Impacts of Surface Morphology on Ion Desorption and Ionization in Desorption Ionization on Porous Silicon (DIOS) Mass Spectrometry

Ordered silicon nanocavity arrays were prepared with e-beam lithography to yield systematically varied pore features and porosity (4-92%). These substrates were used to investigate the effects of substrate morphology on desorption ionization on porous silicon-mass spectrometry (DIOS-MS). Five benzylpyridinium salts, 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and angiotensin III were used as the model molecules in the study. For substrates of the same pore depth, MS results suggested that the pore size and the interpore spacing had little impact on the laser irradiation threshold required for ionization. Instead, the laser threshold was found to be highly dependent on the overall porosity for all substrates investigated-the higher the porosity the lower the threshold. Moreover, the substrates with deeper pores but of similar porosity showed significantly reduced laser thresholds. This close relationship between laser threshold and substrate morphology was attributed to the thermal confinement property of porous structures. Benzylpyridinium salts were used to study molecular fragmentation tendency during desorption and ionization (D/I). The results suggested the presence of two competing D/I processes: direct laser desorption ionization (LDI) dominated for the substrates of low porosities where analytes desorbed directly from hot silicon surfaces; for highly porous substrates, the retained solvent molecules behaved as the pseudo matrix-assisted laser desorption/ionization (pseudo-MALDI) matrix that facilitated analyte desorption and ionization in a MALDI mode.