Many thanks to Eric Ye of the National Ultra-high Field NMR Facility for Solids for contributing this post.
Friday, July 31, 2009
Distortions in QCPMG Spectra from Pulse Miscalibrations
A standard QCPMG NMR pulse sequence consists of a 90o pulse followed by a train of 180o pulses. Ideally, the resulting spikelet envelope should outline the static lineshape from a conventiaonal Hahn-echo experiment. If the first pulse deviates from 90o due to incorrect calibration, the QCPMG spikelet pattern does not change significantly, the only effect is somewhat lower overall intensity (first figure). At the same time the miscalibrated subsequent pulses lead to significantly distorted spikelet patterns (second figure). The 180o pulse misset by as little as 20o-30o, could produce considerable oscillations in the spikelet intensity across the envelope. This illustrates that QCPMG NMR experiments are much more sensitive to proper setup of the 180o pulses than the Hahn-echo experiment. The QCPMG spectra shown were calculated in SIMPSON for a central transition of a spin 3/2 nucleus resonating at 295 MHz (87Rb at 21.1 T), CQ=10 MHz, ηQ=0.7, CS anisotropy= -200 ppm, coincidental EFG and CSA tensors, ωRF/2π= 200 kHz.