Dear AMMRL,
Thanks very much to all who have sent me suggestions for solving our
problem with spin detection. Our problem was due to a bad sensor
located on top of the upper stack insert. Since we have a wide bore and
a 5mm upper stack insert we are equipped with two of this sensors.
Switching them solved our problem (just remove the two screws to take
off the sensor). Bruker also sells this sensor for $335. Please see
the summary below for the replies I received.
My original question:
Dear all,
We have a Bruker DRX300 wide bore magnet which has a 5mm upper stack
insert for doing routine 5mm samples. There is an ongoing problem with
spinning regulation. In the past when samples didn't spin well we could
solve the problem by getting a new spinner or cleaning the O-rings and
spinning turbine on the room temp shim assembly. This time there is no
problem spinning the sample, the problem is with spin detection. If we
look at the spinning side band on 1H spectra the spin rate is actually
25 Hz, but the sensor can only detect up to 7 Hz, higher than that the
sensor will register 0 Hz on the BSMS. Bruker currently doesn't have
another 5mm upper stack insert in stock so we cannot know for sure
whether the sensor is causing problem. The spinners we have are new and
have no problems in our other two regular bore Bruker magnets. Could
the sensor be sensitive to the type of black and white labels on the
spinners in a wide bore system? What other kind of labels could we try?
Have Any of you had similar problems?
Thanks in advance for your suggestions.
The replies:
1. I once wanted to replace the reflective labels. Buying this in a
safety store was too expensive. I went to a local
auto parts store where they sold reflective tape for bumpers. It was
diagonally striped red (nonreflective) and silver
(reflective). I cut out little rectangles and stuck them on and they
worked. Our spinners use four each; some use two.
The black was not necessary.
2. Spinning rate sensing is done with an optical transmitter/receiver
setup, operating in the near IR or red part of the spectrum. Since the
spinners work in other systems, the likely culprit is an aging LED which
would need to be replaced. Another possibility is an accumulation of
dirt on the transmitter and/or receiver so inspection and cleaning would
be in order before replacement. I haven't repaired a Bruker stack, but
the transmitter and receiver are usually fairly standard parts that can
be purchased from electronics suppliers at much lower prices than Bruker
charges; however, Bruker has at times used parts that are only available
in Europe.
3. One thing that can cause this is a dirty lens over the IR sensor or
the gain setting on the detection bd.
4. I don't know a lot about this insert, but you might try cleaning the
optical components to make sure transmission and reception are
occurring. I suspect that this is relatively easy to do and worth a shot
given that Bruker can't help you for awhile.
5. We had a problem with spin speed detection on an AM spectrometer
which turned out to be a marginal output from the phototransistor at the
top of the upper stack. If you have another system you could try a
swap. I managed to tap into the output with a scope so I could monitor
the level. On AM consoles there is a test point at the input; I don't
have DRX schematics to check this. The phototransistor output should be
a square wave about 1 to 4 V and the signal from the 4 reflectors should
be similar. A new phototransistor assembly should be available from
Bruker if necessary.
6. We had some Bruker turbine labels that stopped working. I'm not
sure, but it could have been hexanes that dulled the reflectivity. We
replaced them with labels from Bruker, and now use only Isopropanol for
cleaning.
Since your turbines are new, I'd be sure all of the bearing, eject, and
spinning air passages are open in the upper stack and shims, and that
there is nothing blocking an orifice. A length of wire pulled from a
coaxial braid is useful to clear the small orifices. Also ensure there
are no scratches or burrs in the turbine area, possibly resulted from
broken glass. I would also clean the tachometer optics and inspect the
small O-rings between the stack and shims. If anything inhibits
spinning, the BSMS can compensate by increasing the bearing and spinning
air supplies. The resulting high volumes of air lift the sample to a
position where the tachometer marks are not visible to the tachometer
sensor. This can also affect shimming and decrease the NMR signal
because the sample is no longer centered in the receiver coil.
Regards,
Wei
Wei Wycoff
125 Chemistry Building
601 S. College Ave
University of Missouri-Columbia
Columbia,
MO 65211
USA
Phone: (573) 882-3291
Fax: (573) 882-2754
Email: WycoffW_at_missouri.edu
http://www.chem.missouri.edu/nmr/nmr.html
Received on Fri Nov 05 2004 - 14:51:22 MST