Silica-Based Ionogels: Nanoconfined Ionic Liquid-Rich Fibers for

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Silica-Based Ionogels: Nano-Confined Ionic Liquid-Rich Fibers for Headspace Solid-Phase Microextraction Coupled with Gas Chromatography - Barrier Discharge Ionization Detection Technique Francisco Pena-Pereira, #ukasz Marcinkowski, Adam Kloskowski, and Jacek Namiesnik Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac502666f • Publication Date (Web): 05 Nov 2014 Downloaded from http://pubs.acs.org on November 11, 2014

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Analytical Chemistry

1

Silica-Based Ionogels: Nano-Confined Ionic Liquid-Rich Fibers for

2

Headspace Solid-Phase Microextraction Coupled with Gas

3

Chromatography - Barrier Discharge Ionization Detection Technique

4

5

Francisco Pena-Pereiraa,b, Łukasz Marcinkowskic, Adam Kloskowskic*, Jacek

6

Namieśnikb a

7

Campus As Lagoas-Marcosende s/n, 36310 Vigo, Spain

8 9

b

Department of Analytical Chemistry, Chemical Faculty, Gdańsk University of Technology (GUT), ul. G. Narutowicza 11/12, 80-233 Gdańsk, Poland

10 11 12

Analytical and Food Chemistry Department; Faculty of Chemistry; University of Vigo,

c

Department of Physical Chemistry, Chemical Faculty, Gdańsk University of Technology (GUT), ul. G. Narutowicza 11/12, 80-233 Gdańsk, Poland

13

14

*Corresponding author. Tel.: +48 5824 72110; fax: +48 5834 72694; e-mail address:

15

[email protected]

1

ACS Paragon Plus Environment


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1

ABSTRACT

2

In this work, hybrid silica-based materials with immobilized ionic liquids (ILs) were

3

prepared by sol-gel technology and evaluated as solid-phase microextraction (SPME) fiber

4

coatings.

5

bis(trifluoromethylsulfonyl)imide ([C4MIM][TFSI]) were confined within the hybrid

6

network. Coatings composition and morphology were evaluated using scanning electron

7

microscopy and energy dispersive X-ray spectrometry. The obtained ionogel SPME fibers

8

exhibited high extractability for aromatic volatile compounds, yielding good sensitivity and

9

precision when combined with a gas chromatograph with barrier ionization discharge (GC-

10

BID) detection. A central composite design was used for assessing the effect of

11

experimental parameters on the extraction process. Under optimized conditions, the

12

proposed ionogel SPME fiber coatings enabled the achievement of excellent enrichment

13

factors (up to 7400). The limits of detection (LODs) were found in the range 0.03-1.27 µg

14

L-1, whereas the repeatability and fiber-to-fiber reproducibility were 5.6% and 12.0% in

15

average, respectively. Water samples were analyzed by the proposed methodology,

16

showing recovery values in the range of 88.7-113.9%. The results obtained in this work

17

suggest that ionogels can be promising coating materials for future applications of SPME

18

and related sample preparation techniques.

High

loadings

of

the

IL

1-methyl-3-butylimidazolium

19 20

Keywords: ionogel; ionic liquid; sol-gel; solid-phase microextraction; sample preparation

2


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1

INTRODUCTION

2

Solid-phase microextraction (SPME) is undoubtedly one of the main breakthroughs in

3

analytical sample preparation. Introduced by Arthur and Pawliszyn in 1990,1 SPME

4

significantly contributed to the development of miniaturized and greener analytical

5

methodologies.2 Even though commercially available SPME fibers are widely employed,

6

there is currently a marked interest in the development of novel and reusable SPME fiber

7

coatings that provide both enhanced extractability and good reproducibility.3,4

8

The unique physicochemical properties of ionic liquids (ILs), namely, negligible

9

volatility, non-flammability, high chemical and thermal stability, as well as good solubility

10

for both ionic and molecular solutes, make them exceptional materials for a variety of

11

scientific and technological applications.5–7 In particular, the implementation of ILs in

12

analytical sample preparation has experienced an increasing interest over the last years.

13

Their application as SPME sorbent coatings was initially proposed by Liu et al. in 2005.8 In

14

this pioneering work, disposable IL-coated SPME fibers were prepared by a physical dip-

15

coating method. Several strategies have been subsequently developed with the aim of

16

providing IL-based SPME sorbent coatings with increased robustness and fiber lifetime.

