[RASMB] fringe deviation - and fringe gradient

[Arthur Rowe]

Greetings, Tom and everybody

Just a very quick note on just how much the gradient of fringe increment can
be allowed to rise before you get Wiener skewing effects.

Peter Lloyd in his very useful little book stated many years ago that the
effect of the aberration only kicked in when the gradient exceeded 76
fringes per cm (a resolution of 0.02 fringes was assumed). It is many years
since I checked this calculation out, but as I recall it is a mis-print: it
should be 76 fringes per mm. That is still a figure which one can exceed
near to the base of the column.

Although the estimate was made with the Model E in mind, the term in the
Svensson equation which includes the angular aperture can be made to be zero
in the E, and I presume is zero in the XL-I/A.

Regards to everyone

Arthur


Hi folks,
One more comment regarding gradients and shlieren effects in the XLA.
Steep gradients do not have to produce the "dreaded black bands" (I like
that description!) to cause inaccuracies in the absorbance readings.
Helmut Colfen and Steve Harding studied this phenomena using an older
design of the XLA optics:
H. Colfen and S. E. Harding. A Study on Schlieron patterns derived with
the Beckman Optima XL-A UV-absorption optics. Progr.Colloid Polym.Sci.
99:167-186, 1995.
Two things to note: 1) the newer XLA optics (on everyone's machine) are
far less susceptible to the schlieren effects described in this paper
and 2) one can test for a problem very quickly and easily by scanning at
a non-absorbing wavelength (e.g. 320 nm for protein) and observing
whether there are bumps in the trace at positions where there is a boundary.
Best wishes to all,
Tom

-- 
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


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