Spin-echo pulse editing

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

A simple modification of 2D 1,1-ADEQUATE and chemical shift ojj-refocused 2D 1,1-ADEQUATE has been proposed recently 38 An insertion of a 180° aH pulse into the initial 13C-13C spin-echo yields cross peaks edited by the CHV multiplicity the cross peaks of CH and CH3 groups display opposite phase to those of CH2 groups leading to distinct phase patterns facilitating spectral analysis. Although the length of the pulse sequence is not affected by this modification, losses of sensitivity can occur due to a mismatch between the set and actual value of [fc i coupling constants. 

Lithium-6 edited C-spin-echo methods have been described and can be used with success in an investigation of aggregate formation of lithiumorganyls [12]. Since Li resonates close to deuterium, the experiments can in principle be performed in a standard multinuclear probehead by employing the lock channel as the third pulsed radiofrequency channel. 

Kg. 14. (A) A spin-echo sequence for water suppression based on heteronuclear editing, t = (2J) and the heteronuclear ir pulse is applied only in alternating experiments and the experiments are subtracted. (B) A water suppression module based on heteronuclear-coherence selection which results in full sensitivity. ... 

The poor editing accuracy of spin-echoes in the presence of a wide range of J values and the inability to fully characterise all carbon multiplicities are the major limitations of these techniques. More complex variations on the pulsed J-modulated spin-echo are to be found that do allow a complete decomposition of the carbon spectra into C, CH, CH2 and CH3 sub-spectra [21] and which also show greater tolerances to variations in Jch [22]. Likewise,... 

Figure 13.3-2. NMR spectra of rat serum illustrating the various NMR responses that are possible through the use of different pulse sequences, which edit the spectral intensities (a) standard water suppressed spectrum, showing all metabolites (b) CPMG spin-echo spectrum, with attenuation of peaks from fast relaxing components such as macromolecules and lipoproteins (c) diffusion-edited spectrum, with attenuation of peaks from fast diffusing components such as small molecules and (d) a projection of a 2D J-resolved spectrum on to the chemical shift axis, showing removal of all spin-spin coupling and peaks from fast relaxing species.

Figure 3 Schematization of pulsed-gradient spin-echo (PGSE) data editing for a hypothetical mixture composed by a large (red), slow diffusing, and a small (blue), fast diffusing, species. (Reproduced from Ref. 11. CRC Press, 2011.)...

The experiment is repeated with 0 set to 45°, 90° and 135° and linear combinations of the resulting data are taken to obtain the edited subspectra. The pulse sequence contains staggered and spin echos. This structure results in the cancellation of all chemical shift evolution by the start of acquisition, and consequently nothing is lost by omitting chemical shift evolution from the product operator description. The analysis for a group is given... 

Another very important experiment for spectral editing is a heterocorrelation experiment combining MQMAS (in the indirect dimension) with CP. Introducing CP at the MQMAS echo position following a split-fj MQMAS scheme creates a spin-j detected spectra (in the dimension), which is modulated by the isotropic frequency of the quadrupolar nucleus in F. HETCOR has been demonstrated on Na- P pairs, on 2 A1- P pairs and lately on a- H pairs,where proton resolution during F was obtained with the wPMLGS multiple pulse decoupling technique. ... 

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