employ a combination of stationary phases possessing complementary
selectivities. The PDMS and PEG-based stationary phases provide limited solvation capabilities
and may not fully utilize the peak capacity of GC × GC [10]. Therefore, the development of new
stationary phases possessing alternative selectivities is needed in order to fully exploit the
separation power of GC × GC.
A number of ionic liquid (IL) based stationary phases have been commercialized and can
offer unique selectivity when compared to PDMS or PEG-based GC stationary phases [11, 12].
ILs are typically composed of an organic cation paired with an inorganic or organic counter
anion and have melting points less than 100 °C. ILs have enjoyed increasing popularity in GC
100
due to their numerous advantages including negligible vapor pressure, wide liquid ranges,
tunable viscosity, and high thermal stability [13-15]. Commercial IL-based columns have been
successfully utilized in GC × GC for the analysis of agricultural products, petrochemical
samples, environmental samples, and pharmaceutical compounds [16-19]. Furthermore, IL and
molten salt-based stationary phase can be structurally tailored to engage in multiple solvation
interactions that can provide unique selectivities [20]. Recent studies from our group have
demonstrated that the IL cation can be functionalized with long alkyl group substituents to
improve the separation of nonpolar analytes in kerosene using GC × GC [21-23]. It was
previously reported that imparting long alkyl side chains to the cationic moiety resulted in higher
melting ILs due to the cumulative interchain dispersive forces [24]. For example, N-alkyl-N′-
methyl-imidazolium tetrafluoroborate ILs with alkyl side chains of 6, 12, and 18 carbon atoms
resulted in ILs with melting points of -82 °C, 19 °C, and 50 °C, respectively. The melting point
is an important physical property of ILs when they are used as GC stationary phases as it largely
determines the operating range of the IL stationary phase and is an important factor for the
resulting retention mechanism. Typically, analytes can interact with IL-based stationary phases
through either a partition or adsorption mechanism [25-27]. The partition-type retention
mechanism generally provides greater separation efficiencies. When the melting point of the IL
stationary phase is higher than the operating temperature, the retention mechanism is likely to be
dominated by adsorption. Complex samples like kerosene usually contain a large number of
different analytes possessing vastly different boiling points [28]. As a result, separation programs
that range from lower temperatures and operate up to high temperatures are needed highlighting
the requirement of stationary phases to possess unique selectivity as well as wide operating
temperature ranges.
101
Recently, novel lipid-inspired ILs containing long alkyl side chains (≥ C8) were reported
[29-32]. By strategically incorporating various functional groups (i.e., double bonds, thioether
branched chains, and cyclopropanyl groups), the melting point of the resulting ILs can be
significantly reduced. Lipidic ILs with low melting points have the potential to provide wide
operating ranges when applied as GC stationary phases. In addition, the long alkyl side chain
appended to the IL cation can enhance the selectivity towards aliphatic hydrocarbons. Therefore,
lipidic ILs represent an intriguing class of materials that provide enhanced dispersive type
interactions while still being room temperature ionic liquids, an important requirement in high
efficiency GC × GC studies.
In this study, a total of eleven lipidic ILs were applied for the first time as stationary
phases in GC × GC separations. Compared to linear saturated IL stationary phases, superior
resolution of nonpolar hydrocarbons in kerosene at low separation temperatures was achieved by
employing a lipidic IL column in the second dimension. The palmitoleyl lipidic IL exhibited
excellent selectivity toward nonpolar analytes compared to the other lipidic ILs studied, as well
as the commercial SUPELCOWAX 10 column. Lipidic ILs with different structural features
including thioether branched chains and cyclopropanyl groups were investigated to study their
applicability under high separation temperatures. The solvation parameter model was employed
to examine the solvation properties of the lipidic IL-based stationary phases. This new class of
ILs provides improved separation of nonpolar analytes in complex samples and establishes an
additional basis for the structural design of IL-based stationary phases that has never been
explored.
102
6.3 Experimental Section
6.3.1 Materials
Kerosene was purchased from a local distributor. Lithium
bis[(trifluoromethyl)sulfonyl]imide (LiNTf2) was obtained from SynQuest Laboratories
(Alachua, FL, USA). Bromoethane (98%) and propionic acid (99%) were purchased from Alfa
Aesar (Ward Hill, MA, USA). Butyraldehyde (99%), ethyl acetate (99.9%), and 2-nitrophenol
(99.8%) were purchased from Acros Organics (Morris Plains, NJ, USA). Toluene (99.5%), 1-
butanol (99.9%), N,N-dimethylformamide (99.9%), and 2-propanol (99.6%) were purchased
from Fisher Scientific (Fairlawn, NJ, USA). The compounds p-cresol (99%), m-xylene
(99.5%), o-xylene (99.5%), and p-xylene (99.5%) were purchased from Fluka (Steinheim,
Germany). Cyclohexanol (99%) was purchased from J.T. Baker (Phillipsburg, NJ, USA).
Ethylbenzene (95%) was obtained from Eastman Kodak Company (Rochester, NJ, USA).
Acetophenone (99%), aniline (99.5%), benzaldehyde (99%), benzene (99.8%), benzonitrile
(99%), benzyl alcohol (99%), 1-bromohexane (98%), 1-bromooctane (99%), 1-chlorobutane
(99%), 1-chlorohexane (99%), 1-chlorooctane (99%), 1,2-dichlorobenzene (99%), 1-iodobutane
(99%), nitrobenzene (99%), 1-nitropropane (99%), 1-octanol (99%), octyldehyde (99%), 2-
pentanone (99%), phenetole (99%), phenol (99%), propionitrile (99%), pyridine (99.8%), pyrrole
(98%), 1-decanol (99%), 1-methylimidazole (99 %), 1-bromodecane (98%), cyclohexanone
(99.8%), 2-chloroaniline (98%), 1-pentanol (99%), and dichloromethane (99.8%) were
purchased from Sigma-Aldrich (St. Louis, MO, USA). All chemicals were used as received.
Acetic acid (99.7%), methyl caproate (98%), naphthalene (98%), untreated fused silica capillary
tubing (I.D. 0.25 mm) and a 30 m × 200 µm SUPELCOWAX 10 column (df = 0.20 µm) were
purchased from Supelco (Bellefonte, PA, USA). A 30 m × 0.25 mm Rtx-5 capillary column (df =
0.25 µm) was purchased from Restek (Bellefonte, PA, USA).
103
6.3.2 Synthesis of ionic liquids
The chemical structures of all ILs examined in this study are shown in Figure 6.1. All of
the ILs are [NTf2-] salts in order to obtain the lowest possible melting points and highest thermal
stabilities [33]. A detailed description of all synthetic methods used to prepare the new classes of
lipidic ILs is provided in the appendix E. ILs R1 (1-decyl-3-methylimidazolium NTf2) and R2
(1-hexadecyl-3-methylimidazolium NTf2) were prepared according to previously reported
procedures [34]. The detailed synthetic method and 1H NMR data are provided in the appendix
E. The synthetic procedures used for the lipidic ILs is described in previously published studies
[29, 30]. As shown in Figure 6.1, IL 1 ((Z)-1-(9-hexadecenyl)-3-methylimidazolium NTf2)
(Palmitoleyl) contains a cis-double bond on 9th position of the C16 side chain appended to the
imidazolium cation. IL 2 ((Z)-1-methyl-3-(9-octadecenyl)-imidazolium NTf2) (Oleyl) contains a
cis-double bond on 9th position of the C18 side chain. IL 3 ((Z)-1-methyl-3-(11-octadecenyl)-
imidazolium NTf2) (Vaccenyl) possesses a cis-double bond on 11th position of the C18 side chain.
IL 4 ((E)-1-methyl-3-(9-octadecenyl)-imidazolium NTf2) (Elaidyl) contains a trans-double bond
on 9th position of the C18 side chain. IL 5 (1-methyl-3-((9Z,12Z)-octadecadienyl)-imidazolium
NTf2) (Linoleyl) has two double bonds on the 9th and 12th carbon position of the C18 side chain.
IL 6 is a mixture of the 1-(9-(ethylthio)octadecyl)-3-methylimidazolium and 1-(10-
(ethylthio)octadecyl)-3-methylimidazolium NTf2 salts. IL 7 is a mixture of 1-(9-
(decylthio)octadecyl)-3-methylimidazolium NTf2 and 1-(10-(decylthio)octadecyl)-3-
methylimidazolium NTf2. IL 8 is a mixture of 1-(9,12-bis(ethylthio)octadecyl)-3-
methylimidazolium NTf2, 1-(9,13-bis(ethylthio)octadecyl)-3-methylimidazolium NTf2, 1-(10,12-
bis(ethylthio)octadecyl)-3-methylimidazolium NTf2, and 1-(10,13-bis(decylthio)octadecyl)-3-
104
methylimidazolium NTf2. IL 9 is a mixture of 1-(9,12-bis(decylthio)octadecyl)-3-
methylimidazolium NTf2, 1-(9,13-bis(decylthio)octadecyl)-3-methylimidazolium NTf2, 1-
(10,12-bis(decylthio)octadecyl)-3-methylimidazolium NTf2, and 1-(10,13-
bis(decylthio)octadecyl)-3-methylimidazolium NTf2. IL 10 (1-(3-(dodecylthio)propyl)-3-
methylimidazolium NTf2) possesses a thioether functional group within the alkyl side chain. IL
11 (1-(8-(2-heptylcyclopropyl)octyl)-3-methylimidazolium NTf2) contains a cyclopropyl
functional group within the side chain. All eleven lipidic ILs are liquids at room temperature.
6.3.3 Preparation and characterization of GC columns containing lipidic IL stationary phases
GC columns were prepared using the static coating method. Briefly, all ILs were dried in
a vacuum oven overnight at 70 °C to remove residual water and organic solvent. A 0.25% or
0.45% (w/v) coating solution was prepared by dissolving the IL in dry dichloromethane. A five
meter segment of untreated fused silica capillary (I.D. 0.25 mm) was coated by the static coating
method at 40 °C. The coated columns were conditioned using a temperature program from 40 °C
to 110 °C with a ramp of 1 °C min-1 and held isothermally at 110 °C for 3 h. Helium was used as
the carrier gas at a constant flow of 1 mL min-1. The film thickness of the stationary phase was
calculated using a previously reported method [35]. The column efficiency was determined using
naphthalene at 100 °C; all columns possessed efficiencies of at least 2000 plates per meter (see
Table 6.1). The solvation parameter model was used to characterize the solvation properties of
the stationary phases. Table A1 lists the 46 probe molecules and their respective solute
descriptors that were used for evaluating the IL columns in this study. All probe molecules were
dissolved in dichloromethane and injected separately onto 5 m IL-based columns at three
different temperatures (50, 80, 110 ºC).
105
Figure 6.1 Cation structures of two reference ILs and eleven lipidic ILs. ILs R1 and R2 possess
saturated alkyl side chains. ILs 1-5 contain double bonds. ILs 6-9 are a mixture of ILs with
branched thioether chains; the black dots indicate the positions of the branched side chains. IL 10
possesses a thioether linkage within the linear side chain. IL 11 contains a cyclopropanyl group
within the alkyl side chain. All of the ILs investigated in this study are [NTf2-] salts. The Cn(x[y])
represents the functional group and its position on the alkyl side chain.
106
6.3.4 Instrumentation
The characterization of all IL-based columns was performed on an Agilent 7890B gas
chromatography system with a flame ionization detector (GC-FID). Comprehensive two-
dimensional separations were performed on a homebuilt GC × GC-FID system built from an
Agilent 6890 GC-FID system with a two stage cryogenic loop modulator. A detailed illustration
of the GC × GC FID system is described in previously published work [21].
6.3.5 GC × GC-FID analysis
A 30 m × 0.25 mm Rtx-5 capillary column (df = 0.25 µm) was employed as the primary
column and was connected to second dimension column containing the IL stationary phase (1.2
m × 0.25 mm, df = 0.15 µm or 0.28 µm). In total, all eleven lipidic IL-based stationary phases
were investigated using the Rtx-5 × IL column set (see Table 6.1). For comparison purposes, a
commercial SUPELCOWAX 10 column was evaluated as the secondary column for the analysis
of aliphatic hydrocarbons in kerosene. The kerosene sample was diluted 20-fold (v/v) in
dichloromethane. In all experiments, 0.5 µL of the kerosene sample was injected using a 100:1
split ratio at 250 ˚C. The chromatographic oven was programmed from 40 to 120 °C at 2 °C
min−1, followed by a secondary ramp from 120 to 200 °C at 20 °C min−1. Hydrogen was
employed as the carrier gas at a constant flow of 1.2 mL min−1. A modulation period of 7 s was
utilized for all experiments.
107
Table 6.1 Characteristics of lipidic ionic liquid based stationary phases examined in this study.
IL
No.
