Resonance-enhanced multiphoton ionization mass spectrometric analysis of lignin using laser pyrolysis with entrainment into a supersonic jet

Resonance-enhanced multiphoton ionization (REMPI) as a means of specifically ionizing aromatic compounds is extended to the analysis of lignin pyrolyzates. First, mass-resolved excitation spectra of phenolic model compounds of increasing complexity were measured to determine optimal resonant wavelengths for jet-cooled phenol, guaiacol, 4-methylguaiacol, homovanillic acid, and syringol. The obtained set of resonant wavelengths is unique for each benzene derivative, depending on the number and types of substituents on the aromatic ring, different isomeric conformations, and stable rotational conformers which are frozen out in the supersonic jet. In this work, the one-colour REMPI results are used to identify and quantify the main pyrolysis products of a cottonwood milled wood lignin. The insoluble lignin preparation is introduced directly inside the ion source of a time-of-flight (TOF) mass spectrometer and analyzed without any sample pretreatment. Flash pyrolysis is achieved by means of a pulsed CO₂ laser. Analytical calibration curves are linear over a dynamic range of 4 orders of magnitude. Limits of detection obtained at a resonant wavelength of homovanillic acid are homovanillic acid, 400 pg, coniferyl alcohol, 3 ng, and sinapyl alcohol, 5 ng. Results are compared with high-resolution in-source pyrolysis mass spectrometry data obtained under EI conditions.