Extending the Hydrocarbon Range for the Analysis of Soil Gas Samples Using Automated Thermal Desorption Coupled with Gas Chromatography–Mass Spectrometry

Significance of the Topic

Soil gas sampling and analysis play a crucial role in assessing vapor intrusion pathways and evaluating contaminated sites. Traditional EPA methods limit target compounds to those up to C12, leaving higher boiling components such as diesel-range hydrocarbons and polynuclear aromatic hydrocarbons underrepresented. Extending analytical capabilities to C26 provides a more accurate profile of soil vapors and supports better risk assessment and site remediation decisions.

Objectives and Overview of the Study

This study aimed to optimize thermal desorption chemistry and sorbent selection to recover organic compounds beyond the boiling point of naphthalene (C10) and up to C26 in soil gas samples following Method TO-17. Key goals included:

• Extending the analyte range past naphthalene to include diesel-range hydrocarbons and PAHs.
• Ensuring efficient capture of both highly volatile (e.g., vinyl chloride) and semi-volatile species during sampling.
• Verifying quantitative desorption without artifact formation.
• Implementing rapid tube cleanup for sample recollection and cost reduction.
• Maintaining robust water management under high humidity conditions.

Methodology and Instrumentation

Automated sampling employed thermal desorption tubes coupled to a Peltier-cooled concentrator trap integrated into a PerkinElmer TurboMatrix 650 thermal desorber. Desorbed analytes were transferred to a PerkinElmer Clarus 680 gas chromatograph fitted with a Restek Rxi-624Sil MS column and detected by a Clarus SQ8 mass spectrometer operating in full-scan mode. Key parameters included:

• Tube desorption at 325 °C with 50 mL/min nitrogen flow for 10 min.
• Peltier trap cooling to 10 °C, followed by rapid heating to 330 °C.
• Dry purge at ambient temperature and 50 mL/min for moisture removal.
• GC temperature program ramping from 35 °C to 275 °C.

Key Results and Discussion

Breakthrough experiments with humidified nitrogen (10 L) showed minimal loss; even vinyl chloride exhibited no detectable breakthrough. Spike recovery tests for PAHs demonstrated 98–100% recovery for anthracene, phenanthrene, and fluorene. Pyrene recovery was 90% at 325 °C and improved at 350 °C. The method achieved:

• Linearity (r2>0.999) across four orders of magnitude (0.05–250 µg/m3).
• Reporting limits down to 0.05 µg/m3 for a 1 L sample.
• Internal standard precision with RSDs below 2%.
• Effective moisture removal to background levels without instrument quenching.

These results meet or exceed EPA TO-17 performance criteria and validate the extended range for soil gas analysis under field conditions.

Benefits and Practical Applications of the Method

• Broader hydrocarbon coverage (C3–C26) enables comprehensive risk assessment at contaminated sites.
• Automated water management and leak checks preserve sample integrity in humid environments.
• Rapid tube conditioning allows immediate resampling or retention for reanalysis.
• Use of sorbent tubes reduces shipping and handling costs compared to canister-based methods.
• Full-scan MS detection supports non-target screening and retrospective data analysis.

Future Trends and Potential Applications

• Exploration of sorbent chemistries for polar and emerging contaminants.
• Integration of real-time monitoring with miniaturized desorption units.
• Adoption of higher-efficiency cooling techniques for lower temperature trapping.
• Combination with tandem mass spectrometry for enhanced selectivity.
• Application in indoor air quality, industrial hygiene, and fence-line monitoring.

Conclusion

Optimized automated thermal desorption GC–MS extends Method TO-17 analyte coverage to semi-volatile hydrocarbons up to C26 while maintaining low detection limits and robust performance under high humidity. The validated approach offers a cost-effective, high-throughput solution for comprehensive soil gas analysis and supports improved environmental exposure assessments.

References

• California Environmental Protection Agency, Air Resources Board; “How Much Air Do We Breathe?” Research Note #94-11; 1994.
• New York State Department of Health, Bureau of Environmental Exposure Investigation; Guidance for Evaluating Soil Vapor Intrusion in the State of New York; 2006.
• British Columbia Ministry of Environment; Update on Contaminated Sites: Stage 6 Amendments to the Contaminated Sites Regulation; 2008.
• U.S. EPA, Office of Research and Development; Method TO-15: Determination of VOCs in Air with Summa Canisters; Compendium of Methods; 1999.
• U.S. EPA, Office of Research and Development; Method TO-17: Determination of VOCs in Air Using Sorbent Tubes; Compendium of Methods; 1999.
• Provost R., Marotta L., Thomas R.; LCGC North America; 32(10):810–818; 2014.
• Marotta L., Snow M., Varisco S.; Optimizing Analytical Parameters for Soil Vapor and Indoor Air Samples Using Automated TD/GC/MS; AWMA Annual Symposium; 2009.