When shimming a high resolution probe one distinguishes between on-axis (z) and off-axis (x, y etc.) shims. Spinning the sample parallel to the magnetic field direction averages the off-axis in homogeneity, which may result in spinning side-bands.
In MAS spectroscopy the spinner axis is at an angle m with the magnetic field direction and the distinction between the traditional on-axis and off-axis shims no longer holds. The spinning rates in MAS spectroscopy, however, are typically at least a few kilohertz, which is much larger than the magnetic field in homogeneity. As a result the amplitudes of the sidebands are small and shimming may be done with a set of shims that is cylindrically symmetric about the MAS spinner axis.
Such a set of shims can be constructed from combinations of the standard laboratory frame shims, via a transformation to the tilted magic angle frame 9 . In the simplest implementation the MAS probe is aligned such that the spinner axis is in the laboratory xz-plane (figure 5.4).
Figure 5.4. The magic angle spinner axis. The magic angle spinner axis is aligned with the xz-plane of the shims by
positioning the front plate of the probe parallel to the direction of the shim leads.
The 'magic angle z-shims' are listed below for orders Z through Z 5 . The simplest approach is to use the laboratory shims with the largest coefficient, so for instance the X shim is used as the normal high resolution Z shim. Complete shimming can then in principle be accomplished using only X, ZX, Z 2 X, Z 4 and Z 5 as the surrogate n th order on-axis shims. Note, however, that the coefficients given for the magic angle shims do not take into account the efficiencies of the shim coils and the use of the other shims listed below may be needed.
To shim the HR-MAS probe, a sample of 3%CHCl 3 in Acetone-d6 is suggested, using a rotor with a spherical insert. Make sure the magic angle is adjusted prior to shimming the probe. Spin the sample at a rate suggested in the following section. It is always best to shim up the probe already under the conditions used later for the real samples.
Make sure, that the correct gas is used for temperature control and spinning, the coils can be optimized for air or nitrogen gas. To obtain best results, the gas indicated should be used.
Lock the sample, tune the probe and start shimming using the X, ZX, Z 2 X, Z 4 and Z 5 shims as described above. If one of the shims requires an excessive shim current, reduce the current and continue shimming by adding current to another shim from the same group as shown in the table. For instance if the current in ZX is too high, reduce the value and optimize the line shape by adding current to the (X 2 -Y2 )Z shim. If the probe is not exactly aligned with the xz plane (see figure 5.4), then a small amount of Y and YZ may be needed for optimal shimming.
A second shim set should be obtained for rotors without spacers, and the optimal shim settings for these will be somewhat different. Under normal conditions the shim values obtained as described above, will be close to optimal for all other samples. When changing samples one usually only needs to adjust Z (and ZX).
Both shim value sets should be stored on disk. Make sure when the shim values are stored that the HR-MAS probe is defined as the current probe in the EDHEAD command, then shims can be recalled with the correct lock conditions like phase and power.