Selective pulses, solvent suppression

In this chapter we illustrate a direct method of characterisation of polymer/additive dissolutions by means of (500 MHz) NMR, which takes advantage of selective signal suppression allowing elimination of unwanted signals, such as the ca. 105 x more intensive PE signal. The most effective approach to solvent suppression is the destruction of the net solvent magnetisation by pulsed... 

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... 

Selective probe heads are used for optimal sensitivity for a particular nucleus. Sensitivity of a selective H probe head is normally greater than that of a switchable probe head with indirect observation. With a selective X-nucleus (a nucleus other than proton) probe head, decoupling of protons is normally possible. Because of their limited usefulness, selective probe heads are rare in NMR laboratories. Other probe heads are also available, for example, those for triple resonance experiments and experiments utilizing pulsed-field gradients. In addition to their suitability for 2-D experiments, the gradients are particularly suitable for solvent suppression (20). 

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... 

Many pulse sequence suppression schemes exist and these can be loosely classified into three broad classes (i) saturation based, (ii) magnetization destruction based, and (iii) methods avoiding solvent saturation. An ideal method would suppress the solvent only with no other effect on the spectrum. In reality this is never the case. For example, some suppression sequences which involve frequency-selective excitation result in frequency-dependent artifacts such as phase shifts and amplitude responses. Sometimes the applicability of a suppression method depends on the sample. For example, in addition to creating artifacts, caution must be taken when using suppression methods that involve saturation since cross-relaxation can result in significant signal loss, especially with large molecules. 

Apart from the gradient itself, the degree of suppression is related to the efficiency of excitation by the selective pulse and thus the choice of selective pulse is important with, for example, hyperbolic secant pulses perform better than sine pulses." If the solvent Ti is very short and on the order of the time for the dephasing procedure, then some (unwanted) z-magnetization will be re-established prior to the excitation pulse. One method of circumventing this... 

Fig. 11. (A) A conceptual diagram of the WATERGATE subunit, the ir RF pulse is some type of selective tt pulse. (B) Multiple solvent suppression using the double-pulsed field gradient echo. Ihe selective ir pulses are SLP pulses to enable multiple solvent suppression. To avoid breakthrough of undesired coherences, different values for the gradients are used in the first and second echo. The phase cycling is given elsewhere. ...

---