Discussion Examples for Thermal Analysis | What Can Be Learned from TG-DTA and DSC Curves

What Is Thermal Analysis?

Thermal analysis is an analytical method used to measure changes in the physical properties of a sample while changing the temperature.
Representative methods include TG, DTA, and DSC.
TG measures changes in mass, DTA measures the temperature difference between a sample and a reference material, and DSC measures the amount of heat entering or leaving a sample.

When heated, a sample may lose water or solvent, melt, crystallize, undergo chemical reactions, or decompose.
These changes appear in the curves as mass changes or endothermic and exothermic peaks.
Therefore, thermal analysis is useful for investigating the thermal stability, phase transitions, and reactivity of a sample.

Main Items to Include in the Results

In the results of thermal analysis, organize the measurement method, sample name, sample amount, heating rate, measurement temperature range, atmosphere gas, mass-loss temperature, mass-loss percentage, endothermic peak, exothermic peak, peak temperature, onset temperature, end temperature, and other information.
Because TG-DTA and DSC provide different types of information, it is important to clearly state which curve was used to make each judgment.

Main Items to Include in the Results

• Measurement method
• Sample name
• Sample amount
• Measurement temperature range
• Heating rate
• Measurement atmosphere
• Reference material
• Mass change in the TG curve
• Mass-loss onset temperature
• Mass-loss end temperature
• Mass-loss percentage
• Endothermic and exothermic peaks in the DTA curve
• Endothermic and exothermic peaks in the DSC curve
• Peak temperature
• Onset temperature
• Melting point
• Crystallization temperature
• Glass-transition temperature
• Decomposition temperature
• Sources of error and points for improvement

Reference Experimental Values and Examples of Reading Thermal-Analysis Curves (TG, DTA, and DSC)

Here, reference experimental values are organized for discussing changes in sample mass, endothermic and exothermic reactions, melting, dehydration, and decomposition from TG, DTA, and DSC measurement results.

In thermal analysis, a sample is heated at a constant rate and changes in mass and thermal behavior accompanying temperature changes are examined.
TG can be used to read mass changes, while DTA and DSC can be used to read endothermic and exothermic changes.
By combining these methods, it is possible to estimate not only temperature changes but also physical changes and chemical reactions occurring within the sample.

Reference Experimental Conditions

What Can Be Learned From TG, DTA, and DSC?

TG alone shows that the mass has decreased, but it may be difficult to determine whether the change is caused by dehydration or decomposition.
By reading the TG result together with endothermic and exothermic peaks in DTA or DSC, it becomes easier to estimate the nature of the change.

Example TG Measurement of Copper(II) Sulfate Pentahydrate

A reference example of TG measurement when copper(II) sulfate pentahydrate, CuSO4·5H2O, is heated is shown below.
During heating, water of crystallization is lost stepwise and the mass decreases.

Example Calculation of Mass-Loss Percentage

The mass-loss percentage expresses as a percentage how much mass was lost relative to the mass before heating.

Mass-loss percentage (%) = Mass lost ÷ Initial mass × 100

For copper(II) sulfate pentahydrate, if the initial mass is 10.00 mg and the mass loss from room temperature to 120°C is 1.44 mg,

Mass-loss percentage = 1.44 ÷ 10.00 × 100 = 14.4%

It can be organized that a component corresponding to 14.4% of the total sample was lost within this temperature range.

Theoretical Mass-Loss Percentage of Copper(II) Sulfate Pentahydrate

During dehydration of copper(II) sulfate pentahydrate, the water of crystallization is lost.
If the formula mass of CuSO4·5H2O is taken as 249.7 and the mass of five water molecules as 90.1, the theoretical mass-loss percentage when all of the water of crystallization is lost is calculated as follows.

Theoretical mass-loss percentage = 90.1 ÷ 249.7 × 100 = 36.1%

In the measurement, the mass decreased from an initial value of 10.00 mg to a final value of 6.40 mg, so the overall mass-loss percentage is as follows.

Measured mass-loss percentage = (10.00 − 6.40) ÷ 10.00 × 100 = 36.0%

The measured value of 36.0% is close to the theoretical value of 36.1%, suggesting that almost all of the water of crystallization in copper(II) sulfate pentahydrate was lost.

