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<TITLE>Re: [RASMB] sedimentation</TITLE>
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<FONT COLOR="#800000">Hi Jan<BR>
<BR>
(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.<BR>
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.<BR>
<BR>
(2) Sorry - no ideas on this one!<BR>
<BR>
Regards<BR>
<BR>
Arthur<BR>
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Arthur J Rowe<BR>
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<BLOCKQUOTE><B>From: </B>John Coropisnita <brifcor@yahoo.com><BR>
<B>Date: </B>Thu, 23 Oct 2003 03:33:17 -0700 (PDT)<BR>
<B>To: </B>rasmb@rasmb-email.bbri.org<BR>
<B>Subject: </B>[RASMB] sedimentation<BR>
<BR>
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<BR>
Dear Colleagues,<BR>
<BR>
I would like to ask you the following questions<BR>
(I am performing sedimentation velocity measurements<BR>
on small silica particles).<BR>
<BR>
1. Since the Beckman's software does not give <BR>
reproducible results of sedimentation coefficient<BR>
when using the second moment method, I would<BR>
appreciate<BR>
any suggestion about a good software for determining<BR>
this coefficient as well as the diffusion coefficient.<BR>
<BR>
2. In some cells I noticed that the reference (pure<BR>
grade ethanol, taken from the same vial) has an<BR>
absorbance of -0.2 to -0.5 OD, while in other cells it<BR>
is zero. I noticed this when I measured very diluted<BR>
dispersions (and not when more concentrated were<BR>
measured); The meniscus of the reference<BR>
in the above case has also a strange shape. The<BR>
phenomena was reproducible after the cell was<BR>
refilled.<BR>
The window glass was carefully cleaned.<BR>
<BR>
Have you seen something similar?<BR>
<BR>
Thank you very much,<BR>
<BR>
Jan.<BR>
<BR>
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