Ionic liquid and polymeric ionic liquid coatings in solid-phase microextraction

Trends in Analytical Chemistry, Vol. 45, 2013

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Ionic liquid and polymeric ionic liquid coatings in solid-phase microextraction Honglian Yu, Tien D. Ho, Jared L. Anderson Sample preparation is often considered to be a bottleneck in most analytical methods. Solid-phase microextraction (SPME) consolidates sampling and sample preparation into one step. The unique properties of ionic liquids (ILs) and polymeric ILs (PILs) have been exploited as sorbent coatings in SPME. Due to their tunable structures, these materials exhibit unique selectivity for various analytes. This article discusses the overall progression of IL and PIL-based sorbent coatings in SPME. The structural engineering and design of selective sorbent coatings is discussed to demonstrate the versatility of these materials in numerous applications. ª 2013 Elsevier Ltd. All rights reserved. Keywords: Dip coating; Fiber surface; Ionic liquid (IL); Polymeric ionic liquid (PIL); Sample preparation; Sampling; Selectivity; Solid-phase microextraction (SPME); Solvent; Sorbent coating

1. Introduction Honglian Yu, Tien D. Ho, Jared L. Anderson*, Department of Chemistry, School of Green Chemistry and Engineering, The University of Toledo, Toledo, OH 43606, USA

*

Corresponding author. Tel.: +1 (419) 530 1508; Fax: +1 (419) 530 4033. E-mail: [email protected],

Solid-phase microextraction (SPME) was introduced by Pawliszyn and co-workers in the early 1990s as a technique that combines sampling and sample preparation into one step [1]. It has been widely applied due to its simplicity, costeffectiveness, and solvent-free properties [2]. SPME is a non-exhaustive extraction technique and is based on the partitioning of analytes in a sample matrix to a sorbent phase, which is often coated on a support [2,3]. Analytes are extracted by three different modes, namely, headspace SPME (HS-SPME), direct-immersion SPME (DISPME) and membrane-protected SPME (MP-SPME). In HS-SPME, the sorbent coating is exposed to the headspace of the sample solution containing the analyte(s) of interest. Exposing the coating to the headspace allows for the rapid extraction of volatile analytes without the need to introduce the coating to the matrix. For analytes that are less volatile or possess high affinity to the matrix, DI-SPME or MP-SPME may be employed, wherein the sorbent coating is immersed in the sample solution itself [4]. Subsequent to extraction, analytes are desorbed from the

fiber coating and subjected to separation by gas chromatography (GC) or highperformance liquid chromatography (HPLC) [5]. SPME has expanded into multiple scientific disciplines including trace-level analyses of analytes in food, pharmaceutical, environmental, biological and other real-world samples [6–9]. At present, there are a number of commerciallyavailable SPME sorbent coatings that possess broad selectivity based largely on their polarity for different analyte classes. However, there is a lack of coatings possessing high selectivity towards specific classes of analytes. As the applicability of SPME continues to grow to real-world samples in which analytes are often at extremely low concentrations, it is imperative to investigate new sorbent-coating technologies that allow the analyst greater control over analyte selectivity. Ionic liquids (ILs) have previously been exploited as extractants, porogens, mediators, and/or solvents in various microextraction techniques [10]. Specifically, within the past seven years, ILs and polymeric ILs (PILs) have been promising sorbent-coating materials that can be designed to exhibit high selectivity for target analytes [11]. ILs are salts with

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melting points at or below 100C and are comprised of organic cations and organic/inorganic anions. They possess unique physicochemical properties (e.g., high thermal stability, tunable viscosity and solvation capabilities) and negligible vapor pressures. The main advantage of using ILs as SPME sorbent coatings is the ability to incorporate various substituents into the IL structure, so as to enhance specific interactions between the IL coating and target analytes. PILs are polymers synthesized from IL monomers. They possess a number of advantages over ILs when used as coatings in SPME. PILs often possess higher viscosity and greater mechanical strength compared to ILs while exhibiting similar extraction selectivity [12]. PIL-based coatings are not prone to flowing off the fiber support particularly at high GC desorption temperatures. As a result, PIL-based coatings do not need to be recoated after each extraction and desorption step. DI-SPME is also possible for certain classes of hydrophobic PIL-based coatings [11]. This article focuses primarily on the progression of IL and PIL-based SPME sorbent coatings from the initial stages to the most recent advancements. We focus attention upon structural tuning and enhancement of various aspects of coating technology, including selectivity, physicochemical and mechanical stability, and fiber lifetimes. We highlight the applicability of these sorbent coatings by discussing the types of analytes and matrices that have been studied. Finally, we discuss the current limitations of IL/PIL-based coatings.

2. Advances in IL-based SPME sorbent coatings 2.1. Studies on selectivity and fiber surface modifications The following sections encompass studies that involved utilizing ILs as sorbent coatings in SPME. For convenience, studies employing these types of coatings are summarized in Table 1. Liu and co-workers initially applied ILs as SPME sorbent coatings [13]. The 1-octyl-3-methylimidazolium hexafluorophosphate ([C8MIM][PF6]) IL was used for the headspace extraction of benzene, toluene, ethylbenzene, and xylenes (BTEX) in paints, coupled with GC/flame ionization detection (GC/FID). The viscous IL-based coating allowed for extraction times faster than solid sorbent phases. Also, the dip-coated IL was used as a disposable SPME coating, which avoided analyte carryover [13]. Recently, another study using an analogous dipcoating method for the 1-butyl-3-methylimidazolium PF6 ([C4MIM][PF6]) IL was explored by Ho and coworkers [14]. Good extraction efficiency and precision were observed when the coating was applied to the analysis of chlorophenols (CPs) in landfill leachate. Un220

