Development of dispersive micro-solid phase extraction based on micro and nano sorbents

Trends in Analytical Chemistry 89 (2017) 99e118

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Development of dispersive micro-solid phase extraction based on micro and nano sorbents Tahere Khezeli, Ali Daneshfar* Department of Chemistry, Faculty of Science, Ilam University, Ilam, 69315-516, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 30 January 2017

Dispersive micro-solid phase extraction (D-m-SPE) as a new type of solid phase extraction (SPE) is attractive for a variety of analytical applications including pre-concentration, clean-up and extraction methods. A comparison of D-m-SPE with SPE method from several analytical and economical aspects is presented. Up to now the application of D-m-SPE for the determination of many analytes is increasing due to the simplicity, fastness and low cost of it. Another important feature of D-m-SPE is the diversity of solid sorbents to achieve specific selectivity and high extraction efficiency. Many reports have discussed the synthesis and application of different sorbents in D-m-SPE. With the rise of nanotechnology, a large number of synthetic nano-material is widely used to enhance the extraction efficiency and reduce the extraction time of D-m-SPE. Most studies focused on the magnetic nanocomposites sorbents. Herein, we summarize the type of sorbents and review the latest progress in D-m-SPE. © 2017 Elsevier B.V. All rights reserved.

Keywords: Dispersive micro-solid phase extraction Pre-concentration Sorbent Nano-material Magnetic nanocomposites

1. Introduction The accurate determination of compounds especially at trace levels without any pre-treatment steps is very difficult because the concentration of target compounds in the majority of samples are usually around the limit of detections of the most sensitive techniques and there are high levels of matrices interferences [1]. Therefore, despite the substantial technological advances in the analytical field, instruments cannot handle relatively complex samples directly. These are the reasons why sample preparation prior to instrumental analysis is a very important goal for analytical chemists. Accordingly, the presentation of simple, rapid, sensitive and automated methods in this regard is a subject of great interest. Liquid-liquid extraction (LLE) and solid phase extraction (SPE) are considered as the most commonly used techniques for the preconcentration/separation of trace amounts of target analytes [2e4]. LLE is time-consuming, tedious and uses large amounts of potentially toxic organic solvents [5,6]. Among the classical extraction methods, SPE has been distinguished from other extraction techniques due to factors such as convenience, cost,

* Corresponding author. Fax: þ98 843 2227022. E-mail addresses: [email protected], (A. Daneshfar). http://dx.doi.org/10.1016/j.trac.2017.01.004 0165-9936/© 2017 Elsevier B.V. All rights reserved.

[email protected]

time-saving, simplicity, less consumption of organic solvents and the ability to combine with different detection techniques whether in on-line or off-line mode [7e11]. Although SPE is being applied broadly, it suffers certain shortcomings including solvent loss, large secondary wastes, a long procedure and a need for complex equipment [12,13]. The most popular sorbents used in SPE are silica, activated carbon, cellulose, chelating resins and polyurethane foams [14]. In recent years, many researches have been oriented towards the development of efficient and cost-effective miniaturized sample preparation methods namely microextraction methods. In microextraction techniques, a small volume/amount of extraction medium in relation to the sample volume is used. Solid-phase microextraction (SPME) and liquid-phase microextraction (LPME) are two miniaturized extraction techniques that have emerged in the past 20 years [15,16]. LPME is based on the use of very low volumes (at the level of microliter) of solvent and has its origin in the use of a drop of extraction solvent [17e21]. SPME overcomes the difficulties of conventional extraction methods by eliminating the use of organic solvents and allowing sample extraction and preconcentration to be performed in a single step. Although, SPME and LPME eliminate and/or reduce the volume of consumed organic solvents and allow widespread monitoring of trace level analytes but they are usually time-consuming processes. SPME involves a relatively high cost, contains fragile coating layers and the fibers

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degrade with multi-use. Moreover, SPME is often associated with carryover problems and batch to batch variation of fiber coatings [22e24]. Carboxen/polydimethylsiloxane (CAR/PDMS), polydimethylsiloxane (PDMS), polydimethylsiloxane/divinylbenzene (PDMS/DVB), polyacrylate (PA), divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/PDMS), carbowax/divinylbenzene (CW/DVB) and carbowax/templated resin (CW/TPR) are commercially available materials for SPME from Supelco. Therefore, the limited number of commercially available sorbent coatings is another important drawback of SPME [25]. To overcome these limitations, the recent advances in SPME focused on the synthesis of novel sorbents and creation of a new strategy for reducing the extraction time. In 2003, a rapid, simple, relatively new and very efficient cleanup technique named dispersive solid phase extraction (DSPE) was introduced by Anastassiades et al. [26]. In this technique, the solid sorbent is added directly to a sample solution without processes of sample manipulation such as conditioning, so the clean-up procedure relies only on shaking and centrifugation. DSPE is a quick, easy, cheap, effective, rugged and safe method [27]. More recently, dispersive micro-solid phase extraction (D-mSPE) has been widely developed as a simple and miniaturized modification of DSPE that can be applied to extract and enrich the quinolones [28], tetracyclines [29], organophosphate pesticides [30], polycyclic aromatic hydrocarbons (PAHs) [31], triazines [32], heavy metal ions [33], semi-volatile compounds [34], organic UV filters [35] and nitroaromatic hydrocarbons [36]. Similar to DSPE, in D-m-SPE the extraction is carried in the bulk solution. Usually, DSPE is used for the matrix clean-up purpose; that means after the dispersion of sorbents in the bulk solution or matrix containing the target analytes, any possible matrix interferences is retained on the sorbents. Then sorbents are separated from the bulk solution by centrifugation, and the target analytes are then collected in the supernatant. In contrast, D-m-SPE can be performed by trapping the target analytes in the sorbents. After extraction, the sorbent containing the target analytes is isolated by centrifugation or filtration. The target analytes can then be eluted or desorbed by an appropriate desorption solvent [37,38]. Also, compared with DSPE, D-m-SPE has the following advantages: simpler operation, less solvent consumption and shorter time requirement. A schematic illustration of D-m-SPE procedure is illustrated in Fig. 1. Nanoparticles (NPs) of different sorbents can be applied in D-mSPE to absorb a target analyte in various matrices. In the last decades, the use of NPs offers major advantages due to their unique size and physicochemical properties [39,40]. These advantages of NPs led to widespread applications of them in the fields of physics, chemistry, biology and medicine. Furthermore, the surface properties of NPs allow functionalizing them by various capping ligands to make them able for a range of applications [41e45]. Therefore, to date, much attention has been paid to the synthesis of different kinds of NPs. Herein; we try to describe the advantages of D-m-SPE and type of solid materials used as sorbents in this technique.

2. What is D-m-SPE? D-m-SPE is a miniaturized extraction method based on dispersion of micro- or nanosorbents in sample solution and isolation of solid sorbent by centrifugation, filtration or using an external magnetic field. D-m-SPE is based on the SPE methodology, but a small amount of solid (mg or mg range) is dispersed in a sample solution containing the target analytes without conditioning. Dispersion phenomenon enables the sorbent to interact rapidly and

uniformly with all the target analytes which lead to enhance the precision of method and reduce the extraction time [30,46e50]. Recently, ultrasound [51] and vortex assisted D-m-SPE [52] have developed in order to favor the kinetic of the mass-transfer process of target analytes based on the following equation [53]:

b¼

D

d

Efficient stirring increases the mass-transfer coefficient (b) of analytes from an aqueous phase to solid sorbent by decreasing the thickness of the Nernst diffusion film (d). These phenomena lead to an increment in the extraction efficiency of technique in a minimum amount of time [50]. Like SPE, D-m-SPE is a surface dependent approach because its kinetic depends directly on the contact surface between analytes and solid sorbent [54]. D-m-SPE enables the isolation of target analytes from matrix solution by adsorption on a solid sorbent. Adsorption of analytes is based on many different phenomena including hydrogen bonding, dipoleediploe and pep interactions. In D-m-SPE, nature and physicochemical properties of the solid sorbent are very important in order to achieve an accurate, sensitive and selective determination of target analytes. In practice, the main requirements for a solid sorbent are: (a) the fast and quantitative adsorption and desorption, (b) a high surface area and high capacity and (c) high dispersibility in liquid samples [54]. 3. Comparison of D-m-SPE with SPE The main differences between SPE and D-m-SPE as extraction methods are: a) The difference in the availability of sorbents and selectivity: at present, there are several types of sorbents for SPE including normal-phase, reversed-phase, ionic and other special sorbents. In contrast, the number of solid sorbents in D-m-SPE is much more in progress and there is an increasing interest in the development of synthetic new sorbents. However, in SPE method due to the unsatisfactory selectivity of traditional sorbents, they usually cannot separate analytes efficiently in complex biological or environmental samples [55]. The selectivity and recovery of SPE and D-m-SPE can be improved by using immunosorbents. Immunosorbents can be divided into three categories: 1- Antibody-based sorbents: these types of immunosorbents are produced by immobilization of relatively matched antibodies on a solid-support surface such as silica beads, alumina, polystyrenedivinylbenzene polymers, agarose-gel and C18 [56]. Based on the specific molecular antigen-antibody recognition, antibody based sorbents have afforded the following advantages over conventional solid sorbents: a) they provide higher selectivity, b) antibody cross-reactivity allows establishment of multiresidue procedures targeting a parent compound, its metabolites or a class of structurally related analytes and c) they allow pre-concentration of analytes from large sample volumes resulting in low limit of detections while minimizing the use of organic solvents [57]. 2- Molecularly imprinted polymer-based sorbents: molecularly imprinted polymer is usually developed by mixing a template molecule (target analyte) with functional monomers, a cross-linker and an initiator. After polymerization, the removed template molecules make the binding sites and the cavities, which are complementary

