2-LASER MASS-SPECTROMETRY OF THIOLATE, DISULFIDE, AND SULFIDE SELF-ASSEMBLED MONOLAYERS

Self-assembled monolayers (SAMs) of thiolates, disulfides (RSSR+), and sulfides were studied on Au by N-2 laser desorption followed by vacuu m ultraviolet (VUV) (118-nm) photoionization of secondary neutrals in a time-of-flight mass spectrometer. Dimers (RSSR+) dominated the photo ionization mass spectrum from all chain lengths of alkanethiolates and disulfides studied. Nonmethyl-terminated alkanethiolates with X = (OH and COOH) were detected as dimers without loss of the terminal group. Phenyl-SAMs with X = (H, OH, OCH3, Cl, and NO2) were detected as both monomers and dimers. Thiocholesterol SAMs were detected solely as mon omers. The data suggest that dimerization occurs as a result of the re combination of surface thiolates during desorption. The alkane sulfide s were detected intact, but with additional monomer and dimer species present in the spectra. The appearance of dimers is not a strong funct ion of adsorbate structure or ordering and therefore cannot be taken a s evidence for or against the recently proposed model of thiolate dime rs on Au surfaces. Two receptor adsorbates, resorcin[4]arene tetrasulf ide and beta-cyclodextrin sulfide were examined by two-laser mass spec trometry (L2MS), but only the former gave identifiable high mass peaks . Mixed thiolate and disulfide monolayers generated both pure and mixe d dimers, providing information on nearest neighbor interactions. The mixed disulfide results indicate there is a common adsorption state fo r thiolates and disulfides. The laser desorption and VUV photoionizati on cross sections for these various organosulfur SAMs were found to be similar. L2MS with VUV photoionization was nonselective in its detect ion of these organosulfur species and produced mass spectra with littl e fragmentation.