Microwave-Assisted Extraction

Microwave-Assisted Extraction V Lopez-Avila, Agilent Technologies, Santa Clara, CA, USA MD Luque de Castro, University of Cordoba, Cordoba, Spain ã 2014 Elsevier Inc. All rights reserved.

Theoretical Considerations in Microwave-Assisted Extraction Instrumentation for MAE Commercial MAE Systems Noncommercial MAE Systems Discontinuous Microwave-Assisted Extractors Continuous MAE at Ambient Pressure Dynamic MAE High Pressure Extractant-Free MAE Coupling of Microwave and Ultrasound Energies to Improve Extraction Specific Applications for MAE Polycyclic Aromatic Hydrocarbons Pesticides and Herbicides Polychlorinated Biphenyls Phenols Organometallic Compounds Additives in Polymers Flame Retardants Pharmaceutical and Personal Care Products Perfluorinated Compounds Natural Products Future Trends Acknowledgments References

Abbreviations BFRs ELISA FMASE GPC ILs LOQ MHG MIS MSD MSDf MSPD MUED

Brominated flame retardants Enzyme-linked immunosorbent assay Focused microwave-assisted Soxhlet extraction Gel permeation chromatography Ionic liquids Limit of quantitation Microwave hydrodiffusion and gravity Microwave integrated Soxhlet Microwave steam distillation Microwave steam diffusion Matrix solid-phase dispersion Microwave–ultrasonic synergistic in situ extraction–derivatization

OCPs OPPs PAHs PBDPEs PCBs PPCPs PFRs PFSAs PSFME SFME SPME TMAH VMHD

1 2 2 3 3 5 6 7 8 8 10 10 11 12 12 13 13 14 14 14 15 15 15

Organochlorine pesticides Organophosphorus pesticides Polycyclic aromatic hydrocarbons Polybrominated diphenyl ethers Polychlorinated biphenyls Pharmaceutical and personal care products Phosphor-containing flame retardants Perfluorinated sulfonates Pressurized solvent-free microwave extraction Solvent-free microwave extraction Solid-phase microextraction Tetramethyl ammonium hydroxide Vacuum microwave hydrodistillation

Theoretical Considerations in Microwave-Assisted Extraction Microwaves are high-frequency electromagnetic waves placed between radio frequency and the far infrared light regions of the electromagnetic spectrum (their frequency range from 0.3 to 300 GHz corresponding to wavelengths of 1 m to 1 cm). In contrast to conventional heating where the heat penetrates slowly from the outside to the inside of an object, in microwave-assisted extraction (MAE) the heating appears right in the core of the body that is being heated, and the heat spreads from the inside to the outside of that body. The microwave energy affects molecules by ionic conduction and dipole rotation. In ionic conduction, the ions in solution will migrate when an electromagnetic field is applied. The resistance of solution to this flow of ions will result in friction and, thus, heating of the solution. Dipole rotation means realignment of the dipoles with the applied field. At 2450 MHz, the dipoles align and randomize 4.9
109 times per second; this forced molecular movement results in molecular ‘friction’ and, thus, heating of the solution. An in-depth discussion of dielectric heating can be found in Ref. [1].

Reference Module in Chemistry, Molecular Sciences and Chemical Engineering

http://dx.doi.org/10.1016/B978-0-12-409547-2.11172-2

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Microwave-Assisted Extraction

Table 1

Solvent boiling point and closed vessel temperature

Solvent

Boiling point ( C)

Closed vessel temperature ( C) at 175 psi

Dichloromethane Acetone Methanol Ethanol Acetonitrile 2-Propanol Acetone–hexane (1:1, v/v) Acetone–cyclohexane (70:30, v/v) Acetone–petroleum ether (1:1, v/v) Dichloromethane–acetone (1:1, v/v) Toluene–methanol (10:1, v/v) Toluene–methanol (1:10, v/v)

39.8 56.2 64.7 78.3 81.6 82.4 52+ 52+ 39+

140 164 151 164 194 145 156 160 147 160 110–112 146

a a a

a

Information not available. Source: Adapted from Kingston, H. M.; Haswell, S. J. Microwave-Enhanced Chemistry; American Chemical Society: Washington, DC, 1997.

Selection of proper solvents is the key to a successful extraction. In selecting solvents, consideration should be given to the microwave-absorbing properties of the solvent, the interaction of the solvent with the matrix, and the analyte solubility in the solvent (the principle of ‘like dissolves like’ is still applicable in MAE). The larger the dipole moment of the solvent the faster the solvent will heat under microwave irradiation. For example, hexane (dipole moment is <0.1 Debye) will not heat, whereas acetone with a dipole moment of 2.69 Debye will heat in a matter of seconds. Thus, a mixture of hexane and acetone is an ideal solvent for compounds of environmental significance, and many applications described here use hexane–acetone (1:1). Other important factors under considerations include: (1) the compatibility between the extraction solvent and the analytical method used in the analysis of the extract (the less polar solvents seem to be preferred for gas chromatographic analysis, whereas the more polar ones for liquid chromatographic analysis and immunoassay techniques), and (2) the selectivity of the solvent. Little has been reported in the literature on the selectivity of MAE because the technique is so efficient that it cannot be regarded as a selective extraction technique. ‘Everything gets extracted,’ so a cleanup step after the extraction is needed in almost all cases. When MAE is conducted in closed vessels, the temperature achieved during the extraction will be greater than the boiling points of the solvents. For most of the solvents (e.g., acetone, acetone–hexane, dichloromethane–acetone), the temperature inside the vessel is two to three times the boiling point of the solvent. These elevated temperatures result in improved extraction efficiencies of the analyte from the sample matrix. The reader should refer to Table 1 for a list of solvents and their maximum closed-vessel temperatures achieved at 175 psi. More on the fundamentals of MAE can be found in Ref. [2].

Instrumentation for MAE Commercial MAE Systems The elements of a microwave extraction device consist of: a microwave generator (known as magnetron), a waveguide used to distribute the microwaves, an applicator (where the sample to be extracted is loaded) and a circulator (i.e., a fan-shaped blade).3 All microwave systems must have homogenous electric field profiles (done by either using mode stirrers or by rotating the extraction vessel itself ), must provide for the temperature and pressure control within the extraction vessels, and the vessel geometry needs to consider the penetration depth of the microwaves.3 There are two types of commercial MAE systems: one uses a closed extraction vessel, in which the temperature and the pressure are controlled, and the other is on open vessel containing the sample that is attached to a solvent extractor and the microwaves are focused on the sample. In the closed-vessel microwave category, there are (a) multimode systems that operate at high pressure and temperature and use vessels made out of fluoropolymer or fluoropolymer/ glass, (b) focused systems that operate at pressures as high as 130 atm and 320  C, and (c) dynamic closed-vessel microwave systems.3 In the open-vessel category there are open vessels for microwave-assisted on-line sample pretreatment and special openvessel microwave systems such as: (a) microwave–ultrasound combined, (b) FMASE, (c) microwave-assisted drying system, and (d) microwave-assisted distiller. Details of the various types of systems can be found in Ref. [3]. Despite the fact that the open vessel systems are cheaper and safer that the closed vessel systems, they are less precise, and the sample extraction times are longer. The equipment used for closed-vessel MAE consists of the magnetron tube, an oven where the individual extraction vessels (closed vessels) are set up on a turntable or rotor, monitoring devices for temperature and pressure, and electronic components. It usually includes specific safety features such as rupture membranes for the extraction vessels, an exhaust fan to evacuate air from the instrument cavity, a solvent-vapor detector (monitors the presence of solvent vapor in the microwave cavity and shuts off the microwave energy whenever solvent vapor is detected in the instrument cavity), an expansion container (the extraction vessels are connected to this expansion container through vent tubing; in case the membrane ruptures, due to increased pressure in the vessel, the vapor is removed through the rupture vent tube), and an isolator located in the wave guide that diverts reflected microwave energy into a dummy load to reduce the microwave energy within the cavity. One manufacturer of microwave

