As usual, the AMMRL community is coming through quickly with numerous ideas and suggestions. A huge thanks to everyone who has responded so far – too many to list individually at nearly 20! I'll list most of these below in the hope that they can help someone else in the future, as well as that someone may followup with another useful suggestion.
One very common response is that I may have lost a supercon shim. Below is a link to images of the best lineshape I can get right now on doped D2O (I think 0.1 mg/mL GdCl3 should be 2 Hz, I can get a bit over 4 Hz and there is a slight downfield tail) and the same sample if I set all the shims to zero. The two main reasons I don't think this is the problem are that the helium boiloff was normal and, while a little bit wide (somewhere in the 200-300 Hz range) the line is pretty symmetric. I really don't know the best way to test this for sure. I will compare to the supercon lineshape at installation if I can find it – it was before I started here and the computer has been changed twice. I'm pretty sure it's on a CD or tape somewhere…
https://picasaweb.google.com/afowler13/ShimProblems?authkey=Gv1sRgCObQw_WK36H2Fg&feat=directlink
Other suggestions so far:
The probe may have slipped. I don't really think so. I've removed and reseated the probe, checked for rotation, eject is fine, and the "center" of a gradient shim map hasn't moved.
A problem with the gradient amplifier. We have XYZ but usually run with pfgon='nny'. Unplugging X and Y changed nothing. Setting pfgon='nnn' caused a slight drop in lock, compensated by changing Z1 by 100 DAC units. No major problem here. Gradients are behaving normally.
Something metallic stuck to the magnet and/or in the bore. I can't find anything, and I've cleaned out the bore with some propanol on a large swab. There was a bit of dirt, but nothing extraordinary.
The ribbon cable may not be seated well. I've checked this numerous times.
Fans in the shim supply could be going bad. They're both spinning freely.
A bad DB9 to RJ45 adapter on the back of the shim supply caused a similar problem for one site. The RJ45 plugs directly into this shim supply, but a good thing to check for others.
A failing -9VDC transverse supply under load. I checked all the test points, but not under load. This is easy and I will check this tomorrow when I open up the supply again to replace the original main driver board. This is the only suggestion so far based on my observation that the shim supply seems to cut out early for Z2-Z7 with negative values.
I hope these ideas help someone else in the future, and I'll post any other ideas and/or the resolution when I find one.
Andrew
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Dr. Andrew Fowler | University of Iowa
Associate Director | B291 Carver Biomedical Research Building
Medical NMR Facility | Iowa City, IA 52242
319-384-2937 (office) | 319-335-7273 (fax)
andrew-fowler_at_uiowa.edu
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Received on Mon Jun 27 2011 - 10:57:06 MST