Repetition times

These experiments yield T2 which, in the case of fast exchange, gives the ratio (Aoi) /k. However, since the experiments themselves have an implicit timescale, absolute rates can be obtained in favourable circumstances. For the CPMG experiment, the timescale is the repetition time of the refocusing pulse for the Tjp experiment, it is the rate of precession around the effective RF field. If this timescale is fast witli respect to the exchange rate, then the experiment effectively measures T2 in the absence of exchange. If the timescale is slow, the apparent T2 contains the effects of exchange. Therefore, the apparent T2 shows a dispersion as the... 

H-NMR studies were performed on a Bruker MSL-400 spectrometer operating in the Fourier transform mode, using a static multinuclei probehead operating at 400.13 MEtz. A pulse length of 1 iis is used for the 90° flip angle and the repetition time used (1 second) is longer than five times Tjz ( H) of the analyzed samples. 

Fig. 6. The CP/MAS spectra of cellulose acetate-butyrate (CAB) and of cellulose acetate (CA, degree of substitution = 1.97), 20. The observation frequency was 50.1 MHz and the irradiation frequency 199.5 MHz. The pulse repetition time was 5 s and the contact time 2 ms. For CAB 400 scans and for CA 60 scans were collected...

Fig. 9. Solid-state NMR spectra of stiff chain aromatic polyesters containing sulfur bonds and tentative assignements of their signals, 401. A contact time of 2 ms and a pulse repetition time of 10 s were used...

Fig. 1.20 Gradient-echo based pulse sequences based on low flip angles. When low flip angles and short image repetition times are employed at the expense of transverse magnetization during the course of the complete image acquisition, this represents a FLASH sequence (without ). The combination of flip angle and repetition time can be adjusted in...

Fig. 2.4.5 Profile of a phantom made of three 2-mm thick rubber layers separated by glass slides of 2- and 1-mm thick. The CPMG sequence was executed with the following parameters repetition time = 50 ms, tE = 0.12 ms, number of echoes = 48 and 64 accumulations. The profile was scanned with a spatial resolution of 100 pm in 5 min.

A nominal spatial resolution of 50 pm was set, acquiring an echo window 20-ps long. The position of the sensor was moved in steps of 25 pm requiring 160 points to cover the complete sample thickness. Using 512 scans per point and a repetition time of 150 ms the acquisition time per point was 75 s. 

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.

Fig. 2.4.14 Profile of a multi-layer polymer coating used to protect concrete surfaces from environmental corrosion. The profile is the signal amplitude resulting from the addition of the first 32 echoes acquired with a CPMG sequence with tE = 50 ps. It has an FOV of 8 mm and was measured with a spatial resolution of 100 pm. Using 256 scans per point and a repetition time of 100 ms, the total acquisition time per point was 25 s.

Fig. 3.4.12 Three-dimensional rendered spin-echo image of water filled cracks in a cement paste specimen [13]. Three cracks are visible in the image a large triangular crack in the forefront, a smaller crack in the bottom left corner and a sheet-like structure at the top of the image. Water droplets can also be observed condensing on the cement paste surfaces. The measurement parameters were FOV 20 x 20 x 20 mm, acquisition points 128 x 128 x 64, nominal resolution 156 x 156 x 312 pm, echo time 2.7 ms, repetition time 500 ms and acquisition time 270 min.

Our method is demonstrated with experiments on a Bentheimer sandstone sample. The sample was prepared to be cylindrically shaped with a diameter of 2.5 cm and a length of 2.0 cm. The sample was fully saturated with de-ionized water under vacuum. We performed the CPMG imaging experiment described in the previous section to measure the magnetization intensity at 50 echoes spaced by 4.6 ms for each of 32 x 16 x 16 voxels within the field of view of 3.0 cm x 3.0 cm x 3.0 cm. The corresponding voxel size is 0.938 mm x 1.88 mm x 1.88 mm. We used 1 s of repetition time (TR) and the total imaging time was 4.3 min. 

Proton NMR and deuteron NMR spectra of soluble fractions and spent solvent mixtures were obtained by using a JE0L FX60Q FT NMR Spectrometer. A flip angle of 45° was used which corresponds to 14 ms for and 75 ms for 2H. The pulse repetition times were 6.0 and 9.0 s, respectively. Chloroform-d was used as the NMR solvent, and chloroform was used as the 2H NMR solvent. 

The "decrease of the spin temperature means an increase of population difference between the upper and lower energy spin states and consequently an increased sensitivity of the NMR experiment. From Equation (25), the temperature of dilute spins has been lowered by a factor 7x/y1 h, that is, V4 when X = 13C. This means an increased sensitivity of the FID resonance experiment equal to about 4 for the 13C nuclei. Because the X signal is created from the magnetization of dilute nuclei, the repetition time of NMR experiment depends on the spin-lattice relaxation time of the abundant spin species, protons, which is usually much shorter than the spin-lattice relaxation times of the dilute nuclei. This, a further advantage of cross polarization, delay between two scans can be very short, even in the order of few tens of milliseconds. 

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