In December of 2000 I asked about the possible causes of broken glass
inserts in our Bruker 500 MHz Dual probe. I received a number of responses
with good suggestions but I think the problem was actually due to not
tightening the screws that hold the probehead base to the outer body of the
probe sufficiently. I took the probe apart again and upon re-assembling it
I noticed that the screws which thread from the bottom up into the outer
(cylindrical) body could be tightened considerably, closing a visible gap
between the body and the probehead. Since two of these screws are missing
(a common occurence on old probes!) it's possible that only one was used to
thread into the body, and the probehead was pulled into a bent position.
This time I used two screws and tightened them enough to close the gap all
around the probe body. Result: the samples spin easily and we've had no
broken inserts (so far). I'm still puzzled as to why there is a gap
between the probehead and the body and why it's necessary to tighten the
screws so much to close the gap. But the problem seems to be solved! What
follows is the original message and a summary of responses:
Message:
We have broken the second inner glass insert in two months on our Bruker
5mm Dual (500 MHz) 13C-detect (non-gradient) probe. We use the probe
ocasionally on our DRX-500. Do you have any idea what could be causing
this? I replaced the insert but the spectrometer is clearly having trouble
spinning the sample. The air pressure goes up high and the spinning rate
is erratic, like it's fighting against some kind of friction. Maybe the
tube is rubbing against the insert. For now I will tell people to run 13C
non-spinning. But I can't imagine what could get things so far out of
alignment as to cause the tube to contact the insert. Does anyone have any
ideas?
Responses:
We have had similiar problems when the sample is inserted too far, either
by inattention to the depth guage or the sample slips because the o-ring
around the base of the spinner is loose. It may not be your problem, but
it's a simple thing to try, pull the sample up a few millimeters.
Dave Vander Velde, University of Kansas
Thermocouple depth (try turning off temp control and pull out thermocouple
Sample depth (raise sample higher that the depth gauge indicates and
see if it spins properly) -George D Sukenick
You could try spinning the sample with the probe out to be sure it is the
probe that is causing the spin problems (alternatively try a spinner blocked
with tissue paper or similar but no sample tube). This is perhaps too
obvious to mention (apologies if so) but have you checked the top of the
shim assembly for dirt?..... this is the usual cause of spin problems. Could
this be the cause of very erratic and asymmetrical spinning? Not sure from
your mail how things behave with other probes.... -Tim Claridge
Have you tried painting an NMR tube with a Sharpie marker and then
attempting to spin the tube. This will at least show you where it is
rubbing. It is a place to start. Good luck, Sara Kunz
Are you using a Wilmad NMR tube for your experiments? Ralph Flachsbart
WILMAD-LABGLASS
We had problems when we used just about any tube besides a Wilmad 528-PP.
If you have people using Kontes or Norell tubes, the walls may be OK, but
the bottoms were sometimes out of round and would crack the insert on
insertion. It could also happen is someone is overheating a tube in a
drying oven. -Ron Nieman
NOTE: We are using Kontes 33-220 tubes. -Neil
Neil E. Jacobsen, Ph.D.
NMR Facility Manager
Department of Chemistry
119 Old Chemistry
1306 E. University
University of Arizona
Tucson, AZ 85721
520-621-8146
FAX 520-621-8407
Received on Wed Feb 28 2001 - 15:41:37 MST