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like the single-use IL coating reported by Liu et al. [13], fibers were reported to be reusable up to 120 times. Amini and co-workers explored the selectivity of four different ILs {namely, 1-butyl-3-methylimidazolium tetrafluoroborate ([C4MIM][BF4]), 1-ethyl-3-methylimidazolium ethylsulfate ([C2MIM][EtSO4]), [C8MIM][BF4], and [C8MIM][PF6]} for the extraction of methyl tertbutyl ether (MTBE) in gasoline samples [15]. The disposable IL coatings were loaded on silica-fiber supports and prepared for headspace extraction. A comparison in extraction efficiency showed that the [C8MIM][BF4] coating exhibited the highest sensitivity for MTBE. In order to achieve a higher, more homogeneous IL loading, Nafion was employed in a fiber-pretreatment method [16]. A stable IL coating was achieved on the Nafion pre-coated support as a result of the electrostatic interaction at the IL-Nafion interface. The fiber was applied in the analysis of polycyclic aromatic hydrocarbons (PAHs) in water samples. The extraction efficiencies of three ILs {namely, 1-octyl-3-methyl-imidazolium trifluoromethanesulfonate ([C8MIM][TfO]), 1-benzyl-3methylimidazolium trifluoromethanesulfonate ([BeMIM][TfO]), and 1-methyl-3-phenylpropylimidazolium trifluoromethanesulfonate ([PhproMIM][TfO]} were compared wherein the [C8MIM][TfO] IL coating exhibited the highest extraction efficiency. One disadvantage to using the Nafion-polymer membrane was that it may undergo hydrophobic interaction with PAHs, resulting in unwanted interference during the adsorption/desorption process [16,17]. Huang and co-workers developed a wet-etching method to pretreat the silica support using ammonium hydrogen difluoride prior to coating [17]. This etching technique increased the surface area of the fiber and allowed for a more mechanically stable IL coating on the fiber support, as shown in Fig. 1. Three silica supports, namely, etched, Nafion-supported and bare fused silica were coated with the [C4MIM][PF6] IL and applied to the headspace extraction of PAHs. The etched fiber exhibited the highest extraction efficiencies for the analytes studied, followed by the Nafion-supported fiber, which exhibited extraction efficiencies 3–5-fold higher than the bare fiber. The superior extraction efficiency of the etched-IL coated fiber was attributed to the ability to load a larger volume of IL onto the rough surface of the support. 2.2. Substrate-bonded IL sorbent coatings for improved chemical and mechanical stability In order to allow for improved chemical, thermal, and/or mechanical stability, surface-modification techniques have been developed to immobilize the IL to a fiber substrate. The following studies describe the fabrication of substrate-bonded IL sorbent coatings and their applications in SPME. For convenience, Table 2 lists the various coating immobilization methods used along with potential advantages and disadvantages.

Author [Ref.]

IL coating

Film thickness (lm)

Coating method

Analyte class

Desorption temperature (C)

Fiber lifetime

Summary of study

Liu [13]

[C8MIM][PF6]

–

Dip-coated

BTEX

200

1

Ho [14]

[C4MIM][PF6]

–

Dip-coated

CPs

240

80

Amini [15]

12.7

Dip-coated

MTBE

180

1

–

Nafion pretreated support + dip-coated

PAHs

240

1

Huang [17]

[C4MIM][BF4] [C8MIM][BF4] [C8MIM][PF6] [C2MIM][EtSO4] [C8MIM][TfO] [BeMIM][TfO] [PhproMIM][TfO] [C4MIM][PF6]

IL-based sorbent coating for increased selectivity Investigated the selectivity of ILs towards CPs Investigated the selectivity of ILs towards MTBE

30

Etched support + dip-coated

PAHs

220

1

Amini [18]

[MTPIM][NTf2]

11

Silanization

MTBE

220

16

Yang [19]

[C4MIM][BF4] [C4MIM][PF6] [C6MIM][PF6] [C8MIM][PF6] [C4MIM][CO2CF3] [C4Py][BF4] [C4MIM][SO3CF3] [C2MIM][NTf2] [EeMIM][NTf2]-impregnated silicone polymer

–

IL impregnated filter paper

Acetone

–

–

50

IL impregnated silicone elastomer

MAP, AP

220

>100

Hsieh [16]

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He [20]

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Table 1. Summary of studies employing ionic liquids (ILs) and impregnated ILs as SPME sorbent coatings

Applied an even IL film using a Nafion-coated support Increased the surface area of the fiber support to facilitate higher loading of IL coating Increased the mechanical and chemical stability of the IL-based coatings Investigated the selectivity of ILs towards acetone

Increased the viscosity and mechanical stability of the IL-based coating

BTEX, Benzene, toluene, ethyl benzene, xylene; CP, Chlorophenol; MTBE, Methyl tert-butyl ether; PAH, Polycyclic aromatic hydrocarbon; MAP, Methamphetamine; AP, Amphetamine.

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Figure 1. Optical micrographs of home-made SPME fibers. (a) Bare fused-silica support, (b) Etched fused-silica support, (c) Nafion-coated fusedsilica support. (a 0 –c 0 ) Corresponding supports after coating with [C4MIM][PF6]. (a’’–c’’) Corresponding supports after coating with [C8MIM][PF6]. {Reprinted with permission from [17]}.

Amini and co-workers chemically bonded the 1-methyl-3-(3-trimethoxysilyl propyl) imidazolium bis[(trifluoromethyl)sulfonyl]imide ([MTPIM][NTf2]) IL to a fused-silica support in order to achieve a more thermally-stable coating [18]. The methoxy-functionalized IL was chemically bonded to the silanol groups on the surface of the silica support. The immobilization provided an IL-based coating which exhibited improved mechanical stability and analytical performance in the extraction of MTBE in a gasoline sample by HS-SPME GC/FID. The fiber was reusable up to 16 extractions and exhibited high thermal stability (220C) compared to the single-use, dip-coated IL fibers (180C) based on chromatographic bleeding studies. The limit of detection (LOD) of the chemically bonded IL fiber for MTBE was also lower than that of two commercially-available fibers

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[namely, polydimethylsiloxane/divinylbenzene (PDMS/ DVB) and PDMS/Carboxen]. 2.3. IL-impregnated SPME sorbent coatings Yang impregnated various ILs in filter paper for the analysis of acetone in human plasma using HS-SPME coupled to a cataluminescence (CTL) sensor [19]. The enrichment factor of eight ILs {[C4MIM][BF4], [C4MIM][PF6], 1-hexyl-3methylimidazolium hexafluorophosphate ([C6MIM][PF6]), [C8MIM][PF6], 1-butyl-3-methylimidazolium trifluoroacetate ([C4MIM][CO2CF3]), n-butyl-pyridinium tetrafluoroborate ([C4Py][BF4]), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([C4MIM][TfO]), and 1-ethyl-3-methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([C2MIM][NTf2])} were compared in the extraction of acetone. The [C4MIM] [CO2CF3] IL exhibited the highest extraction efficiency for

