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| {{Infobox chemical analysis
| | With fall slipping by and winter on its way, often yourself reminiscing about the golden events of summer. Why not put that inspiration to use? Now is the time to organize your outdoor makeover - next summer your backyard could become the perfect second area. Redoing your outdoor living space will furthermore give the perfect destination for a relax when the warm weather arrives, it'll add value to real estate. By adding an outdoor room you are effectively an individual's home's size.<br><br> |
| | name = Differential Scanning Calorimetry
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| | image = Inside DSC small.jpg
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| | caption =Differential Scanning Calorimeter
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| | acronym = DSC
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| | classification =[[Thermal analysis]]
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| | analytes =
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| | manufacturers = Linseis Thermal Analysis, [[Mettler Toledo]], Netzsch, Shimadzu, PerkinElmer, Setaram Instrumentation, [http://www.tainstruments.com TA Instruments]
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| | related = [[Isothermal microcalorimetry]]<br>[[Isothermal titration calorimetry]]<br>[[Dynamic mechanical analysis]]<br>[[Thermomechanical analysis]]<br>[[Thermogravimetric analysis]]<br>[[Differential thermal analysis]]<br>[[Dielectric thermal analysis]]
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| | hyphenated =
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| }}
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| '''Differential scanning calorimetry''' or '''DSC''' is a [[thermal analysis|thermoanalytical]] '''technique''' in which the difference in the amount of [[heat]] required to increase the [[temperature]] of a sample and reference is 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.
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| The technique was developed by E.S. Watson and M.J. O'Neill in 1962,<ref>{{US patent|3263484}}</ref> and introduced commercially at the 1963 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. The first [[adiabatic]] differential scanning calorimeter that could be used in biochemistry was developed by P.L. Privalov and D.R. Monaselidze in 1964.<ref>{{cite book|year=1975|title=Molecular biology, vol.6|place=Moscow |pages=7–33 ''(in russian)''}}</ref> The term DSC was coined to describe this instrument which measures energy directly and allows precise measurements of heat capacity.<ref>{{cite book|author=Wunderlich, B. |year=1990|title=Thermal Analysis|place=New York|publisher=Academic Press|pages=137–140|isbn=0-12-765605-7}}</ref>
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| ==Detection of phase transitions==
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| The basic principle underlying this technique is that when the sample undergoes a physical transformation such as [[phase transition]]s, more or less heat will need to flow to it than the reference to maintain both at the same temperature. Whether less or more heat must flow to the sample depends on whether the process is [[exothermic reaction|exothermic]] or [[endothermic reaction|endothermic]]. For example, as a solid sample [[melting|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 [[calorimeter]]s are able to measure the amount of heat absorbed or released during such transitions. DSC may also be used to observe more subtle physical changes, such as [[glass transition]]s. It is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing.<ref name=Dean>{{cite book|author=Dean, John A.|title=The Analytical Chemistry Handbook|place=New York|publisher=McGraw Hill, Inc.|year=1995|pages=15.1–15.5|isbn=0-07-016197-6}}</ref><ref name=Pugnor>{{cite book|author=Pungor, Erno|title=A Practical Guide to Instrumental Analysis|publisher=Boca Raton|place=Florida|year=1995|pages=181–191}}</ref><ref name=Skoog>{{cite book|author=Skoog, Douglas A., F. James Holler and Timothy Nieman|title=Principles of Instrumental Analysis|edition=5|place=New York|year=1998|pages=805–808|isbn=0-03-002078-6}}</ref>
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| ==DTA==
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| 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 measures the energy required to keep both the reference and the sample at the same temperature whereas DTA measures the difference in temperature between the sample and the reference when they are both put under the same heat.