Microwave-Assisted Extraction

Table 2

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Features of commercially available MAE systems

Model/manufacturer Multiwave PRO/Anton Paar GmbH, Austria MARS-6/CEM, USA

Power (W) 1700 1800

Discover SPX/CEM, USAa

300

Ethos EX, Milestone, USA Titan MPS/ Perkin Elmer, USA Initiator Robot 60/Biotage, USAa

1200 1500 400

Sensors Pressure and temperature control in all vessels Infrared temperature and pressure in all vessels (fiberoptic) Infrared temperature and pressure in all vessels (fiberoptic) Pressure and temperature control in all vessels Only temperature control Pressure and temperature control in all vessels

Maximum pressure (bar)

Vessel volume (ml)

Vessel material

Number of vessels

Stirring

40

100

Teflon

16

Yes

20

75/100

Teflon/optional glass

40/24

Yes

28

10/35/80

72/36/24

Yes

30

70/75

41/24

Yes

N/A

75

Glass/quarz with optional Teflon liners Teflon/optional glass Teflon

16

No

28

5/10/20

Glass

60/30

Yes

a

These close-vessel MAE systems process samples sequentially using an autosampler that feeds and removes extraction vessels to and from the microwave cavity.

equipment uses resealable vessels. In this case, vessels are placed on a sample rotor and secured with a calibrated torque wrench for uniform pressure. If the pressure exceeds the vessel limits, a spring device (Milestone’s patented technology) allows the vessel to open and close quickly, thus releasing the excess pressure. These sample rotors are available with perfluoroalkoxy polymer (PFA™) and (tetrafluoroalkoxy)polymer (TFM™) liners with pressure ratings of 435–1450 psi. Another safety feature which was added to the microwave system is the ‘movable wall.’ To prevent the door from being blown away, a door frame on spring-loaded, highimpact steel bars was added, such that the door moves out and in to release pressure from the microwave cavity. The features of commercially available MAE systems are identified in Table 2. Although the batch style microwave systems in use today are not much different to those available prior to 1999, there is a new level of ‘intelligence’ added to the new systems,4 which includes instrument control software, libraries of predefined extraction methods, and the capability to recognize the vessel type and the number of extraction vessels inside the microwave cavity. Thus, the MAE system can apply the needed microwave power for the number of samples being extracted and per the extraction conditions specified in the method. Typical pressures reached with most closed-vessel systems (first generation) were 7 bar, but today’s technology can handle pressures as high as 40 bar. A special rotor, which houses several thick-walled vessels, is available commercially on several systems, including the Anton Paar, CEM’s MARS-6 and Milestone’s Ethos EX. If the operating pressure inside the vessel exceeds the vessel limits, a special spring device will allow the vessel to open and close, thus reducing the pressure. The vessels are typically made of microwave transparent materials such as Teflon and are lined with perfluoroalkoxy or Teflon™ liners. In the first generation of MAE systems, a control vessel was used to allow a pressure-sensing tube and a fiber optic temperature probe. The fiber optic probe was microwave transparent and was positioned in the control vessel to monitor the temperature inside the vessel providing feedback that regulated the microwave power to maintain the set temperature. This temperature control technology has been replaced with a floor-mounted temperature sensor that measure the temperature of all vessels as they rotate inside the microwave cavity.4 Additional features such as magnetic stirring of the extraction solvent inside multiple sample vessels is available to all, except the Perkin Elmer system. Moreover, nonpolar solvents, such as hexane, can now be heated at elevated temperatures by use of magnetic stir bars made of Milestone’s proprietary fluoropolymer Weflon™ (this polymer absorbs the microwave energy and subsequently transfers heat to the surrounding medium). The Anton Paar and the CEM systems use silicone carbide inserts or Carboflon, respectively.4

Noncommercial MAE Systems Microwave-assisted extractors designed by their users can be divided into discontinuous and continuous devices that work either at ambient or higher pressure.

Discontinuous Microwave-Assisted Extractors Users have adapted different types of microwave devices to assist solid–liquid extraction (better expressed as leaching), from household ovens to more or less sophisticated devices that have been assisted even by focused microwaves. The use of household ovens has involved or not the design of the sample container where the sample is located, as well as the use of open and closed vessels; therefore, from the use of a simple precipitate glass, polypropylene containers designed to support

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Microwave-Assisted Extraction

pressures of several atmospheres, to more or less complicated designs for either simultaneous assistance of different samples have been the results of users’ inventiveness.5 Widely used extractors based on the Soxhlet principle have applied either monomode or multimode microwaves. The formers or FMASE differ mainly in four aspects from other microwave-assisted devices, namely: (a) the extraction vessel is open, so it always works under normal pressure; (b) microwave irradiation is focused on the sample; (c) the extraction step is totally or partially performed as in the conventional Soxhlet technique (i.e., with permanent sample–fresh extractant contact); and (d) no subsequent filtration is required. Therefore, it retains the advantages of conventional Soxhlet extraction while overcoming its limitations as regards to throughput, automation, and the ability to quantitatively extract strongly retained analytes. Similarities and differences as compared with conventional Soxhlet are shown in Figure 1. Manual emptying of FMASE by switching the valve (shortcoming overcome in the improved prototype in Figure 2), but shorter tubing system that makes it appropriate for the use of water as extractant.6 Some authors consider these extractors as semicontinuous because the almost continuous circulation of the extractant.

Figure 1 Similarities and differences between (a) a conventional Soxhlet extractor and (b) a first prototype of focused microwave-assisted Soxhlet extractor. Modified from Kingston, H. M.; Haswell, S. J. Microwave-Enhanced Chemistry; American Chemical Society: Washington, DC, 1997, with permission.

Figure 2 Automated focused microwave-assisted Soxhlet extractor. Modified from Kingston, H. M.; Haswell, S. J. Microwave-Enhanced Chemistry; American Chemical Society: Washington, DC, 1997, with permission.

