Water Elimination Fourier Transform

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 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. 

Other approaches to elimination of the water signal have exploited the difference in relaxation times between water and the compounds of interest, and have the advantage that peaks near the solvent are unaffected. For example, WEFT (Water Eliminated Fourier Transform) NMR uses an inversion recovery pulse sequence (designed for measurement). 

Fig. 6. One-dimensional water eliminated Fourier transform spectroscopy (WEFT) spectra of (A) wild-type amicyanin (B) the amicyanin mutant H96D (C) the amicyanin mutant H PFM H QGAGM (D) wild-type Pseudomonas aeruginosa azurin. The diamagnetic region from 9 to 1 ppm has been omitted from the centers of the spectra. The insets show the spectra in H2O, where an additional peak from an exchangeable proton appears. Peak assignments for wild-type amicyanin are given at the top of panel A. All spectra were collected at 32 C on samples containing 50 mAf phosphate, pH 7.0.

The SNIFTIRS approach. The acronym SNIFTIRS means Subtractively Normalized Interfacial Fourier Transform Infrared Spectroscopy. The basic concept of this method involves the fact that the raw data obtained directly from the Fourier Transform process contain components which are undesirable. Firstly, there is material in the solution which may have affected the spectrum. Secondly, unwanted information on certain material on the electrode (adsorbed water, for example) is best eliminated. 

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