Determination of relative response factor

[Robotjock]

Can a RRF be determined indirectly or by association? An impurity (X) in one of our products converts rapidly into another more stable impurity (Y) - I think the compounds are tautomers. We have Y material and have determined its
RF relative to the drug substance using the traditional slope ratio method.

However , how can one, if possible, determine the RF of X relative to the drug substance when no X material is available to make varying solutions of X?

thanks,

[tom jupille]

How rapid is the conversion (and under what conditions)? If you're dealing with tautomers and the interconversion is quick, you will only ever see one peak which represents the average of the two forms at equilibrium.