Microwave Extraction Techniques

1986 Gedye (Canada) and Giguere (USA) First microwave-assisted organic synthesis Ganzler (Hungary) First microwave-assisted extraction 

1994 Strauss (Australia) First continuous microwave reactor Mengal and Monpon (France) First patent on vacuum microwave hydrodistillation 

2004 Loupy (France) and Varma (USA) Microwave green chemistry Chemat and Lucchesi (France) Solvent-free microwave extraction 

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Supercritical fluid extraction (SFE) has also been used for the extraction of MMHg from sediments. ° Lorenzo et al. ° compared manual, microwave assisted techniques and SFE for the extraction of Hg from aquatic sediments. Higher recoveries were obtained with microwave extraction techniques compared to manual extraction techniques and SFE. 

A Microwave Extraction Techniques j 96S Tab. 22.2. Twenty years of microwave chemistry and extraction. 

In the last decade there has been an increasing demand for new extraction techniques, amenable to automation, with shortened extraction times and reduced organic solvent consumption, to prevent pollution and reduce the cost of sample preparation. Driven by these goals, advances in microwave extraction have resulted several techniques such as microwave-assisted solvent extraction (MASE) [32, 36-39], vacuum microwave hydrodistillation (VMHD) [40, 41], microwave hydrodistillation (MWHD) [42, 43], compressed air microwave distillation (CAMD) [44], microwave headspace (MHS) [5], and solvent-free microwave hydrodistillation (SEME) [45, 46]. Table 22.3 summarizes the most common microwave extraction techniques for plant matrices and lists their advantages and drawbacks. Over the years procedures based on microwave extraction have replaced some of the conventional processes and other thermal extraction techniques that have been used for decades in chemical laboratories. 

Specifically for triazines in water, multi-residue methods incorporating SPE and LC/MS/MS will soon be available that are capable of measuring numerous parent compounds and all their relevant degradates (including the hydroxytriazines) in one analysis. Continued increases in liquid chromatography/atmospheric pressure ionization tandem mass spectrometry (LC/API-MS/MS) sensitivity will lead to methods requiring no aqueous sample preparation at all, and portions of water samples will be injected directly into the LC column. The use of SPE and GC or LC coupled with MS and MS/MS systems will also be applied routinely to the analysis of more complex sample matrices such as soil and crop and animal tissues. However, the analyte(s) must first be removed from the sample matrix, and additional research is needed to develop more efficient extraction procedures. Increased selectivity during extraction also simplifies the sample purification requirements prior to injection. Certainly, miniaturization of all aspects of the analysis (sample extraction, purification, and instrumentation) will continue, and some of this may involve SEE, subcritical and microwave extraction, sonication, others or even combinations of these techniques for the initial isolation of the analyte(s) from the bulk of the sample matrix. 

Table 3.4 summarises the main characteristics of a variety of sample preparation modes for in-polymer additive analysis. Table 3.5 is a short literature evaluation of various extraction techniques. Majors [91] has recently reviewed the changing role of extraction in preparation of solid samples. Vandenburg and Clifford [4] and others [6,91-95] have reviewed several sample preparation techniques, including polymer dissolution, LSE and SEE, microwave dissolution, ultra-sonication and accelerated solvent extraction. 

Table 3.6 Relative efficiencies of microwave-assisted and reflux extraction techniques for additives in LDPE ...

The popularity of MAE methods for in-polymer additive analysis is reflected in a limited list of reported applications. This is both on account of the former lack of dedicated microwave equipment designed specially for small analytical samples and the relatively recent commercial introduction of the technique. Microwave extraction for analytical purposes is a relatively new growth area [441]. 

Mahan K.I., Foderaro T.A., Garza T.L., Martinez R.M., Maroney G.A., Trivisonne M.R., Willging E.M. Microwave digestion techniques in the sequential extraction of cadmium, iron, chromium, manganese, lead and zinc in sediments. Anal Chem 1987 59 938-945. 

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. 

To take advantage of the fluorous extraction procedure, or the use of the preparatively simple filtration, the use of highly fluorous reagents is crucial. The usefulness of the microwave heating technique is obvious in these examples. 

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... 

Kaufmann, B. and Christen, P., Recent extraction techniques for natural products microwave-assisted extraction and pressurized solvent extraction, Phytochem. Anal., 13, 105, 2002. 

