Applications of Thermogravimetric Analysis (TGA).

Thermogravimetric techniques have a very wide field of application. The technique can be used in the examination of absorptive surfaces, together with the nature and processes involved in thermal decomposition and oxidation processes. The technique has also been applied to the examination of water of crystallization and in forensic work involving the identification and comparison of varnishes and other surface coating. Thermogravimetric analysis has also been involved in determining the age of art treasures, particularly paintings and in determining the stability of explosives. The technique is commonly used to control the dehydration procedures for crops particularly in the control of the drying processes used for tobacco. Thermogravimetric analysis is also used extensively in the pharmaceutical industry in the examination of drug stability and the rate of degradation of certain drugs when expose to air.

The results from thermogravimetric analyses are usually reported in the form of curves relating the mass lost from the sample against temperature. In this form the temperature at which certain processes begin and are completed are graphically demonstrated. An example of a Thermogravimetric analysis curve obtained from heating a sample of silica gel from 30^oC to 1000^oC is shown in figure 4. The curve shows the loss in weight that occurred at different temperatures, as different types of water are lost from the surface.

Figure 4. The TGA Curve for Silica Gel

Thermogravimetric analysis data can also be presented in a differential form which is often more informative than the normal Thermogravimetric analysis curve. The differential form of a Thermogravimetric analysis curve must not be confused with Differential Thermal Analysis (DTA), which is an entirely different thermal analytical technique. The differential form of the TGA curve shown in figure 4 is shown in figure 5.

Figure 5. The Differential form of the TGA Curve for Silica Gel.

It is seen that the curves demonstrates unambiguously the evolution water from three distinctly different sources and the temperature at which the maximum water loss of each type can be readily identified. The use of thermal methods of analysis will be discussed in considerable data later in this chapter and will include a detailed discussion on the use of different thermal techniques to investigate the nature of the silica gel surface; more informative form of figures 2 and 5 will be included in that discussion.