Pulsed signal attenuation

The flow artifacts detected in the droplet size measurements are similar to those reported by Goux et al. [79] and Mohoric and Stepisnik [80]. In their work natural convection effects led to an increase in the decay of signal attenuation curves, causing over-prediction in the self-diffusion coefficient of pure liquids. In order to avoid flow effects in droplet size distributions, flow compensating pulse sequences such as the double PGSTE should be used. It has been demonstrated recently that this sequence facilitates droplet size measurements in pipe flows [81]. 

Only a few specific operating parameters have a major Impact on the spectra obtained. These include the excitation level (the power of the RF pulse), the filament emission current, the electron beam voltage, the trapping voltage, (which is applied to the cell trapping plates), and the signal attenuation. Each of these has to be tuned specifically to maximize response for any peak(s) of interest, and this is particularly Important for the lower field strength analyses. 

If the z coordinates of the individual spins at the time of the second field gradient pulse differ from those during the first, then the dephasing of the first field gradient is not completely compensated. This leads to a decrease in the signal intensity, which is indicated in Fig. 7c and d by the broken lines. It is this signal attenuation that is analyzed in the NMR diffusion measurements. 

For a quantitative treatment we have to consider that the spins of atoms that have been at positions Zq and Zq + z during the first and second field gradient pulses suffer a net phase shift y8gz, where the z coordinate has been assumed to be determined by the direction of the pulsed field gradients. Hence, summing over the contribution of all spins, one may write for the signal attenuation under the influence of the pulsed field gradients... 

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