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Table 2. Comparison of different coating-immobilization methodologies for ionic liquids (ILs) and polymeric ILs (PILs) Immobilization methodologies Silanization of the substrate using an alkoxy silanefunctionalized IL Gluing IL/PIL-bonded silica particles to the fiber substrate

Advantages

Disadvantages

Simple and cost-effective

Generates porous and stable coatings which possess high robustness

Electrochemical deposition of IL/PANI composites

Combines surface modification and coating into one step; highly controllable coating film thicknesses

Sol–gel technique

Produces organic/inorganic hybrid copolymers which exhibit high thermal and chemical stability Improves the overall mechanical stability of the fiber

Surface radical chain-transfer reaction on a derivatized substrate Free radical copolymerization on a derivatized substrate

Improves the overall mechanical stability of the fiber [41–43,46]; solvent-free and high-throughput fabrication [43]

acetone in different matrices due to its high selectivity. Also, the enrichment factor of the IL-SPME-CTL method was 80fold higher than direct injection analysis and more than six-fold higher than headspace single-drop microextractionCTL. An IL-impregnated silicone elastomer was introduced as a sorbent coating by He and co-workers [20]. The 1ethoxyethyl-3-methylimidazolium bis[(trifluoromethyl) sulfonyl]imide ([EeMIM][NTf2]) IL was mixed with silicone elastomer at a 1:1 (w/w) ratio to increase the viscosity of the IL and trap it within the cross-linked silicone network. No peeling or bleeding was observed at 220C. In order to examine the contribution of the silicone network during extraction, a neat silicone-coated fiber and an [EeMIM][NTf2] IL-impregnated silicone (1:4, w/w)coated fiber were compared in the headspace extraction of methamphetamine (MAP) and amphetamine (AP). The IL-impregnated silicone coating exhibited superior extraction efficiency (4.2-fold higher for MAP and 7.3-fold higher for AP) compared to the neat silicone-based coating. The IL-impregnated silicone fiber (50 lm) exhibited higher extraction efficiency than a 7-lm PDMS fiber and lower extraction efficiency than a 100-lm PDMS fiber. The IL-impregnated coating could also be used for over 100 extractions without loss of extraction efficiency. 3. Advances in PIL-based SPME sorbent coatings 3.1. Selectivity of PIL-based SPME sorbent coatings The following sub-sections explore studies that exploited PILs and their derivatives as sorbent coatings in SPME.

May require additional immobilization steps to improve the durability of coating Silica particles and glue occupy a large volume of the sorbent coatings which can decrease extraction efficiency Requires the use of IL/PILs which possess a suitable viscosity and conductivity under optimized reaction conditions May sacrifice analyte selectivity due to the multi-component composition of the hybrid coating Can be time-consuming due to the multi-step procedures involved in the surface modifications; limited sorbent loading volumes Can be time-consuming due to the multi-step procedures involved in the surface modifications [41,42]; coating film thicknesses vary from fiber-to-fiber [43]

[Ref.] [18]

[31]

[32,33]

[36–38]

[39,40]

[41–43,46]

Studies employing these coatings are summarized in Table 3. PILs were initially examined as selective sorbent coatings for SPME by our group in 2008 [12]. Three PILs {namely, poly(1-vinyl-3-hexylimidazolium bis[(trifluoromethyl)sulfonyl]imide) (poly([ViHIM][NTf2])), poly (1-vinyl-3-dodecylimidazolium bis[(trifluoromethyl)sulfonyl]imide) (poly([ViDDM][NTf2])) and poly(1-vinyl3-hexadecylimidazolium bis[(trifluoromethyl)-sulfonyl] imide) (poly([ViHDIM][NTf2])} were applied as sorbent coatings for the headspace extraction of esters and fattyacid methyl esters (FAMEs) from aqueous solutions and wine samples. The PIL-coated fibers exhibited similar or superior analytical performance compared to a commercial PDMS coating. The PIL coatings also exhibited higher viscosity and enhanced thermal stability compared to traditional IL-based coatings, and that facilitated homogenous coatings and hindered their flow at elevated desorption temperatures in the GC injector. Moreover, these PIL-based coatings were observed to exhibit exceptional fiber lifetimes of approximately 150 extraction and desorption steps [12]. Zhao and co-workers designed two imidazolium-based PILs containing different anions {namely, poly([ViHIM][NTf2]) and poly([ViHIM][taurate])} as selective sorbent coatings in the headspace extraction of CO2 [21]. The extraction of CO2 using the poly([ViHIM][NTf2]) PIL relied on its high selectivity to CO2 via physical interaction, whereas the poly([ViHIM][taurate]) PIL was capable of chemically reacting with CO2 to form a carbamate salt. In the GC injector, the carbamate salt

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Zhao [12]

PIL coating

Film thickness (lm)

Coating method

Analyte class

Desorption temperature (C)

Fiber lifetime

Summary of study Exploited the increased viscosity and stability of PIL-based sorbent coatings Employed a reversible CO2 selective sorbent coating Investigated CO2 selectivity versus CH4 and N2 from simulated flue gas samples Utilized PIL-based sorbent coatings for direct immersion SPME Incorporated aromatic functionality to PILs for enhanced p-p interactions Investigated the selectivity of PILs containing hydrogen bond basic chloride anions relative to NTf2- anions Investigated the selectivity and thermal stability of blended PIL coatings

poly([ViHIM][NTf2]) poly([ViDDIM][NTf2]) poly([ViHDIM][NTf2]) poly([ViHIM][NTf2]) poly([ViHIM][taurate]) poly([ViHIM][NTf2]) poly([ViHIM][taurate])

15

Dip-coated

FAMEs

250

150

10

Dip-coated

CO2

–

10

Dip-coated

CO2

250 180 250 180

Lo´pez-Darias [23]

poly([VBHDIM][NTf2]

12

Dip-coated

PAHs, parabens, alkylphenols,

250

70

Meng [24]

poly([VBHDIM][NTf2]) poly([VHDIM][NTf2])

12

Dip-coated

PAHs

250

70 –

Meng [26]

poly([ViHIM][Cl]) poly([ViHIM][NTf2])

