Ultrasound-assisted extraction of organic contaminants

Trends in Analytical Chemistry 118 (2019) 739e750

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Trends in Analytical Chemistry journal homepage: www.elsevier.com/locate/trac

Ultrasound-assisted extraction of organic contaminants
L. Tadeo*, Rosa A. Pe
rez Beatriz Albero, Jose
n y Tecnología Agraria y Alimentaria (INIA), Ctra de la Corun ~ a Km 7, Departamento de Medio Ambiente y Agronomía, Instituto Nacional de Investigacio 28040, Madrid, Spain

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 15 July 2019

Ultrasound-assisted extraction is an efficient environmental friendly technique, since the solvent volume required in sample preparation is reduced and the extraction time is shortened in comparison to classical extraction procedures. Ultrasound radiation has been primarily applied to the extraction of analytes from solid samples. However, an important increase in the application of ultrasounds in the extraction of liquid samples has been observed lately, coupled to new microextraction techniques. Several improvements have been applied to these techniques, such as the use of ionic liquids or surfactants, as green solvents. This review discusses the application of different ultrasound-assisted techniques in the analysis of frequently occurring organic contaminants in food and environmental samples, mainly focused on the works published in the last five years. © 2019 Elsevier B.V. All rights reserved.

Keywords: Ultrasound assisted extraction Organic contaminants Microextraction techniques Ionic liquids Green solvents Magnetic nanoparticles

1. Introduction Sample preparation has a key role in the analysis of organic pollutants in environmental and food samples, because these compounds are usually present at very low concentrations and their analytical determination is highly influenced by matrix components. Thus, major efforts, particularly at the sample preparation stage, have been undertaken to develop robust analytical methods for a fast and sensitive detection of the target analytes. Classical extraction procedures, such as liquid-liquid extraction or Soxhlet extraction, are time consuming techniques. They require large amounts of glassware and organic solvents that are often toxic and pose an environmental hazard. As an alternative to these classical methods, ultrasound-assisted extraction (UAE) has found its niche as an environmentally-friendly procedure for sample preparation and extraction of analytes from different matrices. The cavitation process produced by the ultrasound radiation reduces considerably the extraction time required in traditional extraction procedures. When the ultrasound radiation is transmitted through a medium, it generates a disturbance that if repeated periodically originates expansion and compression cycles in the molecules of the medium with the formation and collapse of bubbles [1]. Bubble implosion generates changes in the temperature and pressure that enhance the penetration of the solvent into the matrix. As a

* Corresponding author. E-mail address: [email protected] (J.L. Tadeo). https://doi.org/10.1016/j.trac.2019.07.007 0165-9936/© 2019 Elsevier B.V. All rights reserved.

consequence, the mass transfer of the analytes into the solvent is increased [2]. The efficient sample/extractant contact provided by sonication generally results in a good recovery of the analyte. Nevertheless, although UAE is a technique with many advantages over conventional techniques, the experimental conditions should be controlled to avoid analytes degradation that may occur during the extraction procedure [3]. Ultrasound radiation is provided by an ultrasonic water bath or by other devices, such as probes, sonoreactors or microplate horns. The most available and cheapest source of ultrasound irradiation is the ultrasonic bath which allows to carry out easily the simultaneous extraction of several samples. On the contrary, a single sample is usually processed at a time with a probe, although there are multielement probes for the processing of multiple samples at the same time [4]. Several papers describing the principles of UAE have been published in the last years [1e5]. Since the late 1990s, different miniaturized extraction techniques have been developed for the fast and sensitive detection of organic contaminants, particularly in liquid matrices. Dispersive solid-phase extraction (dSPE), dispersive liquid-liquid microextraction (DLLME) or emulsification microextraction (EME) are some examples of these techniques. The microextraction technique to be used and the conditions of application would depend on the matrix and the analytes. These techniques minimize or eliminate the consumption of solvents and the generation of waste, being therefore more safe and environmental friendly (green techniques). The use of green solvents, such as ionic liquids or surfactants, and the assistance of ultrasound radiation are some of the improvements that have been applied to these new

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Abbreviations C18 silica modified with octadecyl groups [C8MIM][PF6] 1-octyl-3-methylimidazolium hexafluorophosphate CPE mixed micelle cloud point extraction DES deep eutectic solvents DLLME dispersive liquid-liquid microextraction dSPE dispersive solid phase extraction Fe3O4 magnetite Fe3O4@G Fe3O4-grafted graphene FUSLE focused ultrasonic solid-liquid extraction HF hollow fibre ILs ionic liquids IL-USA-DLLME ionic liquid based ultrasound-assisted dispersive liquid-liquid microextraction LCeMS/MS liquid chromatographyetandem mass spectrometry

microextraction techniques. Nowadays, UAE is a widely used and consolidated extraction technique for the analysis of solid samples. However, an important increase in the application of ultrasounds in the extraction of liquid samples has been observed lately. Thus, in the last five years, near 60% of the published methods regarding the extraction of organic contaminants assisted by ultrasounds were for liquid samples; whereas about 40% were for solid samples (see Fig. 1). Several review articles have been published on the application of UAE to the determination of organic contaminants in food and environmental samples [5,6], on its use in microextraction techniques [3,7] or as a green extraction technology [2,8]. UAE is a

MAE MIPs MNPs MSPD mSPE SBSE SFO SPE SPME SUPRASs THF UAE UAEME UASEME UEMAE USAEME

microwave assisted extraction molecularly imprinted polymers magnetic nanoparticles matrix solid-phase dispersion magnetic solid phase extraction stir bar sorptive extraction solidification of a floating organic droplet solid-phase extraction solid-phase microextraction supramolecular solvents tetrahydrofuran ultrasound-assisted extraction ultrasound assisted emulsification microextraction ultrasound-assisted surfactant-enhanced emulsification microextraction ultrasonic-enhanced microwave-assisted extraction ultrasound-assisted emulsion microextraction

versatile technique that can be applied to different matrices, diverse target molecules and various extraction methods. This review discusses the applications of UAE in the last five years, alone or in combination with microextraction techniques, to extract frequently occurring organic contaminants from food and environmental samples. 2. Ultrasound assisted extraction for solid samples The use of ultrasounds in the extraction of organic compounds from solid samples has been extensively reported in recent years, mainly because it is an economical and efficient alternative to

Fig. 1. Overview of ultrasound-assisted extraction techniques for the analysis of contaminants in solid and liquid samples published in the last five years.