Example DTA and DSC Peaks of Copper(II) Sulfate Pentahydrate

If an endothermic peak and mass loss are observed in the same temperature range, a component may have been lost while absorbing heat, as in dehydration or volatilization.

Example TG and DTA Measurements of Calcium Carbonate

Calcium carbonate, CaCO3, decomposes at high temperature to form calcium oxide, CaO, and carbon dioxide, CO2.

CaCO3 → CaO + CO2

If the formula mass of CaCO3 is taken as 100.1 and that of CO2 as 44.0, the theoretical mass-loss percentage caused by loss of CO2 is calculated as follows.

Theoretical mass-loss percentage = 44.0 ÷ 100.1 × 100 = 44.0%

The measured value of 43.3% is close to the theoretical value of 44.0%, suggesting that CaCO3 decomposed and released CO2 within this temperature range.

Example Measurement of Polyethylene Melting by DSC

DSC can be used to investigate thermal changes such as melting, crystallization, and glass transition.
A reference example for heating polyethylene is shown below.

If an endothermic peak is observed by DSC but no mass change is observed by TG, the sample is considered to have undergone a physical change such as melting rather than decomposition.

Example Judgments Using a Combination of TG and DSC

In thermal analysis, it is important not only to check whether a peak is present but also to confirm whether a mass change occurs at the same time.

Example Thermal Analysis of an Unknown Sample

A reference example of TG and DSC measurements of unknown sample X is shown below.

Unknown sample X showed a small mass loss at low temperature, suggesting that it may contain moisture or residual solvent.
Because there was no mass change at the endothermic peak near 150°C, the peak is considered to correspond to melting.
Above approximately 300°C, a large mass loss and exothermic peak were observed, suggesting that decomposition or oxidative decomposition of organic components occurred.

Differences in Peak Temperature Caused by Heating Rate

In thermal analysis, the faster the heating rate, the more the reaction or peak may appear shifted toward higher temperatures.
A reference example for measuring the same sample at different heating rates is shown below.

When comparing measurement results obtained at different heating rates, it is necessary to consider the possibility that differences in peak temperature are caused by differences in measurement conditions rather than by differences in the substance.

Differences in Thermal Decomposition Behavior Depending on Atmosphere

Because the presence or absence of oxidation differs between nitrogen and air atmospheres, the TG and DTA/DSC curves also change.

Because oxygen is present in air, oxidative decomposition and combustion occur more readily, the exothermic peak may become larger, and the residue amount may become smaller.

Differences in Peak Shape Caused by Sample Amount

If the sample amount is too large, heat may not be transferred uniformly throughout the sample, and the peak may broaden or shift toward higher temperatures.

If the sample amount or the way the sample is packed differs, the peak shape and peak temperature may change even for the same substance.

Example of How to Write the Results

In the TG measurement of copper(II) sulfate pentahydrate, stepwise mass loss was observed from room temperature to approximately 350°C.
The final mass was 6.40 mg relative to an initial mass of 10.00 mg, giving a measured mass-loss percentage of 36.0%.
The theoretical mass-loss percentage when CuSO4·5H2O loses five molecules of water of crystallization is 36.1%, and the measured value agreed well with the theoretical value.

In DTA and DSC, endothermic peaks were observed near 80–120°C, 130–190°C, and 230–300°C.
Because mass loss was also observed by TG in these temperature ranges, the endothermic peaks were considered to correspond to loss of water of crystallization.
Because mass loss and endothermic peaks occurred simultaneously, the changes can be judged to involve dehydration rather than being simple phase transitions.

For calcium carbonate, a large mass loss and endothermic peak were observed near 650–780°C.
The measured mass-loss percentage was 43.3%, which was close to the theoretical value of 44.0% for loss of CO2 from CaCO3.
From this, calcium carbonate was considered to have thermally decomposed at high temperature to form calcium oxide and carbon dioxide.

Points for Connecting the Results to the Discussion

In a discussion of thermal analysis, it is important to explain not only the peak temperatures but also the relationship between mass changes in TG and endothermic or exothermic changes in DTA and DSC.

