Trace Carbonyl Sulphide and Phosphine in Ethylene or Propylene

Importance of the Topic

Trace levels of carbonyl sulfide and phosphine in polymer feedstocks can poison catalysts and degrade product quality in polyethylene and polypropylene manufacture. Monitoring these impurities at sub-ppm levels is essential to ensure catalyst performance and final polymer properties.

Objectives and Study Overview

This study evaluates a gas chromatography method using a PLOT Q capillary column and pulsed flame photometric detection to quantify trace COS and PH3 in ethylene and propylene matrices. Key goals include achieving low detection limits, stable retention times, and reproducible peak shapes under industrially relevant conditions.

Experimental Methodology and Instrumentation

A 100 microliter gas sampling loop coupled to a six port valve was used to inject samples into a PLOT Q capillary column (50 m x 0.32 mm x 10 micrometers). Carrier gas helium flowed at 3 mL per minute. Oven temperature started at 30 degrees C with a 15 minute hold, followed by a 5 degrees C per minute ramp to 120 degrees C. A pulsed flame photometric detector operated with hydrogen at 13 mL per minute, two air flows at 17 and 10 mL per minute, and separate filters and combustor settings for sulfur and phosphorus channels (200 degrees C with sulfur filter for COS, 300 degrees C with phosphorus filter for PH3). Matrix standards were prepared by diluting certified COS and PH3 gas mixtures in ethylene or propylene.

Main Results and Discussion

Both analytes were resolved from the carrier gases with sharp peak shapes and stable retention times. COS eluted after ethylene but before propylene, while PH3 eluted after ethylene and before propylene, with peak geometry influenced by matrix interactions. Method precision was evaluated over five injections, yielding retention time RSD below 0.2 minute and peak area RSD below 6 percent. Method detection limits were approximately 0.07 to 0.12 ppm, calculated using a student t factor of 3.1 for 99 percent confidence with five replicates.

Benefits and Practical Applications

• Low detection limits enable routine monitoring of COS and PH3 at sub-ppm levels in polymer feedstocks.
• Minimal sample volume reduces matrix effects and extends column lifetime.
• High reproducibility ensures reliable quality control in industrial laboratories.

Future Trends and Potential Applications

Expect integration with automated sampling systems for continuous on-line monitoring. Advances in detector selectivity and column materials may further lower detection limits and improve maintenance intervals. The methodology can be adapted for other trace sulfur and phosphorus species in petrochemical streams.

Conclusion

The GC PLOT Q column combined with pulsed flame photometric detection provides a robust approach for quantifying trace carbonyl sulfide and phosphine in ethylene and propylene. The method achieves sub-ppm detection, excellent reproducibility, and clear separation from matrix gases, offering a practical solution for polymer production quality control.

Reference

• SCION Application Note AN0011 Trace Carbonyl Sulphide and Phosphine in Ethylene or Propylene