Advances in different configurations of solid-phase microextraction and their applications in food and environmental analysis

Accepted Manuscript Title: Advances in different configurations of solid-phase microextraction and their applications in food and environmental analysis Author: Jia Li, Yan-Bin Wang, Ke-Yao Li, Yuan-Qi Cao, Shang Wu, Lan Wu PII: DOI: Reference:

S0165-9936(15)00189-2 http://dx.doi.org/doi:10.1016/j.trac.2015.04.023 TRAC 14487

To appear in:

Trends in Analytical Chemistry

Please cite this article as: Jia Li, Yan-Bin Wang, Ke-Yao Li, Yuan-Qi Cao, Shang Wu, Lan Wu, Advances in different configurations of solid-phase microextraction and their applications in food and environmental analysis, Trends in Analytical Chemistry (2015), http://dx.doi.org/doi:10.1016/j.trac.2015.04.023. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Advances in different configurations of solid-phase microextraction and their applications in food and environmental analysis Jia Li a, *, Yan-Bin Wang a, Ke-Yao Li b, Yuan-Qi Cao c, Shang Wu a, Lan Wu a a

Chemical Engineering Institute, Key Laboratory of Environmental Friendly Composite Materials and

Biomass Utilization, Northwest University for Nationalities, Lanzhou, 73000, China b

No.1 Hospital of People’s Liberation Army, Lanzhou, 730000, China

c

Clinic Department of Lanzhou General Hospital, Lanzhou Military, Lanzhou, 730000, China

HIGHLIGHTS  Hollow-fiber-based solid-phase microextraction  Magnetic dispersive solid-phase microextraction  Ionic liquid-based solid-phase microextraction  Future potential developments of solid-phase microextraction Graphical abstract

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ABSTRACT Major objectives in research for food and environmental samples include improving robustness, sensitivity, selectivity, and extraction efficiency for polar organic compounds, avoiding matrix effects, and developing faster, simpler, more environment-friendly sample-preparation procedures. In this article, we review novel configurations of solid-phase microextraction (SPME), which make the whole process of analysis more selective, more sensitive and more environment friendly. These techniques include hollow-fiber SPME, magnetic dispersive SPME, and ionic liquid-based SPME. Finally, we look ahead to future potential developments of these configurations of SPME. Keywords: Environment Extraction efficiency Food Hollow fiber Ionic liquid Magnetic dispersive solid-phase microextraction

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Magnetic nanoparticle Molecularly-imprinted polymer Sample preparation Solid-phase microextraction * Corresponding author. Tel.: +86 931-2938253; Fax: +86 931-2938032. E-mail address: [email protected] (J. Li)

1. Introduction The development of novel sample-preparation techniques with significant advantages over conventional methods, such as reduction in organic-solvent consumption and in sample degradation, elimination of additional sample clean-up and concentration steps before chromatographic analysis, improvements in extraction efficiency and selectivity, are likely to play an important role in sample pretreatment of analytical chemistry. A successful sample pretreatment method typically has three major objectives: (1) sample matrix simplification; (2) analyte enhancement or concentration; and, (3) sample clean-up [1]. Several new miniaturized extraction procedures, known as liquid-phase microextraction (LPME) and solid-phase microextraction (SPME) techniques, have been introduced and applied with success [2–6]. One of the most popular techniques for preparing samples for analysis to satisfy the requirements of green analytical chemistry is SPME, developed in the 1990s by Pawliszyn and his group [7,8], and implemented by researchers worldwide regarding its fundamental understanding, development of devices and novel applications. As a simple, rapid, practical and effective sample-preparation technique, SPME, coupled with various instrumental analytical methods, especially gas chromatography (GC) and high-performance liquid chromatography (HPLC), has been increasingly and widely used to research and to determine trace or ultra-micro levels of inorganic and organic analytes from samples of complex matrices. The enormous popularity of SPME is due to its undoubted merits: simplicity of operation, relatively short extraction time, solvent-free nature, possibility of full automation, and easy coupling with chromatography (such as GC), all of which reduces contamination of the original sample and loss of analytes. SPME can also be used for on-site analysis, and where extraction occurs on-site and only instrumental analysis is performed in laboratory. In addition, using SPME, samples can be collected in situ, and reliable results can be obtained for analytes present in trace quantities. There have been significant advances over the past five years in the development of methods for environmental analysis, and SPME continues to be one of the leading techniques for the extraction of pollutants from aquatic systems [9]. Current trends in the handling of environmental samples include the development of new sorbents for sorptive extractions, the evaluation of new complexing agents, and novel configurations and strategies in SPME. Food samples are very complex, often containing proteins, fat, salts, acids, bases, and numerous food additives with different chemical properties. A large number of analytical tools, especially chromatography, have been used to analyze the constituents of food in order to control their quality. However, considering the

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complex constituents in food samples, attention should be focused on those novel pre-treatment methods, extraction techniques with enhanced efficiency and sensitivity, and highly selective sorbents that have been applied in pretreating food matrices. With increasing applications in food samples, SPME plays an important role in food-sample pretreatments. In this article, we review several configurations of SPME used in food and environmental matrices, which make the whole analysis process more selective, more sensitive and more environment friendly. These techniques include hollow-fiber SPME (HF-SPME), magnetic dispersive SPME (MD-SPME) and ionic liquid-based SPME (IL-SPME). Emphasis is placed on brief descriptions of the unique capabilities and advantages of each modern extraction technique, and how these techniques were exploited to improve absorption and extraction for a variety of analytes. These techniques, based on the partition or adsorption of analytes, were responsible for extracting the majority of the analytes from the sample matrix prior to analysis. Finally, we look ahead to future developments and potential applications of these configurations of SPME methods in food and environmental analysis.