• Can the temperature range in which mass loss occurs in TG be identified?
• Can endothermic and exothermic peaks in DTA and DSC be distinguished?
• Can the mass-loss percentage be calculated relative to the initial mass?
• Can the theoretical and measured mass-loss percentages be compared?
• Can an endothermic peak accompanied by mass loss be explained in relation to dehydration, volatilization, or decomposition?
• Can an endothermic peak without mass change be discussed as melting or a phase transition?
• Can an exothermic peak be related to oxidation, combustion, crystallization, or a curing reaction?
• Can the effects of heating rate, sample amount, and atmosphere on peak temperature and mass loss be discussed?

Example Discussion

In this experiment, TG, DTA, and DSC were used to investigate mass changes and thermal changes accompanying heating of the samples.
For copper(II) sulfate pentahydrate, a total mass loss of 36.0% was observed in the TG curve up to approximately 350°C.
This value was almost identical to the theoretical mass-loss percentage of 36.1% corresponding to loss of five molecules of water of crystallization.
Therefore, this mass loss was considered to be caused by the loss of water of crystallization.

Endothermic peaks were observed in DTA and DSC within the same temperature ranges.
Because dehydration requires heat to remove water molecules from the crystal, the changes were considered to have been observed as endothermic processes.
The presence of mass loss in TG and an endothermic peak in DSC indicates that the change was not a simple physical phase transition such as melting but a change involving the loss of water molecules.

For calcium carbonate, a large mass loss occurred near 650–780°C, accompanied by an endothermic peak in DTA.
When CaCO3 decomposes to CaO and CO2, CO2 is lost as a gas, causing a mass decrease in TG.
The measured mass-loss percentage of 43.3% was close to the theoretical value of 44.0%, suggesting that thermal decomposition of calcium carbonate occurred.

In the DSC measurement of polyethylene, an endothermic peak was observed near 124°C, while almost no mass change was observed by TG.
This was considered to be because the crystalline regions of the sample melted rather than because the sample decomposed.
An endothermic peak without mass change is likely to correspond to a physical change such as melting or a phase transition.

As an effect of measurement conditions, peak temperatures tended to shift toward higher temperatures as the heating rate increased.
This is because a time lag occurs before the temperature inside the sample catches up with the programmed temperature of the instrument.
In addition, in air, the exothermic peak became larger because of oxidation reactions, and the decomposition behavior differed from that in nitrogen.
Therefore, when comparing thermal-analysis results, measurement conditions such as heating rate, sample amount, and atmosphere must be kept consistent.

Summary

TG can be used to investigate mass changes, while DTA and DSC can be used to investigate endothermic and exothermic changes.
When mass loss and an endothermic peak occur simultaneously, dehydration or decomposition can be considered; when an endothermic peak occurs without mass change, melting or a phase transition can be considered; and exothermic peaks may correspond to oxidation or crystallization.

This reference example dealt with dehydration of copper(II) sulfate pentahydrate, thermal decomposition of calcium carbonate, and melting of polyethylene.
In a report, it is useful to discuss the TG mass-loss percentage, DTA and DSC peaks, comparison with theoretical values, and effects of heating rate and atmosphere in relation to one another.

What Is TG?

TG is thermogravimetric analysis, a method used to measure changes in the mass of a sample while heating or cooling it.
In a TG curve, the horizontal axis represents temperature or time, while the vertical axis represents mass or mass-retention percentage.
When the sample undergoes dehydration, solvent evaporation, decomposition, combustion, or oxidation, its mass changes.

From a TG curve, the temperature at which mass loss begins, the temperature at which mass loss ends, the mass-loss percentage, and the residue amount can be read.
However, melting and phase transitions that do not involve mass changes are difficult to determine from TG alone.
Therefore, TG is analyzed in combination with DTA or DSC.

What Is DTA?

DTA is differential thermal analysis, a method in which a sample and a reference material are heated under the same conditions and the temperature difference between them is measured.
When the sample undergoes an endothermic change, its temperature becomes lower than that of the reference material, while during an exothermic change, its temperature becomes higher than that of the reference material.
This temperature difference appears as a peak in the DTA curve.

DTA can detect thermal changes such as melting, crystallization, phase transitions, dehydration, decomposition, and oxidation.
However, DTA may be less suitable than DSC for quantitative measurement of heat.
Whether a process is endothermic or exothermic is judged from the peak temperature and peak direction.

