Secondary Ion Mass Spectrometry of Monomolecular Layers of Fatty Acids Prepared by Langmuir-Blodgett Techniques

A novel approach to the examination of static secondary ion mass spectrometry (SIMS) sampling and ionization mechanisms is demonstrated by utilizing Langmuir-Blodgett techniques for producing monomolecular layers. This method offers advantages over those reported previously in that a model system can be engineered and the physical state and orientation of large molecules on a polycrystalline metal surface can be precisely controlled. A series of five (saturated and unsaturated) fatty acid monolayers was prepared on cleaned silver surfaces, and positive and negative static SIMS results are reported from argon ion bombardment. Results showed that molecular and quasi-molecular ions are produced and can be detected from a single monolayer. The degree of cationization by silver from the substrate is dependent on the extent of surface oxidation and is only observed for unsaturated fatty acids. The degree of unsaturation was also readily detected from both fragmentation and molecular ion production. Analysis of LB multilayers of stearic acid on silver shows that even with an ordered deposition of 375 A, the silver ion signal is not totally diminished, due to the high vapor pressures of the substrate. These results taken together support a cationization process of attachment in the selvage vapor phase for the oriented molecules.