Particle- and photon-induced mass spectrometry of proteins. A physicist's view

Abstract

In recent years, methods based on particle impact on solid and liquid surfaces have been exploited to extend mass spectrometry to high-molecular-weight organic compounds like biomolecules. An important aspect of these methods is that intact ions of whole large organic molecules can be produced. Ablation processes using UV-lasers have also proved to be very powerful in this application. Here, emphasis is laid on description of the ejection process in plasma-desorption mass spectrometry (PDMS) but slow (keV) primary particles, as used in organic secondary-ion mass spectrometry (SIMS) and fast atom bombardment (FAB), and photons, as in matrix-assisted UV-laser desorption, are also discussed. In the case of fast ion impact, i.e., electronic sputtering, the ejection process is now fairly well understood. Explanation of the experimental data has lead to the discovery of a new sputtering mechanism, pressure-pulse sputtering. The experimental results and molecular dynamics simulations which prompted the development of an analytical model are reviewed. The physical limits to the size of ejected molecules in PDMS and organic SIMS are discussed in the light of recent results from matrix-assisted UV-laser desorption of proteins and finally the ejection process in matrix-assisted laser desorption is discussed in terms of the pressure-pulse model.