Specialized lipidic
side chain feature Abbreviationa
Film
Thickness
(µm)
Efficiency
(Plates/
Meter)
Resolution of selected
analytesb
1 & 2 3 & 4 5 & 6
R1 None C10(0) 0.28 2400 0.96 0.66 1.89
R2 None C16(0) 0.28 2200 0.63 - -
1 Double bond C16(1[9]) 0.28 2300 2.69 1.98 6.88
2 Double bond C18(1[9]) 0.28 2500 1.15 0.68 2.42
3 Double bond C18(1[11]) 0.28 2500 1.04 0.67 1.95
4 Double bond C18(1[9t]) 0.28 3600 1.18 0.77 3.41
5 Double bond C18(2[9,12]) 0.28 2400 0.61 0.42 1.84
6 Branched thioether
chain
C18(C2S[9/10])
0.28 2200 1.15 0.96 2.54
7 Branched thioether
chain
C18(C10S[9/10])
0.28 3400 1.15 1.10 4.43
8 Branched thioether
chain
C18(2C2S[9/10,
12/13])
0.28 4200 1.32 1.02 3.84
9 Branched thioether
chain
C18(2C10S[9/10,
12/13])
0.28 3400 1.25 1.06 3.34
10 Linear thioether
chain C16(S[4]) 0.28 2700 1.21 0.89 2.54
11 Cyclopropanyl C18(Cyclo[9]) 0.28 2400 0.97 0.47 1.94
SUPELCOWAX
10 1.96 1.29 6.81
a The Cn(x[y]) represents the functional group and its position within the side chain. b Selected pairs of analytes are shown in representative GC × GC contour plot in Figure E3.
108
6.4 Results and Discussion
6.4.1 Unique physical properties of lipidic IL-based stationary phases
The viscosity, surface tension, wetting ability on fused silica, thermal stability, and
melting point of ILs are all important for them to be successfully employed as GC stationary
phases [20]. Among these properties, the melting point is critical since it determines the
operating range of the stationary phase and influences the retention mechanism in GC analysis.
In this study, N-alkyl-N′-methyl-imidazolium-based ILs containing different alkyl side chains
were used as model structures to investigate the relationship between the structural features of
alkyl moieties and the physicochemical properties of the ILs (see Figure 6.1). It was previously
reported that the melting points of a homologous series of N-alkyl-N′-methylimidazolium salts
sharply increases with the elongation of the side chain beyond seven carbon atoms [36]. The R1
IL containing a saturated C10 side chain is liquid at room temperature, whereas the R2 IL
containing a saturated C16 side chain is solid at room temperature. The incorporation of double
bonds, thioether linkages, and cyclopropyl groups as symmetry breaking moieties can
significantly decrease the melting points of the lipidic ILs with a C16 or C18 side chain. All of the
lipidic ILs are liquids at room temperature. As shown in Figure 6.2A, the melting point of 1 (-
22.0 ºC) is much lower than R2 (46.9 ºC) despite that both 1 and R2 contain an alkyl side chain
of the same length. This result indicates that lipidic ILs containing long alkyl chains can provide
much lower practical minimum operating temperatures compared to their saturated analogues
when used as GC stationary phases. Furthermore, this suggests that it is possible to design an IL
with an alkyl side chain greater than C18 while maintaining a room temperature liquid.
109
Figure 6.2 Melting point comparison for reference ILs and lipidic IL-based stationary phases and
their applications in the GC × GC separation of kerosene. (A) The melting points of ILs R1, R2,
and 1. GC × GC chromatograms of kerosene using lipidic ILs as 2D columns: (B) Rtx-5 × R1, (C)
Rtx-5 × R2, and (D) Rtx-5 × 1. The melting points were obtained from reference [30].
6.4.2 GC × GC separation of kerosene using lipidic IL based stationary phases
It has been previously reported that incorporating long alkyl chain substituents to the
cationic moiety of ILs can enhance the separation of nonpolar analytes in GC × GC [21]. To
further evaluate this effect, two ILs containing long alkyl side chains appended to the
imidazolium cations were studied. R1 and R2 were applied as secondary columns for the
separation of kerosene by GC × GC. As shown in Figure 6.2B, R1 provided good separation for
the aliphatic hydrocarbons. R2, which possesses 6 more carbon atoms within the saturated alkyl
chain, exhibited poorer selectivity in resolving the aliphatic hydrocarbons, as observed in Figure
6.2C. By comparing the melting points of these two ILs in Figure 6.2A, R2 possesses a higher
melting point (46.9 ºC) compared to R1 (-18.9 ºC); therefore, the melting point of R2 is higher
than the employed starting temperature of the GC separation. As a result, very broad peaks were
110
Figure 6.3 Enlargement of selected parts of GC × GC chromatograms of kerosene employing (A)
Rtx-5 × SUPELCOWAX 10 and (B) Rtx-5 × 1 column sets.
observed eluting from the secondary column (see Figure 6.2C). To further validate this
assumption, IL 1 containing an unsaturated C16 alkyl chain was employed as the second
dimension column. It can be observed in Figure 6.2D that IL 1 exhibited significantly higher
chromatographic efficiency compared to R2. Moreover, compared to R1, which possesses a
shorter alkyl side chain substituent, IL 1 also exhibited a significant enhancement in the
selectivity of nonpolar analytes, as observed in Figure 6.2D.
To further evaluate the resolving power of the lipidic IL-based stationary phase toward
aliphatic hydrocarbons, the separation result for 1 was also compared with a PEG-based column
(SUPELCOWAX 10) commonly employed as a second dimension column for the separation of
nonpolar aliphatic hydrocarbons by GC × GC [7-9]. As shown in Figures 6.3A and 6.3B, a wider
distribution of the analytes within the separation window was observed when employing the Rtx-
5 × 1 column set. In order to better evaluate the selectivity of the second dimension column, a
quantitative measurement was carried out by calculating the resolution values of selected pairs of
analytes within the second dimension column. As shown in Figure E3 (see appendix E), three
pairs of analytes were selected for comparison and their relative resolution values were
111
calculated based on their retention times and peak widths within the secondary column. As
shown in Table 6.1, IL 1 exhibited higher resolution values for all three pairs of analytes
compared to the SUPELCOWAX 10 column. The resolution for the first two pairs of selected
analytes increased from 1.96 and 1.29 to 2.69 and 1.98, respectively. For better comparison of
the two dimensional chromatographic separations, 2D resolution were calculated. As shown in
Table E1, the improved separations of the selected analytes were observed employing Rtx-5 × 1
column set. The 2D resolution values of selected analytes increased from 2.20, 2.05, and 7.23 to
2.81, 2.49, and 7.32, respectively. Moreover, as shown in the highlighted area of Figure 6.3, a
few groups of analytes that could not be separated on the Rtx-5 × SUPELCOWAX 10 column
set were fully resolved on the Rtx-5 × 1 column set. This result indicates that lipidic ILs
containing long alkyl substituents on the imidazolium cation and possessing low melting points
are promising candidates as GC stationary phases to provide further enhanced selectivity toward
nonpolar analytes.
Lipidic ILs containing longer alkyl side chain substituents (i.e., C18 chain) and double
bonds in different positions of the side chain were also examined in this study (see Figures 6.1
and 6.4). ILs 2, 3, and 4 contains a cis-double bond on 9th position, a cis-double bond on 11th
position, and a trans-double bond on 9th position within the C18 side chain, respectively. In
contrast, IL 5 contains two cis-double bonds on the 9th and 12th carbon position within the C18
side chain. It can be observed in Figure 6.4 and Table 6.1 that the position and geometry of the
double bond does not strongly influence the overall GC × GC separation. Very similar
selectivities for the aliphatic hydrocarbons were obtained when applying ILs 2, 3, and 4 as
stationary phases in secondary columns. Interestingly, when IL 5 was applied in the second
dimension, the aliphatic hydrocarbons exhibited much less retention compared to lipidic ILs
112
Figure 6.4 GC × GC contour plots of kerosene separation employing lipidic ILs as 2D columns:
(A) Rtx-5 × 2, (B) Rtx-5 × 3, (C) Rtx-5 × 4, (D) Rtx-5 × 5.
containing only one double bond (see Figure 6.4D). The resolution of the selected pairs of
analytes also decreased when comparing IL 2 (1.15, 0.68, and 2.42) and IL 5 (0.61, 0.42, and
1.84) despite the only structural difference being the additional double bond on the 12th position
of the alkyl side chain. It was previously reported that the geometric change in the alkyl chain
structure imposed by the cis-double bonds leads to a more cohesive IL and reduced capability for
dispersive interactions [37, 38].
6.4.3 Thermal stability of lipidic IL stationary phases
Lipidic ILs with unsaturated side chains exhibit excellent selectivity toward nonpolar
analytes in the GC × GC separation of kerosene. Since many petrochemicals contain analytes
with large molecular weight and high boiling points, high separation temperatures are often
required. Therefore, it is important to thoroughly evaluate the thermal stability of this new class
113
of ILs. The robustness of the lipidic ILs was examined using a previously reported method [23].
The thermal stability of the columns was tested by evaluating the resolution of three pairs of
selected analytes from kerosene after high temperature conditioning (i.e., 200 ºC, 250 ºC, 300 ºC,
and 350 ºC). This approach is employed to detect any viscosity and/or morphology change of the
stationary phases at elevated temperatures by examining the changes in the chromatographic
efficiency and resolution. The best performing lipidic IL (IL 1) was evaluated using this
approach. The stationary phase was found to be thermally stable in the temperature range
between 100 ºC and 150 ºC, as the resolution values for the selected analytes did not change
appreciably. After the column was conditioned at 200 ºC and the separation was performed, a
significant decrease in resolution was observed (see Figure E4, Appendix E). The maximum
allowable operating temperature (MAOT) for this IL was determined to be in the range between
150 ºC and 200 ºC. As previously reported, imidazolium-based ILs containing a double bond
generally possess lower thermal stability compared to their saturated analogues [33]. At elevated
temperatures, they can undergo slow oxidative decomposition at the site of the unit of
unsaturation. Even though IL 1 exhibits excellent selectivity for aliphatic hydrocarbons at low
operating temperature, it lacks the thermal stability needed for general use at higher separation
temperatures.
Several approaches were employed to replace the double bond within the lipidic IL in an
attempt to increase the thermal stability. One approach involves the addition of thiols to the
double bond containing alkyl side chains to obtain ILs with branched chains. ILs 6-9 contain
branched thioether chains and are all liquids at room temperature corresponding to previous
reports that the addition of branched chains reduces the melting point of the ILs [29]. As shown
in Figure 6.5 and Table 6.1, when used as stationary phase in second dimension columns, ILs
114
Figure 6.5 Use of lipidic ILs with branched thioether chains as 2D column stationary phases in
GC × GC separation of kerosene: (A) Rtx-5 × 6, (B) Rtx-5 × 7, (C) Rtx-5 × 8, and (D) Rtx-5 × 9.
possessing branched chains (i.e., IL 6-9) exhibited better resolution for aliphatic hydrocarbons
compared to ILs containing double bond (i.e., ILs 2 and 5). It was also observed that the length
of branched chain strongly influences the retention of the analytes. As shown in Figure 6.5, ILs 7
and 9 containing longer branched thioether side chains (C10S) exhibited stronger retention of
analytes in the second dimension, compared to ILs 6 and 8 (C2S). IL 8, which contains two C2S
branched thioether chains on the C18 side chain, exhibited the best resolution of the selected
analytes (Table 6.1). This result indicates that ILs with two short branched thioether side chains
are beneficial for the separation of nonpolar analytes. IL 8 was found to be thermally stable in
the temperature range between 100 ºC and 150 ºC. However, after the column was conditioned at
200 ºC, a significant decrease in resolution was observed, indicating a MAOT between 150 ºC
and 200 ºC (see Figure E5).
115
Figure 6.6 GC × GC separations of kerosene employing lipidic ILs with improved thermal
stabilities as 2D column stationary phases: (A) Rtx-5 × 10 and (B) Rtx-5 × 11.
Yet another approach for improving the thermal stability of the lipidic IL is to use the
photoinitiated thiol-ene “click” reaction to insert a thioether functional group within the linear
alkyl side chain. [29]. As shown in Figure 6.6, IL 10 exhibited good selectivity for aliphatic
hydrocarbons. Compared to R1 containing a C10 side chain, the resolution of selected pairs of
analytes in kerosene was enhanced from 0.96, 0.66, and 1.89 to 1.21, 0.89, and 2.54. After IL 10
was conditioned up to 200 ºC, the resolution of selected analytes did not change appreciably.
After high temperature conditioning to 250 ºC, the resolution of the selected pairs of analytes
decreased indicating that the MAOT for 10 was between 200 ºC and 250 ºC (see Figure E6). A
final approach to improve the thermal stability of the lipidic IL involved the net addition of a
methylene (CH2) fragment to the double bond within the side chain to obtain an IL possessing a
cyclopropanyl group in the side chain [32]. As shown in Figures E7 and E8, IL 11 exhibited a
116
similar thermal stability to R1. After conditioning to 300 ºC, IL 11 still exhibited good retention
and selectivity for the nonpolar analytes in kerosene.
6.4.4 Effect of stationary phase film thickness on GC × GC separations
The stationary phase film thickness plays an important role in GC × GC separations. It is
well-known that higher film thicknesses can reduce column over loading and offer improved
selectivity in the second dimension. As shown in the previous sections, lipidic IL-based
stationary phases possessing a film thickness of 0.28 µm provided good selectivity toward the
aliphatic hydrocarbons in kerosene. However, thicker films in GC × GC can cause band
broadening and wrap-around due to excessive retention (see Figures 6.2, 6.5, and 6.6). Therefore,
these parameters need to be optimized to preserve selectivity and maximize the overall peak
capacity. As shown in Figure 6.7, stationary phases containing film thicknesses of 0.15 µm were
prepared and applied as second dimension columns using ILs 1, 7, 10, and 11. Wrap-around was
significantly reduced for these separations compared to those columns with the thicker films (see
Figures 6.2D, 6.5B, 6.6A and 6.6B). However, the distribution of analytes within the separation
window was reduced when employing a second dimension column with a film thickness of 0.15
µm (see Figures 6.2D, 6.5B, 6.6A and 6.6B). Columns containing a stationary phase film
thickness of 0.28 µm exhibited higher selectivity for the selected analytes. These results show
that the lipidic IL-based stationary phases with thicker films provide a higher resolution for the
aliphatic hydrocarbons in the second dimension.
