Radiofrequency pulses

Application of a radiofrequency pulse to give an excess of nuclei m the higher spin state... 

The technique for measurement which is most easily interpreted is the inversion-recovery method, in which the distribution of the nuclear spins among the energy levels is inverted by means of a suitable 180° radiofrequency pulse A negative signal is observed at first, which becomes increasingly positive with time (and hence also with increasing spin-lattice relaxation) and which... 

What will happen if the radiofrequency pulse is applied for an unusually long time ... 

Fortunately, an alternative method of excitation exists, instead of the sequential excitation of nuclei by the slow variation of the magnetic field. This involves the application of a short but intense radiofrequency pulse extending over the entire bandwidth of frequencies in which the nuclei to be observed resonate, so that all the nuclei falling within the region are excited simultaneously. As a result the total scan time is made independent... 

Inside the probe is a wire coil that surrounds the sample tube. This wire transmits the radiofrequency pulses to the sample and then receives... 

Figure 1.13 (a) Bulk magnetization vector, M°, at thermal equilibrium, (b) Magnetization vector after the application of a radiofrequency pulse. 

Figure 1.14 Effect of radiofrequency pulses of different durations on the position of the magnetization vector.

M and are magnetization components of the X vector of a heteronuclear AX spin system, shown here at a certain delay after the application of a 90° pulse. Draw the vector positions and their direction of rotation after each of the following radiofrequency pulses ... 

Free induction decay A decay time-domain beat pattern obtained when the nuclear spin system is subjected to a radiofrequency pulse and then allowed to precess in the absence of Rf fields. 

Nuclear magnetic resonance spectroscopy Interaction magnetic fields - nuclei Resonance of radiation quanta, h v Radiofrequency pulses Spectrum in time or frequency (FT) domain ... 

Familiar to most chemists is the notion of spin-lattice relaxation [25]. Labeled as T, the spin-lattice relaxation time is defined as the amount of time for the net magnetization (A/J to return to its equilibrium state (M0) after a spin transition is induced by a radiofrequency pulse ... 

The radiofrequency pulses involved in MRI cause thermal heating of the tissues, and are thus subject to FDA limits on the amount of RF power that is transmitted to a subject during a medical scan. The RF power unit is specified as the specific absorption rate (SAR) and is measured in watts per kilogram of body tissue (W/kg tissue). Powers that exceed this level put the subject at risk of tissue damage incurred as a result of the tissue s inability to remove the heat through blood flow. 

Fig. 5. Pulse sequence for MR detection of vibration using a radiofrequency field gradient. A binomial 1331 radiofrequency pulse (pulse length D, interpulse delay r) is applied in-phase with the mechanical wave. Thus the vibration period 7V is equal to 4(D + r). The number of cycles can be increased to ensure a better frequency selectivity. The constant RF field gradient generated by a dedicated RF coil allows space encoding without using conventional static field gradients (from Ref. 16 with permission from Elsevier).

Al MAS NMR has been demonstrated to be an invaluable tool for the zeoHte sdentist It provides a simple and direct way to quantify the proportions of A1 in four [Al(4)j, five [Al(5)j and six [Al(6)j coordinations. Quantitative determination of these species is an important issue in catalysis, and major effort is devoted on this topic. As mentioned already, for A1 only the central transition (-i-half to —half selective exdtation ) is detected. The central transition is unaffected by first order quadmpolar interaction, but the presence of second order effects causes broadening and complicates the quantitation of the A1 species. Usually hydrated samples and short radiofrequency pulses are employed for quantitative determination of framework and extra framework aluminum species. It is uncertain whether hydration changes the coordination of A1 species. Certain extra framework A1 can have very large quadmpolar interactions resulting in very broad lines ( NMR invisible ) [155, 202]. Unlike Si NMR, Al has a short relaxation time due to its quadmpolar nature, and the Al NMR spectrum with good signal to noise can be obtained in a relatively short time. 

The newer instruments (Figure 2.4c) utilize a radiofrequency pulse in place of the scan. The pulse brings all of the cycloidal frequencies into resonance simultaneously to yield a signal as an interferogram (a time-domain spectrum). This is converted by Fourier Transform to a frequency-domain spectrum, which then yields the conventional m/z spectrum. Pulsed Fourier transform spectrometry applied to nuclear magnetic resonance spectrometry is explained in Chapters 4 and 5. 

In reality the individual lines obtained after the Fourier transformation are composed of both absorptive A(f) and dispersive D(f) components. This non-ideality arises because of a phase shift between the phase of the radiofrequency pulses and the phase of the receiver, PHCO, and because signal detection is not started immediately after the excitation pulse but after a short delay period A. Whereas the effect of the former is the same for all lines in a spectrum and can be corrected by a zero-order phase correction PHCO, the latter depends linearly on the line frequency and can be compensated for by a first-order phase correction PHCl. Both corrections use the separately stored real and imaginary parts of the spectrum to recalculate a pure absorptive spectrum. 

At the end of the radiofrequency pulse, M0 will regain its equilibrium value and its relaxation period will depend on the medium (Fig. 9.10). This relaxation period... 

At the beginning of the analysis, the ensemble of frequencies is incoherent and has no measurable macroscopic effect on the cell. Ions with the same m/z ratio must be made coherent to conduct a frequency analysis. This is achieved by irradiating the cell with a short radiofrequency pulse (ca. 1 ms lifetime) that includes all the frequencies to be determined. During the irradiation pulse, ions will increase their... 

Two types of low resolution solid state NMR techniques can be distinguished a) broad line NMR in which the absorption signal is obtained by sweeping the magnetic induction B in the vicinity of the resonance value B0 (Eq. (1)), and b) pulsed techniques which are based on the possibility of rotating the magnetization under the influence of particular radiofrequency pulses, or pulse sequences. 

Radiofrequency pulse -fornmr [MAGNETIC SPIN RESONANCE] (Vol 15)... 

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