What Is DSC?

DSC is differential scanning calorimetry, a method used to measure the difference in heat required to change the temperatures of a sample and a reference material in the same way.
When a sample undergoes an endothermic change such as melting or dehydration, additional heat is required.
During exothermic changes such as crystallization or oxidation, the sample itself releases heat.

DSC can be used to evaluate melting point, crystallization temperature, glass-transition temperature, heat of reaction, heat of fusion, and heat of crystallization.
It is commonly used to evaluate the thermal properties of polymers, pharmaceuticals, organic compounds, and materials.
The area of a DSC peak corresponds to the amount of heat associated with the change.

Difference Between TG-DTA and DSC

TG-DTA often makes it possible to observe mass changes and thermal changes simultaneously.
TG indicates whether the mass increased or decreased, while DTA indicates whether the process was endothermic or exothermic.
Therefore, it is useful for analyzing phenomena such as dehydration and decomposition that involve mass changes.

DSC is a method that allows heat changes to be handled more quantitatively.
It is suitable for evaluating heat of fusion, heat of crystallization, and glass transition.
On the other hand, DSC alone cannot directly determine whether a mass change occurred, so combining it with TG is effective for confirming dehydration and decomposition.

How to Read a TG Curve

In a TG curve, the amount of mass change and the temperature range in which it occurs are read.
If the mass remains constant, relatively few phenomena involving mass changes, such as volatilization or decomposition, are considered to occur within that temperature range.
If the mass decreases, dehydration, desolvation, volatilization, decomposition, or combustion can be considered.

Mass loss may occur in a single stage or in multiple stages.
A small mass loss at low temperature may correspond to desorption of adsorbed water or water of crystallization, while a large mass loss at high temperature may correspond to decomposition of organic components or inorganic salts.

Discussion of Mass-Loss Percentage

From a TG curve, the mass-loss percentage can be calculated using the difference in mass before and after heating.
The mass-loss percentage can be used to evaluate the amount of water released, the amount of solvent, the amount of component decomposed, and the residue amount.
For hydrates and complexes, comparison with the theoretical mass-loss percentage may make it possible to estimate the number of molecules of crystal water or coordinated water.

If the mass-loss percentage agrees with the theoretical value, this supports the assumed reaction or desorption process.
On the other hand, if it deviates from the theoretical value, impurities, adsorbed water, insufficient drying of the sample, incomplete reactions, or overlapping decomposition processes can be considered.

Mass-loss percentage = Mass lost / Initial mass × 100

Discussion of Endothermic Peaks

An endothermic peak appears when a sample undergoes a change that absorbs heat from the surroundings.
Representative endothermic processes include melting, dehydration, solvent evaporation, sublimation, and phase transitions of crystal structures.
In DSC and DTA, the direction of the peak may differ depending on the instrument and display settings, so it should be confirmed in the laboratory manual.

If an endothermic peak appears simultaneously with a TG mass loss, an endothermic process involving a mass change, such as dehydration, volatilization, or decomposition, can be considered.
On the other hand, if no mass change is observed by TG and an endothermic peak is seen by DSC, melting or a solid-state phase transition can be considered.

Discussion of Exothermic Peaks

An exothermic peak appears when a sample undergoes a change that releases heat.
Representative exothermic processes include crystallization, oxidation, combustion, curing reactions, rearrangements, and some decomposition reactions.
When measurements are performed in air, organic substances or metal components may oxidize and show exothermic peaks.

If an exothermic peak appears together with an increase in TG mass, a mass increase caused by oxidation can be considered.
If an exothermic peak appears together with mass loss, decomposition or combustion may be occurring.
Comparing the results obtained in nitrogen and air atmospheres makes it easier to discuss the effects of oxidation.

Discussion of Dehydration and Desolvation

If mass loss occurs in the low- to medium-temperature range and an endothermic peak appears near the same temperature, dehydration or desolvation may have occurred.
Adsorbed water tends to be lost at relatively low temperatures, while crystal water and coordinated water may be lost at higher temperatures.
In solvates, solvent incorporated into the crystal may escape during heating.