2. SPME techniques in complex matrix As one of the most widely-used sample-preparation techniques for liquid and gas samples, SPME belongs to the group of sorptive-based extraction techniques, in which the sample is placed in contact with a suitable material, so the availability of different materials to carry out the extraction is essential. SPME combines sampling, extraction, separation and concentration in one step. In SPME sampling, the analyte is distributed among two or three phases – the fiber coating (or the sorbent materials), condensed samples (liquid or solid sample), and headspace (HS). The kinetics of mass transport of the analytes within the various media determines the SPME sampling time. A distribution constant establishes the equilibrium-concentration ratios of the analyte across each pair of phases. This distribution constant is temperature-dependent, and is also affected by certain factors (e.g., the nature of sorbents, ionic strength, pH of samples, and organic-solvent content). Heating tends to drive analytes out of the liquid phase, but also alters the partitioning of the analytes between the sample and the sorbent. Extraction time and temperature are interrelated variables, and their effect on SPME should not be examined by simply changing one variable at a time [10]. Advances in SPME, in vivo and in vitro, for the analysis of organic and inorganic compounds from different food and environmental samples, such as beverages, tea, milk, fish, cereals, vegetables, fruits, herbs and medicinal plants, and water samples, which are summarized and discussed below, clearly demonstrate the potential of SPME as a powerful sample-preparation tool in analysis of complex samples. Focus was primarily on the different static configurations of SPME techniques. Research on sample-preparation techniques often therefore focuses on developing new materials to achieve higher selectivity and capacity of the configurations. 2.1. HF-SPME Polypropylene hollow fibers (PP-HFs) were first introduced by Jeannot and Cantwell in 1996 [11] and He and Lee in 1997 [2] in the form of two- and three-liquid-phase microextraction [12]. Due to its low cost, low carry-over effect,

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low organic-solvent consumption, and better sample enrichment and clean-up function, PP-HFs were usually used in HF liquid-phase microextraction (HF-LPME) for the protection of microdroplets of extraction solvents, and proved to be practical for the treatment of samples in various areas of study [13,14]. Although high enrichment, clean-up and low solvent consumption are the major advantages of HF-LPME, relatively long extraction times and low selectivity are perhaps its major disadvantages [12,15]. Since the fused-silica or steel fibers for SPME were still comparatively expensive or the polymer coating was fragile [16], PP-HF seems to be a good alternative for SPME due to its cheapness, toughness and repeatable use. However, there is low selectivity and limited specific surface areas in PP-HF [17], and pure PP-HF did not possess much absorption capacity. PP needs to be immobilized or bonded with other absorptive materials to improve its extraction ability, so, with PP-HF as template, zirconia HF in the macro range was successfully synthesized for the first time via a template method using a sol-gel process [18]. The preparation procedure included repeated impregnation of the template in a proper zirconia sol precursor, and calcination to burn off the template, then zirconia HF was produced. The resulting HF is almost identical to its template in terms of morphology, exhibiting a hollow lumen structure. HF-SPME is a new configuration in terms of an entity. It can avoid tedious procedures associated with the powder as the adsorption phase, such as centrifugation or filtration, or adhesion onto the support materials. Zirconia HF can also be used as a substrate, even in an extreme pH and temperature environment. The work has opened the possibility of fabricating numerous interesting inorganic HF structures with a homogeneous controllable wall and porous substructure. Inspired by Lee’s work, a series of oxide HFs {e.g., TiO2 [17,19], SiO2 [20], Al2O3 [21] and MgO [22]} were fabricated by the sol-gel method and their applications explored in the analysis of natural products [20], food [19,23], environmental [17] and biological [24] samples. Oxide HF even allows direct derivatization on it and could be coupled to ultrasound- or microwave-assisted extraction, and the desorption solution analyzed by GC or HPLC in combination with mass spectrometry (MS). However, inorganic metal oxides still have some common problems, such as relatively low selectivity for analytes in complicated matrices. Thus, further modification, via chemical bonding or coating, to increase the selectivity could be an efficient, easy way to expand its potential applicability. With the development and innovation of functional materials, there have been an increasing number of applications of carbon nanotubes (CNTs). CNTs exhibit extraordinary structural, mechanical and electronic properties, which have made them potentially useful in CNT-reinforced materials, such as sorbents for SPME [25]. Basheer et al. used a small bag made of PP filled with CNTs. This micro-bag membrane was used as a protective barrier for micro-solid-phase extraction (µ-SPE) [26]. They found this µ-SPE method was robust and durable, but it was not easily automated. Es’haghi and his group promoted the HF-SPME technique by inserting CNTs into the pores or the lumen of PP-HF using sol-gel technique [27–31]. The silica sol containing CNTs was injected into a PP-HF segment, and the gel-formation process was implemented in situ. CNTs were held in the lumen or immobilized in the pores of the HF by the sol-gel method, which offered strong adhesion between PP-HF and CNTs, while a porous silica structure provided large surface area and a compatible composition to increase sorbent selectivity. Analytes could be extracted by both porous silica and CNTs to increase the selectivity and the enrichment efficiency.

