Echo pulse sequence

Figure Bl.14.2. Gradient-recalled echo pulse sequence. The echo is generated by deliberately dephasing and refocusing transverse magnetization with the readout gradient. A slice is selected in the z-direction and v- and y-dimension are frequency and phase encoded, respectively.

Figure Bl.14.5. J2-weighted images of the propagation of chemical waves in an Mn catalysed Belousov-Zhabotinsky reaction. The images were acquired in 40 s intervals (a) to (1) using a standard spin echo pulse sequence. The slice thickness is 2 nun. The diameter of the imaged pill box is 39 nun. The bright bands...

The mathematical description of the echo intensity as a fiinction of T2 and for a repeated spin-echo measurement has been calculated on the basis that the signal before one measurement cycle is exactly that at the end of the previous cycle. Under steady state conditions of repeated cycles, this must therefore equal the signal at the end of the measurement cycle itself For a spin-echo pulse sequence such as that depicted in Figure B 1.14.1 the echo magnetization is given by [17]... 

There are a number of NMR methods available for evaluation of self-diffusion coefficients, all of which use the same basic measurement principle [60]. Namely, they are all based on the application of the spin-echo technique under conditions of either a static or a pulsed magnetic field gradient. Essentially, a spin-echo pulse sequence is applied to a nucleus in the ion of interest while at the same time a constant or pulsed field gradient is applied to the nucleus. The spin echo of this nucleus is then measured and its attenuation due to the diffusion of the nucleus in the field gradient is used to determine its self-diffusion coefficient. The self-diffusion coefficient data for a variety of ionic liquids are given in Table 3.6-6. 

Figure 2.1 (A) Spin-echo pulse sequence. (B) Effect of spin-echo pulse sequence...

Figure 5.19 Modulation effects of a doublet and triplet by a spin-echo pulse sequence, 90°-T-180°-T-echo.

MRP can also be performed with spin-echo pulse sequences, which results in sensitivity to contrast in vessels of all sizes that more closely (but not perfectly) approximates the truly uniform sensitivity achieved by However, this... 

Wideline NMR spectra were collected using a Bruker CXP 200 NMR spectrometer, operating at u>o/2n ( H) = 30.7 MHz. To obtain spectra void of spectrometer artifacts, the solid spin echo pulse sequence, n/2) -T-n/2) -x-echo, was used. Unless otherwise noted, the delay between pulses, X, was set at 30 Js. 

The DDIF experiment consists of a stimulated echo pulse sequence [50] and a reference scan to measure and separate the effect of spin-lattice relaxation. The pulse diagrams for these two are shown in Figure 3.7.2. Details of the experiments have been discussed in Ref. [51] and a brief description will be presented here. 

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. 

Fig. 5 The quadrupolar echo pulse sequence and coherence transfer pathways for I = 1. The pathway marked in red is eliminated by the EXORCYCLE but not by the conventional quadrupolar echo...

The 14N spectra of single crystals of h-GaN (Fig. 3, bottom right) are doublets, corresponding to the two transitions — 1 <-> 0 and 0 +1, and also show the expected orientation dependence for a Cq of 24.14 kHz [49]. A 90 — t — 90v x quadrupolar-echo pulse sequence was used to eliminate probe ringdown. The spin-... 

Measurement of a true T2 can be obtained using a spin-echo pulse sequence, such as the Carr-Purcell-Meiboom-Gill (CPMG) sequence, which minimizes the loss of phase coherence caused by inhomogeneities (Kemp, 1986). 

Figure 8.3 Illustration of HR-MAS techniques applied to a resin-bound trisaccharide (a) static XH spectrum of the solvent swollen sample (b) XH spectrum with magic-angle spinning at 3.5 kHz (c) H spectrum with MAS and spin echo pulse sequence.

Figure 8.7 HR-MAS spectra of resin-bound intermediates in the solid-phase synthesis of a trisaccharide using thiodonors. Traces a and b have been recorded using spin echo pulse sequences to suppress resonances of the resin. Traces c and d have been recorded using a standard proton experiment.

