[RASMB] 3mm centerpieces

[Tom Laue]

Hi-
Yes you can calculate (or measure) the correct 2/3rds position for the 
centerpiece, make unequal spacers and drill the cell housing holes at 
that level. Be aware that the 2/3 position is not the mechanical 
location, but the optical. These positions differ by a mm or more, and 
are different for sapphire and quartz window. Those cells would be 
useful for the interference optics, so it would be best to determine the 
2/3 position for sapphire windows. The absorbance optics are focused at 
the midpoint, so the new housings would not be helpful for them.
Best wishes,
Tom

Kristian Schilling wrote:
> Tom and Borries,
>
> I am wondering whether we might modify cell housings to overcome that 
> problem. There is no other need for equal spacers but to align the 
> centerpiece with the filling holes, so we might just drill the filling 
> holes at a modified position. Of course that would mean that these 
> housings can be used only with 3-mm-cps, but the first question is: 
> can we match the ideal position for the centerpiece and enhance data 
> quality?
>
> Kristian
>
> At 01:07 15.09.2006 -0400, you wrote:
>> 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
>>> _______________________________________________
>>> RASMB mailing list
>>> RASMB at rasmb.bbri.org
>>> http://rasmb.bbri.org/mailman/listinfo/rasmb
>>>
>>>
>>
>> -- 
>> Department of Biochemistry and Molecular Biology
>> University of New Hampshire
>> Rudman Hall 379
>> 46 College Road
>> Durham, NH 03824-3544
>> Phone: 603-862-2459
>> FAX:   603-862-0031
>> www.camis.unh.edu
>> www.bitc.unh.edu
>>
>>
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>
>
>
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>
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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