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Advances in Sample Extraction Sheng Tang, Hong Zhang, and Hian Kee Lee Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b04040 • Publication Date (Web): 28 Nov 2015 Downloaded from http://pubs.acs.org on November 29, 2015
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Advances in Sample Extraction Sheng Tanga, Hong Zhanga and Hian Kee Leea,b,c,* a Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore b National University of Singapore Environmental Research Institute, T-Lab Building #02-01, 5A Engineering Drive 1, Singapore 117411, Singapore c Tropical Marine Science Institute, National University of Singapore, S2S, 18 Kent Ridge Road, Singapore 119227, Singapore * Corresponding author. Email:
[email protected]; Tel: +65-6516-2295; Fax: +65-6779-1691
1. Introduction Analyte or sample extraction is the most basic, yet a very important and dominant feature of any sample preparation technique which in general can also include other processes such as pretreatment, cleanup, analyte enrichment, and derivatization (or any other reaction such as complexation). The main aim of sample extraction is to clean up, isolate and concentrate the analytes of interest in a matrix, while rendering them in a form that is compatible with the instrument used for the subsequent analysis.1 For this purpose, two classical sample preparation methods, liquid-liquid extraction (LLE) (since ca. 1870)2 and solid-phase extraction (SPE) (commercially available since 1978),3 have been popular choices throughout the years. Based on solvent- and sorbent-based approaches, various microscale extraction methods have been developed and widely used in the past 25 years, such as solid-phase microextraction (SPME),4 various implementations of liquid-phase microextraction (LPME)5 including single-drop microextraction,6 dispersive solid-phase extraction (DSPE),7,8 stir-bar sorptive extraction (SBSE)9 and microextraction by packed sorbent (MEPS).10 However, the practice of modern sample preparation continues to evolve to address some key challenges, e.g., the preference, or sometimes necessity, for lower sample volumes, consideration of more complex sample matrixes,11 and need for more rapid analysis. To address these challenges, in recent years, researchers have been designing and developing newer and innovative extraction methods, which usually employ new materials to handle emerging chemicals. Whereas most review articles have focused on a specific sub-field of sample extraction (e.g., development
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of new materials as sorbents, or specific methodologies, for instance, liquid-phase extraction), it is equally important and instructive to cover more wide-ranging developments in order to portray the bigger picture of the latest and emerging trends in sample extraction.
Figure 1. The four elements of an extraction technique. A new extraction process may be represented by a combination of one or more of existing and/or new elements.
An extraction procedure is defined by four main factors, the analyte(s), the extraction device, the material (sorbent or solvent), and the process itself (Figure 1). A new extraction procedure would involve one or more of these elements. For example, the consideration of contaminants, either new or known ones that did not elicit any previous concern, in the environment could provide the impetus for the development of an extraction procedure comprising a combination of one or more of existing, improved and/or new elements. An illustration of this scenario is in relation to the current apprehension over contaminants of emerging concern (e.g., pharmaceuticals and personal care products, etc.) and nanomaterials in the environment.
Taking nanomaterials as an example, they are prepared, modified and used as novel chemicals in industrial and domestic products, and have been detected as contaminants in some types of environmental water.12,13 For the determination of these analytes, which cannot be conveniently handled by conventional methods, new extraction approaches based on new devices, materials or processes, or alternatively, a combination of current and new elements, have had to be developed. To an appreciable extent, the extraction efficiency depends on the contact area and time between the sorbent/solvent and the analytes. Thus, the extraction system or device, if applicable, is designed to enhance these two parameters. This may be realized by the judicious choice of an existing or newly designed and synthesized material, as the sorbent or solvent. Particularly as sorbents, these materials usually have special
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properties such as high selectivity towards specific analytes. As will be shown below, sometimes the introduction of new materials with interesting properties can inspire the development of innovative extraction processes, not only for emerging, but also known analytes. It is arguable that consideration of the four elements mentioned above is guiding the current development of, and advances in, sample extraction.
Contemporarily, trends in the advancement of sample extraction are also being driven by
miniaturization,
automation,
universalization
and
specialization,
and
intelligentization. These trends will be discussed towards the end of this article.
In this review, we present and discuss recent (2013-2015) advances in sample extraction focusing on the consideration of the four elements introduced in the preceding paragraphs.
2. Technical advances 2.1. Devices The device represents the base or platform of an extraction technology. Usually, this platform facilitates the interaction between the sample and the sorbent or solvent that leads to the mass transfer of the analyte to the extractant. Following this, the spent sample, and the extractant with the analytes are separated. There may be additional processing to isolate the analytes (e.g., by solvent elution) (as in SPE, etc.), or they may be thermally desorbed (as in SPME and SBSE) and directly analysed. Three important functions: sampling, extraction, and separation, rely on the device. The design of the device has the aim of simplifying these processes, combining some or all of them so that they occur simultaneously, or even eliminating them altogether to allow for a faster, easier and more cost-effective technique overall. The following sections discuss various types of extraction device components and recent advances in their development.
