Ref Std or Area Normalization for Impurities

[Sunjay]

Dear All,

I would like to know that for the impurities determination which method will be preffered and why out of these two: by Area normalization and by using referance standard?

Regards

[Kostas Petritis]

In order to use the reference standard method you need to know the nature of your impurity... furthermore if you can not find it commercially available you will need to synthesize it/them...

[tom jupille]

Reference standard is far better. Only if you cannot buy or synthesize the impurity (as Kostas pointed out) should you use area percent.

The reason: when you use area percent, you are assuming that the absorbance spectra of the impurity and the parent compound are identical.

[S.M.PANDYA]

Dear Sanjay

I fully agree with Tom Jupille that Reference Standard method is always better . If you are really working on detection of impurity , identification of impurity then area normallisation method will not help you .

Detection of Impurity : In this study if you have not match with known concenstration of reference standard fully characterized , it is difficult to know the response towards detection on column & this creates erretic result of impurity quantitation .

Areanormallisation can be considered subject to response factor with reference to known synthetic mixer of product ref. std & imp. ref std is determined .

If you have PDA detector , reference std spectrum is known & response factor is determined initially , area normallisation can be consider based on trend of product profile & column performance check w.r.t. theorytical plates.

[HW Mueller]

S.M., I am having some difficulty with your 2nd - 4th paragraphs. Thus some questions:
2nd paragraph: How does the ignorance of relevant physical parameters of a substance (impurity) produce erratic results?
3rd and 4th paragraph: Are you saying that if one knows the absorbance of the analyte and the impurity in the mobile phase than one can calculate the mol % impurity?

[DR]

In P3, I suspect the intent was to say that if you know the absorbtivity of the analyte, you can calculate concentration (get the ratio of absorbtivities for main peak and related substance once, then use that ratio and just the main peak standard response in the future).

In P4, SM may also suspect that, if the spectrum of a degradant is very similar to that of the parent, one might be able to assume that molar absorbtivities are also similar (I wouldn't assume that, nor would I expect plate counts or other chromatographic properties to have anything to do with it).