From: Les Hicks <hicks@gpu.srv.ualberta.ca>
  To  : RASMB Bulletin Board <rasmb@alpha.bbri.org>
  Date: Fri, 4 Feb 2000 07:38:45 -0700

Notice regarding absorbance scans

Hello all,

    I just had our area Beckman service rep here doing scheduled maintenance
on our XLI and was telling him about the problems I've been having with the
absorbance values jumping up and down in sequential scans of the same cell
without changing the wavelength setting between scans.  He contacted Jerry
Porter and was told by him that  there is no fix for this problem at present
if you are doing single scans rather than method scans.  If I understand it
correctly, he said that when you select a single scan, the pot motor for the
monochromator initially takes the monochromator all the way to one end of
the wavelength range (I assume the high end), and then moves to the set
wavelength position and the scan is performed.  If you then select another
single scan, the monochromator again moves all the way to the end before
again moving to the set wavelength (within 2 nm or so).  I often perform
scans on the shoulder rather than the peak maximum so a difference of even 1
nm can make a big difference in the absorbance measurement.  I have always
assumed that if you don't change the wavelength setting, the monochromator
will stay fixed at the last setting.  He said that it does stay at the set
wavelength after the initial scan if you set up a series of scans in a
method.  We actually tried it out while the service rep was here.  I set up
a method to take 5 sequential scans and the scans all overlaid nicely with
no significant variation in absorbance.  I then performed three or four
sequential scans manually and the absorbance readings were up and down from
scan to scan.  I contacted John Philo about this and he said there are still
occasionally variations in the  absorbance measurements from scan to scan
even in the methods mode, but he thinks this is more a mechanical problem
than a software problem.
    I thought I'd pass this info along to save other users  the grief I've
been going through.

Cheers(?),

Les


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Les Hicks
PENCE / Dept. of Biochemistry
Rm 3-36 Medical Sciences Bldg.
University of Alberta
Edmonton, Alberta, Canada
T6G 2H7

Phone : 780-492-3412
Cell     : 780-975-7741
FAX    : 780-492-0095
email   : hicks@gpu.srv.ualberta.ca
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