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| ==DSC curves==
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| The result of a DSC experiment is a curve of heat flux versus temperature or versus time. There are two different conventions: exothermic reactions in the sample shown with a positive or negative peak, depending on the kind of technology used in the experiment. This curve can be used to calculate [[enthalpy|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:
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| <math>\Delta H = K A</math>
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| 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 calorimetric constant will vary from instrument to instrument, and can be determined by analyzing a well-characterized sample with known enthalpies of transition.<ref name=Pugnor/>
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| ==Applications==
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| Differential scanning calorimetry can be used to measure a number of characteristic properties of a sample. Using this technique it is possible to observe [[melting point|fusion]] and [[crystal]]lization events as well as [[glass transition]] temperatures ''T<sub>g</sub>''. DSC can also be used to study [[oxidation]], as well as other chemical reactions.<ref name=Dean/><ref name=Pugnor/><ref>{{cite journal|author=M. J. O'Neill|title=The Analysis of a Temperature-Controlled Scanning Calorimeter|journal=Anal. Chem.|year=1964|volume=36|pages=1238–1245|doi=10.1021/ac60213a020|issue=7}}</ref>
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| [[Glass transition]]s may occur as the temperature of an [[amorphous]] solid is increased. These transitions appear as a step in the baseline of the recorded DSC signal. This is due to the sample undergoing a [[Kauzmann paradox|change in heat capacity]]; no formal phase change occurs.<ref name=Dean/><ref name=Skoog/>
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| As the temperature increases, an amorphous solid will become less [[viscosity|viscous]]. At some point the molecules may obtain enough freedom of motion to spontaneously arrange themselves into a crystalline form. This is known as the [[crystallization|crystallization temperature]] (''T<sub>c</sub>''). 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<sub>m</sub>''). The melting process results in an endothermic peak in the DSC curve. The ability to determine [[transition temperature]]s and [[enthalpy|enthalpies]] makes DSC a valuable tool in producing [[phase diagram]]s for various chemical systems.<ref name=Dean/>
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| [[Image:CalibratingDSCcurve.png|frame|Top: A schematic DSC curve of amount of energy input (y) required to maintain each temperature (x), scanned across a range of temperatures. Bottom: Normalized curves setting the initial heat capacity as the reference. Buffer-buffer baseline (dashed) and protein-buffer variance (solid).]]
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| [[Image:2and3stDSCcurves.png|frame| Normalized DSC curves using the baseline as the reference (left), and fractions of each conformational state (y) existing at each temperature (right), for two-state (top), and three-state (bottom) proteins. Note the minuscule broadening in the peak of the three-state protein's DSC curve, which may or may not appear statistically significant to the naked eye.]]
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| ==Examples==
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| The technique is widely used across a range of applications, both as a routine quality test and as a research tool. The equipment is easy to calibrate, using low melting [[indium]] at 156.5985 °C for example, and is a rapid and reliable method of thermal analysis.
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| ===Polymers===
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| DSC is used widely for examining polymeric materials to determine their thermal transitions. The observed thermal transitions can be utilized to compare materials, however, the transitions do not uniquely identify composition. Composition of unknown materials may be completed using a technique such as IR. [[Melting point]]s and [[glass transition temperature]]s for most [[polymer]]s are available from standard compilations, and the method can show [[polymer degradation]] by the lowering of the expected melting point, ''T<sub>m</sub>'', for example. ''T<sub>m</sub>'' depends on the [[molecular weight]] of the polymer and thermal history, so lower grades may have lower melting points than expected. The percent crystalline content of a polymer can be estimated from the crystallization/melting peaks of the DSC graph as reference heats of fusion can be found in the literature.<ref>B. Wunderlich, Macromolecular Physics, (1980), Vol. 3, Ch. 8, Table VIII.6.</ref> DSC can also be used to study thermal degradation of polymers using an approach such as Oxidative Onset Temperature/Time (OOT), however, the user risks contamination of the DSC cell, which can be problematic. [[Thermogravimetric Analysis]] (TGA) may be more useful for decomposition behavior determination. Impurities in polymers can be determined by examining thermograms for anomalous peaks, and [[plasticiser]]s can be detected at their characteristic boiling points. In addition, examination of minor events in first heat thermal analysis data can be useful as these apparently "anomalous peaks" can in fact also be representative of process or storage thermal history of the material or polymer physical aging. Comparison of first and second heat data collected at consistent heating rates can allow the analyst to learn about both polymer processing history and material properties.
