Effect of hydrogen carrier gas on Py-GC/MS analysis of polymers Part 2 Hydrogenation of HDPE pyrolyzates in EI source of MS

Significance of the topic

The choice of carrier gas in pyrolysis gas chromatography mass spectrometry (Py-GC/MS) directly impacts the chemical pathways and ionization processes of polymer fragments. Studying hydrogenation effects on high density polyethylene (HDPE) pyrolyzates enhances the reliability of polymer characterization in research and quality control.

Objectives and study overview

This work compares HDPE pyrolysis under helium and hydrogen atmospheres. It aims to determine how hydrogenation of unsaturated pyrolyzates influences mass spectral peak patterns, retention times, and fragment intensity ratios during electron ionization.

Methodology and instrumentation

• Sample: 200 µg HDPE powder placed in a deactivated stainless steel cup (4 mm inner diameter, 8 mm depth)
• Pyrolysis: 600 °C using a Multi-Shot Pyrolyzer EGA/PY-3030D with Auto-Shot Sampler AS-1020E
• Chromatography: Direct split injector GC/MS with UA+-5 capillary column (5% diphenyl/95% dimethylpolysiloxane, 30 m × 0.25 mm, 0.25 µm film) at split ratio 1/100
• Carrier gas: Helium or hydrogen at 1 mL/min flow rate

Main results and discussion

Hydrogen carrier gas reduced retention times for C14 fragments due to its higher linear velocity. Significant enhancement of the M+2 ion (m/z 198) under hydrogen indicates partial hydrogenation of unsaturated bonds either during chromatographic separation or within the MS ion source. Consequently, peak intensity ratios for diolefins, monoolefins, and alkanes differed markedly from those obtained under helium.

Benefits and practical applications

• Improved identification of hydrogenated products enhances structural elucidation of polymers
• Reduced ambiguity in detecting unsaturated species supports accurate quantitation
• Method applicable to routine polymer QA/QC, degradation studies, and materials research

Future trends and possibilities

The demonstrated hydrogenation effect paves the way for broader use of hydrogen carrier gas in Py-GC/MS workflows. Future research may optimize ion source conditions to control in-source hydrogenation, extend the approach to diverse polymer classes, and integrate catalytic hydrogenation modules for targeted fragment analysis.

Conclusion

Using hydrogen as a carrier gas in Py-GC/MS induces partial hydrogenation of unsaturated pyrolysis fragments, altering mass spectral signatures and retention behavior. Recognizing and managing these effects is essential for precise polymer analysis.

References

1. A. Watanabe et al., Analytical Chemistry 88 (2016) 5462–5468