Excitation sculpting solvent suppression

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 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).

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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