Hello everyone,
I posted this question a while ago. Sorry for the late reply.
It turned out that our sample depth gauge was somehow shifted by a few mm. When we reset the gauge, everything worked just right. Steven K. Silber_at_
Texas A&M University had the perfect answer. But it was a good learning experience reading (and trying, before found the gauge as the culprit) the suggestions from AMMRL. Thanks to you all:
Daina Avizonis (Varian)
Chris Kojiro (U of Michigan)
Rainer Wechselberger (Utrecht University)
Tim Burrow (U of Toronto)
Jerry Hirschinger (Purdue)
Thomas C. Stringfellow (U of Wisconsin)
Dr. Sarathy (Texas A&M)
Laurine Galya
Lamy Chopin III (Dow)
Charles G Fry [Wisconsin)
Here is my original questions and the suggestions are summarized following this message. Thanks again,
Tracy
-----Original Message-----
> From: Zhang, Tracy (GE Advanced Materials)
> Sent: Wednesday, April 06, 2005 11:02 AM
> To: ammrl_at_ammrl.org
> Subject: Problem with automatic shimming on Mercury400.
Hello everyone,
We have a Varian 400 Mercury here, and we've been using homospoil auto shimming on H2 for both our 5mm id/pfg and 10mm BB probes. In the past, we usually update our shimfile/shimmaps every 6-8 months, and everything works pretty good--shimming usually converge after 2 iterations, sometimes 3. Last Monday, however, we started to have trouble with autoshimming on both probes. The symptoms were the same--Z5 constantly went out-of-range, and when we turn off Z5 shim, it took 4 or 5 iterations to converge and on the resultant spectra, there was always a broad base. Both shimmaps on these 2 probes were updated in Feb. Here is what I've done in trying to trouble shoot--
(1) checked find z0: autofind z0 value is pretty close to manually found z0, difference is within 5 units;
(2) checked lock channel: locked a sample and let it sit for 2-3 hrs, the lock level was stable;
(3) checked and cleaned out the upper stack.
(3) in an attempt to update the shimfile (probename_hs_d2o), I manually shimmed with the H1 lineshape sample (CHCl3 in Acetone-d6) on all 14 shimming channels. All shims were responsive--Iock level goes up and down when changing the shims, but I was not able to shim this sample into spec. (0.45/6/12 at 50%/.55%/.11% peak heights). The res. numbers I typically got was ~.3/8-15/15-20. So there is always a broadbase on the lineshape peak. I shimmed the higher order axial and non-axial shims quite extensively. No spinning side bands were observed, but the broadbase was always there. In the past, I never had much trouble shimming this sample into spec.
When shimmed on the doped 4 Hz water sample, I was able to get 3/56/86, and make a decent shimmap with gzsize=4. When ran autoshimming using this new shimmap, the same problem occurs--it takes >3 iterations to converge, and the lineshape is not good--either broadbase on bottom of peaks and/or broad peak in general.
I wonder if anyone from AMMRL has experienced a similar problem. Just to let you know, I am an organic/polymer chemist with no formal training in NMR spectroscopy, so I am learning to take care of this instrument. Any help would be much appreciated!
Tracy Zhang, Ph.D.
Analytical Chemist
GE Advanced Materials
Building 1, 1 Lexan Lane, Mt. Vernon, IN 47620
Phone: 812 831-4651, DC: 8*393-4651, Fax: 812 831-4917
tracy.zhang_at_ge.com
-------------------------------------
Definitely have seen similar problems, and usually with a broken glass
insert. Give that a look.
------------------------
Have you checked the bottom of the magnet for paper clips, etc.? Sometimes
something gets stuck to the magnet and can affect the high order shims.
---------------------------
If you also do not manage to get a reasonable shim by shimming manually,
I would suggest
you should check the shim power-supply. I remember, that we once or
twice had the situation that shims 'run wild' because there was a
problem with a resistor in the named power-supply.
