Quantitation of Olefin Content in Plastic-Derived Pyrolysis Oils

Importance of the Topic

The accumulation of plastic waste and the search for sustainable alternatives to fossil fuels make chemical recycling approaches, such as pyrolysis, strategically important. Pyrolysis-derived oils from waste plastics can be processed as fuel replacements, but their detailed compositional characterization is essential because high olefin content affects fuel properties and downstream processing. Robust analytical workflows that can differentiate and quantify olefins within highly complex hydrocarbon matrices are therefore critical for quality control, process optimization, and environmental assessment.

Objectives and Study Overview

This application note demonstrates a practical, cost-effective analytical workflow to identify and quantify olefins in pyrolysis oils from waste polyethylene (PE) foil. The approach combines comprehensive two-dimensional gas chromatography with flame ionization detection (GCxGC-FID), a single-step iodine derivatization to chemically shift olefins on the chromatogram, and advanced comparative software (ChromaTOF and ChromaTOF Tile) to quantify olefin mass fractions by carbon number without relying on mass spectrometry. The study aims to show how derivatization plus GCxGC-FID improves separation and enables carbon-specific olefin quantitation in complex mixtures.

Methodology

Sample preparation:

• Two parallel sample treatments were used: simple dilution in pentane (1:50) and a single-step iodine derivatization intended to chemically modify olefins and change their chromatographic behavior.

Chromatographic analysis and data processing:

• Each sample (underivatized and derivatized) was analyzed in quintuplicate to assess reproducibility.
• Data were processed with ChromaTOF and comparative analysis was performed with ChromaTOF Tile to align chromatograms, detect peak shifts and quantify differences in peak areas corresponding to olefin conversion.
• Quantitation relied on FID peak areas: regions corresponding to monocycloalkanes (MCAs) were used as reference bands by carbon number; differences in normalized peak areas pre- and post-derivatization were attributed to olefin content for each carbon bin.

Used Instrumentation

The analysis used LECO QuadJet thermal modulated GCxGC with FID (QuadJet SD platform). Key operational details summarized from the study:

• Injection: 0.5 µL liquid, split 1:20 at 285 °C.
• Carrier gas: helium at 1.5 mL/min (constant flow).
• 1st-dimension column: Rxi-17SilMS, 29.5 m × 0.25 mm ID × 0.25 µm film.
• 2nd-dimension column: DB-1HT, 1.30 m × 0.25 mm ID × 0.25 µm (0.9 m coiled in secondary oven).
• Oven program: 3 min at 40 °C, ramp 3.0 °C/min to 285 °C, 1 min hold; secondary oven and modulator temperatures were offset (+35 °C and +20 °C relative offsets, respectively).
• Modulation: thermal modulation with 1.7 s modulation period and 0.28 s hot pulse.
• FID: 300 °C, data acquisition at 200 Hz; hydrogen 40 mL/min, air 450 mL/min, make-up helium 50 mL/min.

Data-processing parameters in ChromaTOF Tile were tuned for GCxGC-FID datasets (tile sizes, S/N thresholds, F-ratio-based statistical thresholds) to robustly detect and compare features across derivatized and underivatized runs.

Main Results and Discussion

GCxGC-FID generated structurally ordered chromatograms where hydrocarbon class bands (paraffins/olefins/naphthenes versus aromatics) are resolved into distinct regions. Iodine derivatization caused olefin peaks to shift to different elution regions, making previously coeluting or masked olefins visible in the derivatized chromatogram.

Comparative visualization with ChromaTOF Tile enabled automated alignment, highlight of regions lost or gained after derivatization, and zoomed inspection of individual carbon-number bands. By comparing normalized MCA peak areas before and after derivatization, the study calculated carbon-specific olefin contents. Representative quantitative outcomes from the PE-foil pyrolysis oil include:

• Total calculated olefin content (sum across analyzed carbon numbers C5–C10): 15.47 wt%.
• Example by carbon number: C9 region—pre-derivatization MCA wt% 6.41; post-derivatization MCA (true) wt% 1.58; calculated olefins 4.84 wt% for C9.

These results illustrate the approach’s ability to provide detailed, carbon-resolved olefin quantitation where conventional one-dimensional GC or non-derivatized analysis would struggle due to coelution and class overlap.

Benefits and Practical Applications of the Method

Key advantages demonstrated by the workflow:

• Improved discrimination of olefins in complex hydrocarbon matrices using a simple, single-step derivatization rather than routine MS identification.
• Quantitative capability via FID that, combined with chromatographic relocation of olefins, yields reliable mass-fraction estimates by carbon number.
• Cost-effectiveness relative to routine GCxGC-MS workflows: FID-based quantitation reduces the need for MS for routine quantitation tasks.
• Applicability to process control and fuel characterization: carbon-specific olefin profiles inform refining/upgrading strategies and compliance with fuel specifications.

Future Trends and Applications

Potential directions to build on this work include:

• Integration with mass spectrometry: coupling GCxGC to MS for compound-level confirmation while retaining FID quantitation for streamlined workflows.
• Automation of derivatization and data-processing pipelines for higher throughput and routine QA/QC in recycling or fuel production facilities.
• Extension to a broader range of feedstocks (mixed plastics, polyolefin blends) and broader carbon ranges to map olefin distributions for different process conditions.
• Development of calibrated quantitation protocols (internal standards, response-factor corrections) to increase absolute accuracy across diverse matrices.
• Exploring alternative derivatization chemistries or on-line derivatization to simplify sample handling and reduce operator time.

Conclusion

A single-step iodine derivatization combined with thermal-modulated GCxGC-FID and advanced comparative software provides a robust, reproducible method for detecting and quantifying olefins in highly complex plastic-derived pyrolysis oils. The approach enables carbon-specific olefin quantitation without requiring mass spectrometry and supports detailed fuel characterization useful for recycling and fuel-upgrading workflows.

Reference

1. Gentilcore et al. Journal of Environmental Chemical Engineering, 2024. DOI: 10.1016/j.jece.2024.113836
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4. Carlson, D. A.; Roan, C. S.; Yost, R. A.; Hector, J. Dimethyl disulfide derivatives of long chain alkenes, alkadienes, and alkatrienes for gas chromatography/mass spectrometry. Analytical Chemistry 1989, 61, 1564-1571.

Acknowledgments: Complex Chemical Analysis Laboratory (CCAL), California State University, Los Angeles (Dr. Petr Vozka, Genesis Barzallo, Hung Gieng) and LECO Corporation personnel contributed to the study.