In secondary ion mass spectrometry (SIMS), a sample is bombarded with a primary beam of ions. Secondary ions are emitted from the sample surface and these are mass analysed to produce mass spectra and chemical maps (see Note 1). The technique provides detailed surface chemical information; elements, chemical groups, molecules, polymer groups.

Time-of-Flight SIMS (ToFSIMS) involves the use of a primary ion beam which is pulsed to produce packets of primary ions. Each primary ion packet impacts the sample surface and generates a packet of secondary ions at a well-defined point in time. These secondary ions are then accelerated to the same kinetic energy by the use of an extraction field. For the same kinetic energy, secondary ions of different mass will have different velocities and, therefore, different flight times through the mass analyser to the time-sensitive detector.

ToFSIMS analysis can be carried out routinely under static conditions, using such low primary ion doses that the sample is effectively undamaged by the analysis. It is also highly surface sensitive where, in the static regime. The sampling depth is typically 1-2nm. ToFSIMS is analytically very sensitive (often trace detection levels) but it is not quantitative. Its chemical specificity, however, may be exploited in a semi-quantitative manner where variations in the surface concentrations of different chemical species may be followed by monitoring changes in the relative intensities of their diagnostic ToFSIMS signals.

High resolution ToFSIMS ‘resolves’ the problem of overlapping peaks of secondary ions with the same nominal mass and promotes the technique into the level of accurate mass measurement which is often necessary for the identification of secondary ion signals. High resolution is also important for improved analytical sensitivity since the practical detection limits of some species are restricted by mass overlap.

• Region-of-Interest (ROI) analysis: For defined regions (any shape and size) within the analysed area, it is possible to sum the mass spectra at all of the pixel points within each defined region i.e. total area spectrum, small area spectrum (e.g. feature of interest).
• Imaging analysis: For the analysed area, chemical maps can be created with high spatial resolution for any of the spectral peaks within the mass range. For weak signals, where there is not enough individual peak intensity to produce meaningful images, it is possible to add related peak intensities thereby improving image statistics.

The retrospective feature is particularly useful since it allows data to be analysed/re-analysed at a later date and ROI spectra from different regions of a sample can be produced or different chemical maps can be created, without the need for re-analysing the sample in real time.

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[Article: Organic and molecular imaging by ToFSIMS—at the cutting edge].