B. Albero et al. / Trends in Analytical Chemistry 118 (2019) 739e750

classical extraction techniques. These techniques usually require a high solvent consumption, are time consuming and prone to cause the degradation of thermolabile compounds. Modern extraction technologies have been developed to overcome or minimize these drawbacks. In this way, pressurized liquid extraction, supercritical fluid extraction and microwave assisted extraction (MAE) are carried out applying high temperature and/or pressure to enhance the extraction efficiency, although they use a high cost equipment. An advantage of UAE over other modern extraction techniques is the lower cost of the apparatus, an easy operation and the possibility to be used with any solvent. Fig. 2 shows a schematic view of different approaches for the application of UAE to solid samples: extraction performed in tubes, using a probe or a bath system, followed by centrifugation to collect the extract; or extraction carried out in columns followed by vacuum filtration. In these approaches a cleanup of the extract is usually necessary. 2.1. Ultrasound assisted extraction techniques The application of UAE for the determination of organic contaminants in solid samples (i.e. soil, sediment, sludge, manure, and food of vegetable or animal origin) has increased in the last five years. These studies were mainly focused on the analysis of emerging contaminants or persistent organic pollutants. When UAE was applied to solid samples, with or without a cleanup step, the methods were usually applied to the multiresidue determination of contaminants (from 4 to 148 compounds). Table 1 summarizes recently published methods based on the assistance of ultrasounds for the determination of more than ten organic contaminants in solid samples. The optimization of different UAE parameters, such as the type of solvent and irradiation conditions (temperature and amplitude

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of sonication), is necessary to maximize the extraction yields. Other variables that influence extraction efficiency are: sonication time, sample particle size, solid/liquid ratio and the ultrasound device employed. It has been observed that the application of several cycles can, in some cases, improve the efficiency of extraction. Temperature affects the extraction efficacy because it influences the solubility of analytes in the medium and the cavitation phenomenon, affecting the mass transfer. In addition, the control of temperature improves the reproducibility. An increase in temperature enhances the permeation of solvent into the matrix by reducing solvent viscosity, but the co-elution of interference compounds is also enhanced and target analytes may be degraded. Although UAE processes using ultrasonic water baths are usually carried out at room temperature, sometimes higher extraction temperatures are applied. Thus, Gago-Ferrero et al. [11] described the development and validation of an UAE method for the simultaneous determination of 148 pharmaceuticals, belonging to different classes, in sewage sludge using LC-MS/MS. In the optimization process, the extraction solvent mixture and temperature were the most important conditions established. The extraction was performed at 40, 50 and 60
C and a slight increase in the recovery of some compounds was observed when the extraction was carried out at 50
C. However, at 60
C a significant decrease in the extraction efficiency was observed as a result of the coextraction of interferences. They concluded that a combination of methanol and acidified water, containing ethylenediaminetetraacetic acid, and 50
C were the optimum conditions to perform the extraction. In addition, a cleanup step was not required, which substantially reduced sample preparation time and solvent consumption. In the method described by Pinto et al. [13] for screening of a broad number of priority pesticides in different types of marine sediments, an increase of water bath temperature to 80
C had a

Fig. 2. Diagram of UAE for solid samples using an ultrasonic probe (A) or an ultrasonic bath (B).

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Table 1 Some selected applications of UAE for the multiresidue analysis of organic contaminants in solid samples. Sample

Soil Sewage sludge Sewage sludge

Analyte (number of compounds)

Extractant

Clean-up method

Sonication

Ref

Type (Temp)

Time (min)

EDCs (13) Emerging pollutants (13) Pharmaceuticals and illicit drugs (148) Emerging pollutants (15) Pesticides (27) 29 POPs

MeOH (15 mL) MeOH:water (50:50, v/v) þ0.5% FA (2 mL) MeOH:water (0.5% FAþ0.1% EDTA), (2 mL)

none none none

Bath Bath Bath (50
C)

25 15 15

[9] [10] [11]

Acetone:ACN (70:30, v/v) (2  6 mL) ACN (10 mL) isopropanol (9 mL)þ ethyl ether (1 mL)

dSPE (Florisil) SBSE (PDMS) SPE (florisil)

Bath (30
C) Bath (80
C) Probe

10 30 2

[12] [13] [14]

Perfluoroalkyl and polyfluoroalkyl substances (10) PCBs (19) Fluoroquinolones (10) Perfluorinated compounds (13)

EtOH (24 mL)

LLE (2  1 mL MeOH)

Probe

1/6

[15]

Hexane (150 mL) MeOH/acetic acid (95:5, v/v) (7 mL) ACN:water (9:1 v/v) (2  7 mL)

SPE pipette tip (acidic silica) SPE (Oasis HLB) SPE (Waters Oasis-WAX)

Probe Probe Probe

2/3 0.5 5

[16] [17] [18]

EDCs (11)

Hexane:acetone (30:70,v:v) (10 mL)

Probe

5

[19]

Hexane: DCM (1:1, v/v) (3  15 mL) water/MeOH/acetone (1:2:1, v/v) (15 mL) ACN/water (90:10, v/v) (10 mL)

Bath Bath Bath

15 60 10

[20] [21] [22]

Indoor house dust Lettuce

POPs þ EOCs (86) PFCs þ EDCs (33) Veterinary anti-microbial agents (120) Flame retardants (18) Drugs (13)

dSPE (carrot: Envi-Carb C18; soil: Envi-Carb) pipette column (florisil) SPE (Carbograph-4) SPE (Oasis HLB) SPE (Florisil) On-line SPE (Oasis HLB)

Bath Bath

15 20

[23] [24]

