Investigations Into the Interactions of a MALDI Matrix with Organic Thin Films Using C₆₀⁺ SIMS Depth Profiling

Molecular depth profiling of multilayer organic films is now an established protocol for cluster secondary ion mass spectrometry (SIMS). This unique capability is exploited here to study the ionization mechanism associated with matrix-enhanced SIMS and possibly matrix assisted laser desorption/ionization (MALDI). Successful depth profiling experiments were performed on model bi-layer systems using 2,5-dihydroxybenzoic acid (DHB) as the matrix with dipalmitoylphosphatidylcholine (DPPC) or phenylalanine (PHE). The interaction between the matrix and organic analyte is monitored at the interface of the films. Tri-layer films with D₂O as a thin-film sandwiched between the matrix and organic layers are also investigated to determine what role, if any, water plays during ionization. The results show successful depth profiles when taken at 90K. Mixing is observed at the interfaces of the films due to primary ion bombardment, but this mixing does not recreate the conditions necessary for ionization enhancement.