Comparing the mass-loss percentage with the theoretical value may make it possible to estimate the number of water or solvent molecules that were lost.
However, if adsorbed water, crystal water, and residual solvent are lost in overlapping temperature ranges, it may be difficult to distinguish them clearly.

Discussion of Melting

Melting is an endothermic process in which a solid changes into a liquid.
In DSC, an endothermic peak corresponding to melting is observed.
Highly pure crystalline substances may show relatively sharp melting peaks.
If many impurities are present or crystallinity is low, the melting peak may broaden or the melting point may decrease.

If an endothermic peak appears in DSC without a TG mass change, the peak may correspond to melting.
However, because dehydration and decomposition may also produce endothermic peaks, the result must be judged together with the TG mass change.

Discussion of Crystallization

Crystallization is an exothermic process in which an ordered crystal structure forms from an amorphous state or a supercooled liquid.
In DSC, an exothermic peak corresponding to crystallization may appear.
In polymers and glassy samples, molecular mobility may increase during heating and crystallization may proceed.

If a crystallization peak is observed, the sample may not have been completely crystallized before measurement.
In addition, the crystallization temperature and peak shape change depending on cooling conditions and thermal history.
Differences in thermal history greatly affect DSC results.

Discussion of Glass Transition

Glass transition is a phenomenon in which an amorphous region changes from a rigid glassy state to a soft rubbery state.
In DSC, it often appears as a step in the baseline rather than as a sharp peak.
It is an important indicator for evaluating polymers and amorphous materials.

Glass-transition temperature is affected by molecular mobility, molecular weight, crosslink density, plasticizers, water content, and thermal history.
Because glass transition does not involve a mass change, it is normally not detected by TG.
The baseline change in DSC must be read carefully.

Discussion of Phase Transitions

A phase transition is a phenomenon in which a crystal structure or molecular arrangement changes into another state.
Solid-to-solid phase transitions often occur without mass change and may appear as endothermic or exothermic peaks in DSC or DTA.
Materials having crystal polymorphs may transform to a more stable crystal form during heating.

When discussing a phase-transition peak, it is necessary to check whether there is a mass change in TG.
If there is no mass change, a change in crystal structure or a solid-state phase transition can be considered rather than dehydration or decomposition.
Combining the results with XRD makes it possible to confirm the crystal structures before and after the phase transition.

Discussion of Thermal Decomposition

Thermal decomposition is a phenomenon in which a compound chemically decomposes when heated.
In TG, it appears as a large mass loss and may be accompanied by an endothermic or exothermic peak in DTA or DSC.
In organic compounds and polymers, volatile products are generated during decomposition, causing the mass to decrease.

Thermal-decomposition temperature can be used as an indicator of the thermal stability of a sample.
The higher the decomposition-onset temperature, the more thermally stable the sample is considered to be.
However, decomposition temperature is affected by heating rate, atmosphere, sample amount, and sample shape, so the measurement conditions must be stated when comparing values.

Discussion of Oxidation and Combustion

When measurements are performed in air or an oxygen atmosphere, the sample may undergo oxidation or combustion.
Oxidation tends to appear as an exothermic peak, while TG may show either a mass increase or a mass decrease.
Oxidation of metal powder may increase the mass because oxygen is incorporated.
Combustion of organic matter produces volatile products, carbon dioxide, and water, causing a large mass decrease.

Because oxidation is suppressed in a nitrogen atmosphere, comparison with measurements performed in air can be used to judge the effects of oxidation.
Even for the same sample, the thermal-analysis curve may change greatly depending on the atmosphere gas.

Discussion of Residue Amount

The mass remaining after TG measurement is called the residue amount.
If inorganic components remain after organic matter has decomposed or burned, the amount of ash or inorganic material may be estimated from the residue.
In inorganic salts and metal complexes, the residue may correspond to the final oxide or metal component.

By comparing the residue amount with the theoretical value, the sample composition and decomposition products can be discussed.
However, the result may deviate from the theoretical value if decomposition is incomplete, carbonized material remains, oxidation is incomplete, or the atmosphere affects the process.

Effect of Heating Rate

The heating rate greatly affects thermal-analysis results.
If the heating rate is fast, the temperature inside the sample becomes less uniform and the peak temperature or decomposition-onset temperature may shift toward higher temperatures.
Peaks may also broaden or multiple changes may appear overlapped.

