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Monday, December 3, 2007

Cross Polarization

The sensitivity of NMR spectra for solids containing protons can be increased dramatically by using cross polarization. Cross polarization is a technique where magnetization is transferred from an abundant proton source to a dilute (isotopically of chemically) nucleus, X, during a "contact" period. During the contact time, rf fields for both 1H and X are turned on. The ratio of power levels between 1H and X must be equal to the ratio of gyromagnetic ratios between X and 1H. The enhanced magnetization of the dilute isotope is then detected while the abundant protons are decoupled. The maximum gain in sensitivity is equal to the ratio of gyromagnetic ratios between 1H and X. The technique has the additional advantage in that the relaxation delay can be chosen based on the T1(rho) for 1H rather than the T1 of the dilute isotope, which is often larger by at least an order of magnitude.

4 comments:

Xun Liu said...

Thanks for your work on this blog. It is of great help to my understanding CP. Still i got one question about the power levels. Since "The ratio of power levels between 1H and X must be equal to the ratio of gyromagnetic ratios between X and 1H", then how to decide the first contact power of X or 1H? The higher the better within safety range?
Thanks.

Glenn Facey said...

Xun,

I would start with a safe 1H power level, then with a standard sample, array the X power level until you observe the maximum signal.

Glenn

Juan Carlos said...

Dear Glenn,

is it the power level (Watts) or the field strength (kHz, 1/4*tau90) what you have to consider for CP optimization. I understand it's field strength, i.e., B1.
If we consider a 1H-13C CP, i.e, gamma(1H) = 4*gamma(13C), the HH condition is simplified to:
tau90(13C) = tau90(1H)/4

However, in practice I've always seen people using very similar values of tau90 for both 1H and 13C (2.5 and 2.8, 3 and 3.3, or similar). Is it because we normally use ramp-CP?

I'd very much appreciate your comments.
Regards.
Juan.

Glenn Facey said...

Juan,
I think you should find that the 1H and 13C pulses are equal if the HH condition is met with rectangular matching pulses.
Glenn