[RASMB] 3 mm centerpieces

[Tom Laue]

Hi Bo,
If you change the centerpiece thickness, it will affect the optimal 
focal point. The extent of the effects will depend on the gradient in 
refractive index, not on the concentration per se. The 1964 Yphantis 
paper in Biochemistry includes a thorough analysis of the effects, based 
on the work of Swensson. It is the third-order aberrations that are most 
important, not the second order (even order aberrations tend to be 
minimal). The upshot is that all aberrations up to the seventh order are 
minimized by focusing at the plane in the sample that is 2/3 of the way 
from the light source to the exit plane. The dominant effect, called 
Weiner skewing, results in a distortion of the radial axis. That is, if 
you had a glass ruler in the parallel light beam positioned between the 
lightsource and the sample, the rulings would appear to scrunch together 
in the regions where there were sharp concentration (refractive) 
gradients  in the sample. This effect applies to both the absorbance and 
interference optics. For the interference optics, gradients exceeding 
~50 fringes/mm should be edited out of a data set. For the absorbance 
system, it is a good idea to scan at a wavelength away from any 
absorbance to see if the baseline shows any bulges (due to schlieren 
effects) in the vicinity of a concentration gradient. If the baseline is 
bulged, you should not include those data in an analysis.

The key is to avoid steep concentration gradients if at all possible. If 
you cannot (e.g. at the start of a sedimentation velocity experiment) do 
not trust any information that depends on the shape of the gradient 
(e.g. diffusion coefficient, thermodynamic or hydrodynamic 
nonidealtity). For sed velocity, the position of the boundary is usually 
still fairly well specifiec, so values of s tend to be OK.

CAMIS builds custom centerpieces and window holders for academic 
researchers and the member companies of BITC. Given your extreme 
patience waiting for the fluorescence system (finally done), we'll give 
you an extra good price.
Best wishes,
Tom


Borries Demeler wrote:

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>Hello all,
>
>I have a question about 3 mm centerpieces:
>
>If I were to replace the standard 1.2 cm centerpieces with the 3 mm
>version, and add the appropriate spacers, would I have to worry about
>adjusting the optics of the XLA, or will the spacers assure optimal
>focus for both optical systems (UV abs and IF)?
>
>The other question is about where to get them. Beckman apparently has
>them on back order, does anyone have an extra one they are willing to sell 
>and a set of spacers we can buy? Does anyone else besides Beckman make these?
>
>Thanks, -Borries
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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
www.bitc.unh.edu
www.camis.unh.edu