[RASMB] more systematic differences

[Jack Kornblatt]

Hello John, Andrew, Tom and Boris

Thanks very much for the tips on how not to check for leaks.  I'll put 
an indicator in one chamber and check the absorbance in the other 
chamber at the end of the run.

Re: The meniscus shift:
We have one rotor and three cells. Because we are compulsive and are 
following the instructions that were given to us when the AUC was 
bought, the cells are numbered 1-3; each cell has its own quartz or 
sapphire windows, its own center piece and its own cushions.  Cell 1 
always goes into rotor position 1, etc.  For the sedimentation velocity 
experiments that I am doing, the cells are always filled with the same 
pipetter.  Sample compartments get 0.390 mL, reference compartments 
always get 0.400 mL.  The meniscus in cell 1 during a run is always at 
6.03+/-0.005 cm.  The meniscus in cell 2 is always at 6.095+/- about 
0.007 cm.  In cell 3 the meniscus is always at 6.05+/-. 
This problem is relatively new but I can not say how new.  In 2004 
(according to my notebooks) the meniscuses of the three cells were quite 
similar but diverged after we bought one new cell.  Since then, I've 
logged up perhaps 50 runs (could be 70).  The changes occurred sometime 
between 2004 and now.
The following variation was suggested by Peter S.  This was done only 
one time.  Cell 1 was placed in rotor position 2, cell 2 into rotor 
position 3 and cell 3 into rotor position 1.  The meniscus of (cell 1 in 
position 2) was 6.03+/-.  The meniscus of (cell 2 in position 3) was 
6.095+/-.  The meniscus of (cell 3 in position 1) was 6.05+/-.
The data were very clear.  The problem is the center piece.
I think that the rotor is quite ok.

jack
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