Re: External Address Advance on AC spectrometer

Gerald Pearson (gpearson@blue.weeg.uiowa.edu)
Mon, 24 Oct 1994 17:34:24 -0500


Tim, I don't know if this has anything to do with your problem, but I _have_
noticed a wierd 0.3 millisecond timing limitation in loops, when writing code
for producing pulse trains. For example, in the code fragment

50 (P1 PH1):D ; FAKE A LONG, WEAK 90-DEGREE DECOUPLER PULSE
D2 ; WITH A DANTE-LIKE TRAIN OF 30 HARDER
LO TO 50 TIMES 30 ; 3-DEGREE PULSES.

If D2 is shorter than 300 microseconds, this will NOT WORK!!! Changing the
loop counter from a constant number to "C" does not help. This is the case
both on our AC-300 with Aspect 3000 & process controller running DISRxx, and
on our WM-360 with Aspect 3000 brain transplant & process controller, also
running DISRxx.

-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-
SOFTWARE EXPERTS: IF THERE'S ANY WAY AROUND THIS 300 US TIMING-LIMITATION
(SOFTWARE?) PROBLEM (BUG?), PLEASE LET ME KNOW.
-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-

I _have_ programmed such pulse trains for our MSL-300, with Aspect 3000 &
process controller running DISMSL. No problem here.

So what's going on -- don't both spectrometers have the same computer
hardware???? Is it a bug in the DISRxx programs? The reason might
conceivably be a completely unnecessary software limitation in DISRxx, left
over from the days of the Aspect 2000 with 3-clock pulse controller. The
memory in that controller is a 16-word LIFO, which gets reloaded on the fly
for all but the simplest pulse sequences. The A2000 needs at least 300
microseconds to reload the LIFO (interrupt service routine) without messing up
the timing of the ongoing pulse sequence, so the last instruction in every
loop must contain a delay of at least 300 microseconds. DISNxx -- being nice
and user-friendly -- will simply prevent you from making the mistake of
putting in a shorter delay.



So why do we bother with pulse trains? In general, DANTE-like pulse trains
can be used to "fake" soft, selective pulses with a transmitter which cannot
produce pulses which are soft enough. Short trains can be used as 90's and
180's, and very long trains can be used for presaturation. The only
limitation is that you must ensure that none of the pulse-train sidebands
accidently coincide with the position of any NMR signal in the spectrum.
A couple of examples:

MSL-300 -- A long (several seconds) train of 700-nanosecond F1-transmitter
pulses about 180 microseconds apart allows us to pre-saturate a P-31 signal
which happens to have a long T1 and short T2, so we can remove this offending
signal from the in-vivo spectrum of a perfused rat liver. We do this without
a P-31 decoupler, and without any special hardware to reduce the F1-
transmitter power by maybe 50 dB on the fly, and then ramp it back up to full
power for the sampling pulse. The effective "power" of the transmitter is
controlled by the pulse spacing. [W.J.Thoma, J.A.Snyder, & G.A.Pearson,
"Broad Resonance Edited 31P NMR Spectrum Obtained with a Single Radiofrequency
Channel", NMR in Biomedicine 2, 112-114 (1989)] This trick can obviously be
used with any nucleus which you happen to be observing. BTW: you _do_ have
to flip the multi-pulse switch on the back of the F1 transmitter in the MSL if
the pulse spacing is shorter than a few hundred US; otherwise, an interlock
will disable the transmitter every time you try to do the pulse train.

AC-300, etc. -- SELECTIVE INEPT is a robust, high-sensitivity 1D experiment.
If your spectrometer can't do HMBC, or if you need to sort out long-range
couplings of carbons to only a few protons, then selective INEPT can do the
job very cleanly and nicely. This experiment requires 90-degree decoupler
pulses of about 10 milliseconds length during the pulse sequence, but requires
high power for decoupling during acquisition of the C-13 FID. Neither the
decoupler on our AC-300 nor the one on our WM-360 is capable of gymnastics
quite this extreme, so we do the job with trains of 30 3-degree pulses (about
5 to 9 microseconds each, in our hardware), a little over 300 microseconds
apart. If anyone is interested in the details, I can email them my
SELINEPT.ACQ ".aur" program. If there's enough general interest, I could post
it on BUM.

Hope this helps. -- Gerry
---------------------------------------------------------------
Gerald A. Pearson INTERNET: gerald-pearson@uiowa.edu
Chem. Dept., Univ. of Iowa VOICE: 319-335-1336
Iowa City, IA 52242-1219, USA FAX: 319-335-1270
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