Brominated tyrosine and polyelectrolyte multilayer analysis by laser desorption vacuum ultraviolet postionization and secondary ion mass spectrometry

The small molecular analyte 3,5-dibromotyrosine (Br(2)Y) and chitosan-alginate polyelectrolyte multilayers (PEM) with and without adsorbed Br(2)Y were analyzed by laser desorption postionization-mass spectrometry (LDPI-MS). LDPI-MS using a 7.87 eV laser and tunable 8-12.5 eV synchrotron vacuum ultraviolet (VUV) radiation found that desorption of clusters from Br(2)Y films allowed detection by ≤8 eV single photon ionization. Thermal desorption and electronic structure calculations determined the ionization energy of Br(2)Y to be ~8.3 ± 0.1 eV and further indicated that the lower ionization energies of clusters permitted their detection at ≤8 eV photon energies. However, single photon ionization could only detect Br(2)Y adsorbed within PEMs when using either higher photon energies or matrix addition to the sample. All samples were also analyzed by 25 keV Bi(3)(+) secondary ion mass spectrometry (SIMS), with the negative ion spectra showing strong parent ion signal which complemented that observed by LDPI-MS. However, the negative ion SIMS appeared strongly dependent on the high electron affinity of this specific analyte and the analyte's condensed phase environment.