117
Figure 6.7 Two-dimensional chromatograms showing the separation of kerosene using lipidic
ILs as 2D column stationary phases with film thickness of 0.15 µm: (A) Rtx-5 × 1, (B) Rtx-5 × 7,
(C) Rtx-5 × 10, (D) Rtx-5 × 11.
6.4.5 Characterization of lipidic IL solvation interactions by the solvation parameter model
The separations achieved in this study indicate that the lipidic ILs possess unique
selectivity required to enhance the resolution of nonpolar analytes in kerosene. The numerous
structural features within the lipidic ILs appear to play an important role in the selectivity of the
lipidic IL-based stationary phases. To understand the solvation properties of this new class of
ILs, the Abraham solvation parameter model was employed. This model is a linear free-energy
relationship that describes the contribution of individual solvation interactions of a solvent (i.e.,
IL-based stationary phase) by evaluating the solute/solvent interactions [39-41].
𝐿𝑜𝑔 𝑘 = 𝑐 + 𝑒𝐸 + 𝑠𝑆 + 𝑎𝐴 + 𝑏𝐵 + 𝑙𝐿 (1)
In Eq. (1), k is the retention factor of each probe molecule on the liquid stationary phase
at a specific temperature. The solute descriptors (E, S, A, B, and L) have been previously
determined and are shown in Table A1. The solute descriptors are defined as: E, the excess molar
refraction calculated from the solute’s refractive index; S, the solute dipolarity/polarizability; A,
118
the solute hydrogen bond acidity; B, the solute hydrogen bond basicity; and L, the solute gas
hexadecane partition coefficient determined at 298 K. The system constants (e, s, a, b, and l) are
used to characterize the strength of each solvation interaction and are defined as: e, the ability of
the stationary phase to interact with analytes by electron lone pair interactions; s, a measure of
the dipolarity/polarizability of the stationary phase; a and b, the IL hydrogen bond basicity and
acidity of the stationary phase, respectively; and l describes the dispersion forces/cavity
formation of the IL. The system constants of two reference ILs and four selected lipidic ILs at
three different temperatures (50 ºC, 80 ºC, and 110 ºC) are listed in Table 6.2. As expected, the
interactions between the probe molecules and the stationary phases become weaker at higher
temperature, resulting in a decreased values of the system constants.
To investigate the solvation properties of lipidic ILs, six ILs (R1, R2, 1, 5, 10, and 11)
were examined in this study. The first comparison was made between R1 and R2, where R1
contains a C10 side chain and R2 possesses a longer C16 side chain. As shown in Table 6.2,
similar system constants were observed for the two ILs except for the e and l terms. The l term of
R2 (l = 0.64) is higher compared to that of R1 (l = 0.60) at 80 ºC. This result agrees with
previous reports that have shown longer alkyl side chains within the cationic moiety result in less
cohesive ILs [21-23].
To further explore the solvation properties, IL 1 was compared to R1 where IL 1 contains
a longer alkyl chain with a double bond. IL 1 has a higher l term (l = 0.62) than R1 (l = 0.60) at
80 ºC, indicating that 1 is less cohesive compared to R1. ILs 1 and R2 are structural analogues
with the only difference being a double bond within the alkyl side chain of the IL cationic
moiety. The cis-double bond incorporated in 1 appears to result in a lower l term relative to IL
R2. The l term of 1 (l = 0.62) is lower than R2 (l = 0.64) at 80 ºC (see Table 6.2). This result
119
Table 6.2 System constants of the studied lipidic ionic liquids attained using the Solvation
Parameter Model.
Stationary
Phase
Temperature
(°C)
System constants
c e s a b l n a R2 a F a
IL R1 50 -2.90
(0.09)
-0.21
(0.08)
1.71
(0.10)
1.97
(0.08)
0.26
(0.12)
0.72
(0.02) 39 0.99 549
80 -2.91
(0.07)
-0.06
(0.06)
1.62
(0.08)
1.62
(0.06)
0.19
(0.10)
0.60
(0.01) 38 0.99 691
110 -2.98
(0.06)
-0.07
(0.05)
1.56
(0.07)
1.36
(0.06)
0.11
(0.09)
0.52
(0.01) 36 0.99 632
IL R2 50 -2.97
(0.08)
-0.16
(0.07)
1.52
(0.08)
1.85
(0.08)
0.36
(0.11)
0.79
(0.02) 36 0.99 732
80 -2.81
(0.07)
-0.12
(0.06)
1.44
(0.07)
1.56
(0.06)
0.15
(0.10)
0.64
(0.02) 35 0.99 684
110 -2.81
(0.06)
-0.04
(0.06)
1.23
(0.07)
1.23
(0.06)
0.29
(0.09)
0.55
(0.01) 36 0.99 594
IL 1 50 -2.93
(0.07)
-0.13
(0.07)
1.69
(0.09)
2.01
(0.08)
0.29
(0.10)
0.73
(0.01) 36 0.99 899
80 -2.91
(0.08)
-0.11
(0.07)
1.57
(0.08)
1.74
(0.08)
0.20
(0.11)
0.62
(0.02) 37 0.99 944
110 -2.92
(0.07)
-0.05
(0.06)
1.42
(0.07)
1.44
(0.06)
0.24
(0.09)
0.53
(0.01) 35 0.99 550
IL 5 50 -2.96
(0.07)
-0.11
(0.07)
1.65
(0.08)
2.29
(0.06)
0.37
(0.10)
0.71
(0.01) 35 0.99
101
0
80 -2.53
(0.06)
-0.08
(0.05)
1.49
(0.06)
1.84
(0.06)
0.21
(0.09)
0.55
(0.01) 35 0.99 799
110 -2.96
(0.07)
-0.02
(0.06)
1.45
(0.08)
1.58
(0.06)
0.26
(0.10)
0.50
(0.02) 37 0.99 463
IL 10 50 -2.91
(0.08)
-0.24
(0.07)
1.74
(0.08)
2.16
(0.07)
0.17
(0.10)
0.73
(0.02) 35 0.99 660
80 -2.88
(0.06)
-0.15
(0.05)
1.57
(0.06)
1.78
(0.05)
0.16
(0.08)
0.62
(0.01) 40 0.99
111
5
110 -3.20
(0.07)
-0.09
(0.05)
1.52
(0.07)
1.57
(0.06)
0.24
(0.09)
0.56
(0.01) 35 0.99 682
IL 11 50 -2.93
(0.08)
-0.18
(0.07)
1.48
(0.08)
1.84
(0.07)
0.22
(0.11)
0.77
(0.02) 35 0.99 733
80 -2.91
(0.06)
-0.19
(0.05)
1.41
(0.06)
1.57
(0.06)
0.09
(0.08)
0.66
(0.01) 36 0.99 872
110 -2.93
(0.06)
-0.10
(0.05)
1.30
(0.06)
1.30
(0.05)
0.10
(0.08)
0.56
(0.01) 37 0.99 721
a Note: n, number of probe analytes subjected to multiple linear regression; R2, correlation
coefficient; F, Fisher coefficients.
120
supports the previous observations that alkanes generally have larger l terms than their alkene
homologues [42]. IL 1 provided superior selectivity toward nonpolar analytes compared to R1 as
well as a wider separation range than R2 (see Figure 6.2).
The system constants of IL 5 possessing two double bonds within the alkyl side chain are
shown in Table 6.2. The l term of IL 5 (l = 0.55) decreased significantly compared to the lipidic
IL 1 (l = 0.62) and R1 (l = 0.60), despite the fact that IL 5 possesses two more carbon atoms
within the side chain (C18 side chain). This shows that IL 5 containing two double bonds within
the alkyl side chain is less capable of nonspecific dispersive interaction, compared to the other
lipidic ILs. The low l term for IL 5 may help explain the lower selectivity toward aliphatic
hydrocarbons in the GC × GC separation of kerosene (see Figure 6.4).
The solvation properties of ILs 10 and 11 were examined in this study. IL 10 possesses a
linear thioether side chain. As shown in Table 6.2, IL 10 has the same l term (l = 0.62) with IL 1
(l = 0.62), but higher a term (a = 1.78) than IL 1 (a = 1.74) at 80 ºC. All other system constants
are largely unchanged. IL 11 containing a cyclopropanyl group incorporated within a C18 side
chain is the most thermally stable IL among lipidic ILs. As shown in Table 6.2, IL 11 has a lower
e term (e = 1.41) and a term (a = 1.57) compared to IL 1 (e = 1.57) (a = 1.74) or IL 10 (e = 1.57)
(a = 1.78) at 80 ºC, due to the lack of a double bond or thioether group. IL 11 possesses a high l
term (l = 0.66), compared to IL R1 (l = 0.60), IL R2 (l = 0.64), and IL 1 (l = 0.62) at 80 ºC. This
result agrees with the previous observation that IL 11 exhibits strong retention of nonpolar
analytes in the second dimension compared to IL R1 or IL R2 (Figures 6.1 and 6.6).
6.5 Conclusions
A total of eleven lipidic ILs were evaluated for the first time as GC stationary phases and
were further examined by comprehensive two-dimensional gas chromatography. Lipidic ILs with
unique physicochemical properties provided enhanced selectivity towards nonpolar analytes in
121
kerosene compared to their saturated linear chain analogues. Although most of the lipidic ILs
(ILs 2-11) exhibited lower selectivity compared to a commercial SUPELCOWAX 10 column,
the structurally tuned palmitoleyl lipidic IL (IL 1) provided the highest selectivity compared to
all other lipidic ILs as well as the commercial SUPELCOWAX 10 column. The linoleyl IL (IL
5) with two double bonds exhibited the lowest selectivity since it is more cohesive and less
capable of dispersive-type interactions. The lipidic IL containing a cyclopropanyl group within
the alkyl side chain (IL 11) exhibited a MAOT of 300 °C and the highest thermal stability of all
lipidic ILs studied. The best performing ILs were selected to prepare stationary phases with
different film thicknesses. IL-based columns with thicker films provided improved selectivity for
aliphatic hydrocarbons in kerosene. The solvation parameter model was used to evaluate the
solvation properties of the lipidic ILs with the various symmetry breaking moieties in the alkyl
side chain. Dispersive interactions were found to play a critical role in the separation of nonpolar
analytes within the kerosene sample. Lipidic ILs can be designed to be less cohesive through the
addition of symmetry breaking moieties to result in ILs with low melting points. This study
demonstrates which structural features of lipidic ILs produce the highest thermal stabilities as
well as solvation properties that provide the required selectivity to resolve analytes in complex
samples.
6.6 Acknowledgments
The authors acknowledge funding from the Chemical Measurement and Imaging
Program at the National Science Foundation (Grant number CHE-1413199).
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effects on imidazolium salt melting points: a descriptor modelling study, ChemPhysChem 8
(2007) 690-695.
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solvent properties of a new series of room-temperature liquid tetraalkylammonium
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column thermal modulator interface, J. Chromatogr. Sci. 29 (1991) 227-231.
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[30] S.M. Murray, R.A. O'Brien, K.M. Mattson, C. Ceccarelli, R.E. Sykora, K.N. West, J.H.
Davis, The fluid-mosaic model, homeoviscous adaptation, and ionic liquids: dramatic
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2755-2758.
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biomaterials with attractive properties and applications, in: Ionic Liquids: Science and
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126
CHAPTER 7. GENERAL CONCLUSIONS
The first half of the dissertation describes the development of IL-based stationary phases
with unique selectivities in the conventional 1D GC separations. Firstly, a new series of ZIL-
based stationary phases was developed for the selective separation of volatile carboxylic acids. It
is shown that this class of ZILs exhibit strong retention of VCAs with excellent peak symmetry.
Unique chromatographic selectivity toward VCAs is also demonstrated by tuning the structural
features of the ZILs. Secondly, The solvation properties of eight room temperature ILs
containing various transition and rare earth metal centers (e.g., Mn(II), Co(II), Ni(II), Nd(III),
Gd(III), and Dy(III)) are characterized using the Abraham solvation parameter model. A new
class of highly efficient GC columns were prepared using these metal-containing ILs (MCILs)
and exhibited unique selectivity to a wide range of analytes including amines, carboxylic acids,
and alcohols. Lastly, silver-containing IL-based stationary phases was successfully used for the
separation of paraffin/olefin mixtures. The selectivity of the stationary phase toward olefins can
be tuned by adjusting the ratio of silver ion and the mixed ligands. The maximum allowable
operating temperature of these stationary phases was determined to be between 125 ºC and 150
ºC.
In the second half of the dissertation, development and application of the silver-based and
lipidic IL-based stationary phases in GC × GC was discussed. A silver IL-based stationary phase
was successfully employed as a second dimension column for the GC × GC separation of
mixtures containing alkynes, dienes, terpenes, esters, aldehydes, and ketones. Compared to a
widely used non-polar and polar column set, the silver IL-based column exhibited superior
performance by providing better chromatographic resolution of co-eluted compounds. In
addition, eleven lipidic IL-based stationary phases with long alkyl side chains as well as low
127
melting points was used for the GC × GC separation of aliphatic hydrocarbons. Compared to a
homologous series of ILs containing saturated side chains, lipidic ILs exhibit improved
selectivity toward the aliphatic hydrocarbons in kerosene. The palmitoleyl IL provided the
highest selectivity compared to all other lipidic ILs as well as the commercial
SUPELCOWAX10 column. This study provides the comprehensive examination into the
relation between various IL structures and the resulting solvation characteristics.
