PRESAT

The Presat-180 is simple, easy to implement with a short phase cycle, relatively insensitive to miscalibration, does not add significantly to the length of pulse sequences or introduce artefacts, and is adequately effective (Figure IIC). 

Eight benzimidazoles Animal tissues ACN extn, liq-liq partns, two SPE cleanups 24 Hewlett-Packard RP-18,5 m, with Kontron RP-18 Presat, 25-40 m, guard column O.OIM pentane-sulfonate contg 0.5% TEA, pH 3.5/ACN (50 50) UV 298 nm/ 20-50 ppb/ GC-NPD 39-87% or GC-EI-MS or GC-Pulsed PICI-NICI-MS after derivatization with methyl iodine or pentafluoro- benzyl bromide 358... 

Selective presaturation (presat), with pre referring to implementation prior to execution of the nearly limitless range of pulse sequences (Figure 6A), depends on the inhomogeneity of the induced field (Bi) to result in a loss of coherence (saturation) for the selected resonance. ... 

Figure 6 Presat, WEFT/NOESY, and FLIPSY. The typical (A) presat, (B) WEFT/NOESY and (C) FLIPSY. Any 90 and 180 hard pulses are identified, respectively, by narrow and wide hollow rectangles, respectively, with relative phases indicated inside the shape (where appropriate). Outlined Gaussian shapes indicate selective pulses while gradients are represented on their own horizontal line by filled shapes. For full phase cycle discussions please refer to the referenced manuscripts. The 45 phase shifted pulse discussed in the text is indicated by the asterisk in the NOESY sequence and an V in the FLIPSY shows the variable flip angle pulse. The modified FLIPSY contains a homo-spoil gradient just after the presaturation period.

To address the solvent dynamic range problem, one or multiple signals from the solvent are selectively suppressed in the NMR spectrum. Solvent suppression is not a perfect solution. Compound peaks that are proximal to the solvent are also completely or partially suppressed. Similarly, hydrogens that readily exchange with water are also equally suppressed with the water solvent peak. Additionally, solvent suppression causes artifacts and streaking in 2D NMR spectra. This streaking may obscure cross peaks that fall near the solvent chemical shift in either spectral dimension. The commonly used solvent suppression NMR techniques, such as, PRESAT, WET (Smallcombe and Patt,... 

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.

Upon analysis 720 (il of perchloric extract were added 80 (il of D2O and pH was adjusted to 7.80. Subsequently, the samples were centrifuged 5 min at 14,000 rpm at room temperature and the clear supernatant placed into 5-mm NMR tubes. NMR spectra were acquired at 298 K using a Varian Mercury 400 MHz NMR spectrometer. The flip angle pulse was chosen to be 60° and the standard Varian presat pulse sequence was applied to allow efficient water signal suppression. 

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