Degrees, identity cards and wills: “These are all documents that are likely to attract the attention of forgers. The falsification of security documents or personal identity papers is a tool which can enable terrorists to mobilize, facilitate drugs and arms trafficking and lead to tax fraud etc.” We are reminded of this fact by Laetitia Heudt in the introduction to her doctoral thesis in chemistry which she has just defended at the University of Liege, under the double guidance of Professor Edwin De Pauw (Mass Spectrometry Laboratory) and Professor Bernard Gilbert (Laboratory of Analytical Chemistry and Electrochemistry).

Started in 2005, this PhD research consists in “Molecular analysis of printing inks for forensic purposes”. It is a comparative study of methods for analyzing 'inkjet inks' (black and colored) and aims to help investigators to detect possible falsification of documents. This work has proved to be very useful because Laetitia Heudt has just defended her thesis, under research contract with the National Institute of Criminalistics and Criminology (INCC). Her remit is to develop expertise related to 'Inks'. 

“Ink, which is homogenous in appearance, is in fact a complex mixture made up of a conducting solvent, the ‘vehicle’, dyestuffs (pigments and dyes) and various additives whose function is always specific: promoting flow and adherence, reducing viscosity and ensuring stability of the dyes with regard to oxidation, etc. All the constituents play a role in the properties of the ink and determine its color, its intensity, its brilliance and resistance. The chemistry of a particular ink is an important element for the manufacturers but also for scientists from a completely different area: those document experts who, in a legal context, examine suspect documents with a view to gathering the maximum amount of information. The results can make it possible, for example, to link a threatening letter to the printer of a suspect or to expose a falsified or altered document”, explains Laetitia Heudt. The chemistry of ink is so particular that she is working on determining the methods used to analyze it. Although experts have always focused on the analysis of hand-written documents by means of the thin-layer chromatography technique, that is to say, a method for separating dyes according to their characteristics after the ink has been extracted from the paper, advances in printer technology during the last few decades has complicated their work.

“For a long time, falsified documents were typically hand–written or typed. The analytical sequences and procedures for analyzing such documents are well-established and are routinely used in a large number of forensics laboratories throughout the world. The same does not apply to documents that are printed by inkjet. Indeed, this type of printing is today very popular with forgers”. The inkjet is a printing procedure which consists of projecting ink droplets from a small opening in the ink reservoir to pre-determined positions on a surface in order to create an image. “Available to everyone, high-performance and accessible, inkjet printers make it possible to produce false documents that are closer to the original. However, analysis of documents printed by inkjet which is of great interest in the area of forensics - to identify false documents by comparing the ink of the legal suppliers with that of the documents under scrutiny, or to highlight the common origin of false documents coming from different sources or even to help to connect the printer of a suspect with a specific document - is paradoxically very seldom described in literature”.

This leads to another important observation: the need, where possible, for a systematic use of methods for analyzing paper samples (and therefore the inks they are impregnated with) that are as non-destructive as possible. “ Most methods used up to the present time are in fact quite destructive : it is neccessary to cut into the paper in order to examine it which runs counter to one of the requirements of scientific crime detection: the preservation of the integrity of the sample. A fast, differential and non-destructive technique that makes it possible to characterize these ‘inkjet’ inks that are in contact with the paper would be useful”, Laetita Heudt explains.

(1) Also known as varnish, the vehicle represents around 70 % of the weight of the ink. Its main function is to transport dyes and/or pigments from the reservoir to the surface.

(2) Collectively known as dyestuff, the main purpose of the dyes and pigments is to give dyeing properties to the ink. As well as the particular color of the ink (the dominant dye), the pigments and colorings determine the depth and freshness of the color as well as the degree of opacity (or transparency) of the ink. The resistance of the ink to light, heat and chemical agents depends also on the nature of the dyestuff. The dyes are soluble in the medium they are used in, while the pigments, which are almost totally insoluble, are maintained in colloidal suspension in the fluid phase of the ink.

(3) The underlying principle of inkjet technology is the digital control of the ejection of droplets of ink from a print head to the substrate. The physical procedures and associated technological designs are numerous. It is, however, possible to classify them according to two very different types of procedures: The Continuous Ink Jet or CIJ, principally used in industry, and the Drop-On-Demand or DOD, developed for office applications. Most ‘inkjet’ inks are based on dyes and these do not block up the nozzles.