128
APPENDIX A. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 2
Spectroscopic Data of Compounds
ZIL 1 (OE2IMC4S) 1H-NMR (400 MHz, DMSO-D6) δ 9.14 (s, 1H, NCHN), 7.77 and 7.71 (s,
2H, NCHCHN), 4.30 (t, J = 4.8 Hz, 2H, OCH2CH2N), 4.17 (t, J = 7.1 Hz, 2H, NCH2(CH2)3SO3),
3.73 (t, J = 4.8 Hz, 2H, OCH2CH2N), 3.52-3.35 (m, 4H, CH3OCH2CH2), 3.17 (s, 3H, CH3O),
2.41 (t, J = 7.6 Hz, 2H, N(CH2)3CH2SO3), 1.90-1.79 (m, 2H, NCH2CH2(CH2)2SO3), 1.54-1.44
(m, 2H, N(CH2)2CH2CH2SO3)
ZIL 2 (C8IMC3S) 1H NMR (500 MHz, DMSO-d6) δ 9.18 (d, J = 1.9 Hz, 1H), 7.79 (dt, J = 9.1,
1.9 Hz, 2H), 4.30 (t, J = 7.0 Hz, 2H), 4.15 (t, J = 7.3 Hz, 2H), 2.39 (dd, J = 8.5, 5.9 Hz, 2H), 2.09
(p, J = 7.1 Hz, 2H), 1.85 – 1.64 (m, 2H), 1.25 (s, 9H), 0.86 (t, J = 6.7 Hz, 3H).
ZIL 3 (C8IMC4S) 1H NMR (500 MHz, DMSO-d6) δ 9.22 (s, 1H), 7.80 (d, J = 1.7 Hz, 2H), 4.17
(dt, J = 15.1, 7.1 Hz, 4H), 2.43 (d, J = 7.6 Hz, 2H), 1.88 (p, J = 7.2 Hz, 2H), 1.79 (t, J = 7.2 Hz,
2H), 1.53 (p, J = 7.7 Hz, 2H), 1.25 (d, J = 9.5 Hz, 12H), 0.86 (t, J = 6.7 Hz, 3H).
129
IL R1 ([BMIM+][NTf2-]) The IL R1 was synthesized using a previously published method [1]. A
mixture of 1-methylimidazole (0.05 mol) and 1-chlorobutane (0.075 mol) was added in 15 mL of
isopropanol at 70 ºC for 24 h. The solvent was removed using rotary evaporation. The product
was then dissolved in 10 mL of water and washed using ethyl acetate (3 mL) for three times. The
[BMIM+][Cl-] IL was recovered from the water layer and dried under vacuum at 80 ºC for 24 h.
The halide anion was then exchanged to [BMIM+][NTf2-] by metathesis reaction using one molar
equivalent of [Li+][NTf2-].
1H NMR (500 MHz, DMSO-d6) δ 9.10 (s, 1H), 7.76 (t, J = 1.8 Hz, 1H), 7.70 (t, J = 1.8 Hz, 1H),
4.16 (t, J = 7.2 Hz, 2H), 3.85 (s, 3H), 1.85 - 1.68 (m, 2H), 1.34 – 1.16 (m, 2H), 0.91 (t, J = 7.4
Hz, 3H).
IL R3 ([OE2IMC3+][MeSO3
-]) The IL R3 1-(2-methoxyethyl)-3-propylimidazolium
methanesulfonate was synthesized using a previously published method [2, 3]. Briefly, 1-bromo-
2-(2-methoxyethoxy)ethane (0.06 mol) was first washed with 12 mL of water twice and dried
using 4Å molecular sieves (MilliporeSigma) to remove the stabilizer. After filtration, 1-bromo-2-
(2-methoxyethoxy)ethane (0.04 mol) was added to 1-propylimidazole (0.016 mol) in 100 mL
tetrahydrofuran (THF) under argon gas. The mixture was stirred at 80 °C for 24 h and the THF
solvent was removed by rotary evaporation to obtain [OE2IMC3+][Br-]. After washing with
130
diethyl ether three times, the resulting compound was dissolved in dichloromethane and passed
through aluminum oxide (basic, MilliporeSigma). Dichloromethane was removed by rotary
evaporation and the compound was washed with diethyl ether. An anion-exchange resin
(Amberlite IRN78 hydroxide form) and methanesulfonic acid was used to obtain
[OE2IMC3+][MeSO3
-]. After drying under reduced pressure, a colorless liquid was obtained.
1H NMR (500 MHz, DMSO-d6) δ 9.21 (s, 1H), 7.80 (dt, J = 18.7, 1.9 Hz, 2H), 4.36 (q, J = 4.9
Hz, 2H), 4.16 (t, J = 6.9 Hz, 2H), 3.77 (t, J = 5.0 Hz, 2H), 3.54 (t, J = 4.7 Hz, 2H), 3.40 (dd, J =
5.7, 3.6 Hz, 2H), 2.32 (d, J = 2.7 Hz, 3H), 1.81 (p, J = 7.2 Hz, 2H), 0.84 (t, J = 7.4 Hz, 3H).
131
Table A1 List of all probe molecules and their corresponding solute descriptors used to
characterize ZIL-based stationary phases employing the solvation parameter model Probe molecule E S A B L
Acetic acid 0.265 0.65 0.61 0.44 1.75
Acetophenone 0.818 1.01 0 0.48 4.501
Aniline 0.955 0.96 0.26 0.41 3.934
Benzaldehyde 0.82 1 0 0.39 4.008
Benzene 0.61 0.52 0 0.14 2.786
Benzonitrile 0.742 1.11 0 0.33 4.039
Benzyl alcohol 0.803 0.87 0.33 0.56 4.221
Bromoethane 0.366 0.4 0 0.12 2.62
1-Bromooctane 0.339 0.4 0 0.12 5.09
1-Butanol 0.224 0.42 0.37 0.48 2.601
Butyraldehyde 0.187 0.65 0 0.45 2.27
2-Chloroaniline 1.033 0.92 0.25 0.31 4.674
1-Chlorobutane 0.21 0.4 0 0.1 2.722
1-Chlorohexane 0.201 0.4 0 0.1 3.777
1-Chlorooctane 0.191 0.4 0 0.1 4.772
p-Cresol 0.82 0.87 0.57 0.31 4.312
Cyclohexanol 0.46 0.54 0.32 0.57 3.758
Cyclohexanone 0.403 0.86 0 0.56 3.792
1,2-Dichlorobenzene 0.872 0.78 0 0.04 4.518
N,N-Dimethylformamide 0.367 1.31 0 0.74 3.173
1,4-Dioxane 0.329 0.75 0 0.64 2.892
Ethyl acetate 0.106 0.62 0 0.45 2.314
Ethyl benzene 0.613 0.51 0 0.15 3.778
1-Iodobutane 0.628 0.4 0 0.15 4.13
Methyl caproate 0.067 0.6 0 0.45 3.844
Naphthalene 1.34 0.92 0 0.2 5.161
Nitrobenzene 0.871 1.11 0 0.28 4.557
1-Nitropropane 0.242 0.95 0 0.31 2.894
1-Octanol 0.199 0.42 0.37 0.48 4.619
Octylaldehyde 0.16 0.65 0 0.45 4.361
1-Pentanol 0.219 0.42 0.37 0.48 3.106
2-Pentanone 0.143 0.68 0 0.51 2.755
Ethyl phenyl ether 0.681 0.7 0 0.32 4.242
Phenol 0.805 0.89 0.6 0.3 3.766
Propionitrile 0.162 0.9 0.02 0.36 2.082
Pyridine 0.631 0.84 0 0.52 3.022
Pyrrole 0.613 0.73 0.41 0.29 2.865
Toluene 0.601 0.52 0 0.14 3.325
m-Xylene 0.623 0.52 0 0.16 3.839
o-Xylene 0.663 0.56 0 0.16 3.939
p-Xylene 0.613 0.52 0 0.16 3.839
2-Propanol 0.212 0.36 0.33 0.56 1.764
2-Nitrophenol 1.015 1.05 0.05 0.37 4.76
1-Bromohexane 0.349 0.4 0 0.12 4.13
Propionic acid 0.233 0.65 0.6 0.45 2.29
1-Decanol 0.191 0.42 0.37 0.48 5.628
Data obtained from reference [4].
132
Table A2 A linear free-energy relationship was used in Abraham solvation parameter model to
evaluate the solvation properties of IL-based stationary phases
𝐿𝑜𝑔 𝑘 = 𝑐 + 𝑒𝐸 + 𝑠𝑆 + 𝑎𝐴 + 𝑏𝐵 + 𝑙𝐿
k, retention factor of each probe molecule on the stationary phase at a specific temperature
Unknown values representing the solvation
properties of IL stationary phase
Previously determined values of the probe
molecules (see Table A1)
e
The ability of the stationary phase to
interact with analytes by electron lone pair
interactions
E The excess molar refraction calculated
from the solute’s refractive index
s A measure of the dipolarity/polarizability
of the stationary phase S The solute dipolarity/polarizability
a IL hydrogen bond basicity of the
stationary phase A The solute hydrogen bond acidity
b IL hydrogen bond acidity of the stationary
phase B The solute hydrogen bond basicity
l The dispersion forces/cavity formation of
the IL L
The solute gas hexadecane partition
coefficient determined at 298 K
c The intercept of the regression line
133
Table A3 System constants of ILs obtained by the solvation parameter model
Stationary
Phase
Temperature
(°C)
System constants
c e s a b l n a R2 a F a
40 -2.87 0 1.89 2.02 0.36 0.63 33 0.99 -
IL R1
[BMIM+]
[NTf2-]b
70 -3.02 0 1.67 1.75 0.38 0.56 35 0.99 -
110 -3.13 0 1.60 1.55 0.24 0.49 32 0.98 -
40 -2.43 0 1.86 3.02 0 0.61 30 0.98 -
[BMIM+]
[TfO-]b 70 -2.64 0 1.73 2.71 0 0.52 31 0.99 -
110 -2.76 0 1.39 2.35 0 0.48 32 0.96 -
[TBA+]
[MeSO3-]c
121.4 -0.61 0.33 1.45 3.76 - 0.44 32 0.99 -
50 -2.90
(0.09)
-0.21
(0.08)
1.71
(0.10)
1.97
(0.08)
0.26
(0.12)
0.72
(0.02) 39 0.99 549
[DMIM+]
[NTf2-]d
80 -2.91
(0.07)
-0.06
(0.06)
1.62
(0.08)
1.62
(0.06)
0.19
(0.10)
0.60
(0.01) 38 0.99 691
110 -2.98
(0.06)
-0.07
(0.05)
1.56
(0.07)
1.36
(0.06)
0.11
(0.09)
0.52
(0.01) 36 0.99 632
a n, number of probe analytes subjected to multiple linear regression; R2, correlation coefficient;
F, Fisher coefficients. b [BMIM+][TfO-], 1-butyl-3-methylimidazolium trifluoromethanesulfonate. Data were obtained
from reference [1]. c [TBA+][MeSO3
-], Tetra-n-butylammonium methanesulfonate. Data were obtained from
reference [5]. d Data were obtained from reference [6].
134
Figure A1 Chromatographic separations of volatile acid mixture by ILs R1, R2, and R3
columns. Analytes: 1, formic acid; 2, acetic acid; 3, propionic acid; 4, isobutyric acid; 5, n-
butyric acid; 6, isovaleric acid; 7, n-valeric acid; 8, isohexanoic acid; 9, n-hexanoic acid; 10, n-
heptanoic acid. Formic acid was not observed on the chromatogram. All gas chromatography
measurements were performed on an Agilent 7890B instrument with a flame ionization detector
(FID). Helium was used as the carrier gas with a flow rate of 1 mL min-1. The inlet and FID
detector temperatures were held at 250 °C. A split ratio of 20:1 was used. Five meter columns
with 250 µm inner diameter and 0.28 µm film thickness were used in this study. The FID
detector used hydrogen as a makeup gas at a flow rate of 30 mL min-1 and air flow was held at
400 mL min-1.
135
Figure A2 Column bleed profile (A) and the column efficiency tests (B) of ZIL 1 column after
stepwise heating from 100 °C to 250 °C. The column bleed profile was generated by using a
temperature program (100 °C hold for 5 min; 20 °C min-1 heating up to a higher temperature
ranging from 125 °C to 250 °C and hold for 1 hour; 20 °C min-1 cooling down to 100 °C). The
column efficiency test was performed after each heating step. The naphthalene standard solution
(1 µL) was injected to the 5 m column at isothermal condition (100 °C) with a split ratio of 20:1.
An Agilent 7890B GC system with a FID detector was used for the data collection.
136
Figure A3 Column bleed profile (A) and the column efficiency tests (B) of ZIL 2 column after
stepwise heating from 100 °C to 250 °C. The column bleed profile was generated by using a
temperature program (100 °C hold for 5 min; 20 °C min-1 heating up to a higher temperature
ranging from 125 °C to 250 °C and hold for 1 hour; 20 °C min-1 cooling down to 100 °C). The
column efficiency test was performed after each heating step. The naphthalene standard solution
(1 µL) was injected to the 5 m column at isothermal condition (100 °C) with a split ratio of 20:1.
An Agilent 7890B GC system with a FID detector was used for the data collection.
