Radio frequency field

Very slow exchange. Slow exchange means that the lifetime ta = tb in each site is very long. Thus, a nucleus in site A precesses many times, at frequency (vq i a) in the rotating frame, before it leaves site A, and similarly for a nucleus in site B. Thus, there is time for absorption of energy from the radio-frequency field ffi, and resonance peaks appear at Va nd Vb in the laboratory frame. 

FIG. 18 Schematic representation of an NMR experiment. A sample is placed in a probe containing a radio frequency coil, which is located in a strong applied magnetic field, B0. The coil is tuned to resonate at the frequency of the applied radio frequency field, / ,. 

Roussy, G. and Pearce, J.A. 1995. Foundations and Industrial Applications of Microwaves and Radio Frequency Fields . Wiley, Great Britain. 

Figure 6.3. Levitation of a molten metal in a radio-frequency field. The coil consists of water-cooled copper tubes. The counter winding above the sample stabilizes levitation. The same coils (and possibly additional ones) act as the induction heater. This technique has been applied to container-less melting and zone refining of metals and for drop calorimetry of liquid metals. It can be also used to decarburize and degas in ultrahigh vacuum mono-crystalline spheres of highly refractory metals (adapted from Brandt (1989)). The arrows indicate the instantaneous current flow directions in the inductors.

Solid state NMR spectroscopy is used to determine molecular structures by analyzing the static and dynamic features of the material. In NMR experiments, both a magnetic field and a radio frequency field are applied to a solid sample or a solution resulting in an absorption of energy, which is detected as an NMR. Spectrometers are also available for high resolution solid state NMR. Nuclei in... 

The rehydrated samples were obtained by exposing dehydrated samples to water vapor at least three days over saturated NH4GI solution at room temperature. A duraction of 0.5 s between scans were allowed for nuclear spin to recover to their equilibrium magnetization. The one—dimensional Na NMR spectra were recorded by using the spin—echo technique. The strength of the radio-frequency field for the two dimensional nutation experiments was 80 kHz and 128 ti values were used (0 250 /is). Each two- dimensional experiment took about 12 hours of spectrometer time. 

The cross-sectional view of an inductively coupled plasma burner in Figure 21-12 shows two turns of a 27- or 41-MHz induction coil wrapped around the upper opening of the quartz apparatus. High-purity Ar gas is fed through the plasma gas inlet. After a spark from a Tesla coil ionizes Ar, free electrons are accelerated by the radio-frequency field. Electrons collide with atoms and transfer their energy to the entire gas. maintaining a temperature of 6 000 to 10 000 K. The quartz torch is protected from overheating by Ar coolant gas. 

The assignments are made through a battery of techniques to identify the through-bond and through-space interactions. Multidimensional NMR spectra are taken in which the sample is irradiated by two radio frequency fields in... 

Fig. i.4. (a) Action of a radio-frequency field Bl on a nucleus precessing about direction B0. (b) NMR signal (Av1,2 is the half-maximum intensity line width). 

Continuous irradiation by the radio frequency B1 would soon cause all nuclei to precess against B0, and no further absorption of energy would occur, if there were no processes at work to restore the energetically favored orientation of the spins. In fact, energy absorption from radio frequency fields due to NMR is observed for long periods if the rf power is not too high. The processes responsible are referred to as relaxation. 

This relation tells us that, at resonance, the magnetization vector M precesses about the field vector Bt i of the radio-frequency field (Fig. 1.6(b)). 

In the conventional NMR experiment, a radio-frequency field is applied continuously to a sample in a magnetic field. The radio-frequency power must be kept low to avoid saturation. An NMR spectrum is obtained by sweeping the rf field through the range of Larmor frequencies of the observed nucleus. The nuclear induction current (Section 1.8.1) is amplified and recorded as a function of frequency. This method, which yields the frequency domain spectrum f(ai), is known as the steady-state absorption or continuous wave (CW) NMR spectroscopy [1-3]. 

Figure 4. Schematic explanation of T] and T2 relaxation phenomena. The equilibrium macroscopic magnetization vector, MCi is tipped away from the direction of the magnetic field (z-axis) by application of a radio frequency field. After the rf field is removed, it continues to rotate in x y -plane about the z -axis. Two relaxation processes occur recovery of the magnetization Mz (component along the z-axis) to equilibrium value M and decay of Mxy (magnetization in the x y -plane) to zero due to the loss of phase coherence.

Humbert, F. (2001). Potentials of radio-frequency field gradient NMR microscopy in environmental science. /. Indust. Microbio. Biotech. 26,53-61. 

In a nuclear magnetic resonance (NMR) experiment the magnetic moments of the nuclei are manipulated by suitably chosen radio frequency fields, resulting in a so-called spin-echo signal. The amplitude of this signal is proportional to the number of nuclei excited by the radio frequency field. NMR is a magnetic resonance technique, where the resonance condition for the nuclei is given by ... 

In this equation / is the frequency of the radio frequency field, 7 is the gyro-magnetic ratio (7 = 42.6 MHz/T for 1H, 11.3 MHz/T for 23Na and 4.2 MHz/T for 35C1) and B0 is the externally applied static magnetic field. Because of this condition the method can be made sensitive to one type of nuclei and therefore to hydrogen (and thus to water), sodium or chloride. Because the sensitivity of chloride is very low this ion was not considered in the present study. 

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