Evolved Gas Analysis by TG-FTIR

Significance of the Topic

Thermogravimetric analysis combined with Fourier transform infrared spectroscopy (TG-FTIR) provides a powerful approach to monitor mass changes in samples while simultaneously identifying evolved gaseous species. This dual capability is crucial for elucidating decomposition pathways, assessing material stability, optimizing industrial processes, and ensuring quality control in pharmaceuticals, polymers, minerals, and environmental samples.

Objectives and Study Overview

The aim of this work is to present a TG-FTIR system configuration and to illustrate its analytical power through two representative examples: the thermal decomposition of calcium oxalate monohydrate and the oxidative degradation of polyethylene terephthalate (PET) in air.

Methodology

A thermogravimetric analyzer is directly coupled to an FTIR spectrometer via a heated transfer line to prevent condensation of evolved gases. Samples are heated at a constant rate of 20 °C/min under an air atmosphere. The FTIR records IR chromatograms by monitoring absorbance changes at selected wavenumbers over time. Spectra corresponding to key temperature or time points are extracted for qualitative identification of gaseous products.

Instrumentation Used

• Thermogravimetric analyzer with DTA capability
• Fourier transform infrared spectrometer equipped with a heated gas cell
• DLATGS detector for mid-IR sensitivity
• Temperature-controlled transfer line and gas cell to maintain 150–200 °C
• Data acquisition software for synchronizing weight loss and IR signals

Main Results and Discussion

• Calcium oxalate monohydrate: TG-DTA curves reveal three major weight-loss events: dehydration at ~175 °C (–12%), CO formation and oxidation near 460 °C (–19%), and secondary CO2 evolution at ~720 °C (–30%). IR chromatograms at 1508 cm⁻¹ and 2361 cm⁻¹ confirm H2O and CO2 release, respectively. Extracted spectra demonstrate stepwise gas evolution and oxidation processes.
• PET in air: The TG and derivative curves show two decomposition steps with total mass loss exceeding 93%. IR chromatograms track CO2 (2361 cm⁻¹) and carbonyl fragments (1760 cm⁻¹). Early-stage spectra indicate rapid benzoic acid release, while later stages reveal gradual formation of ester compounds, alongside two distinct CO2 evolution peaks.

Benefits and Practical Applications

• Real-time correlation of mass loss with gas composition for mechanistic insights
• Detection and quantification of trace volatiles in polymers, minerals, and complex mixtures
• Quality control in manufacturing by monitoring decomposition and additive evaporation
• Environmental and forensic analysis through identification of combustion and degradation products

Future Trends and Potential Applications

• Integration with mass spectrometry and gas chromatography for enhanced sensitivity and speciation
• Advanced chemometric approaches to deconvolute overlapping spectral features
• Miniaturized and portable TG-FTIR platforms for in situ and field analyses
• High-pressure and reactive atmosphere studies to mimic industrial conditions

Conclusion

The presented TG-FTIR configuration delivers comprehensive thermal and spectroscopic data, enabling detailed characterization of decomposition pathways and evolved gases. Demonstrations with calcium oxalate and PET highlight its applicability across mineral and polymer analysis, supporting both research and quality control objectives.

References

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