Stability Indicating Assay

[sarasst]

I am developing a HPLC indicating assay for a hormonal drug. I am using the peak purity option to analysis the purity of my peak once the sample has been subjected to 20% degradation under various conditions. The purity results show that my peak is not pure although its 5 overlaid spectrums are more or less identical. The resolution of my drug from its degradation products is greater than 2. I need to know how I can accurately interpret my peak purity results in order to ensure that no other degradation product is co-eluting at the same time as my main peak. I would appreciate any suggestions with regards to this matter.

[tom jupille]

The purity results show that my peak is not pure although its 5 overlaid spectrums are more or less identical.

"more or less identical" is *not* a statement you can submit to a regulatory agency. I'm not a great fan of peak purity measurements, but that is because they tend to give false negative (peak shows as pure even though it is not). If your peak shows as impure, you probably have something else coeluting

The problem is that UV spectra in solution simply don't give a lot of structural information. The only way to be sure is to run LC-MS where the detector *does* give useful information. Next best would be to develop an orthogonal method (different column type, different organic solvent, possibly different temperature and/or pH) and compare results. You will not necessarily see all impurities in either method, but between the two methods, you have a better chance of catching "hidden" peaks.