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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.

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

8 comments:

Victor said...

For more information on the origin of spikelet oscillations in CPMG/QCPMG see

R.Siegel, T.T. Nakashima and R.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

Jim Frye said...

Suggestion: simulate the effect of RF inhomogeneity on CPMG pattern. A 180 pulse in the center of a coil may generate a 90 pulse at the edge (rule of thumb).

Anonymous said...

Hi,

I have a question about the aquisition time. Is the aquisition time the same for all the echoes? I see that the first echo is defined as 1/2 the aquisition time in the literature, but I am assuming that they mean that the actual signal is only present for the 1/2 the time than the other echoes, which is why they define it this way.

thanks.

Glenn Facey said...

Anonymous,

The first signal is not really an echo as it occurs after the first 90 degree pulse. (see http://u-of-o-nmr-facility.blogspot.com/2008/11/qcpmg.html )so the "acquisition time" for the first signal is indeed 1/2 the the "acquisition time" for the the echoes.

Glenn

Anonymous said...

Thanks for your reply. So is the first signal (FID) usually aquired in this experiment? Or can one just aquire the echo maximum as the first signal in the echo train?

Glenn Facey said...

Anonymous,

The particular pulse program I use does not collect the data for the signal immediately following the 90 deg pulse. It only collects the echoes, all of which use the same "acquisition time".

Glenn

Peter said...

Hi,

short question about the experiment:
Do the transmitter frequency and one of the spikeletts always appear at the same frequency?

Or do the spikeletts radiate out from the isotropic chemical shift (as it would be the case when recording a Deuteron Pake Pattern under MAS), independent of the frequency where I set my rf pulses?

Glenn Facey said...

Peter,

The spiklets are symmetric about the transmitter frequency.

Glenn