ANALYSIS OF PROPYLENE IMPURITIES USING SELECT LOW SULFUR COLUMN AND SINGLE TUNE WITH GC-ICP-MS QQQ ORS

Significance of the topic

Accurate determination of trace impurities in high-purity propylene is vital for maintaining polymerization catalyst performance, product clarity, and odor control. Even parts-per-billion levels of phosphine, arsine, hydrogen sulfide, and carbonyl sulfide can impact catalyst lifetime and product quality, making reliable ultra-trace analysis essential for industrial QA/QC and process monitoring.

Objectives and overview of the study

This study aimed to develop a streamlined single-column, single-tune GC-ICP-MS method to quantify key sulfur, phosphorus, and arsenic contaminants in propylene at sub-ppb levels. By leveraging an Agilent Select Low Sulfur column and the ORS mass-shift capability of a triple quadrupole ICP-MS, the authors sought to overcome prior limitations related to coelution and matrix interferences.

Methodology and instrumentation

• Gas chromatograph: Agilent 7890B GC equipped with Select Low Sulfur column (60 m × 0.32 mm) operated isothermally at 35 °C under 20 psig helium.
• Mass spectrometer: Agilent 8800 ICP-QQQ in MS/MS mass-shift mode using oxygen gas to detect P (m/z 31→47), S (m/z 32→48), and As (m/z 75→91) while suppressing interferences.
• Sample introduction: 400 μL split injection (~5:1) via 10-port gas sampling valve for standard additions, dynamic dilution of 10 ppmv gas standards to ppb levels using a fixed restrictor system.
• Matrix handling: High-flow Deans switch to vent propylene matrix and maintain ~800 cc/min argon make-up to protect the plasma and cones.

Main results and discussion

Detection limits based on 3x signal-to-noise were 2.5 ppb for H2S, 1.9 ppb for COS, 0.15 ppb for PH3, and 0.05 ppb for AsH3. The Select Low Sulfur column achieved baseline separation of analytes from propylene and propane, eliminating coelution issues seen with prior columns. Chromatograms revealed that hydrocarbon matrices depress the sulfur signal yet enhance the phosphorus response, likely via plasma stoichiometry effects. The ORS oxygen mass-shift effectively minimized isobaric overlaps (e.g., OO+ on S+) and reduced background noise without retuning or changing cell gas.

Benefits and practical applications

• Single-column, single-tune workflow simplifies method setup and increases laboratory throughput.
• Sub-ppb quantitation of critical impurities ensures robust QA/QC and predictive catalyst life modeling.
• Oxygen-based ORS mass-shift reduces interferences and eliminates the need for multiple GC columns or tune modes.

Future trends and potential applications

Further enhancements may include using a mega-bore column with full-flow injection to push detection limits lower, expanding the method to additional trace contaminants, and integrating automated standard addition routines. The ORS mass-shift approach could be applied to other gas matrices and emerging impurities in petrochemical process streams.

Conclusion

The combination of the Agilent Select Low Sulfur GC column with an 8800 ICP-QQQ in oxygen MS/MS mass-shift mode enables reliable, high-sensitivity quantitation of ppb-level sulfur, phosphorus, and arsenic impurities in propylene using a single analysis. This streamlined approach overcomes previous separation and interference challenges, supporting efficient industrial monitoring and quality control.

Reference

Geiger WM, McElmurry B, Anguiano J. Analysis of propylene impurities using select low sulfur column and single tune with GC-ICP-MS QQQ ORS. Agilent Technologies Application Note, 2016.