2.1.1. Samplers
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Samplers may either be active or passive depending on the requirements of the work. Passive samplers function on the basis of molecular diffusion of the analytes to the sorbent. The diffusion is generally unassisted.14 In contrast, more rapid active sampling is based on the provision of a driving force such as pumping, or forced stirring or agitation, etc.
Active sampling has been very much the preferred approach in most sample extraction applications in recent times. However, passive sampling continues to meet particular needs. For example, a recent work by Liu and co-workers15 involved the design of a unique multisection passive sampler using low-density polyethylene (LDPE) as sorbent phase to profile vertical concentrations of chemicals (dichlorodiphenyltrichloroethane and its metabolites) in sediment porewater. A series of identical sampling cells insulated with seclusion rings made up the sampler. As shown in Figure 2, in each section of the sampler, diffusion of chemicals through the water layer separated from the sediment by a glass fiber filtration membrane and a porous stainless steel shield, enabled their sorption on the LDPE. Bioavailability and mobility of the chemicals in the sediment at different depths could be reliably assessed with this sampler.
Figure 2. Schematic showing the configuration of the multisection passive sampler. LDPE = low density polyethylene, GF/F = glass fiber filtration membrane, CLDPE = chemical concentration in LDPE, C′LDPE = chemical concentration in water adjacent to LDPE, Cpw = chemical concentration in sediment porewater, and C′pw = chemical
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concentration in sediment porewater just within the sampler cavity. (Reproduced from Liu, H.; Bao, L.; Feng, W.; Xu, S.; Wu, F.; Zeng, E. Y. Anal. Chem. 2013, 85, 7117– 7124 (ref 15). Copyright 2013 American Chemical Society)
On the active sampling front, interesting sampling devices in the form of microneedle patches were fabricated to easily acquire dermal interstitial fluid containing biomarkers.16 The patches were prepared from cross-linked hydrogel composed of poly(methyl vinyl ether-alt-maleic acid) and poly(ethylene glycol) produced by micromolding, and achieved danger-free, pain-free, and expertise-free collection of dermal interstitial fluid containing biomarkers, when demonstrated in rats. The analytes were then extracted from the microneedle patches by mounting the patches within the cap of microcentrifuge tubes or forming the top of V-bottom multiwell microplates, after which a drop of water was applied on each patch to dissolve analytes and was subsequently collected at the bottom of the tubes after gentle centrifugation. The demonstrated device provided an attractive platform to collect biological samples and extract analytes in a simple and convenient way. It would seem possible for the patches to be used as passive samplers as well.
2.1.2. Extraction tips, disk, wells or membranes Although SPME has been commercialized for many years now, research on identifying new sorbents and supports for sorbents continue to attract strong attention, and engender innovative approaches in the procedure and its applications. Commercial SPME fiber coatings usually have a hydrophobic surface, which is not amenable to the use of electrospray ionization (ESI) mass spectrometry (MS) if analysis by this technique is considered. This surface is unsuitable for the ESI solvent to adhere to, and may thus result in unstable signals. To overcome this possible drawback,
a
wooden-tip
SPME
probe
with
its
surface
modified
by
n-octadecyldimethyl[3-(trimethoxysilyl)propyl]ammonium chloride was developed by Deng et al.17 Apart from being applied as a solid substrate to induce ESI for rapid and direct MS analysis, this novel probe was highly selective when used to enrich perfluorinated compounds from complex environmental and biological media via both
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reversed-phase and ion-exchange adsorption mechanisms..
Packing sorbent into a pipette tip is an alternative, miniaturized approach to cartridge-based SPE. A hydrazide bead-packed pipette tip was constructed by Chen et al. to achieve rapid extraction of N-linked glycopeptides from glycoproteins.18 The N-linked glycopeptides were selectively extracted by the hydrazide tip, with the extraction time significantly reduced from 3-4 days, using conventional SPE (including the coupling of glycoproteins to a solid support using hydrazide chemistry and removal of non-glycoproteins, proteolysis of captured glycoproteins to hydrazide with trypsin, removal of digested non-glycopeptides with washing, and specific release of N-glycopeptides using peptide-N-glycosidase F), to less than 8 hours. . Recently, Sun et al.19 crafted a self-assembly pipette tip graphene (PT-G) SPE cartridge for fast and cost-effective extraction of hazardous sulfonamide antibiotics (SAs) in environmental water samples. The PT-G-SPE cartridge was assembled by packing 1.0 mg of graphene as sorbent into a 100 µL pipette tip. The device showed superior adsorption/extraction capacity for the SAs due to the unique chemical structure and large surface area of graphene. Another graphene-based SPE device for environmental analysis reported was in the shape of disk developed by Wang et al.20 Based on the strong π–π stacking interaction between the polycyclic aromatic hydrocarbons (PAHs), and graphene, the analytes were specifically extracted by the disk, thereafter eluted by cyclohexane and then determined by GC-MS, rapidly and economically, as claimed by the authors.