17

Thus, the modification of the fiber support by means of a Nafion membrane9 or etching

18

with a solution of ammonium hydrogen difluoride10 prior to the dip-coating with IL has

19

been reported in the literature. In addition, reusable SPME fibers obtained by fixing the IL

20

through cross-linkage of IL-impregnated silicone elastomer on the surface of a fused silica

21

fiber were also proposed.11 Silanization reactions were also exploited to chemically bond an

22

alkoxysilane-functionalized IL to a fused silica substrate.12 Probably, the introduction of

23

polymeric ionic liquids (PILs) involved one of the main advances in this field.13,14 PILs, 3



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1

prepared by polymerization of IL monomers which incorporate one or more polymerizable

2

units, are characterized by their high viscosity and mechanical stability. Recent reviews

3

provide the state of the art on this challenging area.15,16

4

Sol-gel technology provides an invaluable tool for the synthesis of organic-

5

inorganic hybrid materials with advanced properties.17 Malik and co-workers reported the

6

first application of the sol-gel technology towards the development of SPME coatings,18

7

and from this work numerous developments have been reported. ILs have been used in a

8

number of publications as a template to generate sol-gel coatings with porous morphology

9

for SPME and related sample preparation techniques.19,20 It is worth mentioning that the IL

10

is generally removed after preparing the coatings and, therefore, it does not play any role

11

during extractions. Nevertheless, some attempts have been made to implement ILs in

12

SPME fiber coatings as co-extractants. Thus, SPME fiber coatings containing

13

poly(dimethylsiloxane) (PDMS)21 and calixarenes22 were prepared by sol-gel processing

14

using ILs, even though the amounts of IL immobilized were limited and their role in terms

15

of extractability was therefore negligible. Chemical bonding of allyl- or alkoxy-

16

funtionalized ILs to the sol-gel matrix has also been described.23–27 In addition, IRMOF-

17

3@ILs/PDMS fibers were very recently proposed for the headspace enrichment of

18

polycyclic aromatic hydrocarbons.28 Physical coating with IL successfully avoided the

19

substantial cracking that moisture causes on the metal-organic framework. However, an

20

additional PDMS protective film was mandatory to avoid the loss of the IL during thermal

21

desorption.

22

A challenging area of research in materials science is the confinement of ILs in

23

solid-like matrices while keeping their unique properties practically unchanged.29 In this 4


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1

regard, Dai et al. reported the first sol-gel synthesis using an IL, specifically, 1-ethyl-3-

2

methylimidazolium bis(trifluoromethylsulfonyl)imide, as a reaction solvent in 2000.30

3

From this initial work, a plethora of sol-gel processes have been reported in the literature

4

for obtaining solid skeletons with entrapped ILs, where the IL can act as drying control

5

chemical additive, catalyst, porogenous agent and solvent or co-solvent.31 Ionogels are

6

stable hybrid materials formed by nanoconfinement of an IL within an oxide matrix,

7

resulting from the interpenetration of two continuous three dimensional networks of solid

8

and IL.31,32 Importantly, non-leaching IL-containing materials are thus obtained, whereas

9

the main properties of the IL used in the preparation of ionogels are not markedly modified,

10

including the liquid-like nature even at high degrees of confinement. Ionogels have found

11

application as optical devices, electrolytic membranes, catalysts and sensors, as reported in

12

recent reviews.29,31,33 In

13

this

work,

ionogels

based

on

the

IL

1-butyl-3-methylimidazolium

14

bis(trifluoromethylsulfonyl)imide ([C4MIM][TFSI]) were prepared by sol-gel processing

15

using methyltrimethoxysilane (MTMS) as the silicon alkoxide precursor. The obtained

16

ionogels were evaluated as SPME fiber coatings for the headspace enrichment of aromatic

17

compounds. To the best of our knowledge, this work provides the development and

18

application of sol-gel SPME fibers with the highest IL content reported in the literature.

19 20

EXPERIMENTAL SECTION

21

Reagents and materials

22

A standard mixture of volatile organic compounds (VOCs) (EPA VOC Mix 2, 2000 µg mL-

23

1

each component in methanol), including benzene, bromobenzene, n-butylbenzene, 5



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1

ethylbenzene, p-isopropyltoluene, naphthalene, styrene, toluene, 1,2,3-trichlorobenzene,

2

1,2,4-trichlorobenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, and m-xylene was

3

purchased from Sigma-Aldrich (Milwaukee, WI, USA). Deionized water obtained from a

4

Millipore Q system (Millipore, Molsheim, France) was used throughout. Methanol HPLC

5

grade was obtained from Sigma Aldrich. Standard solutions of the above VOCs were

6

prepared by appropriate dilution of the stock solution in methanol, and working solutions

7

were obtained by subsequent dilution of the standard solutions with deionized water.

