Direct observation of hyperpolarization breaking through the spin diffusion barrier

Dynamic nuclear polarization (DNP) is a widely used tool for overcoming the low intrinsic sensitivity of nuclear magnetic resonance spectroscopy and imaging. Its practical applicability is typically bounded, however, by the so-called 'spin diffusion barrier', which relates to the poor efficiency of polarization transfer from highly polarized nuclei close to paramagnetic centers to bulk nuclei. A quantitative assessment of this barrier has been hindered so far by the lack of general methods for studying nuclear-polarization flow in the vicinity of paramagnetic centers. Here we fill this gap and introduce a general set of experiments based on microwave gating that are readily implemented. We demonstrate the versatility of our approach in experiments conducted between 1.2 – 4.2 K in static mode and at 100 K under magic angle spinning (MAS) — conditions typical for dissolution-DNP and MAS-DNP — and for the first time directly observe the dramatic dependence of polarization flow on temperature.