Thermal methods of Analysis

THERMAL METHODS OF ANALYSIS
Rohan Jagdale
Pharmaceutical Analysis II
T. Y. B. Pharm
YTIP, University Of Mumbai
❏ Thermogravimetry (TG)
❏ Differential thermal analysis (DTA)
❏ Differential scanning calorimetry (DSC)

Introduction
▪Thermal analysis is a branch of materials science where the properties of materials are
studied as they change with temperature.
▪Any measurement of a change in properties as the temperature changes qualifies as a
thermal analysis technique
▪When performing thermal analysis procedures, usually a controlled temperature program
heats or cools a sample at a certain rate, and physical or chemical property changes are
monitored as a function of temperature.
▪Thermal analysis methods are well-established techniques in research laboratories of
pharmaceutical industry.
▪Thermal analysis techniques give useful information about polymers, inorganic
compounds, alloys, drugs, and other organic materials.

Types of Thermal analysis
Thermal analysis
Mass Temperature or heat flow
Other parameters
Eg. Dimension
⬇
Thermogravimetry (TG)
⬇
Differential thermal
analysis (DTA)
⬇
Differential scanning
calorimetry (DSC)
Thermomechnical
analysis TMA
Dynamic mechanic
analysis (DMA)
Thermo optical
analysis (TOA)
Thermosonimetry
(TS)
⬇

Sr. No. Name of technique Instrument
employed
Parameter
measured
Graph
1 Thermogravimetry (TG) Thermobalance Mass Mass Vs. Temp or
time
2 Differential thermal analysis
(DTA)
DTA Analyzer or
DTA apparatus
ꕔT ꕔT Vs temp
3 Differential scanning
calorimetry (DSC)
Calorimeter dH, dt dH/dt vs. Temp

Introduction
▪Thermogravimetric analysis (TGA) is a thermoanalytical technique where a
thermo-balance (a combination of an electronic microbalance with a furnace and
appropriate temperature controller) measures changes in sample mass.
▪This technique can be use to study the material weight loss due to e.g.
decomposition,oxidation, or loss of volatiles, such as moisture in a set temperature
range, giving typically a temperature (or time)/mass (or mass percentage) plot.
▪Thermogravimetric Analysis is a technique in which the mass of a substance is
monitored as a function of temperature or time as the sample specimen is
subjected to a controlled temperature program in a controlled atmosphere.

Defination
lt is a technique whereby the weight of a
substance, in an environment heated or
cooled at a controlled rate, is recorded as a
function of time or temperature.

Types of thermogravimetry
▪ Dynamic TGA: In this type of analysis, the sample is subjected to
condition of continuous increase in temperature usually linear with time.
▪Isothermal or Static TGA: In this type of analysis, sample is
maintained at a constant temperature for a period of time during which
change in weight is recorded.
▪Quasistatic TGA: In this technique sample is heated to a constant
weight at each of a series of increasing temperature.

Principle
▪ In thermogravimetric analysis, the sample is heated in a given environment
(air, N2, Co2, He, Ar, etc.) at controlled rate.
▪ The change in the weight of the substance is recorded as a function of
temperature or time.
▪The temperature is increased at a constant rate for a known initial weight of
the substance and the changes in weights are recorded as a function of
temperature at different time interval.
▪ This plot of weight change against temperature is called thermogravimetric
curve or thermogram, this is the basic principle of TGA.

Thermogravimetric curve
▪The instrument used for themogravimetry is a programmed precision
balance for rise in temperature known as Thermobalance.
▪ Results are displayed by a plot of mass change versus temperature
or time and are known as Thermogravimetric curves or TG curves.
▪TG cunves are normally plotted with the mass change (Dm) in
percentage on the y-axis and temperature (T) or time (t) on the x-axis.

Information from TG curve
▪Types of TGA curve
o TG curves are classified according to their shapes into seven types.
▪Type A- this curves shows no mass change over the entire range of temperature. It
can be concluded that the decomposition temperature for sample is greater than the
temperature range of instrument
▪Type B- this curves shows that there is large mass loss followed by mass plateau
and is formed when evaporation of volatil e product(s) during drying desorption or
polymerization takes place. If a non-interacting atmosphere is present in the
chamber, type B curve will change into type A curve.
▪Type C-this curve shows the single-stage decomposition temperatures (Ti and Tf).

