Solvent suppression binomial sequences

Mohebbi and Shaka (1991b) also developed selective homonuclear Hartmann-Hahn experiments based on zero-quantum analogs of DANTE sequences (Bodenhausen et al., 1976 Morris and Freeman, 1978) and binomial solvent suppression methods (Plateau and Gueron, 1982 Sklenaf and Starcuk, 1982 Hore, 1983) (see Section X.C). 

Symmetrically shifted pulses have been proposed as a means of solvent suppression. Symmetrically shifted pulses are symmetrically shifted laminar pulses that contain equal numbers of rectangular pulse components of the same phase at an offset frequency. The basis of the symmetrically shifted pulse family is the SS pulse which is conceptually equivalent to applying simultaneous ir/2 rectangular pulses with two separate, but in-phase, transmitters at offset frequency from the water. On a practical basis an SS pulse is obtained by a complete Itt cosine modulation of a single transmitter (see Fig. 15). An S pulse is half of an SS pulse (i.e. a half-cycle tt pulse) which results in a narrower null and a 180° phase inversion at the transmitter frequency. They are also the soft, continuous equivalent of binomial sequences. The SS and S pulses have broader excitation maxima than the sinusoidal profile of the JR sequence. The method has maximal excitation at an offeet frequency of second-order U-shaped water suppression. The exdtation profile is related to the maximum amplitude modulation and can be determined by numerical evaluation of the Bloch equations. Hence a new pulse shape must be used for each excitation window. The SS pulses give better water suppression than the JR sequence, but at the expense of poorer excitation of resonances closer to the water. Also, there is no phase inversion at zero frequency. The S pulse gives better excitation near the water frequency but with less water suppression. 

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... 

A wide range of other methods for solvent suppression has been developed which may collectively be classed as tailored excitation. These rely on the application of appropriate combinations of pulses to excite protons lying outside a narrow band of frequencies while leaving those within that band (i.e. the solvent) undisturbed. Examples of these are the Redfield pulse [38], the jump-return technique [39] and binomial sequences [40]. 

Binomial pulses, which are predominantly used in signal suppression sequences, such solvent signal suppression experiments will be discussed as a special category. 

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