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Fig. 1. Schematic illustration of D-m-SPE procedure.

to the template in size, shape and functionality, accessible [58,59]. The molecularly imprinted polymers afforded cleaner extracts, due to their high selectivity, which led to a better quantification of the target analytes from complex matrices [60]. However, these types of immunosorbents have the following drawbacks: a) the preparation of them is tedious, b) the template used in the preparation of them is always strongly retained and c) they could recognize a single or a class analytes which have the similar structure with the template. Therefore, the selectivity of the molecularly imprinted polymers was limited because it was based on the complementary of structure and chemical bonds [61,62]. 3- Aptamer-based sorbents: aptamers, single-stranded oligonucleotides of DNA, RNA or polypeptides [63], apply to capture various target molecules including proteins, nucleic acids, peptides, amino acids, cells, viruses, small molecules and ions. In comparison with antibody-based sorbents, aptamers have better affinity towards target analytes, high affinity constant, specificity and stability with targets, and being easily prepared and modified at low cost [64]. Lin et al. developed a novel sorbent based on aptamer-functionalized magnetic metal-organic framework material for selective enrichment of the trace polychlorinated biphenyls from soil sample. The sorbent possessed good mechanical stability which can be applied in replicate at least for 60 extraction cycles with recovery over than 80%. It provided a linear range of 0.02e400 ng/ mL with a good correlation coefficient (R2 > 0.9994) and limit of detection (0.010e0.015 ng/mL) [65]. b) The difference in extraction procedures: in conventional SPE, the liquid sample is passed through a column containing

sorbent that retains the target analytes. The sample loading step for cartridge SPE, irrespective of whether the loading is performed by gravity flow or pressure/vacuum-assisted, requires a relatively long period of time. In the case of D-m-SPE, a small amount of a solid sorbent (mg or mg range) is dispersed in the sample solution to extract the target analytes. c) The difference in extraction times: although, SPE as a wellknown method of separation and pre-concentration has been developed and widely used in different fields but it is a time-consuming method. D-m-SPE as a miniaturized format of SPE has been attracting attentions to overcome the abovementioned drawback of SPE. In D-m-SPE, the extraction is not carried out in the SPE cartridge, but it is performed in the bulk solution. Compared with SPE, D-m-SPE enables the analytes to interact equally with all of the dispersed sorbent particles, to achieve greater capacity per amount of sorbent and to avoid common problems of conventional SPE method such as channeling or blocking of cartridges or discs. D-m-SPE has the following advantages: less organic solvent and sorbent consumption, high extraction efficiency and short time requirement. d) The difference in the applicability of nano-materials: In both cases (SPE and D-m-SPE), the use of suitable sorbents is a critical factor to get high extraction efficiency. However, the practical application of the nano-materials packed into a cartridge of SPE can be hampered because these materials can cause high back pressure and long sample loading time, while nano-materials can easily be applied in D-m-SPE [66]. Nano-materials possess large surface area and short diffusion route, which may result in high extraction efficiency and rapid extraction dynamics of D-m-SPE. As previously reported

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by Jeannot and Cantwell the extraction rate constant is a function of mass-transfer coefficient of analytes, interface area between the aqueous phase and extraction phase [67]. Therefore, compared to the traditional micrometer-sized sorbents used in SPE, nano-materials offer a significantly larger surface area and a shorter diffusion route causing the rapid extraction dynamics and high extraction capacity in Dm-SPE [68]. e) The difference from other aspects: one of the main strategies to reduce the sample preparation steps and allow the development of faster extraction, consequently increasing the sample throughput of SPE is the automation of it [69]. On-line systems generally decrease the degree of manual handling of samples and, thereby, minimize the risk of errors, such as the adsorption of target analytes on glassware, transfer losses and contamination of the sample [70]. However, the difficulty of the on-line process because of the difficulty in replacement and/or reconditioning of on-line cartridges caused SPE to commonly be performed in off-line mode. Although, the off-line mode is somewhat timeconsuming and increases analytical errors but the operation process is simple and easy [71]. According to best of our knowledge, the number of reported literature about the online D-m-SPE is very less than on-line SPE. The users encountered with another drawback of D-m-SPE when they want to synthesize the desired sorbent in a laboratory. Unfortunately, in most cases, the synthesis and preparation of sorbents is a time-consuming process and requires significant chemical consumption as well as energy. However, as previously mentioned, the number of sorbents used in SPE is limited. This apparent drawback of SPE can be considered as an advantage depending on the context; thus, simultaneous extraction of a large number of organic compounds might save cost, time and solvents. Moreover, D-m-SPE also addresses one of other disadvantage of SPE (e.g., analyte memory effect). Therefore, the elution step may be done several times until the acceptable desorption capacity is achieved. The D-m-SPE procedure was also found to have other drawbacks: it needs to an ultrasonication or vortex step to complete dispersion of sorbents in aqueous phase and a tedious centrifugation or filtration step to collect them after adsorption process. Accordingly, the magnetic sorbents have been developed in order to simple and effective phase separation of sorbents. 4. Type of sorbent in D-m-SPE Nowadays, several commercial or synthetic nano-material including functionalized silica, multi-walled carbon nanotubes, graphene, graphene oxide and modified magnetic NPs (MNPs) have been applied as sorbents in D-m-SPE. These nano-materials can be used for the speciation, enrichment and separation of various analytes from different matrices. The larger superficial area of nano-materials (that enhances the extraction kinetic) and the variety of different chemical interaction (that makes wider the applicability to different problems) can be considered among the main reasons. The combinations of different types of nanomaterials (hybrid nano-materials) as well as the combination of nano-materials with micrometric systems (composites) have increased the potential application of them in analytical sciences. Within the composites, the combination of MNPs with different materials such as metal organic frameworks, zeolitic imidazolate frameworks, layered double hydroxides and polymers is especially interesting. In this sense, the composite presents high extraction

capabilities due to the porosity, existence of hydrogen bonding, dipole-diploe and pep interaction and/or polymeric network while maintains a magnetic behavior that simplifies the overall extraction procedure. Based on a review study reported by Castillo-Garcia and coworkers, the distribution percentages of different types of coated MNPs used in solid-based extraction methods in the period 2011e2015 are as follow [72]: Polymers (21.1%), molecularly imprinted polymers (17.2%), silica (13.2%), graphene (12.7%), surfactants (12.7%), carbon nanotubes (10.3%), ionic liquids (7.4%) and metal/metal oxides (5.4%). The use of MNPs has really increased the applicability of magnetic-facilitated extraction procedures since 1996. Analytical extraction approaches based on MNPs are advantageous over more classical SPE. So, MNPs have large constant magnetic moments and can be easily collected by using an external magnetic field placed outside of the extraction container without additional centrifugation or filtration of the sample solution, which makes sampling and collection easier and faster. Fe3O4 NPs are good candidates for magnetic separation which have the following advantages: (1) they can be produced in large quantity using a simple method; (2) it is expected that their adsorption capacity is high due to their large surface area; (3) they have strong magnetic properties and low toxicity; and (4) these particles are super paramagnetic, it means that metal-loaded sorbent can be easily separated from the treated water via an external magnetic field [13,73,74]. 4.1. Functionalized silica NPs Silica NPs have been used in many applications including membranes [75], catalyst supports [76], optical devices [77], chemical/biological sensors [78] and as building blocks of colloidal and NPs crystals [79]. Silica NPs are synthesized predominantly € ber method, resulting in narrow distributions of large using the Sto particles (>200 nm) but broad distributions of smaller particles. Silica-based mesoporous materials such as MCM-41 (Mobil Composition of Matter-41) and UVM-7 (University of Valencia Materials-7) have been applied widely as sorbent because of high performances, excellent mechanical resistance, and good thermal, hydrothermal and chemical stabilities together with large surface area, high porosity, well-defined pore size and well-modified surface properties, which allow easy access to functional groups into the structure [80e83]. Mehdinia et al. reported an efficient nano-composite by incorporation of gold NPs into the magnetic MCM-41 as a sorbent of D-mSPE [84]. For the effective immobilization of gold NPs on the surface of MCM-41, magnetic MCM-41 was functionalized by amine groups. The synthesized nano-composite was used as a sorbent for the rapid and effective extraction of some PAHs from aqueous samples. In this work, to achieve high pre-concentration factor, dispersive liquid-liquid microextraction procedure was coupled with the D-mSPE method. Herein, after applying the D-m-SPE method, the analytes on the sorbent was eluted by desorption solvent (acetonitrile). The desorption solvent containing PAHs was collected in a conical tube and mixed with the extraction solvent (C2Cl4). Finally, by rapidly injection adequate volume of water into the homogenous mixture of desorption and extraction solvents, a cloudy solution was formed. This cloudy solution was centrifuged and the sedimented phase (C2Cl4) was injected into the GC. UVM-7 was used in ultrasound assisted D-m-SPE method as bimodal mesoporous silica NPs by Shirkhanloo and coworkers [12]. UVM-7 is an interesting material which can be considered as a nanometric version of the well-known MCM-41 material. The main purpose of this research was to develop a simple, fast, reliable and

