Hi Wendy et al.
How’s the weather? Literally, has your area been experiencing rapid
changes in barometric pressure?
Years ago, I had an Oxford 600 with a lineshape instability problem. It would
degrade over time, but the rate of decay wasn’t consistent, and sometimes
the linewidth would actually recover (getting sharper with time after having
gotten bad). I ran a long-term experiment acquiring a scan every half hour
(I think), plotted the linewidth over time, which demonstrated it was changing
at varying rates. I found that plotting linewidth as a function of atmospheric
pressure, as recorded at the nearest airport,* showed a very strong dependence.
We installed a manostat to stabilize the helium pressure in the magnet, and
that mostly cleared up the problem.
I’ve encountered other lineshape stability problems on various
instruments, including some pretty bad problems with a new magnet (which has
since mostly stabilized). I’ve got a pretty standard approach for o
gathering data now, developed in part with Bruker service:
* Shim a lineshape standard as sharp as practical
* Run a kinetics experiment for a desired period (4 hours to weekend,
depending on need and other factors)
* Pulse program kx_zg2d, Bruker parameter set KX_PROTON_2D
* One 1H experiment with lineshape acceptance test parameters every
15 or 30 minutes
* Keep locked or unlocked, depending on whether you think there’s
significant coupling of a shim to Z0, like if your B0 field was also drifting strongly
* Keep Autoshim on, selecting suspected problem shims (for me, usually u
Z, Z2, Z3, X, Y, XZ, YZ), interval = 3 sec, delta = 1 unit for each *
While the kinetics experiment is running, also run a script that tracks your
shim values (writes them to a text file at specified intervals, like every 10
or 15 minutes, depending on how fast you see your lineshape degrade). Earlier
in this thread (Aug 12 for me), Mike Groves mentioned such a script. I think
he tried attaching it, but my system gave me warning message that said an
attachment was removed. While troubleshooting a new magnet, Bruker Service sent
me a script called “followshims” that performs this function,
and works OK. I don’t feel right passing along their script, but I
suggest you reach out to them for a copy.
* Analysis: After determining whether the Autoshim routine faithfully kept
the lineshape on track, plot the values for each shim against time, and see if
any were drifting. With this data, you can identify which shims are
more problematic and perhaps try correlating changes with possible causes,
especially those that can be associated with time, like atmospheric pressure,
room temperature, elevator traffic, etc.
* I imagine if you’re having a problem with a specific cryoshim,
it will show up as a strong change in the corresponding RT shim.
For extra certainty with respect to the environment, I suggest getting a
temperature/humidity/pressure logger (which is already a good idea for tracking
HVAC problems). Placed near the magnet, you can plot shim values as a function
of these parameters with whatever frequency you choose, like every 15 minutes.
This is the kind I use:
https://lascarelectronics.com/data-loggers/pressure/el-sie-6-plus/
I hope this helps. Good luck!
* Josh
* For records of recent weather observations in the U.S., see
https://www.ncei.noaa.gov/access/search/data-search/global-hourly?pageNum=1
Josh Kurutz, PhD
NMR Facility Manager, Chemistry Dept.
University of Chicago
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> From: main_at_ammrl.groups.io on behalf of Matthias Brandl
> Date: Friday, August 16, 2024 at 3:34 AM
> To: main_at_ammrl.groups.io <main_at_ammrl.groups.io>
> Subject: Re: [AMMRL] Off axis shims bad after 24 hours on 40-year-old Oxford magnet #Bruker #Hardware
Hi Wendy,
We have had partial and full nitrogen port blockages several times even with
heat exchangers installed on the turrets, probably due to some combination
of bad seals and suboptimal filling practices.
While I have not heard any complaints about bad lineshapes from that (though
that might not mean much), I have used a piece of large-gauge copper building
wire (the kind that contains a small number of relatively large individual
strands) to check for a blockage and, if one is there, remove it. I have cut
a piece of that about 30-40 cm long, stripped it and put a dogleg in one end
to avoid dropping the whole thing into the nitrogen tank, and also keep the
strands together a bit better.
I use this to rasp the ice away (rotate at first, then at some point scrape
the walls of the tube). The copper should be sufficiently soft to not hurt
anything, at least that is my hope.
Cheers
Matthias
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Received on Sat Aug 17 2024 - 08:28:17 MST