A slower heating rate makes it easier to measure under conditions closer to thermal equilibrium, but the measurement takes longer.
When comparing different experimental results, it is important to use the same heating rate.

Effect of Measurement Atmosphere

In thermal analysis, atmosphere gases such as nitrogen, argon, air, and oxygen greatly affect the results.
In inert atmospheres such as nitrogen and argon, oxidation reactions are suppressed and mainly dehydration, volatilization, and thermal decomposition are observed.
In air or oxygen atmospheres, oxidation and combustion occur more readily.

Even for the same sample, the temperature of exothermic peaks and mass changes and the amount of residue may differ between nitrogen and air.
The measurement atmosphere should always be stated when discussing thermal stability and decomposition mechanisms.

Effect of Sample Amount

If the sample amount is too large, heat is not readily transferred uniformly throughout the sample.
As a result, peaks may broaden, peak temperatures may shift, or decomposition may appear to occur in stages.
In addition, gases generated during dehydration or decomposition may have difficulty escaping from inside the sample and may affect the TG curve.

If the sample amount is too small, the signal becomes weak and more susceptible to noise.
In thermal analysis, it is important to select an appropriate sample amount according to the instrument and measurement purpose.

Peak Temperature and Onset Temperature

In DSC and DTA, the temperature at which a peak reaches its maximum or minimum is read as the peak temperature.
On the other hand, the temperature at which a change begins may be treated as the onset temperature.
When discussing melting points and decomposition-onset temperatures, care must be taken not to confuse the peak temperature and onset temperature.

Peak temperature is readily affected by heating rate and sample amount.
The onset temperature may be used as a value indicating the beginning of a change, but it may vary depending on how the baseline is drawn.
It is important to clearly state which temperature was used.

Discussion of Peak Area

In DSC, the area of an endothermic or exothermic peak corresponds to the amount of heat associated with the change.
The heat of fusion can sometimes be determined from the area of a melting peak, while the heat of crystallization can be determined from the area of a crystallization peak.
The larger the peak area, the greater the energy associated with the thermal change is considered to be.

However, accurate evaluation of peak area requires appropriate baseline setting.
If multiple peaks overlap or the baseline is sloped, an error occurs in the calculated heat.
The heat may also be divided by the sample mass and compared as heat per unit mass.

Discussion When Peaks Overlap

In thermal analysis, peaks may overlap when multiple changes occur within similar temperature ranges.
For example, if dehydration and melting, crystallization and decomposition, or a phase transition and a reaction occur at similar temperatures, they may be observed as one broad peak or as a complex peak shape.
In such cases, it is risky to interpret the result as a single phenomenon.

Combining TG, DTA, and DSC is effective for discussing overlapping peaks.
Checking whether the peak is accompanied by a mass change makes it easier to distinguish dehydration, volatilization, and decomposition from melting, phase transitions, and crystallization.

Discussion of the Baseline

In DSC and DTA, baseline stability greatly affects peak analysis.
If the baseline is sloped, wavy, or drifting, errors occur in reading peak temperature and peak area.
Baseline disturbance is affected by instrument stability, sample containers, atmosphere gas, heating rate, sample amount, and other factors.

Changes such as glass transition, which appear as steps in the baseline, become particularly difficult to read if the baseline is unstable.
Baseline setting is also important when quantifying heat.

Effect of Thermal History

The heating and cooling conditions a sample has experienced in the past are called its thermal history.
Thermal history greatly affects crystallinity, glass transition, melting peaks, and crystallization peaks.
Particularly in polymers and amorphous materials, DSC curves change depending on cooling rate and prior heating.

In DSC, the first heating may show results that include the sample’s previous thermal history, while the second heating may show results for a state standardized by the measurement conditions.
To erase thermal history, the sample may be heated once, cooled, and then reheated for measurement.

Discussion of Thermal Analysis of Polymer Materials

In thermal analysis of polymer materials, glass transition, crystallization, melting, and thermal decomposition are important.
In amorphous polymers, glass transition appears as a major change, while crystalline polymers may show melting and crystallization peaks.
The thermal properties of polymers are affected by molecular weight, crystallinity, crosslinking, plasticizers, additives, and thermal history.

