Differential scanning Colorimetry (DSC).

Differential scanning Calorimetry
Mrs. Poonam Sunil Aher (M.Pharm, PhD)
Assistant Professor
Sanjivani College of Pharmaceutical Education and
Research (Autonomous),
Kopargaon, Ahmednagar-423603 (M.S.), INDIA

Thermal Analysis
• Thermal analysis includes a group of techniques
in which specific physical properties of a material
are measured as a function of temperature.
• Thermal Analysis is useful in both quantitative
and qualitative analysis.
• In Qualitative analysis samples are identified and
characterized by thermal behavior.
• In Quantitative analysis results are recorded with
changes of thermal reaction in weight and
enthalpy

Summary Thermal Analysis Techniques
Technique Quantity Measured Application
Differential scanning
Calorimetry (DSC)
Heat and temperatures of
transitions and reactions
Study kinetic reactions
Purity analysis
Polymer study
Polymer curves study
Differential thermal
Analysis ( DTA)
temperatures of
transitions and reactions
Phase diagram
Thermal stability
Thermo gravimetric
analysis (TGA)
Weight changes Thermal stability
Compositional analysis
Thermomechanical
Analysis (TMA)
Dimensions and viscosity
changes
Study of temperatures
Expansion coefficients
Dynamic mechanical
analysis (DMA)
Modules , damping and
viscoelastic behavior
Impact resistance
Mechanical stability
Evolved gas analysis( EGA) Amount of gaseous
products of thermally
induced reactions
Analysis of volatile organic
components of shale

Differential scanning Calorimetry

• Differential scanning calorimetry, or DSC, is a thermo
analytical 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.

History
• The technique was developed by E. S. Watson and M. J.
O'Neill in 1962, 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 at Institute of
Physics in Tbilisi, Georgia.
• The term DSC was coined to describe this instrument,
which measures energy directly and allows precise
measurements of heat capacity.

Principle
• 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 less or more 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 heat absorbed or released during such transitions.
• DSC may also be used to observe more suitable
physical changes, such as glass transitions. It is widely
used in industrial settings as a quality control
instrument due to its applicability in evaluating sample
purity and for studying polymer.

• DSC, change in enthalpy ∆H of the sample is
equal to the difference between the heat flow
of sample and reference
• ∆H= Q s- Q r
• According to thermal analogs of Ohm’s law,
• Q = T2-T1
Rth
Where Q s and Q r is heat flow from sample and
reference
Rth is thermal resistance

• The heat flow is proportional to the driving force.
• Driving force is the force required for
temperature difference T1 anT2
• Heat flow is inversely proportional to thermal
resistance
• ∆H= Q s- Q r = Te- Ts _ Te – T r
R th R th
Where Te is external temperature
• ∆H = _ Ts - T r
R th
The measured signal of voltage from thermocouple
is proportional to (Ts – T r )

Instrumentation
• A typical DSC cell uses a constantan (Cu-Ni) disk
as the primary means of transferring heat to the
sample and reference positions and also as one
element of temperature sensing thermoelectric
junctions.
• The sample and reference are placed in separate
pans that sit on raised platforms on the disk.
• Heat is transferred to the sample and reference
through constantan disk

• The differential heat flow to the the samples and
reference is monitored by the chromel or constantan
thermocouples. Which are formed by the junction of
the constantan disk and the chromel wafer covering
the underside of each platforms.
• Chromel and aluminium wires are connected to the
underside of the wafers from thermocouple, which
directly monitor the sample temperature.
• Constant calorimetric sensitivity is maintained by
computer software, which linearizes cell calibration
coefficients
• DSC provides maximum calorimetric accuracy from -
170 to 750 ◦ C
• Sample size ranges from 0.1 to 100 mg
• It measure enthalpy.

DSC curves
• 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 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:
∆H =KA
• where ∆H is the enthalpy of transition,
• K is the calorimetric constant, and A is the area under the
curve.

Instrumentation Basics:
• Types of DSC Instruments:
1. Power compensated DSC
2. Heat flux DSC
3. Modulated DSC
4. High sensitivity DSC (HS-DSC)
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• In power compensated DSC, the base of the sample holder unit is in direct
contact with a reservoir of coolant.
• Resistance sensor measures the temperature of the base of the holder and a
resistance heater.
• Phase changes in sample temperature difference is detected
electrical power gets supplied to bring temperature difference below
threshold value.
• Temperatures of both are increased or decreased linearly.
• The power needed to maintain the sample temperature equal to the
reference temperature is measured.
 Advantages over Heat flux DSC:
 Its response time is more rapid than heat flux DSC.
 It is also capable of higher resolution.
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• In heat flux DSC, the difference in the heat flow into the sample and
reference is measured while the sample temperature is changed at a
constant rate.
• Both sample and reference are heated by a single heating unit.
• Heat flows into both the sample and reference material via an
electrically heated constant thermoelectric disk is given by
where,
f (T, t) is the kinetic response of the sample in J mol ˉˡ
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3.Modulated DSC Instruments (MDSC) :
• In MDSC, a sinusoidal function is superimposed on the overall temperature
program.
• And produces a micro heating and cooling cycle as the overall temperature
is increased or decreased.
• The overall signal is mathematically deconvoluted into two parts
1. A reversing heat flow signal- Associated with the heat capacity component
of the thermogram .
2. Non- reversing heat flow signal- Associated with the kinetic processes.
 Step transitions appear only in the reversing heat flow signal.
 Exothermic and endothermic events may appear in either or in both
signals.
• It can be useful in resolving polymer glass transitions, which are difficult to
analyze owing to concomitant solvent evaporation.
23

Sample Preparation
• If possible, clean and dry your sample prior to DSC analysis.
• Wear gloves or use forceps when handling your DSC sample.
• DSC samples should be small enough.
• Powder samples or flat solid samples (less than 2 mm tall) work best.
• Sample weight should be between 0.5 and 100 mg.
DSC sample preparation procedures :
• Weigh sample about 0.5 mg with the analytical balance. Record the weight.
• Use forceps to place sample.
• Use forceps to place the aluminium pan lid on top of your sample.
• Use forceps to load the aluminium pan and sample into the sample encapsulating
press .
• Align the sample pan in the encapsulating press, and press down on the handle to
seal the aluminium pan.
• Use the empty pan as a reference sample.
• Need inert gas like nitrogen to avoid oxidation or decomposition.
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Precaution :
Sample Decomposition
 The DSC is capable of heating samples to 600°C.
 Many materials may decompose during the heating, which can
generate hazardous by-products.
WARNING: If you are using samples that may emit harmful gases,
vent the gases in an appropriate manner. In general, samples
should not be heated above their decomposition temperatures to
prevent the release of hazardous materials.
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Instrument Operation
• Powering up the instrument,
• Loading your sample,
• Setting your testing conditions,
• Running a scan and collecting
data
• Analyzing your results.
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Applications
 Quantitative application includes the determination of heat of fusion and the extent
of crystallization for crystalline materials.
 Glass transition temperatures and melting points are useful for qualitative
classification of materials.
 DSC is used to study ‘aging’ and shelf life of pharmaceuticals, as well as other basic
research and development.
 In the Food Industry, DSC has numerous applications to monitor thermal events
discussed earlier such as melting, crystallization, etc, as well as decomposition,
denaturation , dehydration, polymorphism, oxidation, etc.
 Evaluation of metal catalysts by pressure DSC .
 Measurement of heat capacity
 Measurement of thermal conductivity
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Differential scanning Colorimetry (DSC).

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