Open-Vessel Microwave Devices

FIGURE 2.27 Focused microwave-assisted Soxhlet extractor. (Reprinted from Luque-Garcfa, J. L. and Luque de Castro, M. D., Trends Anal. Chem., 22, 90-98, 2003. With permission from Elsevier.) 

Eskilsson et al. ° have written an excellent review covering the theory and applications of closed vessels MAE up till the year 2000. More recently, Nobrega et al. have compiled another review, this time dealing with focused-microwave-assisted techniques. In the following sections a brief overview of some recent work is given, divided into three fields (analysis of air, water and solids), and more detailed information is given in Table 2.8 and Table 2.9. 

Every year regulatory institutions decrease the amount of various toxic species allowed in air. In air analysis, air-borne particles are collected using various types of filters and MAE has been utilized 

The superiority of microwave extraction compared to traditional extraction methods for the determination of polychlorinated biphenyl compounds in indoor air samples was also shown. Again a decrease in the extraction time was highlighted the microwave procedure needed only 10 min and, followed by GC-electron capture detection, was claimed as a valuable alternative to the Soxhlet method for the extraction of six noncoplanar PCBs associated with fly ashes. 

Another interesting application of microwave energy was reported by Mueller et al The authors studied the amount of benzene and alkylated benzenes (BTX) in ambient and exhaled air by microwave desorption coupled to GC-MS. Microwave desorption was proved as an effective sample preparation technique for the analysis of BTX in air samples. A similar desorption technique was applied for the GC analysis of nicotine in indoor air as well.  

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Usually, multi-mode systems use closed type vessels and focused systems use open vessels. /. Closed-Vessel Microwave Devices... 

Although microwave-heated organic reactions can be smoothly conducted in open vessels, it is often of interest to work with closed systems, especially if superheating and high-pressure conditions are desired. When working under pressure it is strongly recommended to use reactors equipped with efficient temperature feedback coupled to the power control and/or to use pressure-relief devices in the reaction vessels to avoid vessel rupture. Another potential hazard is the formation of electric arcs in the cavity [2], Closed vessels can be sealed under an inert gas atmosphere to reduce the risk of explosions. 

Dynamic systems for high-pressure microwave treatment were developed much later than open-vessel systems. Operating under a high pressure reduces the flexibility afforded by working at atmospheric pressure. However, some recently developed devices allow microwave-assisted high-pressure digestion and extraction in a dynamic manner [33,34]. 

As noted earlier, not all open-vessel systems (viz. those that operate at atmospheric pressure) are of the focused type. A number of reported applications use a domestic multi-mode oven to process samples for analytical purposes, usually with a view to coupling the microwave treatment to some other step of the analytical process (generally the determination step). Below are described the most common on-line systems used so far, including domestic ovens (multi-mode systems) and open-vessel focused systems, which operate at atmospheric pressure and are thus much more flexible for coupling to subsequent steps of the analytical process. On the other hand, the increased flexibility of open-vessel systems has promoted the design of new microwave-assisted sample treatment units based on focused or multi-mode (domestic) ovens adapted to the particular purpose. Examples of these new units include the microwave-ultrasound combined extractor, the focused microwave-assisted Soxhlet extractor, the microwave-assisted drying system and the microwave-assisted distillation extractor, which are also dealt with in this section. Finally, the usefulness of the microwave-assisted sample treatment modules incorporated in robot stations is also commented on, albeit briefly as such devices are discussed in greater detail in Chapter 10. 

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]. 

Safety features are essential to a microwave apparatus. An exhaust fan draws the air from the oven to a solvent vapor detector. Should solvent vapors be detected, the magnetron is shut off automatically while the fan keeps running. Each vessel has a rupture membrane that breaks if the pressure in the vessel exceeds the preset limit. In the case of a membrane rupture, solvent vapor escapes into an expansion chamber, which is connected to the vessels through vent tubing. To prevent excessive pressure buildup, some manufacturer use resealable vessels. A spring device allows the vessel to open and close quickly, releasing the excess pressure. 

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