Low temperature solid-state NMR experiments of half-integer quadrupolar nuclides: caveats and data analysis

Solid-state NMR spectroscopy of half-integer quadrupolar nuclides has received a lot of interest recently with the advent of new methodologies and higher magnetic fields. We present here the extension of our previous low temperature method to an 18.8 T system. This new probe entailed a total redesign including a cross coil and variable capacitors that are operational at cryogenic temperatures. The limitations to sensitivity are also discussed; including a new diode network, the utilization of a cryogenic band pass filter, and the consequences of the RF profiles of the coil. Further, details of the spectroscopy of quadrupolar nuclei in a protein are discussed, such as the observation of the outer transitions and how to distinguish them from the desired ±1/2 transition.