[RASMB] sedimentation

[Arthur Rowe]

Hi Jan

(1) I imagine that your small silica particles have a reasonably tight
distribution, but even so, you cannot readily determine translational
diffusion coefficients in the AUC for a non-monodisperse system. Dynamic
light scattering (DLS) is a better option, as that is basically a
D-determining method. And modern software allows you to retrieve a
distribution, of sorts at least.
As regards s values, the 2nd moment method makes terribly inefficient use of
your data. Grab all the data you can, scanning as fast as your XL-A/I will
allow, then use Peter Schuck's SEDFIT (go to
http://www.analyticalultracentrifugation.com/ for the SEDFIT Home Page).
Fitting a c(s) distribution ought to give nice results for your system.

(2) Sorry - no ideas on this one!

Regards

Arthur

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Arthur J Rowe
Professor of Biomolecular Technology
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School of Biosciences
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Leicestershire LE12 5RD   UK

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From: John Coropisnita <brifcor at yahoo.com>
Date: Thu, 23 Oct 2003 03:33:17 -0700 (PDT)
To: rasmb at rasmb-email.bbri.org
Subject: [RASMB] sedimentation


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Dear Colleagues,

I would like to ask you the following questions
(I am performing sedimentation velocity measurements
on small silica particles).

1. Since the Beckman's software does not give
reproducible results of sedimentation coefficient
when using the second moment method, I would
appreciate
any suggestion about a good software for determining
this coefficient as well as the diffusion coefficient.

2. In some cells I noticed that the reference (pure
grade ethanol, taken from the same vial) has an
absorbance of -0.2 to -0.5 OD, while in other cells it
is zero. I noticed this when I measured very diluted
dispersions (and not when more concentrated were
measured); The meniscus of the reference
in the above case has also a strange shape. The
phenomena was reproducible after the cell was
refilled.
The window glass was carefully cleaned.

Have you seen something similar?

Thank you very much,

Jan.

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