▪Type D-this curve shows the multi-stage decomposition processes
where reaction is resolved.
▪Type E-this curve shows the multi-stage decomposition reaction wherereaction is
not resolved.
▪Type F-this curve shows the increase in mass in the presence of an interacting
atmosphere e.g. surface oxidation reactions.
▪Type G-this curve shows multiple reactions one after the other e.g. surface
oxidation reaction followed by decomposition of reaction product(s).

Instrumentation of Thermogravimetry

Block diagram of Thermobalance

The balance
▪ The balance is the most important component of thermobalance.
▪ A good balance must fulfil
▪ Accuracy, sensitivity, reproducibility and capacity should be similar to those of
analytical balance.
▪Should have an adequate range of automatic weight adustment.
▪ Should have high degree of mechanical and electronic stability.
▪ Should have rapid response to weight changes
▪Should be unaffected by vibration.
▪Simple to operate and versatile.

Types of recordings balances: 1.Deflection type
▪These are of following types :-
▪Beam type - in these balances, the conversion of defiected beams takes place into
the weight change.
▪Helical type - in these balances, elongation or contraction of spring occurs with
change in weight which is recorded by the help of transducers.
▪The cantilevered beam - in these balances, one end of beam is fixed and on other end
sample is placed. It undergoes deflection which can be recorded by the help of
photographic recorded trace
▪Torsion wire- In these balances, the beam is attached to hard torsion wire which acts
as fulcrum. The wire is attached to one or both ends of balance to make the deflection
of beam proportional to weight changes.

2.Null type balances
▪ It has sensor to detect the deviation of the balance from its null position.
▪ Then a restoring force is applied ( electrical or mechanical) to the beam to
restore its null position.
▪This force is proportional to weight change.

Sample Holder
▪The geometry, size and material with which it is made have an important effect on the TGA Curve.
▪Materials used for construction are glass, quartz, alumina, stainless steel, graphite, etc.
▪ Types:
▪Shallow pans- :-used for substances where it becomes necessary to eliminate diffusion as rate
controlling step. the sample is placed after forming a thin layer of it so that as soon as volatile
Substance is formed, it will escape.
▪ Deep crucibles :-These are used in such cases where side reactions are required such as in study of
industrial scale calcinations, surface area measurements, etc.
▪Loosely covered crucibles :-These are used in self-generated atmospheric studies. Rate of
temperature or weight loss is not important because the studies are done isothemally.
▪Retort cups:- These are used in boiling point studies. It provides single plat of reflux for a boiling
point determination

Furnace
▪ The furnace and control system( furnace controller) should be designed to
produce a linear heating rate over the whole working temperature range of
furnace.
▪ The furnace heating coil should be wound in such a way that there is no
magnetic interaction between coil and sample or there can cause apparent mass
change.
▪ Coils used are made of different materials with variant temperature changes viz.
Nichrome wire or ribbon for T<1300 K, Platinum for T>1300 K, Platinum-10%
rhodium Alloy for T<1800 K and Silicon Carbide for Te1800 K.

▪ The size of furnace is important. A high mass furnace may have a high
range of temperature and obtain uniform hot zone but requires more time to
achieve the desired temperature. Comparatively, a low mass furnace may
heat quickly but it's very difficult to control rise in temperature and maintain
hot zone.
▪ The position of furnace is also important.
▪Quartz spring balance has the weighing system below the furnace but the
beam balance has weighing system at several positions.

Temperature measurement
▪ lt is done with the help of thermocouple.
▪ Different materials are used for measuring different ranges of
temperatures i.e.
▪ Chromel or alumel (alloys of Platinum) for T=11000C.
▪tungsten or rhenium thermocouples are used for higher
temperature.

▪The position of thermocouple is important. It can be adjusted in following
ways:-
▪Thermocouple is placed near the sample container and has no contact with
sample container. This arrangement in not preferred in low-pressures.
▪The sample is kept inside the sample holder but not in contact with it. It
responds to small temperature changes only.
▪ Thermocouple is placed either in contact with sample or with sample
container. This method is best and commonly employed.

Recorder
O Two types:
O Time - based potentiometric strip chart recorder, and
O X-Y recorders. We get ourves having plot of weights directly against
temperatures
O In some light- beam -galvanometer, photographic paper recorders or one
recorder with two or more pens are used.