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strength of sample solution, the volume of eluent solvent (mL), vortex and ultrasonic times (min) were investigated by PlacketteBurman design. The significant variables were optimized by a Box-Behnken design combined by a desirability function. Gal an-Cano and coworkers used the methylimidazoliumhexafluorophosphate (an ionic liquid) functionalized silica under a D-m-SPE approach for the extraction of organophosphate pesticides from water samples [30]. After its synthesis, the sorbent is chemically characterized by paying special attention to the type of interaction that it can establish with the target analytes. Moreover, different dispersion approaches were evaluated in order to clarify the main parameters affecting the extraction. The ionic liquid-modified silica allows the isolation and the preconcentration of analytes (phosmet, parathion, triazophos and phoxim) with enrichment factors in the range from 74 (phoxim) to 111 (triazophos) in a simple procedure. Table 1 summarizes some information concerning the particle size of sorbent, limit of detection, recovery (%), pre-concentration factor and extraction time of different silica-based sorbents used in D-m-SPE [12,30,84e87].

cost-effective method for the speciation and pre-concentration of trace amounts of Mn (II)/Mn (IV) ions in water samples by using NH2-UVM-7 as an efficient sorbent. Experimental parameters including sample pH, amount of sorbent, sample volume, eluent type and volume, time of ultrasound, etc., have been optimized. The developed method has enough sensitivity and simplicity for the determination of metal ions in real water samples. Lu et al. reported a simple and straightforward strategy for coating molecularly imprinted polymers layer at the surface of silica NPs with a controllable shell thickness by tuning the amount of surface end vinyl groups [85]. The chlorpyrifos (CP, target analyte) was first preassembled to noncovalent complex with methacrylic acid (MAA) due to the strong hydrogen bonds effect. The 3(methacryloxy)propyltrimethoxysilane was modified onto the surface of silica NPs to produce the end vinyl bonds that acted as reactive sites to induce the selective occurrence of imprinting polymerization at the particles surface. It has been demonstrated that the amount of end vinyl bonds can be adjusted to control the thickness of imprinted shell. With an optimal shell thickness, a large capacity, a high selectivity and fast kinetics of binding target CP were achieved. The sorbent was applied to the efficient extraction and fast enrichment of CP from the complex matrix such as green vegetables, cucumber, pear and jujube. Peng and coworkers demonstrated a new programmed heating approach to imprint sulfonylurea herbicides at the surface of silica NPs [86]. Prior to the molecular imprinting, the silica NPs were chemically modified by 3-(methacryloxy)propyltrimethoxysilane to obtain vinyl end groups. Methacrylic acid was used as a functional monomer for imprinting metsulfuron-methyl (MSM) due to its hydrogen bond interaction. It was demonstrated that programmed heating in imprinting polymerization led to the formation of uniform imprinting layer at the surface of silica NPs. Meanwhile, the thickness of coating layer can be efficiently adjusted in a suitable condition of polymerization reaction. The large adsorbing capacity and high selectivity to sulfonylurea analogs were achieved when the layer-coated particles were used as the sorbents of D-m-SPE before high-performance liquid chromatography (HPLC) analysis. The method can be applied to the simultaneous analysis of mixed sulfonylurea herbicides in the spiked soil, rice, soybean and corn samples. SiO2@3-mercaptopropyltriethoxysilane (MPTES)@Au core-shell NPs were used as a sorbent for the extraction of hydroquinone, resorcinol, pyrocatechol and phenol from water samples by our group [87]. During the core-shell preparation, the surface of SiO2 NPs was treated with mercaptopropyl groups. Mercaptopropyl groups are able to attract metal species and facilitate the formation of Au NPs shell. The extraction procedure was based on D-m-SPE. Variables such as the amount of sorbent (mg), pH and ionic

4.2. Carbon nanotubes and multi-walled carbon nanotubes Carbon nanotubes, which are classified as single-walled carbon nanotubes and multi-walled carbon nanotubes on the principle of presence of carbon atom layers in the walls of nanotubes, have been recently applied in a laboratory as effective sorbents for the preconcentration of trace concentration of analytes because of strong adsorption power, exceptional mechanical properties, high chemical stability and large specific surface area [88e90]. Besides the mentioned advantages, due to strong van der Waals interaction of multi-walled carbon nanotubes, they tend to form bundles and hard to disperse in aqueous solutions. To overcome this drawback, modifications have been used to improve their dispersion in water [91]. Surface modification can greatly increase the interaction of multi-walled carbon nanotubes with the analytes and thereby can increase the adsorption capacity of sorbent in pre-concentration of them. Recently, multi-walled carbon nanotubes have been characterized as sorbents for removing environmental contaminants such as dioxins, phenols, phthalate esters, chlorobenzenes and drugs. Eshaghi et al. used the magnetic functionalized multi-walled carbon nanotubes modified with silica compound and a new triazene ligand (1-(2-ethoxyphenyl)-3-(4-ethoxyphenyl) triazene (EET)), (multi-walled carbon nanotubes-Fe3O4 MNPs-silica-EET) to selective Hg (II) adsorption from the solution [92]. In this study, during the solegel reaction of 3-(trimethoxysilyl) 1-propane thiol with triazene solutions and multi-walled carbon nanotubes-Fe3O4

Table 1 Comparison of different silica-based sorbents used in D-m-SPE method from different aspects. Sorbent

Particle size

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

Magnetic-Au-NH2-MCM-41 NH2-UVM-7 Silica-CP-MIPc Silica-MSMd-MIP SiO2@MPTESe@Au SiO2-MIMf-PF6

<150 nm 30e40 nm e e e e

5 min 2 min 30 min 25 min 5 min 1 min

0.002e0.004 mg/L 0.007e0.008 mg/L e 0.004e0.013 mmol/L 540e1240 ng/L 295e560 ng/L

91.4e104.2 >96.0 76.1e93.5 73.8e110.8 >93.5 94.0

5519e6271 (100 mL) 98.8e102.3 (100 mL) (5 mL) (1 mL) (4 mL) 74e111 (8 mL)

[84] [12] [85] [86] [87] [30]

a b c d e f

LOD ¼ limit of detection. PF ¼ pre-concentration factor. CP-MIP ¼ chlorpyrifos-molecularly imprinted polymer. MSM ¼ metsulfuron-methyl. MPTES ¼ 3-mercaptopropyltriethoxysilane. MIM ¼ methylimidazolium.

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MNPs, the silica particles formed interconnecting rigid networks and immobilized on the surface of the multi-walled carbon nanotubes-Fe3O4 MNPs, while EET and multi-walled carbon nanotubesFe3O4 MNPs were simultaneously fixed on the surface of silica particle. Xu et al. reported an effective and rapid ionic liquid-coated multi-walled carbon nanotube based ultrasound assisted D-m-SPE method for the one step extraction and the pre-concentration of trace levels of Rhodamine B in food samples [93]. The aim of this work was to simplify the analytical step, reduce the consumption of toxic solvents and improve the sensitivity. Several experimental conditions such as type of ionic liquids, ratios of ionic liquids and multi-walled carbon nanotube, amount of sorbent, extraction time, pH, ionic strength of solution and desorption conditions were studied and optimized. The performances of developed method were evaluated under optimized condition. A D-m-SPE method reported by Barbara Feist for the selective pre-concentration of trace lead ions on oxidized multi-walled carbon nanotubes with complexing reagent 1,10-phenanthroline [94]. Flame and electrothermal atomic absorption spectrometry (were used for detection of lead ions). In order to synthesize the oxidized multi-walled carbon nanotubes, briefly, 2 g of multi-walled carbon nanotubes were suspended in 100 mL of concentrated HNO3 and refluxed for 6 h at 100 C. Finally, the mixture was filtered and washed with deionized water until pH of filtrate was 7. The filtered solid was dried in an oven at 60 C. The suspension of oxidized multi-walled carbon nanotubes (5 mg/mL) was prepared in high purity water. Before the use, oxidized multi-walled carbon nanotubes suspension was sonicated for 30 min to obtain a homogeneous dispersion. Amoli-Diva and coworkers developed a D-m-SPE procedure coupled with surfactant-enhanced spectrofluorimetric detection for the determination of ofloxacin and lomefloxacin from biological and environmental samples [95]. D-m-SPE procedure was performed by magnetic Fe3O4 NPs grafted multi-walled carbon nanotubes as an efficient sorbent. The main factors affecting the signal enhancement (including surfactant concentration and pH) and extraction efficiency (including pH, extraction time, sample volume, amount of magnetic sorbent and desorption conditions) were investigated in detail. Zhao et al. developed a fast, convenient and simple magnetic Dm-SPE procedure for the pre-concentration of several steroid hormones (17-a-hydroxyprogesterone, 17-a-hydroxyprogesterone acetate, norgestrel, melengestrol acetate, levonorgestrel, dydrogesterone, hydroxyprogesterone caproate, norethindrone, 21-a-hydroxyprogesterone, ethisterone, medroxyprogesterone, megestrol acetate, chlormadinone acetate, medroxyprogesterone acetate, progesterone, boldenone, nandrolone propionate, nandrolone, metandienone, 17-methyltestosterone, testosterone, trenbolone, testosterone propionate, and stanozolol) in river water [96]. Ethylenediamine-functionalized magnetic carbon nanotubes (EDA@Mag-carbon nanotubes) were synthesized by a simple onepot reaction and were used as a sorbent in extraction procedure. The obtained results demonstrated the higher extraction capacity of EDA@Mag-carbon nanotubes with recoveries between 82 and 113%. Kocot and coworkers proposed the combination of D-m-SPE, using multi-walled carbon nanotubes as solid sorbents, with totalreflection X-ray fluorescence spectrometry (TXRF) for the preconcentration and the determination of lead and cadmium ions in water samples [54]. The proposed sample preparation is quite simple and economic. After the adsorption processes of the metal ions on the surface of multi-walled carbon nanotubes, the aqueous sample is separated by centrifugation and the metal ions loaded

