Doing the Dirty Work: Combined TGA-IR and Transmission Studies of Soil

Significance of the topic

The combined use of thermogravimetric analysis (TGA) and Fourier-transform infrared spectroscopy (FT-IR) for soil analysis augments routine bulk measurements by identifying volatile and semi-volatile organics evolved during controlled heating. This capability is highly relevant to forensic investigations, environmental monitoring and remediation, and agricultural soil characterization because it links quantitative mass-loss data with chemical identity of off-gassed species, improving contaminant detection and enabling insights into nitrogen speciation and organic composition that single-method approaches cannot provide.

Objectives and overview of the study

The application note demonstrates how coupling a TGA instrument to an FT-IR gas cell (TGA-IR) and combining these evolved-gas measurements with conventional KBr pellet transmission spectra before and after pyrolysis extends the information obtainable from simple total organic content loss. Key aims were to (1) detect and identify soil-bound contaminants (illustrated by spiking with 1,2,4-trichlorobenzene, TCB), (2) observe principal evolved species (water, CO2, ammonia), and (3) show the potential to use evolved-gas signals to infer nitrogen content and other compositional features.

Methodology

Sample preparation and thermal program:

• Garden soil samples were air-dried, ground, and sieved to remove coarse debris. One batch was spiked with 250 μL 1,2,4-trichlorobenzene (TCB) to model a bound contaminant.
• Small aliquots (~100–130 mg) of thoroughly dried soil were placed into platinum TGA pans.
• Heating ramp: 20 °C min−1 from ambient to final temperatures of either 650 °C or 800 °C.

FT-IR and evolved-gas data acquisition:

• Transmission spectra of pre- and post‑heated soils were produced as KBr pellets (200 mg KBr mixed with 10–15 mg soil, pressed at ~8 tons).
• Evolved-gas FT-IR spectra were recorded during the TGA run as time-series: 6 scans per time point at 4 cm−1 resolution (~7.8 s time resolution); transmission pellet spectra were collected using 32 scans at 4 cm−1.
• Data capture used Thermo Scientific OMNIC Time Series software; experiments were run under nitrogen or dry air purge.

Used instrumentation

• Thermo Scientific Nicolet Series FT-IR spectrometer (illustrated with Nicolet iS 10)
• TGA accessory with platinum sample pans and gas interface to FT-IR
• OMNIC Time Series software for synchronized time-resolved data collection

Main results and discussion

• TGA traces showed high signal-to-noise mass-loss profiles with dominant early weight loss attributable to moisture; an additional small-weight-loss “hump” was observed above ~650 °C in several samples.
• Evolved-gas FT-IR spectra were dominated by water and CO2 bands in most runs; in the TCB-spiked soil, characteristic TCB absorption peaks were observed in the evolved-gas stream at moderate oven temperatures (~240 °C), enabling contaminant identification via spectral search.
• An ammonia doublet was detected reproducibly in evolved-gas spectra from multiple soils, most prominently in a sample from a yard with a domestic pet, suggesting release of nitrogenous species during heating. Ammonia evolution occurred under both nitrogen and dry-air purge, indicating decomposition sources within the soil matrix rather than the purge gas.
• Transmission KBr pellet spectra before and after pyrolysis provided complementary information: subtraction/difference approaches show which organic absorptions are lost by heating, consistent with evolved-gas identifications.

Interpretation and limitations:

• Detection of TCB in evolved gas demonstrates the method's sensitivity to soil-bound semi-volatile contaminants that desorb or decompose on heating, supporting forensic and remediation applications.
• The low-magnitude weight loss at higher temperatures indicates only small amounts of refractory organics or bound contaminants are released above 650 °C, but TGA-IR still resolves their spectral signatures.
• Although ammonia detection indicates potential for assessing soil nitrogen content via evolved-gas analysis, the application note did not perform quantitative calibration; further work is required to translate spectral signal to mass or concentration.
• Carrier gas, heating rate, and matrix effects will influence evolved-gas profiles and must be standardized for comparative or quantitative studies.

Benefits and practical applications of the method

• Combines quantitative mass-loss (TGA) with qualitative/speciative identification (FT-IR) of evolved gases in a single experiment.
• Enables rapid screening for bound contaminants (e.g., chlorinated aromatics), useful in forensic evidence comparison and preliminary site contamination assessments.
• Provides a route to detect low-level nitrogenous emissions (ammonia) that could inform soil fertility, pollution source tracing, or biological activity indicators.
• Offers a non-destructive complement to chromatographic techniques for initial identification; evolved-gas FT-IR is fast and requires minimal sample handling compared with solvent extraction methods.

Future trends and potential uses

• Develop calibrated protocols to quantify evolved species (e.g., ammonia, specific organics) using suitable standards and transfer line efficiency corrections.
• Integrate complementary detectors (GC-MS, FT-IR with higher sensitivity, or tunable laser spectroscopy) downstream of the TGA to improve identification and sensitivity for complex matrices.
• Apply method systematically for nitrogen budgeting in soils and for differentiating sources of nitrogen (organic vs. inorganic) by combining thermal fractionation with spectral markers.
• Expand to monitoring remediation progress by periodic TGA-IR screening of site soils to detect residual bound contaminants and to assess treatment efficacy.
• Translate laboratory workflows to portable or field-adapted evolved-gas analysis platforms for on-site screening where feasible.

Conclusion

The application note demonstrates that TGA-IR combined with pre- and post-pyrolysis transmission FT-IR significantly enhances soil analysis by linking mass-loss events to chemically specific evolved-gas signatures. This approach improves contaminant detection (illustrated by TCB), reveals trace nitrogenous emissions (ammonia), and provides a foundation for further quantitative method development. Limitations include the need for calibration, control of experimental variables, and potential matrix interferences, but the technique shows clear promise for forensic, environmental, and agricultural applications.

References

1. Cox RJ, Peterson HL, Young J, Cusik C, Espinoza EO. Forensic Science International 2000;108:107–116.
2. Thermo Fisher Scientific. Applications Note AN50716_E; analysis of museum materials and polymer failure using TGA-IR.
3. U.S. EPA. Toxicological review and information on trichlorobenzenes; EPA technical information.
4. Thermo Fisher Scientific. Applications Note AN50753_E; ammonia detection with FT-IR evolved-gas analysis.