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CNT-reinforced HFs can also be used in complex matrices, especially in food and biochemical samples, as the pore in HF prevents the entrance of large molecules. As “a molecular shape sorter”, β-cyclodextrin can entrap and retain analytes with appropriate sizes and polarities in its cavity, so it has remarkable ability to recognize certain analytes in a highly selective and sensitive manner. In Song’s work, β-cyclodextrin was covalently linked to carboxylic CNTs, forming a new sorbent material, which was then immobilized into the lumen of the HF. This β-cyclodextrin/CNT-reinforced HF was used for extraction and determination of naphthalene-derived plant hormones in complex matrices with satisfactory results [32]. However, the interaction between HF and CNTs was mainly physical, and not as stable as chemical binding. Accordingly, researchers explored methods for improving the stability of CNTs in the HF-SPME. Multi-walled CNTs (MWCNTs) were first processed to be functionalized with –OH, –COOH, –NO2 and –HSO3, and dispersed in sol by sonication. Modified MWCNTs showed a higher extracted amount than raw MWCNTs, because the surface of modified MWCNTs has more oxygenated functional groups, which makes modified MWCNTs more hydrophilic, thus more suitable for the extraction of comparatively polar compounds [33]. Apart from modifying MWCNTs with different hydrophilic functional groups, Li et al. prepared a composite of MWCNTs and SiO2 on the porous surface of the HF using the layer-by-layer (LBL) self-assembly technique [34,35]. Not only do these composite materials help to enhance the surface area and the pore-size distribution of PP-HF controllably, but they also avoid centrifugation. Besides, the composite HFs might be selective to some organic compounds due to electrostatic attraction, π-π stacking, hydrophobic interaction and hydrogen bonding. The MWCNT/SiO2-reinforced HF was applied to determine trace phthalates in food samples [35]. The extracted amount of phthalates increased due to the strong hydrophobic and π-π interactions between the target analytes and MWCNTs. Since the fibers can be reused, this technique is cost effective for the quality control of phthalate residues in food samples. The MWCNT/SiO2-reinforced HF was also applied for the extraction of metronidazole in milk, and proved that the MWCNT/SiO2-HF has higher extraction efficiency for polar compounds in milk samples than those of MWCNT/ZrO2-HF and MWCNT/TiO2-HF [34]. Fig. 1 shows comparisons of SEMs for ZrO2-HFs, TiO2-HFs, SiO2-HFs and MWCNT/ZrO2-HFs, MWCNT/TiO2-HFs, and MWCNT/SiO2-HFs. Graphene (G) has become one of the worldwide research hotspots since its discovery [36]. It can form a strong π-π stacking interaction with the benzene ring due to its large delocalized π-electron system, which causes it exhibit strong sorption for benzenoid compounds. In addition, due to its high specific surface area, G has an adsorption capacity for certain analytes higher than that of CNTs [37]. A Graphene-oxide-silica (GO-SiO2) composite-coated HF was prepared for the first time and used for the extraction of trace heavy metals in various water samples followed by inductively-coupled plasma MS (ICP-MS) with good results [38]. Since MWCNTs and inorganic metal oxides have good affinities for certain metal ions, HF-supported sol-gel combined with MWCNTs, coupled with differential pulse anodic stripping voltammetry (DPASV) was employed in the simultaneous extraction and determination of heavy-metal ions {e.g., lead [29,39], arsenic [40], cadmium [41], zinc [42] and copper [38]}. The method involves microextraction and pre-concentration of metal ions on the pseudo-stir-bar HF. DPASV using a hanging mercury-drop electrode (HMDE) was applied. Desorption was then done using a

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suitable solvent containing a suitable ligand as complexing agent. The PP porous membrane showed high stability and could be used in a method based on CNTs reinforced sol-gel combined with ASV for the extraction and determination of metal ions in a single stage, with extraction and back-extraction occurring at the same time. Furthermore, the extraction efficiency of reinforced HFs can be further enhanced {e.g., by molecularly imprinted polymers (MIPs) [30], ILs [39], magnetic nanoparticles (MNPs) [43,44]}. Table 1 shows the most recent of these applications of HF-SPME. No matter which kind of absorptive material is immobilized or bonded on the surface or in the lumen of HF, PP-HF usually plays the role of carrier for absorptive materials to avoid centrifugation or filtration. 2.2. MD-SPME Among the available adsorbents, MNPs, particularly magnetite (Fe3O4), have made rapid and significant progress in extraction [45], due to their large surface-area-to-volume ratio, low toxicity and biocompatibility [46]. With respect to MNP adsorbents, satisfactory results were therefore obtained by using a few NPs as adsorbent and high activities could be caused by the size-quantization effect. As known, magnetic materials as sorbents have several advantages over traditional sorbents [47,48]. The separation process can be performed directly in sample solution containing solid sorbent, and the MNPs can be collected and separated from the liquid phase using a magnetic field, which avoids tedious filtration or centrifugation [49]. Due to the large surface-area-to-volume ratios of nanometer (nm)-sized sorbents, MNPs are particularly useful for extracting and enriching a large volume of target analytes without tedious centrifugation. And the target analytes absorbed on the surfaces of MNPs can easily be eluted by means of liquid desorption with an appropriate solvent. However, with common problems faced by metal oxides, pure nm-sized Fe3O4 particles could easily form large aggregates and show less selectivity for most analytes in complicated matrices. Fortunately, easy surface modification of MNPs makes a suitable coating essential to overcome such limitations [50]. Unlike PP-HF, MNPs are absorptive materials in MD-SPME, and different kinds of modified coating can increase extraction capacity and selectivity. Hybrid MNPs (HMNPs) have been widely used to develop new sorbents for SPME techniques. Since MWCNTs exhibit good affinity to compounds containing polycyclic conjugated systems, this is beneficial to the formation of π-π stacking interaction between them [51]. Furthermore, hydroxyl and carboxyl groups on MWCNT surfaces can serve as chelation sites to form hydrogen bonds. To avoid the separation step of MWCNTs from a sample solution, magnetic carrier technology is introduced to overcome the intrinsic flaw of MWCNTs and to improve the extraction efficiency of MNPs. A magnetic carbon nanomaterial for Fe3O4 enclosure of hydroxylated MWCNTs (Fe3O4-MWCNTs-OH) was prepared by the aggregating effect of Fe3O4-NP on hydroxylated MWCNTs, and combined with HPLC-diode-array detection (HPLC-DAD) to determine the aconitines (aconitine, hypaconitine and mesaconitine) in serum samples. After the extraction process, MWCNTs with target analytes were collected from the sample solution and resolved in desorption solvent using MNPs as carriers under an external magnetic field [52]. Fig. 2 shows the experimental process of a typical MD-SPME method. Among carbon materials, G was also considered the most widely-used material because of some typical characteristics. A G-based MD-SPME coupled with GC was

