[RASMB] sedimentation equilibrium negative absorbance

[Beld, Joris]

Dear people,

I'm completely new to Analytical Ultracentrifugation but our lab just acquired a Beckman XL-A and I try to study a 37 kDa protein (in 50 mM PIPES, 5 mM MgCl2) running it at 15000 rpm and 4 C. Strangely enough the obtained spectrum, after 20h, shows flatline, meniscus, flatline, meniscus and than a curve downwards into negative absorbance values and than again a meniscus. Seeing this, I'm starting to doubt that I put the sample+FC43 and reference in the right cell compartments?
If I did it wrong (is this assumption correct?), can you still use the results?
Many thanks in advance.
Kind regards,

Joris Beld
ETH Hönggerberg
Zürich, Switzerland
beld at org.chem.ethz.ch