Pressurized MAE

Microwave-assisted extractions (MAE) can be performed in open (focused MAE) or closed (pressurized MAE) flasks. This technique is commonly used for extractions from complex and difficult sample matrices, replacing time- and solvent-intensive Soxhlet extractions or hydrodistillations.46 MAE is also widely applied to environmental samples, for example, for extracting polycyclic aromatic hydrocarbons (PAH) from soil, methylmercury from sediments, and trace metals and pesticide residues from plant material47 48 The use of microwave treatment instead of hydrodistillation offers a solvent-free separation technique essential oils are heated and dry-distilled 46... 

Pressurized MAE in closed vessels This technique employs a microwave-transparent vessel for the extraction and a solvent with a high dielectric constant (electrical permittivity). Such solvents absorb microwave radiation and can thus be heated to a temperature exceeding solvent boiling points under standard conditions. Boiling does not occur, however, because the vessel is pressurized. This mode of operation is very similar to ASE—the elevated pressure and temperature facilitate extraction of the analyte from the sample. 

The fundamentals and practical use of MAE have been described in detail in several review articles (68-70) and books (18, 71, 72). The following text focuses on closed-vessel (pressurized) MAE, which permits extractions at elevated temperatures. A major difference of MAE compared to SEE and PLE, in addition to its unique heating performance, is that the commercially available MAE systems today operate in batch mode. The possibility of built-in clean-up is therefore difficult to perform and related to the design of the instrumentation. Both automated SEE and PLE are most commonly used in a dynamic or semidynamic mode, which simplifies the development of combined extraction/clean-up strategies. 

By using MAE, the organic solvent and the sample are subjected to radiation from a magnetron in either a sealed vessel (pressurized) or an open vessel under atmospheric pressure. Pressurized MAE allows a large number of samples to be extracted simultaneously, whereas atmospheric pressure MAE is limited to six simultaneous vessels. The main drawback of this technique is that the solvent needs to be removed from the sample matrix upon extraction completion prior to analysis. 

Microwave energy may be applied to samples using either of two technologies closed vessels (under controlled pressure and temperature) or open vessels (at atmospheric pressure).These two technologies are commonly termed pressurized MAE (PMAE) or focused MAE (EMAE), respectively. Whereas in open vessels the temperature is limited by the boiling point of the solvent at atmospheric pressure, in closed vessels the temperature may be elevated by simply applying the correct pressure. The latter system seems more suitable in the case of volatile compounds. However, with closed vessels, one needs to wait for the temperature to decrease after extraction before opening... 

On the other hand, MAE affords the opportimity of performing several simultaneous extractions with a closed-vessel system (pressurized MAE). The cell containing the sample and the liquid solvent is subjected to microwave radiation that enables instantaneous and efficient heating in the presence of... 

The dynamic high-pressure MAE system described previously also allows the extraction to be coupled to a subsequent stage by connecting the extractor outlet to the unit concerned. Figure 5 illustrates one possible coupling connecting the extractor to a fluori-metric detector allows the kinetics of the extraction of PAHs from sediments to be monitored. 

General trends are focused on reduced-solvent extractions or adsorption-based methods — enviromnentaUy friendly solvents for both solid and liquid samples. In recent decades, advanced techniques like supercritical fluid extraction (SFE), ° pressurized liquid extraction (PLE)," microwave-assisted extraction (MAE), ultrasound-assisted extraction, countercurrent continued extraction (www.niroinc.com), solid... 

The solvent, microwave energy applied, and extraction time selected are the main parameters controlled in MAE. The user should use proper extraction vessels and equipment in MAE because very high pressures can be achieved and explosions may result if appropriate precautions are not taken. 

Soxtherm , FEXTRA and intermittent extractions. The newer methods (SFE, ASE , MAE) invariably operate at pressures above atmospheric. Micromethods of solvent extraction have also been described [57,58]. 

The open-vessel MAE approach can be contrasted to closed-vessel MAE (Section 3.4.5.1). Focused microwave-atmospheric pressure technology was reviewed by Mermet [473]. 

Dean [272] has compared various extraction methods. The use of ASE has made a large initial impact due to its acceptance as an EPA method. Inter-extraction competition is expected to favour the technique even further. Vandenburg et al. [457] have compared PFE and MAE with atmospheric pressure methods for extraction... 

In mercury speciation studies, pressurized liquid extraction (PLE), microwave-assisted extraction (MAE), and supercritical fluid extraction (SEE) are employed [33]. In particular, methyl-mercury is extracted by the Westoo method [33,34], which consists in a leaching process with hydrochloric acid, the extraction of the metal chloride into benzene or toluene, the addition of ammonium hydroxide that converts the metal species to hydroxide and the saturation with sodium sulfate. Most of the HPLC methods reported in literature for the determination of organomercury compounds (mainly monomethyhnercury, monoethyhnercury, and monophenylmercury) are based on reversed... 

For the determination of organotin compounds (tributyltin, triphenyltin, triethyltin, and tetra-ethyltin) a MAE is proposed before the normal phase (NP) HPLC/UV analysis [35], In organotin and arsenic speciation studies, hydride generation is the most popular derivatization method, combined with atomic absorption and fluorescence spectroscopy or ICP techniques [25,36], Both atmospheric pressure chemical ionization (APCI)-MS and electrospray ionization ESI-MS are employed in the determination of butyltin, phenyltin, triphenyltin, and tributyltin in waters and sediments [37], A micro LC/ESI-ion trap MS method has been recently chosen as the official EPA (Environmental Protection Agency) method (8323) [38] it permits the determination of mono-, di-, and tri- butyltin, and mono-, di-, and tri-phenyltin at concentration levels of a subnanogram per liter and has been successfully applied in the analysis of freshwaters and fish [39], Tributyltin in waters has been also quantified through an automated sensitive SPME LC/ESI-MS method [40],... 

In order to accelerate sample preparation, new extraction methodologies such as accelerated solvent extraction (ASE) and MAE, based on the use of elevated temperature and pressure to heat the mixture sample-solvent, have been recently developed and applied for PAH extraction from meat [695] and vegetables [696-698]. Garda Falcon et al. [699] used microwave treatment with hexane to accelerate PAH extraction from freeze-dried foods. The fat extracted in this way underwent microwave assisted saponification with ethanolic KOH. Hernandez-Borges et al. [700] combined microwave-assisted hydrolysis and extraction to isolate organic pollutants from mussels, while... 

The basic components of a microwave system include a microwave generator (magnetron), a waveguide for transmission, a resonant cavity, and a power supply. For safety and other reasons, domestic microwave ovens are not suitable for laboratory use. There are two types of laboratory microwave units. One uses closed extraction vessels under elevated pressure the other uses open vessels under atmospheric pressure. Table 3.12 lists the features of some commercial MAE systems. 

Generally, recovery increases with the increase in temperature and then levels off after a certain point. For thermally labile compounds, analyte degradation occurs at high temperatures and results in low recovery. Excessively high temperatures lead to matrix decomposition in polymer extractions and should be avoided. In general, pressure is not a critical parameter in MAE. It changes with the solvent system and the temperature used and is acceptable below a preset limit. 

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