Applicator, multi-mode

Microwave heaters. Increasing interest is being shown towards applications in chemistry of microwave heating, both for solution and solid-state chemistry. Domestic ovens are so-called multi-mode instruments in which the microwaves are reflected by the walls of the cavity. This kind of equipment can irradiate several vessels in a cavity, whereas in a single-mode instrument there is one vessel at a fixed distance from the radiation source. 

Negishi, K. and Ri, H. U. (1987). Propagation of multi-mode ultrasonic pulses in non-destructive material evaluation. In Ultrasonic spectroscopy and its application to materials Science (Ed. Y. Wada), pp. 70-4. Ministry of Education, Science, and Culture, Japan. [100,102]... 

Sherar, M. D., Noss, M. B., and Foster, F. S. (1987). Ultrasound backscatter microscopy images the internal structure of living tumour spheroids. Nature 330,493-5. [174] Shimada, H. (1987). Propagation of multi-mode ultrasonic pulses in non-destructive material evaluation. In Ultrasonic spectroscopy and its application to Materials science (ed. Y. Wada), pp. 50-6. Ministry of Education, Science and Culture, Japan. [148] Shotton, D. M. (1989). Confocal scanning optical microscopy and its applications for biological specimens. J. Cell. Sci. 94,175-206. [177,200]... 

This chapter describes the principal applications of microwaves to the pretreatment of solid samples, with special emphasis on digestion and extraction, which are their two main uses in analytical chemistry. The description is preceded by a discussion of the fundamentals of microwave energy and its interaction with matter, and also of the equipment used by analytical laboratories, which can be of the open or closed type depending on whether they operate at atmospheric pressure or a higher level and whether they use multi-mode or focused microwaves. Selected designs developed for specific purposes are also commented on. 

The applicator is the zone in which microwaves are applied to the sample. It can be a multi-mode cavity where microwaves are randomly dispersed or the waveguide itself, in which case the sample vessel is placed directly inside it to focus the microwave radiation onto the sample. 

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. 

The application of microwave energy to organic compounds, such as solvents, can pose serious hazards. The hazard is greater with flammable Uquids. Two main types of apparatus are available commercially. They make use of singlemode or of multi-mode technology. The readers are invited to review commercial literature from CEM Corp. [9] and Prolabo [10], currently the only two manufacturers that offer legitimate MAP instrumentation. 

The present conjugated function, characteristic of a multi-mode chemical transducer, may be applicable for dual-mode memory One is a "deep" memory mode (hydroquinone-type photochromism, hard to transform) and the other is a "shallow" memory mode (quinone-type photochromism, easy to transform). 

Microwave applicators may appear in many different shapes and dimensions and in fact their design is critical to microwave processing since within applicators microwave energy is transferred (coupled) to processed material. Most common microwave applicators belong to three main types multi-mode cavity, single-mode cavity including TEM structures, and travefing wave applicators. 

The simplest applicator is a rectangular metal box that can accommodate the target load. When microwaves are launched into such a box via a waveguide launcher, the waves undergo multiple reflections from the walls. The reflected waves interfere and establish a distribution of electrical held within the internal space (including the load) which, within the band of frequencies, correspond to many different stable modes of oscillations. And for this reason it is called a multi-mode applicator. The most common example of such a device is a cavity in domestic microwave oven. 

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