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| ===Liquid crystals===
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| DSC is used in the study of [[liquid crystal]]s. As some forms of matter go from solid to liquid they go 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.<ref name=Pugnor/>
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| ===Oxidative stability===
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| Using differential scanning calorimetry to study the stability to [[oxidation]] 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 analysis can be used to determine the stability and optimum storage conditions for a material or compound.<ref name=Dean/>
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| ===Safety Screening===
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| DSC makes a reasonable initial safety screening tool. In this mode the sample will be housed in a non-reactive crucible (often [[gold]], or gold plated steel), and which will be able to withstand [[pressure]] (typically up to 100 [[bar (unit)|bar]]). The presence of an [[exothermic]] event can then be used to assess the [[chemical stability|stability]] of a substance to heat. However, due to a combination of relatively poor sensitivity, slower than normal scan rates (typically 2-3 °/min - due to much heavier crucible) and unknown [[activation energy]], it is necessary to deduct about 75-100 °C from the initial start of the observed exotherm to ''suggest'' a maximum temperature for the material. A much more accurate data set can be obtained from an [[Calorimeter#Adiabatic_calorimeters|adiabatic calorimeter]], but such a test may take 2–3 days from [[Ambient temperature|ambient]] at a rate of a 3 °C increment per half hour.
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| ===Drug analysis===
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| DSC is widely used in the [[pharmaceutical]] and [[polymer]] industries. For the polymer chemist, DSC is a handy tool for studying [[curing (chemistry)|curing]] processes, which allows the fine tuning of polymer properties. The [[cross-link]]ing 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.<ref name=Dean/><ref name=Pugnor/><ref name=Skoog/>
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| In the pharmaceutical industry it is necessary to have well-characterized [[Medication|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.<ref name=Pugnor/>
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| ===General chemical analysis===
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| [[Freezing-point depression]] can be used as a '''purity analysis''' tool when analysed by differential scanning calorimetry. This is possible because the temperature range over which a mixture of compounds melts is dependent on their relative amounts. Consequently, less pure compounds will exhibit a broadened melting peak that begins at lower temperature than a pure compound.<ref name=Pugnor/><ref name=Skoog/>
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| ==See also==
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| {{colbegin|3}}
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| *[[Chemical thermodynamics]]
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| *[[Calorimetry]]
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| *[[Endothermic]]
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| *[[Exothermic]]
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| *[[Forensic engineering]]
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| *[[Forensic polymer engineering]]
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| *[[Glass transition temperature]]
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| *[[Phase transitions]]
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| *[[Polymer]]
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| {{colend}}
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| ==References==
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| {{reflist|2}}
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| ==External links==
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| *[http://pslc.ws/macrog/dsc.htm www.pslc.ws/macrog/dsc.htm] Differential Scanning Calorimetry information from the Polymer Science Learning Center
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| ==Bibliography==
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| *Brydson, J A, ''Plastics Materials'', Butterworth-Heinemann, 7th Ed (1999).
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| *Ezrin, Meyer, ''Plastics Failure Guide: Cause and Prevention'', Hanser-SPE (1996).
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| *Wright, D.C., ''Environmental Stress Cracking of Plastics'' RAPRA (2001).
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| *Lewis, Peter Rhys, and Gagg, C, ''Forensic Polymer Engineering: Why polymer products fail in service'', Woodhead/CRC Press (2010).
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| {{DEFAULTSORT:Differential Scanning Calorimetry}}
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| [[Category:Materials science]]
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| [[Category:Biophysics]]
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| [[Category:Scientific techniques]]
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| [[Category:Calorimetry]]
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