Cross-polarization magic-angle spinning

Cross-polarization magic-angle spinning, abbreviated CPMAS, combines two operations in solid-state nuclear magnetic resonance. Cross-polarization (nuclear magnetic resonance) transfers nuclear spin polarization from a more strongly detected spin population, commonly protons, to a less sensitive one such as carbon-13. Magic angle spinning rotates the sample to average appropriate orientation-dependent interactions. Signal enhancement and spectral narrowing address different experimental obstacles.

The angle is fixed by a geometrical condition:This nulls the relevant second-rank angular factor under the averaging assumptions; it does not remove every interaction or every source of linewidth. Chemical shift anisotropy and sample motion must still be interpreted within the actual experiment.

In their 1976 paper, Jacob Schaefer and Edward O. Stejskal compared carbon-13 spectra of solids obtained with cross-polarization (nuclear magnetic resonance), with and without magic angle spinning, against solution spectra. The polysulfone example showed how the combined method could reveal much more resolved structural detail in a solid.^[ref]

For an original comparison, consider two carbon sites with equal populations but different efficiencies of polarization transfer. Their measured peak areas can differ even when their populations are equal. Changing contact time or relaxation conditions can change that weighting. NMR signal integration in a CPMAS spectrum therefore requires a justified acquisition model before peak areas are treated as direct composition measurements.