Ionic Liquids in Sample Preparation

Ionic Liquids in Sample Preparation

CHAPTER SEVEN Ionic Liquids in Sample Preparation Rafael Lucena and Soledad C ardenas* University of C ordoba, C ordoba, Spain *Corresponding auth...

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CHAPTER SEVEN

Ionic Liquids in Sample Preparation Rafael Lucena and Soledad C ardenas* University of C ordoba, C ordoba, Spain *Corresponding author: E-mail: [email protected]

Contents 1. Introduction 2. Ionics Liquids as Extraction Solvents 2.1 Ionic liquids in dispersive microextraction formats

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2.1.1 Ionic liquids as solvents in dispersive liquideliquid microextraction 2.1.2 External energyeassisted ionic liquids-based dispersive liquideliquid microextraction 2.1.3 In situ ionic liquids formation 2.1.4 Dispersive liquideliquid microextraction by magnetic ionic liquids

2.2 Hollow fiber protected ionic liquids-liquid phase microextraction 2.3 Ionic liquidsebased single drop microextraction 3. Ionic Liquids in Solid Phase Microextraction 3.1 Classical in fiber solid phase microextraction 3.2 Ionic liquids in combination with nanoparticles in the solid phase microextraction realm 4. Concluding Remarks Acknowledgements References

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1. INTRODUCTION Sample preparation is the first step in any analytical process. In general, the laboratory sample, obtained after a well-defined sampling protocol, is usually incompatible with the direct analysis by the selected instrumental technique because of three main reasons, namely (1) low analyte concentration; (2) presence of potential interferences, mainly due to the matrix components and (3) aggregation state of the sample or need for solvent changeover due to instrumental requirements. Ideally, the substeps involved in any sample treatment should be simple, automated, miniaturized, cheap and safe. Therefore, several research efforts have been aimed at implementing these requirements. Among the strategies available, the use of new extractant phases, more efficient than Comprehensive Analytical Chemistry, Volume 76 ISSN 0166-526X http://dx.doi.org/10.1016/bs.coac.2017.01.007

© 2017 Elsevier B.V. All rights reserved.

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the conventional ones, is a topic of great relevance in this context. Nanostructured solids and new liquid media such as supramolecular solvents, switchable solvents or ionic liquids (ILs) can be cited among the most evaluated in the micro solid-phase extraction (m-SPE) and liquid phase microextraction or dispersive liquideliquid microextraction (LPME or DLLME) formats. ILs are a wide group of semiorganic salts composed entirely by ions which are liquid in a temperature range 180e600K. The existence of at least one constituent with a delocalized charge is behind the impossibility of forming stable crystal lattices [1]. Some ILs are also liquid at room temperature. The so-called room temperature ILs exhibit greater applicability in the microextraction context as they can be used as liquid extractant in m-SPE or LPME without any additional requirement. Although the green character of ILs is at present under discussion (see chapter: Green Analytical Chemistry: The Role of Green Extraction Techniques by Armenta et al. [1a] for further information), their interest as new solvents providing new features in the analytical process justifies the inclusion in the present book. The exceptional properties that have raised the ILs to play a prominent role in the microextraction context are their negligible vapour pressure, thermal stability and tunable viscosity and miscibility with other solvents. The nature of the cation and anion are responsible for the final physicochemical properties of the ILs and thus of their potential applicability in microextraction [2]. This chapter will present a general overview of the ILs in liquid and solidphase microextraction (SPME). As extractant solvent, they have been used in dispersive formats, supported on a hollow fibre or in the single drop configuration. The use of external energies [temperature and ultrasounds (USs)], in situ solvent formation or magnetic ILs (MILs) is also included in this chapter. In solid phase (micro)extraction, ILs have been successfully combined with nanoparticles (NPs) to design special sorbents. In addition, polymeric ILs have found great application in conventional in fibre SPME.

