A well-configured solution is essential to completing solid-state NMR experiments effectively. The magic angle setting is a crucial factor in a magic angle spinning (MAS) probe, pivotal for achieving the highest possible resolution.
This article details which TopSpin setup can be employed to control and readjust the magic angle. Either the prosocial table must be accurately filled, or the operator must understand the right pulse length and power pairs for the MAS probe.
A KBr-filled rotor with a diameter corresponding to the probe must be used to conduct these experiments. The angle setting’s spinning speed must be 5 kHz for most probes, but it should be 40 kHz for smaller diameters (1.3 mm, 0.7 mm, 0.4 mm).
Consistently monitoring the magic angle position is crucial for conducting high-sensitivity experiments employing MAS probes. If the position is incorrectly altered, it must be readjusted utilizing “KBr” as a reference compound.
The magic angle is deemed ideal if the main peak’s line width in the rotating KBr spectrum is close to the line width of a spinning sideband, i.e. within ± 10 % of the full width at half maximum (FWHM) of a chosen spinning sideband.
This conventionally encompasses assessing the center band alongside a sideband at 20 or 40 kHz. The TopSpin software incorporates a parameter set named “SETUPMAS,” which can be opened utilizing the command “rpar SETUPMAS all” (see Figure 1).
Figure 1. Set up the experiment by reading the predefined parameter set. Image Credit: Bruker BioSpin - NMR, EPR and Imaging
The dataset is arranged when “getprosol” is called and a first reference spectrum can be obtained utilizing “zg” (see Figure 2).
Figure 2. KBr spectrum as detected using the predefined parameter set. Image Credit: Bruker BioSpin - NMR, EPR and Imaging
If the above ± 10% criteria are not fulfilled, the probe has an inaccurate magic angle position and must be rearranged. The AU program “Angle_Check” can be utilized for confirmation while the rotor continues running at the earlier chosen MAS speed (see Figure 3). The program calculates the KBr spectrum and records a stability statement in the experiment's title.
Figure 3. AU program for checking the +/- 10 % criteria for an optimal angle setting. Image Credit: Bruker BioSpin - NMR, EPR and Imaging
Two choices are available to recalibrate the MAS probe’s angle, depending on the probe type used:
- iProbe-based probes: An automatic routine is available.
- Non-ATMA probes: A manual procedure for revision is required.
For iProbe-based probes, the automated angle adjustment feature is in the equivalent tab of the MASDISP software (see Figure 4). When the routine finishes, the ideal organization is stored in the probe’s permanent memory by clicking the “Save” button, which appears upon routine completion.
Figure 4. In the “MAS display”, the tab “Magic Angle” is shown, if an iProbe is connected. The button “start automatic adjustment” calls the routine to readjust the angle position based on the rotating reference sample. Image Credit: Bruker BioSpin - NMR, EPR and Imaging
A fully automated routine is unavailable for non-ATMA-based probes, and the angle must be modified physically by rotating the magic angle handle underneath.
To do this, load the parameter set, as detailed above, insert the equivalent pulse lengths/power pairs, and write “gs” in the open dataset to begin the live acquisition.
The length of the 79Br FID or the intensity of the spinning sideband pattern of the KBr spectrum must then be optimized. Adjusting the magic angle involves monitoring the live acquisition on the PC screen while turning the adjustment rod at the bottom.
This rod has a black end marked with an “A” for Angle in current probe designs. The magic angle is changed correctly when the FID continues for the longest time, or when the real-time Fourier transformation delivers the maximum spinning sideband intensities.
When the adjustment is finished and the results are satisfactory, enter “stop” and obtain a new reference spectrum using “zg” to save the final result in the dataset. For automated and manual methods, running the “Angle_Check” program again will verify if the adjustment is correct or if readjustment is required.
The FWHM comparison between the center peak and the spinning sideband at 20 or 40 kHz is the criteria for a well-set angle position.
If the two peaks have identical linewidth within a ± 10 % error margin, the KBr-based angle setting is precise enough to conduct 13C-based CP experiments.
For experiments with quadrupolar nuclei, setup samples with higher accuracy than KBr, including Rubidium nitrate or Rubidium sulfate, must be employed.
This information has been sourced, reviewed and adapted from materials provided by Bruker BioSpin - NMR, EPR and Imaging.
For more information on this source, please visit Bruker BioSpin - NMR, EPR and Imaging.