Wednesday, November 21, 2018

Magic angle spinning spheres

This article is indirectly related to DNP. It describes an exciting new idea from the Barnes lab how to spin samples in a MAS-NMR experiments and the perspective of integrating DNP.

Chen, Pinhui, Brice J Albert, Chukun Gao, Nicholas Alaniva, Lauren E Price, Faith J Scott, Edward P Saliba, et al. “Magic Angle Spinning Spheres.” SCIENCE ADVANCES, 2018, 8.

Magic angle spinning (MAS) is commonly used in nuclear magnetic resonance of solids to improve spectral resolution. Rather than using cylindrical rotors for MAS, we demonstrate that spherical rotors can be spun stably at the magic angle. Spherical rotors conserve valuable space in the probe head and simplify sample exchange and microwave coupling for dynamic nuclear polarization. In this current implementation of spherical rotors, a single gas stream provides bearing gas to reduce friction, drive propulsion to generate and maintain angular momentum, and variable temperature control for thermostating. Grooves are machined directly into zirconia spheres, thereby converting the rotor body into a robust turbine with high torque. We demonstrate that 9.5–mm–outside diameter spherical rotors can be spun at frequencies up to 4.6 kHz with N2(g) and 10.6 kHz with He(g). Angular stability of the spinning axis is demonstrated by observation of 79Br rotational echoes out to 10 ms from KBr packed within spherical rotors. Spinning frequency stability of ±1 Hz is achieved with resistive heating feedback control. A sample size of 36 ul can be accommodated in 9.5-mm-diameter spheres with a cylindrical hole machined along the spinning axis. We further show that spheres can be more extensively hollowed out to accommodate 161 ul of the sample, which provides superior signal-to-noise ratio compared to traditional 3.2-mm-diameter cylindrical rotors.