Since current methods used in document examinations are not suited to associating or discriminating between sources of paper and gel inks with a high degree of certainty, the current project employed nearly non-destructive, laser-based methods - using laser induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) - to improve the forensic comparisons of gel inks, ballpoint inks, and document papers based on similarities in elemental composition.
Some of the advantages of these laser-based methods include minimum sample consumption/destruction, high sensitivity, high selectivity, and excellent discrimination between samples from different origins. Figures of merit are reported, including limits of detection, precision, homogeneity at a micro-scale, and linear dynamic range. The variation of the elemental composition in paper was studied within a single sheet, between pages from the same ream, between papers produced by the same plant at different time intervals, and between 17 paper sources produced by 10 different plants. The results show that elemental analysis of paper by LIBS and LA-ICP-MS provides excellent discrimination (> 98 percent) between different sources. Batches manufactured at weekly and monthly intervals in the same mill were also differentiated. The ink of more than 200 black pens was analyzed to determine the variation in the chemical composition of the ink within a single pen, between pens from the same package, and between brands of gel inks and ballpoint inks. Homogeneity studies show smaller variation of elemental compositions within a single source than between different sources (i.e. brands and types). It was possible to discriminate between pen markings from different brands and between pen markings from the same brand but different model. Discrimination of ~ 96–99 percent was achieved for sets that otherwise would remain inseparable by conventional methods. The results show that elemental analysis, using either LA-ICP-MS or LIBS, provides an effective, practical, and robust technique for the discrimination of document paper and gel inks with minimum mass removal (9–15 μg) and minimum damage to the document's substrate. (publisher abstract modified)
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