development of sorptive extraction techniques based on...
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Organic Chemistry
Separation Science Group (SSG)
Development of Sorptive Extraction Techniques
Based on Immobilized Ionic Liquids for Selective
Chromatographic Analysis
Thesis submitted to obtain
the degree of Master of Science in Chemistry by
Kevin ROELEVELD
Academic Year 2011-2012
Promotor: prof. dr. Frederic Lynen
Daily Supervisor: Mike De Vrieze
Acknowledgements I would like to express my special thanks to prof. dr. F. Lynen for giving me this
opportunity to join the world of separation sciences under his guidance and expertise. I also
would like to thank prof. dr. em P. Sandra for bringing me the love for chromatography.
Without the help of my daily supervisor, Mike De Vrieze, this thesis wouldn’t have come to
an end. You were there when I needed you.
My appreciation also goes to Prof. Dr. F. David for his help and experience with sample
preparation and stir bars in particularly. Karine Jacq I would like to thank for her help with
the usage of my stir bars and for providing me some data. Thanks to the RIC, I could test my
stir bars using their state-of-the-art equipment. Also the other people from RIC, I would like
to thank you all for the warm welcome they gave me when I was with them.
Bernhard De Meyer, Stéphanie Deman and Frederique Backaert have earned a place in
these acknowledgements for helping me with the TGA, DSC and NMR analyses. Guys from
the polymer department were so kind for helping me with some polymer related problems,
for this I would like to thank Jarne, Jonas and Frank for their support during this year!
All other people within PARC, including my fellow students who were doing their
thesis, I would like to thank you for the nice atmosphere, the good music and the nice
football games! Thanks for all this to Lesley, Joachim, Thomas, Sander, Maarten, Piotr,
Seppe… In this respect, a special word to Pieter, for solving all kind of GC related problems.
Birgit, Jarne, Robin, thanks for all the lunches, while playing cards and for eating some
nice ‘kersekreemkoeken’!
Naturally I would like to thank my parents for supporting me with this crazy thing
called chemistry!
Last but not least I would like to thank my girlfriend Heleen for her support although
she was under the same stress as I was! I love you!
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Table of contents 1. General introduction and goals....................................................................................... - 1 -
2. Sorptive sample preparation methods ........................................................................... - 4 -
Principles of sorptive extraction .............................................................................. - 5 - 2.1
Solid phase micro extraction ................................................................................... - 6 - 2.2
Stir bar sorptive extraction ...................................................................................... - 8 - 2.3
Comparison between SPME and SBSE .................................................................. - 11 - 2.4
Characteristics of coating materials suitable for sorptive extraction ................... - 12 - 2.5
2.5.1 Polydimethylsiloxane ..................................................................................... - 13 -
2.5.2 Polyacrylates .................................................................................................. - 14 -
2.5.3 Polyurethanes................................................................................................. - 15 -
2.5.4 Polymer mixtures ........................................................................................... - 15 -
2.5.5 Monoliths ....................................................................................................... - 16 -
Complimentary techniques for the extraction of polar solutes ............................ - 17 - 2.6
2.6.1 Derivatisation ................................................................................................. - 17 -
2.6.2 Salting-out effect and sequential SBSE .......................................................... - 18 -
2.6.3 Dual-phase twisters ........................................................................................ - 19 -
3. Ionic liquids as new material for sorptive extraction.................................................... - 20 -
Ionic liquids: definition & properties ..................................................................... - 20 - 3.1
Ionic liquids and sorptive extraction ..................................................................... - 22 - 3.2
Polymerized ionic liquids ....................................................................................... - 23 - 3.3
3.3.1 Ionic liquid moiety in the side chain .............................................................. - 23 -
3.3.2 Applications of polymerized ionic liquids with IL moiety in side chain ......... - 25 -
3.3.3 Ionic liquid moiety in the main chain ............................................................. - 26 -
Thermal properties of ILs and PILs ........................................................................ - 27 - 3.4
4. Rationale ....................................................................................................................... - 29 -
5. Experimental ................................................................................................................. - 30 -
Polymer synthesis .................................................................................................. - 30 - 5.1
5.1.1 Chemicals ....................................................................................................... - 30 -
5.1.2 Synthesis of 1-(3-chloropropyl)imidazole ...................................................... - 30 -
5.1.3 Synthesis of polymer ...................................................................................... - 31 -
5.1.4 Ion Exchange .................................................................................................. - 31 -
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Characterization .................................................................................................... - 32 - 5.2
5.2.1 Thermogravimetric analysis ........................................................................... - 32 -
5.2.2 Differential scanning calorimetry ................................................................... - 32 -
Coating stir bars ..................................................................................................... - 33 - 5.3
5.3.1 Materials ......................................................................................................... - 33 -
5.3.2 Coating procedure .......................................................................................... - 33 -
Sample preparation ............................................................................................... - 34 - 5.4
Chromatographic instrumentation ........................................................................ - 35 - 5.5
6. Results and discussion ................................................................................................... - 37 -
Synthesis ................................................................................................................ - 37 - 6.1
Characterization of the polymers .......................................................................... - 39 - 6.2
6.2.1 Thermogravimetric analysis ........................................................................... - 39 -
6.2.2 Differential scanning calorimetry ................................................................... - 40 -
Coating ................................................................................................................... - 42 - 6.3
6.3.1 Glass plate ...................................................................................................... - 42 -
6.3.2 Stir bars ........................................................................................................... - 44 -
Conditioning........................................................................................................... - 45 - 6.4
6.4.1 Poly-NTf2 coated stir bar ................................................................................ - 45 -
6.4.2 Poly-PF6 coated stir bars ................................................................................ - 46 -
6.4.3 PDMS/poly-NTf2 and VPDMS/poly-NTf2 coated stir bars. ............................. - 47 -
6.4.4 PEG/poly-NTf2 coated stir bars....................................................................... - 48 -
6.4.5 Selecting the best conditioning conditions .................................................... - 49 -
Extraction of coffee ............................................................................................... - 50 - 6.5
Analysis of whisky .................................................................................................. - 54 - 6.6
Analysis of bath cream .......................................................................................... - 55 - 6.7
Pesticide analysis ................................................................................................... - 56 - 6.8
7. Conclusion and future perspective ............................................................................... - 58 -
8. Appendix A: Methods .................................................................................................... - 60 -
9. Appendix B: Additional representative chromatograms .............................................. - 62 -
10. Appendix C: Molecules and their log Ko/w value ........................................................... - 63 -
11. List of abbreviations ...................................................................................................... - 68 -
12. Bibliography .................................................................................................................. - 70 -
- 1 -
1. General introduction and goals
Over the last decades, analytical separation methods have become increasingly faster
and sensitive through improved system and column design by clever exploitation of the
fundamental principles of chromatography and other separation techniques. As prior
cleanup allows to selectively extract and pre-concentrate solutes for easier achievement of
these goals accordingly much effort has been placed in the development of improved
sample preparation techniques. However, because modern chromatographic techniques
today often allow analysis within minutes which is rarely the case in sample preparation, the
latter has increasingly become the time limiting step. Therefore a growing need for
improved, more generic approaches for these issues has to be addressed. In this way many
approaches have been developed to pre-treat samples to improve the robustness and
sensitivity of the overall methodologies. In this work emphasis is set on the development of
improved sample preparation strategies for gas chromatography (GC).
Sample preparation for GC can essentially be divided in two essentially different, but
equally important parts: derivatisation and selective extraction. With derivatisation,
thermally labile compounds are transformed into their more stable analogues in order to
obtain more accurate results for these compounds. Derivatisation can, for example, be done
using trimethylsilylimidazole in order to silylate active hydrogens on hydroxyl (OH), thiol (SH)
and amine (NH) groups.
Before analysing a sample, some extraction is also required in order to make the
sample more suitable for analysis or to selectively remove the desired analytes from their
matrix. Several approaches have been developed for extraction, which can be divided into
two types: the ‘wet’ approach and the ‘dry’ approach.
Liquid-liquid extraction (LLE) and solid phase extraction (SPE) are the two most applied
‘wet’ approaches. LLE is a standard procedure used in organic chemistry, where two solvents
are brought into contact. In this way, molecules will distribute themselves according to their
affinity for both solvents. The solvent containing the desired solutes is then concentrated
and subsequently injected into a GC system. With solid phase extraction (SPE), which uses a
small column packed with a selective stationary phase, complex samples like blood and urine
are cleaned by removing the matrix from analytes. This can be done selectively by choosing
- 2 -
the appropriate stationary phase and mobile phase. The retained molecules are then eluted
by a selected solvent, which is subsequently concentrated and injected into the GC system.
Solid phase microextraction (SPME) and stir bar sorptive extraction (SBSE) do not
require any solvent to achieve extraction and are therefore called ‘dry sampling’. Here,
extraction is done by means of an absorptive polymer, where solutes ‘dissolve’ in. With
SPME, a fiber, coated with an extractive polymeric phase, is brought into contact with a
sample or its headspace whereby partitioning between the polymeric phase and the sample
occurs. With SBSE, a similar approach to SPME, a stirring bar is coated with an extractive
polymeric phase. As the amount of polymeric phase is thereby about 100 times larger than
with SPME, larger quantities of the solutes can be extracted from the solution in comparison
to SPME. After extraction, desorption from a SPME fiber or a stir bar is obtained by heating
such that the analytes become volatile and are removed in this way from the extractive
phase. This vapour phase is then directed into the analytical instrument. Because of the
relatively high temperatures needed for desorption of solutes which can be analysed by GC,
the polymeric phase needs to withstand temperatures up to about 300°C to be of any
practical use.
Therefor various polymeric phases have been developed over the last decade.
Polydimethylsiloxane (PDMS) is the most used extractive polymer. This polymer, with
excellent thermal stability, is well understood (as it has been used for decades in GC) and is
very suitable for the extraction of relatively hydrophobic volatile species from many types of
matrices. A drawback of PDMS is that it is rather nonpolar, which makes the extraction of
polar analytes difficult. Polar coatings for both SPME and SBSE are available, like
polyacrylates, polyurethanes or polyethyleneglycol, but none of them have the desired
thermal stability.
In an attempt to get thermally stable extractive phases that are polar as well, ionic
liquids (IL) are increasingly used. These are molten organic salts that generally melt below
100°C and have virtually no vapour pressure. Ionic liquids are used in all kinds of
applications, going from green solvents 1, solvents in catalysis 2, to usage in electrochemistry
3 and analytical chemistry 4. In chromatography 5, ionic liquids are already used as stationary
phase in GC, as extraction solvent, as modifier in liquid chromatography (LC) or as extractive
phase in SPME. A drawback of ionic liquids as extractive phase in SPME is that at elevated
- 3 -
temperatures (during desorption), the ionic liquid can drip of the fiber thus decreasing the
extractive phase volume. This makes it difficult to get reproducible results.
For this reason polymerized ionic liquids (PIL), which depict the same characteristics as
above, but with higher viscosity have been introduced. In this way different types have been
developed and have already been used as coating in SPME. In this work, different PILs are
synthesized with the intention of developing extractive phases which are able to extract
polar compounds and to resist the high temperatures needed for desorption. These
polymers are then coated on stir bars, a support which allows to increase the amount of
extractive phase. These newly coated stir bars are then evaluated for their expected
improved sensitivity which they are expected to offer in this way. The stir bars are tested on
several samples, including coffee and pesticides.
- 4 -
2. Sorptive sample preparation methods
Sampling and sample preparation techniques are today an integral and often an
essential part of an analytical procedure. The general purpose is thereby to transfer the
molecules of interest from their matrix into a form suitable for introduction into an
analytical instrument. This can include extraction, purification (clean-up, fractionation),
preconcentration and/or derivatisation. The relevance of the development of better
performing sample preparation procedures has gradually been increasing due to the
enhancing speed of the instrumental analysis. As a consequence, the sample preparation has
often become the time limiting factor and therefore necessitates additional research to
improve the overall performance of analytical strategies.
When involved with gas chromatography (GC), derivatisation is typically used to
convert polar molecules to less polar analogues, allowing easier extraction from their matrix
and to reduce their boiling point to values suitable for analysis with this technique. Many
approaches have been developed for the extraction of solutes from solid, liquid and gaseous
samples but more recently the development of miniaturised extraction techniques has
grown significantly. The most important ones nowadays are solid phase extraction (SPE),
micro liquid liquid extraction (µLLE), solid phase microextraction (SPME) and stir bar sorptive
extraction (SBSE). With SPE, a complex sample can be cleaned and desired analytes can be
separated by means of a small column packed with a certain stationary phase and a mobile
phase. In the framework of green chemistry, the reduction of solvent use or even solventless
techniques have emerged, contrary to SPE and µLLE. SPME and SBSE can be seen as
solventless sample preparation techniques. No solvent is thereby required in order to
extract the molecules and subsequently analysis by GC, with minimal background
contamination.
In the first part of this chapter, the principles of sorptive extraction, together with the
main experimental methods such as SPME and SBSE will be discussed. Subsequently, an
overview is provided of the ways which have thus been developed to address the often
problematic extraction and analysis of polar analytes by GC.
- 5 -
Principles of sorptive extraction 2.1
All extraction techniques require the use of an extractive phase (solid, liquid, liquid
like, gas), which is not miscible with the phase to be extracted. In sorptive extraction, solutes
can be extracted from a liquid or gaseous phase via an ab- or adsorption process. The latter
is thereby not preferable because irreversible adsorption is possible, isotherms are not
always linear or the material can depict catalytic activity 6. Materials in which molecules are
retained via a partitioning mechanism are therefore much to be preferred. This mechanism
occurs via weaker interactions, as the molecules are basically dissolved in a polymer which
depicts liquid like behaviour. This allows easier and complete subsequent recuperation of
the molecules out of the absorbing material. Because desorption from an absorbent involves
overcoming weaker forces, lower temperatures can for example be used and less
degradation of thermolabile compounds is therefore occurring.
Materials in which this partitioning absorption mechanism is applied are mostly
synthetic polymers. These polymers exhibit a very low glass transition point (Tg), meaning
that above this temperature they are present in a gum-like or liquid-like state (Figure 1). The
material then shows similar behaviour to organic solvents, and solutes will dissolve in the
polymer material. At higher temperatures, the polymer will become liquid like, but with very
high viscosity. This point is called the flow temperature (Tf). This should not be confused with
the melting temperature (Tm) of a semi-crystalline polymer. At the melting point, the
polymer loses its crystallinity.
As polydimethylsiloxane (PDMS) is the most used coating material, the distribution
coefficient between PDMS and water (log Kpdms/w) is an important value. Studies have shown
that this value correlates very well over a specific polarity range with the octanol-water
Figure 1: Different transition states of an amorphous polymer expressed as modulus of elasticity in function of temperature
- 6 -
partition coefficient log Ko/w, especially for low-molecular weight analytes 7. For high-
molecular weight and very nonpolar analytes, the correlation seems no longer valid.
The log Ko/w value can give an idea of how good the extraction will be with PDMS
coatings. The theoretical recovery η is therefore defined as the ratio of the extracted
amount of analytes (mPDMS) over the original amount of analytes present in water (m0 =
mPDMS + mw). (Eq. 1)
Eq. 1
With knowledge of the phase ratio β (β = Vw/VPDMS) and the octanol-water distribution
coefficient, the recovery η can be calculated. The Ko/w can for example be calculated using
the SRC- KOWWIN software package (Syracuse Research, Syracuse, NY, USA).
Based on this equation, it is clear that the more polar the solute, the lower the
recovery will be, as Ko/w for polar molecules is smaller than for nonpolar ones. Besides this,
the phase ratio plays an important role. The amount of extracted solutes is thereby directly
related to the volume of the PDMS extractive phase.
Recently, Sandra et al. published that the partitioning between PDMS and air follows
similar correlations as the partitioning between octanol and water for a range of volatile
compounds. The Ko/w value can thus be used to predict the partitioning when performing
headspace analysis 8.
