First comparisons

By way of preliminary examination, it was necessary at first to compare different types of ‘inkjet’ inks in the liquid state, that is to say without being on their paper surface and therefore as they are found in the cartridge: two series of three ink types – the three primary colors — which are very different from each other are studied (Epson and Hewlett-Packard) one based on pigments and the other on dyes. “The aim of the approach was to demonstrate the feasibility of Raman, DESI and LDI techniques and to assess the capacity of these techniques to supply rapid, useful, varied and reliable information about the chemical composition of all ‘inkjet’ inks”. While the first analyses confirm the relevance of analysis by Raman Spectroscopy for the two types of ink and the analysis by Mass Spectrometry LDI in the case of pigment-based ink, they reveal, contrary to all expectation, the necessity to use the MALDI method to analyze dye-based inks. “It was an important part of my research: understanding why a matrix was neccessary even though the literature did not mention this in the case of, for example, the analysis of the ink of a ball-point pen. The thesis opens up some avenues in this regard: the physicochemical nature of the inks used by printers and therefore their viscosity, their capacity for fixation onto paper etc. is fundamentally different from the inks used for ball-point pens. These are all particular properties which probably require the use of a matrix in the context of analysis by laser desorption/ionization in mass spectrometry”. Similarly, the researcher observes, this study of inks has made it possible to gather a lot of very precious information about each of them. For example, the presence of synthetic polymers which constitute their identity card so to speak, and are characteristics of a brand of ink. One kind of polymer in the Cyan-Magenta-yellow inks of Hewlett-Packard, another kind of polymer in Epson inks, etc. “A first interesting area of study for distinguishing two types of ink”.

On paper

It remained to apply these procedures to paper ink, during a second series of experiments carried out on sheets printed by Epson and HP printers. One of the main questions posed at this stage of research is that of the influence of paper on the ink spectra. “This problem had, up to the present day, barely been dealt with in literature on the subject”. Various papers are used ‘randomly’: different brands and colors  and various grammages. “We show that this type of paper has no effect on the results of analyses by Raman Spectroscopy, unless the fluorescence of the paper is too high and masks the Raman signal of the ink. Fluorescence is an extraneous luminous emission and is a much more intense phenomenon than Raman. It could be explained by the presence in the paper of compounds known as optical brighteners. These absorb the light at the ultra-violet level, and then reemit this energy into the visible”, at wavelengths varying between blue-violet and blue-green. The function of these optical brighteners is to accentuate the whiteness of the sheet of paper. “This technique is frequent, particularly in the case of extra-white paper. However, when the paper’s signal is too powerful, it masks the Raman spectrum, especially when the wavelength of the laser used to irradiate the sample is located close to the ultra-violet, that is, in the wavelengths absorbed by the optical brighteners”. The MALDI analyses reveal that the properties of the paper are not problematic. “We observe peaks that are characteristic of the paper that are more or less visible according to the printers, some of them use more ink than others, but they do not present a problem on condition that they have been identified beforehand, with an analysis of the white sheet itself. This is interesting: imagine a search of a suspect’s house; you remove the printer and blank sheets taken from the site. It is then possible to compare not only the inks, but also the papers used, as additional exhibit". However, the MALDI analysis has been shown to be particularly rich in data relative to dyes, pigments and additives. Analysis of seven Hewlett Packard yellow-cyan-magenta models show peaks with regard to additives. This makes for better characterization of the sample, and therefore a better differentiation: two inks could very well be identified during analysis of identical dyes, and yet be radically different from each other due to their additives, the latter being the more specific to each brand.

Illustration: A. Hudd, Chapter 1 : Inkjet Printing Technologies, in: S. Magdassi, The chemistry of inkjet inks, World Scientific Publishing, Israel, 2009.