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Pulse Sequences

QCPMG

The Carr-Purcell-Meiboom-Gill sequence for quadrupolar nuclei (QCPMG) is a technique that can generate an order of magnitude sensitivity enhancement when compared to Hahn-echo and solid-echo experiments. The pulse sequence is composed of 3 parts;

• Application of a standard solid-echo pulse sequence and recording of the FID.
• A series of refocusing p/2 pulses are applied with the complete echo being recorded after each pulse in the train.
• Signal that remains is allowed to decay while being recorded before starting all over again.

The resulting spectrum is a manifold of spin-echo sidebands where both the shape of the overall envelope and individual spikelets contain information on molecular motion.

 

Quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence dramatically improves signal-to-noise in solid-state NMR experiments of quadrupolar nuclei, here for 87Rb (I=3/2).


October, 2005

 

In materials ultrahigh magnetic fields are beneficial when studying low-gamma (and) quadrupolar nuclei. These 39K NMR spectra of KNO3 were recorded on our 900 instrument at the resonance frequency of 42 MHz (compare with 23 MHz at 11.7 T). Experiments performed by I. Moudrakovski (SIMS-NRC) with the wideline static probe built by J. Bennett (NRC). (don't mind S/N in the Hahn-echo spectrum, only few scans were accumulated !)

February, 2006

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A standard QCPMG NMR pulse sequence consists of a 90 pulse followed by a train of 180 pulses (more). Ideally, the resulting spikelet envelope should outline the static lineshape (middle spectrum, 90-180). If the first pulse deviates from 90 due to incorrect calibration, the QCPMG spikelet pattern does not change significantly, the only effect is somewhat lower overall intensity (Figure A).

While the miscalibrated 90 pulse alone has little impact on the QCPMG lineshape, of course it is often used to calculate the 180 pulse. As can be seen both, experimentally and using SIMPSON calculations, the miscalibrated 180 pulse leads to significantly distorted spikelet patterns (Figure B). The 180 pulse misset by as little as 20-30 degrees could produce considerable oscillations in spikelet intensity across the envelope. This illustrates that the QCPMG NMR experiments are much more sensitive to proper setup of the 180 degree pulse than the Hahn-echo experiments.

QCPMG spectra shown were calculated by Eric Ye (900 Facility) using the SIMPSON software for a central transition of a spin 3/2 nucleus resonating at 295 MHz, CQ=10 MHz, etaQ=0.7, CS anisotropy -200 ppm, coincidental EFG and CSA tensors.

For more information see

Renée Siegel, Thomas T. Nakashima and Roderick E. Wasylishen, "Signal-to-Noise Enhancement of NMR Spectra of Solids Using Multiple-Pulse Spin-Echo Experiments", Conc. Magn. Reson. 26A (2005) 62-77. http://dx.doi.org/10.1002/cmr.a.20038