TG can be used to evaluate the decomposition temperature and residue amount of polymers.
DSC can be used to evaluate glass-transition temperature, melting point, crystallization temperature, and heat of fusion.
In polymer materials, combining DSC and TG makes it possible to separately discuss thermal properties and thermal stability.

Discussion of Thermal Analysis of Inorganic Materials

In thermal analysis of inorganic materials, desorption of adsorbed water, loss of water of crystallization, dehydration of hydroxides, decomposition of carbonates, oxidation, phase transitions, and crystallization through calcination may be observed.
TG can be used to confirm mass changes caused by dehydration and decomposition, while DTA and DSC can be used to confirm endothermic and exothermic changes associated with phase transitions and crystallization.

In inorganic salts and hydrates, the number of molecules of water of crystallization may be estimated from the mass-loss percentage.
In metal hydroxides, dehydration may produce oxides, and the mass loss may correspond to the theoretical value.

Discussion of Thermal Analysis of Complexes

In thermal analysis of metal complexes, loss of crystal water or coordinated water, decomposition of ligands, and formation of metal oxides may be observed.
Mass loss at low temperature may correspond to loss of crystal water or solvent, mass loss in the intermediate-temperature region may correspond to loss of coordinated water or part of the ligand, and large mass loss at high temperature may correspond to decomposition of organic ligands.

If the final residue corresponds to a metal oxide, comparison of the residue amount with the theoretical value makes it possible to evaluate the validity of the complex composition.
However, if the decomposition process is complicated and occurs in multiple overlapping stages, simple assignment may be difficult.

Discussion of Thermal Analysis of Pharmaceuticals and Organic Compounds

In thermal analysis of pharmaceuticals and organic compounds, melting point, crystal polymorphs, desolvation, decomposition, purity, and thermal stability are important.
DSC can be used to observe melting peaks, crystal transitions, and glass transitions.
TG can be used to confirm desolvation of solvates and thermal decomposition.

A sharp melting peak may indicate a relatively uniform crystalline phase.
If multiple endothermic peaks are present, crystal polymorphism, melting after desolvation, or melting accompanied by decomposition can be considered.
The TG mass change is used together with DSC to distinguish melting from desolvation.

Reasons Quantitative Values and Temperatures Deviate

Causes of differences between measured temperatures, mass-loss percentages, or heat quantities and theoretical or literature values in thermal analysis include heating rate, sample amount, measurement atmosphere, sample purity, particle size, thermal history, baseline, instrument calibration, and differences in sample containers.
Because thermal-analysis values depend strongly on measurement conditions, differences in conditions must be checked when comparing results with literature values.

Peak temperatures are particularly sensitive to heating rate, and faster heating rates may shift peaks toward higher temperatures.
Mass-loss percentages may deviate from theoretical values because of adsorbed water, residual solvent, impurities, or overlapping decomposition processes.

When Thermal Analysis Can Be Judged to Have Given Good Results

Thermal analysis can be judged to have given good results when the mass changes in the TG curve are clear, the temperature ranges correspond to endothermic and exothermic peaks in DTA or DSC, and the mass-loss percentages and peak temperatures generally agree with theoretical or literature values.
It is also important that the baseline be stable, noise be small, and changes in the sample be readable stepwise.

However, thermal-analysis results are readily affected by measurement conditions, so complete agreement does not need to be demanded.
What is important is that changes in TG, DTA, and DSC do not contradict one another and can be explained as chemical or physical changes in the sample.

Example Discussion When the Experiment Did Not Go Well

When thermal analysis does not go well, possible causes are considered from results such as unclear peaks, disturbed baselines, mass changes that do not agree with theoretical values, overlapping peaks, decomposition temperatures that differ greatly from literature values, and poor reproducibility.
Organizing the causes according to sample amount, heating rate, atmosphere, sample-drying condition, purity, thermal history, and instrument calibration makes the discussion easier.

How to Write Points for Improvement

In a discussion of thermal analysis, including not only sources of error but also points for improvement makes the report easier to organize.
Points for improvement can be divided into sample preparation, measurement conditions, and analysis methods.

