Pulse selective

Closer examination of equation B 1,14,3 reveals that, after the slice selection pulse, the spin isocln-omats at different positions in the gradient direction are not in phase. Rather they are rotated by i exp jyC. )tind... 

In TOF-SARS [9], a low-keV, monoenergetic, mass-selected, pulsed noble gas ion beam is focused onto a sample surface. The velocity distributions of scattered and recoiled particles are measured by standard TOF methods. A chaimel electron multiplier is used to detect fast (>800 eV) neutrals and ions. This type of detector has a small acceptance solid angle. A fixed angle is used between the pulsed ion beam and detector directions with respect to the sample as shown in figure Bl.23.4. The sample has to be rotated to measure ion scattering... 

A simple, non-selective pulse starts the experiment. This rotates the equilibrium z magnetization onto the v axis. Note that neither the equilibrium state nor the effect of the pulse depend on the dynamics or the details of the spin Hamiltonian (chemical shifts and coupling constants). The equilibrium density matrix is proportional to F. After the pulse the density matrix is therefore given by and it will evolve as in equation (B2.4.27). If (B2.4.28) is substituted into (B2.4.30), the NMR signal as a fimction of time t, is given by (B2.4.32). In this equation there is a distinction between the sum of the operators weighted by the equilibrium populations, F, from the unweighted sum, 7. The detector sees each spin (but not each coherence ) equally well. 

Selective experiments can also be performed by the tailored excitation method of Tomlinson and Hill. The selective pulse is frequency-modulated with a function designed to yield zero effective field at the resonance offset of the neighboring nuclei. Although this technique is especially promising for studies of more-complex spin systems, its use is as yet very limited, in part because the instrumentation needed is not yet commercially available. 

Two additional systematic errors may arise in selective pulse experiments associated with pulse selectivity and relaxation during excitation. Pulse... 

The frequency bandwidth, in Hz, of a weak, selective pulse is approximately given by Av— 1/Tp, where Tp is the pulse duration in ms. 

Figure 6.2 Pulse sequences for some common 3D time-domain NMR techniques. Nonselective pulses are indicated by filled bars. Nonselective pulses of variable flip angle are shown by the flip angle )8. Frequency-selective pulses are drawn with diagonal lines in the bars. (Reprinted from J. Mag. Reson. 84, C. Griesinger, et al, 14, copyright (1989), with permission from Academic Press, Inc.)...

There are three main requirements of good-quality selective pulses ... 

Reproduced with permission, from H. Kessler, S. Mronga and G. Gemmecker, (1991). Multi-Dimensional NMR Experments Using Selective Pulses, Mag. Reson. Chem. 29, 527... 

Figure 7.1 Selective excitation of only one multiplet by a selective pulse transforms a 2D experiment into a ID technique. A selective pulse generates the transverse magnetization. The result is a trace of the corresponding 2D spectrum. (Reprinted from Mag. Reson. Chem. 29, H. Kessler ei al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)...

SELINQUATE (Berger, 1988) is the selective ID counterpart of the 2D INADEQUATE experiment (Bax et al., 1980). The pulse sequence is shown in Fig. 7.4. Double-quantum coherences (DQC) are first excited in the usual manner, and then a selective pulse is applied to only one nucleus. This converts the DQC related to this nucleus into antiphase magnetization, which is refocused during the detection period. The experiment has not been used widely because of its low sensitivity, but it can be employed to solve a specific problem from the connectivity information. 

This is a variation of the proton-detected shift-correlation experiment via long-range couplings proposed by Bax and Summers (Bax and Summers, 1986), with the difference that the first C pulse is substituted by a frequency selective pulse (Fig. 7.14) (Bermel et al., 1989 Kessler et al., 1989b,1990). This significantly increases resolution in the F dimension. For example, this can be used to remove the overlap between the cross-peaks of the carbonyl resonances of peptide bonds in proteins that all occur within a... 

G. Multi-dimensional NMR experiments using selective pulses. Magn. Reson. Chem. 1991, 29, 527-557. 

Another approach to obtain spatially selective chemical shift information is, instead of obtaining the entire image, to select only the voxel of interest of the sample and record a spectrum. This method called Volume Selective spectroscopY (VOSY) is a ID NMR method and is accordingly fast compared with a 3D sequence such as the CSI method displayed in Figure 1.25(a). In Figure 1.25(b), a VOSY sequence based on a stimulated echo sequence is displayed, where three slice selective pulses excite coherences only inside the voxel of interest. The offset frequency of the slice selective pulse defines the location of the voxel. Along the receiver axis (rx) all echoes created by a stimulated echo sequence are displayed. The echoes V2, VI, L2 and L3 can be utilized, where such multiple echoes can be employed for signal accumulation. 

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