Dried fish

OPFRs (18)

hexane/acetone (1:1, v/v) (3  1.5 mL) ACN-MeOH (1:1, v/v) (2  16 mL) Hexane:acetone (1:1 v/v) (2  15 mL)

Bath

15

[25]

Leafy and root vegetables (6)

Emerging pollutants (35)

ACN (3  3 mL)

SPE (tandem basic alumina with C18) dSPE (C18)

Bath

10

[26]

Tree leaves Marine sediments Liver tissue of giant toad Packaging and popcorn Biological tissues Fish tissues Fish, vegetables, amended soil Carrot, lettuce, amended soil Suspended sediments Sediments Tissues, milk, and eggs

ACN, acetonitrile; C18, silica modified with octadecyl groups; DCM, dichloromethane; EDCs, endocrine-disrupting compounds; EDTA, ethylenediaminetetraacetic acid; EOCs, emerging organic contaminants; EtOH, ethanol; FA, Formic acid; LLE, liquid-liquid extraction; MeOH, methanol; PDMS, polydimethylsiloxane; PFCs, Perfluorinated compounds; OPFRs, organophosphorus flame retardants; SBSE: stir bar sorptive extraction.

significant effect on the extraction of pesticides with high octanolwater partitioning coefficients. They evaluated several parameters such as the frequency, power intensity and signal modulation of the ultrasound radiation, as well as the water bath temperature. It was observed that, to reach maximum extraction, a compromise between the intensity and the frequency must be met. The combination of ultrasounds irradiation with high temperatures was evaluated. Thus, it was found that extractions at low frequencies and low intensities with temperatures near the boiling point of acetonitrile (82
C) showed a positive effect on the extractability of the evaluated pesticides, especially for those that tend to bind tightly to soil and sediments. Furthermore, the application of ultrasounds at high temperatures reduced the number of extraction cycles. When UAE is carried out with probes instead of water baths it is usually known as focused ultrasound extraction (FUSE) or focused ultrasonic solid-liquid extraction (FUSLE). The power of the FUSE technique is 100 times higher than the conventional UAE in a water bath [19] and as a result the extraction is significantly shorten with sonication cycles that range from 10 s to 5 min, whereas with the water bath this step is longer, from 10 to 60 min. A probe systems is comprised of 3 major components: a generator, a converter and a horn (also known as a probe). This allows to amplify and concentrate the energy of the ultrasound in comparison with bath systems. In sample extraction, the solid sample and the extractant are placed in the container, and the probe is then immersed for direct sonication. Cavitation intensity within the sample is increased by increasing the amplitude (intensity) setting. The diameter of the probe's tip dictates the liquid volume that can be effectively processed. Smaller tip diameters deliver high intensity sonication and the energy is focused within a small, concentrated area. On the contrary, larger tip diameters can process larger volumes, but offer lower intensity. Boosters and high

gain horns can be used to increase the output of large diameter probes. The use of ultrasonic probes may cause cross-contamination and probe erosion, resulting in a loss of efficiency [4]. In addition, volatile compounds may be lost when UAE is carried out with a probe as a result of the enhanced degassing effect and the heating of the medium. In the bath system these problems are eliminated or diminished because there is an indirect sonication, the need for a probe in contact with the sample is eliminated. On the contrary, the higher ultrasonic energy transmitted from the horn could be sometimes needed to obtain good recoveries. The assistance of ultrasound radiation to solid samples has been also applied in combination with other extraction techniques, such as matrix solid-phase dispersion (MSPD), to improve the extraction efficiency of analytes from the matrix [27]. Recently, Lu et al. [28] developed a method using ultrasonic-enhanced microwave-assisted extraction (UEMAE) for the determination of fourteen fluoroquinolone antibiotics in cattle manure-based biogas residue. In this method, the UEMAE was performed with a mixed solution of sodium dihydrogen phosphate and disodium ethylenediamine tetraacetic acid avoiding the use of organic solvents. Different extraction strategies have been applied to reduce the consumption of organic solvents and shorten the extraction time. As an example, ultrasound-assisted surfactant-enhanced emulsification microextraction (UASEME) is a method where the extraction is performed under the synchronized actions of ultrasound irradiation and surfactants. Surfactants are amphiphilic organic compounds that are soluble in organic solvents and water. They serve as emulsifiers to improve the dispersion of a water-immiscible phase in the aqueous phase. This may favors the mass transfer of analytes from the aqueous to the organic phase, accelerating the development of small solvent droplets with ultrasound irradiation and reducing the extraction time of the process in this way [29].

