Radio frequency generator, pulsed

Fourier Transform NMR. In Fourier transform NMR (FTNMR), a repetitive radio frequency (RF) pulse is applied in order to excite all of the nuclei of the particular nuclear species being studied. The sum of the free induction decay (FID) curves from each pulse is analyzed by a Fourier transform method in order to generate the familiar frequency domain spectra. Fundamentally, parameters such as the frequency, intensity, application time of the appropriate RF pulse, and time intervals between these pulses are important variables when using this technique. The principle of the pulsed Fourier transform technique can be found in books covering the fundamental concepts of NMR spectroscopy (58,59). 

In an alternative approach (42) the sample is coated on a ferromagnetic wire in the inlet of the gc machine, surroimded by a coil connected to a radio frequency generator. A short pulse of radio frequency power causes the temperature of the wire to rise rapidly to its Curie point, where it remains imtil the power is switched off. In theory the great advantages of this approach are its self-thermostat action and the absence of direct contact between the power imit and the pyrolysis wire and this has made Curie point p5Tolysis popular, with commercial imits available. However, it has been pointed out (12) that neither the speed of temperatime rise nor the self-thermostat action is as reliable as is often claimed. 

Precisely controllable rf pulse generation is another essential component of the spectrometer. A short, high power radio frequency pulse, referred to as the B field, is used to simultaneously excite all nuclei at the T,arm or frequencies. The B field should ideally be uniform throughout the sample region and be on the order of 10 ]ls or less for the 90° pulse. The width, in Hertz, of the irradiated spectral window is equal to the reciprocal of the 360° pulse duration. This can be used to determine the limitations of the sweep width (SW) irradiated. For example, with a 90° hard pulse of 5 ]ls, one can observe a 50-kHz window a soft pulse of 50 ms irradiates a 5-Hz window. The primary requirements for rf transmitters are high power, fast switching, sharp pulses, variable power output, and accurate control of the phase. 

Figure 7.1 Most modern NMR techniques are based on the fact, that the phase (p of the precessing transverse magnetisation M t) kann be measured. By use of the Fourier transformation the phase provides access to NMR spectra, images, and parameters of translational motion like velocity v and acceleration a. Spectroscopic parameters as well as components of translational velocity and acceleration can be used for generating contrast in NMR imaging. In the drawing the magnetisation M(t) has been generated from Mz by use of a 90° pulse of the B1 radio-frequency (rf) field in y direction...

Fourier transform NMR spectroscopy. The FT NMR spectrometer delivers a radio-frequency pulse close to the resonance frequency of the nuclei. Each nucleus precesses at its own resonance frequency, generating a free induction decay (FID). Many of these transient FIDs are accumulated and averaged in a short period of time. A computer does a Fourier transform (FT) on the averaged FID, producing the spectrum recorded on the printer. 

An alternative method for excitation of nuclei over a range of chemical shifts is by irradiation with a weak, noise-modulated radio-frequency, instead of with strong r.f. pulses. In one realization of this method, protons were irradiated with repetitive sequences of noise that was truly random,162 and, in another,163 fluorine nuclei were excited by pseudo-random noise generated by amplitude modulation of the r.f. with maximum-length sequences of pulses from a computer or shift register (a series of flip-flop devices connected by feedback loops). With the carrier wave suppressed, the latter process is equivalent to phase modulation of the r.f. by+7r/2 radians when the pulse is turned on, and by —ir/2 radians when it is turned off. This method is identical with that used in most broadband, heteronuclear, noise decouplers, except that greater power is required for decoupling. 

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