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Friday, May 2, 2008

Cross Polarization Using Ramped Pulses

Cross polarization is routinely used for sensitivity improvement when measuring the solid state MAS NMR spectra of spin I = 1/2 nuclei (X). This technique relies on dipolar coupling between 1H and X as a means to transfer magnetization. When the dipolar coupling is small or averaged due to molecular motion or fast MAS, the Hartman Hahn matching condition required for cross polarization is very sensitive. This sensitivity means that even minor missettings or instrumental instability can lead to a dramatic (or complete) loss in signal. One way of overcoming this sensitivity is to use a ramped contact pulse on either the X or the 1H channel as in the figure below.
The effect of using such ramped pulses is shown in the figure below where the dipolar coupling is averaged by fast MAS.

One can see that the matching condition is spread out over a broad area and the matching sidebands and troughs are no longer a problem. It is easy to see how using such a scheme makes the CPMAS experiment less susceptible to instrumental instability.

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