Thermobalance
▪Capable of recording continuously the wt changes of the sample as function of
time and temperature.
▪ Should cover wide range of temperature.
▪ Temperature and weight loss should be recorded to an accuracy range of better
than +/ 1.
▪ Linear heating should be there.
▪ Radiation and convection currents, and magnetic effects due to furnace heaters
must not affect the weighing system.
Points to be taken in mind while purchasing a Thermobalance

▪ Sensitivity of the balance should be commensurate with the size of the samples
being used.
▪ There should not ne any chemical attacks of volatile products on the apparatus.
▪ Crucible should be located within the hot zone.
▪Balance has to be protected from furnace.
▪ Capable of adjusting various speeds of the chart that is being used to record the
mass lose or temperature rise.
▪ Should facilitate rapid heating or cooling of the furnace to record several TG
curves in short span of time.

Atmosphere controller
OTo stop the reaction of gases present in the furnace with the
sample atmospheric controller is required.
O Inert gases will be circulated through that atmosphere to stop the
reactions.

Factors affecting Thermogravimetric curve
Two main types :-
1. Instrumental effects or factors
2. The characteristics of samples

Instrumental factors
▪Heating rate:
O The temperature at which the compound (or sample) decompose
depends upon the heating rate.
o When the heating rate is high, the decomposition temperature is also high.
O A heating rate of 3.5 C per minute is usually recommended for reliable and
reproducible TGA
▪Furnace atmosphere:
O The atmosphere inside the furnace surrounding the sample has a
profound effect on the decomposition temperature of the sample.

o The common atmospheres invoved are:
O Static air: air from atmosphere is allowed to flow through the furnace.
O Dynamic air: compressed air from cylinder is allowed to pass through the
furnace at a measured flow rate.
O Inert atmosphere. A pure N2 gas from a cylinder passed through the
furnace which provides an inert atmosphere.

Sample Characteristics
▪Weight of the sample
O A small weight of the sample is recommended usinga small weight
eliminates the existence of temperature gradient through the sample.
▪Particle size:
Various particle sizes of the sample alter the reaction rate and hence the curve shape.
O Smaller dimensions - decomposition earlier
O Larger size decomposition proceeds at higher temperatures.
O The particle size of the sample should be small and uniform generally.

▪Heat of reaction:
O It alter the difference between the sample temperature and furnace
temperature.
O If the heat effect is exothermic or endothermic, this will cause the sample
temperature to lead or lag behind the furnace temperature.
▪Compactness of the sample:
O A compressed sample will decompose at higher temperatures than a
loose sample.

Applications
▪Thermal Stability: related materials can be compared at elevated
temperatures under the required atmosphere. The TG curve can help
toelucidate decomposition mechanisms.
▪Material characterization: TG curves can be used to "fingerprint" materials
for identification or quality control.
▪ Compositional analysis: by careful choice of temperature programming
and gaseous environment, many complex materials or mixtures may be
analyzed by decomposing or removing their components. It is used to
analyze e.g. filler content in polymers; carbon black in oils; ash and carbon
in coals, and the moisture content of many substances.

▪Simulation of industrial processes: the thermobalance furnace is
thought as mini-reactor and has ability to perform operations like some
types of industrial reactors.
▪ Kinetic Studies: by understanding the controlling chemistry or predictive
studies, a variety of methods can be used to analyze the kinetic features of
weight loss or gain.
▪ Corrosion studies: TG provides a means of studying oxidation or some
reactions with other reactive gases or vapors.
▪To study purity
▪ To determine decomposition temperature. Forced degradation study

Differential Thermal Analysis (DTA)
DTA Analyzer

Differential thermal analysis (DTA)
▪Differential thermal analysis (DTA), in analytical chemistry, a technique
for identifying quantitatively and analyzing the chemical composition of
substances by observing the thermal behavior of a sample as it is
heated.
▪The technique is based on the fact that as a substance is heated, it
undergoes reactions and phase changes that involve absorption or
emission of heat.

Principle
▪A Technique in which the temperature difference between a substance and reference
material is measured as a function of temperature, while the substance and reference
are subjected to a controlled temperature programme.
▪The Difference in temperature is called as Differential temp(△ t) is plotted against
temp. or a function of time.
▪Physical changes usually result in Endothermic peak, whereas chemical reactions
those of an oxidative nature are exothermic.
▪Endothermic reaction (absorption of energy) includes vaporization, sublimation, and
absorption & gives downward peak.
▪Exothermic reaction (liberation of energy) includes Oxidation, polymerization, and
catalytic reaction & gives upward peak.