2. IONICS LIQUIDS AS EXTRACTION SOLVENTS 2.1 Ionic liquids in dispersive microextraction formats Dispersive extraction techniques in both solid and liquid formats, have been extensively used in the last decade due to the extremely high efficiency of the analytes transference from the matrix to the extractant phase. This fact

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is attributed to the boasting of the contact surface between phases achieved by the dispersion process, which result in a faster kinetics. In the DLLME realm, first proposed by Rezaee et al. [3], a mixture of extractant and disperser solvents is injected into an aqueous sample. A cloud of fine droplets is immediately formed. It favours the enrichment of the analyte in the extractant, which is finally recovered by centrifugation and analyzed using the most appropriate instrumental technique. ILs have been used as extractants in DLLME, but the efficiency of the extraction can be improved by means of an external auxiliary energy such as temperature or USs. Also, a great simplification of the process is achieved by using MILs as the extractant is recovered by means of an external magnet. Finally, the solubility of the IL in the sample can be tuned by the proper selection of the anion. These approaches will be discussed in the following subsections. 2.1.1 Ionic liquids as solvents in dispersive liquideliquid microextraction ILs have been used to extract either organic [4e7] or inorganic [8,9] compounds in different matrices although the latter approach requires the derivatization of the analytes (Co, Pb or Cd). In general, 1-alkyl-3methylimidazolium is the preferred choice. Other ILs, such as 1,3-dipentylimidazolium, have been proposed [10]. In addition to these conventional uses, ILs have also been used as disperser solvent instead of methanol or acetone. In this case, the main condition is that the IL used must be miscible with the aqueous sample matrix [11]. Concerning other alternatives available for the IL-based DLLME, the isolation procedure can be implemented using plastic disposable syringes. The so-called in-syringe DLLME allows the easy separation of the IL after the extraction without centrifugation taking advantage of the different density between the IL and the aqueous sample. In this configuration, the IL is separated by the simple and controlled movement of the plunger [12]. This in-syringe DLLME can be easily automated by using flow configurations which is an added value of the technique. UV-filters and Tl have been extracted from environmental waters using this automated workflow [13,14]. 2.1.2 External energyeassisted ionic liquids-based dispersive liquideliquid microextraction It is well-known that the disperser solvent has a negative effect on the thermodynamics of the DLLME as it directly increases the analyte solubility in

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the sample, making difficult their transference to the extractant solvent. Several alternatives have been evaluated to reduce and even eliminate the need for the disperser solvent in the DLLME. Among them, the uses of external energies such as temperature or USs seem to be ones of the most useful. Temperature-based DLLME modulates the solubility of the IL in the aqueous sample matrix. Both increasing and decreasing the temperature are respectively used to solubilize or induce the separation of the IL enriched with the analyte after the extraction. Temperature-controlled IL-DLLME was proposed by Zhou et al. for the determination of pyrethroid pesticides in water samples [15]. They used a water immiscible IL for the extraction. After dispersion, the temperature of the mixture is increased being the IL completely solubilized (viz. dispersed) as a result. Next, the temperature of the mixture is cooled down using an ice-bath being the two phases separated as the consequence of the lower solubility of the IL under this thermal condition. After a centrifugation step, the IL enriched with the analyte is separated and analysed by liquid chromatography with ultraviolet detection. Phenols [16], hydrocarbons [17] and UV-filters [18] have been identified and quantified in environmental samples using liquid chromatography coupled with different detectors with very promising results. Cold induced aggregation microextraction was proposed in 2008 by Baghadadi and Sheminari to determine Hg in water [19]. The main difference with the workflow previously described is that they use a chemical disperser to assist the dispersion of the IL in the sample. Some other authors have used similar approaches to determine organic [20,21] and inorganic [22,23] contaminants in environmental samples. The beneficial effect of using US to assist any LPME process is a very well-known issue. Application of US not only affects the homogeneous dispersion of the two immiscible phases involved in the extraction process, but also the kinetics of the potential reaction that takes place during the extraction. Concerning the first effect, the use of US to facilitate the dispersion of the extractant reduces or even avoids the need for a disperser solvent. This fact positively affects the extraction efficiency as the solubility of the analyte in the sample is not altered (usually increased). The cavitation induced the formation of fine droplets of the extractant solvent with enhanced surface to volume ratio. US-assisted IL-DLLME has been used to extract different contaminants in environmental samples although it can be extended to other analytes and samples. In general,

