Isothermal microcalorimetry for quantifying amorphous content in processed pharmaceuticals

Many processing steps can result in generation of partially amorphous materials. While the fraction of disorder may be low (typically up to 5% w/w) its location primarily on particle surfaces means its effects might be significant, especially in regard to powder flow and force of adhesion. Quantification of small amorphous contents is thus becoming an important part of product development. Isothermal microcalorimetry can be used as an assay for amorphous content by controlling the relative humidity or relative vapour pressure in the sample ampoule. The technique is very sensitive (typically detecting less than 1% w/w amorphous content) and universally applicable to pharmaceutical powders. However method design and data interpretation are critical factors in successful assay design. This article discusses methods and techniques and reviews current pharmaceutical applications to aid assay design.