[RASMB] s values as function of cell position

[John Philo]

Jack,
 
I agree with Jack Correia that you should try swapping cells to see whether
this variation is really associated with the particular cell or the position
in the rotor. If it is the cell, then also try swapping centerpieces between
housings.
 
This issue might be related to convection. First be sure your centerpieces
have no scratches or roughness inside the channels (this often happens when
people fill them using syringe needles). Sometimes too when you have a
severe leak at high speed the center rib gets permanently bowed, which is
not always easy to see by eye.
 
One other possibility might be that the alignment marks on your rotor or
cell housings are incorrect (although if this is truly a problem that did
not exist 2 years ago that cannot be the cause). I don't know the age of
your rotor but there was definitely a period some years back where the marks
on the rotors were not positioned properly. The cell alignment tool sold by
BITC at U.N.H. would take care of that issue (assuming the slots in the cell
housings are properly located). Similarly, at another time some centerpieces
were produced where the key slot was not properly centered between the
channels.
 
John
 
 -----Original Message-----
From: rasmb-bounces at rasmb.bbri.org [mailto:rasmb-bounces at rasmb.bbri.org] On
Behalf Of jack kornblatt
Sent: Tuesday, August 08, 2006 2:08 PM
To: rasmb at server1.bbri.org
Subject: [RASMB] s values as function of cell position



I have recently come across a problem the answer to which I cannot find in
the Archives.
The problem is this: If I run the same sample in the AUC at the same
temperature and speed over the course of 5 days, I get a distribution of s
values that has a spread of plus or minus <0.4%. The sample is quite stable
and shows no suggestion of degradation between days 1 and 4 but a small peak
(< 2% of the total) shows up on day 5. The perplexing thing is that there
are systematic differences between cells 1, 2 and 3. The s values in cell 1
are always highest, the s values in cell 2 are lowest and the s values in
cell 3 intermediate. The differences are small. Cell 1 s values are about
0.4% less than those of cell 1. 
The samples in the three cells are the same; they were taken from the same
batch. The buffer is the same. 
The problem has only become evident recently. It was not present two years
ago but I cannot state unequivocally when it first started to occur.
Has anyone suggestions as to cause and what might be done to correct the
problem?
 
thanks for any help you can give
jack (kornblatt)

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