Thermal Methods

Thermal methods of analysis discussed in this section are differential scanning calorimetry (DSC), thermogravimetry (TG), and hot-stage microscopy (HSM). All three methods provide information upon heating the sample. Heating can be static or dynamic in nature, depending on the information required.

Differential scanning calorimetry monitors the energy required to maintain the sample and a reference at the same temperature as they are heated. A plot of heat flow (W/g or J/g) versus temperature is obtained. A thermal transition which absorbs heat (melting, volatilization) is called endothermic. If heat is released during a thermal transition (crystallization, degradation), it is called exothermic. The area under a DSC peak is directly proportional to the heat absorbed or released and integration of the peak results in the heat of transition.

Samples are loaded into pans for DSC analysis. Pan configuration (open, crimped, hermetically sealed, hermetically sealed with a pinhole, etc.) and scan rate can result in variations in position and intensity of the thermal events. These variations can be used to gain further information about the sample as well as other crystal forms.

The observance of thermal transitions by DSC is insufficient to fully characterize the behavior of a substance on heating. It is not known if an endothermic transition observed in the DSC is a volatilization or a melt without corroborating information, such as TG or HSM data. It is important to understand the origin of the DSC transitions to fully characterize the system and understand the relationship between various solid forms.

Thermogravimetry measures the weight change of a sample as a function of temperature. A total volatile content of the sample is obtained, but no information on the identity of the evolved gas is provided. The evolved gas must be identified by other methods, such as gas chromatography, Karl Fisher titration (specifically to measure water), TG-mass spectroscopy, or TG-infrared spectros-copy. The temperature of the volatilization and the presence of steps in the TG curve can provide information on how tightly water or solvent is held in the lattice. If the temperature of the TG volatilization is similar to an endothermic peak in the DSC, the DSC peak is likely due or partially due to volatilization. It is usually necessary to utilize multiple techniques to determine if more than one thermal event is responsible for a given DSC peak.

Hot-stage microscopy is a technique that supplements DSC and TG. Events observed by DSC and/or TG can be readily characterized by HSM. Melting, gas evolution, and solid-solid transformations can be visualized, providing the most straightforward means of identifying thermal events. Many polymorphic systems have been investigated using only these thermal methods, as illustrated by the publications of Kuhnert-Brandstatter (7). Details of the methodologies used in hot-stage microscopy have also been reviewed (8).

Thermal analysis can be used to determine the melting points, recrystal-lizations, solid-state transformations, decompositions, and volatile contents of pharmaceutical materials. DSC can also be used to analyze mixtures quantitatively.

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