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

Significance of the topic

Polycyclic aromatic hydrocarbons (PAHs) are widespread environmental contaminants formed by incomplete combustion. Due to their mutagenic and carcinogenic potential, regulatory agencies monitor PAHs at trace levels in water, waste, food and other matrices. High sensitivity and selectivity are essential for reliable PAH analysis, motivating the development of advanced soft ionization techniques coupled to high-resolution mass spectrometry.

Objectives and study overview

This study evaluates the feasibility of gas chromatography coupled to soft ionization via dielectric barrier discharge (SICRIT®) and high-resolution mass spectrometry (GC-SICRIT®-HRMS) for trace analysis of 16 EPA priority PAHs. Key aims include demonstrating compound separation, assessing ion species formation under different plasma carrier gas conditions, and determining detection sensitivity.

Methodology

A certified PAH mix (CRM47930) dissolved in dichloromethane was diluted in MS-grade acetonitrile down to 10 ppb. Samples (2 µL splitless injections) were separated on a 20 m SLB-5ms column with helium carrier gas (constant flow 30 cm/s) and a temperature program from 55 °C to 320 °C. A SICRIT® dielectric barrier discharge source operated at 1.6 kV and 10–15 kHz introduced soft ionization plasma using either dry or humidified nitrogen. The plasma-ionized effluent was analyzed on a Thermo LTQ Orbitrap XL in positive full-scan mode (m/z 50–500) at 30 000 FWHM resolution.

Instrumentation

• Gas chromatograph: Shimadzu GC-2014
• Column: Supelco SLB-5ms, 20 m, 0.18 mm ID, 0.18 µm film
• Ion source: SICRIT® dielectric barrier discharge plasma
• Mass spectrometer: Thermo LTQ Orbitrap XL
• Carrier gas: Helium (GC), Nitrogen (plasma)

Main results and discussion

All 16 PAHs were baseline-separated and confirmed by accurate mass measurements. Variation of plasma carrier gas humidity significantly affected ion species distribution:

• Humidified N₂ generated a mix of radical cations, protonated species and minor oxidation products.
• Dry N₂ favored clean protonation pathways, producing predominantly [M+H]+ ions with minimal oxidized by-products.

Under optimized dry N₂ conditions, calibration for pyrene and other PAHs demonstrated detection at 10 ppb in full-scan mode, indicating potential ppt-level limits under MRM optimization.

Practical benefits and applications

GC-SICRIT®-HRMS combines soft ionization with high mass resolution, enabling straightforward molecular ion detection, reduced fragmentation and improved signal-to-noise. This approach complements conventional GC-MS and LC-MS techniques, offering a robust alternative for environmental monitoring, food safety testing and industrial quality control of PAHs.

Future trends and opportunities

Further enhancements may include:

• Incorporation of dopants to boost ionization efficiency
• Optimized carrier gas compositions for broader analyte classes
• Integration with triple quadrupole systems under MRM for sub-ppt quantification
• Expansion to other neutral organic pollutant families

Conclusion

GC-SICRIT®-HRMS demonstrates effective soft ionization and high-resolution detection of PAHs at trace levels. Dry nitrogen as plasma gas yields optimal [M+H]+ ion formation, supporting quantitative analysis down to ppt concentrations. This method offers a competitive, sensitive alternative for routine PAH monitoring.

References

• [1] A.K. Huba, M.F. Mirabelli, R. Zenobi, Anal. Chem. 2019, 91, 10694-10701.