Solid phase micro extraction 2.2
Solid phase micro extraction (SPME) was developed by
Arthur and Pawliszyn in 1990 9. The extractive phase is thereby
composed of a fused silica fiber of about 1 cm length and an
outer diameter of, typically, 150 µm. The fused silica is coated
with a sorbent layer with a thickness of 5 to 100 µm. This layer
corresponds to a volume of typically 0.5 µl. This fiber is
anchored on a metal support which is subsequently screwed
onto the fiber holder. The fiber holder contains a needle to
purge through a septum, a barrel and a plunger to protect the fiber when not used, by
Figure 2: Set-up of a SPME syringe
- 7 -
retracting it into the device. The needle can thereby easily perforate a septum, typically used
to close of a sampling device. The plunger is then actuated such that the fiber comes out of
the needle for sampling (Figure 2). Here, partitioning between the sorptive polymer and the
sample occurs. When the fiber is in direct contact with the (liquid) sample, the technique is
called immersion or direct mode sampling. When the fiber is kept in the headspace (the
vapour phase above the liquid in a closed top reservoir), it is called headspace SPME. (Figure
3) The fiber is then kept within this environment for a well-defined period, in order to
achieve an equilibrium between both phases.
Subsequently, the fiber is transferred into an instrument for desorption and
subsequent analysis in a gas chromatograph. Since SPME is basically solvent free and it
resembles a standard GC injection needle, it can easily be introduced into a standard GC
injector with split/splitless configuration. In this hot inlet, thermal desorption takes place.
Essential here is that desorption is efficient, which depends on the analyte volatility,
thickness of the coating, injector temperature and exposure time. Therefore the injector
needs to be closed carefully and shoved surround the needle tightly with a septum. By using
a narrow bore liner, an increased flow around the fiber is obtained, which results in a faster
and more efficient desorption and reduces peak broadening phenomena 10. Desorption can
also be achieved when introducing the fiber into HPLC compatible interfaces. Here liquid-
liquid-extraction principles extract the molecules into the solution for subsequent HPLC
analysis.
Figure 3: Immersion SPME versus Headspace SPME
- 8 -
Stir bar sorptive extraction 2.3
In this approach the extractive phase is immobilized on a stirring bar of which, after
sampling, the solutes are back-extracted or evaporated and transferred to the GC via a
thermal desorber (TD) equipped with a programmable temperature vaporizing (PTV) unit for
refocusing at the head of the column prior to analysis.
Stir bar sorptive extraction (SBSE) was introduced at the university of Technology in
Eindhoven by Sandra et al. 11. It can be seen as an improved version of SPME, which allows
up to a 1000-fold improved sensitivity. As the amount of material coating on a SPME fiber is
rather low (0.5 µl PDMS), recovery of especially polar solutes is often also rather low. On stir
bars (SB), the quantity of the coating material is significantly larger, typically 20-200 µl,
depending on the size of the stir bar. As the phase ratio for stir bars is favoured compared to
SPME, so the extraction efficiency is accordingly significantly improved. The stir bar support
is composed of a magnetic stirring rod which is covered with a thin jacket of deactivated
glass. The magnet is necessary to apply a rotating movement from a stirring plate to speed
up the extraction process. The glass is necessary to prevent the metal from catalysing the
degradation of the coating material 12. On this glass wall, the coating material is immobilized.
This coating is about 0.5 – 1 mm thick. The size of a stir bar varies between 10 mm and 40
mm. In some cases, direct immobilization on the glass wall is impossible. For these cases, stir
bars (with the glass wall) contain an extra layer of stainless steel (Figure 4). This aids in the
immobilization of the polymer phase on the stir bar and increases the mechanical strength.
Stir bars can also be used in immersion mode as well as in headspace mode. In the former
mode, it is stirred in the solution for a certain time (a few minutes up to some hours), before
desorption is performed. When headspace sorptive extraction (HSSE) is used, a special
device is needed to mount the stir bar above the liquid or solid phase in a closed
thermostated sampling vial. All these approaches have been commercialised (Figure 5).
After removing the stir bar from the sample, it is rinsed with a small amount of Milli-Q
water to remove unwanted adsorbed molecules like sugars or proteins. Water droplets are
then removed with lint-free tissue. Rinsing of the stir bar thereby doesn’t cause loss of
solute as the sorbed molecules are present inside the coating 12, 13.
- 9 -
Desorption of the trapped compounds can subsequently be done via liquid desorption
or thermal desorption. Each method presents advantages and disadvantages 6. When using
liquid desorption, the stir bar is washed with a small amount of organic solvent (liquid back-
extraction). Generally, sensitivity is lost in this way as significant dilution is thereby
occurring. Although this approach can be coupled to HPLC or large volume injection GC,
thermal desorption is more widely used for introduction of the solutes from the stir bar in
the analytical system. Here the analytes which were evaporated when heating the stir bar
are transported under a flow of helium. However, in contrast to the use of SPME fibers,
desorption inside the inlet of a gas chromatograph is not feasible, since the sorptive material
on stir bars is much larger than on SPME fibers. This causes the desorption times (10 min) to
be much longer and higher desorption flows (100 ml/min) are needed. Therefore dedicated
thermal desorption instruments have been developed by Gerstel GmbH (Müllheim a/d
Ruhr).
The system which has been developed first was the thermal desorption system (TDS)
(Figure 6 left). Stir bars are thereby introduced in a glass tube of about 20 cm long with an
outer diameter (OD) of 6 mm and an inner diameter (ID) of 4 mm or 5.5 mm depending on
the type of stir bar used. This glass tube can contain a frit (to immobilize the content) and/or
some types of adsorbents like Tenax ™ or Chromosorb™. The glass tube is positioned
horizontally in the desorption oven. Here the stir bar is heated from ambient temperature to
about 300°C and is held there for 10 minutes. The ramp at which the temperature increases
can be up to 60°C/min. The helium flow is thereby kept constant at 50-100 ml/min. In this
way, no oxygen will be present to degrade the analytes and desorption is efficient. The
Figure 4: Stir bar coated with PDMS and stir bar
with steel grid
Figure 5: SBSE versus HSSE
- 10 -
analytes are then transferred via a transfer capillary towards a dedicated cooled injection
system (CIS). As desorption takes 10 minutes, focussing of the analytes in the latter is
essential before injecting into the column to avoid peak broadening due to injection. With
the TDS, the helium flow containing the analytes is cooled as low as -150°C using liquid
nitrogen cooling. Depending on the volatility of the analytes, temperatures up to -50°C are
allowed when correct packing material in the inlet is used. The CIS is thereby used as a
programmable temperature vaporizing (PTV) inlet. After complete desorption and focusing
in the CIS, this device is heated rapidly (up to 12°C/s) to about 300°C and held for 10 minutes
for complete desorption of all analytes. In this way sharp peaks in the chromatogram will be
obtained as all analytes are injected at quasi the same moment.
A thermal desorption unit (TDU) (Figure 6 right) has also been developed. This
desorption device works according to the same principles as the TDS system with some small
differences in setup. Here, the glass tube in which the stir bar is introduced is much smaller,
only 60 mm long, with an OD of 6 mm and ID of 5 mm. Introduction in the desorption oven is
done vertically. The system is much smaller and can be positioned directly on top of the
CIS injector. This will reduce flow related problems and reduced loss of analytes as no
Figure 6: Thermal Desorption System (left) and Thermal Desorption Unit (right) combined with a Cooled Injection System
- 11 -
transfer line is thereby needed. As desorption is done vertically, this system can also
thermally desorb other materials than stir bars in an easier way. It also allows to perform
direct headspace analysis of solid samples or liquid samples containing a dirty matrix.
Thermal desorption is used in most cases to desorb stir bars as molecules that where
absorbed by the coating can be desorbed and reach the detector. This explains its high
sensitivity compared to what is possible with liquid desorption. Various studies have also
shown that SBSE can easily outperform LLE in several ways. Recovery rates, for example, are
higher and simultaneous extraction is possible, which reduces time needed drastically. As
mentioned, much less or no organic solvent is needed to perform the extractions 14.
Comparison between SPME and SBSE 2.4
In Figure 7, a comparison of the extraction efficiency between SPME and SBSE is shown
when using PDMS as a coating material. Due to the larger amount of PDMS sorbent on a stir
bar compared to what is present on an SPME fiber, the recovery for molecules with a log
Ko/w value between 1 and 6 is better on a stir bar, as the shifted phase ratio (β) is thereby
promoting more extraction. In this example the SPME fiber and the stir bar are respectively
coated with 0.5 µl and 24 µl PDMS.
Figure 7: Theoretical recovery (%) as a function of log Ko/w for solutes in a 10 ml water sample using SPME and SBSE, both coated with PDMS. Equilibrium sampling is thereby assumed
0
10
20
30
40
50
60
70
80
90
100
0 1 2 3 4 5 6 7 8
Re
cove
ry η
(%
)
log Ko/w
SPME (0.5 µl)
SBSE (24 µl)
- 12 -
Thiobencarb is used to demonstrate the
superior possibilities of stir bars (Figure 8). This
molecule is relatively polar with a log Ko/w value of
only 3.90. According to the above prediction, using
an SPME fiber, the recovery will be 28.3%. By
contrast, SBSE is much more suitable as it yields a recovery of 95%. Note that stir bars will
recover nearly 100% of molecules with a log Ko/w of 4 or more. Stir bars with more PDMS will
recover even more polar molecules more efficient. However, when the log Ko/w value is
above 6, there is no significant difference between SPME and SBSE.
When the molecules of interest have a log Ko/w below 2, stir bars coated with 24 µl
PDMS will only recover about 20%. This is too low for reproducible results, especially in trace
analysis. For this reason a lot of research is being done on the use of new coatings to
improve the extraction efficiency of these solutes by sorptive extraction.
Another advantage of stir bars compared to SPME fibers is their robustness. SPME
fibers are relatively delicate to work with as the fused silica support can break under stress.
On the other hand, SPME has the advantage that currently a broader variety of coating
materials is commercially available for this technique. As mentioned before, SPME requires
only a standard GC instrumentation, whereas SBSE needs extra apparatus when thermal
desorption is used. Both stir bars and SPME fibers can be reused up to a 100 times.
Characteristics of coating materials suitable for sorptive extraction 2.5
Good extractive phases for sorptive extraction depict following characteristics. Most
important is the glass transition temperature (Tg) of the polymer, which should preferably be
below room temperature. In this way, the polymer will behave as a liquid and ensure that
absorption occurs instead of adsorption. Additionally, a polymer at a temperature above its
glass transition temperature is more user friendly. The ones below that temperature tend to
be more brittle and could break more easily under mechanical stress.
The polymer also needs to be inert, of high purity and straightforward synthesis is to
be favoured as this promotes reproducibility and upscaling to commercial applications. Fast
diffusion of molecules in and out of the material is required. If the coating is used in
combination with thermal desorption, it needs also to be highly thermally stable. Since
desorption can take place at temperatures up to 400°C, the polymer should withstand these
Figure 8: Thiobencarb Cl
S
O
N
- 13 -
temperatures. Degradation will be immediately detected by, for example, mass
spectrometry, which will observe a raise in the noise levels, which is contaminating the
system and affecting the limits of detection in a negative way.
Below the coating materials which are most used today for sorptive extraction will be
discussed.
2.5.1 Polydimethylsiloxane
The properties of polydimethylsiloxane
(PDMS)(Figure 9) are well known, since it has been widely
as stationary phase in gas chromatography for several
decades. PDMS can easily be synthesized and obtained
with high purity. The polymer itself is non-porous and a high degree of homogeneity will be
achieved. As PDMS itself is hydrophobic, it will mainly extract nonpolar molecules with high
log Ko/w values (Figure 7). The degradation pattern of this polymer is well known and can
easily be discriminated from target solutes by mass spectrometry. It exhibits a very high
maximum allowable operating temperature (MAOT) of about 320°C, making it very good to
be used in combination with thermal desorption. As the Tg of this polymer is very low (-
125°C) the necessary conditions are met for PDMS to be an excellent material for sorptive
extraction. Its retaining capacity is not influenced by the presence of even high amounts of
water in the sample making it ideally suitable for aqueous extraction techniques. In order to
improve the extraction efficiency to more polar compounds than mentioned above, PDMS
can also be mixed with other polymers or molecules to obtain more polar characteristics.
This aspect is also studied in this work and therefore discussed further. On the other hand
when dealing with very volatile molecules (e.g. compounds with carbon chains up to C4), the
retaining capacity of PDMS is also affected, but then due to unfavourable headspace/PDMS
partition coefficients 15. At this moment only two types of stir bars are commercially
available, one is coated with pure PDMS while the second one is coated with PDMS/EG
(ethylene glycol) which is immobilized on an inert metal grid. The former is thereby the most
widely used.
PDMS coated stir bars have successfully been used for a multitude of applications
including environmental or food analysis and in biomedical applications 13, 16, 17, 18. The
analysis of polycyclic aromatic hydrocarbons (PAH) 19, polychlorinated biphenyls (PCB) 18, 20,
Figure 9: Polydimethylsiloxane
Si
O
Si
O
Si
n
- 14 -
volatiles in coffee 21, fruit or plant material 21, pesticides in water 17 and vegetables 22 have in
this way been described making use of PDMS coated stir bars. In some of these examples,
the extraction was performed in combination with derivatisation or the salting-out effect,
which will be discussed further.
2.5.2 Polyacrylates
The use of polyacrylates (PA) for sorptive extraction and
desorption has also been investigated 23, 24. Typically
polybutylacrylate (PBA) is thereby used (Figure 10). The glass
transition temperature of PBA is -54°C 6 and it can be used in its
pure form without mixing with PDMS. Polyacrylates have the
ability to recover more polar solutes than is possible with PDMS. However, as the thermal
stability is significantly lower, desorption needs to be done at correspondingly lower
temperatures (200°C) to avoid the appearance of a large background signal in the
chromatogram. Another drawback of PBA is that diffusion in- and out of the phase is very
slow, requiring hours to obtain satisfactory extraction and therefore acceptable recoveries.
PBA is coated on a stir bar using a metal grid on which 25 µl is deposited, as this ensures the
best immobilization of the polymer. It is also frequently used on SPME fibers for the
extraction of polar semi-volatiles.
These acrylates have, for example, been used for the analysis of herbicides and
insecticides using commercially available SPME fibers 25. Making use of the stir bars coated
with PBA, TD-GC-MS was used for the extraction of benzothiazole in untreated wastewater.
Adding sodium chloride (NaCl) aided this extraction 24. Other studies showed that the
extraction on a coffee sample with PBA coated stir bars didn’t perform better than when
using PDMS coated ones. The latter was thereby able to extract more compounds, with
higher recoveries, except for the very polar molecules like caffeine (log Ko/w = 0.16). Note
that a relationship between the log Ko/w and the extraction efficiency, like is the case with
PDMS, is no longer available 26.
Figure 10: Polybutylacrylate OO
C4H9
n
- 15 -
2.5.3 Polyurethanes
Polyurethanes (PU) is another candidate, which has
been significantly investigated to extract polar molecules
from a sample with SBSE. PU is easily synthesized by
combining a polyol, silicone oil and a catalyst in a
polyethylene flask under vigorous stirring. To this mixture an
isocyanate is then added while stirring is applied for a short period (15 seconds). Now the
formation and growth of the foam occurs, after which it is cut into a cylindrical shape and is
positioned around a glass stir bar 27, 28. These polymers also show a high thermal stability,
although liquid desorption is mostly applied. PU is mechanically resistant enough in order to
be reused several times.
PU has, for example, been used for the analysis of atrazines. Much higher recoveries
compared to PDMS were thereby noted. Analysis was done using liquid desorption, followed
by HPLC 27. Similar foams were used the determination of acidic pharmaceuticals, including
acetylsalicylic acid, ibuprofen and naproxen in water prior to analysis with HPLC. Better
limits of detection were thereby found compared to PDMS based stir bars 29.
2.5.4 Polymer mixtures
To improve the recovery of polar solutes, polymer mixtures can also be used. PDMS is
thereby mostly used in combination with divinylbenzene (DVB) or polyethylene glycol (PEG).
In both cases, this leads to a partial loss of the absorption mechanism.
The Tg of pure PEG, which is commercially available as carbowax™, is 70°C, therefore
this polymer is used in the glass state when working under ambient conditions. Both pure
PEG as mixtures with PDMS are used whereby it is unclear if the Tg is positively affected in
the latter case 6. PEG is thermally stable to temperatures up to 250°C, more polar than
PDMS, somewhat hygroscopic and it mixes easily with water. It also dissolves in ethanol,
chloroform and acetone. When used in sorptive extraction, the kinetics of extraction and
desorption are fast 30. PEG has, for example, successfully been used for the sampling of
alcohols and polar molecules with low molecular mass.
