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.
![](http://up.wiki.x.io/wikipedia/commons/thumb/c/cb/Cross-polarization.png/200px-Cross-polarization.png)
![](http://up.wiki.x.io/wikipedia/commons/thumb/b/b1/Hartmann_Hahn.png/220px-Hartmann_Hahn.png)
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]
where are the gyromagnetic ratios, is the spinning rate, and 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 T1ρ(X) relaxation, i.e. the decay of the magnetisation during a spin lock.
References
edit- ^ Pines, A.; Gibby, M. G.; Waugh, J. S. (1972-02-15). "Proton Enhanced Nuclear Induction Spectroscopy. A Method for High Resolution NMR of Dilute Spins in Solids". The Journal of Chemical Physics. 56 (4): 1776–1777. Bibcode:1972JChPh..56.1776P. doi:10.1063/1.1677439. ISSN 0021-9606.
- ^ US 3792346, "Proton-enhanced nuclear induction spectroscopy"
- ^ Hartmann, S. R.; Hahn, E. L. (1962). "Nuclear Double Resonance in the Rotating Frame" (PDF). Phys. Rev. 128 (5): 2042–2053. Bibcode:1962PhRv..128.2042H. doi:10.1103/PhysRev.128.2042.