Instrumentation
1. FURNACE
2. SAMPLE HOLDER
3. DC AMPLIFIER
4. DIFFERENTIAL TEMPERATURE DETECTOR
5. FURNACE TEMPERATURE PROGRAMMER
6. RECORDER
7. CONTROL EQUIPMENT

Block diagram of DTA apparatus

Furnace
▪In DTA apparatus ,one always prefers a tubular furnace.
▪This is constructed with an appropriate material (wire or ribbon) wound on
a refractory tube.
▪These are fairly inexpensive .Generally , the choice of the resistance
material as well that of refractory is decided from the internal maximum
temperature of operation and gaseous environments.

Sample holders
▪Both metallic as well as non-metallic are employed for the fabrication of
sample holders.
▪Metallic materials generally include nickel, stainless steel, platinum and
its alloys.
▪Non-metallic material generally includes glass, vitreous silica or
sintered alumina.
▪Metallic holders give rise to sharp exotherms and flat endotherms.On
the other hand non-metallic holders yield relatively sharp endotherms
and flat exotherms.

DC Amplifier
▪It is used for amplification of signals obtained from (T)c.
▪It is gain and low noise circuit.

Differential temperature detector
▪In order to control temperature , the three basic elements are
required.
- These are sensor, control element and heater.
▪ON-OFF CONTROL-In this device, if the sensor signal indicates the
temperature has become greater than the set point, the heater is
immediately cut off.
Not used in DTA

▪PROPORTIONAL CONTROL-In on-off controllers there
occurs fluctuations of temperature around the set value.
These can be minimized if the heat input to the system is
progressively reduced as the temperature approaches the
desired value.
▪Such a controller that anticipates the approach to the set
value is known as proportional controller.

Furnace temperature programmer / sensors :-
It provides smooth heating or cooling at a linear rate by changing
the voltage through heating component. Modern DTA instruments
incorporate electronic temp controller in which the signal from
thermocouple in furnace is compared electically against ref.potential
which can be programmed to corresponds to a variety of heating
modes & heating rates.

Recorder
▪In thermo analytical studies, the signal obtained from the sensors can be
recorded in which the signal trace is produced on paper or film , heating
stylus, electric writing or optical beam.
▪There are two types of recording devices similar to
the TG-
DEFLECTION TYPE
NULL-POINT TYPE

Control equipment
For some type of samples the atmosphere must be controlled to
suppress and undesirable reaction such as oxidation by flowing an
inert gas.

Working of DTA
▪The sample and reference standard are placed in the furnace on flat, highly
thermally conductive pans and the thermocouples are physically attached to the
pans directly under the sample.
▪This procedure avoids or reduces any thermal lag resulting from the time required
for the heat to transfer to the sample and reference materials then to the
thermocouples.
▪The thermocouple are connected in opposition.
▪In a similar manner any change in state that involves a latent heat of transition will
cause the temperature of the sample to lag or lead that of the reference standard
and identify the change of state and the temperature at which it occurred.

Thermogram
A differential thermogram consists of a record of the difference in sample
and reference temperature (ꕔ T) plotted as a function of time t, sample
temperature (Ts), reference temperature (Tr) or furnace temperature (T).

In most of the cases physical changes give rise to endothermic curves
whereas chemical reaction gives rise to exothermic .
▪Sharp endothermic-change in crystallinity or fusion
▪Broad endothermic-dehydration reaction.
▪Exothermic- mostly oxidative reaction.

Factors affecting DTA curve
The various factors affecting the DTA curve are as follows:
▪Environmental factors.
▪Instrumental factors.
▪Sample factors.

