Karen Ann,
Is it possible you have a bad connection to one of the shims, perhaps at the
base of the shim coil asssembly? If this were the case, when you have the
bad lineshape, one of the shims would have no effect. I think you have
Brukers, right? From my experience with Bruker shim stacks, the soldered
connections where the wires of the shim coils are attached to the PC board
where the wires to the shim cable attach, at the base of the shim stack, are
prone to fail. I have seen this a couple of times. I believe the culprit
is electrolytic corrosion that occurs when there is water condensed in this
area during low temperature operation, when the shim assembly is cooled
below the dew point. A small amount of mechanical disturbance can break or
open the connection, and when a connection is remade it can "weld" slightly
due to the current density at the point of contact. The first time I
experienced this, it was maddening until we removed the shim stack,
disassembled the base, and examined the connections. Lots of corrosion
products were evident, and an open connection.
If this is the cause of the problem you are experiencing, it is easily
fixed. You would need to remove the shim assembly to the bench, disassemble
the base to examine the connections, clean off any corrosion products, and
repair any break. I have long thought Bruker ought to passivate these
connections because of the potential corrosion problem, perhaps with a
coating of lacquer of some sort.
Of course these comments may be way off the mark in this case; if so,
"never mind!"
Ben
*****************
Karen Ann Smith wrote:
> 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 - 09:35:08 MST