ON THE CAPABILITY OF MULTIPHOTON IONIZATION MASS-SPECTROMETRY FOR ISOMER DISCRIMINATION - MASS-SPECTRA OF POSITIONAL ISOMERS OF AROMATIC-MOLECULES

Multiphoton ionization (MPI) mass spectra of positional isomers of aromatic molecules including substituted polycyclic aromatic hydrocarbons, anilines, and phenols are studied in a reflection time-of-flight mass spectrometer. It is shown that isomer discrimination is difficult based on the MPI mass fragmentation patterns for most positional isomers. However, compared with the electron impact (EI) ionization technique, MPI is found to be a more versatile method in that both soft and hard ionization mass spectra can be readily obtained. The ability of generating high intensity low-mass fragments with MPI is proven to be very useful for the discrimination of some positional isomers based on the fragmentation patterns even though these isomers have identical EI mass spectra. It is demonstrated that if aromatic ring isomerization results in a different precursor from the isomer pair, then MPI can distinguish these isomers.