Cross-polarization experiment basic pulse sequence

Cross-polarization NMR relies on the presence of an abundant spin system ( H) to enhance the observation of a signal from a dilute spin system ( C). The idea is to transfer polarization (hence signal intensity) from the abundant H spins to the dilute C spins. The CP experiment consists of four basic timed sequences of radio-frequency (r.f.) pulses (Fig. 4). The left portions of Fig. 4 represent the experimental timing sequence and the right portions depict what happens to the nuclei in the sample under the action of the pulses. The four-part procedure consists of (1) polarization of the H spin system. 

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