Cryogenic-compatible spherical rotors and stators for magic angle spinning dynamic nuclear polarization.

Cryogenic magic angle spinning (MAS) is a standard technique utilized for dynamic nuclear polarization (DNP) in solid-state nuclear magnetic resonance (NMR). Here we describe the optimization and implementation of a stator for cryogenic MAS with 9.5 mm diameter spherical rotors, allowing for DNP experiments on large sample volumes. Designs of the stator and rotor for cryogenic MAS build on recent advancements of MAS spheres and take a step further to incorporate sample insert and eject and a temperature-independent spinning stability of $\pm 1$ Hz. At a field of 7 T and spinning at 2.0 kHz with a sample temperature of 105-107 K, DNP enhancements of 256 and 200 were observed for 124 and 223 $\mu$L sample volumes, respectively, each consisting of 4 M ${}^{13}$C, ${}^{15}$N-labeled urea and 20 mM AMUPol in a glycerol-water glassy matrix.