Has anyone successfully removed their shim stack from a Bruker Ascend magnet
with a SampleXpress sample changer mounted on it in order to clean the air
bearing on the top of the shims, and put everything back together? My AV
III HD has been in very heavy use for 7 years, and I am beginning to see
bothersome inconsistency in sample spinning.
I have removed Bruker shims to clean the air bearing in the past, on older
systems that didn't have a sample changer. In that case, it required loosen
ing 3 screws on the top of the upper stack, plus a clamp at the bottom of
the magnet, to get the shims out. With the SampleXpress, the top of the
upper stack is completely covered, and I'm not even sure the system is assembled
the same way as in the past. Plus, the SampleXpress manual does not contain
any directions about how to remove or (re)install it and get the correct
alignment.
I have tried the time tested trick of trying to clean the air bearing from
the top using some Kimwipes taped to a plastic dowel and wetted with a little
solvent, without extracting anything visible, and with at most a very temporary
improvement in spinning. After doing this same thing for decades on Varian
systems, I have concluded this method is the triumph of hope over experience.
The Varian air bearings seem to foul more easily than Bruker's, but on the
other hand, they are drastically simpler to pull out of the magnet and
thoroughly clean. The Varian air bearing is at the bottom of the upper
barrel rather than at the top of the shims. There are just 2 knurled knobs
holding the upper barrel in the magnet. Once that is taken out and the air
bearing is unscrewed from the upper barrel, it is obvious by inspection
there can be extensive stubborn deposits on the horizontal surface of the
air bearing which are completely untouched by the Kimwipe on a dowel method.
Plus, there can be deposits of crud on the lower vertical surface on the
air bearing that are probably even more effective brakes on spinning than
whatever is on the horizontal surface. Those cannot be reached, much less
removed, by a Kimwipe on a dowel.
For what it's worth, on the 6 spectrometers I have with warm probes, I have
turned spinning completely off on 2 of them and made not spinning the default
on another, and I'm satisfied on those 3 with the trade of a small improvement
in homogeneity for streamlining everything by not bothering with spinning. The
system in question here has a Prodigy cryoprobe. With the sample not spinning,
I get errors from topshim about convection wrecking the shim map when the sample
contains a low viscosity solvent. The 'convcomp' option in topshim eliminates
those errors. However, the dropoff in shimming performance nonspinning is too
pronounced for my taste. I am really, really happy with the spectrum quality on
a well prepared spinning sample that I get from this probe.
David VanderVelde
Manager, Liquids NMR Facility, Caltech
davidv_at_caltech.edu
Received on Fri Nov 19 2021 - 07:12:52 MST