Thermogravimetric analysis, or in short, TGA, is a thermal analytical method used for the characterization of a substance by weight loss. It is a technique used to measure the weight of a sample as a function of temperature. TGA involves heating the sample at a constant temperature in a controlled environment and measuring the weight change over time.
TGA gives a clear image of the composition of the analyte. The main focus is to measure how much weight is gained or lost, at a constant heating rate. In fact, these weight changes occur due to evaporation, oxidation, and decomposition of the analyte.
In a thermal gravimetric analyzer, the sample is placed on an ultra-microbalance in a controlled environment. The balance is considered the heart of TGA because it gives precise measurements with connected thermocouples. The thermocouple determines the changes that occur during thermal analysis. It is often used in the fields of materials science, chemistry, and pharmaceuticals.
The graph obtained in TGA is known as a thermogram.
Principle
The working principle of thermogravimetry is that the mass of a substance is directly related to its chemical composition and physical structure. By heating the substance and measuring the changes in its mass over time, it is possible to determine the physical and chemical nature of the substance.
In TGA, the weight of the sample analyte changes at an increasing temperature at a constant rate. Note that, the atmospheric conditions and pressure remain constant.
Moreover, a similar technique differential thermal gravimetry (DTG) using the same principle, measures the rate at which the mass changes occur with temperature. However, DTG is more useful in interpreting overlapping curves in thermograms.
What thermal gravimetric analysis can tell about the analyte?
- Thermal stability
The fingerprint thermograms and trends of thermal decomposition explain the thermal stability of the compounds.
- Activation energy
The activation energy of the given sample by values at which the mass changes. For example, the activation energy of corn stalk is found by pyrolysis in TGA. For this purpose, the corn stalk is heated at five different rates, then the reciprocal of temperature (1/T) is directly plotted against the natural log of rate constants “ln(k)” to deduce the slope.
- Oxidation studies
The TGA of an analyte can be performed in an oxygen environment to study its oxidation process. It is also used to study the following reactions.
- Corrosion
- Dehydration
- Removal of water of crystallization
- Desorption
- Decomposition kinetics
Thermogravimetric anlyser
The instrument used for TGA mainly consists of thermobalance and a recorder. A schematic diagram shows the main components of this instrument. It shows the heating furnace and the microbalance is the main discriminator. However, the temperature is controlled with the cooling chamber.
Instrumentation
TGA consists of the following components:
Heating Furnace
It is a compartment for heating sample analyte in a controlled environment. It can be opened and closed as per need. It ranges from -20ºC to 1600ºC . Gases are introduced through three inlets, while effluents are removed from the other side. It is to be noted that the heating furnace contains the sample holder.
Weighing balance
The weighing balance is present above the furnace of TGA in the main chamber/cell. The sample holder is counterbalanced with it.
Cooling system
The heating furnace is connected through a pipeline of the cooling system, which controls the temperature of the cell. It significantly regulates gradual heating and cooling condition to avoid quenching.
The common coolants employed in TGA are; a 19% solution of Ethylene glycol and liquid nitrogen.
Data representation system
A dedicated PC displays the working and changes in the curve. it is used for the evaluation of thermogravimetric analysis curves. The software connects the TGA module with the PC. Temperature programming and data acquisition can be conveniently performed here.
Pan/ Crucibles and their types
The common types of pans used in the TGA furnace are explained below:
- Alumina Pans
Aluminum crucibles are inert pans of aluminum oxide. They remain intact up to 1700ºC, therefore most commonly used in pyrolysis. They are reused after cleaning after every analysis. For cleaning purposes, they are placed in a strongly alkaline solution for several hours and then placed in a muffle furnace for heat treatment.
- Platinum pans
They are sophisticated pans designed especially for the analysis of TGA/ DSC because of their high thermal conductivity.
- Aluminum pans
Aluminum has a low melting point so its max usage range is 650ºC. They are used in TG/ DSC analysis where conductivity is the main concern to quantify the substance.
