Upon stability, the assay reduces to 91 to 92% but the remaining 8 to 9% is not being compensated by impurities. Assay and RS, both methods have been developed using HPLC. To identify non-chromophoric or volatile substances, we've analysed using LC-MS and GC. But no significant conclusions could be drawn. Are there any other techniques to identify the impurities, if that is the reason? What could be the root cause? Is there any chance for LDPE to absorb the drug, if so, how can we identify it?
You have a 'new' RS that is reaction product between the LDPE and the API! Thus, this molecule does not appear on your chromatogram because it elutes in the solvent front or is completely retained by the column.
How much your weighing? are you use analitical balance or semicro or microbalance?
There is another possibility! This new RS may be a gas (which cannot be recovered for your mass balance equation) and the LDPE is just a catalytic surface.
The possible reasons for the impurities not eluting in your HPLC method could be attributed to the detection mechanism or type of detector used.
You can try using CAD - charged aerosol detectors - to verify for the species that account for the loss of mass balance.
Solution stability studies should also guide to some information on the degradation pathway.
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