Radio frequency principles

The principle of the ENDOR method is illustrated in Fig. 1. It refers to the most simple spin system with an electron spin S = 1/2 and a nuclear spin I = 1/2 for which an isotropic hf interaction, aiso, is considered. In a steady state ENDOR experiment4, an EPR transition (A, D), called the observer, is partly saturated by microwave radiation of amplitude B while a driving rf field of amplitude B2, called the pump, induces nuclear transitions. At frequencies vj and v2, the rf field tends to equalize the populations within the ms-states. This alters the degree of saturation of the observer so that, in the display of the EPR signal height versus the radio frequency, two ENDOR lines at transition frequencies vj = aiso/2 - vn (A, B) and v2 = ais0/2 + v (C, D) will be observed (v = / NgnBo denotes the nuclear Zeeman frequency for a static field B0). 

Fig. 3. Principle of linear accelerator (linac). Partially accelerated electrons from a source, such as a Cockcroft-Walton generator, arc further accelerated by stages as rhe electrons pass through radio-frequency cavities, powered by if oscillators. Each paiticle receives a small push as it passes from one cavity to the next until the final desired accelerated beam is produced Tile machine must be carefully synchronized CSG = Cockcroft-Walton generator, RFO = radio-frequency oscillator RFC = radio-frequency cavity...

Storage has mainly been achieved in four types of traps (11 the radio frequency or Paul trap (2) the Penning trap (3) the Kingdon electrostatic trap and (4) the magnetostatic (magnetic bottle) trap. The principles, advanlages, and disadvantages of these traps are detailed by DJ. Wineland (Science, 226, 395-400, Oct. 26, 1984). 

The basic set-up and compounds of an ICP-AES and ICP-MS are shown in Fig. 2. The ICP part is almost identical for AES and MS as detection principle. The ICP torch consists of three concentric quartz tubes, from which the outer channel is flushed with the plasma argon at a typical flow rate of 14 1 min-1. This gas flow is both the plasma and the cool gas. The middle channel transports the auxiliary argon gas flow, which is used for the shape and the axial position of the plasma. The inner channel encloses the nebulizer gas stream coming form the nebulizer / spray chamber combination. This gas stream transports the analytes into the plasma. Both the auxiliary and the nebulizer gas flow are typically around 1 1 min-1. The plasma energy is coupled inductively into the argon gas flow via two or three loops of a water-cooled copper coil. A radio frequency of 27.12 or 40.68 MHz at 1-1.5 kW is used as power source. The plasma is... 

Similar to the principles of optics, materials interact with radio frequency and microwave radiation in three ways they reflect radiation impinging upon them, they transmit radiation, and finally absorb some of the energy which is being transmitted through them. 

The precessional motion can be maintained by a suitable radio frequency field superimposed on the steady field. For example, in Fig. 9.38(b), when a steady field Hz is applied along the z axis and a radiofrequency field //,., is applied in the x-y plane and rotates in the same sense and at the same frequency as the precession, resonance occurs. Gyromagnetic resonance as outlined above is in principle the same as ferrimagnetic resonance referred to earlier (Section 9.3.1), except that in the former case the material is magnetically saturated by a strong applied field. In practice the steady field, which determines the Larmor frequency, is made up of the externally applied field, the demagnetizing field and the anisotropy field, and is termed the effective field He. Figure 9.39 shows the He values at which resonance occurs in some of the important communications and radar frequency bands. 

FIGURE 9.23 Radio frequency encoding basic principles. 

This laboratory long ago devised [120] the use of radio-frequency dielectric spectroscopy [121, 122] for the on-line and real-time estimation of microbial and other cellular biomass during laboratory and industrial fermentations. The principle of operation is that only intact cells (see [123] for what is meant in this context by the word viable ), and nothing else likely to be in a fermentor, have intact plasma membranes and that the measurement of the electrical properties of these membranes allows the direct estimation of cellular biomass (Fig. 4). 

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