Hi Wendy,
Many years ago my 500 spectrospin magnet (now 40 years old coincidently)
would suddenly go to a terrible line shape (and this was before Topshim so
it was very painful to shim it back.) Turns out the source was a nitrogen
port that was getting partially or fully blocked and this caused the
magnet/dewar to shift and the homogeneity to change. I didn’t have the metal
heat exchanger fins on the exhaust ports but only tubing over the port. The
seal between tubing and port was not perfect so when we did helium fills,
air would sometimes get sucked back into the LN2 can, blocking the port.
Replacing the tubing with metal fins solved the problem and the magnet has
been stable ever since.
Another possibility (remote likely) could be changes in the RT shims during
VT experiments. Do you do VT often? I find my shims get hot or cold during
VT which causes homogeneity shifts and then they drift back to RT slowly and
that is with a large amount of air purge down the bore.
And of course, make sure that RT shim stack is tight as others have suggested!
To me it doesn’t sound like a potential quench but something is moving
or perhaps as Charlie has suggested, an unstable power supply or bad contacts.
john
> On Aug 15, 2024, at 5:32 PM, Karel Klika <karkli_at_utu.fi> wrote:
>
> Hi Wendy,
>
> Your problem reminds me of a problem we had many, many years ago. We had
> an NMR for which the shims would suddenly go bad, as in 20% spinning sidebands.
> This would happen intermittently and seemingly out of the blue for no
> apparent reason. Readjusting the shims (even by the instrument manufacturer
> engineers since the instrument was still under warranty) would resolve the
> problem for a period of time but sooner or later the jump to bad shims would
> happen. Looked at every possible cause you could imagine... But one day
> when I was tuning and matching the probe by physically getting down on the
> floor and adjusting the tuning knobs I noticed the shim stack could move.
> Turns out the problem was simply a loose holding screw, tightened it, adjusted
> the shims, and that was the problem solved. So if you are lucky, whilst
> you might be getting driven crazy by the problem as I am sure you are, it
> could be something very simple.
>
> Regards,
> K. Klika
> ________________________________________
John Decatur Ph.D.
Director of Chemical Instrumentation
Director of Precision Biomolecular Characterization Facility
3000 Broadway
Department of Chemistry
Columbia University
New York, NY 10025
212-854-2155
jdd13_at_columbia.edu
decatur.john_at_gmail.com
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Received on Thu Aug 15 2024 - 16:13:25 MST