Two Headspace Solutions for the Analysis of Dissolved Gases in Water by Method RSKSOP-175

Importance of the Topic

Dissolved gas concentrations in water provide vital information for environmental quality assessment, hydrogeological studies and leakage monitoring in gas extraction sites. The RSKSOP-175 headspace method has become especially relevant due to increased natural gas production from shale formations and the need to monitor methane and light hydrocarbons in groundwater.

Aims and Overview of the Study

This application note evaluates automated headspace sampling using the Teledyne Tekmar HT3 and Versa instruments to streamline the US EPA RSKSOP-175 procedure for quantifying dissolved methane, ethene, ethane and propane in groundwater. The goal is to compare performance metrics, reproducibility and detection limits across both systems.

Instrumentation Used

• Teledyne Tekmar HT3 Automated Headspace Analyzer
• Teledyne Tekmar Versa Automated Headspace Analyzer
• Agilent 6890 Gas Chromatograph with Flame Ionization Detector
• Restek Rt-U-Bond capillary GC column (15 m × 0.53 mm ID, 20 μm film)

Methodology

Groundwater samples were prepared by displacing 10% of sample volume with helium, followed by a 30-minute equilibration at 64 °C. Headspace aliquots were injected into the GC/FID using a 21-minute cycle time. Calibration standards were generated by diluting saturated gas-water stock solutions (prepared at 0 °C) to cover target ranges from roughly 0.02 to 24 mg/L for methane, 0.14 to 169 mg/L for ethene, 0.07 to 79 mg/L for ethane and 0.07 to 88 mg/L for propane. Seven replicates of the lowest level assessed method detection limits (MDLs).

Main Results and Discussion

• Linearity: Correlation coefficients exceeded 0.996 for all analytes on both HT3 and Versa systems, demonstrating excellent calibration performance.
• Precision: Replicate RSD values at the lowest calibration level ranged from 1.3% to 5.9%, with comparable reproducibility between instruments.
• Detection Limits: MDLs were below 0.03 mg/L for all gases, enabling trace-level quantification in environmental samples.

Benefits and Practical Applications

Automating RSKSOP-175 headspace sampling reduces manual labor, improves throughput and ensures consistent equilibration. The high precision and low MDLs make these systems ideal for routine monitoring of dissolved hydrocarbons in groundwater around fracking operations, landfill sites and natural gas pipelines.

Future Trends and Applications

Broader adoption of automated headspace analyzers is expected in:

• On-site monitoring using portable GC and headspace modules
• Integration with real-time data reporting platforms
• Expansion to additional dissolved volatile organic compounds and other environmental matrices

Conclusion

The Teledyne Tekmar HT3 and Versa Automated Headspace Analyzers effectively implement the EPA RSKSOP-175 method, delivering robust calibration, precision and sensitivity for dissolved gas analysis in water. Their automation capabilities enhance operational efficiency and data quality in environmental testing laboratories.

References

• Hudson F. RSKSOP-175, Rev. 2, May 2004; Sample Preparation and Calculation for Dissolved Gas Analysis in Water Samples Using GC Headspace Equilibration Technique.