We report on design of solid-state NMR probes for the 1.5 GHz NMR magnet at NHMFL facility in Florida.1 This magnet is open to external NMR users, offering sensitivity and resolution enhancements, and the new opportunities in NMR of quadrupolar and low-γ nuclei. We will discuss strategy for making higher-field solid-state NMR probes for materials and biological applications, while preserving the large sample volumes that NMR community grew accustomed to in the sub-GHz NMR range. We constructed multi-resonant direct-detection NMR probes, MAS and static, with sample sizes in 2, 3, 4, 5 mm range. These probes are part of the 1.5 GHz Bruker NEO spectrometer. We will report performance and NMR spectra.
Both 1HXY and 1HX probes use Low-E coils designs that separate high- and low-frequency RF circuits2. This allows efficient direct detection at higher 1H frequencies with far less trade-off in sample size or sensitivity. The triple-resonance 1HXY circuit is arranged on tune cards1 for quick interchange of X/Y isotopes. The static probe has modular slide-in sample coils that accommodate different sample shapes and sizes: 3, 4, 5 mm round and 4x4 mm square profiles.
Each probe required additional hardware to perform NMR in the hybrid magnet consisting of series-connected superconducting coil and resistive DC insert.1 Active field-regulation compensates B0 fluctuations, enabling signal averaging and 2D NMR experiments. An inductive sensor tracks B0 field and drives an outer correction coil to cancel fast 60 Hz fluctuation and harmonics. An external 7Li NMR field lock is incorporated to correct slower B0 drift. A MnCl2-doped LiCl solution serves as the lock sample. Spatial B0 inhomogeneity of < 0.9 ppm over 1cm3 DSV is achieved by combining active shims with the passive ferroshims in the probehead.
1Gan Z. et al., J.Magn.Reson. 2017; 284, 125.
2Gor’kov P. et al., J.Magn.Reson. 2007; 185, 77.