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2.2. Combination with cleanup techniques After the extraction, an effective cleanup step is usually performed, especially when analyzing complex matrices (e.g. sediments or animal tissues). This is often necessary in order to have a good sensitivity and accuracy in the analytical determination of the target compounds (see Table 1). When numerous compounds with different physicochemical characteristics have to be determined, the cleanup of the extract is probably the most challenging step of the analysis. The procedure most often used is solid-phase extraction (SPE) with a wide variety of sorbents in bulk or pre-packed in cartridges, columns and pipette tips. In the reported methods, the cleanup was generally performed by SPE using Oasis HLB cartridges [17,22,24], although other sorbents such as Florisil or graphitized carbon black have also been employed [14,21,23,30]. SPE is usually performed manually but there are automated workstations that allow the simultaneous processing of multiple samples. Recently, Chen et al. [22] reported a screening method for the multiresidue analysis of 120 antibiotics in food from animal origin (milk, eggs, muscles and liver) based on UAE followed by an automated SPE cleanup with Oasis HLB cartridges. On-line SPE is another powerful technique to improve sample throughput while minimizing sample manipulation. This approach was selected by Lu et al. [28] for cleanup of the extracts obtained by UEMAE. Two SPE columns in parallel were employed, so while a sample was being eluted from the extraction column, the next sample could be loaded onto the other column. As a result, lower solvent consumption and a shorter sample preparation time were achieved. Conventional SPE was the starting point of an alternative methodology, dispersive solid phase extraction (dSPE). This technique is based on the addition of a bulk amount of SPE sorbent into the extract before the mixture is shaken and centrifuged. When magnetic particles are used as sorbent, the separation is done using a magnet and the method is named magnetic SPE (mSPE). The most attractive advantage of dSPE is the reduction of the amount of sorbent and the sample treatment time. Other advantages of this technique are its simplicity, adaptability and easy handling, in comparison to SPE. The application of dSPE for the multiresidue analysis of emerging contaminants [19,26] and pesticides [31] have been reported. Thus, Mijangos et al. [19] developed an analytical method based on FUSLE followed by dSPE cleanup and liquid chromatographyetandem mass spectrometry (LCeMS/MS) for the analysis of 11 endocrine disrupting compounds, including alkylphenols, bisphenol A, triclosan and several hormones in vegetables (lettuce and carrot) and amended soil samples. After FUSLE employing a mixture of hexane:acetone (30:70,v:v), a cleanup based on dSPE was carried out. Different dSPE cleanup sorbents, such as Florisil, Envi-Carb, primary-secondary amine bonded silica (PSA) and silica modified with octadecyl groups (C18) were evaluated. In a first approach, Envi-Carb was chosen because of its capacity to remove pigments and sterols and then different combinations of Envi-Carb with other sorbents were evaluated. Envi-CarbeC18 and Envi-Carb dSPE approaches were finally chosen as optimum cleanup procedures in terms of efficiency, cleanliness of the extracts and low matrix effect for the analysis of endocrine disrupting compounds in carrot and amended soil extracts, respectively. Modern liquid-phase microextraction techniques, developed for the analysis of organic compounds in liquid samples, such as DLLME, have been successfully applied in the purification of the extracts obtained after UAE extraction of solid samples. Environmental samples, such as soil [29,32,33], sediments [34,35] or sludge [36] were mainly studied. Cacho et al. [33] carried out a DLLME of the methanol extract obtained by UAE for the determination of nitrophenols in soil samples. The methanolic extract was used as

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dispersant solvent adding dichloromethane and acetic anhydride as extraction solvent and derivatization reagent, respectively. This mixture was rapidly injected into an aqueous solution of K2HPO4 to form a cloudy solution that was then centrifuged before collecting the dichloromethane phase containing the derivatized nitrophenols suitable for gas chromatographic analysis. Although less frequently, other new extraction techniques have been also applied in the cleanup of extracts from solid samples, such as microsolid-phase extraction [37], mSPE [38] and solidphase microextraction (SPME) [39,40]. 3. Ultrasound assisted extraction for liquid samples In the last five years, a significant increase in the use of ultrasound energy in the extraction of organic compounds from liquid samples has been observed, although its application was initially limited to solid samples. Fig. 1 shows that, during this period, the number of published methods applying ultrasounds for the extraction of organic contaminants was higher for liquid samples (59%) than for solid samples (41%). In general, miniaturized extraction techniques assisted by ultrasounds with a reduced solvent and sorbent consumption have been developed in order to achieve more economical, quick or environment friendly analytical methods. Fig. 1 summarizes the main ultrasound assisted extraction techniques used in liquid sample extraction, including their use in the cleanup of solid samples extracts, found in published articles cited in the Web of Science (WOS) from 2013 to 2018. DLLME and emulsion microextraction assisted by ultrasound energy are the two techniques more commonly reported in the last five years. In the reported methods, new solvents (surfactants and ionic liquids (ILs)), novel sorbents (molecularly imprinted polymers (MIPs) or magnetic nanoparticles (MNPs)) and innovative materials (hollow fiber, HF) have been applied in the extraction process [5,6]. The main extraction techniques used for the determination of organic contaminants in liquid samples are discussed below, including some representative applications. 3.1. Ultrasound assisted dispersive liquid-liquid microextraction (UA-DLLME) DLLME has become a widely used environmental friendly sample preparation procedure for the extraction of organic compounds. This could be due to its simplicity, speed, low cost and high enrichment factors with low sample volumes. In this technique, the extraction solvent is mixed with a dispersive solvent before their rapid injection into the aqueous sample. The cloudy solution formed with microdroplets of the extraction solvent dispersed in the aqueous sample allows the partitioning of the target analytes from the aqueous phase to the extraction solvent. Finally, the separation of phases is obtained after a centrifugation step, and the extraction solvent containing the target compounds is collected with a microsyringe for analysis. The application of ultrasound energy to this technique (UA-DLLME) has gained acceptance in recent years, because it allows an increased rate of mass transfer of analytes from the aqueous phase to the extraction solvent. Recoveries of the target compounds using UA-DLLME depend on several factors, such as the choice of the extraction and dispersion solvents, the volume of solvents, the amount of sample, the ionic strength or the pH. The design of green, efficient and sustainable extraction methods has been a hot research topic over the last years. A general scheme of the different methods based on UADLLME is represented in Fig. 3. Ultrasonic radiation is an efficient tool to facilitate the emulsification, increasing the speed of the mass transfer process between the two immiscible phases, so a dispersion solvent is not required.

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In addition, the absence of the dispersant prevents a decrease in the partition coefficient of the analytes in the extraction solvent. Other modifications to the conventional DLLME are focused in the use of less toxic solvents in the procedure. The aim is to replace toxic halogenated organic solvents (e.g. chloroform, carbon tetrachloride, chlorobenzene and dichloromethane) used as extraction solvents, as well as other organic solvents (such as methanol,

A

acetone and acetonitrile) employed as dispersants. Recent trends in DLLME are the use of low density organic solvents, ionic liquids (ILs) and surfactants in search of greener extraction methods. Thus, the use of solvents less dense than water and the solidification of a floating organic droplet (SFO) in DLLME allows the use of non-toxic solvents and facilitates an easy collection after extraction. However, the need of centrifugation for phase separation is a drawback of this

Sedimented ES

UA-DLLME

Floated ES

centrifugation

ultrasonication

dispersion

cooling

mSPE Magnetic sorbent

magnetic separation

extractant retrieval

B

Magnetic sorbent

extraction

desorption

UA-mSPE

magnetic separation

desorption

Fig. 3. Diagrams of UAE for liquid samples: A. dispersive liquid-liquid microextraction procedures; B. magnetic solid-phase extraction.

