[RASMB] Measurable limits of the interference system of the XL-I

[Tom Laue]

Hi Christine-
While there is no practical upper concentration limit to the 
interference optics, there are very real limits to the concentration 
gradients it can follow. The horizontal pixel spacing of the camera and 
the "splicing" routine used to convert fractional fringe displacement to 
the total fringe displacement limit the concentration gradient to values 
less than 0.5 fringes/10 um (500 fringes/cm). While this seems like a 
lot, it is in the range of concentration gradients where focusing 
affects the accuracy of the concentration determinations (e.g. Weiner 
skewing may be significant).
My experience is that with 12 mm centerpieces, the system has difficulty 
tracing gradients in solutions with concentrations > ~20 mg/ml 
(depending on how much or little diffusional broadening there is).
If you are less concerned about the accuracy of the concentration 
readings, you may increase the concentration range by using thinner 
centerpieces. Be aware, though, that the focus of the XLI assumes 12 mm 
centerpieces are being used. You will still get good fringes, and the 
concentrations will be reasonably close to the correct values so long as 
the gradients are not very steep. You can get an idea of how much error 
to expect from David Yphantis's 1964 paper:
D. A. Yphantis. Equilibrium Ultracentrifugation of Dilute Solutions. 
Biochemistry 3:297-317, 1964.
Best wishes,
Tom


christine.wandrey at epfl.ch wrote:
> Dear all,
>
> I started analyzing micelle/particle polymer systems at relative high
> concentrations by interference optics. The aggregation is concentration
> dependent and needs certain minimum concentration.
> During the first screening experiments I realized strong fringe shifts 20-40
> fringes.
> I know that the Rayleigh optics has some limitations but I could not find
> quantitative numbers.
> Thank you for help.
> Best,
> Christine
>
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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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