Dear AMMRL'ers,
I have had exactly the experience that Ben describes below. We do a
lot of VT work and I have fixed these connections more times than I would
like to remember. I agree 100% with Ben that Bruker should consider making
these connections a lot more robust.
Jane Strouse
>Date: Thu, 11 Apr 2002 09:05:16 -0400
>From: Ben Bangerter <ben.bangerter_at_yale.edu>
>X-Mailer: Mozilla 4.76 [en] (Win98; U)
>X-Accept-Language: en
>To: Karen Ann Smith <karenann_at_unm.edu>
>CC: ammrl <ammrl_at_chemnmr.colorado.edu>
>Subject: Re: Another shimming weirdness
>
>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."
Dr. Jane Strouse
Department of Chemistry & Biochemistry
UCLA
Los Angeles, CA 90095-1569
(310)-825-9841 - voice
(310)-825-0393 - FAX
strousej_at_chem.ucla.edu
Received on Thu Apr 11 2002 - 14:02:11 MST