Chemical Surface Characterization with ToF-SIMS Analysis
Mass spectrometry is a well-known chemical analysis technique. Analytical chemists use it to determine the structure of molecules and the chemical composition of mixtures. Since mass spectrometry works in a vacuum chamber, typically gases and vapors are analyzed with mass spectrometry. ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) is an exception. It is a technique for the chemical analysis of the outside of a material, its surface.
Like other mass spectrometry, a ToF-SIMS measurement is carried out in a vacuum chamber. In it, an ion (an electrically charged atom or molecule) is aimed at the surface of the sample. The energy of this primary ion is so high that it flies straight into the material. For high energy particles, materials seem very “open and empty”. When the particle hits an atom inside the material, this atom will move, but with a much lower energy. For this particle, the material doesn’t seem as open and it will start colliding with the atoms and molecules around it and an avalanche of atom-atom collisions arises that is spreading through a part of the material.
When this avalanche reaches the surface, there is nothing to collide with anymore, and the atoms, or even molecules, are thrown into the vacuum. Most of these secondary atoms and molecules are neutral, but some are ions: they have a charge. With electrical fields, these secondary ions can be accelerated into the Time-of-Flight analyzer. There, they are simply flying, without any force from the outside. Light ions are flying fast and heavy ions are flying slowly. At the end of the Time-of-Flight analyzer there is a detector. The light, fast ions will arrive first and the heavy, slow ions will come later. This means that the mass of the secondary ions can be determined by measuring the time between the firing of the primary ion and the arrival of the individual secondary ions at the detector.
The firing of ions is repeated to get stronger signals. A computer keeps track of the number of ions of the different masses are detected and the result is a mass spectrum of the surface of the sample. When the beam of primary ions is scanned over the surface, and the position of the beam is stored together with the signals for the secondary ions, it is even possible to create secondary ion images of the location of the various compounds that are detected.
ToF-SIMS analysis is a valuable technique for the surface analyst, since it gives the chemical structure of the outermost few layers of atoms of a material. With it, the surface chemist can tell why a coating failed to adhere to a material, what the surface of a catalyst is made of or what that ugly stain is and where it is coming from.