[RASMB] SedVel60K [was: intensity issues]

[John Philo]

Sorry Tom but I beg to differ on the SedVel60K issue. I have seen raw data
from at least 4 instruments myself where this was a problem (including one
of yours). There may indeed be instruments with hardware trouble where the
extra noise and distortion of the baselines occurs 100% of the time, but in
other instruments the problem is only intermittent (and may not be obvious).


The delay calibration shifts slightly from run to run even when the
counterbalance alignment is constant, and even in a correctly functioning
instrument, potentially allowing variable amounts of light to hit the walls
of the SedVel60K centerpiece. My own careful testing shows that the timing
is marginal at 60K even with the standard centerpieces. In some runs, and
especially at the highest rotor speeds, I think every user will see data
distortion with the SedVel60K. Whether that distortion and extra noise is
significant or not depends of course on your particular applications and
needs. 

This issue could, I think, be put to rest if Spin Analytical would create
and market window holders that incorporate an appropriate mask to prevent
the light from hitting the walls of the SedVel60K centerpieces. These would
be similar to the "interference masks" already available, but would need a
sector-shaped hole with appropriate orientation rather than parallel slits.
If you make them, I think the extra cost will easily be made up if the
centerpiece lifetime is much longer as we all expect.

John

-----Original Message-----
From: Tom Laue [mailto:tml at cisunix.unh.edu] 
Sent: Thursday, April 03, 2008 9:22 AM
To: jphilo at mailway.com
Cc: 'RASMB'
Subject: Re: [RASMB] intensity issues

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
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