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As compared with similar commercial extractors as Soxwave-100 that uses a single heating source (focused microwaves) acting on both the sample and solvent, the FMASE uses two energy sources (microwaves for sample irradiation and electrical heating for the extractant). This latter dissimilarity has led to a number of differences in performance, namely: (i) because the heating source of the solvent is microwaves, the dielectric constant of the extractant is of paramount importance in the Soxwave-100; thus, polar extractants are more efficient here than are nonpolar and low-polar extractants, so this is not the most appropriate choice for lipid extraction, for example. By contrast, extractant distillation in FMASE is accomplished by electrical heating and is thus unaffected by the solvent polarity. (ii) Because the amount of energy required by the extractant differs from that needed to remove the target analytes from the sample, a compromise must inevitably be made in this respect in Soxwave-100 extraction. This is not the case with FMASE, where the operating conditions can be optimized independently at each temperature. (iii) Operation with the Soxwave-100 involves a preliminary step where the sample is immersed in the boiling extractant, followed by lifting of the cartridge over the solvent and continuous dropping of the condensate on the cartridge. In this step, a matrix–extractant partitioning equilibrium of extractable species is established while microwave radiation acts on both the sample and extractant. In the second step, the partitioning equilibrium is displaced to extraction completion by effect of the sample coming into contact with fresh extractant in the absence of microwave irradiation. In FMASE, clean extractant and microwave irradiation are simultaneously supplied, which facilitates mass transfer and shortens extraction times as a result. (iv) Whereas the Soxwave-100 has retained its original design, FMASE has been the subject of continuous improvements based on alterations of the initial prototype or its subsequent versions, as shown in Figure 2. Operationally, the FMASE is identical to a conventional Soxhlet apparatus, except that it affords irradiation with focused microwaves for a preset time during each extraction cycle while fresh extractant is recirculated through the solid sample. The operational variables amenable to optimization in the FMASE prototype are irradiation power, irradiation time and number of cycles. Unlike a conventional Soxhlet extractor, the microwave-assisted Soxhlet system allows up to 75–85% of the total extractant volume to be recycled. Electrical heating of the extractant, the efficiency of which is independent of its polarity, is also crucial here. Moreover, the efficiency is unaffected by the moisture content of the sample. Relevant applications of the prototypes in Figures 1(b) and 2 are described and discussed in references.7–9 The Chemat team developed a microwave-assisted extractor that was deemed similar to a Soxhlet extractor – in fact, they call it ‘microwave integrated Soxhlet’ (MIS) – but in fact differs markedly from the classic extractor in operational terms.10 Thus, as can be seen in Figure 3, the sample is never brought into contact with fresh extractant and the extract is not siphoned; also, the extractant is heated by microwaves (similarly to the Soxwave-100) and a filtration step is required as the sample is not held in a cartridge, but rather dispersed in the extractant. Other characteristics of MIS are as follows: it uses a Milestone ETHOS multimode microwave oven with a twin magnetron (2
800 W, 2.45 GHz) delivering a maximum power of 1000 W in 10 W increments. Time, temperature, pressure and power are controlled with the software ‘easy WAVE.’ The flask holding the solid material, (1) in Figure 3, is suitable for microwave radiation and contains a polytetrafluoroethylene (PTFE)/graphite stir bar capable of absorbing microwaves and diffusing the heat to the surroundings. This is essential with solvents transparent to microwave radiation. The vessel contains an inner support (3) for placing the solid material (2) to be extracted. The support is preferentially made of PTFE and placed at a preset distance from the vessel bottom. After the method has been applied, the solid material (2) placed on the said support is separated from residual solvent, which is collected at the bottom of the vessel. A condenser (7) is placed on top of the extraction tube (5), in which switching valves allow the solvent present in the base vessel to reflux upon microwave irradiation and either repeatedly percolate the sample to ensure thorough extraction or be removed from the extractor to concentrate the extract. The solid material is extracted by immersing the sample into the vessel containing the solvent under reflux and repeated percolation with the same organic solvent. The four stages of the process are preceded by placement of the sample onto a preset amount of raw material for lipid extraction and the addition of extractant (usually n-hexane), into which the sample is immersed, the condenser being placed on the extraction tube and extraction started. First, the solvent is heated up to its boiling point by microwave irradiation and stirred with the PTFE/graphite magnetic stirrer (13). The solvent vapors penetrate through the sample and condense on the condenser. Then, the condensate drips down onto the sample and extraction is done for a preset time. Second, the level of solvent is lowered below the sample (4b) by switching the 3-way valve accordingly for a given time. Third, repeated leaching with only clean, fresh solvent follows for a preset time with the valve adjusted such that the condensate is driven back into the extraction tube. Finally, the solvent level is lowered to concentrate the extract. The main difference between these laboratory-designed extractors, which are more or less similar to a conventional Soxhlet extractor, and the commercial devices from CEM or Milestone is that the former use open vessels. As a result, the maximum temperature reached in the extraction vessel in these open systems is strongly dependent on the boiling point of the extractant. Also, extraction takes more time than in closed vessels – which, however, require waiting for the vessel to cool before it can be opened. Finally, the open-vessel systems are better suited to thermolabile compounds.

Continuous MAE at Ambient Pressure Most online procedures involving microwaves and working at ambient pressure that are conducted to couple a microwave extraction with another step of the analytical process (usually detection, with or without prior cleanup) use either a household oven or a commercial focused system. Figure 4 shows representative examples of the former. That in Figure 4(a) is an open vessel device proposed by Burguera and Burguera,11 consisting of six test tubes containing the samples and located in a Pyrex jar. Each tube contained a tubing–valve system for aspirating the treated sample to a flow injection manifold that droves it to an atomic

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Microwave-Assisted Extraction

Figure 3 The so-called ‘microwave-assisted integrated Soxhlet extractor.’ For details, see text. Reproduced from Veggi, P. C.; Martinez, J.; Meirells, M. A. A. In Microwave-Assisted Extraction for Bioactive Compounds: Theory and Practice; Chemat, F., Cravotto, G., Eds.; Springer: New York, 2013.

absorption spectrometer for determination of zinc and cadmium in kidney and liver tissues. Figure 4(b) shows the coupling of single extraction of metals from a soil sample to subsequent filtration, liquid–liquid extraction of the leachate and preconcentration by liquid–solid extraction prior to location of the eluate in an autosampler vial for injection into GC–MS equipment.12 A more recent design for continuous MAE uses up to three in-series household ovens placed on top of each to impart a continuous operation, as shows Figure 4(c). The use of this device to extract isoflavones from soy flour doubled the efficiency as compared with a conventional method.13 The main advantages of online designs using household ovens are their low cost, automation, reduced delay between sample delivery and analysis, and isolation of samples from the environment. On the other hand, their main problem is the inherent inhomogeneity of microwave power distribution within the cavity: with only a small area occupied by the reaction coil, a high proportion of the microwave power within the cavity is not absorbed. Unlike household ovens, where solid samples are usually introduced as slurries, in commercial focused microwave devices the sample is placed directly in the vessel (10) or in an extraction cartridge that is in turn placed in the vessel (11). Figure 5 depicts two such systems in which the microwave device is either coupled or not to a subsequent step of the analytical process. The dynamic approach in Figure 5(a) allows several consecutive extraction cycles to be performed to ensure quantitative removal of the target compounds.14 The extract provided by each cycle is aspirated through a cellulose filter to retain solid particles. Figure 5(b) shows as the use of a pump as an interface between the extractor and a flow injection manifold allows MAE to be coupled to filtration, cleanup, preconcentration, individual chromatographic separation, and detection, and hence automation of the whole analytical process.15

Dynamic MAE High Pressure Dynamic microwave-assisted extractors appeared much later than open-vessel systems, possibly because operating under a high pressure reduces the flexibility afforded by working at atmospheric pressure. The highly complex first design required solid samples

Microwave-Assisted Extraction

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to be slurried and the assistance of a nitrogen bomb to supply N2 at a constant pressure up to 5 bar.16 A subsequent design avoided the need for the nitrogen bomb and enabled direct introduction of solid samples, through which fresh extractant was continuously pumped, and connection to a subsequent step of the analytical process.17 Recently, a commercial system based on the Discover platform has been developed by implementing a module named Voyager. This module is an automated flow system intended to scale up microwave-assisted treatments for both continuous flow and stop-flow processing. Thus, the same parameters used with the Discover system can be used to scale up from milligram amounts to approximately 1 kg with identical results. A dynamic stirring device ensures homogeneous, uniform mixing.

Extractant-Free MAE Solvent-free microwave extraction (SFME) is a combination of microwave heating and dry distillation, performed at atmospheric pressure without adding any organic solvent or water.18 Isolation and concentration of volatile compounds, usually essential oils from plants, are performed by a single stage. The approach involves placing plant material in a microwave reactor, without any added solvent or water. The internal heating of the water within the plant material distends the plant cells and leads to rupture of the glands and oleiferous receptacles. The process frees essential oils that are evaporated by the in situ water of the plant. A cooling system outside the microwave oven continuously condenses the distillate. The excess of water is refluxed to the extraction vessel and restores the in situ water to the plant material. Comparison of the results obtained by applying the approach to several aromatic plants with hydrodistillation showed that 30 min application of SFME provides essential oils that are quantitatively and qualitatively similar to those obtained by hydrodistillation for 4.5 h.19 In the case of oregano the efficiency of essential oils extraction increased by 80%, without differences in composition, as demonstrated by GC–MS analysis.20

Figure 4 (a) Assembly for the simultaneous treatment of up to six samples. (b) Online development of leaching, liquid–liquid extraction, and sorption/ cleanup with manual transportation to the GC–MS equipment. Continued

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Microwave-Assisted Extraction

Figure 4, cont’d (c) Schematic depiction of the continuous microwave system. Adapted from Luque-Garcia, J. L.; Luque de Castro, M. D. Trends Anal. Chem. 2003, 22(2), 90–98; Luque de Castro, M. D.; Priego-Capote, F. In Microwave-Assisted Extraction in Enhancing Extraction Processes in the Food Industry; Lebovka, N., Vorobiev, E., Chemat, F., Eds.; CRC Press: Boca Raton, FL, 2012; Luque-Garcı´a, J. L.; Luque de Castro, M. D. J. Chromatogr. A 2003, 998, 21–28, with permission.

