We used 3H (tritium, a radio active material) incorporated label in a mol. to investigate why we observed a 10% drop in the active in a drug product over 24 mo at 25°C (or 3 mo at 40°C) without observing a 10% increase in total impurities.We prepared a lab batch of product with labeled active added to cold active to achieve the concentration of the active that is present in the drug product.These prepared samples were then stored at 25°C for 24 mo and at 40°C for 3 mo.The product was assayed using reversed-phase liquid chromatog. with both UV-detection and RAD (Radio Active Detection) at 0-mo, 3-mo, 6-mo and 12-mo time points for samples stored at 25°C and at 0, 1.5-mo and 3-mo time points for samples stored at 40°C.Commonly used in the pharmaceutical industry, reversed-phase liquid chromatog. by UV-detection provides a convenient means for assay and degradant anal.However, RAD provides addnl. detail on degradants in cases where a degradant is very weakly absorbing or impurities are hidden under the UV detector due to absorption of excipients.Addnl., the determination of absorption to glass vials and stoppers (used in sterile injectable products) can be readily performed using RAD.As expected in our study, RAD showed a good mass balance while the traditional anal. (UV-detection), run in parallel on the same samples, still did not demonstrate the 10 % missing mass balance.Some of the radioactivity was found to elute quite late in the method.The 3H labeled mol. also demonstrated that the missing mass balance was not due to loss of the active mol. to the glass vials or the stoppers.While the study has been completed at the 40°C condition, it is still underway for samples stored at 25°C.