Surface and in-depth characterization of commercial papers by LIBS technique: Parameters and features for their classification, and complementary information from Raman spectroscopy

In examination of documents by LIBS technique, the paper material is usually probed by surface scanning and the averaged spectra are processed by chemometric methods. The scope of this work is to have insight into LIBS analysis of common writing/printing papers, considering both the laser depth profiling and surface measurements. The sample set consisted in 14 papers (copy, notebook and envelope papers), some of them made of recycled materials. Examination of the laser induced craters showed a preferential ablation of paper fillers while the cellulose fibres were displaced at surface and swallowed around the centre of laser spot. Beside material inclusions, found both on the paper surface and in depth, the not uniform ablation also contributes to large intensity fluctuations of the element's line intensities, which standard deviations differ from one sample to another. By studying correlations between the line intensities from different elements at paper surface and in depth, it was possible to distinguish some top coatings (e.g., kaolinite) from bulk fillers (e.g., containing alumina-silicates), to hypothesize the use of NaOH in the industrial processing, as well as to exclude presence of certain types of fillers or coatings. Raman spectroscopy was performed both on sample surfaces and inside the laser induced craters (bulk material), showing the differences among papers regarding relative contents of CaCO₃ filler, lignin, and cellulose. We tested different chemometric models based on the LIBS measured composition of paper bulk or surface, obtaining up to 100 % correct classification for six similar copy papers of Italian manufacture.