8

MTMS was obtained from Fluka (Buchs, Switzerland). Trifluoroacetic acid (TFA) and

9

[C4MIM][TFSI] were purchased from Sigma-Aldrich. Optical fibers with a 150 µm

10

diameter glass core and protective polyimide coating (Cezar Int., Poland) were used to

11

prepare the ionogel SPME fibers. Sulfuric acid (POCH, Gliwice, Poland), hydrochloric acid

12

(POCH, Gliwice, Poland), and sodium hydroxide (POCH, Gliwice, Poland) were used for

13

the pretreatment of the glass fibers. Acetonitrile (ACN) was purchased from Sigma Aldrich

14

(Poland). Sodium sulfate (Sigma Aldrich, Poand) was used to evaluate the salting-out

15

effect.

16 17

Instrumentation

18

All analyses were carried out on a Shimadzu Tracera system that consists of a Shimadzu

19

GC-2010 Plus capillary gas chromatograph coupled with a Shimadzu barrier ionization

20

discharge (GC-BID) detector (BID-2010 Plus) (Shimadzu Scientific Instruments, Inc.,

21

Columbia, MD). The GC was equipped with a Durabond DB-VRX high resolution GC

22

column (60 m x 0.32 mm, film thickness 180 µm). The injector was operated in the splitless

23

mode and was maintained at 220 ºC. Helium was used as the carrier gas. The column oven 6


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1

was initially set at 35 ºC for 10 min, programmed to 75 ºC at 7 ºC min-1, then increased to

2

175 ºC at 20 ºC min-1, and finally to 220 ºC at 30 ºC min-1, in which it was held for 5 min.

3

The temperature of the BID detector was set at 250 ºC.

4

A scanning electron microscope (SEM) (Hitachi, model S-3400N) and an energy

5

dispersive X-ray spectrometer system (EDX) (Thermo Fisher, model NSS 312) were used

6

to evaluate the surface morphology and elemental composition of ionogel fiber coatings,

7

respectively.

8 9

Preparation of the SPME coatings

10

Ionogel-based SPME fibers were obtained by a procedure involving four steps, namely,

11

pretreatment of the glass fiber, preparation of the sol solution, sol-gel coating of the SPME

12

fiber, and thermal conditioning.

13

Firstly, the protective polyimide coating of the optical fiber was removed by means

14

of conc. H2SO4. A conventional pretreatment procedure was then followed. Briefly, the

15

glass fiber was dipped in 1 mol L-1 NaOH for 1 h in order to obtain a high density of

16

surface silanol groups, and rinsed with distilled water. Thereafter, the fiber was dipped in

17

0.1 mol L-1 solution of HCl for 30 min to neutralize the excess NaOH, washed with

18

distilled water, allowed to dry at room temperature and stored in a desiccator for no more

19

than 12 h before use.34

20

The sol solution was prepared by adding 56.4 µL of TFA (95% v/v), 407.2 µL of

21

[C4MIM][TFSI] and 50.0 µL of MTMS (corresponding to a MTMS/TFA/IL molar ratio of

22

1:2:4) into a plastic Eppendorf tube, mixing thoroughly after each addition. A clear sol

23

solution was thus obtained. A dipping-coating method was used for obtaining the ionogel 7



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1

fiber. The pretreated glass fiber was dipped vertically into the sol solution for 1 min and

2

subsequently withdrawn with the pulling rate set at 160 mm min-1 before drying in air for 1

3

min. Several dipping and drying steps were carried out with the same sol solution for 30

4

min, so as to obtain a layer of ionogel on the outer surface of the glass fiber with the

5

required film thickness. It is worth mentioning that the film thickness of the obtained

6

ionogel can be controlled by means of the number of coating cycles. Thus, ionogel fibers

7

with a film thickness of about 65-70 µm were obtained by repeating the above mentioned

8

procedure with a freshly prepared sol solution. The fiber was then placed in a desiccator for

9

12 h and thermally conditioned at 230 °C for 1 h in the injection port of the GC.

10 11

SPME procedure

12

Headspace solid-phase microextraction (HS-SPME) was carried out with the ionogel fiber

13

installed into a commercial SPME device (Supelco, Bellefonte, PA). 12 mL of aqueous

14

standard solution or water sample containing 20% (w/w) Na2SO4 was placed into a 15 mL

15

glass vial together with a glass-coated stir bar and sealed with a screw cap with a Teflon

16

faced septum. Before extraction, the sample solution was thermostated at 45 ºC for 15 min

17

to reach thermal equilibrium. Then, the ionogel fiber was exposed to the headspace above

18

the sample solution stirred at the highest stirring rate (1800 rpm) for 30 min. After

19

extraction, the fiber was removed from the vial and introduced immediately into the GC

20

injector port for thermal desorption at 220 ºC for 10 min, which proved to be sufficient to

21

ensure complete desorption with no carryover.

