Wendy,
It sounds like your magnet is seriously drifting, and unhappily that
suggests you're headed toward a quench. There are things one can do,
and others here are smarter about the practical things you can do (such
correcting degradation of the magnet vacuum). Confirming the size and
type of drift will help with that. You need to check both main field
drift, and the possibility of a cryoshim drift. Main field drift can
be checked on a daily (or weekly) basis with a quick measurement, but I
like to do occasional overnight measurements that also check cryoshims
(this is especially important after a magnet energization to check when
stability is good enough to take longterm data). Make a full
drift measurement as follows:
1. Redo your good shim on the test sample (do this toward the end of the
workday). As soon as you get a good shim with a normal locked sample,
take a 1 scan 1H spectrum with the lock off (in topspin turn off in the
BSMS panel, and set LOCNUC to 'off'). The spectrum should look the same
as when locked.
2. Staying unlocked, take 1H 1D spectrum every 30 mins or so from this
running through the overnight. (For topspin use multizg to do this.)
3. The change in the center position of one peak through the spectra
gives the Hz/hr drift. It should be < 10 Hz/hr.
4. If the center position drift is < 10 Hz/hr, but the line shape
significantly degrades during the overnight, then there is a cryoshim
drifting that may quench (but is easier to fix, assuming it doesn't take
the main field when it goes).
The other primary question I'd have is cryogen consumption/blowoff. If
blowoff is high and you're doing fills more frequently, then the magnet
is losing vacuum. I'm curious as to when the last rebuild was done to
the magnet. Possibly the o-rings are at the point that they need
replacement through a rebuild (typically on such magnets every 15-20 years)
This data will enable more practical suggestions to be made about what
can/should be done. Sooner is better right now, and hopefully you can
correct things enough to avoid a quench. Vendors experienced with
rebuilds can directly help although of course for a fee. If it's been
quite a while since the last rebuild, you'll likely have to go to one of
them for the rebuild, but you want to avoid a quench.
Good luck,
Charlie
On 8/9/2024 5:28 PM, Wendy Breyer wrote:
> Hello,
> We have an Avance NEO with a Helium cryoprobe in our ~40-year-old
> unshielded 500 MHz magnet (vintage Oxford 500/51). Recently I've
> noticed that I need to do a 3D shim more and more frequently. So I
> did a test: I 3D-shimmed the magnet on the Solvent Suppression test
> sample (i.e. 2 mM sucrose, etc., 90% H2O/10% D2O), wrote the shim
> file, inserted our test sample (sealed 30% Menthol in CDCl3), locked,
> topshimmed (params: convcomp, ordmax=8) and collected a 1D proton
> without spinning. Everything looked good. 24 hours later, I inserted
> the Menthol test sample, read the saved shim file, and collected a 1D
> proton without spinning (using the same steps outlined above) and it
> looked like garbage. I collected a 1D proton with spinning and the
> data improved a bit but didn't return to the quality of the original
> spectrum.
> If you have an ideas what might be going on, please let me know.
> Thanks!
> Wendy
> *Wendy Breyer, Ph.D.*
> Director of Instrumentation
> Department of Chemistry
> Lehigh University
> 6 E. Packer Ave.
> Bethlehem, PA 18015
> web216_at_lehigh.edu
>
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Received on Fri Aug 09 2024 - 18:17:21 MST