8 12

Dip-coated

VFAs, alcohols

200

30–40 –

Graham [28]

poly([ViHDIM][NTf2]): A poly([ViHDIM][Cl]): B poly([ViHIM][NTf2]): C poly([ViHIM][Cl]): D Six Mixtures: A 100%; A 75%/B 25%; A 50%/B50%; C 100%; C 75%/D 25%; C 50%/D 50%) poly([VBHDIM][NTf2]) poly([ViHIM][Cl])

20

Dip-coated

Benzonitrile, ethyl phenyl ether, octylaldehyde, 1,2-dichlorobenzene, naphthalene, alcohols, xylenes

250

200

160 160 250 160 160

– – – – 100

250 170

–

175 175 250 175 200 200 250 250

>70

– – – >50

Increased thermal, chemical, and mechanical stability of PIL and IL-based coatings

Exploited electrochemically deposited IL-PANI composite for enhanced chemical stability Investigated the selectivity of IL-PANI composites towards organochlorine pesticides Utilized silica sol-based PIL sorbent coating for increased thermal and chemical stability Investigated the selectivity of IL-sol materials towards PAEs

Zhao [21] Zhao [22]

Lo´pez-Darias [29]

Dip-coated

poly([DDMGlu][MTFSI]) poly([VPPIM][Cl])) poly([VBHDIM][NTf2]) poly([ViHIM][Cl]) ILs: [(HeIM)2PEG3] 2[NTf2] [(HeIM)2PEG3] 2[TfO] PILs: [(StyrIM)2C6, 2NTf2]n ([(StyrIM)2C6, 2TfO]n PANI-[C4MIM][BF4] composite

9 9 9 9 50

Dip-coated

Gao [33]

Ho [30]

Wanigasekara [31]

Alcohols, aldehydes; heterocyclic aromatics, acids, miscellaneous GTIs: alkyl halides and aromatics

Covalently bonded to silica particles + glued to support

Short-chained alcohols, small amines

80

Electrochemical deposition

Benzene derivatives

220

–

PANI-[C4MIM][PF6] composite

15

Electrochemical deposition

Organo-chlorine pesticides

250

>250

Liu [36]

[AMIM][PF6]-OH-TSO [AMIM][NTf2]-OH-TSO [AMIM][NTf2]

77 or 45 67 or 45 45

Sol-gel + UV-initiated polymerization

PEEs, aromatic amines

280

– – –

Zhou [37]

[AMIM][NTf2]-OH-TSO

77

Sol-gel + UV-initiated polymerization

PAEs

360

122

Zhao [32]

Examined PIL coatings towards volatile aromas from coffee samples Studied the selectivity of PILs towards GTIs and structurally alerting compounds

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14 8

–

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Table 3. Summary of studies employing polymeric ionic liquids (PILs) and PIL derivatives as SPME sorbent coatings

[A(Benzo15C5)HIM][PF6] [AMIM][PF6]-OH-TSO

52, 77, 69 or 62  50 or 71

Sol-gel + UV-initiated polymerization

Alcohols, PAEs, PEEs, fatty acids and aromatic amines

300 300

–

Feng [39]

poly([ViOIM][PF6])

5

BTEX, phenols, PAHs

250

56

Feng [40]

poly([ViOIM][NapSO3]) poly([ViOIM][PF6])

5

PAHs, PAEs

250 250

>100 –

Feng [41]

poly([ViOIM][Br]) cross-linked with ([(ViIM)2C6] 2[Br]); non-cross-linked poly ([ViOIM][Br]) Poly([ViBIM][NTf2])

35

Surface derivatized + surface free radical-chain transfer polymerization Surface derivatized + surface free radical-chain transfer polymerization Surface derivatized + thermal-initiated free radical copolymerization + crosslinking

Alcohols

200

60

200

30

Pang [42]

– 20

Surface derivatized + thermal-initiated free radical copolymerization

PAHs

280

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Ho [43]

Poly([ViHIM][Cl]) cross-linked with ([(ViIM)2C8] 2[Br]) or ([(ViIM)2C12] 2[Br])

3–9

Etched + surface derivatized + UV-initiated free radical copolymerization + crosslinking

Alcohols, aldehydes, esters, naphthalene

175

>90

Zhao [44]

poly([ViHIM][NTf2])

15

Dip-coated

Chiral alcohols

250

–

Meng [45]

poly([ViHDIM][NTf2])

10–15

Dip-coated

Low-volatility aliphatic hydrocarbons, and FAMEs

250

30

Zhang [46]

poly([ViHDIM][PF6])

–

Surface derivatized + thermal-initiated free radical polymerization

Pyrethroids

260

–

Incorporated benzyl and crown ether-functionality to a PIL for improved p-p and hydrogen bonding interactions PIL-based coating produced by surface free radical-chain transfer polymerization Investigated the selectivity of PIL coatings towards PAHs and PAEs Increased thermal stability and selectivity by forming cross-linked PIL-based sorbent coatings for headspace SPME Enhanced the mechanical stability of PIL-based coatings for direct immersion SPME Employed a solvent-free UVinitiated ‘‘on-fiber’’ copolymerization approach to produce cross-linked PIL sorbent coatings for headspace and direct immersion SPME Utilized a rapid ‘‘on-fiberÕ derivatization step for enantiomeric excess determination Exploited PILs in high temperature headspace SPME Exploited a PIL for the direct immersion SPME of pyrethroids

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Zhou [38]

FAME, Fatty acid methyl ester; VFA, Volatile fatty acid; GTI, Genotoxic impurity; PEE, Phenolic environmental estrogen; PAE, Phthalate ester; BTEX, Benzene, toluene, ethyl benzene, xylene.