Coupling of Microwave and Ultrasound Energies to Improve Extraction The first design of a microwave–ultrasound combined extractor was constructed by Lagha et al.21 in 1999 from a Prolabo Maxidigester and a cup-horn Branson Sonifier at the base of the microwave oven for indirect ultrasonic agitation of the sample under focused microwaves. A dramatic shortening of the MAE time in the presence of ultrasonic agitation (from 3 to 1 h) was observed. Domini et al.22 reported a 10 min digestion for the determination of Kjeldahl nitrogen using an ultrasonic probe that enters the microwave cavity via a nonmetallic horn. The combination of electromagnetic energy (2.45 GHz) and mechanical energy (20 kHz) to carry out sample digestion, dissolution and extraction appears very promising.23

Specific Applications for MAE Several review articles have been published in the last 5 years that contain detailed information on sample pretreatment and MAE conditions (i.e., solvent type, solvent volume, operating temperature, and power) for specific compound classes that can provide the reader sufficient information for a particular application involving MAE. For example, Sanchez-Prado et al.24 reviewed 116 articles on flame retardants, surfactants (i.e., linear alkylbenzene sulfonates and alkylphenol ethoxylates), pharmaceutical and personal care products or PPCPs (i.e., synthetic musk fragrances, triclosan, antibiotics, and anti-inflammatory drugs), and estrogens, and report that compared to other extraction techniques, MAE is easier to optimize because it has fewer variables (i.e., matrix moisture content, solvent, time, microwave power, and temperature) and what makes it so attractive is the short duration of the extraction process with the minimum solvent consumption. Zuloaga et al.25 reviewed the extraction of PPCPs from different type of sludges (i.e., sewage, raw, digestive and compost, primary and secondary, etc.) and reports detailed extraction conditions for MAE. The majority of applications reported in Ref. [25] deal with pressurized MAE and few use focused MAE. A review of 138 articles on the use of MAE in biological sample preparation for small molecule analysis, done by Teo et al.,26 provides detailed MAE conditions for extraction of foods (i.e., cucumber, beets, seaweed, pet food, smoked meat, olive oil, cereal, etc.) and plants.

Microwave-Assisted Extraction

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Figure 5 (a) Dynamic focused microwave-assisted extractor. (b) Experimental setup used to integrate microwave-assisted extraction with the subsequent steps of the analytical process. Adapted from Garcı´a-Ayuso, L. E.; Sa´nchez, M.; Ferna´ndez de Alba, A.; Luque de Castro, M. D. Anal. Chem. 1998, 70, 2426–2432; Morales-Mun˜oz, S.; Luque-Garcı´a, J. L.; Luque de Castro, M. D. J. Chromatogr. A 2004, 1026, 41–48.

Tadeo et al.27 and Chung and Chen28 reviewed sample preparation for determination of pesticides in soil and fatty foods, respectively. The 102 references in the review by Tadeo et al. refer to the use of MAE, accelerated solvent extraction, and ultrasound-assisted extraction and provide details on sample size, extraction time, solvent type, and specific operating conditions for MAE (i.e., temperature, microwave power, pressure) for many different types of pesticides. The authors concluded that the choice of the extraction technique is up to the user and may to some extend depend on the matrix–analyte combination.27 In the review by Chung and Chen of 109 publications, the focus was on the analysis of organochlorine pesticides (OCPs) in fatty foods.28 Despite the fact that there were many extraction techniques besides MAE in the 109 publications, the conclusions were that a fast and efficient extraction method is yet to be developed.28 Garcia-Jares et al.29 reviewed 163 articles on analysis of emerging

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Microwave-Assisted Extraction

contaminants in indoor air and gave detailed extraction conditions specifically for the MAE of polybrominated diphenyl ethers (PBDPEs), phosphate esters, and pyrethroid insecticides. Although the review does not focus specifically on MAE, it provides a wealth of information on other extraction techniques and the concentrations of these type of contaminants in indoor air. Use of ionic liquids (ILs) to extract pollutants from foods and environmental samples discussed in 135 publications was reviewed by RuizAceituno et al.30 ILs are salts usually consisting of organic cations such as imidazolium, pyridinium, pyrrolidinium, phosphonium, or quaternary ammonium) and organic or inorganic anions such as bromide, acetate, trifluoroacetate, tetrafluoroborate, or hexafluorophosphate.30 Their advantages and disadvantages in contrast to conventional solvents and their applicability to extract aminoacids, proteins, carbohydrades, phenols, fatty acids, alcohols, amines, and selected pharmaceuticals are covered in details in this review. Use of MAE in cosmetic analysis has been reviewed by Caballeiro et al.31 and although the authors reported that its use has been limited to extraction of sunscreen compounds from creams and lipstick, its implementation led to much faster extraction times than conventional procedures and reduced solvent usage. A review of use of microwave-assisted techniques such as hydrodistillation, steam distillation, vacuum hydrodistillation, SFME and microwave hydrodiffusion and gravity was recently published for essential oil extraction.32

Polycyclic Aromatic Hydrocarbons Work done by Lopez-Avila et al.33 indicated that polycyclic aromatic hydrocarbons (PAHs), with the exception of more volatile compounds such as naphthalene, can be extracted quantitatively (recovery > 80%) from soil and sediment matrices with hexane– acetone (1:1, v/v) at temperatures of 115  C. Typical extraction times for batches of up to 12 samples (5 g each) were 10 min at 100% power (1000 W). The lower recoveries of naphthalene, acenaphthene, and acenaphthylene were attributed to the presence of water in the soil matrix (to prepare a representative aged soil sample, water was added to the soil matrix to bring its water content to 30). Other successful MAE conditions for extracting PAHs from soils, sediments, and fly ash have been reported with hexane–acetone (1:1, v/v), acetone alone, dichloromethane alone, dichloromethane–toluene (50:50, v/v), acetone–petroleum ether (1:1, v/v), methanol–toluene (9:1, v/v), and toluene–water.34–41 Dean and Barnabas34 reported on a direct comparison between Soxhlet, MAE, and supercritical fluid extraction for PAHs and concluded that the major advantage of MAE is the speed of extraction, but they also acknowledged that without additional cooling after extraction it takes approximately 30 min until the vessels can be opened and extracts processed. Barnabas et al.40 also investigated the effects of pressure, temperature, extraction time, and percent of methanol modifier added to the extraction solvent in order to optimize the extraction. Chee et al.36 reported a 5-min heating at 115  C with 30 ml hexane–acetone (1:1, v/v) as the optimum extraction conditions for a 5 g sample, conditions which are very similar to those reported by Lopez-Avila et al.33 Optimization of MAE of PAHs using open-vessel technology was conducted by Budzinski et al.,41 who reported that the optimum conditions are 30% water, 30 ml dichloromethane, and 10 min heating at 30 W power. A heating time of 10 min was more than sufficient to extract PAHs quantitatively from the matrix, especially when adding water, which is supposed to cause swelling of the matrix. Accurate quantification of PAHs in dust samples was investigated by Itoh et al.42 using isotope dilution MS with deuterated PAHs. The MAE was performed at 160  C with methanol–toluene (1:3, v/v) for 40 min and the authors reported recoveries <40% for PAHs with molecular weight > 264 u and a second extraction under the same conditions contributed about half of the amounts reported in the first extraction for the deuterated compounds.42 In an earlier publication by Itoh et al.43 using 13 C-labeled PAHs and sediment samples, the recovery yields were 60–80% when using dichloromethane or ethyl acetate or a mixture of both at 100  C or 150  C for 30 min. A mixture of PAHs, nitrated PAHs, and heavy n-alkanes were extracted by MAE from a carbonaceous material taken out from a diesel engine44; aromatic solvents such as xylene and chlorobenzene increased recoveries of PAHs and heterocyclic aromatic solvents such as pyrrole and pyridine facilitated the extraction of nitro-PAHs. MAE with 1 M KOH in methanol and pressurized liquid extraction with toluene or dichloromethane–ethyl acetate (1:1, v/v) were used in parallel to certify 18 PAHs in a fresh lake sediment material.45 The reader may refer to specific references in this review46 related to extraction of PAHs from environmental samples such as atmospheric particulates and different types of sediments. Recently, environmentally friendly ILs such as 1-hexadecyl-3-methylimidazolium bromide and 1-hexadecyl-3-butyl limidazolium bromide have been used to extract PAHs from sediments with recoveries of 91% and extraction times of 6 min.47,48

