Thermogravimetry
Thermogravimetry or thermogravimetric analysis (TGA) is a method for the quantification of organic and inorganic components of solid and liquid samples.
Table of contents
How does thermogravimetry work?
During thermogravimetry, a sample is heated in a controlled manner in an inert crucible (often made of platinum or aluminium oxide). Here temperatures of up to 1,500 °C are possible. During heating, the sample chamber is, as a rule, purged with an inert gas such as nitrogen to prevent the undesirable oxidation of the sample by the oxygen in the air. This gas flow can be controlled via a computer connected to the TGA instrument, as can the heating rate or the maximum temperature for example.
The increase or reduction in the mass that the sample experiences during this process is determined with the aid of a microbalance. The relationship between a known temperature and the weight change that occurs at that temperature can be specified for a certain substance and can be used to detect the presence of the material.
How do mass changes in thermogravimetry occur? The mass changes in thermogravimetry can have various causes. A loss of mass occurs due to physical processes such as vaporisation, the decomposition of a sample or a chemical reaction (specifically a reduction), an increase in mass occurs, as a rule, as a result of oxidation, therefore also a chemical reaction.
Thermogravimetry at Quality Analysis
At Quality Analysis, we have a modern thermobalance for the precise assessment of the thermally-related mass changes in the form of the Netzsch TG 209 F1 Libra. Our experts from chemical analytics take the time to specify together with you the analytical method that answers your questions specifically and efficiently. After the analysis you receive a detailed report with an informative interpretation of the results. This information then helps you to identify and rectify the sources of defects and to design your processes more efficiently. Practicality and the highest precision go hand in hand with us.
You can always rely on the quality of our work: We are an accredited test laboratory according to DIN EN ISO/IEC 17025.
Parts of a TGA instrument
A TGA instrument is also termed a thermobalance, a term that also identifies its two most important components: an oven in which the exactly controllable increase in temperature occurs and a balance that measures exactly the mass changes induced by the temperature changes. Obtaining reliable results with the thermogravimetric method also requires that the sample does not react unintentionally with the ambient air during the measurement, for this reason the third relevant part is a gas supply line via which an inert gas can be fed into the oven.
The oven in a thermobalance
The oven in a TGA instrument is used to generate the homogeneous temperature field necessary for the analysis. Unintentional temperature fluctuations must not occur in the instrument because even minor deviations can distort the measurement result. The higher the temperature increase, the smaller the tolerance range becomes.
What is the purpose of the gas supply line?
As already mentioned, the oven is flooded with an inert gas to prevent undesirable oxidation. Here nitrogen is normally used. However, if there is a mass spectrometer after the TGA instrument, helium is also used because it is not in the same detection range as carbon monoxide and therefore does not cause an incorrect CO spectrum.
If the oxidation of the sample is required
For some analyses, the oxidation of the sample can be expressly required, then of course an inert gas is not used, a reactive gas is used instead. Depending on the application, this gas is mostly (synthetic) air or pure oxygen. Other gases or gas mixtures can also be used for special samples.
Determination of the mass
The crucible with the sample is at one end of a beam, at the other end there is a counterweight. If the weight of the sample increases or reduces during the heating, the beam is deflected. This deflection is acquired by a photoelectric sensor. The deflection is then corrected by varying the voltage applied to the magnetic coils such that the beam returns to its initial position. This process is repeated for each change in mass.
The balance: central element for thermogravimetry
During thermogravimetric analysis, the exact determination of the change in the mass is crucial for a reliable measurement result. A very wide range of balance types is used for this purpose. The most widely used are those that operate based on the principle of electromagnetic compensation. A balance beam made of metal is held in position by electromagnetic coils.
Advantages and disadvantages of thermogravimetric analysis
One of the most important advantages of thermogravimetric analysis is the small quantity of sample required. Even a few milligrams are, as a rule, sufficient for a reliable analysis. It is possible to analyse both solid and liquid samples. Also minimal preparation of the sample is necessary.
However, TGA also has limits: it is not able to actively verify the presence of a specific substance. Verification is only indirect because a change in mass is explained by means of corresponding prior knowledge of the temperature-dependent behaviour of a specific substance.
Practical
application
Thermal analytical methods play a major role, above all in plastics analytics, however the method is also used in other areas. As such TGA is used for the quantification of the material composition , for instance to determine the polymer or plasticiser fractions or the filler material content (glass fibres, carbon black, chalk, other inorganic filler materials). It is also possible to determine the residual mass or ash using the method.
The analysis of the thermal decomposition behaviour, for instance the determination of the decomposition temperatures (initial/middle/end), is possible using TGA.
Plastics analytics
Thermogravimetry plays a role during the analysis of plastics because with its aid it is possible to quantify the material composition, in particular in relation to the polymer and plasticiser fractions of the sample, and also in relation to the inorganic filler material content (glass fibres, carbon black, chalk, and others); it is also possible to determine the residual mass or the ash. If more accurate conclusions about the composition of the plastic sample analysed are necessary, thermogravimetry is coupled to FTIR spectroscopy or GC-MS.
Materialography
Materialographic questions can also be answered with the aid of thermogravimetry. For example, it is possible to analyse the maximum temperature load a plastic-coated material is allowed to be exposed to before it oxides, or when oil fractions evaporate from a plastic.
In summary: thermogravimetry
Thermogravimetry (TG) or thermogravimetric analysis (TGA) is a method for analysing samples thermally. Due to the temperature-specific increase or reduction in mass during the heating process, it is possible to draw conclusions as to the composition of the sample. The comparatively straightforward implementation of the method without complex preparation of the sample and the possibility of coupling to other analytical methods are significant advantages of this method.