8


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1

RESULTS AND DISCUSSION

2

Preparation of ionogel SPME fibers

3

The sol-gel process involves two sets of reactions, namely, hydrolysis of the sol-gel

4

precursor and polycondensation of the hydrolyzed products.20 A macromolecular network

5

structure of sol-gel materials is thus formed.20 Condensation reactions can also take place

6

with the silanol groups of the fiber surface, then yielding a chemically bonded sol-gel

7

coating on the glass fiber.18 One of the aims was to prepare fiber coatings with a high

8

loading of IL confined into the sol-gel network for their evaluation in SPME experiments.

9

Ionogels were obtained by using MTMS as the sol-gel precursor, and TFA as the sol-gel

10

catalyst. It has been reported that alkyl derivatives of a tetraalkoxysilane precursor

11

minimize the cracking of sol-gel coatings, MTMS being commonly used in the preparation

12

of sol-gel based SPME fibers.20 TFA is a strong organic acid that serves as a sol-gel

13

catalyst and usually facilitates the dissolution of sol-gel ingredients. Besides, a controlled

14

amount of water is commonly used to hydrolyze the sol-gel precursor. [C4MIM][TFSI] was

15

the IL of choice, due to its relative hydrophobicity, high thermal stability, and relatively

16

low viscosity.

17

Prior to the preparation of the ionogel fibers, a separate set of in-vial gelation

18

experiments was preliminarily performed. Thus, TFA and water contents were carefully

19

selected to obtain a sol composition that yielded reasonable gelation times (e.g., ~1h) in the

20

presence of [C4MIM][TFSI] while keeping constant the amount of MTMS. A solution of

21

TFA containing 5% water was thus used at a TFA/MTMS molar ratio of 2 in the

22

preparation of ionogel fibers. Notably, the presence of small amounts of [C4MIM][TFSI] in

23

the sol led to an important decrease on the gelation time. Besides, gelation times increased 9



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Page 10 of 33

1

when larger amounts of IL were involved, even though gelation times were still much faster

2

than observed in the absence of IL. This behavior has been experimentally observed in a

3

non-aqueous

4

bis(trifluoromethylsulfonyl)imide ([C6MIM][TFSI]), with tetramethoxysilane as precursor

5

and formic acid as the catalyst.35 The effect of [C6MIM][TFSI] on the gelation time has

6

been attributed to two competitive roles of the IL. On the one hand, the IL destabilized the

7

colloidal dispersion when present at low concentrations, thus accelerating particle

8

aggregation, whereas the dilution effect, at high concentrations of IL, kept the particles

9

apart.35

sol-gel

process

involving

1-hexyl-3-methylimidazolium

10

The relationship between coating thickness and withdrawal speed has been reported

11

for sol-gel processes.36 Accordingly, the pulling rate at which the fiber is retracted from the

12

sol solution has a direct effect on the film thickness of the formed coating. Thus, the highest

13

available pulling rate (160 mm min-1) was used to obtain ionogel fiber coatings with the

14

greatest possible thickness and sufficient film regularity.

15

Three ionogel SPME fibers with [C4MIM][TFSI]/MTMS molar ratios in the range 2

16

to 4, with film thicknesses around 20 µm in average, were subsequently obtained and used

17

in preliminary studies to evaluate the extraction efficiency (EE) of ionogels with different

18

IL content. The prepared ionogel SPME fibers were exposed to the headspace above

19

aqueous solutions containing 50 µg L-1 of each VOC compound. It can be deduced from

20

Fig. 1 that the larger the content of [C4MIM][TFSI] in the SPME fibers, the higher the

21

extractability achieved for the selected compounds. Therefore, a [C4MIM][TFSI]/MTMS

22

molar ratio of 4 was selected for subsequent studies.

10


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1

As explained above, the film thickness of the obtained ionogel SPME fibers

2

depends on the number of coating cycles performed. Thus, an additional ionogel fiber with

3

increased film thickness (69.3 µm in average) was obtained by carrying out two cycles of

4

the procedure described in the experimental section, and evaluated as extractant phase. As

5

expected, increased EEs were obtained with the thicker ionogel fiber, in accordance with

6

the results shown in Fig. 1. Therefore, ionogel SPME fibers with the optimal film thickness

7

(65-70 µm) were employed along the work. Ionogel fibers with higher thicknesses were not

8

evaluated due to the limitation imposed by the inner diameter of the SPME assemble

9

needle.

