Dear Colleagues,
When my 2.5mm probe heads have trouble like that, it is time for a new
stator. Unfortunately, they are not cheap. I never had luck with cleaning
them, so if someone has had success, I would like to hear how it is done.
In my experience, not being able to eject a rotor can have several reasons,
originating from the probe head (loose or leaking silicone tubing) or the
stack (top broken or leaking).
The instability from the spinning most likely stems from the problem of not
spinning smoothly. If you hear rubbing or can prove it by marking the rotor
along the lengths with several lines and the checking of scratches in the
lines after the experiment, you will not get smooth spinning. With a good
stator, in my experience the older MAS units do fine to stabilize the
spinning above 10 kHz, at least most of the time. Definitely an empty rotor
and rotors that have many "flight" hours on their back, like a KBr
standard.
Happy resonating,
Ulli
On Fri, Apr 26, 2024 at 10:01 AM Michael Groves via groups.io wrote:
> I have two thoughts about this--it does sound like the probe would
> definitely benefit from going in for inspection/repair. I'm not sure how
> much cleaning you can actually do on a 2.5mm probe without disassembling
> things to a higher degree than you probably want to. The sample eject
> difficulty is also suspicious--it just sounds like overall things don't
> quite work right and it would benefit from a good going-over.
>
> And the MAS/2 with that size probe likely won't regulate well at "low"
> speeds. So if your first test is at 5kHz I suspect it's not going to
> regulate well. The MAS/2 doesn't have the fine adjustment capability of
> the newer MAS/3 so for rotors smaller than 3.2mm it tends to have problems
> regulating well at "lower" spinning speeds. It could also depend on the
> firmware on the MAS/2. Newer versions of the firmware might have better
> regulation profiles for the smaller rotors but it is likely not trivial to
> update.
>
> Cheers,
> Mike
>
> On Friday, April 26, 2024 at 04:40:35 AM MDT, Fares, Christophe
>
> Dear solids-ammrlers,
>
> we have a MAS VTN 500WB BL2.5mm (13C-31P/1H) probehead from Bruker. It
> was installed in 2000.
>
> I don’t know much about its history before my time in the lab… only that
> it was rarely used. But in 2 years I have never had any success with this
> probe, and each time I put it in, I run into major problems. For one, a
> majority of samples that I prepare do not rotate stably. The bearing/drive
> regulation from our MAS Unit does not seem to be able to find the right
> pressure for a given desired MAS rate. Very often it oscillates from too
> fast to too slow and eventually crashes the rotor (we hear the rotor
> rubbing the side). I have tried several rotors now, including brand new
> ones. The second issue is that it is very difficult to eject the sample and
> require several attempts.
>
> I should however mention that, with a bit of patience and after a few
> tries, I am able to get my adamantane and KBr test-rotors to rotate stably
> up to 25kHz (I don’t dare go faster at this point).
>
> I am running out of ideas and I wonder what you recommend for me to try at
> this point? Should we first clean the stator? How do I do this? Or should
> we even send the probehead in for testing/repair? On the other hand,
> perhaps it is the regulation which is faulty. We are still using an old MAS
> II unit (H13000 EC:04.01 1057) which works flawlessly with all our 4mm and
> 7mm MAS probeheads.
>
> I also heard that 2.5mm is an awful size to work with in the first place
> (bigger or smaller give better results). Is this a widespread opinion?
>
> Looking forward to your suggestions.
>
> Best regards
>
> ---------------------------------------------------
> Christophe Farès, Ph.D.
> ---------------------------------------------------
> NMR Department/NMR Abteilung
> Max-Planck Institut für Kohlenforschung
> Kaiser-Wilhelm Platz 1
> 45470 Mülheim an der Ruhr
> Tel: +49 208 306 2130
> E-mail: fares [a] mpi-muelheim [.] mpg [.] de
> https://www.kofo.mpg.de/en/research/services/nmr
> ---------------------------------------------------
>
--
Dr. Ulrike Werner-Zwanziger
Adjunct Professor
Department of Chemistry, NMR-3,
Dalhousie University
6245 Castine Way (!!!new address!!!)
PO BOX 15000
Halifax, NS B3H 4R2
e-mail: Ulli.Zwanziger_at_Dal.ca
Tel. 1 902 494 8085
FAX: 1 902 494 1310
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Received on Fri Apr 26 2024 - 06:20:15 MST