''One of the problems in forensic work is examination of authenticity of different documents such as contracts, testaments, etc. In such documents there are often intersections between printed pages and signatures where determination of deposition order between different writing tools such as toners and pens can help to spot the forgeries or alterations done at the documents.
For that purpose, many scientific methods are conventionally employed for the forensic work, among them various microscopies and spectroscopies, video analysis tools, digital imaging etc.,'' explains Dr Ivančica Bogdanović Radović, the corresponding author and HORIZON2020 project leader.
In the article recently published in Analytical Chemistry, scientists from the Ruđer Bošković Institute in Zagreb, Croatia (RBI) together with the scientists from the University of Surrey, UK have explored the potential of two accelerator based Ion Beam Analysis (IBA) techniques Secondary Ion Mass Spectrometry with MeV Ions (MeV-SIMS) and Particle Induced X-Ray Emission (PIXE) to solve some of the difficult cases where standard forensic methodology fails. Six different cases involving one ballpoint pen, three laser printers and two inkjet printers were studied.
MeV-SIMS detects secondary molecular ions sputtered from the uppermost surface layers after bombardment with a heavy energetic ion beam, hence it is an excellent choice for such a study. As the molecules from the surface are detected, the trace made last will yield a continuous line in a 2D MeV-SIMS image and will not be interrupted.
Conversely, the trace made first will show a break in the line, the reason being the molecules from underneath are not sputtered and detected. By using only MeV-SIMS, they were able to determine deposition order for cases involving ballpoint pen and laser printer toner (2 out of 6 combinations).
However, all the cases involving inkjet ink and other writing tool (laser printer or ballpoint pen) shown in 2D MeV-SIMS images break in the inkjet line, leading to the conclusion that inkjet ink was deposited first in all cases, which was not correct. Inkjet ink tends to be mainly composed of water, meaning it penetrates deep into the paper and any remaining ink on the surface evaporates.
Because MeV-SIMS is a surface technique, this leads to a hypothesis that the inkjet ink was not adhering at the intersection at all if some other writing tool was already deposited there.
To investigate this further, they used PIXE, which provides information about elemental composition from deeper layers in the sample through the emitted X-rays. For PIXE, 2 MeV proton microbeam was used. PIXE can resolve the behavior of the inkjet ink if it is characterized by unique elements (via characteristic X-rays) not present in the paper and another writing tool.
In contrast to the MeV-SIMS, in the case when inkjet ink is deposited first there should be no break in the 2D X-ray map of inkjet ink while inkjet ink is adhering well to the paper. If the laser printer toner or ballpoint pen trace is already deposited on the paper, and the inkjet ink is deposited second, there will be a break in the elemental maps corresponding to the inkjet ink, where it does not adhere to other already deposited writing tool.
By using combination of MeV-SIMS and PIXE, authors managed additionally to predict the correct deposition order in 2 out of 4 problematic cases. In the other 2, the result was inconclusive. The only reason why PIXE was not able to resolve 2 remining cases was due to a lack of a characteristic element only present in inkjet and not in paper or other writing tool.
The results of the two methods combined contributed to understanding the behaviour of inkjet ink at the intersection and helped resolve the issue in most cases.
"It is certain that both methods have potential in cases of suspicious documents, especially where standard forensic methods fail, but their full potential in forensics has yet to be explored and we need to do further analysis using more inkjet inks in combination with different writing tools in order to establish the wider benefits of MeV-SIMS and PIXE," concludes Dr Ivancica Bogdanovic Radovic.
Katherine Moore, a student at the University of Surrey, also participated in this research while spending a year in the Laboratory for Ion Beam Interactions within the ERASMUS + project.
Part of the research was also supported by the HORIZON2020 project 'RADIATE' worth HRK 4,394,837. Laboratory researchers are also participating in one COST action (CA16101) as well as the IAEA project on forensics. Furthermore, the team has established close cooperation with the Croatian Centre for Forensic Testing, Research and Expertise "Ivan Vučetić" on the same topics.