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developed for the determination of five chloroacetanilide herbicides in water and green-tea samples with a satisfactory result [53]. Sol-gel or electrostatic self-assembly technology was also used to prepare the composite of MNPs and MWCNTs or G [54]. Silica supported or coated on Fe3O4-NPs by sol-gel techniques were developed as new sorbents for MD-SPME. Silica-supported Fe3O4-NPs were applied as sorbent phase for MD-SPME combined with capillary LC-DAD to determine organophosphorus compounds at trace level [55,56]. A core-shell structure of Fe3O4/SiO2/TiO2 composite was prepared by coating magnetite core particles with silica and titania layers. Thus, the sol-gel-derived Fe3O4/SiO2/TiO2 core-double shell nanocomposite as a novel high efficiency sorbent, coupled with HF-SPME, was used for extraction and determination of six non-steroidal anti-inflammatory drugs in hair samples [43]. Molecular imprinting is an attractive synthesis approach to mimic nature in preparing robust materials with specific recognition characteristics. The synthesis of MIPs starts by positioning the functional monomers around template molecules. The monomers interact with sites on the template via covalent or non-covalent interactions. They are then polymerized and cross-linked around the template, leading to a highly cross-linked, three-dimensional network polymer. The trapped template molecules are then extracted and binding sites are established with shape, size and functionalities complementary to the target analyte. The simplicity of in-situ immobilization promotes the application of MIPs in SPME coatings. In addition, monoliths of MIPs were also used in fiber or in-tube SPME configurations [57], so, when MNPs are coated with MIPs, these suspended magnetic dummy molecularly-imprinted microspheres (mag-MIMs) can not only selectively recognize analytes in the sample solution, but also be easily isolated from the sample using a magnetic field [58]. Moreover, the mag-MIM adsorbents exhibit strong recognition of MIPs, super-paramagnetism, and a greater surface area of magnetic composite materials [59]. One of the trends in development of magnetic sorbents in SPME is therefore combination of MIPs and MNPs to increase the selectivity for analytes. A vortex-assisted MD-SPME method coupled with GC-electron-capture detection was developed for rapid screening and selective recognition of dicofol in tea products [60]. The mag-MIMs synthesized by aqueous suspension polymerization using dichlorodiphenyltrichloroethane (DDT) as a dummy template showed high selectivity and affinity to dicofol in aqueous solution and were successfully applied as special adsorbents of MD-SPME for rapid isolation of dicofol from a complex tea matrix. In another example, a high-performance magnetic MIP coating using zeolite imidazolate framework-8-coated magnetic iron oxide (Fe3O4@ZIF-8) as carrier was also developed for simultaneous SPME of four estrogens in 24 food samples [61]. The coating material was synthesized through time-efficient LBL assembly of ZIF-8 and MIP film on Fe3O4 particles. A novel microextraction technique combined the principles of stir-bar sorptive extraction (SBSE) and MD-SPME [62]. The main feature of the method was the use of a neodymium-core stir bar physically coated with a hydrophobic magnetic nanosorbent. Depending on stirring speed, the magnetic sorbent acted as a coating material to the stir bar, thus affording extraction like SBSE, or as a dispersed nanosorbent medium for the collection and extraction of the target analytes. Once the stirring process is finished, the strong magnetic field of the stir bar prevails again and rapidly retrieves the dispersed MNPs. Like SBSE, the stir bar is collected and the analytes are back-extracted by liquid desorption into an appropriate organic solvent, which is then used for analysis. This enrichment technique is easy to prepare, since it

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does not require special surface-modification procedures, uses low volumes of non-toxic organic solvents and most importantly provides SBSE with additional functionalities against a wide range of analytes (since nanosorbents with various coatings can be employed) while it affords additional benefits to magnetic SPME in terms of extraction and post-extraction treatment. Table 2 shows studies that involved utilizing modified MNPs as sorbents in MD-SPME. 2.3. IL-based SPME ILs are inorganic and organic salts with melting points below 100oC. Most ILs are a combination of organic cations (e.g., imidazolium, pyridinium, pyrrolidium ammonium and phosphonium) and anions, which can be inorganic (e.g., Cl–, PF6–, BF4– and NTf2–) or organic (e.g., trifluoromethylsulfonate or trifluoroethanoate). They have unique properties (e.g., negligible vapor pressure, high thermal stability, high viscosity, and good conductivity and solubility). One important feature of ILs is that varying the cation or the anion can significantly affect their physical and chemical properties. As a result of their exceptional properties, ILs have attracted interest as green solvents for chemical processes, including organic catalysis, and, more recently, analytical chemistry and inorganic synthesis. ILs also exhibit a range of solvent properties, especially for the isolation of compounds from aqueous solutions, and they tend to be highly viscous, which limits their use as conventional solvents. In analytical chemistry, they are therefore good candidates for use in conventional solvent-based extraction, and as stationary phases in sorptive extractions [63]. In recent years, ILs were immobilized onto silica or polymeric supports, known as supported IL phases (SILPs) in order to take advantage of the chemical functionality that ILs can impart and, as a result, new groups of stationary phases with different fields of application in different extraction techniques emerged (e.g., SPME and SPE). SILPs can therefore be considered another class of sorptive material. With respect to sorptive extraction techniques, SILPs were first applied as an SPME coating in 2005, in a study where ILs were merely adsorbed on the fiber surface [64], involving fiber re-coating after each extraction. Subsequently, in order to circumvent this limitation, polymeric ILs (PILs) were developed by Anderson’s research group [65]. As opposed to ILs, PILs typically exhibit higher viscosity, preventing them from flowing into the GC injector at higher operating temperatures. We highlight that the liquid state of ILs is lost when immobilized onto a solid support [66]. Nevertheless, under these conditions, multi-modal type interactions can still be exploited. The following sub-sections encompass studies that involved utilizing ILs and PIL-based coatings as sorbents in SPME. Researchers in Iran developed an IL-mediated MWCNT-poly(dimethylsiloxane) (PDMS) hybrid coating by covalent functionalization of MWCNTs with hydroxyl-terminated PDMS using a sol-gel technique [67]. The prepared fiber was successfully used for separation and determination of trace amounts of polycyclic aromatic hydrocarbons in urine samples using HS-SPME coupled to GC-FID. In another work, two sol-gel reactive crown-ether ILs with [N(SO2CF3)2]−as anions, namely, 1-(trimethoxysily) propyl 3-(6’-oxo-benzo-15-crown-5 hexyl) imidazolium bis(trifluoromethanesulphonyl) imide {[TMSP(Benzo15C5)HIM][N(SO2CF3)2]} and 1-allyl-3(6’-oxo-benzo-15-crown-5 hexyl) imidazolium bis(trifluoromethanesulphonyl) imide {[A(Benzo15C5)HIM][N(SO2CF3)2]}, were synthesized and used as selective coating materials to prepare crown-ether IL-based

