Diffusion filters

Fig. 5. 500 MHz HRMAS H NMR spectra of FMOC-isoleucine on Wang resin swollen in DMF- 7 and spun at 4 kHz. Spectrum A shows the presence of dissolved phenylalanine peaks, particularly around 3.0 ppm from its methylene protons, and residual protons of the solvent. Application of a diffusion filter using gradients selectively removes these signals in spectrum B. Reproduced with permission from Ref. 50. Copyright 1999 Elsevier.

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.

This diffusion-filtered approach has been extended in application to macroscopic supports, specifically a Synphase lantern.77 The advantage of the diffusion-filtered experiment was extended to suppression of the macroscopic support signals, not just those of the protonated solvents. Although, the lanterns required some modification to fit in the MAS rotors, it is proposed that manufacturers account for this size restriction in future formats of these supports. 

J. Chin, A. Chen and M. J. Shapiro, Improved high-resolution diffusion filtered H MAS NMR, Magn. Reson. Chem., 2000, 38, 782-784. 

Weickert J 1997 A review of nonlinear diffusion filtering In Scale-Space Theory in Computer Vision (eds. ter Haar Romeny B, Florack L, Koenderink J and Viergever M), pp. 3-28. Springer-Verlag, Berlin. 

Weickert J, ter Haar Romeny B and Viergever MA 1998 Efficient and reliable schemes for nonlinear diffusion filtering. IEEE Transactions on Image Processing 7(3), 398-410. 

Regulating Transformer 2- Lamp Housing 3 - Hot Mirror, Diffusion Filter... 

Figure 6.1.2 Diffusion filtered H hr-MAS spectra, from top to bottom, of [P-H]a t)[P-(CH2)nCI]t, [P-H]( t)[P-(CH2)uSn-n-BuPh2]t and [P-H]( t)[P-(CH2)uSn-n-BuCl2]f (Reproduced from Chemistry - A European Journal 2005, 11, 4552-4561. Copyright (2005) with permission from Wiley-VCH.)...

Another way to introduce diffusion contrast is to acquire two separate images, one with and one without diffusion gradients for example in a spin-echo diffusion filter (combination of schemes (a) and (c) in Fig. 7.2.13). In the quotient of both images, the relaxation weight is eliminated as well as spin density,... 

In applying suppression methods to multidimensional sequences, it is important to consider what preconditions the suppression scheme makes on the water magnetization. For example, multidimensional sequences are generally run with a recycle delay less than sufficient to allow the water magnetization to relax back to equilibrium before the start of the next scan. Thus the water is always at least partially saturated but the theory behind most suppression methods assumes that the water magnetization starts from thermal equilibrium in each instance. We note in particular that spin-lock, WATERGATE and diffusion filters can be used to suppress the water irrespective of whether the solvent magnetization is at equilibrium. 

Figure 18.5. H spectra of 0.15 mg of polystyrene/1 % divinylbenzene resin (0.27 mmol/g) carrying the tet-rapeptide Giy-Asn-Leu-Ile through an HMBA linker (A) in h7-DMF. (B) in h7-DMF with diffusion filter (10% d7-DMF is added as lock signal).

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