TOF-SIMS mass spectrometry imaging for the analysis of ancient paintings

Mass spectrometry imaging involves recording a two-dimensional map of a surface, for which each point contains a mass spectrum. These mass spectra, which represent the intensity of the ions detected as a function of their mass-to-charge ratio, are themselves generated by an analysis technique that can produce ions characteristic of the surface with very high spatial accuracy, down to the micrometer. In this case, the analysis is carried out using the SIMS (Secondary Ion Mass Spectrometry) method, in which a beam of so-called "primary" ions, consisting of bismuth aggregate ions accelerated to a kinetic energy of 25 keV, focused and chopped into short pulses of less than one nanosecond duration, strikes the sample surface point after point. Secondary ions, characteristic of the surface being analyzed, are emitted. These secondary ions are then accelerated to a kinetic energy of 2 keV, for mass analysis using a Time-of-Flight (TOF) analyzer. With this type of mass analyzer, precise measurement of the time of flight of secondary ions in a field-free tube, between the instant of their production and their detection, gives access to their mass-to-charge ratio, the square root of which is proportional to the time of flight. This method of surface analysis, which is then called "TOF-SIMS", gives access, provided the dose of ions irradiating the surface is limited, to the organic molecular composition as well as the mineral composition. In some cases, a second ion beam, consisting of massive argon aggregates accelerated to 10-20 keV, can be used to sputter the surface monolayer by monolayer, and thus carry out an analysis of the sample to a depth of several microns. This is known as "dual beam depth profiling". TOF-SIMS mass spectrometry imaging, which will be described in this article, is a method particularly well suited to the analysis of cultural heritage samples, such as antique paintings, which are generally only a few tenths of a millimetre in size. Spatial resolution of up to 400 nm, with the option of surface or volume analysis, is ideally suited to very small samples. The technique gives access to both mineral and molecular composition in a single analysis, providing invaluable information on both pigments and oils and binders. The description of the instrument and the analysis method will then be followed by three examples of analyses of samples taken from old paintings, namely paintings by Rembrandt van Rijn (1606-1669), Matthias Grünewald (1470-1528) and Nicolas Poussin (1594-1665).