From: JOHN PHILO <JOHN.PHILO@amgen.com>
  To  : rasmb <rasmb@bbri.eri.harvard.edu>
  Date: 25 Jul 1995 10:23:54 U

finding the right cells...

...for equilibrium studies.

Like Steve Harding, many of us are excited about the new 
refractometric XL-A and anxious to see it expand our capabilities.
Steve's message about the need for cells/rotors with longer 
pathlength for the new refractometric XL-A prompted me to alert you 
to another problem/deficiency I view as a serious impediment to 
use of the new refractometric machine:

Beckman told me they currently have no plans to produce "external 
loader" 6-channel equilibrium cells for refractometric work.  As I 
understand it, this type of cell is what most people use for such 
work in the Model E.  In my view, it is essential for Beckman to 
make these cells available---let me explain why.

One drawback to interference data is that the baseline offset 
is relatively much larger than for absorbance.   A buffer/buffer 
blank will give a fringe count which varies significantly 
across the cell (~0.5 fringe) due to small variations in the window 
thickness.  Thus to take advantage of the high precision of the 
interference data, for all types of velocity or equilibrium 
analysis (except Walt Stafford's dc/dt method) one needs to do 
a careful baseline subtraction.  Without subtracting a baseline, 
even if you have a total signal of ~10 fringes the signal/noise may 
be only ~20, i.e., much worse than you get with absorbance.

The problem with the conventional 6-channel equilibrium cells 
is that the interference baselines change significantly every time 
you put the cell together, and thus one cannot get a good 
buffer/buffer baseline to correct the sample data.  The "external 
loader" cells have channels and plugs in the sides of the cell, 
similar to those in velocity cells.  While this makes them  
difficult to fill, they do allow one to replace what is in the 
sample channel without dis-assembling the cell, and therefore 
to record a buffer/buffer baseline before and/or after the sample 
has been run.

I have been talking to the Beckman folks about this for over a 
year, but the best response I have gotten is that they "will look 
into it".  If you agree with me, I urge you to contact them and 
express your concerns.  We all realize they are trying to devote 
their engineering resources to the new instrument development; 
however, it will be a shame if people are unable to really use 
the hardware optimally due to lack of appropriate cells and 
rotors.

Best regards to all,

John Philo, Amgen