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<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
11.0pt'>Dear RASMB members,<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
11.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
11.0pt'>I have recently been struggling with a recurring problem when I
centrifuge 10 and 20nm gold particles that causes my XLA to generate poorly
formed OD data scans that cannot be fit by Sedfit. I had initially been getting
well formed scans that were easily fit by Sedfit using the continuous c(s)
model however after half of the sample side in one cell (in a 4 cell rotor)
leaked out during a run I was no longer able to get the nice sigmoidal scans
that I originally had even though I tightened and refilled the leaking cell. I
asked the Beckman service rep to investigate and he cleaned the optics and told
me to have the XLA perform a radial calibration and the scans were back to a sigmoidal
shape using the same sample of gold nanoparticles and Sedfit could fit the data
well again. However after another cell leaked in the centrifuge, the scans
could not be fit with Sedfit even though the cell was tightened and refilled
and no longer leaked. I have been performing a radial calibration before each
run and have not turned off the delay calibration hoping the instrument will
recalibrate and improve the scans however, this has not happened. I have
attached screen shots showing the quality of the data before and after the cell
leak. It looks like it could be the optics but I am not convinced this is the
whole story since it seems extreme that one cell leak can alter the XLA
performance this much until the optics are cleaned.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
11.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face="Times New Roman"><span style='font-size:
11.0pt'><o:p> </o:p></span></font></p>
<p class=MsoAutoSig><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>James B. Falabella<o:p></o:p></span></font></p>
<p class=MsoAutoSig><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Process Sensing Group<o:p></o:p></span></font></p>
<p class=MsoAutoSig><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>National <st1:PlaceType w:st="on">Institute</st1:PlaceType> of <st1:PlaceName
w:st="on">Standards</st1:PlaceName> and Technology <st1:place w:st="on"><st1:City
w:st="on">Gaithersburg</st1:City>, <st1:State w:st="on">MD</st1:State></st1:place><o:p></o:p></span></font></p>
<p class=MsoAutoSig><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Tel: 301-975-5041<o:p></o:p></span></font></p>
<p class=MsoAutoSig><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Fax: 301-975-2643<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
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