An ion microprobe is an instrument that uses a finely focused primary ion beam to erode, or ‘sputter’ a solid sample and to collect secondary ions ejected during that process into a mass spectrometer generating a spatially resolved mass spectrum. The underlying technique, Secondary Ion Mass Spectrometry (SIMS), has become a standard tool for the in-situ study of trace-element concentrations and isotope ratios in the fields of geochemistry, geochronology, biogeochemistry and cosmochemistry. an overview of the most recent developments in SIMS is given by Chabala et al. (1995), Ireland (1995), Mac Rae (1995), Becker (2005), Betti (2005), Deloule & Wiedenbeck (2005), Deloule (2006) and McPhail (2006). Secondary ion mass spectrometry offers parts per million (ppm) or better detection limits for almost all elements, imaging capabilities, periodic table coverage (h–U), and isotope analyses of major and trace elements. The following three examples illustrate the unique power of the SIMS technique in measuring and imaging isotope ratios and trace element distributions.

Firstly, the lateral distribution of elements of interest and isotope ratios can be measured. Figure 1 demonstrates the lateral resolution of SIMS imaging with the Cameca NanoSIMS 50. a spatial resolution of 50 nm is possible, even for biological samples. Scans of a cell culture were taken at appropriate mass number to recognize bacterial cells (CN⁻, major molecular ion image) on a nucleopore polycarbonate filter, to identify photosynthetic active cells by their incorporation of ¹³C-labelled bicarbonate (¹³C/¹²C ratio, isotope ratio image), and to recognize species with the help of a halogen marker (¹⁹F⁻, trace element ion image) that binds to the ribosome of the cell.