Molecular Sieve PLOT Columns for the Capillary GC Separation of Permanent Gases

Significance of Topic

The analysis of permanent gases and light hydrocarbons is critical across environmental monitoring, petrochemical processing, and quality control in industrial laboratories. High-resolution, reproducible separations enable accurate quantitation of trace impurities and rapid profiling of gas samples. Porous layer open tubular (PLOT) columns based on molecular sieves offer enhanced performance over traditional packed columns, combining high efficiency with direct injection capability for concentrated gas mixtures.

Objectives and Study Overview

This study evaluates two classes of PLOT columns—Carboxen-1006 carbon molecular sieve and aluminosilicate Molecular Sieve 5A—for capillary gas chromatography of permanent gases and C2/C3 hydrocarbons. Key goals include assessing reproducibility, separation efficiency under temperature-programmed and isothermal conditions, and the capacity for direct injection of high-concentration samples.

Methodology and Instrumentation

Both column types feature a 30µm porous layer on capillaries of 0.32mm and 0.53mm internal diameter. Carboxen-1006 uses carbon particles with hierarchical porosity (macro, meso, and micropores) bonded to the wall, while Molecular Sieve 5A employs submicron aluminosilicate beads. Separation performance was characterized using temperature programming and constant-temperature methods. Key parameters included:

• Oven temperatures ranging from subambient conditions up to 300°C
  
• Carrier gas helium at flows of 10mL/min or linear velocities of 22–76cm/sec
  
• Detectors: thermal conductivity detector (TCD) at 230–260°C and flame ionization detector (FID) at 230°C
  
• Direct injection of sample volumes up to 230°C inlet temperature for concentrated streams

Main Results and Discussion

Reproducibility tests over multiple runs yielded retention factor standard deviations below 1 for carbon dioxide on Carboxen-1006 and carbon monoxide on Molecular Sieve 5A columns. Van Deemter analysis demonstrated superior efficiency at linear velocities exceeding the optimum due to the presence of macropores in Carboxen-1006, facilitating faster analyses without peak broadening. Representative separations include:

• Baseline resolution of nitrogen, CO, methane, CO2, and C2/C3 hydrocarbons on Carboxen-1006 under temperature programming (35–250°C)
• Clear separation of formalin components (water, formaldehyde, methanol) at elevated isothermal temperature (220°C)
• Detection of propylene impurities at the 1% level using FID on Carboxen-1006
• Efficient separation of permanent gases (H2, O2, N2, CO) and light hydrocarbons on Molecular Sieve 5A at 65°C

Benefits and Practical Applications

PLOT capillary columns with Carboxen-1006 and Molecular Sieve 5A sorbents deliver:

• High resolution for trace-level impurities in petrochemical streams
• Direct injection of concentrated gas samples without pre-dilution or trap steps
• Rapid temperature programming for fast cycle times
• Stable performance with minimal bleed and long column lifetimes

This makes them well suited for environmental analysis, process monitoring, and regulatory compliance testing.

Future Trends and Potential Uses

Emerging directions include integration of PLOT columns with mass spectrometry for enhanced identification, development of new sorbent chemistries for selective capture of volatile organic compounds, and miniaturization for portable GC systems in field analysis. Advances in column fabrication may further improve thermal stability and reduce analysis times.

Conclusion

Carboxen-1006 and Molecular Sieve 5A PLOT capillary columns represent a significant advancement for capillary GC separation of permanent gases and light hydrocarbons. They offer reproducible retention, high efficiency at elevated flow rates, and direct injection capability, providing a robust alternative to conventional packed columns in diverse analytical applications.

References

1. Betz W., Keeler M. Molecular Sieve PLOT Columns for the Capillary GC Separation of Permanent Gases. Supelco Reporter Vol.15, No.3, 1996.
2. US Patent No. 4,839,331.
3. van Deemter J.J., et al. A Study of the Separation Process in Chromatography, Chem. Eng. Sci., 1956.