multi-walled carbon nanotubes are suspended in a small volume of an internal standard solution and analyzed directly by TXRF. D-mSPE-TXRF was applied to the analysis of different types of water samples including sea water, river water and wastewater. Cao et al. described the use of trace-chitosan-wrapped multiwalled carbon nanotubes (CS-multi-walled carbon nanotubes) as a sorbent in D-m-SPE, which was combined with ultra-HPLC coupled with quadrupole time-of-flight tandem mass spectrometry (ultraHPLC-Q-TOF/MS) to analyze seven phenolic compounds (chlorogenic acid, luteolin-7-O-b-D-glucoside, linarin, diosmetin, apigenin, luteolin and acacetin) in chrysanthemum tea and a chrysanthemum beverage [97]. The high specific surface areas of multiwalled carbon nanotubes and cationic surfactant properties of chitosan caused the composites to exhibit excellent performance for the adsorption of above-mentioned analytes. Grijalba developed a sensitive pre-concentration method based on D-m-SPE technique combining multi-walled carbon nanotubes and an ionic liquid for determination of inorganic arsenic species in garlic [98]. The method proposes a thorough simplification by allowing the direct injection of analyte-containing dispersed multiwalled carbon nanotubes into the graphite furnace of electrothermal atomic absorption spectrometer. Thus, no digestion or back-extraction procedures are needed for arsenic injection in detection instrument, as multi-walled carbon nanotubes are pyrolized before the atomization step. Bahadir et al. demonstrated the usefulness of the combination of D-m-SPE using modified multi-walled carbon nanotubes with the anionic exchanger Aliquat 363 with benchtop TXRF instrumentation for the determination of trace amounts of Cr (VI) in drinking waters [99]. Parameters affecting the extraction process (pH and volume of the aqueous sample, amount of multi-walled carbon nanotubes and extraction time) and TXRF analysis (volume of internal standard, volume of deposited suspension on the reflector, drying mode and instrumental parameters) have been carefully evaluated to test the real capability of the developed methodology for the determination of Cr (VI) at trace concentration levels. Barfi and coworkers developed a novel method called syringeassisted D-m-SPE based on repeated withdrawing and pushing out a mixture of an aqueous sample including some chelated potentially toxic metal ions with bis-(acetylacetone) ethylenediimine and a low level of a suitable sorbent (1.6 mg of multi-walled carbon nanotubes) in a test tube using a syringe [100]. Since maximum contact surface areas were simply provided between the chelated ions and sorbent with no need to essentially be off-line the accelerating mass transfer (including sonication and vortex) and a centrifugation step, maximum efficiency was achieved within a short period of time. The proposed method was successfully applied to the extraction of Pb (II), Cd (II), Co (II), Ni (II) and Cr (III) from different water (tap and wastewater), fruit juice (apple, pear, grape, and grapefruit) and biological fluid (saliva and urine) samples. Table 2 gives information about the sorbent and analytical aspects of D-m-SPE method based on carbon nanotubes and multiwalled carbon nanotubes [54,92e100]. 4.3. Graphene and graphene oxide Among the carbon nano-material, graphene and graphene oxide seem to be the most intensively studied materials in analytical chemistry [11,101]. Graphene, the single-layer sheet composed of sp2 carbon atoms with a two-dimensional honeycomb lattice, has a large specific surface area due to the availability of both sides of the planer sheets for sorption [102e105]. Chemical oxidation of graphite, chemical vapor deposition and reduction of graphene oxide are common fabrication methods of graphene. Hydrazine,

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sodium bore hydride, ascorbic acid and iron powder are the most frequent materials used as a reductant for the chemical preparation of graphene from graphite as starting material [106]. Sizes and qualities of graphene depend on the synthetic or processing methods of graphene [107]. Graphene has attracted great attention in D-m-SPE due to its unique physicochemical properties and superb adsorption capacity. The special pep electrostatic stacking property also allows a strong attraction between graphene and target analytes in sample solution [108]. The researchers believe that the performance of graphene sorbent is better than that of carbon nanotubes. This is related to the special morphology of graphene in which both sides of its planar sheets area are available to molecule adsorption and lead to fast adsorption equilibrium and analyte elution [109e113]. Sitko and coworkers proposed an analytical methodology based on D-m-SPE and novel sorbent (mercapto-modified graphene oxide) for multi-elemental ultra-trace determination of heavy metal ions and arsenic species. The sorbent demonstrates selectivity toward arsenite in the presence of arsenate [14]. Due to such features of sorbent nanosheets as wrinkled structure and excellent dispersibility in water, it seems to be ideal for fast and simple preconcentration and determination of Co (II), Ni (II), Cu (II), As (III), Cd (II) and Pb (II) using TXRF measurement. A D-m-SPE with graphene as a solid sorbent and with graphene as a solid adsorbent and ammonium pyrrolidinedithiocarbamate (APDC) as a chelating agent was applied to the speciation and determination trace and ultra-trace amounts of Se (IV) and Se (VI) ions by Kocot and coworkers [114]. Although graphene is insoluble and hard to disperse in aqueous solutions, but this researcher group found that application of nonionic surfactant such as Triton-X-100 gives the possibility to use graphene as a solid sorbent. In a typical experiment, 200 mL of graphene/APDC/Triton-X-100 suspension was injected into 50 mL of sample solution (pH ¼ 1) rapidly. In the next step, the sample was filtered through the membrane filter with the use of filtration assembly of 5 mm in diameter. Finally, the membrane filter with graphene and Se (IV)-APDC complex adsorbed on its surface was dried under an IR heater and measured using energy-dispersive X-ray fluorescence spectrometry. Graphene oxide as the oxidized derivative of graphene has attracted great attention for the application in sample clean-up procedures. The existence of hydroxyl, epoxy and carboxyl groups, large specific surface area and high water solubility has made graphene oxide as an ideal sorbent [115]. The oxygenated

105

lattice of graphene oxide provides a good solubility and dispersibility of this material in many solvents and provides the possibility of hydrogen bonding formation and/or electrostatic interactions with organic materials or metal ions. Because of high water solubility of graphene oxide, the separation of it from aqueous phase is difficult [116]. The introduction of magnetic properties into graphene oxide can efficiently eliminate good solubility and dispersibility of graphene oxide. Unfortunately, when graphene oxide is applied as a sorbent, the determination of analytes in a complex matrix can be seriously hampered. Since graphene oxide is not selective and the competitive adsorption can be observed. Therefore, adsorption ability depends on the difference in the affinity of target analytes toward graphene oxide as well as on the concentration of coexisting analytes. Accordingly, the improvement of the sorbent selectivity can be realized by application of selective chelating agent or through chemical functionalization of graphene oxide. Hu et al. described the use of graphene oxide as sorbent in D-mSPE, together with ultra- HPLC-Q-TOF/MS for the determination of phenolic compounds (protocatechuic aldehyde, caffeic acid and rosmarinic acid) in dietary supplements [117]. In a typical experiment 1 mL of a 0.2 mg/mL graphene oxide suspension was added to an aliquot of sample solution containing target analytes. Subsequently, the mixture was agitated using an orbital shaker, favoring the extraction of the phenolic components. Then, the sorbent was isolated from the sample by a 0.45 mm nylon filter previously conditioned passing 2 mL of methanol and 2 mL of ultrapure water. The sorbent enriched with the analytes was washed with 100 mL of methanol. The elution solvent was collected in a glass vial for the further ultra-HPLC-Q-TOF/MS analysis. Kazemi and coworkers developed a novel, sensitive, fast, simple and convenient method for separation and pre-concentration of trace amounts of fluoxetine before its spectrophotometric determination [118]. The method was based on the combination of magnetic graphene mixed hemimicelles solid phase extraction and D-m-SPE using 1-hexadecyl-3-methylimidazolium bromide coated magnetic graphene as a sorbent. Hemimicelles are formed when the charged groups of ionic surfactants or ionic liquids were attracted to the opposite charge of substrates (herein magnetic graphene surface). The driven force for the formation of hemmicelles is Coulombic force between opposite charges and nonCoulombic force between the surfactant and/or ionic liquid chain-chain [119]. Because hemmicelles are not covalently bound

Table 2 Analytical aspects of D-m-SPE method obtained from MWCNTs as sorbent. Sorbent c

d

MWCNTs -Fe3O4 MNPs-silica-EET [C4MIM][PF6]-coated MWCNTs Oxidized-MWCNTs Magnetic-MWCNT EDA@Mag-CNTse Oxidized-MWCNTs CSf-MWCNTs ILg-assisted MWCNTs MWCNTs modified with the anionic exchanger MWCNTs a b c d e f g

