Introduction

Differential scanning calorimetry (DSC) is one of the most widely used thermoanalytical techniques.^[1] Structural transformations, such phase transitions or crystallization can be studied with DSC.^[2] Advantages of DSC include wide analysis temperatures and therefore transitions can be detected in a broad range. DSC is used for example to investigate thin films, solid state chemistry of pharmaceuticals or to determine free energy of protein folding.^[2][3][4] A DSC graph describes heat flow as a function of temperature. An example of a DSC curve is presented in figure 1. Positive signal describes exothermic and negative endothermic transformation. Integration of a DSC curve gives amount of the total heat exchanged. In addition, energies of final and initial states can be determined. Kinetics of structural transformations can also be studied with DSC.^[1]

Figure 1. DSC curve for exothermic and endothermic change. (Figure: Saija Pajari)

Working principle

In DSC instrumentation, there are two chambers in a thermally isolated container. The chambers are identical but only the other contains the sample and the other one is the reference.  Concentration of the sample is known: the goal is to determine the change of material's heat capacity.^[3] Heat capacity C_p (J/K), corresponds the amount of heat added or removed from the sample to cause a temperature change. Heat capacity can be determined from the heat flow, by dividing it with heating rate of the sample.^[5] In DSC measurement temperature of both chambers is increased or decreased gradually, which is called temperature scan. There is a difference between heat absorbed by the sample and the reference to maintain the same temperature.^[3] Apparatus is presented in figure 2.

Figure 2. DSC instrumentation by Kodre et al.^[2] (License: CC BY 4.0)

Applications

Thin films

Thin films can be investigated by DSC. Roura et al. (2009) performed quantitative DSC analysis for silicon thin films. They recorded the DSC graph for hydrogenated amorphous silicon. Heat was released in the experiment, resulting in exothermic DSC signal. Dehydrogenation, structural relaxation and crystallization could be determined from the graph. In addition, kinetics and energies of crystallization were analyzed for different Si materials.^[1]

Pharmaceutical industry & biomolecules

DSC has a number of applications also in pharmaceutical industry.  It has been used for purity determinations, to study polymorphism of drugs and to quantify pharmaceutical crystal forms. Impurities cause depression of the melting point and therefore purity of the pharmaceuticals can be estimated by determining melting point with DSC.^[6] Advantages of DSC include automation of the instrumentation and sensitivity of the experiment.^[7] However, DSC can be used only for compounds which have sharp melting points.^[6]

Another application area of DSC includes proteins. Melting temperature and unfolding of the protein can be investigated with DSC.^[3] DSC can be also used to study phase transitions of lipids.^[8] DSC provides a fast and straightforward way to determine thermal stability of the proteins. However, impurities can cause a shift in melting temperature when they interact with the protein in question.^[7]

References