Mixtures of PDMS with DVB have also been used for the analysis of insecticides. This
coating was compared to pure PDMS and polyacrylate (PA) based coatings. The mixture
Figure 11: Urethane repeating unit
N
R''
R'
O
O
R
- 16 -
thereby provided the best results, although when analysing more complex matrices, some of
its sensitivity was lost 30.
2.5.5 Monoliths
Monoliths are porous polymers, possessing a good permeability. Once developed, the
synthesis is quite straightforward as it can then usually be performed in one step. The
benefit of this strategy is that the porosity and surface properties can relatively easily be
modified, for example, by varying the amount of porogen. Both silica based as non-silica
based monolithic polymers have been widely described. In this way, many types of
monolithic HPLC columns have been developed. All these columns are characterised by a
continuous (monolithic) stationary phase intertwined with a large number of connected
pores through which mobile phase is flowing. A major drawback of the monolithic approach
is that their reproducibility is not very satisfactory. Every time, the distribution of the sizes
and the connection points of the pores are different, making it difficult to ensure constant
retention, column efficiency and back pressure. As the realisation has developed that the
best application of monoliths might therefore be in sample preparation strategies, in the last
few years (non-silica based) monolithic polymers have increasingly been developed for
extractive purposes.
Huang et al. were in this way the first to introduce monolithic polymers for the
extraction of polar analytes using SBSE 31. A wide variety of monoliths have been produced
so far for this purpose, ranging from monoliths used for the extraction of PAHs to polar
solutes such as phenols 32, 33, 34. Even heavy metal ions could be extracted using these
polymers 35.
The use of monoliths in sample preparation has thus far mostly been described in
combination with liquid desorption, followed by direct HPLC analysis. Due to low mechanical
strength of this type of monoliths which have thus far been explored, stirring speed needed
to be kept fairly low (400-900 rpm), slowing the extraction down 36. Examples of non-silica
based monolithic polymers used for SBSE in combination with HPLC are copolymers of
vinylpyrrolidone (VPL) or vinylimidazole (VI) with divinylbenzene (DVB). These were used for
the extraction of phenols and aromatic amines in aqueous samples 32, 33, 34.
A few thermally desorbable monolithic polymers have also been described.
Poly(acrylamide-4-vinylpyridine-N,N’-methylenebisacrylamide) was for example synthesized
- 17 -
and used in both HSSE and SBSE mode 37. In SBSE the robustness was poor. In headspace,
this was no longer the problem, however the bleeding pattern was thereby still very
problematic, as could be noticed by large pyridine containing background signals. These
results therefore put doubts on the viability of the use of this type of polymers in SBSE in
combination with TD-GC-MS.
Complimentary techniques for the extraction of polar solutes 2.6
Next to the development of new stationary phases for stir bars, efforts have been
made towards the introduction of new procedures allowing the extraction of more polar
solutes using stir bars coated with conventional PDMS by changing the physical
characteristics of the extractive phase, of the sampling environment or of the solutes to be
extracted. Three approaches to increase the recovery of these standard twisters towards
more polar analytes are discussed.
2.6.1 Derivatisation
First of all, derivatisation of polar and thermally labile compounds can be used. It can
be performed before, during or after extraction and a plethora of reactions has been
developed for all kinds of polar functionalities.
In situ derivatisation is thereby the easiest approach. To the sample containing the stir
bar, a derivatising agent is added and reaction occurs before extraction. As stir bar based
sampling occurs almost exclusively in aqueous environments, this approach is not applicable
to moisture-sensitive reactions. (Figure 12A) 16.
Phenolic moieties can, in this way, be acylated using acetic acid anhydride, carboxylic
acid and amine functions can be converted into its esters and amides with
ethylchloroformate (ECF) and aldehydes or ketones are reacted with
pentafluorobenzylhydroxylamine (PFBHA) to form oximes 13, 38, 39.
The stir bar can also be pre-impregnated with the derivatisation agent to overcome
the problems with moisture-sensitive reactions. (Figure 12B) 38. Reaction is then performed
simultaneously with the extraction itself. Derivatisation can finally also be done after the
extraction by exposing the stir bar to the vapour of the derivatisation agent which can also
be performed in the thermal desorption tube itself.
- 18 -
Next to increasing the hydrophobicity of the solutes, derivatisation usually also
improves the thermal stability of some compounds. This ensures the introduction of larger
amounts of (not degraded) material on the column, better peak shapes, correspondingly
improved sensitivity. Silylation is typically used in this case. Many silylating agents have been
developed including N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA), hexamethyldisiloxane
(HMDS) and chlorotrimethylsilane. They are used to convert, for example, hydroxyl or
carboxylic acid functional groups into their silyl ether or silyl ester.
2.6.2 Salting-out effect and sequential SBSE
The extraction efficiency of polar molecules can also be improved by altering the ionic
strength through the addition of salt (NaCl) to the phase to be extracted. This approach is
better known as the salting-out effect. This will reduce the recovery for molecules with a log
Ko/w larger than 3.5 but will improve the extraction for more polar molecules. This
phenomenon has for example successfully been used for the analysis of volatile compounds
in vinegar and wine 40, 41.
Sequential SBSE is a strategy which applies this salting-out effect in a clever way 42. A
first PDMS coated stir bar is thereby used to extract the apolar compounds. After this
extraction, the stir bar is removed and salt is added together with another stir bar. Now the
more polar compounds are extracted from the solution. Both stir bars are then thermally
desorbed simultaneously using the TDU system. In this way, a chromatogram covering a
broad log Ko/w range of solutes can be obtained. Making use of this approach, over 80
Figure 12: Different derivatisation modes in SBSE: In situ derivatisation (A) and on-stir-bar, with the derivatisation agent preloaded before exposure to sample (B)
16
- 19 -
pesticide molecules with octanol-water partitioning coefficients ranging from 1.7 to 8.35
were analysed. The limits of detection (LOD) were all in the low ppb range 42.
2.6.3 Dual-phase twisters
PDMS can also be used in combination with adsorptive
material. An example of this approach is dual-phase twisters.
Here, PDMS tubes are filled with 20 mg of carbon material
and sealed with two small Teflon-coated magnets (Figure 13)
15. Dual-phase twisters are based on the combination of absorption and adsorption at the
same time. The extraction involves three steps. First, the absorption of the molecule onto
the PDMS, followed by its diffusion through the PDMS phase and finally the adsorption onto
the carbon. However the latter must be reversible in order to achieve successful thermal
desorption.
Different carbon based materials have been studied for this application 15, 21.
Adsorbents like carbopack B and Tenax GC, a 25:75 bisphenol-PDMS copolymer, were
thereby used successfully for the headspace analysis of coffee powder. Using this approach,
compared to standard twisters, the extraction efficiency of polar and more volatile
molecules could be much improved, especially when the stir bar was brought in direct
contact with the sample 26. A drawback is the inherent lower reproducibility and high cost of
this strategy. Due to the adsorptive aspect of the carbon based materials, broad generic use
of this type of stir bar is, however, not an option.
Figure 13: Schematic drawing of a dual-phase twister
15
- 20 -
3. Ionic liquids as new material for sorptive extraction
Ionic liquids: definition & properties 3.1
Ionic liquids (IL) are essentially a class of charged organic salts. They have no notable
vapour pressure and can be thermally stable up to temperatures of 300°C, while being
chemically and electrochemical stable as well. ILs are non-flammable and possess a high
heat capacity. These salts generally melt below 100°C due to the larger size of the cation
which implements the impossibility to fit in a crystalline structure. In some cases, the salts
are present in the liquid aggregation state at room temperature (room-temperature ionic
liquids, RTIL). These liquid salts are able to dissolve a wide range of organic and inorganic
molecules, while being miscible with water (hygroscopic) and most organic solvents. The first
ionic liquids were synthesized in the 1980s 43. These were mainly chloroaluminate ionic
liquids. 10 years later, air and moisture stable ionic liquids were developed. Over the last
decade, the development of task specific ionic liquids has become more important. A
plethora of ionic liquids can be made by the versatility of counter ions that are available.
Some representative ions are shown in Figure 14 44, 45. Each of these ions depict unique
physicochemical properties. Subtle changes to these properties can be introduced by, for
example, altering the side chains on the imidazole moiety.
In terms of thermal stability, imidazolium (IM) based salts are currently considered the
best choice. The side chains of the cation appear to have only a minor influence on the
stability. However the choice of counter ion has also a very pronounced effect on the
thermal stability 46. In combination with imidazolium, nucleophilic anions like chloride and
bromide lower the thermal stability due to the occurrence of nucleophilic substitution
reactions. On the other hand, counter ions like bis[(trifluoromethyl)sulfonyl]imide (NTf2) and
hexafluorophosphate (PF6) do not show this behaviour and therefore give rise to better
thermal stability 44, 47.
The type of anion also largely influences the hydrophobicity of the organic salt. NTf2
and PF6 increase the hydrophobicity of the ionic liquid, halide and carboxyl anions raise the
hydrophilicity and the use of counter ions such as BF4 and mesylate (CF3-SO3) results in ILs
with intermediate hydrophobicity. Depending on the intended application, the viscosity of
the IL can play a very important role. For imidazolium based ionic liquids, the latter has been
- 21 -
shown to depend on the anionic radius, whereby an increase of the latter is correlated to a
decreasing viscosity.
N NR1 R2
NR2
R1
N R
N
R4
R2R1
R3
P
R4
R2R1
R3
Imidazolium Pyrrollidinium
Tetraalkylammonium Tetraalkylphosphonium
Cl Br
O
OF3C
S
O
O
O
CF3
S
N
S
O
OF3C
O
O
CF3
O
OR
O
P
OR2
R1
F
B
FF
F
F
P
F
FF
FF
N
NN
S
O
O
O
Chlorine Bromine Trifluoroacetate Carboxyl
Trifluoromethylsulfonate (TfO) Phosphinate Dicyanamide Tetrafluoroborate
Hexafluorophosphate (PF6) Tosylate Bis[(trifluoromethyl)sulfonyl]imide (NTf2)
Pyridinium
AlCl4
Chloroaluminate
Figure 14: Overview of some representative cations and anions used in ionic liquids
Ionic liquids are already used to some extend in chromatography. One of these
developments is the use of ILs as stationary and extractive phases. Columns for gas
chromatography 48 and liquid chromatography 49 have in this way been coated with ionic
liquids. The benefit of using ILs is that they allow to retain and separate both polar and
nonpolar structures on the same column 50. Drawbacks of this approach are, however, the
lower maximum operating temperatures (MAOT) and the reduced homogeneity of the
coating thickness. Ionic liquids can also be used as mobile phase additive for LC 51.
- 22 -
Figure 15: Chemically bonded IL, as proposed by Amini
54
Ionic liquids and sorptive extraction 3.2
The use of ILs as extractive phase for sorptive extraction has already been investigated
to some extend in SPME. Fused silica fibers were thereby physically or chemically coated
with ILs. The former was performed by dipping a fiber, whereby the silica surface was priorly
enhanced via an etching process, into the ionic liquid. The etching process can, for example,
be done by treating the fiber with a saturated solution of ammonium hydrogen difluoride in
methanol 52. Drawbacks of this straightforward approach is that during the desorption inside
a heated GC inlet, the ionic liquid becomes less viscous and can drip into the liner. Regular
Liner swapping or cleaning is thereby necessary and the reproducibility of this approach is
therefore severely affected.
The first application of physically coated SPME fibers, with 1-octyl-3-
methylimidazolium hexafluorophosphate ([OMIM][PF6]), was used for the headspace
analysis of paint. Somewhat surprising, hydrophobic solutes such as benzene, toluene,
ethylbenzene and xylenes (BTEX) were thereby most favourably extracted and analysed with
GC-FID 53. After each extraction the ionic liquid was removed from the fiber by washing with
an organic solvent and subsequently recoated.
In order to prolong the lifetime of ionic liquid
coated fibers, Amini et al. introduced chemically
bonded IL fibers 54. 1-methyl-3-(3-trimethoxysilyl
propyl) imidazolium bis[(trifluoromethyl)sulfonyl]
imide ([MTPIM][NTf]) was therefore synthesized
(Figure 15). This molecule was then allowed to react
with the fused silica. Using this approach, the coated fiber could tolerate temperatures up to
220°C. It was used for the analysis of methyl-tert-butylether (MTBE) in gasoline. Detection
limits of 0.1 µg/l were obtained and the fiber could be reused up to 16 times.
Other ionic liquids, coupled to the support via chemical linkage, were introduced by
Huang et al. 55. These ionic liquids contained two imidazolium moieties, linked together with
PEG. These were then bonded to modified silica, via the formation of an isocyanate bond.
These fibers were used in combination with salting-out, in order to maximize the extraction
of the polar solutes. Detection limits ranging from 10 µg/l to 40 µg/l were obtained in this
way for the analysis of lower alcohols 56. These fibers could also be reused several times and
no loss of the IL was detected at conventional operating temperatures of a GC inlet.
- 23 -
Recently, an innovative approach was introduced for the chemical bonding of IL to
fused silica 57. 1-allyl-3-methylimidazolium (AMIM) cation was thereby incorporated in a sol-
gel network. A schematic drawing of the structure of the chemically coated IL is shown in
Figure 16. This structure proved to be stable up to 320°C with PF6 and 400°C with NTf2 as
counter ion. This fiber was tested on its capability towards the extraction of phenols.
Recovery rates were much higher compared to PDMS and similar extraction compared to PA
is observed. A drawback of the sol-gel approach is, similarly to the use of monoliths, the
difficulties involved in providing very reproducible coatings.
Polymerized ionic liquids 3.3
3.3.1 Ionic liquid moiety in the side chain
To overcome the problems concerning the decreasing viscosity of IL when heated in a
GC inlet and to allow the use of chemically more homogeneous phases, polymerized ionic
liquids (PIL) have therefore increasingly been introduced. Additionally, polymeric ionic
liquids have much improved processability and are more resistant to mechanical stress
compared to ILs 58. Also in this new type of polymers, a broad variation has already been
developed, ranging from vinyl based polymers to main chain cationic polymers. In most
polymers an imidazolium moiety is incorporated 59. As was the case in the ILs, also in PILs the
imidazolium ring offers high thermal stability, combined with chemical stability, even
Figure 16: Simplified structure of the sol-gel coating used by Lui et al. 57
.
- 24 -
towards harsh acids and bases. These polymers aren’t liquids but the ionic liquid is
polymerized or the ionic liquid moiety is introduced during polymerization.
An important category within the former are those made from ionic liquid monomers
which contain a vinyl moiety. Just like with other vinyl moieties in polymer chemistry, these
monomers can be polymerized by using free radical polymerization with a thermal initiator
like 2,2′-azobis(2-methylpropionitrile) (AIBN), but the polymerization can also be controlled
via ATRP or RAFT. Monomers used for this type of polymerization are shown in Figure 17 60.
1-vinyl-3-alkyl-imidazole (Figure 17A) is obtained out of a reaction of vinylimidazole
with an alkyl halide. When the alkyl chain is short, the ionic liquid dissolves in water, while
with increasing length, it becomes insoluble in water.
(Meth)acryloyl functionalized IL monomers (Figure 17B) are synthesized by reacting
methacryloylchloride with a hydroxyl-containing alkyl halide. This alkyl halide is then further
coupled to an alkyl-imidazole. This type of monomer can be varied in many ways. Next to the
different alkyl chains, the spacer between the acryloyl and the imidazolium moiety can also
be differentiated.
Styrenic IL monomers (Figure 17C) are produced out of the reaction of 4-
chloromethylstyrene with imidazole. To the molecule which is formed in this way, an alkyl
chain is subsequently connected, by means of an alkyl halide.
All these monomers can readily be cross-linked with divinylbenzene. Crosslinking can
also be achieved in other ways 59. A polycation can, for example, be physically cross-linked
Figure 17: Vinyl based PIL monomers: 1-vinyl-3-alkyl-imidazole (A), (meth)acryloyl functionalized alkyl-imidazole (B), styrenic functionalized
alkyl imidazole (C)
- 25 -
with a poly anion (Figure 18A) or an imidazolium containing divinylic monomer can be
synthesized, giving a cross-linked polycation (Figure 18B). Other crosslinking molecules can
contain more than one cation (Figure 18C). Crosslinking is done in order to increase its
stability towards moisture and solvents.