Particle size

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

~36.92 nm e e 11 mm e 6-9 nm e e e

2 min 3 min 10 min 10 min 10 min 5 min 3 min 5 min 20 min

1.5 ng/mL 0.28 ng/mL 0.0064e0.26 mg/L 12e15 ng/mL 0.0067e0.33 ng/L 1.0e2.1 ng/mL 0.22e16.19 ng/mL 7.1 ng/L 2e3 mg/L

96.6e103.4 85.1e96.0 >95.0 92.0e99.0 82.1e113.0 93.0e116.0 89.0e106.0 94.0e104.0 98.4e111.0

(10 mL) 451 (50 mL) 100e200 (100 mL) 188e192 (100 mL) 1000 (100 mL) (20 mL) 12e233 (20 mL) 70 (5 mL) (100 mL)

[92] [93] [94] [95] [96] [54] [97] [98] [99]

e

~1 min

0.3e2.0 mg/L

94.0e102.0%

29e31 (10 mL)

[100]

LOD ¼ limit of detection. PF ¼ pre-concentration factor. MWCNTs ¼ multi-walled carbon nanotubes. EET ¼ (1-(2-ethoxyphenyl)-3-(4-ethoxyphenyl) triazene. EDA@Mag-CNTs ¼ ethylenediamine-magnetic carbon nanotubes. CS ¼ chitosan. IL ¼ ionic liquid.

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Fig. 2. The SEM images of graphene (a) and magnetic graphene (b) (reprinted from Ref. [118] with permission of Elsevier 2015).

to the surface of substrates, there is concern about the reusability of the hemmicelles sorbents. In this work, the magnetic graphene was synthesized by a simple coprecipitation method and characterized by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). Fig. 2 shows the SEM images of graphene (a) and magnetic graphene (b) [118]. Later, the same group developed a simple and rapid D-m-SPE combined with mode-mismatched thermal lens spectrometry as well as fiber optic linear array spectrophotometry for the separation, extraction and determination of sulfadiazine [120]. Graphene oxide was synthesized using the modified Hummers method and functionalized with Fe3O4 NPs by means of a simple one step chemical coprecipitation method. The synthesized Fe3O4 functionalized graphene oxide was successfully utilized as an efficient sorbent in D-m-SPE of sulfadiazine. Pytlakowska reported a new procedure for multi element preconcentration of heavy metal ions, specifically Cr (III), Co (II), Ni (II), Cu (II), Zn (II) and Pb (II) ions [121]. The method is based on D-mSPE of the metal complexes of 2-(5-bromo-2-pyridylazo)-5diethylaminophenol by graphene oxide NPs prior to energydispersive X-ray fluorescence determination. Mahpishanian and coworkers established a simple method for the analysis of nicotine in biological and environmental water samples based on the graphene oxide-D-m-SPE [122]. The effects of adsorption and desorption factors on the extraction recovery of the analyte were studied systematically. Table 3 gives information

about the analytical figures of merit and extraction condition of Dm-SPE procedure based on graphene and graphene oxide [14,114,117,118,120e122]. 4.4. Metal organic frameworks Metal organic frameworks or porous coordination polymers are a novel class of highly porous materials. These porous materials are generally comprised of metal ions (as inorganic moieties) or clusters of zinc, copper, chromium, aluminum, zirconium and other metals, and organic components of carboxylates or N-containing aromatic groups connected by coordination covalent bonds with a typical pore size ranging from ultra-microscale to mesoscale [123e125]. To date, thousands of metal organic framework structures have been reported. The typical feature of metal organic frameworks is the presence of ordered and well defined (pore size and pore shape) micropores (<2 nm). Based on the availability of diverse metal ions, organic ligands as well as chelating ratio difference, metal organic frameworks have shown excellent properties of numerous structures, tunable pore sizes as well as large surface areas [126e128]. A variety of potential applications of metal organic frameworks such as gas adsorption, separations, catalysis and sensing has been reported [129e131]. Recently, the application of metal organic framework-199, MIL-53 (MIL: Material Institute Lavoisier), MIL-88B and metal organic framework-199/graphene oxide as novel composites for D-m-SPE has received considerable attention.

Table 3 Analytical figures of merit and extraction condition of D-m-SPE procedure based on graphene and graphene oxide as sorbent. Sorbent

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

Mercapto-modified graphene oxide Graphene/APDCc/Triton-X-100 Graphene oxide 1-hexadecyl-3-methylimidazolium bromide coated magnetic graphene Fe3O4 functionalized graphene oxide Graphene oxide Graphene oxide

10 min e 2 min <10 s e 15 min 10 s

0.054e0.11 ng/mL 0.032 ng/mL 0.07e0.21 ng/mL 0.21 mg/L 0.34 mg/L 0.07e0.25 ng/mL 1.5 ng/mL

91.8e108.4 97.7e99.2 90.1e96.4 95.3e100.6 94.3e100.7 94.4e103.5 88.7e109.7

150 (75 mL) 1013 (50 mL) (10 mL) 167 (50 mL) 200 (100 mL) (50 mL) 93 (10 mL)

[14] [114] [117] [118] [120] [121] [122]

a b c

LOD ¼ limit of detection. PF ¼ pre-concentration factor. APDC ¼ ammonium pyrrolidinedithiocarbamate.

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Fig. 3. (a) SEM micrograph of the Fe3O4@MIL-100 core-shell magnetic microspheres; (b) TEM micrograph of the Fe3O4@MIL-100 core-shell magnetic microspheres (reprinted from Ref. [132] with permission of Elsevier 2013).

Chen et al. developed the application of Fe3O4@MIL-100 coreshell magnetic microspheres for D-m-SPE of polychlorinated biphenyls from environmental water samples [132]. Fig. 3(a) shows the SEM image of the magnetic microspheres. Fig. 3(b) shows the transmission electron microscopy (TEM) image of the synthesized microspheres, which were found to be composed of a Fe3O4 core and a metal organic framework shell with a thickness of approximately 100 nm. Rocío-Bautista and coworkers reported the utilization of the metal organic framework HKUST-1 in combination with MNPs, in a D-m-SPE method with ultra-HPLC and fluorescence detection for the determination of eight heavy PAHs [133]. The analytical applications are carried out with environmental waters (tap water and wastewaters) and with fruit tea infusion in which PAHs have been reported. It is important to highlight that the preparation of the hybrid material HKUST@Fe3O4 is accomplished without ensuring any kind of chemical binding through a reaction; both materials are simply mixed with the aqueous sample. Thus, the overall method is quite simple, not only for the use of a magnetic-based approach but also for the easiness of preparation of hybrid sorbent material. We reported a selective and sensitive method based on D-m-SPE and green deep eutectic solvents for the extraction of dopamine, epinephrine and norepinephrine from biological samples prior to HPLC [134]. The Fe3O4@MIL-100 (Fe) core-shell NPs were grafted with pyrocatechol. The synthesized sorbent was characterized using SEM, TEM, vibrating sample magnetometry (VSM) and FT-IR spectroscopy. The fractional factorial design and central composite design of response surface methodology were used in the experimental design and the optimization of the extraction efficiency.

A novel automatic strategy for the use of micro- and nanomaterial as sorbents for D-m-SPE based on the lab-in-syringe concept was reported by Maya et al. [135]. A hybrid material based on submicrometric crystals MIL-100 (Cr) containing Fe3O4 NPs was prepared. In a preliminary step, adequate amounts of sorbent were loaded in the syringe already containing a magnetic bar. The extraction procedure is based on the loading of the malachite green sample into the syringe at a high flow rate while stirring, achieving the total dispersion of the magnetic metal organic framework. Then, the sample matrix was discarded while sorbent containing the extracted analyte was gradually deposited onto the magnetic bar. Once the sample was discarded, the procedure was repeated with pure water in order to wash the metal organic framework and remove the remaining sample matrix. Finally, the sorbent was eluted by loading 0.5 mL of eluent solvent into the syringe. After elution, a volume of the eluent was loaded into the loop of the injection valve and subsequently injected towards the spectrophotometer detector. Li et al. developed D-m-SPE based on MIL-101 (Cr) for the extraction of triazine and phenylurea herbicides in vegetable oils [136]. MIL-101 was prepared with 1,4-benzenedicarboxylic acid ligand and Cr ions as a metal center. The herbicides were extracted directly with MIL-101 from diluted vegetable oils and no further clean-up operation is needed. The fat remained in sample solution was easily removed after centrifugation. The herbicides were separated and determined by HPLC. The analytical performance data, extraction condition and particle size of sorbents for D-m-SPE method based on metal organic frameworks are listed in Table 4 [132e136].

Table 4 Analytical performance data, extraction condition and particle size of sorbent for D-m-SPE method based on metal organic frameworks. Sorbent c

Fe3O4@MIL -100 Fe3O4@HKUST-1 Fe3O4@MIL-100 (Fe) grafted with pyrocatechol Fe3O4@MIL-100(Cr) MIL-101 a b c

LOD ¼ limit of detection. PF ¼ pre-concentration factor. MIL ¼ Material Institute Lavoisier.

