Hot Stage Microscopy
Hot-stage microscopy is thermal microscopy which is also referred to as thermomicroscopy. It is an analytical tool which involves thermal analysis along with microscopy. Characterization of pharmaceutical materials is performed as the function of temperature and time.
The technique of thermomicroscopy was first developed by Ludwig and Adelheid Kofler. The characterization of pharmaceutical compounds was first observed by Maria Kuhnert Brandstatter.
This helps to observe the physical appearance of the material, solid-solid transformations, incompatibility, melting upon heating, solidification upon cooling, charring or decomposition.
Microscope setup comprises of the optical stereomicroscope, polarizer, hot stage, digital programmable temperature controller, digital camera, computer and software for the capture and analysis of micrographs. It is a simple technique where the sample is placed on glass discs and the sample is heated by heat transfer from a metal block. The heating and cooling are maintained by the regulation of hot and cold gases flow rate.
The hot stage microscopy can be merged with Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC) and also with scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman and confocal microscopy.
It is used for a single as well as multi-component characterization. In single-component pharmaceutical systems, it is used to find out the difference in crystalline and amorphous forms and for identification of polymorphs of a crystalline substance. Multi-component pharmaceutical systems can be characterized for studying the compatibility with active pharmaceutical ingredients and excipients and to discover new co-crystals formed by the fusion of contacted components.
It is a simple thermal analytical technique along with DSC and thermogravimetric analysis (TGA). Further advancement in this technique can include better performance in image capturing, improvement in software for its image analysis and statistical calculations.