In this technique (TG or TGA), changes in the mass of a sample are studied while the sample is subjected to a controlled temperature programme. The temperature programme is most often a linear increase in temperature, but isothermal studies can also be carried out, when the changes in sample mass with time are followed. (There is also a family of newer control techniques - i.e. Controlled Rate methods, and Hi-Res™ TG, described elsewhere.)

The main processes amenable to study are listed below.

TG is inherently quantitative, and therefore an extremely powerful thermal technique, but gives no direct chemical information. The ability to analyse the volatile products during a weight loss is of great value.

The essential components of the equipment used, called a thermobalance, are a recording balance, furnace, temperature programmer, sample holder, an enclosure for establishing the required atmosphere, and a means of recording and displaying the data.

Balance sensitivity is usually around one microgram, with a total capacity of a few hundred milligrams. A typical operating range for the furnace is ambient to 1000°C, with heating rates up to 100°C/min. The quality of the furnace atmosphere deserves careful attention, particularly the ability to establish an inert (oxygen-free) atmosphere, and it is useful to be able to quickly change the nature of the atmosphere. Compatibility between the materials of construction and the sample and its decomposition products, and the gaseous atmosphere, must be considered. Sample holder materials commonly available include aluminium, platinum, silica, and alumina.

Indication of the sample temperature is by a thermocouple close to the sample. Careful calibration for temperature is important, especially for kinetic studies. Various means are available for temperature calibration, which is not a trivial matter, though reproducibility is often more important than absolute accuracy. Weight calibration is readily achieved using standard weights.

Many factors influence the form of the TG curve, both sample- and instrument-related, some of which are interactive. The primary factors are heating rate and sample size, an increase in either of which tends to increase the temperature at which sample decomposition occurs, and to decrease the resolution between successive mass losses. The particle size of the sample material, the way in which it is packed, the crucible shape, and the gas flow rate can also affect the progress of the reaction. Careful attention to consistency in experimental details normally results in good repeatability. On the other hand, studying the effect of deliberate alterations in such factors as the heating rate can give valuable insights into the nature of the observed reactions.

TG has been applied extensively to studying analytical precipitates for gravimetric analysis. One example is that of calcium oxalate monohydrate, as shown below. This material has become popular for demonstrating thermobalance performance, as it gives three distinct weight losses over a wide temperature range.

The plot also shows the derivative of the TG curve, or the DTG curve, which is often useful in revealing extra detail, such as the small event around 400°C, which would not have been seen on the TG curve itself. The DTG curve is sometimes used to determine inflection points on the TG curve, to provide reference points for weight change measurements in systems where the weight losses are not completely resolved.

The measured losses above agree well with the theoretical losses, according to the usual scheme

CaC₂O₄.H₂O ® CaC₂O₄ ® CaCO₃

apart from that due to decomposition of the anhydrous oxalate to calcium carbonate around 500°C. The curve gives a value of 18.7% lost, as compared to the theoretical value of 19.2%. This is due to the disproportionation of carbon monoxide into carbon dioxide and carbon. CO₂ has been seen to evolve in parallel with CO by evolved gas analysis. The residual calcium oxide from an experiment such as that above is pale grey, due to the carbon deposited. Changing the atmosphere in the thermobalance to air after the decomposition of the carbonate around 700°C would initiate a weight loss due to the oxidation of this carbon. The extent to which the disproportionation reaction occurs is influenced by several factors including the nature and cleanliness of the material from which the sample container is made, and it is difficult to obtain reproducible results for this reaction step. It is therefore unwise to use this material as a weight change standard, as has been sometimes suggested, when operating in inert atmosphere. Even in an oxidising atmosphere, difficulties may arise in obtaining a reproducible result. In this case, the CO produced on decomposition is oxidised quantitatively to CO₂, in a strongly exothermic process. The heat generated raises the sample temperature, and accelerates the reaction, resulting in a sharper DTG peak. The sharpness of the peak however depends on the catalytic activity of the sample holder material. Careful cleaning is needed to restore the activity of a new platinum holder, which is the most commonly used pan material.

The absence of a grey residue after running calcium oxalate in an inert atmosphere would probably be due to traces of air in the system. A good test of the inert atmosphere quality in a thermobalance is to hold a sample of carbon black at 1000°C, and measure the rate of weight loss. A well-made instrument, after thorough initial purging with inert gas, should be able to give a result of below 1m g/min. For the best results with highly sensitive materials (e.g. finely-powdered metals) it may well be necessary to deliver the inert gas via metal pipes, and to use an oxygen scrubber.

The ability of TG to generate fundamental quantitative data from almost any class of materials, has led to its widespread use in every field of science and technology. Key application areas are listed below:

• Thermal Stability: related materials can be compared at elevated temperatures under the required atmosphere. The TG curve can help to elucidate decomposition mechanisms.
• Kinetic Studies: a variety of methods exist for analysing the kinetic features of all types of weight loss or gain, either with a view to predictive studies, or to understanding the controlling chemistry.
• Material characterisation: TG and DTG curves can be used to "fingerprint" materials for identification or quality control.
• Corrosion studies: TG provides an excellent means of studying oxidation, or reaction with other reactive gases or vapours.
• Simulation of industrial processes: the thermobalance furnace may be thought of as a mini-reactor, with the ability to mimic the conditions in some types of industrial reactor.
• Compositional analysis: by careful choice of temperature programming and gaseous environment, many complex materials or mixtures may be analysed by selectively decomposing or removing their components. This approach is regularly used to analyse e.g. filler content in polymers; carbon black in oils; ash and carbon in coals, and the moisture content of many substances.