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The NMR signals of isotopically or chemically dilute nuclear spins S in solids can be enhanced by repeatedly transferring polarization from a more abundant species I of high abundance (usually protons) to which they are coupled. The gain in power sensitivity as compared with conventional observation of the rare spins approaches NII(I+1)γI2/NSS(S+1)γS2, or ∼ 103 for S = 13C, I = 1H in organic solids. The transfer of polarization is accomplished by any of a number of double resonance methods. High-frequency resolution of the S -spin signal is obtained by decoupling of the abundant spins. The experimental requirements of the technique are discussed and a brief comparison of its sensitivity with other procedures is made. Representative applications and experimental results are mentioned.
Pines et al. (Sun,) studied this question.