Rapid analysis of organochlorine pesticides (OCPs) in soil samples using the EXTREVA ASE accelerated solvent extractor

Significance of the topic

Soil contamination by organochlorine pesticides (OCPs) remains a persistent environmental and public-health concern because these compounds are chemically stable, bioaccumulative, and historically widespread in agricultural and urban use. Reliable, rapid, and reproducible quantification of OCPs in complex matrices such as soil is essential for environmental monitoring, regulatory compliance, site assessment, and remediation decision-making. Improvements in sample-preparation throughput, solvent consumption, and automation directly impact laboratory capacity and data quality for large-scale monitoring programs.

Objectives and study overview

The primary objective was to develop and validate a rapid, end-to-end method for determining 20 targeted organochlorine pesticides in soil using the Thermo Scientific EXTREVA ASE accelerated solvent extractor. The study sought to demonstrate high extraction recovery, low carryover, minimal thermal degradation of labile analytes, and a short turnaround time (TAT) through an integrated workflow that automates both pressurized solvent extraction and solvent evaporation prior to GC analysis.

Applied methodology and workflow

The workflow combined gas-assisted dynamic accelerated solvent extraction (GA-dASE) with automated endpoint-controlled evaporation to produce instrument-ready extracts for GC analysis. Key procedural elements included:

• Sample preparation: 2 g of clean loam soil mixed 1:1 with diatomaceous earth (DE) as dispersant; 10 mL stainless-steel extraction cells packed with cellulose filters at top and bottom.
• Extraction solvent and conditions: acetone:hexane 1:1 (v/v); cell fill 50% (~7.5 mL real volume); oven temperature 100 °C; system pressure ~200 psi; nitrogen gas assistance at 20 mL/min per channel during extraction; solvent flow 0.5 mL/min; extraction time configured to 5 min for a 4-sample sequence.
• Automated evaporation/enrichment: fixed-volume concentration to 1.0 mL final volume using nitrogen-assisted evaporation at 40 °C, increased nitrogen flow (200 mL/min per channel during evaporation, 800 mL/min total across four channels), and vacuum set at 8 psi to accelerate solvent removal while endpoint level sensing assured reproducible final volume.
• Calibration and quantification: five-point calibration (0.01–0.2 µg/mL); internal standards and surrogate spikes used; linear regression of analyte/internal standard area ratios for quantification; QC checks included fortified soils (250 µg/kg for recovery studies and 500 µg/kg for carryover assessment).
• Confirmation of method robustness: carryover assessments, replicate precision tests (multiple runs, n=4 for recoveries), and thermal degradation studies at 100 °C and 150 °C for thermally labile analytes (endrin, 4,4’-DDT).

Used instrumentation

• EXTREVA ASE system (Thermo Scientific) implementing GA-dASE with integrated solvent reservoirs, gas/liquid manifold, multi-cell oven (10 mL stainless-steel cells), automated evaporation with endpoint level detection and vial adaptors for 2 mL collection vials.
• Thermo Scientific TRACE 1310 Gas Chromatograph equipped with an electron capture detector (GC-ECD) for OCP detection.
• Consumables and standards: Rtx-CL Pesticides column (30 m × 0.25 mm × 0.25 µm), PTV injector with Topaz liner, Restek organochlorine pesticide mix, pesticide surrogate and internal standard mixes, Ottawa sand and DE dispersant, cellulose filters.

Main results and discussion

Key analytical performance outcomes and observations were:

• Extraction efficiency: Average recoveries for the 20 target OCPs ranged approximately from 80% to 122% at a 250 µg/kg spike level. Relative standard deviations (RSD) for recoveries were below 20% for all compounds, meeting typical EPA recovery acceptance ranges (70–130%) and indicating good intra-method precision.
• Chromatography and sensitivity: GC-ECD analysis achieved clear resolution (resolution >2.0) of target peaks in under 27 minutes per run. Calibration linearity across the five-point curve (0.01–0.2 µg/mL) produced percentage errors <10% between measured and nominal values.
• Carryover and cross-contamination: After extracting a heavily fortified sample (500 µg/kg) and performing a 10 mL hexane rinse, carryover in a subsequent blank extraction was very low—generally <0.6% and for many analytes effectively zero—demonstrating the effectiveness of the rinse strategy and the low solvent-volume GAS-dASE approach in limiting cross-contamination.
• Turnaround time (TAT) and throughput: The method reproducibly achieved an average analysis time of ~32 minutes per sample sequence; with the EXTREVA ASE capacity and process optimizations, laboratories can process up to 24 soil samples per instrument in approximately four hours under the revised workflow, representing a substantial TAT improvement over prior ASE configurations.
• Thermal stability: Thermal degradation tests for endrin and 4,4’-DDT at extraction temperatures of 100 °C and 150 °C revealed breakdown percentages well below the 15% threshold suggested by regulatory guidance, indicating minimal thermally induced analyte loss under the chosen extraction conditions.

