An Alternative Method to RSK 175 using a Purge and Trap Concentrator and GC/FID

Significance of the Topic

The monitoring of dissolved light hydrocarbons in water has gained importance with the rapid expansion of hydraulic fracturing for natural gas extraction. Accurate, sensitive, and high-throughput analysis of methane, ethane, ethene and propane is essential to assess environmental impacts and ensure drinking water safety.

Objectives and Study Overview

This study evaluates an alternative analytical approach to EPA method RSK 175 and Pennsylvania DEP method BOL6019. It aims to demonstrate an automated purge-and-trap concentration combined with GC/FID for four dissolved gases, comparing performance metrics—such as linearity, detection limits and reproducibility—against established headspace techniques.

Methodology and Instrumentation

The analytical system comprised a Stratum Purge and Trap Concentrator coupled to an AQUATek 100 autosampler, with sample temperature controlled below 10 °C. A GC equipped with a Restek Rt-U-BOND capillary column and FID operated under constant pressure and makeup flow conditions was used. Calibration employed aqueous standards prepared by gas saturation in chilled water; serial dilutions covered ranges from parts-per-billion to parts-per-million. Method detection limits were established from low-level replicates.

Main Results and Discussion

Calibration curves exhibited excellent linearity (r2 ≥ 0.999) across all analytes. Relative standard deviations were below 14 % for ethane and under 5 % for other compounds. Method detection limits ranged from 0.4 ppb (methane) to 31 ppb (ethene). Carryover remained negligible (< 0.04 %). Automated purge-and-trap provided comparable or improved precision relative to headspace equilibration, while eliminating manual sample handling.

Benefits and Practical Applications

The proposed method leverages instrumentation common in environmental laboratories, reducing capital investment in specialized headspace devices. Full automation minimizes human error and increases sample throughput. Its sensitivity meets or exceeds regulatory requirements for groundwater monitoring near fracking sites.

Future Trends and Opportunities

Advancements in trap materials and detector technologies could further lower detection limits and expand analyte scope to higher hydrocarbons or volatile organics. Integration with real-time data processing and field-deployable purge-and-trap modules may enhance remote environmental surveillance.

Conclusion

The purge-and-trap GC/FID method validated here offers a robust, precise and efficient alternative to existing headspace protocols for dissolved gas analysis. It aligns with regulatory performance criteria while improving laboratory workflow and resource utilization.

References

• Kampbell, D.H.; Vandegrift, S.A. Analysis of Dissolved Methane, Ethane, and Ethylene in Ground Water by a Standard Gas Chromatographic Technique, J. Chromatogr. Sci. 36, 253–256 (1998).
• Pennsylvania DEP. C1 to C3 Dissolved Gases via Headspace and GC/FID, Revision 5, Feb 2011.
• Energy Information Administration. Annual Energy Outlook 2009.