Resonance-Enhanced Multiphoton Ionization Spectroscopy of Monocyclic and Polycyclic Aromatic Hydrocarbons in the Gas Phase

Abstract

Rationale

Aromatic hydrocarbons (AHs) and polycyclic aromatic hydrocarbons (PAHs) pose significant risks to human health and the environment due to their toxic and carcinogenic properties. These depend strongly on molecular structure, with even isomers exhibiting different characteristics. Consequently, when conducting a risk assessment of a sample, a rapid and reliable detection technique capable of differentiating between isomers is crucial.

Methods

Time-of-flight mass spectrometry (TOFMS) combined with (1 + 1) resonance-enhanced multiphoton ionization ((1 + 1)-REMPI) has proven to be a promising approach due to its wavelength selectivity for different structures. An optical parametric oscillator generated UV radiation from 213 to 300 nm from the third harmonic (355 nm) of a Nd:YAG laser beam. A thermogravimetric system was applied to transfer the substances into the gas phase.

Results

We performed REMPI spectroscopy of 48 monocyclic and polycyclic aromatic hydrocarbons, including compounds with various substituents (alkyl groups, -OCH₃, -SH, -OH, -Cl) and heteroatoms (N, O, S). The observed spectral shifts correlate with ring number as well as the type, number, and position of substituents and heteroatoms. While these shifts are comparable to trends observed in absorption spectra, variations in intensity arise due to differences in excited-state lifetimes and the cross sections of both absorption steps. It was further demonstrated that the selected wavelength range, extending to a lower limit of 213 nm, is especially beneficial for the naphthalenes. The relative photoionization cross sections of the investigated compounds have been calculated, showing that the aforementioned structural dependencies also influence the ionization efficiency.

Conclusions

In common applications, these results may be used to determine a suitable laser wavelength for the substances of interest in order to achieve a higher level of sensitivity. For tunable laser applications, they serve as a reference for distinguishing and quantifying isomers in complex mixtures based on spectral shifts.