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it requires the addition of typically 100 mL of the IL to the sample. The mixture is then sonicated (in a bath or by means of a probe) during an optimized time to ensure complete dispersion of the IL. Depending on the nature of the IL, the time and power of US used, a subsequent cooling step would be necessary to facilitate the recovery of the IL after the extraction, usually by centrifugation. Following this general procedure, with slight modifications, aromatic amines [24], pharmaceuticals [25] and mercury species [26] have been extracted from environmental water samples with excellent results. 2.1.3 In situ ionic liquids formation Another alternative to modify the solubility of an IL in a liquid sample is the nature of the anion included in its structure. This property can be exemplified for [bmim] cation, which is highly soluble in water as chloride while it is almost insoluble when combined with [PF6] anion. The substitution of the anion in an IL is known as metathesis reaction, and it is behind the so-called in situ solvent formation microextraction (ISFM). This technique was first proposed in 2009 [22], and its advantages over other approaches clearly pointed out [27]. ISFM does not require the use of a disperser solvent as the IL is added in its soluble form to the sample (viz. maximum dispersion). Next, only a few seconds later (e.g., 30 s), a proper reagent is added to induce the formation of the insoluble IL via metathesis reaction. The IL enriched with the target compounds is further recovered by centrifugation (typically 5 min). The competitiveness of ISFM as regards other microextraction techniques is demonstrated by the enrichment factors achieved (10e20 times higher) in relatively short times (less than 10 min) if compared with those obtained with, for example, IL-single drop microextraction (SDME) after 60 min of extraction. In general, ILs are hard to be recovered after centrifugation. Sometimes, the use of a syringe or solidification by cooling the solution after centrifugation can be used to facilitate the recovery. The use of a syringe cannot prevent from separating a low sample volume together with the IL, which reduces the method precision. The second alternative permits the complete separation of the sample from the solid extract which can be further redissolved for instrumental analysis [28]. The high saline content of some samples, mainly of environmental nature, can also influence the metathesis reaction, displacing the equilibrium to the soluble form of the IL. This fact hinders the quantitative recovery of

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the IL. However, this limitation can be overcome by increasing the concentration of the metathesis reagent [29]. Both organic [30e32] and inorganic [28,33] compounds have been determined using ISFM. 2.1.4 Dispersive liquideliquid microextraction by magnetic ionic liquids In the DLLME context, the use of extractants with superparamagnetic properties is of relevance as the extractant phase can be easily recovered after the isolation of the analytes by the simple use of an external magnet located on the walls of the extraction vessel. This fact avoids the need for tedious steps such as centrifugation or filtration. MILs are a particular case of IL which successfully combine the tunable physicochemical properties of the ILs with the strong susceptibility to external magnetic field. This synergic combination makes MILs ideal solvents for DLLME. Fe(III), Gd (III) or Dy (III) have been proposed for the synthesis of MIL [34e36]. Trihexyl(tetradecyl)phosponium tetrachloroferrate has been proposed for the isolation and preconcentration of phenols from waters [37]. Moreover, it seems that MILs can play a crucial role in the extraction of biomolecules such as DNA [38]. The advantage over magnetic NPs is the enhanced extraction efficiency as the IL structure can be tailored to maximize the interactions with the phosphate backbone of the DNA without losing the superparamagnetic behaviour. The challenge in this analytical problem is the synthesis of an MIL, hydrophobic enough to allow phase separation after the extraction. The authors deeply discussed the influence of the use of a hydrophobic anion or a functionalized cation to improve the hydrophobicity of the MIL while allowing the incorporation of paramagnetic anions such as FeCl4  . Finally, three different MILs were evaluated in the dispersive format for the extraction of DNA from a simple matrix, containing metals and other proteins, and from more complex ones such as bacterial cell lysate. A clear influence of the MIL used in the DLLME was observed depending on the length of the DNA chain. Also, the composition of the matrix would affect to the extraction recovery through a competitive mechanism.

2.2 Hollow fiber protected ionic liquids-liquid phase microextraction An alternative to DLLME involves the immobilization of the IL in a solid support. This specific approach is the so-called hollow fibre protected IL