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technique. Recently, Arghavani-Beydokhti et al. [41] proposed the addition of a salting-out reagent to solve this drawback. These authors coupled an ultrasound-assisted dispersive micro solidphase extraction (UA-DmSPE) with a salting-out UA-DLLME-SFO for the detection of three non-steroidal anti-inflammatory drugs (diclofenac, ibuprofen, and mefenamic acid) in human plasma, urine and waste water samples. A layered double hydroxide-carbon nanotube nanohybrid (8 mg) was used in the UA-DmSPE and, in the next step, the solution obtained was diluted with 5 mL of deionized water, the pH was adjusted to 3.0 and 1-undecanol (60 mL) was added to carry out the UA-DLLME. Then, the mixture was passed through a small column filled with NaCl (5 g) and, due to the salting-out effect, fine droplets of 1-undecanol were formed. Since the solvent has a lower density than water, the droplets went up to surface of the solution. In both steps, the application of ultrasounds accelerated the mass transfer and decreased the extraction time. Ionic liquids (ILs) are replacing chlorinated toxic solvents commonly used in DLLME and USAEME, due to their unique physicochemical properties, such as variable viscosity, high thermal stability and negligible vapor pressure [42]. In addition, the combination of organic cations with different anions provides ILs with hydrophobic or hydrophilic properties. In ILs-DLLME, 1-alkyl-3methylimidazolium hexafluorophosphate ([CnMIM]PF6) is the IL most commonly employed, with 2e8 carbons in the alkyl chain, being hexyl and octyl the most usual alkyl groups. Ultrasoundassisted-ILs-DLLME (UA-ILs-DLLME) has been applied in order to obtain a more efficient dispersion of the extraction solvent with
zquez short sonication times (usually from 2 to 8 min). Parrilla-Va et al. [43] developed an UA-ILs-DLLME for the extraction of nine pharmaceuticals in wastewater using the IL 1-octyl-3methylimidazolium hexafluorophosphate ([C8MIM][PF6]) and acetonitrile, as extraction and dispersion solvent, respectively. Different factors affecting the extraction efficiency, such as the type and volume of ionic liquid, type and volume of dispersion solvent, cooling in ice-water, sonication time, centrifugation time, sample pH and ionic strength, were optimized. Supramolecular solvents (SUPRASs) are nanostructured solvents generated from amphiphiles through a sequential, self-assembly process. This process gives, at first, three-dimensional aggregates, mainly reverse micelles or vesicles that coacervate. In a second stage, it produces water-immiscible liquids made up of large supramolecular aggregates dispersed in a continuous phase, generally water. Reverse micelle aggregates (size 3e500 nm) of alkanols or carboxylated acids, in tetrahydrofuran (THF) dispersed in an aqueous solution, make up a SUPRAS that can be used as an extraction solvent in the DLLME process. The extraction is favored by the different interactions of the SUPRASs (e.g., hydrogen bonding and hydrophobic interactions) with the analytes in the aqueous phase. The process at which non-ionic surfactants start to undergo phase separation is known as cloud point extraction. An ultrasound-assisted supramolecular-solvent-based microextraction technique combined with HPLC-UV was developed by Karimiyan et al. [44] for the determination of five chlorophenols in environmental water samples. In this method, different parameters were evaluated and optimized in the generation of a SUPRA with decanoic acid (DeA) and THF. Thus, the amount of DeA and THF, pH, sonication time and ionic strength were evaluated. THF (1.5 mL) containing 60 mg DeA was rapidly injected using a 2.5 mL syringe into a tube containing 7 mL of sample, without salt addition. Then, a cloudy solution made of DeA reverse micelles was formed and the mixture was sonicated to favor analyte partition. The solution was then centrifuged at 3000 rpm for 10 min to accelerate the complete separation of the two immiscible phases. For the analysis of high polar compounds by GC, a derivatization step is necessary to increase their volatility. In these cases, an

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advantage of DLLME is the potential ability to perform a simultaneous extraction and derivatization of analytes. As an example, a fast UA-DLLME method, integrating the derivatization in the extraction step, was developed by Carro et al. [45] for the determination of the chloropropanols in water and beverages by GC-MS. The main parameters of this method (type and volume of derivatization agent, dispersant solvent and extractant volumes, ultrasonic bath temperature and salt addition) were investigated using experimental design to obtain optimal conditions of the UA-DLLME. The extraction-derivatization and preconcentration were simultaneously performed by adding acetonitrile as dispersion solvent, Nheptafluorobutyrylimizadole as derivatization agent and chloroform as extraction solvent. UA-DLLME with simultaneous silylation has been reported for the determination of five salicylate and benzophenone-type UV filters in aqueous samples [46]. In this work, a central composite design was chosen to evaluate the influence of different parameters in the UA-DLLME procedure. The optimal silylation and extraction conditions involved the rapid injection of a mixture of acetone as dispersant (750 mL), tetrachloroethylene as extractant (15 mL) and BSTFA as the derivatizing agent (20 mL) into the sample (10-mL, pH 7.0) containing NaCl (0.5 g). Then, the mixture was sonicated and centrifuged (for 2 min and 10 min at 5000 rpm, respectively). Finally, 5 mL of the organic phase was injected into the GCeMS and LOQs <6 ng/L were obtained. The use of ultrasounds accelerated the DLLME process and increased the rate of the silylation reaction. A method for the determination of fourteen emerging contaminants in water and different herbal infusions was developed carrying out the derivatization before the UA-DLLME process [47]. The derivatization of the target analytes was achieved using ethyl chloroformate, pyridine and ethanol that also acted as dispersant in the UA-DLLME with 100 mL of chloroform. The limits of detection of the target compounds ranged from 0.01e0.03 ng/mL in water and from 0.06 to 0.15 ng/mL in different herbal infusions. In some works, DLLME is assisted by ultrasounds combined with other techniques, such as vortex agitation, to improve the dispersion process, and it is named ultrasound vortex assisted dispersive liquid-liquid microextraction (USVADLLME). Thus, Cinelli et al. [48] reported a method based on USVADLLME for the sensitive determination of seven organophosphorus and triazine pesticides in wines. In this method, no dispersion solvent was added because the matrix has between 10 and 15% alcohol by volume. This alcohol content plays a co-surfactant effect, and the ultrasound bath provides sufficient energy to obtain a finely dispersed phase. The vortex plays an essential role in the initial dispersion of the extraction solvent. In order to achieve good extractions, five solvents with different density, polarity and water solubility were evaluated as extraction solvents, and finally 1,2-dichloroethane was selected. The final extraction method allows enrichment factors of the analytes from 210 to 232 fold, with recoveries from 92.0% to 103.4%. Another example of USVADLLME has been recently developed by Montevecchi et al. [49] for the determination of three phthalate esters in distillates using GC-MS. Vortex was used to dissolve a certain amount of NaCl in the diluted sample and obtain an emulsion by the fine dispersion of the extraction solvent (without the use of dispersion solvent), followed by 10 min of sonication. Sometimes, the preconcentration factors achieved with DLLME are not high enough for trace determination of some organic contaminants, and larger sample volumes are required in order to improve the detection limits of the method. With that goal, a method for the determination of nine polycyclic aromatic hydrocarbons in large volumes of surface water was recently developed by Avino el al [50]. The main innovation of the developed DLLME

