33S MAS NMR of a disordered sulfur-doped silicate: Signal enhancement via RAPT, QCPMG and adiabatic pulses
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Three different signal enhancement techniques have been applied to 33S magic-angle spinning nuclear magnetic resonance (MAS NMR) of a disordered silicate containing 1.15 wt% 33S. Partial saturation of the satellite transitions was achieved using a rotor-assisted population transfer (RAPT) pulse sequence, resulting in a signal enhancement of 1.63, albeit with a slight distortion of the line shape due to selective excitation. Adiabatic inversion of the satellite transitions by various amplitude-and frequency-modulated pulse shapes (such as hyperbolic secant and wideband uniform-rate smooth truncation) was also attempted, resulting in a signal enhancement of up to 1.85, with no apparent line shape distortion. Quadrupolar Carr-Purcell-Meiboom-Gill (QCPMG) and RAPT-QCPMG sequences were also used, both of which yielded spikelet spectra that accurately reflected the MAS line shape with a greatly improved signal-to-noise ratio. It is hoped that this study demonstrates that 33S solid-state MAS NMR is now feasible even on disordered, low-sulfur-content systems.
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