Environmental Factors :-
▪The DTA technique is more sensitive to the gaseous environment around the sample
▪Reaction of atmospherie gases with the sample may also produce extra peaks in the curve.
▪In DTA two types of gaseous environment are used
▪Static gaseous atmosphere
- The atmosphere surrounding the sample is changing in concentration chemically due to
evolved gases and physically due to convection currents.
- Studies in it are imprecise.
▪Dynamicgaseous atmosphere
- The gases are swept past the sample in a controlled way.
- Reliable and reproducible.
- Sweep gases can be inert or reactive. But should not contain any of the product gases

Instrumental Factors
▪The geometry and material with which it is made of affects the DTA curve.
▪If material has
- High thermal conductivity sharp exothermic peaks and flat endothermic peaks are
obtained.
🔹Eg. Metals
- Poor thermal conductivity - reverse is true.
🔹Eg Ceramic
▪the size of holder and the amount of sample should be
as small as possible for better resolution.
Sample holders

Differential temperature sensing devices :
▪The thickness of thermocouple wires affect the intensity of the peaks, shape
of the peaks and the baseline.
▪If wires used are much thick
- More distortion of peak heights and peak temperatures may take place.
▪If thinner wires are used
- Less distortion of peak heights and peak temperatures may take place.
- But the resistance is high and may be unstable in impedance matching

Furnace characteristics
▪The type of winding shows a direct effect on DTA curves.
▪It should be uniform, hand wound are not uniform and are not useful.
▪Machine wound are uniform.
▪Grooved muffled cores and time biflar winding is preferred.
▪The entire length of the differential thermocouple should be shielded.

Temperature programmer controller :-
▪On-off type controllers are not used because switching off or on or
full power, considerable noise may occur particularly at
temperatures above 700°C.
▪If one has to measure small differential temperature, one should
maintain highest accuracy, control and precision in temperature
measurement.

Thermal regime :-
▪The heating rate has a great influence on the DTA curves.
▪Higher the heating rates, higher the peak temperature and sharper the
peaks with greater intensity.
▪Generally, heating rates of 10 to 20° per minute are employed
▪If the sample temperature is used as a reference material, this
minimizes the shift in the peak temperature to higher values with faster
heating rates.

Recorder
▪DTA curve is greatly influenced by the type, span, chart-speed and
pen-response of a recorder.
▪If proper sensitivity is not selected, weaker signals would not be
recorder whereas the stronger signals might undergo damping.
▪If faster charts speeds are used, DTA peaks get flattened out.

Sample Characteristics
▪Packing density.
▪Particle size
- Peak area decreases with increase in size.
- Peak T shifts to higher values with increase in size.
- Completion T decreases with decrease in size.
▪Degree of crystallinity.
▪Amount of sample influence peak area.
- As wt of the sample increases peak intensity and temperature.
▪In order to maintain the heat capacity nearly constant during heating, the sample is generally mixed
with diluents. Generally, diluents affects the area, temperature and even resolution of the DTA peaks.
Physical

Chemical
▪The chemical reactivity of the sample, the sample holder,
thermocouple material, the ambient gaseous environment and
added diluents greatly alter the DTA peaks.
▪Therefore, one should make every effort to select these
materials as inert chemically as possible with the sample.

Applications
▪A DTA curve can be used only as a finger print for identification purposes
but usually the applications of this method are the determination of phase
diagrams, heat change measurements and decomposition in various
atmospheres.
▪DTA is widely used in the pharmaceutical and food industries.
▪DTA may be used in cement chemistry, mineralogical research and in
environmental studies.
▪DTA curves may also be used to date bone remains or to study
archaeological materials. Using DTA one can obtain liquidus & solidus lines
of phase diagrams.

▪Used to study the characteristic of polymeric material.
▪This technique is used for testing the purity of the drug
sample and also to test the quality control of number of
substances like cement, soil, glass, etc.
▪Used for the determination of heat of reaction, specific heat
and energy change occurring during melting etc.
▪Trend in ligand stability (thermal stability of then ligands)
gives the information about the ligands in the coordination
sphere.

Differential Scanning Calorimetry (DSC)

Defination
Differential scanning calorimetry is a 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

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.

Principle
▪It is a technique in which the energy necessary to establish a zero temp.
difference between the sample & reference material is measured as a
function of temperature
▪Here, sample & reference material are heated by separate heaters in such
a way that their temp are kept equal while these temp. are increased or
decreased linearly
▪During a thermal event in the sample, the system will transfer heat to or
from the sample pan to maintain the same temperature in reference and
sample pans

▪Endothermic reaction: if sample absorbs some amount of heat
during phase transition then reaction is said to be endothermic. In
endothermic reaction more energy needed to maintain zero temp
difference between sample & reference.
E.g. Melting, boiling, sublimation, vaporization, de-solvation
▪Exothermic reaction: if sample released some amount of heat
during phase transition, then reaction is said to be exothermic. In
exothermic reaction, less energy needed to maintain zero temp
difference between sample & reference.
E.g crystallization, degradation, polymerization.