10

Sol-gel coatings containing covalently attached methyl groups have been previously

11

reported in the literature for the extraction of aromatic compounds.37 Thus, the latter

12

ionogel fiber was subjected to solvent extraction with ACN for 48 h in order to remove the

13

IL from the coating, and the extraction capability of the thus obtained fiber coating was

14

evaluated. The ionogel fiber yielded, in average, seven-fold higher EEs than the fiber

15

subjected to IL elution (Fig. 1). Remarkably, the extractability with the tested fibers was

16

dependent on the analyte of interest, thus demonstrating that the confined IL had effectively

17

a supreme role in the extraction of the target VOCs. Specifically, the ionogel fiber yielded

18

two- to three-fold higher extractability than the fiber devoid of IL when dealing with the

19

headspace enriching of the more hydrophobic compounds, while an increase of about

20

seven-eight times the amount of analytes extracted was achieved in the case of

21

ethylbenzene, m-xylene, styrene and bromobenzene, and even extractability ratios above

22

twenty were obtained for benzene and toluene. SEM images of both the ionogel fiber and

23

the fiber obtained after elution of the IL are shown in Fig. 2. 11



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1

Characterization of ionogel SPME fibers

2

The surface characteristics of the ionogel SPME fiber were investigated by SEM. Fig. 2

3

shows the SEM images of both the ionogel and the washed gel SPME fibers. As can be

4

seen from Fig. 2A and B, the ionogel fiber possessed a homogeneous and rough surface.

5

The thickness of the ionogel fiber coating was typically in the range 65-70 µm after two

6

coating cycles.

7

Cracking of the fiber treated with ACN is also apparent from Fig. 2C. After

8

replacement of the IL with ACN, large capillary stresses are produced in the pore walls

9

during the evaporation step, then resulting in the cracking of the obtained washed gel.

10

Interestingly, the bulk of the coating revealed a high porosity that can be attributed to the

11

confinement of the IL (Fig. 2D). This agrees with reports were ILs have been employed as

12

templates and reaction media for the preparation of microporous and mesoporous

13

materials.38,39

14

The elemental composition of the ionogel SPME fiber was assessed by EDX

15

analysis. The EDX spectrum and the corresponding element mapping images are depicted

16

in Fig. 3. It can be deduced from the EDX analysis the presence of C, O, F, Si and S in the

17

ionogel fiber. The obtained results confirm the successful immobilization of

18

[C4MIM][TFSI] within the three-dimensional solid-like network, as the EDX analysis

19

yielded the presence of homogeneously distributed S and F elements that can be attributed

20

to the TFSI anion of the IL object of study.

21 22 23 12


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1

Optimization of the HS-SPME procedure

2

Determination of optimal conditions for sample preparation is essential for analytical

3

method development. There are several factors, influencing extraction efficiency,

4

which should be considered in the case of HS-SPME technique. Among them,

5

extraction time, sample temperature, agitation method, stirring rate and size of the

6

stir bar (if magnetic stirring is chosen), pH, headspace volume, salt addition, as well

7

as desorption time and temperature, are the most often studied. However,

8

considering available literature and previous experience, we decided to reduce the

9

number of parameters to the following input variables:

10 11

− extraction temperature, which has a significant influence on the values of partition coefficients;

12

− extraction time, due to the time-dependent convective-diffusive mass transport;

13

− salt content, due to the potential effect of electrolytes on the distribution of

14

target analytes between the sample and the headspace.

15

The reduction of the number of parameters has a critical impact on the

16

number of experiments which have to be performed during optimization, particularly

17

in full factorial design procedures. Magnetic stirring was chosen in advance to avoid

18

problems with sample temperature control that might occur when applying

19

ultrasound irradiation. Furthermore, based on literature, the highest available stir bar

20

rotation speed was applied (1800 rpm). Sample pH was not optimized, since the

21

investigated analytes do not reveal acid-base properties. In addition, bearing in mind

22

the basic relationship describing analytes partitioning among three phase systems,

23

the headspace volume was kept at the minimum necessary for the fiber exposure.40 13



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1

Desorption parameters were also excluded from the optimization of the extraction

2

process. In our opinion, desorption conditions ensuring quantitative transfer of

3

analytes from the coating to the GC column should be determined before other

4

parameters are optimized. Otherwise, the results of the latter could be not valid.

5

Therefore, the desorption conditions, namely, desorption temperature and time, were

6

optimized first. The desorption temperature was set at 220°C, that is 20°C below the

7

temperature in which [C4MIM][TFSI] exhibits long-term thermal stability.41. Under

8

these conditions, a desorption time of 10 min was sufficient to avoid carryover

9

effects.

10

A central composite design (CCD) was used for optimization of extraction

11

parameters. 7 star points were added to make the design orthogonal and rotatable.

