Pushing the limit of liquid-state dynamic nuclear polarization at high field.

Dynamic nuclear polarization in the liquid state was predicted more than 50 years ago by Overhauser. Its application for NMR sensitivity enhancement has been limited because of intrinsic and experimental problems to apply this method at high magnetic fields. Here we report on 95 GHz DNP experiments using the common TEMPO radical dissolved in water. In an efficient non-radiative microwave resonator, we observe average experimental enhancement factors up to -65. The local enhancement in the center of the resonator is calculated to reach a level of -94 at the highest microwave power. At high microwave power, the DNP enhancement shows a linear increase with no tendency to saturation. The results indicate that a substantial sensitivity enhancement is possible for liquid state NMR in nL sample volumes.