Energy, microwave

Table 6. Microwave Energy Absorption and the Factor of Merit for Various Types of Glasses and Glass-Ceramics...

The nature of potential exposure ha2ards of low level microwave energy continues to be investigated (116—118). In the United States, leakage emission from microwave ovens is regulated to the stringent limit of 5 mW/cm at 5 cm (119). There is no federal limit on emission from industrial systems but the IMPI has set a voluntary standard which specifies 10 mW/cm at 5 cm (120). Emission values are equivalent to personnel exposures at several meters, well below limits that had previously prevailed in eastern Europe. This conclusion, derived for microwave ovens, should be vaUd for all microwave systems (121). 

The dielectric properties of most foods, at least near 2450 MH2, parallel those of water, the principal lossy constituent of food (Fig. 1). The dielectric properties of free water are well known (30), and presumably serve as the basis for absorption in most foods as the dipole of the water molecule interacts with the microwave electric field. By comparison, ice and water of crystaUi2ation absorb very Httie microwave energy. Adsorbed water, however, can retain its Hquid character below 0°C and absorb microwaves (126). 

Waste Treatment. Microwave energy has been studied for the desulfurization of coal (qv) and treatment of wastes (190). Developments in microwave incinerators for medical and radioactive wastes have occurred (191,192). Even a consumer unit for consumption of sohd household waste has been proposed (193). Economic factors remain a key barrier in these developments. 

The use of infrared heat (gas-frred ceramics) or microwave energy to speed up the roasting process while providing a more even roast has also been patented (26,27). 

Figure 2. Penetration depth of microwave energy as a function of specific resistivity or conductivity of the semiconductor material for microwave radiation of 10 and 40 Gc/s.11...

Color mimicking by means of electrochemistry, 361 Completion of oxidation for polymers and diffusion control, 414 Concentration effects of microwave energy, 442... 

YOSHIDA H and kajimoto g (1989) Effects of microwave energy on the tocopherols of soybean seeds , J Food Sci, 54 (6) 1596-600. 

The intensity of the absorption of microwave energy is a measure the abundance of that isotope. The potency of the NMR spectroscopy is not only its ability to quantify the concentration of an isotope, but to check the enviromnent into which an isotope is embedded. This is possible because the magnetic resonance and thus the absorption frequency prove to be sensitive to the spins of neighboring atoms and to structural features of the probe. Therefore, NMR spectroscopy is more a tool for scientific structural analyses than for daily food (colorant) inspection. For a detailed study of the NMR techniques used in food science we recommend books by Macomber and Pochapsky. - ... 

MAE simply involves placing the sample with the solvent in specialized containers and heating the solvent using microwave energy. MAE is also sometimes called MASE, which can stand for microwave-assisted solvent extraction or microwave-accelerated solvent extraction. In any event, the extraction process is more rapid than Soxhlet extractions, can be run in batches, and reduces solvent consumption. As in the case of sonication, MAE may overcome retention of the analyte by the matrix, but analyte degradation can be a problem at higher temperatures in certain applications. 

The solvent, microwave energy applied, and extraction time selected are the main parameters controlled in MAE. The user should use proper extraction vessels and equipment in MAE because very high pressures can be achieved and explosions may result if appropriate precautions are not taken. 

This is a relatively new technique that is used for PCBs and other nonpolar, volatile and semi-volatile organic compounds. Typically, a small aliquot of soil sample (0.5-20 g) is used for the extraction. Soil samples are extracted with one or more organic solvents using microwave energy at elevated temperature (100-115 °C) and pressure (50-175 psi). This method uses less solvent and takes significantly less time than Soxhlet extraction but is limited to thermally stable compounds. 

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