After polymerization the anion can easily be exchanged, as after addition of the NTf2-
or PF6- salts in aqueous medium precipitation readily occurs 59. As mentioned this ion
exchange will influence, next to the solubility properties, the thermal stability of the polymer
and is therefore an important step during the synthesis.
3.3.2 Applications of polymerized ionic liquids with IL moiety in side chain
This kind of PILs have to some extend already been used in the headspace analysis of
fatty acid methyl esters (FAMEs) with SPME 61. Polymerized vinylimidazole (PVI)(Figure 17A)
based PILs containing a different length of alkyl chains were thereby employed. The polymer
was dip-coated on a fused silica fiber, resulting in an SPME fiber with a lifetime of about 150
extractions. It was thereby observed that different counter ions not only changed the
hydrophobicity of the polymer and its thermal stability, but also its extraction efficiency 62.
Chloride polymers exhibit higher extraction efficiencies compared to NTf2 polymers. For the
solutes which were analysed, this can be related to the bigger hydrogen-bond accepting
capacity of a chloride anion. PVI can also be used in immersion SPME 63. The sensitivity
achievable with PVI was thereby better than what could be achieved with commercially
available PDMS coated SPME fibers for the analysis of PAHs and phenols in water. However,
Figure 18: Crosslinkable monomers
- 26 -
commercially available PA fibers still managed to reach a better sensitivity. The PIL based
fibers could be reused up to 50 times.
Another application was developed by the group of Armstrong 56. They coated a SPME
fiber with styrenic dicationic monomers (Figure 18B) which were polymerized on
azobisisobutyronitrile-derivatized silica. These fibers were used in headspace and immersion
and compared to their IL counterpart, PA fibers and PDMS-DVB fibers for the extraction of
small alcohols, acetonitrile, acetone and ethyl acetate. The polymeric ionic liquids were
synthesized with NTf2 and trifluoromethanesulfonate (triflate, TfO-) as counter ion. Both PILs
were able to reach significant lower LODs compared to their corresponding IL. For most
analytes, the PILs performed better than PA and PDMS-DVB, especially when using the
polymer containing triflate as counter ion. The alcohol level in beer and whisky was sampled
with this PIL in headspace. Deviation of the alcohol percentages on the bottle and those
measured was thereby in the range of only 2-6%, with a relative standard deviation of 7-12%
on 4 measurements. These fibers were used more than 50 times in the study.
Another group showed that PILs containing chlorine as counter ion were able to
extract more efficiently polar compounds compared to the ones with NTf2 62. While using the
salting-out effect, molecules with log Ko/w ranging from 1 to 3 were extracted three times
more efficiently when chlorine was used as counter ion, leading to limits of detection in the
low µg/l range. They stated that chlorine anions possess high hydrogen bond basicity and are
able to form hydrogen bonds with the polar solutes, whereas NTf2 could not induce this
effect.
3.3.3 Ionic liquid moiety in the main chain
In another PIL design the imidazolium group is present in
the main chain 64, 65. The polymerization occurs via a
condensation reaction, whereby the cation is formed. A typical
monomer for this reaction is shown on Figure 19. Very
recently, crosslinking of this monomer was demonstrated 58. The synthesis of the branching
agent is thereby performed in 2 steps. First 1,3,5-hydroxybenzene is reacted with 1-bromo-
4-chlorobutane in the presence of sodium hydroxide (NaOH). This is then allowed to react
with sodium imidazolide to produce 1,3,5-tris-(4-imidazol-1-yl-butoxy)-benzene (Figure 20).
Figure 19: Main chain PIL monomer
N N Cln
- 27 -
This molecule is then used in a subsequent step as a crosslinker together with the
polymerization of the monomer shown in Figure 19.
This type of polymer can, depending on the counter ion, withstand temperatures of
about 400°C. This makes them very good candidates to be used as coating materials on stir
bars followed by thermal desorption.
Not cross-linked poly(hexylimidazolium) with NTf2 as counter ion has been evaluated
as stationary phases in GC 48. It thereby showed a very good bleeding profile compared to
commercially available 5% phenyl 95% PDMS column. This PIL based column could withstand
temperatures up to 380°C without an increased background due to bleeding of the column.
Both polar and apolar molecules can be separated on this stationary phase, but it should be
mentioned that the results for polar molecules were thereby more convincing. Also for the
analysis PAHs including very low volatility molecules like dibenzo[a,i]pyrene, good separation
could be obtained. Especially for these high boiling compounds the thermal stability of the
stationary phase is of great importance in order to be able to purge them out of the column.
Thus far cross-linked main chain PILs haven’t been used in combination with
chromatography. In sample preparation techniques, no main chain PILs have been used so
far.
Thermal properties of ILs and PILs 3.4
Below a small overview is provided of ionic liquids and of polymerized ionic liquids,
with their reported onset degradation temperatures. When comparing imidazolium based
ionic liquids, it becomes clear that when bis[(trifluoromethyl)sulfonyl]imide was used as a
counter ion this systematically resulted in the most temperature stable ionic liquids. For
example with 1,2,3,4,5-pentamethyl-imidazolium, the onset degradation temperature is 303
°C, 401 °C and 470 °C for chlorine, hexafluorophosphate and NTf2 respectively 44. The same
trend can be seen in all polymeric types. Crosslinking vinylic poly(ionic liquids) with dicationic
Figure 20: synthesis of 1,3,5-tris-(4-imidazol-1-yl-butoxy)-benzene
- 28 -
crosslinkers clearly improved the thermal stability. The main chain poly-Cl also appears more
stable than the corresponding ionic liquid 1-n-butylmethylimidazolium chloride, which is
reported to decompose already at 254 °C. Branched main chain poly-Cl is slightly less stable
compared to this linear analogue.
Weber et al provided some glass transition temperatures for styrenic based
polymerized ionic liquids 66. A comparison for poly-vinylbenzyl-hexyl-imidazolium with
different counter ions was thereby made. The Tg’s were measured as 109 °C, 87 °C, 79 °C and
9 °C respectively for chlorine, PF6, tetrafluoroborate (BF4) and NTf2, respectively.
Counter ion Tonset (°C)
Imidazolium ionic liquid 44 Cl- 280-300
PF6- 330-400
NTf2- 400-470
Sol-gel coated ionic liquid 57 PF6- 320
NTf2- 402
Vinylic PIL (no crosslinking) 67 NTf2- 300-340a
NTF2-/Cl- b 190-250
Vinylic PIL (with crosslinking) 50 NTf2- 350-380 c
Styrenic PIL 66 d
Cl- 233 e
BF4- 291 e
PF6- 295 e
NTf2- 345 e
Main chain PIL 65 Cl- 300
PF6- 370
NTf2- 450
Branched main chain PIL 58 Cl- 291
a onset bleed temperature as GC stationary phase
b in various ratios
c depending on the crosslink density
d based on poly-vinylbenzyl-hexyl-imidazolium (PVBnHexIM)
e Temperature where 5% weight loss is observed
Table 1: Ionic liquids and polymerized ionic liquids with their degradation temperature
- 29 -
4. Rationale
In this work, polymeric ionic liquids were used for the first time in combination with
stir bar sorptive extraction. The combined influence of the much increased amount of
extractive phase in SBSE compared to SPME and the expected polar selectivity of the
polymerized ionic liquids should therefore lead to the development of a superior extraction
technique in GC-MS effectively addressing the problem of the analysis of relatively polar
solutes, especially at the trace analysis level. The strategy was thereby the use the PIL with
the best reported thermal stability for immobilization on stir bars and evaluation thereof.
The main chain PIL backbone with composition shown in Figure 19 was therefore selected.
The synthesis is done with respect to the desired properties in terms of chain length,
viscosity, glass transition temperature and degradation temperature. Characterisation of the
synthesised polymers is done using techniques like differential scanning calorimetry (DSC),
thermogravimetric analysis (TGA) and infrared spectroscopy (IR) to ascertain the chemical
and physicochemical properties of the material. Next to synthesising and evaluating the pure
polymers, also mixtures with PDMS and PEG are tested. Polymer mixtures with appreciable
properties are then coated on glass plates to evaluate the physical behaviour as this provides
an indication on how they perform on a stir bar. The tested coating procedure is then
applied when coating stir bars. Subsequently thermal desorption of these stir bars is
performed, to check for residual solvents and other impurities and if bleeding occurs.
Subsequently, the stir bars are used for the extraction of several model samples containing a
variety of representative molecules in the polar range. The performance of the coated stir
bars is finally compared to commercially available PDMS coated stir bars by thermal
desorption GC-MS.
- 30 -
5. Experimental
Polymer synthesis 5.1
5.1.1 Chemicals
Lithiumhydride (95%), 1-bromo-3-chloropropane (99%), lithium
bis[(trifluoromethyl)sulfonyl] imide (99,95%) and potassium hexafluorophosphate (98%)
were purchased from Sigma-Aldrich Chemie (Steinheim, Germany). Imidazole (99%) was
delivered by Acros Organics (Geel, Belgium). Dichloromethane (DCM) and acetone were
purchased from Fisher Scientific (Loughborough, Leicestershire, UK). Methanol and
tetrahydrofuran (THF) were obtained from Biosolve (Valkenswaard, the Netherlands). THF
was dried over sodium. All solvents are HPLC grade. 1N silver nitrate was obtained from
Merck (Darmstadt, Germany). Milli-Q water was made in-house.
5.1.2 Synthesis of 1-(3-chloropropyl)imidazole
The synthetic procedure was adapted from Amarasekara et al. 64 and Hsieh et al. 65 and
is therefore briefly outlined here again. To a suspension of 1.35 g lithium hydride (0.170 mol)
in 30 ml dry THF, 11.56 g imidazole (0.170 mol) dissolved in dry THF was added dropwise at
0°C under nitrogen atmosphere. This was stirred for 0.5 hours. To this mixture, 13.9 ml 1-
bromo-3-chloropropane (0.140 mol) was added slowly at 0°C under N2-atmosphere.
Reaction was continued for 24 hours at room temperature giving a white-yellow emulsion.
40 ml of water was subsequently added very slowly to terminate the reaction. THF was
removed using a rotary evaporator with a water bath at 40°C. The resulting yellow solution,
containing white lithium salts, was extracted with dichloromethane (DCM) (4 x 50 ml). This
was dried over Na2SO4. DCM was removed using a rotary evaporator at room temperature,
resulting in a yellow viscous oil. The yield was 93%. The molecular weight of the monomer is
144.60 g/mol. This oil was stored in the freezer to prevent self-polymerization. The NMR
spectrum was measured on a Bruker 300 MHz Ultrashield™ with CDCl3 as deuterated
solvent, which shows chemical shifts at 7.38 (1H), 6.92 (1H), 6.82 (1H), 4.05 (2H), 3.33 (2H),
2.05 (2H), confirming the identity of the molecule.
Figure 21: Synthesis of 1-(3-chloropropyl)imidazole
NHN +ClBr
LiH NN Cl
- 31 -
5.1.3 Synthesis of polymer
Polymerizing the previously synthesized monomer was done in a straightforward way
as no initiator was thereby needed. It could be performed in a solvent like ethylene glycol or
even without solvent, by simple heating of the liquid monomer. Since this polymerization is
a step-growth polymerization, controlling the reaction time is essential for the molecular
weight. Polymerizations ranging from 30 minutes up to 24 hours have therefore been tested
in order to obtain the desired molecular weight. Since the reaction is an addition
polymerization, no by-products are formed. Therefore no subsequent clean-up is required.
One gram of monomer was polymerized by heating to 90°C for various times as
described. When polymerization was performed in a solvent under reflux, ethylene glycol
was used. After cooling down, the white-yellow solid was dissolved in 10 ml of methanol.
This solution is then poured into 200 ml of acetone and the resulting white precipitation was
filtered off on a por. 5 glass filter (poresize 1.0 µm – 1.6 µm) obtained from Robu® Glasfilter-
Geräte GmbH (Hattert, Germany). The yield was depending on the reaction time, ranging
from 80% to 97%. After washing with acetone, the polymer was dried for 1 day in an oven at
60°C. The white powder (poly-Cl) was stored in a desiccator under vacuum, as the polymer is
hygroscopic by nature.
Figure 22: Polymerization of 1-(3-chloropropyl)imidazole
5.1.4 Ion Exchange
Exchanging the chlorine counter ion for a more suitable one was done by adding a
fivefold excess of the desired counter ion. This was done by dissolving the chlorine polymer
(0.5 g) in 25 ml milli-Q water to which an aqueous solution (50 ml of 0.35M) of the counter
ion was added. Some molecules used to perform this metathesis reaction are listed in Table
2. The mixture was stirred for 12 hours at room temperature during which precipitation
takes place. Afterwards the water was decanted or filtered off and the polymer was washed
with water until all chlorine is removed. The latter was checked via reaction with silver
nitrate. As the yield was 95% and 96% for PF6- and NTf2
- respectively, not all anions were
N N Cl 90°C N N
nCl
Cl
- 32 -
thereby exchanged. After washing with water, the polymer was dried for 3 days in an oven
at 60°C. For convenience, the polymer was dissolved in acetone and dried in a vial.
Desired counter ion Molecule added
NTf2- Lithium bis[(trifluoromethyl)sulfonyl] imide
PF6- Potassium hexafluorophosphate
Table 2: Molecules for ion exchange
Characterization 5.2
5.2.1 Thermogravimetric analysis
With thermogravimetric analysis (TGA), the thermal stability of polymers was
measured. A certain amount of polymer (mg range) was weighted accurately on an
aluminium pan and placed in an oven equipped with a microbalance. Here the sample was
heated gradually, while being weighted continuously. Due to evaporation, degradation and
other effects, low molecular weight compounds were produced. These are transported away
with the flow of inert gas surrounding the pan. In this way, the mass decreases with
increasing temperature. The decrease depends on the thermal stability of the polymer itself.
Thermogravimetric analysis was done on TGA/SDTA 851e from Mettler Toledo
(Zaventem, Belgium). Analysis was done in the range of 25°C to 800°C. The heating rate was
10°C/min. The sample was kept under inert atmosphere (N2) with a flow of 100 ml/min, to
avoid oxidative pyrolysis of the polymer. Initial mass ranges varied from 3 to 14 mg.
5.2.2 Differential scanning calorimetry
Differential scanning calorimetry was used in this work to measure the glass transition
temperature of the synthesized polymers. A polymeric sample and a reference sample
(indium) were heated gradually with two separate ovens and kept at exactly the same
temperature during analysis. When a transition within the polymer occurs, for which heat is
needed (endothermic) or heat is produced (exothermic), the oven will have to correct for
this, by providing more or less heat to the sample compared to the oven for the reference.
This difference is a measure for the difference in heat capacity. A polymer evolving from the
glassy state to its rubber state, correlates with a measured increase in the heat capacity.
- 33 -
DSC measurements were done on a DSC 2920 modulated DSC from TA instruments
(Zellik, Belgium). Samples were weighted on an aluminium pan. Then liquid nitrogen cooling
was applied to -50°C and heated at a rate of 10°C/min to 150°C where it was held for 3
minutes. Then it was cooled at a rate of 10°C/min to -50°C again and kept there for 5
minutes. Subsequently the second heating experiment was done at a rate of 10°C/min to
150°C. The helium purge flow was thereby set at 25 ml/min and the N2 flow on the outside
of the furnace, used to prevent condensation at lower temperatures was set at 19 ml/min.
Coating stir bars 5.3
5.3.1 Materials
Silicone OV-1 (PDMS) was obtained from Chrompack (Varian, Bergen op Zoom, The
Netherlands). Dimethyl-(0.5-1%) methyl-vinylsiloxane copolymer (VPDMS) was purchased
from ABCR GmbH & Co. (Karlsruhe, Germany). Bondable PEG was delivered by Innophase
(Portland, CT, USA). This was stored under nitrogen atmosphere at 4°C. As both silicones are
not yet cross-linked, this was done with dicumylperoxide (DCP) as radical initiator. DCP was
obtained from Sigma-Aldrich Chemie (Steinheim, Germany).
