Characterisation of laser-ablated craters of different painting materials and evaluation with modified LA-APCI-MS system.

The cultural heritage materials (CH) are the most difficult to analyse as they are complex, aged multicomponent mixtures. For the analysis of the composition of organic substances, chromatographic and mass spectrometric (MS) techniques are preferred, but they usually require a small sample piece from the object (risking damaging the artefacts) and additional sample preparation. We have developed a novel flexible laser ablation (LA) coupled to the atmospheric pressure chemical ionisation-MS (LA-APCI-MS) method that could analyse solid organic material directly on the object under atmospheric conditions without removal of the sample piece and sample preparation with minimal visual damage to the surface. The main aim of this work was to investigate the performance of LA-APCI-MS by analysing the pulsed 355 nm Nd:YAG laser's impact on hand-made painting materials (oil and acrylic paints and varnish) by characterising the area and volume of laser-ablated craters (measured with an optical microscope and 3D profilometer) obtained with different laser energies and the number of pulses at 90°, 70°, and 45° incidence angles and evaluating the corresponding MS signals obtained with APCI-MS. The areas of the craters of all the samples obtained with different laser energies and a number of laser pulses at 90°, 70°, and 45° incidence angles were between 4.1·10 to 4.2·10 μm and volumes 6.9·10 to 4.3·10 μm. From a cultural heritage perspective, these craters were visually barely noticeable. For analysed materials, the optimal laser pulse fluence (energy density) for achieving maximum MS signals ranged from 1.8 to 2.7 J/cm.