[RASMB] intensity issues

[Tom Laue]

Hi-
I think the two problems, low intensity and the periodic fluctuations, 
are related. There is a small offset voltage that is not intensity 
related but that shows up on the A/D. At low intensities, this offset 
becomes a greater fraction of the total signal. Fluctuations of the sort 
you are seeing could be the result of small fluctuations in timing, the 
offset voltage, sensitivity of the reference detector.
The underlying cause, though, is the low intensity... the machine is not 
in spec.
This is not due to using SedVel60K centerpieces. The timing issue with 
SedVel60Ks result in increased stochastic noise, typically at one or the 
other end of the channels. Also, out of 100+ instruments using SedVel60K 
centerpieces, we have encountered the timing problem with only 3 
instruments. With two of those three cases, the noise problem 
disappeared after the instrument was serviced.
Best wishes,
Tom

John Philo wrote:
> Bo,
>
> First, what is the wavelength? If this is up in the visible that low mean
> intensity might be perfectly normal.
>
> Second, are you sure these folks aren't using the SedVel 60K centerpieces
> that have the lamp timing issue? That might produce an oscillation as the
> lamp timing varies slightly. The oscillation does suggest some sort of
> timing issue, so you might ask them to check whether the delay calibration
> is in a normal range (you can see it from Service...Absorbance...Cal
> Parameters).
>
> Third, regarding the low intensity, is it possible there is a problem with
> the 400 nm cutoff filter? Even if the control lever is in the right
> position, I believe the filters have been known to become detached.
>
>
> John 
>
> -----Original Message-----
> From: rasmb-bounces at rasmb.bbri.org [mailto:rasmb-bounces at rasmb.bbri.org] On
> Behalf Of Borries Demeler
> Sent: Thursday, April 03, 2008 8:07 AM
> To: RASMB
> Subject: [RASMB] intensity issues
>
> Dear Colleagues,
>
> I have an issue with an XLI at FSU - it has a strange problem with
> intensity. First, it is very low, and second, it has a regularly spaced
> intensity oscillation. Please see the attached image. It was generated by
> scanning a channel containing just water in intensity mode and averaging the
> intensity in the flat region over all points. The plot shows the average
> intensity of the scans over a number of scans. The entire run shows this
> consistent intensity fluctuation, and it is apparent in all cells. The run
> was done at 50,000 rpm. Does anyone have a good suggestion as to what flaw
> in the XLI could potentially cause such a pattern? Is this a lamp firing
> issue, or a voltage issue with the PM tube or flash lamp? The machine was
> just newly installed and configured by Beckman service, and this is the
> pattern it produces.
>
> Thanks, -borries
>
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>   

-- 
Department of Biochemistry and Molecular Biology
University of New Hampshire
Durham, NH 03824-3544
Phone: 603-862-2459
FAX:   603-862-0031
E-mail: Tom.Laue at unh.edu
www.bitc.unh.edu
www.camis.unh.edu