Factors affecting the results of thermogravimetric analysis

Factors Affecting Thermogravimetric Analysis (TGA) Results

Let's take a closer look at the factors that may influence the results of a thermogravimetric analysis. If you are interested in this topic, read on!

1. Sample Mass and Crucible
In thermogravimetry, the sample mass should be small, typically 2–5 mg. This is partly because the instrument's balance is highly sensitive (up to 0.1 μg). Moreover, a larger sample mass increases mass transfer resistance and the internal temperature gradient. The heat effect generated by the sample itself may even cause its temperature to deviate from the linear programmed heating, altering the TG curve. Finer particle size is preferred, and the sample should be spread as evenly as possible. Large particle sizes can shift decomposition reactions to higher temperatures.

The crucible material must be heat-resistant and inert to the sample, intermediates, final products, and the atmosphere - meaning it must lack reactive or catalytic activity. Common crucible materials include platinum, ceramic, quartz, glass, and aluminum.

Crucially, different samples require different crucible materials; otherwise, the crucible may be damaged. For instance, sodium carbonate reacts with SiO₂ in quartz or ceramic at high temperatures to form sodium silicate. Therefore, for alkaline samples like sodium carbonate, do not use aluminum, quartz, glass, or ceramic crucibles.

Platinum crucibles are active towards organic compounds involving hydrogenation or dehydrogenation and are unsuitable for polymer samples containing phosphorus, sulfur, or halogens. Selection must be made with care.

2. Heating Rate
The faster the heating rate, the more severe the temperature lag. For example, when polystyrene decomposes in N₂, if we define the decomposition point as 10% weight loss, the temperature measured at 1°C/min is 357°C, whereas at 5°C/min it is 394°C—a difference of 37°C. A faster heating rate reduces the resolution of the curve and may cause the loss of information regarding certain intermediates. For example, slow heating of hydrated compounds allows for the detection of intermediates formed by stepwise dehydration.

3. Atmosphere Influence
Changes in the atmosphere surrounding the thermobalance significantly affect the TG curve. The TG curves of CaCO₃ in vacuum, air, and CO₂ atmospheres show a decomposition temperature difference of nearly 600°C. This is because CO₂ is a decomposition product of CaCO₃; the presence of CO₂ in the atmosphere inhibits the decomposition of CaCO₃, thereby raising the decomposition temperature.Polypropylene shows a distinct weight gain in air at 150–180°C due to oxidation, whereas no weight gain occurs in N₂. The gas flow rate is generally 40 ml/min; a higher flow rate is beneficial for heat transfer and the diffusion of evolved gases.

4. Volatile Condensation
Decomposition products volatilized from the sample often re-condense in cooler areas. If they condense on the suspension wire or crucible, it results in a lower measured weight loss. As the temperature rises further, the re-volatilization of these condensates creates "false weight loss," distorting the TG curve. The solution is generally to increase the gas flow rate to ensure volatiles leave the crucible immediately.

5. Buoyancy
Buoyancy changes occur because heating causes the gas surrounding the sample to expand, reducing its relative density and thus the buoyancy, which leads to an apparent weight gain of the sample. For instance, buoyancy at 300°C can drop to half of that at room temperature, and at 900°C, it drops to about 1/4. A practical correction method is to perform a blank test (empty load thermogravimetry) to eliminate the apparent weight gain.

That concludes our sharing on the factors affecting thermogravimetric analysis results. We hope this helps you gain a better understanding of the instrument.