Microwave-assisted extraction closed-vessel

Weichbrodt et reported on the use of focused open-vessel microwave-assisted extraction (EOV-MAE) for the determination of organochlorine pesticides in high-moisture samples such as fish. The results were comparable to those with closed-vessel microwave-assisted extraction (CV-MAE) and ASE. The main advantage of FOV-MAE is that the use of Hydromatrix is unnecessary as the solvent mixture of ethyl acetate and cyclohexane allows the removal of water from the sample matrix via azeotropic distillation. 

Table 3.26 Characteristics of closed-vessel microwave-assisted extraction...

FIGURE 11.33 Closed-vessel microwave-assisted extraction system (reprinted with permission from H. M. Kingston and L. B. lassie, eds.. Introduction to Microwave Sample Preparation, American Chemical Society, 1998). 

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. 

Most studies about the microwave-assisted extraction of PAHs from solid samples have been conducted using closed-vessel systems [12,214,226,236,239-246] and only a few with open-vessel focused microwave devices [57,247-252]. Because open-vessel systems operate at atmospheric pressure, the extraction vessel can be used as a reactor in order to perform on-line purification pretreatments of the total extracts (reagents can be readily added to the medium) [53] or directly introduce the extract into the determination instrument, as in the focused microwave-assisted extractor with on-line fluorescent monitoring of Fig. 5.10, which provides a matrix-independent approach to the extraction of PAHs [61]. 

In microwave assisted extraction (MAE), microwave energy accelerates the partition, i.e. the mass transfer of an analyte from a sample matrix into a solvent, by directly heating the solution. The extraction is performed at an elevated temperature in a closed vessel. The major benefits are the shorter extraction time, reduced consumption of organic solvents and increased sample throughput. However, there is a need for an additional filtration step and, if the extract is dilute, further concentration, e.g. by evaporation or SEE, may be needed before analysis. 

Microwave assisted wet digestion has attracted considerable attention and has been successfully applied to plant material. Both open and closed vessels have been used, but the most popular approach is the sealed bomb method (Kingston and Jassie, 1988 Sulcek and Povondra, 1989 Matusiewicz, 1991). Karanassios et al. (1991) describe microwave stopped flow digestion systems that can give rapid (ie, less than 5 min) reproducible extractions of elements of environmental concern from plant samples. 

In microwave-assisted solvent extraction (MASE), the sample and solvent are heated directly rather than in more conventional schemes where the vessel is heated to extract the sample. The sample solvents are placed into a closed vessel that does absorb microwaves. This facilitates the extraction of the samples... 

Li, K., J. M. R. Belanger, M. Uompart, R. Turpin, R. Singhvi, and J. R. J. Pare. 1996. Evaluation of Rapid Solid Sample Extraction Using the Microwave-Assisted Process (MAP) under Closed-Vessel Conditions, in Spectroscopy, vol. 13, pp. 1-13. 

Two main types of MAE systems are commercially available, in both cases agitation is provided during extraction to improve the mass transfer phenomenon. In closed extraction vessels, extraction is performed under controlled pressure and temperature. In focused microwave-assisted solvent extraction (FMASE), only a part of the extraction vessel containing the sample is irradiated with microwaves. Instrumental setups like Soxwave combine both the features of Soxhlet and the advantages of microwave, thus making extraction an even more attractive alternative to traditional solid—liquid extraction. 

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