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“TG-GC” as Thermogravimetric and Quantitative Reacted-gas Analyses

Teruki Motohashi, Yoshiteru Kawahara, Kenji Arai, Kenta Suzuki and Miwa Saito

Summer 2020, Volume 36, No. 2 , 01-05

Thermogravimetry (TG) is regarded as one of the most powerful techniques to investigate the fundamental characteristics of inorganic compounds. For a certain compound, information on the temperature-vs-mass relationship leads to a deep understanding of thermal behaviors as well as interactions with the surrounding atmosphere. Nevertheless, the interpretation would not be straightforward only with TG, when multiple gas species are involved in the thermal behaviors. Then, TG combined with a gas analyzer will effectively work.  The combination of thermobalance and quadrupole mass spectrometer (Q-MS), the so-called “TG-MS,” is commercially available, but this technique has the following drawbacks relating to Q-MS: (i) poor quantitative accuracy in the gas amounts, (ii) the complexity caused by fragmentations of gas molecules in the ionization process, (iii) strict limitations of the measuring atmosphere due to the necessity of differential evacuation. For detailed studies on inorganic materials, an alternative analytical method that is capable of quantitative gas analyses under various atmospheres is highly desirable to compensate for the drawbacks of TG-MS.

The authors have recently designed and developed a novel system, “TG-GC,” which employs gas chromatography (GC) as a gas analyzer instead of Q-MS. As mentioned, TG-GC has several advantages, making this system a complementary tool to conventional TG-MS. In this article, we will show a basic concept and the capability of TG-GC. Then, some case studies with the use of TG-GC will be reviewed.
 

Highlights

  • TG-GC combines thermogravimetric analysis with high-speed micro gas chromatography to provide quantitative identification of evolved gases while maintaining close correlation with sample weight changes.
  • The technique accurately measures multiple gas species, including oxygen, carbon dioxide, water vapor, carbon monoxide, nitrogen, and methane, offering advantages over conventional TG-MS for many inorganic materials.
  • Quantitative gas analysis reveals reaction pathways and compositional details that are difficult to obtain from mass loss data alone, enabling improved characterization of mixed-anion materials, catalysts, and complex hydroxide compounds.

Summary

Thermogravimetric analysis provides valuable information about the thermal behavior of materials by measuring changes in mass during heating, but interpretation becomes challenging when multiple gases evolve simultaneously. Integrating thermogravimetry with high-speed micro gas chromatography enables direct, quantitative identification of evolved gases while preserving the temperature resolution needed to correlate gas evolution with thermal events. This approach overcomes several limitations associated with quadrupole mass spectrometry, including fragmentation effects, limited quantitative accuracy, and restrictions on operating atmospheres.

The system employs precisely controlled gas flow rates and rapid gas chromatography, allowing gas analyses every few degrees during heating. Validation using well-characterized reactions, including copper oxide reduction, calcium carbonate decomposition, and calcium oxalate dehydration, demonstrates excellent agreement between measured mass loss and the quantified amounts of oxygen, carbon dioxide, water vapor, and carbon monoxide. These results confirm that the technique can accurately measure multiple gases released during complex thermal reactions.

Applications illustrate how quantitative gas evolution provides insight beyond conventional thermogravimetric measurements. Analysis of cobalt oxyhydroxide reveals distinct water and oxygen evolution steps, suggesting a more complex decomposition pathway than previously assumed while confirming nearly stoichiometric composition. In aluminum hydroxide-bicarbonate materials, separate quantification of water and carbon dioxide enables determination of composition and reveals multiple desorption states associated with different bonding environments. The technique is particularly valuable for mixed-anion materials and reactions involving gases with similar molecular weights, such as nitrogen and carbon monoxide, which are difficult to distinguish using conventional thermogravimetric mass spectrometry. Overall, the combination of thermogravimetry and micro gas chromatography provides a practical, quantitative method for correlating thermal events with gas evolution while offering flexibility, affordability, and broad applicability for advanced materials characterization.

Frequently asked questions

TG-GC replaces the quadrupole mass spectrometer with a high-speed micro gas chromatograph, improving quantitative measurement of evolved gases while avoiding issues caused by molecular fragmentation during ionization. It also allows measurements under a wider range of gas atmospheres because it does not require differential vacuum conditions. Although TG-MS generally offers higher sensitivity and faster time resolution, TG-GC provides superior quantitative accuracy and greater experimental flexibility.

The system uses precisely controlled gas feed and exhaust rates so that the flow of reacted gas entering the gas chromatograph is accurately known. High-speed micro gas chromatography separates and quantifies each gas species individually, allowing integrated gas amounts to be directly compared with thermogravimetric weight loss. Validation experiments show excellent agreement between calculated gas quantities and measured mass changes for oxygen, carbon dioxide, and water.

Conventional gas chromatography typically requires 10 to 20 minutes per analysis, resulting in temperature intervals that are too large to accurately correlate gas evolution with thermal events. Micro gas chromatography completes each analysis in approximately 1 to 2 minutes, enabling gas measurements every 5 to 10 K during typical heating experiments. This preserves the temporal resolution needed to study reaction mechanisms.

The system can simultaneously detect gases including nitrogen, hydrogen, oxygen, carbon monoxide, carbon dioxide, water vapor, and methane using parallel chromatographic columns. This capability makes it suitable for studying decomposition, dehydration, oxidation, reduction, and other thermally driven reactions involving multiple gaseous products.

By measuring the quantity and timing of each evolved gas, TG-GC allows individual reaction steps to be distinguished even when the thermogravimetric curve alone appears ambiguous. For example, separate detection of water, carbon monoxide, and carbon dioxide during calcium oxalate decomposition clearly identifies sequential dehydration, decarbonylation, and decarboxylation processes.

Mixed-anion compounds frequently undergo thermal reactions involving multiple gaseous species, making mass-loss measurements alone insufficient to identify reaction pathways. Quantitative gas analysis enables researchers to correlate specific gases with individual reaction steps, determine material stoichiometry, and better understand decomposition mechanisms that influence catalytic, electrochemical, and structural properties.

Beyond identifying evolved gases, TG-GC can determine chemical composition, quantify volatile components, distinguish different bonding environments, and reveal previously unrecognized reaction pathways. These capabilities make it valuable for studying catalysts, battery materials, adsorbents, mixed-anion ceramics, and other advanced inorganic materials where thermal behavior strongly influences performance.

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