[RASMB] laser delay difference between sample and water?

[Frank Niesen]

Hello Tom,

Thank you very much for your reply. Does the rotor speed need to be constant to be able do the laser delay setting? I.e. can I do it quickly while the centrifuge is running up to the measurement speed?
Steve Eyles (many thanks for this) also pointed out the possibility that a ill-shaped overspeed disk of oil vapor depositions (sadly, always a worry with XL-i) could be the culprits in the described case.

Best regards,

Frank

>>> Tom Laue <Tom.Laue at unh.edu> 05/11/2008 14:45 >>>
Hi Frank-
The software controlling the laser delay has to compensate for the 
effects of signal propagation. For a fixed propagation delay, the rotor 
angle changes more at high speed than at low speed. Over much of the 
speed range, the software can use a single value to account for the 
propagation delay. Unfortunately, the propagation delay is not fixed at 
low speeds (due to frequency-dependent phase shifts in the Hall effect 
circuitry), so the software incorporates some empirical parameters at 
low speed. I haven't been involved with the hardware/software 
maintenance of the Beckman interference optics in 15 years, so I am not 
sure how they handle the effects on the propagation delays resulting 
from any seemingly innocuous changes in the hardware (e.g. the TE board 
electronics, DAB electronics).
My recommendation is, if possible, to do the laser delay setting at 
speeds >= 6000 rpm, where the propagation delay nonlinearity is minimal.
Best wishes,
Tom Laue

Frank Niesen wrote:
> Hello everybody,
>
> I wonder if you have experience with a phenomenon that I experienced on several occasions, using interference optics:
>
> I adjust the laser delay for all cells while running blanks, i.e. filled with water. I do this at the high speed, when I record the blank scans for all cells. The reason is that I noticed that a longer laser delay is necessary at higher than at lower speed.
> Then I remove the water from the cells and fill in my samples. I test those with a scan at 3000 rpm and when everything seems alright I start the experiment after equilibration.
> However, I frequently see that at the high speed later on the signal in one of the cells has become too faint and the experiment needs to be repeated/re-started after mixing etc.
>
> It seems wrong to me that it would be required to adjust the delay with the sample at high speed: Apart from the fact that one potentially loses valuable information there is the fact that it is not happening for all cells in general.
>
> Many thanks in advance for any advice you might be able to give me.
>
> Best regards,
>
> Frank
>
>
> Dr. Frank H. Niesen
>
> Chemical Biology
> Structural Genomics Consortium
> University of Oxford
> ORCRB, Roosevelt Drive
> Oxford, OX3 7DQ
> United Kingdom
>
> frank.niesen at sgc.ox.ac.uk 
> skype ID: frankniesen 
> phone +44 (0)1865 617578
> fax      +44 (0)1865 617575
> http://www.sgc.ox.ac.uk/ 
>
>
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-- 
Department of Biochemistry and Molecular Biology
University of New Hampshire
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Phone: 603-862-2459
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E-mail: Tom.Laue at unh.edu 
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