New ASTM Method Dissolved Gas Sampling Technique Comparison

Significance of the Topic

Monitoring dissolved gases in water is critical for environmental quality control, industrial process monitoring and regulatory compliance. The reliability of headspace sampling methods directly impacts data integrity, particularly at low analyte concentrations. Ensuring sample integrity during preparation can reduce cross-contamination and improve detection limits, making the method more robust for routine laboratory use.

Goals and Study Overview

This study compares two sample handling approaches for ASTM Method D8028: keeping water samples sealed throughout analysis versus opening and pouring into headspace vials. Key objectives include evaluating calibration linearity, precision, accuracy, method detection limits (MDLs) and cross-contamination potential for four light hydrocarbons: methane, ethane, ethylene and propane.

Methodology and Instrumentation

Samples were prepared by dissolving pure gases into water at 20 °C and performing serial dilutions to generate five-point calibration curves without relying on Henry’s constant. Two workflows were tested:

• Sealed Samples: 40 ml vials filled completely without air bubbles, analyzed in DGA mode on the LGX50.
• Poured Samples: 10 ml aliquots decanted into headspace vials, analyzed in Screen mode on the LGX50.

Both sets were analyzed by GC/FID using an Agilent 7890 instrument equipped with a Restek RT®-QS Bond column. Calibration curves, seven-replicate MDLs (per 40 CFR 136, App. B) and precision/accuracy tests were performed for each gas.

Major Findings and Discussion

Calibration performance:

• Sealed samples exhibited lower response %RSD (5–17 %) compared to poured samples (22–28 %), while R² values remained high (>0.998) for both.
• MDLs improved by up to two-fold with sealed handling, indicating enhanced sensitivity at low levels.

Precision and accuracy at mid-range concentrations:

• Both methods achieved acceptable relative standard deviations (<7 %) and recoveries within 88–99 %.

Cross-contamination was observed in poured samples at the lowest calibration points, likely due to analyte transfer between vials during decanting. This adversely affected linearity and MDL determinations.

Benefits and Practical Applications

Maintaining sealed conditions throughout sampling:

• Minimizes cross-contamination and improves detection limits for trace‐level analytes.
• Provides more consistent peak responses with lower variability.

At higher concentration ranges, both sealed and poured approaches yield comparable precision and accuracy, allowing flexibility based on laboratory throughput and equipment availability.

Future Trends and Opportunities

Advancements may include automated online headspace systems that further reduce human intervention and potential contamination. Integration of real-time monitoring sensors with autosampler platforms could enable continuous field analysis. Emerging materials for vial seals and improved loop designs may enhance sample integrity for ultratrace applications.

Conclusion

The sealed sample handling approach for ASTM D8028 delivers superior analytical performance, particularly for low-level dissolved gas measurements. It offers significantly better precision, lower MDLs and eliminates cross-contamination risks. Laboratories seeking to ensure data quality for environmental and industrial testing should adopt sealed sampling techniques where feasible.

Used Instrumentation

• EST Analytical Liquid Gas Extractor (LGX50) with 1 ml headspace loop
• Agilent 7890 Gas Chromatograph with Flame Ionization Detector
• Restek RT®-QS Bond 30 m × 0.53 mm × 20 µm column

References

1. Dean JA. Lange’s Handbook of Chemistry. 14th Ed. R.R. McGraw-Hill; 1992.
2. Gas Encyclopaedia. Air Liquide; 2009.
3. Hudson F. RSKSOP-175, Rev. 2; May 2004.
4. PADEP. Light Hydrocarbons in Aqueous Samples via Headspace GC/FID (PADEP 3686); Oct 2012.