Radiofrequency transmitter

The essential features of an NMR spectrometer shown m Figure 13 5 are not hard to understand They consist of a magnet to align the nuclear spins a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one a receiver to detect the absorption of rf radiation and a recorder to print out the spectrum... 

A pulse is a burst of radiofrequency energy that may be applied by switching on the Rf transmitter. As long as the pulse is on, a constant force is exerted on the sample magnetization, causing it to process about the Rf vector. 

The basic components of the solid state spectrometer are the same as the solution-phase instrument data system, pulse programmer, observe and decoupler transmitters, magnetic system, and probes. In addition, high-power amplifiers are required for the two transmitters and a pneumatic spinning unit to achieve the necessary spin rates for MAS. Normally, the observe transmitter for 13C work requires broadband amplification of approximately 400 W of power for a 5.87-T, 250-MHz instrument. The amplifier should have triggering capabilities so that only the radiofrequency (rf) pulse is amplified. This will minimize noise contributions to the measured spectrum. So that the Hartmann-Hahn condition may be achieved, the decoupler amplifier must produce an rf signal at one-fourth the power level of the observe channel for carbon work. 

Ten years ago, most nmr spectrometers operated for protons with radiofrequency (rf) transmitters set at 60 MHz (6 x 107 cycles per sec) but there has been a proliferation of different proton-operating frequencies and now 30, 60, 90, 100, 220, 270,300, and 360 MHz machines are commercially available. The cost of these machines is roughly proportional to the square of the frequency, and one well may wonder why there is such an exotic variety available and what this has to do with the chemical shift. High operating frequencies are desirable because chemical shifts increase with spectrometer frequency,... 

In c.w.-n.m.r. spectroscopy, a relatively weak, but rapidly oscillating, magnetic field is produced on the x axis by the application of a continuous, low-powered radiofrequency (r.f.) to the transmitter coil(s). As this radiofrequency approaches the resonance frequency, the magnetization vector is very slightly tipped out of the z axis, and precesses about this axis. When this frequency of precession is matched by the r.f. applied (the resonance condition), some of the individual, nuclear moments undergo transitions to the less-stable energy-level represented by precession about the — z direction, accompanied by absorption of energy from the transmitter coil. 

The first radiofrequency (RF) pulse is at the proton NMR frequency and is of a duration tp chosen for a 90° displacement of the magnetization vector away from the static field. At the end of tp, the proton transmitter is left on but the oscillation is phase-shifted through 90°. The RF field now oscillates in synchronization with precession of the proton magnetization vector instead of displacing the vector further from the static field, it counteracts any tendency for the vector to drift away from synchronized precession. The magnetization is then said to be spin locked . 

Special effects may be routinely and elegantly created by using sources of radiofrequency energy in addition to the observation frequency (uj = yBi) y = y/2i ). The technique is called multiple irradiation or multiple resonance and requires the presence of a second transmitter coil in the sample probe to provide the new irradiating frequency ( 2 = yBi). When the second frequency is applied, the experiment, which is widely available on modern spectrometers, is termed double resonance or double irradiation. Less often, a third frequency (U3 = yBf) also is provided, to create a triple-resonance experiment. We already have seen several examples of multiple-irradiation experiments, including the removal of... 

It is widely appreciated that modem NMR spectrometers use a short pulse of radiofrequency energy to excite nuclear resonances over a range of frequencies. This pulse is supplied as monochromatic radiation from the transmitter, yet the nuclear spin transitions giving rise to our spectra vary in energy according to their differing Larmor frequencies and so it would appear that the pulse will be unable to excite all resonances in the spectmm simultaneously. However, Heisenberg s Uncertainty principle tells us that an excitation pulse of duration At has associated with it a frequency uncertainty or spread of around 1/At Hz... 

The radiofrequency transmitter is the part of the spectrometer which generates the pulses. We start with an RF source which produces a stable frequency which can be set precisely. The reason why we need to be able to set the frequency is that we might want to move the transmitter to different parts of the spectrum, for example if we are doing experiments involving selective excitation (section 3.11). 

Radiofrequency (RF) transmitters generate frequencies of a few MHz to almost 1 GHz, which irradiate the sample molecules. If the energy difference between the relevant spin states is matched by the RF pulse, the nuclei will move to the higher spin state and be in resonance with the magnetic field. In older instruments, either the frequency sweep... 

In this technique, the frequency of a second radiofrequency transmitter (the decoupler) is set either upheld or downheld from the usual sweep width of a normal proton spectrum (i.e., off resonance). In contrast, the frequency of the decoupler is set to coincide exactly with the range of proton resonances in a true decoupling experiment. Furthermore, in off-resonance decoupling, the power of the decoupling oscillator is held low to avoid complete decouphng. 

As can be seen in Fig. 3, the pressure-broadening of the ICR lines is marked. The corresponding ion cyclotron double-resonance spectrum in Fig. 4 depicts the changes in SF5 signal intensity while sweeping the radiofrequency of the rf transmitter over the range of SFJ and SFg resonance frequencies. 

Bore of magnet contains a probe that acts as a transmitter of radiofrequency (RF) pulses and receiver of signals from the sample. The transmitter is housed in a console along with other electronic equipment. 

---