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Tuesday, December 4, 2007

Sensitivity Improvement from Cross Polarization

Yesterday's post showed the pulse scheme for cross polarization in solids. Below is a comparison of the single scan 39.7 MHz 29Si MAS spectra for solid tetrakis(trimethylsilyl)silane. The trace on the left was acquired with a simple one pulse sequence with high power CW proton decoupling during the acquisition (Bloch decay). The trace on the right was acquired with cross polarization and high power CW 1H decoupling. In both cases the sample was spinning at 2 kHz. The signal-to-noise improvement resulting from cross polarization is obvious.

4 comments:

Anonymous said...

Perhaps you could also post the recycle delays used in each case?

Dave

Glenn Facey said...

Dave,

The spectra were acquired with one scan after complete relaxation.

Kenneth Knott said...

Interestingly, I believe the proton transfer also depends upon the rigidity of the solid. For example, we have looked at insoluble silicones which are rubbery. These show excellent lineshape and signal in single pulse hpdec sequences and show little benefit from CP - perhaps even a reduction in signal. I wasn't expecting this. I wonder if there is another explanation.

Glenn Facey said...

Kenneth,
Cross polarization relies on the dipolar interaction between the protons and the X nucleus. Mobile materials like silicones have a much reduced dipolar interaction due to motional averaging. Small dipolar couplings require a very precisely set Hartman Hahn match. If the Hartman Hahn match is not set precisely, then CP will be inefficient. This problen can be addressed by using ramped CP contact pulses.

Glenn