Enormous sample scale-up from nanoliter to microliter in high field liquid state dynamic nuclear polarization

Dynamic nuclear polarization (DNP) enhances nuclear magnetic resonance (NMR) signals by transferring electron spin polarization to nuclei. As DNP requires microwave magnetic fields B1 strong enough to saturate electron spins, microwave resonators are generally used to achieve a sufficient B1, at the expense of restricting the sample size. Higher fields improve NMR sensitivity and resolution. However, resonators at 9 T for example can only hold nano-liters (nL). Larger volumes are possible by avoiding resonators, but the higher power needed to reach B1 is likely to evaporate the sample. Here, we demonstrate a breakthrough in liquid state DNP at 9 T, boosting the sample size to the microliter range. We could use high-power (70 W) microwaves thanks to a planar probe designed to alleviate dielectric heating. We enhanced the 1H NMR signal intensity of 2 uL of liquid water by a factor of 14, while maintaining the water temperature below 40 degree Celsius.