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reversibly decomposes to release the sequestered CO2 for analysis. The extraction efficiency of CO2 using both PILbased coatings was compared with a PDMS and a PDMS/ Carboxen fiber. The two PIL-based fibers showed comparable extraction efficiency in spite of their smaller film thicknesses. The PIL-based coatings also exhibited competitive CO2-storage capability, especially in the case of the poly([ViHIM][taurate]) PIL [21]. In a subsequent study [22], CO2 was extracted from a simulated flue-gas sample matrix consisting of CO2 and CH4 or N2. The poly([ViHIM][NTf2]) PIL exhibited superior sensitivity for CO2 compared to the poly ([ViHIM][taurate]) and PDMS/Carboxen fibers. In the case of the poly([ViHIM][taurate]) PIL, a morphology change was observed only after exposure to CO2, illustrating the unique selectivity of this sorbent coating. The poly([ViHIM][NTf2]) PIL exhibited the highest selectivity in a CO2/N2 and CO2/CH4 gas mixture compared to the poly([ViHIM][taurate]) PIL and PDMS/Carboxen fibers. However, the PDMS/Carboxen fiber demonstrated higher CO2/N2 selectivity and similar CO2/CH4 selectivity, when compared to the poly([ViHIM][taurate]) PIL [22]. By synthetically designing a highly hydrophobic PIL coating, DI-SPME was applied for the analysis of PAHs and substituted phenols in water using the poly([ViHDIM][NTf2]) PIL [23]. The analytical performance of the PIL fiber (20 lm) was compared to two PDMS (30 lm and 100 lm) fibers and the polyacrylate (PA, 85 lm) fiber. The PIL-based coating exhibited superior sensitivity for all analytes compared to the 30-lm PDMS fiber while higher sensitivity was obtained for some analytes compared to the 100-lm PDMS fiber. The PIL fiber showed higher affinity for non-polar analytes while the PA fiber exhibited higher affinity for the more polar analytes. A benzyl-functionalized PIL, namely, poly((1-4-vinylbenzyl)-3-hexadecylimidazolium bis[(trifluoromethyl) sulfonyl]imide) (poly([VBHDIM][NTf2])), was synthesized and applied for the extraction of PAHs in aqueous solutions via DI-SPME [24]. This PIL sorbent coating exhibited high selectivity towards PAHs due to the incorporation of benzyl-moieties which allowed for an enhancement in p-p interaction. The benzyl-functionalized PIL coating also showed higher extraction performance for PAHs compared to a 7-lm PDMS coating and an analogous nonbenzyl functionalized PIL coating. Lo´pez-Darias and co-workers extended the application of the poly([VBHDIM][NTf2] PIL sorbent coating for the DI-SPME analysis of endocrine-disrupting chemicals (EDCs), including PAHs, alkylphenols, and parabens, from various aqueous matrices [25]. The 12-lm PIL coating exhibited superior extraction efficiency of these analytes compared to a 30-lm PDMS and 85-lm PA coating. Analogous to a previous study [23], higher partition coefficients for all the target analytes were obtained using the benzyl-functionalized PIL sorbent coating compared to the PDMS and PA fibers. 226

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The role of counteranions in PIL-based coatings was explored by comparing the poly([ViHIM][Cl]) and poly([ViHIM][NTf2]) PILs [26]. The two PIL-based fibers were employed for the extraction of polar analytes including phenols, volatile fatty acids (VFAs), and alcohols. The enrichment factor (EF), defined as the ratio of the chromatographic peak area obtained from SPME with respect to direct injection, was studied for all analytes. The EF for all analytes using the poly([ViHIM][Cl]) PIL coating was superior to those of the poly ([ViHIM][NTf2]) PIL coating. The effect was pronounced for hydrogen bond acidic analytes, such as VFAs, 2fluorophenol, phenol and p-cresol, due to the hydrogen bond basic nature of the chloride anion. Thermo-gravimetric analysis (TGA) revealed that the poly ([ViHIM][NTf2]) PIL exhibited higher thermal stability than the poly([ViHIM][Cl]) PIL, due to the susceptibility of the chloride anion to undergo nucleophilic substitution with the alkyl substituent of the imidazolium cation [27]. As a result, the poly([ViHIM][Cl]) PIL-based coating was usable for 30–40 extractions and desorptions at 200C, exhibiting a relatively shorter lifetime compared to the poly([ViHIM][NTf2])-based coating. Mixtures of PILs containing different compositions of the poly([ViHIM][NTf2]), poly([ViHIM][Cl]), poly([ViHDIM][NTf2]) and poly([ViHDIM][Cl]) PILs were used as sorbent coatings for the headspace extraction of various analytes [28]. The sensitivity of many analytes was not significantly affected by the addition of the chloridecontaining PILs. However, increasing the weight percentage of the chloride anion in the sorbent coating did improve the extraction efficiency for alcohols. The poly[ViHDIM][NTf2] PIL-based coating possessed a fiber lifetime of over 200 extractions while the 50% poly[ViHIM][NTf2]/50% poly[ViHIM][Cl] coating was reusable up to approximately 100 extractions. The sensitivity of the poly([VBHDIM][NTf2]) and poly([ViHIM][Cl])-based coatings was studied by Lo´pezDarias and co-workers [29]. The sorbent coatings were employed to extract 49 volatile analytes from coffee samples by HS-SPME GC/MS and compared with an 85 lm PA fiber. The poly([VBHDIM][NTf2]) PIL fiber resulted in higher extraction efficiencies for aldehydes and acids, while the poly([ViHIM][Cl]) fiber exhibited superior extraction efficiency for aromatic alcohols. However, the PA fiber exhibited higher extraction efficiencies for heterocyclic aromatics. The application and the development of analyte-selective PIL-based coatings was broadened into pharmaceutical analysis by our group in the determination of genotoxic impurities (GTIs) and structurally alerting compounds (namely, alkyl halides and aromatics) [30]. Two novel PIL coatings – N,N-didecyl-N-methyl-D -glucaminium poly(2methyl-acrylic acid 2-[1-(3-{2-[2-(3-trifluoromethanesulf onylamino-propoxy)-ethoxy]-ethoxy}-propylamino)-vinyl amino]-ethyl nester) (poly([DDMGlu][MTFSI])), and

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Figure 2. SEM images of the (a, b) electrodeposited PANI-[C4MIM][PF6] coated fiber, and (c) the neat PANI coating. {Reprinted with permission from [33]}.

