the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Cryogenic-Compatible Spherical Rotors and Stators for Magic Angle Spinning Dynamic Nuclear Polarization
Lauren E. Price
Nicholas Alaniva
Marthe Millen
Till Epprecht
Michael Urban
Alexander Däpp
Alexander B. Barnes
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 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/eject and temperature-independent spinning stability of +/- 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 μL and 223 μL sample volumes, respectively, each consisting of 4 M 13C, 15N-labelled urea and 20 mM AMUPol in a glycerol-water glassy matrix.
Lauren E. Price et al.
Status: open (until 05 Jun 2023)
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RC1: 'Comment on mr-2023-6', Kong Ooi Tan, 11 May 2023
reply
The manuscript presents the first DNP results using MAS spheres at cryogenic temperatures, which is a significant achievement towards the development of novel DNP instrumentation. I recommend the manuscript for publication subject to minor revisions (see below). Additionally, I would like to urge the authors to share the CAD files for the stators, spheres, and other relevant parts. This could encourage the community to attempt or start developing NMR instrumentation, which is an important but often neglected aspect in the field.
Regarding the legs (line ~47), are the FDM-printed (Prusa) parts airtight? In my experience, FDM-printed parts tend to be porous and don't maintain a good vacuum or airtight seal. I believe resin-printed (Formlabs) parts would be better suited for this application.
Fig. 2a: It would be helpful to include the x, y, and z axes in the figure to help readers identify which 2D plane they are looking at. Additionally, it would significantly improve clarity and impact if the authors could upload the CAD files of all relevant parts to the supplementary information or an online repository.
Why keep saying maco is 'ceramic-like’? Macor is a glass ceramic material. To avoid confusion, I suggest replacing 'ceramic-like' with 'glass ceramic' or simply using the term 'Macor.' Furthermore, it is worth mentioning that another advantage of using Macor as the stator instead of 3D-printed plastics is that Macor is relatively 1H-free, resulting in less 1H background.
On pages 3-4, the authors mention that 'sphericity' is a critical feature for stable spinning. Can the authors comment on how it is quantified, how much sphericity is required, and what has been achieved by the manufacturer?
On page 5, could the authors provide more information on how the hollowing of the sphere was performed? Was it done in-house, outsourced to a third-party machining company, or purchased as is from Sandoz Fils SA? Does the internal hollowed sphere also require high sphericity?
It is known that conventional cylindrical rotors require precisely machined internal and external diameters (with a precision of a few microns). I am curious about the tolerance for the sphere.
On page 5, line 112, what is the loss tangent of Vespel? Is it lower than sapphire? Can the cap be machined from the same ceramic material (sapphire) instead of Vespel? Wouldn't using sapphire be better in terms of microwave transmission (lower loss tangent), hardness, and thermal conductivity?
In Fig. 3, there appear to be some local spots where the speed exceeds 350 m/s, which is above the speed of sound. Would this not result in a shock wave and potentially cause some issues? Can the authors comment on this?
In Table 1, could the authors comment on how the inductance was measured? Was it done using an LRC meter or an impedance analyzer? From my experience, most standard LRC meters perform measurements at 100 kHz, which may not yield accurate inductance values for NMR applications in the high MHz range. Additionally, the references provided on self-resonance are not directly relevant to NMR applications. I suggest adding the following two references: (1) 'A large-inductance, high-frequency, high-Q, series-tuned coil for NMR' by Bruce Cook and Lowe, and (2) 'NMR coils with segments in parallel to achieve higher frequencies or larger sample volumes' by Roeder, Fukushima, and Gibson. These references discuss the issues of self-resonance when making coils for large-volume samples, which is exactly what you experience here with the 9.5 mm sample. Moreover, the authors should also elaborate more (in a few extra sentences) on the origin of self resonance, and how to characterize them. Although this is known to people who have built NMR probes, not everyone in the field is familiar with this topic.
On page 8, section 3.1, if available, could the authors provide a histogram of the spinning frequency for a certain time interval? Additionally, could the authors comment on the reproducibility of the results? It would be valuable to know if the authors machined only one sapphire rotor and one Macor stator, or if such apparatus can be successfully replicated in 1 out of 3 or 10 trials.
Minor issues:
Abstract: It could be useful to indicate the size of the sphere somewhere in the abstract, such as '9.5 mm' if I am not mistaken.
Line 25: long relaxation ‘times'
Line 29: replace 'directionally-dependent' with 'anisotropic’
Lines 45-46: Rephrase for improved clarity, such as 'one for spinning, one for pneumatic magic angle adjustment (not used in this work), and one for cooling'
Citation: https://doi.org/10.5194/mr-2023-6-RC1 -
RC2: 'Comment on mr-2023-6', Ilia Kaminker, 25 May 2023
reply
The manuscript by Price et.al desribes the next step of the magic angle spinning spheres technology. For the first time sphere spinning was achieved under cryogenic temeratures and this setup was used to demonstrate DNP experiments with state of the art signal enhancements. As such this paper describes a significant advancement of interest to the MR community.
Major comments:
In lines 30-34 The conventional cylindrical rotor is MAS setup described. VT gas is not mentioned though it is ubiquitos to cool down using a separate stream of a VT gas. This allows to either keep drive and beating gasses warm to ease on the spinning or to make them cold if even lower temperature is needed. This should be mentioned and discussed on how this is different / similar to the approach presented by the authors.
I would urge authors to expand the section 2.1 Probe Design: For example: it is not clear what is the puprpose of the cooling gas and how if at all it interacts with the rotor. Similarly, no details about the cooling gas pressure, temperature and flow are given in section 3.1.
I would suggest to expand the discussion of the CFD simulation and especially the Figure 2b. Most readers of MR a non-experts in CFD simualtions. I would suggest adding an explanation what the readers should look at on Figure 2. Perhaps make arrows that point into the crucial differences. (Similar to the arrow pointing at turbulence on Figure 3.)
Some crucial dimensions are missing such as channel apertures on Figure 2a. Semisphere diameter on the stator. Other dimensions that authors deem critical for the spinning performance should be added as well.
There is an excellent parapgraph on the material properties in sections 2.3. I suggest also adding a discussion on the acceptable machining tolerances.
Discussion in lines 111-113 mentions that Vespel is mm-wave transparent. How much this is relevant in the presented waveguide geometry? It appears that the irradiaion from the top results in most of the mm-wave beam going through the saphire sphere wall rather than the Vespel plug.
Minor comments:
Line 158 - Nutation freuqencise are given for both 13C and 1H cahnnels and only one power level of 800 W is given. Was 800 W used in both 1H and 13C channels?
Line 175 - "reflection of a mirror" - why does reflection of a mirror reduces power? A properly designed mirror will be practically losless.
Citation: https://doi.org/10.5194/mr-2023-6-RC2
Lauren E. Price et al.
Lauren E. Price et al.
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