Microwave systems focused

A microwave-heated, flow-through digestion container (coiled Teflon tubing) was design for a commercial (Prolabo A300) focused microwave system (instead of microwave oven) and applied to the on-line preparation of biological samples, including milk, blood, and urine [108]. 

Matusiewicz, H. Development of a high-pressure asher focused microwave system for sample preparation. Anal. Chem. 71, 3145-3149 (1999)... 

Santelli, R.E., Bezerra de Almeida, M., de SantAna, O.D., Cassella, R.J., Eerreira, S.L.C. Multivariate technique for optimization of digestion procedure by focused microwave system for determination of Mn, Zn and Ee in food samples using EAAS. Talanta 68, 1083-1088 (2006)... 

Fig. 5.3. Scheme of a focused microwave system. (Reproduced with permission of Springer-Verlag.)... 

In some commercially available focused microwave systems, the sample is placed directly in the vessel [45] or in an extraction cartridge that is in turn placed in the vessel [43]. Figure 5.8 depicts two such systems. In the example of Fig. 5.8A, the microwave system was not coupled to a subsequent step of the analytical process rather, a dynamic... 

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. 

Matusiewicz [63,68] has reported the development of a high-pressure, high-temperature, focused-microwave-heated acid (HNO3) digestion system. This microwave technique requires only about 3 % of the time necessary for the thermal high-pressure (HPA) technique. The technique of microwave heating samples in sealed containers to speed up acid digestion has been in widespread use for the past few years [69,70]. 

With the exception of GC-MIP-AES there are no commercial instruments available for speciation analysis of organometallic species. Recently, a prototype automated speciation analyser (ASA) for practical applications was described [544,545], which consists of a P T system (or focused microwave-assisted extraction), multicapillary GC (MC-GC), MIP and plasma emission detection (PED). MCGC-MIP-PED provides short analysis times ([Pg.676]

Hetero-Diels-Alder reactions provide an attractive means of rapidly constructing complex heterocyclic ring systems. Cycloaddition of pyrazolyl imines with a variety of electron-deficient dienophiles has been used to assemble pyrazolo[3,4,1 ]pyridines in a focused microwave reactor under solvent-free conditions53. The reactions proceeded in modest to excellent yield, depending upon the choice of diene and dienophile (Scheme 3.32). 

Ramon, R., Valero, R, and del Valle, M. 2003. Rapid determination of chemical oxygen demand using a focused microwave heating system featuring temperature control. 

The hetero-Diels-Alder reaction is amongst the most efficient processes for the synthesis of six-membered heterocyclic ring systems. Solvent-free conditions have been used to improve reactions of heterodienophiles and heterodynes with low reactivities. Cado et al. (1997) have described the hetero-Diels-Alder reaction of ethyl lH-perimidine-2-acetate as heterocyclic ketene aminal with ethyl propiolate nnder solvent-free conditions with focused microwave irradiation. The new fused perimi-dines (23) were obtained in good yields (67-98%). 

Krushevska, A., Barnes, R.M., Amarasiriwaradena, C.J. Decomposition of biological samples for inductively coupled plasma atomic emission spectrometry using an open focused microwave digestion system. Analyst 118, 1175-1181 (1993)... 

The analytical pervaporator can be used in combination with a flow-injection manifold, either in the upper chamber when the pervaporated species must be derivatized for adaptation to the detector and/or in the lower chamber for the pervaporation of analytes from liquid samples or slurries. Alterations of either the auxiliary dynamic manifold or the pervaporator itself are required when the pervaporation step is assisted by focused microwaves, the separation step assists in the continuous monitoring of an evolving system, untreated solid samples are used or pervaporation is integrated with detection. 

Pervaporation of the analytes. An amount of ca. 0.5 g of sample was placed in the pervaporator s donor chamber, which was then closed, connected to the system and placed either in a water bath or in a vessel of a focused microwave device depending on the polar or non-polar nature of the target analytes. These were evaporated into the gas layer above the sample and then diffused through the membrane to the argon acceptor stream. 

Even though the principle of focused microwaves is efficient in terms of energy transfer, it has allowed for the use of only one flask at a time in most designs. Automation in some systems can enable the sequential use of flasks, but this characteristic can be viewed as a constraint. One recent development involves the use of four flasks at a time by symmetrically splitting the microwave energy among the flasks at the end of each waveguide. 

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