Automated Shimming Techniques for MAS Probes

This article details a shimming routine for magic angle spinning (MAS) probes employed in TopSpin version 4.3. Conventional shimming routines for liquid samples depend on gradient shimming and cannot be utilized with most solid-state nuclear magnetic resonance (NMR) probes.

This automatic MAS shimming routine applies a sophisticated algorithm, shimming on the free induction decay of an adamantane sample, Bruker’s reference sample for cross-polarization (CP)MAS probe shimming.

The routine depends on a ¹³C observe and ¹H decoupled acquisition at elevated MAS rates. It works for Bruker’s routine solid-state NMR probes for wide-bore (WB) and standard-bore magnets and perfectly complements the automation elements of Bruker’s CPMAS iProbes.

An added advantage of the shimming routine, particularly for new solid-state NMR clients, is that it references the B₀-Zeeman field of the magnet to the 38.48 ppm resonance of adamantane, proportionate to tetramethylsilane.

Bruker’s MAS iProbes have been developed to consider the prerequisites for solid-state automation, usability, and remote operation. They support automated tuning and matching, automatic magic angle alteration, and automatic sample changes.

This article demonstrates another crucial aspect—implementing an automatic shimming algorithm. Fully automated solid-state NMR experiments are now achievable using Bruker’s renowned IconNMR software, enhancing the efficiency of solid-state NMR analysis and ease of use when handling it.

TopSpin Commands

The commands presented for the MAS shimming routine include:

Source: Bruker BioSpin - NMR, EPR and Imaging

The parameters for MASshim help accomplish the following functions:

Source: Bruker BioSpin - NMR, EPR and Imaging

More than one option can be employed with the “MASshim” command, for example:

MASshim full --FWHM 7.0

Note: If “FWHM” remains undefined, the shimming routine will operate continually until the “haltshim” command is called. If “angle” remains undefined, a dialog box will open and the “angle” parameter value must be input physically.

When the MAS shimming routine is utilized in IconNMR, it is strongly recommended to use the optional “FWHM” and “angle” parameters to guarantee a clear stop criterium is defined and ensure a continuous automation workflow.

Orientation of MAS iProbes

The parameter “angle” describes the stator’s rotor axis position in the MAS probe in the room-temperature shim system’s X-Y coordinates. Measuring the angle precisely is not required. In practical use, working with one of three values: 0, 90, or 40 ° is usually sufficient.

• 0: Y gradient is the applicable component
• 90: X gradient is the most applicable gradient
• 40: The stator is aligned at app. 40° to the Y-axis

Values for the “angle” parameter for some standard orientations of an iProbe are illustrated in Figure 1. For probes not based on the iProbe platform, the stator’s positioning in the probe may differ. Figure 1 may not be valid for such probes.

It is also only required to input values between 0–90 °, and the same value can be utilized for the “angle” parameter even if the probe is mounted 180° rotated as presented in Figure 1. All Bruker WB probes possess the 90 angle.

Figure 1. Recommended values for the “angle” parameter for some typical orientations of MAS iProbes. Image Credit: Bruker BioSpin - NMR, EPR and Imaging

Application Example

The following example highlights the power of the MASshim algorithm. Figure 2 illustrates the ¹³C spectrum of an adamant sample with every room-temperature shim at zero. The lineshape is not within specification.

Figure 2. With all room temperature shims set to zero, the lineshape (¹³C adamantane) is not within specification. Image Credit: Bruker BioSpin - NMR, EPR and Imaging

If the orientation of the MAS probe is recognized according to the room temperature shim system, the shimming process can be initiated from the TopSpin command line. If the probe is mounted in the X-direction, the following command can be used:

MASshim full --FWHM 7.0 --angle 0

This command incites TopSpin to shim the probe until it reaches a linewidth of 7 Hz FWHM and employs only shim gradients oriented along the Z- and X-axes.

The shimming routine will show a message when the probe is successfully shimmed. Figure 3 illustrates the line-shape spectrum accomplished by the shimming algorithm in this example.

The shimming procedure in figures 2 and 3 took fewer than 10 minutes, conducted with a 600 MHz 3.2 mm HX CPMAS iProbe and at a MAS rotation rate of 15 kHz utilizing ¹H low power decoupling frequency swept TPPM decoupling at 3.75 kHz, a quarter of the MAS rotation frequency.

Figure 3. Lineshape spectrum after successful completion of the MASshim routine. The lineshape specification of 7 Hz FWHM is easily reached. Image Credit: Bruker BioSpin - NMR, EPR and Imaging

More strict linewidth targets can always be set, e.g., 2 Hz rather than 7 Hz:

MASshim full --FWHM 2.0 --angle 0

Halting the shimming routine before the target linewidth has been reached is achievable by inputting:

haltshim

This will stop the routine and set the best shim values found throughout the shimming process. If MASshim is summoned without the FWHM selection, the routine will run continually until it is ceased with the “haltshim” command.

Process if Angle Parameter is Unspecified

If the stator’s position in the MAS probe according to room temperature is unspecified, the following method can be performed:

1. Build an hpdec dataset (rpar SETUPSHIM) with adamantane operating at MAS > 10 kHz
2. Load rf-power parameters with “getprosol”
3. Begin the continual acquisition mode by entering “gs” in the command line
4. Type “BSMSDISP” in the TopSpin command line
5. Adjust the X shim to 10,000 units and monitor the shim reaction in the live display, then change X to 0 6.
6. Adjust the Y shim to 10,000 units and monitor the reaction in the live display.

The constraint that should be utilized with the “angle” option can be decided from the outcome of the above method:

1. If just the X shim triggered a response, the angle is 90
2. If just the Y shim triggered a response, the angle is 0
3. If the X and Y shims triggered a response, the angle is between 0–90 and can be changed to 40 in the first step.

If the X and Y shim both caused a reaction (i.e. case 3), for example, the shim routine can be initiated with the following command:

MASshim full --FWHM 7.0 --angle 40

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.