Differential scanning calorimeter

A differential scanning calorimeter is a scientific instrument that is used to observe the heat capacity of a sample as a function of temperature.

Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. Both the sample and reference are maintained at very nearly the same temperature throughout the experiment. Generally, the temperature program for a DSC analysis is designed such that the sample holder temperature increases linearly as a function of time. The reference sample should have a well-defined heat capacity over the range of temperatures to be scanned. The basic principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transitions, more (or less) heat will need to flow to it than the reference to maintain both at the same temperature. Whether more or less heat must flow to the sample depends on whether the process is exothermic or endothermic. For example, as a solid sample melts to a liquid it will require more heat flowing to the sample to increase its temperature at the same rate as the reference. This is due to the absorption of heat by the sample as it undergoes the endothermic phase transition from solid to liquid. Likewise, as the sample undergoes exothermic processes (such as crystallization) less heat is required to raise the sample temperature. By observing the difference in heat flow between the sample and reference, differential scanning calorimeters are able to measure the amount of energy absorbed or released during such transitions. DSC may also be used to observe more subtle phase changes, such as glass transitions. DSC is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing^1,2,3

An alternative technique, which shares much in common with DSC, is differential thermal analysis (DTA). In this technique it is the heat flow to the sample and reference that remains the same rather than the temperature. When the sample and reference are heated identically phase changes and other thermal processes cause a difference in temperature between the sample and reference. Both DSC and DTA provide similar information; DSC is the more widely used of the two techniques.^1,2,3

DSC Instrumentation

A typical differential scanning calorimeter consists of two sealed pans: a sample pan and a reference pan (which is generally an empty sample pan). These pans are often covered by or composed of aluminum, which acts as a radiation shield.¹ The two pans are heated, or cooled, uniformly while the heat flow difference between the two is monitored. This can be done at a constant temperature (isothermally), but is more commonly done by changing the temperature at a constant rate, a mode of operation that is also called temperature scanning.¹

During the experiment, the instrument detects differences in the heat flow between the sample and reference. This information is sent to an output device, most often a computer, and results in a plot of the differential heat flow between the reference and sample cell as a function of temperature. When there are no thermodynamic chemical processes occurring the heat flow difference between the sample and reference varies only slightly with temperature, and shows up as a flat, or very shallow base line on the plot. However, an exothermic or endothermic process within the sample results in a significant deviation in the difference between the two heat flows. The result is a peak in the DSC curve. Generally, the differential heat flow is calculated by subtracting the sample heat flow from the reference heat flow. When following this convention, exothermic processes will show up as positive peaks (above the baseline) while peaks resulting from endothermic processes are negative (below the baseline).¹

The sample (in a condensed form such as powder, liquid, or crystal) is generally placed in an aluminum sample pan, which is then placed in the sample cell. The reference consists of a matched empty aluminum sample pan that is placed in the reference cell of the instrument. The sample pans are designed to have a very high thermal conductivity. Sample sizes generally range from 0.1 to 100 mg. The instrument cells are often airtight in order to shield the sample and reference from external thermal perturbations. This also allows experiments to be performed under variable pressures and atmospheres.¹

Heat Flux DSC

There are two main types of differential scanning calorimeters: heat flux DSC and power compensation DSC. In a heat flux calorimeter, heat is transferred to the sample and reference through a disk made of the alloy constantan. The heat transported to the sample and reference is controlled while the instrument monitors the temperature difference between the two. In addition to its function in the heat transfer, this disk serves as part of the temperature-sensing unit. The sample and reference reside on raised platforms on the disk. Under each of these platforms there is a chromel (chromel is an alloy containing chromium, nickel and sometimes iron) wafer. The junction between these two alloys forms a chromel-constantan thermocouple. The signal from these sensors is the used to measure the differential heat flow. The temperature is typically monitored by chromel-alumel thermocouples attached beneath the chromel wafers.^1,3

Power Compensated DSC

In power compensated calorimeters, separate heaters are used for the sample and reference. This is the classic DSC design pioneered by the Perkin-Elmer® (http://www.perkinelmer.com) company. Both the sample and reference are maintained at the same temperature while monitoring the electrical power used by their heaters. The heating elements are kept very small (weighing about 1 gram) in order to ensure that heating, cooling, and thermal equilibration can occur as quickly as possible. The sample and reference are located above their respective heaters, and the temperatures are monitored using electronic temperature sensors located just beneath the samples. Generally platinum resistance thermometers are used due to the high melting point of platinum.