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method was the use of a large volume of water (1 L). The extraction was carried out with 300 mL of isooctane, but without a dispersion solvent. Ultrasound radiation provided sufficient energy for the emulsification that was then broken with NaCl at 10 g/L. The use of such large sample volume improved the detection limits due to the high pre-concentration factor achieved, up to 100,000 times. 3.2. Ultrasound assisted emulsification microextraction (USAEME) USAEME is one of the most common procedures used in the analysis of organic contaminants in liquid samples (Fig. 3). The main difference between USAEME and UA-DLLME is the absence of a dispersion solvent in the former, being ultrasound radiation the only means of enhancing the partition between the sample and the extraction solvent. In USAEME, an exact volume (usually  200 mL) of extraction solvent is added to the sample solution (from 1 to 12 mL, being 5 and 10 mL the most frequent volumes). Then, the mixture is exposed to ultrasonic irradiation for short periods of time, usually 5 min, although sonication cycles from 0.5 to 20 min have been reported. Afterwards, as in conventional DLLME, the emulsion is separated by centrifugation. Then, the sedimented or floating solvent, containing the target compounds, is drawn off using a syringe for its subsequent analysis. The most frequently used extraction solvents in USAEME are hazardous organic solvents, such as chloroform, carbon tetrachloride, chlorobenzene, dichloromethane, and tetrachloroethylene, although only a few microliters are used (usually  100 mL). Solvents lighter than water (such as 1-undecanol, 1-dodecanol or 1-decanol) are used when SFO approach is employed to collect the extraction solvent. ILs have been also selected as extraction solvents in USAEME to achieve a more environmentally friendly procedure. Liang et al. [51] developed an IL-USAEME method, using [C8MIM][PF6] as solvent, for the determination of four fungicides in environmental waters. Various parameters that affect the extraction efficiency, such as the kind and volume of IL, ultrasound emulsification time, extraction temperature and salt addition were investigated and optimized. The optimized conditions were 5.0 mL of sample, 1% NaCl, 40 mL of extraction solvent [C8MIM][PF6] and 15 min of sonication. New green solvents, such as deep eutectic solvents (DES), have attracted the interest of analytical chemists. DES are the result of the complexation of a quaternary ammonium salt, usually choline chloride, and a hydrogen donor solvent. Hydrogen bonds formed between the chloride anion and the functional groups of the hydrogen donor solvent give a eutectic system with a melting point much lower than either of the individual components. Khezeli et al. [52] developed an emulsification liquideliquid microextraction method using DES (also referred to as bio-ionic liquids) as a watermiscible extraction solvent for the determination of benzene, toluene, ethylbenzene and seven polycyclic aromatic hydrocarbons in water samples. The DES was synthetized by mixing choline chloride and phenol (1:2 M ratio) and stirring until a clear liquid was formed. The method was based on self-aggregation and emulsification of the DES in aqueous medium. Firstly, a homogenous solution was formed by adding 100 mL of DES to 1.5 mL of sample, but the injection of 100 mL of tetrahydrofuran as emulsifier agent lead to the aggregation of the DES and a turbid solution was obtained. Then, the turbid solution was sonicated for 20 min in order to entirely disperse the aggregated DES droplets into the aqueous phase. After extraction, the mixture was centrifuged for 10 min at 3000 rpm to carry out the separation of two clear phases. The upper phase, containing the target compounds was withdrawn with a micro-syringe and analyzed by HPLC-UV. To evaluate the effects of several factors on the extraction efficiency of analytes, a response surface methodology was used. Firstly, the selection of an adequate molar ratio (1:2, 1:3 or 1:4) of choline chloride:phenol

