From: STEVE HARDING <SCZSTEVE@szn1.agric.nottingham.ac.uk>
  To  : rasmb@bbri.harvard.edu
  Date: Tue, 25 Jul 1995 16:47:57 GMT

finding the right path.....

... length for the cells for the new XL-A Interference model.

I don't know what others think but I was a little concerned that the 
interference optics on the new XL-A will only accomodate cells of 
maximum path length of 12mm.

I thought one of the purposes of the new model was to make it 
available to polymer and glycopolymer/polysaccharide communities who 
can't make much use of the absorption optics.  From a polysaccharide 
standpoint, the dn/dc of these molecules is such that to get a decent 
fringe increment from sedimentation equilibrium a minimum loading 
concentration of 1mg/ml is required.  This is too high, not only 
because of the strong thermodynamic non-ideality of these molecules 
but also the critical overlap concentrations of some of them (such as 
xanthan) is as low as 0.5mg/ml.  Whereas I appreciate that 
for sedimentation velocity the methods of Walter Stafford mean that 
we can use much lower concentration, for sedimentation equilibrium I 
am afraid it won't work  - so I'll be obliged to continue to use 
Model E's and their 30mm pathlength cells.

I would be interested to hear the views of others on this, and 
also - if they're tuning in - from the Beckman home team about the 
prospects of a modification to allow use of a rotor that will take 
the larger path length cell

Cheers
Steve Harding