Magic-angle spinning NMR at near-liquid-helium temperatures

Helium-cooling and -spinning dynamic nuclear polarization for sensitivity-enhanced solid-state NMR at 14T and 30K

“…2 illustrates the essential components of the helium-cooling and -spinning DNP NMR probe system. The system utilizes two separate streams of cold helium gas to supply both the rotor bearing and driving energy, similarly to the cryogenic MAS systems previously described by other groups [40,46,47], and has no independent VT gas line [48,49]. Cold helium gas was generated by evaporation of liquid helium in a pressurizable dewar (<500 kPa, Suzuki Shokan).…”

Fluid flow dynamics in MAS systems

“…This has an impact on the development of suitable heat exchanger systems as well as the features of the MAS turbine itself [13,33,[83][84][85][86][87][88][89][90]. Early attempts to work with helium for achieving very low temperatures under MAS go back to Yannoni et al [83,84]. Apart from pure helium systems [13,33,85,86,89], where all MAS gas flows -bearing, drive, VT -are helium flows, a system following a different approach has been proposed which works with nitrogen gas for bearing and drive and uses cryogenic helium for VT [87,88].…”

“…Even at cryogenic temperature below 80 K the properties of helium gas (e.g., condensation point at 4.2 K at standard conditions, its density and speed of sound) are notably different from nitrogen or air at ambient temperature. This has an impact on the development of suitable heat exchanger systems as well as the features of the MAS turbine itself [13,33,[83][84][85][86][87][88][89][90]. Early attempts to work with helium for achieving very low temperatures under MAS go back to Yannoni et al [83,84].…”

Ultra-Low Temperature Nuclear Magnetic Resonance

“…Innovative studies by Yannoni and coworkers showed that cold helium (He) gas can successfully be used to cool spinning samples at temperatures <<100 K [23,24], others followed the idea with their own technological designs [22]. The construction of a device to access this "ultra"-low temperature spinning (ULTMAS) regime is far from straightforward and specific limitations as cost and loss of (precious) He or low kinematic viscosity of He gas inhibiting stable MAS [25].…”