Chemistry
Principles of Thermogravimetric Analysis for Studying Decomposition Pathways
Quick fact
TGA can detect a mass loss of just 1 microgram, allowing you to follow the loss of water, decomposition of a polymer, or oxidation of a metal in real time as a function of temperature.
Why this is interesting
Have you ever wondered why a plastic bottle becomes brittle and discolored when heated, or how scientists know the exact temperature at which a material gives up its water?
Read the full explanation
Understanding Principles of Thermogravimetric Analysis for Studying Decomposition Pathways
Thermogravimetric analysis (TGA) is a technique that measures how the mass of a sample changes as it is heated, cooled, or held at a constant temperature. Imagine placing a tiny sample on a high-precision balance inside a furnace. As the temperature rises, any process that releases a volatile product—water, carbon dioxide, or a decomposition fragment—causes the balance to record a lower mass. The instrument continuously plots mass versus temperature, producing a mass-loss curve. Each step in this curve corresponds to a distinct decomposition reaction, and the temperature at which the step occurs tells you about the material's thermal stability. By controlling the atmosphere (e.g., nitrogen, air, or inert gas), you can even distinguish between simple evaporation, oxidative degradation, and inert pyrolysis. For a beginner, the key is to see TGA as a 'weighing experiment' that reveals the temperature windows where a material changes its composition.
A deeper explanation
The underlying principle of TGA is the precise measurement of a sample's mass while its temperature is programmatically controlled. A furnace heats the sample, and a sensitive thermobalance records the mass. As the temperature increases, thermal energy overcomes the activation energy of chemical bonds, leading to reactions that release volatile species. The mass-loss curve is a direct readout of these events: the cumulative mass loss at any temperature is the sum of all volatiles lost up to that point. The derivative of the mass-loss curve (DTG) helps separate overlapping steps by showing peaks at the temperatures of maximum mass-loss rate. The shape of the curve, the onset temperature, and the residue amount provide a 'fingerprint' of the decomposition pathway. For example, a hydrated salt may show a gradual loss of water of crystallization, followed by decomposition to an oxide at a higher temperature. By varying the heating rate, one can derive kinetic parameters (activation energy, pre-exponential factor) using model-fitting or model-free methods. The atmosphere is critical: in nitrogen, decomposition follows a pyrolysis pathway; in oxygen, oxidative degradation occurs, often at lower temperatures and with exothermic effects. TGA is widely used to compare thermal stability of polymers, determine filler content in composites, and probe oxidation resistance of metals.