Bruker NMR iProbe HR-Liquids, User Manual, Page: 38 / 56

Operation
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7.6.3 Variable Temperature Unit (VTU)
Since the MAS unit regulates the gas flow of the bearing gas, the VTU will only regulate the
temperature but not the flow. Therefore, the VTU must be configured to external gas flow
control in the Temperature Control Suite (VTU user interface) as highlighted in
Figure 7.19
[ } 38] .
Figure 7.19: Temperature Control Suite (VTU user interface)
7.6.4 Gradient Amplifier
In order to minimize possible artefacts coming from gradient recovery time, it is
recommended to switch on the pre-emphasis of the gradient amplifier.
Note that the pre-emphasis should be set to OFF with compensated gradient coils (typically
the case of iProbe-Liquids). Never exceed the allowed maximum current given in the TopSpin
Gradient System Parameters window.
Figure 7.20: TopSpin Gradient System Parameters.
7.6.5 Tuning & Matching
Tuning & Matching of each channel is done with the atma or atmm command (see also
ATMA [
}
29] ). Note that in order to see the wobble curve the stator must be in the insert
position.
7.6.6 Shimming
In HR-MAS spectroscopy the rotor axis is oriented at the magic angle with respect to the
magnetic field direction. As the spinning rates are typically at least a few kilohertz, which is
much larger than the magnetic field inhomogeneity, the amplitudes of the sidebands
generated from the magnetic field inhomogeneity are small. Therefore, shimming may be
done with a restricted set of shims that is cylindrically symmetric about the MAS rotation axis
(Z', Z2', Z3', etc.). To generate these
magic angle shims a linear combination of the standard