Mass Spectrometric Investigation of the aging processes of ballpoint ink for the examination of questioned documents

摘要

In the forensic examination of documents the legitimacy of the age of an ink entry is often an essential question. Since its beginnings, the forensic questioned document field is concerned with the dating of inks. Inks aging processes follow complex paths and disagreements about the feasibility of actual methods raised worldwide among the scientific community. This controversy has been the starting point of the present work. Ball-point ink contains three main classes of compounds: solvents (50%), dyes (25%) and resins (25%). After the ink is deposited on paper, its composition begins to change qualitatively and quantitatively. While the dyes fade mainly through photochemistry processes, the aging behaviour of solvents are characterised by diffusion and evaporation processes. The aims were to actually study the aging processes of dyes and solvents found in ballpoint pens by modern mass spectrometric method in order to evaluate the potentiality and limitations of dynamic dating methods.Two major methods, Laser Desorption Ionisation - Mass Spectrometry (LDI-MS) for dyes and Gas Chromatography / Mass Spectrometry (GC/MS) for solvents were developed for this purpose and validated. The aging processes of reference substances were studied under defined storage conditions (light, heat, humidity). The same processes were investigated for the ballpoint dyes and solvents on the paper matrix. Therefore ballpoint ink strokes were exposed during several months to daylight and compared to strokes stored in the dark. On the one hand, small pieces of paper were cut, glued to a metallic sample holder with a carbon tape, and introduced into the ionisation chamber for direct analysis by LDI-MS. On the other hand, ink strokes were cut from the paper and extracted for subsequent GC/MS analysis. Moreover scanning microprobe LDI-MS was tested to microscopically visualize concentration profiles of ink dyes on the paper surface, i.e. it provides images of the chemical compound distribution. The determination of the writing sequence of crossing strokes was evaluated for questioned document expertises.Many factors influencing the fading of dye and the drying of solvents over time have been identified, to some extent evaluated. They can be divided in two mains categories: ballpoint ink composition and storage conditions of the document. Precise knowledge of these variables and their influence are necessary to deliver a scientific determination of the age of an ink entry. This work intend to give forensic scientists a better understanding of the complexity of the processes involved in the aging of a ballpoint ink matrix on a porous substrate such as paper. An overview of the ink dating issues is needed to comprehend how a dating method should be developed and validated in order to provide useful and accurate results. This has not been documented so far. Guidelines are proposed to compensate the limitations and set up valid ink dating methods. Any established dating method based on the fading of dyes or the drying of solvent has to be checked for reliability through indications of reproducibility, valid answers through blind testing and measurements errors lower than predictable variations.