poly(1-vinyl-3-propylphenylimidazolium) chloride (poly ([VPPIM][Cl])) – were synthesized and applied alongside two other PIL coatings – namely, poly([VBHDIM][NTf2]) and poly([ViHIM][Cl]) – for the headspace extraction of GTIs. PIL-based coatings containing the chloride anion exhibited superior selectivity for analytes containing hydrogen-bond acidic moieties. The glucaminium-based PIL coating showed favorable selectivity towards longchained aliphatic alkyl halides [30]. Nevertheless, the two aforementioned polar coatings were not recommended for DI-SPME, since they are susceptible to dissolution in the aqueous sample matrix. The poly([VBHDIM][NTf2]) PILbased coating exhibited superior sensitivities for aliphatic and aromatic analytes. 3.2. Improvements to the chemical, mechanical, and thermal stability of PIL-based SPME sorbent coatings 3.2.1. IL-functionalized silica particles as sorbent coatings. Two ILs derived from 1,1 0 -(1,6-hexanediyl)bisimidazole, namely, ([(HeIM)2PEG3] 2[NTf2], [(HeIM)2 PEG3] 2[TfO]), and two styrene-based dicationic PILs ([(StyrIM)2C6, 2NTf2]n, ([(StyrIM)2C6, 2TfO]n) were synthesized, chemically bonded to 5-lm silica particles, and glued to a fiber support for the headspace extraction of short-chain alcohols and amines [31]. The PIL-based coatings exhibited superior sensitivities compared to the IL-based coatings. Higher absolute sensitivities for the IL and PIL-based coatings (50 lm) were obtained relative to an 85 lm PA and a 65 lm PDMS/DVB fiber for ace-

tonitrile, methanol, and ethanol. It was noted that the silica particles occupied a large percentage of the sorbent coating, thus making the effective sorbent volume significantly smaller than the commercial coatings. In order to take into account only the effective sorbent volume, the relative response was obtained by normalizing the absolute response using an internal standard. The IL and PIL-based coatings exhibited higher relative extraction efficiency compared to the commercial fibers, particularly for short-chain alcohols. The PIL-based coatings were also employed for the extraction of polar and basic amines via DI-SPME in a pH 11 aqueous matrix, wherein no coating damage was observed [31]. 3.2.2. Electrochemically-deposited PIL composite. Zhao and co-workers combined the [C4MIM][BF4] IL with polyaniline (PANI) to produce a PANI-IL composite SPME sorbent coating on a platinum fiber by an electrochemical-deposition method [32]. SEM images revealed a more porous and even morphology of PANI[C4MIM][BF4] coating compared to that of the neat PANI coating. A chromatographic bleeding profile also showed that the PANI-[C4MIM][BF4] coating possessed high thermal stability (up to 320C). The PANI-[C4MIM][BF4] coating exhibited higher extraction efficiency in the headspace extraction of benzene derivatives compared to the neat PANI coating and a 100-lm PDMS fiber. Moreover, the PANI-[C4MIM][BF4] coating showed good resistance to organic solvent.

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Electrochemical-deposition was also applied to the production of a nano-structured IL-PANI composite SPME sorbent coating using the [C4MIM][PF6] IL [33]. SEM images revealed a porous, even morphology of the coating, as shown in Fig. 2. The IL-PANI composite coating exhibited thermal stability up to 350C and was applied in the headspace extraction of organochlorine pesticides. The IL-PANI composite coating exhibited superior analytical performance compared to a neat PANI sorbent coating in addition to a 30 lm PDMS fiber. The IL-PANI composite fiber was used up to 250 times, indicating high durability and robustness. 3.2.3. PIL-based composites derived from sol-gel chemistry. Chemically-bonded PIL-based SPME sorbent coatings prepared by sol-gel methods, based on work stemming from Malik et al. [34,35], were initially introduced by Liu and co-workers [36]. Two allylfunctionalized ILs {namely, 1-allyl-3-methylimidazolium hexafluorophosphate ([AMIM][PF6]) and 1-allyl-3methylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([AMIM][NTf2])} were copolymerized with hydroxyterminated silicone oil (OH-TSO) to form polymeric coatings. The resulting silica-PIL based coatings ([AMIM] [NTf2]-OH-TSO and [AMIM][PF6]-OH-TSO) were used to extract phenolic environmental estrogens (PEEs) and aromatic amines via DI-SPME and HS-SPME, respectively. The coatings exhibited good selectivity for the target analytes due to the strong electrostatic, hydrogen bonding and p-p interactions. The sol-gel coating method also produced PIL coatings with reproducible film thicknesses while exhibiting high thermal stability and tolerance to large pH ranges and various solvents. The TGA data for the two coatings indicated that the [AMIM][NTf2]-OH-TSO PIL possessed higher thermal stability (up to 402C) compared to the [AMIM][PF6]-OH-TSO PIL (up to 320C). Scanning electron microscopy (SEM) revealed smaller pores were formed for the [AMIM][PF6]-OH-TSO coating surface than for [AMIM][NTf2]-OH-TSO, resulting in larger surface area contributing to higher extraction efficiencies. Zhou and co-workers employed the [AMIM][NTf2]OH-TSO silica-PIL based sorbent coating for the extraction of phthalate esters (PAEs) from a methanolic matrix using ultrasonic extraction (UE) in DI-SPME [37]. This coating was able to withstand an elevated desorption temperature (360C), which significantly minimized the extraction-to-extraction carry-over. The silica sol-based PIL fibers also exhibited comparable or superior extraction efficiencies for many PAEs when compared to commercial PDMS, PDMS/DVB, and PA fibers. A crown ether-functionalized IL – 1-allyl-3-(6Õ-oxobenzo-15-crown-5 hexyl) imidazolium hexafluorophosphate ([A(Benzo15C5)HIM][PF6]) – was copolymerized with OH-TSO via the sol-gel method for DI-SPME of PAEs and HS-SPME of fatty acids, alcohols, and aromatic 228

