Distortions in QCPMG Spectra from Pulse Miscalibrations

A standard QCPMG NMR pulse sequence consists of a 90^o pulse followed by a train of 180^o pulses. Ideally, the resulting spikelet envelope should outline the static lineshape from a conventiaonal Hahn-echo experiment. If the first pulse deviates from 90^o 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 180^o pulse misset by as little as 20^o-30^o, 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 180^o 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), C_Q=10 MHz, η_Q=0.7, CS anisotropy= -200 ppm, coincidental EFG and CSA tensors, ω_RF/2π= 200 kHz.

Many thanks to Eric Ye of the National Ultra-high Field NMR Facility for Solids for contributing this post.