Solid echo train

Fig. 2.4.7 Profile of a PE gasoline tank wall. T-, and T2 were measured across the sample, and uniform values of about 90 ms and 300 ps, respectively, were obtained, except for EVOH where T2 drops to 30 [is. The profile amplitude is the coefficient at zero frequency of the FT of the signal obtained as the direct addition of the first 8 echoes generated with a solid-echo train.

Figure 2.4.11 Profiles of paintings where different layers can clearly be resolved. A solid-echo train was used with tE = 40 (is, and the first 4 echoes were used to calculate the amplitude. The profiles were reconstructed by moving the sensor in steps of 50 pm in the paint and canvas regions, and 100 pm in the gypsum and wood layers. Using 128 scans per point and a repetition time of 100 ms the total acquisition time per point was 16 s. Profiles of paint based on tempera ( ) and oil ( ) binders show appreciable difference.

The slice selection procedure can be combined with a number of pulse sequences to spatially resolve NMR parameters or to contrast the profiles with a variety of filters. The most commonly used acquisition schemes implemented to sample echo train decays are the CPMG [(jt/2)0—(Jt)90] or a multi-solid echo sequence [(jt/ 2)0-(jt/2)9o]. In these instances, the complete echo train can be fitted to determine... 

Fio. 9.3.4 Excitation with a surface coil of 9 mm diameter in a Bo gradient of the order of lOT/m by the NMR-MOUSE (a) Series train of CPMG echoes from a carbon-black filled SBR section of an intact car tyre with a steel belt. A fit of the echo envelope with an exponential decay function yields a transverse relaxation time Ti [Eidl]. (b) Variation of the pulse duration in an a — te/l — Ta — ts/ i- pulse sequence for different rf frequencies. For each frequency maxima and minima are observed which define the nominal 90° and 180° pulse widths. With decreasing rf frequency the distance of the sensitive volume from the rf coil increases. A frequency of 17.5 MHz correspond to depths of 0-0.5 mm, 16MHz to 0.5-1.0mm, and 16.5 MHz to 1.0-1.5 mm into the sample [Gut3], (c) Hahn- and solid-echo envelopes for a sample of carbon-black filled cross-linked SBR. The Hahn-echo decay is faster because of residual dipolar couplings which are partially refocused by the solid-echo [Gut3]. (d) Multi-echo excitation. 

For quadrupolar nuclei with integral spins STRAFI studies have been reported for H and N (both 7= 1). In this case there is no central transition and the full effects of quadrupolar broadening should be expected when solids are imaged. Deuterium has only a relatively low quadrupolar coupling constant (e.g. Cq < 200 kHz in heavy water ice) and there was little appreciable effect on the echo shapes produced by either the odd or even pulse sequences. Heavy ice was produced by freezing and maintaining heavy water samples ( H enriched to 99.8%) at 268 K, while deuteriated samples of copper sulphate and silica gel were obtained by the addition of heavy water to the anhydrous samples. The echo trains for the last two samples decayed relatively rapidly and only about 16 echoes could be obtained for each train. In contrast, very long echo trains (up to 9000 echoes) were obtained for both... 

Figure 8. Double logarithmic plot of T2<, the long decay constant ( = effective) of the spin-echo train, vs. t, the spacing of the first two pulses in the SLSE excitation sequence. The transition is the v line of NaN02 at 77 K at 3757 kHz. The solid line is a fit to the data of t5T2 = constant. Also shown are the relaxation times Tj and T2 for the transition.

However, wider exploitation of multidimensional solid-state NMR methods is hindered by low Si sensitivity, which results from low natural abundance of spin-1/2 isotope (4.7%), small gyro-magnetic ratio, and unusually slow longitudinal relaxation. Fortunately, the presence of a long relaxation time in inorganic solids is often accompanied by a long transverse dephasing (decoherence) time defined here as the decay time of the echo train due to time-dependent interactions... 

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