As suitable biomarkers of sympathetic activity, plasma catecholamines need to be selectively isolated and sensitively detected. Ordinary methods based on SPE to analyze plasma catecholamines either lack sensitivity or involve use of potentially hazardous reagents for catecholamine derivatization before tandem MS detection. To address this problem, a specific 96-well microplate device for plasma catecholamine extraction was introduced by Dunand et al.21 Catecholamines eluted in small amounts
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were collected by a 96-well microplate, and were then injected into an ultra-high-performance liquid chromatography (UHPLC)-tandem MS system directly without need of solvent evaporation. This example also represented an improvement in sample throughput with simultaneous multiple-sample processing, another recent trend in the development of extraction procedures due to the wider availability of instruments with such capability.
Yeung et al22 developed a miniaturized SPE platform, sorbent membrane funnel, which permits in situ cleanup prior to membrane funnel-based spray analysis (Figure 3). The sorbent was fabricated as a funnel on a membrane to extract the target analyte, vildagliptin, an anti-diabetic drug, from human plasma. The analytes were then washed by solvent and passed through the funnel. The solvent was then directly introduced to an ESI MS spray source. Owing to this sorbent funnel, the whole analytical process was simple and straightforward.
Figure 3. Schematic of extraction procedure using a sorbent membrane funnel. (Reproduced from Yeung, H. S.; Chen, X.; Li, W.; Wang, Z.; Wong, Y. L. E.; Chan, T.-W. D. Anal. Chem. 2015, 87, 3149–3153 (ref 22). Copyright 2015 American Chemical Society)
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Zhang et al. synthesized and fabricated molecularly imprinted membranes (MIMs) bags for the recognition of pyrethroid insecticides in fish by membrane-assisted solvent extraction. Compared to conventional membrane materials, as might be expected, MIMs showed higher extraction recoveries (83.8% to 100.6%).23
A divinylbenzene particle-loaded polydimethylsiloxane membrane with high extraction capacity was prepared by Jiang and Pawliszyn24 using the bar coating method. The membrane was applied to extract benzene in outdoor air. After extraction, the membrane was introduced to the thermal desorption unit coupled to a GC for benzene determination.
2.1.3. Microfluidic system A microfluidic chip is a potentially powerful analytical platform, in which miniscule amounts of samples are isolated and selected by SPE or liquid-phase extraction. For example, a microfluidic liquid-phase nucleic acid purification chip was fabricated by Zhang et al.25 to selectively isolate DNA or RNA from small amounts of bacterial cells. The bacterial lysate in aqueous phase was first isolated in an array of microwells. An organic phase (phenol-chloroform) was then added to a headspace (HS) channel connected with the array of microwells. Target DNA or RNA could be selectively extracted into the aqueous phase when the flow rate of the organic phase reached its optimal value. The remaining organic phase was then evaporated under vacuum. Finally, in order to maximize the sensitivity of the detection, on-chip quantitative PCR (qPCR) was performed in the same microwells where the DNA or RNA was isolated to avoid any further liquid transfer since this would potentially lead to sample loss.
In another chip-based SPE application, Mao et al.26 developed a novel microdevice engaging an on-chip SPE and “Surface Tension Plug” (STP) to study cell-to-cell communication in vitro. The device had three essential functional sections, including a cell co-culture channel, and target pretreatment and target detection sections. The STP, which stopped the flow going through the channel, was defined as a channel
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with the gas−liquid interface in it. It did not need any extra equipment or energy support, apart from several polytetrafluoroethylene (PTFE) cylinders, and was easily integrated on the microfluidic device.
Wu et al.27 devised a fast in vivo equilibrium microextraction method comprising a microfluidic device with a stable and biocompatible core-sheath electrospun nanofiber membrane sandwich (from top to bottom: a PTFE/silicone septum, one piece of electrospun
nanofiber
membrane,
a
PTFE
substrate).
The
utilized
polystyrene/collagen core-sheath nanofiber membrane was coaxially electrospun and had excellent mechanical strength after in situ glutaraldehyde cross-linking treatment. After linking to the living system (rabbit), blood sample was introduced to the device to contact with the membrane repetitively, and equilibrium microextraction could be achieved within 2 minutes. This method was used to monitor the pharmacokinetic profiles of the tricyclic antidepressant drug, desipramine.
In the preconcentration and analysis of peptides at low abundance, Gasilova et al.28 designed a microchip electrospray emitter with a magnetic bead trap for SPE-gradient elution-MS. The reversed-phase-coated magnetic beads inside of the microchip served as a sorbent trap. Upon passing through the microchip, the peptides in the sample were retained and enriched in the trap. They were then released sequentially by stepwise gradient elution and subjected to ESI-MS analysis. Sample desalting, enrichment, gradient elution and MS detection were successfully accomplished by this approach without the additional separation step after SPE.
