Accelerating the quantitation of polycyclic aromatic hydrocarbons (PAHs) in soil samples using the EXTREVA ASE system

Importance of the topic

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants with carcinogenic and mutagenic potential. Reliable, fast, and reproducible quantitation of PAHs in soils is crucial for regulatory monitoring, site assessment, and remediation decision-making. Improving sample-preparation throughput and reducing manual handling without sacrificing data quality directly addresses laboratory capacity constraints and regulatory compliance requirements.

Objectives and study overview

This application note describes the development and demonstration of a complete workflow for extracting and quantifying 16 priority PAHs from soil using the Thermo Scientific EXTREVA ASE Accelerated Solvent Extractor. The goals were to: optimize extraction and automated evaporation to reduce solvent use and turnaround time (TAT); assess extraction recoveries and reproducibility; evaluate blank carryover after high-concentration samples; and validate performance using a certified reference material (CRM).

Methodology and workflow

• Sample preparation: 2 g of clean loam soil mixed 1:1 with diatomaceous earth dispersant (ASE Prep DE). Cells were 10 mL stainless-steel bodies with cellulose filters at top and bottom to retain particulates.
• Fortification: Spike levels of 250 µg/kg for recovery/TAT reproducibility and 12,500 µg/kg for carryover tests. Surrogate standards (2-fluorobiphenyl, p-terphenyl-d14) and deuterated internal standards were used for quantitation and QA/QC.
• Extraction: Gas-assisted dynamic ASE (GA-dASE) at ~200 psi (~14 bar) and 100 °C. Solvent: hexane:methylene chloride (1:4, v/v). Cell fill volume 50% with a solvent flow of 0.5 mL/min. Nitrogen assistance during extraction at ~20 mL/min per channel. Typical extraction time for four samples was 5 minutes.
• Automated concentration: Integrated evaporation in the EXTREVA system using controlled heating (40 °C), vacuum (8 psi / ~420 torr), and directed nitrogen flow (200 mL/min per channel; 800 mL/min total across four channels) with endpoint-level sensing to achieve a fixed final volume (1.0 mL). A 3 mL hexane rinse of the collection bottle was used prior to final concentration.
• Instrumental analysis: GC–MS (timed-SIM) using a PTV splitless injection (1 µL) onto a Trace TR-5MS column, helium carrier gas at 1.2 mL/min, with a temperature program rising from 60 °C to 310 °C. Electron ionization at 70 eV and source/transfer temperatures ~275–280 °C. Calibration used six points (0.1–2.0 µg/mL) with analyte/internal standard area ratios and linear or quadratic fits.

Used instrumentation

• Thermo Scientific EXTREVA ASE Accelerated Solvent Extractor (parallel GA-dASE with integrated evaporation and endpoint level sensing)
• 10 mL stainless-steel ASE extraction cells and cellulose filters
• Thermo Scientific TRACE 1310 Gas Chromatograph coupled to ISQ single-quadrupole GC–MS
• Consumables and reagents: hexane (Optima), dichloromethane (for HPLC), ASE Prep DE, Ottawa sand, CRM soil (PAH), PAH calibration mixes, surrogate and internal standard mixes

Key results and discussion

• Extraction recoveries: At the 250 µg/kg fortification level, recoveries ranged predominantly between ~74% and 114% across the 16 PAHs; specific values included naphthalene ~74%, fluoranthene ~98%, pyrene ~101%, benzo[a]pyrene ~103%, and benz[a]anthracene ~114%. All recoveries met the EPA acceptance band of 70–130% for the method. Relative standard deviations (RSDs) were below 20% for all analytes, indicating good reproducibility across channels and runs.
• Carryover: After extracting a heavily fortified sample (12,500 µg/kg), a follow-up extraction using clean sand and a 10 mL solvent rinse per channel showed carryover values <0.5% for all analytes; many were at or near 0.01–0.05%, demonstrating effective decontamination of flow paths with the implemented rinse.
• Validation with CRM: Analysis of a certified PAH soil material using the same 10 mL cell and conditions produced measured values within the CRM acceptance ranges for all target compounds, confirming trueness of the combined extraction/concentration workflow.
• Throughput and turnaround time: Sequence reproducibility (n=8) produced an average per-sample analysis time of approximately 40 minutes, with RSDs for repeated runs under 1.3% for TAT measurements. Compared to the prior instrument version, the improved EXTREVA system showed a roughly threefold decrease in TAT.

Benefits and practical applications

• Significant reduction in hands-on time and solvent consumption due to parallelized GA-dASE extraction and integrated automated evaporation, enabling load-and-go operation for unattended batches.
• High extraction efficiency and reproducibility that meet EPA method criteria (e.g., Method 3545A/8270E workflows), suitable for routine environmental monitoring, site assessment, and regulatory reporting.
• Low carryover supports serial processing of samples with a wide range of contamination levels while maintaining data integrity.
• Automated endpoint-volume sensing reduces variability in final concentrate volumes, improving consistency for downstream GC–MS quantitation.

Future trends and potential uses

• Further hardware and software integration could expand parallelization (higher channel counts) and enable remote monitoring and scheduling to maximize instrument utilization in high-throughput labs.
• Adapting GA-dASE workflows to additional analyte classes (e.g., pesticides, PCBs, polar semi-volatiles) and to different matrices (sediment, biosolids, food) can broaden the system’s applicability for environmental and food-safety laboratories.
• Optimization of solvent-minimization strategies and faster evaporation with advanced flow control and sensor fusion will further shorten TAT and reduce laboratory carbon footprint and solvent waste.
• Combining the EXTREVA ASE front end with higher-sensitivity detectors (e.g., high-resolution MS) or automated sample cleanup modules could enable lower reporting limits and streamlined workflows for complex matrices.

Conclusion

The EXTREVA ASE system, employing gas-assisted dynamic accelerated solvent extraction and integrated automated evaporation with enhanced flow control and endpoint sensing, provides a rapid, reproducible, and low-solvent workflow for quantifying 16 priority PAHs in soil. The method demonstrated acceptable recoveries, minimal carryover, CRM validation, and substantially reduced turnaround times, making it well suited for high-throughput environmental laboratories following EPA-style protocols.

References

• US EPA Method 8270E. Semivolatile Organic Compounds by Gas Chromatography/Mass Spectrometry.
• US EPA Method 3500C. Acid digestion (for sample preparation) or related extraction guidance.
• US EPA Method 3545A. Pressurized Fluid Extraction (PFE), formerly called Accelerated Solvent Extraction.
• US EPA Method 8100. Polycyclic aromatic hydrocarbons (guidance for PAH determinations).
• K. Srinivasan and R. Ullah. Method and Device to Extract Analyte from a Sample with Gas Assistance. U.S. Patent No. US 9,440,166 B2, 2016.
• K. Srinivasan and R. Ullah. Apparatus for Parallel Accelerated Solvent Extraction. U.S. Patent No. US 11,123,655 B2, 2021.
• SW-846 Test Method 8000D. Determinative Chromatographic Separations (GC–MS quantitation guidance).