Closed-vessel microwave extraction systems

A closed vessel microwave digestion system (MLS-1200 mega, Mileston s.r.l, Italy) was used for the extraction of the standard-spiked PUF and filter samples... 

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. 

Generally, two set-ups for conducting MAE are applied in laboratories closed vessels systems allowing for temperature and pressure adjustment and control, and open vessel systems for procedures carried out under atmospheric pressure. In the open vessel systems, maximal temperature is determined by the temperature at which the extractant boils. In those systems, absorption of microwave radiation occurs in the whole sample therefore, heating is effective and homogeneous. The main disadvantage of open vessel systems is the possibility of volatile compound loss. This can be reduced by application of a reflux system fitted into the top of the extraction vessel. 

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]. 

However, the use of domestic microwave ovens can pose serious hazards in the application of microwave energy to flammable organic compounds (such as solvents). For that reason, ad hoc commercial devices were developed using only the 2450 MHz frequency. Noticeable contributions to expand the use of microwaves as an alternative to conventional methods have been reported by Lopez-Avilla. The authors used a MES-1000 microwave sample extraction system (GEM Corp., Matthews, NC) as shown in Figure 22.7. It uses the technology with closed vessels under... 

Microwave energy may be applied to samples using either of two technologies closed vessels (under controlled pressure and temperature) or open vessels (at atmospheric pressure).These two technologies are commonly termed pressurized MAE (PMAE) or focused MAE (EMAE), respectively. Whereas in open vessels the temperature is limited by the boiling point of the solvent at atmospheric pressure, in closed vessels the temperature may be elevated by simply applying the correct pressure. The latter system seems more suitable in the case of volatile compounds. However, with closed vessels, one needs to wait for the temperature to decrease after extraction before opening... 

On the other hand, MAE affords the opportimity of performing several simultaneous extractions with a closed-vessel system (pressurized MAE). The cell containing the sample and the liquid solvent is subjected to microwave radiation that enables instantaneous and efficient heating in the presence of... 

The first completely re-engineered laboratory-focused microwave system was introduced by Prolabo in 1986. Most commercial open-vessel microwave systems manufactured since then are of the focused-microwave type, i.e., they use the waveguide as a single-mode cavity. Since their introduction, they have widely been used for sample extraction, substituting in most cases the closed-vessels systems, which as of now are used mainly for carrying out sample digestions. 

The basic components of a microwave system include a microwave generator (magnetron), a waveguide for transmission, a resonant cavity, and a power supply. For safety and other reasons, domestic microwave ovens are not suitable for laboratory use. There are two types of laboratory microwave units. One uses closed extraction vessels under elevated pressure the other uses open vessels under atmospheric pressure. Table 3.12 lists the features of some commercial MAE systems. 

A common commercial closed system is the Microwave Accelerated Reaction System (MARS ) 5, as supplied by the CEM Corporation, USA (Figure 5.7). This system allows up to 14 extraction vessels (XP-1500 Plus ) to be irradiated simultaneously. In addition, other features include a function for monitoring both pressure and temperature, and most notably, the system is equipped with... 

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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