NMR in Resistive Magnets

HR MAS at 1.065 GHz

This 1065 MHz 4 mm High Resolution MAS probe was built for the ongoing efforts to develop NHMFL resistive magnets for high resolution NMR spectroscopy. The ¹H–²H double-tuned probe is designed for the 25 T, standard 52 mm bore Bitter resistive magnet. Inherent instability of static magnetic field in resistive magnets is addressed by two techniques:

1. Flux Stabilization

Magic angle spinning combined with active feedback to reduce magnetic field noise in resistive magnets allows for dramatic improvements in NMR spectroscopy. The feedback is accomplished with internal flux sensor coil, situated above the MAS stator. The water peak in the spectra below was narrowed about 30-fold from ~ 2 ppm to 66 parts per billion.

Sample: 50% H₂O, 50% D₂O 4 mm rotor, 3.5 kHz rotation.

2. HENPEC

HEteroNuclear PhasE Corrected (HENPEC) spectroscopy is a technique developed by Dr. Z. Gan for obtaining spectra in unstable static field of resistive Bitter magnets. Phase correction is achieved by mixing sample with well known reference, such as deuterium, for example. Custom RF coil, made of susceptibility-matched wire in collaboration with Bruker Biospin, further enchances spectral resolution.

1.06 GHz ¹H HRMAS spectra of ethanol before and after HENPEC correction