137
References
[1] J.L. Anderson, J. Ding, T. Welton, D.W. Armstrong, Characterizing ionic liquids on the basis
of multiple solvation interactions, J. Am. Chem. Soc., 124 (2002) 14247-14254.
[2] E. Alcalde, I. Dinarès, A. Ibáñez, N. Mesquida, A Simple Halide-to-Anion Exchange Method
for Heteroaromatic Salts and Ionic Liquids, Molecules, 17 (2012) 4007-4027.
[3] M. Abe, K. Kuroda, D. Sato, H. Kunimura, H. Ohno, Effects of polarity, hydrophobicity, and
density of ionic liquids on cellulose solubility, Phys. Chem. Chem. Phys., 17 (2015) 32276-
32282.
[4] M.H. Abraham, Scales of solute hydrogen-bonding: their construction and application to
physicochemical and biochemical processes, Chem. Soc. Rev., 22 (1993) 73-83.
[5] T.O. Kollie, C.F. Poole, Influence of fluorine substitution on the solvation properties of
tetraalkylammonium alkanesulfonate phases in gas chromatography, Chromatographia, 33
(1992) 551-559.
[6] H. Nan, C. Zhang, R.A. O'Brien, A. Benchea, J.H. Davis, Jr., J.L. Anderson, Lipidic ionic
liquid stationary phases for the separation of aliphatic hydrocarbons by comprehensive two-
dimensional gas chromatography, J. Chromatogr. A, 1481 (2017) 127-136.
138
APPENDIX B. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 3
Synthesis of MCILs
The MCILS (ILs 1-6) were synthesized according to previously published procedures [1]. 10
mmol of ammonium hydroxide were first mixed with 30 mL of ethanol. After sealing the
reaction flask with a rubber septum, 10 mmol of hexafluoroacetylacetone or were added
dropwise using a syringe. Next, 3.3 mmol of nickel(II) chloride, cobalt(II) chloride hexahydrate,
or manganese(II) chloride tetrahydrate were added. For the rare earth-based MCILs, 2.5 mmol of
dysprosium(III) chloride hexahydrate, gadolinium(III) chloride hexahydrate, or neodymium(III)
chloride hexahydrate were added to the mixture. The reaction was stirred at room temperature
for 5 hours after which the solvent was removed using rotovap. Liquid-liquid extraction was
performed using diethyl ether and deionized water until the aqueous fraction yielded no
precipitate when subjected to the silver nitrate test. The salt was then dried at 50°C overnight
under vacuum. A 1 mmol quantity of the metal salt was added to 1 mmol of
trihexyl(tetradecyl)phosphonium chloride and stirred for 24 h in methanol at room temperature.
The methanol was evaporated and the crude product dissolved in diethyl ether and extracted with
several 5 mL aliquots water. The solvent was evaporated and the resulting MCIL dried overnight
at 50 °C in the vacuum oven. The synthesis of IL 7 possessing
trifluoromethylphenylacetylacetonate ligands was performed using the same method.
IL 8 with a [MnCl42-] anion was prepared using a previously reported method [2]. Briefly, 0.5
mmol MnCl2·4H2O was added to a solution of 1 mmol [P66614+][Cl−] in dichloromethane. The
reaction was performed for 24 h at room temperature under constant agitation. Afterwards, the
dichloromethane solvent was removed by rotary evaporation. The obtained product was dried at
50 °C overnight.
139
Thermal Stability of MCIL-Based Columns
The thermal stability of six MCILs with hexafluoroacetylacetonate (hfacac) ligand (ILs 1-6)
were previously reported [1]. The thermal stability of IL 7 and IL 8 were studied using the same
method. Briefly, a three meter column possessing an approximate 0.28 µm film thickness was
heated slowly in a GC oven and an ultra-sensitive flame ionization detector (FID) was used to
detect any volatilization/decomposition of the MCIL-based stationary phase. As shown in Figure
B1, the IL 7 with trifluoromethylphenylacetylacetonate (tfmphacac) ligands exhibited stronger
column bleed compared to IL 8 with the [MnCl42-] anion. Compared with the thermal stability of
IL 3 with hfaccac ligands which was reported in previously published paper [1], IL 7 possess
higher column bleed than IL 3. These results indicate that the MCILs with hfacac or tfmphacac
ligands are less thermally stable compared to the MCILs without any chelating ligand.
Figure B1 Thermal stability diagram of IL 7 ([P66614
+][Mn(tfmphacac)3-]) and IL 8
([P66614+]2[MnCl4
2-]) constructed by coating a thin layer of MCIL on the wall of untreated fused
silica capillary followed by heating under a constant flow of helium and detecting any
volatilization/decomposition products using a FID detector.
140
Table B1 System constants of the studied metal-containing ionic liquids obtained using the
solvation parameter model.
Stationary
Phase/Tempera
ture (°C)
System constants
c e s a b l n a R2 a F a
Trihexyl(tetradecyl)phosphonium tris(hexafluoroacetylaceto)manganate(II) (IL 3)
50 -3.18
(0.10) b
-0.46
(0.09)
1.51
(0.11)
2.66
(0.15)
0.89
(0.15)
0.82
(0.03) 31 0.99 467
80 -2.97
(0.07)
-0.40
(0.06)
1.33
(0.08)
2.34
(0.10)
0.51
(0.11)
0.69
(0.02) 31 0.99 633
110 -3.03
(0.09)
-0.38
(0.07)
1.33
(0.10)
1.75
(0.09)
0.25
(0.14)
0.60
(0.02) 26 0.99 463
Trihexyl(tetradecyl)phosphonium tris(trifluoromethylphenylacetylaceto)manganate(II) (IL 7)
50 -3.09
(0.09)
-0.43
(0.08)
1.66
(0.10)
2.91
(0.14)
-0.25
(0.13)
0.82
(0.02) 36 0.99 463
80 -3.04
(0.09)
-0.35
(0.07)
1.51
(0.08)
2.19
(0.11)
-0.28
(0.11)
0.70
(0.02) 34 0.99 400
110 -3.05
(0.07)
-0.23
(0.05)
1.27
(0.07)
1.38
(0.10)
-0.08
(0.10)
0.60
(0.02) 31 0.99 507
Trihexyl(tetradecyl)phosphonium tetrachloromanganate(II) (IL 8)
50 -3.00
(0.09)
-0.29
(0.08)
1.94
(0.11)
3.88
(0.14)
-0.73
(0.14)
0.76
(0.02) 31 0.99 565
80 -2.96
(0.07)
-0.18
(0.06)
1.75
(0.09)
3.30
(0.11)
-0.71
(0.12)
0.63
(0.02) 33 0.99 614
110 -3.11
(0.07)
-0.14
(0.06)
1.63
(0.08)
2.76
(0.09)
-0.66
(0.10)
0.56
(0.01) 32 0.99 696
Trihexyl(tetradecyl)phosphonium chloride c
70 -3.63
(0.23)
-0.15
(0.18)
1.51
(0.19)
6.60
(0.31)
-0.58
(0.24)
0.83
(0.06) 26 0.97 152
Trihexyl(tetradecyl)phosphonium trifluoromethanesulfonate c
70 -3.28
(0.11)
-0.24
(0.10)
1.55
(0.12)
2.85
(0.12)
-0.36
(0.15)
0.76
(0.03) 33 0.99 358
Trihexyl(tetradecyl)phosphonium bis[(trifluoromethyl)sulfonyl]imide c
70 -3.29
(0.09)
-0.28
(0.08)
1.55
(0.09)
1.55
(0.8)
-0.15
(0.11)
0.75
(0.02) 34 0.99 537
Trihexyl(tetradecyl)phosphonium tetrachloroferrate c
50 -3.09
(0.08)
-0.27
(0.08)
1.51
(0.10)
1.53
(0.09)
0.12
(0.13)
0.79
(0.02) 45 0.99 556
80 -3.19
(0.07)
-0.26
(0.07)
1.43
(0.09)
1.23
(0.08)
0.02
(0.11)
0.69
(0.02) 45 0.99 577
110 -3.29
(0.08)
-0.21
(0.08)
1.31
(0.10)
1.03
(0.08)
-0.04
(0.13)
0.61
(0.02) 41 0.99 344
a Note: n, number of probe analytes subjected to multiple linear regression analysis; R2,
correlation coefficient; F, Fisher coefficients. b The values in brackets are the reported standard
deviations. c Data obtained from Ref. [3] and [4].
141
References
[1] S.A. Pierson, O. Nacham, K.D. Clark, H. Nan, Y. Mudryk, J.L. Anderson, Synthesis and
characterization of low viscosity hexafluoroacetylacetonate-based hydrophobic magnetic ionic
liquids, New J. Chem, 41 (2017) 5498-5505.
[2] H. Yu, J. Merib, J.L. Anderson, Faster dispersive liquid-liquid microextraction methods using
magnetic ionic liquids as solvents, J. Chromatogr. A, 1463 (2016) 11-19.
[3] Z.S. Breitbach, D.W. Armstrong, Characterization of phosphonium ionic liquids through a
linear solvation energy relationship and their use as GLC stationary phases, Anal Bioanal Chem,
390 (2008) 1605-1617.
[4] L.W. Hantao, A. Najafi, C. Zhang, F. Augusto, J.L. Anderson, Tuning the Selectivity of Ionic
Liquid Stationary Phases for Enhanced Separation of Nonpolar Analytes in Kerosene Using
Multidimensional Gas Chromatography, Anal. Chem., 86 (2014) 3717-3721.
142
APPENDIX C. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 4
Synthesis of silver-based ionic liquids
The mixed-ligand silver-based IL was synthesized using a previously reported procedure [1, 2].
Briefly, 0.05 mol of Ag2O was added to acetonitrile with 0.1 mol
bis[(trifluoromethyl)sulfonyl]amine and the mixture was stirred at room temperature for 2 h. The
solvent was removed in vacuo and the remaining solid was placed in a vacuum oven to obtain
[Ag+(ACN)][NTf2-] as a white solid. The 1-methylimidazole and 1-butylimidazole ligands were
added into [Ag+(ACN)][NTf2-] in acetonitrile with at a molar ratio of 1:1:1 and the mixture was
left to stir for 2 h. The acetonitrile was removed in vacuo to yield [Ag+(MIM)(BIM)][NTf2-]. IL
mixtures were obtained using [Ag+(ACN)][NTf2-], 1-methylimidazole, and 1-butylimidazole
with molar ratios of 2:1:1 and 4:1:1, respectively. The mixed-ligand silver-based IL
[Ag+(MIM)(BIM)][NTf2-] possesses a melting point of 30 °C, which is much lower than its
corresponding [Ag+][NTf2-] without any ligand (248.8 °C) (see Table C1). The melting point
data were obtained from a previously published report [1, 3].
Synthesis of 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide IL
The 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [BMIM+][NTf2-]
IL was prepared using a previously published method [4]. Briefly, 0.05 mol of 1-
methylimidazole and 0.075 mol of 1-chlorobutane were mixed in 15 mL of isopropanol at 70 ºC
for 24 h. The product was then dissolved in 10 mL of water and washed three times with 3 mL of
ethyl acetate. The water layer containing the IL was recovered and dried under vacuum at 80 ºC
for 24 h to yield [BMIM+][Cl-]. The halide counter anion was then exchanged to [BMIM+][NTf2-
] by metathesis reaction using one equivalent of [Li+][NTf2-].
143
Preparation of GC columns containing silver based IL stationary phases
GC columns were prepared using the static coating method. Briefly, all ILs were dried in
a vacuum oven overnight at 70 °C to remove all residual water and organic solvent. A 0.45%
(w/v) coating solution was prepared by dissolving the IL in dry dichloromethane. Three, five,
and ten meter segments of untreated fused silica capillary (I.D. 250 µm) were coated by the static
coating method at 40 °C. The coated columns were conditioned using a temperature program
from 40 °C to 110 °C with a ramp of 1 °C min-1 and held isothermally at 110 °C for 3 h. Helium
was used as the carrier gas at a constant flow of 1 mL min-1. The film thickness of the stationary
phase was calculated using a previously reported method [5]. The column efficiency was
determined using naphthalene at 100 °C.
Sample preparation and instrumentation
A gas mixture of hexane and 1-hexene was prepared by adding 1 µL of each compound
into a 20 mL sealed headspace vial. A gas mixture of hexane, 1-hexene, trans-2-hexene, cis-2-
hexene, 1-hexyne, 2-hexyne, 3-hexyne, 2,4-hexadiene, and 1,5-hexadiene was prepared using the
same procedure. A 1 µL volume of the headspace was subjected to GC separation. All
separations were performed on an Agilent 7890B gas chromatograph equipped with a flame
ionization detector (GC-FID). The inlet temperature was maintained at 250 ºC, while the FID
was held at 250 ºC. Helium was employed as the carrier gas at a constant flow of 1 mL min−1.
Solution and Solid-State NMR Spectroscopy
1H solution NMR experiments were performed on a 16.4 T Bruker Avance III NMR
144
spectrometer equipped with a double resonance HX broadband probe.
In the solid-state NMR spectra, 1H chemical shifts were referenced with respect to neat
tetramethylsilane via a secondary standard of adamantane (iso = 1.82 ppm). 109Ag chemical
shifts were referenced using the established relative frequency scale.[6] 1H and 109Ag solid-state
NMR experiments were performed on a 9.4 T (400 MHz) Bruker Avance III HD spectrometer
equipped with a double resonance fast MAS 1.3 mm HX broadband probe. All solid-state NMR
experiments were performed with MAS frequencies of 50 kHz. The previously described
forwards and backwards cross-polarization (CP) pulse sequence [7] was used for the acquisition
of proton detected 2D 1H-109Ag HETCOR spectra. 1H-109Ag CP was initially setup on silver
methanesulfonate.[8] The 2D 1H-109Ag HETCOR spectra were obtained with forwards and
backwards CP contact times between 15 ms and 18 ms. 2D 1H-109Ag HETCOR spectra were
obtained with 4 to 8 scans per increment, t1 was incremented in steps of 40 s to 100 s and 128
to 768 individual t1 increments were acquired. Recycle delays between 1.8 s and 14.6 s were
used. The CP contact time was directly optimized on the samples to obtain maximum sensitivity.
