Radiofrequency pulses measure

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... 

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. 

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... 

A1 MAS NMR spectra were measured at 104.26 MHz using powerful 0.6 ps radiofrequency pulses with a 0.2 s recycle delay. Samples were spun at 3.4 kHz in an aluminium-free probehead and Vespel rotors. Chemical shifts are quoted in ppm from external A1(H20)63+. 10000 transients were acquired for each spectrum. 

Figure 3.4. H-Decoupled CNMR spectra of 35% enriched I-[ CJ- and 2-[ C]pyruvate, in the presence of (A) Mn -pyruvate carboxylase, and (B) Mg -pyruvate carboxylase. Each spectrum, except the normal Fourier transform (NFT) spectrum, is the result of applying two radiofrequency pulses to the sample the delay between them is indicated in sec. From measurements of peak heights the T, values can be calculated, and a resonance has zero intensity at about 0.69 T,. Note that Mn shortens T, and has a differential effect on the two...

The Solomon equations are an excellent way of understanding and analysing experiments used to measure the nuclear Overhauser effect. Before embarking on this discussion it is important to realize that although the states represented by operators such as Iz and Sz cannot be observed directly, they can be made observable by the application of a radiofrequency pulse, ideally a 90° pulse... 

The length of time in which the radiofrequency pulse excites the nuclei is the pulse width, which is measured in p,s but usually expressed in terms of pulse angles. Pulse angles between 30° and 90° are used... 

Radiofrequency pulses are also utilized to measure relaxation times. Three relaxation times have been measured in TPEs, and each is sensitive to different phenomena. Ti, the spin-lattice relaxation time in the laboratory frame, is the relaxation from the nonequilibrium population distribution created by the pulse to the equilibrium Boltzmann distribution. Ti is sensitive to molecular motions that rate in the range of 10 -10 Hz. T2, the spin-spin relaxation time, is the relaxation caused by the establishment of equilibrium between nuclear spins within the system. Spin-spin relaxation measurements also probe motions with rates in the range of 10M0 Hz however, low frequency motions (lOMtPHz) also affect T2. Generally,T2 is one to three orders of magnitude smaller than Ti in solid polymers. Tip, the spin-lattice relaxation time in the rotating frame, probes motions with rates on the order of lO -KfHz. Cross polarization is usually used in Tip measurements. 

Figure 8 Pulse sequences commonly used for PGSE measurements sequences with (a) spin-echo (b) stimulated echo (c) stimulated echo and longitudinal eddy-current delay (LED) (d) stimulated echo with bipolar pulsed field gradients and LED. Narrow and wide black rectan es represent 7t/2 and tt radiofrequency pulses, respectively. Narrow and wide open rectangles are field-gradient pulses of duration /2 and...

Pulsed ENDOR. In both the inversion recovery (Fig. 5b) and stimulated echo experiment (Fig. 5c), the echo amplitude is influenced by a radiofrequency pulse applied during the interpulse delay of length T, if this pulse is on-resonance with a nuclear transition. In the former experiment, such a pulse exchanges magnetization between inverted and noninverted transitions, so that echo recovery is enhanced (Davies ENDOR) (32). In the latter experiment the on-resonance radiofrequency pulse induces artificial spectral diffusion, so that the echo amplitude decreases (Mims ENDOR) (33). These pulsed ENDOR experiments exhibit less baseline artifacts and are easier to set up compared with CW ENDOR experiments, as the required mean radiofrequency power is smaller and the ENDOR effect does not depend on a certain balance of relaxation times. Davies ENDOR is better suited for couplings exceeding 1-2 MHz, while Mims ENDOR is better suited for small couplings, for instance matrix ENDOR measurements. 

A simple modification of a conventional Fourier transform n.m.r. spectrometer has made it feasible to apply selective, radiofrequency pulses at the resonance frequency of one or more proton resonances and to monitor the effects of these perturbations in the usual way. 2, 3 -0-Isopropylideneuridine was used to illustrate how this technique can be used to measure selective spin-4attice relaxation rates, to eliminate unwanted resonances by selective saturation or by selective partial relaxation, and to carry out the pulse equivalent of a INDOR... 

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