From: John Philo <jphilo@mailway.com>
  To  : Rasmb <rasmb@alpha.bbri.org>
  Date: Thu, 18 Nov 1999 09:57:16 -0800

unexpected monochromator movement during velocity runs

I am experiencing an instrument problem and I am wondering whether
others are seeing the same thing.

During velocity runs I find that the wavelength occasionally changes
during the middle of a run, even though all cells are set to the same
wavelength. This causes the apparent concentration to suddenly go up
or down (most noticeable in the plateau, of course). This produces
major problems for all methods of analysis that I can think of, but is
particularly bad for DCDT analysis.

This effect is especially large and noticeable when you are working on
a steep part of the absorption curve rather than a peak. Examination
of the file headers shows that indeed the recorded wavelength actually
does sometimes change back and forth by a nanometer.

This problem was present some years back (? '92 or '93) because the
instrument control software was asking for a wavelength 'seek'
operation before every scan. The software was then altered to leave
the wavelength alone during velocity scans when all cells were the
same wavelength. I am suspicious that this same error has crept back
in again.

If it's a software problem it may of course differ for different
versions of the instrument software. I am running version 3.01h of the
instrument control program (the "GUI").

I'd appreciate hearing from others whether you believe you have seen
this problem, and also whether you are sure your system does NOT have
this problem (and please include your GUI version).

Best regards to all,

John Philo, Alliance Protein Laboratories
www.ap-lab.com