GC Analysis of Permanent Gases

Importance of Permanent Gas Analysis

Permanent gases such as hydrogen, oxygen, nitrogen, carbon monoxide, methane and carbon dioxide play critical roles in industrial processes, environmental monitoring and equipment health diagnostics. Their low molecular weight, small size and similar physical properties make chromatographic separation challenging. Optimizing analyte selection, column chemistry and detection techniques is essential for accurate quantification in quality control, biogas analysis, transformer oil monitoring and other applications.

Study Objectives and Overview

This work reviews analytical strategies for mission-critical permanent gases. It covers methods to simplify solute lists, advanced sample introduction, comparison of capillary and porous columns, approaches to include CO₂ and higher hydrocarbons, low-level hydrogen detection, field-deployable micro gas chromatography and specialized analysis of ammonia.

Methodology and Instrumentation

• Sample introduction by gas-tight syringes and multiport valves for reproducible injections.
• Wall-Coated Open Tubular (WCOT) versus Porous Layer Open Tubular (PLOT) columns: PLOT phases (molecular sieves, alumina, porous polymers, carbon; e.g. HP-PLOT Al₂O₃, CP-SilicaPLOT) provide enhanced retention and selectivity for non-cryogenic separations.
• Molecular sieve (5Å) columns for H₂, O₂, N₂, CH₄ and CO; exclude CO₂, water and larger hydrocarbons.
• Cryogenic separation (GasPro at –80 °C) and packed columns (ShinCarbon ST) for simultaneous C1–C3 and CO₂ analyses.
• Column isolation and parallel column setups using switching valves to direct specific fractions onto molesieve and PLOT-Q phases.
• Detection by Thermal Conductivity Detector (TCD) with helium, argon or nitrogen carriers; use of methanizer upstream of FID to convert CO/CO₂ to CH₄ for enhanced sensitivity.
• Micro gas chromatography with up to four parallel channels, TCD detection, dynamic range to 10⁵ and field-portable configurations.
• Ammonia analysis on specialised WCOT (CP-Volamine) or short PLOT-U columns coupled with FID/TCD or Nitrogen Chemiluminescence Detector (NCD) to mitigate adsorption and decomposition.

Main Results and Discussion

PLOT columns enable baseline resolution of C2–C3 hydrocarbons and CO₂ at ambient temperatures, while molecular sieves excel for light permanent gases but require dry samples and periodic reconditioning. Analyzing CO₂ alongside smaller gases involves dual injections, cryogenic cooling or splitting flow across columns. Hydrogen detection by TCD faces a sensitivity trade-off: He carrier yields low H₂ response, whereas Ar/N₂ carriers degrade detection of other gases. Coupling a methanizer with TCD-FID permits simultaneous low-level measurement of H₂, CO and CO₂. Micro GC demonstrates sub-ppm detection for many permanent gases in portable formats. Ammonia requires fast columns and selective detectors to overcome reactivity and achieve practical detection limits near 100 ppm.

Benefits and Practical Applications

• Flexible analytical schemes tailored to target gas mixtures by combining column chemistries and detectors.
• High sensitivity and selectivity for light gases in laboratory and field environments.
• Rapid transformer oil dissolved gas analysis (TOGA) and on-line industrial monitoring.
• Integrated micro GC solutions for process control and environmental compliance.
• Improved ammonia measurement in air and industrial streams.

Future Trends and Applications

Advances in column materials with higher selectivity, miniaturized and multi-detector micro GC systems, laser-based detection and real-time data analytics are poised to enhance permanent gas analysis. Automated valve switching, machine-learning for chromatogram deconvolution and hybrid detector arrays will further streamline multi-component separation and quantitation.

Conclusion

Effective permanent gas analysis relies on judicious selection of analytes, column types and detection modes. Combining molecular sieves, porous phases, cryogenic and packed columns with optimized detectors addresses common challenges in sensitivity and selectivity. Emerging technologies promise greater portability, automation and analytical performance for industrial and environmental applications.

Reference

• S. Sinnott, Analysis of Permanent Gases: More Challenging Than You Might Think, Agilent Technologies, May 2023.
• NIST Chemistry WebBook, Thermal Conductivity of Gases at 400 K.