Onsite operations of analytical procedures is an emerging trend, and several studies contributing to this were reported recently. For the onsite detection of three representative types of trinitroaromatic explosives, a novel microfluidic paper-based analytical device (µPAD) coupled with confirmation by a lab-on-chip analysis was developed by Pesenti and coworkers.29 On the surface of µPADs, potassium hydroxide was primarily deposited to form an active spot. In the presence of
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explosives, reactions leading to a color change occurred, allowing subsequent colorimetric detection. For confirmatory analysis, positive µPADs were sampled using a 5-mm hole-punch, followed by extraction of the explosives from the punched-out chad in 30 s by solvent (20 µL borate/sodium dodecyl sulfate buffer). The extracted explosives were then analyzed with the lab-on-a-chip bioanalyzer device. The fabrication of µPADs involved the construction of patterns of hydrophobic barriers on filter paper. The hydrophobic barriers allow controlled fluid movement that segregated chemical reactions. Not only were the portable µPADs ideal for in-field applications but also their simple fabrication allowed low cost production in less advanced laboratories, the authors claimed.
Enzyme inhibition, a technique which involves multiple steps, is usually used to detect pesticide residues in vegetable or soil samples. Recently, Duford et al.30 integrated these multistep reactions into centrifugal microfluidic devices and thus managed to automate the simultaneous analysis of several samples or of replicates. Due to the inherently small sample size requirement of microfluidic devices, the expensive enzyme, which was essential to this approach, could also be economized. Liquid−solid magnetically actuated extraction, filtration, sedimentation, and detection were all integrated on the devices to determine the pesticide residue (carbofuran, or 2,3-dihydro-2,2-dimethyl-7-benzofuranyl methylcarbamate) in this study. The devices met the desirability for portability and inexpensive field analyses of environmental solid samples.
While microfluidic-based technologies, which have proven valuable in more or less continuous-flow fluidic situation, are usually performed using microchannels, a new strategy termed digital microfluidics (DMF) has appeared as a powerful alternative recently.31–33 In DMF, separated nano- to microscale droplets of sample fluid on a hydrophobic surface are manipulated by applying electrical potential to an array of electrodes. Owing to its advantages like facile handling process and good compatibility, DMF has attracted attention for a wide range of chemical, biological,
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and medical applications.32 Nonetheless, a challenge facing DMF is the handling of real samples, which are generally composed of larger fluid volumes containing analytes at low concentrations. In an attempt to address this problem, Jebrail et al.33 fabricated a new “world-to-DMF” interface. In this innovative device, a companion module drove a large-volume sample through a 10-µL droplet region to accomplish analyte (in this case, RNA from human whole blood lysates) recovery with the help of magnetic beads.
Lab-on-a-disc platforms have attracted particular interest amongst all the lab-on-a-chip concepts. This is conceivably because the entire set of important elements and functions are integrated onto a thin disc, and thus enables complete automation by using only a simple spinning motor to rotate the disc. Kim et al.34 demonstrated the extraction of DNA from whole blood spiked with Hepatitis B virus and Salmonella bacterium, a major food-borne pathogen, in spiked milk samples using a lab-on-a-disc device (Figure 4). All the DNA extraction, signal amplification and detection were achieved on the single disc.
Figure 4. (a) Expanded view of the lab-on-a-disc showing top and bottom plates made of polycarbonate, strip sensors, adhesive layer, and the metal heater. (b) Top view of a section of the disc featuring the chambers for cell lysis, isothermal amplification, metering, dilution, and detection. (c) Schematic illustration of the experimental setup. Computer-controlled unit includes a spinning motor, a laser for the operation of the ferrowax microvalves, local heating of the amplification chamber, and a strobe light and a CCD camera for the visualization of the fluidic transfer on the rotating disc. (Reproduced from Kim, T.; Park, J.; Kim, C.; Cho, Y. Anal. Chem. 2014, 86, 3841–3848 (ref 34). Copyright 2014 American Chemical Society) In another disease analytics application using chip-based technology, Liu et al.35
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reported a microfluidic capillary-array system for tuberculosis diagnosis. The presented microsystem was fully integrated with loop-mediated isothermal amplification analysis, during which the liquid plugs containing sample or reagents were consecutively introduced into the capillaries to form droplets. The entire diagnostic process including DNA extraction, isothermal amplification and determination were accomplished in the formed droplets coupled with magnetic beads. The high throughput, small reaction volume and low-cost reportedly enabled the capillary-array microsystem to perform tuberculosis diagnosis efficiently.
