[RASMB] XL-I laser wavelength, magnitude versus pixels per fringe and mag netic field(s)

[Tom Laue]

Hi all-
1- The original Toshiba laser used by Beckman had a nominal wavelength 
of 672 nm. That laser had a habit of mode skipping during at the edges 
of the pulse, leading to short periods where the wavelength could be 
anywhere from 670-678 nm. The mode skipping resulted in no noticeable 
degradation of the data (there are plenty of other sources of noise). 
Over the years, Beckman may have changed laser vendors and used lasers 
with slightly different wavelengths. I believe the wavelength that 
appears in the data files is "hard coded" in the program and cannot be 
changed. Its value may or may not match the laser, depending on the 
relative age of your laser and the program. I would trust the number 
stamped on the light source.
2- The kink is not significant, and you want to use the maximum. Slight 
variations in the amplitude of the Fourier are a consequence of the 
mismatch of the sine wave and the fringe pattern. The appearance of 
kinks will also depend on the column of pixels chosen to determine the 
pixels per fringe. Any blemish or problem with pixels (e.g. differences 
in sensitivity) will cause wiggles in the curve (which is essentially a 
power spectrum). The fewer the pixels in a column, the more sensitive 
the analysis will be to flaws in individual pixels. The same is true for 
determining the phase, which is used to calculate the fringe 
displacement. David Yphantis and Jeff Lary showed that increasing the 
number of pixels per column from 96 (Beckman's camera) to 1024 or 2048 
results in a ~100-fold improvement in the precision of the fringe 
displacements.
3- The magnetic field is negligible at the position of the cells. I 
recall that David Yphantis and, I believe Walter Stafford, have 
sedimented magnetic particles in the XLI without any problems.
Best wishes,
Tom


Karel Planken wrote:
>
> Dear RASMB'er,
>
>  
>
> Here are a few questions regarding our XL-I.
>
>  
>
> Question 1:
>
> On the laser tube it reads for the wavelength of the laser 675 nm. 
> Since we have a new computer the reported wavelength for the recorded 
> scans is 655 nm (before it was 660 nm!?). Why are these different and 
> how can we change or set this back to 675 nm?
>
>  
>
> Question 2:
>
> If I set up the laser (delay and duration) and check the magnitude 
> versus pixels per fringe (see attachment) I often get a pretty smooth 
> curve but with a kink half way, just before the maximum. Is it very 
> important to obtain a completely smooth curve? Adjusting the laser 
> delay, the kink can be minimized but usually doesn't disappear completely.
>
>  
>
> Question 3:
>
> The rotor is driven by three coils underneath the rotor chamber that 
> produce a (alternating?) magnetic field(s). Since I measured the 
> sedimentation velocity of magnetic particles I would like to raise the 
> following question: can somebody provide me information on how strong 
> this magnetic field is (or better: how strong is the magnetic field 
> that the sample sector of a standard d.s. centerpiece experiences?) 
> and how the magnetic field lines in the rotor chamber and in the 
> sample sector of a standard d.s. centerpiece look like?
>
>  
>
> Thank you in advance for your answer(s) (and time),
>
>  
>
> Kind regards, Karel Planken.
>
>  
>
> K. L. Planken
>
> Van 't Hoff Laboratory for Physical and Colloid Chemistry
>
> Utrecht University
>
> H.R. Kruytgebouw
>
> Padualaan 8
>
> 3584 CH Utrecht, The Netherlands
>
>  
>
> Phone: +31-30-253 2888
>
> Fax: +31-30-253 3870
>
>  
>
> Please visit my webpage! 
> <http://www.chem.uu.nl/fcc/www/peopleindex/karel/karel.htm>
>
>  
>
>
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>
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>
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