The acquisition of images in which the brightness of each pixel re¯ects the abundance of speci®c mass selected ions and thereby the local surface concentration, is a rather unique feature in molecular MS. It allows direct visualization of phases with distinct chemical composition in, e.g., polymer blends and minerals, the coverage of the surface by speci®c compounds, anti-corrosion coatings and paint layers etc. It also permits fast survey analysis of a large number of individual micro-objects, e.g., micrometer size silver halogenide crystals from the photographic industry. To create an imaging technique, it is not suf®cient to achieve an adequate lateral resolution, but the mass spectral information must be recorded from each point on the sample without operator's intervention. For instance, LMMS yields chemical information on a 1 mm spot, but the critical dependence of the signal intensities on the exact focusing of the laser beam prevents ``unattended'' operation except in special cases (Van Vaeck et al., 1994a and 1994b).
Nevertheless, S-SIMS imaging is not at all obvious. Section VI.C of part I outlined how the lateral resolution, number of molecules available in each surface spot, the ion dose and ion yield together with the instrumental transmission de®ne the limits of static imaging. Mapping of especially organic molecules is often beyond the capabilities of modern instrumentation. In contrast, elemental ions can be imaged at a lateral resolution of 100 nm. The high transmission and inherent panoramic recording of TOF analyzers make these instruments preferable for imaging. Within a single analysis run, full information on all m/z is available so that examination of the selected ion images can be done after analysis. A full mass spectrum for each pixel creates massive data ®les, of which the handling can be cumbersome so that data reduction during analysis is often preferred. In quad S-SIMS, the scanning mass analyzer points to the fact that the ions to be imaged must be de®ned before analysis, considering the minimum number of counts per m/z, the analysis time, the lateral resolution and the sample area to be covered.