2.1.4. Specific extraction devices Zhang et al.36 designed a novel and economic DNA extraction device which was integrated with a modified visual loop-mediated isothermal amplification assay. The DNA extraction procedure could be realized in a silica gel membrane filtration column with a modified syringe. Such simple experimental instruments meet the requirement of on-field tests for genetically modified organisms. Utilizing this device, high-quality genomic DNA appropriate for isothermal amplification and thermal amplification like polymerase chain reaction (PCR) could be successfully isolated and extracted from plant tissues within 15 min.
For sensitive and automated detection of plutonium in a relatively large volume of human urine, Qiao et al.37 developed a bead injection extraction chromatographic microflow system, based on a high-capacity lab-on-a-valve (LOV) platform. In the LOV processing unit, the attached column was first loaded with chromatographic resin for the extraction step through programmable (accurate handling of solutions via bidirectional and discontinuous programmable flow) bead transport. After pretreatment (preliminary purification), the sample was then forced through by pressure and the plutonium was selectively captured and purified. The developed system was well suited for the expedient low-level analysis of plutonium in the urine of exposed individuals.
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Speed of processing and analysis has always been a topic drawing the interest of those in the analytical field. To this end, the use of a surface acoustic wave (SAW) atomizer to handle sample rapidly in matrix-assisted laser desorption ionization (MALDI) MS protein and peptide profiling of the Islets of Langerhans was the subject of a study by Bllaci and coworkers.38 A 2 µL buffer solution, containing typically 2−6 freshly prepared islets, was pipetted onto a dry, unpretreated membrane placed on the SAW atomizer. The latter was used to extract insulin and other peptide hormones acoustically from the islets, stimulated directly on the membrane. The membrane sampling was fast, simple, economical and noninvasive, and would seem to be a useful strategy for limited amounts of biofluids.
MS imaging is a research field that has made great progress in localizing metabolites in and around bacterial colonies. However, several superior techniques for the detection of small metabolites, such as nanostructure-initiator mass spectrometry (NIMS), cannot yet be used for direct microbe imaging due to the fact that desorption/ionization occurs on the bottom of the sample. In order to solve this problem, Louie et al.39 developed a “replica-extraction-transfer” (REX) technique that transferred biomolecules from agar cultures of spatially arrayed bacterial colonies onto NIMS surfaces. In this technique, a solvent-laden semisolid like a gel was firstly used to extract metabolites from a microbial sample like a biofilm or agar culture. The metabolites were then replica “stamped” onto the NIMS surface (Figure 5). This integrated REX-NIMS technique was capable of detecting a series of small compounds from agar gels. This interesting procedure extends the applicability of MS in chemical analysis to study and characterize microbial interactions, another field that MS is making promising inroads into.
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Figure 5. Workflow for “replica-extraction-transfer” (REX) NIMS imaging. (A) Microbes are grown on agar media or other surface. For spatial patterning with acoustic printing, 1−10 nL droplets of microbes in liquid media are acoustically printed onto an agar surface according to a predetermined custom template. (B) In preparation for REX, the biofilm or agar gel with cultured microbes is first dehydrated. For biomolecule extraction, a solvent-laden gel is placed upon the dried microbial surface with slight pressure; for biomolecule transfer, the gel is removed, then the same side is contacted to a NIMS chip surface. (C) For NIMS imaging, a laser is rastered across the spatially transferred biomolecules to generate an individual mass spectrum at each position. (Reproduced from Louie, K. B.; Bowen, B. P.; Cheng, X.; Berleman, J. E.; Chakraborty, R.; Deutschbauer, A.; Arkin, A.; Northen, T. R. Anal. Chem. 2013, 85, 10856–10862 (ref 39). Copyright 2013 American Chemical Society)
Efficient sample treatment remains a major challenge in compound-specific stable isotope analysis. In a very recent development in this topic, Herrero-Martín et al.40 creatively integrated a HS extraction approach on a commercial autosampler, with a programmed temperature vaporizer-equipped gas chromatographic (GC) system, for carbon (δ13C) and hydrogen (δ2H) isotope analysis of volatile organic compounds in water. The limits of detection (LODs) for δ13C and δ2H were measured to be at tens of micrograms per liter of water.
2.2. Novel materials
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Analytical Chemistry
Materials science has become an important companion to sample preparation research. To extract analytes from primarily aqueous samples, more and more new materials,41,42 including alternative solvent systems,43 are being introduced. These materials, whether by design or by discovery, are usually reported to have significant and selective interaction with the analytes under study. Studies on such extraction materials mainly cover: (1) the exploration of new sorbents that have high selectivity towards target analytes; (2) the application of known materials to different modes of extraction; (3) the modification of known materials to generate new structures or morphologies with specific properties for extraction, e.g., high surface area, magnetic properties, increased water dispersibility, amphiphilicity, etc.; and (4) the design of new extraction procedures by using materials with specific properties. The following paragraphs highlight some new advances in this particular facet of sample extraction/preparation.
