Dear Bruker Users,
I have been experiencing some real problems with VT measurements on a TBO
probe (and to a slightly lesser extent on a BBI probe) for an AVANCE 500
spectrometer. The narrow bore z gradient probes are fitted with spacers
so they will be accomodated in the 500 MHz wide bore magnet. I observe
the following:
1. When the temperature is being regulated on the a non spinning sample
the lock level is not stabe making it very difficult to the shim. This
problem is much reduced when the sample is spinning. The problem is
evident even when regulating a temperature within a few degrees of ambient
temperature.
2. When the temperature is not being regulated (i.e. VT heater turned
off) the lock level is stable.
3. At elevated temperatures (50 to 80 degrees C), the problem is very
severe for a nonspinning sample truly making shimming on the lock signal
impossible. The instability in the lock signal is on a time scale of
seconds to tens of seconds and does not appear regular. Furthermore when
the sample is spinning the lock level increases VERY significantly but is
stable enough to shim the on-axis shims. It appears as if one or more of
the transverse shims is changing drastically with temperature as the
spectrum of the nonspinning sample is very broad and the spectrum of the
spinning sample has spinning sidebands.
4. I have tried various flow rates of air over the heater and found no
difference so I believe the sample position is stable.
5. I have measured all of the shim currents (using the BSMS service tool)
at high temperature and find that all coils have current so apparently all
shim coils have continuity.
6. When the sample is being heated the shims slowly change, i.e. the
change is continuous - not abrupt.
7. When the sample is cooled back to room temperature the lock level
"drifts" (for lack of a better word) back to a reasonable level and
becomes stable. High resolution can once again be attained.
My questions are these:
1. Could the problem be with the current in the heater producing unwanted
magnetic fields? Has this been seen before by others? 2 Are the problems
with lock stability and loss of resolution related or separate? 3. Are the
shims getting too warm? - This is hard to imagine as the probes are made
for a 51 mm bore while our magnet has an 89 mm bore. 4. Is something
moving in the probe as a function of temperature affecting the shims? 5.
Could unwanted currents somehow get induced into the gradient coil of the
probe from the heater?
I would be grateful for any and all input to help me resolve this problem.
Glenn
Glenn A. Facey, Ph.D.
NMR Facility Manager
University of Ottawa
mailto:gafacey@science.uottawa.ca
http://www.science.uottawa.ca/~nmr
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