Development of ionic liquid-based stationary phases for ...

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

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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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17

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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



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




n-Hexanoic acid 2.63 3.69 2.03

HP-FFAP Acetic acid 1.11 1.57 7.64




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.


3.3.1 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)


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.


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


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


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



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.


4.3.1 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.


4.4.1 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-]


0.28 1500 1.37 11.27

6

[Ag+(MIM)(BIM)][NTf2-] +

[Ag+][NTf2-]


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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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.


5.3.1 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.


5.4.1 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) ×


-] 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 ×


-] 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



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.

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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.

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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


chain

C18(C10S[9/10])

0.28 3400 1.15 1.10 4.43


chain

C18(2C2S[9/10,

12/13])

0.28 4200 1.32 1.02 3.84


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.

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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].


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


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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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


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.



[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


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


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.


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


(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


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


1 21.5


3

(A)


(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



[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)


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



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



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



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



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


-]: 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).


-]: 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


-] 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) ×


-] 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.),




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.),


175



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.

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