12

The final plan consisted of 23 runs, i.e. 8 factorial points, 6 star points, and 9 central

13

points. The plan was generated by random sampling using Statistica 10 software

14

package (StatSoft, USA). In the study, the following low (-1) central (0) and high

15

(+1) parameter values were used: fiber exposure, 12, 25 and 38 min; sample

16

temperature, 28, 35°C and 42°C; and salt concentration 5, 10 and 15% (w/w). The

17

values of independent variables corresponding to specific experiments are shown in

18

Table 1. The table also contains the values of the response variable for the extraction

19

procedure, i.e. sum of the chromatographic peak areas, obtained using the ionogel

20

fiber.

21

The statistical significance of variables was evaluated by using the analysis of

22

variance (ANOVA); the results are presented as Pareto chart in Fig. 4. For the

23

assumed 95% confidence level (p=0.05) depicted in the diagram, all parameters have 14


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1

statistically significant linear influence on extraction efficiency. Based on the

2

standardized values of main effects, it can be stated that the amount of retained

3

analytes (sum of peak areas) depended strongly on temperature and extraction time,

4

whereas the influence of salt concentration was less significant. All parameters

5

studied had positive effects, which indicated that the extraction efficiency increased

6

with increasing values of these parameters.

7

Response surface functions were fitted to the obtained data using the model

8

including linear main effects, quadratic terms and two-factor interactions. The

9

response surfaces plotted for the three combinations of independent variables are

10

presented in Fig. 5. Each response surface was calculated by assuming the mid-range

11

value for the third variable. For example, 25 min extraction time was used to

12

calculate the response surface for the relationship between temperature and salt

13

concentration. The models obtained for ionogel fibers showed quite a good fit

14

described by the respective R2 values of 0.92. However, their analysis suggests that

15

optimal values of the investigated parameters are outside of the analyzed range. This

16

is especially evident in case of temperature and time. Thus, an additional set of

17

experiments involving only those two parameters, but in shifted range, was

18

performed. Temperature and time were evaluated in the range of 30-60°C and 30-60

19

min, respectively. A salt concentration of 20% (w/w) was chosen for practical

20

reasons, as samples were stored at room temperature (20°C) in which the solubility

21

of Na2SO4 is close to 20% (w/w). The obtained response surface function is shown

22

in Fig. 6. It is well known that temperature shows an impact on the partition

23

coefficients between the three involved phases, namely, sample, headspace and fiber 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

coating. Thus, high temperatures enable a more efficient transfer of analytes from

2

the sample to the headspace, whereas a reduction of the analyte extraction can be

3

produced at high temperatures due to the exothermic nature of sorption processes. It

4

can be noticed from Fig. 6 that the highest extractability was achieved at a

5

temperature of 45ºC. In addition, the amount of retained analytes increased slightly

6

with time under these conditions. After 30 min, the extraction efficiency reached ca.

7

90% of that at 60 min. Based on these results, a temperature equal to 45°C and 30

8

min of extraction time were chosen for further experiments.

9 10

Evaluation of quality assurance and quality control parameters. Analysis of real

11

samples.

12

The analytical characteristics of the proposed methodology are summarized in Table 2. The

13

enrichment factors (EFs) were calculated under optimal extraction conditions as the ratio of

14

the analyte concentration in the ionogel fiber and the analyte concentration in the aqueous

15

sample. EFs in the range 275-7400 were obtained for the target compounds with the

16

proposed ionogel fiber, corresponding to EEs ranging from 1.3-35.9%.

17

Calibration curves were obtained using up to seven concentration levels of VOCs in

18

the range 0.025-75 µg L-1. All the analytes showed acceptable linearity from the limits of

19

quantification (LOQ) to 75 µg L-1, with correlation coefficients above 0.9984. The limits of

20

detection (LODs) were calculated on the basis of a signal-to-noise ratio of 3, and the LOQs

21

as ten times of the above mentioned ratio. LODs and LOQs were found to be between 0.03-

22

1.27 µg L-1 and 0.11-4.24 µg L-1, respectively. The repeatability of the method, evaluated

23

by five replicate analyses of aqueous samples fortified at a 5 µg L-1 level with the aromatic 16


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1

compounds object of study, varied between 1.7 and 18.7%. The fiber-to-fiber

2

reproducibility was also evaluated using three different ionogel fibers prepared using the

3

same procedure, and was found in the range 8.6-31.4%. Remarkably, a single ionogel fiber

4

could be used with no obvious decline of performance for about 60 extraction/desorption

5

cycles. After 60 cycles the EEs start to be systematically below established average value

6

from previous experiments, thus we assumed that 60 extraction/desorption cycles is the

7

admissible lifetime of the fiber.

