Resonance-Enhanced Multiphoton Ionization inside the C-trap of an Orbitrap Mass Spectrometer

Importance of the Topic

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants with well-documented toxic and carcinogenic effects. Sensitive and selective trace analysis of PAHs in gas, liquid, and particle phases is critical for monitoring exposure and understanding emission sources. Resonance-enhanced multiphoton ionization (REMPI) coupled with high-resolution mass analyzers offers a route to achieve both selectivity and sensitivity, addressing challenges of isobaric interferences in complex samples.

Aim and Study Overview

This work demonstrates the first implementation of a 1+1 REMPI scheme directly inside the C-trap of an Orbitrap mass spectrometer. The study aims to evaluate the performance of on-trap vacuum photoionization for aromatic compounds, assess limits of detection (LODs) at ppb levels, and preserve the high mass accuracy and resolving power inherent to the Orbitrap platform.

Experimental Methodology and Instrumentation

The experimental setup integrates:

• A UV laser system (Nd:YAG fundamentals and harmonics at 248 or 266 nm, Ø 0.7 mm beam) directed into the C-trap via a custom MgF2 flange.
• Orbitrap Exactive series mass analyzer operated at 140,000 resolution (m/Δm at m/z 200), with dynamic range preserved.
• FTMS Booster X2 data acquisition for full-profile capture and deep evaluation of the resonance ionization events.
• Optional gas chromatograph (GC) coupling with heated and non-heated inlets for volatile and semi-volatile species.

Main Results and Discussion

Calibration with standard mixtures of benzene, toluene, xylene, and 18 PAH compounds demonstrated high resolving power (R > 1.6×10^5) and mass accuracy within ±1 ppm. LODs ranged from 13.95 ppb for m/z 106 to 70 ppb for trimethylbenzene. Complex matrices such as heavy fuel oil (HFO), marine gas oil (MGO), and wood combustion aerosols yielded clear separation of isobaric species with mass splits down to 3.4 mDa. GC-REMPI-Orbitrap coupling resolved isomeric interferences and maintained chromatographic retention time fidelity.

Benefits and Practical Applications

On-trap REMPI provides:

• Selective photoionization of aromatic compounds, reducing chemical noise.
• Preservation of Orbitrap resolution and mass accuracy under resonance ionization conditions.
• Compatibility with GC and potential coupling to thermal or desorption devices for particle and aerosol studies.
• Field-deployable configuration enabling online and in situ measurements.

Future Trends and Opportunities

Emerging directions include exploiting alternative laser wavelengths (KrF excimer, OPO) to target a broader range of intermediates, applying high-repetition-rate lasers to increase ion yield and duty cycle, and integrating with hyphenated techniques (thermal analyzers, desorption electrospray) for comprehensive molecular mapping.

Conclusion

This work establishes a novel REMPI implementation within the C-trap of an Orbitrap mass spectrometer, combining high selectivity for aromatic species with the superior resolution and accuracy of FT-MS. The approach enables ppb-level quantitation in complex samples and opens avenues for advanced environmental and petrochemical analyses.

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