Pesticides and Herbicides Many applications involving MAE of pesticides and herbicides were reported during the past decade. The reader may refer to specific references in the review by Belanger and Pare´46 related to extractions of OCPs from environmental samples such as marine sediments, soils, and semipermeable membrane devices. Likewise, the review article by Tadeo et al.27 provides specific details for the MAE of OCPs, organophosphorus pesticides (OPPs), chlorophenoxy acid and quaternary ammonium herbicides, pyrethroids, carbamates, urea and chloroacetanilide pesticides from soils. Typical sample sizes for MAE are 5 g but 1.0–2.5 g have also been used and typical volumes for the extracting solvent are 20–25 and 50 ml has been used for a 10 g soil sample. Typical extraction solvents are hexane–acetone (1:1, v/v) for OCPs from soil, methanol–water (1:1, v/v) or ethyl acetate for OPPs and chloroanilide pesticides, ethyl acetate for pyrethroids, and acetonitrile for carbamate and urea herbicides.27 Critical parameters that affect the recoveries of pesticides such as pyrethroids and OPPs are extraction temperature, addition of water to the extraction solvent, and solvent/soil

Microwave-Assisted Extraction

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ratio.49 Time, temperature and volume of solvent were the main parameters affecting the extraction of 30 pesticides from airborne particulate matter.50 The optimized conditions for the air particulate study were 50  C, 20 min at 1200 W, and 30 ml ethyl acetate. Onuska and Terry51 extracted selected OCPs such as aldrin, dieldrin, and DDT from soils and sediments using acetonitrile, isooctane, or a mixture of isooctane–acetonitrile (1:1, v/v) and achieved quantitative recoveries using five or seven 30-s irradiations with microwave energy. They also reported that MAE recoveries increase as the moisture content of the soil increases up to 15%. Fish and Revesz52 used hexane–acetone as extraction solvent and reported that OCP recoveries improved when changing from hexane–acetone (1:1, v/v) to hexane–acetone (2:3, v/v). The latter solvent has a composition similar to the azeotropic vapor in the Soxhlet extractor. Lopez-Avila et al.53 extracted 45 OCPs from freshly spiked and 24-h aged soil samples with hexane–acetone (1:1, v/v). For the freshly spiked soil, 38 compounds had recoveries between 80% and 120%, six compounds had recoveries between 50% and 80%, and the recovery of captafol was above 120%. For the spiked soil samples aged for 24 h, 28 compounds had recoveries between 80% and 120%; 12 compounds had recoveries between 50% and 80%; three compounds including captafol, captan, and dichlone were poorly recovered; and chloroneb and 4,40 -DDT had recoveries above 120%. When recoveries from freshly spiked soil were compared to those from aged spiked soil, it was found that the recovery of captafol dropped from 122% to 36%, the recovery of captan dropped from 106% to 21%, and the recovery of dichlone dropped from 78% to 10%. Captafol and captan appear to be quite stable upon irradiation of soil–solvent suspensions, but dichlone was found to disappear upon irradiation of the solvent. (The recovery of dichlone from solvent was only 5.5% after heating at 145  C for 5 min and 2.6% after 20 min at the same temperature.) Microbial degradation may be responsible for the low recoveries of captafol and captan, whereas in the case of dichlone, it is quite likely that this compound is not stable under the conditions used. Nonetheless, these recoveries are higher than those obtained by Soxhlet or sonication extraction. Triazine herbicides have been successfully extracted from soil by MAE with water, methanol, acetone–hexane (1:1, v/v), dichloromethane, acetonitrile–0.5% ammonia in water (70:30, v/v), dichloromethane–water (50:50, v/v), methanol– dichloromethane (10:90, v/v).54 Water seems to be preferred since it is a very polar solvent and can interact strongly with polar matter in soils to enhance the desorption of triazines55; it is a cheap, safe, and environmentally friendly solvent; and it heats up very quickly when irradiated with microwave energy. Xiong et al.55 reported that direct heating of soil with water gave a 73.4% recovery for atrazine from soil and, therefore, stated that ‘MAE is not only a simple heating.’ Imidazolinones (e.g., imazapyr, imazmetapyr, imazethapyr, imazaquin, etc.) are extracted from soil with 0.1 M ammonium acetate–ammonium hydroxide (pH 9–10) in a 10-min extraction. A variety of soil samples fortified at 1–50 ppb exhibited an average recovery of 92% (standard deviation 13%).56–58 Sulfonamides such as sulfadiazine, sulfadimidine, sulfathiazole, sulfachloropiridazine, and sulfadimethoxine have been extracted from agricultural soils (aged for 3 months) using acetonitrile–buffer pH 9 (20:80, v/v) and, although MAE was found to be the most suitable extraction technique, the recoveries were quite low (i.e., 15–25% for sulfathiazole).59 Water samples can also be extracted by MAE; however, they have to be preconcentrated first on a membrane disk or some adsorbent material. Chee et al.60 used C18-membrane disks and then extracted the disks with 20 ml solvent (acetone and dichloromethane) in a closed-vessel MAE system at 80  C, 100  C, and 120  C for 1, 3, 5, and 10 min. Acetone was found to give higher recoveries than dichloromethane. This approach would allow extremely low detection limits as several disks generated by processing a large volume of sample can be extracted in one vessel. A review by LeDoux61 on extraction of pesticides residues from foods of animal origin mentions extraction of 18 OCPs and two OPPs from beef meat by MAE with acetonitrile and from eggs/milk/mackerel fillet/cod liver by MAE with ethyl acetate–hexane (1:12, v/v). For fatty tissues, MAE and accelerated solvent extraction gave comparable recoveries.61 Vetter et al.62 extracted OCPs from fatty tissues (e.g., seal blubber) with solvents such as hexane–ethyl acetate (1:1, v/v). To transfer heat to hexane, which is microwave transparent, disks of Weflon™ (2.5 cm in diameter
0.3 cm thickness) were used in the extraction vessel. The yield of extractable fat and recoveries of OCPs after seven irradiation cycles were comparable to those obtained by Soxhlet extraction. Since ethyl acetate–cyclohexane (1:1, v/v) seems to extract more fat than hexane, a gel permeation chromatography (GPC) step after extraction is a must. A critical review on determination of OCPs in fatty foods has been published and was mentioned above.28 ILs were also investigated for extraction of OCPs from soil.63 1-Butyl-3-methylimidazolium hexafluorophosphate was found to perform best and recoveries of four OCPs ranged from 73% to 99%. The optimized conditions were: temperature 70  C, 400 W, 10 min, and solvent–soil ratio was 5.63