8

Finally, the proposed method was applied to the analysis of three real water

9

samples, namely, mineral, tap and ground water. The contents of VOCs were below the

10

LOQ of the proposed method in all the analyzed samples. In order to evaluate potential

11

matrix effects, water samples were subsequently fortified at 1 µg L-1 with the target

12

compounds and analyzed by the HS-SPME-GC-BID method. Typical chromatograms

13

obtained by the proposed methodology can be shown in Fig. 7. Relative recoveries were

14

obtained by subjecting aqueous samples spiked with 1 µg L-1 of each VOC to the proposed

15

method and comparing the analytical response to that obtained from aqueous standards

16

subjected to the same procedure. In accordance with the analytical results presented in

17

Table 3, the obtained recoveries were found in the range 88.7-113.9 %, then indicating that

18

matrix effects were not significant.

19 20

CONCLUDING REMARKS

21

In this work, high loadings of [C4MIM][TFSI] were physically confined into a sol-gel

22

network by gelation of MTMS and TFA. The obtained ionogel SPME fibers were evaluated

23

for the headspace enrichment of VOCs in aqueous samples. EFs in the range 275-7400 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

were obtained under optimum conditions. Remarkably, IL-rich SPME fibers yielded up to

2

twenty-fold higher extractability than the SPME fiber obtained after removal of the IL by

3

solvent extraction.

4

The very promising results obtained with the herein presented ionogel fibers make

5

us consider them as an excellent option for the preparation of advanced SPME coatings. A

6

broad range of ionogel coatings with tunable properties could potentially be prepared by

7

using a large variety of readily available ILs. Due to their advantageous properties, we

8

consider that ionogels open-up new horizons in a variety of extraction and microextraction

9

techniques.

10 11

Acknowledgements

12

F. Pena-Pereira thanks Xunta de Galicia for financial support as a post-doctoral researcher

13

of the I2C program.

14

The authors are indebted to Prof. K. Darowicki and Dr. J. Ryl (Gdańsk University of

15

Technology) for scanning electron microscopy measurements and analysis.

16 17

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21



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1 2 3

Table 1 Matrix of the CCD plan, and the responses (sum of peak areas) obtained for extraction of investigated VOCs using the ionogel fiber Run

4

Page 22 of 33

Temperature (°C)

Time (min)

4 28 38 10 46.8 25 6 42 12 2 28 12 15 C 35 25 22 C 35 25 20 C 35 25 18 C 35 25 19 C 35 25 16 C 35 25 9 23.2 25 21 C 35 25 3 28 38 23 C 35 25 12 35 46.9 11 35 3.1 7 42 38 8 42 38 14 35 25 13 35 25 1 28 12 5 42 12 17 C 35 25 a VOCs concentration, 10 µg L-1

Na2SO4 concentration (% w/w) 15 10 15 15 10 10 10 10 10 10 10 10 5 10 10 10 5 15 18.4 1.6 5 5 10

22


Total areaa [a.u.]x107 1.42 1.79 1.73 0.82 1.27 1.34 1.31 1.20 1.25 1.27 0.82 1.28 1.06 1.27 1.20 0.30 1.58 2.07 1.97 0.65 1.11 1.06 1.27

Page 23 of 33


1 2 3 4 Table 2 5 Analytical characteristics of the developed HS-SPME-GC-BID methodology 6 7 8 Fiber-to-fiber LOD LOQ Repeatability 9 Compound EE(%) EF reproducibility (µg L-1) (µg L-1) (RSD %, n=5) 10 (RSD %, n=3) 11 Benzene 1.27 4.24 1.3 275 18.7 31.4 12 0.30 0.99 5.2 1069 3.4 8.6 13Toluene 14Ethylbenzene 7.7 10.4 0.03 0.11 13.3 2736 15m-Xylene 0.10 0.33 12.8 2639 2.5 8.8 16 Styrene 0.04 0.13 16.0 3304 3.4 9.0 17 4.3 11.7 0.05 0.18 14.3 2956 18Bromobenzene 1,3,5-Trimethylbenzene 0.14 0.47 21.3 4384 5.3 9.9 19 201,2,4-Trimethylbenzene 0.12 0.41 26.4 5442 4.2 8.9 21p-Isopropyltoluene 0.08 0.26 29.6 6109 1.7 10.0 22n-Butylbenzene 0.04 0.13 33.2 6850 5.5 12.8 23 0.09 0.28 32.1 6605 6.7 12.7 241,2,4-Trichlorobenzene 4.4 9.6 Naphthalene 0.05 0.18 35.9 7400 25 0.14 0.47 34.0 7008 5.5 11.7 261,2,3-Trichlorobenzene 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 23 43 44 45 46 ACS Paragon Plus Environment 47 48

Slope ± S.D.

Intercept ± S.D.

