Microwave-assisted distillers

P. Bermejo Barrera, M. Aboal Somoza, A. Bermejo Barrera, M. L. Cervera, M. de la Guardia, Microwave assisted distillation of iodine for the indirect atomic absorption spectrometric determination of iodide in milk samples, J. Anal. Atom. Spectrom., 16 (2001), 382-389. 

Munoz, D. Velez, M. L. Cervera, R. Montoro, Rapid and quantitative release, separation and determination of inorganic arsenic (As(III) + As(V)) in seafood products by microwave-assisted distillation and hydride generation atomic absorption spectrometry, J. Anal. Atom. Spectrom., 14 (1999), 1607-1613. 

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. 

Microwave-assisted distiller Microwave energy has also been used to facilitate the distillation of various compounds from solid samples [11,63-65]. Figure 5.12A depicts a typical microwave-assisted distillation apparatus. It consists of a laboratory-made poly-tetrafluoroethylene (PTFE) vessel of 120 ml inner volume and 10 mm wall thickness, and a 60 ml collector flask. The screw cap of the vessel includes a drilled PTFE disc located on its upper part that avoids sample ejection during distillation. A hole made in the vessel cap provides a means for evacuating volatile compounds via a PTFE tube that is passed through the vent holes of the microwave cavity and is then adapted to a glass line finished by a bubbler which is inserted in the collector flask. A domestic microwave oven equipped with a 2450 MHz magnetron is normally used for this purpose, altered only as required to connect the sample vessel with the collector flask. 

Fig. 5.12. (A) Microwave-assisted distillation set-up - C collector TL tansfer line V vessel WR water reservoir. (Reproduced with permission of Elsevier.) (B) Microwave-assisted distillation/ solid-phase extraction apparatus. The parts designated by capital letters are described in the text. (Reproduced with permission of the American Chemical Society.)...

Conte, E. D., Shen, C. Y., Perschbacher, P. W., and Miller, D. W. 1996. Determination of geosmin and methylisoborneol in catfish tissue (Ictalurus punctatus) by microwave-assisted distillation—solid phase adsorbent trapping, J. Agricul. Food Chem., 44 829-835. 

Fuge, R., Johnson, C. C. (1986).The geochemistry of iodine - a review. Environmental Geochemistry and EIealth,Yo. 8, No. 2, pp 31-54, ISSN 0269-4042 Gamallo-Lorenzo,D., Barciela-Alonso, M. C., Moreda-Pineiro, A., Bermejo-Barrera, A. Bermejo-Barrera, P. (2005). Microwave-assisted alkaline digestion combined with microwave-assisted distillation for the determination of iodide and total iodine in edible seaweed by catalytic spectrophotometry. Analitka Chimica Acta, Vol. 542, pp 287-295, ISSN 0003-2670... 

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. 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

Microwave-assisted extractions (MAE) can be performed in open (focused MAE) or closed (pressurized MAE) flasks. This technique is commonly used for extractions from complex and difficult sample matrices, replacing time- and solvent-intensive Soxhlet extractions or hydrodistillations.46 MAE is also widely applied to environmental samples, for example, for extracting polycyclic aromatic hydrocarbons (PAH) from soil, methylmercury from sediments, and trace metals and pesticide residues from plant material47 48 The use of microwave treatment instead of hydrodistillation offers a solvent-free separation technique essential oils are heated and dry-distilled 46... 

During, R.A., X. Zhang, H.E. Hummel, J. Czynski, and S. Gath. 2003. Microwave-assisted steam distillation with simultaneous liquid/liquid extraction of pentachlorophenol from organic wastes and soils. Anal. Bioanal. Chem. 375 584—588. 

Before applications are dealt with, the main variables governing microwave-assisted processes and the parameters characterizing specific microwave treatments are examined. The applications discussed include not only microwave-assisted digestion and extraction — which are the two most widely implemented and hence those with the highest potential interest to readers — but also others of special significance to solid sample treatment such as microwave-assisted drying, distillation and protein hydrolysis. Finally, some safety recommendations on the use of microwave equipment are made. 

Finally, other salient uses of microwaves for treating solid samples such as microwave-assisted drying, distillation and protein hydrolysis are also briefly described. 

Extraction of essential oils is one of the most time- and effort-consuming processes in the analysis of the constituents of plants. Various extraction methods were traditionally employed, depending on the material or the available devices. The most commonly used methods are steam distillation and distillation-solvent extraction. The introduction of innovative extraction methods, such as microwave-assisted extraction (MAE) and supercritical fluid extraction (SEE), has led to significant improvement, not only in the analytical performance, but also in the accuracy and reproducibility of methods. 

Many investigations have been carried ont of procednres for improving the analytical qnality of GC methods by changing the matrix, increasing the concentration of the pertinent analytes and redncing the interference of other componnds present in the sample. Preconcentration by LLE, before or after derivatization, is most freqnently apphed in GC trace analysis however, other techniqnes, snch as SPE, sample stacking (see Section V.A.l) and some of their modifications, snch as simnltaneons distillation and extraction (SDE) and SPME, are also mentioned. Application of microwave-assisted processes (MAP) dnring sample preparation seems to improve recoveries. 

Despite the often large increase in the reaction rate the use of microwave-assisted reactions has still not been implemented on an industrial scale. One of the main barriers for industrial applications is reliable scale-up of microwave reactors [116], but there are also other engineering problems that have to be solved. The use of microwaves to speed-up distillation processes has also been indicated [123]. 

Treatment of -lonone (13) in Water. Traditional methods for the dehydration of a- and p-ionones to give ionene (14) have involved catalysis by hydriodic acid along with small amounts of phosphorus, or distillative heating in the presence of 0.5% iodine (18). This latter procedure is now known to co-produce about 10% 1,1,6-trimethyl-1,2-dihydronaphthalene (19). However, we have found that the cyclization could be effected by merely heating p-ionone in water at 250 °C for twenty minutes in the MBR. The microwave-assisted reaction was not optimized, and proceeded with only moderate conversion. Unlike in the literature method (18), however, the work-up did not require exhaustive washing procedures. 

Numata, M., Yarita, T., Aoyagi, Y., and Takatsu, A., Microwave-assisted steam distillation for simple determination of polychlorinated biphenyls and organochlorine pesticides in sediments, Anal. Chem., 75, 1450-1457, 2003. 

FIGURE 22.9 Principle of the microwave-assisted steam distillation. (From Japenga, J., Wagenaar, W. J., Smedes, F., and Salomons, W., Environ. Technol. Lett., 8, 9-29, 1987. With permission.)... 

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