Cross-polarization

{{Short description|Spectroscopy technique}} {{For|Cross-polarized light|Polarized light microscopy}} thumb|200px|right|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 magnetization from H to X. Finally, the free induction decay (FID) of the X nuclei is detected, typically with ¹H decoupling. '''Cross-polarization''' ('''CP'''), originally published in 1962 as '''nuclear double resonance in the rotating frame''' by Hartmann and Hahn<ref name=":0" /> 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<ref>{{Cite journal |last1=Pines|first1=A.|last2=Gibby|first2=M. G.|last3=Waugh|first3=J. S.|date=1972-02-15|title=Proton Enhanced Nuclear Induction Spectroscopy. A Method for High Resolution NMR of Dilute Spins in Solids|url=http://aip.scitation.org/doi/10.1063/1.1677439|journal=The Journal of Chemical Physics|language=en|volume=56|issue=4|pages=1776–1777|doi=10.1063/1.1677439|bibcode=1972JChPh..56.1776P |issn=0021-9606|url-access=subscription}}</ref><ref>{{cite patent|country=US|number=3792346|title=Proton-enhanced nuclear induction spectroscopy}}</ref> who adapted a variant of the Hartmann and Hahn experiment designed by Lurie and Slichter.<ref>{{Cite journal |last=Lurie |first=Fred M. |last2=Slichter |first2=Charles P. |date=1964-02-17 |title=Spin Temperature in Nuclear Double Resonance |url=https://journals.aps.org/pr/abstract/10.1103/PhysRev.133.A1108 |journal=Physical Review |volume=133 |issue=4A |pages=A1108–A1122 |doi=10.1103/PhysRev.133.A1108|url-access=subscription }}</ref> The technique is now widely known as CPMAS. thumb|When the Hartmann Hahn condition is matched, energy levels align in the RF rotating frame, allowing the magnetization transfer. 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:<ref name=":0">{{cite journal|last1=Hartmann |first1=S. R. |last2=Hahn |first2=E. L. |title=Nuclear Double Resonance in the Rotating Frame |journal= Phys. Rev. |volume=128 |year=1962 |issue=5 |pages= 2042–2053 |doi=10.1103/PhysRev.128.2042 |bibcode=1962PhRv..128.2042H |url=http://astrophysics.fic.uni.lodz.pl/100yrs/pdf/09/065.pdf}}</ref>

:<math>\gamma_H B_1(^{1}\text{H}) = \gamma_X B_1(\text{X}) \pm n \omega_R</math>

where <math>\gamma</math> are the gyromagnetic ratios, <math>\omega_R</math> is the spinning rate, and <math>n</math> 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== {{Reflist}}

Category:Nuclear magnetic resonance Category:Spectroscopy