Hi Ken
for D2O samples I would not suspect convection. Water is too viscous for
convection to occur.
A temperature gradient is the more likely cause.
If you can bring back resolution by increasing the value of Z1 then this would
confirm that a temperature gradient is present.
You will not see much of an effect with organic solvents. But in water the
gradient shimming will interpret the spread of water shift as a Z1 in homogeneity.
We try to keep the temperature gradient to 0.1 to 0.2 degrees in cryoprobes. But
that is still 0.001 ppm or 0.5 Hz on a 500.
Typical Z1 corrections are in the order of 10 - 20 units in Z1. if it is much
larger you have a larger temperature gradient.
the main factor influencing the temperature gradient is airflow. It should
be around 500 l/h. Lower airflow will increase the temp gradient.
Much higher airflow will eventually lift the sample a bit.
If this is a Avance II you might have one of the older VT units that allows
flow adjustment in discrete steps. One way to check at the sample is to start
"wobb" and then adjust the airflow. Up to 640 l/h all should be stable at some
flow rate above the wobble curve should become unstable and eventually the dip
jumps when the sample lifts a bit.
If these events happen at much higher flowrates it is possible that not all
the air actually makes it into the probe.
Best regards
Clemens
On 1/8/2024 12:38 PM, Kenneth Sharp-Knott wrote:
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)
--
==========================
==========================
====================
Clemens Anklin Ph.D. Vice President
Bruker BioSpin NMR Applications & Training
15 Fortune Drive NEW Phone: 978-313-5NMR(5667)
Billerica MA 01821 twitter: _at_canklin
web: http://www.bruker.com
e-mail: clemens.anklin_at_bruker.com
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Received on Tue Jan 09 2024 - 08:16:36 MST