Polychlorinated Biphenyls MAE of polychlorinated biphenyls (PCBs) was reported by Lopez-Avila et al.,64 Onuska and Terri,65 Pastor et al.,66 Dupont et al.,67 and Kodba and Marsel.68 Lopez-Avila et al.64 used hexane–acetone (1:1, v/v) and reported that the average recoveries from typical soil matrices were greater than 70% for the Aroclors 1016 and 1260 and the method precision was better than 7%. Furthermore, there was no degradation of PCBs upon heating of solvent–soil suspensions with microwave energy. Three reference materials and 24 soils from a Superfund site, most of which contained Aroclors, were extracted by MAE and analyzed by both GC–ECD and enzyme-linked immunosorbent assay (ELISA). Because ELISA is very sensitive and its detection range is quite narrow, the hexane– acetone extracts were first diluted with methanol and subsequently with the assay buffer (which contained 50% methanol) to bring the Aroclor concentrations to less than 5 ng ml1. These data indicate excellent agreement between the certified Soxhlet–GC/ECD

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data and the MAE–ELISA data (correlation coefficient 0.9986; slope 1.0168) and the MAE–GC/ECD data and the MAE–ELISA data (correlation coefficient 0.9793; slope 1.0468). Other solvents used successfully to extract PCBs from environmental samples include hexane, ethyl acetate–cyclohexane (1:1, v/v), acetone and dichloromethane, and toluene–water (1:5, v/v).62,69,70 A study by Wang et al.70 concluded that extraction of PCBs and PBDPEs from soil and fish by MAE and accelerated solvent extraction gave comparable results to Soxhlet extraction but for PBDPEs MAE extraction conditions need to be carefully optimized for the particular matrix. The reader may refer to specific references in the Ref. [46] related to extractions of PCBs from environmental samples such as marine sediments, soils, and ash samples.

Phenols Previous MAE studies of phenolic compounds were reported by Lopez-Avila et al.,53 Llompart et al.,71,72 Chee et al.,73 and Egizabal et al.74 Acetone–hexane seems to be the preferred solvent for 16 phenolic compounds and dichloromethane, acetone–petroleum ether (1:1, v/v) were reported to work well for extraction of nonylphenol. The only compounds found to degrade during MAE are 2,4-dinitrophenol and 4,6-dinitro-2-methylphenol. MAE recoveries for phenolic compounds are usually higher than the classical extraction method recoveries, and the method precision is significantly better for MAE (e.g., coefficient of variation of 3% for MAE as compared to 15% for Soxhlet and 20% for sonication). Mahugo Santana et al.75 used a nonionic surfactant, polyethylene lauryl ether, for the MAE (700 W, 3 min) of 15 phenols from a soil matrix and achieved average recoveries >80%. Use of IL-based surfactants (i.e., 1-hexadecyl-3-methyl-imidazolium bromide and 1-hexadecyl-3-butyl-imidazolium bromide), as extraction solvents for MAE, allowed efficient extraction of five alkylphenols from marine sediments at 40 ppb in 25 min.48 Focused microwaveassisted micellar extraction using the nonionic surfactant POLE (polyoxyethylene 10 lauryl ether) coupled with solid-phase microextraction (SPME) and GC/MS was reported for the determination of chlorophenols in wood.76 Use of micellar media as extractant for MAE and coupling this procedure with a preconcentration step such SPME, and followed by a thermal desorption step represents a new direction in the use of MAE, making it into a faster and environmentally friendly extraction method. Similarly, MAE was coupled with headspace SPME and GC/MS in the determination of octyl and nonyl phenols in paper.77 Two grams of paper were extracted with 15 ml water in a Soxwave Map system for 5 min, the aqueous extract was acidified at pH 2 after addition of NaCl and was heated at 65  C in a sealed vessel with a headspace of 5 ml. A divinylbenzene–carboxen–polydimethylsiloxane was employed to sample the headspace for 45 min in order to extract the phenols at low ppb concentrations.77 Coupling of MAE with dispersive liquid–liquid extraction was reported for the determination of five chlorophenols in soils and sediments.78 In this case, water at pH 10 was used to extract soil (2 ml water for 1.2 g soil) in a sealed MAE vessel for 90 s. Subsequently, the pH of the solution was adjusted to pH 6 and was followed by injection of an acetone solution containing chlorobenzene, which was the extracting solvent and was recovered by centrifugation. The review article by Teo et al.26 gives detailed operating conditions for the MAE of phenolic compounds from foods such as potatoes, tomatoes, grapes, spices, peanut skins, etc. Optimization of MAE-assisted headspace SPME conditions of phenol from cigarette pad samples was investigated by Sha et al.79 When investigating MAE for a particular type of application, consideration needs to be given not only to the analyte recovery and the method sensitivity but to the matrix interferences. This was addressed in detail by Perez et al.80 in the determination of several alkyl phenols in which the authors report that the matrix solid-phase dispersion (MSPD) is preferred to MAE due to lower matrix interferences in MSPD.

Organometallic Compounds Methods reported in the literature for the determination of organotin compounds in soils use extraction with organic solvents (i.e., hexane, toluene, dichloromethane)81 in the presence of a complexing agent (i.e., tropolone and carbamates),81 or leaching with acetic or hydrochloric acid assisted by sonication or some sort of shaking. Open-vessel MAE was recommended to accelerate the leaching with 50% acetic acid aqueous solution, and the data showed that a 3-min irradiation at 60 W was sufficient to recover tributyl tin from certified reference sediments.82 Ethanoic acid (0.5 M in methanol) was also reported for MAE of organotin compounds.83 After extraction, organotins are ethylated with NaBEt4 to the tetrasubstituted species for GC separation. Huang et al.84 used 1 M CaCl2, 0.1% tropolone and glacial acetic acid to extract organotin compounds from plant litter and C-rich soils. When dealing with biological matrices (e.g., tuna tissue, mussel tissue), solubilization with tetramethylammonium hydroxide (TMAH) for a 3 min at 90  C, 115  C, and 130  C in a closed vessel was demonstrated to be as efficient as the hot-plate procedure.85 Schmitt et al.86 reported on the integration of the solubilization step with the derivatization–extraction step by using 11 M acetic acid for solubilization and NaBEt4 for derivatization using an open vessel MAE system. Organotin degradation during MAE has been reported in the literature and was attributed to the fast temperature rise of the extractant that induces desalkyl and desarylation of the analytes leading to low recoveries and poor detection limits.87 Quantitative extraction of butyl, phenyl and octyl compounds was obtained by using glacial acetic acid and mechanical stirring for 16 h or sonication for 30 min88 or by use of acetic acid and methanol (3:1, v/v) and mechanical stirring.89 Use of self-tuning single-mode microwave cavity with direct pressure control (max pressure set at 300 W) and temperature control, plus use of magnetic stirring and isotope dilution allowed quantitative MAE of several organotin and organomercury compounds from a certified marine sediment and oyster and mussel reference material.90 Use of low power (60 W) with 50% glacial acetic acid for 4 min was reported for extraction of organotin and organomercury compounds from certified reference sediments.91 The novelty in this application was the use of SPME to enrich the organometallic

