Trace Analysis of PAH´s and PCB´s in Soil through On-Line Direct Thermal Desorption

Significance of the Topic

Rapid and accurate assessment of soil contamination by volatile organic pollutants such as PAHs and PCBs is essential for environmental monitoring and risk assessment. Traditional solvent-based extraction methods are time-consuming and labor-intensive, creating demand for faster, automated screening approaches.

Objectives and Study Overview

This study evaluates a fully automated direct thermal desorption system coupled with cryofocusing and GC–MS for semi-quantitative trace analysis of PAHs, PCBs, and mineral oil hydrocarbons in soil. It aims to demonstrate method performance, simplify sample preparation, and compare results with established reference techniques.

Instrumentation

• Thermal Desorption System (TDS 2, Gerstel GmbH) for controlled heating of soil samples
• Cooled Injection System (CIS 3, Gerstel GmbH) with programmable liner (–150 °C to 400 °C)
• Gas Chromatograph (HP 5890 Series II, Hewlett-Packard)
• Mass Selective Detector (HP 5972, Hewlett-Packard)

Methodology

• Soil samples crushed (<4 mm) with no drying to preserve native moisture
• Samples placed in glass tubes precooled to subambient temperature
• Thermal desorption to 350 °C under carrier gas flow transfers volatiles into the CIS
• Cryofocusing at subzero temperatures followed by rapid CIS heating to 400 °C for injection onto a DB-5 column
• Oven program: 60 °C to 250 °C at 10 °C/min, then 250 °C to 320 °C at 5 °C/min
• Mass spectrometric detection for selective identification and quantification

Main Results and Discussion

• Recoveries of 16 EPA PAHs on sand spikes exceeded 86% for most analytes, including high-boiling dibenzo[a,h]anthracene (mean 101%)
• Real soil samples yielded PAH recoveries of 95–102% in moderately contaminated matrices but dropped to ~17% in clay-rich soils
• PCB recoveries varied from 21% to 50%, reflecting adsorption of higher-boiling congeners
• Chromatograms demonstrated that mass spectrometric modes (extracted-ion and selected-ion monitoring) are essential for resolving trace PAHs and PCBs
• Detection limits were comparable to standard Soxhlet and ultrasonic extraction methods

Benefits and Practical Applications

• Minimal sample preparation: only grinding to <4 mm, no solvent extraction or drying
• Rapid analysis: desorption and transfer completed in approximately 30 minutes
• Small sample requirements (~500 mg) enable efficient screening of limited samples
• Broad boiling-point coverage allows analysis of both low- and high-boiling volatiles in various soil types
• Fully automated workflow enhances throughput and reproducibility

Future Trends and Potential Applications

• Integration with high‐resolution MS for precise compound identification
• Extension to other environmental matrices (sediment, sludge, airborne particulates)
• Development of field‐portable units for on‐site screening
• Refinement of quantitative accuracy via matrix‐matched calibration and internal standards
• Automation of data processing for real‐time reporting and decision support

Conclusion

The combined thermal desorption and cryofocusing GC–MS approach provides a fast, reliable, and semi-quantitative screening method for volatile contaminants in soils. It eliminates lengthy solvent extractions while delivering trace-level detection across a wide boiling-point range with minimal manual intervention.

References

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2. Ministry of Environment decree on PAK analysis, RdErl. III A 5 – 567, 25.3.1988.
3. DIONEX application notes on supercritical fluid extraction (1992).
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7. EPA Method 610: Determination of 16 PAHs, 49 Fed. Reg. 209 (1984).
8. VDI Guideline 3872 – Thermal Desorption Techniques.
9. State Office for Water and Waste Abfallwirtschaft, Method PAK, Nr. 13/1987.
10. Sludge Ordinance (AbfKlärV), Annex 1, Bundesgesetzblatt 21/1992.