Category Archives: Heat Exchanger

A versatile custom cryostat for dynamic nuclear polarization supports multiple cryogenic magic angle spinning transmission line probes #DNPNMR

This article describes a heat exchanger used to generate cold gas for MAS-NMR experiments. Several different designs of this type of device have been described in the literature such as:

  1. The original design reported by Girffin et al.: https://doi.org/10.1016/0022-2364(91)90357-Y
  2. A counter flow heat exchanger reported by Zilm et al.: http://dx.doi.org/10.1016/j.jmr.2004.03.002

Scott, Faith J., Nicholas Alaniva, Natalie C. Golota, Erika L. Sesti, Edward P. Saliba, Lauren E. Price, Brice J. Albert, Pinhui Chen, Robert D. O’Connor, and Alexander B. Barnes. “A Versatile Custom Cryostat for Dynamic Nuclear Polarization Supports Multiple Cryogenic Magic Angle Spinning Transmission Line Probes.” Journal of Magnetic Resonance 297 (December 2018): 23–32.

https://doi.org/10.1016/j.jmr.2018.10.002

Dynamic nuclear polarization (DNP) with cryogenic magic angle spinning (MAS) provides significant improvements in NMR sensitivity, yet presents unique technical challenges. Here we describe a custom cryostat and suite of NMR probes capable of manipulating nuclear spins with multi-resonant radiofrequency circuits, cryogenic spinning below 6 K, sample exchange, and microwave coupling for DNP. The corrugated waveguide and six transfer lines needed for DNP and cryogenic spinning functionality are coupled to the probe from the top of the magnet. Transfer lines are vacuum-jacketed and provide bearing and drive gas, variable temperature fluid, two exhaust pathways, and a sample ejection port. The cryostat thermally isolates the magnet bore, thereby protecting the magnet and increasing cryogen efficiency. This novel design supports cryogenic MAS-DNP performance over an array of probes without altering DNP functionality. We present three MAS probes (two supporting 3.2 mm rotors and one supporting 9.5 mm rotors) interfacing with the single cryostat. Mechanical details, transmission line radio frequency design, and performance of the cryostat and three probes are described.

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

Albert, B.J., et al., Instrumentation for cryogenic magic angle spinning dynamic nuclear polarization using 90L of liquid nitrogen per day. J. Magn. Reson., 2017. 283(Supplement C): p. 71-78.

https://doi.org/10.1016/j.jmr.2017.08.014

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.

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