Hi Kirk,
This problem has been noted earlier and only arises in systems that have
a third amplifier being shared between
channel 3 and channel 4, typically for 15N and 2H. The ampmode='dddp' is
present in those parameter sets in
which 2H decoupling is available in the pulse sequence. The 'p' is
"forced" to make sure the amplifier does not
contribute noise in the (separate) lock channel, also tuned to 2H. This
is fine for the case of 4 amplifiers, but
does give the problem you note for a shared 3rd amplifier.
Normally dn3 for channel 4 is set to '', except for those sequences in
which there might be 2H decoupling.
Our advice in these cases is to "destroy" the ampmode parameter,
restoring the operation to a default of "cw",
allowing decoupling. Since in most cases channel 4 is not used for 2H
decoupling there will be no additional
noise in the lock channel.
BioPack macros for experiment setup of experiments in which there might
be 2H decoupling (specified by the pulse sequence) handle ampmode
directly, destroying it for 3-channel systems. Unfortunately, for a
4-channel system with 3 amplifiers, ampmode is not destroyed.
There are several ways in which you could automatically destroy ampmode.
However, you'd have to avoid doing this
for sequences in which you want to do 2H decoupling. The macro fixpar
could be modified to destroy ampmode since this
is run by "rt" which is run by BPrtppar, but ampmode would be needed
when you wanted to do 2H decoupling. Perhaps this
is the preferable way to handle the situation globally. (Of course, you
could modify BPrtppar to do the destroy without the
need of modifying fixpar).
My advice is to modify the BioPack macro "BPsetampmode" which is run
when the experiment setup macro runs for those
experiments in which 2H decoupling is possible. This macro destroys
ampmode for 3-channel systems and checks to see if
you have a normal channel or lock/decoupler, so you could just add a
line at the bottom that destroys ampmode.
I will take a look at making some decision based on the amplifier
configuration in the setup macros to avoid this situation and will
update BioPack accordingly. Check MR News for any new version of
BioPack or check the Varian Forum BioPack threads.
Regards,
George
Kirk Marat wrote:
> 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 - 13:09:55 MST