Analysis of Permanent Gases: More Challenging Than You Might Think

Importance of the Topic

The reliable separation and quantification of permanent gases (H₂, O₂, N₂, CH₄, CO, CO₂, and light hydrocarbons) underpins critical applications in environmental monitoring, industrial process control, transformer oil diagnostics, and quality assurance. The low molecular weight and similar physical properties of these analytes pose significant chromatographic challenges, demanding tailored column chemistries, optimized detectors, and innovative sampling approaches.

Objectives and Article Overview

This work reviews advanced gas chromatography strategies for comprehensive permanent gas analysis on a single GC platform. Key goals include: demonstrating the strengths and limitations of molesieve and PLOT columns; outlining techniques to incorporate CO₂ and C₂–C₃ hydrocarbons; addressing low-level hydrogen detection; and proposing practical workflows involving series, parallel, and cryogenic separations.

Methodology and Instrumentation

Sample introduction was performed via gas-tight syringes or multiport valves. Chromatographic separation relied on:

• Molesieve PLOT columns for molecular-size-based separation of H₂, O₂, N₂, CH₄, and CO.
• PLOT-Q and GasPro columns for retention of CO₂ and C₂⁺ hydrocarbons, with cryogenic trapping (–80 °C) where needed.
• ShinCarbon ST packed columns as an alternative single-column separation.

Detection modes included:

• Thermal Conductivity Detector (TCD) with He, Ar, or N₂ carrier gases.
• Flame Ionization Detector (FID) coupled after a methanizer (Ni catalyst at 350 °C) to convert CO/CO₂ into CH₄ for enhanced detection.

Main Results and Discussion

Molesieve columns offered high resolution for permanent gases except CO₂ and water, which are irreversibly adsorbed. Pairing a molesieve with a PLOT-Q column in series or parallel enabled simultaneous analysis of CO₂ and light hydrocarbons. Valve-based column isolation allowed flexible elution timing, while cryogenic trapping on GasPro delivered baseline separation of C₂ and C₃ species. ShinCarbon ST provided a single-injection, packed-column alternative, albeit with trade-offs in resolution. Hydrogen detection by TCD exhibited a carrier-dependent sensitivity trade-off (He carrier yields ~10 % sensitivity; Ar/N₂ carriers improve H₂ detection but reduce response for other gases). Incorporating a methanizer upstream of an FID overcame this “catch-22,” enabling low-ppm hydrogen determination alongside CO/CO₂ analysis in transformer oil gas analyzer (TOGA) configurations.

Benefits and Practical Applications

This integrated approach offers:

• Modular column configurations to adapt to target analytes without multiple instruments.
• Improved throughput through single-injection methods with column switching or parallel flow paths.
• Enhanced detection limits for trace hydrogen and CO₂ using methanizer-FID combinations.
• Compatibility with existing GC systems and valve manifolds for retrofit applications.

Future Trends and Potential Applications

Emerging opportunities include:

• Development of advanced porous materials and hybrid stationary phases for broader analyte coverage at ambient temperatures.
• Integration of micro-GC modules and automated column-switching valves within compact analysers.
• Application of pulsed discharge or laser-based detectors to further lower detection limits for permanent gases.
• AI-driven method optimization and real-time data analytics for on-line process monitoring.

Conclusion

Comprehensive permanent gas analysis demands a suite of chromatographic strategies combining size-selective PLOT phases, cryogenic traps, column isolation techniques, and dual-detector configurations. By judiciously selecting column chemistries and leveraging methanizer-FID coupling, laboratories can achieve robust, high-throughput profiling of both major and trace gaseous species on a single GC platform.

References

• International Zeolite Association. Type KFI molecular sieve overview. izo-online.org.
• NIST Standard Reference Data. Thermal conductivity of gases at 400 K.