Open-vessel microwave extraction systems

To date, little work has been cited in the literature with respect to arsenic speciation of polluted soil. A feasibility study on the identification and monitoring of arsenic species in polluted soil and sediment samples (Thomas etal. 1997) has been reported. In this study, polluted soil samples were extracted in phosphoric acid media using an open vessel microwave-assisted extraction system. The determination of arsenic species was investigated using an on-line system involving HPLC-ICP-MS system. The speciation was performed to identify As(III), As(V) and monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA). The proposed method had the potential to form the basis of a routine procedure for monitoring the behaviour of arsenic species in soils. This extraction procedure was recently applied to contaminated... 

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. 

Atmospheric MAE system This second technique employs solvents with low dielectric constants. Such solvents are essentially microwave-transparent they thus absorb very little energy, and extraction can therefore be performed in open vessels. The temperature of the sample increases during extraction because it usually contains water and other components with high dielectric constants the process is thereby enhanced. Because extraction conditions are milder, this mode of operation can be used to extract thermolabile analytes. 

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. 

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. 

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]. 

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]. 

Microwave-assisted extraction has also been used as a solid sample treatment prior to speciation analysis [264-266], leaving the organometallic compound moiety intact. This is a prerequisite for a successful extraction procedure to be applied prior to speciation analysis and can be met by careful optimization of the conditions of the microwave attack. Open-vessel treatment is preferred to pressurized bomb systems commonly used in the analysis for total metals because it offers milder reaction conditions — the increase in temperature is governed to a great extent by the boiling point of the solvent — and easier control of process variables [266]. 

Metals such as Fe, Cu, Zn, Pb, Hg, Se, As, Co, Cd, Cr and Ni have been extracted using an acid solution (usually HNO,) in a focused open-vessel system for periods ranging from 3 min for soil samples [68] to 50 min for coal (where metals are much more strongly retained) [46] the system of Fig. 5.8A allowed the sample to be brought into contact with fresh solvent in four cycles, which resulted in the relatively short extraction time achieved. One clear example of the dramatically reduced extraction times afforded by microwaves is the extraction of Pb, Zn and Cu from calcareous soil [277], where the sequential procedure used proved considerably faster than conventional extraction (2 h versus 20 h). 

The examples described in this section 10.6.1 constituted the first report ever of ill situ reaction-extraction work involving foodstuffs. We had reported earlier on a related approach in a different field, namely a derivatisation-extraction procedure whereby phenols and methylated phenols were acetylated-extracted from environmental matrices in a one-step MAP procedure (15). The latter procedure, however, was performed under much harsher conditions that could not be used with foodstuffs where the potential of creating artefacts is a prime concern. This approach of one-pot, multiple-step procedures opens the avenue to numerous applications of direct interest to the food analysts and are especially versatile and valuable when using microwave-assisted extraction performed in open-vessel systems. This, along with solvent-less extraction (such as MAP gas-phase applications ) is believed... 

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... 

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. 

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