was evaluated. Three emulsifier solvents (THF, acetone or 1,4dioxane) were evaluated and authors observed that the type of emulsifier solvent had no significant effect on the total peak area of target analytes. THF was selected for further experiments because it provided an efficient phase separation employing a lower volume. The influence of the main parameters affecting the extraction of analytes (volume of DES, volume of THF and ultrasonication time) were studied employing simultaneously a Box-Behnken design with three central points and analysis of variance in order to evaluate the significance and fitness of the model. In comparison with imidazolium-based ILs, DES saves time and cost due to its fast synthesis at room temperature without the need of reflux condition. A modification of the USAEME procedure is the addition of a surfactant as emulsifier to enhance the dispersion of the extraction solvent into the aqueous phase accelerating the formation of droplets under ultrasound radiation. Thus the extraction process is shortened and the degradation of the analytes is avoided. This alternative approach is known as UASEME (ultrasound-assisted surfactant-enhanced emulsification microextraction). In a UASEME method developed by Vichapong and Burakham [53], 20 min of ultrasound radiation were necessary to have an efficient recovery of five carbamate pesticides in fruit juices. The recovery decreased when the sonication time increased from 20 to 30 min. This was attributed to the increase of the temperature due to sonication, which promoted the formation of micelles hindering the mass transfer. The introduction of manual shaking before UASEME has been reported as an improvement, because it enhances the extraction efficiency of the process without the need of long sonication cycles. It was applied to the extraction of three fungicides from fruit juices [54]. Manual shaking for 15 s ensured that the IL used as extraction solvent, 1-ethyl-3-methylimidazolium, was mixed with the sample before the sonication treatment (1 min) using NP-10 as surfactant. Then, the extraction time of the method (1 min) was substantially shortened with the use of surfactant and a previous manual shaking in comparison to former ultrasound procedures (5e30 min). USAEME-SFO is another alternative process that has been used for the extraction of organic contaminants from liquid samples. This technique presents several advantages, such as a large contact surface between the aqueous solution and the droplets of extraction solvent, and the use of an organic solvent with a melting point close to room temperature (in the range of 10e30
C), which facilitates that the drop can be easily collected by its solidification by cooling after the USAEME procedure. Methods based on the combination of SFO with USAEME have been reported for the simultaneous determination of 13 organochlorine pesticides in water samples [55] and for the quantitative determination of 19 antibiotics in waste water samples [56]. In the latter, better results were reported in terms of sensitivity, cost-effectiveness and detection limits, as well as a more ecological approach, in comparison with previously published methods. When biological samples were analyzed (urine or plasma), the target compounds studied were usually emerging contaminants, such as parabens [57] or some pharmaceutical compounds, for example ibuprofen and metabolites [58]. In these methods, the volume of sample used was lower (generally 1 or 2 mL) than that used for other aqueous samples (often 5 or 10 mL), and the sample was generally pretreated, either with an enzymatic de-conjugation using a solution of b-glucuronidase and sulfatase enzymes [57] or with the hydrolysis of acyl glucuronic acid conjugates with NaOH, followed by neutralization with HCl [58]. In the case of complex matrices, especially biological fluids, a subsequent cleanup step is required. Therefore, Bazregar et al. [59]

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combined two dispersive microextraction methods in tandem, USAEME and air-agitated liquid-liquid microextraction (AALLME) for the determination of 3 tricyclic anti-depressant drugs in complex samples (wastewater and human plasma). In the first step, the pH of the sample was basified. The target analytes (basic drugs) were extracted by USAEME with only 95 mL of organic solvent (1,2dichloroethane). In this step, the analytes were extracted, although the extract contained many interferences which were soluble in the organic solvent. Therefore, a second step was carried out in which 50 mL of an acidic aqueous solution (pH 1.75) was used. This allowed the ionization of the analytes and their extraction in the aqueous solution by AALLME, in less than 2 min. Then, the aqueous extract was analyzed by HPLC-UV without the need for additional treatments. The response surface methodology was used to obtain the optimal extraction conditions of the volume of the organic solvent and acceptor phase in the first and second steps, respectively, and the pH values for the donor and acceptor phases. The results obtained showed that the combination of these two dispersive microextraction techniques provided clean extracts in a few minutes. 3.3. Ultrasound assisted magnetic solid-phase extraction (UAmSPE) The use of MNPs as solid support for the extraction of organic contaminants in liquid samples has increased in recent years. Although MNPs can be synthesized with a wide range of magnetic materials (such as iron, nickel, cobalt, magnetite or maghemite), the iron oxide Fe3O4 (magnetite) is the material for MNPs most commonly used in mSPE. This is due to its stability, low toxicity, and biocompatibility, as well as its high magnetic moment and the simplicity of preparation that allows its production in large quantities. In addition, MNPs are ready to be coated with silica and modified with a wide variety of functional groups. They are attracted by a magnetic field, so they can be isolated by means of a magnet, a key point for their ease of use. In UA-mSPE, the sorbent is added directly to the solution, and after sonication, the MNPs with the captured analytes are isolated by placing a magnet on the wall of the flask. Then, the aqueous solution is discarded and the target compounds can be eluted from the sorbent using a low quantity of an adequate organic solvent. Again, MNPs are separated from the eluate using a magnet. Sonication may be applied in different stages of the mSPE procedure: to disperse the MNPs into the aqueous sample, during the extraction step, or to desorb the analytes from the sorbent. This technique is described as simple and rapid because the easy separation of the sorbent using a magnet overcomes the need for centrifugation and manual collection of the extractant. Fig. 3B shows the schematic diagram of the mSPE procedure. Nano- or microparticles of Fe3O4 with different surface modifications have been usually employed in mSPE. Thus, C18functionalized Fe3O4 mesoporous silica microspheres, graphenecarbon nanotube-Fe3O4 nanocomposite and n-octyl-modified magnetic iron oxide nanoparticles have been described as good magnetic sorbents for the extraction of organophosphorus pesticides from water [60e62]. The application of MNPs in a two-step microextraction technique, DLLME or USAEME combined with mSPE, has been described (See Fig. 3A). In these methods, the combination of liquid-phase microextraction techniques with mSPE overcame the need for time consuming steps, such as centrifugation or cooling; because the organic phase containing the analytes is the target of the magnetic retrieval step rather than the analytes directly [62e64]. Tay et al. [63] reported the use of n-octyl-triethoxysilane surfacemodified magnetite in a DLLME method assisted by mSPE for the