Particle size

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

~100 nm e e

40 min 5 min 5 min

1.07e1.57 ng/L 0.02e0.08 mg/L 0.22e0.36 mg/L

80.0e111.9 22.0e98.0 91.4e103.4

(10 mL) (20 mL) 164 (10 mL)

[132] [133] [134]

0.20e0.25 mm e

30 s 15 min

0.012 mg/L 0.58e1.04 mg/L

95.0e107.0 87.3e107.0

120 (40 mL) (12 mL)

[135] [136]

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4.5. Zeolites and zeolite imidazolate frameworks Zeolite imidazolate frameworks were first reported in 2006 by Park et al. These materials as a subclass of metal organic frameworks have been demonstrated to have great potential recently in diverse fields [137e140]. Zeolite imidazolate frameworks consist of metal nodes (usually zinc or cobalt) connected to imidazolate linkers and exhibit zeolite-like structures, perhaps because the transition metal-imidazolate angle is 145 like the SieOeSi angle in many zeolites [141]. As a novel type of microporous crystalline materials, zeolite imidazolate frameworks combine highly desirable properties of both zeolites and metal organic frameworks, such as crystallinity, unimodal micropores, high surface areas, exceptional thermal and chemical stability and flexibility in surface modification, which make zeolite imidazolate frameworks very promising candidate materials for many technological applications. Zeolite imidazolate framework-8 is one of the most widely investigated zeolite imidazolate frameworks. This material was demonstrated to be a suitable sorbent for water samples because of its high surface area, hydrophobic property and water stability [16,142]. Salisaeng and coworkers developed a vortex-assisted D-m-SPE based on cetyltrimethylammonium bromide (CTAB)-modified zeolite NaY for pre-concentration of carbamate pesticides (oxamyl, methomyl, aldicarb, propoxur, bendiocarb, carbaryl, isoprocarb, methiocarb and promecarb) in fruits, vegetables and natural surface water prior to analysis by HPLC with photodiode array detection [52]. Zeolite NaY was prepared in the same manner as that was conducted by Patdhanagul and coworkers [143]. To modify the zeolite NaY surface, an aqueous solution of CTAB solution was added to an Erlenmeyer flask containing the adequate amount of zeolite NaY sorbents. The suspension was shaken mechanically for 24 h at 150 rpm. After the supernatant was decanted, the remained materials were washed with water to ensure that excess or loosely bound CTAB surfactant was removed. The obtained CTAB-modified zeolite NaY sorbents then were air-dried and stored in closed bottles for subsequent uses. Liu et al. described the preparation of nanoporous carbon using a metal organic framework as a template and furfuryl alcohol as the source of carbon [144]. The metal organic framework consists of a zeolite imidazolate framework-8 that was obtained from 2methylimidazole and Zn (II) ions. Zeolite imidazolate framework8 was soaked in furfuryl alcohol which then was carbonized at 900 C. The resulting nanoporous carbon (metal organic framework-C) exhibits a high specific surface area and a large pore volume. It was used as an efficient sorbent for the preconcentration of benzoylurea insecticides diflubenzuron, triflumuron, hexaflumuron and teflubenzuron from water and tangerine samples. Under optimized conditions, the method exhibits excellent extraction performance. Ghazaghi et al. described a hybrid nano-sorbent prepared by depositing graphene on the zeolite clinoptilolite (G-CL) by chemical vapor deposition [145]. The resulting sorbent is well suited for the pre-concentration of Pb (II) and Cd (II) by ultrasound assisted D-m-SPE. The surface morphology of the G-CL hybrid was characterized by SEM, which is presented in Fig. 4. It shows the presence of porous CL structure and graphene sheets around it. As a part of this study, the extraction efficiency of G-CL hybrid was compared to CL and the results showed that the synthesized nano-sorbent has a better ability for extraction from complex matrices. The analytical characteristic, extraction condition and particle size of sorbent used in D-m-SPE method based on zeolite and zeolite imidazolate frameworks are summarized in Table 5 [52,144,145].

Fig. 4. SEM image of G-CL hybrid (reprinted from Ref. [145] with permission of Springer 2015).

4.6. Polymeric sorbents Polymers due to their multifunctional properties including hydrophobicity, acidebase character, pep interaction, polar functional groups, ion exchange property, hydrogen bonding and electro-activity have attracted a great deal of attention. Hybrid MNPs with polymers are widely used in analytical chemistry considering that they are powerful materials in sample preparation. They can be classified into two types according to their structure. In most of cases, a polymeric layer is coated on the MNPs surface and the resultant hybrid maintains the nanometric size. On the other side, the MNPs are embedded in a polymer network giving rise to composites which combine nano- and micrometric structures. In the last case, the immobilization of NPs depends on the functional groups of the polymer and can involve dispersive or van der Waals, electrostatic, hydrogen or covalent bonds [31,146]. Reyes-Gallardo and co-workers reported the synthesis, characterization and potential uses of MNPs-nylon 6 composites in sample preparation for the first time [31]. The composite is easily synthesized by a solvent changeover playing with a different solubility of the polymeric network in formic acid and water. As it can be seen from the SEM images of nylon 6 and MNPs-nylon 6 composites (Fig. 5), after the synthesis, the surface of the polymer becomes rougher due to the formation of the composite with MNPs in the polymeric network. The determination of four PAHs (benzo[b] fluoranthene, fluoranthene, indeno[1,2,3-cd]pyrene and phenanthrene), as representatives of atmospheric pollutants, in aqueous samples has been used as a model analytical problem. The recovery study was performed in three different water samples. Zhao et al. synthesized an inexpensive and effective sorbent namely tetraethylenepentamine-functionalized Fe3O4 magnetic polymer (TEPA-MP) [147]. TEPA-MP was prepared by suspension polymerization according to preparation procedure which is illustrated in Fig. 6. The object of this study was to prepare a novel amino-functionalized magnetic polymer and substantiate its D-mSPE ability for the analysis of phenolic environmental estrogens, i.e., bisphenol A, diethylstilbestrol, dienestrol, hexestrol, 4-(tertoctyl)-phenol(4-tOP), 4-nonylphenol, estrone and estradiol valerate in blood. Chen et al. reported the use of a new D-m-SPE procedure using core-shell nanoring amino-functionalized magnetic polymer (CS-NRMP) as sorbent combined with ultrafast liquid chromatographytandem quadruple mass spectrometry [148]. The method has been developed for the detection of trace dicyandiamide in powdered milk. The excellent sensitivity and selectivity of this method for dicyandiamide were investigated in laboratory batch tests, and it can be

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Table 5 Analytical characteristics, extraction condition and particle size of sorbent for D-m-SPE method based on zeolites and zeolite imidazolate frameworks. Sorbent

Particle size

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

CTABc-modified zeolite NaY Nanoporous carbon obtained from ZIF-8d Graphene-zeolite clinoptilolite

9.7 mm e e

2 min 15 min 1 min

0.004e4.000 mg/kg 0.10e0.23 ng/mL 0.004e0.07 mg/L

79.5e124.0 91.7e107.9 97.0

1e34 (7 mL) (50 mL) 20 (2 mL)

[52] [144] [145]

a b c d

LOD ¼ limit of detection. PF ¼ pre-concentration factor. CTAB ¼ cetyltrimethylammonium bromide. ZIF-8 ¼ zeolite imidazolate framework-8.

Fig. 5. SEM images of nylon 6 (left) and MNP-nylon 6 composites (right) at 500 and 1600 (reprinted from Ref. [31] with permission of Elsevier 2014).

Fig. 6. The preparation procedure of TEPA-MP (reprinted from Ref. [147] with permission of Elsevier 2013).

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Fig. 7. Schematic procedures of CS-NR-MP NPs (reprinted from Ref. [148] with permission of Elsevier 2014).

applied to the routine analyses for the determination of trace dicyandiamide residue in powdered milk samples. The preparation procedure of CS-NR-MP is illustrated in Fig. 7. In the literature presented by Asgharinezhad et al. pyrrole conductive polymer in the presence of two different dopants was coated on the surface of Fe3O4 NPs [66]. The capability of these sorbents and bare Fe3O4 NPs for the simultaneous preconcentration and the determination of two widely used antidepressant drugs (sertraline and citalopram) as model compounds were investigated by a D-m-SPE. The average diameter of these NPs was about 40 nm. Socas-Rodríguez and coworkers prepared and characterized core-shell Fe3O4@poly(dopamine) NPs as sorbents for D-m-SPE of twelve estrogenic compounds of interest (i.e. 17a-estradiol, 17bestradiol, estrone, hexestrol, 17a-ethynylestradiol, diethylstibestrol, dienestrol, zearalenone, a-zearalanol, b-zearalanol, a-zearalenol and b-zearalenol) from different water samples [149]. Dopamine has attracted wide interest due to its self-polymerization capacity in aqueous phase under weak alkaline conditions. The sorbent was prepared by dispersing Fe3O4 NPs at a concentration of 2.75 g/L in a 15 mmol/L dopamine solution of phosphate buffered saline at pH ¼ 8.3 and maintaining the polymerization process for 6 h under magnetic stirring (850 rpm) at room temperature. Finally, the desired sorbent was washed with acetonitrile:water and dried at 40 C. Reyes-Gallardo et al. presented the combination of commercial polymeric microparticles (OASIS MCX) and MNPs (CoFe2O4). As model analytical problem, six selected nitroaromatic hydrocarbons (4-nitrobenzaldehyde, 1,4-dinitrobenzene, nitrobenzene, 2,4-