Acetone and hexane were obtained from Fisher Scientific (Loughborough,
Leicestershire, UK). Methanol was purchased from Biosolve (Valkenswaard, the Netherlands)
and dimethylsulfoxide (DMSO) was supplied by Sigma-Aldrich Chemie (Steinheim, Germany).
Stir bars containing a metal grid were kindly supplied by the Research Institute for
Chromatography (RIC).
5.3.2 Coating procedure
Coating of the polymers was performed on a stir bar containing a steel grid as seen in
Figure 4, as well as on glass plates. Synthesized pure polymers were coated as well as
mixtures of these polymers with other polymers, like PDMS and PEG. Below, solvents used
for the coating of each of the polymers are listed. When the radical initiator DCP was
needed, it was dissolved in hexane.
- 34 -
Solvent
Poly-NTf2 Acetone
Poly-PF6 DMSO
PDMS Hexane
VPDMS Hexane
PEG Methanol
Table 3: Solvent for polymers
50 mg of each polymer was dissolved in the least possible amount of solvent, making
sure that the solution was not too viscous. For the mixtures with PDMS or VPDMS, 5 w% of
dicumylperoxide was added to ensure crosslinking of the PDMS. Within the PEG mixture, an
appropriate crosslinker was added by the manufacturer. This solution was then
homogeneously distributed on the metal grid with a syringe. After the addition of a few
droplets, the solvent was allowed to evaporate, to avoid overloading the metal grid with
solvent. This process was repeated several times until a sufficient amount of polymer was
immobilized on the stir bar. When crosslinking was necessary (i.e. when using the PIL/PDMS
and the PIL/PEG mixtures), the stir bar was heated under nitrogen atmosphere to 150°C for
30 minutes. This procedure was repeated until grid was fully covered with polymer.
Before being used for extraction, stir bars were conditioned at elevated temperatures
under a flow of helium in order to remove any residual solvent and other volatile molecules.
The empty TDU tube was also conditioned before inserting the stir bar in it.
Sample preparation 5.4
Depending on the sample which was selected, different extraction
conditions were chosen. In Table 4 the corresponding extraction
temperatures and times which were used are listed for each material. All
extractions were done in headspace mode except for the analysis of
pesticides, these were extracted in solution. The exact amounts of
sample used will be discussed in the results and discussion. In Figure 23 a
typical setup is presented for the headspace analysis of a coffee sample.
The stir bar is thereby positioned in a glass insert inside a vial above the sample.
Figure 23: Headspace analysis of coffee
- 35 -
Sample Extraction T Extraction time
Coffeea RT; 50°C; 70°C 0.5h; 2.5h
Whiskyb 70°C 0.5h
Bath creamc RT 3h
Pesticidesd RT 2.5h
a HB (Efico, Antwerp, Belgium) or Bruynooghe superior
(Miko, Turnhout, Belgium)
b Suntory pure malt Whisky 12 years (Japan)
c Yves Rocher (Kain, Belgium)
d Pesticide mix 34; 100 ng/µl acetonitrile (Dr Ehrenstorfer
GmbH, Germany)
Table 4: Conditions used for the extraction of different samples
Chromatographic instrumentation 5.5
All measurements were performed on an Agilent 7890A (Agilent Technologies, Little
Falls, DE, USA) equipped with an Agilent 5975c triple axis mass spectrometer (MS).
Additionally, a thermal desorption unit (TDU) was used to desorb the stir bars equipped with
a CIS 4 programmable temperature vaporizing inlet. Helium was used as carrier gas.
Chemstation (Agilent Technologies) in combination with Maestro® (Gerstel GmbH) were
used as software to operate the system. This controlled the MultiPurpose Sampler (MPS,
Gerstel GmbH) as well (Figure 24).
Depending on the application, different columns were used. One column was a DB-
5MS, 30 m long, with an internal diameter of 0.250 mm and a film thickness of 0.25 µm, with
an additional 10 m DuraGuard column. Temperatures between -60°C and 325°C could be
used on this type of column. The second column, a DB-WAX, 30 m long, with an internal
diameter of 0.250 mm and a film thickness of 0.25 µm was used. The minimal allowable
operating temperature (MiAOT) and maximum allowable operating temperature (MAOT) on
that column were 20°C and 250°C, respectively. Both columns were delivered by J&W
Scientific. Helium was used as carrier gas in both columns.
Thermal desorption was performed in a TDU. A temperature program was used,
starting at 35°C, followed by a raise up to 200°C or higher (depending on the experiment) at
- 36 -
a rate of 60°C/min. This maximum temperature was held for 5 minutes. The temperature in
the transfer line between the TDU and the PTV was 300°C and transport from the TDU to the
CIS was done in the splitless mode. Desorbed molecules were cryo-trapped in a PTV using
liquid nitrogen at -50°C. After desorption from the stir bar, the PTV was heated very fast
(12°C/s) to 300°C and the temperature was kept there for 8 minutes. Split-ratios of 1/50 to
1/20 were used to inject the sample into the column.
Different temperature programs for the GC column were used to obtain sufficient
separation for the best comparison between the different profiles obtained for each
extraction. All conditions are listed in appendix A.
The transfer line between the end of the column and the MS detector was set at
300°C. There was no solvent delay used and the MS scanned mass/charge ratios going from
40 to 400 amu.
Figure 24: GC-MS instrument equipped with TDU, CIS 4 and MPS
- 37 -
6. Results and discussion
Synthesis 6.1
The NMR spectrum (Figure 25) revealed the presence of some impurities. The most
important impurity was thereby unreacted imidazole (shift G-J). However, the presence of
residual imidazole couldn’t give problems during the polymerization reaction. This was
tested by adding an excess amount of imidazole to the monomer. This mixture still resulted
in the generation of polymers with similar viscosity as when imidazole was not added. GC
analysis revealed that the monomer mixture contained about 5% unreacted imidazole.
Other impurities were residual solvent (THF, shift S) and chloroform-d (shift S’).
The corresponding electron impact (EI) mass spectrum (Figure 26) showed a molecular
ion of 144 and 146 in a 3 to 1 ratio, characteristic for a chlorine containing molecule. This
molecular weight is in good agreement with the predicted molecular weight of 144.60. The
main spectral peaks within the mass spectrum are 81 and 82 amu, corresponding to the loss
of ethylchloride, with the possible formation of a methylimidazole intermediate and its
proton adduct. The ion with a mass over charge ratio of 56 corresponds to a degradation of
the latter 68.
Figure 25: NMR spectrum with interpretation
- 38 -
Figure 26: Mass spectrum of 1-chloropropyl-imidazole
Polymerization steps were carried out for 0.5h, 1h, 12h and 24h. Since low molecular
weight polymers possess lower glass transition temperatures compared to higher molecular
weight polymers, the shorter reaction times were focused on. It became clear that their
viscosity was very low compared to the higher molecular weight. Since viscosity needed to
be high enough to overcome dripping from the stir bar, 12h was used for further
characterisation. The polymer with chlorine as counter ion was a bright white powder. This
powder was insoluble in acetone, but completely soluble in water. After ion-exchange, both
NTf2 and PF6 polymers were no longer soluble in water, NTf2 formed an emulsion and PF6 a
precipitation. Poly-NTf2 dissolved easily in acetone, whereas poly-PF6 only upon heating. PF6
only dissolved well in DMSO at room temperature. The NTf2 polymer was still white, but
somewhat transparent. The PF6 polymer became blue-violet and very hard, while the NTf2
polymer had a softer texture, indirectly indicating that the Tg of NTF2 was closer to room
temperature.
0
10000
20000
30000
40000
50000
60000
70000
0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150
Co
un
ts
m/z ratio
Molecular ion
- 39 -
Characterization of the polymers 6.2
6.2.1 Thermogravimetric analysis
Through thermogravimetric analysis, the important thermal stability of polymers could
be measured. Below the TGA thermographs are represented of the polymer without prior
ion exchange (poly-Cl) and both polymers whereby the Cl- has been ion exchanged by PF6
and NTf2 (Figure 27).
An initial decrease below 100°C is visible in the thermograph of the polymer containing
chlorine as counter ion, indicating the presence of residual solvent, most likely water. The
hygroscopicity of the polymer could also be detected by the eye. Therefore 9% of the total
mass was measured as water and other impurities. Between 100°C and 250°C, the polymer
lost another 1% of its mass and at 250°C, the actual degradation process of the polymer
started, as can be seen in the thermograph. Only a fifth of the initial mass was left at 351°C.
From 400°C on, mainly inorganic residue was removed with increasing temperature.
The polymer containing hexafluorophosphate as counter ion depicted no significant
loss at low temperatures. No residual solvents or big amounts of impurities evaporated
below 250°C, as indicated by the point where the first 1% is lost (Table 5). The real polymer
degradation occurs between 300°C and 400°C. After 450°C, inorganic residue is removed
with increasing temperature.
Figure 27: TGA thermographs for the main chain polymer with different counter ions
0
20
40
60
80
100
0 100 200 300 400 500 600 700 800
We
igh
t (%
)
Temperature (°C)
Poly-Cl
Poly-PF6
Poly-NTf2
- 40 -
The best thermal stability was achieved on the polymer with
bis[(trifluoromethyl)sulfonyl]imide as counter ion. As with PF6, no significant losses were
detected at low temperatures, indicating no residual solvents or impurities. The first 1% of
the initial mass is lost at 360.8°C, which is about 100°C higher compared to PF6. Polymer
degradation occurred between 400°C and 470°C.
1% loss 5% loss
Poly-Cl 39.1°C 65.5°C Poly-PF6 247.5°C 306,6°C Poly-NTf2 360.8°C 415.0°C
Table 5: Temperatures were 1% and 5% of initial weight is lost
As mentioned in chapter 2 the extra thermal stability provided by the new counter ions
is coming from the fact that when chlorine is present as counter ion, nucleophilic attacks
from the chlorine can already happen at moderate temperatures, with polymer degradation
as a consequence 66. The hexafluorophosphate counter ion provided a lower thermal
stability, compared to NTf2, due to an anion-mediated mechanism which occurs at higher
temperatures compared to the nucleophilic attacks of chlorine 69. In this way the C-N bond is
cleaved. Poly-NTf2 only degraded due to decomposition of the polymer backbone along C-C
bonds. The imidazolium moiety has been reported to be stable up to 500°C 69.
6.2.2 Differential scanning calorimetry
DSC measurements were done on both poly-PF6 and poly-NTf2. Only second heating
curves are shown, as these give the more reproducible and therefore reliable results
compared to the first heating process. This is because after the first cycle, the surface
between the polymer and the aluminium pan is more homogeneous than before the first
heating. In this case, the cooling rate was set at 10°C/min. This implicated that controlled
crystallization could occur because of the slow cooling. This would give rise to an additional
and less informative peak in the thermograph during first cooling as a crystallization
transition and consequent a melting peak during second heating process.
However, the second heating DSC-thermograph of poly-PF6 (Figure 28) did not provide
any information on the glass transition temperature or any other transition. As mentioned
before, this polymer is very hard, implicating that its Tg is above room temperature. This
- 41 -
thermograph shows that the glass transition is even above 150°C. Therefore it must be
concluded that this polymer cannot be a very good candidate for sorptive extraction.
Sampling at temperatures above 150°C, to obtain the wanted extraction mechanism, is
difficult to implement as it could involve a lot of compound degradation and would lead to
boiling of aqueous matrices and is therefore not recommended.
The second heating thermograph of poly-NTf2 provided more information (Figure 29).
At first sight a large transition was visible between 20°C and 90°C. A corresponding transition
can also be seen in the cooling curve, indicating that this transition corresponds to the
crystallization of the polymer, taking place at the melting temperature (Tm). More important
for sorptive extraction is the transition visible at -15°C. Since there is a baseline shift during
this transition, this is a second-order transition. This implicates that this transition is the
glass transition and that this polymer should therefore be applicable for sorptive extraction
at room temperature, as it will be in its rubber state.
Figure 28:Second heating DSC thermograph of poly-PF6
- 42 -
Coating 6.3
Both poly-PF6 and poly-NTf2 were evaluated as coating material in this work. Poly-Cl
was thereby not used because it is hygroscopic and the thermal stability is too low compared
to the other two polymers to be of any use. First, both polymers, as well as mixtures of these
polymers with conventional PDMS and PEG, were coated on glass plates for some
straightforward but important physical testing. In the second part, these polymers were then
coated on stir bars equipped with a metal grid. For all coatings, polymerization reaction time
was 12 hours.
6.3.1 Glass plate
As mentioned in the experimental part, all polymers used were dissolved in another
solvent. Five different coatings were made. Both pure poly-PF6 and poly-NTf2 were coated.
Additionally, poly-NTf2 was also mixed with PDMS, dimethyl-(0.5-1%) methyl-vinylsiloxane
copolymer (VPDMS) and polyethylene glycol (PEG). For all mixtures a ratio of 1:1 was
thereby used. Although acetone and hexane are completely miscible, when adding the poly-
NTf2 containing acetone solution to the PDMS containing hexane solution, phase separation
occurred. For this reason, the subsequent solution was shaken before being used for coating
purposes.
Figure 29: Second heating DSC thermograph of poly-NTf2, with first cooling inserted
Tg
- 43 -
These polymer solutions were then placed on a microscopic slide with a syringe and
dried in an oven for 0.5h under nitrogen atmosphere, in order to evaporate the solvent. To
all coatings except pure poly-NTf2, a temperature of 150°C was applied in order to evaporate
DMSO (for poly-PF6) or to assure crosslinking of PDMS or PEG. Poly-PF6 was also coated by
partially dissolving the polymer in acetone with subsequent solvent evaporation at 60°C.
Pictures of these coatings are shown in Figure 30. It can thereby be seen that the coating for
poly-PF6 in acetone was not as homogeneous as when the same polymer was dissolved in
DMSO. This was probably due to the fact that the hot acetone was already slowly cooling
down in the syringe and in this way was decreasing the solubility of the polymer leading to
an inhomogeneous coating. In any case, all poly-PF6 coatings were very brittle and came of
the glass slide easily. The poly-NTf2 coatings were more robust and stuck very well to the
glass. With the poly-NTf2 mixtures with PDMS, VPMDS and PEG transparent, sticky coatings
were obtained.
Afterwards these glass plates were immersed in water to simulate the conditions
under which sorptive extraction normally takes place. All polymers were immersed in water
for 30 minutes at room temperature. It could thereby be observed that poly-PF6 was
released from the glass plate and a thin, very brittle film remained in the water. Also the PEG
+ poly-NTf2 coating was easily released from the glass plate under these conditions, most
probably because polyethylene glycol is quite soluble in water. This also implicated that
poly-PF6 and the mixture with PEG cannot be used in immersion SBSE and only headspace
sorptive extraction remained a possibility. For the coatings (pure poly-NTf2, poly-NTf2/PDMS
and poly-NTf2/VPDMS) that are still eligible for immersive sorptive extraction, no significant
Figure 30: coated microscopic slides with various polymers
- 44 -
weight increase was noted after immersion, indicating virtually no water uptake by these
polymers.
As a final test, poly-NTf2 was coated on a narrow glass plate that fits in the liner of a GC
inlet. 20 mg of the polymer was therefore dissolved in 50 µl acetone. This solution was
added dropwise to the glass plate. The acetone was evaporated at 70°C in an oven and the
glass plate was introduced into a cold GC liner (60°C). The GC inlet was then heated to 200°C
and this temperature was held for 10 minutes. The inlet was then allowed to cool down and
the liner, containing the glass plate was removed. However, due to the higher temperature,
the polymer became too fluid and dripped off the glass plate.
In an attempt to increase grip of the polymer to the glass, the glass plate was therefore
first etched using a 3 M NaOH solution. The glass plate was inserted for 3 hours in this
solution and afterwards rinsed with water and dried at 80°C for 2 hours. The same
procedure for coating this pre-treated glass plate was used, with the same GC inlet setup.
However, even under these conditions the polymer didn’t sufficiently stick to the glass plate.
From these first, straightforward experiments some important conclusions could be
drawn. Some polymers were not compatible with water, making immersion SBSE not
possible. More importantly was the observation that at elevated temperatures, the polymer
was too fluid to stick to glass, making it necessary to rely on other ways of immobilization.
