Instrumentation for cryogenic magic angle spinning dynamic nuclear polarization using 90 L of liquid nitrogen per day

Abstract

Cryogenic sample temperatures can enhance NMR sensitivity by extending spin relaxation times to improve dynamic nuclear polarization (DNP) and by increasing Boltzmann spin polarization. We have developed an efficient heat exchanger with a liquid nitrogen consumption rate of only 90L per day to perform magic-angle spinning (MAS) DNP experiments below 85K. In this heat exchanger implementation, cold exhaust gas from the NMR probe is returned to the outer portion of a counterflow coil within an intermediate cooling stage to improve cooling efficiency of the spinning and variable temperature gases. The heat exchange within the counterflow coil is calculated with computational fluid dynamics to optimize the heat transfer. Experimental results using the novel counterflow heat exchanger demonstrate MAS DNP signal enhancements of 328±3 at 81±2K, and 276±4 at 105±2K.

Recent developments in MAS DNP-NMR of materials

“…This functionality is accomplished by blowing on the rotor not only cold bearing and drive gas, but also an additional cooling gas. A heat exchanger is employed to cool the gas [174,175], which is transported to the MAS stator by thermally insulated transfer lines. Furthermore, the probe can be equipped with a pneumatic insert and eject system, which allows the change of the sample at low temperature [176].…”

Improved waveguide coupling for 1.3 mm MAS DNP probes at 263 GHz

“…This is not only reflected in the abundant scientific literature, but also in the patent literature [51,54,[56][57][58][59][60][64][65]70], and is an indicator for the active research and development in the field of microwave instrumentation for MAS DNP NMR. Specifically for the MAS geometry the microwave beam coupling has been studied for the case of axial irradiation (i.e., along the rotor axis or cylindrical sample axis, [52,56,61,[63][64][65][66]) and for the case of radial irradiation (i.e., perpendicular to the MAS rotor axis, [34,46,[48][49]59,62,[69][70][71]).…”

Pulsed Electron Decoupling and Strategies for Time Domain Dynamic Nuclear Polarization with Magic Angle Spinning

“…(b) Heat exchanger that is used to cool drive, bearing, and VT fluids to 80–100 K. These figures were reproduced with permission from the Journal of Magnetic Resonance. 32,55 …”

“…A lower temperature of the incoming spinning gases reduces the amount of boil-off of the liquid nitrogen cooling reservoir and results in a liquid nitrogen consumption of only 90 L/day when conducting MAS experiments. 32 …”

“…Cryogenic technology used in DNP MAS NMR experiments from RT to 4.2 K. (a) NMR probe head showing the path of the variable-temperature (VT) helium fluid (shown by the blue arrow) onto the sample. (b) Heat exchanger that is used to cool drive, bearing, and VT fluids to 80–100 K. These figures were reproduced with permission from the Journal of Magnetic Resonance. , …”