ABSTRACT
Cross-polarization-the process of generating rare-spin polarization at the expense of the abundant spins-was also an old concept. Although introduced ear lier with a different emphasis, cross-polarization was first applied to naturally abundant 13C by Pines et al. in 1972 [4,5]. This technique circumvents the long T\ values normally encountered in solid materials and usually results in an enhance ment of the signal of the observed rare spin. As we shall see in Section 7.4, the en hancement depends on the ratio of the magnetogyric ratios of the two spins.
