We have a weird problem on our INOVA 600, and we are not sure if it is a just (!) a software problem
or whether we also have an underlying hardware problem with a software work-around. The system
configuration is an INOVA console with VNMRj 2.1B with the latest patches installed. We are using
the latest BioPack with totally unmodified parameters and sequences. The probefile is one created
in the standard fashion by BioPack.
Recently, we tried some BioPack triple resonance experiments (e.g. HNCO, HNCA... etc.), and
everything seemed fine _except_ that there was no 15N decoupling in the NH dimension. Every peak
was a ~93 Hz doublet. Looking at the output of the third channel amplifier on a scope confirmed
that 15N pulses were coming out of the amplifier just fine, but that there was no decoupling during
the acquisition period. Going one step further back in the RF chain (J290 on the atten. board)
showed a _proper_ decoupling signal, so the problem was either failure of the AMT to unblank
during the decoupling period, or switch to CW mode, or both. The strange thing is, other (2D)
BioPack sequences like gNhsqc worked just file - decoupling and all.
After about two months of poking away at this problem, it was finally discovered that the offending
BioPack 3D sequences had the 'ampmode' parameter set to 'dddp' which is default operation on the
first three channels and pulse mode on the 4th (dedicated 2H) channel. The functioning (mostly 2D)
sequences did not have this parameter defined at all. It turned out that we could get the errant 3D
sequences to work by "destroying" the 'ampmode' parameter.
This does give us a work-around, but the work-around has some annoying consequences:
- I have to either modify virtually all of the BioPack 3D sequences (and maybe some of the 2D
sequences) by removing the 'ampmode', or instruct students to remember to destroy the parameter
before running an experiment.
- Presumably, the 'ampmode' setting was in there for a purpose. I'm guessing that the 'p' setting
for the 4th channel is for proper control of the 2H decoupler. Will destroying 'ampmode' have an
effect on 2H decoupling?
What puzzles me is why having 'ampmode' set with 'd' or default on the 3rd (15N) channel should
cause the problem at all. Isn't default mode the same as what happens when 'ampmode' is destroyed?
This is what leads me to suspect that we might have a problem on the board that controls the AMT
amplifiers (The AMP. ROUT, board), or perhaps a software bug.
Has anyone else seen a problem like this?
Can anyone with a similar system check it out? The easiest way is to use a high power attenuator
(at least 30 dB) and a scope on the third channel. Simply call the HNCO experiment from the
Experiments menu, and look to see if there is decoupling in the sequence (you will see a series of
short pulses followed by the decoupling at a lower power level). If you see only the pulses,
destroy 'ampmode' and see if the decoupling returns.
Many thanks
-Kirk
Kirk Marat, Ph. D., NMR Facility Manager
Dept. of Chemistry
University of Manitoba
Winnipeg, MB, R3T 2N2, CANADA
C#, What C++ should have been
ph. (204) 474-6259 FAX: (204) 474-7608
kirk_marat_at_umanitoba.ca
ALL SPAM forwarded to Spam Cop
Received on Thu Feb 12 2009 - 07:24:34 MST