From: H. Olin Spivey <ospivey@bmb-fs1.biochem.okstate.edu>
  To  : RASMB@BBRI.ORG
  Date: Sat, 25 Mar 2000 16:24:55 -0600

Aberrant Intensities

3/25/00


>Colleagues,

>

>For about the last two years, we have had a persistent problem with
our XL-A instrument.  Radial intensity scans on blank cells (cells with
no windows spun at 3,000 rpm) decrease in intensity from the beginning
to about the middle of the radial range and then increase again to the
end (cell bottom).  Examples of our recent intensity I vs. r and I vs.
lambda scans can be seen at

>

>http://osucau.okstate.edu/plot1.gif

>and

>http://osucau.okstate.edu/plot2.gif

>

>Two separate Beckman service engineers have invested several days of
effort and replaced virtually all components in our instrument to cure
this problem, but to no avail.  We agreed to live with this situation
since we can get (after vigorous cleaning of the lamp) the sample and
reference beams to superimpose, and the intensity at the minimum
position (about 10,000 at 280 nm) has been acceptable for our needs. 
However, these signals deteriorate with use giving both lower
intensities and separation of the sample and reference beam intensities
(variable ratios with r).  This forces us to invest many hours of lamp
cleaning (one attempt is rarely sufficient) after every few
experiments.  This requirement is more frequent for us since the light
intensities near the middle r values are so low to begin with.  Current
intensity at 228nm is 19188 and at 542nm is 1775.  Intensity at 280nm
is ca. 19679 at both 5.9 cm and 7.1 cm, but drops to 7932 at 6.53cm
(refer to graphs above).  Our questions are:

>

>1) Is anybody else experiencing the same problem?  If so were you able
to alleviate it, and if so how?

>

>2) What could be the cause of this aberrant behavior?

>

>Since this problem is seriously reducing our productivity, we would be
grateful for any suggestions.


With much appreciation,

Frank Hays and Olin Spivey

H. Olin Spivey                       Phone: (405) 744-6192

Dept. Biochem. & Molec. Biology      Fax:   (405) 744-7799

246 NRC                              Email:
OSpivey@Biochem.Okstate.Edu

Oklahoma State University

Stillwater, OK 74078-3035