Automated Sampling of RSK-175 Samples

Importance of the Topic

Dissolved gas analysis in water is critical for environmental monitoring, particularly around hydraulic fracturing sites where methane and other hydrocarbons may migrate into drinking water. The RSK-175 standard provides a framework for quantifying these gases, but manual sample preparation is labor intensive and time consuming. Automating this workflow can improve throughput, consistency, and safety in laboratories tasked with rigorous water quality testing.

Goals and Study Overview

This study evaluates an automated sample preparation technique for dissolved gases in water based on the RSK-175 SOP. The main objectives are:

• Implement automation using the EST LGX50 autosampler.
• Maintain sample integrity during headspace generation and transfer.
• Assess accuracy, precision, linearity, and carryover for methane, propane, ethane, and ethylene.

Methodology and Instrumentation

The LGX50 autosampler was configured with two 40 mL vial trays—one holding empty vials with stir bars, and the other holding water samples with no headspace. The system:

• Evacuates the empty vial and transfers sample liquid to create a closed headspace.
• Heats (65 °C) and stirs samples for a fixed equilibration time (10 min).
• Pressurizes the vial to fill a 2 mL sample loop and transfers headspace gas directly to a GC/FID.

GC/FID conditions:

• Agilent 5890 GC; Restek RT Q-bond 30 m × 0.53 mm × 20 µm column.
• Oven program from 45 °C hold to 180 °C ramp; total runtime ~10.5 min.
• Inlet at 250 °C, 9 psi He carrier, split ratio 20:1; FID at 250 °C.

Main Results and Discussion

Calibration curves were generated using saturated water standards prepared via serial dilution. Key findings include:

• Methane: 20 ppb–19.8 ppm range, R² = 0.999, curve RSD = 13.3%.
• Propane: 8 ppb–38.4 ppm, R² = 1.000, curve RSD = 12.5%.
• Ethane: 6.6 ppb–32.9 ppm, R² = 1.000, curve RSD = 8.95%.
• Ethylene: 5.6 ppb–70 ppm, R² = 0.999, curve RSD = 8.23%.
• Internal standard (MtBE-d3) precision RSD = 2.6–6.0% across compounds.
• Carryover after high-level injections < 0.1% for all gases.

These results demonstrate excellent linearity, low carryover, and robust reproducibility across a wide dynamic range.

Benefits and Practical Applications

Automating RSK-175 sample preparation with the LGX50 delivers:

• Significant labor savings by replacing manual headspace creation.
• Improved consistency and reduced human error in equilibration and transfer steps.
• Enhanced throughput for environmental and QA/QC laboratories monitoring natural gas migration or water quality.

Future Trends and Potential Applications

Further developments may include expanding the automated workflow to additional volatile organics, integrating real-time field sampling modules, and miniaturizing headspace systems for on-site monitoring. Advances in sensor technology and data connectivity could enable continuous dissolved gas surveillance in remote locations.

Conclusion

The LGX50 autosampler effectively automates the RSK-175 dissolved gas procedure, offering reliable, accurate, and reproducible results while reducing manual labor. This approach enhances laboratory efficiency and supports rigorous environmental monitoring of hydrocarbons in water.

References

1. Solubility of Gases in Water. Engineering Toolbox. Retrieved November 15, 2011.
2. Gas Encyclopaedia. Air Liquide. Retrieved November 15, 2011.
3. ConocoPhillips Company. Drilling and Completion. Retrieved January 20, 2012.
4. Hudson, F. RSKSOP-175, Revision No. 2, May 2004.
5. EPA New England. Technical Guidance for the Natural Attenuation Indicators: Methane, Ethane, and Ethene. Revision 1, July 2001.