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amines [38]. The sorbent coating exhibited superior extraction performance for alcohols, PAEs, and PEEs compared to a neat OH-TSO-coated fiber and a [AMIM][PF6]-OH-TSO silica-sol PIL-based fiber. Similar extraction performance was achieved when the crownether-functionalized coating was compared to PDMS, PDMS/DVB, PA fibers, and that was attributed to the incorporation of the benzo-15-crown-5 functional group. The crown ether-functionalized PIL coating also exhibited high extraction efficiency for alcohols due to the strong hydrogen-bond basicity of the electronegative oxygen heteroatoms within the crown-ether ring. High extraction efficiency of aromatic analytes was achieved, probably due to p-p interactions offered by the benzyl moiety with the sorbent coating. 3.2.4. Substrate bonded PIL-based sorbent coatings on surface-modified supports. Surface free radical chaintransfer polymerization was employed to polymerize and to immobilize 1-vinyl-3-octylimidazolium hexafluorophosphate (poly([ViOIM][PF6])) to a surface -modified stainless -steel wire [39]. Prior to loading the PIL, the stainless-steel support was modified with a layer of microstructured silver via the silver-mirror reaction. The silver-deposited support was subsequently modified with a self-assembled monolayer of a chain-transfer agent, 1,8-octanedithiol, and subsequently immersed in a solution containing the [ViOIM][PF6] IL monomer and 2,2 0 -azobis(2-methylpropionitrile) (AIBN) to initiate radical chain-transfer polymerization. The analytical performance of this substrate-bonded PIL-based coating was evaluated by HS-SPME of BTEX and DI-SPME of phenols and PAHs. Superior extraction efficiency towards the selected analytes was obtained compared to the disposable [C8MIM][PF6] coating. The substratebonded PIL-based coating also possessed high durability with a lifetime of 56 extractions [39]. An aromaticallyfunctionalized PIL sorbent coating {namely, poly (1-vinyl-3-octylimidazolium sodium 2-naphthalene-sulfonate) (poly([ViOIM][NapSO3]))} was prepared by an analogous method in subsequent work [40]. The poly([ViOIM][NapSO3]) PIL exhibited improved selectivity towards PAHs due to enhanced p-p interaction stemming from the aromatic anion. The coating was successfully applied for recovery studies of PAEs in diluted hair-spray and nail-polish solutions via DI-SPME. Recently, cross-linked PIL-based copolymers were used as SPME coatings for the headspace extraction of polar alcohols [41]. The monocationic IL monomer, 1-vinyl-3octylimidazolium bromide ([ViOIM][Br]), was copolymerized with a dicationic IL cross-linker, 1,1 0 -(1,6-hexanediyl)bis(1-vinylimidazolium) dibromide ([(ViIM)2C6] 2[Br]). First, the stainless-steel fiber with microstructured silver was coated with a self-assembled monolayer of 3-mercaptopropyltrimethoxysilane (MPS). Subsequently, the MPS coating was hydrolyzed and further derivatized

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Figure 3. SEM images of the cross-linked copolymeric PIL-based coating at (a) 400-fold magnification, and (b) 600-fold magnification. {Reprinted with permission from [41]}.

with vinyltrimethoxysilane (VTMS), which facilitated the immobilization of the cross-linked PIL during AIBNinitiated polymerization in dimethyl sulfoxide (DMSO). The fiber was repeatedly dipped in the AIBN/IL monomer DMSO solution until an acceptable film thickness was obtained. SEM images of the cross-linked PIL-based coating are shown in Fig. 3. SEM images of the cross-linked PIL coating revealed a porous surface, which may improve the extraction efficiency of the target analytes due to larger surface area. The cross-linked PIL-based coating was used up to 60 times with good precision, while an analogous non-cross-linked coating was only used 30 times [41]. A different surface-modification approach was investigated, wherein a stainless-steel support was derivatized with silica in order to allow covalent bonding of a PIL to the support surface [42]. Subsequent to silanization, the 1-vinyl-3-(3-triethoxysilypropyl)-4,5-dihydroimidazolium chloride IL was immobilized on the silica surface to impart vinyl functionality onto the fiber. The bonded IL was then copolymerized with 1-vinyl-3-butylimidazolium

chloride ([ViBIM][Cl]) via AIBN-initiated free-radical polymerization. Finally, the chloride anion of the PIL copolymer was exchanged ‘‘on-fiber’’ to NTf2 in order to obtain a hydrophobic coating that was applicable for DI-SPME in the aqueous matrices. The developed PILbased coating was evaluated by DI-SPME in the extraction of PAHs from water, and the coating exhibited comparable analytical performance to a 7-lm PDMS fiber. The PIL-immobilized fiber possessed a relatively thick coating (20 lm), in addition to exhibiting high thermal stability (up to 280C) and a fiber lifetime of 40 extractions. Very recently, our group described a solvent-free ‘‘onfiber’’ photoinitiated polymerization method that produced highly-stable cross-linked PIL-based sorbent coatings [43]. The method eliminated the need for organic solvent, which is typically required for dip-coating the SPME fiber (see Fig. 4A). As shown in Fig. 4B, a mixture containing the monocationic IL monomer ([ViHIM][Cl]), dicationic IL cross-linker (1,8-di(3-vinylimidazolium)

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Figure 4. (A) AIBN-initiated PIL approach used in previous studies to produce linear PILs, and (B) the UV-initiated IL polymerization approach for the preparation of cross-linked copolymeric PIL-based sorbent coatings. {Reprinted with permission from [43]}.

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octane dibromide ([VIM)2C8] 2[Br]) or 1,12-di(3-vinylimidazolium) octane dibromide ([VIM)2C12] 2[Br])), and UV initiator (DAROCUR 1173) was dip-coated on an etched and VTMS-derivatized fused silica fiber. The coated fiber was then placed in a UV reactor to initiate copolymerization and immobilization of the PIL to the fiber support. This approach allowed for the production of mechanically and chemically stable cross-linked PILs containing hydrogen-bond basic halide anions, which do not dissolve in organic or aqueous solvents. As a result, the application of neat polar PILs for DI-SPME was realized for the first time in the extraction of alcohols, aldehydes, and esters in complex water matrices. These coatings exhibited high extraction efficiency and sensitivity to the target analytes in addition to lower coating bleed compared to an analogous non-cross-linked PIL produced via AIBN-initiated polymerization (see Fig. 4A). The coatings also exhibited long lifetimes and robustness of fibers after 90 DI studies in samples of deionized, well and river waters. 3.3. Miscellaneous applications of PIL-based SPME sorbent coatings An ‘‘on-fiber’’ derivatization approach was developed using the poly([ViHIM][NTf2]) PIL coating for the rapid determination of chiral alcohols dissolved in the [C4MIM][NTf2] IL solvent using HS-SPME [44]. Following extraction, the enantiomers were derivatized ‘‘onfiber’’ using acetic anhydride and then subjected to the GC inlet. The derivatization process greatly improved the enantioselectivity obtained using chiral GC. The 15-lm poly([ViHIM][NTf2]) PIL coating exhibited analytical performance comparable to commercial 100-lm PDMS and 85-lm PA fibers. The versatility of ILs and PILs in separation science was demonstrated in the determination of low-volatility aliphatic hydrocarbons and FAMEs by HS-SPME GC/FID [45]. The 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([C6MIM][FAP]) IL was applied as a thermally stable solvent wherein a high extraction temperature was applied to increase the analyte concentration in the headspace. The poly([ViHDIM][NTf2]) PIL was used as a selective SPME sorbent coating for the extraction of the target analytes due to the high thermal stability of the sorbent coating. Due to the extreme extraction and desorption conditions, the poly([ViHDIM][NTf2]) PIL coating was used up to 30 times. A PIL-based coating {namely, poly([ViHDIM][PF6])} was chemically bonded to a pretreated silica fiber and applied for DI-SPME to determine seven pyrethroids in water, cabbage, and cucumber. The bare silica fiber was initially derivatized by c-methacryloxypropyltrimethoxysilane and subsequently exposed in the polymerization solution consisting of IL monomer, AIBN and methanol. The coating exhibited high extraction

