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