The almost universal applicability of DTA and DSC has led to their use in nearly every field of science, with a strong emphasis on solving problems in materials technology and engineering, as well as "pure" scientific investigations. The approach taken here is to divide the subject according to the basic features of a DTA/DSC curve, and to illustrate their uses by specific examples.

The DSC curve above shows most of the general features likely to be encountered. At the start of heating, an offset, O, is usually apparent, which is due to an imbalance in the thermal capacities of the sample pan and its contents, and the reference pan and contents.

In the absence of any discrete physical or chemical transformations, the baseline signal, as at B above, is related to the heat capacity of the sample. DSC allows this parameter to be determined with good accuracy over a wide temperature range. The conventional approach is to compare the signal obtained for the sample above that given by an empty pan, with the signal obtained for a standard material, usually sapphire, under the same conditions. Careful experimental technique is required to obtain accurate results, but heat capacities can be routinely measured to accuracies better than ±1%. The measurement principle is shown below.

The instrument is programmed to heat between the initial and final isothermal stages at Ti and Tf, first with empty pans. A perfect instrument would show no deflection from the isothermal baseline, but this is never the case, and a small signal is given which can be used to correctly measure the deflections given by the sample and sapphire standard when run under precisely the same conditions. The deflections obtained are directly dependent on the heating rate; linearity and reproducibility of the temperature programme are therefore essential for accurate work.

The ratio of the deflections shown as the coloured arrows in the figure, is equal to the ratios of the heat capacities of the sample and sapphire:

Other techniques are available for heat capacity measurement by DSC. Accurate data can be obtained in narrow temperature intervals by using a non-equilibrium pulse technique, which is particularly useful when measurements need to be made in a region constrained by adjacent complicating transitions. Less time-consuming experiments than those described above can generate data more rapidly, but at the expense of accuracy and precision, which may be adequate for a given purpose. MTDSC, a recent enhancement of DSC, routinely generates Cp data from a standard experiment, and can in fact measure this in a nominally isothermal condition. The classical method above however is recommended for the best quality results.

The DSC/DTA curve may show a step change, as at S in the curve, reflecting a change in heat capacity not accompanied by a discrete enthalpy change. The most common example, and a major application area of DSC, is the glass transition (Tg) seen in amorphous polymers. This important region, in which the material changes from a rigid glassy state to a rubber, or very viscous liquid state, may be analysed to give a wealth of information about the material.

The temperature Tg may be used to identify polymers, as it varies over a wide range for commonly used materials. The amount or effectiveness of a plasticiser may be judged by how much it reduces Tg or affects the shape of the transition. Examination of the transitions in polymer blends gives information as to their compatibility. Curing reactions result in an increase in Tg and measurements can be used to monitor the extent of cure. Tg also varies with chain length for a related group of polymers. Additional features occurring in the glass transition region, often a superimposed endothermic peak, are related to the aging undergone by the material in the glassy state, and can sometimes obscure the transition, making precise temperature measurement difficult or futile. MTDSC offers considerable benefits in this respect, being able to separate the two effects.

A standardised technique is important for Tg measurements, as the measured values depend on the thermal history of the material. The Tg changes with the rate of cooling from the melt, and is therefore not a fixed value. For comparison purposes, it is common to record the event on first heating, melt the sample, cool at a chosen standard rate, and then reheat through the transition.

The DSC/DTA curve may show an exothermic or endothermic peak, as at EX and EN in the curve above. The enthalpy changes associated with the events occurring are given by the area under the peaks. In general, the heat capacity will also change over the region, and problems may arise in the correct assignment of the baseline. In many cases the change is small, and techniques have been developed for reproducible measurements in specific systems.

Peaks may be characterised by:

1. Position (i.e., start, end, extrapolated onset and peak temperatures)
2. Size (related to the amount of material and energy of the reaction)
3. Shape (which can be related to the kinetics of the process)

Some possible processes giving enthalpic peaks are listed below.

• Probably the commonest use of the DSC curve is in "fingerprinting", in which simple or complex materials can be compared for identification, or quality control purposes, using measurements of peak positions, sizes, or shapes as appropriate
• The temperature at which peaks occur can lead to an identification of a particular component, and the size (usually the area, though the height is sometimes used) can give a measure of the amount of that component. Examples include the determination of quartz in clays, which is difficult by other methods, and the analysis of polymer blends.
• Studying the DTA/DSC curves of mixtures can allow the construction of phase diagrams.
• Analysis of the form of the fusion peak of a fairly pure (>98%) substance can, with certain restrictions, lead to a determination of its purity. This approach is used routinely with pharmaceuticals and fine chemicals in general.
• Solid-solid reactions, such as those occurring in pyrotechnics, may be examined, as well as the decomposition characteristics of high explosives. The ability of DSC to handle small samples is clearly of benefit here. Provided the heating rate and/or sample size is kept small enough, the rapid "runaway" reaction typical of these materials can be carried out in a controlled manner, and studied in detail.
• Potentially hazardous substances or mixtures can be conveniently and safely screened by DSC using only small amounts of sample. Often, it is sufficient in the first instance to obtain an indication of the likelihood of an exothermic reaction, and then its magnitude and temperature of occurrence.
• The examination of the exothermic processes above, as well as those of a less energetic nature such as curing, may be amenable to kinetic analysis. A useful method is the subject of the standard ASTM E698, which has the virtue of an in built check on the derived parameters. Polymer crystallisation kinetic studies are usually carried out isothermally.
• The relative oxidative stability of materials is an important area of application, and can be used to determine the effectiveness of an antioxidant, or assess the amount present in e.g. oils, fats, waxes and polymers. The preferred method consists in raising the sample temperature to a pre-determined level, while in an inert atmosphere, then switching the atmosphere to air or oxygen. The time to the onset of exothermic reaction is measured. The experiment can be carried out under pressure (up to 100 bar) to avoid volatilisation of some materials.