Howdy,
This happens often. The split field is coming from a change in Z2.
Isolating why is more fun.
One way is having a drifting magnet with a supercon Z2 shim. As the main
field drifts it is coupled to supercon Z2 and induces current into this
shim and thereby generates a different Z2 correction and a split field.
This is the main reason why magnet manufacturers prefer not to use Z2
supercon shims. The best way to approach this problem is to have the
computer add a measured amount of Z2 per unit of time much like people
with drifting magnets can run unlocked if the computer puts in a small
amount of Z0 per unit of time.
Another way is to have a magnet field plot with a lot of change per mm
at the position where the probe is positioned. This is often on one
shoulder of a Z2 field plot of the main magnet. As cryofluids boil off
there are very very small movements of the magnet inside the Dewar. This
movement of the magnet relative to the probe changes Z2 necessary for
the correct shim. The way to correct this is to make sure the probe is
centered at the flattest point of the magnet field plot and not at a
steep slope of the magnet field plot.
There are other reasons but these are the most common in my experience.
woody_at_acornnmr.com
-----Original Message-----
> From: Wei Li [mailto:wli_at_utmem.edu]
> Sent: Sunday, April 28, 2002 1:31 PM
> To: ammrl_at_chemnmr.colorado.edu
> Cc: bmoore_at_utmem.edu
> Subject: peak splitting
Dear friends,
We seemed to have a problem with our Z1 in our new Inova 500. Here is
what we found: we took 1D proton spectrum every hour for 15 hours with
either lineshape or sensitivity sample. The temperature is regulated
well and lock is not saturated. To our surprise, we found the initially
nice siglet split and shifted after a few hours. Our magnet drifted
about 6Hz/hour, and I am suspecting the large Z0 current change affect
the Z1 and mess up with the Z1 shims. Also our Z1 constantly drift to
larger values, and Varian had to come to adjust the cyroshim recently to
prevent room temperature Z1 reach its extreme.
I am curious to find out if anyone has seen such behaviour before, and
if you have, how could you do any long time 2D or 3D experiment, since
after a few fids, the peaks position will shift and split?
Any comment will be highly appreciated.
Regards
Wei Li
Received on Mon Apr 01 2002 - 09:29:51 MST