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SPME fibers by sol-gel technology. The sol compositions, such as the crown-ether ILs and hydroxyl-terminated silicone oil (OH-TSO), were optimized. The physical and chemical characteristics of these two crown-ether IL-based coatings, including surface morphology, thermal and chemical stability, coating preparation reproducibility, selectivity and extraction efficiency, were also evaluated and compared with each other [68]. With PIL-based coatings, sampling via direct immersion mode is possible, as the hydrophobic coatings are capable of maintaining structural integrity in aqueous matrices [69,70] Furthermore, utilizing a highly viscous material as an extractive phase can enable an even coating on the bare fiber in addition to a larger loading capacity [71]. In comparison to commercial coatings, PILs possess the major advantage of structural tunability for the selective extraction of target analytes. The benefits of PILs as SPME sorbent coatings include the effect of structural tenability on selectivities, extraction efficiencies, and detection limits for chosen analytes. Fig. 3 shows process for preparing cross-linked copolymeric PIL-based sorbent coatings. Anderson and his group expanded the applicability of PIL-based SPME sorbent coatings to the analysis of volatile compounds originating from coffee beans and investigated the performance of different PIL coatings, namely, poly(1-(4-vinylbenzyl)-3-hexadecylimidazolium bis[(trifluoromethyl) sulfonyl]imide) and poly(1-vinyl-3-hexylimidazolium chloride) {poly(VBHDIm+ NTf2−)} + − {poly(ViHIm Cl )}. By evaluating the extraction selectivity of different volatile analytes using PIL-based sorbent coatings that differ in their structural make-up and cation/anion combination, further insight can be obtained to provide understanding into how these new coatings behave in complex samples [72]. As novel classes of sorbent coatings, ILs and PILs exhibit valuable characteristics that can significantly broaden the applicability of IL-SPME. Due to their tunable physico-chemical properties, such as negligible vapor pressure, variable viscosity, high thermal stability, and tunable solvation interactions, ILs and PILs have been exploited as highly selective sorbent coatings for analyte-specific extractions. So far, these sorbent coatings have been used to extract a wide variety of analytes from analytical matrices ranging from wines to gas samples both in HS and direct immersion (DI) modes. While ILs and PILs offer selectivity advantages compared to some commercially available SPME coatings, relatively few IL/PIL-based coatings have been explored. Functionalizing ILs and PILs with polar and/or hydrogen-bonding-capable substituents allowed these sorbent materials to extract polar analytes selectively from aqueous matrices. It is mainly functional ILs and PILs attached on SPME fiber or other sorbent materials that play the vital role in selective extraction. New IL/PIL materials and surface-modification methods are desperately needed to develop various components of a complex matrix without compromising analyte selectivity and sensitivity. Apart from being immobilized onto silica or polymeric fiber supports, ILs could be bonded to Fe3O4-NPs, combining the advantages of ILs and MD-SPME. Developments and applications of magnetizable ILs (MILs) have become a new field and a hotspot of research in sample-preparation techniques [73]. Generally, ILs/PILs were bonded or immobilized on the surface of magnetic supports to form solid materials and used as magnetic adsorbents in SPME [74,75]. MILs can be uniformly dispersed in sample solutions through ultrasound irradiation and can be isolated from the solutions by means of an external magnetic field. In SPME using MILs, ILs play an important role in extracting and absorbing the analytes in the sample matrix, while magnetic Fe3O4-NPs serve as carrier for ILs to avoid centrifugation. These examples

