Microwave EDLs

3 Microwave EDLs. In the microwave EDLs the pressure of the inert gas must be about 0.13 kPa when the temperature is 480 to 680 K. The material placed in the tube is pure metal or its chloride or iodide. The addition of 1 to 2 mg of mercury or saturation of the tube by mercury vapour prevents the adsorption of the metal, and hence improves the radiation properties of the lamp and adds to its lifetime. The intensity of the radiation is controlled by the temperature of the lamp, because the vaporization of metal is mainly thermally induced. Therefore, the microwave EDL must be temperature regulated by a thermostat in order to make the intensity stable enough for analytical use. 

These lamps are available for about 50 elements, but the best of them are prepared for arsenic, antimony, bismuth, selenium, and tellurium. On the other hand, the hollow cathode lamps of these elements have the reputation of being the worst lamps which means the hollow cathode lamps and EDLs complement each other. 

Fig ure 23 Construction of an electrodeless discharge lamp (Perkin Elmer Corp.) 

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UV radiation, certainly not sufficient to disrupt the bonds of common organic molecules. We therefore assume that, essentially, photoinitiation is responsible for a chemical change and MW radiation subsequently affects the course of the reaction. The objective of microwave photochemistry is frequently, but not necessarily, connected to the electrodeless discharge lamp (EDL) which generates UV radiation when placed in the MW field. 

The spectral characteristics of EDL are of a general interest in microwave photochemistry. The right choice of filling material can provide a desirable ultraviolet radiation. Atomic fills usually furnish line emission spectra (e.g. that of an Hg-EDL is... 

The microwave photochemical reactor is an essential tool for experimental work in this field. Such equipment enables simultaneous irradiation of the sample with both MW and UV/VIS radiation. The idea of using an electrodeless lamp (EDL), in which the discharge is powered by the MW field, for photochemistry was bom half a century ago [46, 68]. The lamp was originally proposed as a source of UV radiation only,... 

Cirkva and Hajek have proposed a simple application of a domestic microwave oven for microwave photochemistry experiments [86]. In this arrangement, the EDL (the MW-powered lamp for this application was specified as a microwave lamp or MWL) was placed in a reaction vessel located in the cavity of an oven. The MW field generated a UV discharge inside the lamp that resulted in simultaneous UV and MW irradiation of the sample. This arrangement provided the unique possibility of studying photochemical reactions under extreme thermal conditions (e.g. Ref. [87]). 

Fig. 14.5 A modified MW oven for microwave photochemistry experiments. A. magnetron, B. reaction mixture with the EDL and a magnetic stir bar, C. aluminum plate, D. magnetic stirrer, E. infrared pyrometer, F. circulating water in a glass tube, G. dummy load inside the oven cavity [88]. With permission from Elsevier Science.

Fig. 14.6 Photochemistry in a microwave oven (the EDL floats on the liquid surface).

A microwave-assisted, high-temperature, and high-pressure UV digestion reactor has been developed by Florian and Knapp [44] for analytical purposes. The apparatus consists of the immersed electrodeless discharge lamp operating as a result of the MW field in the oven cavity (Fig. 14.8). An antenna fixed to the top of EDL enhanced the EDL excitation efficiency. Another interesting MW-UV reactor has... 

Simultaneous UV and MW irradiation of the sample Possibility of performing photochemistry at high temperature Good photochemical efficiency — the EDL is inside the sample Simplicity of the experiment and the low cost of the EDL Use of a commercially available microwave oven Wireless EDL operation... 

For some elements such as arsenic and selenium, which have their main atomic absorption wavelengths lying on the edge of the vacuum UV, the performance of hollow cathode lamps is often poor, the lamps displaying low intensity and poor stability. This, plus the search for more intense sources for AFS (see Chapter 1, section 10), resulted in the development of microwave-powered electrodeless discharge lamps (EDLs) as spectral line sources towards the end of the 1960s.3-5... 

Figure 9 Effect of increasing applied microwave power upon the source intensity (U) and zinc atomic fluorescence intensity ( ), using a vacuum-jacketed zinc EDL source...

Klan [56] described the photo-Fries rearrangement of phenyl acetate (37) under the action of microwaves and when irradiated with an electrodeless discharge lamp (EDL) (Chapter 19 of this book). The reaction provides two main products, 2- and 4-hydroxyacetophenone (Scheme 5.13 38 and 39, respectively). Product distributions are given in Table 5.3. 

The construction of microwave-excited EDL is relatively straightforward but there are several operating conditions in their preparation which must be considered to produce an intense light source. The desired characteristics and requirements for EDL are high intensity, high stability, long lifetime, and, to a lesser extent, low cost and high versatility. In practice, it is very difficult to meet all these characteristics simultaneously. 

The dimensions and properties of the lamp envelope are based on the discovery that the volume of Hg is critical for effective UV operation [50]. Higher Hg pressures result in the need to use higher microwave power levels. To focus the MW field efficiently into the EDL, a special Cd low-pressure lamp with a metal antenna (a molybdenum foil) was developed for experiments in MW-absorbing liquids [51]. The envelope material must be impermeable to gases, an electrical insulator, and chemically resistant to the filling compounds at the temperature of operation. High quality quartz is the most widely used lamp envelope material but early manufacturers of EDL used glass, Vycor, or Pyrex [52]. 

Microwave energy is widely used for excitation of EDL because it is usually more efficient than radiofrequency energy for generation of intense light. Microwave radiation for excitation of gas discharges is usually generated by use of a fixed-frequency (2.45 GHz) magnetron oscillator. 

Fig. 19.4. A vacuum system for manufacture of EDL 1, rotary vacuum pump 2, mercury manometer 3, tilting-type McLeod pressure gauge 4, EDL blank 5, modified microwave oven ...

The spectral characteristics of EDL are of general interest in microwave-assisted photochemistry experiments. The right choice of EDL envelope and fill material can be very useful in planning an efficient course of the photochemical process without the need to filter out the undesirable part of the UV radiation by use of other tools, for example glass or solution filters or monochromators [59, 60]. 

Gunning, Pertel, and their coworkers reported the photochemical separation of mercury isotopes [92-95] in a flow reactor which consisted of a microwave-operated discharge lamp [52, 96] cooled by a flowing film of water. A filter cell and a circulation system, to prevent heating of the filter solution and the cell, were placed concentrically and coaxially with the lamp. A similar reactor, for small-scale laboratory photolysis of organic compounds in the solution or gas phase, has been proposed by Den Besten and Tracy [91]. In this arrangement the EDL was placed in a reaction solution and was operated by means of an external microwave field from a radio or microwave-frequency transmitter (Fig. 19.11). The quantum output of the lamp was controlled by changing the output of the trans-... 

The use of a domestic microwave oven appeared in a patent [97], according to which gaseous reactants were irradiated with microwave and UV-visible radiation to produce desired photoproducts (the EDL was positioned inside the MW cavity, although outside the reaction vessel). Several similar reactors have been proposed for UV sterilization [98-100] or for treatment of waste water containing organic pollutants [101-103]. 

Cirkva and Hajek have studied the photochemically or microwave-induced addition of tetrahydrofuran to perfluorohexylethene (Scheme 19.2) [105]. Whereas the thermal reaction was too slow, photochemical activation was very efficient, with no apparent thermal effects of MW radiation. Combined UV and MW radiation (Fig. 19.12) has principally been used to initiate EDL operation in the reaction mixture. Another illustration of the MW-UV-assisted reaction has been demonstrated by Niichter et al. [22] on dehydrodimerization reactions of some hydrocarbons. 

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