Surface Chemistry Effects and Sampling Depth of Molecular Ions from Static Secondary Ion Mass Spectrometry of Langmuir-Blodgett Fatty Acid Films

The effects of surface chemistry on the probability of formation of secondary molecular ions in static secondary ion mass spectrometry (SIMS) are studied. Langmuir-Blodgett (L-B) films and solution deposition of neutral fatty acids on polycrystalline silver substrates are chosen as model systems to explore the effects of orientation, coverage, and primary ion beam dosage on the detection of molecular ions. Alternating layers within a five layer L-B multilayer system are used to explore the depth from which such molecular ions originate in a organized assembly. Results presented show the effects of specific orientation on the formation of (M + H)⁺, which requires intermolecular proton transfer in head to head configurations. Deprotonation of molecules to (M − H)⁻ is a simple path to a stable structure. High signal intensity of this ion is observed from a variety of treatments and is shown to be related to proximity to the metal interface. Cationization to form (M + Ag)⁺ and (M − H + 2Ag)⁺ is very sensitive to the concentration of silver ions near the surface. The coverage of multiple layers attenuates detection of these ions. The sampling depth of molecular ions is very different. For (M + H)⁺, ions are detected from the first three molecular layers. Cationized molecular ions only originate from the topmost molecular layer. Deprotonation to form the carboxylate anion (M − H)⁻ can occur from molecules within the first five layers. Ion beam dosage can severely restrict the detection of molecular ions, which are sensitive to orientation and surface chemistry.