Abstract
Determining the sequence of inks in a questioned document is important in forensic science. Conventional and micro beam-based ion beam analysis using Rutherford backscattering spectrometry (RBS) and particle-induced X-ray emission were employed to study the depth distribution of chemical elements in plain paper and inks/toner deposited by different pens as well as inkjet and laser printers. Composition depth profiling with high lateral resolution was performed with focus on areas where two different coloring agents overlapped. We identify under which conditions the sequence of inks deposited can be reconstructed, analyzing the continuity of characteristic contributions to the obtained signals, with a focus on the depth-resolved RBS data. The order of deposition was correctly determined for combinations of two different laser printers and in certain cases for pens. Results indicate a potential for analysis, depending on the composition of staining agent, that is, in particular if heavy species are present in sufficiently high concentration. In such cases, also characters obscured or modified by an agent of different composition can be revealed. Changing the probing depth by modifying the beam energy could yield additional information.
Highlights
Determining the deposition sequence of inks, that is, if a questioned document is a document with overlapping lines from, for example, signatures and print, or possibly multiple printing instances, finding the first printed ink can be useful to identify forged documents
Atomic force microscopy has been used to measure height profiles of inks deposited on paper, and the sequence of inks has been predicted by the height profiles at the intersection [8]
Many other methods have been reported to be capable to determine the chronological deposition order inks on paper, such as using adhesive taps [9], FTIR spectroscopy [10], laser desorption ionization mass spectrometry (LDI-MS) [11], and scanning electron microscope [12]
Summary
Determining the deposition sequence of inks, that is, if a questioned document is a document with overlapping lines from, for example, signatures and print, or possibly multiple printing instances, finding the first printed ink can be useful to identify forged documents. Optical methods using light microscopes have been used for identifying the sequence of inks in a document. Raman spectroscopy and Raman imaging techniques have been used for discriminating inks and finding the sequence of inks in a questioned document [3–5]. Mass spectrometry imaging using easy ambient sonic-spray ionization has been used to find the sequence of inks at intersection of stamp and pen ink [7]. Many other methods have been reported to be capable to determine the chronological deposition order inks on paper, such as using adhesive taps [9], FTIR spectroscopy [10], laser desorption ionization mass spectrometry (LDI-MS) [11], and scanning electron microscope [12]
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