Cross-polarization

Cross-polarization (CP), originally published as nuclear double resonance in the rotating frame by Hartmann and Hahn[1] is a solid-state nuclear magnetic resonance (ssNMR) technique used to transfer nuclear magnetization from different types of nuclei via heteronuclear dipolar interactions.

The 1H-X cross-polarization dramatically improves the sensitivity of ssNMR experiments of most experiments involving spin-1/2 nuclei, capitalizing on the higher 1H polarization, and shorter T1(1H) relaxation times.

CP was crucially adapted to magic angle spinning (MAS) by Michael Gibby, Alexander Pines and Professor John S. Waugh at the Massachusetts Institute of Technology[2][3] who adapted a variant of the Hartmann and Hahn experiment designed by Lurie and Slichter.

In CP, the natural nuclear polarization of an abundant spin (typically 1H) is exploited to increase the polarization of a rare spin (such as 13C, 15N, 31P) by irradiating the sample with radio waves at the frequencies matching the Hartmann–Hahn condition:[1] where

The power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetization transfer.

The CP pulse sequence. The sequence starts with a 90º pulse on the abundant channel (typically H). Then CP contact pulses matching the Hartmann-Hahn condition are applied to transfer the magnetization from H to X. Finally, the free induction decay (FID) of the X nuclei is detected, typically with 1 H decoupling.
When the Hartmann Hahn condition is matched, energy levels align in the RF rotating frame, allowing the magnetization transfer.