Dynamic nuclear polarization at high magnetic fields in liquids

Dynamic nuclear polarization (DNP) spectrometers for high-field liquid NMR (nuclear magnetic resonance) studies have been successfully constructed and used to experimentally probe the maximum obtainable DNP enhancement at high magnetic fields. MR relaxation dispersion experiments were conducted from 0 to 14 T magnetic field strength. EPR experiments were performed at X-band, Q-band, W-band and G-band microwave frequencies to characterize the electron spin parameters of the paramagnetic molecules used as polarizing agents. The DNP enhancement at the low field position was optimized by monitoring the water proton NMR signal with a Bruker Minispec spectrometer. At the low field position, the samples were irradiated continuously for variable times between 1 and 20 s and then pneumatically transferred to the high field position, where a 90° pulse was applied to record the NMR spectrum. Substantial DNP enhancements of the NMR signals were observed for the HF-DNP as well as the Shuttle-DNP approach.