Ken,
Based on the fact that the blue and red spectra do not line up AND that H2O
is a hydrogen-bonding solvent, I think the issue is temperature-related.
If the red and blue solvent signals were better aligned, all the sucrose
signals would be even more poorly aligned with one another – this makes
me believe that the temperatures at which the two spectra were collected are
not the same, because an H-bonded NMR signal will shift a lot more when the
temperature changes.
This suggests that there is a large temperature gradient – if you have
a cobalt-59 sample to toss in there (usually folks do not) you would see the
temperature variation more clearly.
Perhaps there is a gas leak associated with the cooled NMR coils? This would
account for the observed behavior, I believe. You could test this by pulling
the prodigy probe and installing a normal (RT) probe. Not fun, but at least it
would answer the question of whether or not the cooling gas is leaking onto the
sample tube.
Jeff
> From: main_at_ammrl.groups.io <main_at_ammrl.groups.io> On Behalf Of Kenneth Sharp-Knott
> Sent: Monday, January 8, 2024 12:38 PM
> To: ammrl_at_groups.io
> Subject: [AMMRL] Shimming issues related to oscillating lock level
There appears to be more at work here than originally thought...
I believe this all started after a routine refill of the Prodigy N2 dewar.
Shims are terrible. Far worse in D2O with very broad lines. The Lineshape
standard will sometimes meet specs, but samples in D2O are consistently bad.
This is both with topshim and after attempting to manually correct the shims.
I have 3D shimmed repeatedly. This system, an Avance II 500MHz, generally
needs convection compensation. My usual topshim sequence has been
'topshim convcomp ordmax=6 tuneaz'.
The Bruker engineer took a look remotely and didn't find evidence of a dirty
probe or cracked insert. He suggests the issues are related to an oscillating
lock level and blames the oscillation on convection. He suggested a blockage
in the VT gas. I have traced the line and there is no blockage, kink or break
in the line. I have removed the line and can feel VT gas flow which in increases
and decreases normally with the VT Gas Flow setting.
By lock oscillation, I mean that the lock level will be steady and then increase
or decrease 5-10% over a maybe 20 seconds and then stabilize. This happens
repeatedly to varying degrees.
Does anyone have any suggestions for a cause for this lock oscillation? .
30mg Sucrose in D2O - red is usual - blue is current state.
Ken Sharp-Knott
Manager of Analytical Services and the NMR Facility
Department of Chemistry
Virginia Tech
(540)267-6502 (Cell)
(540)231-0885 (Office)
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Received on Mon Jan 08 2024 - 09:57:22 MST