Resonant two-photon ionization mass spectrometry of jet-cooled phenolic acids and polyphenols

A method for analyzing phenolic acids and polyphenols by means of resonant two-photon ionization (R2PI) mass spectrometry coupled with laser desorption and supersonic jet cooling is described. The R2PI spectra of gallic acid, 3-O-methylgallic acid, protocatechuic acid, syringic acid, vanillic acid, and trans-resveratrol are vibronically resolved and distinct to allow for unambiguous identification. For vanillic acid, its R2PI spectrum can be separated into contributions of two rotational isomers based on UV-UV and IR-UV double-resonance spectroscopy. Since R2PI spectra display sharp and well-resolved peaks, the laser wavelength can be tuned for selective ionization of targeted molecules. The mass spectrum recorded under jet-cooled conditions and at the resonant wavelength displays only the molecular ion peak with no fragmentation or background peaks. Picogram sensitivity and linear response over a nanogram range allows trace quantitative measurements of target molecules in complex matrixes. These techniques were applied to detect syringic acid in a model archaeological wine vessel.