Thermogravimetric analysis (TGA) measures how sample mass changes with temperature or time under a controlled atmosphere. It is widely used to study dehydration, volatilization, decomposition, oxidation, combustion, thermal stability, and compositional changes in materials.
Step Isothermal TGA is an adaptive thermogravimetric analysis method that inserts isothermal holds when the real-time mass-loss rate crosses a defined threshold. The hold gives an active dehydration or decomposition event more time to progress before the furnace enters the temperature range of the next event. This can improve separation of overlapping TGA and derivative thermogravimetry (DTG) features while preserving a practical overall test workflow.
In a TGA experiment, a sample is placed on a sensitive balance inside a programmable furnace. The instrument continuously records mass while temperature and atmosphere are controlled. Processes such as evaporation, desorption, dehydration, thermal decomposition, oxidation, and combustion can produce measurable mass changes.
The primary thermogravimetric curve is commonly plotted as mass or mass percent versus temperature or time. Derivative thermogravimetry (DTG) plots the rate of mass change, making individual events appear as peaks. DTG is especially useful when small or partially overlapping mass-loss steps are difficult to distinguish on the main TGA curve.
For a broader overview of how TGA fits alongside DSC, STA, and TMA, see AMI’s thermal properties analysis overview.
Two components do not need to decompose at exactly the same temperature to overlap. If the first process is still occurring while a second process begins, their mass-loss signals can merge into one broad transition. This is common in crystalline hydrates, polymer blends and composites, pharmaceutical materials, cementitious systems, and other multi-component samples.
Variable-heating-rate TGA has been shown in published work to improve separation and efficiency for multi-step mass-loss profiles, supporting the broader principle that adapting the thermal program can improve resolution.
Step Isothermal TGA is an adaptive temperature-control workflow implemented in AMI’s InfinityPro TGA software. Instead of following a fixed temperature ramp throughout the experiment, the method monitors the real-time derivative weight-loss rate and changes the temperature program when a mass-loss event is detected.
|
Attribute |
Conventional Linear-Ramp TGA |
Step Isothermal TGA |
|---|---|---|
|
Temperature program |
Fixed ramp |
Ramp with event-triggered isothermal holds |
|
Response to active mass loss |
Continues heating |
Holds temperature when threshold is exceeded |
|
Main resolution strategy |
Method-development choices such as heating rate, sample mass, and atmosphere |
Adaptive thermal program plus normal method-development controls |
|
Best use |
General thermal stability, decomposition, and composition studies |
Samples with closely spaced or overlapping mass-loss events |
|
DTG behavior |
Overlapping peaks may remain merged |
Event separation may improve when the hold allows one process to progress before the next temperature range is reached |
AMI’s application note describes two control parameters:
A practical method-development sequence is:
Thresholds are method-development parameters, not universal constants. They should be validated for the material and measurement goal.
AMI’s Step Isothermal application note demonstrates the method on a barium chloride hydrate sample. In a conventional experiment at 20°C/min, two dehydration-related mass-loss events near approximately 80°C and 110°C appear as a broadened, poorly separated transition.
With Step Isothermal control, the first event triggers an isothermal hold. The temperature remains constant while that mass-loss process progresses, then the ramp resumes before the second event. In the AMI data, this produces two more clearly separated mass-loss steps from the same material and provides a clearer basis for quantifying each event.
This example illustrates the purpose of Step Isothermal TGA: not to identify chemistry by mass loss alone, but to improve separation of thermal mass-change events that may otherwise overlap. Where chemical identification of evolved species is required, complementary techniques such as FTIR or mass spectrometry may be appropriate.
For a related experiment that measures both mass change and heat flow on barium chloride dihydrate, see AMI’s simultaneous thermal analysis of barium chloride dihydrate.
Sequential loss of water from different binding environments can generate closely spaced dehydration events. AMI’s TGA crystalline hydrates study shows how isothermal TGA can also be used to investigate dehydration kinetics and mechanism.
Polymer components, additives, char formation, and filler-related processes can generate multi-step mass-loss profiles. Improving separation can make the contributions of individual stages easier to interpret.
Water, residual solvents, hydrate transitions, and decomposition can occur over nearby temperature ranges. Step Isothermal control can be useful when the analytical objective is to distinguish overlapping mass-loss processes.
Hydration, dehydroxylation, carbonate decomposition, and other mineral-related processes can occur in adjacent thermal regions. Adaptive holds can complement careful atmosphere and sample-preparation control when improved event separation is required.
Resolution depends not only on temperature programming but also on atmosphere control. For experiments that require strongly inert conditions, see AMI’s guide to oxygen-free TGA analysis and purge-gas verification.
AMI documents Step Isothermal operation within the InfinityPro software used with its TGA platform. The AMI TGA 1000/1200/1500 thermogravimetric analyzers combine a 0.1 μg balance resolution with temperature-range options from ambient to 1000°C, 1200°C, or 1500°C depending on the model.
The TGA Series also supports conventional weight-loss analysis, derivative analysis, residual-mass calculations, programmed atmosphere control, and additional high-resolution TGA workflows. Configuration details vary by model, so current specifications should be confirmed on the product page or with AMI before selecting a system.
Relevant commercial option: If your work requires separation of overlapping decomposition or dehydration events, review the AMI TGA 1000/1200/1500 series or explore the broader range of thermal analysis systems.
Thermogravimetric analysis is a thermal-analysis technique that measures sample mass as a function of temperature or time under a controlled atmosphere. It is used to study mass-change processes such as evaporation, dehydration, decomposition, oxidation, combustion, moisture loss, and residual ash or inorganic content.
Derivative thermogravimetry is the derivative of the TGA mass curve with respect to temperature or time. DTG converts mass-loss steps into peaks, which makes the rate and separation of individual thermal events easier to see.
Overlap occurs when one mass-loss process is still active as another begins. Heating rate, sample mass, sample geometry, atmosphere, gas flow, particle size, and reaction kinetics can all influence how strongly the events overlap.
Step Isothermal TGA is an adaptive temperature-control method that monitors the derivative mass-loss rate during a TGA experiment. When the rate exceeds a selected threshold, the method inserts an isothermal hold; when the rate falls below a lower resume threshold, the temperature ramp continues.
AMI recommends using a conventional TGA or DTG result to estimate the expected peak mass-loss rate, then using that information as a starting point for threshold selection. The final settings should be validated for the sample, noise level, event shape, and analytical objective rather than treated as universal values.
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