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liquid phase microextraction (HF-IL-LPME). In this configuration, the IL is located in the lumen and/or the pores of a hollow fibre which is immersed in the sample (DI-HF-IL-LPME) or exposed to its headspace (HS-HF-ILLPME). The presence of the IL in the hollow fibre modifies the physical structure of the membrane affecting the extraction behaviour. When the IL is incorporated into both, the lumen and the pores of the hollow fibre, the extraction is named two phase HF-IL-LPME (2D-HF-ILLPME). This modality is especially useful for the extraction of nonpolar or moderately nonpolar compounds as they are highly soluble in the ILs typically used in this configuration. UV-filters [39] and BTEX [40] have been extracted from waters with high extraction recoveries. As it has been described for DLLME, the extraction of inorganic targets (e.g., Pd or Ni) in 2D-HF-IL-LPME, requires the use of a derivatization reaction to facilitate the transference of the analytes from the aqueous phase to the IL [41]. The IL can be exclusively situated in the pores of the fibre, being the lumen filled with an acceptor (aqueous or organic) phase in the so-called three phase HF-IL-LPME (3P-HF-IL-LPME). This configuration involves a double extraction, from the sample to the IL and back extraction to the acceptor phase. This fact increases the application field as ionizable organic compounds can be extracted from aqueous phase by establishing a convenient pH gradient at both sides of the membrane to promote the analytes transference in their neutral form to the IL phase (viz. pores of the hollow fibre) and charged to the acceptor aqueous phase (inside the hollow fibre) [42]. The extraction of polar compounds is still a challenge in both 2D and 3D HF-IL-LPME as the mass transference is hindered by the low solubility of the polar compounds in the IL. Some alternatives, such as the use of a carrier to solubilize the analytes in the IL, have also been proposed to extract sulfonamides from environmental samples with favourable analytical figures of merit [43]. IL can play additional roles in the 3P-HF-IL-LPME. For example, they can be used to minimize or avoid losses of the organic acceptor phase miscible with the aqueous phase. It permits the extraction of aliphatic and aromatic hydrocarbons in storm water samples, being the extract compatible with the instrumental technique used and eliminating the need for evaporation-redissolution steps [44]. Their use as pseudophase in 3P-HF-IL-LPME promotes the extraction of neutral compounds by increasing their solubility in the acceptor phase. The miscibility between both phases is the key factor in this process [45].

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It is also an elegant alternative to increase the extraction of Cr (VI) and Cr (III) from different samples [46].

2.3 Ionic liquidsebased single drop microextraction The reduction of the volume of the extractant phases is a key factor to increase the sensitivity of the measurements. The use of a single droplet of an extractant phase was proposed simultaneously by Cantwell [47] and He [48]. Despite the clear advantages of the SDME in terms of sensitivity improvement, its practical development reveals some shortcomings such as the instability of the droplet or its losses by evaporation or solubilization in the sample matrix. Therefore, the research in this field has been focused on solving these limitations. The physical properties of the ILs, mainly related to their high density, viscosity and low vapour pressure make them a promising alternative in this field. The droplets generated with ILs are more stable than those obtained with organic solvents. Also, the miscibility with the aqueous sample can be reduced by the proper selection of the ions of the IL used in the extraction. Typical applications of IL-SDME involve the isolation and preconcentration of organic compounds either in the DI or HS configurations. In the case of DI, it can be implemented by suspending the droplet in a needle tip [49] or directly adding the droplet to the sample [50]. The HS configuration is preferred for the determination of volatile and semivolatile compounds. The use of ILs is especially relevant in this modality because of the higher stability of the droplet even at relative high temperatures (viz. 90 C) without losses by evaporation. Liquid chromatography coupled to different detectors is usually selected as instrumental technique in spite of the volatility of the analytes due to the incompatibility of the IL with gas chromatography. Our research group designed a removable interface (Fig. 1) that permits the repetitive injection of the IL after SDME while preventing it from entering into the chromatographic system [51]. A cotton-bead is used as physical barrier that retains the IL and allows analyte evaporation by the simple heating of the interface at an appropriate temperature. The main advantage of using ILs as extraction solvent in gas chromatography is that even volatile analytes (viz. with short retention times) can be determined as no solvent peak (usually a broadening one) is obtained. Another interesting alternative has been proposed by Zhao et al. [52]. The authors modified the liner of the GC to allow the thermal desorption of the analytes from the IL droplet inside the liner. Then, the IL is withdrawn in the syringe and removed from the injection port.

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Figure 1 Schematic diagram of the proposed interface for the direct coupling of ionic liquidebased single drop microextraction and gas chromatography. PFA, Perfluoroalkoxy; SDME, Single drop microextraction; SS, Stainless steel. Reproduced with permission of American Chemical Society from E. Aguilera-Herrador, R. Lucena, S. rdenas, M. Valca rcel, Direct coupling of ionic liquid based single-drop microextraction Ca and GC/MS, Anal. Chem. 80 (2008) 793e800.