Low power double quantum (DQ) CP conditions with spin lock rf fields of 1,H ≈ 34 kHz and
1,Ag ≈ 16 kHz were employed for all experiments in order to minimize the probe duty. The rf
field of the 1H contact pulse was linearly ramped from 90% to 100% of the maximum value to
broaden the CP match condition [9].
Estimation of Silver-Olefin Binding Constants by Solution 1H NMR
As shown in Figure C7, the 2D heteronuclear multiple-quantum correlation (HMQC)
NMR spectra of 1-hexene (A) and 1-hexene with equal molar of [Ag+][NTf2-] salt (B) are shown.
The proton and carbon chemical shift were observed. Due to the silver olefin complexation, a
145
downfield chemical shift of the proton on the double bond is observed. The coupling constant
between two protons on the double bond also decreased. These results have good correlation
with previously published work [10]. Meanwhile, a decrease in chemical shift of the carbon on
the double bond was observed, due to the hybridization change for the carbon on the double
bond of 1-hexene from sp2 to sp3. These results are in a good agreement with previously
published work [11, 12].
The change of proton chemical shift on the double bonds of 1-hexene and cis-2-hexene
are shown in Figure C8. The binding constant of silver ion with the olefins was calculated using
a previously reported method [13]. The following equation is used to calculate the binding
constants, where [Ag+] and [Olefin] are the total molarities of silver ion and olefin in the
complex.
𝐾 =
∆𝛿𝑜𝑏𝑠𝑑
∆𝛿𝑚𝑎𝑥[𝑂𝑙𝑒𝑓𝑖𝑛]
([𝐴𝑔+] − ∆𝛿𝑜𝑏𝑠𝑑
∆𝛿𝑚𝑎𝑥[𝑜𝑙𝑒𝑓𝑖𝑛])([𝑂𝑙𝑒𝑓𝑖𝑛] −
∆𝛿𝑜𝑏𝑠𝑑
∆𝛿𝑚𝑎𝑥[𝑜𝑙𝑒𝑓𝑖𝑛])
146
Figure C1 1H NMR (500 MHz, DMSO-d6) δ 9.10 (s, 1H), 7.76 (t, J = 1.8 Hz, 1H), 7.70 (t, J =
1.8 Hz, 1H), 4.16 (t, J = 7.2 Hz, 2H), 3.85 (s, 3H), 1.83 – 1.69 (m, 2H), 1.33 – 1.19 (m, 2H).
Figure C2 1H NMR (500 MHz, DMSO-d6) δ 8.04 (d, J = 1.2 Hz, 1H), 7.97 (s, 1H), 7.44 (d, J =
1.4 Hz, 1H), 7.37 (d, J = 1.4 Hz, 1H), 7.10 (dt, J = 6.5, 1.2 Hz, 2H), 4.06 (t, J = 7.1 Hz, 2H), 3.74
(s, 3H), 3.32 (s, 6H), 1.77 – 1.64 (m, 2H), 1.22 (h, J = 7.4, 6.7 Hz, 2H), 0.89 (t, J = 7.4 Hz, 3H).
147
0 0.40.2 0.6 10.8
Retention time (min)
1
2
Re
lative
re
sp
on
se
(p
A)
0 0.40.2 0.6 10.8
Retention time (min)
(A)1
2
Re
lative
re
sp
on
se
(p
A)
(B)
Figure C3 Gas chromatographic separation of a gas mixture containing hexane and 1-hexene
using different columns. (A) column prepared using [Ag+][NTf2-] and [BMIM+][NTf2
-] IL
mixture at a molar ratio of 1:4. (B) column prepared using [Ag+][NTf2-] and [BMIM+][NTf2
-] IL
mixture at a molar ratio of 4:1.
1 21.5
Retention time (min)
3
Rela
tive r
esponse (
pA
)
30 m SUPELCOWAX10 Column
2.5
Figure C4 Gas chromatographic separation of nine probe molecules on 30 m SUPELCOWAX10
column (I.D. 250 µm, df, 0.25 µm). Analyte mixture containing hexane, trans-2-hexene, 1-
hexene, cis-2-hexene, 1,5-hexadiene, 2,4-hexadiene (mixture of three possible isomers), 3-
hexyne, 2-hexyne, and 1-hexyne. Separation conditions: flow rate, 1 mL min-1; oven
temperature, 100 ºC.
148
Retention time (min)
1 21.5
Retention time (min)
3
(A)
Retention time (min)
(B)
(C)
Rela
tive r
espon
se (
pA
)R
ela
tive r
espon
se (
pA
)R
ela
tive r
espon
se (
pA
)
30 m HP-5ms Column
30 m Alumina PLOT Column
10 m Silver-Based IL Column
2.5
1 21.5 32.5
0 10.5 21.5
Figure C5 Separation of 2,4-hexadiene mixture using different columns. (A) 30 m HP-5ms
column, (B) 30 m alumina PLOT column, and (C) 10 m column prepared using silver-based IL
consisting of ([Ag+(MIM)(BIM)][NTf2-] and [Ag+][NTf2
-]) stationary phase at a molar ratio of
1:1. Separation conditions: (A) and (C) flow rate, 1 mL min-1; oven temperature, 100 ºC; (B)
flow rate, 10 mL min-1; oven temperature, 160 ºC.
149
Figure C6 Overlaid chromatograms of 1-hexene separation on a silver-based IL column Two
hundred injections of sample mixture containing hexane and 1-hexene was performed on a 5 m
column consisting of ([Ag+(MIM)(BIM)][NTf2-] and [Ag+][NTf2
-]) stationary phase at a molar
ratio of 1:1. Separation conditions: flow rate, 1 mL min-1; oven temperature, 100 ºC.
A
B
4.99, 114.95
5.06, 111.59
1-Hexene
1-Hexene with Silver
6
Figure C7 Two-dimensional 1H-13C HMQC NMR spectra of (A) 1-hexene and (B) 1-hexene
with equal molar [Ag+][NTf2-].
150
Mole Ratio of Ag+ and alkene
0:1 5:1 15:1 25:110:1 20:1
Rela
tive c
hem
ical shift (p
pm
)
1-Hexene
Cis-2-hexene
0.1
0.2
0.3
0.4
0.5
0.6
Figure C8 Effect of mole ratio of silver ion and alkene on the chemical shift of vinylic protons.
The protons on the double bond of 1-hexene (■) and cis-2-hexene (●) were used.
Figure C9 Proton detected 1H-109Ag cross polarization 2D HETCOR solid state NMR spectra of
(A) [Ag+][CH3SO3-] (B) [Ag+(ACN)][NTf2
-] and (C) [Ag+(MIM)2][NTf2-].
151
Figure C10 Solid-state NMR spectra at 50kHz MAS of the [Ag+(MIM)2][NTf2
-] IL as
synthesized, heated at 150 ºC and at 200 ºC temperatures. 1D 1H spin echo spectra (A), 1D 13C
detected 1H-13C cross polarization (CP) spectra (B), 1D 19F spin echo spectra (C), and proton
detected 1H-109Ag cross polarization 2D HETCOR solid state NMR spectra.
Figure C11 Positive ion mode ESI-MS for the [Ag+(MIM)2][NTf2
-] ionic liquid before (A) and
after heating up to 200 ºC for 6 hours (B).
152
Table C1 Chemical structures and melting points of the reference IL ([BMIM+][NTf2-]), silver
salt ([Ag+][NTf2-]), and silver IL ([Ag+(MIM)(BIM)][NTf2
-]) based stationary phases examined
in this study.
Stationary phase Chemical structure Melting point
[BMIM+][NTf2-]
-4 °Ca
[Ag+][NTf2-]
248.8 °Ca
[Ag+(MIM)(BIM)][NTf2-]
30 °Ca
aThe melting point data was reported previously [1, 3].
Table C2 List of IL stationary phases prepared using [Ag+][NTf2-] and [BMIM+][NTf2
-] mixture.
Column
No. Stationary phase
Retention factor
of 1-hexene Resolution of
hexane and 1-hexene
1 [Ag+][NTf2-] + 4[BMIM+][NTf2
-]a 0.37 2.36
2 4[Ag+][NTf2-] + [BMIM+][NTf2
-]a 0.79 1.58
aIL mixtures are based on molar ratios of the two salts.
153
Table C3 Comparison of retention factors of alkane/alkene/alkyne probe molecules on 30 m
commercial columns and 10 m silver-based IL column.
No. Probe Molecules
Physical properties Retention factor
MW (g/mol) BP
(°C) HP-5msa Aluminab
[Ag+(MIM)(BIM)]
[NTf2-] +
[Ag+][NTf2-]a
1 Hexane 86.18 68.5-69.1 0.08 0.62 0.05
2 Trans-2-hexene 84.16 68-69 0.09 0.74 0.24
3 1-Hexene 84.16 63 0.08 0.85 0.70
4 Cis-2-hexene 84.16 68-70 0.10 0.91 0.65
5 1,5-Hexadiene 82.14 60 0.08 1.35 22.26
6 2,4-Hexadiene 82.14 82
0.10 2.06 0.90
0.13 2.14 1.26
0.14 2.28 1.47
7 3-Hexyne 82.14 81-82 0.13 2.44 2.50
8 2-Hexyne 82.14 84-85 0.14 2.59 1.96
9 1-Hexyne 82.14 71-72 0.10 3.20 -
a Measured isothermally at 100 °C, flow rate 1 mL min-1. b Measured isothermally at 160 °C, flow rate 10 mL min-1.
154
References
[1] N.R. Brooks, S. Schaltin, K. Van Hecke, L. Van Meervelt, J. Fransaer, K. Binnemans,
Heteroleptic silver-containing ionic liquids, Dalton Trans., 41 (2012) 6902-6905.
[2] S. Schaltin, N.R. Brooks, L. Stappers, K. Van Hecke, L. Van Meervelt, K. Binnemans, J.
Fransaer, High current density electrodeposition from silver complex ionic liquids, Phys. Chem.
Chem. Phys., 14 (2012) 1706-1715.
[3] P. Bonhôte, A.-P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Hydrophobic,
Highly Conductive Ambient-Temperature Molten Salts, Inorg. Chem., 35 (1996) 1168-1178.
[4] J.L. Anderson, J. Ding, T. Welton, D.W. Armstrong, Characterizing Ionic Liquids On the
Basis of Multiple Solvation Interactions, J. Am. Chem. Soc., 124 (2002) 14247-14254.
[5] J.L. Anderson, D.W. Armstrong, Immobilized ionic liquids as high-selectivity/high-
temperature/high-stability gas chromatography stationary phases, Anal. Chem., 77 (2005) 6453-
6462.
[6] R.K. Harris, E.D. Becker, S.M.C. De Menezes, R. Goodfellow, P. Granger, NMR
nomenclature. Nuclear spin properties and conventions for chemical shifts - (IUPAC
recommendations 2001), Pure Appl. Chem., 73 (2001) 1795-1818.
[7] Y. Ishii, R. Tycko, Sensitivity Enhancement in Solid State 15N NMR by Indirect Detection
with High-Speed Magic Angle Spinning, J. Magn. Reson., 142 (2000) 199-204.
[8] G.H. Penner, W. Li, A standard for silver CP/MAS experiments, Solid State Nucl. Magn.
Reson., 23 (2003) 168-173.
[9] O.B. Peersen, X.L. Wu, I. Kustanovich, S.O. Smith, Variable-Amplitude Cross-Polarization
MAS NMR, J. Magn. Reson., Ser. A, 104 (1993) 334-339.
155
[10] A.A. Bothner-By, C.N. Colin, H. Gunther, The proton magnetic resonance spectra of
olefins. II. Internal rotation in alkylethylenes, J. Am. Chem. Soc., 84 (1962) 2748-2751.
[11] S. Sakaki, Electronic-Structures of Organo-Transition-Metal Complexes .1. Silver(I)-Olefin
Complexes, Theor. Chim. Acta, 30 (1973) 159-167.
[12] J.P.C.M. Van Dongen, C.D.M. Beverwijk, Alkene- and arene-π-complex formation with
silver(I); A 13C NMR study, J. Organomet. Chem., 51 (1973) C36-C38.
[13] J. Solodar, J.P. Petrovich, Behavior of silver(I)-olefin complexes in organic media, Inorg.
Chem., 10 (1971) 395-397.
