Vacuum Assisted Sorbent Extraction (VASE) and a Dual-Column Thermal Desorption Approach for GC-MS Analysis of Trace-Level Polycyclic Aromatic Hydrocarbons

Significance of the Topic

Polycyclic aromatic hydrocarbons (PAHs) represent a class of environmental contaminants with potential human health risks, even at trace concentrations. Accurate and sensitive analysis of PAHs in water matrices is essential for regulatory compliance, environmental monitoring, and risk assessment.

Objectives and Study Overview

This work introduces a combined Vacuum Assisted Sorbent Extraction (VASE) and dual-column thermal desorption GC-MS method to quantify PAHs at parts-per-trillion levels from microliter-scale aqueous samples. The study demonstrates recovery and reproducibility for a range of PAHs from naphthalene to benzo(g,h,i)pyrene using only 1 mL of sample.

Methodology

A disposable headspace sorbent pen (HSP) packed with ~80 mg Tenax TA is placed in a sample vial under a two-stage vacuum, inducing diffusive headspace extraction.

• Sample volume: 0.5–1 mL water spiked with PAHs (5 ppt to 10 ppb).
• Extraction: vacuum-assisted headspace at ambient temperature, optional agitation.
• Thermal desorption: preheat 70 °C for 30 s, desorption 325 °C for 6 min, bakeout 325 °C for 21 min.
• Dual-column configuration: inert Silonite-coated megabore precolumn for retention during splitless desorption and analytical DB-5MS column for separation.
• Split/splitless operation: split mode during VOC analysis, splitless during SVOC bakeout.

Instrumentation

• Sample Preparation Rail (SPR) and Sorbent Pen Thermal Desorption Unit (SPDU) by Entech Instruments.
• GC-MS: Thermo TRACE 1310 with ISQ mass spectrometer, 50–300 amu scan range, 15:1 split ratio for VOCs.
• Columns: Agilent DB-5MS (30 m×0.25 mm×0.5 µm) analytical column; Quadrex UAC-1MS/HT precolumn or equivalent inert loop tubing.

Main Results and Discussion

The VASE plus dual-column TD-GC-MS approach achieved quantitative recovery of six-ring PAHs at 5 ppt levels from 1 mL water.

• Limits of detection: as low as 5 ppt for high-boiling PAHs.
• Reproducibility: RSDs below 10% (n=5) across all tested compounds.
• Carryover: minimal sorbent pen residuals after bakeout, enabling successive runs without significant contamination.
• Split/splitless cutoff: detection thresholds adjustable via desorption time and flow settings.

Benefits and Practical Applications

• Ultra-trace detection in minimal sample volumes reduces sample preparation time and waste.
• Combined VOC/SVOC analysis in a single run streamlines laboratory workflows.
• High reproducibility and low carryover enhance data reliability for environmental and regulatory laboratories.

Future Trends and Applications

Emerging sorbent materials and microfabricated headspace pens may further lower detection limits. Integration with portable GC-MS systems and automated workflows will expand on-site monitoring capabilities. Machine-learning-driven optimization of extraction parameters could enhance method robustness.

Conclusion

The presented VASE and dual-column thermal desorption GC-MS approach offers a robust, sensitive, and reproducible solution for trace-level PAH analysis in water. Its capacity for simultaneous VOC and SVOC profiling from sub-mL samples makes it a valuable tool for environmental analytics and quality control.

References

• Belardi R.G. and Pawliszyn J. The application of chemically modified fused silica fibers in the extraction of organics from water matrix samples and their rapid transfer to capillary columns. Water Pollut. Res. J. Can. 24 (1989) 179–191.