Depth-resolved analysis of historical painting model samples by means of laser-induced breakdown spectroscopy and handheld X-ray fluorescence

Paintings represent composed materials arranged in successive layers. Development of a suitable method for a depth-profiling analysis is essential for acquiring the information on the stratigraphy as well as on the chemical composition of individual layers, revealing the pigments which had been used. In this study, a depth-resolved analysis of multi-layered model samples of historical easel paintings was performed by means of laser-induced breakdown spectroscopy (LIES) in combination with non-invasive X-ray fluorescence (XRF). The LIBS analysis was carried out using modified laser ablation system, UP-266 MACRO, equipped with a Czemy-Turner spectrometer. The XRF analysis was performed by handheld spectrometer, Delta Premium. In LIBS experiments, a set of six spots was examined with 5, 10, 15, 20, 25 and 30 pulses respectively, for each of the studied paint samples. Digital and 3D optical microscopy was employed to measure individual layer thickness and to obtain the information on average ablation rates of each sample. The chemical composition of the model samples with each layer partly uncovered was known, and this enabled to directly compare the results obtained by LIBS depth profiling with a fast analysis carried out with the handheld XRF spectrometer. The LIBS depth profiling proved to be a suitable method to distinguish layers of a different material composition and estimate their thickness. The combined use of LIBS and XRF analyses offered essential complementary information on the elemental composition of analysed multi-layered samples.