Influence of temperature and storage time after light exposure on the quinine monohydrochloride chemical actinometric system.

The ICH guideline on photostability has proposed quinine monohydrochloride chemical actinometric system as a standard method for measuring light expos ure during photostability testing. A change in the absorption at 400 nm of quinine monohydrochloride after light exposure corresponds to a defined dos e of light. The present work investigated the effect of temperature, light exposure level and the dark reactions following light exposure on the chang e of absorbance obtained. The change in the absorbance was linear with resp ect to time, the rate increased threefold in the temperature range of 25-52 degrees C, and the calculated activation energy was 30 kJ/mol as calculate d by the Arrhenius equation. For the dark reactions the change in absorbanc e was non-linear with respect to time. The rate of the dark reactions was s maller than during light exposure and dependent on the light exposure level prior to the dark reactions. The calculated activation energy of the dark reactions was 18 kJ/mol when calculated by the Arrhenius equation on the in itial reaction rates. The different activation energy of the light reaction and the dark reactions indicated different degradation patterns of the two reactions. The present study shows that the absorbance change of quinine m onohydrochloride chemical actinometric system is dependent on temperature d uring light exposure and on storage time and storage temperature after ligh t exposure. The method proposed in the ICH guideline should therefore be op timized in terms of definition of temperature and limitations in storage ti me after light exposure. (C) 2000 Elsevier Science B.V. All rights reserved .