From: Joel Mackay <j.mackay@biochem.usyd.edu.au>
  To  : rasmb@alpha.bbri.org
  Date: Tue, 10 Oct 2000 09:47:52 +1000

negative XL-A data

hi all,
 From time to time, when we run standard equilibrium scans on protein 
samples, we see that the 280 nm scans dip down to negative values at the 
meniscus end of the cell. We were thinking that this might be due to having 
DTT in the buffer, and the DTT (for some reason that is not clear to me) is 
becoming oxidized more in the reference cell than the sample cell. Have 
other people seen this sort of effect?
Does this explanation sound plausible?
If not, does anyone have any other ideas?

Actually, while i am 'here', we also sometimes see the scans 'jump around', 
such that there are two distinct sets of overlapping scans in the one cell. 
I have explained this effect to myself by supposing that the monochromator 
is not selecting exactly the same wavelength each time it goes to that 
cell. The same questions as above apply here!

Thanks in advance,
joel

PS My apologies if these are old chestnuts that have been answered many 
times in the past - i have only been in this game for a couple of years...
*****************************************************************
Dr Joel Mackay
ARC Research Fellow
Department of Biochemistry
University of Sydney,
Sydney, NSW 2006
Australia
ph +61-2-9351-3906
fax +61-2-9351-4726
WWW: http://www.biochem.usyd.edu.au/staff/mackay/
Sydney Protein Group Website:
http://www.biochem.usyd.edu.au/spg/
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