Seminar on “THERMOGRAVIMETRIC ANALYSIS

AND

DIFFERENTIAL SCANNING CALORIMETRY”

Thermal Analysis

Differential Scanning

Calorimetry (DSC)

Measure heat absorbed or

liberated during heating

or cooling

Thermal Gravimetric

Analysis (TGA)

Measure change in weight

during heating or cooling

Differential Scanning

Calorimetry (DSC)

Calorimeter-Heat flow in sample

Differential calorimeter-heat flow in sample vs referance as function of time

mJ/sec.

• Differential Scanning Calorimetry (DSC) measures the

temperatures and heat flows associated with transitions in

materials as a function of time and temperature in a

controlled atmosphere.

• These measurements provide quantitative and qualitative

information about physical and chemical changes that

involve endothermicor exothermic processes, or changes

in heat capacity.

DSC: The Technique

Conventional DSC

Metal

1

Metal

2

Metal

1

Metal

2

Sample Empty

Sample

TemperatureReference

Temperature

Temperature

Difference =

Heat Flow

•A “linear” heating profile even for isothermal methods

Modes and principles of operation (1)

(b) Power compensated DSC: Temperature differences between the sample and

reference are ‘compensated’ for by varying the heat required to keep both pans at the

same temperature. The energy difference is plotted as a function of sample temperature

(c) Heat flux DSC ultilizes a single furnace. Heat flow into both sample and reference

material via an electrically heated constantan thermoelectric disk and is proportional to

the difference in output of the two themocouple junctions

Modes and principles of operation (3)

a) DTA: difference in temperature between the sample and reference is plotted

against sample temperature

• A DSC apparatus is built

around

- a differential detector

- a signal amplifier

- a furnace

- a temperature controller

- a gas control device

- a data acquisition device

Diagram of a DSC apparatus

Sample Reference

Gas control

Furnace

controller

four

Data acquisition

Microvolt amplifier

Detectors

Furnace

DSC measures:-

•Glass transitions

•Melting and boiling points

•Crystallisation time and temperature

•Percent crystallinity

•Heats of fusion and reactions

•Specific heat capacity

•Oxidative/thermal stability

•Rate and degree of cure

•Reaction kinetics

•Purity

6

DSC Thermogram

Temperature

Hea

t F

low

->

ex

oth

erm

ic

Glass

Transition

Crystallisation

Melting

Cross-Linking

(Cure)

Oxidation

Main Sources of Errors

•Calibration

•Contamination

•Sample preparation – how sample is loaded into a pan

•Residual solvents and moisture.

•Thermal lag

•Heating/Cooling rates

•Sample mass

•Processing errors

Sample preparation

Form of sample: bulk solid, powder (pressed), liquid.

Amount of sample: 3-5mg.

DSC Pan: Al, Pt, stainless steel, Ag,

Cu, Al2O3

99

Sample Preparation : Shape

• Keep sample as thin as possible (to minimise thermal

gradients)

• Cover as much of the pan bottom as possible

• Samples should be cut rather than crushed to obtain a

thin sample (better and more uniform thermal contact

with pan)

Operation procedures

Calibration of instrument

• Temperature, heat of reaction, heat capacity scale using

high purity standards (In, Sn, Bi, Pb,Au)

•Baseline correction for a given scan rate (1 - 40K/min).

• Weight samples before (and maybe after) experiment.

Type of DSC experiments

1. Dynamic heating - thermodynamic properties

2. Isothermal heating - kinetic parameters

1. Dynamic heating:-

Constant heat rate mode.

(e.g. heat flow vs. temperature).

What can we characterise?

Information:

1. Transformation temperature (e.g. onset, peak).

2. Transformation enthalpy (e.g. area under the peak).

3. Activation energy for transformation (e.g. Kissinger

analysis)

2.)Isothermal heating mode:-Heating at a fixed temperature over a time interval.(e.g. heat flow vs.

time)

THERMOGAVIMETRIC ANALYSIS

(TGA)

Principle: TGA measures the amount and the rate of weight change

of a material with respect to temperature or time in controlled

environments.

A TGA consists of three major parts a furnace,

1. A microgram balance,

2. An auto sampler and

3. A thermocouple.

.

Instrument: Instrument used for thermogravimetry is “Thermobalance”.

Data recorded in form of curve known as ‘Thermogram’

The furnace can raise the temperature as high as 1000°C

which is made of quartz.

The auto sampler helps to load the samples on to the

microbalance.

The thermocouple sits right above the sample.

Care should be taken at all times that the thermocouple is not

in touch with the sample which is in a platinum pan.

• A technique that permits thecontinuous weighing of asample as a function oftemperature and/or as afunction of time at a desiredtemperature

TGA Curve of Calcium Oxalate

Sample Preparation

Sample preparation has a significant effect in obtaining good data.

It is suggested that maximizing the surface area of the sample in a

TGA pan improves resolution and reproducibility of weight loss

temperatures.

The sample weight affects the accuracy of weight loss

measurements.

Typically 10-20mg of sample is preferred in most applications.

Whereas, if the sample has volatiles 50-100mg of sample is

considered adequate.

It is to be noted that most TGA instruments have baseline drift of

±0.025mg which is ±0.25% of a 10mg sample.

Experimental Conditions

Heating Rate

Purge gas

Experimental Conditions -Heating Rate

Samples are heated at a rate of 10 or 20°C/min in most

cases.

Lowering the heating rates is known to improve the

resolution of overlapping weight losses.

Experimental Conditions -Purge gas

• Nitrogen is the most common gas used to purge samples in TGA due

to its inert nature.

• Whereas, helium provides the best baseline.

• Air is known to improve resolution because of a difference in the

oxidative stability of components in the sample.

• Vacuum may be used where the sample contains volatile components,

which helps improve separation from the onset of decomposition

since the volatiles come off at lower temperatures in vacuum.

• e.g. oil in a rubber tire product.

Calibration

Blank test

Calibration of mass changes

Calibration of temperature

Applications of TGA

Determination of the bound and unbound water in the

suspension of Milk of Magnesia (MoM), used as a laxative.

In an overview of thermal analysis testing it is always

preferable to do a TGA experiment on unknown samples

before doing a DSC experiment (especially for

pharmaceuticals).

Decomposition of pharmaceuticals renders products which are

insoluble and generally sticky on the inside of a DSC cell.

These products will lower the life use of a DSC cell.

Therefore, know the decomposition temperatures of all drugs

and heat in a DSC evaluation to 50°C below those

temperatures.

Applications of TGA

Evaporation of free (unbound) water begins at roomtemperature due to dry gas flowing over the sample.

Dehydration/Desolvation of bound water almost alwaysbegins at temperatures above room temperature andtypically 125°C.

Decomposition can have multiple stages (weight losses)but the presence of multiple weight loss steps can alsoindicate the presence of multiple components in thesample.

leparlouer@thermalconsulting.com

Referances:-

www.evitherm.com

www.ictac.org

Therm@l Consulting

http://www.tainst.com Mettler Toledo Thermal Analysis Systems