Trace carbonyl sulfide and phosphine in ethylene or propylene

Importance of the topic

The accurate monitoring of trace sulfur and phosphorus species in ethylene and propylene is critical for the polymer industry. Carbonyl sulfide (COS) and phosphine (PH3) at sub-ppm levels can poison polymerization catalysts and degrade polymer properties. Reliable quantification of these contaminants supports quality control, process optimization and regulatory compliance.

Aim and study overview

This application note describes a gas chromatographic method using a PoraPLOT Q capillary column coupled with a pulsed flame photometric detector (PFPD) for the determination of COS and PH3 in ethylene and propylene. The study demonstrates separation, detection sensitivity, precision and detection limits for each analyte in two different hydrocarbon matrices.

Methodology and instrumentation

The experimental setup employs a six-port gas sampling valve with a 100 µL Silcosteel® sample loop as the injector. Samples and standards are delivered through 1/16″ Silcosteel tubing to prevent analyte adsorption. Two primary gas standards (10 ppmv COS in N2 and 10 ppmv PH3 in N2) were diluted into ethylene or propylene to prepare matrix standards. Chromatographic separation was achieved on an Agilent PoraPLOT Q column (0.32 mm × 50 m, 10 µm film) with helium carrier at 3 mL/min. The PFPD operated with hydrogen (13 mL/min), two air flows (17 mL/min and 10 mL/min), a 3 mm combustor and appropriate optical filters (BG-12 for COS, GG-495 for PH3). Oven programs were tailored for each analyte to optimize retention and peak shape.

Main results and discussion

Chromatograms show clear resolution of COS and PH3 from ethylene and propylene background peaks. COS elutes after ethylene and just before propylene, exhibiting symmetric peaks unaffected by the matrix. PH3 elutes between ethylene and propylene with sharper profiles when preceding the matrix gas. Precision over five replicate injections yielded relative standard deviations (RSD) for retention times below 0.2% and for peak areas between 2.6% and 5.1%. Method detection limits (MDLs) were determined at approximately 0.07–0.12 ppm (70–120 ppb) for both analytes in both matrices.

Benefits and practical applications

• High sensitivity for trace COS and PH3 down to ~100 ppb.
• Robust separation in ethylene and propylene matrices.
• Low sample volume minimizes matrix overload and enhances peak shape.
• Method precision suitable for routine quality control in polymer feedstocks.

Future trends and potential applications

Emerging detector technologies and coupling with mass spectrometry could further lower detection limits and improve selectivity. Online sampling and automation of loop injections may enhance throughput. The method could be extended to other olefins or refined to quantify additional sulfur/phosphorus species, supporting broader petrochemical process monitoring.

Conclusions

A PoraPLOT Q column with PFPD provides a reliable and sensitive approach for measuring trace COS and PH3 in ethylene and propylene. The method achieves detection limits below 0.12 ppm with excellent reproducibility, meeting industry requirements for catalyst protection and product quality.

Reference

Agilent Technologies, Inc. Application Note A01557, published prior to May 11, 2010.