Automated Determination of Dissolved Gases in Water

Significance of the Topic

Rapid and accurate measurement of dissolved gases in water is critical for monitoring potential groundwater contamination associated with hydraulic fracturing. Automated sample preparation reduces labor intensity and improves data reliability.

Objectives and Study Overview

This study aims to validate a calibration approach using serial dilutions of saturated gas solutions and an automated headspace sampling system (LGX50) coupled with GC/FID, eliminating manual steps and the need for Henry’s constant corrections.

Methodology and Instrumentation

The LGX50 autosampler performs vial evacuation, sample transfer, heating, stirring and pressurization to generate a headspace sample loop for GC/FID analysis. Key instrument settings include:

• Sample volume: 20 ml, equilibration: 10 min at 60 °C
• GC/FID: Agilent 5890, Restek RT Q-bond column (30 m × 0.53 mm × 20 µm)
• Oven program: 45 °C hold 1 min, ramp 16 °C/min to 180 °C
• Internal standard: Deuterated MtBE

Main Results and Discussion

Calibration curves for methane, ethane and ethylene showed excellent linearity (R² ≥ 0.999). Method detection limits ranged from 1.94 to 5.92 ppb. Precision and accuracy metrics:

• Methane: precision 5.9% RSD, recovery 98.8%
• Ethane: precision 6.9% RSD, recovery 96.7%
• Ethylene: precision 6.0% RSD, recovery 104.9%

Internal standard reproducibility was below 6% RSD and carryover less than 0.1% at high standard levels.

Benefits and Practical Applications

The automated workflow significantly reduces hands-on time, minimizes contamination risks and improves reproducibility. Laboratories can directly report field sample results without complex back calculations, enhancing throughput for environmental monitoring and quality control.

Future Trends and Potential Applications

Advances may include portable autosamplers for in situ analysis, integration with real-time data management platforms, expansion to additional volatile compounds and coupling with alternative detectors to broaden analytical capabilities.

Conclusion

The LGX50 automated headspace system delivers reliable, high-throughput dissolved gas analysis with robust performance metrics. Its adoption can streamline laboratory workflows, reduce costs and support comprehensive environmental monitoring initiatives.

References

1. Dean J. A. Lange’s Handbook of Chemistry, 14th ed., McGraw-Hill, 1992.
2. Gas Encyclopaedia. Air Liquide, 2009.
3. ConocoPhillips Company. Drilling and Completion, 2012.
4. Hudson F. RSK SOP-175, Rev. 2, May 2004.
5. EPA New England. Technical Guidance for Methane, Ethane, Ethene, Rev. 1, 2001.
6. PADEP Method 3686, Rev. 0, April 2012.