The use of desorption tubes [53] or completely automated configurations [54] are also valuable contributions in this field. The DI-IL-SDME can also be opened to the determination of inorganic species, mainly metals, if a neutral chelate, soluble in the IL is previously formed. Either molecular or atomic absorption techniques can be selected for analytes determination. A speciation analysis can be carried out including a previous chromatographic separation of the different chelates formed. In this context, the speciation of Hg in waters has been proposed by Pe~ naPereira et al. by the extraction of the corresponding dithizone chelates and further liquid chromatographic separation with UV detection [55]. If only the total amount of the element is required, atomic absorption with electrothermal atomization is the preferred choice because the IL is destroyed during the analysis and does not generate any irreproducibility

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due to variations in the aspiration flow rate or efficiency of the nebulization step that can occur in flame atomic absorption spectrometry, where the use of ILs is not recommended.

3. IONIC LIQUIDS IN SOLID PHASE MICROEXTRACTION 3.1 Classical in fiber solid phase microextraction Although bulk ILs, due to their aggregation state, have found a great applicability in the LPME context, they have been also proposed as stationary phase in SPME. In the earlier approaches, bulk ILs were physically coated to bare silica fibres [56] generating a nonstable coating. This coating tended to blow out, especially in the GC injector, making necessary the recoating of the fibre after the extraction and increasing the potential appearance of ghost peaks in the chromatograms due to the thermal IL degradation. The use of special supports, like nafion membranes [57], improved the performance but the main challenges still remained. The development of polymeric ionic liquids (PILs) resulted to be a milestone in this context and opened a door to exploit the ILs properties in SPME technique. PILs are polymers obtained by the controlled reaction of monomers that contains typical ILs moieties (usually cations), resulting in a solid materials that maintain the extraction capabilities of ILs. These solids present a higher mechanical and thermal stability than bulk ILs extending therefore their application scope. The research group of Prof. Anderson, which is a reference in this context, proposed for the very first time the use of PILs in SPME [58]. For this purpose, the PIL is previously synthesized by a free radical polymerization. After its purification, the PIL is dissolved in an appropriate solvent forming a solution where a fused silica fibre (previously sealed by a torch) is immersed several times. The evaporation of the solvent allows the formation of a PIL film over the fibre, the thickness of this coating being directly dependent on the number of dipping cycles. Fig. 2 shows scanning electron micrographs of the fused silica fibre before and after the deposition of the PIL coating. The enhanced mechanical stability allows not only the development of SPME in its headspace mode but also in the direct immersion one [59] providing normalized sensitivity factors better than the conventional PDMS coating. PILs share with bulk ILs their great versatility since several interaction chemistries can be developed with the

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Figure 2 Scanning electron pictures of a bare fused silica fibre (left panel) and the same fibre after the polymeric ionic liquid coating (right panel). Reproduced with permission of Elsevier from F. Zhao, Y. Meng, J.L. Anderson, Polymeric ionic liquids as selective coatings for the extraction of esters using solid-phase microextraction, J. Chromatogr. A 1208 (2008) 1e9.

targets analytes. In fact, PILs can also be tuned introducing special moieties, both in the polycation [60,61] or anion [62], to boost their interaction with the target analyte. In addition the combination of different PILs has also been proposed to increase the extraction capacity [63]. Although the potential of PILs deposition on the silica capillary is out of discussion, several efforts have been done to enhance the mechanical strength of the coating. The covalent bonding of the PIL to the fibre support is the main strategy which can be complemented with other alternatives like the selection of the support (different to the classic silica-based one) or the cross-linking of the polymer. Feng et al. have proposed the use of stainless steel (SS) as support for the covalent bonding of PIL [64,65]. Due to the inertness of SS, a previous treatment of the surface is necessary to immobilize the PIL. In this sense, SS is previously coated with silver modifying in this way not only the chemical composition of the surface but also its area. The silver layer is modified using a thiol monolayer that is finally involved, with the PIL monomer, in an in situ via surface radical chain-transfer polymerization yielding the polymeric coating. SS can be also modified in other ways. Pang and Liu proposed the precoating of SS with gold (thanks to replacement reaction in chloroauric acid) which is finally coated by a silica layer that acts as the real support for the PIL growth [66]. In addition, silicon layer formed by magnetron sputtering has been suggested as a way of SS modification [67]. Cross-linking of the PIL also enhances the mechanical and thermal stability of the coating [68]. The cross-linker proportion plays an important role in the final state of the coating. It modifies the state of the system which

