Water suppression, with WEFT

Some methods take advantage of a difference in a particular property between water and the molecule to be studied. In particular, a macromolecule usually has a shorter value for proton Tt than water and a much lower diffusion coefficient. One of the oldest methods for water signal suppression is WEFT (water elimination Fourier transform), in which an inversion recovery sequence is applied (see Fig. 2.12) with r chosen to be the time that the water signal goes through zero (Tj In 2), just as in the BIRD pulse sequence. Another method makes use of the technique described in Section 9.3 to measure diffusion coefficients. 

The WEFT/NOESY style experiment provides substantially better water suppression than simple presaturation with four transients, which is the minimum for the phase cycle to take effect. With proper optimization of the mixing period the WEFT/NOESY still yields suppression even when considering only the first transient, and before pulsed field gradients were used in the mixing period. Therefore, while important, volume selection alone cannot be the sole answer. 

The order of review was intended to be alphabetical with the exception of presaturation and water-eliminated Fourier transform (WEFT)/NOESY due to the importance of these sequences and comparisons to subsequent developments. Presaturation and WEFT (commonly referred to as ID-NOESY) remain, for many groups, the gold standard to which all other forms of suppression are compared. Suppression techniques prior to or during acquisition are considered. Suppression in the solid state and post-acquisition mathematical suppression techniques (e.g. WAVEWAT and others °) are beyond the scope of this review. 

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