TGA finds extensive application in quantifying thermal stability, decomposition modes, and compositions, by monitoring weight loss with temperature or time as the independent variable. TGA, however, does not give the chemical identity of the volatiles released. The evolved gases may also be analysed directly by coupling with QMS. Quadrupole Mass Spectrometry separates and measures gases based on mass-to-charge ratio (m/z), which allows identification of molecules including CO2, H2O, NH3, or organic fragments.
Instrumentation for TGA-QMS
It is built up as a thermobalance, a high-precision furnace, and a quadrupole mass spectrometer combined by a heated transfer line. The TGA quantifies very low weight variations when the sample is heated in a controlled atmosphere (air, oxygen, nitrogen, argon, etc.) and the gases evolved are continuously swept into the QMS. The quadrupole analyser uses oscillating electric fields that selectively filter ions of various mass-to-charge ratios, which are then detected by a multiplier of electrons. Condensation or further reactions of a volatile product do not occur because a heated capillary is used.
Principle and Methodology
The TGA principle is grounded on the gravimetric determination of mass change of the sample during heating under a controlled environment, whereas QMS is founded on the ionization of gas molecules and the separation of these by mass-to-charge ratio. A sample may be thermally decomposed, oxidized, or desorbed during an experiment. These gases are partially ionized, fragmented, and analyzed in the QMS, which gives molecular fingerprints of the volatile species.
Strengths
- Gives thermal and chemical data at the same time.
- Able to detect and identify even trace-level volatile species.
- In real-time analysis, there is no sample handling contamination.
Limitations
- Expensive instrumentation and the requirement of trained staff.
- Fragment peaks can be generated by ionization in QMS, and their interpretation should be done with caution.
- Inadequate sensitivity of very high molecular weight or low-volatility compounds.
Uses of TGA-QMS
The polymer research applies TGA-QMS to provide an understanding of thermal degradation processes and the release of additives. It assists in identifying residues of solvents and decomposition products in pharmaceuticals. It is used in geology and materials science to examine minerals, carbonates, and clays with respect to water and CO2 release.
Related Techniques to TGA-QMS
Other combined methods are TGA-FTIR, where evolved gases are detected by infrared spectroscopy, and TGA-GC-MS, where gas separation is done by gas chromatography before MS analysis. Complementary thermal data, like enthalpic changes, can be provided by Differential Scanning Calorimetry (DSC).
