Microwave source

V. Granatstein and I. Alexeff, eds.. High Power Microwave Sources, Artech House, Boston, Mass., 1991. 

It all started almost 60 years ago when P. Spencer, studying high-power microwave sources for radar applications, observed the melting of a chocolate bar in his pocket at least that is the story told. The first patent in this field was filed by him in 1946 and one year later the first commercial microwave oven appeared on the market. We had to wait until 1955 for domestic models, but by 1976 almost 60% of US households already had a microwave oven. 

The formal definition of this quality factor, Q, is the amount of power stored in the resonator divided by the amount of power dissipated per cycle (at 9.5 GHz a cycle time is l/(9.5 x 109) 100 picoseconds). The dissipation of power is through the resonator walls as heat, in the sample as heat, and as radiation reflected out of the resonator towards the detector. The cycle time is used in the definition because the unit time of one second would be far too long for practical purposes within one second after the microwave source has been shut off, all stored power has long been dissipated away completely. 

When the microwave bridge is in tune mode, the microwave source is at high voltage, and its guaranteed lifetime is ticking away (therefore, switch to off for a lunch break). 

A microwave power meter may perhaps be a handy gadget every now and then to check output levels of microwave sources. On the other hand, we have seen that power leveling is used to retain a constant output power over 10000 operating hours (many years), and if the unleveled power has dropped to below the level specification, then the source can be expected to leave for eternal hunting grounds any time now. 

The next question to address is to what extent does the study of a (deeply) frozen biological sample provide information that is relevant for an understanding of its functioning in a living cell at whatever the ambient temperature of this cell happens to be First and foremost, let us state the fact of experience that solutions of biomacromolecules such as metalloproteins can be frozen and thawed many times without any detectable deterioration of their biological activity. Combined with the rather low intensity (<0.2 W) of the microwave source of an EPR spectrometer, this leads to the proposition that EPR spectroscopy is a nondestructive technique. 

We have previously defined the relative dB scale in Equation 2.11. The power in EPR is expressed in decibels (dB) attenuation (or alternatively in -dB amplification) of a maximum value. X-band microwave sources (either klystrons or Gunn diodes) have a constant output that is usually leveled off at 200 mW. This value then corresponds... 

The basic features of an epr spectrometer are shown in Figure 2.95. The microwave source is a Klystron tube that emits radiation of frequency determined by the voltage across the tube. Magnetic fields of 0.1 — 1 T can be routinely obtained without complicated equipment and are generated by an electromagnet. The field is usually modulated at a frequency of 100kHz and the corresponding in-phase component of the absorption monitored via a phase-sensitive lock-in detector. This minimises noise and enhances the sensitivity of the technique. It is responsible for the distinctive derivative nature of epr spectra. Thus, the spectrum is obtained as a plot of dA/dB vs. 

Radiation is derived from a sealed quartz tube containing a few milligrams of an element or a volatile compound and neon or argon at low pressure. The discharge is produced by a microwave source via a waveguide cavity or using RF induction. The emission spectrum of the element concerned contains only the most prominent resonance lines and with intensities up to one hundred times those derived from a hollow-cathode lamp. However, the reliability of such sources has been questioned and the only ones which are currently considered successful are those for arsenic, antimony, bismuth, selenium and tellurium using RF excitation. Fortunately, these are the elements for which hollow-cathode lamps are the least successful. 

The microwave source used in this study was a microwave network analyzer model IFR 6845 shown in Fig. 15.2b (Microwave network analyzer). Integrated into this single instrument is a synthesized source, a three-input scalar analyzer, and a synthesized spectrum analyzer. Complete engineering details of this equipment is beyond the scope of this document, but the basic function of this instrument is to generate a constant... 

MICROWAVE SPECTROSCOPY. A type of adsorption spectroscopy used in instrumental chemical analysis that involves use of that portion of the electromagnetic spectrum hav ing wavelengths in the range between the far infrared and the radiofrequencies, i.e.. between 1 nun and. 111 cm. Substances to be analyzed are usually in the gaseous state. Klystron tubes are used as microwave source. 

This unit provides three protocols for which there are established procedures for various matrices. The Basic Protocol describes water removal and quantitation after a sample is placed in a convection oven. It is probably the method of choice when one does not know which method to choose when dealing with an unknown matrix, or when one looks at samples that foam excessively in the vacuum oven method or react, such as popcorn under vacuum. Alternate Protocol 1 describes water removal and quantitation after a sample is placed in a vacuum oven. Because it is at reduced pressure, drying times are slightly reduced compared to the convection method. In addition, drying temperatures < 100°C are possible, which is important for samples that may decompose at higher drying temperatures. Alternate Protocol 2 describes water removal using a microwave source where such analyzers measure and calculate loss automatically. 

The average relaxation time is of course temperature dependent and maybe related to a rate constant k for the relaxation process of the molecules in solution. Table 1.3 gives some representative data for EtOH and illustrates the extent to which the relaxation time decreases with temperature. It is noteworthy that the relaxation time decreases from 270 to 49 ps as the temperature rises from 10 to 70°C, and therefore, as the temperature increases the alcohol couples more effectively with the microwave source at 2.45 GHz. Such a situation is ripe for superheating the solvent, since the extent of conversion increases as the temperature rises. It also follows that some organic solvents with very long relaxation times at room temperature may appear to be unsuitable candidates for dielectric heating, but since the match becomes more favourable with temperature then they may behave as effective couplers as the temperature rises, that is, after a slow start they may very well heat very rapidly. 

Figure 10.2 is a schematic diagram of a helium MIP-MS system, with gaseous sample introduction, developed by the Caruso group. This is the most popular method of sample introduction to date for MIP-MS analysis as the MIP at low pressures is not tolerant to liquid samples. A commercial ICP-MS system may be modified by mounting an MIP discharge source in place of the ICP source. A Beenakker cavity is commonly used as the microwave source and serves to focus the microwave energy. Cavity construction and dimensions have been described in detail by Evans et al. [18]. 

The Fig. 2 experimental points result from the averaging of 20 pulses recorded for each frequency. The frequency scan Is performed randomly to avoid systematic drift effects. The width at half maximum of the resonance 120 kHz, corresponds to the theoretical limit due to the finite transit time across the waist of the microwave Gaussian beam. The observed Zeeman components have not the same weight. This is just related to the resonant 447 GHz microwave source whose polarization is partially elliptical. 

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