Soft ionization GC-HRMS of Polycyclic Aromatic Hydrocarbons (PAHs)

Significance of the Topic

Polycyclic aromatic hydrocarbons (PAHs) are toxic, carcinogenic compounds formed during incomplete combustion of organic materials. Their persistence and bioaccumulation pose serious risks to human health and the environment, making sensitive trace-level detection in water, waste, and food matrices essential for regulatory and safety monitoring.

Objectives and Study Overview

This study demonstrates the applicability of gas chromatography coupled with soft ionization SICRIT® high-resolution mass spectrometry (GC-SICRIT®-HRMS) for qualitative and quantitative analysis of PAHs. Emphasis is placed on the influence of plasma carrier gas conditions on ion species distribution and on achieving low detection limits.

Methodology and Instrumentation

Samples consisted of a certified PAH standard mixture in dichloromethane, diluted down to 10 ppb for sensitivity studies.

Used instrumentation:

• Gas chromatograph: Shimadzu GC-2014 equipped with SLB-5ms column (20 m × 0.18 mm ID, 0.18 μm film)
• Cold plasma ion source: SICRIT® dielectric barrier discharge operated at 1.6 kV and 10–15 kHz
• Mass spectrometer: Thermo LTQ Orbitrap XL in positive full-scan mode, resolution 30 000 FWHM (m/z 50–500)
• Carrier gas: helium, plasma supplied by dry or humidified nitrogen (90 % RH)

Key GC parameters:

• Injection: 2 μL splitless, 4 mm ID liner
• Oven program: 55 °C (1.2 min), ramp 25 °C/min to 270 °C, then 35 °C/min to 320 °C (3 min hold)
• Transfer line temperature: 280 °C, constant carrier gas flow of 30 cm/s

Main Results and Discussion

• All 16 priority PAH compounds were baseline separated and identified by exact mass.
• Humidified nitrogen plasma produced both radical cations and protonated ions alongside oxidation by-products; dry nitrogen strongly favored formation of [M+H]+ ions and minimized oxidized species.
• Protonation under dry conditions improved signal-to-noise ratios and simplified mass spectra.
• Detection of all PAHs at 10 ppb in full-scan mode suggests potential limits of detection down to low ppt levels under optimized MRM conditions.

Benefits and Practical Applications

• Soft plasma ionization preserves molecular ions, enhancing specificity and identification confidence.
• High-resolution MS allows direct mass confirmation without extensive fragmentation.
• The streamlined workflow and high sensitivity support environmental monitoring, food safety testing, and regulatory compliance.

Future Trends and Potential Uses

• Exploration of dopants and alternative gas compositions to further boost ionization efficiency.
• Application to other non-polar and semi-volatile organic contaminants.
• Coupling with tandem MS (MRM) methods for ultra-trace quantitation.

Conclusion

GC-SICRIT®-HRMS using dry nitrogen plasma offers a robust, sensitive approach for trace PAH analysis, achieving clean [M+H]+ spectra and detection limits in the ppt range. This technique represents a competitive alternative to conventional GC–MS protocols.

References

1. A.K. Huba, M.F. Mirabelli, R. Zenobi, Analytical Chemistry 2019, 91, 10694–10701.