Instrumentation for cryogenic magic angle spinning dynamic nuclear polarization using 90L 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.

Keywords: Cryogenic MAS; Dynamic nuclear polarization; Electron decoupling; Heat exchanger; Magic-angle spinning; Pulsed DNP; Solid-state NMR.

Figure 1. Cryogenic system setup for low temperature MAS NMR

a) CAD image exhibits the incorporation of the new heat exchanger in a 7 T DNP MAS NMR system (see Fig. S1). b) Flowchart shows the path of nitrogen gas as it cools to < 85 K for MAS and sample cooling. c) Flowchart shoes the generation of pure dry nitrogen from atmospheric air (see also Fig. 2). The stages of the two-stage set of filters between the dryers, models 2312N-1B1-DX and 2312N-1B1-BX, are labeled DX and BX respectively. Nitrogen surge tanks and air surge tanks are labeled NST and AST.

Figure 2. Physical layout of nitrogen generation system

CAD image displays the physical layout of the entire nitrogen generation system from Fig. 1c and its coupling to the nitrogen cooling system and NMR probe. The components labeled as filters contain both the BX and DX filters.

Figure 3. Heat exchanger assembly

a) CAD image displaying the insertion of the individual modules into the liquid nitrogen dewar with the lower section submerged in liquid nitrogen bath. b) Top section view displaying the eclipse of the intermediate stage with the final stage (blue dashed circle) projected to the top surface of the intermediate stage. c) Top section of the heat exchanger assembly highlights the installment of the exhaust return manifold.

Figure 4. Mechanical design of a single heat exchange module incorporating the counterflow coil

a) CAD image of a single module highlights the intermediate and final stages. Detailed view of the inner vacuum space of the intermediate stage b) displays the jog in the tubing necessary for axillary installation of the modules, the rigid thin-walled tube which helps resist thermal compression induced strain, and the inner and outer tubes of the concentric tube counterflow heat exchange coil. c) Photo of a 316-stainless-steel bellow included in this design, indicated by yellow stars in Fig. 3b. For photos of the heat exchanger module assembly, see Fig. S3.

Figure 5. Computational fluid dynamics simulations determine the counterflow coil to be > 98% effective

a) Simulated counterflow coil temperature profiles. b) Representative Finite Element Analysis mesh applied in the computational fluid dynamics simulations. c) Distribution of bulk fluid temperatures along the incoming (red) and exhaust return (blue) nitrogen gas streams. Arrows indicate the direction of nitrogen gas flow as up or down the coil axis.

Figure 6. Liquid nitrogen consumption of the heat exchanger

Plot of the volume of liquid nitrogen bath in the heat exchanger dewar over three days. Positive slopes correspond to fill periods, while negative slopes correspond to heat exchanger boil-off periods. Individual boil-off periods were linearly fit and averaged over the 10 cycles shown above to find an average boil-off rate of 3.8 L/h or 91 L/day. Sample temperatures were maintained below 85 K while the MAS rotor frequency held constant at 5.2 kHz.

Figure 7. Temperature dependent DNP enhancements of 1 M [U-¹³C, ¹⁵N] Urea

a) ¹³C CP-MAS DNP NMR spectra of 1 M [U-¹³C, ¹⁵N] urea with 20 mM AMUPol dissolved in cryoprotecting glassy matrix of d₈-glycerol/D₂O/H₂O (60/30/10 v/v/v%) under 6000 ± 15 Hz MAS and a sample temperature of 81 ± 2 K are shown with (black line) and without microwave irradiation (blue line). b) The same sample is under 6300 ± 15 Hz MAS and a sample temperature of 105 ± 2 K.