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show a novel direction for IL-based SPME. There were reports about the application of MILs to separation of target analytes. Soylak and his team reported an IL-linked dual magnetic microextraction procedure for cadmium(II) in fruit and vegetables. Cadmium was complexed with pyrolidine dithiocarbamate and the chelate was extracted into the fine droplets of 1-butyl-3-methylimidazolium hexafluorophosphate using vortex mixing with Fe3O4-NPs [76]. An IL-based ultrasound-assisted dual magnetic microextraction was also developed for the preconcentration and extraction of cadmium from environmental samples [77]. ILs could also be coupled to a magnetic stir bar. One example is that a novel IL-bonded sol-gel stir-bar coating was prepared by chemically binding N-vinyl imidazolium-based ILs to the surface of a bare stir bar with KH-570 as bridging agent, and a method of SBSE coupled to HPLC was developed for the determination of three non-steroidal anti-inflammatory drugs in environmental, food, and biological samples [78]. Another example is IL magnetic bar SPME [79], which was developed for the determination of sulfonamides in butter samples by HPLC. The IL magnetic bar was prepared by inserting a stainless-steel wire into the hollow of an HF and immobilizing the IL in the micropores of the HF. In the extraction process, the IL magnetic bars were used to stir the mixture of sample and extraction solvent and to enrich the sulfonamides in the mixture. After extraction, the analyte-adsorbed IL magnetic bars were readily isolated with a magnet from the extraction system, which combined the advantages of ILs and magnetic SPME. For convenience, Table 2 also shows studies employing IL types of coating. 2.4. Other static SPME Apart from the above configurations of SPME, SBSE is a well-accepted sorptive extraction technique for preconcentrating a great variety of compounds from many different complex matrices. Despite the large number of SBSE applications in the literature, until very recently, the availability of commercial coatings for SBSE was limited to PDMS, apolar in nature. In recent years, promising strategies were proposed to overcome this limitation, such as sol-gel technology and monolithic materials. These novel approaches made SBSE very versatile, since a broad range of commercial monomers with different polarities can be used during coating synthesis, promoting the extraction of more polar compounds. The stir bar can be magnetic. It is reported that magnetic stirrer-induced dispersive microextraction was applied for the determination of vanadium in water and food samples [80]. Furthermore, in order to increase extraction capacity, dual-phase/hybrid coatings were also used for plants in recent years [81]. A number of studies demonstrated the feasibility of SBSE for the extraction of analytes from environmental and food matrices, and discussed the advantages of SBSE [82–84]. Another important configuration of SPME, thin-film microextraction (TFME), utilizes a membrane carrying the extraction phase. The robustness of the film is better than that of fiber-SPME. TFME has higher extraction efficiency and loadability due to the larger volume of its extraction phase and its higher surface-area-to-volume ratio. A typical thin film is a polymer (e.g., PDMS). Just as in SBSE, the thermal desorption of analytes adsorbed in the film needs the assistance of special units (e.g., a liner). In on-site application, after extraction, the film is usually transported to the laboratory for analysis. A recent development of TFME for on-site extraction involves a coated HF membrane [85]. The inner and outer surfaces of the HF are coated with a thin

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layer of polymer that has strong affinity with chlorinated and brominated compounds. The coated membrane can be placed directly into a sample vial containing seawater to extract analytes on site. Apart from the advances of new sorbent materials with unique properties, the development of analytical methods could also be a direction for innovation. For example, traditional HF in combination with direct current electric field could greatly accelerate the rate of mass transfer. This newly proposed mode is called electro-membrane extraction (EME) in the following studies. In this mode, electro-kinetic migration, instead of passive diffusion, is the main driving force, and dramatically shortens the extraction time (Fig. 4). This mode can be a valuable, reliable option for determination of alkaloid concentrations in biological samples [86].

3. Conclusions and future prospects SPME has received special attention due to its simplicity, sensitivity and efficiency in the analysis of analytes in complex matrices, especially environmental and food samples. It can be used for wide ranges of polarities and structures of compounds. In most cases, centrifugation and filtration can be avoided, and less organic solvents are needed. However, the configurations discussed above are still in their early stages of development, and there were few automated sampling systems in applications. Therefore, further improvements should be made in the automation of these SPME configurations and good GC/HPLC compatibility. All of the traditional areas of analytical chemistry have clearly been greatly affected by novel absorbing materials and nanotechnology research. The most recent advances include the use of their unique properties for analysis, the use of new types of materials, new synthetic strategies, or new approaches with novel sorbents for detecting different types of analyte. Small size and large surface area lead to very miniaturized systems for fast, selective and sensitive detection of analytes. Novel sorbent materials (e.g., carbon materials, MNPs, nano inorganic oxides, ILs, reinforced HFs, MIPs, mesoporous organic-inorganic hybrid materials) usually possess larger specific surface area and controllable pore size, and are some interesting nano-scale materials being explored for applications in separations. Another trend in exploiting novel sorbents comprises combinations of two or more sorbent materials. Moreover, the combination not merely mixes two materials, but involves binding at the molecular level to some extent. Sometimes, it is hard to determine to which kind of configurations of SPME a combination belongs, especially when two or more kinds of sorbent materials are used at the same time to extract analytes. All we can conclude is that combination of configurations is a potential trend for future development of SPME. Besides, by providing an elegant means to integrate different kinds of new sorbents for SPME, it has led to the development of a wide variety of hybrid SPME systems in various formats. SPME is a sampling and sample-preparation technique with great potential, because of its inherent advantages over conventional procedures, such as simple operation, low cost, low solvent consumption, speed and high enrichment. SPME can be coupled to different instrument configurations (UV, GC, GC/MS, HPLC, LC/MS, UPLC, and UPLC/MS), and has been widely used in the analysis of organic or inorganic compounds in complex samples. However, future development of SPME technology still has a long road ahead, full of great challenges – advancing with new functional materials, combining several

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configurations of SPME into one step, applying novel analytical methods, innovating analytical instruments, and providing the possibility of automation and high-throughput. Acknowledgements Financial support from National Nature Science Foundation of China (No.21467027 No.31260500 and No.21464013) is gratefully acknowledged. References [1] M.H. Ma, F.F. Cantwell, Solvent microextraction with simultaneous back–extraction for sample cleanup and preconcentration: preconcentration into a single microdrop, Anal. Chem.

