Differential Scanning Calorimetry

This is one of the most frequently used methods to study solid-state properties. The flux type DSC involves heating the sample and reference samples at a constant rate using thermocouples, to determine how much heat is flowing into each sample and thus finding the differences between the two. Examples of such DSC instrumentation are those provided by Mettler and duPont. The power compensation DSC (e.g., Perkin-Elmer), an exothermic or endothermic event, occurs when a sample is heated, and the power added or subtracted to one or both of the furnaces to compensate for the energy change occurring in the sample is measured. Thus, the system is maintained in a thermally neutral position at all times, and the amount of power required to maintain the system at equilibrium is directly proportional to the energy changes that are occurring in the sample. In both types of DSC measurements only a few milligrams of the compound suffices. The sample can be heated in an open pan or in hermetically sealed chambers, where there may or may not be vents to release moisture or solvents; the compound may be subjected to pyrolysis in the testing phase.

Whereas the instrumentation available in the recent years has become very sophisticated, making such analysis possible with great consistency, the interpretation of the results is highly dependent on a keen understanding of the factors that affect the results. For example, such subtle factors as the type of pan, the heating rate used, the nature and mass of the compound, the particle size distribution, packing and porosity, pretreatment and dilution of the sample, and the use of the nitrogen cover can significantly alter the DSC profile obtained, and should be controlled to secure consistency in the repeat results.

A well designed and properly replicated DSC profile would yield such physical properties as melting (endothermic), solid-state transitions (endothermic), glass transitions, crystallization (endothermic), decomposition (exothermic) and dehydration or desolvation (endothermic), purity (of high purity compounds; though much less reliable than high-performance liquid chromatography, HPLC).

A heating rate of 10°C/min is a useful compromise between the speed of analysis and detecting any heating rate-dependent phenomena. If any heating rate-dependent phenomena are evident, the experiments should be repeated by varying the heating rate, in order to identify the nature of the transition that might be the result of polymorphism or particle size. It is noteworthy that milling the powder size may alter the profile significantly, and can be confused with polymorphic changes. Using different heating rates often resolves this problem.

A number of parameters can be measured from the various thermal events detected by DSC. For example, for a melting endotherm, the onset, peak temperatures, and enthalpy of fusion can be derived. The onset temperature is obtained by extrapolation from the leading edge of the endotherm to the baseline. The peak temperature is the temperature corresponding to the maximum of the endotherm, and the enthalpy of fusion is derived from the area of the thermogram. It is an accepted custom that the extrapolated onset temperature is taken as the melting point; however, some users report the peak temperature in this respect. We tend to report both for completeness.

Recycling experiments can also be conducted, whereby a sample is heated and then cooled. The thermogram might show a crystallization exotherm for the sample, which on subsequent reheating might show a melting point different from the first run. In a similar way, amorphous forms can be produced by cooling the molten sample to form a glass.

The calibration of a DSC employs the use of standards; the most common ones are listed in Table 6. These standards must meet a certain criterion of purity. A two-point calibration is often needed, for example, using indium and lead.

A variation of DSC is the MDSC (modulated DSC), wherein heat is applied sinusoidally, such that any thermal events are resolved into reversing and nonreversing components to allow complex and even overlapping processes to be deconvoluted. The heat flow signal in conventional DSC is a combination of

Table 6 Standards for Thermal Analysis in the Order of Increasing Melting Point

Temperature (°C)

Substance

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