6.3.2 Stir bars
The proposed solution to overcome the problems concerning the fluidity of the
polymer at elevated temperature, was the use of stir bars containing a metal grid composed
out of stainless steel. These stir bars have already been used for coatings based on
polyacrylates 26 and are now commercially available with an PDMS/EG coating. All polymer
mixtures were coated on a stir bar with an empty metal grid. The possible benefits of this
metal grid are that its high surface and different chemical composition could help to prevent
the polymer from flowing away from the stir bar when higher temperatures are applied. The
polymer solution was thereby added dropwise while evaporating the solvent regularly,
preventing overloading of the metal grid. The solvent was then evaporated with a nitrogen
flow. When half of the solution was added, the solvent was also more efficiently removed by
heating the stir bar to 60°C under a nitrogen flow. Depending on the polymer solution which
was applied, different amounts could be immobilized on the grid as can be seen in Table 6.
- 45 -
For some later experiments, smaller amounts were coated. It could be visually detected
when the metal grid was fully loaded. On some of the stir bars coated with a PDMS mixture,
the solvent evaporated faster than the viscous solution could enter the grid. This results in
stir bars where the polymer was present around the grid.
Average weight
Poly-PF6 25.5 mg
Poly-NTf2 47 mg
Poly-NTf2 + PDMS (1:1) 18 mg
Poly-NTf2 + VPDMS (1:1) 21.5 mg
Poly-NTf2 + PEG (1:1) 27 mg
Table 6: Average weights of the coatings for fully loaded stir bar
Conditioning 6.4
Before using the stir bars for analysis, all five stir bars were conditioned twice by
introducing them in the TDU where the temperature was increased to 200°C and GC analysis
of what was released from the stir bar (bleeding) was performed with method 1 (Appendix
A). In this way, the stir bar was cleaned by removing any residual solvents and other
impurities and so further information on the stability could be obtained.
6.4.1 Poly-NTf2 coated stir bar
The stir bar coated with poly-NTf2 resulted in the chromatogram shown in Figure 31
after a first and second conditioning step. During the first 2 minutes, the volatile residual
solvent (acetone) eluted and after 10 minutes some impurities coming from (manual)
handling the stir bar eluted, including Z-octa-9-decenamide (13.2 min) and squalene (15
min). Z-octa-9-decenamide probably leached out of polypropylene plastic 70 while squalene
is ubiquitously present in the stratum corneum and can therefore be easily picked up by the
stir bar when handling it 71. After the second conditioning, all these contaminations were
greatly reduced except for 9-octadecenamide-(Z), which remained present in the
chromatogram. However, during further usage of the stir bar, this peak continued to
decrease. In contrast to what was observed on the glass plates, no visual changes could be
- 46 -
observed on the stir bar after it went through these various temperature cycles, indicating
that the stir bar equipped with the steel grid could function as a stable support for the NTf2
based polymer
Figure 31: Chromatogram for conditioned poly-NTf2 coated stir bar (A = acetone; B = n-hexadecanoic acid; C = hexadecanamide; D = 9-octadecenamide; E = squalene)
6.4.2 Poly-PF6 coated stir bars
When conditioning poly-PF6 coated stir bars, initially and as expected large amounts of
DMSO eluted from the system (Figure 32). However, more importantly during this process it
appeared that the PF6 base polymer was not immobilized in the steel support in a sufficient
way and leakages occurred during conditioning. Some polymer was thereby released in the
glass tube, where the stir bar was positioned in during thermal desorption. Therefore this
type of stir bar was not usable for extraction purposes. For these reasons, together with the
(as explained above) undesired Tg, no further analysis have been done with these coatings.
0
2
4
6
8
10
12
14
0 5 10 15
Co
un
ts (
10
6 )
Time (min)
conditioning 1
conditioning 2
A
B C
D
E
- 47 -
Figure 32: Chromatogram for conditioned poly-PF6 coated stir bar (A = DMSO)
6.4.3 PDMS/poly-NTf2 and VPDMS/poly-NTf2 coated stir bars.
Both stir bars coated with PDMS/poly-NTf2 and VPDMS/poly-NTf2 mixtures provided
similar chromatograms. In Figure 33, chromatogram corresponding to the first conditioning
for VPDMS is shown. After 12 minutes, several oligomeric siloxanes were thereby eluting. As
PDMS is less easily crosslinked by DCP than VPDMS, the latter should have depicted less of
these oligomeric siloxanes, but this was not observed. Also in later conditioning steps, these
oligomeric structures were still present. It can therefore be hypothised that either the
crosslinking reaction wasn’t performed properly or that the metal wire, serving as support
material on the stir bar wasn’t entirely inert and therefore led to catalytic degradation of
these polymers. Based on these results, the decision was made to discontinue experiments
making use of PDMS/PIL mixtures.
0
5
10
15
20
25
30
0 5 10 15
Co
un
ts (
10
6 )
Time (min)
conditioning 1
conditioning 2
A
A
- 48 -
Figure 33: Chromatogram for conditioned VPDMS with poly-NTf2 (1:1) coated stir bar
6.4.4 PEG/poly-NTf2 coated stir bars
The chromatograms of the stir bars coated with the mixtures of PEG and poly-NTf2
showed a large number of artefacts after the first conditioning, which were however gone
after the second conditioning. (Figure 34). Again Z-octa-9-decenamide and squalene, but
also many other solutes with no conclusive mass spectrum for identification were thereby
comprised. All impurities were, however, gone after second conditioning. These stir bars
were therefore also considered valuable candidates for further use.
Figure 34: Chromatogram for conditioned PEG with poly-NTf2 (1:1) coated stir bar (A = Z-octa-9-decenamide; B = squalene)
0
1
2
3
4
5
6
7
8
9
10
0 5 10 15
Co
un
ts (
10
6 )
Time (min)
oligomeric siloxanes
0
0,5
1
1,5
2
2,5
3
3,5
4
0 5 10 15
Co
un
ts (
10
6)
Time (min)
conditioning 1
conditioning 2
A B
- 49 -
6.4.5 Selecting the best conditioning conditions
It should also be noted that both stir bars (coated with poly-NTf2 or with PEG/poly-
NTf2), that were still usable at this stage, showed no notable weight loss after these two
conditioning steps. The stir bar coated with poly-NTf2 was further conditioned at 220°C and
260°C to check if thermal stability was maintained on the metal grid even at higher
temperatures, because higher desorption temperatures are needed in order to desorb all
extracted compounds due to some adsorption effect or due to the presence of a broader
number of intermolecular forces than is the case with PDMS based stir bars. This experiment
was performed using method 4 (Appendix A), with different TDU end temperatures, in this
case 220°C and 260°C. From Figure 35 it is clear that at 220°C, no degradation occurs as no
additional peaks were visible. On the other hand, at 260°C, a large number of peaks
appeared. None of them depicted a conclusive mass spectrum, indicating that it is likely that
the poly-NTf2 already started to degrade at 260°C. This was somewhat strange, compared to
the TGA results as these indicated that the polymer is stable until about 400°C. It should be
mentioned that this stir bar was heated and cooled several times before performing the
conditioning at 260°C which might have weakened the structure. Phillips et al. described that
steel, as is present in the metal grid, can react at higher temperatures with the counter ion,
with the formation of FeF2 72. The loss of the counter ion then deteriorates the polymer
structure, as only the cations are left over, with a huge Coulomb repulsion effect as a result,
leading to pyrolysis and breaking of the polymer.
Figure 35: Conditioning of a poly-NTf2 coated stir bar at different temperatures
0
2
4
6
8
10
12
14
16
0 5 10 15 20 25 30 35
Co
un
ts (
10
6 )
Time (min)
220°C
260°C
- 50 -
Compared to a conditioned commercially available PDMS stir bar (Appendix B), stir
bars coated with pure poly-NTf2 as well as with the mixture of PEG and poly-NTf2 showed
acceptable background after the second conditioning step at 200°C. The stir bar coated with
PEG/NTf2 was therefore not conditioned at 220°C, as the PEG will start to degrade around
this temperature. Although 200°C is considered a bit too low for complete desorption, all
following desorptions were done at 200°C, with some exceptions at 220°C. The polymers
were kept in place by the metal grid even at 200°C, as it was supposed to do, but
degradation phenomena occur.
Extraction of coffee 6.5
Grounded coffee is an interesting sample on which the new synthetic polymers can be
tested on their extraction capacity because it contains a variety of polar molecules, with log
Ko/w values below 2 such as pyrazines, furanmethanol and acetic acid. PDMS coated stir bars
are not able to extract these compounds. For example Bicchi et al. were in this way able to
extract these compounds up to 6 times more efficiently using dual phase stir bars (cf supra)
compared to conventional stir bars.
In a first experiment, a stir bar coated with only 8 mg of poly-NTf2 was conditioned at
200°C. Headspace extraction performed on 0.5 gram of grounded coffee (Bruynooghe) for
2.5 hours at room temperature was compared to what was possible when using a PDMS
based stir bar. Thermal desorption was done at 200°C for poly-NTf2 and 260°C for the
conventional PDMS twister. The analyses were performed making use of method 4,
described in appendix A. Some typical compounds like pyridine (A), methyl pyrazine (B),
acetic acid (F) and 2-furanmethanol (I) (Figure 36, Table 7) were watched more closely as
these are typical compounds with low log Ko/w which could particularly benefit from the
improved extraction efficiency of PIL based stir bars. In Appendix C, the corresponding log
Ko/w values of some of these molecules are listed.
- 51 -
Both pyridine (A) and methyl-pyrazine (B) were more efficiently extracted by the PDMS
stir bar compared to the PIL stir bar. More promising results were measured for 2-
furanmethanol (I), where equal intensity was achieved, and acetic acid (F), which showed a
5-fold improvement in extraction efficiency on the poly-NTf2 based stir bar compared to the
PDMS based one. Additionally it should be noted that about 3 times less coating material
was used in this experiment compared to the amount of coating on a conventional PDMS stir
bar. This makes especially the results for 2-furanmethanol and acetic acid promising.
Poly-NTf2
Pyridine 33.0
Methyl-pyrazine 29.8
Acetic acid 522.2
2-furanmethanol 122.4
Table 7: Comparison between a PDMS coated stir bar and a poly-NTf2 coated stir bar for the analysis of coffee (PDMS = 100 as reference)
0
1
2
3
4
5
6
7
8
9
10
7 9 11 13 15
Co
un
ts (
10
6 )
Time (min)
PIL
PDMS
A
B
C D
Figure 36: Extraction of coffee with PIL stir bar and PDMS stir bar (A = pyridine; B = methyl-pyrazine; C = 2,5-dimethyl-pyrazine; D = 2,6-dimethyl-pyrazine; E = 2-ethyl-6-methyl pyrazine; F = acetic acid;
G = 2-furanmethanol acetate; H = 5-methyl-2-furaldehyde; I = 2-furanmethanol)
E
F
I
G
H
- 52 -
In a second experiment, 2 gram of coffee (Bruynooghe) was analysed with both poly-
NTf2 (48 mg) and PEG/poly-NTf2 mixture (25 mg) coated stir bar (Figure 37). Headspace
extraction was done at 50°C for 30 minutes while desorption was done at 200°C for both stir
bars, using method 1 (cf appendix A). As can be seen in Figure 37, virtually no extraction was
achieved with the pure NTf2 polymer coated stir bar. Probably, some degradation
(mentioned before) occurred over time, deteriorating the extractive capacity of the polymer.
The polymer mixture of PEG and poly-NTf2 could still achieve some extraction, but overall
less extraction was measured compared to conventional PDMS coated stir bars. One
exception here is acetic acid, which is twice as well extracted by the PEG/PIL coated stir bar
compared to the PDMS based one.
Figure 37: Extraction of coffee with different stir bars (A = acetic acid; B = 5-methyl-2-furancarboxaldehyde; C = 2-furanmethanol, acetate)
In a final experiment using coffee as sample, the extraction of a stir bar coated with 29
mg of PEG/NTf2 was compared to a PDMS coated stir bar (Figure 38, Table 8). The extraction
was performed on 2 gram of coffee (HB) for 30 minutes at 70°C with subsequent analysis
using method 4 with the desorption at 200°C. Acetic acid (E) and 2-furanmethanol (H) were
thereby extracted significantly better with the polymer mixture compared to the PDMS stir
bar, whereby a 20-fold improvement was observed for the extraction of the former
compared to the use of conventional PDMS based stir bars. Other compounds were equally
extracted.
0
0,2
0,4
0,6
0,8
1
1,2
1,4
0 2 4 6 8 10
Co
un
ts (
10
6)
Time (min)
PDMS
NTf2
PEG/NTf2
A
A
C
B
- 53 -
Figure 38: Extraction of coffee with PEG/NTf2 stir bar and PDMS stir bar (A = pyridine; B = methyl-pyrazine; C = 2,5-dimethyl-pyrazine; D = 2,6-dimethyl-pyrazine; E = acetic acid; F = 2-furanmethanol acetate;
G = 5-methyl-2-furaldehyde; H = 2-furanmethanol)
PEG/Poly-NTf2
Pyridine 93.5
Methyl-pyrazine 71.5
Acetic acid 2339.3
2-furanmethanol 233.2
Table 8: Comparison between a PDMS coated stir bar and a PEG/poly-NTf2 coated stir bar for the analysis of coffee (PDMS = 100 as reference)
When extracting coffee, it became clear that poly-NTf2 shows potential to be a good
alternative for PDMS for the analysis of polar solutes. With equal amounts of coating,
comparable extraction efficiencies can be obtained for all solutes, with some significantly
better extractions , for example for 2-furanmethanol and acetic acid. The group of Anderson
demonstrated that PILs could be used for the selective extraction using SPME of esters
whereby better extraction efficiency was obtained than is possible with PDMS coated SPME
fibers 61. Imidazolium based ionic liquids are also used for the extraction of CO2 in air for
filtration applications 73. CO2 is efficiently extracted due to favourable interactions between
the CO2 molecule and the imidazolium ring and its counter ion. Esters, CO2 and carboxylic
acid all possess the same structural elements, which appear to depict favourable interactions
0
10
20
30
40
50
60
70
9 10 11 12 13 14 15 16 17
Co
un
ts (
10
6 )
Time (min)
PEG/NTf2
PDMS
E
H
A
B
C D
F
G
- 54 -
with the imidazolium ring. This could explain the very satisfactory extraction of acetic acid
with PIL based stir bars compared to the use of standard twisters.
Analysis of whisky 6.6
Whisky has been analysed because of its interesting solutes as it contains mainly
esters. As mentioned above, imidazolium based ionic liquids could possess a great affinity for
the ester moiety, which has been described already when using PIL coated SPME fibers for
the analysis of esters in wine 61. These PILs contained the imidazolium moiety in the side
chain, while here the imidazolium moiety is in the main chain.
For this experiment, the mixture of PEG and poly-NTf2 was used in headspace as PEG
could help the extraction in this sample. The headspace of 1 ml of whisky was extracted for
30 minutes at 70°C and subsequently desorbed at 200°C using method 2 (Appendix A). In
Figure 39 ethanol (A), 3-methyl-1-butanol (B) and a range of ethyl esters (C-H) were eluted.
These esters ranged from the semi-polar ethyl hexanoate to the nonpolar ethyl
hexadecanoate. All these esters were extracted, even though ethanol is the main solute and
competition could occur in favour of ethanol. Desorption wasn’t performing very well as for
ethyl octanoate (D) and ethyl decanoate (E) some peak splitting occurred. This could have
occurred due to the low desorption temperature of 200°C.
Figure 39: Chromatogram for the analysis of Japanese whiskey with PEG/poly-NTf2 coated stir bar (A = Ethanol; B = 3-methyl-1-butanol; C = hexanoic acid ethyl ester; D = octanoic acid ethyl ester; E = decanoic acid ethyl ester;
F = dodecanoic acid ethyl ester; G = tetradecanoic acid ethyl ester; H = hexadecanoic acid ethyl ester)
0
1
2
3
4
5
6
7
8
9
0 5 10 15 20
Co
un
ts (
10
6 )
Time (min)
A
B
C
D
D
E
F
G H
- 55 -
Analysis of bath cream 6.7
Bath cream is also an interesting representative sample as it contains a lot of different
samples, both very polar and less polar ones. A lot of these compounds are thereby volatile
as they are used as fragrance, meaning that these solutes are excellent to be analysed in
combination with headspace sorptive extraction (HSSE).