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determination of 4-n-nonylphenol in water. In this work, DLLME was performed by injecting 1 mL of the dispersion solvent (methanol) containing 10 mL of extraction solvent (1-octanol) into 15 mL of water sample. The MNPs (10 mg) were immediately added to the solution that was sonicated for 4 min for the retrieval of the extraction solvent and the target compound. Then, a magnet was held next to the vial to attract and isolate the MNPs which were freeze-dried, and the analyte was desorbed from the MNP by sonication with methanol. In the method developed by Soon and Tay [62] for the determination of organophosphorus pesticides in water, DLLME was coupled with mSPE using n-octyl-modified magnetite nanoparticles to enhance the efficiency. The DLLME was carried out with 15 mL of water, 10 mL of chlorobenzene (extraction solvent) and 1 mL of methanol (dispersion solvent), followed by the addition of 10 mg of dried MNPs. The vial was then sealed and sonicated for 8 min before the MNPs were isolated from the aqueous solution using a magnet. Then, the MNPs were washed with 5 mL of water and vacuum dried prior to the addition of acetonitrile (3  0.5 mL) to extract the pesticides from the MNPs by sonication for one minute. The method developed by Wang et al. [64] for the analysis of six PAHs in environmental water samples combined USAEME with magnetic retrieval. In this case, 10 mL 1-octanol was injected rapidly into a 20 mL water sample and the mixture was sonicated for 4 min to form a homogeneous cloudy solution. After that, 20 mg of highly hydrophobic magnetic particles (silver nanoparticles on the surface of Fe-polydopamine particles) were added into the solution for magnetic retrieval of 1-octanol droplets, and vortexed for another 4 min. Subsequently, the MNPs were separated rapidly from the solution by applying an external magnetic field. After discarding the supernatant solution, PAHs and 1-octanol were eluted from the particles with acetonitrile. Although magnetite is usually employed as sorbent in mSPE, other magnetic sorbents without Fe3O4 core shell have been recently reported. Thus, zero valent-reduced graphene oxide quantum dots have been described as a new and effective sorbent for extraction of organophosphorus pesticides in water and fruit juice samples [65]. The developed method presented lower extraction and desorption times, some lower LODs, relatively high enrichment factors and low sorbent consumption than previously reported methods. However, one disadvantage of the method was the lower precision of the results in comparison to most of the previous works cited. Fe3O4-grafted graphene (Fe3O4@G) possesses a large surface area and super-paramagnetism, being a suitable sorbent to aggregate ionic liquids on its surface. UA-mSPE with ILs-Fe3O4@G was reported for the simultaneous determination of five nitrobenzene compounds in environmental samples [66]. Fe3O4@G MNPs and [C7MIM][PF6] (used as IL) were added to 10 mL water and sonicated for 20 min. Then, the supernatant was discarded while the MNPs containing the analytes were gathered with a magnet and were then eluted with 0.3 mL of methanol sonicating for 15 s. In the optimization of the extraction method, several parameters that affect the extraction efficiency were evaluated, such as the types of MNP and ILs, the volume of IL and the amount of MNPs, the extraction time, the ionic strength, and the pH of the solution. It was concluded that with the assistance of ultrasound radiation, the extraction of the target compounds by the IL and the self-aggregation of the IL onto the surface of the Fe3O4@G proceeded synchronously; allowing that the extraction takes place in 20 min using 144 mL of ILs ([C7MIM][PF6]) and 3 mg of Fe3O4@G. DES, discussed in Section 3.2, are an interesting alternative to ILs to coat MNPs for their application in mSPE. Lamei et al. [67] modified magnetic graphene oxide nanoparticles with a DES that

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was prepared with choline chloride and 5,6,7,8-tetrahydro-5,5,8,8 tetramethylnaphthalen-2-ol, as hydrogen-bond donor, for the preconcentration of methadone from water and biological fluids (urine and plasma). The pH of the sample was adjusted to 10 before adding 15 mg of sorbent and diluted to 10 mL with deionized water. The MNPs were dispersed with ultrasound radiation for 2 min. The solution was discarded and methadone retained in the sorbent was eluted with 0.5 mL of ethanol for its analysis by gas chromatography. The retrieval of the extraction solvent by means of mSPE has been also applied to mixed micelle cloud point extraction (CPE). Gao et al. [68] applied the combination of both techniques in the determination of doxazosin and alfuzosin, two a1-blockers, in urine and plasma. CPE is considered an environmentally friendly technique because surfactants instead of toxic organic solvents are employed. Most CPE methods require a centrifugation to separate the sample from the surfactant phase. In addition a cooling step is usually required, so the surfactant becomes more viscous and can be separated from the sample by decantation. The retrieval with mSPE avoided this step employing Fe3O4 modified with diatomite. The analytes were eluted from MNPs with 200 mL of ethanol by 2 min sonication. 4. Conclusions and future trends The determination of organic contaminants at trace levels generally requires the use of analytical procedures with an adequate analyte isolation. This makes sample preparation to play a key role in the analysis of contaminants. The application of green sample preparation techniques that use ultrasound energy requiring minimal amounts of solvents or employing safe and nontoxic extraction media, such as ionic liquids, and supramolecular solvents, constitute nowadays an important topic in analytical chemistry, according to the numerous publications in the last years. The assistance of ultrasounds in the extraction process is very useful for developing methods with low volumes of organic solvents and short extraction times. The application of ultrasounds facilitate the formation of fine extraction droplets or the emulsification when surfactants, ILs or DES are used, providing an increase in the speed of the mass transfer process. In addition, new solvents, such as switchable solvents and natural DES or hydrotropes are promising green solvents. Moreover, the use of novel solid sorbents, such as MNPs or MIPs, can provide high enrichment factors. These methods compare favorably with classical extraction procedures because they contribute to decrease the environmental impact of analytical methods and reduce the analysis time, while yielding similar efficiency and sensibility. In addition, the comparison of ultrasound assisted extraction with other modern extraction techniques is also positive, because of its low cost and easy operation. Due to these advantages, the use of ultrasound assisted extraction in the analysis of organic contaminants will continue to grow in the next years in combination with other microextraction techniques and novel materials. The application of new nanomaterials and green solvents will be of great interest to reduce the environmental impact and extraction time in the analysis of contaminants. The availability of effective and low cost ultrasound instrumentation will continue to play an important role in the determination of organic contaminants by environmental friendly techniques. Acknowledgements Authors wish to thank the Spanish Ministry of Economy, Industry and Competitiveness for financial support, project (RTA2014-00012C03).

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