T2 measurements usually employ either Carr-Purcell-Meiboom-Gill (CPMG) [7, 8] spin-echo pulse sequences or experiments that measure spin relaxation (Tlp) in the rotating frame. The time delay between successive 180° pulses in the CPMG pulse sequence is typically set to 1 ms or shorter to minimize the effects of evolution under the heteronuc-lear scalar coupling between 1H and 15N spins [3]. 

The NMR experiments were performed using the quadrupolar echo pulse sequence 7i/2x—Ti—7i/2y—T2—acquisition with phase-cycling and quadrature detection. A Bruker MSL 400 spectrometer was used for the high pressure studies operating at a resonance frequency of 61.4 MHz. In the liquid-crystalline phase, perdeuterated lipids display NMR spectra, which are superpositions of axially symmetric quadrupolar powder patterns of all C-D bonds.From the sharp edges, the quadrupolar splittings... 

In DFS, this is accomplished by applying an RF pulse with amplitude modulation so that, in the frequency domain, the RF frequency changes linearly and adiabatically as a function of time on both sides of the CT. This "DFS pulse" is applied at the beginning of the pulse sequence, being followed by a selective pulse applied solely to excite and detect the CT (or by a conventional spin-echo pulse sequence, also with CT-selective... 

The former is a protein of 14.7 kDa involved in the multienzyme nucleotide excision repair (NER) pathway with a determined NMR solution structure . In this protein, the Zn + possesses rather a structural than a catalytic role. Zn NMR spectra were acquired using a rather sophisticated probe (for details, see Reference 87) and operating at temperatures 5-250 K. Data acquisition was performed with the application of spin-echo methods for enhanced sensitivity . Specifically, experiments were carried out at 25 K using a combination of CP (cross-polarization) and spikelet echo pulse sequences which provide a considerable increase in signal-to-noise ratio (of the order of 30) relative to a classical quadrupole echo pulse sequence. The proton field strength applied to the above measurements was 60 kHz with a matching field of 20 kHz for zinc and a contact time... 

Fig. 5. A spin-echo pulse sequence used to determine T, . (a) A (it/2)y pulse puts Mq into the /-direction, and (b) the spin isochromats dephase with time. At a time Ij later, a it pulse is applied along the j -axis, causing the spins to rotate through it radians (c) such that they refocus along the /-aids to form an echo at time 21 (d). The decrease in magnitude of the magnetization vector between stages (a) and (d) provides a measure of (Eq. (4)). All reversible contributions to the T2 process are removed by the application of the it pulse.

Fig. 6. (a) Schematic representation of a simple slice-selective 2-D spin-echo pulse sequence. In this pulse sequence the magnetic field gradient (G, ) is varied for successive acquisitions of different rows of the k-space raster, (b) The corresponding k-space raster used to show how we interpret the pulse sequence. Following a sufficient Ti-relaxation period, the sequence is repeated to acquire a second row of the k-space raster. Acquisition of each row of k-space requires a separate r.f. excitation and application of a G,-gradient of different magnitude. 

By chosing short values of x, the effect of diffusion may be eliminated, because the diffusion is effective in reducing the echo intensity only during the 2x period. However, there is a modified version of spin-echo pulse sequences which provides the measurement of the spin diffusion coefficients (D) by NMR, as it will be shown later. 

Fig. 2.37. Spin-echo pulse sequence (a) components of transverse magnetization at time z after the 90 pulse (b) 180" rotation of the vectors...

This procedure is based on the unique response of the hydrogen nuclei (protons) present in bound moisture versus free oil. The physical state of the moisture and oil is a critical factor in experiment design. Since the oil in typical oilseeds is present in a liquid state and moisture is bound under conditions in a mature seed, a simple spin echo pulse sequence can determine the quantities separately. 

Figure A1.3.2 Hahn Echo (Spin Echo) pulse sequence and schematic NMR signal from an oilseed. Label A1 indicates the position of the water plus oil sampling window. The label A2 indicates the oil-only sampling window.

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