Laser induced breakdown spectroscopy as a tool for in-situ cultural heritage applications: from portable to handheld instrumentations

Over the last years mobile Laser Induced Breakdown Spectroscopy (LIBS) instrumentations have been developed and improved for in-situ cultural heritage investigations. LIBS offers many competitive features for the analysis of cultural heritage objects such as simultaneous multi-element analysis under atmospheric conditions in a very short time, micro-invasiveness, simplicity of instrumentation, almost no sample preparation, and no restriction of sample shape and size. Most artifacts are unique and precious, thus non-invasive techniques are required for analytical purposes. Further, some of them cannot be transferred to the analytical laboratory. This implies the need to dispose of mobile instruments for in-situ analysis. As already well known, LIBS is a plasma-based atomic emission spectroscopy technique that permits rapid qualitative and quantitative multi-elemental analysis. It relies on ablating and evaporating a material by focusing the radiation from a pulsed laser onto the surface of the target so forming a hot plasma in which atoms and ions are excited and emit characteristic lines that can be analyzed spectroscopically. Thus, elemental information can be obtained via specific atomic or ionic transitions and associated emanating photons[1] . LIBS spectra can provide quantitative information on the element concentration in the sample also, if a predictive model, i.e. a calibration curve, is constructed by plotting the peak intensities versus the known concentrations of standard materials. A conventional LIBS equipment consists of a laser, a spectrometer, a number of lenses and optical fibers, a data acquisition system, and a control and synchronization system between the laser and the spectrometer. Mobile LIBS instruments have shown a promising performance in micro-destructive, in-situ, multilayer diagnostic analysis and in-depth elemental profiling of encrustations and impurities even for highly inhomogeneous layered crusts prior to conservation treatments. Repetitive laser shots at the same spot can ablate a thin weathered (~100 mm thick) layer of the material so recording changes in composition as the underlying bulk material is approached. Handheld LIBS instrumentations recently introduced in the cultural heritage field[2] appear to be very promising for in-situ measurements on outdoor monuments, because of their capabilities (spectral ranges from ultraviolet to near-infrared, argon purging, rastering and portability) that rival traditional bench-top instruments.