From: Jeff Hansen <hansen@bioc02.uthscsa.edu>
  To  : rasmb@bbri.eri.harvard.edu
  Date: Mon, 9 May 94 12:15 GMT

Upward sloping baselines

We definitely DO NOT see this problem at absorbance maximas (at both and UV
and vis wavelengths) with either of our two machines.  We definitely DO see
this problem occasionally when the sample(s) contain a small amount of
heterogeneously-sized aggregates.  This may not be the origin of a specific
person’s problem, but Tom is certainly correct in pointing out that it is
possible, and in many cases probable, explanation for upward sloping
baselines.  As Bo Demeler first mentioned, we also routinely see the problem
when a samples is scanned at the shoulder of the absorbance peak (but not at
its peak). 
It looks to me like there are many different causes of this phenomenon -
some based in the sample and some based in the XL-A.  For those of you who
are seeing this problem at absorbance maximas, and can positively rule out
aggregation, CALL YOUR BECKMAN SERVICE ENGINEER AND INSIST THAT THEY FIX
YOUR MACHINE.
Finally, Jack’s original message stated that upward sloping plateaus cause
a problem with the van Holde-Weischet analysis.  If your machine or sample has
upward sloping plateaus, you'll have a problem with any sophisticated
sedimentation velocity analysis method.  However, I feel compelled to point
out that we have collectively analyzed hundreds of different samples under
all kinds of different conditions using Bo Demeler's version of the van
Holde-Weischet method without any problems.  In many cases other techniques
have confirmed the results of the sedimentation analysis.  The problem is
with the sample/machine, NOT the analysis method.