156
APPENDIX D. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 5
Instrumentation
Two-dimensional separations were performed on a GC×GC-FID prototype, assembled on an
Agilent 6890 GC-FID equipped with a split/splitless injector and a two-stage cryogenic loop
modulator. The first dimension was equipped with a Rtx-5MS capillary column (30 m, 0.25 mm
ID, 0.25 μm – 5% diphenyl / 95% dimethyl polysiloxane), with a second dimension column
comprised of the silver-based IL (1.2 m, 0.25 mm ID, 0.15 μm). Modulation was achieved by
alternating hot or cold pulses of N2 (g) to a delay loop (1.0 m untreated capillary column, 250
µm I.D.). An Asco three-way solenoid valve (Florham Park, NJ, USA) was used to toggle the
continuous N2 (g) stream. The N2 (g) was continuously and periodically directed to the heating
system or cooling system, which was cooled by liquid nitrogen. A solid-state relay and a low-
cost 8-bit Arduino Uno microcontroller board was employed to control the actuator of the
solenoid valve. The microcontroller board was monitored the GC remote start/stop to
synchronize the modulation events. Agilent ChemStation was used for data acquisition and
processing and GC Image 2.04 (Zoex Corporation, Houston, TX, USA) will be used for data
visualization. Microsoft Visual Basic 6.0 will be used to create a graphical interface to control
the modulator.
Ionic Liquid Synthesis
Silver-based ionic liquids. The silver-based IL was synthesized based on a previously reported
procedure from the literature 1–3. In the first step, 0.05 mol of silver oxide was dissolved in
acetonitrile with 0.1 mol bis[(trifluoromethyl)sulfonyl]amine and the mixture was stirred at room
temperature for 2 h. The solvent was removed in vacuo and the remaining solid was dried in a
vacuum oven at room temperature to obtain [Ag+(ACN)][NTf2-]. In the second step, the dried
157
[Ag+(ACN)][NTf2-] was mixed with 1-methylimidazole (MIM) and 1-butylimidazole (C4IM), 1-
decyl-2-methylimidazole (C10MIM) and MIM, and C10MIM and C4IM ligands in acetonitrile at a
molar ratio of 1:1:1 and stirred for 2 h. Acetonitrile was removed in vacuo to yield
[(MIM)(C4IM)Ag+][NTf2-], [(C10MIM)(MIM)Ag+][NTf2
-], and [(C10MIM)(C4IM)Ag+][NTf2-]
(Figure D1). IL mixtures were obtained using [Ag+(ACN)][NTf2-], MIM, and C4IM with molar
ratio of 2:1:1. All silver-based IL were dried overnight in a vacuum oven at room temperature
prior coating on the untreated fused silica capillaries.
Conventional ionic liquids. 1-butyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide
[C4IM+][NTf2
-], 1-octyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C8IM+][NTf2
-
], and 1-decyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide [C10IM+][NTf2
-] ILs
were prepared by reacting 1 molar equivalent of MIM with 1 molar equivalent of 1-
chlorobutane, 1-bromooctane, and 1-bromodecane, respectively, in 15 mL of isopropanol at 70
ºC for 24 h. The product was then dissolved in 10 mL of water and washed three times with 5
mL of dichloromethane and three times with 5 mL of ethyl acetate. Then, water was removed
and the remaining ILs containing the halide anion were dried under vacuum at 80 ºC for 48 h.
The halide anion was exchanged for the bis[(trifluoromethyl)sulfonyl]imide (NTf2-) anion by
reaction of 1 molar equivalent of [Li+][NTf2-] dissolved in water.
Preparation of IL-based GC Columns
The ILs were dried in a vacuum oven overnight under ambient temperature to remove all
residual water and organic solvent prior coating. Three meter segments of untreated fused silica
capillary (I.D. 250 µm) were coated by the static coating method at 40 °C 4. The capillary
columns possessing 0.28 µm and 0.15 µm film thickness were prepared using a 0.45 and 0.24%
(w/v) coating solution, respectively. These solutions were prepared by dissolving the IL in dry
158
dichloromethane. Capillaries coated with different amount of silver-based IL were obtained by
mixing the [(MIM)(C4IM)Ag+][NTf2-] with [C4IM
+][NTf2-] in proportions 1:1, 1:10, 1:20, 1:30,
1:40, and 1:50 (w/w). The coated columns were conditioned using a temperature program from
40 °C to 110 °C with a ramp of 1 °C min-1 and held isothermally at 110 °C for 1 h. Helium was
used as the carrier gas at a constant flow of 1 mL min-1.
Table D1 List of all probe molecules used to characterize the silver-based IL columns
Number Compound Chemical structure
ALDEHYDES
1 Propionaldehyde H
O
2 Butyraldehyde H
O
3 Pentanal H
O
4 Hexanal H
O
5 Heptanal H
O
ALDEHYDES
1 Propionaldehyde H
O
2 Butyraldehyde H
O
3 Pentanal H
O
159
Table D1 (continued)
Number Compound Chemical structure
4 Hexanal H
O
5 Heptanal H
O
6 Octanal H
O
7 Benzaldehyde H
O
KETONES
8 Acetone
O
9 2-Butanone
O
10 2-Pentanone
O
11 3-Pentanone O
12 2-Hexanone
O
13 Cyclohexanone
O
160
Table D1 (continued)
Number Compound Chemical structure
ESTERS
14 Ethyl acetate O
O
15 Methyl acetate O
O
16 Methyl butyrate O
O
17 Ethyl butyrate O
O
20 Isopropyl butyrate O
O
21 Methyl 4-pentenoate O
O
22 Methyl pentanoate O
O
23 Methyl 2,4-pentadienoate O
O
24 Methyl 3-pentenoate O
O
161
Table D1 (continued)
Number Compound Chemical structure
25 Methyl tiglate O
O
26 Ethyl pentanoate O
O
27 Isoamyl acetate O O
28 Propyl tiglate O
O
29 Ethyl hexanoate O
O
30 Propyl tiglate O
O
31 Isopropyl tiglate O
O
32 Ethyl heptanoate O
O
33 Heptyl acetate O O
162
Table D1 (continued)
Number Compound Chemical structure
ALKANES, ALKENES, ALKYNES, DIENES
34 n-Pentane
35 2,4-Hexadiene
36 3-Methyl-1,4-pentadiene
37 1,5-Hexadiene
38 1,3-Hexadiene
39 1-Hexene
40 Cis 2-hexene
41 3-Hexene
42 n-Hexane
43 2,3-Dimethyl-1,3-butadiene
44 Benzene
45 2-Hexyne
46 1-Hexyne
47 3-Hexyne
48 Toluene
49 n-Octane
50 m-Xylene
163
Table D1 (continued)
Number Compound Chemical structure
51 o-Xylene
52 p-Xylene
53 1,8-Nonadiene
54 1-Nonene
55 n-Nonane
TERPENES
56 myrcene
57 α-Terpinene
58 γ-Terpinene
59 Terpinolene
164
Table D2 Separation performance of selected pairs of analytes by GC × GC-F
ID using a silver-based IL column or a SUPELCOWAX10 column as second dimension column.
Compounds 2D Resolution
Silver-based IL Columna SUPELCOWAX10
2 and 9 2.19 0.87
3 and 16 3.77 1.58
4 and 12 6.30 4.86
7 and 28 10.16 1.63
21 and 22 2.12 0.96
34 and 36 4.09 0.69
37 and 40 3.58 1.02
46 and 47 16.52 1.75
53 and 54 2.56 1.05
54 and 55 2.60 0.81
aSilver-based IL column was prepared using [(C10MIM)(MIM)Ag+][NTf2-] dissolved
in [C10MIM+][NTf2-] at ratio of 1:30 (w/w) for compounds 2-22 and 1:20 (w/w) for
compounds 34-55.
Note: For compound identification: butyraldehyde (2), pentanal (3), hexanal (4),
benzaldehyde (7), 2-butanone (9), 2-hexanone (12), methyl butyrate (16), Methyl 4-
pentenoate (21), Methyl pentanoate (22), propyl tiglate (28), n-pentane (34), 3-
methyl-1,4-pentadiene (36), 1,5-hexadiene (37), cis 2-hexene (40), 1-hexyne (46), 3-
hexyne (47), 1,8-nonadiene (53), 1-nonene (54), and n-nonane (55).
165
Figure D1 Chemical structures of the silver-based ILs and conventional ILs examined in this
study.
Figure D2 1H NMR (500 MHz, DMSO-d6) spectra of [C4MIM+][NTf2
-]: δ 9.10 (s, 1H), 7.76 (t, J
= 1.8 Hz, 1H), 7.69 (t, J = 1.8 Hz, 1H), 4.15 (t, J = 7.2 Hz, 2H), 3.84 (d, J = 1.8 Hz, 3H), 1.77 (q,
J = 7.5 Hz, 2H), 1.30 – 1.24 (m, 2H), 0.90 (t, J = 7.4 Hz, 3H).
166
Figure D3 1H NMR (500 MHz, DMSO-d6) spectra of [C8MIM+][NTf2
-]: δ 9.09 (s, 1H), 7.76 (t, J
= 1.8 Hz, 1H), 7.69 (t, J = 1.8 Hz, 1H), 4.14 (t, J = 7.2 Hz, 2H), 3.84 (s, 3H), 1.78 (q, J = 7.4 Hz,
2H), 1.25 (s, 10H), 0.89 – 0.81 (m, 3H).
Figure D4 1H NMR (500 MHz, DMSO-d6) spectra of [C10MIM+][NTf2
-]: δ 9.09 (s, 1H), 7.76 (t,
J = 1.9 Hz, 1H), 7.69 (d, J = 2.0 Hz, 1H), 4.14 (t, J = 7.2 Hz, 2H), 3.84 (s, 3H), 1.77 (q, J = 7.4
Hz, 2H), 1.24 (s, 14H), 0.85 (t, J = 6.7 Hz, 3H).
167
Figure D5 Chemical structures of polyunsaturated fatty acids (PUFAs).
168
Figure D6 GC × GC-FID chromatograms of an analyte mixture containing (1) n-hexane, (2) 2-
hexene, (3) methyl 4-pentenoate, (4) methyl pentanoate, (5) methyl 3-pentenoate, and (6) methyl
2,4-pentadienoate obtained using the column sets: Rtx-5MS (30 m, 0.25 mm ID, 0.25 μm) ×
[(C4IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] (30 m, 0.25 mm ID, 0.25 μm). The silver based IL
stationary phases used in the second dimension separation were prepared using the following
proportions of [(C4IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] a) 1:0, b) 1:10, c) 1:20, d)1:30, e)
1:40, f) 1:50, and g) 0:1 (w/w).
169
Figure D7 Chromatographic resolution of n-hexane (1), 2-hexene (2), methyl 3-pentenoate (5),
and methyl 2,4-pentadienoate (6), in the second dimension, using a) capillary columns coated
with mixtures of silver-based IL ([(C4IM)(MIM)Ag+][NTf2-]) and different conventional ILs at a
ratio of 1:30 (w/w) and b) columns coated with mixtures of silver-based ILs comprising different
ligands and [C10MIM+][NTf2-] IL at a ratio of 1:30 (w/w).
Figure D8 Evaluation of a) film thickness and b) capillary column length for the separation of
methyl 3-pentenoate and methyl 2,4-pentadienoate. The second dimension column was the
[(C10IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 (w/w) silver-based IL column.
170
Figure D9 a) Column bleed diagram illustrating the thermal stability of the column consisting of
the [(C10IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 (w/w) silver-based IL stationary phase.
The temperature was increased from 40 ºC to 250 ºC at 10 ºC min-1; b) Evaluation of the
chromatographic resolution between methyl 2,4-pentadienoate and methyl 3-pentenoate, after
conditioning the column for 1 h at different temperatures.
Figure D10 GC × GC-FID chromatogram showing the separation of esters, aldehydes, and
ketones obtained using the following column sets: a) Rtx-5MS (30 m, 0.25 mm ID, 0.25 μm) ×
[(C10IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:30 w/w (1.2 m, 0.25 mm ID, 0.15 μm) and b)
Rtx-5MS (30 m, 0.25 mm ID, 0.25 μm) × [C10MIM+][NTf2-] (1.2 m, 0.2 mm ID, 0.2 μm).
Temperature program: 40 °C to 44 °C at 2 °C/min; then ramped to 100 °C at 5 °C/min and held
for 3 min. Modulation time: 10 s. For peak identification, refer to Table D1.
171
Figure D11 GC × GC-FID chromatograms showing the separation of alkanes, alkenes, alkynes,
dienes, cycloalkanes, and terpenes obtained using the following column sets: a) Rtx-5MS (30 m,
0.25 mm ID, 0.25 μm) × [(C10IM)(MIM)Ag+][NTf2-]/[C10MIM+][NTf2
-] 1:20 (w/w) (0.9 m, 0.25
mm ID, 0.15 μm) and b) Rtx-5MS (30 m, 0.25 mm ID, 0.25 μm) × [C10MIM+][NTf2-] (0.9 m, 0.2
mm ID, 0.2 μm). Temperature program: 40 °C to 44 °C at 2 °C/min; then ramped to 100 °C at 5
°C/min and held for 3 min. Modulation time: 10 s. For peak identification, refer to Table D1.
Figure D12 a) GC-FID chromatogram and b) GC × GC-FID chromatogram of a polyunsaturated
fatty acid sample obtained using a) SUPELCOWAX10 column (30 m, 0.25 mm ID, 0.25 μm)
and b) the SUPELCOWAX10 (30 m, 0.25 mm ID, 0.25 μm) × [(C10MIM)(MIM)Ag+][NTf2-
]/[C10MIM+][NTf2-] 1:30 (w/w) (0.4 m, 0.25 mm ID, 0.15 μm) column set. The peaks labelled
with (*) refer to interferent compounds present within the purchased sample.
