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The use of mass spectrometry (MS) in the identification of evolved gaseous species in thermogravimetric (TG) experiments is well established. The mass spectrometer can be attached directly to the outgas port of the thermogravimetric instrument. Mass spectrometers have the ability to detect species to 1 ppm or better, and the detection is done in real time as the TGA scans. Typically, the mass spectrometer is used for identification only. However, there are situations where it would be desirable to quantify the MS data. For example, when two or more species are evolved during a weight transition, the total weight loss is known from the TGA, but the weights for the individual components are unknown. Calibration techniques for quantifying MS data are already well established. Following one of these techniques, we show that trend data from a mass spectrometer can be used to approximate the amount of material evolved for low m/e ratio species such as water and carbon dioxide. The process involves first calibrating the system using a sample that only gives off the gaseous species of interest. Multiple runs at different initial weights provide the data for the calibration. A correlation between mass loss as measured by the TGA and ion current increase as measured by the MS is then constructed. In this study, the technique is applied not only to overlapping transitions but also to the quantification of reaction products from gases released during a TGA experiment. An attempt is also made to quantify the accuracy of the technique.
Thermogravimetry, Mass Spectrometry, Compositional Analysis
Slough, Carlton G.
Applications Chemist, TA Instruments, New Castle, DE