For the analysis of the headspace of bath cream, 1.1 gram was weighted in a
headspace vial, to which 200 µg benzyl alcohol, which has a low vapour pressure, was
added. The stir bar was inserted and extraction was done for 3 hours at room temperature.
A comparison between a stir bar coated with poly-NTf2 (20 mg) and a standard PDMS twister
was made. Analysis was done using method 4, with a maximum desorption temperature of
200°C for poly-Ntf2 and 260°C for PDMS. In Figure 40 the chromatogram for both stir bars is
shown. However, most compounds were slightly less extracted by the PIL stir bar. Only
benzaldehyde (A) (retention time = 13.05) was extracted with similar efficiency and benzyl
alcohol (B) (retention time = 17.07) was significantly better extracted by the poly-NTf2
coated stir bar (Table 9).
Figure 40: Analysis of bath cream with PIL stir bar and PDMS stir bar (A = benzaldehyde; B = benzoic acid methyl ester; C = p-menth-1-en-8-ol; D = acetic acid, benzyl ester; E = benzyl alcohol; F = 2-phenyl-ethanol
G = 2-(phenylmethylene)-heptanal)
0
10
20
30
40
50
60
10 12 14 16 18 20
Co
un
t (1
06 )
Time (min)
PIL
PDMS
A
B C
D
E
F
G
- 56 -
Table 9: Comparison between a PDMS coated stir bar and a PEG/poly-NTf2 coated stir bar for the analysis of bath cream (PDMS = 100 as reference)
Pesticide analysis 6.8
The analysis of pesticides have earned a lot of research over the last years 74, 39, 23, 38.
Pesticides are toxic compounds for animals, and as they dissolve well in water, problems
concerning the toxicity of water streams is important. For this reason pesticides need to be
monitored as sensitively as possible in a reliable way, and for this reason the applicability of
PIL coated stir bars for pesticide extraction was investigated here.
From a pesticide mixture, containing 27 compounds with a concentration of 100 ng/µl
in acetonitrile, 1 µl was added to 10 ml tap water, resulting in a final concentration of 10
ppb. On this sample, immersion stir bar sorptive extraction was performed using a poly-NTf2
coated stir bar. For comparison, a PDMS twister was used and 1 µl of the pure pesticide
mixture was injected. SBSE was performed for 2.5 hours at room temperature. Thermal
desorption of poly-NTf2 was done at 200°C and for the PDMS twister at 260°C. The
separation was done according to method 4.
Figure 41: Extraction of pesticides by PIL stir bar, PDMS stir bar and comparison with liquid injection (A = propham; B = crimidine; C = chlorpropham; D = propazine; E = terbutylazine; F = metolachlor; G = atrazine; H = prometryn;
I = terbutryn)
0
2
4
6
8
10
12
14
16
18
20
20 22 24 26 28 30
Co
un
ts (
10
6)
Time (min)
Liquid
PDMS
PIL
A B C D
E+F
G H
I
PEG/NTf2
Benzaldehyde 110.5
Benzylalcohol 370.4
- 57 -
The method used was able to separate 15 pesticides. PDMS was able to extract 9 of
these. The extraction efficiency of PDMS was below 23% for 7 out of 9 analytes, only for
chlorpropham (24.33 min) and terbutryn (26.31 min) the efficiency was above 50%. Poly-
NTf2 could only extract 5 of the pesticides, but all with a very low extraction efficiency as can
be seen in Table 10. Possible carry over from previous analyses could influence these results.
This may be the case for prometryn and terbutryn, which are very similar structures with
comparable polarity. A reason why the poly-NTf2 coated stir bars weren’t performing very
well in solution hasn’t been established yet as this coated stir bar didn’t go through any heat
cycle, so degradation could not have occurred yet. However, it is possible that due to
unfavourable log D (distribution coefficient), which is similar to the log P but taking into
account the pH of the solution, extraction was not possible under these conditions with the
synthesized polymeric coating.
PDMS/liquid PIL/liquid PIL/PDMS
Propham / 3.7% /
Chlorpropham 63.1% 10.7% 17.0%
Metolachlor 21.2% 8.4% 39.3%
Prometryn 32.5% 1.6% 4.9%
Terbutryn 51.6% 14.2% 27.4%
Table 10: Comparison between PIL stir bar, PDMS stir bar and liquid injection
- 58 -
7. Conclusion and future perspective
The extraction of polar solutes using commercially available stir bars through sorptive
extraction has long been challenging because polydimethylsiloxane, which is mostly used as
extractive phase possesses unfavourable characteristics towards the extraction of polar
solutes. For this reason, a new type of polymer, applicable for sorptive extraction, was
developed during the course of this work. It consists of an interlinked imidazolium based
cationic polymeric backbone. This polymer depicted excellent thermal stability, ranging
between 300-400°C depending on the type of counter ion used. The PIL-NTf2 combination
proved particularly useful as it depicts a sub-ambient glass transition temperature
(Tg = -15°C), making it ideal for room temperature sorptive extraction in combination with
the highest measured thermal stability, allowing efficient thermal desorption.
Stir bars coated with poly-NTf2 were in this way successfully used for the analysis of
the headspace of e.g. coffee and whisky, whereby it was observed that particularly for the
extraction of esters and carboxylic acids, this new extractive phase could outcompete the
commercial PDMS based stir bars. In this way acetic acid could, for example, be extracted 50
times more efficiently using a PEG/NTf2 based-coating. However, thus far a correlation
between the extraction efficiency and the log Ko/w couldn’t be established and in immersion,
the performance of the polymer proved to drop dramatically.
Due to viscosity reasons, the polymer required immobilization on a steel grid.
However, the latter, in combination with elevated temperatures, sometimes gave rise to
polymer degradation. Although exact degradation patterns need a more detailed study, it
thereby appears that especially the counter ion reacts with iron support when heated. In this
way the coating could not be used yet in a reproducible way. A possible solution for this
problem can be mixing of the synthesised polymer with PDMS, as in this way, viscosity
problems can be solved and the steel grid is not required anymore. New supporting
materials should be looked into as well such as covalent bonding of the polymer to the glass
wall of a stir bar or the problem could be addressed through polymer synthesis including the
use of crosslinkers or of block copolymer designs.
Although the proof of principle of ionic liquid coated stir bars could effectively be
demonstrated in this work, further research is necessary to improve the robustness and
practicality of this type of new stir bars. The various IL immobilisation approaches are
- 59 -
therefore expected to provide additional mechanical stability and increased polymer
viscosity.
- 60 -
8. Appendix A: Methods
Method 1
Column DB-5MS
Purge flow to split vent 60 ml/min
Flow in Column 1.2 ml/min
Split ratio 1/50
Temperature program 50 °C (1 min) 20 °C/min 320 °C (2 min)
Run time 16.5 min
Method 2
Column DB-5MS
Purge flow to split vent 25 ml/min
Flow in Column 1 ml/min
Split ratio 1/25
Temperature program 50 °C (1 min) 10 °C/min 200 °C (0 min)
25 °C/min 300°C (0 min)
Run time 20 min
Method 3
Column DB-WAX
Purge flow to split vent 20 ml/min
Flow in Column 1 ml/min
Split ratio 1/20
Temperature program 40 °C (5 min) 10 °C/min 250 °C (4 min)
Run time 30 min
- 61 -
Method 4
Column DB-WAX
Purge flow to split vent 50 ml/min
Flow in Column 1 ml/min
Split ratio 1/50
Temperature program 40 °C (5 min) 10 °C/min 250 °C (9 min)
Run time 35 min
- 62 -
9. Appendix B: Additional representative chromatograms
Figure 42: Chromatogram for a conditioned PDMS stir bar using method 1
Figure 43: Chromatogram for a conditioned empty TDU tube using method 4
0
0,1
0,2
0,3
0,4
0,5
0,6
0 5 10 15
cou
nts
(1
06 )
Time (min)
0
0,5
1
1,5
2
2,5
3
3,5
0 5 10 15 20 25 30
cou
nts
(1
06 )
Time (min)
- 63 -
10. Appendix C: Molecules and their log Ko/w value
Name Structure Log Ko/wa
2,5-dimethyl-pyrazine
N
N
1.03
2,6-dimethyl-pyrazine
N
N
1.03
2-ethyl-6-methyl
pyrazine
N
N
1.53
2-furanmethanol
O
HO
0.45
2-furanmethanol
acetate O
O
O
1.45
2-phenyl-ethanol HO
1.57
2-(phenylmethylene)-
heptanal O
4.33
5-methyl-2-
furaldehyde
OO
1.38
- 64 -
3-methyl-1-butanol
HO
1.26
Acetic acid
O
HO
0.09
Acetic acid benzyl
ester
O
O
2.08
Atrazine
N
N
N
Cl
NH
NH
2.82
Benzaldehyde O
Benzoic acid methyl
ester
O
O
1.83
Benzyl alcohol HO
1.08
Chlorpropham
NH
O
O
Cl
3.30
- 65 -
Crimidine
N
NCl N
1.31
Ethanol HO
-0.14
Ethyl decanoate O
O 8
4.79
Ethyl dodecanoate O
O 10
5.78
Ethyl
hexadodecanoate
O
O 14
7.74
Ethyl hexanoate O
O 4
2.83
Ethyl octanoate O
O 6
3.81
Ethyl
tetradodecanoate
O
O 12
6.76
Metolachlor
N
O
Cl
OCH3
3.24
- 66 -
Methyl pyrazine
N
N
0.49
p-menth-1-en-8-ol
OH
3.33
Prometryn
N N
NS
HN
HN
3.73
Propazine
N
N
N
Cl
NH
NH
3.24
Propham
NH
O
O
2.66
Pyridine N
0.80
- 67 -
Terbutryn
N N
NS
HN
HN
3.77
Terbutylazine
N
N
N
Cl
NH
NH
3.27
a calculated using EPI suite KOWWIN v1.68
- 68 -
11. List of abbreviations
β phase ratio
µLLE micro liquid liquid extraction
AIBN 2,2′-Azobis(2-methylpropionitrile
AMIM 1-allyl-3-methylimidazolium
Amu atomic mass unit
BSTFA N,O-bis(trimethylsilyl)trifluoroacetamide
BTEX Benzene, toluene, ethylbenzene, xylene
CIS Cooled injection system
DCM Dichloromethane
DCP Dicumylperoxide
DMSO Dimethylsulfoxide
DSC Differential scanning calorimetry
ECF Ethylchloroformate
EG Ethylene glycol
EI Electron impact
FAMEs Fatty acid methyl esters
GC Gas chromatography
HPLC High performance liquid chromatography
HSSE Headspace sorptive extraction
IL Ionic liquid
IM Imidazolium
Ko/w Partition coefficient between octanol and water
Kpdms/w Partition coefficient between PDMS and water
LiH Lithium hydride
LLE Liquid liquid Extraction
LOD Limits of detection
MAOT Maximum allowable operation temperature
MS Mass spectrometry
MTBE Methyl-tert-butylether
MTPIM 1-methyl-3-(3-trimethoxysilylpropyl)imidazolium
- 69 -
NMR Nuclear magnetic resonance
NTf2 Bis[(trifluoromethyl)sulfonyl]imide
PA Polyacrylates
PAH Polycyclic aromatic hydrocarbon
PBA Polybutylacrylate
PCB Polychlorinated biphenyl
PDMS Polydimethylsiloxane
PEG Polyethylene glycol
PF6 Hexafluorophosphate
PFBHA Pentafluorobenzylhydroxylamine
PIL Polymerized ionic liquid
PTV Programmed temperature vaporization
PU Polyurethane
PVBnHexIM Poly-vinylbenzyl-hexyl-imidazolium
PVI Polymerized vinylimidazole
RTIL Room-temperature ionic liquid
SBSE Stir bar sorptive extraction
SPE Solid phase extraction
SPME Solid phase micro extraction
TD Thermal desorption
TDS Thermal desorption system
TDU Thermal desorption unit
Tf Flow temperature
TfO Trifluoromethanesulfonate
Tg Glass transition temperature
TGA Thermogravimetric analysis
THF Tetrahydrofuran
Tm Melting temperature
TOF Time of flight
VI Vinylimidazole
VPDMS Dimethyl-(0.5-1%) methyl-vinylsiloxane copolymer
VPL Vinylpyrrolidone
- 70 -
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Ontwikkeling van Sorptieve Extractietechnieken op Basis van Geïmmobiliseerde
Ionische Vloeistoffen voor Selectieve Chromatografische Analyse
K. Roelevelda, M. De Vrieze
a, F. David
a,b, F. Lynen
a
a Laboratorium voor Scheidingstechnieken, Vakgroep Organische Chemie, UGent, Krijgslaan
281 (S4), 9000 Gent, België b
Research Institute for Chromatography, President Kennedypark 26, 8500 Kortrijk, België
De extractie van polaire componenten met behulp van stir bar sorptive
extraction is uitdagend aangezien enkel polydimethylsiloxaan als
stationaire fase commercieel verkrijgbaar is en deze apolaire
eigenschappen bezit. Daarom worden constant nieuwe stationaire fases
ontwikkeld die een betere affiniteit vertonen voor polaire analieten. Het
probleem van de huidige gesynthetiseerde extractieve fases is dat geen
enkele verenigbaar is met thermische desorptie wegens een te lage
thermische stabiliteit. In dit werk wordt een nieuwe extractieve fase
gesynthetiseerd die gebaseerd is op ionische vloeistoffen. Deze
polymeren hebben een zeer hoge thermische stabiliteit en een polair
karakter.
Trefwoorden: gepolymeriseerde ionische vloeistoffen, stir bar sorptive extraction
Inleiding
De laatste decennia zijn staalvoorbereidingstechnieken steeds belangrijker geworden bij elke
analytische procedure. De instrumentatie is namelijk zo snel en nauwkeurig, dat de
voorbereiding van het staal vaak de zwakke schakel is bij een dergelijke procedure. De
staalvoorbereiding is vaak een arbeidsintensief onderdeel, waarbij nog al te vaak
onzuiverheden worden ingevoegd. Daarenboven zijn voor veel van deze technieken veel
solventen nodig, wat de natuur niet ten goede komt. Staalvoorbereidingstechnieken op basis
van sorptieve extractie bieden hier een voordeel, aangezien ze, in combinatie met thermische
desorptie, geen solventen vereisen.
Sorptieve extractie is gebaseerd op het absorberen of oplossen van analieten in een polymeer.
Hiervoor dient het polymeer in de rubberfase voor te komen, met andere woorden: de
temperatuur bij extractie moet hoger zijn dan de glastransitie temperatuur (Tg)van het
polymeer. Absorptie is een reversibel proces waarbij, door ofwel verhoogde temperatuur,
ofwel door middel van een solvent, de geabsorbeerde moleculen kunnen gedesorbeerd
worden. Bij desorptie door verhoogde temperatuur, ook wel thermische desorptie genoemd,
worden de analieten verdampt uit de polymeer fase, waarna ze kunnen geanalyseerd worden
door middel van gaschromatografie.
De twee voornaamste technieken gebaseerd op sorptieve extractie zijn vaste-fase
microextractie (SPME) en stir bar sorptive extraction (SBSE). Bij SPME is een vezel gecoat
met het extraherende polymeer. Deze vezel wordt ofwel in de oplossing (immersion SPME)
ofwel in de gasfase boven het staal (headspace SPME) gebracht. Hier vindt dan partitie plaats
tussen de vaste/vloeistof/gasfase en de polymeerfase, waarbij de analieten met de meeste
affiniteit voor het polymeer meer zullen geëxtraheerd worden dan deze met weinig affiniteit.
Bij SPME vindt de thermische desorptie plaats in de inlet van een gaschromatograaf, waarna
de analyse volgt. Op stir bars kan een grotere hoeveelheid extraherende fase aangebracht
worden waardoor meer analieten kunnen opgenomen worden, wat de detectielimiet ten goede
komt. Ook met stir bars is het mogelijk om zowel in oplossing (SBSE) als in de gasfase
boven het staal te extraheren (headspace sorptive extraction, HSSE).
Commercieel beschikbare stir bars zijn gecoat met polydimethylsiloxaan (PDMS). PDMS
beschikt over een goede thermische stabiliteit (tot 320°C) en past uitstekend in toepassingen
voor sorptieve extractie, daar de glas transitie temperatuur van dit polymeer -125°C bedraagt.
