Pulsed excitation sculpting

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 ... 

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). 

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... 

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 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. 

Figure 7 Excitation sculpting and SOGGY pulse sequence. (A) shows a cartoon representation of the basic DPFGSE module while (B) shows the SOGGY sequence. Cartoon sequence components are the same as described previously.

Most small molecule NMR will make use of highly deuterated solvents. However, there are times when protonated solvents have to be used, for example when examining intact biofluids, or in LC/NMR, and in these cases efficient suppression of the protonated solvent signals is imperative if the solutes are to be sensitively detected. This is a key use of PFGs and a series of pulse sequences have been devised, such as WATERGATE [28], WET [29], and excitation sculpting [30] to achieve solvent suppression. More will be said about these methods in relation to LC/NMR in Section 4.3.1.3. 

Solvent suppression is a particular problem in LC/NMR and has been a theme throughout its development. Early methods for suppression of the pro-tonated solvent signals which otherwise dominate the NMR spectrum made use of binomial pulse sequences [124-126]. Methods in use today either use fully deuterated solvents, or make use of solvent suppression schemes such as the NOESY presaturation technique [127], WATERGATE [28,128], WET [29,129], or excitation sculpting [30,130,131]. These methods have for some time made it possible to study relatively low-level (several %) impurities [132,133]. The need... 

Fig. 4.9 (A)-(C) H spectra recorded at 500 MHz on a 4 mM solution of a compound in a 60 40 (v/v) mix of D2O and CH3CN. (A) Simple pulse and collect ID H spectrum showing the C satellites of acetonitrile (B) NOESY presat spectrum with presaturation of the MeCN signal apphed during the relaxation delay (2 s) and during the mixing time (200 ms) using a field of 90 Hz (C) Excitation sculpting sequence using a 1 s presaturation of water and a selective proton JC pulse of 4.25 ms. Compare the spectral region around the residual acetonitrile resonance in (B) and (C) - excitation sculpting results in the obliteration of far less of the spectrum near the suppressed solvent resonance, which in this case contains many solute resonances.

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