Thanks to all of you for sending quick responses. I am posting the
comments and suggestions that people have made so far. I am also adding
some clarifications so that you know what all I have done.
Original Message:
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I have been having a really hard time for the last couple of days
shimming on a 4:1 mixture of CDCl3 and CD3OD. After extensive shimming
the line shapes look unacceptable. I have tried locking on both CDCl3
and the CD3 peaks. When I use a mixture of 1:1 mixture of CDCl3 and
CD3OD the spectrum looks perfect. Also, the line shape sample give very
good results, so I can rule out that there is a problem with the current
shimset. Has anyone faced a problem like this before? Any suggestion
will be greatly appreciated.
Clarifications:
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1. I am trying to do a 1H NMR.
2. I don't have any sample in the mixed solvent system right now. After
I get a good set of shims ( I hope!) I will be adding a calixarene
sample in there.
3. I tried a 4:1 mixture again and it looked about the same as before
(which is UGLY). Then when I added more methanol to it to make the
concentration 1:1 it shimmed up quite good.
4. I have changed the NMR tube several times.
5. I have about 0.5 ml of solvent in there.
6. I have tried using a weighted average of the shim values of CDCl3 and
CD3OD as the starting point (assuming magnetic susceptibility is
linear). That failed too!
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That sounds pretty subtle.
You do not mention what your sample is, whether you are doing 1H or 13C
or ?....I have used a solvent like that for 31P in lipids.
In any case my thought was that it may have little to do with the
shimming and perhaps some exchange dynamics in your samples... which
change with solvent. The lipids 31P lineshapes I worked
with certainly cared about the solvent.
I would be interested to know how this works out.
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I don't know what else may be in your sample, but it is possible that your
are making a two phase sample with "wet" CDCl3/CD3OD or your solute is more
or less soluble thus aggregating at 4:1 mixture.
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I don't know about those particular solvents, but definitely, some solvent
mixtures that do not phase separate tend to stratify in the NMR tube.
Acetonitrile and water are a good example. As you develop a concentration
gradient, in order to maximize the lock, you apply a field gradient that
counteracts it. If you consider the solvent signals only, you can get the
one you are shimming on to look good, and the other will always look bad.
What happens to the solute in that case will depend on whether it also has
a concentration gradient. The situation when the sample is spinning may
be better than when it is nonspinning, but there is no workaround that I
know of, since this is a physical phenomenon in the tube. If you FID
shim, you can do the best job with solute peaks, in which case BOTH
solvent peaks will look bad.
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I only can imagine, that you work with very low
filling height within your tube. Otherwise there
is no reasonable explanation for your effect.
If you have a filling height of 4 cm, try to shim
with CDCl3 alone and change the sample then.
But be careful to have the same filling height
(+/- 0.5 mm). Using this procedure only Z and Z2
have to be adjusted.
But once again, in the case of low filling heights
there could be some trouble as you described.
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If I assume these solvents are miscible, you're still trying to shim on a
very dilute deuterium signal. Pick either the 7.24 ppm or the CD3 and
increase lock power until saturation becomes a problem, then shim.
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Have you tried a new tube? I've had a tube or two in the past that
wouldn't shim regardless of solvent.
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Hi, I had some problems shimming on mixed cdcl3/cd3od too. I found the
problem was unmixed solvents. Shake the tubes to mix the solvents.
Also, lock and shim on a pure cd3od sample, then shim on the mixed
solvent.
hope this helps,
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perhaps you may be running into an increasing mismatch of magnetic
susceptibility [Chi(m)] going from 1:1 to 4:1 since X(m) for chcl3 is
more than twice that for methanol ??? In that event, which one you
choose to lock on would be irrelevant.
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Dipanjan Nag, Ph.D.
Director of Instrumentation
Department of Chemistry
University of Nebraska-Lincoln
Phone:402-472-6255
Fax:402-472-6964
Received on Tue Apr 22 2003 - 09:27:12 MST