As others have said, the controller may not be up to the task of regulating
the spinning of a rotor that small at lower MAS rates. If you do want to
spin slowly for any reason, you may get better stability by setting the
bearing and drive pressures manually instead of trying to regulate,
although you probably won't hit the exact frequency you are targeting.
If you are having problems with the rotors rubbing and also with ejecting
that may be an indication of a problem with the alignment of the coil in
the stator, which unfortunately is difficult to fix without sending it in
to have the stator replaced. It's probably worth trying to clean it before
resorting to that, since cleaning is much easier/cheaper. In the past I
have taken the cover off, put the probe upside down, and soaked the probe
head in a beaker of IPA so that the stator is immersed. Just make sure to
dry it thoroughly (leave it overnight/blow some gas through the drive and
bearing) before you try to spin a rotor in it afterwards.
Good luck.
Brian
On Fri, Apr 26, 2024 at 6:40 AM Fares, Christophe wrote:
> 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
> ---------------------------------------------------
>
-=-=-=-=-=-=-=-=-=-=-=-
Groups.io Links: You receive all messages sent to this group.
View/Reply Online (#1198): https://urldefense.com/v3/__https://ammrl.groups=
.io/g/main/message/1198__;!!PvDODwlR4mBZyAb0!SyqcLl5atcnaXQjCf58j97A3ivGU9Y=
RymwTjtsSsMSkw6aSivetbU_y88SHe6q8Pq5AwSrYUTVF4wdnvUc8pk9m-TKmU$
Mute This Topic: https://urldefense.com/v3/__https://groups.io/mt/105747724=
/7559972__;!!PvDODwlR4mBZyAb0!SyqcLl5atcnaXQjCf58j97A3ivGU9YRymwTjtsSsMSkw6=
aSivetbU_y88SHe6q8Pq5AwSrYUTVF4wdnvUc8pk5eVEZzl$
Group Owner: main+owner_at_ammrl.groups.io
-=-=-=-=-=-=-=-=-=-=-=-
Received on Fri Apr 26 2024 - 07:11:47 MST