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drives from liquid to a hard semisolid form. It also increases the fibre lifetime, due to a reduction of the coating thermal bleeding, but according to Feng et al. the extraction reproducibility decreases at higher cross-linker proportions. The support is key to achieve a high mechanical stability on the final fibre but it is also important to define the coating coverage. Titanium wires are especially remarkable in this sense since their surface can be modified to increase the superficial area. Anodization provides titanium wires with a rougher surface increasing the amount of PIL that can be immobilized [69]. Anodization is also able to create titanium dioxide nanostructures [70] to boost even more this superficial area. In this sense, the use of nanostructures in combination with PILs has been proposed by some researchers to improve the extraction capacity of the fibres. NPs may play a double role in this context as they may introduce new interaction chemistries on the resulting fibre, and they may change the inner structure of the fibre increasing the porosity since the inclusion of the nanoparticle modifies the normal polymeric growth. Carbon nanotubes, due to their nanometric size and their ability to interact pep with aromatic compounds, have been exploited in this context [71e73]. Fig. 3 shows the differences between neat PIL coating (upper panels) and the nanotubes-PIL coating (lower panels). As it can be observed, the superficial structure of the coating dramatically changes forming a rougher surface. The porosity of the PIL coating may be further increased by using monolithic materials [74,75]. The increase of the porosity, which involves an increase in the superficial area, has a very positive effect on the sorption capacity and also in the extraction kinetics. The use of thick monolith PILs modifies a little bit the geometry of the SPME unit as it can be observed in Fig. 4 [76]. In this approach, a waterlogged PIL monolith is pierced by an SS wire allowing its direct immersion into the sample. After a defined extraction time, the monolith is withdrawn from the sample and chemically eluted for the subsequent HPLC analysis. These monoliths can be easily modified with graphene oxide to improve the extraction ability [77]. Other approaches, different to the synthesis of a conventional PIL coating over a fibre support, have been explored too. Silica particles grafted with PIL have been glued to a conventional SPME support [78] or packed into a needle [79] for microextraction purposes with excellent results. In addition, hollow fibres can be also used as support for the synthesis of PIL-based SPME devices [80]. In this case, a PVDF hollow fibre is immersed into a prepolymerization allowing the wetting of the pores and lumen with the monomers. The polymerization induces the formation of PIL capsules

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(B)

(C)

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Figure 3 Scanning electron pictures of a neat polymeric ionic liquids (PIL) coating (A, B) and a nanotubes-PIL coating (C, D). Reproduced with permission of Elsevier from C. Zhang, J.L. Anderson, Polymeric ionic liquid bucky gels as sorbent coatings for solid-phase microextraction, J. Chromatogr. A 1344 (2014) 15e22.

Figure 4 Solid-phase microextraction device using polymeric ionic liquid monoliths. Reproduced with permission of Springer from J. Feng, M. Sun, Y. Bu, C. Luo, Development of a functionalized polymeric ionic liquid monolith for solid-phase microextraction of polar endocrine disrupting chemicals in aqueous samples coupled to high-performance liquid chromatography, Anal. Bioanal. Chem. 407 (2015) 7025e7035.

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(B)

Figure 5 Scanning electron pictures of the hollow fibre membrane coated polymeric ionic liquid (PIL) capsules (A) and the PIL sorbent (B) at magnification 500. Reproduced with permission of Springer from J. Feng, M. Sun, Y. Bu, C. Luo, Hollow fiber membrane-coated functionalized polymeric ionic liquid capsules for direct analysis of estrogens in milk samples, Anal. Bioanal. Chem. 408 (2016) 1679e1685.

where the sorptive phase is protected by the hollow fibre increasing the selectivity of the extraction and allowing the processing of complex samples. Fig. 5A shows the hollow fibre protected PIL coating while the superficial structure of the PIL is shown in Fig. 5B.

