Evolved Gas Analysis by TG-FTIR

Importance of the Topic

Evolved gas analysis by coupling thermogravimetry (TG) with Fourier transform infrared spectroscopy (FTIR) enables simultaneous measurement of mass changes and qualitative identification of gaseous products during thermal events. This approach is critical for understanding decomposition pathways, assessing material stability, investigating pyrolysis mechanisms, and supporting quality control in various industries.

Objectives and Study Overview

This application study demonstrates TG-FTIR analysis of two representative samples: calcium oxalate monohydrate and polyethylene terephthalate (PET). The goals are to correlate thermal transitions detected by TG-DTA with the evolving gas species identified by FTIR, and to illustrate how spectral data reveal detailed reaction mechanisms.

Methodology and Instrumentation

The evolved gas analysis system transfers gases from the TG furnace via a heated, temperature-controlled line into an FTIR gas cell. Infrared absorption is monitored over time to generate chromatograms and extract spectra at key temperatures.

• Thermogravimetric Analysis (TG-DTA) conducted under air with a heating rate of 20 °C/min.
• FTIR detection using a DLATGS detector at resolutions of 8 cm⁻¹ for calcium oxalate and 4 cm⁻¹ for PET.
• Spectral acquisition intervals of 15 seconds (calcium oxalate) and 30 seconds (PET) with gas cell temperatures maintained at 150 °C and 200 °C, respectively.

Main Results and Discussion

Calcium oxalate monohydrate:

• TG-DTA reveals three main mass loss events: water release near 175 °C, partial decomposition with CO/CO₂ formation around 460 °C, and further CO₂ evolution close to 720 °C.
• FTIR chromatograms at 1508 cm⁻¹ and 2361 cm⁻¹ confirm H₂O and CO₂ evolution profiles in line with observed thermal events.

Polyethylene terephthalate (PET):

• Two decomposition peaks appear in the derivative TG curve under air at around 360 °C and 480 °C.
• FTIR spectra display distinct CO₂ release at both stages, rapid release of benzoic acid in the first decomposition step, and a gradual increase of ester-related bands during the second step.

Benefits and Practical Applications of the Method

TG-FTIR provides a comprehensive view of material behavior by linking quantitative thermal data with qualitative gas identification. Applications include:

• Analysis of decomposition and pyrolysis mechanisms.
• Quality control of polymers, pharmaceuticals, and additives.
• Detection of trace volatile components in complex matrices.
• Investigation of reaction pathways in research and development.

Future Trends and Applications

Advancements may include integration with mass spectrometry for enhanced selectivity, development of faster detectors, improved automated data processing, and wider adoption in environmental analysis, food safety, and forensic science. Ongoing improvements in transfer-line design and cell temperature control will further increase sensitivity and reproducibility.

Conclusion

The combination of TG and FTIR offers a powerful, versatile tool for elucidating thermal decomposition processes and identifying evolved gases in real time. The case studies of calcium oxalate and PET highlight the technique’s ability to resolve complex reaction sequences, making it invaluable for academic research and industrial applications.