<HTML>
<HEAD>
<TITLE>Re: [RASMB] fringe deviation - and fringe gradient</TITLE>
</HEAD>
<BODY>
<BLOCKQUOTE><FONT COLOR="#800000">Greetings, Tom and everybody<BR>
<BR>
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. <BR>
<BR>
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 <U>per cm</U> (a resolution of 0.02 fringes was assumed<U>)</U>. 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 <U>mm</U>. That is still a figure which one can exceed near to the base of the column. <BR>
<BR>
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. <BR>
<BR>
Regards to everyone<BR>
<BR>
Arthur<BR>
</FONT><BR>
<TT><BR>
Hi folks,<BR>
One more comment regarding gradients and shlieren effects in the XLA. <BR>
Steep gradients do not have to produce the "dreaded black bands" (I like <BR>
that description!) to cause inaccuracies in the absorbance readings. <BR>
Helmut Colfen and Steve Harding studied this phenomena using an older <BR>
design of the XLA optics:<BR>
H. Colfen and S. E. Harding. A Study on Schlieron patterns derived with <BR>
the Beckman Optima XL-A UV-absorption optics. Progr.Colloid Polym.Sci. <BR>
99:167-186, 1995.<BR>
Two things to note: 1) the newer XLA optics (on everyone's machine) are <BR>
far less susceptible to the schlieren effects described in this paper <BR>
and 2) one can test for a problem very quickly and easily by scanning at <BR>
a non-absorbing wavelength (e.g. 320 nm for protein) and observing <BR>
whether there are bumps in the trace at positions where there is a boundary.<BR>
Best wishes to all,<BR>
Tom<BR>
<BR>
-- <BR>
Department of Biochemistry and Molecular Biology<BR>
University of New Hampshire<BR>
Durham, NH 03824-3544<BR>
Phone: 603-862-2459<BR>
FAX: 603-862-0031<BR>
E-mail: Tom.Laue@unh.edu<BR>
www.bitc.unh.edu<BR>
www.camis.unh.edu<BR>
<BR>
<BR>
_______________________________________________<BR>
RASMB mailing list<BR>
RASMB@rasmb-email.bbri.org<BR>
http://rasmb-email.bbri.org/mailman/listinfo/rasmb<BR>
</TT></BLOCKQUOTE><TT><BR>
</TT>
</BODY>
</HTML>