Weight Loss Determined from Mass Spectrometry Trend Data in a Thermogravimetric/Mass Spectrometer System

Importance of the Topic

The ability to quantify individual gas species evolved during thermogravimetric analysis (TGA) greatly enhances material characterization. Combining TGA with mass spectrometry (MS) allows both weight change and gas composition to be monitored simultaneously. This integrated approach provides insight into decomposition mechanisms, reaction pathways and gas–gas as well as gas–solid interactions, critical for polymer, inorganic and industrial sample analysis.

Objectives and Study Overview

This work presents a straightforward calibration protocol to convert MS trend data for low mass-to-charge ratio ions into semi-quantitative weight losses. The study aims to resolve the contributions of overlapping gas evolutions—specifically water, carbon monoxide and carbon dioxide—in multi-step decomposition events. Calcium oxalate monohydrate and calcium carbonate serve as model systems for method development and validation.

Methodology and Used Instrumentation

• A TA Instruments Q50 TGA, featuring a quartz-lined furnace, was interfaced via a 200 °C heated capillary to a Pfeiffer ThermoStar quadrupole MS.
• Electron-impact ionization generated ions, which were filtered by m/e and detected by a secondary electron multiplier.
• Calibration involved running pure samples: calcium carbonate for CO₂ quantification and calcium oxalate’s first step for H₂O calibration.
• Trend scans tracked selected m/e values (H₂O at m/e 18, CO at 28, O at 16, CO₂ at 44) and integrated peak areas against TGA weight losses.

Main Results and Discussion

• Calcium oxalate exhibited three decomposition stages: dehydration (H₂O release), oxalate to carbonate conversion (CO and CO₂ release) and carbonate decomposition (CO₂ release).
• Calibration curves for CO₂ and H₂O displayed linear relationships between MS peak area and TGA weight loss, with negligible intercepts.
• Using the CO₂ calibration, the second step in calcium oxalate produced 0.46 mg CO₂, implying 0.29 mg CO based on molecular weight ratios.
• Accuracy checks showed agreement within 22% for CO₂ and within 20% for H₂O when compared to independent TGA results.
• Key considerations include matching purge gas, using fine powders for linear response and assuming consistent fragmentation ratios.

Benefits and Practical Applications

• Enables semi-quantitative determination of individual evolved gases from complex TGA weight loss events.
• Offers additional quantitative insight beyond qualitative identification in hyphenated TGA/MS analyses.
• Applicable for reaction mechanism studies, quality control in materials processing and assessment of gas interactions.

Future Trends and Possible Applications

• Extension to higher m/e species and complex organics for broader applicability.
• Integration with automated baseline correction algorithms to improve accuracy.
• Combination with other spectroscopic detectors (e.g., FTIR) for comprehensive multi-modal quantification.
• Development of standardized calibration materials and protocols for industrial QC environments.

Conclusion

A simple calibration procedure for converting MS trend data into weight loss estimates was demonstrated for H₂O and CO₂ with accuracies within 25%. The approach facilitates distribution of TGA-measured weight losses among multiple evolved species. Future work may enhance precision, extend to higher masses and integrate advanced data correction methods.

Reference

1. Edith A. Turi, Thermal Characterization of Polymeric Materials, 2nd Ed., 1997, Vol. 1, pp. 49–73.
2. Clement Duval, Inorganic Thermogravimetric Analysis, 2nd Ed., 1963, pp. 281–282.
3. Wesley W. Wendlandt, Thermal Methods of Analysis, 2nd Ed., 1974, p. 16.