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Captions Fig. 1. SEMs of oxide and MWCNT-reinforced oxide hollow fibers (inset figure: cross-sectional image of corresponding hollow fiber; main SEMs: detailed images of outside wall for each corresponding hollow fiber). Fig. 2. Experimental process of the Fe3O4-MWCNTs-OH SPME method. The black powder is Fe3O4-MWCNTs-OH, which was used to extract aconitines in human serum samples {Reprinted with permission from [52]}. Fig. 3. Preparation of UV-initiated IL polymerization approach (AIBN: polymerization initiator). Fig. 4. Set-up of electro-membrane extraction and IL-electro-membrane extraction and the structure of strychnine and brucine. Three sets of extraction set-up are connected in parallel to one DC power supply. The porous green part is polypropylene hollow fiber {Reprinted with permission from [86]}.

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Table 1. Applications of hollow-fiber SPME in food and environmental samples Analyte

Matrix

Sorbent

Configuration of SPME

Detection method

Limits of detection

Recoveries

Ref.

Pinacolyl methylphosphonic

Lake water

Zirconia hollow fiber

HF-SPME

LC-MS

0.07 ng/mL

94.2%

[18]

Milk and milk

Zirconia hollow fiber

HF-SPME

GC-MS

0.3 ng/mL

64.9–105.6%

[19]

Zirconia hollow fiber and

HF-SPME

GC-FID

300 ng/mL

89.4%

[17]

HF-SPME

GC-MS

α-BHC: 4 ng/g

α-BHC: 80.9%

[20]

β-BHC: 5 ng/g

β-BHC: 73.0%

γ-BHC: 3 ng/g

γ-BHC: 73.4%

δ-BHC: 3 ng/g

δ-BHC: 83.8%

DDE: 2.5 ng/g

DDE: 89.9%

DDD: 2.5 ng/g

DDD: 80.2%

DDT: 1.3 ng/g

DDT: 84.9%

acid Melamine

powder N,N-dimethylacetamide

Water samples

titania hollow fiber Organochlorine residues

Radix et Rhizoma

Silica hollow fiber

Rhei

Pesticide residues

Grape samples

Silica hollow fiber

HF-SPME

GC-MS

0.9–8.4 ng/mL

61–108%

[23]

caffeic acid

Echinacea

CNTs reinforced hollow

HF-SPME

HPLC

0.00005 ng/mL

89.1–94.6%

[27]

purpurea herbal

fiber HF-SPME

GC-FID

0.49–0.61 ng/mL

74–87%

[28]

HF-SPME

HPLC

Piroxicam:4.58ng/mL

Piroxicam:70–111%

[87]

Diclofenac:0.4 ng/mL

Diclofenac:71.1–114.

extracts Benzene, toluene,

Hair and waste

CNTs reinforced hollow

ethylbenzene, and xylenes

water samples

fiber

Piroxicam and diclofenac

Water

CNTs reinforced hollow fiber

1% Strychnine and brucine

Urine

CNTs reinforced hollow

HF-SPME

HPLC

fiber Carbamate pesticides

Apples

CNTs reinforced hollow

Strychnine:0.7 ng/mL

83.81–116.14%

[88]

Brucine:0.9 ng/mL HF-SPME

HPLC

0.09–6.00 ng/g

91.7–112.5%

[89]

HF-SPME

HPLC

0.32 ng/mL

102%

[90]

fiber Phenobarbital

Water samples

CNTs reinforced hollow

Page 20 of 28

fiber Aflatoxins

Cereals

CNTs reinforced hollow

HF-SPME

HPLC–DAD

fiber Triazines

Water sample

Oxidized single–walled

B1:0.074 ng/mL

47.4–106.8%

[91]

B2: 0.062 ng/mL HF-SPME

GC-MS

0.05–0.1 ng/mL

90%

[92]

HF-SPME

GC–MS

10 ng/mL

69–96%

[34]

HF-SPME

GC–MS

DEP: 0.03 ng/mL

DEP: 77.5–85.4%

[35]

DBP: 0.05 ng/mL

DBP: 80.2–115%

DEHP: 0.02 ng/mL

DEHP:68.2–114%

CNTs reinforced hollow fiber Metronidazole

Milk

MWCNTs reinforced hollow fiber

Plasticizers

Pesticide residues

Juice, milk and

MWCNTs reinforced

carbonated drink

hollow fiber

Water and hair

IL–MWCNTs–HF

HF-SPME

HPLC

0.003–0.095 ng/mL

82–94%

[22]

heteropolyacid–based

HF-SPME

HPLC

7.4-1300 ng/g

86–95.2%

[31]

HF-SPME

HPLC

0.08 ng/mL

84.8–94.2%

[30]

HF-SPME

HPCL

1-naphthaleneacetic

95.7–108.2%

[32]

samples Organophosphorus residues

Hair samples

hollow fiber Chlorogenic acid

Medicinal plants

CNTs reinforced molecularly imprinted materials

plant hormones

Vegetables

β–cyclodextrin modified carboxylic CNTs

acid: 0.8 ng/g 2-naphthoxyacetic acid: 1.5 ng/g

non-steroidal

Human hair

anti-inflammatory drugs nine flavonoids lead, cadmium and copper

Heavy metals

magnetic SiO2/TiO2

HF-SPME

HPLC

10–10000 ng/mL

79–86%

[43]

HF-SPME

UPLC-MS

0.17 ng/mL

95.2–99.8%

[44]

–

[29]

85–119%

[38]

reinforced hollow fiber Polygonum

Fe3O4 powder reinforced

hydropiper L.