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efficiency and precision. Compared to a commercial PDMS fiber, the PIL-based coating possessed 20% higher extraction efficiency for pyrethroids as a result of enhanced p-p and dispersive interactions. Furthermore, the [PF6] anion enabled the PIL-based coating to possess high thermal stability [46]. It should be noted that special measures should be taken when employing IL/PILs coatings containing the [PF6] or [BF4] anion as these coatings can undergo hydrolysis at high temperatures to form HF [47,48]. As a result, the analytical performance of these coatings may be compromised.

4. Future outlook Within the past six years, there has been an explosive increase in the study of IL/PIL-based sorbent coatings in SPME. New sorbent-loading methodologies and fibersurface modifications have improved the stability of these coatings, although improvements in coating robustness are still desperately needed. Structural modifications to the IL monomer and the synthesis of IL/PIL composites have greatly improved their chemical and thermal stabilities. The ability to tune the structures of ILs and PILs has resulted in significant improvement to selectivity, sensitivity, and LODs towards specific target analytes. Functionalizing ILs and PILs with polar and/or hydrogen-bonding-capable substituents allowed these sorbent materials to address the challenging tasks of selectively extracting polar analytes from aqueous matrices. This is especially important for the expansion of SPME, as current commercial coatings exhibit limited affinity towards highly polar analytes. Furthermore, employing polar cross-linked PILs will play an important role in the DI-SPME of these analytes, which can potentially allow for the coupling of IL/PIL-based SPME coatings to LC provided that excessive swelling of the sorbent coating can be minimized. As SPME technology is becoming more widely available for real-world applications, IL/PIL-based sorbent coatings must also become an integral part of this rapid growth. Further improvements in IL/PIL coatings will allow these coatings to be used for complex analyses, in which broad and analyte-specific selectivities are desired. Investigations into the fouling of these coatings by particulate matter and biological macromolecules are desperately needed. It is very important that new IL/PIL materials and surface-modification methods are developed to tolerate the various components of a complex matrix without compromising analyte selectivity and sensitivity. In addition to synthesizing IL/PILs that possess high matrix resistance, sorbent-fabrication technology should also be further developed. Currently, dynamic or static dip-coating are popular choices for loading IL/PILs to the fiber substrate. Although these approaches are less complicated, dynamic dip-coating http://www.elsevier.com/locate/trac

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may not achieve highly precise IL/PIL loading from fiberto-fiber while static dip-coating can be time-consuming and require many repetitions of loading. Special care should also be taken when choosing a solvent as a dispersive media for IL/PIL loading. Since residual solvent within the sorbent coating can contribute to elevated background and decrease the sensitivity of the technique, it is imperative that these solvents be fully removed prior to using the fiber. Thus, it is recommended that dispersive solvents that exhibit sufficient volatility be used during the coating process. Other routes to using organic solvents as a dispersive media for IL/PIL loading should also be explored. Finally, it is vital that all IL-based materials used in SPME be extremely pure and free of any impurities (e.g., starting materials or intermediates), which can have a detrimental effect on the stability of the coating during DI-SPME or decrease the thermal stability during the desorption step. References [1] C.L. Arthur, J. Pawliszyn, Anal. Chem. 62 (1990) 2145. [2] C.L. Arthur, L.M. Killam, S. Motlagh, M. Lim, D.W. Potter, J. Pawliszyn, Environ. Sci. Technol. 26 (1992) 979. [3] D. Louch, S. Motlagh, J. Pawliszyn, Anal. Chem. 64 (1992) 1187. [4] H. Lord, J. Pawliszyn, J. Chromatogr. A 885 (2000) 153. [5] H. Prosen, L. Zupancˇicˇ-Kralj, Trends Anal. Chem. 18 (1999) 272. [6] C. Duan, Z. Shen, D. Wu, Y. Guan, Trends Anal. Chem. 30 (2011) 1568. [7] H. Bagheri, H. Piri-Moghadam, M. Naderi, Trends Anal. Chem. 34 (2012) 126. [8] S. Risticevic, J.R. DeEll, J. Pawliszyn, J. Chromatogr. A 1251 (2012) 208. [9] B. Bojko, E. Cudjoe, M. Wasowicz, J. Pawliszyn, Trends Anal. Chem. 30 (2011) 1505. [10] E. Aguilera-Herrador, R. Lucena, S. Ca´rdenas, M. Valca´rcel, Trends Anal. Chem. 29 (2010) 602. [11] T.D. Ho, A.J. Canestraro, J.L. Anderson, Anal. Chim. Acta 695 (2011) 18. [12] F. Zhao, Y. Meng, J.L. Anderson, J. Chromatogr. A 1208 (2008) 1. ˚ . Jo¨nsson, M.-J. Wen, J. [13] J.-F. Liu, N. Li, G.-B. Jiang, J.-M. Liu, J.A Chromatogr. A 1066 (2005) 27. [14] T.-T. Ho, C.-Y. Chen, Z.-G. Li, T.-C. Yang, M.-R. Lee, Anal. Chim. Acta 712 (2012) 72. [15] R. Amini, A. Rouhollahi, M. Adibi, A. Mehdinia, Talanta 84 (2011) 1. [16] Y.-N. Hsieh, P.-C. Huang, I.-W. Sun, T.-J. Whang, C.-Y. Hsu, H.-H. Huang, C.-H. Kuei, Anal. Chim. Acta 557 (2006) 321. [17] K.-P. Huang, G.-R. Wang, B.-Y. Huang, C.-Y. Liu, Anal. Chim. Acta 645 (2009) 42.

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