Improving the sensitivity of MAS spheres using a 9.5 mm spherical shell with 219 μL sample volume spinning in a spherical solenoid coil

[Display omitted] •The spherical solenoid coil does not only provide the RF field but also the appropriate flow dynamics for stable spinning.•Ruby spherical shells were fabricated using 5-axis milling technique to hold a sample volume of 219 μL, leading to six times greater sensitivity than that of...

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Bibliographic Details
Published in:Journal of magnetic resonance (1997) 2022-10, Vol.343, p.107305-107305, Article 107305
Main Authors: Gao, Chukun, Chen, Pin-Hui, Däpp, Alexander, Urban, Michael A., Gunzenhauser, Ronny, Alaniva, Nicholas, Barnes, Alexander B.
Format: Article
Language:English
Subjects:
Magic angle spinning
NMR filling factor
NMR sensitivity
Radiofrequency coil
Spherical rotor
Spherical solenoid coil
Variable-pitch solenoid coil
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Summary:[Display omitted] •The spherical solenoid coil does not only provide the RF field but also the appropriate flow dynamics for stable spinning.•Ruby spherical shells were fabricated using 5-axis milling technique to hold a sample volume of 219 μL, leading to six times greater sensitivity than that of the first prototype.•The apparatus produces an RF field along the coil axis that is perpendicular to the external magnetic field, regardless of rotor orientation, and allows magic angle adjustment without coil movement.•Simulations of the RF performance of spherical and straight solenoid coils show that both generate similar RF intensities over the same sample volume but more homogeneous RF intensity within the spherical coil. Spherical rotors in magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments have potential advantages relative to cylindrical rotors in terms of ease of fabrication, low risk of rotor crash, easy sample exchange, and better microwave access. However, one major disadvantage so far of spherical rotors is poor NMR filling factor due to the small sample volume and large cylindrical radiofrequency (RF) coil. Here we present a novel NMR coil geometry in the form of a spherical coil. The spherical coil best fits the spherical sample to maximize sensitivity, while also providing excellent RF homogeneity. We further improve NMR sensitivity by employing a spherical shell as the rotor, thereby maximizing sample volume (219 μL in this case of 9.5 mm outer diameter spheres). The spinning gas is supplied by a 3D-printed ring stator external to the coil, thereby introducing a simplified form of MAS stators. In this apparatus, the RF field generated along the coil axis is perpendicular to the external magnetic field, regardless of rotor orientation. We observe a linear increase in sensitivity with increasing sample volume. We also simulate the RF performance of spherical and cylindrical solenoid coils with constant or variable pitch for spherical and cylindrical rotors, respectively. The simulation results show that spherical solenoid coils generate comparable B1 field intensities but have better homogeneity than cylindrical solenoid coils do.
ISSN:1090-7807
1096-0856
DOI:10.1016/j.jmr.2022.107305