Cross-polarization (CP), originally published as proton-enhanced nuclear induction spectroscopy (PENIS) [1][2] is a solid-state nuclear magnetic resonance (ssNMR) technique 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 polarisation, and shorter T1(1H) relaxation times.It was developed by Michael Gibby, Alexander Pines and Professor John S. Waugh at the Massachusetts Institute of Technology.In this technique 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:[3] whereThe power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetisation 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 magnetisation 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 magnetisation transfer.
