Spin echo Hahn echoes

Spin- spin relaxation is usually determined applying spin echoes (Hahn-Echo, CPMG method), which delete other influences such as Bo inhomogeneity. The spin-lattice relaxation time Ti is measured by detecting the relaxation after inverting Mo by a 180° pulse (inversion recovery method). 

Hahn spin echoes Hahn, Bull. Am. Phys. Soc. 24 13... 

Rowan L G, Hahn E L and Mims W B 1965 Electron-spin echo envelope modulation Phys. Rev. 137 A61-A71... 

Figure 1.2 High-fleld (aliphatic) region of 500 MHz H Hahn spin-echo NMR spectra of (a) a typical inflammatory synovial fluid sample (b) as (a) but following 7-radiolysis (5.00 kGy). Typical spectra are shown. A, acetate-CHs ...

The intense water signal and the broad protein resonances were suppressed by a combination of continuous secondary Irradiation at the water frequency and the Hahn spin-echo sequence (0[90 x-t-180 y-t-collect]). 

Figure 1.5 High-field (aliphatic) regions of 400 MHz H Hahn spin-echo NMR spectra of (a) E-199 culture medium (b) as (a) but following a 2 h incubation with neutrophils at 37°C (c) as (b) but incubated in the presence of 1.00 x 10 mol/dm phorbol 12-myrlstate 13-acetate (PMA). For abbreviations, see Fig. 1.2 with Met, methionine-S-Ctl3 group resonance MetSO, methionine sulphoxide-SO-CHs group resonance. The 2.245 p.p.m. singlet detectable in spectrum (c) arises from the -CH3 groups of acetone, the solvent in which PMA was...

Fig. 2.9.2 Radiofrequency, field gradient and current distributions requires a three-dimen-ionic current pulse sequences for two-dimen- sional imaging sequence [see Figure 2.9.1(a)] sional current density mapping. TE is the Hahn and multiple experiments with the orientation spin-echo time, Tc is the total application time of the sample relative to the magnetic field of ionic currents through the sample. The 180°- incremented until a full 360°-revolution is pulse combined with the z gradient is slice reached. The polarity of the current pulses...

Fig. 2.9.7 Hahn spin-echo rf pulse sequence combined with bipolar magnetic field gradient pulses for hydrodynamic-dispersion mapping experiments. The lower left box indicates field-gradient pulses for the attenuation of spin coherences by incoherent displacements while phase shifts due to coherent displacements on the time scale of the experiment are compensated. The box on the right-hand side represents the usual gradient pulses for ordinary two-dimensional imaging. The latter is equivalent to the sequence shown in Figure 2.9.2(a).

In order to avoid flow artifacts it may be advisable to replace the spatial encoding pulses (right-hand box) by velocity compensated pulses such as shown in Figure 2.9.4(e) for phase encoding. The amplitude of the Hahn spin-echo is attenuated by hydrodynamic dispersion. Evaluation of the echo attenuation curve for fixed intervals but varying preparation gradients (left box) permits the allocation of a hydrodynamic dispersion coefficient to each voxel, so that maps of this parameter can be rendered. 

Both find their origin in the spin-echo sequence, devised hy Hahn in 1952 and used for the determination of relaxation times. 

A few relatively recent published examples of the use of NMR spectroscopy for studying polymer degradation/oxidation processes will now be discussed briefly. At the early stages of degradation, the technique can be used to provide chemical identification and quantification of oxidised species for polyolefins, oxidation sites can be identified by the chemical shifts of -CH2- groups a and ji to carbons bonded to oxygen [85]. Spin-spin relaxation times may be determined by a pulse sequence known as the Hahn spin-echo pulse sequence. 

As increasing the time interval between the single-shot DNP operation and the aH NMR measurement, the intensity of the magic echo was found to decrease. Instead, the intensity of the 1H Hahn echo grew up, as demonstrated in Figure 19B-D). The fact that the inhomogeneous character in the aH spectra increased with time shows that the spin polarization had been transferred to the place where the H spins were dilute, that is surrounded by the 2H spins in the heavily deuterated host material. 

In this section, three experiments are going to be discussed. Two of them, a broadband inversion and a Hahn spin echo, are well-known in the rotating frame. They need to meet the requirement of the phase coherence in PIPs in order to work properly in the Eigenframe. The third is a composite pulse with offset modulation. 

For the basic PFGE experiment a spin-echo experiment (either the two-pulse Hahn echo sequence, Fig. la, or the three-pulse stimulated echo sequence. Fig. lb) is combined with two magnetic field gradient pulses with duration 8 and separated by the time duration A. The gradient pulses generate a magnetic... 

IR, inversion recovery SR, saturation recovery PS, progressive saturation STEAM, progressive saturation with stimulated echo acquisition mode SE, Hahn spin-echo sel. SE, frequency selective spin-echo. 

SR, saturation recovery ss ST, steady state saturation transfer SE, Hahn spin-echo. 

Fig. 14. Dependence of the relaxation times T2. and the fractions of protons with different mobility (f.) for unsaturated polyester on the curing time, as measured from broad line NMR ( ), Hahn spin-echo ( ) and Carr-Purcell pulse sequence (O)- Symbol x indicates the initial distribution of styrene and unsaturated polyester protons (adapted from Ref. S5))...

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