Pulse sequences, solvent suppression

The NOESY sequence proved to be very effective for the reduction of one particular signal such as the methyl group of acetonitrile. However, very often the mobile phase has a composition of several solvents, together with up to six solvent signals. Here, the application of the soft pulse multiple solvent suppression technique is advisable. 

Other even more cunning methods have been devised to suppress the water signal in samples that have a large water content (e.g., bio-fluid samples) such as the WET and the WATERGATE pulse sequences. Other sequences have been devised to cope with signals from carbon-bound hydrogens. Some of these actually collapse the 13C satellites into the main 12C peak prior to suppression. Such a sequence would be useful if you were forced to acquire a spectrum in a nondeuterated solvent. 

Fig. 7. 300 MHz H HRMAS NMR spectra of a resin suspension swollen with DMF-dq from a reaction mixture and spun at 4 kHz. The spectrum (A) was obtained with a single-pulse sequence. The spectra in (B), liberated from the reaction vessel at the times indicated, were obtained using a diffusion filter to reduce the signals from non-bound species. Note the excellent suppression of the solvent DMF peak at 8 ppm. Reproduced with permission from Ref. 75. Copyright 2000 American Chemical Society.

Figure 1.16 Representation of the WET pulse sequence for multiple solvent suppression...

Direct on-line coupling of an NMR spectrometer as a detector for chromatographic separation, analogous to the use of MS for such applications, has required the development of technical features such as flow-probe hardware, efficient NMR solvent suppression pulse sequences and new software. 

This main difficulty in coupling HPLC to NMR spectroscopy is faced by methods known as solvent suppression techniques, where the large solvent signals are reduced by special pulse sequences, switched prior to the information-selecting and acquisition pulses. Therefore, many efforts have been made to develop effective and minor-disturbing pulse sequences, such as presaturation, zero excitation and PFG-pulse sequences (WET) (see Chapter 1 and the following chapters). Despite the possibility of also suppressing several of the... 

H and 19F NMR spectra are recorded with a normal one-pulse sequence or, alternatively, the XH spectra are recorded with a sequence that allows simultaneous solvent suppression with presaturation (31) or a sequence that includes some other method of suppression 13C 1H and 1P 1H spectra are recorded with proton broadband (composite pulse) decoupling (32), and 31P spectra with gated proton decoupling (33). 

Fig. 46. — Structural-reporter-group Regions of the Resolution-enhanced, 500-MHz, H-N.m.r. Spectrum of Compound 63. [The bold numbers and letters in the spectrum refer to the corresponding residues in the structure. The relative-intensity scale of the IV-acetyl-proton region (see insertion) differs from that of the other parts of the spectrum, as indicated. For solvent-peak suppression, a w.e.F.t. pulse-sequence was used.]...

Figure 14.4. Pulse sequence of a solvent suppression experiment.

For structural identification of the fractions, the XH-NMR spectrometer was directly coupled via capillary tubing to the chromatograph. The injection of the sample into the HPLC system was automatically initiated by the NMR console via a trigger pulse when starting the acquisition of NMR data. Using an appropriate pulse sequence, both solvent resonances (ACN at 2.4 ppm and water at 4.4 ppm) could be suppressed simultaneously. As a result of the on-line HPLC-NMR experiment, a contour plot XH chemical shift vs. retention time was generated (see Fig. 39). Due to the efficient solvent suppression, the obtainable structural information relates to the entire chemical shift region. From the contour... 

Based on the correspondence between rf pulses in the usual rotating frame and Hartmann-Hahn transfer in the zero-quantum frame (see Sections II and VIII.C), Mohebbi and Shaka (1991b) introduced an alternative approach for the construction of (band-) selective Hartmann-Hahn transfer. In direct analogy to DANTE sequences (Bodenhausen et al., 1976 Morris and Freeman, 1978) and binomial solvent suppression... 

The BIS HMQC pulse sequence may give relatively intensive residual parent lines resulting from imj>erfections of the 180° pulse. These can be removed using the post-acquisition solvent suppression techniques. 

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