Cross polarisation

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A basic Cross Polarisation (CP) Experiment goes as follows:

1H 13C cross-polarization NMR experiment

In this example the abundant nucleus was chosen to be 1H and the observed nucleus is 13C. This also works with other combinations of nuclei. The abundant nucleus is excited, and its energy is then transfered to the observed nucleus by using a long low power pulse on both channels. The RF power ratio of these pulses needs to be tuned so that the transition energy for both nuclei is the same. So that for instance for the polarization transfer from a 1H to a 13C, the rotating field B1 must be 4 times weaker for the proton channel than for the carbon channel.

This method often gives a much stronger signal than direct excitation, allow faster repetition rate (it now depends on the T1 of the proton instead the one of lower gyromagnetic ratio nuclei). The major limitation of CP method is the requirement of high power irradiation, that could deteriorate the sample or the probe.

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