Benefits and practical applications

This integrated method provides multiple practical advantages for environmental analytical labs:

• High throughput: Parallel ASE and automated evaporation reduce manual handling and enable processing of multiple samples per run, increasing sample throughput for monitoring programs and contract laboratories.
• Reduced solvent use and faster evaporation: GA-dASE lowers solvent consumption per sample and dynamic gas-assisted evaporation shortens concentration times, lowering solvent costs and waste handling.
• Reproducibility and data quality: Endpoint-level sensing for evaporation and controlled extraction parameters yield reproducible final volumes and consistent recoveries, supporting robust quantitation with standard GC-ECD instrumentation.
• Operational efficiency: Automation minimizes operator intervention, reducing potential human error and labor costs while enabling unattended operation.

Future trends and potential uses

• Broader adoption of integrated extraction–evaporation platforms will likely expand for other semivolatile and persistent organic pollutants in diverse environmental matrices (sludge, sediments, biota), especially where throughput and solvent minimization are priorities.
• Combining GA-dASE with mass-spectrometric detection (single-quadrupole or higher-resolution MS) could improve compound confirmation and extend applicability to complex or co-eluting matrices while retaining the automation benefits.
• Further miniaturization of extraction cells and enhanced endpoint sensing could reduce solvent consumption and TAT even more, enabling fit-for-purpose workflows for screening and emergency-response monitoring.
• Method transfer and harmonization with regulatory methods will facilitate routine use in compliance testing, provided inter-laboratory validation and round-robin studies confirm equivalence to traditional methods.

Conclusion

The EXTREVA ASE GA-dASE workflow provides a rapid, automated, and reliable method for the quantification of 20 organochlorine pesticides in soil. The method demonstrated acceptable recoveries and precision, negligible carryover after appropriate rinsing, limited thermal degradation for labile analytes, and significantly reduced turnaround times. These attributes make the approach well-suited for environmental monitoring programs requiring high throughput, reproducible results, and reduced solvent usage while maintaining compatibility with standard GC-ECD analytical platforms and EPA method frameworks.

Reference

1. Blocksom KA, Walters DM, Jicha TM, Lazorchak JM, Angradi TR, Bolgrien DW. Persistent organic pollutants in fish tissue in the midcontinental great rivers of the United States. Science of The Total Environment. 2010;408(5):1180–1189.
2. Stockholm Convention on Persistent Organic Pollutants. Secretariat information and convention overview.
3. US EPA Method 8270E: Semivolatile Organic Compounds by Gas Chromatography/Mass Spectrometry (GC/MS).
4. US EPA Method 8081B: Organochlorine Pesticides by Gas Chromatography.
5. US EPA Method 3540C: Soxhlet Extraction.
6. US EPA Method 3550C: Sonication Extraction.
7. US EPA Method 3546: Microwave-Assisted Extraction (MAE).
8. US EPA Method 8081B (duplicate reference in original document).
9. US EPA Method 3545A: Pressurized Fluid Extraction (Accelerated Solvent Extraction).
10. Patent: Srinivasan K, Ullah R. Method and Device to Extract Analyte from a Sample with Gas Assistance. US Patent 9,440,166 B2, 2016.
11. Patent: Srinivasan K, Ullah R. Apparatus for Parallel Accelerated Solvent Extraction. US Patent 11,123,655 B2, 2021.
12. US EPA Method 8000D: Analytical Methods for Organic Compounds.
13. US EPA Method 3500C: Sample Preparation (General Guidance).
14. Thermo Fisher Scientific application notes: Method transfer to the EXTREVA ASE and investigation of thermal degradation during extraction (internal Thermo Fisher technical white papers referenced in the original study).