Electronic Sputtering and Its Application to SIMS Analysis of Biomolecular Monolayers

Fast ions, i.e. ions with a velocity larger than the Bohr velocity, interact primarily with the electrons in a medium. In a good conductor like a metal, no late effects of the interaction will be found. However if the medium is an insulator, where the relaxation of the deposited electronic energy is slow, fast ion impact will lead to atomic displacement effects, i.e. local bulk damage of atomic order and surface erosion. In 1974 MACFARLANE and coworkers found that fast ions like fission fragments can induce desorption of intact ions of fairly large fragile and thermally labile molecules like small biomolecules [1]. They called their method Plasma Desorption Mass Spectrometry (PDMS). During the last few years that method has been used to study larger organic molecular ions than ever in the history of mass spectrometry [2,3]. About seven years ago it was established that PDMS is based on electronic sputtering from a layer of biomolecules [4].