Hello All,
Some time ago, I posted a query to you all about problems I had with a
Bruker Avance 400 MHz Micro-bay from which I was occasionally receiving
data that was "doubled."
After trying many of your suggestions, the problem was finally resolved
by swapping in a pair of new lock transmitter & receiver boards.
Thank you all for your suggestions and efforts!
Special thanks to:
Clemens Anklin & the rest of the Bruker App Lab & Service Center
George Sukenick, Joseph Dumais, Brian Cutting, Jens Knudsen, Daina
Avizonis, Kurt Wollenberg, Borlan Pan, David Redwine, Andrew Jensen, Tim
Claridge, Richard Fitch, Robert Santini, Joyce James, Dinu Iuga, Phil
Dennison, Perry Pellechia, and Rob Kleps.
Happy New Year!
Patrick Wheeler
Pfizer La Jolla Labs
Discovery Platform Chemistry
Chemical Analysis and Purification
Open Access Team
10770 Science Center Dr.
San Diego, CA 92121
858-526-4849 Office
858-395-2023 Mobile
858-678-8156 Fax
The original post:
I have a problem that has been stumping me for some time, and has
perplexed a few Bruker engineers, as well.
We have a two-year old Bruker Avance 400 MicroBay with a QNP probe
running TopSpin 1.3 with the latest patch level under Windows2000 Pro.
We run it full time in IconNMR automation.
Intermittently, perhaps once or twice in a hundred spectra, we have a
peak doubling problem. It looks as if the lock jumps part way through
the experiment, such that the spectrum is run properly referenced, then
each peak has a shadow a few hundred Hertz off. The "jump" can vary from
a few dozen to a few hundred Hertz.
I have never been able to observe the phenomenon while running manually.
If the spectrum is run again under identical conditions it almost
certain to run properly.
The effect does not correlate with solvent type. It happens with any of
the solvents we use, and most commonly occurs with DMSO-d6, as that is
used most often. Temperature in the magnet and in the lab are
relatively stable. The effect has been seen in 1H & 13C spectra. It
might occur in 31P & 19F, too, but this instrument runs these nuclei
rarely.
Suggestions (identification deleted, comments added):
(1) Nice problem. Are you sure there isn't something really big and
made of metal that occasionally zips by?? Is the magnet located over
the garage - or possibly under it?? J
[The system is next to a garage, but problems did not correlate with
passing cars.]
(2) Is the jump random for both H1 and C13 regarding Hz and ppm for both
nuclei. My fuzzy thinking goes delta Hz = frequency problem and delta
ppm = magnet problem.
[Jumps were random for any affected nuclei, and might occur during the
1H spectrum, but not the 13C, or vice-versa.]
(3) We saw a similar problem on our DXR 400. The shift was around 110Hz
and sometimes you could see ringing around the base line of one of the
peaks. It only occurred during ICON NMR runs too. We would see the
problem maybe once or twice a week for about two months.
While I am not 100% certain to the cause, I am pretty sure it was a
problem with our Accustar tri-ax gradient amp. As we were trying to get
an idea what the problem was I discovered that the +/-15V PS in the
Accustar had failed. Since the Accustar was (first shut off and then)
repaired we have not seen the problem. This has been about six months
ago.
(4) Causes of doubling would be
a shift in the field due to:
residual gradients
running unlocked with sweep on
acquiring a few scans before the sample is fully locked
the BOSS Z0 shim changing for some reason
big external disturbance
someone switching off the lock when no one is looking delay in
acquisition for a few scans (?)
First step I would try is if Icon-NMR code is accessible put in some
delays such as via the ii command when switching between shimming
(gradient shimming?) and particularly before the ZG command.
Or are they just powering on that old rusty iron core magnet next to
your lab only when you use Icon-NMR?
.. or the gradient coil in the probe
or lock switch if you have one.
or the magnet is right over a seismic fault that magma rolls through
only when you run Icon-NMR :-)
[Given the solution to the problem, it seems likely that we were running
partly without lock. This was difficult to observe directly, due to
intermittency.]
(5) I wonder if you simply have instability in the Z4 shim. I know of
another case were a user had a "doubling of peaks" and thought he had an
isomer - lost it all in re-purification too - but really all he had was
Z4 way out. I am guessing the second set has much lower intensity -
right?
[In our case, the chemical shift differences between the sets of peaks
were much to large to support this hypothesis.]
(6) I think we had a similar problem although we are using varian INOVA.
Our problem was that the spectra shifted with kHz from one scan to
another. It took us a while to understand the cause because the problem
appeared and disappeared. Over the holidays I let the spectrometer
running one scan every 5 or 6 minutes and finally we were able to see
and locate the problem. Our lock tranciver was bad and it drove in a
strange way the z0 shim. (we notice this in both solution and solids
mode and in solids the z0 was set to zero and lock not connected,
however the lock tranceiver was "controlling" the z0 shims even if it
was not suppose to do so).
To locate the problem we did such stability measurements with the shims
on and off, with the lock transceiver connected and disconnected and so
on.
I think is important that you measure only one scan because otherwise
you average over several acquisitions and is difficult to see the
problem.
[This was probably close to the nature of our problem.]
(7) My first guess is that the temperature is shifting. Do you have VT
on?
