Definitions & qualifiers

The following tables summarise the definitions and units used for the different thermal analysis techniques. The definitions have been established by IUPAC (International Union for Pure and Applied Chemistry) and are recognised by ICTAC (International Confederation for Thermal Analysis and Calorimetry).

Definitions in the field of thermal analysis




Thermal analysis comprises a diverse range of techniques and experimental types that may be considered collectively if the total measurement conforms to the total definition


A technique is characterised by the property that is under investigation


Implies that time is an integral part of the thermal analysis experiment and the total experiment, and the interpretation and discussion of the measured data are included


Implies that either the sample property can be measured as a function of temperature (controlled-temperature program), or the temperature can be measured as a function of the sample property (sample controlled heating)


The material under study during the entire experiment (starting material, intermediates and final products) and its ambient atmosphere. This is equivalent to the thermodynamic system


Any physical or chemical property of the sample


Can be directly programmed by the user, or controlled by a property of the sample. The program may include an increase, or decrease in temperature, a periodic change, or a constant temperature or any combination of these


Definition of terms used in thermal analysis techniques




Indicating that the experiment is carried out so that no heat enters or leaves the system


The gaseous environment of the sample, which may be controlled by the instrumentation or generated by the sample


Techniques in which heat is measured


The application of two or more techniques to different samples at the same time. This can include thermal and non-thermal analytical techniques e.g. TGA-FTIR

Controlled temperature program

The temperature history imposed on the sample during the course of the thermal analysis equipment

Cooling curve

The experimental result of measuring the temperature of the sample as a function of time during cooling


Pertaining to the first derivative (mathematical) of any curve with respect to temperature or time


Pertaining to a difference in measured or measurable quantities, usually between a sample and a reference or standard material

Gas flow

The passage of the gas from one part of the system to another, either by sorption by the sample, evolution from it, or chemical reaction


The experiment is carried out at constant pressure


The temperature is maintained constant throughout the experiment


Prefix indicating that the technique measures small quantities, either with respect to the amount of sample studied, or with respect to the change in the properties measured


Indicating that a parameter changes in a periodic manner during the experiment

Modulated temperature

Applied to the technique name to indicate that a temperature modulation has been applied to the temperature program


A prefix used to denote that the technique measures very small quantities, either with respect to the amount of sample studied, or with respect to the change in the properties measured


A prefix to indicate that the experiment involves illumination of the sample or measures the amount of light emitted from a sample

Sample controlled

Indicates that a property of the sample is used either continuously to control the sample heating. With no prefix, it is assumed that the experiment is following a controlled-temperature program


A term used to describe the data produced from a thermal analysis experiment. More correct usage is thermal analysis curve or, for a specific technique, thermogravimetric curve, etc.


Indicating a specified experimental, usually temperature, is changed in a controlled manner


The measurement of two or more properties of a single sample at the same time


Indicating discrete, discontinuous changes in an experimental parameter, e.g. force or temperature

Thermal curve

A graph of any combination of property, time, temperature derived from a thermal analysis technique


A prefix indicating the use of changing temperature during the experiment


Definition of terms related to DTA/DSC curves



Calibration material (for DSC)

A material for which one or more of the thermal properties is sufficiently uniform and well established to be used for the calibration of a DSC apparatus, for assessment of a measurement method or for assigning values to materials

DSC baseline

Part of the recorded curve without any reactions or transitions. This can be an isothermal baseline when the temperature is maintained constant or a dynamic baseline when the temperature is changed according to a controlled temperature program

DTA baseline

The baseline corresponds to the portion or portions of the DTA curve for which ∆T is approximately zero, ∆T being the difference between the sample and reference temperatures

DTA endothermic peak

An endothermic peak or endotherm, is a peak where the temperature of the sample falls below that of the reference material, i.e., ∆T is negative

DTA exothermic peak

An exothermic peak, or exotherm, is a peak where the temperature of the sample rises above that of the reference material, i.e., ∆T is positive

DTA peak width

Peak width is the time or temperature interval between the points of departure from and return to the base line. There are several ways of interpolating the baseline

DTA peak height

Peak height is the distance, vertical to the time or temperature axis, between the interpolated baseline and the peak tip. There are several ways of interpolating the baseline

Extrapolated onset

The extrapolated onset is the point of intersection of the tangent drawn at the point of greatest slope on the leading edge of the peak with the extrapolated base line


A peak is that portion of the curve which departs from and subsequently returns to the baseline

Peak area

Peak area is the area enclosed between the peak and the interpolated baseline


Definition of terms related to calorimetry 



Boiling point

The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid. A liquid may change to a gas at temperatures below the boiling point through the process of evaporation (expressed in K)

Enthalpy of crystallisation

The heat released by the crystallisation process of the material at constant pressure, measured in J/g or J/mol

Enthalpy of fusion

The heat required to melt material at constant pressure, measured in J/g or J/mol

Enthalpy of vaporisation

The energy required to change a gram of a liquid into the gaseous state at the boiling point is called the "heat of vaporisation", expressed in J/g or J/mol

Heat flow rate or heat flux (thermal power)

Heat transferred per unit time, expressed in watts. The energy can be transferred by means of conduction, convection or radiation

Melting and freezing points

The melting point of a solid is the temperature at which it changes state from solid to liquid. Conversely, the temperature of the reverse change, going from liquid to solid, is called the freezing point (expressed in K)

Phase change

Phase changes are transitions between solid, liquid, and gaseous phases that typically involve large amounts of energy compared to the specific heat (expressed in J/g or J/mol)

Power (Watt)

Equivalent to kgm2 s-3 in SI base units. The watt is the standard unit of power (or energy per unit time) and is equivalent to one joule per second

Quantity of heat (Joule)

A unit of energy equal to the work done when a current of one ampere passes through a resistance of one ohm for one second. Equivalent to  kg m2 s-2 in SI base units. One joule is also the equivalent to one watt of power radiated or dissipated for one second

Specific heat capacity at constant pressure

Quantity of heat necessary to raise the temperature of unit mass of material by 1K at constant pressure, expressed in joules per mol Kelvin [J / (mol . K)], or joules per gram Kelvin [J / (g . K)]

Triple point

The triple point of a substance is the temperature and pressure at which the three phases (gas, liquid, and solid) of the substance may coexist in thermodynamic equilibrium (expressed in K)