Microwave Assisted Extraction MAE

MAE is thus another special case of liquid extraction accelerated by increasing the temperature of the solvent. It differs from traditional solvent extraction (including the Soxhlet method) by the fact that the heat is generated internally within the solvent-solid matrix system wherever the microwaves can interact with solvent molecules, including within any pores in the solid. MAE differs from SAE by the fact that no spectacular localized hot spots are involved. In addition, because of the mechanism involved, different solvents have different properties with respect to microwave radiation and this provides a degree of selectivity for MAE by adjusting the nature of the solvent. The principal molecular properties involved in response to a microwave field are the dipole moment of the solvent and the extent of nonbonding interactions 

In practice, different configurations of extraction vessel and microwave oven are employed. In one approach the vessel is open to the atmosphere and the microwave energy is delivered in a focused manner via a waveguide 

Solvent Dielectric Constant, E (Fm- ) Loss Factor, e (Fm- ) Dissipation Factor, S( x iff ) 

Of course, thermal stability of analytes becomes a concern for all extraction methods (Sections 4.3.2a-d) that involve extended exposure to boiling solvents. A somewhat different approach is discussed below. 

This is a book about analytical chemistry but, as can be seen in other Chapters, most of the fundamental concepts underpinning the techniques and methods that are used are derived from classical (i.e., non-quantum) physical chemistry, possibly the least favorite topic of science students who require some chemistry courses in then-degree program. 

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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... 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

Microwave-assisted extraction (MAE) of analytes from various matrices using organic solvents has been operative since 1986 [128], In this process microwave energy is used to heat solvents in contact with a solid sample uniformly and to partition compounds of analytical interest from the sample matrix into the solvent. The way in which microwaves enhance extraction is not fully understood. The main factors to consider include improved transport properties of molecules, molecular agitation, the heating of solvents above their boiling points and, in some cases, product selectivity. 

For solid samples PLE, microwave-assisted extraction (MAE) and supercritical fluid extraction (SFE) [39, 40] are the preferred extraction methods. 

In recent years, extraction methods for PhACs have usually been based on liquid partitioning with ultrasonic extraction (USE) [43-47], microwave-assisted extraction (MAE) [48], or the more advanced PLE [49-52]. When compared to the other extraction techniques, PLE provides good recoveries, saves time and organic solvent, which makes it become currently a preferred technique for PhAC analyses. 

Vryzas Z, Papadopoulou-Mourkidou E. Determination of triazine and chloroacet-anilide herbicides in soils by microwave-assisted extraction (MAE) coupled to gas chromatographic analysis with either GC-NPD or GC-MS. J. Agric. Food Chem. 2002 50 5026-5033. 

Supercritical fluid extraction (SFE), microwave-assisted extraction (MAE) and Soxhlet extraction under various experimental conditions were applied for spiked poly(vinyl) chloride samples. Extracted dyes were separated in an ODS column (250 X 4.6 mm i.d. particle size 5 jum) using methanol as the mobile phase. Dyes are well separated by this method as demonstrated in Fig. 3.59. The optimal parameters of the extraction methods are compiled in Table 3.23. Recoveries depended on both the type of extraction method and the chemical structure of the dye. It was found that the highest recovery can be obtained by MAE and the extraction efficacy was the lowest for Solvent red 24 [129],... 

The carcinogenic aromatic amines released from azo dyes in leather were investigated by using microwave-assisted extraction (MAE) or supercritical fluid extraction (SFE) followed by RP-HPLC. The chemical structures of dyes and aromatic amines are listed in Fig. 3.69. The flow schemes for SFE and MAE are shown in Figs 3.70. and 3.71. 

Fig. 3.71. Flow scheme for the analytical procedure based on microwave-assisted extraction (MAE). Reprinted with permission from C. S. Eskilsson et al. [140].

Microwave-assisted extraction (MAE) is a recent extraction technique, which combines microwave and traditional solvent extraction. The MAE approach have many advantages, such as shorter time, less solvent, higher extraction rate and better products with lower cost. Soxhlet... 

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... 

A nnmber of phenolic snbstances were identified by Sterbova et al. [363] in different plants by a BDS C18 colnmns with 3 pm particle size, and a comparison was carried out on different extraction methods, by the comparison of microwave-assisted extraction (MAE) to the traditional approach. 

Microwave-assisted extraction (MAE) has been described for the extraction of various compounds from different matrices. It is a simple technique that can be completed in a few minutes. Microwave energy is applied to the sample suspended in solvent, either in a closed vessel or in an open cell. The latter allows larger amounts of sample to be extracted. A certain degree of heating is involved. 

Microwave-assisted extraction (MAE) it has attracted growing interest, as it allows rapid extraction of solutes from solid matrices by employing microwave energy as a source of heat [42], The portioning of the analytes from the sample matrix to the extractant depends upon the temperature and the nature of the extractant. 

Bayen, S., Lee, H.K., Obbard, J.P., 2004b. Determination of polybrominated diphenyl ethers (PBDEs) in marine biological tissues using microwave assisted extraction (MAE). J. Chromatogr. A. 1035, 291-294. 

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... 

This chapter covers techniques for the extraction of semivolatile organics from solid matrices. The focus is on commonly used and commercially available techniques, which include Soxhlet extraction, automated Soxhlet extraction, ultrasonic extraction, supercritical fluid extraction (SFE), accelerated solvent extraction (ASE), and microwave-assisted extraction (MAE). The underlying principles, instrumentation, operational procedures, and selected applications of these techniques are described. In a given application, probably all the methods mentioned above will work, so it often boils down to identifying the most suitable one. Consequently, an effort is made to compare these methodologies. 

It should be noted that microwave-assisted extraction (MAE) discussed in this chapter is different from microwave-assisted acid digestion. The former uses organic solvents to extract organic compounds from solids, while the latter uses acids to dissolve the sample for elemental analysis with the organic contents being destroyed. Microwave-assisted digestion of metals is covered in Chapter 5. 

OF samples were collected with the commercial device Salivette, which consists of a cotton swab that is inserted into the mouth for 2-3 min. Toxitubes A was compared to microwave assisted extraction (MAE). A volume of 1 ml of saliva was poured into each Toxitube and treated similarly as seen previously [109], For MAE different solvents (chloroform, dichloromethane, hexane), temperatures (80, 90, and 100 °C), and time periods (5, 10, and 15 min) have been tested finally 1 ml of saliva was mixed with 10 ml of chloroform and placed in the oven vessel for extraction at 100 °C for 10 min. Recoveries were found to range from 53 to 95 % with Toxitubes and from 83 to 100 % with MAE, so authors demonstrated that microwave-assisted extraction provides recoveries higher than those obtained for opiates with SPE. 

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