Sample preparation and insertion
A sample in the solid (grounded) or liquid form can be used in TGA. After inserting the sample in the crucible, this crucible is placed on the sample holder.
There are several methods of sample placement practiced as per the requirement or type of sample. For example, autosampler.
Detector
The sample holder is connected with a highly sensitive balance and also with a gold or platinum thermocouple. It measures the initial temperature and the respective weight and then continuously records the reading after small intervals. These collected data points are then plotted at mass versus temperature graph.
Calibration
Calibration is an important step before starting the analysis. Since It can remove errors/variations during TGA.
TGA instrument is calibrated in 3 steps which are detailed below:
- Balance calibration
It can be done in two ways; internal calibration and external weight calibration. The internal calibration is automatically done if you press the internal calibration button after closing the window of the cell. While the external weight calibration is performed by removing the pan, and zero or tare the balance. Then the standard weights are placed as a reference. This procedure should be repeated three times.
- Furnace calibration
Furnace calibration is a point calibration routine that has limits defined by the user. Therefore, after assigning the limits it is allowed to heat and calibrated accordingly.
- Temperature calibration
Temperature calibration is done by using curie point reference materials. Such materials lose their affinity of magnetism beyond a specific temperature. For example, nickel has a specific curie point and at that temperature, its magnetic properties are diminished and thus can be used as a standard.
Mechanism of weight changes in TGA
The weight changes that occur in TGA (at elevated temperatures) are due to a physical or chemical change. There can be weight loss as well as weight gain. The process that causes these changes is given below:
The processes that cause weight loss:
- Evaporation: At high temperatures, the volatile substances evaporate.
- Thermal decomposition: The breaking down of chemical compounds
- Reduction: The loss of oxygen
- Desorption: The removal of adsorbed substances
The processes that cause weight gain in TGA:
- Oxidation: It happens when the sample interacts with atmospheric oxygen
- Absorption
- Adsorption
- Nitride formation
Thermo gravimetric analysis (TGA) vs. differential thermal analysis (DTA)
Thermal gravimetric analyzer (TGA) and differential thermal analyzer (DTA) both are thermal analysis modules. Their instrumentation and method of analysis are almost the same but the TGA only measures the weight changes upon heating at a constant rate whereas, DTA can record the changes in the sample where even no weight gain or loss occur, for example, crystal lattice change, glass transition, melting and boiling, etc.
Types of TGA curves
The curves obtained as a result of weight changes are classified on the basis of shapes. some standard curves are discussed below:
1. No significant change in mass is observed over the provided range of temperature
2. Drying or desorption occurs
3. Single-step decomposition
4. Multistep decomposition
5. Oxidation
6. Oxidation or any other atmospheric reaction followed by decomposition
Factors affecting Thermograviteric (TGA) curves
- Heating rate
- Sample size
- The particle size of the sample
- Crucible shape and type
- Gas flow rate
- Gas type
Effect of heating rate
The sample specimen is affected in different ways by the various heating rates. For example, at a heating rate 2-degree per minute, the decomposition of polytetrafluoroethylene occurs at 556ºC, while at the rate of 20ºC/min, the same happens at 650ºC.
TG/DSC: Coupled instruments
Thermal gravimetric-Differential scanning calorimetry (TG/DSC) is a versatile thermal instrument in which both weight change and heat transfer are observed simultaneously. In this instrument, a sample holder containing two places for the crucible is used; one pan is a reference and the other is for a sample crucible.
Highly sensitive thermocouples are used to monitor the minor heat transfer and both graphs are plotted together; one graph show enthalpy changes and the other represents the weight loss.
Quantitative analysis with TGA
All chemical substances have unique thermal behaviors. When they are heated in a controlled environment at a constant rate, the weight changes can be used for quantitive aspects. As weight loss and temperature are the main variables here, they can be monitored precisely with TGA.
