Bornet, A., et al., Cross Polarization for Dissolution Dynamic Nuclear Polarization Experiments at Readily Accessible Temperatures 1.2 < T < 4.2 K. Appl. Magn. Reson., 2012: p. 1-11.
Cross polarization can provide significant enhancements with respect to direct polarization of low-γ nuclei such as 13 C. Substantial gains in sample throughput (shorter polarization times) can be achieved by exploiting shorter build-up times τ DNP ( 1 H) < τ DNP ( 13 C). To polarize protons rather than low-γ nuclei, nitroxide radicals with broad ESR resonances such as TEMPO are more appropriate than Trityl and similar carbon-based radicals that have narrow lines. With TEMPO as polarizing agent, the main Dynamic Nuclear Polarization (DNP) mechanism is thermal mixing (TM). Cross polarization makes it possible to attain higher polarization levels at 2.2 K than one can obtain with direct DNP of low-γ nuclei with TEMPO at 1.2 K, thus avoiding complex cryogenic technology.