Hahn spin echo

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. 2.9.7 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. 

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. 

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. 

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))...

Figure 2. Pulse sequence diagram of a Hahn spin-echo experiment with field gradient pulses. Rf- and field gradient pulses are denoted by 90°, 180° and FGP, respectively. The FGP pulses have a length 5 and are separated by an interval A as in the spin-echo sequence given in Fig. 1. VD is a time delay which may be variable in which case also A is variable. A PFG NMR experiment may also be performed with variable 5 or gradient strength (G) and fixed A. Normally, 6 is chosen between 0 and 10 ms and A between 0 and 400 ms. The time delay t depends on the T1 relaxation time of the pure oil of the emulsion but is normally between 130 and 180 ms.

Figure 3. High resolution proton NMR spectra of cheese, obtained by application of a Hahn spin echo pulse sequence with and without field gradient pulses. Measurements were performed on a Bruker MSL-300 spectrometer, operating at 300 MHz. The field gradient unit used with this spectrometer was home-built and the strength was calibrated to 0.25 T/m, using a 1-octanol sample for which the diffusion coefficient is known at several temperatures.

The self-diffusion coefficients in supercritical ethylene were measured using the pulsed NMR spectrometer described elsewhere (9,10), automated for the measurement of diffusion coefficients by the Hahn spin echo method (11). The measurements were made at the proton resonance frequency of 60 MHz using a 1 1.2 kG electromagnet. 

In principle, Ti and T2 can be measured experimentally by the inversion recovery sequence (180°—x—90°) and the Hahn spin-echo sequence, respectively. In practice, these experiments can be easily performed only when the 33S signal is very narrow. If the signal is broad, the difficulties in obtaining 33S spectra with a good S/N make the direct measurements very time-consuming and less precise. The problem can be easily circumvented because T2 (and Ti) can be obtained with good precision directly from line width. 

The rf part of the pulse sequence generates a Hahn spin echo at time 2x. In imaging jargon the time from the center of the 90° pulse to the center of the echo is called the echo time (or time to echo) TE = 2x (where x is the spectroscopist s usual symbol for the time from the 90° pulse to the 180° pulse). The time from the center of the 90° pulse to the center of the next 90° pulse is called the repetition time (or time to repeat) TR. Spectroscopists know TR as the time equal to the recycle delay plus the time taken by the pulsing and data sampling. 

The Hahn spin echo technique may be used to remove the inhomogeneous broadening. A n pulse at tJ2 changes to —q, allowing a refocusing of the coherence at ti for all orders. However, this n pulse also removes the separation of the orders which was a result of the frequency offset. 

Fig. 1. The 500-MHz Hahn spin-echo H NMR spectra of a cell culture medium, Dulbecco s minimal essential medium, before (A) and after (B) reaction with 400 iiM cisplatin for 24 hr at 37°C, and after (C) reaction with Pt(en)Cl2l. Note the disappearance of the singlet at 2,14 ppm for the S-methyl of L-Met, which has formed Pt-Met complexes, giving new peaks at —2.7 ppm. (Adapted from Ref. 23.)...

Static spin echo decay spectroscopy also forms the basis for the measurement of magnetic dipole-dipole interactions between two unlike nuclei I and S. While this interaction is refocused by the Hahn spin echo, it can be recoupled by applying a 7i-pulse to the S-spins during the dipolar evolution period [12]. This manipulation inverts the sign of the heterodipolar Hamiltonian, and thereby interferes with the ability of the Hahn spin echo technique to refocus this interaction. The corresponding pulse sequence, termed SEDOR spin echo double resonance) shown in Fig. 4, compares the I-spin echo intensity as a function of dipolar evolution time (a) in the absence and (b) in the presence of the ti(S) pulses. Experiment (a) produces a decay F(2ti)/Fo, which is dominated by homonuclear dipole-dipole interactions, while experiment (b) results in an accelerated decay, reflecting the contribution from the heteronuclear I-S dipole-dipole interaction, which is now re-introduced into the spin Hamiltonian. For multi-spin systems, a Gaussian decay is expected ... 

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