Differential scanning calorimetry (DSC) measures the difference between the heat flow to the sample and the reference pan that flows undergo a controlled temperature program. Heat flow corresponds to transmitted power and is estimated in watts (W). The change in enthalpy after absorbing the energy is termed an endothermic reaction and when the sample releases the energy is termed an exothermic reaction.
Different thermal events are measured by DSC such as crystallization, the onset of oxidation, melting, cure reaction, and heat of transitions i.e. enthalpy.
Pharmaceutical Applications
DSC is widely used to measure reaction kinetics, glass transition temp, specific heat capacity, compatibility, the stability of samples, the effect of aging, the impact of additives on crystallization, and the characterization of the drug substance. DSC is useful for the measurement of glass transition temperature (Tg). Glass transition is the stage at which the substance changes its states from a glass-like form to a more elastic form.
DSC is a useful technique to identify the polymorphic form conversions mainly due to its capability to study the sample under different heating and cooling conditions required to impact the polymorph formation and following are the events:
- Endothermic Events: When the sample absorbs some amount of energy during phase conversion then the reaction is termed an endothermic event. The endothermic reaction requires additional energy to sustain the zero-temp difference between sample & reference. Following are the different endothermic events:
- Glass Transition,
- Melting,
- Evaporation/Volatilization,
- Enthalpic Recovery,
- Polymorphic Transitions,
- Decompositions
- Exothermic Events: When the sample evolves some amount of energy during phase conversion, then the reaction is termed an exothermic event. The exothermic reaction requires less energy to sustain a zero-temp difference between sample & reference. Following are the different exothermic events:
- Crystallization,
- Cure Reactions,
- Polymorphic Transitions,
- Oxidation,
- Decomposition,
- Freezing
- Calorimetric Purity by DSC can be used to measure the absolute purity of the crystalline drug substance sample with high sensitivity for detecting the low level of impurity. This is due to the melting point depression caused by the impurity, which lowers and enlarges the temperature range of the melting point. A sharp symmetrical curve shows the relative purity.
DSC is used mainly to study the thermal stability of the sample. DSC is very useful in the characterization of drug substances and drug products. DSC is a speedy, simple, and consistent technique which allows fast estimation of polymorphic forms, pharmaceutical drug substance/excipient compatibilities, endotherms, and exotherms in the corresponding enthalpies of reaction.
References:
- Physical Characterisation of Pharmaceutical Solids, Edited by Harry G. Brittain,
- Various Aspects of The Estimation of Impurities in Drugs, Sdndor Gorog,
- Craig, D.Q.M., Reading, M. Thermal Analysis of Pharmaceuticals. Boca Raton, FL: CRC Press,
- Sestak, J. Thermophysical properties of solids. In: Svehla, G. ed. Comprehensive Analytical Chemistry: Volume XII, Thermal Analysis Part D. New York,
- Ford, J.L; Timmins, P. Pharmaceuticals Thermal Analysis – Techniques and Applications,
- Ahuja S and Scypinski S, Eds Handbook of Modern Pharmaceutical Analysis,
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