Dear Keith,
You have piqued my curiosity with this message. Do you get the same
behavior on more than one probe? I have an ARX400 and and ARX500 that were
installed at about the same time. With these systems, I started to notice
similar behavior on some (but certainly not all) probes. The most
interesting case was the proton channel of the QNP probe of the ARX400.
Initially we had to use at least 6 dB of attenuation or we would observe
arcing. However, while using 6 dB, a smooth curve was obtained when the
pulse time was stepped by 2 usec at a time. But the 90 deg pulse was 6 usec
and the 180 pulse was 16 usec. This is not as extreme as what you are
observing but the trend is in the same direction. If the 180 was much less
than twice the 90, there is a substantial dead time to the rise of the
amplifier. But what we are seeing here is a negative dead time (the
"undead" time, I guess). We had the capacitors replaced on the proton
channel and now the 180 is virtually 2X 90. I think that the only logical
explanation of the "undead" time is that there is minor breakdown of power
in the capacitors.
I have the feeling that I am not the only person with probes that
experience this. Interestingly enough, if you happen to have the Bruker NMR
Training Course Manual called "A Practical Guide to Setup and Operation of
the AMX/ARX Spectrometers - CPU/4 Version" (mine is dated Feb. 25, 1994},
look on page 4 of Chap. 3. The plot looks as though the proton 90 deg pulse
is ~9 usec and the 180 deg pulse is about 22.5 usec. Also on page 9 of the
same chapter, the C-13 90 deg pulse is about 10 or 11 usec and the 180 deg
pulse is about 26 usec. I've always been curious as to whether other Bruker
users have observed this sort of thing.
Jane Strouse
Dept. of Chemistry and Biochemistry
UCLA
(310)-825-9841
mjs@argon.chem.ucla.edu