Document examination is a forensics field focused on the analysis of a disputed document. One way to analyze a document is to focus on the composition of the ink used or the materials from which documents are produced. Nowadays, three major classes of inks are studied: pen inks, inkjet printer inks, and toner. If the analysis of pen inks is already well developed in the literature and in the forensic lab, the analysis of inkjet printer inks is reported only to a limited degree and is principally focused on color inks. Fraudulent-minded people use more and more inkjet printer technology to produce falsified documents. So, it is a necessity to develop an analytical method for inkjet-printed document examination. Due to the particular chemical composition of these inks, different or more complicated analytical methods could and must be reviewed.
Four analytical methods are evaluated in this thesis as possible tools for the analysis of inkjet-printed documents in terms of discriminating power, chemical information quality, and nondestructive capability: Raman spectroscopy and mass spectrometry coupled with Laser Desorption Ionization (LDI), Matrix Assisted Laser Desorption Ionization (MALDI), and Desorption ElectroSpray Ionization (DESI).
The results obtained from two color inkjet printer cartridges (either fresh or printed) show that Raman spectroscopy and MALDI (9-aminoacridin as matrix) are the two most powerful methods for the analysis of color inkjet inks. Both methods have the advantage to perform analysis of inks directly on paper. The complementarity of the Raman spectroscopy and the MALDI-MS techniques is demonstrated based on the analysis of 10 different inkjet cartridges. The Raman method is a nondestructive approach while MALDI mass spectrometry has a better discrimination power and can also lead to information about some ink additives and paper composition. If it is necessary to determine the spatial distribution of one compound, then molecular imaging can be performed using Raman and (MA)LDI mass spectrometry. These techniques have been used successfully on a banknote or a stamp.
Black inks are also under investigation in this work. The discrimination of black inkjet-printed documents is more difficult because of the common use of carbon black as the principal black pigment. LDI mass spectra recorded on black-printed inks give polyethylene glycol (PEG) peaks that are characteristics of a particular brand. Moreover, unknown characteristic peaks are detected in the low mass range of these spectra, giving additional information for discrimination of black inks. A software has been developed for a rapid and objective comparison of the low mass range of these positive mode LDI spectra.