For example, the calcium oxalate monohydrate thermogram clearly describes its multi-step decomposition upon heating. In the first step water of crystallization is removed, and a dip in the weight is observed. Afterward, carbon monoxide (CO) is released as a gas and the calcium oxalate is converted into CaCO3. In the last step, carbon dioxide (CO2) is eliminated and calcium oxide (CaO) remains as ash.
Applications
- Determination of accurate drying temperature
- Analysis of the burning point, and ash content
- Deduction of the composition of materials and their characterization
- Thermal stability
- Material identification and purity assessment
- Corrosion studies of chemical substances
- Aging studies
Concepts Berg
Why is thermogravimetric analysis important?
Thermal analysis is important for a comprehensive study of the thermal behavior of chemical compounds. When a substance is heated at a constant rate its decompositional kinetics tells about the activation energy, thermal stability, and diagnosis of impurities.
What is TGA in polymer?
TGA can analyze polymers. The chemical changes with temperature may be used to identify the structures and chemical composition.
What is the use of thermo gravimetric analysis?
Thermal gravimetric analysis is used for the thermal characterization of an analyte with respect to weight loss.
Why do we use nitrogen gas in thermogravimetric analysis?
In thermogravimetric analysis nitrogen gas is used in three ways:
- Purge gas
- Protective gas
- Method gas
What does ‘Thermo-chemical analysis’ mean?
The thermochemical analysis known as thermochemistry is the study of enthalpy changes during a chemical reaction.
What is gravimetric water content?
During gravimetric analysis, water content is lost near 100ºC. This can be identified as weight loss.
What is the purpose of using Thermogravimetric analysis and how to interpret the TGA graph?
TGA graph can be interpreted as the weight changes as a function of temperature. The weight on the y-axis (ordinate) is normalized. While the temperature change is measured on the x-axis. The weight changes appear as a dip or increase in the curve.
How is a TGA Thermal Curve displayed?
TGA curve is weight as a function of temperature. It is also known as a thermogram. Weight is normalized as 100% at starting temperature, and changes in it can be seen as the change in the curve.
What is reproducibility and why is it important?
IN TGA reproducibility has key importance. Its most important application is a comparative study of two or more products at a given temperature range.
How do I clean the TGA furnace?
TGA furnaces can be cleaned by heating at 1000ºC or more temperature with a stay of 30 minutes at this particular elevated temperature in the excessive supply of oxygen. This process will remove the carbon deposits that remain after numerous analyses.
How can the TGA be used as a QA/QC tool to ensure products meet their material specifications?
TGA can be used to ensure the product meets its required specifications. For example, fiberglass is a mixture of glass and resin. The final product should have 44% of resin and 56% of glass. This can be determined by using the TGA curve. In addition, the durability of rubbers can be easily evaluated by it.
Can the TGA be used to determine carbon content?
TGA can be used to calculate the carbon content by using a coupled TGA with Fourier transform infrared spectroscopy (FT-IR) or mass spectrometry (MS).
Can the TGA be used to identify safe operating temperatures in various gases?
TGA describes the degradation pattern of the analyte at a range of temperatures. It can therefore be used to identify the safe operating temperature of the chemical product.
How can the TGA be used to enhance product formulation processes?
TGA can be used to determine the analyte during its production such that the impurities and reactants can be identified in the final product. Therefore, TGA is a good tool for quantitative-level analysis of the production unit.
How can the TGA be used to reverse engineer a product?
TGA gives a complete view of weight changes during the heat at a range of temperatures. It can be used to reverse engineer a product and its composition.
What is evolved gas TGA analysis?
In TGA analysis oxides are formed in an oxygen environment such as CO2 and CO. Whereas, in a nitrogen environment, the sample reacts to form nitrides and other compounds of nitrogen.
Can TGA analyze nanomaterials?
TGA gives a figure print thermogram for a material. The scientist used TGA for the study of nanomaterials by using its thermal behavior.
References
- Thermogravimetry (Mettler toledo)
- Thermogravimetric analysis (PerkinElmer)
