[RASMB] linearity of radial scan on XL-A

[Seth Fraden]

At Brandeis, we have a XL-A circa 1995 and I suspect that the radial
scan coordinates are not being read correctly. The evidence for this
assertion is that the sedimentation velocity depends on the radial
position of where it is measured in the cell, with a larger Svedberg
measured at the bottom. This is not a radial dilution effect because of
the following two measurements. In the first measurement we evaluated S
at the top of the cell in a dilute sample of concentration c. In the
second measurement an initially more concentrated sample was used and S
was determined towards the bottom of the cell, but because of radial
dilution the concentration was reduced to the same concentration c as in
the first measurement. We found that S was always greater at the bottom
of the cell. The plot of ln(radius) vs. time was quadratic in time,
instead of linear in time. This could be explained if the radial scan
was non-linear. The coefficient of the quadratic term was independent of
sample concentration, consistent with the effect being an instrumental
artifact. We have published part of these results in 

"Concentration Dependent Sedimentation of Colloidal Rods",
Z. Dogic et. al., J. Chem. Phys. 113, 8368 - 8380 (2000), 

available at the following link:
http://www.elsie.brandeis.edu/pub/sedimentation.pdf


Questions: Have any users experienced non-linear ln(radius) vs. time
problems? How does one calibrate the linearity of the radial scan? Are
there windows with 10 line pairs per mm (like a Moire grating) that one
can insert in a cell?

-- 
Yours,

Seth Fraden                      website: http://www.elsie.brandeis.edu/
Department of Physics            phone:   (781) 736-2888
MS-057                           fax:     (781) 736-2915
Brandeis University              email:   fraden at brandeis.edu
Waltham, MA 02454  USA