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Alexander Pines coauthored a method enhancing nuclear magnetic resonance signals from dilute nuclear spins in solids. Repeated transfer of polarization from abundant spins, usually protons, strengthens the rarer species' signal. Decoupling the abundant spins during observation then improves spectral resolution. The technique separates the problem of building a detectable spin polarization from the problem of measuring a narrow resonance, making otherwise weak solid-state spectra accessible.^[ref]

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]

J. S. Waugh helped develop proton-enhanced solid-state nuclear magnetic resonance with Alexander Pines and M. G. Gibby. Their method transfers polarization from abundant proton spins to more dilute nuclear spins, making otherwise weak signals easier to observe.^[ref]

This cross-polarization (nuclear magnetic resonance) technique separates creating a strong spin signal from reducing spectral broadening during its detection.

M. G. Gibby, with Alexander Pines and J. S. Waugh, developed enhanced detection of dilute spins in solids through proton polarization transfer. The work connected spin manipulation with both signal enhancement and resolved spectra.^[ref]

Abundant spins serve as a polarization resource, while their interactions must also be managed so that the dilute-spin spectrum remains interpretable. The cross-polarization (nuclear magnetic resonance) mechanism therefore addresses sensitivity without making line broadening irrelevant.