Field-gradient pulses gradients

Figure 7.25 illustrates the power of magnetic field gradient pulses to eliminate unwanted coherences. The double-quantum filtered COSY spec-... 

Figure 7.25 Homoniiclear double-quantum filtered COSY spectrum (400 MHz) of 8-mMangiotensin II in H,0 recorded without phase cycling. Magnetic field gradient pulses have been used to select coherence transfer pathways. (Reprinted from J. Mag. Reson. 87, R. Hurd, 422, copyright (1990), with permission from Academic Press, Inc.)...

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).

Fig. 5.3.4 (A) Stimulated echo dynamic NMR microscopy pulse sequence. The first field gradient pulse (g,) of duration 8 serves to encode spatial positions of spins and the second field gradient pulse has a refocusing effect.

Self-Diffusion of Desmopressin and Monoolein by NMR. The self-diffusion coefficient was measured with the NMR diffusion technique using a Bruker MSL 100 spectrometer. Two magnetic field gradient pulses were applied at either side of the 180-degree pulse in a [90x-T-180y-T-echo] spin echo sequence (7,8) (Figure 2). Due to diffusion, the amplitude of a component in the spin-echo spectrum is attenuated according to (7)... 

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... 

PFGNMR measures the ratio T of an NMR spin-echo signal A, which is observed after two field gradient pulses g are applied in a pulse sequence n/2-g-tt/2, compared to the signal Aq from the same sequence without gradient pulses ... 

Other types of PGSE experiments, including those involving alternating or sinusoidal field gradient pulses 2S), have particular applications which may be useful in polymer systems. 

The PGSE experiment can detect the presence of two or more diffusivities in the species at resonance. The intensity of the spin echo is a function of the duration 8, magnitude G and the time A between the applied magnetic field gradient pulses ... 

Fig. 2.28. Motion of the magnetization vector during a saturation-recovery experiment the first 90J pulse rotates the magnetization vector M0 to the x y plane (a -> b). where the resultant transverse magnetization is dispersed by a field gradient pulse (homo-spoil) after r s (d), a second 90 pulse monitors the partially relaxed magnetization Mt (d - f), and the resultant FID signal is Fourier transformed to the NMR signal with amplitude Ax. (Reproduced by permission of the copyright owner from E. Breitmaier and G. Bauer ...

Figure 1. Pulse sequence diagram of a spin-echo experiment with field gradient pulses. The rf-pulses are denoted by 90° and 180° and the field gradient pulses by FGP. TheFGP pulses have a length 5 and are separated by an interval A.

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.

Figure 5. Echo attenuation R versus the time interval A between field gradient pulses for different widths 5 of the field gradient pulses in the case of unrestricted (A) and restricted (B) diffusion.

Figure 12.4 The pulse scheme of the three-pulse echo sequence to determine Xe diffusion coefficients in polymers and other porous systems. The shaded areas are magnetic field gradient pulses with amplitude g and length 8. The time between the two gradient pulses A determines the time during which the diffusion path length is...

Figure 1 Diffusional spin echo attenuation of Boc-Phe in MPS1 gels with DMF-d7 as solvent on A at 30 °C by varying field-gradient pulse duration (diffusing time) A, where the Boc-Phe concentration is 10 wt%. (A) A = 10 ms and (B) A=100 ms.

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