Maybe you can monitor the temperature in EDTE and see if the temperature
is shifting at the same time as the peak shifts.
[We did have VT on. I would have expected this to give a more gradual
progression of peaks, but I have been wrong before.]
(8) I don't think that the situation you describe is a hardware
problem. DMSO-D6 typically produces a lock spectrum that is a
multiplet coupling pattern at deuterium frequencies. This is a side
affect of water absorption that initiates a proton-deuterium exchange
reaction on the methyl groups. Thus, a proton coupling pattern appears
at the lock frequency. You probably see a "wiggle" in the pulsed lock
display after the spectrometer is locked-up. The wiggle is the 1H to 2H
coupling constant as seen on the time axis.
You describe exactly what would be expected if you were locked on one
peak in the multiplet at the start of the experiment. The spectrometer
is jumping to another peak in the multiplet during data acquisition and
then jumping back to the original peak. While it is locked on the
alternate peak, the actual proton spectrum is shifted by the ratio of
the peak separation (at 2H frequencies) multiplied by 6. The factor of
6 is the ratio of 1H to 2H resonance frequencies.
DMSO-D6 is not the ideal lock solvent for this reason. It probably
should be avoided unless there is no other alternative to dissolve a
particular class of molecule. If you must use DMSO try to keep it very
dry.
As far as other lock solvents go, if you are recovering and re-purifying
the solvents for multiple use, a similar argument will apply. You would
gradually pick up water over time.
You can test your solvents by running proton spectra. A fully
deuterated lock solvent will exhibit no proton spectrum at all. If you
see a proton spectrum with deuterium coupling, then there has been some
proton exchange. The same thing is happening at the lock frequency with
a spin 1/2 coupling pattern.
The lock receiver is not selective enough to reject the (small) coupling
frequency differences. If the lock multiplet peaks are of similar
intensity a random burst of noise will "jump" the spectrometer during a
long data acquisition. At some point, the spectrum will jump back to the
preferred peak.
[We observed this phenomenon in all solvents that we had in regular use:
DMSO, CD3CN, CDCl3, MeOD, C6D6. All solvents were new from the ampoule,
or were in sealed NMR standards.]
(9) I think we have experienced exactly the same problem on an almost
identical instrument here (the only difference being that we are running
XWIN 3.5), otherwise same hardware and we also run under ICONNMR with
QNP. The attached spectrum shows what we were seeing. Does this match
yours? Again we saw this in only 1-2% of spectra.
We found a solution to this which was to remove & interchange the two
SGU boards. We did this at the end of december 2005 and since then the
problem has not reappeared and the instrument is operating fine. It may
be, of course, that simply reseating the boards could cure the problem;
we plan to switch back the SGUs when we feel confident the problem has
not reoccured (and the instrument is less busy!). We have seen on a
number of occasions that reseating boards in our Bruker instruments
cures spurious problems such as this, but it may be that we have a
faulty SGU (the instrument is ca. 4.5 years old).
[I had high hopes for this suggestion, but the problems turned out to be
slightly different.]
(10) I am sure the Bruker engineers have checked it, but it seems like
it could be electrical/timing. Maybe the TCU is having a pause every
once and a while?
(11) Look for large steel objects that come and go. Elevators, cars,
gas cylinders. The last time I saw this it turned out to be a lawn
mower. They thought the concrete pad outside the NMR lab door was a
good place to take their break.
It could also be an intermittent drop-out in the lock circuit. The lock
creates a small field that adds to Zo to maintain constant field. If
the lock dies then the field jumps to the value set by Zo alone. You
might be able to make the symptom go away if you carefully adjust Zo
("Field" on a Bruker) so that the lock channel is on resonance before
you lock.
(12) IT seems to me that we have seen this on our AV 500 running xwinnmr
as well. It is rare but we have seen this in the past. When it occurs we
just run the sample over. We also have a AV300 w/microbay running
Topspin 1.3 but I do not remember seeing this since we have installed
Topspin 1.3. I will ask the others in our group to see if they have seen
this as well.
[It's always less lonely to know I'm not the only one with a given
problem.]
(13) I am wondering if you may have a sticky check valve on the magnet.
If the pressure builds and then releases you would see some effect on
the magnetic field. (I think I would try replacing the check valve
straight off) I would also want to see a detailed list of the shifts
that you have seen, does it appear to have any discrete increment to the
jump? Is the jump always towards the same direction? Does the "twinned"
peak appear at the unlocked position? Does Icon indicate a lost lock
condition? You could shim the magnet under lock and then release the
lock, stop the sweep acquire data and see if that appears to correlate
to the shift. If an event shifts the magnetic field faster than the
lock can track it then it will lose the lock but I think that ICON will
show that lost lock condition but I am not sure that the trouble flag is
not just tied to the act of initially acquiring the lock. If the
"twinned" peak is shifted enough to resolve it does the lineshape appear
to be ok or is it spoiled? Have you updated your Field value lately? Do
you have a record of the field position over time. Another thing you
might try is to run a series of experiments (short runs with long delays
between them) over the weekend in a shimmed but unlocked state and see
if the magnet is showing any discrete jumps or does it appear to be well
behaved.
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Received on Mon Jan 08 2007 - 11:35:28 MST