7.87 eV postionization of peptides containing tryptophan or derivatized with fluorescein

Chemical tags such as anthracene can be attached to a molecular analyte and serve as chromophores for 7.87 eV laser postionization by lowering the overall ionization potential of the tagged molecular complex. Fluorescein and tryptophan are demonstrated as two new tags for 7.87 eV laser postionization of various amino acids and peptides. Other molecular species that are efficient fluorescence probes should also serve as tags for 7.87 eV postionization since they display highest occupied molecular orbitals with extended π-conjugation that lead to ionization potentials below this photon energy and an ability to stabilize the net positive charge of the radical cations. This technique is demonstrated here for laser desorbed species, but is also applicable to keV ion sputtered neutrals. Overall, 7.87 eV laser postionization of derivatized species promises to expand the capabilities of mass spectrometric surface analysis.