A differential scanning calorimeter is a scientific instrument that is used to observe the heat capacity of a sample as a function of temperature. A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes. ...

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 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 A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes. ... Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. ... Heat (abbreviated Q, also called heat change) is the transfer of thermal energy between two bodies which are at different temperatures. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Temperature is the physical property of a system which underlies the common notions of hot and cold; the material with the higher temperature is said to be hotter. ... Heat capacity (abbreviated Cth or just C, also called thermal capacity) is the ability of matter to store heat. ... In physics, a phase transition is the transformation of a thermodynamic system from one phase to another. ... In chemistry, an exothermic reaction is one that releases heat. ... In chemistry, an endothermic reaction is one that requires heat to break the bonds of the reactants. ... Physics In physics, melting is the process of heating a solid substance to a point (called melting point) where it turns liquid. ... In physics, a phase transition is the transformation of a thermodynamic system from one phase to another. ... Crystal (disambiguation) Insulin crystals A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... A simplistic view of a materials glass transition temperature (Tg) is the temperature below which molecules have very little mobility. ...

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 General Name, Symbol, Number aluminium, Al, 13 Chemical series poor metals Group, Period, Block 13 (IIIA), 3, p Density, Hardness 2700 kg/m3, 2. ... An isothermal process is a thermodynamic process in which the temperature of the system stays constant; ΔT = 0. ...

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 tower of a personal computer (specifically a Power Mac G5). ... Thermodynamics (Greek: thermos = heat and dynamic = change) is the physics of energy, heat, work, entropy and the spontaneity of processes. ...

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 In physics, thermal conductivity, λ, is the quantity of heat transmitted, due to unit temperature gradient, in unit time under steady conditions in a direction normal to a surface of unit area, when the heat transfer is dependent only on the temperature gradient thermal conductivity = heat flow rate / (distance × temperature...

Heat Flux DSC

Figure1. Diagram of a heat flux differential scanning calorimeter

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 This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... An alloy is a combination, either in solution or compound, of two or more elements, at least one of which is a metal, and where the resultant material has metallic properties. ... Constantan is an alloy usually consisting of 55% of copper and 45% of nickel (Cu55Ni45). ... In electronics, thermocouples are a widely used kind of temperature sensor. ...

Power Compensated DSC

Figure 2. Diagram of a power compensated differential scanning calorimeter

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. This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... Transmission lines in Lund, Sweden Electric power, often known as power or electricity, involves the production and delivery of electrical energy in sufficient quantities to operate domestic appliances, office equipment, industrial machinery and provide sufficient energy for both domestic and commercial lighting, heating, cooking and industrial processes. ... The gram or gramme, symbol g, is a unit of mass, and is defined in the SI system of units as one one-thousandth of a kilogram (i. ... In thermodynamics, a thermodynamic system is in thermodynamic equilibrium if its energy distribution equals a Maxwell-Boltzmann-distribution. ... General Name, Symbol, Number Platinum, Pt, 78 Chemical series transition metals Group, Period, Block 10 , 6, d Density, Hardness 21. ... Electrical resistance is a measure of the degree to which an electrical component opposes the passage of current. ... A thermometer is a device used to measure temperatures or temperature changes. ...

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 An electrical network or electrical circuit is an interconnection of analog electrical elements such as resistors, inductors, capacitors, diodes, switches and transistors. ...

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:

ΔH = KA

where ΔH is the enthalpy of transition, K is the calorimetric constant, and A 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

Figure 3. A schematic DSC curve demonstrating the appearance of several common features

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 (T_g). In addition to these applications DSC can be used to study oxidation as well as other chemical reactions.^1,2,3 This image has been released into the public domain by the copyright holder, its copyright has expired, or it is ineligible for copyright. ... The melting point of a solid is the temperature at which it changes state from solid to liquid. ... Quartz crystal A crystal is a solid in which the constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions. ... The most fundamental reactions in chemistry are the redox processes. ...

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 An amorphous solid is a solid in which there is no long-range order of the positions of the atoms. ... The Pitch Drop Experiment at the University of Queensland. ...

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 (T_c). 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 (T_m). 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 Schlieren texture of Liquid Crystal nematic phase Liquid crystals are substances that exhibit a phase of matter that has properties between those of a conventional liquid, and those of a solid crystal. ... This article is being considered for deletion in accordance with Wikipedias deletion policy. ... Isotropic means independent of direction. Isotropic radiation has the same intensity regardless of the direction of measurement, and an isotropic field exerts the same action regardless of how the test particle is oriented. ...

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 General Name, Symbol, Number Nitrogen, N, 7 Chemical series nonmetals Group, Period, Block 15 (VA), 2, p Density 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 Pharmacology (in Greek: pharmacon is drug, and logos is science) is the study of how chemical substances interfere with living systems. ... A polymer is a generic term used to describe a substantially long molecule. ... In polymer chemistry and process engineering, curing refers to the toughening or hardening of a polymer material by cross-linking of polymer chains, brought about by chemical additives, ultraviolet radiation or heat. ... Categories: Chemistry stubs | Chemistry ...

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 Many drugs are provided in tablet form. ...

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 Freezing-point depression is the difference between the freezing points of a pure solvent and a solution of a nonelectrolyte in that solvent. ... A substance is soluble in a fluid if it dissolves in the fluid. ... Dissolving table salt in water In chemistry, a solution is one or more substance (the solute) dissolved in another substance (the solvent) forming a homogenous mixture. ...

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 A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes. ...

Thermal analysis Thermal analysis is a branch of materials science where the properties of materials are studied as they change with temperature. ...