dinitrotoluene, 4-nitrotoluene and 3-nitrotoluene) were determined in water samples [36]. In this report, the high affinity of the polymeric material toward the target analytes as well as the magnetic behavior of CoFe2O4 NPs are combined in a synergic way to develop an efficient and simple D-m-SPE approach. Zhang and coworkers applied a novel D-m-SPE method based on a kind of polymer cation exchange material sorbent to the simultaneous determination of five triazine pesticides (ametryn, atrazine, prometryn, simazine, terbutryn) in honey. An ultra-HPLC-high resolution isotope dilution mass spectrometry was used as instrument detection [150]. They found that the total time required for one sample clean-up was approximately 5 min, and the cost of the proposed method for one sample was low enough (approximately 0.30 US dollars) compared to that of a solid phase extraction column (at least 3.0 US dollars for each). D-m-SPE procedure using polyaniline/silica coated nickel NPs (PANI/SiO2/Ni NPs) as sorbent along with GC-flame ionization detector has been developed for detection and pre-concentration of phenolic compounds from water samples. The sorbent was prepared by in situ chemical polymerization of aniline on the surface of silica-modified nickel NPs and was characterized by FTIR spectroscopy, TEM, X-ray powder diffraction, SEM, energydispersive X-ray spectrometry and VSM [151]. Polyaniline polymer is promising for extraction applications because of its good environmental stability, facile synthesis, extraction capability of polar compounds and relatively low cost. Analytical figures of merit of D-m-SPE procedure, extraction condition and particle size of polymer-based sorbents were reported in Table 6 [31,36,66,147e151].

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Table 6 Analytical figures of merit of D-m-SPE procedure, extraction condition and particle size of polymer based sorbents. Sorbent

Particle size

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

MNPs-nylon 6 composite TEPA-MPc CS-NR-MPd Fe3O4@PPye Fe3O4@poly(dopamine) CoFe2O4@OASIS MCXf PCXg PANIh@SiO2@Ni NPs

10 nm ~800 nm 150 nm 40 nm e e e e

30 min <10 min <10 min 7 min 30 s 2 min 30 s 5 min

0.05e0.58 mg/L 0.022e0.140 mg/L 0.02 mg/kg 0.2e1.0 mg/L 0.21e4.51 mg/L 0.11e1.26 mg/L 0.02e0.2 mg/kg 10e23 mg/L

80.0e111.0 85.0e105.0 99.8e105.6 94.0e99.0 70.0e119.0 71.0e103.0 89.8e116.2 96.0e105.0

18e43 (25 mL) (100 mL) e 46.7e49.5 (6 mL) (25 mL) 161e292 (200 mL) (1 mL) 91e117 (5 mL)

[31] [147] [148] [66] [149] [36] [150] [151]

a b c d e f g h

LOD ¼ limit of detection. PF ¼ pre-concentration factor. TEPA-MP ¼ tetraethylenepentamine-magnetic polymer. CS-NR-MP ¼ core-shell nanoring amino-magnetic polymer. PPy ¼ polypyrrole. OASIS MCX ¼ a commercial polymeric microparticles. PCX ¼ a polymer cation exchange material. PANI ¼ polyaniline.

4.7. Layered double hydroxides Layered double hydroxides as a new type of sorbents are lamellar inorganic materials which can be represented by the general formula [M2þ1xM3þx (OH)2]zþ (An)z/n$mH2O. M2þ and M3þ are divalent and trivalent metal ions, respectively, that occupy octahedral sites in the hydroxide layers, An is an exchangeable anion such as CO3 2 or Cl, and x is the ratio of M3þ/(M2þ þ M3þ) [152e154]. Layered double hydroxides are traditionally synthesized by co-precipitation reactions from an aqueous solution of the chip and available materials. For example, MgeAl layered double hydroxide with CO3 2 and OH as intercalating anions was prepared as follow:

6MgCl2 ðaqÞ þ 2AlCl3 ðaqÞ þ 16NaOHðaqÞ þ Na2 CO3 ðaqÞ þ 4H2 OðlÞ/Mg6 Al2 ðOHÞ16 CO3 $4H2 OðsÞ þ 18 NaClðaqÞ When layered double hydroxides are synthesized by conventional co-precipitation techniques, they have a hexagonal platelet morphology as shown in Fig. 8(a,b) [155,156]. Due to their unique anion exchange capacities, large surface areas, variable size of the interlayers, good thermal stabilities and water resistant structures,

layered double hydroxides have stimulated intense research activities dedicated to a large range of applications including removal, anion exchange, catalysts, sensing and so on. Saraji and coworkers presented the dissolvable MgeAl layered double hydroxide coated Fe3O4 NPs for the extraction and the determination of five phenolic acids (p-hydroxy benzoic, caffeic, syringic, p-coumaric and ferulic) by D-m-SPE [157]. After the extraction, the magnetic sorbents were separated from the aqueous solution by means of an external magnetic force. The analytes were then eluted by dissolving layered double hydroxides in acidic solution. Dissolution of layered double hydroxides coating containing the analytes led to the omission of analyte elution step and simplified the extraction procedure. In this way, the use of a large volume of elution solvent and thus, the tedious time-consuming drying step could be avoided. Tang and Lee synthesized three types of MgeAl layered double hydroxides and employed them as sorbents to extract several aromatic acids (protocatechuic acid, mandelic acid, phthalic acid, benzoic acid and salicylic acid) from aqueous samples [158]. In the key adsorption process, both dispersion and co-precipitation extraction with the sorbents were conducted and experimental parameters such as pH, temperature and extraction time were optimized.

Fig. 8. (A) Schematic structural representation of layered double hydroxides. M2þ and M3þ represent di- and trivalent cations; (B) an example, Zn8Al2(OH)20[CO2 3 ] layered double hydroxides (Zn: gray; Al: pink; O: red; C: black; H: white) (reprinted from Refs. [155] and [156] with permission of Wiley-VCH 2010 and Royal Society of Chemistry 2014, respectively).

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Fig. 9. TEM image of Ni/Cu-Al-layered double hydroxides nanocomposites (reprinted from Ref. [160] with permission of Elsevier 2017).

Later the same group synthesized and employed layered double oxide hollow spheres (LDO-HSs) as a dissolvable sorbent to extract eleven United States Environmental Protection Agency's priority phenols from aqueous samples [159]. In order to obtain LDO-HSs, first D-glucose and boric acid were dissolved in 40 mL of pure water. The aqueous solution was sealed in a Teflon-lined stainless steel autoclave at 180 C for 6 h. The obtained powder was centrifuged and dried at 60 C overnight. Magnetically separable sorbent named Ni/Cu-Al-layered double hydroxide was synthesized and employed as a sorbent in ultrasonic-assisted D-m-SPE to extract several carboxylic acids (namely propionic, butyric, pentanoic, hexanoic, heptanoic, octanoic and decanoic) from non-alcoholic beer samples [160]. The TEM image (Fig. 9) illustrates that Ni NPs were coated with layered double hydroxide. The ridge-like structures confirm the layered double hydroxide coating on the Ni NPs. Effective variables such as the amount of sorbent (mg), pH and ionic strength of sample solution, volume of eluent solvent (mL), vortex and ultrasonic times (min) were investigated via fractional factorial design. The significant variables were optimized by a Box-Behnken design and combined by a desirability function. Table 7 shows the summary of results from D-m-SPE obtained by layered double hydroxides sorbents [157e160]. 4.8. Other sorbents Asgharinezhad et al. developed the MNPs based D-m-SPE method for the simultaneous pre-concentration and the determination of diclofenac (an acidic drug) and diphenhydramine (as a basic drug) in biological fluids (plasma and urine) and wastewater samples for the first time [161]. These drugs were adsorbed on CTAB-coated Fe3O4@decanoic acid NPs in a batch extraction procedure. The MNPs were then gathered using a supermagnet; which makes them particularly suitable for sample preparation since no

centrifugation or filtration is needed after extraction. Afterward, the extracted drugs were eluted with very low amount of organic solvent from the surface of the sorbent under fierce vortex and were determined simultaneously using HPLC. Huang and coworkers employed microwave-assisted extraction (MAE) coupled with D-m-SPE as an efficient method to sensitively and reliably extract N-nitrosodimethylamine and six representative volatile N-nitrosamines: N-nitrosodiethylamine, N-nitrosopyrrolidine, N-nitrosopiperidine, N-nitrosodibutylamine, Nnitroso-n-methylethylamine and N-nitrosodi-n-propylamine, in meat products at trace-levels, prior to their determination by GCMS with chemical ionization [162]. The evaluation procedures consisted of dual experimental protocols in which the effects of MAE parameters (i.e., extraction temperature and time, concentration and volume of sodium hydroxide) and D-m-SPE parameters (i.e., the type of Carboxen sorbent and desorption solvent, extraction time and amount of sorbent used), were simultaneously investigated and evaluated. The accuracy and precision of the method were evaluated, and its suitability for use in determining trace-levels of target compounds in various meat products was demonstrated. Wang et al. developed the MNPs based D-m-SPE method combining with HPLC-diode array detector for the preconcentration and determination of estrogens in pork samples. CTAB-coated Fe3O4@caprylic acid (CA) was used as sorbent and vortex was utilized as an assistance approach to accelerate the mass transfer [163]. In addition, the sorbent was separated from the aqueous samples by an external magnet. Estradiol, estrone and diethylstilbestrol were selected as model compounds to examine the feasibility of method. The affecting factors of the CTAB coated Fe3O4@caprylic acid based D-m-SPE of three target estrogens (sorbent type, sorbent dosage, CTAB amount, sample pH, extraction time and salt concentration) were investigated and optimized. The proposed method was successfully applied to the extraction and

