Elemental analysis of multilayer samples by means of combined laser ablation and x-ray fluorescence spectrometry

Abstract

Even though absolutely noninvasive techniques are ideally required for the analysis of valuable works of art, in some cases, the historical study or the restoration would benefit from deep investigations that require some kind of alterations on the sample. This is the case when a stratigraphic analysis of a paint has to be carried out to determine pigments and ligants used by the painter. To perform this task, a fully automated device working in situ without physical contact with the sample is certainly the best choice in terms of minimal damage, ease of operation, and quickness of analysis. Such a device could result from the combination of the state of the art portable x-ray fluorescence (XRF) analyzer with a digging tool capable of micrometric control of positioning and depth of operation. In this article, we propose a new method for the determination of chemical element distribution in multilayer samples, based on the use of an alternate sequence of laser photoablations and XRF analysis. The laser source used in this work is a flash lamp pumped Er:YAG laser (2.94 μm), assembled in our laboratories. It operates at a repetition rate of 8 Hz in a Q-switching regime and generates an average power of 60 mW. The XRF spectrometer is a portable instrument based on a Peltier-cooled silicon drift detector (operated at about −10 °C) which is characterized by an energy resolution of about 155 eV full width at half maximum at 6 keV. An x-ray tube with a tungsten anode is used as excitation source. In this article, the measurement technique is described and the first experimental results obtained on samples composed by layers of different thickness and composition are presented.