Dynamic nuclear polarization (DNP) at high magnetic fields (9.2 T, 400 MHz ¹H NMR frequency) requires high microwave power sources to achieve saturation of the EPR transitions. Here we describe the first high-field liquid-state DNP results using a high-power gyrotron microwave source (20 W at 260 GHz). A DNP enhancement of −29 on water protons was obtained for an aqueous solution of Fremy's Salt; in comparison the previous highest value was −10 using a solid-state microwave power source (maximum power 45 mW). The increased enhancements are partly due to larger microwave saturation and elevated sample temperature. These experimentally observed DNP enhancements, which by far exceed the predicted values extrapolated from low-field DNP experiments, demonstrate experimentally that DNP is possible in the liquid state also at high magnetic fields.

A series of 12 asphaltene samples extracted from heavy oils and the oxidized bitumen of different origin has been studied with high-frequency W-band (94 GHz) pulsed electron paramagnetic resonance (EPR) spectroscopy. Transverse (T_2e) and longitudinal (T_1e) relaxation times of the free radical (FR) and the vanadyl porphyrin (VO²⁺) were measured for each sample. A significant contribution of the spectral diffusion to T_2e has been revealed and ascribed to the dipole–dipole interaction between the FR and VO²⁺. This indicates that the distance between the FR and VO²⁺ does not exceed a few nanometers, which means, in turn, that VO²⁺ can participate in construction of the asphaltene aggregates via the intermolecular interactions.