Purge Trap for GC Analyses of Volatile Aromatic (BTEX) Compounds in Soils and Sediments

Significance of the topic

The accurate monitoring of benzene, toluene, ethylbenzene and xylene isomers (BTEX) in soil and sediment matrices is critical for environmental assessment of hazardous waste sites and for compliance with regulatory frameworks such as the US EPA SW-846 methods. Volatile organic compounds (VOCs) pose health and ecological risks even at low concentrations, so sensitive, robust, and interference-free analytical techniques are essential.

Objectives and study overview

This application note evaluates the performance of a purpose-designed purge trap (BTEXTRAP) for GC analysis of BTEX compounds in soil and sediment extracts prepared according to US EPA Method 5030. The primary goals are to demonstrate consistent retention of analytes, effective passage of excess methanol, and reproducible quantitation across a range of analyte concentrations and methanol levels.

Methodology

• Sample preparation: Soil/sediment samples dispersed in methanol to dissolve VOCs, then diluted with water to achieve analyte levels of approximately 10–200 ppb.
• Purge and trap protocol: 5 mL methanolic extract purged for 11 minutes at 25–40 mL/min, followed by a 5 minute dry purge to remove residual methanol.
• Adsorbent configuration: 30 mg Carbopack B (benzene retention) followed by 300 mg Carbopack C (heavier aromatics) assembled in 1/8" OD stainless steel traps.
• Thermal desorption: 260 °C for 6 minutes, bake at 270 °C for 4 minutes.
• Chromatography: SUPELCOWAX 10 capillary column (60 m × 0.75 mm ID, 1.0 μm film), oven ramp from 50 °C (8 min) to 100 °C at 4 °C/min; helium carrier at 10 mL/min; FID detection.
• Performance evaluation: Analyte mixtures spiked at five concentrations (1, 5, 10, 50, 100 ng/mL) in low (<1 µL/mL) and high (5 µL/mL) methanol matrices; each level analyzed in triplicate.

Used Instrumentation

• Purge trap models: BTEXTRAP, compatible with CDS Peakmaster, Dynatech “Dyna”, O.I. Analytical, Tekmar (LSC series).
• Gas chromatograph: Fitted with SUPELCOWAX 10 capillary column and FID detector.
• Auxiliary: Stainless steel trap housings with integrated thermocouple on selected models.

Main results and discussion

• Chromatographic separation: Baseline resolution of benzene through o-xylene with minimal solvent peak interference even at high methanol levels.
• Reproducibility: Relative standard deviations below 3% for all analytes at both methanol concentrations, demonstrating high precision.
• Linearity: Correlation coefficients exceeding 0.9992 across the calibration range, indicating negligible matrix or solvent impact on detector response.
• Methanol tolerance: The trap design effectively excludes methanol from the adsorbent beds, preventing large solvent peaks and signal quenching at the FID.

Benefits and practical applications

• Reliable quantitation of BTEX in complex solid matrices following methanol extraction.
• Compatibility with standard EPA purge and trap protocols and a range of purge flow rates.
• Reduced downtime and analysis time due to minimized solvent interference and rapid trap drying.
• Versatile integration with multiple GC systems and detectors.

Future trends and potential applications

• Extension to more volatile VOCs using multi-bed traps incorporating Carboxen and other high-surface adsorbents.
• Integration with mass spectrometric detectors for enhanced specificity and lower detection limits.
• Development of field-deployable purge and trap modules for on-site environmental monitoring.
• Automation of sample introduction and thermal desorption cycles for high-throughput laboratories.

Conclusion

The BTEXTRAP purge trap delivers robust, reproducible, and linear performance for the GC analysis of benzene, toluene, ethylbenzene, and xylene isomers in soil and sediment samples prepared with methanol. Its unique adsorbent configuration allows high methanol throughput without compromising analyte retention or chromatographic quality, making it an ideal choice for environmental VOC monitoring.

References

1. US EPA Solid Waste Test Methods SW-846 — Purge and Trap for Volatile Organics (Method 5030A), National Technical Information Service, Springfield, VA.