[RASMB] radial shift

[John Philo]

Joris,

That is such a large shift I think the only likely explanation is a
mechanical problem in the radial drive motor-potentiometer. Perhaps
something is loose and is producing a variable backlash when the motor
changes directions. This sort of thing has been seen for the wavelength
drive, which has a similar design. 

To convince Beckman this is a real problem and not something to do with your
samples I suggest that you do repeated intensity scans of the counterbalance
and show that the precision edges at 5.85 and 7.15 cm appear to move.

With regard to analyzing the data you have, in effect your radial
calibration is uncertain by at least 0.2 to 0.4 mm, so the molecular mass
values will have a fractional error of roughly (.4/65)^2, which is probably
smaller than the uncertainty in your knowledge of the correct vbar.

John

-----Original Message-----
From: rasmb-admin at server1.bbri.org [mailto:rasmb-admin at server1.bbri.org] On
Behalf Of Beld, Joris
Sent: Wednesday, June 29, 2005 5:39 AM
To: RASMB at server1.bbri.org
Subject: [RASMB] radial shift


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Dear all,
 
Lately we have encountered a strange problem with our XL-A. For a SE run we
normally scan three times (with ten averages) per cell per wavelength. In
principle these three scans are not further than 10-20min apart. Still, we
can see a relatively big radial shift (0.2-0.4 mm). The scans are
superimposable if shifted over the x-axis. I was doing two consecutive runs
(with different proteins at different speeds, from 4k to 30k) with scans
both at 235 and 280nm and this strange behavior is more pronounced at 235nm
than at 280nm. Of course, one could think of samples that have not reached
equilibrium yet but we estimated the time to equilibrium with ultrascan and
let the system equilibrate varying from three days to 20h. So, I think the
samples are at equilibrium, and moreover, within 10-20min one should not see
such big radial shifts.

We have actually two questions:
- do you think we can still use this data? (in principle fitting should not
be influenced by radial shifts, or am I making a mistake in that
assumption?!)
- does anyone has some experience with this problem and/or knows a solution
(before I call Beckman)?

Thanks a lot in advance.

To comment quickly on lamp cleaning. We clean the lamp monthly (with
toothpaste) and so far that seems to be sufficient.

Kind regards,

Joris Beld

Swiss Federal Institute of Technology
Zürich, Switzerland _______________________________________________
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