Excitation sculpting

In the early days of selective excitation, spectrometers were not equipped to generate amplitude-modulated rf pulses and the DANTE method (Delays Alternating with Nutation for Tailored Excitation) was devised [62] requiring only short, hard pulses. Although largely superseded by the amplitude-modulated soft pulses, DANTE may still be the method of choice in older instrumentation. 

The basic DANTE sequence is composed of a series of N short, hard pulses of tip angle a where a 90°, interspersed with fixed delays, r, for free precession 

The total length of the selective pulse is the product Nrtp where tp is the duration of each hard pulse, and the net on-resonanee tip angle is the sum of the individual pulses. The effect of such a sequence is illustrated in Fig. 10.27. Following each hard pulse, spins are able to 

The major difference between soft-shaped pulses and DANTE methods is the occurrence of strong sideband excitation windows on either side of the principal window with DANTE. These occur at offsets from the transmitter at multiples of the hard-pulse frequency 1/r. They arise from magnetisation vectors that are far from resonance and that precess full circle during the r period. Since this behaviour is precisely equivalent to no precession, they are excited as if on-resonance. Further, sidebands at +2/r, 3/r and so on also occur by virtue of trajectories completing multiple full circles during r. Such multi-site excitation can at times be desirable [63, 64] but if only a single excitation window is required, the hard-pulse repetition frequency must be adjusted by varying r to ensure that the sideband excitations do not coincide with other resonances. 

For use in the laboratory, it is convenient to choose a simple, robust inversion pulse as the element S, and the Gaussian pulse is well suited to routine use. Example excitation profiles for this are illustrated in Fig. 9.20 and offer guidance on selection of pulse duration for a desired excitation window. For proton spectroscopy, a Gaussian pulse of around 40 ms proves suitable for many applications. 

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Hwang TL, Shaka AJ, Water suppression that works. Excitation sculpting using arbitrary wave-forms and pulsed-field gradients, J. Magn. Reson., 112 275-279, 1995. 

COMBINING SHAPED PULSES AND PULSED FIELD GRADIENTS EXCITATION SCULPTING ... 

Stott K, Stonehouse J, Keeler J, Hwang T-L, Shaka AJ. Excitation sculpting in high-resolution nuclear magnetic resonance spectroscopy application to selective NOE experiments. J. Am. Chem. Soc. 1995 117 4199-4200. 

HOHAHA experiment with minimal water saturation was developed by Schleucher et al. (1995a) and Dhalluin et al. (1996) proposed a water-flip-back TOCSY where water-selective pulses are used to flip a maximal fraction of the water magnetization back along the +z axis at the start of the acquisition time. TOCSY experiments with excellent water suppression based on excitation sculpting (Stott et al., 1995 Hwang et al., 1995) were reported by Callihan et al. (1996). 

Excitation sculpting also can be used for solvent suppression in the DPFGSE version of WATERGATE [Gl-S-Gl-G2-S-G2-/(acquire)], in which the solvent peak is selected for dephasing during S and all other resonances are refocused. Figure 6-42 illustrates the removal of the solvent resonance for 2-mM sucrose in 9 1 H2O/D2O. 

Figure 6-42 (a) Water suppression with excitation sculpting on 2-mM sucrose in 9 1 H2O/D2O. (b) The residual solvent peak has been eliminated by further processing. (Reproduced from T. D. W. Claridge, High-Resolution NMR Techniques in Organic Chemistry, Pergamon Press, Amsterdam, 1999, p. 365.)... 

Figure 2 Gradient-echo-based water suppression pulse sequences, (a) WATERGATE (b) water-flip-back (c) excitation sculpting (d and e) examples of the S pulse train that is sandwiched between the gradient echo (d) water-selective inversion...

The excitation sculpting method was expended to the selective inversion of z-magnetisation. The method utilises DANTE train of hard pulses combined with gradient suppression of transverse magnetisation. Application of the selective inversion to z-magnetisation reduces relaxation loses and extends selective methods to larger molecule. Use of shaped pulses in DANTE train produces band-selective inversion. The proposed inversion method can be... 

A modification of GOESY experiment was proposed by Dixon et al In the new version of the experiment called M-GOESY the target proton is selectively excited using excitation sculpting method but not decoded before NOE... 

Many different ways of effecting water suppression in the NOESY sequence have been implemented, for example, presaturation, jump and return, l-T-echo, WATERGATE, SS pulses and excitation sculpting. The basic NOESY sequence using presaturation and two variations using WATERGATE are illustrated in Fig. 24. In the basic NOESY sequence (Fig. 

Fig. 23. Excitation-sculpting TOCSY sequence. The shaped pulses have SEDUCE profiles.

Figure 5.71. One-dimensional TOCSY spectra of the tetrameric carbopeptoid 5.9 in CDCI3. Each amide proton was selectively excited and used as the starting point for coherence transfer. Selective excitation was achieved with the excitation sculpting method and mixing used a 97 ms MLEV-17 spin-lock.

Figure 9.27. Solvent suppression schemes employing pulsed field gradients based on (a) WATERGATE (single-echo) and (b) excitation sculpting (double-echo) principles. The pulse element S has zero net effect on the solvent resonance but inverts all others.

Figure 9.30. Solvent suppression with the excitation sculpting scheme using the approach of Fig. 9.28b with 4.1 ms 90° Gaussian pulses and gradients of 0.1 0.1 0.03 0.03 T m . The sample is 2 mM sucrose in 9 1 H2O D2O. In (b) the small residual solvent signal has been completely removed through additional processing of the FID (see text).

The excitation sculpting procedure is usually implemented in ID and 2D pulse sequences using a selective 180° refocusing element. However as shown in Table 2.9 there are also a number of applications where non-selective pulses are used. In all cases the choice of a particular pulse sequences depends upon the phase distortions the refocusing units introduce into the spectrum. 

Table 2.9 Examples of pulse sequence using Excitation sculpting.

In Check it 2.3.3.11 the excitation sculpting procedure is demonstrated using the GBIRD sequence, an interesting alternative for suppressing the IH signals of isotopomers. Finally the performance of the iH-GBIRD pulse sequence is compared to the alternative BIRD-tnun sequence. 

Further examples of the use of excitation sculpting to minimize the phase distortions introduced by a spin echo unit are discussed in section 5.2.3 on the WATERGATE sequence and in section 5.4.1.1 for a ID selective COSY experiment. 

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