[RASMB] 3mm centerpieces

[Tom Laue]

Hi-
The spacers for the 3 mm centerpieces place the centerpiece at the 
mid-point of the cell, which is out of focus for the interference 
system, which is focused at the 2/3 plane of a 12-mm centerpiece, with 
sapphire windows and a solvent refractive index of water. Mis-focusing 
will lead to mis-registration of the true radial position with the 
apparent radial position in regions of the image with steep gradients in 
the concentration (1). For the 3 mm centerpieces, the focal plane is 
below the sample 2/3 plane, causing steeper fringes than might be 
expected... i.e. analysis will reveal what appears to be a weak 
association or a heterogeneous system. This problem would also affect 
the absorbance system, but the absorbance optics cannot trace a steep 
enough gradient.
So long as you keep gradients low enough (as I recall, about 50 fringes 
displacement per mm), your data will be OK.
Best wishes,
Tom

(1)    D. A. Yphantis. Equilibrium Ultracentrifugation of Dilute 
Solutions. Biochemistry 3:297-317, 1964.

Borries Demeler wrote:
> Question about 3 mm centerpieces - the spacers that come with them seem
> to take it out of the focal plane for the interference system, at least
> we cannot get 3 mm CPs to work in interference mode. Are there spacers
> that will correct this issue, or is there something else we should try?
>
> thanks, -Borries
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