3.2 Ionic liquids in combination with nanoparticles in the solid phase microextraction realm The combination of ILs and NPs is a research line into continuous development. NPs can be defined as those materials with one or more dimensions in the nanometer range (using 100 nm as reference) and possessing novel characteristics compared to those observed at the macroscale [81]. The huge impact of NPs in sample treatment [82] is due to several facts. On one hand, there is a wide range of NPs, either of inorganic or carbon origin, providing different interaction chemistries which make possible their proper selection attending to the analytical problem under study. On the other hand, NPs can be chemically modified to improve the selectivity and/or the efficiency of the analyte extraction. In addition, NPs may confer special properties to the resulting materials. Among these properties, the large surface to volume ratio involves a dramatic increase of the sorption capacity and the extraction kinetics. Paramagnetic nanoparticles (MNPs), which are attracted by external magnetic fields, have been extensively used in combination with ILs to assist the extraction of target analytes from liquid samples. These MNPs act as inert support for IL, the real sorptive phase, which can be anchored to the

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nanoparticle surface in different ways. The superficial charge of the MNPs can be used to retain the IL cation by electrostatic forces. Cheng et al. used this approach for the retention of 1-hexadecyl-3-methylimidazolium over the surface of Fe3O4 NPs working at higher pH than the NPs isoelectric point (6.3 for uncoated Fe3O4) [83]. The concentration of the IL plays a key role since it defines the number of IL cation layers over the NPs surface, therefore defining the interaction chemistry (nonpolar, ionic or mixed) with the target analytes. This workflow can be extended to other MNPs just requiring the previous evaluation of the MNP isoelectric point [84]. The MNP-IL interaction remains until the elution step when it is broken, releasing the IL to the elution media. Covalent bonding avoids this release since the IL is strongly immobilized on the MNPs surface during the whole extraction process. This approach is easily compatible with LC [85,86] but it extends the applicability to GC [87] as no IL remains in the final eluted fraction. Covalent bonding has been also proposed to anchor PIL over magnetic NPs [88e90]. The combination of ILs and magnetic NPs goes beyond the SPME context. In fact, Li et al. proposed this combination in ionic liquid-basedeDLLME to assist the recovery of the solvent after the extraction [91]. Although MNPs-ILs have been extensively used for microextraction purposes, other combinations have been also proposed. Ríos et al. used gold NPs coated with IL to isolate and preconcentrate sulfonylurea herbicides from water samples [92]. As these NPs cannot be recovered by an external magnetic field, the authors preferred to pack them in a conventional SPE cartridge to make easier the processing of the samples. Polo-Luque et al. have also presented the potential of the carbon nanotubes-ionic liquid [93,94]. The IL enhances the dispersion of nanotubes, which tend to aggregate, and the resulting soft material combines the excellent properties of both components.

4. CONCLUDING REMARKS ILs play a crucial role in the development of new and efficient microextraction approaches in both the liquid and solid phase formats. Their physicochemical properties together with their tunable synthesis can be highlighted as the most favourable features. They overcome some of the limitations of common organic solvents in LPME, increasing the reproducibility of the measurement while maintaining or enhancing the sensitivity

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and selectivity of the microextraction step. The possibility of including magnetic properties also simplifies the analytical process without the need of additional synthetic step. As far as SPME is concerned, they are also competitive phases and PILs which clearly contribute to the robustness of the coatings. As far as LPME is concerned, the main role of ILs will be played in the dispersive context. The in situ solvent formation and the use of magnetic ILs are the main alternatives. The use of a metathesis reaction to modify the miscibility of the IL with the sample matrix eliminates the need for a disperser solvent, which maintains the distribution constants of the analytes unaltered during the isolation step. The yield of the microextraction is increased as result. The use of magnetic ILs is competitive with the hybrid NPs-IL coreeshell structures. The synthetic process is dramatically reduced while the extractant exhibits a superparamagnetic behaviour which simplifies its recovery after extraction. As a main shortcoming, the reduced availability of MILs reduces up to now the application field. Regarding micro m-SPE, no doubt, the synthesis of novel hybrid nanomaterials resulting of the coverage of NPs or polymers with ILs is of special relevance. The great potential of polymeric ILs in formats different to SPME will open a door to their extensive use in this microextraction modality.

ACKNOWLEDGEMENTS Financial support from the Spanish Ministry of Economy and Competitiveness (CTQ201452939R) is gratefully acknowledged.

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