Signal enhancement cross-polarisation

Signal enhancement in liquids (NOE) and solids (H-C cross-polarisation). 

Presence of protons in solid materials can be useful for the enhancement of the signal of rare spins via cross-polarisation. Major disadvantages... 

In cross-polarisation experiments two nuclides are excited at the correct frequencies to satisfy the Hartmann-Hahn condition (Chapter 2) so that the magnetisation of the more abundant spin system (typically H) is transferred to the system with the smaller signal ( Si). The FID of the nuclide of interest ( Si) is acquired with a stronger signal and at the usually shorter T i value of the protons, allowing the spectrum to be obtained more quickly and with an enhanced signal/noise level. 

As well as providing improved sensitivity, cross-polarisation experiments between H and Si can also be used to provide additional structural information, since the Si atoms in closest proximity to protons are preferentially enhanced. The data from CPMAS experiments should, however, be regarded as qualitative rather than quantitative. If improved quantification is required, it is essential to ensure that the signal is not... 

Such cross-polarisation procedures to enhance the signal intensity of Si are expected to be of use in making two-dimensional isotropic-anisotropic correlation spectroscopy more accessible to systems without protons. 

Figure 6.8. A. O MAS NMR spectra of boehmite, with and without CP. Asterisks denote spinning sidebands. Note the significant enhancement of the Al-OH signal under CP conditions. From Walter et al. (1988). B. cross-polarisation curve for the hydroxyl group singularity...

Cross Polarisation is a widely used solid state NMR technique that enhances signal from one type of nucleus by the transfer of magnetisation from another, typically H. It is applicable to both spin and quadrupolar nuclei, though the high natural abundance of NMR active nuclei such as and Al tend to... 

IH spectra due to the wider range of chemical shifts. MAS spectra of 13C are mostly acquired under cross-polarisation (CP-MAS), i.e. the dipolar interaction is employed to transfer magnetisation from excited IH nuclei to the 13C nuclei and thus enhance the signal. 

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