Evaluation of a ‘Soil Gas’ sorbent tube for improving the measurement of volatile and semi-volatile fuel vapors in soil contaminated land

Significance of the Topic

Soil gas analysis of volatile and semi-volatile petroleum vapors is critical for assessing vapor intrusion risks, human health impacts and guiding remediation strategies. A representative total petroleum hydrocarbon (TPH) profile and detection of key toxic compounds, such as benzene and naphthalene, are essential for accurate site characterization and contaminant source identification.

Study Objectives and Overview

The study aimed to design and evaluate a dedicated 'soil gas' sorbent tube combining Tenax TA and Carbotrap X for capturing fuel vapors ranging from C4 to C26 under variable temperature (30–50°C) and humidity. A secondary goal was to validate compatibility of these tubes with an existing Air Toxics thermal desorption GC/MS system (TO-15/TO-17 methods) to enable soil gas monitoring without additional capital equipment.

Methodology and Instrumentation

Samples were prepared by spiking sand with 1 μL of gasoline, kerosene or diesel, then equilibrating 48 h in sealed vials at ambient conditions. Both dry and wet sand replicates were placed in a Micro-Chamber Thermal Extractor (µ-CTE) set at 30 and 50°C. Vapors were purged at 50 mL/min for 5 min onto stainless steel tubes packed with Tenax TA (front) and Carbotrap X (rear).

Analysis was performed on a Series 2 ULTRA-UNITY-CIA 8 thermal desorber interfaced to GC/MS under TO-17 compliance:

• Prepurge: 3 min; tube desorb: 300°C for 5 min;
• Cold trap: 25°C/310°C; split flow: 50 mL/min;
• GC: VF-5ms column, He carrier, 50→140°C @5°C/min, then 15°C/min to 300°C;
• MS scan range: 35–600 amu.

Main Results and Discussion

The sorbent tubes effectively captured the full volatility range of fuel vapors, including light hydrocarbons (e.g. 1,3-butadiene) through heavy fractions up to n-C18 and polyaromatic hydrocarbons. Repeat desorption tests showed negligible carryover (<0.1%) under all conditions. Profiles matched or surpassed canister (TO-15) sampling, especially for middle distillate fuels, confirming representative TPH fingerprinting and robust performance in humid and high-temperature scenarios.

Benefits and Practical Applications

• Improved accuracy in vapor intrusion risk assessments by comprehensive recovery of volatile and semi-volatile fuel components.
• Hydrophobic sampling allows direct use on wet soils without water interference.
• Minimal carryover and automated conditioning ensure immediate reuse over >100 cycles, reducing operational costs.
• Compatibility with existing TO-15/TO-17 thermal desorption systems enables laboratories to expand soil gas services without new equipment.

Future Trends and Opportunities

Potential developments include:

• Expansion of sorbent combinations for reactive or polar contaminants using Silcosteel tubes.
• Integration with automated high-throughput sampling systems and real-time monitoring technologies.
• Coupling with high-resolution MS or isotopic analysis for source apportionment.
• Broader application in indoor air quality monitoring and vapor intrusion screening across diverse environmental matrices.

Conclusion

The tailored soil gas sorbent tubes provide a reliable, quantitative and reusable solution for monitoring volatile to semi-volatile petroleum vapors in contaminated land. Their compatibility with standard thermal desorption GC/MS workflows enhances analytical efficiency and accuracy in environmental risk assessments.

References

1. US EPA Compendium Method TO-15: Determination of VOCs in air collected in SUMMA® canisters by GC/MS.
2. US EPA Compendium Method TO-17: Determination of VOCs in ambient air using sorbent tubes.
3. Hayes HC et al., Evaluation of sorbent methodology for petroleum-impacted site investigations, Air & Waste Mgmt. Assoc. Conf., 2007.
4. Caputi M, Monitoring VOCs and PAHs in the atmosphere, PhD Thesis, Univ. of Bari, 2004.
5. Clausen P et al., Emission of DEHP from PVC flooring, Environ. Sci. Technol., 2004, 28, 2531–2537.
6. Markes TDTS Notes on quantitative recovery of semi-volatiles and high-boiling compounds using thermal desorption.
7. Coutant R, McClenny W, Competitive adsorption in canisters, US EPA/A&WMA Symp., 1991.
8. OSHA Method PV2120 – VOCs in air using canisters.
9. Markes TDTS Notes on air monitoring with canisters vs. sorbent tubes.