negative dead times?

Jane Strouse (mjs@chem.ucla.edu)
Mon, 10 Jun 1996 10:48:19 -0700

At 10:46 AM 6/10/96 -0500, Vikram Roongta wrote:
>
>Hi Bruker Users:
>
>I want to find out if anybody out there has seen the following problems
>on the AMX-600 and AM-250 instruments.
>
>AMX-600: Calibration of the proton pulse is asymmetric. For ex: if HL1=3
>and using PAROPT with p1 starting at 1u and incremented by 1u for 32
>increments. The 90 maximum is reached early at about 8u and 180 is at
>about 20u. The pulse width I usually get is 10u for 90 pulse. I see this
>at all different power levels of the proton e-coupler. It is really bad
>at low power levels like HL1=40 for a 1ms pulse for saturating water in
>watergate sequence. The 90 I get in the last case is at about 700u
>instead of 1000u, but 180 is at 2000u. I have tried everything. Please
>help.
>

I have seen exactly this sort of behavior on several (but not all)
Bruker probes. In one case it went away when a capacitor was replaced for
another reason. It is my feeling that the capacitors are experiencing very
minor arcing as the pulses get long. Although people at Bruker generally
say they have not seen this type of behavior, I would refer you to the
paropt diagrams in chapter 3 (called "Optimizing Paramters") of their NMR
Training course manual. The proton paropt example looks to me like the 90
deg pulse is about 9 usec and the 180 deg pulse is about 22.5 usec. The 13C
example looks like about 11 and 26 usec. I am certainly not surprised to
hear that I am not the only person who has this problem. After many
discussions about it that got nowhere, I finally gave up and live with it.
In pulse programs that are delivered with the system, I comment out the
line that creates 180 pulses by simply doubling the 90 degree pulse. This
is clearly not a valid assumption on these probes.

Jane Strouse
Dept. of Chemistry & Biochemistry
UCLA
Los Angeles, CA 90095-1569
(310)-825-9841
(310)-825-0393 FAX
strousej@chem.ucla.edu