Matsuki, Yoh, and Toshimichi Fujiwara. “Cryogenic Platforms and Optimized DNP Sensitivity,” 7:16, 2018.
Modern high-field DNP NMR spectrometers are typically based on a cryogenic magic-angle sample spinning (MAS) capability. Conventionally, sample temperatures of T ∼100 K have been widely used, enabling substantial NMR signal enhancement with DNP at high external field conditions such as B0 =9.4 T. Today, however, the need for performing MAS DNP at much lower temperatures (T ≪100 K) is receiving growing attention for its ability to recover the rapidly degrading efficiency of the cross-effect (CE)-based DNP at even higher magnetic fields, B0 >10 T. In this article, we describe three contemporary cryogenic DNP MAS NMR probe systems: one is N2 based for T ∼100 K, and the other two are helium based for T ≪100 K. Principal requirements important in designing the cryogenic MAS NMR systems include long-term stability, cost efficiency, and readiness of operation. All the described setups incorporated various modifications and novel features to meet these challenges. In particular, the novel closed-cycle helium MAS system realizes all the requirements to a high standard, establishing an efficient and practical platform for ultralow sample temperature (T ∼30 K) MAS DNP. The resulting dramatic increase in sensitivity gain suggests the regained promise for the CE-based DNP at very high-field conditions (B0 >10 T). The experimental DNP data and effective sensitivity gain obtained with the described systems operating at 14.1 and 16.4 T are also discussed.