Cross-polarization

Cross-polarization (CP), originally published in 1962 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.

In 1972 CP was crucially adapted to magic angle spinning (MAS) by Michael Gibby, Alexander Pines and 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.^[4] The technique is now widely known as CPMAS.

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]

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

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 magnetization transfer.

The evolution of the X NMR signal intensity during the cross polarization 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 magnetization occurs, during which the transfer of ¹H magnetization from nearby spins (and remote spins through proton spin diffusion) to X occurs. For longer CP contact times, the X magnetization decreases from T_1ρ(X) relaxation, i.e. the decay of the magnetization during a spin lock.