Cross-polarization

Cross-polarization (CP), originally published as proton-enhanced nuclear induction spectroscopy[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.

When the Hartmann Hahn condition is matched, energy levels align in the RF rotating frame, allowing the 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 ¹H decoupling.

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]

${\displaystyle \gamma _{H}B_{1}(^{1}{\text{H}})=\gamma _{X}B_{1}({\text{X}})\pm n\omega _{R}}$

where ${\displaystyle \gamma }$ are the gyromagnetic ratios, ${\displaystyle \omega _{R}}$ is the spinning rate, and ${\displaystyle n}$ is an integer. This process is sometimes referred to as "spin-locking". The power of one contact pulse is typically ramped to achieve a more broadband and efficient magnetisation transfer.

The evolution of the X NMR signal intensity during the cross polarisation is a build-up and decay process whose time axis is usually referred to as the "contact time". At short CP contact times, a build-up of X magnetisation occurs, during which the transfer of ¹H magnetisation from nearby spins (and remote spins through proton spin diffusion) to X occurs. For longer CP contact times, the X magnetisation decreases from T_1ρ(X) relaxation, i.e. the decay of the magnetisation during a spin lock.