Effect of hydrogen carrier gas on Py-GC/MS analysis of polymers Part 1 Hydrogenation of HDPE pyrolyzates

Significance of the Topic

The choice of carrier gas in pyrolysis gas chromatography–mass spectrometry (Py-GC/MS) can influence the chemical composition of pyrolysates. Understanding hydrogenation reactions during pyrolysis is essential for accurate polymer identification and reliable library searches in analytical chemistry.

Objectives and Study Overview

This study investigates how hydrogen (H2) as a carrier gas affects the mass spectra of high density polyethylene (HDPE) pyrolysis products, particularly in the presence of residual metal catalysts. Part 1 focuses on the hydrogenation of unsaturated hydrocarbons formed during pyrolysis.

Methodology and Instrumentation

100 µg of freeze-pulverized HDPE powder was placed in a deactivated stainless steel cup. A Frontier Multi-Shot Pyrolyzer (EGA/PY-3030D) was directly interfaced with a GC/MS system. Separations were performed on a 30 m Ultra ALLOY⁺-5 capillary column (5% diphenyl, 95% dimethylpolysiloxane, 0.25 mm i.d., 0.25 µm film). Pyrolysis was conducted at 600 °C, and the GC oven was programmed from 40 °C (2 min hold) to 320 °C at 20 °C/min (3 min hold). Helium or hydrogen was used as carrier gas at 1 mL/min with a 1/100 split ratio.

Main Results and Discussion

Comparison of pyrograms in He and H2 atmospheres revealed significant changes in the relative peak areas of diolefins (Cn”) and alkanes (Cn). Under He, diolefin C14” predominates over alkane C14, whereas in H2 the opposite trend occurs. This shift is attributed to stepwise hydrogenation of diolefins to mono-olefins and ultimately to alkanes, catalyzed by trace metal residues (Mg, Ti, Cr, Zr, Al) in HDPE. Low density polyethylene (LDPE), free of metal catalysts, did not exhibit hydrogenation under H2, confirming the catalytic role of residual metals. Despite these reactions, the limited extent of hydrogenation has negligible impact on library search outcomes.

Benefits and Practical Applications

• Maintains compatibility with existing spectral libraries when using H2.
• Enables faster GC separations due to higher diffusivity of hydrogen.
• Provides insight into catalyst effects on polymer degradation pathways.

Future Trends and Potential Applications

Advances may include development of metal-free pyrolysis protocols to eliminate hydrogenation artifacts, exploration of hydrogen carrier gas benefits for other polymer types, and integration with machine learning for enhanced spectral interpretation.

Conclusion

The study demonstrates that hydrogen carrier gas induces partial hydrogenation of HDPE pyrolysates in the presence of metal residues, altering the ratio of unsaturated to saturated hydrocarbons. However, the effect is minor and does not compromise spectral library matching, supporting the use of H2 for accelerated Py-GC/MS analyses.

References

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