The preceding chapter has outlined the many possibilities afforded by thermogravimetric analysis (TGA), together with a brief overview of the practicalities associated with such measurements. However, in relating the technique to current pharmaceutical practice, it can be reasonably argued that, at present, TGA is almost certainly underexploited, with the method used largely as a means of estimating water content. This is clearly an important application from a drug design and delivery viewpoint but does not reflect the full range of possibilities afforded by the approach, particularly in terms of understanding the nature of the binding of the volatilized component to the substrate or assessing the kinetics of the weight loss process, from which information on longer-term stability may potentially be obtained. One of the contributing factors to the poor crossover between the basic chemistry and pharmaceutical fields in this respect may be the difference in emphasis placed on the technique within the two scientific cultures. The majority of work using TGA outside the drug-
related arena is concerned with the examination of thermal decomposition reactions, and a wealth of literature exists in which the chemical pathway, loss profile, and kinetics have been interrelated. However, thermal decomposition induced by temperature ramping (with the important exception of dehydration) is usually not a major issue within the pharmaceutical sciences owing to the materials in question being processed and stored at temperatures well below those at which such reactions are discernible by TGA. Similarly, extrapolation of thermally induced decomposition rate data to normal storage temperatures to predict long-term stability is unreliable because of the strong possibility of the mechanisms involved being different.