Table 7 Summary of results obtained from D-m-SPE based on layered double hydroxide sorbents. Sorbent c

Fe3O4@Mg-Al LDH MgeAl LDH LDO-HSsd Ni/Cu-Al LDH nanocomposite a b c d

Particle size

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

e 200 nm 500e600 nm e

5 min 5 min 30 min 10.5 min

0.44e1.3 mg/L 0.03e1.47 mg/L 0.005e0.153 mg/L 16-40 mg/L

93.5e104.0 81.3e128.5 95.1e124.0 87.0e110.0

12e24 (5 mL) ~1e100 (10 mL) 36e459 (100 mL) 34e128 (5 mL)

[157] [158] [159] [160]

LOD ¼ limit of detection. PF ¼ pre-concentration factor. LDH ¼ layered double hydroxide. LDO-HSs ¼ layered double oxide-hollow spheres.

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113

Fig. 10. TEM image of Fe3O4@decanoic acid NPs (reprinted from Ref. [166] with permission of Springer 2015).

pre-concentration of estrogens in pork samples. It was found that CTAB plays a prominent role in the extraction mechanism of estrogens and affects the adsorption efficiency of estrogens. Nyaba et al. developed a rapid and efficient pre-concentration method for speciation of Mo (VI), Sb (V) and V (V) that is based on ligandless ultrasound-assisted D-m-SPE using Fe3O4@Al2O3 NPs as a sorbent [164]. Fe3O4@Al2O3 NPs were prepared by adding an adequate amount of AlCl3 into alkaline dispersed Fe3O4 solution. The resulting sol-gel was left to mature at room temperature for 30 h and then dried for 24 h at 100 C. Finally, the sol-gel was calcined by heating at 1000 C in the furnace. This preconcentration method demonstrated excellent qualities such as high sensitivity, low cost, high sample throughput, relatively low limit of detections, high enrichment factors as well as high precision and accuracy. Trujillo-Rodríguez and coworkers described the use of mixed hemimicelles of mono-cationic ionic liquid and double-salt (DS) ionic liquid-based surfactants in a magnetic D-m-SPE approach for the extraction of 2-chlorophenol, 2,4-dimethylphenol, 4-tertButylphenol, 4-cumylphenol, bisphenol A, phenol and 4nonylphenol [165]. The performance of Fe3O4-DS ionic liquids was compared with that of Fe3O4-ionic liquids-based surfactant as a sorbent in D-m-SPE. The best results were obtained with the DS ionic liquid formed by C12C12Im-Br (molar fraction 0.5) and 1hexadecyl-3-methylimidazolium bromide (C16MIm-Br), after proper optimization of the overall method in combination with HPLC-diode array detection. Wang and coworkers synthesized a novel, simple and low cost sorbent through coating Fe3O4 NPs with decanoic acid [166]. The functionalized MNPs showed excellent dispersibility in aqueous solution to D-m-SPE followed by HPLC analysis for 4 phthalate esters

including benzyl butyl phthalate, dicyclohexyl phthalate, di-nbutyl phthalate and di-n-octyl phthalate from liquor samples. The morphology and particle size of Fe3O4@decanoic acid NPs were investigated by TEM (Fig. 10). Accordingly, the average particle sizes of sorbent were measured to be in the range of 10e20 nm. The results of D-m-SPE based on above-mentioned sorbents were summarized in Table 8 [161e166]. Table 9 shows a summary of applications of D-m-SPE for quantitation of a wide range of analytes in different matrices [167e181]. As it can be seen, various sorbents can be employed in D-m-SPE. 5. Conclusion and future This review discussed D-m-SPE as a new extraction method. We have reviewed the most important advantages of D-m-SPE in comparison with SPE method. The type of sorbents commonly used in this method including silica NPs, carbon nanotubes, multiwall carbon nanotubes, graphene, graphene oxide, metal organic frameworks, zeolite imidazolate frameworks, polymeric sorbents and layered double hydroxides are summarized and discussed. Significant works have been achieved over the past several years in the creation and synthesis of new sorbents. NPs and related composites have attracted considerable attention due to their extraordinary properties. With attention to the progress in different sciences, it is highly expected that the number of synthetic sorbents will increase more in the future and more applications of D-m-SPE will be envisaged. Unfortunately, apart from the discussions above, the application of D-m-SPE is limited to the analytical scale in spite of the advantages of it. In our opinion, for widespread application of D-m-SPE, researchers and industry must team up. As we know, to achieve an accurate, sensitive and selective determination of target

Table 8 Summary of results obtained from D-m-SPE based on different sorbents. Sorbent

Particle size

Extraction time

LODa

Recovery (%)

PFb (Volume of sample)

Ref.

Fe3O4@decanoic acid NPs Carboxen Fe3O4@caprylic acid NPs Fe3O4@Al2O3 NPs Fe3O4@DSILc NPs Fe3O4@decanoic acid NPs

10e20 nm e 10e20 nm 80e100 nm 14.9 nm 10e20 nm

5 min 30 min 2 min 5 min 2.5 min 60 s

1.8e3.5 mg/L 0.01e0.12 ng/g 0.02e0.03 ng/mL 0.17e0.18 ng/L 0.12e0.55 mg/L 0.91e2.43 ng/mL

89.6e97.4 74.0e107.0 93.3e106.7 98.3e103.0 62.2e130.0 88.9e105.4

47e77 (5 mL) e (5 mL) 215e270 (50 mL) 105 (100 mL) (5 mL)

[161] [162] [163] [164] [165] [166]

a b c

LOD ¼ limit of detection. PF ¼ pre-concentration factor. DSIL ¼ double salt ionic liquid.

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Table 9 List of different sorbents applied in D-m-SPE. Type of sorbent

Target analytes

Instrument

Matrix

LODa

Recovery (%)

Ref.

Chitosan Graphitic-C3N4/Fe3O4 NPs Mercapto- and polyaminophenol-bifunctionalized MCM-41 Diatomaceous earth-supported M NPs MIP-NPs PCX (a kind of cation exchange polymer material)

Flavonols PAHs Ni(II)

Ultra-HPLC-UV GC-FIDb ICP-OESc

Tea samples Saliva and blood Water samples

0.22e0.36 ng/mL 0.3e0.6 ng/mL 0.006e0.019 mg/L

95.2e102.4 94.9e98.6 97.7e98.5

[167] [168] [169]

HPLC-UV Capillary electrophoresis-UV Ultra-HPLC-HRMSd

85.7e105.5 70.5e83.7 78.1e107.1

[170] [171] [172]

20 mg/L

70.9e108.4

[173]

Zein NPs

Chlorophenols

GC-ECDe

0.08e0.6 ng/mL

91.0e109.0

[174]

Carbon coated TiO2 nanotubes

HPLC-UV

34.1e110.0 mg/L

96.1e119.0

[175]

MIL-101(Cr) Multiwall carbon nanotubes

Naproxen and ketoprofen Herbicides Cd (II) and Pb (II)

Water samples Liver Milk and milk powder Orange juice and mussels Water and honey samples Saliva and urine

0.5e5.0 ng/mL 1.1 mg/g 0.05e0.6 mg/kg

Sulfonated nanocellulose

Phthalate esters Coenzyme Q10 Melamine and cyromazine Ag NPs

Soybeans Water samples

1.56e2.00 mg/kg 0.001e0.03 mg/L

91.1e106.7 85.0e103.0

[176] [177]

Fe3O4 NPs MIL-101(Cr)@graphene oxide TiO2 NPs

ortho-phosphate ions Sulfonamides Hg (II) and CH3Hg (I)

Water samples Milk Water samples

<0.01 mmol/L 0.012e0.145 mg/L 0.004 ng/mL

91.5e104.8 79.83e103.8 97e118

[178] [179] [180]

Cu@SnS/SnO2

Atorvastatin

Plasma and urine

0.0608 mg/L

98.7e105.2

[181]

a b c d e f

Capillary electrophoresis

HPLC-DADf Atomic absorption spectrometry UVevis spectrophotometer Ultra-HPLC-MS/MS Cold vapor atomic absorption spectrometry HPLC-UV

LOD ¼ limit of detection. FID ¼ flame ionization detector. ICP-OES ¼ inductively coupled plasma-optical emission spectrometry. HRMS ¼ high resolution mass spectrometry. ECD ¼ electron capture detector. DAD ¼ diode array detector.

analytes with D-m-SPE, the nature and properties of the solid sorbent are a key factor. Therefore, to achieve the industrial success of D-m-SPE, it is crucial that researchers present new strategies for economic and large-scale production of efficient sorbents. In fact, more efforts are needed to the production of eco-friendly and green sorbents.

Acknowledgment The authors would like to acknowledge Ilam University (32/403) for funding this work.

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