Microwave-assisted extraction closed-vessel systems

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]. 

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 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. 

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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