Improvements to Sample Preparation

• Dry the sample sufficiently
• Use an appropriate sample amount
• Make the sample uniform
• Standardize particle size
• Place the sample uniformly in the sample container
• Reduce impurities and residual solvent
• Standardize thermal history

Improvements to Measurement Conditions

• Set an appropriate heating rate
• Clearly define the measurement atmosphere
• Keep the gas flow rate constant
• Use a sufficiently wide measurement temperature range
• Calibrate the instrument
• Check empty-cell measurements and blanks
• Repeat the measurement when necessary

Improvements to Analysis

• Read TG together with DTA and DSC
• Calculate the mass-loss percentage
• Distinguish peak temperature from onset temperature
• Classify the causes of endothermic and exothermic peaks
• Set the baseline appropriately
• Check measurement conditions when comparing with literature values
• Judge the results together with other analytical methods

Difference Between a Superficial Discussion and a Good Discussion

In a discussion of thermal analysis, simply writing that “the mass decreased” or that “there was a peak” results in a superficial discussion.
A good discussion relates the temperature range, mass-loss percentage, endothermic or exothermic behavior, correspondence between TG and DSC, and effects of measurement conditions.

Examples of Expressions That Can Be Used in Reports

The following expressions can be used when writing the results and discussion of thermal analysis.
Adjust the necessary parts according to your own experimental results.

• The TG curve makes it possible to confirm changes in sample mass accompanying an increase in temperature.
• Mass loss in the low-temperature region is considered to originate from desorption of adsorbed water or residual solvent.
• Large mass loss in the high-temperature region may correspond to thermal decomposition of the sample.
• An endothermic peak in DTA or DSC may originate from melting, dehydration, evaporation, or a phase transition.
• An exothermic peak may originate from crystallization, oxidation, combustion, or a curing reaction.
• If a peak appears in DSC without a mass change in TG, melting or a phase transition is highly likely.
• Comparison of the mass-loss percentage with the theoretical value makes it possible to evaluate the number of water molecules released or the residue amount.
• Peak temperature is affected by heating rate, sample amount, and measurement atmosphere.
• The area of a DSC peak corresponds to heat quantities such as heat of fusion and heat of crystallization.
• The type of change should not be determined from thermal analysis alone but should be judged together with other results such as XRD, IR, and mass spectrometry.

Points to Check When Discussing Thermal Analysis

Checking the following points before writing the report makes the discussion easier to write.

• Is the measurement method clearly stated?
• Is the heating rate recorded?
• Is the measurement atmosphere clearly stated?
• Is the sample amount recorded?
• Has the temperature of TG mass loss been read?
• Has the mass-loss percentage been calculated?
• Are endothermic and exothermic peaks in DSC or DTA distinguished?
• Are peak temperature and onset temperature kept separate?
• Are the changes in TG related to those in DSC and DTA?
• Are dehydration, melting, crystallization, and decomposition distinguished?
• Are the effects of heating rate and atmosphere considered?
• Do the points for improvement correspond to the sources of error?

Summary

Thermal analysis is an analytical method used to measure changes in sample mass and heat flow accompanying changes in temperature and to investigate dehydration, volatilization, melting, crystallization, phase transitions, thermal decomposition, oxidation, and other phenomena.
TG evaluates mass changes, DTA evaluates the temperature difference between a sample and a reference material, and DSC evaluates changes in heat flow.
By combining these methods, the thermal stability and thermal properties of a sample can be discussed in detail.

Mass loss in a TG curve may correspond to adsorbed water, crystal water, residual solvent, decomposition, or combustion.
Endothermic peaks in DSC and DTA may correspond to melting, dehydration, evaporation, or phase transitions, while exothermic peaks may correspond to crystallization, oxidation, curing reactions, or decomposition reactions.
Checking whether a TG mass change occurs within the same temperature range makes it easier to determine the cause of a peak.

In a report, rather than simply writing that “the mass decreased” or that “a peak appeared,” organize and discuss the temperature range, mass-loss percentage, direction of endothermic or exothermic change, correspondence among TG, DSC, and DTA, heating rate, measurement atmosphere, and effects of sample amount.
Thermal analysis is a powerful method, but identifying a phenomenon with certainty requires judgment in combination with other analytical results such as XRD, IR, and mass spectrometry.