172
APPENDIX E. SUPPLEMENTAL INFORMATION ACCOMPANYING CHAPTER 6
I. General Procedures and Materials
Commercial reagents were obtained from Acros Organics, Nu Check Prep and Aldrich Chemical
and used without further purification. 1H and 13C NMR were recorded on a 500 MHz JEOL
spectrometer using CDCl3 as a solvent at room temperature. All chemical shifts for 1H and 13C
NMR were reported downfield using tetramethylsilane (TMS, at = 0.00 ppm). Solvents such as
dichloromethane, hexanes, diethyl ether, and chloroform were used as received without further
purification. The photochemical apparatus employed was an ACE Glass Photochemical System
with an immersion lamp (#7825-34), which yielded a total radiated energy of 175.8 watts and
utilized #15 quartz photochemical tubes with a 0.75” ID x 7.5” length.
Compound 6: 1-Methyl-3-((9/10-thiaethyl)octadecyl)-imidazolium bistriflimide
To a quartz photochemical tube were added ((Z)-1-methyl-3-(9-octadecenyl)-imidazolium
bistriflimide (0.830 g, 0.00135 mol, 1.0 equiv.), ethanethiol (0.960 g, 0.0155 mol, 11.5 equiv.),
2,2-dimethoxy-2-phenylacetophenone (0.310 g, 0.00121 mol, 0.90 equiv.), dichloromethane (15
mL) and methanol (15 mL). The contents were shaken in the sealed tube at ambient temperature
until homogeneous. The contents were irradiated without stirring at ambient temperature for 10
hours. The solvents were removed from the crude product via evaporation. The residue was then
washed with hexanes (6 × 30 mL). 1H and 13C NMR showed the disappearance of the alkene and
photo initiator. Removal of residual solvent in vacuo yielded the product (0.77 g, 93% yield) as a
viscous, yellow liquid.
173
1H NMR (CDCl3, 500 MHz): δ 8.77 (s, 1H), 7.28 (d, 1H), 7.24 (d, 1H), 4.14 (t, 2H), 3.92 (s, 3H),
2.50-2.58 (m, 1H), 2.44-2.50 (q, 2H), 1.78-1.88 (m, 2H), 1.06-1.54 (series of m, 31H), 0.85 (bt,
3H) ppm.
13C NMR (CDCl3, 125 MHz): δ 136.20, 123.58, 122.06, 121.03, 118.47, 115.92, 50.22, 45.54,
36.38, 34.85, 34.78, 34.76, 31.86, 30.04, 29.65, 29.57, 29.54, 29.46, 29.38, 29.29, 29.23, 28.82,
26.78, 26.74, 26.69, 26.08, 24.20, 22.65, 14.95, and 14.09 ppm.
Compound 7: 1-Methyl-3-((9/10-thiadecyl)octadecyl)-imidazolium bistriflimide
To a quartz photochemical tube were added ((Z)-1-methyl-3-(9-octadecenyl)-imidazolium
bistriflimide (0.790 g, 0.00129 mol, 1.0 equiv.), decanethiol (2.680 g, 0.0154 mol, 11.9 equiv.),
2,2-dimethoxy-2-phenylacetophenone (0.350 g, 0.00136 mol, 1.1 equiv.), and methanol (30 mL).
The contents were shaken in the sealed tube at ambient temperature until homogeneous. The
contents were irradiated without stirring at ambient temperature for 10 hours. The solvents were
removed from the crude product via evaporation. The residue was soluble in hexanes so flash
column chromatography (hexanes followed by 1:1 hexanes:diethyl ether) was employed to purify
the product. 1H and 13C NMR showed the disappearance of the alkene and photo initiator. Removal
of residual solvents in vacuo yielded the product (0.66 g, 65% yield) as a viscous, yellow liquid.
1H NMR (CDCl3, 500 MHz): δ 8.69 (s, 1H), 7.29 (d, 1H), 7.26 (d, 1H), 4.13 (t, 2H), 3.90 (s, 3H),
2.46-2.54 (m, 1H), 2.40-2.46 (q, 2H), 1.76-1.86 (m, 2H), 1.00-1.55 (series of m, 45H), and 0.84
(tt, 6H) ppm.
13C NMR (CDCl3, 125 MHz): δ 136.86, 136.12, 135.78, 135.03, 124.09, 124.03, 123.52, 123.23,
122.64, 121.57, 121.53, 120.97, 118.41, 115.86, 50.13, 49.69, 49.07, 46.75, 45.98, 45.69, 36.30,
174
31.84, 30.81, 30.54, 30.36, 30.27, 30.01, 29.89, 29.54, 29.26, 29.00, 26.76, 26.06, 22.62, 14.98,
14.05, and 13.14 ppm.
Compound 8: 1-Methyl-3-((9/10-thiaethyl)(12/13-thiaethyl)octadecyl)-imidazolium bistriflimide
To a quartz photochemical tube were added ((Z, Z)-1-methyl-3-(9, 12-octadecenyl)-imidazolium
bistriflimide (0.770 g, 0.00126 mol, 1.0 equiv.), ethanethiol (0.990 g, 0.0159 mol, 12.6 equiv.),
2,2-dimethoxy-2-phenylacetophenone (0.310 g, 0.00121 mol, 0.96 equiv.), and methanol (30 mL).
The contents were shaken in the sealed tube at ambient temperature until homogeneous. The
contents were irradiated without stirring at ambient temperature for 10 hours. The solvents were
removed from the crude product via evaporation. The residue was then washed with hexanes (6 ×
30 mL). 1H and 13C NMR showed the disappearance of the alkene and photo initiator. Removal of
residual solvent in vacuo yielded the product (0.84 g, 91% yield) as a viscous, yellow liquid.
1H NMR (CDCl3, 500 MHz): δ 8.81 (bs, 1H), 7.27 (bd, 1H), 7.24 (bd, 1H), 4.15 (bt, 4H), 3.94 (s,
3H), 3.91 (bt, 2H), 2.44-2.58 (bq, 2H), 1.05-1.90 (series of m, 28H), and 0.85 (t, 9H) ppm.
13C NMR (CDCl3, 125 MHz): δ 136.36, 128.10, 123.58, 123.49, 121.98, 121.03, 118.47, 115.92,
50.25, 50.16, 45.55, 45.46, 36.42, 36.30, 34.91, 34.80, 31.78, 30.93, 30.04, 29.51, 29.37, 29.27,
29.22, 28.81, 26.75, 26.08, 24.22, 24.07, 22.61, 14.97, 14.92, and 14.06 ppm.
Compound 9: 1-Methyl-3-((9/10-thiadecyl)(12/13-thiadecyl)octadecyl)-imidazolium bistriflimide
To a quartz photochemical tube were added ((Z, Z)-1-methyl-3-(9, 12-octadecenyl)-imidazolium
bistriflimide (0.810 g, 0.00132 mol, 1.0 equiv.), decanethiol (2.300 g, 0.0132 mol, 10.0 equiv.),
2,2-dimethoxy-2-phenylacetophenone (0.460 g, 0.00179 mol, 1.35 equiv.), and methanol (25 mL).
175
The contents were shaken in the sealed tube at ambient temperature until homogeneous. The
contents were irradiated without stirring at ambient temperature for 10 hours. The solvents were
removed from the crude product via evaporation. The residue was then washed with hexanes (6 ×
30 mL). 1H and 13C NMR showed the disappearance of the alkene and photo initiator. Removal of
residual solvent in vacuo yielded the product (0.68 g, 54% yield) as a viscous, yellow liquid.
1H NMR (CDCl3, 500 MHz): δ 8.81 (bs, 1H), 7.27 (bd, 1H), 7.24 (bd, 1H), 4.15 (bt, 4H), 3.94 (s,
3H), 3.91 (bt, 2H), 2.44-2.58 (bq, 2H), 1.78-1.90 (bm, 2H), 1.00-1.75 (series of m, 59H), and
0.85 (t, 9H) ppm.
13C NMR (CDCl3, 125 MHz): δ 136.65, 123.45, 123.55, 123.33, 121.97, 121.04, 118.48, 115.93,
50.33, 50.11, 45.90, 45.80, 36.53, 36.36, 31.89, 30.07, 29.96, 29.92, 29.56, 29.32, 29.09, 28.84,
26.12, 26.03, 22.67, and 14.10 ppm.
Synthesis of IL R1 (1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and IL
R2 (1-hexadecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide)
ILs were prepared using a previously reported method [1]. Briefly, 0.05 mol of 1-methylimidazole
and 0.075 mol of 1-bromodecane were mixed in 15 mL of isopropanol at 70 ºC for 24 h. The
product was then dissolved in 10 mL of water and washed with 3 mL of ethylacetate for three
times. The water layer containing the IL was recovered and dried under vacuum at 80 ºC for 24 h
to yield [MDIM][Br]. The halide counter anion was then exchanged to [NTf2] by metathesis
reaction using one equivalent of lithium bis(trifluoromethylsulfonyl)imide. The IL R2
[MHDIM][NTf2] was prepared in the same method.
176
Figure E1 1H NMR (500 MHz, Chloroform-d) of IL R1 (1-decyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide) δ 9.05 – 8.72 (m, 1H), 7.31 (q, J = 1.8 Hz, 1H), 7.29 (d, J =
1.9 Hz, 1H), 4.21 (t, J = 7.5 Hz, 2H), 4.00 (s, 3H), 1.97 – 1.79 (m, 2H), 1.41 – 1.24 (m, 14H),
0.91 (t, J = 6.9 Hz, 3H).
Figure E2 1H NMR (500 MHz, Chloroform-d) of IL R2 (1-hexadecyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide) δ 8.84 (s, 1H), 7.32 (d, J = 1.8 Hz, 1H), 7.29 (d, J = 2.0 Hz,
1H), 4.20 (t, J = 7.5 Hz, 2H), 3.99 (s, 3H), 1.90 (p, J = 6.4, 5.4 Hz, 2H), 1.29 (s, 26H), 0.91 (t, J
= 6.8 Hz, 3H
177
1
2
3
46
5
Rtx-5 × SUPELCOWAX 10
1tR (min)
6 18
2t R
(s)
0
7
8 10 12 14 16
Figure E3 GC × GC contour plot showing the selected pairs of analytes (1 and 2, 3 and 4, 5 and
6) for determining the resolving power of the stationary phases in the second dimension.
Feb 14, 2016
Feb 14, 2016200 C
110 C
IL 1C16(1[9])
1tR (min)
6 18
2t R
(s)
0
7
8 10 12 14 16
1tR (min)
6 18
2t R
(s)
0
7
8 10 12 14 16
(A)
(B)
Figure E4 Thermal stability test of IL 1 column. GC × GC contour plots of kerosene using Rtx-5
× IL 1 column set after the column was conditioned up to (A) 110 °C and (B) 200 °C.
178
200 C
110 C
IL 8
1tR (min)
6 18
2t R
(s)
0
7
8 10 12 14 16
2t R
(s)
0
7
(A)
(B)
C18(2C2S[9/10, 12/13])
4
1tR (min)
6 188 10 12 14 164
Figure E5 GC × GC separation of kerosene using Rtx-5 × IL 8 column set after the column was
conditioned up to (A) 110 °C and (B) 200 °C.
200 C
110 C
IL 10
1tR (min)
6 18
2t R
(s)
0
7
8 10 12 14 16
2t R
(s)
0
7
(A)
(B)
4
1tR (min)
6 188 10 12 14 164
C16(S[4])
Figure E6 Two-dimensional chromatograms using Rtx-5 × IL 10 column set after the column
was conditioned up to (A) 110 °C and (B) 200 °C.
179
(A) 110 C
IL 11
(B) 200 C
(C) 225 C (D) 250 C
(E) 275 C (F) 300 C
(G) 325 C
C18(Cyclo[9])
Figure E7 GC × GC contour plots using Rtx-5 × IL 11 column set after the column was
conditioned up to (A) 110 °C, (B) 200 °C, (C) 225 °C, (D) 250 °C, (E) 275 °C, (F) 300 °C, and
(G) 325 °C.
180
IL R1
C16(0)
(H) 350 C
(A) 110 C (B) 200 C
(C) 225 C (D) 250 C
(E) 275 C (F) 300 C
(G) 325 C
Figure E8 Two-dimensional chromatograms of kerosene separation using Rtx-5 × IL 11 column
set after the column was conditioned up to (A) 110 °C, (B) 200 °C, (C) 225 °C, (D) 250 °C, (E)
275 °C, (F) 300 °C, (G) 325 °C, and (H) 350 °C.
181
Table E1 The 2D resolutions of selected analytes on two dimensional chromatograms using
different column sets.
Column sets 2D resolution of selected analytesa
1 & 2 3 & 4 5 & 6
Rtx-5 × R1 1.38 1.98 3.14
Rtx-5 × R2 1.18 - -
Rtx-5 × 1 2.81 2.49 7.32
Rtx-5 × 5 1.62 2.04 2.89
Rtx-5 × 10 1.31 1.49 3.69
Rtx-5 × 11 1.26 1.67 3.30
Rtx-5 × SUPELCOWAX 10 2.20 2.05 7.23
a Selected pairs of analytes are shown in representative GC × GC contour plot in Figure E3.
a The 2D resolution was calculated according to the previously published paper [2]. The
calculation equation is 𝑅𝑠2𝐷 = √ 𝑅𝑠1 2 + 𝑅𝑠2 2, where 1Rs and 2Rs are the resolutions between
the two solutes along the first and the second dimension, respectively.
References
[1] J.L. Anderson, R. Ding, A. Ellern, D.W. Armstrong, Structure and properties of high stability
geminal dicationic ionic liquids, J. Am. Chem. Soc. 127 (2005) 593-604.
[2] J.C. Giddings, Concepts and comparisons in multidimensional separation, J. High Resolut.
Chromatogr. 10 (1987) 319-323.