2.2.1. Solvents Hoogerstraete et al.44 introduced ionic liquid trihexyl(tetradecyl)phosphonium chloride as solvent to extract and separate transition metals from rare earths. Without using the organic diluents or extra extraction agents, the solvent extraction process was applied as a sustainable hydrometallurgical method for removing transition metals from neodymium–iron–boron or samarium–cobalt permanent magnets. In another study, hybrid silica-based materials with immobilized ionic liquid 1-methyl-3-butylimidazolium bis(trifluoromethylsulfonyl)imide were prepared by sol−gel technology and coated on an SPME fiber.45 This ionogel SPME fiber exhibited high extractability for volatile aromatic compounds. Clark et al.46 reported that they synthesized and employed three types of hydrophobic magnetic ionic liquids and use them as solvents in microdroplet form (20 µL volume) for extraction of DNA from aqueous solution. Each DNA-enriched microdroplet was then removed by a magnetic field, for further processing.
Monoethylene glycol was reported as an interesting alternative extraction medium for
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removing phenol, and alkyl phenols such as cresols and xylenols in organic matrixes, in a microfluidic device and with multi-dimensional GC-MS analysis.47 Apparently, this solvent has never been used for extraction of these compounds which are important in the pharmacological and chemical industries. The biggest advantage is that the monoethylene glycol is amenable to GC analysis. Since it is a stable extractant, there were no ensuing issue(s) related to solvent evaporation after the extraction was completed.
Use of materials that subscribe to green chemistry principles is a growing area of interest in sorbent-based extraction. Dai et al.48,49 have developed a series of natural ionic liquids and deep eutectic solvents composed of primary metabolites common in living cells, named natural deep eutectic solvents (NADES). These solvents exhibit very low volatility at room temperature (and hence are safer than volatile organic solvents), high solubilization ability, and tunable selectivities. In certain molar ratios, these NADES show strong intermolecular interactions, demonstrating suitability of the solvents for extraction of potentially bioactive secondary metabolites, e.g., phenolic metabolites in the safflower, Carthamus tinctorius L.48
Unusually, methyl tert-butyl ether (MTBE) was employed by Pizarro et al as extractant in lipid extraction for exhaustive lipid fingerprinting of human blood plasma samples.50 MTBE is almost exclusively used as an additive in gasoline (petrol) engine fuel, and rarely as a solvent. Compared to conventional chlorinated solvents, it has a lower density than water, which facilitated the collection of the extract; no less importantly, comparable extraction results were obtained.
2.2.2. Graphene Graphene is a remarkable material, which has been studied and utilized in many different areas, including analytical chemistry.51 In the past few years, it has also played an important role in SPE as an efficient sorbent thanks to the following advantages:52 (1) It has a large specific area due to its single layer structure, and both
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sides of its planar sheets are available for adsorption; (2) It can form strong π-stacking interaction with other compounds with its π-electron system (e.g., benzene rings); and (3) It is easily modified with functional groups to generate new materials for particular applications, and for use in specific devices.
Guan et al.53 synthesized amine-modified graphene as a reversed-DSPE material to extract pesticides in oil crops. In another study, Zhang et al.54 immobilized graphene layers inside PTFE tubing as a substrate for in-tube SPME. Sitko et al. attached 3-mercaptopropyl trimethoxysilane groups onto graphene oxide nanosheets and use the material to extract divalent metal ions Co(II), Ni(II), Cu(II), Cd(II), and Pb(II), and Ar(III) species, followed by total-reflection x-ray fluorescence spectrometric analysis.55 Recently, various magnetic graphene composites, such as iron II/III oxide (Fe3O4)–graphene,56
Fe3O4@SiO2–graphene,57
Fe0/iron
oxide-oxyhydroxide/graphene,58 and Fe3O4@SiO2@polyaniline–graphene oxide,59 have been developed for magnetic SPE. These magnetic graphene composites have several advantages, e.g., large surface area, fast separation ability, and high extraction ability for aromatic analytes, which are attributed to their specific structural features comprising π-conjugated networks with highly exposed surfaces.