Electronically, the instruments consist of two temperature control circuits. An average temperature control circuit is used to monitor the progress of the temperature control program. This circuit is designed to assure that the temperature scanning program set by the operator is the average temperature of the sample and reference. A differential temperature control circuit is used to determine the relative temperatures of the sample and reference, and adjust the power going to the respective heaters in such a way as to maintain both at the same temperature. The output of the differential temperature control circuit is used to generate the DSC curve.^1,3

DSC Curves

The result of a DSC experiment is a heating or cooling curve. This curve can be used to calculate enthalpies of transitions. This is done by integrating the peak corresponding to a given transition. It can be shown that the enthalpy of transition can be expressed using the following equation:

<math>\Delta H = K A<math>

where <math>\Delta H<math> is the enthalpy of transition, <math>K<math> is the calorimetric constant, and <math>A<math> is the area under the curve. The calometric constant will vary instrument to instrument, and can be determined by analyzing a well-characterized sample with known enthalpies of transition.²

Applications

Differential scanning calorimetry can be used to measure a number of characteristic parameters of a sample. Using this technique it is possible to observe fusion and crystallization events as well as glass transition temperatures (<math>T_g<math>). In addition to these applications DSC can be used to study oxidation as well as other chemical reactions.^1,2,3

Glass transitions occur as the temperature of an amorphous solid is increased. A glass transition is characterized by a decrease in viscosity. These transitions appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing a change in heat capacity, but no formal phase change occurs.^1,3

As the temperature increases, an amorphous solid will become less viscous. At some point the molecules will obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This is known as the crystallization temperature (<math>T_c<math>). This transition from amorphous solid to crystalline solid is an exothermic process, and results in a peak in the DSC signal. As the temperature increases the sample eventually reaches its melting temperature (<math>T_m<math>). The melting process results in an endothermic peak in the DSC curve. The ability to ascertain transition temperatures and enthalpies makes DSC an invaluable tool in producing phase diagrams for various chemical systems.¹

DSC may also be used in the study of liquid crystals. As matter transitions between solid and liquid it often goes through a third state, which displays properties of both phases. This anisotropic liquid is known as a liquid crystalline or mesomorphous state. Using DSC, it is possible to observe the small energy changes that occur as matter transitions from a solid to a liquid crystal and from a liquid crystal to an isotropic liquid.²

Using differential scanning calorimetry to study the oxidative stability of samples generally requires an airtight sample chamber. Usually, such tests are done isothermally (at constant temperature) by changing the atmosphere of the sample. First, the sample is brought to the desired test temperature under an inert atmosphere, usually nitrogen. Then, oxygen is added to the system. Any oxidation that occurs is observed as a deviation in the baseline. Such analyses can be used to determine the stability and optimum storage conditions for a compound.¹

DSC is widely used in the pharmaceutical and polymer industries. For the polymer chemist, DSC is a handy tool for studying curing processes, which allows the fine tuning of polymer properties. The cross-linking of polymer molecules that occurs in the curing process is exothermic, resulting in a positive peak in the DSC curve that usually appears soon after the glass transition.^1,2,3

In the pharmaceutical industry it is necessary to have well-characterized drug compounds in order to define processing parameters. For instance, if it is necessary to deliver a drug in the amorphous form, it is desirable to process the drug at temperatures below those at which crystallization can occur.²

DSC curves may also be used to evaluate drug and polymer purities. This is possible because the temperature range over which a mixture of compounds melts is dependent on their relative amounts. This is effect is due to a phenomenon known as freezing point depression, which occurs when a foreign solute is added to a solution. (Freezing point depression is what allows salt to de-ice sidewalks and antifreeze to keep your car running in the winter.) Consequently, less pure compounds will exhibit a broadened melting peak that begins at lower temperature than a pure compound.^2,3

References

1. Dean, John A. The Analytical Chemistry Handbook. New York. McGraw Hill, Inc. 1995. p15.1-15.5
2. Pungor, Erno. A Practical Guide to Instrumental Analysis. Boca Raton, Florida. 1995. p181-191.
3. Skoog, Douglas A., F. James Holler and Timothy Nieman. Principles of Instrumental Analysis. Fith Edition. New York. 1998. p805-808.

See Also

Calorimeter

Thermal analysis