Atmospheric Pressure Laser Ionization Mass Spectrometry with Tunable UV Wavelength Utilizing an Optical Parametric Oscillator

To our knowledge, this study presents the first implementation of wavelength-resolved resonance-enhanced multiphoton ionization (REMPI) spectroscopy under atmospheric pressure ionization conditions using a high-resolution mass spectrometric system. Atmospheric pressure laser ionization MS spectroscopic measurements were conducted on over 70 different polycyclic aromatic hydrocarbons (PAHs) and hetero-PAHs (N, S, and O) in standard solutions, as well as three complex PAH-containing samples. The results demonstrate the successful transfer of REMPI spectroscopy from vacuum to atmospheric pressure conditions, maintaining spectral integrity without significant band broadening. The obtained spectral data add an orthogonal dimension to mass spectrometry, providing structural insights into aromatic core motifs and enabling isomeric differentiation. This differentiation is further enhanced by linear regression algorithms, which allow for the semiquantitative analysis of isomer mixtures. Comparison between standard spectra and complex sample spectra reveals high correlation values for certain peaks, strongly indicating the presence of specific PAHs and distinguishing between phenanthrene and anthracene in fossil- and biobased samples.