2.2.3. Metal organic frameworks Metal organic frameworks (MOFs) represent a class of hybrid materials which are formed by metal (oxide) cations of clusters and polydentate bridging ligands. Due to their properties, such as large surface area, diverse structural topologies and tunable pore sizes, MOFs demonstrate good potential as sorbents in SPE and other solid-based extraction methods,60 as has been reported extensively in the past several years.61 The different pore sizes and diverse topologies of MOFs can be obtained by adjusting the structure and size of the guest molecules. Moreover, specific properties of MOFs can be achieved by in-pore functionalization, outer-surface modification or magnetization to suit various types of analytes.62
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Recently, the application of the MOF, MIL-88B, for SPME was reported by Wu et al.63 MIL-88B with nanosized bipyramidal cages and large surface area was coated on an etched stainless steel fiber and was utilized to extract polychlorinated biphenyls. Li et al.64 applied a series of MOFs as sorbents for analyte enrichment. They introduced multiwalled carbon nanotubes (MWCNTs) with MOF-199 coating. Due to interactions such as the “molecular sieving effect,” hydrogen bonding, open metal site interaction, and π−π affinity, this coating represented significant extraction superiority over commercial SPME coatings. Benefiting from the hybridization of MWCNTs with MOF-199, the enrichment capability of sorbent was further improved. The hybridization also functioned as a hydrophobic “shield” to prevent the open metal sites of MOF-199 from being filled by water molecules. This enhanced the moisture resistance of the material. The authors also immobilized MOF-5 on a porous copper support.65 The MOF-5 bar was then applied to the HS sorptive extraction of volatile organic sulfur compounds in Chinese chives and garlic sprout. Due to the extraordinary porosity of the MOF as well as the interaction between the S-donor sites and the surface cations at the crystal edges, the MOF-5 bar exhibited high extraction ability and good selectivity to the volatile sulfides. In another work, a hybrid magnetic MOF-5 material was reported.66 Combining the favorable attributes of both the magnetic characteristics of Fe3O4 nanoparticles and the high MOF porosity, this hybrid material was shown to be an excellent candidate as sorbent for the enrichment of trace analytes, specifically, PAHs and gibberellic acids.66 A magnetic MOF Fe3O4–pyridine nanocomposite has been applied as a novel sorbent for the fast separation and preconcentration of Cd(II) and Pb(II) ions in various matrixes.67 The sorbent exhibited selectivity towards these heavy metals due to the presence of the pyridinyl group. Hao et al. developed a novel magnetic nanoporous carbon (MNPC) material,68 which was synthesized by a one-step direct carbonization of Co-based MOF. The Co-MNPC has several excellent advantages as a sorbent, such as a high specific surface area, large pore volume, and super paramagnetism. To evaluate its performance, it was used to extract some neonicotinoid insecticides from
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water and a fruit, fatmelon after the usual pretreatment (homogenization), by magnetic SPE.
2.2.4. Molecularly imprinted polymers A molecularly-imprinted polymer (MIP) is a polymeric material that is synthesized according to a target molecule and has extremely strong binding capacity and selectivity against the latter. Since the binding site is tailor-made for the analyte, the specificity towards it is the biggest advantage of MIP.69 Besides, stability and relative ease of preparation also make MIPs outstanding sorbents in SPE,70 and there have been many reports on their applications. Some more recent studies are highlighted below.
While many MIPs are prepared for single-analyte applications, some can be applied to multiple compounds. For example, Duan et al.71 prepared a multi-templated MIP that was used to extract the five acidic pharmaceuticals simultaneously by SPE.
For simultaneous determination of several explosive nitroaromatic compounds (NTs), MIPs based on thin films were prepared by Huynh et al.72 The sensing MIPs were based on the use of bis(2,2′-bithienyl)-(4-aminophenyl)methane as functional monomer allowing for π−π stacking recognition of the NTs. In another study by the same authors,73 a 6-aminopurine (adenine) derivative of bis(2,2′-bithienyl)-methane, vis., 4-[2-(6-amino-9H-purin-9-yl)ethoxy]phenyl-4-[bis(2,2′-bithienyl)methane], was designed and synthesized for recognition of 5-fluorouracil, an antitumor chemotherapy agent, by RNA-type (nucleobase pairing)-driven molecular imprinting.
For the isolation and enrichment of fluoroquinolones (FQs) from egg samples, Xiao et al.74 introduced a MIP on the surface of magnetic carbon nanotubes (MCNTs@MIP). By using ofloxacin as a pseudo template, methacrylic acid as the functional monomer, and ethylene glycol dimethacrylate as the cross-linker, the MCNTs@MIP were prepared. By using an external magnetic field, the sorbent could be collected and
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separated rapidly. It also had a high specific surface area, strong mechanical properties and specific recognition for FQs. Similarly, in Mehdinia et al.’s study,75 at the surface of modified magnetic nanoparticles, a magnetic MIP was polymerized by using methacrylic acid as the functional monomer, 4-nitrophenol as the template and ethylene glycol dimethacrylate as the cross-linker. The obtained MIP presented high adsorption capacity, selectivity and fast kinetic binding with respect to 4-nitrophenol.
