400 MHz/263 GHz ultra-low temperature MAS-DNP using a closed-cycle helium gas cooling system and a solid-state microwave source

Abstract

Dynamic nuclear polarization (DNP) is widely used in a wide range of applications in solid-state NMR nowadays due to recent advancements of magic-angle spinning (MAS) DNP. Conventionally, an MAS-DNP system employs a gyrotron as a microwave source and operates at $\sim$100 K using nitrogen gas. As an alternative, we present a 400 MHz/263 GHz MAS-DNP system utilizing a compact solid-state microwave source and an ultra-low temperature (ULT) helium MAS probe equipped with a cryogenic preamplifier. Compared to gyrotrons, solid-state microwave sources are compact, cost-effective, and frequency agile. The ULT compensates for the decreased DNP efficiency resulting from the lower microwave power of the solid-state source. Additionally, the large Boltzmann polarization at ULT and the improved signal-to-noise ratio provided by the cryogenic preamplifier enhance the sensitivity of the MAS-DNP system. The system is tested using a DNP standard sample of proline in a mixture of deuterated glycerol and partially deuterated water doped with AMUPol, achieving a DNP enhancement of 85 using a 2 mm-diameter rotor at a sample temperature of 30 K and microwave power of 160 mW. Experimental data show that the Boltzmann polarization and the cryogenic preamplifier contribute an additional sensitivity gain of 11$\times$ at 30 K compared to 100 K. Overall, the ULT-DNP related sensitivity gain of this system is estimated to be roughly twice that of a 100 K gyrotron system, although the DNP enhancement factor alone is smaller using a solid-state microwave source.