Chromatography Forum

At my place of worship (oops, I mean work!) we use the following equation for calculation of assigned purity:

Assigned purity % = (HPLC %Purity/100) x [100-(Sum of Non-HPLC %Impurities)] x Salt Factor. Salt factor is for conversion to free acid or base.

This seems to work but my question has to do with COA's that have a test for the salt (e.g., chloride for HCl salts, maleate for maleic acid salts). The question is "do you subtract this value out as an impurity and not use the calculated salt factor?" If so, it seems the "chloride" or "maleate" tests are only measuring the anion and thus ignoring the proton which doesn't correct to the free base but the protonated base.

What am I missing? Thanks.

My response would be to standardize on one way of doing the calculation. the way listed seems fine and unless all CoA's include the data on the salt it is the only "standard" calculation method you have.

Thanks JGK. So what about the last question about subtracting out only the value for the anion, if one is given (e.g., Cl-) when in fact you want to correct for the HCl salt? Can the proton be ignored even though you are left with a correction to the protonated base?

Or, am I missing something?