Instrumentation
There are four different types of DSC instrument
• Heat flux DSC
• Power compensated DSC
• Modulated DSC
• Hyper DSC
• Pressure DSC

Heat flux DSC
▪In heat flux DSC, the difference in heat flow into the sample and reference is
measured while the sample temperature is changed at the constant rate
▪The main assembly of the DSC cell is enclosed in a cylindrical, silver heating black,
which dissipates heat to the specimens via a constantan disc which is attached to the
silver block. The disk has two raised platforms on which the sample and reference
pans are placed.
▪A chromel disk and connecting wire are attached to the underside of each platform,
and the resulting chromel-constantan thermocouples are used to determine the
differential temperatures of interest.
▪Alumel wires attached to the chrome discs provide the chromel-alumel junctions for
independently measuring the sample and reference temperature.

A separate thermocouple embedded in the silver block serves a temperature
controller for the programmed heating cycle. And inert gas is passed through
the cell at a constant flow rate of about 40 ml/min
In heat flux DSC, we can write the total heat flow dH/dt as,
Where, H = enthalpy in J mol-1
Cp=specific heat capacity in JK-1 mol-1
f (T,t )= kinetic response of the sample in J mol-1

In power compensation DSC, the temperatures of the sample and reference are kept
equal to each other while both temperatures are increased or decreased linearly. The
power needed to maintain the sample temperature equal to the reference temperature is
measured. In power compensation DSC two independent heating units are employed.
These heating units are quite small, allowing for rapid rates of heating, cooling and
equilibration. The heating units are embedded in a large temperature- controlled heat
sink.
The sample and reference holders have platinum resistance thermometers to
continuously monitor the temperature of the materials. Both sample and reference are
maintained at the programmed temperature by applying power to the sample and
reference heaters.
The instrument records the power difference needed to maintain the sample and
reference at the same temperature as a function of the programmed temperatures. Power
compensated DSC has lower sensitivity than heat flux DSC, but its response time is
more rapid.

This makes power compensated DSC well suited for kinetics studies in
which fast equilibrations to new temperature settings are needed. it is
also capable of higher resolution then heat flux DSC
All PC DSC are in basic principles the same. But, one of the special PC
DSC is photo DSC. Where direct measurements of radiation flow
occurunder a light source. This way the degradation of material can also
be observed. The maximum heating rate for not modified PC DSC is up
to 500K/min and the maximum cooling rate is up to 400 K/min.
Temperature range of measurement is up to 400 °C with time constant of
only 1.5 s or lower. Sample masses are around 20 mg. Crucibles of
different volumes (lower than several ten cubic millimetres) are made
mostly of aluminium

Modulated DSC
▪Modulated DSC uses the same heating and cell arrangement as
the heat – flux DSC method. it is a new technique introduced in
1993
▪The main advantage of this technique is the separation of
overlapping events in the DSC scans. In MDSC the normally
linear heating ramp is overlaid with the sinusoidal function
(MDSC) defined by a frequency and amplitude to produce a sine
wave shape temperature versus time function.

Hyper DSC
The high resolution of PC-DSC or new type of power compensating DSC provides the
best results for an analysis of melting and crystallisation of metals or detection of glass
transition temperature (Tg) in medications.
Fast scan DSC has the ability to perform valid heat flow measurements with fast linear
controlled rates (up to 500 K/min) especially by cooling, where the rates are higher than
with the classical PC DSC. Standard DSC operates under 10 K/ min.
The benefits of such devices are increased sensitivity at higher rates (which enables a
better study of the kinetics in the process), suppression of undesired transformation like
solid – solid transformation etc
It has a great sensitivity also at a heating rate of 500 K/min with 1 mg of sample material.
This technique is especially proper for the pharmaceutics industry for testing
medicaments at different temperatures where fast heating rates are necessary to avoid
other unwanted reactions etc

Pressure DSC
In pressure DSC, the sample can be submitted to different
pressures, which allows the characterisation of substances at
the pressures of processes or to distinguish between
overlapping peaks .Applications of this technique includes
studies of pressure sensitive reactions, evaluation of catalysts
, and resolution of overlapping transitions.

Typical DSC Curve
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.

Typical DSC Curve
It can be shown that the enthalpy of transition can be expressed using the
following equation:
ꕔ H = KA
where H is the enthalpy of transition, K is the calorimetric
constant, and A 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.

