By Jose Uribe
Achieving homogenous radiofrequency (rf) magnetic fields in solid-state NMR transceiver coils is crucial for maximizing sensitivity during experimentation. Methods to measure coil homogeneity successfully and accurately have been a time-consuming and error-prone task. Conventionally this has been performed using a manual ball-shift assay to map the magnetic field of a resonant cavity, rf coil, by measuring perturbations in the tuning frequency during sequential advancements of a small conductor. However, with this manual apparatus it is difficult to achieve adequate precision to ensure uniform steps, as this is done by hand and turns are estimated by eyeballing. This can cause errors in data collection due to the sensitivity of perturbations from small movements. Furthermore, assessing accurate results from this manual method is difficult, thus, reproducibility is hard to attain. To improve this assay, an automated method that uses inexpensive and open-source equipment to create a modular, yet specialized tool, is presented. Components holding and joining the apparatus together were designed using Computer Aided Design (CAD) software and made using 3D printing additive manufacturing methods. The use of an A4988 stepper driver chip, capable of various driving techniques to regulate power going to the motor, allows for small increment capabilities, with the lowest being 1/16th step-mode or 0.0016-inch increments. The hardware components are fully controlled by an Arduino UNO; designating pins, controlling direction, turn speed, etc. In combination to the assay, an automatized data collection scheme that uses Python scripting is used. This makes for a hands-free method which returns parsed rf coil data ready for mapping. The overall difference is not only smaller increments compared to the manual method, but accurate rf coil inhomogeneity results due to precise step control and resolution in data collection. Moreover, this automated method will serve as quality control for our theory-driven parameterized and optimized coil designs, which have similar appearance but very distinct field profiles.
In addition, we present fully 3D-printed spinning assemblies for 3.2 mm MAS ssNMR probes. These printed assemblies require no machining and are printed using fluorinated filaments which reduces, or completely eliminates, their proton background. These assemblies are designed to scale and meant to print for easy assembly, having an air-tight seal. Moreover, computational fluid dynamic (CFD) simulations are conducted to ensure the proper airflow for spinning sample rotors. The increasing modularity of 3D printing, along with low-cost filaments, makes it feasible to design and make spinning assemblies for an experiment’s needs. Here, we discuss parameter optimization schemes for printing with fluorinated filaments and limitations.