I have an impurities assay (HPLC) test method on which I obtained an OOS (high) result for an unknown impurity for Morphine Sulfate API. This happened over 2 years ago. Two injections are run on the same sample preparation.
We ran a retest protocol on 5 new sample preparations. This unknown impurity is still present (in all of these preps) but at a much lower conc (approx 1/3 that of the OOS conc.). A different HPLC system, columns, and solutions were used. The method is gradient with a UV detector at 284 nm, inj. volume 30 ul, and flow rate 1.0ml/min. The run time is 72 min. Column type is 4um C18 dim = 4.6 x 150 mm. A USP reference standard of Morphine sulfate is used to calculate impurities conc (based on average area count of 6 injects).
I was originally thinking that contamination of the orig sampe prep was the root cause. But since this same impurity appears in the 5 resample preps, over 2 years later, I am thinking there is some other cause.
Do you have any ideas?