-----------------------------
There are a couple of things I would try in addition to what you have
already tried. I assume you have optimized your lock phase. If you are
running this in automation, it would be a good idea to check your z0
position on d2o and compare this with what is in the probe file. If it
far off, you will need to update your probe file. I have seen similar
thing to what you are describing when either my z0 or my lock phase is
off. Even with my z0 off enough to cause problems the find z0 can
usually find the lock.
You said that you have cleaned the upper barrel. How about the probe?
If you have dirt or grease in the probe it could give you this trouble.
I clean my probe with a cotton swab soaked in methanol and very, very
gently swab the inside glass of the probe. Varian might not like you
doing this because you can dislodge the coil but I do it frequently. If
there is dirt stuck to the glass I can feel it as I swab the glass.
Also, do you see the same problem on both probes that you have?
When you shim manually do you get to the end of one of the shims? If so
you may have lost a cryoshim. Somewhere in the original installation
file there should be a spectrum of d2o with all the shims set to 0.
This is your cryoshimmed line shape and can be compared with a current
plot of the same thing (d2o with all shims set to 0) and lets you know
if the cryoshims have changed a lot. If you don't have one of these,
once you get the spectrometer working again I would encourage you to
make one on d2o with all the shims set to 0 and put it in your
installation file for future reference.
------------------------------------
I had a problem once with my Inova500 not shimming suddenly. Found that
the depth gauge had been shifted (Varian had it mounted on the console with
cable ties) and the tubes were about 10mm high in the probe. Readjusted the
depth and all was well.....
-----------------------------------
I am wondering if you are updating the Z0
value(for doped D2O) in the probe table if the drift is significant. With the wrong Z0, grad.
autoshim may not work. But this does not explain the specs on H1 lineshape sample.
-----------------------------------
Tt seems that you've been systematically characterizing your problem. A
couple of other possibilities that come quickly to mind are:
1) Check for a damaged or contaminated probe, such as a cracked insert or
obvious signs of sample tube breakage and subsequent spill contamination.
This is one of the first things that I would check, but since you make
similar observations with two probes, this is a much less likely---though
not impossible---scenario.
2) Check the cryoshim lineshape to see if you've "lost" one or more of the
cryoshims. This is easily done if you've previously recorded a baseline
cryoshim lineshape in preparation for these kinds of events. If you have no
previous record for comparison, then you may need to make a qualitative
judgement. (Just set all the room-temperature shim DAC values to zero and
record a spectrum of water.)
-------------------------------------
I'm using homospoil shimming on a Mercury 400 too, with good results.
You might shorten d3 and see if that helps. But I can think of two
things that might be the cause: a cracked insert (the glass holding the
coils) or a metallic particle in the bore. If it affects both probes,
then it maybe the latter.
-------------------------------------
Be sure the probe position is correct, and that the sample position is
correct. Either one can seriously affect the shims. Check near the edges
of the fringe field to see if someone moved a refrigerator or something else
massively magnetic directly above, below or to the side of the magnet.
Here once a refrigerator moved in below a 500, and every time someone opened
the refigerator door, the lock level changed. Same thing when we changed a
lab from a 200 to a 300; an electrical panel door on the other side of the
wall affected it. When the lab below the 500 was renovated they tore out
all of the tin ductwork, necessitating resetting all cryoshims. The effect
is worse below the magnet than above.
After you are sure the RT shims are working, everything is in the correct
position, and you still can't get it, then you might have to hire Varian or
an NMR service company like Triangle Analytical
-------------------------------------------
It sounds like you are doing everything correctly. Is this shim
problem also seen with your other probe? That could indicate a probe
problem.
I would check to see if anything magnetic has been pulled into the
magnet bore (e.g. paper clip, staple) during a probe change.
The other possibility is a quenched cryo-shim. If you have the
original spectra when the magnet was installed, there should be a water
spectrum with the cryo-shims before the room-temperature shims were set.
You just set all your room-temperature shims to zero and see if you have
the same water resolution values.
Finally, there could have been some change in your shim module.
But, I would have expected a complete re-shim to have come up with a good
set of shims.
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Received on Mon May 23 2005 - 15:33:52 MST