Secondary Ion Mass Spectrometry of Automotive Coating Systems

By Steven J. Simko,, Steven L. Kaberline , Larry P. Haack

Secondary ion mass spectrometry (SIMS) is a specialized form of surface analysis that is used primarily to deter­mine the elemental and chemical struc­ture of solid materials.1•2 It is based on mass spectrometry which is a family of techniques where elements, molecules, and molecular fragments are sorted based on their atomic/molecular weights. The technique employs a source to excite the material of interest into the gas phase with some portion of it as ionized spe­cies. The ions then pass through a mass filter which sorts them based on their mass-to-charge ratio. The resulting mass spectrum gives a representation of the elemental and/ or molecular composi­tion of the material of interest.1-3 Often, the list of molecular fragments in the spectrum can be pieced together to obtain a detailed understanding of the chemical structure of the materia 1.1•3•4.

In SIMS, the excitation source con­sists of a vacuum chamber and an ion gun. Placing the sample in a vacuum keeps the surface clean and allows for easy transport of ions through the instru­ment. The ion gun produces a stream of ions, called primary ions, that illuminate the sample. As the primary ions reach the sample surface, they transfer energy to the atoms in the solid through elastic collisions in a process called a “collision cascade.”5 A schematic diagram of this process is shown in Figure 1. Enough en­ergy is transferred in the collision cascade to break chemical bonds and to displace atoms from their positions in the solid lattice. A small fraction of these atoms and molecular fragments obtain enough energy to escape the solid and enter the gas phase. Some of the ejected particles are electrically charged (ionized). These particles, called secondary ions, can be passed through a mass spectrometer to generate the SIMS spectrum of the solid.