2.2.5. Fe3O4 magnetic nanoparticles The use of Fe3O4 nanoparticles has formed the basis of the synthesis of many magnetic
sorbents.76
A layer
of
hydrophobic
coat or adlayer such as
hemimicelles/admicelles of ionic surfactant or alkyl carboxylates groups, phenyl or carbon materials, or polymers can be used to modify the highly hydrophilic property of Fe3O4 to provide extraction capability towards new analytes.77
Zhang et al.78 prepared a 3-aminopropyltriethoxysilane (APTES)-functionalized magnetic nanoparticles by a one-step silanization reaction. Since the APTES could conjugate with the aldehydes from oxidized glycopeptides, it could be used to extract the latter. By using this material, the desalting step was eliminated. Thus, the LODs of glycopeptides were improved by two orders of magnitude, compared to those obtained by conventional hydrazide chemistry-based SPE. Similarly, they also synthesized aminooxy-functionalized magnetic nanoparticles (Fe3O4@SiO2-ONH2) for selective extraction of glycopeptides.79 The attached aminooxy groups on the nanoparticles
exclusively
reacted
with
the
oxidized
glycan
chain
from
glycoproteins/glycopeptides; thus, glycopeptides were selectively bound to the nanoparticles.
Multicore magnetic nanoparticles (MMNPs) doped with cesium and fluorescein isothiocyanate (FITC) were synthesized by Ko and Lim80 for the extraction and determination of biomarkers. The application of the MMNPs was demonstrated to extract carbohydrate antigen 19-9 in serum nonspecifically by using magnetic
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separation, and the doped cesium was used as an internal standard for the radiometric measurement of the tagged particle. The resulting MMNPs showed multifunctional character, e.g., the preservation of superparamagnetic property for magnetic separation, multicores with a size of a few tens of nanometers for activity, the doped metal for an interference-free signal in inductively-coupled plasma (ICP) MS, the FITC for fluorescence monitoring, and the silica shell for surface modification.
A type of core/shell structured magnetic Fe3O4/polydopamine (Fe3O4/PDA) nanoparticles was applied as sorbent in the determination of trace levels of PAHs.77 The PDA functioned as an affinity material for organic pollutants. It has several advantages, like biocompatibility, dispersibility in water, multifunctionality (with amino and catechol groups), and can provide π-π stacking interaction with the target analytes. In another study, Wang et al.81 coated cetyltrimethylammonium bromide on Fe3O4 nanoparticles. The material was used as an SPE sorbent to extract the bioaccessible fraction of PAHs in spiked river water, rainwater, wastewater and tap water
Based on the surface molecular imprinting technique, Jia et al.82 synthesized a polydopamine-based molecular imprinted film coating on silica–Fe3O4 nanoparticles for recognition and separation of bovine hemoglobin. The polydopamine coating could react with biomacromolecules and overcome the limitations of mass transfer and the non-quantitative recovery of template molecules to a greater extent, which usually happens in molecular imprinting. Similarly, Li et al.83 prepared surface-imprinted magnetic nanoparticles with an epitope anchored to His-tag (an amino acid motif composed of at least six histidine residues), as the template. This material showed specific recognition of the target protein, human serum albumin.
2.2.6. Carbon nanotubes Benefitting from their special structural features, carbon nanotubes (CNTs) possess some useful properties, such as distinctive electron transport characteristics, largest
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elastic modulus, high thermal stability and substantial available surface area.41 These properties, especially high surface area and ability to establish π-π interactions, pre-dispose CNTs to be good sorbents in SPE.84 Moreover, they can be covalently or non-covalently functionalized and, therefore, tailored to adapt to different analytes (whether polar or non-polar).
A novel poly(methacrylic acid-co-ethylene dimethacrylate-co-single-wall CNT monolith was prepared by Wang et al.85 through the incorporation of single-wall CNTs into a polymer monolith containing methacrylic acid and ethylene dimethacrylate. The material was applied in in-tube SPME to the enrichment of six triazine herbicides from water samples. Polymeric monoliths within capillary tubings have been widely utilized for microextraction due to the advantages of easy preparation, large surface area, and easily-controlled porosity. The marriage of the unique properties of CNTs and the mentioned features of polymer monoliths has broadened the appeal of these mixed materials in recent years, of which the above is just one recent example of an application in sample preparation.
By using magnetic multiwall CNTs as an intracellular probe, Zhang et al.86 developed a strategy to extract intracellular proteins from living cells. Due to the specific intracellular localization of the magnetic CNTs around the nuclei and the strong interaction with nucleic acids, this material exhibited highly efficient extractability for cellular nucleic acid associated proteins from living cells. In another study by Rastkari and Ahmadkhaniha,87 magnetic CNTs were prepared by assembling magnetic nanoparticles onto the acid-treated multiwall CNTs. Owing to their excellent adsorption capability, the magnetic CNTs were utilized as sorbent in the magnetic SPE of phthalate monoesters, the main biomarkers of phthalate exposure, from human urine.
2.2.7. Layered double hydroxides Layered double hydroxides (LDHs) are relatively recent promising sorbents for
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enriching anions owing to their excellent anion-exchange capacity, high porosity, and high specific surface area, as has been shown in many reports in the past few years.88 They have been particularly used in SPE to extract various analytes, such as As(III)/As(V),89 chromate,90 PAHs,91 phthalate esters,92 metal ions93 and phenols94 etc.
An interesting property of LDHs is that they can be dissolved through pH control of their solutions (
