Cross-polarization (CP), also known as proton-enhanced nuclear induction spectroscopy is a

solid-state nuclear magnetic resonance Solid-state NMR (ssNMR) spectroscopy is a technique for characterizing atomic level structure in solid materials e.g. powders, single crystals and amorphous samples and tissues using nuclear magnetic resonance (NMR) spectroscopy. The anisotropic pa ...

(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 Alexander Pines (born June 22, 1945) is an American chemist. He is the Glenn T. Seaborg Professor Emeritus, University of California, Berkeley, Chancellor's Professor Emeritus and Professor of the Graduate School, University of California, Berke ...

and Professor John S. Waugh at the

Massachusetts Institute of Technology The Massachusetts Institute of Technology (MIT) is a Private university, private Land-grant university, land-grant research university in Cambridge, Massachusetts. Established in 1861, MIT has played a key role in the development of modern t ...

In this technique the natural nuclear

polarization Polarization or polarisation may refer to: Mathematics *Polarization of an Abelian variety, in the mathematics of complex manifolds *Polarization of an algebraic form, a technique for expressing a homogeneous polynomial in a simpler fashion by ...

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 ratio In physics, the gyromagnetic ratio (also sometimes known as the magnetogyric ratio in other disciplines) of a particle or system is the ratio of its magnetic moment to its angular momentum, and it is often denoted by the symbol , gamma. Its SI u ...

\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 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.

References

Nuclear magnetic resonance Spectroscopy {{Chemistry-stub