Karen,
This sounds very much like a problem I saw on a 400 some years back. If a
probehead was left in the magnet then shimming remained good but if it was
removed then the homogeneity disappeared out f the window (even if the same
probehead was put straight back in). After much investigation - including a
very good look up the shim bore - we found a staple that had 'migrated' up
the bore. A brown staple is very difficult to see up a dark shim bore!! Our
assumption at the time was that each probehead change cause a small movement
in the position of the staple which had obvious consequences for magnet
homogeneity.
I suggest you remove the shimbore (with a probehead in place if you have
sufficient clearance) and get it as far away from the magnet as you can
before seeing if there is anything in there that shouldn't be.
Best of luck
Regards
Ian
Ian Marshall
Analytics Group , SCYNEXIS Europe Ltd.
email: ian.marshall_at_scynexis.com
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-----Original Message-----
> From: Karen Ann Smith [mailto:karenann_at_unm.edu]
> Sent: Wednesday, April 10, 2002 8:28 PM
> To: ammrl
> Subject: Another shimming weirdness
AMMRLers,
I have been seeing a pretty weird problem on my 500. Sometimes when I
swap probes, the shim goes to (really bad). As in linewidths of (many)
10s of Hz. But only sometimes.
The most recent example is that last week the 5 m probe was in and
shimmed pretty well. Swapped it for the 2.5 on friday and all was
well. Swapped the 5 back in on Monday and again, all was well. I did a
little shimming on Monday and saved the file. Tues. I swapped the 2.5
back in. The lineshape was terrible, the field had to be changed to get
the system to lock, and the o1 necessary to put water on resonance was
off by about 1100 Hz. Today I put the 5 back in, read in Monday's shims-
and the field was fine, but the shims... not so good.
Last month, this happened about every probe swap. I looked up the bore
and didn't see anything. I cleaned the bore anyway, and the problem
seemed to go away - until yesterday. When this first started, I blamed
the surrounding- esp. as it seemed that the nearby (touching 5 gauss
lines) 250 also seemed to be changing. Now, I don't think so. I can't
find any changes in the nearby labs- and the shims are fine as long as a
probe is left in the magnet. I am sure the probes are aligned the same
every time- I have marks on the probe and magnet.
Last month, z4 was the big changer. Today, z4 was not too bad, but
lower x,y,z all needed some adjustment.
Ideas, thoughts, suggestions??? Please note that not-swapping probes is
not an option.
kas
--
Karen Ann Smith karenann_at_unm.edu
Director, NMR Facility Adj. Asst. Prof.
Dept. of Chemistry Clark Hall
University of New Mexico Albuquerque, NM 87131
505.277.4031 url: http://www.unm.edu/~karenann
Out of this world screensaver: http://setiathome.ssl.berkeley.edu/
"There is nothing a good day of cycling won't cure."
Received on Thu Apr 11 2002 - 10:56:01 MST