Cell imaging by dynamic secondary ion mass spectrometry (SIMS): basic principles and biological applications

Abstract

The development of analytical microscopes based on various physical principles has allowed increasingly precise elementary analysis. Among these various methods, mass spectrometry imaging (ion microscopy) offers remarkable possibilities insofar as it can apply with extreme sensitivity to the quasi totality of the elements and their isotopes. Originally introduced in the early sixties by Castaing & Slodzian [1], this technique is based on the mass spectrometry of secondary ions (SIMS) extracted from the surface of a solid sample under the impact of an energetic beam of primary ions. In an ion microscope the sample is brought to a potential of several kV. Thus, secondary ions resulting from the impact of the primary beam, and bearing a charge of the same sign as the sample, can be readily extracted through a first electrostatic lens. This secondary beam is then focused and guided to the entrance of a double focusing magnetic sector (mass spectrometer) using several transfer lenses. Secondary ions are then sorted in energy in the electrostatic sector before undergoing specific deviation by the magnetic field according to their m/z ratio (mass/charge) while keeping the topological information of the origin of the emission. Ions with the same m/z ratio can be selected and directed on the exit slit of the mass spectrometer in a beam which could be guided and widened by a set of projection lenses either to a Faraday cylinder for total ion current measurement or to a visualization screen for imaging purpose. Numerous applications have emerged in such diverse fields as surface analysis, materials science, geochemistry and microelectronics, in which SIMS is now a central analytical tool. However, in spite of sporadic application of this technique in biology, over more than 30 years ago [2], ion microscopy has been for a long time considered only as a marginal method for solving problems in the life sciences, due mainly to poor lateral resolution (1-0.5 µm) and insufficient mass separation power. Many technological and conceptual improvements led to significant progress in both lateral resolving power, i.e. at the level of the image itself, and mass resolution, the parameter on which the precision of the analysis depends. The first breakthrough came with the use of a finely focused primary ion beam rastering across the sample surface [3], thus improving the "theoretical" side resolution to the size of the primary beam. Enhancement of the mass resolving power was achieved by using various mass analyzers (double focusing magnetic sector, quadrupole mass analyzer). However, with such tools, only one specific ion image can be acquired at a time. Because SIMS analysis is a destructive method, this major restriction was