[RASMB] s values as function of cell position

[jack kornblatt]

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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