Hi Christophe,
I agree with Ulrike. Watching the wobb curve (especially on 1H or 19F) is
useful to check first if the sample get inside the stator and then if the
spinning is working well. You should see no oscillations or jump on the wobb
curve.
If you didn't have stable spinning in automatic mode, switch to manual and
see if it's stable at constant pressure. If not, then the problem is either
the stator or the rotor. Try an empty rotor, it should be stable without any
problem.
When spinning manually, I start with a bearing of 1500 mbar, then slowly
increase DP. 1500 mbar was the starting value used by the MAS I unit when
you try to spin automatically. Then I use the following table to increase
DP and BP. That table was made with a glycine sample, so spinrate will be
lower for heavy sample and with an old stator or rotor cap.
vr (kHz), DP (mbar), BP (mbar)
6, 110, 1520
10, 230, 1520
15, 480, 2120
20, 790, 2740
24, 1110, 3240
28, 1560, 3720
30, 1860, 3920
32, 2220, 3920
34, 2660, 3940
I have noticed the spinning was usually stable at 10-15 kHz, sometimes a little
bit unstable around 20kHz, but get better at 25 kHz and above.
For the eject problem, it could be some dirt inside the stator or the part
on the top of the stator. You can try to clean with isopropanol. Is the eject
pressure high enough?
Sincerely,
Cedric
Cédric Malveau, PhD
Conseiller de recherche
Laboratoire de RMN
Département de Chimie
Université de Montréal
Tél. 514-343-6111 #1966
De : main_at_ammrl.groups.io De la part de Fares, Christophe via groups.io
Envoyé : 26 avril 2024 06:30
À : main_at_ammrl.groups.io
Objet : [AMMRL] MAS VTN 500WB BL2.5mm Probleme
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
---------------------------------------------------
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Received on Tue Apr 30 2024 - 08:06:51 MST