Optimization of sensitivity in pulsed 13C NMR of coals

Pulsed C-13 NMR analyses (SPE) were performed on Argonne Premium coals using a modified version of the ring-down elimination (RIDE) pulse sequence that included high-power proton decoupling. Employing RIDE in combination with the use Of O-2 as a relaxation agent and appropriate considerations of the Ernst-Anderson angle produced high-quality spectra devoid of baseline artifacts in reasonable measuring times. Employing this approach reduced the total acquisition time over the standard procedure of applying a 90degrees excitation pulse by as much as a factor of 1/17 in some instances. Carbon aromaticity values determined in this manner were similar to those obtained previously by SPE using a 90degrees pulse and long recycle delay times. Values determined by both methods are generally found to be higher than those by cross polarization.