[RASMB] XL/I Absorbance Problem (follow-up)

[Leech, AP]

Hello John,

I'm not quite clear on this - does the XL before each scan move the
slit to 6.5 cm, adjust the PMT voltage, then do the required radial
scan? Presumably this is only done on the reference sector?

I guess this could have a significant effect on attempts to do
"differential"  experiments, where there is something actually
sedimenting in the reference sector (not to mention cells which
have sample and reference in the wrong sectors!)

Kindest regards,

Andrew

John Philo wrote:
> I think a couple of points regarding the use of pseudo-absorbance were 
> not sufficiently brought out by either Bo Demeler or Peter Schuck:
>  
> (1) With pseudo-absorbance there are vertical shifts of the scans from 
> one scan to another, somewhat similar to the TIN or 'jitter' in 
> interference data. Treating this systematic noise as a true constant 
> vertical displacement certainly helps, but if you look closely at the 
> structure of these shifts they do not appear to be truly constant across 
> the cell. One source of this effect is clearly the adjustment of the 
> photomultiplier voltage based on the light levels at 6.5 cm, as Peter 
> mentioned, and thus the shifts tend to be largest as the boundary moves 
> past 6.5 cm.

[...]

> With respect to (1) it would be useful for Beckman to provide an 
> option that suppresses the photomultiplier voltage adjustment after the 
> first scan of each cell, and that also drops the assumption that the 
> voltage should be initially optimized for measuring sample - reference.
>  
> Best regards,
>  
> John

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-- 
Dr Andrew Leech                   *  Laboratory Manager
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