Photocalorimetry : design, development and test considerations
The goal of the project is to design and build a photocalorimeter capable of carrying out photostability testing in the pharmaceutical industry. A current challenge is to develop methods of testing the photostability of solid materials which are non-invasive, non-destructive and allow real time observations to be made. Calorimetry represents one such method. Solution-phase systems are relatively straight-forward and have been applied to calorimetry for many years requiring few assumptions about the reaction system under study to be made. Previous studies have developed iterative methods to fit experimental data to established calorimetric equations. A new approach allowing the direct calculation of reaction parameters is described in chapter two. For solids, the situation is more complicated than that for solutions and equations are more difficult to develop since additional parameters (such as the solid state fitting parameters m and ri) have to be accommodated. New theoretical approaches to data fitting in the solid state are also described in chapter two. Having previously established that the imidazole-catalysed hydrolysis of triacetin reaction is robust and reliable, new applications, such as the effect of fill-volume on the calorimetric output and thus the reaction parameters, are described in chapter three in preparation for use of the system on solid state systems. Chapter four describes the design and development of the photocalorimeter, together with improvements and modifications made to it as the project progressed, whilst chapter five describes the development of actinometric techniques. An actinometer provides a means of measuring the amount of light-energy being delivered to a target sample. Actinometric techniques are described in the literature and studies carried out on candidates for use with the photocalorimeter are outlined. Particular success was achieved with 2-nitrobenzaldehyde, where results within 2% error were achieved for the actinometric experiments. The final stage of the project involves the application of the newly-developed photocalorimeter to the testing of solid samples such as nifedipine. Studies were carried out both on the photodegradation caused by white light and also using monochromatic light. This allowed "causative wavelengths" of photodegradation to be investigated using the photocalorimeter - a significant area of interest in the pharmaceutical industry and the first time quantitative data has been obtained for a solid state material using non-classical techniques. Finally, studies were carried out into establishing the photostability of an unknown solid state test material.