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

Abstract

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.

Figure 1

Stator design. (a) CAD of the stator demonstrating the flow for the spinning gas and the magic angle (MA) adjustment gas. The diameter of the fluid exhaust is also given. (b) CAD of the stator sliced to show the half-section of the tangent plane and the channel for spinning fluid. The diameter of the hemispherical cup is also given. (c) Zoom-in from above (view a) highlighting the tangent plane and dimensions of the aperture. (d) Zoom-in of the aperture as shown in view (b), with dimensions of the tangent plane called out.

Figure 2

Cryogenic stator design. CFD of a CAD of the stator both with and without the tangent plane. The critical features include the forward, backward and normal fluid flows in the simulations. Changes in fluid velocity and distribution that are altered by the presence or absence of the tangent plane have been highlighted using arrows.

Figure 3

CFD of a sphere with flat caps. CFD demonstrating the results of imprecise caps in the stator's hemispherical cup. The red arrow highlights the area of turbulence.

Figure 4

Coil and sphere design. (a) CAD of a “one-and-a-half”-turn saddle coil. Blue depicts one wire and red a separate wire. The orange arrows indicate the flow of the current through the coil. (b) CAD of the “blind-hole” cylindrical-chamber spherical rotor and a Vespel^® cap that has a sample volume of 124  $\mathrm{\mu }$ L. (c) CAD of the blind-hole spherical-shell rotor and a Vespel^® cap that has a sample volume of 223  $\mathrm{\mu }$ L.

Figure 5

Probe head design. (a) CAD of the probe head. The gases for spinning and pneumatic magic angle adjustment enter from above through the 3D-printed legs. Gas next travels through the pivot, into the channel in the adapter and then through the channel in the stator providing both lift and spin to the sphere. The cooling gas (variable temperature) is directed at the underside of the Macor^® stator via a 3D-printed channel. The center hole at the top allows microwaves to shine directly on the sample. It also doubles as an insert and eject tube for the sphere. A fiber optic holder directs and secures the fiber optics, which are used to detect the spinning frequency of the sphere. The “rotation lock” between the adapters and the stator ensures concurrent movement for manual magic angle adjustment. (b) Picture of the probe head with the one-and-a-half-turn saddle coil included.

Figure 6

DNP results using a small-volume sphere (124  $\mathrm{\mu }$ L sample volume). (a) DNP enhancement of 256 on ${}^{\mathrm{13}}$ C, ${}^{\mathrm{15}}$ N urea with 20 mM AMPUPol in 60  $/$  30  $/$  10 d ${}_{\mathrm{8}}$ –glycerol  $/$  D ${}_{\mathrm{2}}$ O  $/$  H ${}_{\mathrm{2}}$ O at a spinning frequency of 2 kHz and a temperature of 107 K. (b) DNP cross-effect saturation using an enhancement vs. power curve showing saturation at 6.3 W of power.

Figure 7

DNP results using a spherical-shell rotor (223  $\mathrm{\mu }$ L sample volume). (a) DNP cross-effect saturation using a power vs. enhancement curve showing saturation at 9 W. (b)  $T_{\mathrm{1}}$ DNP experiment showing the optimal $T_{\mathrm{1},\mathrm{DNP}}$ of 3.7 s as the DNP transfer period.

Figure 8

Microwave heating spherical-shell rotor. Sample temperature vs. microwave power during DNP acquisition with the spherical-shell rotor. Sample heating reaches 118 K at 16 W of microwave power.