CPMG spin echo sequences

More conventional CPMG spin echo sequences [(90°), -t - ((180°), - t— echo - T - ) ] can also be used with consequent advantages in signal-to-noise ratio." In this case, uniform slice selection is achieved by the use of a 180° pulse of twice the 90° amplitude, rather than twice its length, so that the frequency bandwidth of the pulses is identical. 

A NMR Mouse (Mobile Unit for Surface Exploration) and a CPMG spin echo sequence were used to assess T2 as a function of tin content. Figure 6 and Figure 7 show that as the catalyst levels in the reaction mixture are increased, the level of chain restiction is increased and the T2 is lowered. These results suggest that the tin affects the mobility of the more rigid environments (short T2, such as crosslinks) as well as the mobility of chain ends and mid chain components (long T2). 

CPMG Carr-Purcell-Meiboom-Gill T2-dependent spin-echo sequence 2.4.4... 

T-90°-data acquisition [9] in which the time interval, x, is chosen such that the water resonance, which is expected to have the longest T, in the sample, has zero magnetization after the 90° pulse [113, 114], The CPMG spin-echo pulse sequence 90°-(t-180°-t) n-data acquisition (n = number of repetitions), has been used with the pulse interval, r, adjusted to attenuate the water signal, for example, from erythrocyte and protein suspensions [113], The technique is improved by the addition of ionic species such as ammonium chloride which increases the chemical exchange of the water protons and thus shortens T2 relative to the compounds of interest. This method is known as WATR (Water Attenuation by T2 Relaxation) [114]. Solvent suppression can also be achieved by selective excitation of the spectrum with special pulses such that the water resonance occurs at a point of null excitation [115-119]. However, distortion of peaks near the null point may occur. 

Figure 13.3-2. NMR spectra of rat serum illustrating the various NMR responses that are possible through the use of different pulse sequences, which edit the spectral intensities (a) standard water suppressed spectrum, showing all metabolites (b) CPMG spin-echo spectrum, with attenuation of peaks from fast relaxing components such as macromolecules and lipoproteins (c) diffusion-edited spectrum, with attenuation of peaks from fast diffusing components such as small molecules and (d) a projection of a 2D J-resolved spectrum on to the chemical shift axis, showing removal of all spin-spin coupling and peaks from fast relaxing species.

F ure 2A7. Spin-echo sequences for measuring relaxation times, (a) A basic spin-echo, (b) the Carr-Purcell sequence and (c) the Carr-Purcell-Meiboom iU (CPMG) sequence. 

Fig. 25. Free-induction decay (FID) after a 90° pulse at t=0 and a series of Carr/Purcell/ Meiboom/Gill (CPMG) spin echoes in linear polyethylene (M, =92,300) at 150 °C. The 180° pulses of the CPMG sequence are indicated by vertical lines. The homogeneity of the external magnetic flux density is demonstrated by an FID of water at 30 °C. The data demonstrate the existence of strong internal field gradients arising from voids [152]...

Fig. 1.22 RARE sequence. Here the formation of the first spin echo is conventional. The CPMG form of spin echo is used to avoid the accumulation of flip angle errors over the echo train. However, before the second echo can be acquired, the phase-encoding has to be rewound to undo the dephasing of the spins. Therefore, a phase encoding step of equal...

Fig. 2.7.2 Diffusion-relaxation correlation se- The detection (2nd) segment for both is a quences using pulsed field gradients, (a) The CPMG pulse train that is similar to that in first segment is a spin-echo with the echo Figure 2.7.1. The amplitude or the duration of appearing at a time 2tcpi after the first pulse, the gradient pairs in both sequences is (b) The first segment is a stimulated echo incremented to vary the diffusion effects, appearing at a time tcpi after the third pulse.

Figure 15B shows a screenshot displaying the spin echoes of 195Pt in platinum powder acquired by the CPMG sequence. The experiments were performed at a carrier frequency of 50.74 MHz. The 19SPt signal... 

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). 

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 measurements were performed on systems composed of ca. 25 wt. % samples of aPS (M = 6.6x 10 g/mol, PD = 1.1, Pressure Chemical Company) in either reagent grade toluene (Aldrich) or toluene-d0 (Aldrich). The protonated solvent was used for the diffusion measurements and the deuterated solvent for the relaxation studies. At this concentration, the Tg j for the system was determined to be about -65 °C (i). The NMR spectra were run on a JEOL FX-90Q NMR spectrometer operating at 90 and 14 MHz for protons and deuterons, respectively. The T and T2 measurements were made with the standard inversion-recovery and spin-echo (CPMG) sequences, respectively. 

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