Hi Hoon,
Using a reference value is not always a good idea, because experimental effects can play a role.
BUT, S02 should not change during charge-discharge on a single sample, or a series of samples prepared and measured similarly. Instead, it's likely something correlated with S02 in the fit is changing, and so the fitting routine is getting a bit confused and attributing part of the change to S02. (That's not a knock on the fitting routine; it doesn't know any better unless you tell it!)
I think the best recommendation is to do a simultaneous fit on multiple spectra, constraining the S02 to be the same for each. So you're still fitting S02, but forcing all the spectra to use the same value.
Second best is to fit one spectrum and allow S02 to vary, and then constrain all the other fits to use that value.
--Scott Calvin
Sarah Lawrence College
On Sep 18, 2014, at 7:06 AM, HOON Kim