Pushing sensitivity limits: cryogenic sample spinning combined with DNP

Pushing the sensitivity of MAS-DNP beyond its current boundaries requires addressing the following points:

  • 2 to 4 orders of magnitude of additional experimental time-savings are still to be gained because the DNP mechanism is still rather inefficient compared to the theoretical limits.
  • Faster sample spinning is clearly desirable (for systems with high anisotropic interactions or more generally to successfully extend the technique to magnetic fields >15 T)
  • Cheaper alternative to the widely used high-power microwave source (gyrotron) is also an important parameter to accelerate the spread of this technology.

Decreasing further the sample temperature is not only a direction towards solving the first point, but the other two as well, as we recently showed (Ref. 20). Several groups (mainly three including us (see Ref. 22)) have recently tried to access spinning temperatures well below the current standard of 100 K and demonstrate its feasibility.

Thanks to a huge instrumental development, we were able to recently report a strategy to push the limits of solid-state NMR sensitivity far beyond its current state-of-the-art. This leap-forward was made possible thanks to the development of a prototype cryostat (called NUMOC) employing cryogenic helium as the gas to power MAS for DNP-enhanced NMR experiments. These experimental conditions far exceed what is currently possible both in terms of minimal sample temperature and maximum spinning frequencies. For instance, using a 3.2 mm diameter rotor, we can achieve safely 14 kHz @30K and 25 kHz @85 K.

Sustainable cryogenic helium sample spinning significantly enlarges the realm and possibilities of the MAS-DNP technique and is the route to transform NMR into a versatile but also sensitive atomic-level characterization tool. Thanks to the construction of this equipment, we were able to reduce by more than a factor 50 the required experimental time of a DNP experiment compared to one performed at around 100 K (conditions commercially available and used in almost all MAS-DNP labs).

Based on these first results we have assembled a second-generation cryostat (SACRYPAN) to go further with this approach. This new device is based on a different design, which allows reducing the running costs – only electricity costs in this case – without compromising the specifications of the instrument. Moreover, our approach towards cryogenic helium sample spinning is fully compatible with the use of smaller diameter rotors (1.9/1.3 mm, etc.) and should thus allow performing in the near future experiments at ultra-fast MAS frequencies. In addition, the access to sustainable sample spinning at ultra-low temperature will allow testing lower power and cheaper microwave sources. These are key points to address in order to take this emerging technique to a more mature state and to spread its use to many more laboratories.

Related contributions (see list of publications):

1- MAS-DNP with cyrogenic helium to power sample cooling and rotation. See ref. 20, 22