Cross polarisation

From NMR Wiki

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

In this example the abundant nucleus was chosen to be ¹H and the observed nucleus is ¹³C. This also works with other combinations. 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 power ratio of these pulses needs the 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 fiend B₁ must be 4 time 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's depend on the T₁ of the proton instead the one of lower ratio nuclei). The major limitation is that this experiment require high power irradiation, that could deteriorate the sample or the probe.