Correlation coefficient

5113±88 32351±904 81812±1524 80759±2037 88838±2170 48273±1471 126927±4156 150820±4186 171791±3782 189119±4205 120454±4349 163370±5394 130308±4895

269±2109 -981±18771 -21758±28319 -4637±42318 3472±40326 -17093±27331 -28544±86333 22475±86951 -10273±70278 -18396±78128 14103±90348 4481±100222 19143±101688

0.9999 0.9992 0.9995 0.9994 0.9991 0.9986 0.9989 0.9992 0.9993 0.9993 0.9987 0.9984 0.9986


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3

Table 3 Analytical results for the determination of VOCs in natural water samples Compounds

4

Toluene Ethylbenzene m-Xylene Styrene Bromobenzene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene p-Isopropyltoluene n-Butylbenzene 1,2,4-Trichlorobenzene Naphthalene 1,2,3-Trichlorobenzene a Spiking level: 1 µg L-1

Relative recoveries (%)a Mineral water Tap water Ground water 93.9±10.4 108.9±7.7 94.4±5.6 99.1±5.7 100.6±6.0 95.7±11.6 96.9±6.3 109.4±12.7 90.6±12.1 93.8±6.5 105.0±8.7 95.9±12.5 96.8±6.6 96.1±8.7 88.7±10.0 102.7±5.0 100.1±11.1 99.6±12.2 96.7±8.6 104.0±6.5 99.7±12.7 107.7±5.1 100.2±7.0 98.3±8.6 97.3±10.1 97.6±6.4 93.0±5.9 106.8±11.0 97.8±5.6 98.2±11.4 94.1±5.8 113.9±6.7 101.2±10.0 105.2±15.4 104.1±9.4 93.6±8.9

5

24


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1

Figure captions

2

Figure 1: Evaluation of the composition of the ionogel fiber in the HS-SPME of VOCs.

3

Experimental conditions: VOCs concentration, 50 µg L-1; sample volume, 12 mL; sample

4

temperature, 30 ºC; Na2SO4 concentration, 20% (w/w); equilibration time, 5 min; extraction

5

time, 15 min; desorption temperature, 220 ºC; desorption time, 10 min.

6

Figure 2: SEM images of the ionogel fiber (A and B) and the fiber subjected to solvent

7

extraction (C and D).

8

Figure 3: Typical EDX spectrum of an ionogel fiber and the corresponding elemental

9

mapping of C, O, F, Si, and S.

10

Figure 4: Standardized effect Pareto chart for the central composite plan.

11

Figure 5: Response surfaces as functions of optimized parameters, calculated for the

12

ionogel fiber for (A) time vs temperature, (B) temperature vs salt concentration, and (C)

13

salt concentration vs time.

14

Figure 6: Response surfaces as functions of time and temperature, calculated for the

15

ionogel fiber.

16

Figure 7: GC-BID chromatograms of (A) blank, (B) unspiked ground water, and (C)

17

ground water spiked with VOCs at 1 µg L-1 after enrichment by the ionogel SPME fiber

18

under optimal conditions. Peaks: (1) toluene, (2) ethylbenzene, (3) m-xylene, (4) styrene,

19

(5) bromobenzene, (6) 1,3,5-trimethylbenzene, (7) 1,2,4-trimethylbenzene, (8) p-

20

isopropyltoluene, (9) n-butylbenzene, (10) 1,2,4-trichlorobenzene, (11) naphthalene, (12)

21

1,2,3-trichlorobenzene. 25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2 3

For Table of Contents Only

4

26


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Figure 1: Evaluation of the composition of the ionogel fiber in the HS-SPME of VOCs. Experimental conditions: VOCs concentration, 50 µg L-1; sample volume, 12 mL; sample temperature, 30 ºC; Na2SO4 concentration, 20% (w/w); equilibration time, 5 min; extraction time, 15 min; desorption temperature, 220 ºC; desorption time, 10 min. 180x119mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2: SEM images of the ionogel fiber (A and B) and the fiber subjected to solvent extraction (C and D). 254x190mm (300 x 300 DPI)


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Figure 3: Typical EDX spectrum of an ionogel fiber and the corresponding elemental mapping of C, O, F, Si, and S. 239x119mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4: Standardized effect Pareto chart for the central composite plan. 124x79mm (300 x 300 DPI)


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Figure 5: Response surfaces as functions of optimized parameters, calculated for the ionogel fiber for (A) time vs temperature, (B) temperature vs salt concentration, and (C) salt concentration vs time. 83x246mm (300 x 300 DPI)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

84x80mm (300 x 300 DPI)


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203x290mm (300 x 300 DPI)


Silica-Based Ionogels: Nanoconfined Ionic Liquid-Rich Fibers for

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