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compounds onto the SPME fiber as fully alkylated species. Organotin speciation in textiles and plastics using MAE with 60% methanol in water was reported by Wang et al.92 Various solvents including: aqueous methanol with 0.1 M HCl, 0.5 M acetic acid in methanol, or 25% aqueous solution of TMAH were also used to extract organotin compounds from ERM-CE477 certified mussel tissue reference material using MAE.93 Organomercury compounds can be extracted from sediments with 6 M hydrochloric acid at 120  C for 10 min in closed vessel or 2 M nitric acid and 2 M hydrochloric acid after 3 min irradiation at 60 W in open vessel86 and from soil and litter by MAE with a mixture of 1 M CaCl2, 0.1% tropolone, and glacial acetic acid.84 Pure acetic acid and 1 M sulfuric acid could only extract 85% and 55%, respectively. Species-specific isotope dilution analysis (i.e., 119Sn and 201Hg) has been reported for MAE of organotin and organomercury compounds from solid environmental matrices.90 Microwave-assisted digestion has been successfully used to prepare solid samples for organometallic speciation analysis82,94,95 and the microwave-assisted digestion of the biological tissue with 25% TMAH for 2–4 min at 40–60 W gave quantitative recovery of both organomercury and inorganic mercury.95 Speciation of mercury and lead in fish samples using MAE and LC–ICPMS was reported by Chang et al.96 The extractant was ethylene diamine tetraacetic acid at 10 g l1 and 0.2% (v/v) 2-mercaptoethanol and the extraction was performed at 65  C for 2 min.96 Additional information on the extraction of arsenic, zinc and copper containing compounds from food and marine organisms can be found in the review by Teo et al.26

Additives in Polymers Antioxidants such as the Irganox 1010, Irganox 1076, and Irgaphos 168, which are added to polymers to protect them during end-use applications, can be extracted with >95% efficiency by MAE with n-heptane–acetone in a few minutes.97 Higher temperatures (e.g., 140  C) were used by Jordi et al.98 with cyclohexane–chloroform–triethylamine (45:45:10, v/v/v) to dissolve polyethylene and extract compounds such as Tinuvin 770, Tinuvin 622, Tinuvin 144, and Chimasorb 81. A group of compounds, known as plasticizers, are the monoesters of phthalic acid, used as coatings for polyvinyl chloride, which is used in food packaging and medical devices. As these compounds could migrate from packaging material into food, their levels in both the packaging materials and foods exposed to such packaging materials need to be known. Sun et al.99 reported on the MAE of 24 phthalate esters in edible vegetable oils. The oil extraction was done with methanol for 15 min at 100  C. The extracts were fractionated by GPC and a C18 solid-phase extraction cartridge. The overall recoveries were in the range of 95–105% at a spike level of 40 mg kg1. Cano et al.100 used MAE to extract six different adipate esters, also used as plasticizers, from polyvinyl chloride (PVC) plastics. Methanol, ethanol, 2-propanol, and cyclohexane–acetone (1:1, v/v) were evaluated and methanol gave the best recoveries. Other variables that were optimized in this study included extraction temperature and time, microwave power, and amount of sample. The optimized conditions were: temperature 120  C, extraction time 10 min, microwave power 20%, solvent volume 25 ml, and sample size 0.5 g. When dealing with plasticizers in aqueous solutions, cloud point extraction coupled with MAE gave recoveries > 95% for water containing 3% (w/v) acetic acid, and >85% for 10% (v/v) aqueous ethanol.101 In the cloud point extraction the plasticizers are entrapped in micelles of nonionic surfactant Triton X-114 and removed from the aqueous phase by centrifugation.101

Flame Retardants There are two categories of flame retardants: brominated (BFRs) and phosphorus-containing (PFRs). PBDPEs belong to the group of BFRs and are considered an emerging class of contaminants. For a detailed review on PBDPEs, the reader should consult Refs [102,103] for a detailed review of PFRs. MAE of PBDPEs concentrated on semipermeable membrane devices with 60 ml hexane– acetone (1:1, v/v) for 1 min at 85  C (two cycles) gave recoveries of 72–91%.104 PBDPEs were extracted from electrical and electronic equipment by MAE with hexane (10 ml) and ultrapure water (4 ml) at 100  C for 10 min. Recoveries were 72–108% and LODs ranged from 0.17 to 68 ng g1 using GC–MS in the SIM mode.105 Isopropanol/methanol (1:1, v/v), isopropanol–hexane (1:1, v/v) at 130  C were used successfully to extract BFRs (i.e., TBBPA and decabromodiphenyl ether) from waste electrical and electronic equipment.106 Extraction of PBDPEs from dust by MAE was performed with hexane and reagent water at basic pH, 80  C,107 but required additional purification of extracts using Florisil activated at 130  C. Twelve PBDPEs were extracted quantitatively from air particulates collected on PM 2.5 filters using MAE with 50 ml hexane–acetone (1:1, v/v) at 75  C for 2 min at 600 W power.108 The determination of decabromodiphenyl ether is challenging because of the thermal degradation108 and because recovery of deca-BDPE was reported to be low for MAE as well as for ultrasonication,109 multistage liquid–liquid extraction with different polarity solvents was suggested.106 Extraction of PBDPEs from soil and fish samples according to Environmental Protection Agency method 3546 was reported.70 Detailed MAE conditions for many PBDPEs from marine samples, human adipose tissue, sediments, sewage sludge, house dust, liver, and muscle tissue can be found in Ref. [24]. To extract PFRs from dust samples and sediments several solvents including ethyl acetate, dichloromethane, acetone, acetonitrile were tested. The best solvents were found to be acetone followed by a second extraction with acetonitrile.103,110 Acetone provided higher recoveries than ethyl acetate and dichloromethane but, because the recoveries of tripropyl phosphate tris-chloroethyl phosphate were still low, an additional extraction with acetonitrile was implemented to achieve recoveries higher than 80% for ten PFRs tested.110 Other combination of solvents for extracting PFRs from biological samples (i.e., fish, domestic birds) included ethyl acetate– dichloromethane (1:1, v/v), ethyl acetate–acetone (1:1, v/v) at 100  C for 30 min.111 Selected PFRs were extracted from sediments by MAE with acetone at 120  C for 20 min with recoveries ranging from 62% to 106%.112 An on-line method for performing

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dynamic MAE followed by solid-phase extraction and on-line transfer of the effluent to a GC injector was reported by Erricson and Colmsjo.113

Pharmaceutical and Personal Care Products Pharmaceutical compounds include human and veterinary drugs that are being released to the environment either as wastes from manufacturing processes, spills, or improper disposal. Examples include antibiotics and anti-inflammatory drugs, stimulants, antimicrobial drugs, as well as legal or illegal drugs. Personal care products include polycyclic musks and nitromusks, triclosan, and many synthetic chemicals used in soaps, lotions, and toothpaste.24 Detailed MAE conditions for extraction of samples containing PPCPs (i.e., synthetic musks, triclosan, caffeine, carbamazepine, ciprofloxin, fluoroquinolones, flumequine, ibuprofen, naproxen, norfloxacin, phenazone, and others) can be found in Ref. [24]. Extraction solvents for pharmaceutical compounds include dichloromethane–methanol (2:1, v/v), water containing a nonionic surfactant, 1 M phosphoric acid (pH 2) and dichloromethane, acetonitrile–water (6:1, v/v), and acetonitrile–water (4:1, v/v). The extraction temperatures were 115  C or 150  C. Extraction solvents for personal care products include hexane–acetone (1:1, v/v), dichloromethane/methanol (2:1, v/v), acetone–methanol (1:1, v/v), and dichloromethane. The extraction temperatures were 110  C or 130  C.24 MAE of polycyclic and nitro musks from sewage sludge and sediments has been described in detail in several references included in the review article by Zuloaga et al.25 It has been reported that, when dealing with synthetic musks, the presence of water increases the extraction efficiency and, therefore, there is no need to dry the sample prior to extraction. A recent publication114 on the MAE of benzotriazoles UV P, UV 326, UV 327, UV 328, UV 329, UV 360, and UV 571, which was validated for several sediments and sludge samples, allows on-line SPE and LC–MS/MS analysis with limit of quantitation of 0.2–0.4 ppb for sediments and 0.2–0.5 ppb for sludges.113 For a side-by-side comparison of extraction methods for legal and illegal drugs from environmental samples the reader should refer to a recent review by Vasquez-Roig et al.115 MAE recoveries of 21 pharmaceuticals from lyophilized sludge, soil and sediment samples using methanol–water (3:2) ranged from 91% to 101% and fluoroquinolones recoveries from air-dried marine sediment with water containing 5% HTAB ranged from ranged from 73% to 96%. Plasma and urine samples spiked with six drugs (metamizol, acetamidofen, salicylic acid, tramadol, ibuprofen, and diclofenac) were subjected to MAE after mixing the 0.5 ml sample with 4 ml phosphate buffer at pH 4 and 12 ml ethyl acetate in a closed vessel and heating it at 67  C (for plasma) and 115  C (for urine) for 9 min (plasma) and 4 min (urine) with continuous magnetic stirring. Excellent recoveries were reported for both matrices.116 A review article on MAE and cloud point extraction of drugs and other bioactive compounds with detailed extraction conditions was published by Madej.117 Triton X-114 has been widely used for cloud point extraction in these applications.