G. LeBlanc, Microwave-Accelerated Techniques for Solid Sample Extraction, LCGC 1999,17, S30 (June 1999 supplement). 

Camel, V. 2001. Recent extraction techniques for solid matrices-supercritical fluid extraction, pressurized fluid extraction and microwave-assisted extraction Their potential and pitfalls. Analyst 726 1182-1193. 

Hubert et al. [101] state that accelerated solvent extraction compared to alternatives such as Soxhlet extraction, steam distillation, microwave extraction, ultrasonic extraction and, in some cases, supercritical fluid extraction is an exceptionally effective extraction technique. Hubert et al. [ 101 ] studied the effect of operating variables such as choice of solvent and temperature on the solvent extraction of a range of accelerated persistent organic pollutants in soil, including chlorobenzenes, HCH isomers, DDX, polychlorobiphenyl cogeners and polycyclic aromatic hydrocarbons. Temperatures ofbetween 20 and 180 °C were studied. The optimum extraction conditions use two extraction steps at 80 and 140 °C with static cycles (extraction time 35 minutes) using toluene as a solvent and at a pressure of 15 MPa. 

Various other extraction techniques have been used to recover hydrocarbons from soil including microwave-assisted extraction [19] and supercritical fluid extraction coupled with on-line infrared spectroscopy detection [20,21], The on-line SFA infrared procedure produced results similar to those obtained by Soxhlet extraction. 

Ultrasonic and microwave extractions are relatively simple and inexpensive techniques for greening extractions. 

Microwave extraction uses microwaves to heat solvents that are placed in the sample container. The microwaves heat the solvent and also add pressure to the sample vessel. The solvent is then removed and treated. The advantages of the microwave technique over Soxhlet extraction include reduced solvent consumption (—40 ml per sample), rapid extractions (12 samples/hr), and apparent ease of use 63 The disadvantages include the initial capital investment, lack of approval from government agencies, and the need to use solvents that are microwave compatible 63 Applications include those in the food and environmental fields. 

Chiba et al. [749] used atmospheric pressure helium microwave induced plasma emission spectrometry with the cold vapour generation technique combined with gas chromatography for the determination of methylmercuiy chloride, ethylmercury chloride and dimethylmercury in sea water following a 500-fold preconcentration using a benzene- cysteine extraction technique. 

Aminoquinolines 62 have been prepared in a two-step, one-pot, three-component reaction of 2-azidobenzophenones, secondary amines and arylac-etaldehydes [110]. The microwave-assisted reaction proceeded via the initial formation of enamines 59. Subsequent addition of 2-azidobenzophenones 60 afforded the triazoline intermediates 61, which underwent thermal rearrangement and cyclocondensation to furnish 2-aminoquinolines 62 (Scheme 41). Direct comparison with conventional thermal conditions demonstrated the superiority of microwave dielectric heating in terms of yields (73% vs. 31% of heterocycle 63 after 10 min at 180 °C). Furthermore, the formation of by-products due to decomposition of azide 60 was diminished in the microwave-assisted synthesis. Purification of the products was achieved using solid-phase extraction techniques. 

Combined with expedient purification techniques (e.g., scavengers, reagents on solid support, and solid phase extraction techniques) [ 14], microwave-assisted synthesis is leading the way towards genuine high-throughput chemistry that will hopefully ease the chemistry-related bottleneck in the drug development process. 

Lucchesi, M.E., Smadja, J., Bradshaw, S., Louw, W. and Chemat, F. (2007) Solvent free microwave extraction of Elletaria cardamomum L. a multivariate study of new technique for the extraction of essential oil. journal of Food Engineering 79, 1 079-1 086. 

Solvent-free microwave extraction (SFME) is a recently developed green technique, performed in atmospheric conditions without adding any solvent or water and being applied to the extraction of essential oil from fresh plant or dried materials. The essential oil is evaporated by the in situ water in the plant materials. Wang et al. 

A. M. Carro, R. A. Lorenzo, M. J. Vazquez, M. Abuin, R. Cela, Different extraction techniques in the preparation of methylmercury biological samples classic extraction, supercritical-Buid and microwave extraction, Int. Lab., 28 (1998), 23D28. 

A variety of solvent extraction techniques have been used to extract antioxidants from food matrices. The most commonly used is maceration or homogenization of the sample with an extraction solvent however, alternative procedures have been developed including pressurized fluid extraction (PFE), ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and matrix solid-phase dispersion (MSPD), among others. The principles of each extraction technique will be briefly discussed. 

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