hollow fiber

Rice

Functionalized MWCNTs

HF-SPME with pulse

Metrohm Model

Pb: 0.025 ng/mL

reinforced hollow fiber

anodic stripping

797 VA

Cd: 0.01 ng/mL

voltammetry

computerace

Cu: 0.0073 ng/mL

HF-SPME

Plasma MS

0.00039–0.028 ng/mL

Environmental

Graphene oxide–silica

Page 21 of 28

lead and cadmium

water samples

reinforced hollow fiber

Water

IL mediated hollow fiber

HF-SPME

Metrohm Model

Pb: 0.061 ng/mL

Pb:94.4–110.7%

797 VA

Cd: 0.019 ng/mL

Cd:96.1–103.3%

[39]

0.003 ng/mL

–

[40]

[41]

computerace Arsenic

Water

nanoparticles

HF-SPME

Atomic fluorescence spectroscopy

Lead and Cadmium

Water

Ligand assisted pseudo-stir

pseudo-stir bar

polarography

Pb: 0.015 ng/mL

Pb: 102%

bar hollow fiber

HF-SLPME

stand Metrohm

Cd: 0.012 ng/mL

Cd: 98%

0.015 ng/mL

94.2–95%

Model 757 VA computrace Zinc

Water

Ligand assisted pseudo-stir

pseudo-stir bar

polarography

bar hollow fiber

HF-SLPME

stand Metrohm

[42]

Model 757 VA computrace

Page 22 of 28

Table 2. Applications of magnetic-based SPME and IL-based SPME in food and environmental samples Analyte

Matrix

Sorbent

Configuration of SPME

Detection method

Limits of detection

Recoveries

Ref.

Aconitines

Human serum samples

Fe3O4-MWCNTs

MD-SPME

HPLC

3.1–4.1 ng/mL

98–103%

[52]

Acetanilide Herbicides

Green tea

Fe3O4-Graphene

MD-SPME

GC-MS

0.01–0.03 ng/mL

80.2–105.3%

[53]

Organophosphorus

Water samples

Fe3O4-silica

MD-SPME

HPLC

Clofenvinphos: 0.1

Chlorfenvinphos:89

[55]

ng/mL

–99%

Chlorpyrifos:0.05

Chlorpyrifos:

ng/mL

91–103%

–

–

[93]

compounds

pH value

Acetate buffer

carboxylic magnetite core

MD-SPME

shell nanoparticles Polycyclic aromatic

Water samples

hydrocarbons Dicofol residues

Au-NH2-magnetic

electrophoresis MD-SPME

GC-FID

0.002–0.004 ng/mL

91.4–104.2%

[56]

MD-SPME

GC-ECD

0.05 ng/g

83.6–94.5%

[60]

MD-SPME

HPLC

0.4–1.7 ng/g

E1: 77.4–91.3%

[61]

MCM-41 Tea samples

Magnetic molecular imprinted

Estrogens

capillary

Fish and pork samples

microspheres

molecularly imprinted polymer Fe3O4@ZIF-8

E2: 83.5–96.7% E3: 80.9–95.2% EE2: 73.8–88.4%

Hydrophobic organic

Seawater samples

Magnetic Fe3O4@stir bar

magnetic SBSE

LC-UV

2.4–30.6 ng/mL

87–120%

[80]

Urine samples

IL-mediated

HS-SPME

GC

0.0005–0.004 ng/mL

89.3–107.2%

[67]

IL-SPME

GC-FID

–

–

[68]

MD-SPME

Atomic absorption

0.018–0.125 ng/mL

97–99%

[75]

0.32 ng/mL

96–100%

[76]

compounds Polycyclic aromatic hydrocarbons

MWCNTs-poly(dimethylsi loxane) fiber

PAEs

Water samples

crown ether functionalized ionic liquids based fiber

Vanadium Cadmium

Food and water

Magnetic stirrer induced

samples

ILs

Fruit and vegetables

ILs Fe3O4 nanoparticles

spectrometry IL-magnetic SPME

Flame atomic absorption

Page 23 of 28

spectrometry Cadmium

Water, vegetables, and

ILs Fe3O4 nanoparticles

IL-magnetic SPME

hair samples

Flame atomic

0.40 ng/mL

98.1–101%

[77]

absorption spectrometry

Non-steroidal

Water, urine and milk

anti-inflammatory

samples

IL-bonded stir bar

IL-SBSE

HPLC-UV

0.23–0.31 ng/mL

76.9–116%

[78]

drugs Sulfonamides

Butter samples

ionic liquid magnetic bar

IL-MD-SPME

HPLC

1.2–2.17 ng/g

73.25–103.85%

[79]

Triazine herbicides

Vegetable oils

Magnetic ILs

IL-SPME

HPLC

1.31–1.49 ng/mL

81.8–114.2%

[94]

Environmental

Water samples

ILs-based sol-gel coating

IL-SPME

GC-FID

0.0030–0.1248

89.1–97.1%

[95]

estrogens and aromatic

fiber

ng/mL

amines Chlorophenols

Landfill leachate

IL coated SPME fiber

HS-SPME

GC-MS

0.008 ng/mL

87%

[96]

Antidepressant drugs

Water samples

IL-zeolite imidazolate

IL-DLLME-SPME

HPLC

0.3–1 ng/mL

94.3–114.7%

[97]

In-tube SPME

HPLC

1.2–13.5 ng/mL

85.4–98.3%

[98]

framework 4 Acidic food additives

Coca–Cola

ionic liquid-modified organic polymer monolith

Volatile fatty acids

Water samples

PIL coated SPME fiber

IL-SPME

GC-FID

0.02–7.5 ng/mL

–

[99]

Water samples

PIL coated SPME fiber

IL-SPME

GC-FID

0.003–6 ng/mL

–

[100]

Pomelo and orange

MWCNTs-PILs fiber

HS-SPME

GC

0.20–0.94 ng/g

81.9–110%

[101]

and alcohol polycyclic aromatic hydrocarbons 2-naphthol

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