Accelerated stability studies of morphine injections in plastic ampoules

Although oxygen sensitive, morphine injections are produced in plastic ampoules. In this study, the shelf life of sodium metabisulphite stabilised morphine injections in plastic ampoules was evaluated by accelerated studies at temperatures ranging from 50 to 80 degrees C. A derived model based on the rate equation and the Arrhenius equation was used for extrapolation. Determination of morphine and oxidation products was done by reversed phase ion pair HPLC. The concentration of morphine remained constant during the stability studies. The oxidation of morphine was partly compensated for by diffusion of water through the ampoules. A lag phase was observed for the formation of pseudomorphine. The shelf life is limited to 15 months by diffusion of water through the ampoules.