Cross-polarization (nuclear magnetic resonance)

Cross-polarization (nuclear magnetic resonance) transfers nuclear spin nuclear spin polarization between different nuclear populations, commonly from abundant hydrogen nuclei to dilute carbon-13 nuclei in a solid. Applied radio frequency fields bring the two spin populations into a condition that permits polarization exchange through magnetic dipolar coupling.^[ref]

For on-resonance spin locking, an ideal matching condition is $|{\gamma }_I|B_{1I}=|{\gamma }_S|B_{1S}$, where $\gamma$ is the gyromagnetic ratio and $B_1$ is the applied transverse field amplitude for each population. The fields match motion in the respective rotating frames, even though the nuclei have different laboratory resonance frequencies. This can enhance a weak carbon signal, but intensity also depends on transfer and relaxation, so peak areas need not directly count atoms without further controls.^[ref]