Perfluorinated Compounds Perfluorinated alkyl substances include fully fluorinated compounds containing between 4 and 18 carbon atoms and a hydrophilic functional group such as sulfonate in sulfonates (PFSAs) or carboxylate in perfluorinated carboxylates. The extraction of PFSAs from filters and adsorbents was reported by Beser et al.118 Because the MAE vessels are made of Teflon, the authors performed the extractions in quartz vessels and did a thorough investigation of the main parameters affecting MAE (i.e., time, temperature, volume of solvent). Methanol was used for MAE and the optimized parameters were a 2-min extraction time at 120  C with 25 ml methanol for a 25 mm in diameter quartz fiber filter.118 Identification of perfluorinated compounds in foodstuff by MAE was reported by Bugey et al.119 A 5-g sample was extracted with 10 ml methanol for 5 min at 25 W power in a focused MAE system and the extract was subjected to solid-phase extraction prior to LC–MS/MS analysis. Twenty five perfluoroalkoxy compounds including carboxylates, sulfonates, and sulfonamides were targeted in this study of 200 samples of common food products that were analyzed for possible fluorinated compounds released from the packaging materials during microwave cooking.

Natural Products For the past 10 years there has been an exponential increase in the use of MAE to extract biologically active compounds from plant material. The book by Chemat and Cravotto120 focuses specifically on the MAE of bioactive compounds of interest to food, cosmetics, pharmaceutical, and neutraceutical industries. Extraction of oils from mint leaves and other materials of biological origin is a patented process known as the ‘microwave-assisted process’.46 MAE was listed among the emerging green technologies (i.e., ultrasound extraction, supercritical fluid extraction, and accelerated solvent extraction) for the chemical standardization of botanicals and herbal preparations and various MAE processes (i.e., vacuum MAE, ultrasonic MAE, dynamic MAE, SFME, VMHD, MSD, MSDf, PSFME, MHG) have been implemented to accommodate specific applications in this field.121 Its distinct advantages are high efficiency, lower cost than supercritical fluid extraction, high reliability, high stability, and great reproducibility.122 Among its disavantages, poor performance with nonpolar solvents as they do not absorb microwaves, and low selectivity, thus requiring additional fractionation of extracts.122 Extraction of essential oils, flavonoids, coumarins, phenolic compounds, alkaloids, and saponins by MAE have been discussed in detail in Ref. [123]. A review article124 on MAE of flavonoids with 128 references discusses specific applications published in recent years and discusses advanced methods (i.e., high-pressure MAE, dynamic MAE coupled with on-line derivatization and UV or liquid chromatography, dynamic or vacuum MAE and solvent-free microwave hydrodiffusion). Coupling of MAE with the cloud-point extraction was mentioned for licorice root and coupling of MAE with solid-phase microwave extraction was mentioned for the extraction of volatile compounds from plant materials.124 Optimization strategies and

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specific examples for a variety of medicinal plants, including the analytes, the extraction solvent and time for both open vessel MAE and closed vessel MAE were published by Mandal et al.125 Other reports on MAE of natural products include that of Young,126 Bichi et al.,127 and Mattina et al.128 Young126 extracted ergosterol from fungi and spores by MAE with methanol and 2 M sodium hydoxide. Bichi et al.127 extracted pyrrolizidine alkaloids from Senecio paludosos and Senecio cordatus dried plants by MAE with methanol at 65–100  C for 20–30 min. Mattina et al.128 reported on the extraction of taxanes from Taxus biomass by MAE with ethanol. Using 5 g of freshly harvested needles (moisture content 55–65%) soaked in 5 ml of water prior to MAE and 10 ml ethanol at 85  C for 9 min resulted in about 90% recovery. This procedure would reduce significantly the costs of the extraction of taxanes from biomass with no reduction in the extraction yields. A MAE method for extraction of aflatoxins B1, G1, B2, and G2 from grains and grain products using acetonitrile at 80  C for 15 min was reported by Chen and Zhang.129 By using aflatoxin M1 as internal standard, the relative recoveries of the aflatoxins ranged from 91% to 106% for corn and 88% to 103% for wheat. Use of ILs has been reported for extraction of kaempherol and quercetin from Toona sinensis and Rosa chinensis,130 essential oil and oxygenated monoterpenes from Fructus forsythia seed,131 camptothecin and 10-hydroxy-camptothecin from Camptotheca accuminata,132 carnosic and rosmarinic acid from Rosmarinus officinalis,133 Senkyunolide I and H and Z ligustilide from Lugusticum chuanxiong Hort,134 podophyllotoxin from Dysosma versipellis,135 rutin from Saururus chinensis,136 dehydrocavidine from C. saxicola,137 N- and O-nornuciferine from Lotus leaves,138 proanthocyanidins from Cortex cinnamon,139 quercetin/kaempferol from Bauhinia championii,140 phloroglucinols from Dryopteris fragrans,141 and phenolic compounds (i.e., chlorogenic acid, caffeic acid, and quercetin) from burdock leaves.142 ILs are gaining popularity as environmentally friendly solvents due to their negligible vapor pressure and good thermal stability.143,144 Unfortunately, there have been no systematic studies on the use of ILs for MAE and most studies reported so far used trial-and-error to select the best extraction conditions. Very recently, Liu et al.145 analyzed fatty acids in raw nuts and seeds by microwave–ultrasound synergistic in situ extraction–derivatization (MUED) and GC–MS, and reported that the total content of fatty acids using MUED is significantly higher than conventional Soxhlet extraction. Although there are reports on MAE coupled with ultrasonics,21–23,122,142 there have been no studies published on coupling the derivatization with the MAE-ultrasonic extraction.

Future Trends Emerging trends in MAE, especially for botanicals, include SFME, which was reported by Chemat et al.120 for extraction of natural products from plants using the in situ water within the plant material. In this case, a cooling system outside the microwave oven is needed for the collection of the distillate that contains the water and the essential oils. Variations of the SFME that have been reported include: (a) improved SFME, in which a microwave-absorption medium is added to the sample matrix to absorb part of the emitted microwave energy and store it as heat, (b) pressurized SFME involving extraction under pressure without addition of solvents, (c) coupling of microwaves to saturated steam to release compounds of interest from the plant tissue, (d) microwave hydrodiffusion or dry-diffusion and gravity. Coupling of MAE with other extraction methods such as headspace SPME,146 on-line solid-phase extraction113,114,147 has already been demonstrated and more applications are expected given the popularity of MAE.

Acknowledgments The authors would like to acknowledge the contribution of Kim Mooney of Agilent Technologies who performed the database searches for this review.

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