Dear Colleagues,
I agree with Yannick that an individual pressure protocol is helpful.
Establishing the pressures is a matter of trial and error. Watching the
wobb curve gives a good diagnosis.
In my case, even the automatic, initial bearing pressure of 1520 mBar is
too high and the rotors get pushed out of the stator. I start with 400 mBar
bearing, drive up to about 100 mbar, (about 3-5kHz MAS) then bearing of
approx 800 mBar, drive up to 150-200 mBar, (order of 7kHz MAS), then go to
1520 mBar bearing (the initial automatic value), drive such that
spinning is just under 10 kHz. Then I hand it over to automatic spinning
control to get (more or less stable) 10 kHz MAS. None of these values are
precise, and they are fluent. I don't wait for the spinning to stabilize
until I get to 10 kHz, but I watch how the spinning reacts to the pressures
and the stability of the wobb curve.
Good luck.
Happy resonating,
Ulli
On Tue, Apr 30, 2024 at 8:01 AM Yannick COPPEL via groups.io
<yannick.coppel=lcc-toulouse.fr_at_groups.io> wrote:
> Dear Christophe,
>
> I have the same problem. But I have a procedure that allows me to rotate
> the 2.5 mm rotors with my MAS II unit.
> I start by rotating the rotor manually, using the MAS unit's screen
> directly. At first I put in a lot of bearing (around 1600 mBars), then I
> gradually increase the drive to 200 mB (the frequency should reach 10 kHz=
).
> Then I gradually increase the bearing and drive in parallel up to a
> frequency of 15 kHz. From this point, I ask the Topsin's MAS interface to
> set the frequency to the one I am at. Then, as far as I am concerned, I can
> use Topspin's interface to continue increasing the frequency.
>
> Good luck,
>
> Yannick
>
> Le 26/04/2024 à 12:29, Fares, Christophe a écrit :
>
> 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 Tue Apr 30 2024 - 07:02:12 MST