Date: Wed, 13 Jan 2010 12:37:34 -0500
AMMRLers,
I have an instrument that is down. The problem came on gradually when doing
1H detection, but now the problem is there with 13C detection as well. The
problem is this: the FID blanks for a period of time and, when it resumes
its tracking of the signal, the sinusoidal signal no longer lines up. That
is, a random phase error is introduced. This suggests that it is a
detection side problem, because I don't see any way for the precessing
magnetization to suddenly jump from one spot in the rotating frame to
another discontinuously. Here is a picture of the problem:
Putting a sample in the instrument that contains a single peak is useful in
that it shows clearly the nature of the problem. Oh yeah, the frequency of
the 'resets' is 120 Hz. This problem is observed with dsp='n', 'i', or 'r'
and with various sw's, np's, etc.
IMG_0612a.JPG
If anybody has an idea as to what might be wrong, I would love to hear them.
I was thinking that it might be an error having to do with the triggering of
the a/d, or it might be a transcription error associated with moving the
data around prior to sending it via the Ethernet connection to the host
computer, but so far all the board swaps (I have two Inova 500 consoles of
this vintage, 1998, so I have tried swapping many, many boards).
Thanks in advance for any consideration you give this.
Jeff
Jeff Simpson, Director
Department of Chemistry Instrumentation Facility
Massachusetts Institute of Technology
77 Massachusetts Avenue, 18-0090
Cambridge, MA, 02139
617-253-2016 (1806 lab)
(image/jpeg attachment: image001.jpg)