Dear Friends,

How I determine the Assay of an Impurity of my Product if the Standard of this impurity is unavailable?

Is it possible validate the quantitative test of this Impurity?

Tanks

The best thing you can do is assume a 1:1 response factor for your impurity and product and calculate its amount based on %area of your product peak. By being accurate with your sample weighings and dilutions you can at least be consistent with your quantitative determinations of this impurity, and hopefully at a later date determine the correct response factor for it when you identify it. Hope this helps

deST:

Are you talking about finding a qualified reference standard for this product?

Mel

For a stability indicating assay of your PRODUCT you can estimate the impurity as Daren suggests until the impurities are characterized.

If you want to assay the impurity quantitatively then you have no choice, you need to obtain a standard. If you can generate the impurity then you run your separation in semiprep mode and collect the impurity and obtain a standard, often a fraction of a milligram is sufficient to establish a response factor.

If your impurity is a degradant you can sometimes see possible degradation pathways and order the possible degradants and ID by hplc. If you have information on the synthetic route to your product this can help in identifying possible impurities. Intermediates can probably be ordered. Good luck.

My Dear Friends (Mizukami, Daren, Mel);

Tanks for your precious help. I do not have standard of my Impurity.

I will do your sugestions.

Happyness

Dear deST;

The recommendations above are good, but there is one more thing you can try.

If the separation of the impurity and main compound can be done isocratically, then you could use a RI detector. This is known to give very much an equal response for all compounds and therefore a simple Area % analysis will be enough.

Obviously, you will have to increase sample load and injection volume to compensate for the reduced sensitivity of RI detectors. If you follow this idea then I also recommend you, once you have an idea of the impurity content in your samples, to spike your solution with a known % of a related compound and confirm that indeed the response is about equal for all signals. This will also make you feel better about your results.

I hope this helps. Good Luck;

Benjamin

dear benjamin:
you said that RI gives very much an equal response for all compounds .
it is very interesting to me and i would like to ask; if the equality in response is based on concentrations in mg/ml or by nmole/ml.
i.e. does the solution of a concentration of e.g. 10nmole/ml of any substance give the same response in RI ? or the solution of 1mg/ml does?

I diagree with Benjamin's statement that "This is known to give very much an equal response for all compounds ". In differential RI detectors, the response is based on the RI difference between the sample and reference cell. If a compound of interest has the same (or similar) RI as the MP, response will be lower relative to the same amount of a compound that produces very different RI than the MP alone.

Anonymouns, Benjamin, Mel, etc;

Your discussion is very good. I like it. But I do not have Standard of my Impurity, so I prefer work with the Area %.

Thanks four your attention and helps.