Factors Affecting DSC Curve
Two types of factors affect the DSC curve
▪Instrumental factors
• Furnace heating rate
• Recording or chart speed
• Furnace atmosphere
• Geometry of sample holder/location of sensors
• Sensitivity of the recording system
• Composition of sample containers

▪Sample characteristics
• Amount of sample
• Nature of sample
• Sample packing
• Solubility of evolved gases in the sample
• Particle size
• Heat of reaction
• Thermal conductivity

Applications
▪Metal alloy melting temperatures and heat of fusion.
▪Metal magnetic or structure transition temperatures and heat of
transformation.
▪Intermetallic phase formation temperatures and exothermal
energies.
▪Oxidation temperature and oxidation energy.
▪Exothermal energy of polymer cure (as in epoxy adhesives), allows
determination of the degree and rate of cure.

▪Determine the melting behavior of complex organic materials, both
temperatures and enthalpies of melting can be used to determine purity of a
material.
▪Measurement of plastic or glassy material glass transition temperatures or
softening temperatures, which change dependent upon the temperature
history of the polymer or the amount and type of fill material, among other
effects.
▪Determines crystalline to amorphous transition temperatures in polymers
and plastics and the energy associated with the transition.
▪Crystallization and melting temperatures and phase transition energies for
inorganic compounds.

Applications
▪Oxidative induction period of an oil or fat.
▪May be used as one of multiple techniques to identify an unknown
material or by itself to confirm that it is the expected material.
▪Determine the thermal stability of a material.
▪Determine the reaction kinetics of a material.
▪Measure the latent heat of melting of nylon 6 in a nylon Spandex fabric
to determine the weight percentage of the nylon

Liquid crystals
DSC is used in the study of liquid crystals. Some materials go from
solid to liquid; they go through a third state, which displays
properties of both the phases. This anisotropic liquid is known as a
liquid crystalline state or mesomorphous state. Using DSC, observe
the small energy changes that occur as matter transitions from a
solid to a liquid crystal and from a liquid crystal to anisotropic liquid.

Oxidative stability
To study the stability to oxidation of samples generally requires an airtight
sample chamber. Usually, 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.

Drug analysis
DSC is widely used in the pharmaceutical and polymer industries. For
polymers, DSC is a 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 peak in DSC curve
that usually appears soon after the glass transition.
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 which crystallization can occur.

Hyphenated Techniques
DSC is not normally hyphenated as frequently as is TGA but hyphenation has been used.
DSC-IR has been used to look at the evolved solvents from pharmaceuticals while DSC-MS has
been used to look at the composition of meteorites and lunar rocks. It is also used for the
determination of the purity of materials (polymers, inorganic compounds, pharmaceutical
products, etc).
DSC has also been coupled to FT-IR microscopy to look at changes in a sample during a DSC
run. Probably the most promising hyphenated technique is DSC-Raman, where a sample is
irradiated by a Raman laser as the sample is run in DSC profile. Because of the nature of the
Raman spectrometer, it is ideally suited for this as it does not require any processing of neither
reflectance spectra nor the use of a special transmission path cell. DSC-Raman shows great
potential for the study of polymorphic materials, polymeric-crystallization, and chain movements
at the glass transition, and for hydrogen bonding polymers.

Reference
➢ http://www.photometrics.net/DSC.html
➢ http://personal.cityu.edu.hk/~bhtan/PerkinElmerThermal.ppt
➢ http://www.ices.a-star.edu.sg/ices/ documents/MicroDSC.pdf
➢ Czichos.Saito,Smith. Materials Measurement Methods. Springer, 2006.p. 411–413.
➢ http://www.google.co.in/url?q=http://pslc.ws/macrog/dsc
➢ www.perkinelmer.com/.../4474542GDE_DSCBeginnersGuide.pdf
➢ http://www.uzaktanegitimplatformu.com/UEP/uep_ylisans/ey2/ey2_download/DSC%
2520 Thermal2.pdf&sa=U&EI=8YxXU6Xsl9TTsAS8qoHIBA&ved=
OCA0QFjAB&usg=AFQjCNEen7ydZppWT17EAOJVyCO2ouomZw
➢ ResearchGate Articles
➢ Wikipedia
➢ Google /Google search images

Thermal methods of Analysis

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