Vacuum Thermogravimetric Analysis and Evolved Gas Analysis by Mass Spectroscopy
The combined techniques of vacuum thermogravimetric analysis (TGA) and evolved gas analysis (EGA) by mass spectrometry (MS) can be productive tools for mineral studies. In its simplest configuration, a TGA-EGA system correlates quantitative thermal weight-loss with semiquantitative evolved gas data. Thus, each weight-loss event can be assigned to the volatile species identified in that temperature interval. More sophisticated systems are capable of quantitative EGA data, with volatile abundances determined directly by integration of evolved gas peaks and verified with TGA measurements. Alternatively, the ratio of evolved gases may be used to apportion weight losses.
The vacuum TGA-EGA results from cyanotrichite, Cu4Al2(SO4)(OH)12.2H2O , illustrate the importance and power of the combined technique. Even the small size sample (1.18 mg) used in this experiment produces good thermogravimetric information (Figure 1) because the losses during heating are relatively large. The TGA curve consists of five distinct weight-loss steps plus minor inflections that suggest further complexities. Except for assuming that H2O will be lost at a lower temperature than OH, such a curve is impossible to interpret with any certainty without further information. The EGA curves (Figure 1) provide this information. The curves of ionized waterl , H2O+, and one of its fragments, O+, which peak at 61°C, indicate that the first weight-loss event is due to the loss of H20. Similarly, the second two events, which peak at 168°C and 308°C, represent the evolution of H2O during dehydroxylation of the mineral. The sensitivity of the instrument is demonstrated by the detection
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Thermal analysis involves the observation of a physical property of a sample and how that property changes in response to a change in temperature. Thus, the essence of this group of techniques includes the measurement of a physical property, e.g. mass, temperature, and volume, and the control of temperature. Inasmuch as heating objects is a very ancient practice, one should not be surprised that the first observations of the response of certain materials to heat were made quite some time ago. Such observations might be considered as a form of thermal analysis (Mackenzie, 1981), but serious investigations required that the temperature be known with reasonable accuracy. Temperature measurements, especially of a solid material that is being heated rapidly, was first ccomplished with a thermocouple. Two events, then, mark the beginning of thermal analysis. The first was the invention of the thermocouple. This led directly to the study of the thermal properties of a group of clay minerals. In fact, thermal analysis, in the modern sense, started with a simple description: “Si I 'on echauffe rapidement unepetite quantite d'argile, il seproduit, au moment de la deshydratation, un relentissement dans l'elevation de temperature...” (if one heats rapidly a small quantity of clay, there is, at the point of dehydration, a slowing in the increase in temperature...) (Le Chatelier, 1887). The temperature at which the dehydration occurred was determined for each of the clay minerals examined by Le Chatelier, and he pointed out that the temperature at which dehydration occurred could be used to distinguish between and to identify different clay minerals.