De extractie met PDMS kan goed voorspeld worden aan de hand van de partitie coëfficiënt
tussen octanol en water (log Ko/w). Er werd reeds aangetoond dat deze coëfficiënt een sterke
correlatie vertoont met partitiecoëfficiënt tussen PDMS en water (immersion SBSE/SPME,
log Kpdms/w) (1) en met de partitiecoëfficiënt tussen PDMS en lucht (headspace analyse, log
Kpdms/air) (2). Door het grotere volume polymeer op een stir bar in vergelijking met een SPME
vezel, verschuift de faseverhouding β (= Vw/VPDMS). Aan de hand van (1), met stijgende
hoeveelheid polymeer en dus dalende β, zal voor een bepaalde component, met gekende log
Ko/w, het extractiepercentage η stijgen. Hierdoor is het mogelijk om meer polaire
componenten toch te kunnen extraheren met SBSE, waar dit niet meer mogelijk is met SPME.
Toch is het met stir bars, gecoat met PDMS, niet mogelijk om analieten met een log Ko/w
lager dan 3 op voldoende representatieve wijze te extraheren.
η =
m
m m =
β o
[2]
Om deze reden worden nieuwe polymeerfasen ontwikkeld die betere eigenschappen vertonen
om meer polaire moleculen te kunnen extraheren. Enkele voorbeelden hiervan zijn
polyacrylaten, polyurethaanschuimen en monolithische polymeren. Elk van deze heeft zijn
nadelen, waarbij vooral het gebrek aan thermische stabiliteit een probleem is. Zo zijn
polyacrylaten slechts thermisch desorbeerbaar tot 200°C.
In dit werk wordt een nieuw type polymeer gesynthetiseerd dat thermisch zeer stabiel is (tot
400°C). Deze polymeren bevatten een polykation met een niet gepolymeriseerd tegenion.
Deze polymeren worden ook wel gepolymeriseerde ionische vloeistoffen genoemd wegens
hun structurele gelijkenis met ionische vloeistoffen. Deze polymeren zullen worden
gekarakteriseerd met differentiële scanning calorimetrie (DSC) - om de glas transitie
temperatuur van het polymeer te bepalen - en thermogravimetrische analyse (TGA) om de
thermische stabiliteit te meten. Hierna worden deze polymeren aangebracht op een stir bar,
waarmee vervolgens extracties worden uitgevoerd.
Experimenteel
Lithiumhydride (95%) en 1-broom-3-chloorpropaan (99%) werden aangekocht bij Sigma-
Aldrich Chemie (Steinheim, Germany). Imidazool werd aangekocht bij Acros Organics (Geel,
België). Lithium bis[(trifluoromethyl)sulfonyl] imide werd geleverd door 3M België.
Dichloormethaan en aceton werden aangekocht bij Fisher Scientific (Loughborough,
Leicestershire, VK). Methanol en tetrahydrofuran (THF) werden bekomen bij Biosolve
(Valkenswaard, Nederland). THF werd gedroogd over natrium. 1N zilver nitraat werd
bekomen bij Merck (Darmstadt, Duitsland). Milli-Q water werd in het labo gemaakt.
Crosslinkbaar polyethylene werd aangekocht bij Innophase (Portland, CT, VSA) en werd
bewaard bij 4°C onder stikstof atmosfeer. Stir bars, met een stalen netje erop werden
aangeboden door het Research Institute for Chromatography (RIC). Koffiestalen werden
geleverd door Miko (Turnhout, België) en Efico (Antwerpen, België).
De synthetische procedure werd reeds besproken door Amarasekara et al. (3) en Hsieh et al.
(4). Aan een suspensie van lithium hydride in droge THF werd een imidazool oplossing (in
droge THF) traag toegedruppeld bij 0°C onder stikstof atmosfeer. Deze oplossing werd dan
gedurende 0,5 uur geroerd. Hierna werd 1-broom-3-chloorpropaan traag toegedruppeld bij
0°C onder stikstof atmosfeer. De reactie werd na 24 uur gestopt door een overmaat water toe
te voegen. THF werd verwijderd en de bekomen oplossing werd geëxtraheerd met
dichloormethaan. Deze fase werd gedroogd over Na2SO4 en vervolgens ingedampt. Een gele
olie werd bekomen (rendement = 93%). 1H NMR (CDCl3, 300 MHz) 7,38 (1H) 6,92 (1H)
6,82 (1H) 4,05 (2H) 3,33 (2H) 2,05 (2H)
Deze olie werd vervolgens opgewarmd tot 90°C en gedurende 12 uur op deze temperatuur
gehouden. In deze fase vond de stapsgewijze additiepolymerisatie plaats. Het bekomen wit
product werd opgelost in methanol en daarna neergeslaan in aceton, waarna de bekomen witte
neerslag werd afgefilterd. Het rendement van deze reactie was 95%. Hierna werd bij het
bekomen polymeer het tegenion (chloor) nog uitgewisseld voor een ander tegenion, hier
bis[(trifluoromethyl)sulfonyl] imide. Door de ionuitwisseling werd de thermische stabiliteit en
de polariteit beïnvloed. De reactie werd bewerkstelligd door het polymeer met chloor als
tegenion op te lossen in water en hieraan een overmaat van een waterige oplossing van
lithium bis[(trifluoromethyl)sulfonyl] imide (NTf2) toe te voegen. Deze oplossing werd
gedurende 12 uur geroerd bij kamertemperatuur. Hierdoor werd een neerslag gevormd van het
ionuitgewisselde polymeer. Het polymeer werd gewassen met water tot alle chloorionen
verwijderd waren. De neerslag werd vervolgens 3 dagen gedroogd.
Thermogravimetrische analyse (TGA) werd uitgevoerd op een TGA/SDTA 851e van Mettler
Toledo (Zaventem, België). Hierbij werd een temperatuursschema van 25°C-800°C gebruikt
met een temperatuursgradient van 10°C/min. Het staal werd continu onder stikstof gehouden
om oxidatieve pyrolyse van het polymeer te voorkomen.
Differentiële scanning calorimetrie (DSC) werd uitgevoerd op een DSC 2920 modulated DSC
van TA instruments (Zellik, België). Hierbij werd het polymeer eerst gekoeld tot
-50°C waarna het werd opgewarmd aan 10°C/min tot 150°C. Hierna werd het staal afgekoeld
tot -50°C aan 10°C/min. Daarna werd een laatste keer opgewarmd tot 150°C aan 10°C/min.
Gecoate stir bars werden geconditioneerd in een thermisch desorptie eenheid (TDU) die
gekoppeld was aan een GC-MS systeem. Het TDU systeem was afkomstig van Gerstel GmbH
en het GC-MS systeem was een Agilent 7890A met een Agilent 5975c massa spectrometer
(Agilent Technologies, Little Falls, DE, VSA). Dit systeem bevatte een Cooled Injection
System (CIS) die diende als programmeerbare temperatuur vaporisator (PTV). Hierbij werden
2 methodes toegepast. (zie Tabel I) Kolommen werden bekomen bij J&W Scientific.
Tabel I: Gebruikte methodes voor analyses
Methode 1 Methode 2
Kolom DB-5MS (30 m x 0,250 mm x 0,25
µm) + 10 m Duraguard
DB-WAX (30 m x 0,250 mm x 0,25
µm)
TDU 35°C 60°C/min 200°C (5 min) 35°C 60°C/min 200°C (5 min)
CIS -50°C 12°C/s 300°C (8 min) -50°C 12°C/s 300°C (8 min)
Split ratio 1/50 1/20
Temperatuur programma 50 °C (1 min) 20 °C/min 320
°C (2 min)
40 °C (5 min) 10 °C/min 250
°C (4 min)
Resultaten en discussie
Synthese van de gepolymeriseerde ionische vloeistof
De monomeer synthese was eenduidig. Enige voorzichtigheid was nodig bij het verwijderen
van THF, daar de temperatuur laag diende gehouden te worden om voortijdige polymerisatie
te voorkomen. Om deze reden werd het waterbad van de rotavapor op maximum 40°C gezet.
Voorts werd het monomeer bewaard in bij -15°C om dezelfde reden.
Figuur 1. Synthese van 1-(3-chloorpropyl)imidazool
Na het neerslaan van het poly-Cl in aceton werd de neerslag afgefilterd op een por. 5 glasfilter
(Robu-glasfilter geraete GmbH, Hattert, Germany). De neerslag werd niet geretenteerd door
filters met grotere poriën. De neerslag werd bewaard in een desiccator
Figuur 2. Synthese van het polymeer (met chloor als tegenion) uit 1-(3-
chloorpropyl)imidazool
Na de ion-uitwisseling en het wassen van de neerslag, werd het polymeer opgelost in aceton
en daarna 3 dagen gedroogd in een oven op 60°C. Hierna was het polymeer klaar voor
gebruik
Karakterisatie van het polymeer
Het bekomen polymeer met NTf2 als tegenion werd opgemeten met TGA en DSC. Het
polymeer blijft stabiel tot 380°C (Figuur 3) en de glas transitie temperatuur is -15°C (Figuur
4). Dit zijn zeer interessante eigenschappen aangezien dit een zeer goede thermische stabiliteit
is, maar belangrijker is de gunstige Tg die onder kamertemperatuur ligt. Hierdoor is sorptieve
extractie mogelijk onder deze condities.
Figuur 3. TGA thermogram van het polymeer met bis[(trifluoromethyl)sulfonyl] imide
(NTf2) als tegenion.
0
20
40
60
80
100
25 125 225 325 425 525 625 725
gew
ich
tsp
erc
en
tage
Temperatuur (°C)
NHN +ClBr
LiH NN Cl
N N Cl 90°C N N
nCl
Cl
Figuur 4. DSC thermogram van de tweede warmtecyclus met de koelingscyclus ingevoegd
van het polymeer met bis[(trifluoromethyl)sulfonyl] imide (NTf2) als tegenion.
Coaten van de stir bars met het polymeer
Een hoeveelheid polymeer werd opgelost in een weinig aceton. Deze oplossing werd
vervolgens gedruppeld op het metalen netje van de stir bar. Na een aantal druppels, werd door
een stikstofstroom het solvent verdampt. Dit proces werd herhaald tot voldoende polymeer
geïmmobiliseerd is op de stir bar. Op deze manier kon 48 mg poly-NTf2 gecoat worden.
Voorts werd ook een mengsel van PEG (in aceton) en poly-NTf2 (in aceton) gemaakt in een
1:1 verhouding. Dit mengsel werd op analoge manier geïmmobiliseerd op de stir bar, met 27
mg coating als resultaat.
Deze twee geproduceerde stir bars werden vervolgens geconditioneerd bij 200°C in een TDU.
Hierdoor werd de stir bar gereinigd van resterend solvent en andere onzuiverheden die kunnen
opgenomen zijn door de coating. In beide gevallen zijn de voornaamste onzuiverheden
squaleen en 9-octadeceenamide-(Z). Deze werden geïncorporeerd door (manuele)
behandeling van de stir bar, maar waren bij een tweede conditioneringscyclus volledig
verdwenen.
Extractie van koffie met de gecoate stir bars
Koffie bevat een groot aantal polaire componenten wat het een uitstekend staal maakt om de
capaciteit van de geproduceerde coatings te testen.
In een eerste experiment werd een stir bar, die slechts 8 mg coating bevatte, vergeleken met
een commercieel verkrijgbare PDMS gecoate stir bar (figuur 5) Hiervoor werd 0,5 gram
gemalen koffie in een headspace vial gebracht. Hieraan werd de stir bar in de headspace
aangebracht. De extractie werd gedurende 2,5 uur gedaan bij kamertemperatuur. De analyse
werd gedaan volgens methode 1. Enkele typische moleculen zoals pyridine (A), methyl-
pyrazine (B), azijnzuur (C) en 2-furanmethanol (D) werden in detail vergeleken. Al deze
moleculen hebben een zeer lage log Ko/w, wat maakt dat PDMS amper in staat is ze om te
extraheren.
Figuur 5. Analyse van de headspace van koffie met een stir bar gecoat met de
gepolymeriseerde ionische vloeistof en met een conventionele PDMS gecoate stir bar. (A =
pyridine; B = methyl-pyrazine; C = azijnzuur; D = 2-furanmethanol)
Pyridine en methyl-pyrazine werden beter geëxtraheerd door PDMS terwijl
2-furanmethanol eenzelfde extractie-efficiëntie had. Azijnzuur werd significant beter
geëxtraheerd door de gesynthetiseerde gepolymeriseerde ionische vloeistof. Er werd reeds
beschreven dat de imidazoliumgroep een goede affiniteit bezit voor esters en CO2 (5), (6). Dit
kan de goede extractie van azijnzuur verklaren. Er dient hierbij wel vermeld te worden dat er
slecht een hele kleine hoeveelheid coating werd aangebracht voor deze extractie. Het is
mogelijk om zo’n 6 keer meer polymeer te immobiliseren in het stalen net. Dit kan de
extractie-efficiëntie van alle componenten alleen maar ten goede komen.
In een tweede experiment werd 2 gram gemalen koffie geëxtraheerd met een PEG/PIL
gecoate stir bar en met een PDMS gecoate stir bar gedurende 0,5 uur bij 70°C (figuur 6). De
analyse werd gedaan met methode 2. Ook hier werd azijnzuur veel beter geëxtraheerd (50
keer meer) dan wanneer gebruik gemaakt werd van een conventionele stir bar. Voorts werd
nu ook 2-furanmethanol met een hogere extractie-efficiëntie bekomen. Enkele pyrazines en
pyridine werden nu met een gelijke efficiëntie geëxtraheerd.
0
1
2
3
4
5
6
7
8
9
10
7 9 11 13 15
Telle
n /
s (
10
6 )
Tijd (min)
PIL
PDMS
A
B
C
D
Figuur 6. Analyse van de headspace van koffie met een stir bar gecoat met een mengsel van
gepolymeriseerde ionische vloeistof/polyethyleenglycol en met een conventionele PDMS
gecoate stir bar. (A = azijnzuur; B = 2-furanmethanol)
Samenvatting
In dit werk werd gezocht naar een nieuw polymeer dat gebruikt kan worden voor sorptieve
extractie op stir bars. Hierbij diende dit polymeer polaire karakteristieken te vertonen én
thermisch stabiel te zijn om de tot hiertoe gekende polymere fases te overtreffen bij het
gebruik in stir bar sorptive extraction. Het geopperde polymeer is een gepolymeriseerde
ionische vloeistof. Dit is een op imidazolium gebaseerd polymeer met een kation in de
hoofdketen, waarrond een vrij anion aanwezig is. Dit polymeer werd met succes gemaakt
waarbij de thermische stabiliteit voor het polymeer met bis[(trifluoromethyl)sulfonyl] imide
als tegenion zeer goed was, aangezien het polymeer slechts degradeert bij 380°C. Ook is de
glas transitie temperatuur gunstig (-15°C), waardoor sorptieve extractie met dit polymeer
mogelijk is. Dit polymeer werd met succes geïmmobiliseerd op een genette stir bar. Deze stir
bar werd vervolgens toegepast voor de analyse van de headspace van een koffiestaal. Hierbij
werd een goede extractie-efficiëntie bekomen voor 2-furanmethanol en azijnzuur.
Dankwoord
Ik zou graag Karine Jacq (RIC) willen bedanken voor de hulp bij de analyses alsook
Stephanie Deman voor de hulp bij de TGA en DSC thermogrammen.
Referenties
(1) Baltussen, E.; Sandra, P.; David, F.; Janssen, H.; Cramers, C. Study into the
Equilibrium Mechanism Between Water and Poly ( dimethylsiloxane ) for Very Apolar
olutes : Adsorption or orption ? Analytical Chemistry 1999, 71, 5213-5216.
(2) De Coensel, N.; Desmet, K.; Górecki, T.; Sandra, P. Determination of
Polydimethylsiloxane-air Partition Coefficients Using Headspace Sorptive Extraction.
Journal of Chromatography A 2007, 1150, 183-189.
0
10
20
30
40
50
60
70
9 10 11 12 13 14 15 16 17
Telle
n/s
(1
06 )
Tijd (min)
PEG/NTf2
PDMS
A
B
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