Spin-echo pulse sequences

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. 

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. 

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]. 

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... 

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.

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.

Spin-spin relaxation times T2 are determined by the Carr-Purcell-Meiboom-Gill spin echo pulse sequence and provide information about slower molecular motions (66,67). 

Resonance offset effects on spin-echo pulse sequences... 

In Figures 2 and 3 are shown the GAMMA C++ code for a simulated spin-echo pulse sequence using ideal RF pulses and a simulated PRESS pulse sequence that accounts for a non-ideal RF pulse envelope, crusher gradients and spatially varying RF refocusing due to resonance group chemical shift offset and... 

Figure 2 shows the code for the spin-echo pulse sequence. In lines 1-7, NMR object variables are declared. In lines 9-16, these objects are assigned values or specific NMR operations. A lactate spin system is read in from a file using a standard GAMMA text file format. Lines 20-24 are the code for the actual 90°Y-delay-180°Y-delay-acquire pulse sequence. In line 26, the density matrix is parsed into a transition table for the specified observation operator. And the "write results" function in line 27 converts the GAMMA transition table into three arrays of ppm, area and phase values one value for each line found in the transition table. This is a fairly trivial example, and the use of ideal pulses is often not sufficient to account for real-world artefacts in a simulation, but it shows how the object-oriented style of coding results in short amounts of code that is easy to read and comprehend. When compiled with the Visual Studio C++ compiler on a... 

Figure 2 A simple example of GAMMA C++ code that initializes objects and organizes them into a spin-echo pulse sequence simulation that uses ideal RF pulses.

Let us investigate what happens when we alter the normal, 3C ( H experiment in Figure 12.2 by using the /-modulated spin echo pulse sequence shown in Figure 12.8a. This pulse sequence is closely related to the spin echo technique described in Section 3.6.3. 

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