cross-polarization modulation

Cross-polarization (CP), originally published in 1962 as nuclear double resonance in the rotating frame by Hartmann and Hahn 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 who adapted a variant of the Hartmann and Hahn experiment designed by Lurie and Slichter. 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:

:$\gamma_H B_1(^\text) = \gamma_X B_1(\text) \pm n \omega_R$

where $\gamma$ are the gyromagnetic ratios, $\omega_R$ is the spinning rate, and $n$ 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.

References

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Nuclear magnetic resonance
Spectroscopy