Automation in solid state NMR

Abstract

Automation in solid state NMR (ssNMR) requires appropriate hardware, from rotor loading mechanisms over highly stable rf-transmitters and probe circuitry to automatic tuning and matching capabilities including automatic magic angle adjustment for ssNMR probes. While these hardware capabilities are highly desirable and are, to various degrees, provided by manufacturers, we focus herein on automating experiment setup using radio frequency (rf) fields, which are key parameters in solid state NMR experiments. Specifically, these include spinlock fields during cross polarization (CP), or rf-fields for homo- or heteronuclear spin recoupling or decoupling. Often, these fields have specific relationships to the magic angle spinning (MAS) frequency. Relying on a well-maintained spectrometer, the experiment setup shifts from traditionally required optimization of rf-power values for each element of an experiment sequence to automatically setting all parameters correctly without any need for optimization. The proposed approach allows executing an experiment by reading its rf-amplitude requirements based on the actual MAS rotation frequency just before starting data acquisition, while all other hardware-related parameters are automatically provided through global tables and scripts. Under modest MAS frequencies, no further rf-power optimization is required while providing optimal sensitivity of better than 90% of the optimal signal to noise. Any optional parameter optimization relates only to adjusting rf-nutation frequencies to the requirements of the sample and the sample rotation frequency rather than the spectrometer hardware. Fast MAS CP experiments with MAS frequencies above 40 kHz require a semi-automated setup by optimizing Hartmann-Hahn (HH) matched rf-fields that are synchronously varied relative to the MAS-frequency. This allows for a significant reduction of setup steps by up to one order of magnitude for such experiments, avoiding the traditional grid search for optimal CPMAS conditions. The approach presented here can also be applied to decoupling or recoupling sequences, requiring rotor synchronized rf-fields, reducing the setup to a few steps addressing the spin system’s properties rather than the spectrometer hardware. Our approach permits automating all basic solid state NMR experiments for high throughput analytical tasks.